WO2025205840A1 - Additive for lithium-ion secondary battery positive electrode using electrolyte solution, lithium-ion secondary battery positive electrode material, and lithium-ion secondary battery - Google Patents

Abstract

Provided are: an effective additive for a lithium-ion secondary battery electrode, other than a ferroelectric material, which can improve input-output characteristics by lowering the electrode-electrolyte solution interfacial resistance R₂ in a lithium-ion secondary battery positive electrode material that contains one or more of Mn, Fe and Ni; and a lithium ion secondary battery positive electrode material and a lithium ion secondary battery containing same.　An additive for a lithium ion secondary battery positive electrode is characterized by containing a composite oxide that is present together with a lithium ion secondary battery positive electrode material containing cations of one or more elements selected from among Mn, Fe, Ni and Co, and in that the composite oxide does not contain alkali metal ions or alkaline earth metal ions, has an acid point and a base point, and has a relative dielectric constant of 20 or more. Also provided are a lithium ion secondary battery positive electrode material and a lithium ion secondary battery containing the additive.

Description

Additive for positive electrode of lithium ion secondary battery using electrolyte, positive electrode material for lithium ion secondary battery, and lithium ion secondary battery

The present invention relates to an additive for a lithium-ion secondary battery positive electrode that uses an electrolyte, a lithium-ion secondary battery positive electrode material, and a lithium-ion secondary battery.

Lithium-ion secondary batteries are widely used as batteries for electronic devices, communications equipment, and other devices. They are also used in automobiles and energy storage facilities. One technology for improving the performance of lithium-ion secondary batteries is the use of additives (composite oxides) in lithium-ion secondary batteries.

Patent Document 1 discloses a positive electrode material for lithium-ion secondary batteries that includes positive electrode active material particles and a ferroelectric, with the ferroelectric being disposed on at least a portion of the surface of the positive electrode active material particles. This is said to result in excellent output characteristics in low-temperature environments.

Patent Document 2 discloses a non-aqueous electrolyte secondary battery that uses a ferroelectric material with a relative dielectric constant of 500 or more sintered onto the surface of a positive electrode active material. This is said to enable sufficient output characteristics to be obtained even in low temperature ranges.

Patent Document 3 discloses a positive electrode material for lithium ion secondary batteries, in which cation A, which has an ionic radius smaller than that of lithium ions and a valence of 2 or more, and cation B, which has an ionic radius larger than that of lithium ions and a valence of 2 or more, are present between the layers of a positive electrode material having a layered crystal structure. Furthermore, typical elements of cation A and cation B are disclosed for producing the positive electrode material for lithium ion secondary batteries. It is believed that this enables the production of a positive electrode for lithium secondary batteries with high output characteristics.

Japanese Patent Application Laid-Open No. 2018-181614 JP 2011-210694 A JP 2023-52895 A

In order to improve the input/output characteristics (rate characteristics) of a lithium-ion secondary battery, it is important to reduce the internal resistance. The internal resistance _Rt can be roughly decomposed into the resistance _R1 within the electrode material (active material), the interface resistance _R2 between the electrode and the electrolyte, and the electrolyte (diffusion) resistance _R3 .
R _t =R ₁ +R ₂ +R ₃

Patent Document 3 proposes improving input/output characteristics by reducing the particle internal resistance _R1 of the positive electrode material. While it is important to improve input/output characteristics by reducing the particle internal resistance _R1 of the positive electrode material, the largest of the internal resistances is the electrode-electrolyte interface resistance _R2 , and reducing this resistance can effectively improve the input/output characteristics of a lithium ion secondary battery.

Patent Documents 1 and 2 aim to improve output characteristics at low temperatures by reducing the interfacial resistance _R2 between the electrode and the electrolyte, but the inventors have discovered a problem in that a ferroelectric material is coexisted to reduce the interfacial resistance _R2 , and because the ferroelectric material contains alkaline earth metal ions, when combined with a positive electrode material containing Mn, Ni, Fe, Co, etc., the components react with the alkaline earth metal ions. Similarly, they have discovered that alkali metal ions also react with the components.

Therefore, reducing the interfacial resistance _R2 between the electrodes and the electrolyte is an effective way to improve the input/output characteristics of lithium-ion secondary batteries, and new materials other than ferroelectrics that can reduce the interfacial resistance _R2 between the electrodes and the electrolyte are desired.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an effective lithium ion secondary battery electrode, particularly an additive for a positive electrode, other than a ferroelectric, that can reduce the interfacial resistance _R2 between the electrode and the electrolyte of a lithium ion secondary battery positive electrode material containing one or more cations selected from the elements Mn, Fe, Ni, and Co, thereby improving the input/output characteristics.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the input/output characteristics can be improved by using a composite oxide containing both acid and base sites and having a relative dielectric constant of 20 or more in combination with a positive electrode material for a lithium-ion secondary battery. The present invention was made based on the above findings and through further research, and is summarized as follows.

