WO2025164374A1 - Positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery - Google Patents

Abstract

This positive electrode active material contains a lithium transition metal composite oxide that contains a total amount of Ni and Mn that is 80 mol% or more with respect to the total molar amount of metal elements excluding Li. The lithium transition metal composite oxide has a single particle shape, has a volume-based median diameter of 0.5–5.0 μm, and a crystallite size of 370–1500 Å. A sulfur compound and a boron compound are present on the particle surface of the composite oxide. In an XPS spectrum of the positive electrode active material, in a region including the particle surface of the composite oxide and a depth of less than 100 nm from said surface, a peak derived from S is present in the range of 172–176 eV and a peak derived from B is present in the range of 194–198 eV or less.

Description

Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

This disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery using the positive electrode active material.

In non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, the positive electrode has a significant impact on battery performance, including input/output characteristics, capacity, cycle characteristics, and thermal stability, and therefore much research has been done on the positive electrode. For example, Patent Document 1 proposes a non-aqueous electrolyte secondary battery that contains a sodium salt and at least one of a sulfate and a sulfite, with the aim of improving high-temperature durability. Patent Document 1 describes fabricating a positive electrode by applying sodium sulfate, sodium sulfite, etc. to the surface of the positive electrode mixture layer.

JP 2018-60693 A

As the charging voltage of a battery increases, the resistance of the positive electrode increases, with the increase in resistance becoming particularly noticeable in the high state of charge (SOC) range. Therefore, in order to achieve a higher charging voltage, it is important to suppress this increase in resistance. In particular, to extend the life of a battery, it is important not only to reduce the initial resistance, but also to suppress the increase in resistance that accompanies charge/discharge cycles, which will also lead to improved capacity retention.

The positive electrode active material according to the present disclosure comprises a lithium transition metal composite oxide in which the total content of Ni and Mn is 80 mol% or more relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide having a single particle shape, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less, sulfur compounds and boron compounds are present on the particle surfaces of the lithium transition metal composite oxide, and a photoelectron spectrum obtained by X-ray photoelectron spectroscopy (XPS) measurement shows a peak derived from S in the binding energy range of 172 eV or more and 176 eV or less and a peak derived from B in the binding energy range of 194 eV or more and 198 eV or less at the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface.

The nonaqueous electrolyte secondary battery according to the present disclosure comprises a positive electrode containing the above-described positive electrode active material, a negative electrode, and a nonaqueous electrolyte.

The positive electrode active material disclosed herein makes it possible to realize a nonaqueous electrolyte secondary battery in which resistance increases during high-potential charging are suppressed.

1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; FIG. 1 is a cross-sectional view of a test cell prepared in an experimental example. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F9, showing the binding energy range in which a peak derived from B appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G3, showing the binding energy range in which a peak derived from B appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G8, showing the binding energy range in which a peak derived from B appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F9, showing the binding energy range in which a peak derived from S appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F4, showing the binding energy range in which a peak derived from S appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G8, showing the binding energy range in which a peak derived from S appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F9, showing the binding energy range in which a peak derived from Ni appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G3, showing the binding energy range in which a peak derived from Ni appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F4, showing the binding energy range in which a peak derived from Ni appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G8, showing the binding energy range in which a peak derived from Ni appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F9, showing the binding energy range in which a peak derived from Mn appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G3, showing the binding energy range in which a peak derived from Mn appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F4, showing the binding energy range in which a peak derived from Mn appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G8, showing the binding energy range in which a peak derived from Mn appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F9, showing the binding energy range in which a peak derived from Na appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F4, showing the binding energy range in which a peak derived from Na appears. 1 is a backscattered electron image of the positive electrode active material prepared in Experimental Example F9. 1 is a backscattered electron image of the positive electrode active material prepared in Experimental Example F2. 1 is a backscattered electron image of the positive electrode active material prepared in Experimental Example G8.

In recent years, as non-aqueous electrolyte secondary batteries such as lithium-ion batteries have become more widely used in automotive and energy storage applications, there has been a demand for higher charging voltages to meet the demands for higher capacity and faster charging. However, increasing the charging voltage to, for example, 4.5 V (vs. Li metal) results in a significant increase in the resistance of the positive electrode, particularly in the high SOC range. Furthermore, there is a demand for further improvements in the charge/discharge efficiency and durability of non-aqueous electrolyte secondary batteries. In addition, there is a demand for reduced manufacturing costs, and positive electrode active materials primarily composed of high-capacity, relatively inexpensive Ni and Mn are preferred.

The inventors have succeeded in suppressing resistance increases during high-potential charging while maintaining excellent charge-discharge efficiency and durability by adding predetermined amounts of boron and sulfur compounds to the particle surface of a single-particle composite oxide containing Ni and Mn and in a region less than 100 nm deep from the surface. Use of the positive electrode active material disclosed herein effectively suppresses resistance increases in the high SOC region. The effect of suppressing resistance increases by adding boron and sulfur compounds is unique to single-particle composite oxides. In other words, the presence of boron and sulfur compounds on the particle surface of a secondary-particle composite oxide, which is made up of an aggregation of numerous primary particles, does not effectively suppress resistance increases.

Below, with reference to the drawings, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure, and a non-aqueous electrolyte secondary battery using the positive electrode active material, will be described in detail. Note that configurations formed by selectively combining the components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

The following describes an example of a nonaqueous electrolyte secondary battery 10, which is a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom, but the battery outer can is not limited to a cylindrical outer can. Other embodiments of the nonaqueous electrolyte secondary battery according to the present disclosure include, for example, a prismatic battery with a prismatic outer can, a coin-shaped battery with a coin-shaped outer can, and a pouch-shaped battery with an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, and may be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10, an example of an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that houses the electrode assembly 14 and the nonaqueous electrolyte. The nonaqueous electrolyte secondary battery 10 is, for example, a lithium-ion secondary battery. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom and an open axial end, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-shaped bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer in the length direction (longitudinal direction) and width direction (transverse direction) than the positive electrode 11. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

Insulating plates 18 and 19 are arranged above and below the electrode body 14. In the example shown in Figure 1, the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, while the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom of the outer can 16. The positive electrode lead 20 is connected to the underside of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16, which is crimped against the sealing body 17.

The sealing body 17 has a structure in which, from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are layered. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and all members except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When abnormal heat generation causes the internal pressure of the battery to increase, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure increases further, the upper valve body 26 breaks, and gas is released from the opening of the cap 27.

The positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that make up the nonaqueous electrolyte secondary battery 10 will be described in detail below, particularly the positive electrode active material that makes up the positive electrode 11.

[Positive electrode]
The positive electrode 11 has a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode core can be a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

Examples of conductive agents contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. A single type of conductive agent may be used alone, or multiple types may be used in combination. The amount of conductive agent contained is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer; and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The binder content is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

The positive electrode active material includes a lithium transition metal composite oxide in which the combined content of Ni and Mn is 80 mol% or more relative to the total molar amount of metal elements excluding Li. The lithium transition metal composite oxide has a single particle shape, a volume-based median diameter (D50) of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less. Furthermore, predetermined amounts of boron compounds and sulfur compounds are present on the particle surfaces of the lithium transition metal composite oxide. This ensures excellent charge/discharge efficiency and durability while effectively suppressing resistance increases during high-voltage charging. In particular, the effect of suppressing resistance increases in the high SOC range and improving capacity retention are remarkable.

As will be described in more detail below, the photoelectron spectrum obtained by X-ray photoelectron spectroscopy (XPS) measurement of the positive electrode active material shows a peak attributable to S in the binding energy range of 172 eV to 176 eV at the surface of the lithium transition metal composite oxide particles and in a region less than 100 nm deep from the surface, and a peak attributable to B in the binding energy range of 194 eV to 198 eV. Furthermore, the photoelectron spectrum shows a peak attributable to Ni in the binding energy range of 858 eV to 862 eV at the surface of the lithium transition metal composite oxide particles and in a region less than 50 nm deep from the surface, and a peak attributable to Mn in the binding energy range of 645 eV to 649 eV. Unless otherwise specified, the peak positions in the XPS spectrum refer to the peak top positions.

When the lithium transition metal composite oxide has a single particle shape, the addition of a boron compound and a sulfur compound can achieve a resistance-reducing effect. In this specification, a single particle refers to a particle formed from a single primary particle, rather than a secondary particle formed from an aggregation of a large number (e.g., 1,000 or more) of primary particles. In other words, there are substantially no primary particle interfaces within the particle. Note that particles formed from an aggregation of 10 or fewer primary particles approximate a single particle shape and can be considered to be substantially single particles. A single particle may be a single crystal particle with substantially no crystal grain boundaries within the particle, or a polycrystalline particle with several crystal grain boundaries within the particle.

In a lithium transition metal composite oxide having a boron compound and a sulfur compound attached to the particle surface, it is preferable that, for example, 80% or more of all particles have a single particle shape. In other words, the composite oxide may contain secondary particles formed by aggregation of primary particles in an amount of 20% or less of all particles, and at this level, the same effect as when all particles have a single particle shape can be obtained.