[1] An additive for a positive electrode of a lithium ion secondary battery using an electrolyte solution,
The additive comprises a complex oxide that coexists with a positive electrode material for a lithium ion secondary battery, the complex oxide containing cations of one or more elements selected from Mn, Fe, Ni, and Co, and is characterized in that the complex oxide does not contain alkali metal ions or alkaline earth metal ions, has both acid sites and base sites, and has a relative dielectric constant of 20 or more.

[2] The additive according to [1], characterized in that the composition formula of the composite oxide is M _x Zr _1-x O _2-y , the element M contains a trivalent metal ion, and the molar ratios x and y satisfy 0<x<1 and 0<y<0.5, respectively.

[3] The additive described in [2], wherein M contains one or more trivalent metal ions selected from La, Pr, Sm, Nd, Gd, Yb, Ho, and Y.

[4] The additive described in [2] or [3], wherein M contains a metal ion of Ce.

[5] The additive according to any one of [1] to [4], wherein the specific surface area of the composite oxide is 35 m ² /g or more.

[6] A positive electrode material for a lithium ion secondary battery, characterized by containing the additive described in any one of [1] to [5].

[7] A lithium ion secondary battery comprising the lithium ion secondary battery positive electrode material described in [6].

The present invention makes it possible to provide an additive for the positive electrode of a lithium ion secondary battery that can improve the input/output characteristics of a lithium ion secondary battery that uses an electrolyte.

FIG. 1 is an equivalent circuit diagram for determining the interfacial charge transfer resistance (LiCoO ₂ = LCO standard). FIG. 2 is an equivalent circuit diagram for determining the interfacial charge transfer resistance (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ =NCM standard).

<Additive for lithium-ion secondary battery positive electrodes>
The additives for the positive electrode of a lithium ion secondary battery using the electrolyte solution of the present invention will be described below.

The additive for the positive electrode of a lithium ion secondary battery of the present invention (hereinafter also simply referred to as "additive") is used in assembling a lithium ion secondary battery, producing a positive electrode sheet for a lithium ion secondary battery, and manufacturing a positive electrode material for a lithium ion secondary battery.
The additive of the present invention is a composite oxide that coexists with a positive electrode material for a lithium ion secondary battery, the composite oxide containing one or more cations selected from the elements Mn, Fe, Ni, and Co, and the composite oxide does not contain alkali metal ions or alkaline earth metal ions, has both acid sites and base sites, and has a relative dielectric constant of 20 or more.

The positive electrode material for lithium ion secondary batteries is a lithium-containing transition metal composite oxide containing one or more cations selected from the elements Mn, Fe, Ni, and Co, such as LiMn ₂ O ₄ , LiNi _x Co _y Mn _z O ₂ (x + y + z = 1), LiNi _x Co _y Al _x O ₂ (x + y + z = 1), LiNi _x Mn _y O ₄ (x + y = 1), LiCoO ₂ , LiFePO ₄ , Li( _{FexMny )} PO ₄ (x + y = 1), xLi ₂ MnO ₃ -(1-x)LiMeO ₂ (Me = Ni, Co, Mn), Li ₂ MnO _x F _y Examples of the additive of the present invention include Li-containing compounds such as (x + y = 3). The additive of the present invention can be used not only for _LiCoO2 but also for other positive electrode materials to obtain sufficient effects, and can be used widely for various positive electrode materials.

The additive of the present invention comprises a complex oxide that does not contain alkali metal ions or alkaline earth metal ions. Complex oxides that contain alkali metal ions or alkaline earth metal ions will react when coexisting with a positive electrode material that is a lithium-containing transition metal complex oxide that contains one or more cations selected from the elements Mn, Fe, Ni, and Co. Therefore, the additive of the present invention is a complex oxide that does not contain alkali metal ions or alkaline earth metal ions. The Mn, Fe, Ni, and Co cations contained in the positive electrode material are components that form acidic oxides, and therefore, in complex oxides that contain alkali metal ions or alkaline earth metal ions, which are components that form basic oxides, the component that forms the acidic oxide and the component that forms the basic oxide undergo an acid-base reaction.

The composite oxide further has both acidic and basic sites. It is believed that the acidic and basic sites contribute to the effects of the present invention through the following mechanism.