As mentioned above, the D50 of the lithium transition metal composite oxide is 0.5 μm or more and 5.0 μm or less. In this case, the resistance-reducing effect of adding boron compounds and sulfur compounds is obtained. On the other hand, even if boron compounds and sulfur compounds are added to single particles whose D50 is outside this range, or to secondary particles formed by the aggregation of many primary particles, the resistance-reducing effect is not substantially obtained. Based on the resistance-reducing effect of adding boron compounds and sulfur compounds, the D50 of the lithium transition metal composite oxide is preferably 0.7 μm or more and 3.5 μm or less, and more preferably 0.8 μm or more and 3.0 μm or less.

In this specification, D50 refers to the particle size at which the cumulative frequency of the smallest particle size in a volume-based particle size distribution is 50%. The particle size distribution of lithium transition metal composite oxides can be measured using a laser diffraction particle size distribution analyzer (for example, the MT3000II manufactured by Microtrac Bell Co., Ltd.) with water as the dispersion medium.

The crystallite size of the lithium transition metal composite oxide is 370 Å or more and 1500 Å or less, preferably 370 Å or more and 1000 Å or less, more preferably 370 Å or more and 750 Å or less. If the crystallite size is within this range, the resistance reduction effect due to the addition of the boron compound and the sulfur compound can be obtained. The crystallite size is calculated from the half-width of the diffraction peak of the (104) plane in the X-ray diffraction pattern by X-ray diffraction using the Scherrer formula represented by the following formula. In the formula, s is the crystallite size, λ is the wavelength of the X-ray, B is the half-width of the diffraction peak of the (104) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this specification, K is 0.9.
s = Kλ/B cos θ

The X-ray diffraction pattern is obtained by powder X-ray diffraction using a powder X-ray diffractometer (RINT-TTR manufactured by Rigaku Corporation, Cu-Kα radiation source) under the following conditions.
Measurement range: 15-120°
Scan speed: 4°/min
Analysis range: 30-120°
Background: B-spline Profile function: Split pseudo-Voigt function Constraint: Li(3a) + Ni(3a) = 1
Ni(3a) + Ni(3b) = α (α is the Ni content of each element)
ICSD No.:98-009-4814

The BET specific surface area of the lithium transition metal composite oxide is, for example, 0.5 m ² /g or more and 4 m ² /g or less, more preferably 0.9 ^{m 2} /g or more and 3 m ² /g or less. Secondary particles containing many primary particles have voids within the particles, so their specific surface area is relatively large even if their particle size is large. On the other hand, single particles have no voids within the particles, so the larger the particle size, the smaller the BET specific surface area.

The BET specific surface area can be measured using a Tristar II 3020 manufactured by Shimadzu Corporation under the following conditions.
Number of measurement points: 11 points (P/P0: 0.05-0.3)
Warm Free Space: Measured
Equilibration Interval: 5s
Analysis Adsorptive:N2
Analysis Bath Temp.: 77.3K (liquefied nitrogen temperature)
Cold Free Space: Measured
Low Pressure Done: None
Analysis method: BET multi-point method

The lithium transition metal composite oxide preferably has a layered rock salt structure. When a boron compound and a sulfur compound are applied to a composite oxide with a layered rock salt structure, the effect of reducing resistance during high-voltage charging becomes more pronounced. Examples of layered rock salt structures include layered rock salt structures belonging to the space group R-3m and layered rock salt structures belonging to the space group C2/m. Of these, layered rock salt structures belonging to the space group R-3m are preferred from the standpoint of achieving high capacity and stable crystal structure.

The lithium transition metal composite oxide contains Ni and Mn as essential metal elements. The total content of Ni and Mn is 80 mol% or more of the total molar amount of metal elements excluding Li. The proportion of Ni and Mn in the metal elements excluding Li needs to be 80 mol% or more, in which case the resistance reduction effect of adding a boron compound and a sulfur compound can be obtained, and a high-capacity composite oxide can be obtained relatively inexpensively. When the lithium transition metal composite oxide contains Ni, Mn, and Co, the total content of these elements is preferably 90 mol% or more of the total molar amount of metal elements excluding Li. The lithium transition metal composite oxide may contain only Ni and Mn as metal elements excluding Li.

Ni is preferably contained in the largest amount among the metal elements other than Li that constitute the lithium transition metal composite oxide. From the viewpoint of achieving high capacity, the Ni content in the lithium transition metal composite oxide is preferably 50 mol% or more, more preferably 70 mol% or more, and particularly preferably 75 mol% or more, relative to the total molar amount of metal elements excluding Li. The upper limit of the Ni content is, for example, 95 mol%. Examples of suitable ranges for the Ni content are 70 mol% to 95 mol%, or 75 mol% to 95 mol%, or 75 mol% to 90 mol%, or 80 mol% to 90 mol%.

Among the metal elements other than Li that constitute the lithium transition metal composite oxide, Mn is preferably the second most abundant element after Ni. Mn stabilizes the crystal structure of the lithium transition metal composite oxide. The Mn content in the lithium transition metal composite oxide is, for example, 3 mol% to 50 mol%, or 5 mol% to 30 mol%, relative to the total molar amount of metal elements excluding Li. The lithium transition metal composite oxide may also contain Co in a proportion less than Ni. The Co content is preferably equal to or less than the Mn content, for example, 15 mol% or less, 10 mol% or less, 5 mol% or less, or 3 mol% or less.

The lithium transition metal composite oxide may further contain small amounts of elements other than Ni, Mn, and Co. Suitable other elements include at least one selected from the group consisting of Mg, Al, Ca, Nb, Sr, Zr, and W. These elements may be contained inside the lithium transition metal composite oxide particles or may be present on the particle surface. When these elements are contained, for example, side reactions with the electrolyte are suppressed, improving battery durability. The lithium transition metal composite oxide may contain these elements in an amount of 0.01 mol % to 5 mol % relative to the total amount of Ni and Mn. The content of the elements contained in the positive electrode active material can be measured using an ICP optical emission spectrometer (e.g., CIROS-120 manufactured by SPECTRO).

As described above, a predetermined amount of boron compound is present on the particle surface of the lithium transition metal composite oxide. The content of the boron compound is preferably 0.005 mol% or more and 2.0 mol% or less, more preferably 0.1 mol% or more and 1.5 mol% or less, and particularly preferably 0.3 mol% or more and 1.0 mol% or less, relative to the total molar amount of metal elements excluding Li that constitute the lithium transition metal composite oxide. If the content of the boron compound is within this range, the resistance reduction effect is significant. However, even if more than 2.0 mol% of the boron compound is added, the resistance reduction effect will plateau, and it is expected that this will lead to problems such as a decrease in capacity. Therefore, the content of the boron compound is preferably 2.0 mol% or less.

The boron compound is preferably present over a wide area of the particle surface of the lithium transition metal composite oxide, rather than being concentrated in a specific area. The boron compound is present evenly over the particle surface of the composite oxide. The boron compound may be present in the form of a thin film or particles on the particle surface of the composite oxide. The boron compound may also be in solid solution with the composite oxide and bonded to the sulfur compound via oxygen. The average particle size of the boron compound on the particle surface of the lithium transition metal composite oxide is, for example, 1 nm to 500 nm, or 2 nm to 100 nm, and is smaller than the average particle size of the sulfur compound. The average particle size of the boron compound can be measured using the same method as the average particle size of the sulfur compound described below.

The boron compound is preferably boron oxide, boric acid, or a borate salt. Suitable borates are lithium salts, sodium salts, and potassium salts. The boron compound is, for example, at least one selected from the group consisting of boron oxide, boric acid, and a borate salt. Note that the boric acid may be metaboric acid. When at least one selected from these is used, the effect of suppressing the increase in resistance becomes more pronounced.

The boron compound is preferably present on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface, and may be present substantially only in a region less than 100 nm deep from the particle surface. Since the presence of the boron compound on or near the particle surface of the composite oxide is thought to suppress an increase in resistance, in this case, resistance increases can be efficiently suppressed. The content of the boron compound increases from the interior of the particle toward the surface, and substantially the entire amount of the boron compound may be present only in a region 50 nm deep or less from the particle surface.

The elements present on or near the particle surface of the lithium transition metal composite oxide can be confirmed by X-ray photoelectron spectroscopy (XPS). An ESCA 5600 manufactured by Ulvac Phi, Inc. can be used for XPS measurement. The depth from the particle surface obtained by XPS measurement of the positive electrode active material is a value converted to _SiO2 . The obtained data was smoothed (SG9) and then corrected to have a C1s of 284.8 eV.

The XPS measurement conditions are as follows.
X-ray source: Mg-conventional
(Fixed target X-ray tube type, 1253.6 eV)
Neutralization: electron gun only Etching conditions: Ar ion gun Acceleration voltage: 4 kV, 2.3 nm/min ( _SiO2 equivalent)
Measurement conditions: pass energy, 58.70 eV;
Step 0.25 eV (top surface, depth direction)

In the photoelectron spectrum (XPS spectrum) obtained by XPS measurement of the positive electrode active material, as shown in FIG. 3A described below, a peak derived from B exists in the binding energy range of 194 V or more and 198 eV or less at the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface. The intensity of the peak derived from B increases as it approaches the surface from the inside of the particle. The peak derived from _B may exist only in a region less than 100 nm deep from the particle surface, or only in a region 50 nm or less deep from the particle surface, in terms of _SiO2 . Alternatively, the peak derived from B may exist only in a region 30 nm or less deep from the particle surface, or only in a region 20 nm or less deep from the particle surface, in terms of SiO2. In addition, at the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface, it is sufficient that a peak derived from B exists in the binding energy range of 194 V or more and 198 eV or less, but in addition to the peak derived from B, another peak derived from B may exist in a lower binding energy range (for example, 190 eV or more and 193 eV or less).

A predetermined amount of sulfur compound is present on the particle surface of the lithium transition metal composite oxide. The sulfur compound may be any compound containing sulfur, preferably a sulfur oxide, and more preferably a compound containing a functional group represented by formula (1).
The functional group represented by formula (1) is generally called a sulfate ester group. In the formula, R is an alkyl group having 5 or less carbon atoms, preferably an alkyl group having 3 or less carbon atoms. By having a predetermined amount of the sulfur compound present on the surface of the composite oxide particles together with the boron compound, an increase in the resistance of the positive electrode 11 during high-potential charging, particularly in a high SOC region, is effectively suppressed.

The content of sulfur compounds is preferably 0.005 mol% or more and 2.0 mol% or less, more preferably 0.1 mol% or more and 1.5 mol% or less, and particularly preferably 0.3 mol% or more and 1.0 mol% or less, relative to the total molar amount of metal elements excluding Li that constitute the lithium transition metal composite oxide. If the content of sulfur compounds is within this range, the resistance reduction effect will be significant. However, even if sulfur compounds are added in an amount exceeding 2.0 mol%, the resistance reduction effect will plateau, and it is expected that this will lead to problems such as a decrease in capacity. Therefore, the content of sulfur compounds is preferably 2.0 mol% or less.

The sulfur compound is preferably present over a wide area of the particle surface of the lithium transition metal composite oxide, rather than being concentrated in a specific area. As shown in Figure 8 (described below), the sulfur compound is present in granular form and is evenly scattered across the particle surface of the composite oxide. The granular sulfur compound allows for adequate contact between the surface of the lithium transition metal composite oxide particles and the electrolyte, making it easier to achieve resistance reduction. The average particle size of the sulfur compound is preferably 0.005 μm to 0.5 μm, more preferably 0.007 μm to 0.2 μm, and particularly preferably 0.01 μm to 0.1 μm. By using sulfur compounds with an average particle size within this range, the resistance reduction effect of adding the sulfur compound becomes more pronounced. The average particle size of the sulfur compound can be determined by observing the particle surface of the composite oxide using a scanning electron microscope (SEM). The average particle size of the sulfur compounds can be calculated by selecting 100 sulfur compounds from an SEM image of the particle surface, measuring the diameters of the circumscribed circles, and averaging the measured values.

The SEM measurement conditions are as follows.
Equipment: FE-SEM manufactured by JEOL Ltd.
Detector: UED (Upper Electron Detector)
Acceleration voltage: 1 kV
WD: 3mm

It is preferable that the sulfur compounds are substantially present only on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface. In other words, it is preferable that the sulfur compounds are substantially absent from the interior of the particles 100 nm or more from the particle surface. Since the presence of sulfur compounds on or near the particle surface of the composite oxide is thought to suppress an increase in resistance, in this case, the increase in resistance can be efficiently suppressed. The content of sulfur compounds increases from the interior of the particle toward the surface, and substantially the entire amount of sulfur compounds may be present only in a region 50 nm or less deep from the particle surface.

In the XPS spectrum of the positive electrode active material, as shown in FIG. 4A described below, peaks derived from S exist in the binding energy range of 172 eV to 174 eV at the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface. The intensity of the peak derived from S increases as _one approaches the surface from the interior of the particle. The peak derived from S may be substantially present only in a region less than 100 nm deep from the particle surface, or only in a region 50 nm or less deep from the particle surface, in terms of SiO _2. Alternatively, the peak derived from S may exist only in a region 30 nm or less deep from the particle surface, or only in a region 20 nm or less deep from the particle surface, in terms of SiO 2. On the other hand, as shown in FIG. 8, no peak is observed if a sulfur compound is not contained. In addition, at the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface, it is sufficient that a peak derived from S exists in the binding energy range of 172 eV to 174 eV, but in addition to the peak derived from S, another peak derived from S may exist in a lower binding energy range (for example, 168 eV to 171 eV).

Since sulfate is used as the coprecipitated raw material when synthesizing lithium transition metal composite oxides, S may be detected more than 100 nm from the surface of the composite oxide particles, but even if the raw material contains around 2000 ppm of sulfate ions, it will not be detected as a peak in the XPS spectrum. Similarly, Na is contained in the raw material at around 80 ppm, but will not be detected as a peak in the XPS spectrum more than 100 nm from the particle surface. XPS spectra can be used to qualitatively characterize the elements present on the particle surface.

At least one of Na and K may further be present on the particle surface of the lithium transition metal composite oxide. At least one of Na and K is present only on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface, or in a region 50 nm or less deep from the particle surface. In the XPS spectrum of the positive electrode active material, as shown in FIG. 7A described below, a peak derived from Na is present in the binding energy range of 1074 eV or more and 1077 eV or less on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface. The peak derived from Na may be present only in a region _less than 100 nm deep from the particle surface, or in a region 50 nm or less, 30 nm or less, or 20 nm or less deep from the particle surface, in terms of SiO2. In the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface, it is sufficient that a peak derived from Na exists in the binding energy range of 1074 eV or more and 1077 eV or less, but in addition to the peak derived from Na, another peak derived from Na may exist in a range of lower binding energy.

The sulfur compound is preferably a salt of a sulfate ester having 5 or less carbon atoms, particularly lithium salt, sodium salt, and potassium salt. Suitable sulfur compounds include at least one selected from the group consisting of lithium methyl sulfate, lithium ethyl sulfate, lithium propyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, sodium propyl sulfate, potassium methyl sulfate, potassium ethyl sulfate, and potassium propyl sulfate. When at least one selected from these is used, the effect of suppressing resistance increase becomes more pronounced.

As shown in Figures 5A and 6A (described below), the XPS spectrum of the positive electrode active material shows a peak derived from Ni in the binding energy range of 858 eV to 862 eV and a peak derived from Mn in the binding energy range of 645 eV to 649 eV at the surface of the lithium transition metal composite oxide particles and in a region less than 50 nm deep from the surface. The peaks derived from Ni and Mn present in each of these ranges may be present only in a region 30 nm or less, 20 nm or less, or 10 nm or less deep from the surface of the composite oxide particles. The peak derived from Ni is present, for example, in the range of 853 eV to 856 eV in a region more than 50 nm deep from the surface of the composite oxide particles. Furthermore, the peak derived from Mn is present, for example, in the range of 640 eV to 643 eV in a region more than 50 nm deep from the surface of the composite oxide particles. At the particle surface of the lithium transition metal composite oxide and in a region less than 50 nm deep from the surface, a peak derived from Ni may be present in the binding energy range of 858 eV to 862 eV. However, in addition to the peak derived from Ni, another peak derived from Ni may be present in a lower binding energy range (e.g., 853 eV to 856 eV). Furthermore, another peak derived from Ni may also be present. Similarly, at the particle surface of the lithium transition metal composite oxide and in a region less than 50 nm deep from the surface, a peak derived from Mn may be present in the binding energy range of 645 eV to 649 eV. However, in addition to the peak derived from Mn, another peak derived from Mn may be present in a lower binding energy range (e.g., 640 eV to 643 eV). Furthermore, another peak derived from Mn may also be present.

In other words, at the particle surface of the lithium transition metal composite oxide and in a region less than 50 nm deep from the surface, the peaks derived from Ni and Mn are significantly shifted to higher energy compared to the interior of the particles. This large peak shift indicates that the Ni and Mn on the particle surface have a higher binding energy of 2p orbital electrons than the Ni and Mn inside the particles, or are in a high acid value state. It is believed that the use of a positive electrode active material that exhibits this peak shift, i.e., a positive electrode active material with high activity of Ni and Mn on the particle surface, will significantly suppress the increase in resistance. Note that the peaks derived from Ni and Mn gradually shift to higher energy toward the particle surface, for example, in a region more than 10 nm but less than 50 nm deep from the particle surface of the composite oxide.

The large shift in the peaks due to Ni and Mn appears specifically when B and S coexist on the particle surface of the lithium transition metal composite oxide. Figures 5B-5D and 6B-6D show XPS spectra when B, S, or both are not present on the particle surface of the lithium transition metal composite oxide; in this case, no large peak shift like that shown in Figures 5A and 6A is observed.