It is known that lithium ions in the electrolyte of a lithium ion secondary battery are solvated with organic solvent molecules. Specifically, during the charge transfer process at the interface between the positive electrode material and the electrolyte of a lithium ion secondary battery, the solvated lithium ions are desolvated when the lithium ions in the electrolyte migrate to the positive electrode, and solvated when the lithium ions leave the positive electrode into the electrolyte. This desolvation and solvation of lithium ions causes charge transfer resistance at the interface between the positive electrode material and the electrolyte, accounting for a large proportion of the internal resistance in lithium ion secondary batteries. In other words, this resistance hinders the high-power characteristics (rate performance) of lithium ion secondary batteries. In the present invention, by incorporating the additive of the present invention into the positive electrode material, the charge transfer resistance at the interface between the positive electrode material and the electrolyte can be reduced, thereby improving the high-power characteristics of lithium ion secondary batteries. That is, since the additive of the present invention is a composite oxide having both acid and base sites, the positively charged lithium ions of the solvated lithium ions tend to adsorb to the base sites (negatively polarized), and the negatively polarized portions of the solvated solvent molecules tend to adsorb to the acid sites (positively polarized). In other words, when both adsorb to the additive, the charge and polarized portions of the solvated lithium ions tend to be neutralized by the additive (the positive charge of the lithium ions receives electrons from the additive, and the negatively polarized portions of the solvated molecules receive electrons from the additive). As a result, the electrostatic attraction between the solvation and the lithium ions weakens, making desolvation easier.
On the other hand, when a positive electrode material receives lithium ions (when discharging), electrons are injected into the positive electrode material via the current collector, which tends to make the material negative. This is convenient for attracting lithium ions, but makes it difficult to desolvate the solvated molecules of the lithium ions. When the additive of the present invention, which is an electronic insulator, is present, the injected electrons do not reach the additive, so the material does not become negative like the positive electrode material.
When charging (when lithium ions are released from the positive electrode material into the electrolyte), the effect of the additive of the present invention can be interpreted as being the opposite of the above explanation.

The complex oxide contained in the additive of the present invention has a dielectric constant of 20 or greater. It is believed that when a lithium-ion secondary battery is charged and discharged, the electric field polarizes the surface of the complex oxide of the additive, adsorbing solvated lithium and promoting desolvation and solvation. That is, if the complex oxide is an insulator with a high dielectric constant, large polarization occurs due to the electric field gradient within the lithium-ion secondary battery. This effect, along with the acid and base sites, acts synergistically to facilitate desolvation and solvation of lithium ions, thereby achieving the effects of the present invention. If the complex oxide has a dielectric constant of less than 20, the electric field gradient within the lithium-ion secondary battery does not cause sufficient polarization, and the synergistic effect with the acid and base sites is not achieved. As a result, the input/output characteristics of the lithium-ion secondary battery are not sufficiently improved. Therefore, the higher the dielectric constant of the complex oxide, the more preferable it is, and there is no upper limit. However, there are currently no paraelectric oxides with a dielectric constant of 500 or greater.

The relative dielectric constant referred to in this invention is the ratio of the dielectric constant of each complex oxide based on the dielectric constant of a vacuum. The dielectric constant is an indicator of the ease of polarization, and the higher the dielectric constant, the higher the relative dielectric constant. In other words, the higher the relative dielectric constant, the more easily the additive (containing the complex oxide) polarizes. For these reasons, a relative dielectric constant of 30 or more is more preferable, and 40 or more is even more preferable.

In the present invention, acid sites and base sites refer to those measured by adsorption and desorption of ammonia (NH3 ₎ using temperature-programmed desorption (TPD), while base sites refer to those measured by adsorption and desorption of carbon dioxide (CO2 ₎ . The ammonia or carbon dioxide standard substance is first adsorbed onto the measurement sample, and the amount of the standard substance desorbed is then measured while the temperature is increased, thereby measuring the acid sites and base sites and their amounts. Specifically, the measurement sample is pretreated by heat treatment at 370°C for 30 minutes in a 20% _O2 /He atmosphere, and then the sample is exposed to the ammonia or carbon dioxide standard substance at 50°C for 30 minutes to adsorb the standard substance. The temperature is then increased from 50°C to 500°C at a rate of 20°C/min, and the amount of standard substance desorbed during this process is measured.

In the present invention, having acid sites and basic sites means that the amount of acid sites measured at 50°C to 500°C is 0.25 mmol/g or more, and the amount of basic sites measured at 50°C to 500°C is 0.18 mmol/g or more. The amount of acid sites measured at 50°C to 500°C is more preferably 0.30 mmol/g or more, and even more preferably 0.40 mmol/g or more. The amount of basic sites measured at 50°C to 500°C is more preferably 0.25 mmol/g or more, and even more preferably 0.35 mmol/g or more.

Furthermore, the maximum value (maximum peak) of the NH ₄ -TPD curve is more preferably 3.0×10 ⁻⁴ mmol/g·sec or more, and the maximum value (maximum peak) of the _{CO 2} -TPD curve is more preferably 2.5×10 ⁻⁴ mmol/g·sec or more.

The measurement device used was a BELCAT-A manufactured by Japan BEL, and the standard substances used were _NH3 for evaluating acid sites and _CO2 for evaluating basic sites. A TCD (Thermal Conductivity Detector) was used as the detector.