In other words, it is believed that by mixing a composite oxide, a boron compound, and a sulfur compound and heat-treating them at a predetermined temperature, the state of Ni and Mn on the surface of the composite oxide particles changes, resulting in a highly active state. The effects of the boron compound and sulfur compound are believed to be due to the bonding between the boron compound and an oxoacid, and boron oxide is preferably used as the boron compound, with boric acid and borate salts being particularly preferred. Preferred oxoacids include titanic acid, molybdic acid, phosphoric acid, and alkali salts thereof. It is believed that the presence of these on the surface of composite oxide particles, or their bonding with Ni and Mn, results in a positive electrode active material that exhibits a large peak shift.

The positive electrode active material may contain a positive electrode active material other than the lithium transition metal composite oxide of this embodiment in which a boron compound and a sulfur compound are present on the particle surface, as long as the object of the present disclosure is not impaired. Multiple types of positive electrode active materials can be used in the nonaqueous electrolyte secondary battery 10, depending on, for example, the required battery performance. Even when other composite oxides are used in combination as the positive electrode active material, such as composite oxides in which a boron compound or a sulfur compound is not present on the particle surface, or composite oxides in which the total amount of Ni and Mn is less than 80 mol%, the above-mentioned resistance reduction effect can be obtained depending on the content of the positive electrode active material of this embodiment.

Below, an example of a method for producing a positive electrode active material is described. The method for producing a positive electrode active material includes, for example, a lithium transition metal composite oxide synthesis process, a washing process, a drying process, a crushing process, and a boron compound and sulfur compound addition process.

In the synthesis of a lithium transition metal composite oxide, a metal hydroxide containing 80 mol % or more of Ni and Mn in total relative to the total molar amount of metal elements excluding Li is mixed with a Li compound, and the mixture is calcined to obtain a _lithium transition metal composite oxide. Examples of the Li compound include _Li2CO3 , _LiOH , _Li2O2 , _Li2O , _LiNO3 , _LiNO2 , _Li2SO4 , _LiOH.H2O , LiH, and _LiF .

Metal hydroxides can be obtained, for example, by adding an alkaline solution such as sodium hydroxide dropwise to a stirred solution of metal salts containing Ni, Mn, and optional elements, adjusting the pH to the alkaline side (for example, 8.5 to 12.5), and allowing precipitation (co-precipitation). Note that metal oxides obtained by heat-treating metal hydroxides may also be used instead of metal hydroxides. Since the smaller the particle size of the metal hydroxide, the easier it is for primary particles to grow and single particles to be obtained, the D50 of the metal hydroxide is preferably 7 μm or less, and more preferably 5 μm or less.

The metal hydroxide and Li compound are mixed, for example, in a molar ratio of metal elements excluding Li to Li of 1:0.98 to 1:1.05. When mixing the metal hydroxide and Li compound, Mg compounds, Al compounds, Ca compounds, Nb compounds, Sr compounds, Zr compounds, W compounds, etc. may also be added. These compounds are, for example, oxides, hydroxides, or carbonates, and may also be composite compounds containing other metal elements such as Li.

The mixture of metal hydroxide and Li compound, etc., is fired, for example, in an oxygen atmosphere (under a gas flow with an oxygen concentration of 80% or more). The firing process may be multi-stage firing. One example of firing conditions is a temperature rise rate of 1.0°C/min to 5.5°C/min in the temperature range of 450°C to 680°C, with a maximum temperature of 850°C to 1100°C. The temperature rise rate from 680°C to the maximum temperature may also be 0.1°C/min to 3.5°C/min. The holding time at the maximum temperature may also be 1 hour to 30 hours. Adjusting the firing conditions allows the production of single particles, and the particle size to be adjusted. For example, increasing the maximum temperature makes it easier to obtain single particles, and the particle size tends to be larger.

In the washing step, the lithium transition metal composite oxide obtained in the synthesis step is washed with water and dehydrated to obtain a cake-like composition. The washing and dehydration can be performed using known methods and conditions. Mg compounds, Al compounds, Ca compounds, Nb compounds, Sr compounds, Zr compounds, W compounds, etc. may also be added to the cake-like composition. In the drying step, the cake-like composition obtained in the washing step is dried to obtain a powder-like composition. The drying step may be performed in a vacuum atmosphere. For example, the drying temperature is 150°C or higher and 400°C or lower, and the drying time is 0.5 hours or higher and 15 hours or lower. The washing step may also be omitted.

The powder composition obtained in the drying step is pulverized using a pulverizer such as a jet mill. Pulverization using a jet mill can be performed using, for example, a PJM-80 manufactured by Nippon Pneumatic Mfg. Co., Ltd. under the following conditions. The pulverization step can also be omitted.
Compressed air consumption: 0.5 Nm ³ /min Supply gas pressure: 0.53 MPa
Processing capacity: 2000 g/hour

A boron compound and a sulfur compound are added to and mixed with crushed lithium transition metal composite oxide, followed by heat treatment, to obtain a positive electrode active material in which the boron compound and sulfur compound are attached to the particle surface of the composite oxide. For example, the lithium transition metal composite oxide, boron compound, and sulfur compound are dry mixed, and then heat treated at a temperature of 250°C to 600°C for 2 to 5 hours. This heat treatment causes the boron compound and sulfur compound to adhere to the particle surface of the composite oxide. For dry mixing, a mixer such as a planetary mixer, rocking mill, or high-speed mixer can be used. The heat treatment is carried out in an oxygen atmosphere, but may also be carried out in air.

After adding the boron compound and carrying out the heat treatment, the sulfur compound may be added, but preferably the boron compound and sulfur compound are mixed together and then the heat treatment is carried out. In this case, the resistance reduction effect is more pronounced. It is also conceivable that the boron compound and sulfur compound may be added before the lithium transition metal composite oxide is calcined (as a raw material), but in this case the surface condition of the composite oxide particles obtained will differ from that of the composite oxide particles disclosed herein, making it difficult to obtain a positive electrode active material with excellent capacity retention after charge/discharge cycling.

[Negative electrode]
The negative electrode 12 has a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be a foil of a metal stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with such a metal disposed on its surface. The negative electrode mixture layer contains a negative electrode active material and a binder and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be fabricated, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core. Metallic lithium foil can also be used as the negative electrode 12. Alternatively, the negative electrode 12 may be composed only of a negative electrode core, with metallic lithium being deposited on the core surface during battery charging.

There are no particular restrictions on the negative electrode active material, as long as it can reversibly absorb and release lithium ions, and carbon materials such as graphite are generally used. The negative electrode active material may also be an element that alloys with Li, such as Si or Sn, or a material containing such an element. Of these, silicon-containing materials containing Si are preferred. The negative electrode active material may also be lithium titanate, which has a higher charge/discharge potential relative to metallic lithium than carbon materials. One type of negative electrode active material may be used alone, or multiple types may be used in combination.

The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among these, it is preferable to use artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, or mixtures of these. Examples of silicon-containing materials that function as the negative electrode active material include silicon alloys, silicon compounds, and composite materials containing Si. A suitable silicon-containing material is a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase.

As with the positive electrode 11, the binder contained in the negative electrode mixture layer can be fluororesin, olefin resin, PAN, polyimide, polyamide, acrylic resin, etc., but polyvinyl acetate, styrene-butadiene rubber (SBR), etc. may also be used. Among these, SBR is preferable. A single binder may be used, or multiple binders may be used in combination. Furthermore, the negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. These function as thickeners in the negative electrode mixture slurry. The binder content is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less, relative to the mass of the negative electrode mixture layer. The negative electrode mixture layer may also contain a conductive agent such as CNT.

[Separator]
A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[Non-aqueous electrolyte]
The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity) and may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

The electrolyte solution contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents have been replaced with halogen atoms such as fluorine. Examples of halogen-substituted compounds include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as gamma-butyrolactone (GBL) and gamma-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

Examples of the above ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, Examples of such chain ethers include ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiClO4, _LiBF4 , _LiPF6 , _LiAlCl4 , _LiSbF6 , _LiSCN , _LiCF3SO3 , _LiCF3CO2 , _LiAsF6 , _LiB10Cl10 , lower aliphatic _lithium carboxylates, LiCl, LiBr, LiI, _phosphates , borates, _and imide salts. Examples of phosphates include lithium difluorophosphate ( _LiPO2F2 ), lithium difluorobis(oxalato)phosphate ( _LiDFBOP ), and lithium tetrafluoro(oxalato)phosphate. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), and the like. Examples of imide salts that can be used include lithium bisfluorosulfonylimide (LiN( _FSO2 ) ₂ ), lithium bistrifluoromethanesulfonyl imide (LiN( _CF3SO2 ) ₂ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonyl imide ( _LiN ( _CF3SO2 )( _C4F9SO2 )), and lithium bispentafluoroethanesulfonyl imide (LiN( _C2F5SO2 ) ₂ ). Of these, _LiPF6 is preferably used from _the viewpoints of ionic conductivity, electrochemical stability, and _{the like} . The concentration of the lithium salt may be, for example, 4 _mol or less, or 3 mol _or less, preferably 1.8 mol or less, and more preferably 0.8 mol or more and 1.8 mol or less per liter of nonaqueous solvent.