After pretreatment at 370°C for 30 minutes in a 20% _O2 /He atmosphere, the sample was exposed to a standard substance at 50°C for 30 minutes to adsorb the standard substance. The temperature was then increased from 50°C to 500°C at a rate of 20°C/min, and the amount or temperature of the standard substance desorbed was evaluated.

As described above, the coexistence of the complex oxide contained in the additive of the present invention with the lithium-ion secondary battery positive electrode material refers to a state in which the complex oxide is added during the assembly of a lithium-ion secondary battery, the preparation of a lithium-ion secondary battery positive electrode sheet, or the manufacture of a lithium-ion secondary battery positive electrode material. Generally, the lithium-ion secondary battery positive electrode material may be dispersed on the surface or within the positive electrode layer formed on the current collector. Therefore, preferably, the complex oxide additive coexists in the vicinity of the lithium-ion secondary battery positive electrode material on the surface or within the positive electrode layer. More preferably, the complex oxide additive coexists in contact with the lithium-ion secondary battery positive electrode material on the surface or within the positive electrode layer. Ideally, all of the added complex oxide would be in the above-described state; however, the effects of the present invention are sufficiently achieved even if only a portion of the complex oxide is in the above-described state.

The proximity of the lithium ion secondary battery positive electrode material refers to, for example, the distance at which lithium ions desolvated on the surface of the added composite oxide can reach the lithium ion secondary battery positive electrode material without resolvating in the solvent. For example, this is 100 nm or less, more preferably 50 nm or less, and even more preferably 25 nm or less.

The composite oxide contained in the additive of the present invention preferably contains particles smaller than those of the positive electrode material of a lithium ion secondary battery. For example, it is more preferable that the respective average particle diameters _D50 satisfy the relationship _D50 (composite oxide) < _D50 (positive electrode material). Even more preferably, the relationship _D50 (composite oxide) < [ _D50 (positive electrode material)/10]. Note that _D50 is the particle diameter at which the cumulative volumetric frequency is 50%, and is also called the median diameter.
The composite oxide contained in the additive of the present invention preferably contains 35% or more, more preferably 45% or more, of particles with a diameter of 1 μm or less by volume. Since the effect of the composite oxide is greater with smaller particles, the content of such fine particles is preferred. That is, it is more preferred that there are many fine particles attached to or in close proximity to each particle of the positive electrode material. The above-described state is more preferred because desolvation and solvation of lithium ions occur on the surface of the composite oxide, which is the additive of the present invention, and lithium ions are efficiently supplied to each particle of the positive electrode material, as described above.

The composite oxide of the present invention refers to an oxide containing two or more types of cations, such as ( ^M1 , ^M2 , ^M3 , ^M4 , ..., ^Mn ) _Ox . However, for the reasons mentioned above, ^M1 , ^M2 , ^M3 , ^M4 , ..., ^Mn do not contain alkali metal ions or alkaline earth metal ions. A simple _oxide of _MxOy cannot have both acidic and basic sites, and therefore the effects of the present invention cannot be achieved. Possessing both acidic and basic sites can be achieved by appropriately combining multiple cations. This is because combining multiple cations with different valences and electronegativities creates a large polarization state on the surface of the composite oxide, i.e., the formation of acidic and basic sites. However, because the polarization state varies depending on the coordination state of the cations and the crystalline structure of the composite oxide, there is no set rule for the combination. Therefore, to achieve the effects of the present invention, the acidic and basic sites must be determined as described above. More preferably, oxygen vacancies are formed. The presence of oxygen vacancies makes it easier to form acidic and basic sites.

The composite oxide is
Compositional formula M _x Zr _1-x O _2-y ...Compositional formula (1)
It is more preferable that M contains a trivalent metal ion. Zirconium (Zr) ions are preferred as an additive component for batteries because they are stable against oxidation and reduction and are difficult to dissolve as a single cation. Furthermore, by substituting a portion of zirconium oxide (ZrO ₂ ) with a trivalent cation M, oxygen vacancies are formed, which is more preferable because acid sites and base sites can be effectively formed as described above. While 0<x<1 and 0<y<0.5 are sufficient, from the perspective of the degree of association of the oxygen vacancies formed, 0<x<0.9, 0<y<0.45, and even more preferably 0.1<x<0.6 and 0.05<y<0.3 are preferred. Furthermore, the composite oxide of composition formula (1) is preferably cubic or pseudo-cubic.

M is preferably a trivalent cation of a rare earth element, scandium, or yttrium. More preferably, M is a composite oxide containing one or more trivalent metal ions selected from La, Pr, Sm, Nd, Gd, Yb, Ho, and Y. Oxygen vacancies formed by these cations are more likely to form acidic and basic sites when effectively arranged. In addition, composite oxides in which M contains multiple types of cations may be preferred.