The non-aqueous electrolyte may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfate compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

Examples of unsaturated cyclic carbonates include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. One or more unsaturated cyclic carbonates may be used alone or in combination. Some of the hydrogen atoms in the unsaturated cyclic carbonates may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by intermolecular condensation of multiple carboxylic acid molecules, but is preferably an acid anhydride of a polycarboxylic acid. Examples of acid anhydrides of polycarboxylic acids include succinic anhydride, maleic anhydride, and phthalic anhydride.

Phenol compounds include, for example, phenol and hydroxytoluene. Benzene compounds include, for example, fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

Nitrile compounds include adiponitrile, pimelonitrile, propionitrile, succinonitrile, etc. Isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), bisisocyanatomethylcyclohexane (BIMCH), etc. Sultone compounds include propane sultone and propene sultone. Sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, lithium fluorosulfate, etc. Borate ester compounds include trimethyl borate and tris(trimethylsilyl)borate. Phosphate ester compounds include trimethyl phosphate and tris(trimethylsilyl)phosphate. Phosphite ester compounds include trimethyl phosphite and tris(trimethylsilyl)phosphite.

As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known for all-solid-state lithium-ion secondary batteries (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte contains, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. Examples of polymer materials include fluororesin, acrylic resin, polyether resin, etc.

The present disclosure will be further explained below using experimental examples, but the present disclosure is not limited to these experimental examples.

<Experimental Example A1>
[Preparation of Positive Electrode Active Material]
LiOH and _Ni0.8Mn0.2 (OH) ₂ powder obtained by coprecipitation were mixed so that the molar ratio of Li to the total amount of Ni and Mn was 1.05:1 to obtain a mixture. This mixture was then fired in two stages under an oxygen stream _with an oxygen concentration of 90% or higher (a flow rate of 0.15 L/min to 0.2 L/min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then from 650°C to 880°C over 2 hours. The mixture was then held at 880°C for 3 hours to obtain a lithium transition metal composite oxide.

The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, dried, and then crushed using a jet mill. Sodium ethyl sulfate was added to the crushed lithium transition metal composite oxide and mixed, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere (flow rate: 4 L/min), to obtain a positive electrode active material in which sodium ethyl sulfate adhered to the particle surfaces of the lithium transition metal composite oxide. The amount of sodium ethyl sulfate added was adjusted so that the sodium ethyl sulfate content was 0.7 mol% relative to the total molar amount of metal elements in the composite oxide excluding Li.

Observation of the positive electrode active material using an SEM confirmed that most of the lithium transition metal composite oxide particles were single particles composed of a single primary particle. The D50 and crystallite size of the lithium transition metal composite oxide (positive electrode active material) were measured using the above-mentioned method, and the D50 was 1.1 μm, the crystallite size was 459 Å, and the BET specific surface area was 1.9 m ² /g. The crystalline structure of the lithium transition metal composite oxide was a layered rock salt structure belonging to the space group R-3m. It was also confirmed that sodium ethyl sulfate particles were scattered on the particle surface of the composite oxide. In the XPS spectrum of the positive electrode active material, peaks derived from S and Na were confirmed at the particle surface of the composite oxide and in a region shallower than 100 nm from the surface.

[Preparation of test cell]
The test cell shown in FIG. 2 was fabricated by the following procedure. The working electrode 30 was a positive electrode using the above-described positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 80:10:10, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone as a dispersion medium. This slurry was applied to an aluminum foil positive electrode core, and the coating was vacuum-dried at 110°C to obtain the working electrode 30.

In dry air with a dew point of -50°C or below, an electrode group was prepared by placing a separator 34 between each of the working electrode 30, counter electrode 31 (negative electrode), and reference electrode 32, each of which had an electrode lead 38 attached, and then encased in an outer casing 35. Then, an electrolyte solution 36 was poured into the outer casing 35, and the outer casing 35 was sealed to obtain a test cell.

Details of each component of the test cell are as follows:
Counter electrode 31: lithium metal Reference electrode 32: lithium metal Separator 34: polyethylene separator Non-aqueous electrolyte: A non-aqueous solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 (25°C) was used, and _LiPF6 was dissolved as an electrolyte salt to a concentration of 1.0 mol/L.

The following performance evaluations were conducted on the test cell of Experimental Example A1, and the evaluation results, along with the additive composition and amount added, are shown in Table 1-1.

[Evaluation of charge/discharge efficiency and capacity retention rate]
In a temperature environment of 25°C, the test cell was charged to 4.5 V (vs. Li metal) at a constant current of 0.2 C, and then charged to 0.02 C at a constant voltage of 4.5 V. Then, it was discharged to 2.5 V at a constant current of 0.1 C. The charge capacity and discharge capacity at this time were measured, and the discharge capacity was divided by the charge capacity to calculate the charge/discharge efficiency. From the second cycle onwards, the current value during discharge was changed to 0.2 C, and this charge/discharge cycle was repeated 30 times, and the discharge capacity at the 30th cycle was divided by the discharge capacity at the first cycle to calculate the capacity retention rate.

[Evaluation of IV resistance]
The IV resistance (internal resistance) of the test cell after the initial charge/discharge and after 30 charge/discharge cycles was measured at SOCs of 10%, 50%, and 100% (4.45 V). The test cell was placed in each SOC state, then rested for 15 minutes, and the voltage at that state was designated as V0. The voltage drop at each time the cell was discharged for 10 seconds was designated as ΔV. The IV resistance was calculated from the straight line obtained by each current value and each ΔV.

<Experimental Example A2>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that sodium methyl sulfate was used instead of sodium ethyl sulfate, the amount added was adjusted to 0.5 mol %, and the crushing step of the composite oxide was omitted. The evaluation results, along with the composition and amount of additive, are shown in Table 1-1 (similarly for Experimental Examples B1 to B23). The positive electrode active material had a D50 of 4.9 μm and a BET specific surface area of 1.1 m ² /g. It was confirmed that sodium methyl sulfate particles (average particle size: 0.09 μm) were scattered on the particle surface of the composite oxide. Furthermore, in the XPS spectrum of the positive electrode active material, peaks derived from S and Na were confirmed on the particle surface of the composite oxide and in a region shallower than 100 nm from the surface.

<Experimental Examples B1 to B23>
Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A2, except that the compounds shown in Table 1-1 were used instead of sodium methyl sulfate, and the performance evaluations were carried out as described above. The presence or absence of a crushing step for the composite oxide is as shown in Table 1-1.

<Experimental Examples B24 to B41>
Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A1, except that instead of sodium ethyl sulfate, compounds shown in Table 1-2 were used and the amounts added were adjusted to those shown in Table 1-2, and the performance evaluations were carried out as described above. The evaluation results are shown in Table 1-2. The presence or absence of a crushing step for the composite oxide is shown in Table 1-2.

<Experimental Example B42>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

The results shown in Tables 1-1 and 1-2 show that in the test cells of Experimental Examples B1 to B42, the increase in positive electrode resistance increases as the number of cycles increases, but that the increase in resistance is effectively suppressed in the test cells of Experimental Examples A1 and A2. Furthermore, the test cells of Experimental Examples A1 and A2 have a higher capacity retention rate after charge/discharge cycling and superior cycle characteristics compared to the test cells of Experimental Examples B1 to B42. Furthermore, even when sodium ethyl sulfate is used, if the amount added is less than 0.1 mol%, the resistance reduction effect cannot be obtained (see Experimental Example B30).

<Experimental Example C1>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that in the sodium ethyl sulfate addition step, sodium ethyl sulfate was mixed so that the content was 0.5 mol % relative to the total molar amount of metal elements excluding Li in the composite oxide, and heat treatment was performed for 3 hours at 500°C under an oxygen atmosphere, and the composite oxide crushing step was omitted. The evaluation results, along with the composition and amount of additive added, are shown in Table 2 (the same applies to Experimental Examples D1 to D15). It was confirmed that sodium ethyl sulfate particles were scattered on the particle surface of the composite oxide. Furthermore, in the XPS spectrum of the positive electrode active material, peaks derived from S and Na were confirmed at the particle surface of the composite oxide and in a region shallower than 100 nm from the surface.

<Experimental Example D1>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example C1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

<Experimental Examples D2 to D15>
Positive electrode active materials and test cells were prepared and the performance evaluations were carried out in the same manner as in Experimental Example C1, except that the compounds shown in Table 2 were used instead of sodium ethyl sulfate. The amount of each compound added was 0.5 mol % relative to the total molar amount of metal elements in the composite oxide excluding Li.

From the results shown in Table 2, it can be seen that the test cell of Experimental Example C1 had lower positive electrode resistance compared to the test cells of Experimental Examples D1 to D15. Furthermore, these results show that the resistance reduction effect can be achieved regardless of whether the heat treatment temperature in the sulfur compound addition process is 300°C or 500°C. The difference is particularly noticeable in the high SOC range (Spectris).

<Experimental Examples E1 to E3>
In the sodium ethyl sulfate addition step, sodium ethyl sulfate was added in amounts of 0.3 mol %, 0.5 mol %, and 1.0 mol %, respectively, relative to the total molar amount of metal elements excluding Li in the composite oxide. Heat treatment was performed in air at 300°C for 3 hours, and the composite oxide crushing step was omitted. The positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A1, and the performance evaluation was carried out. The evaluation results are shown in Table 3. It was confirmed that sodium ethyl sulfate particles were scattered on the surface of the composite oxide particles in all positive electrode active materials. Furthermore, peaks due to S and Na were confirmed in the XPS spectrum of the positive electrode active materials at the surface of the composite oxide particles and in a region shallower than 100 nm from the surface.

<Experimental Examples E4 to E6>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that boric acid was added together with sodium ethyl sulfate, and the performance evaluation was carried out as described above. The amounts of boric acid and sodium ethyl sulfate added are shown in Table 3 along with the evaluation results.

In all of the positive electrode active materials of Experimental Examples E4 to E6, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the surface of the composite oxide particles. Furthermore, in the XPS spectrum of the positive electrode active material, peaks attributable to B, S, and Na were confirmed on the surface of the composite oxide particles and in a region shallower than 100 nm from the surface. Furthermore, a peak attributable to Ni was confirmed in the binding energy range of 858 eV to 862 eV, and a peak attributable to Mn was confirmed in the binding energy range of 645 eV to 649 eV, on the surface of the composite oxide particles and in a region shallower than 50 nm from the surface.

<Experimental Example EE1>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example E1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

The results shown in Table 3 show that an excellent effect of suppressing resistance increase can be achieved even when the mixture is heat-treated in air during the sodium ethyl sulfate addition process. In particular, the effect of suppressing resistance increase was remarkable when the positive electrode active materials of Experimental Examples E4 to E6 were used. The oxygen concentration during the heat treatment process is preferably 5% or higher, more preferably 10% or higher, and particularly preferably 18% or higher.

<Experimental Examples F1 to F7>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that the amount of sodium ethyl sulfate added was set to the amount shown in Table 4, and the water-washed composite oxide was dried, and then the crushing step using a jet mill was omitted. The D50 of the lithium transition metal composite oxide was 4.9 μm.

Figure 4B is an XPS spectrum of the positive electrode active material of Experimental Example F4, showing the binding energy range in which peaks derived from S appear. As shown in Figure 4B, peaks derived from S were observed in the binding energy range of 168 eV to 172 eV on the surface of the lithium transition metal composite oxide particles and in the region 20 nm deep from the surface (particularly noticeable at 10 nm).

Figure 7B is an XPS spectrum of the positive electrode active material of Experimental Example F4, showing the binding energy range in which peaks derived from Na appear. As shown in Figure 7B, peaks derived from Na were observed in the binding energy range of 1070 eV to 1073 eV on the particle surface of the lithium transition metal composite oxide and in the region 100 nm deep from the surface.

Figure 5C (a) shows the XPS spectrum of the positive electrode active material of Experimental Example F4, indicating the binding energy range in which peaks derived from Ni appear. For comparison, Figure 5C (b) shows the XPS spectrum of the positive electrode active material of Experimental Example G8, described below, which does not contain S. Figure 6C (a) shows the XPS spectrum of the positive electrode active material of Experimental Example F4, indicating the binding energy range in which peaks derived from Mn appear. For comparison, Figure 6C (b) shows the XPS spectrum of the positive electrode active material of Experimental Example G8.

As shown in Figures 5C and 6C, the XPS spectrum of the positive electrode active material of Experimental Example F4, when compared with the spectrum of the positive electrode active material of Experimental Example G8, which does not contain S, shows a shift to higher energy of the peaks derived from Ni and Mn near the particle surface of the composite oxide (0.4 eV for Ni, 0.2 eV for Mn). The depth at which this peak shift is observed roughly coincides with the depth (10 nm) at which the peak derived from S shown in Figure 4B clearly appears, indicating that S is dissolved at the particle surface of the composite oxide and in a depth range of at least 10 nm nearby, forming a surface layer in which the bonding state of Ni and Mn has changed.

Furthermore, in the XPS spectrum of the positive electrode active material produced in Experimental Example F2, clear peaks due to S were confirmed in the binding energy range of 168 eV to 172 eV on the surface of the composite oxide particles and in the region 70 nm deep from the surface. As with Experimental Example F4, S is present in solid solution in the composite oxide at a depth of 70 nm from the surface, forming a surface layer in which the bonding state of Ni and Mn has changed. Furthermore, an XPS peak due to Na was confirmed even at 100 nm.

Figure 9 is a backscattered electron image of the positive electrode active material produced in Experimental Example F2. As shown in Figure 9, the small particles present on the surface of the lithium transition metal composite oxide particles are sodium ethyl sulfate. The sodium ethyl sulfate particles are not concentrated in one part of the composite oxide particle surface, but are scattered evenly over a wide area of the particle surface. The average particle size of the sodium ethyl sulfate was approximately 0.02 μm. Figure 10 is a backscattered electron image of the positive electrode active material produced in Experimental Example G8, and no granular deposits are visible on the particle surface.

<Experimental Examples F8 to F21>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example F1, except that boric acid was added together with sodium ethyl sulfate, and the performance evaluation was carried out. The amounts of boric acid and sodium ethyl sulfate added are shown in Table 4, along with the evaluation results. In Experimental Examples 11 to 13, boron was added to an uncrushed composite oxide, and the resulting mixture was heat-treated at 300°C for 3 hours in an oxygen atmosphere. Then, sodium ethyl sulfate was added, and the resulting mixture was heat-treated at 300°C for 3 hours in an oxygen atmosphere. Furthermore, in the XPS spectrum of the positive electrode active material, peaks derived from B, S, and Na were observed at the particle surface of the composite oxide and in a region 100 nm deep from the surface. Furthermore, a peak derived from Ni was observed in the binding energy range of 858 eV to 862 eV, and a peak derived from Mn was observed in the binding energy range of 645 eV to 649 eV, at the particle surface of the composite oxide and in a region less than 50 nm deep from the surface.

Figures 3A, 4A, 5A, 6A, and 7A are XPS spectra of the positive electrode active material of Experimental Example F9, showing the binding energy ranges in which a peak derived from B appears, the binding energy range in which a peak derived from S appears, the binding energy range in which a peak derived from Ni appears, the binding energy range in which a peak derived from Mn appears, and the binding energy range in which a peak derived from Na appears. In the XPS spectrum, new peaks derived from B, S, and Na were observed at the surface of the composite oxide particles and in a region shallower than 100 nm from the surface, ranging from 194.5 eV to 197 eV, from 172 eV to 174 eV, and from 1074 eV to 1077 eV, respectively. Furthermore, new peaks derived from Ni were observed at the surface of the composite oxide particles and in a region 50 nm deep from the surface, ranging from 858 eV to 862 eV in binding energy, and new peaks derived from Mn were observed at the surface of the composite oxide particles and in a region 50 nm deep from the surface, resulting in roughly two peaks being observed. Furthermore, by incorporating both B and S, the lower energy peaks weaken closer to the particle surface, and the new higher energy peaks become stronger. Therefore, new bonds consisting of B and S are formed on the particle surface, and their presence suppresses structural destruction and improves resistance and capacity retention. By bonding with oxygen between B and S, the energy is shifted to the higher energy side.

In the XPS spectra (see Figures 5B-5D and 6B-6D) of Experimental Examples G3, F4, and G8 (positive electrode active materials containing no B, no S, or both), a peak attributable to Ni can be seen in the binding energy range of 853 eV to 855.5 eV, a peak attributable to Mn in the binding energy range of 640 eV to 643 eV, a peak attributable to S in the binding energy range of 167 eV to 170 eV, a peak attributable to B in the binding energy range of 190 eV to 193 eV, and a peak attributable to Na in the binding energy range of 1071 eV to 1072 eV.

When comparing the spectra of the positive electrode active material of Experimental Example F4 and Experimental Example G8, which does not contain S, a shift to higher energy of the peaks derived from Ni and Mn near the particle surface of the composite oxide (0.4 eV for Ni, 0.2 eV for Mn) is observed. The depth at which this peak shift is observed roughly coincides with the depth (10 nm) at which the peak derived from S clearly appears, indicating that S is dissolved in the composite oxide particle surface and in a depth range of at least 10 nm nearby, forming a surface layer in which the bonding state of Ni and Mn has changed.

Meanwhile, in the XPS spectrum of the positive electrode active material of Experimental Example F9, in addition to the peaks attributable to Ni and Mn mentioned above (a Ni-attributed peak in the binding energy range of 853 eV to 855.5 eV and a Mn-attributed peak in the binding energy range of 640 eV to 643 eV), new peaks attributable to Ni appear in the binding energy range of 858 eV to 862 eV and a Mn-attributed peak in the binding energy range of 645 eV to 649 eV, indicating the presence of at least two Ni and Mn bonding states. On the particle surface of the composite oxide, the newly emerged peaks are observed in a range shallower than 50 nm from the surface (clear at 30 nm). The depth at which this peak shift is observed roughly coincides with the depth (50 nm) at which the new boron peak shown in Figure 3A appears, indicating that B, S, and Na are bonded via oxygen at the particle surface of the composite oxide and within a depth range of at least 50 nm nearby. Therefore, within a depth of less than 50 nm from the surface of the composite oxide particles, a new layer consisting of at least three of S, B, Ni, Mn, and Na is formed or dissolved. Its thickness is at least 5 nm, preferably 10 nm or more, and more preferably 30 nm or more. This composite oxide, whose particle surface contains both B and S, is highly effective in suppressing electrolyte decomposition during high-potential charging, suppressing resistance increases, and maintaining a high capacity retention rate.