It is more preferable that the M contains a Ce metal ion. The valence of the Ce metal ion (cation) is preferably tetravalent. Tetravalent M cannot form oxygen vacancies, but has a higher electron density than Zr ions, making it more preferable as it increases the dielectric constant of the composite oxide. The Ce contained in the composite oxide is more preferably in a Ce/(Ce+Zr) molar ratio of 0.05 to 0.9, even more preferably 0.1 to 0.8, and even more preferably 0.1 to 0.6.

The composite oxide contained in the additive of the present invention preferably has a specific surface area of 35 m ² /g or more. By making the specific surface area 35 m ² /g or more, the number of sites for solvation and desolvation of lithium ions increases, making the effects of the present invention more pronounced. Alternatively, a sufficient effect can be obtained with a small amount of additive. For the above reasons, the specific surface area is preferably even larger, more preferably 55 m ² /g or more, and even more preferably 60 m ² /g or more, 70 m ² /g or more, 80 m ² /g or more, 90 m ² /g or more, 100 m ² /g or more, or 150 m ² /g or more. Although it is experimentally possible to further increase the specific surface area of the composite oxide, the maximum industrial value is approximately 200 m ² /g.

As a pretreatment, approximately 0.3 g of sample is placed in a flask-type sample cell, and degassed using a FloVac degasser (manufactured by Anton Paar Japan) at 370°C for 40 minutes under a nitrogen gas flow. Specific surface area can then be measured using a surface area measuring device (NOVAtouch NX-4LX-1, manufactured by Anton Paar Japan) using the BET method (single-point method) based on nitrogen gas adsorption.

Next, we will explain a method for producing an additive for a lithium-ion secondary battery positive electrode using the electrolyte solution of the present invention.

The additive for the positive electrode of a lithium ion secondary battery using the electrolyte solution of the present invention may be prepared by any method that can produce a composite oxide that satisfies the requirements of the present invention, such as a solid-state reaction method, a chemical vapor deposition method, a hydrothermal method, a wet synthesis method, a sol-gel method, or a spray pyrolysis method.
An example of wet synthesis is shown below.

First, the raw materials for the composite oxide contained in the additive are dissolved in water to prepare a raw material aqueous solution. Examples of the raw materials include water-soluble salts such as nitrates, chlorides, sulfates, and acetates. Specific examples of raw materials include zirconium oxychloride, and chlorides of cations other than zirconium, such as La, Pr, Sm, Nd, Gd, Yb, Ho, Ce, and Y. By dissolving these raw materials in water (preferably pure water or ion-exchanged water) at a specified molar ratio, an aqueous solution containing all the raw materials can be obtained. There are no particular restrictions on the dissolving and stirring methods.

Next, a precipitant is added to the raw material aqueous solution to co-precipitate the composite oxide raw materials, obtaining a precipitate (slurry). The precipitant, for example, controls the pH to a pH range in which the metal ions precipitate. Precipitation can typically be achieved by adding a basic compound such as ammonia water or a hydroxide (NaOH, KOH, etc.) to raise the pH (to make it basic). The resulting slurry is preferably heated to 70-90°C. The heating method is not particularly limited. Heating the slurry promotes olation and oxidation reactions within the precipitate. For example, the resulting slurry is heated to between 70-90°C and maintained at this temperature for 4-10 hours (aging). During aging, industrially standard agitation procedures, such as stirring with a stirring blade or circulation using a pump, can also be performed, which is preferred.

Alternatively, the raw material aqueous solution can be heated and the precipitant added. Heating the raw material aqueous solution changes the behavior of nucleation, which can alter the aggregation structure of the precipitate.

After aging, the slurry is subjected to solid-liquid separation and washing. There are no particular restrictions on the means of solid-liquid separation, and industrially used equipment such as a centrifuge or filter can be used. The washing operation is an operation to reduce chloride ions from the hydroxide slurry containing zirconium and other cations, which is the raw material for the composite oxide. For example, after dispersing the hydroxide in pure water, it is carried out using a centrifuge and filter. It is preferable to continue this washing operation until the chloride ion content in the hydroxide is 3% or less by mass.

After washing, the precipitate containing zirconium and other cations is dried. The drying is carried out using an industrially used externally heated or internally heated drying device, for example, at a temperature of about 100-200°C using hot air or a heater. Spray drying is also possible.

Following drying, the product is fired. The temperature of the firing furnace is determined appropriately to meet the requirements of the present invention in accordance with the precipitation and drying conditions, for example, 400 to 900°C. The firing time is usually between 30 minutes and 10 hours, and, like the firing temperature, is determined appropriately to meet the requirements of the present invention. The above drying and firing operations can be performed independently, or as a continuous series of operations. The precipitate can also be fired directly without going through the drying process.

In the case of the composition formula M _x Zr _1-x O _2-y , the firing conditions are preferably such that a pseudocubic crystal is formed, for example, at a temperature of 900° C. or less, more preferably 600° C. or less.

The calcination process dehydrates the hydroxides containing zirconium and other cations that serve as the raw materials for the composite oxide, turning them into composite oxides. The composite oxide obtained in this manner may be pulverized to adjust the particle size, if necessary. The pulverization can be carried out using industrially used mills such as stamp mills, roller mills, jet mills, or ball mills, as long as the desired particle size can be achieved.

In one embodiment of the present invention, a lithium ion secondary battery positive electrode material is provided that contains the above-mentioned lithium ion secondary battery positive electrode additive. The composite oxide obtained above is used as an additive and mixed with a lithium ion secondary battery positive electrode material containing one or more cations selected from the elements Mn, Fe, Ni, and Co to obtain a positive electrode active material. If the additive coexists with the positive electrode material, mixing may be performed using an industrial mixer such as a V-type mixer, screw mixer, or air mixer. Furthermore, heat treatment at 200°C to 400°C may be performed after mixing.

In one embodiment of the present invention, a lithium-ion secondary battery is provided that includes the above-described lithium-ion secondary battery positive electrode material. When the lithium-ion secondary battery positive electrode material containing the above-described additive is used as the positive electrode active material, a material capable of absorbing and releasing lithium, such as a carbon material or a lithium-absorbing alloy, is used as the negative electrode active material, and a non-aqueous electrolyte solution in which a lithium salt is dissolved in a non-aqueous electrolyte solution or resin can be used as the electrolyte. For example, lithium hexafluorophosphate (LiPF ₆ ) is used as the lithium salt, and a mixed solution of ethylene carbonate and diethyl carbonate is used as the non-aqueous electrolyte. Other examples of lithium salts that can be used include LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , LiN(SO ₃ CF ₃ ) ₂ , and mixtures thereof. Furthermore, diethyl carbonate, propylene carbonate, vinylene carbonate, and mixtures thereof can also be used as the non-aqueous electrolyte.

The present invention will be explained below using examples and comparative examples. Note that the present invention is not limited to these examples.

Example 1
Zirconium oxychloride and yttrium chloride adjusted to a composition formula of Y _0.3 Zr _0.7 O _1.85 were mixed to prepare an aqueous solution. Ammonia water was added dropwise to the aqueous solution while stirring to raise the pH and cause coprecipitation, obtaining a precipitate. The solution slurry containing the precipitate was aged at 80°C for 6 hours. The slurry was subjected to a filtration-washing procedure five times using a filtration device to obtain a precipitate cake. The obtained cake was placed in a sagger and dried at 120°C for 10 hours. The dried sample was calcined at 600°C for 10 hours. The calcined sample obtained was pulverized using a jet mill to form an additive for the positive electrode of a lithium-ion secondary battery.

Examples 2 to 11
In Examples 2 to 11, the aqueous solutions were prepared to have the compositions shown in Table 1, and the additives for the positive electrodes of lithium ion secondary batteries were prepared under the same conditions as in Example 1.

(Comparative Example 1)
In Comparative Example 1, no additive was added, and only the positive electrode material was used as the positive electrode active material.

(Comparative Example 2)
_ZrO2 was prepared as a comparative oxide. _ZrO2 was prepared by dissolving zirconium oxychloride in water to prepare an aqueous solution, and then adding ammonia water dropwise to the aqueous solution while stirring to raise the pH and cause co-precipitation, obtaining a precipitate. The solution slurry containing the precipitate was aged at 80°C for 6 hours. The slurry was subjected to a filtration-washing procedure five times using a filtration device to obtain a precipitate cake. The obtained cake was placed in a sagger and dried at 120°C for 10 hours. The dried sample was calcined at 600°C for 10 hours. The calcined sample was then pulverized using a jet mill.

(Comparative Example 3)
_BaTiO3 was used as a reagent KZM-50 manufactured by Sakai Chemical Industry Co., Ltd.

(Comparative Examples 4 and 5)
Comparative Examples 4 and 5 were prepared under the same conditions as Example 1, except that zirconium oxychloride and chlorides containing each element were used as starting materials so as to have the compositions shown in Table 1.

<Crystal structure analysis>
The obtained sample was evaluated by X-ray diffraction. A sample of the composite oxide (=additive) for use as a positive electrode active material in a lithium-ion secondary battery was placed in a sample holder, and X-ray diffraction measurement was performed using a Rigaku MiniFlex 600 desktop X-ray diffractometer (CuKα radiation source) under the conditions of a 2θ measurement angle range of 20 to 80°, a step of 0.02°, and a scan speed of 5°/min.