Figure 10 is a backscattered electron image of the positive electrode active material of Experimental Example F9. The backscattered electron image of the positive electrode active material confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the surface of the composite oxide particles. The average particle size of the sodium ethyl sulfate was approximately 0.05 μm.

In addition, in all of the positive electrode active materials of Experimental Examples F8, and F10 to F21, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the surface of the composite oxide particles. Furthermore, in the XPS spectrum of the positive electrode active material, peaks attributable to B, S, and Na were confirmed on the surface of the composite oxide particles and in a region shallower than 100 nm from the surface. Furthermore, a peak attributable to Ni was confirmed in the binding energy range of 858 eV to 862 eV, and a peak attributable to Mn was confirmed in the binding energy range of 645 eV to 649 eV, on the surface of the composite oxide particles and in a region shallower than 50 nm from the surface.

<Experimental Example G1>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example F5, except that sodium ethyl sulfate was added before calcining the lithium transition metal composite oxide and the water washing step was omitted, and the performance evaluation was carried out. Because the surface condition of the composite oxide particles differed from that of the composite oxide particles according to the present disclosure, it was difficult to obtain a positive electrode active material with excellent capacity retention after charge/discharge cycling. Note that, if water washing was not performed, the resistance value would be small, so evaluation of IV resistance was not performed.

<Experimental Examples G2 and G3>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Examples F1 and F3, except that boric acid was used instead of sodium ethyl sulfate, and the performance evaluation was carried out. Figures 3B, 5B, and 6B show the XPS spectra of the positive electrode active material of Experimental Example G3, as described above, and show the binding energy ranges in which the peaks derived from B, Ni, and Mn appear, respectively.

<Experimental Examples G4 to G7>
Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example F1, except that the compounds shown in Table 4 were used instead of sodium ethyl sulfate and the amounts added were adjusted to the amounts shown in Table 4. The performance evaluations were then carried out. The evaluation results are shown in Table 4.

<Experimental Example G8>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example F1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above. Figures 3C, 4C, 5D, and 6D are XPS spectra of the positive electrode active material of Experimental Example G3, as described above, and show the binding energy ranges in which the peaks derived from B, Ni, and Mn appear, respectively.

The results shown in Table 4 indicate that the test cells of Experimental Examples F1 to F21, and especially Experimental Examples F8 to F21, have lower positive electrode resistance compared to the test cells of Experimental Examples G1 to G8. This difference is particularly pronounced in the high SOC range. Even when sodium ethyl sulfate is used, no resistance-reducing effect is obtained when added during the calcination of the lithium transition metal composite oxide. Furthermore, adding boron alone does not result in a resistance-reducing effect. Furthermore, when sodium dodecyl sulfate is used in combination with boron, the capacity is lower and the resistance-reducing effect is also smaller than when nothing is added (Experimental Example G8). Sodium dodecyl sulfate has a long carbon number of 12, which results in a coating of carbon on the surface of the composite oxide particles, inhibiting the bond between boron and sulfur and preventing the synergistic effect. Therefore, it is desirable to use a sulfate ester with a carbon number of 5 or less.

<Experimental Examples H1 to H8>
LiOH and _{Ni0.91Co0.04Al0.05} (OH) ₂ powder obtained _by coprecipitation were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Al was 1.03:1 to obtain a mixture. This mixture was then fired in two stages under _an oxygen stream with an oxygen concentration of 90% or higher (a flow rate of 0.15 L/min to 0.2 L/min per 1 L of furnace volume). Specifically, the mixture was heated from room temperature to 670°C over 5 hours, and then heated from 670°C to 720°C over 50 minutes. The mixture was then held at 720°C for 3 hours to obtain a lithium transition metal composite oxide. The resulting composite oxide particles have a secondary particle shape consisting of a large number of aggregated primary particles.

The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dried. The dried lithium transition metal composite oxide was mixed with the compounds shown in Table 5, and then heat-treated in an oxygen atmosphere at 300°C for 3 hours to obtain a positive electrode active material in which the compounds shown in Table 5 were attached to the particle surfaces of the lithium transition metal composite oxide. The D50 of the positive electrode active material was 11 μm. Except for using this positive electrode active material, a positive electrode active material and test cell were prepared in the same manner as in Experimental Example A1, and the performance evaluation was carried out as described above. The performance evaluation of the test cell was performed by charging up to 4.4 V (vs. Li metal) at the temperature environment (25°C or 45°C) shown in Table 5. The IV resistance was evaluated with a cell voltage of 4.4 V defined as 100% SOC.

From the results shown in Table 5, it can be seen that adding sodium ethyl sulfate to a lithium transition metal composite oxide in the form of secondary particles that does not contain Mn does not achieve a resistance-reducing effect, and instead results in a decrease in discharge capacity and an increase in resistance.

<Experimental Examples J1 to J25>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that the water-washed composite oxide was dried, the crushing step using a jet mill was omitted, and boric acid was added and mixed with sodium ethyl sulfate, followed by heat treatment for 3 hours at 300°C in an oxygen atmosphere (flow rate: 3 L/min). The amounts of boric acid and sodium ethyl sulfate added are shown in Table 6, along with the evaluation results.

In all of the positive electrode active materials of Experimental Examples J1 to J25, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the surface of the composite oxide particles. Furthermore, XPS measurements confirmed peaks attributable to B, S, and Na on the surface of the composite oxide particles and in regions shallower than 100 nm from the surface. Furthermore, peaks attributable to Ni were confirmed in the binding energy range of 858 eV to 862 eV, and peaks attributable to Mn were confirmed in the binding energy range of 645 eV to 649 eV on the surface of the composite oxide particles and in regions less than 50 nm from the surface.

<Experimental Examples K1 to K3>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that boric acid was added instead of sodium ethyl sulfate, the jet mill crushing step was omitted, and heat treatment was performed at 300°C for 3 hours in an oxygen atmosphere (flow rate: 3 L/min). The amount of boric acid added is shown in Table 6 along with the evaluation results.

<Experimental Example K4>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that the step of adding sodium ethyl sulfate, the step of crushing using a jet mill, and the heat treatment was performed at 300°C for 3 hours in an oxygen atmosphere (flow rate: 3 L/min) were omitted, and the performance evaluation was carried out.

From the results shown in Table 6, it can be seen that the test cells of Experimental Examples J1 to J25 have lower positive electrode resistance than the test cells of Experimental Examples K1 to K3. This difference is particularly noticeable after charge-discharge cycling. The test cells of Experimental Examples J1 to J25 also have excellent charge-discharge cycle characteristics.

<Experimental Example L1>
[Preparation of Positive Electrode Active Material]
LiOH, _{Ni0.9Co0.05Mn0.05} (OH) ₂ powder obtained by coprecipitation, and Zr(OH) ₄ were mixed so _that the molar ratio of _Li to the total amount of Ni, Co, and Mn was 1.05:1:0.003 to obtain a mixture. This mixture was then fired in two stages under an oxygen stream with an oxygen concentration of 90% or more (a flow rate of 0.15 L/min to 0.2 L/min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 650°C to 810°C over 2 hours. The mixture was then held at 810°C for 3 hours to obtain a lithium transition metal composite oxide.

Observation of the obtained lithium transition metal composite oxide using an SEM confirmed that most of the particles of the composite oxide were single particles consisting of a single primary particle. The D50 and crystallite size of the lithium transition metal composite oxide were measured using the above-mentioned method, and the D50 was 1.8 μm, the BET specific surface area was 1.1 m ² /g, and the crystallite size was 528 Å. The crystalline structure of the lithium transition metal composite oxide is a layered rock salt structure belonging to the space group R-3m.

The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, dried, and then crushed using a jet mill. Boric acid and sodium ethyl sulfate were added to the crushed lithium transition metal composite oxide and mixed, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere to obtain a positive electrode active material in which boric acid and sodium ethyl sulfate adhered to the surface of the lithium transition metal composite oxide particles. The amounts of boric acid and sodium ethyl sulfate added were adjusted to 0.50 mol % and 0.10 mol %, respectively, relative to the total molar amount of metal elements in the composite oxide excluding Li.

Except for using the above positive electrode active material, a test cell was prepared in the same manner as in Experimental Example A1, and the performance evaluation was carried out as described above.

<Experimental Examples L2 to L25>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example L1, except that the amounts of boric acid and sodium ethyl sulfate added were adjusted to the amounts shown in Table 7. The performance of the test cell was evaluated by charging up to 4.4 V (vs. Li metal). The state where the cell voltage was 4.4 V was defined as 100% SOC, and the IV resistance was evaluated.