<Reactivity with positive electrode materials>
To evaluate whether the composite oxide coexists with the positive electrode material of a lithium-ion secondary battery, the base positive electrode material and the additive composite oxide were mixed, and it was confirmed by X-ray diffraction whether only the diffraction peaks of the positive electrode material and the composite oxide were observed.
_LiCoO2 or _{LiNi0.5Co0.2Mn0.3O2} was mixed with additives such _as composite oxides in _a mortar at a mass ratio of 5:1, and then fired at 400°C for 10 hours. X-ray diffraction measurements were performed on the fired product using a Rigaku _MiniFlex600 desktop X-ray diffractometer with a CuKα radiation source. The X-ray diffraction measurements were performed under the following conditions: 2θ measurement angle range of 20-80°, step of 0.02°, and scan speed of 5°/min.

<Evaluation of acid and base sites>
The acid and base sites were measured using a BELCAT-A manufactured by BEL Japan, and the amount of desorption of the standard substance or the temperature was evaluated. _NH3 was used as the standard substance for evaluating the acid sites, and _CO2 was used for evaluating the base sites. A TCD (Thermal Conductivity Detector) was used as the detector. As a pretreatment, the prepared sample was treated at 370°C for 30 minutes in a 20% _O2 /He atmosphere, and then exposed to the standard substance at 50°C for 30 minutes to adsorb the standard substance. The temperature was then raised from 50°C to 500°C at a rate of 20°C/min, and the amount of desorbed standard substance or the temperature was evaluated.

<Particle size distribution measurement>
The particle size distribution was measured by dispersing a sample in a sodium hexametaphosphate solution using an ultrasonic homogenizer (US-300AT, manufactured by Nippon Seiki Seisakusho) and evaluating the dispersion using a particle size distribution analyzer MT3300EXII (manufactured by Microtrac Bell).

<Specific surface area measurement>
The specific surface area was measured by pretreatment, in which approximately 0.3 g of the sample was placed in a flask-type sample cell and degassed using a FloVac degasser (manufactured by Anton Paar Japan) at 370°C for 40 minutes under a nitrogen gas flow. Thereafter, the specific surface area was measured by the BET method (single-point method) using nitrogen gas adsorption using a surface area measuring device (NOVAtouch NX-4LX-1, manufactured by Anton Paar Japan).

<Dielectric constant measurement>
The dielectric constant of the obtained composite oxide was measured at 25°C. An Agilent 4294A (manufactured by Agilent Technologies) was used as the measurement device. Evaluation was performed in the frequency range of 40 Hz to 110 MHz. The Cole-Cole function was used for fitting, and the powder dielectric constant was calculated using the Bruggman model. The relative dielectric constant was calculated as the ratio of the dielectric constant of each composite oxide based on the dielectric constant of a vacuum.

<Battery manufacturing>
The positive _electrode active materials were prepared by mixing the composite oxides and other additives synthesized in each example and comparative example _with the positive electrode materials _LiCoO2 and _{LiNi0.5Co0.2Mn0.3O2 ,} respectively.
In Comparative Example 1, a battery was fabricated without adding any composite oxide and using only the positive electrode material as the positive electrode active material.
In Examples 1 to 11 and Comparative Examples 2 to 5, 1 mol % of an additive such as a composite oxide was weighed out relative to the positive electrode material, and they were mixed in a mortar. The mixed powder was kept at 400°C for 10 hours, and the resulting fired powder was used as the positive electrode active material to fabricate a positive electrode.

Acetylene black was used as the conductive additive, and PVdF was used as the binder.
The positive electrode active material, conductive additive, and binder were weighed in a weight ratio of 7:2:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode slurry.
The obtained slurry was applied to an aluminum current collector, dried, punched into a disk, and pressed to prepare a positive electrode.

In the examples and comparative examples, a disc-shaped piece of metallic lithium was used as the positive electrode, a negative electrode, and an electrolyte solution in which 1 mol/L of solute _LiPF6 was dissolved in a solvent obtained by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 3:7 (layered), respectively, and a coin-type battery CR2032 type (diameter 20 mm, height 3.2 mm) was assembled to perform battery evaluation and measurement.

<Charge/Discharge Test>
A charge-discharge test of a coin-type lithium-ion secondary battery fabricated using _LiCoO2 as the positive electrode material was performed using a TOSCAT-3100 (manufactured by Toyo Systems). The coin-type lithium-ion secondary battery was placed in a thermostatic chamber at 25°C, with a reference capacity of 1C set to 160 mA/g, an upper voltage limit of 4.5 V, and a lower voltage limit of 3.3 V. The charge rate was 1C, and the discharge rate ranged from 0.1C to 100C. 41 cycles of charge-discharge tests were performed. Two cycles were performed at 0.1C, one each at 0.2C and 0.5C, and five cycles at 1C or higher. A four-hour wait was allowed before the test began. A one-hour rest period was allowed after each charge and discharge.

<Deterioration test>
A degradation test was conducted on a coin-type lithium _- ion secondary battery fabricated using _{LiNi0.5Co0.2Mn0.3O2} as the positive _electrode material. _The test was conducted using an Electrofield ABE1024-5V device. The coin-type lithium-ion secondary battery was placed in a thermostatic chamber at 45°C and charged at a constant current rate of 0.2C with a reference capacity of 1C set to 140mA/g. After reaching a voltage of 4.2V, constant voltage charging was continued for 336 hours.

<Impedance measurement>
After the charge/discharge test and the deterioration test, the coin-type lithium ion secondary battery was subjected to impedance measurement in a thermostatic chamber at 20° C. using VSP-300 (manufactured by biologic).

For coin-type lithium-ion secondary batteries using _LiCoO2 as the positive electrode material, measurements were performed at a potential of 4.5 V, an AC amplitude of 10 mV, and a frequency range of 5 mHz to 7 MHz. The obtained data was fitted to the equivalent circuit shown in Figure 1 to calculate the interfacial charge transfer resistance R _LCO of the coin cell using LiCoO2 _alone as the positive electrode active material, and the charge transfer resistance R _ct-LCO of each battery. The ratio R _{ct - LCO} /R _LCO to the interfacial charge transfer resistance R _LCO of the coin-type lithium-ion secondary battery using LiCoO2 alone as the positive electrode active material was calculated.

For coin-type lithium _{- ion} secondary batteries using _{LiNi0.5Co0.2Mn0.3O2} as the positive _electrode material, measurements were performed at a potential of 4.2 V, an AC amplitude of 10 mV, and a frequency range of 10 mHz to 200 kHz. The obtained data was fitted to the equivalent circuit shown in Figure 2 to calculate the interfacial charge transfer resistance _Rct-NCM . The interfacial charge transfer resistance _Rncm of the coin-type lithium-ion secondary battery using _{only LiNi0.5Co0.2Mn0.3O2} as the positive _electrode active material was used as the reference, and the ratio Rct _{-NCM /} Rncm of the _Rct - _NCM of each sample was calculated.

As shown in Table 1, Examples 1 to 11 have a relative dielectric constant of 20 or greater and contain both acid and basic sites, confirming the effects of the present invention. The specific amounts of acid and basic sites in Examples 1 to 11 are 0.25 mmol/g or greater when measured at 50°C to 500°C, and 0.18 mmol/g or greater when measured at 50°C _{to 500} °C. When these composite oxides were mixed with _{LiNi0.5Co0.2Mn0.3O2} in a mass ratio of 5: ₁ and calcined at 400°C for 10 _hours , only diffraction peaks attributable to _LiCoO2 or _{LiNi0.5Co0.2Mn0.3O2} and the composite _oxides of each Example were observed. This confirmed that these composite oxides do _not react with positive electrode materials containing Ni, Co, Mn, or Fe. On the other hand, in Comparative Examples 4 and 5, peaks other than those attributable to _the base material _LiCoO2 or _{LiNi0.5Co0.2Mn0.3O2} and the _additives were also observed, confirming _{that these additives} react with _{LiCoO2 or LiNi0.5Co0.2Mn0.3O2} .
Composite oxides having a composition formula of M _x Zr _1-x O _2-y , where M is an element in Table 1 and x=0.1, 0.5, 0.7, 0.9, etc., that satisfy the requirements of the present invention can be produced, and the effects of the present invention have been confirmed.

The interfacial charge transfer resistance ratio _Rct /R was calculated for each battery, and _it was confirmed that when _{LiCoO2 or LiNi0.5Co0.2Mn0.3O2} was used as the positive _electrode material, the batteries using the additives of the examples showed smaller values than the comparative examples. The smaller _{the Rct} /R, the smaller the charge transfer resistance, indicating improved rate characteristics. From this, it was confirmed that the composite oxides listed in the examples have the effect of improving the rate characteristics of coin-type lithium-ion secondary batteries.

Claims (7)

An additive for a positive electrode of a lithium ion secondary battery using an electrolyte solution,
the additive material includes a composite oxide that coexists with a lithium ion secondary battery positive electrode material that includes cations of one or more elements selected from Mn, Fe, Ni, and Co;
The additive is characterized in that the composite oxide does not contain alkali metal ions or alkaline earth metal ions, has both acid sites and base sites, and has a relative dielectric constant of 20 or more. The composition formula of the composite oxide is
M _x Zr _1-x O _2-y
2. The additive according to claim 1, wherein the element M contains a trivalent metal ion, and the molar ratios x and y satisfy 0<x<1 and 0<y<0.5, respectively. The additive described in claim 2, characterized in that M contains one or more trivalent metal ions selected from La, Pr, Sm, Nd, Gd, Yb, Ho, and Y. The additive according to claim 2 or 3, characterized in that M contains metal ions of Ce. 2. The additive according to claim 1, wherein the specific surface area of the composite oxide is 35 m ² /g or more. A positive electrode material for a lithium ion secondary battery, comprising the additive described in claim 1. A lithium ion secondary battery comprising the lithium ion secondary battery positive electrode material described in claim 6.