In all of the positive electrode active materials of Experimental Examples L1 to L25, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the surface of the composite oxide particles. Furthermore, XPS measurements confirmed peaks attributable to B, S, and Na on the surface of the composite oxide particles and in regions shallower than 100 nm from the surface. Furthermore, peaks attributable to Ni were confirmed in the binding energy range of 858 eV to 862 eV, and peaks attributable to Mn were confirmed in the binding energy range of 645 eV to 649 eV on the surface of the composite oxide particles and in regions less than 50 nm from the surface.

<Experimental Example M1>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example L1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

From the results shown in Table 7, it can be seen that the test cells of Experimental Examples L1 to L25 have lower positive electrode resistance than the test cell of Experimental Example M1. Furthermore, the test cells of Experimental Examples L1 to L25 also have excellent charge-discharge cycle characteristics. It was also confirmed that the effect is still present even when the Zr content is 0.05 mol% or less.

<Experimental Example N1>
[Preparation of Positive Electrode Active Material]
LiOH and _{Ni0.75Co0.05Mn0.20} (OH) ₂ powder obtained _by coprecipitation were mixed so that the molar ratio _of Li to the total amount of Ni, Co, and Mn was 1.05:1 to obtain a mixture. This mixture was then fired in two stages under an oxygen stream with an oxygen concentration of 90% or higher (a flow rate of 0.15 L/min to 0.2 L/min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then from 650°C to 900°C over 2 hours. The mixture was then held at 900°C for 3 hours to obtain a lithium transition metal composite oxide.

Observation of the obtained lithium transition metal composite oxide using an SEM confirmed that most of the particles of the composite oxide were single particles consisting of a single primary particle. The D50 and crystallite size of the lithium transition metal composite oxide were measured using the above-mentioned method, and the D50 was 1.6 μm, the BET specific surface area was 1.5 m ² /g, and the crystallite size was 529 Å. The crystalline structure of the lithium transition metal composite oxide is a layered rock salt structure belonging to the space group R-3m.

The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, dried, and then crushed using a jet mill. Boric acid and sodium ethyl sulfate were added to the crushed lithium transition metal composite oxide and mixed, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere to obtain a positive electrode active material in which boric acid and sodium ethyl sulfate adhered to the surface of the lithium transition metal composite oxide particles. The amounts of boric acid and sodium ethyl sulfate added were adjusted to 0.50 mol % and 0.10 mol %, respectively, relative to the total molar amount of metal elements in the composite oxide excluding Li.

Except for using the above positive electrode active material, a test cell was prepared in the same manner as in Experimental Example A1, and the performance evaluation was carried out as described above.

<Experimental Examples N2 to N25>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example N1, except that the amounts of boric acid and sodium ethyl sulfate added were adjusted to those shown in Table 8, and the performance evaluation was carried out as described above.

In all of the positive electrode active materials of Experimental Examples N1 to N25, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the surface of the composite oxide particles. Furthermore, XPS measurements confirmed peaks attributable to B, S, and Na on the surface of the composite oxide particles and in a region 100 nm deep from the surface. Furthermore, peaks attributable to Ni were confirmed in the binding energy range of 858 eV to 862 eV, and peaks attributable to Mn were confirmed in the binding energy range of 645 eV to 649 eV on the surface of the composite oxide particles and in a region less than 50 nm deep from the surface.

<Experimental Example P1>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example N1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

From the results shown in Table 8, it can be seen that the test cells of Experimental Examples N1 to N25 have lower positive electrode resistance than the test cell of Experimental Example N25. Furthermore, the test cells of Experimental Examples N1 to N25 also have excellent charge-discharge cycle characteristics. From this, it is estimated that even a Ni content of 50% of the total transition metal amount is effective.

<Experimental Examples R1 to R8>
A positive electrode active material and a test cell were prepared and the performance evaluation was carried out in the same manner as in Experimental Example A1, except that boric acid was added together with sodium ethyl sulfate, the performance evaluation was carried out in a temperature environment of 45°C, and the crushing step using a jet mill was omitted. The amounts of boric acid and sodium ethyl sulfate added are shown in Table 9, along with the evaluation results.

<Experimental Examples S1 to S8>
Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example R1, except that additives shown in Table 9 were used instead of boric acid and sodium ethyl sulfate, and the amounts added were adjusted to the amounts added shown in Table 9, and the performance evaluations were carried out.

<Experimental Example S9>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example R1, except that the steps of adding boric acid and sodium ethyl sulfate were omitted, and the performance evaluation was carried out as described above.

From the results shown in Table 9, it can be seen that the test cells of Experimental Examples R1 to R8 have lower positive electrode resistance than the test cell of Experimental Example S9. Furthermore, the test cells of Experimental Examples R1 to R8 also have lower positive electrode resistance than the test cells of Experimental Examples S1 to S8. Furthermore, the test cells of Experimental Examples R1 to R8 also have excellent charge-discharge cycle characteristics.

<Experimental Examples T1 to T8>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example L1, except that the amounts of boric acid and sodium ethyl sulfate added were adjusted to those shown in Table 7, and the performance evaluation was performed in a temperature environment of 45°C.

<Experimental Example U1>
A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example T1, except that the steps of adding boric acid and sodium ethyl sulfate were omitted, and the performance evaluation was carried out as described above.

From the results shown in Table 10, it can be seen that the test cells of Experimental Examples T1 to T8 have lower positive electrode resistance than the test cell of Experimental Example U1. Furthermore, the test cells of Experimental Examples T1 to T8 also have excellent charge-discharge cycle characteristics.

The present disclosure is further illustrated by the following embodiments.
Configuration 1: A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide having a combined content of Ni and Mn of 80 mol % or more relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide having a single particle shape, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less, a sulfur compound and a boron compound are present on the particle surfaces of the lithium transition metal composite oxide, and a photoelectron spectrum obtained by X-ray photoelectron spectroscopy (XPS) measurement shows a peak attributable to S in a binding energy range of 172 eV or more and 176 eV or less and a peak attributable to B in a binding energy range of 194 eV or more and 198 eV or less at the particle surfaces of the lithium transition metal composite oxide and in a region at a depth of less than 100 nm from the surface.
Aspect 2: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Aspect 1, wherein the lithium transition metal composite oxide further contains at least one element selected from the group consisting of Mg, Al, Ca, Nb, Sr, Zr, and W.
Configuration 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein at least one of Na and K is present on the particle surface of the lithium transition metal composite oxide and in a region at a depth of less than 100 nm from the surface.
Configuration 4: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the sulfur compound is a sulfur oxide.
Configuration 5: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the sulfur compound contains a functional group represented by formula (1).
In the formula, R is an alkyl group having 5 or less carbon atoms.
Aspect 6: The positive electrode active material for a nonaqueous electrolyte secondary battery according to any one of Aspects 1 to 5, wherein the sulfur compound is at least one selected from the group consisting of lithium methyl sulfate, lithium ethyl sulfate, lithium propyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, sodium propyl sulfate, potassium methyl sulfate, potassium ethyl sulfate, and potassium propyl sulfate.
Configuration 7: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the boron compound is boric acid.
Configuration 8: A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material according to any one of configurations 1 to 7, a negative electrode, and a non-aqueous electrolyte.

10. Non-aqueous electrolyte secondary battery, 11. Positive electrode, 12. Negative electrode, 13. Separator, 14. Electrode assembly, 16. Outer can, 17. Sealing body, 18, 19. Insulating plate, 20. Positive electrode lead, 21. Negative electrode lead, 22. Grooved portion, 23. Internal terminal plate, 24. Lower valve body, 25. Insulating member, 26. Upper valve body, 27. Cap, 28. Gasket, 30. Working electrode, 31. Counter electrode, 32. Reference electrode, 34. Separator, 35. Outer can, 36. Electrolyte, 38. Electrode lead

Claims (8)

The lithium transition metal composite oxide contains a total content of Ni and Mn of 80 mol % or more relative to the total molar amount of metal elements excluding Li,
the lithium transition metal composite oxide has a single particle shape, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less;
a sulfur compound and a boron compound are present on the particle surfaces of the lithium transition metal composite oxide,
a photoelectron spectrum obtained by X-ray photoelectron spectroscopy (XPS) measurement shows a peak attributable to S in a binding energy range of 172 eV or more and 176 eV or less, and a peak attributable to B in a binding energy range of 194 eV or more and 198 eV or less, at the particle surface of the lithium transition metal composite oxide and in a region at a depth of less than 100 nm from the surface. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide further contains at least one element selected from the group consisting of Mg, Al, Ca, Nb, Sr, Zr, and W. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein at least one of Na and K is present on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfur compound is a sulfur oxide. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfur compound contains a functional group represented by formula (1):
In the formula, R is an alkyl group having 5 or less carbon atoms. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfur compound is at least one selected from the group consisting of lithium methyl sulfate, lithium ethyl sulfate, lithium propyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, sodium propyl sulfate, potassium methyl sulfate, potassium ethyl sulfate, and potassium propyl sulfate. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the boron compound is boric acid. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 7;
a negative electrode;
a non-aqueous electrolyte;
A non-aqueous electrolyte secondary battery comprising: