WO2024195260A1 - Secondary battery - Google Patents

Abstract

The present invention improves load characteristics. This secondary battery comprises a positive electrode, a negative electrode, a separator, and an electrolyte solution. The positive electrode comprises a positive electrode current collector layer and a positive electrode active material layer. The positive electrode active material layer contains particles of a positive electrode active material having a layered rock salt type crystal structure. The particles of the positive electrode active material have a plate shape. In an X-ray diffraction pattern for the main surface of the positive electrode active material layer, when the peak intensity of the (003) plane is defined as I₀₀₃ and the peak intensity of the (110) plane is defined as I₁₁₀, I₁₁₀/I₀₀₃≥10, and the porosity of the positive electrode active material layer is 10% or less.

Description

Secondary battery

The present invention relates to a secondary battery.

Patent Document 1 describes a lithium ion battery having a positive electrode section composed of a positive electrode side current collecting layer and a positive electrode electrically connected to the positive electrode side current collecting layer, and a negative electrode section composed of a negative electrode side current collecting layer and a negative electrode layer electrically connected to the negative electrode side current collecting layer, in which the positive electrodes are each composed of a lithium composite oxide having a layered rock salt structure, and are made of a sintered plate composed of a plurality of plate-shaped primary particles bonded to each other, each of the plurality of primary particles is capable of conducting lithium ions parallel to the plate surface, the average orientation angle of the plurality of primary particles with respect to the plate surface direction parallel to the plate surface is greater than 0° and less than 30°, and in a cross section perpendicular to the plate surface, the total area of the plurality of primary particles having an aspect ratio of 4 or more is 70% or more of the total area of the plurality of primary particles.

JP 2019-71301 A

However, the secondary battery shown in Patent Document 1 may not have sufficient load characteristics.

The present invention was made in consideration of the above problems, and aims to improve load characteristics.

A secondary battery according to one aspect of the present invention includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive electrode current collector layer and a positive electrode active material layer. The positive electrode active material layer contains particles of a positive electrode active material having a layered rock salt crystal structure. The particles of the positive electrode active material are plate-shaped. In an X-ray diffraction pattern for a main surface of the positive electrode active material layer, when a peak intensity of a (003) plane is I ₀₀₃ and a peak intensity of a (110) plane is I ₁₁₀ , I ₁₁₀ /I ₀₀₃ ≧10, and the positive electrode active material layer has a porosity of 10% or less.

The present invention can improve load characteristics.

FIG. 1 is a cross-sectional view illustrating an example of a secondary battery according to a first embodiment. FIG. 2 is an enlarged cross-sectional view showing a part of the cross section of the electrode body shown in FIG. FIG. 3 is a diagram showing X-ray diffraction peak data of a compound having a layered rock salt type crystal structure. FIG. 4 is a cutaway view showing a different example of the secondary battery according to the first embodiment. FIG. 5 is a schematic cross-sectional view taken along line VV in FIG.

The following describes an embodiment of the present invention. Note that the present invention is not limited to this embodiment.

(Secondary battery)
Fig. 1 is a cross-sectional view showing an example of a secondary battery according to a first embodiment. The secondary battery 1 shown in Fig. 1 is a laminated lithium ion secondary battery. As shown in Fig. 1, the secondary battery 1 includes a battery element 20, an exterior member 30, and an adhesive 32.

The battery element 20 is provided inside the exterior member 30. As shown in FIG. 1, the battery element 20 includes an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal drawn from a positive electrode 210 (described later) to the outside of the exterior member 30. In other words, the positive electrode lead 21 is a terminal that serves as the positive electrode of the secondary battery 1. In FIG. 1, the positive electrode lead 21 is provided on an end surface of the electrode body 200. The negative electrode lead 22 is a terminal drawn from the inside of a negative electrode 220 (described later) to the outside of the exterior member 30. In other words, the negative electrode lead 22 is a terminal that serves as the negative electrode of the secondary battery 1. In FIG. 1, the negative electrode lead 22 is provided on an end surface of the electrode body 200. Details of the electrode body 200 will be described later.

The exterior member 30 is a case in which the battery element 20 is housed. The exterior member 30 includes two exterior sheets 30a and 30b. The exterior sheets 30a and 30b each include an insulating layer, a metal layer, and an outermost layer. In the example of FIG. 1, the exterior sheet 30a has a recess 31. As a result, the battery element 20 is housed in the exterior member 30 by housing the battery element 20 in the recess 31 and bonding the peripheral portions of the exterior sheets 30a and 30b.

The exterior sheets 30a, 30b are structured such that an insulating layer, a metal layer, and an outermost layer are laminated in this order from the inside, i.e., from the side where the battery element 20 is provided, and then pasted together by lamination or the like. The insulating layer of the exterior sheets 30a, 30b is made of a resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a polyolefin resin containing ethylene or propylene as a monomer. This allows the exterior sheets 30a, 30b to reduce the moisture permeability of the secondary battery 1 and improve airtightness. The metal layer of the exterior sheets 30a, 30b is a metal plate material or foil material such as aluminum, stainless steel, nickel, or iron. The outermost layer may be made of any material, but is preferably made of a material with high strength against tears, punctures, etc., such as the same resin as the insulating layer, or nylon.

The adhesive 32 is a member for making the exterior member 30 airtight. The adhesive 32 is provided between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22. It is preferable that the material of the adhesive 32 has adhesion to the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metal material, the adhesive 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. As a result, the adhesive 32 can seal the gap between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22, making the interior of the exterior member 30 airtight.

FIG. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode body according to FIG. 1. More specifically, FIG. 2 is a cross-sectional view showing a portion of one layer of a positive electrode 210 and one layer of a negative electrode 220 of the electrode body 200. As shown in FIG. 2, the electrode body 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode body 200 has a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 interposed therebetween. The positive electrode 210 and the negative electrode 220 included in the electrode body 200 are layered members for the charge/discharge reaction of the secondary battery according to the first embodiment.

The positive electrode 210 includes a positive electrode collector layer 211 and a positive electrode active material layer 212. In the positive electrode 210, the positive electrode collector layer 211 is laminated between the positive electrode active material layers 212.

The positive electrode collector layer 211 is a conductive layer, and may be made of, for example, aluminum foil. In the example of FIG. 1, the shape of the positive electrode collector layer 211 is a rectangular sheet with a protrusion on the positive electrode lead 21 side when viewed in a plan view in the thickness direction. The protrusion of the positive electrode collector layer 211 is connected to the positive electrode lead 21.

The positive electrode active material layer 212 is a layer containing particles of positive electrode active material. In the first embodiment, the positive electrode active material layer 212 is a sintered body of positive electrode active material. In other words, the positive electrode active material layer 212 is a bulk layer in which the grain boundaries of the particles of the positive electrode active material are in contact with each other. The shape of the particles of the positive electrode active material is plate-like, that is, flat with a length in one direction shorter than in the other direction. Here, the shape of the grain boundaries of the particles of the positive electrode active material layer can be examined by observing the cross section of the positive electrode active material layer 212 with a scanning electron microscope (SEM).

The positive electrode active material has a layered rock-salt type crystal structure (space group: R-3m). Examples of the positive electrode active material having a layered rock-salt type crystal structure include lithium-containing composite oxides. The lithium-containing composite oxide is an oxide containing _lithium and one _{or more} elements other _{than lithium} as _constituent elements _, such _{as LiNiO2 , LiCoO2 , LiCo0.98Al0.01Mg0.01O2} , _{LiNi0.5Co0.2Mn0.3O2 , LiNi0.8Co0.15Al0.05O2 , LiNi0.33Co0.33Mn0.33O2 , Li1.2Mn0.52Co0.175Ni0.1O2 , Li1.15 ( Mn0.65Ni0.22Co0.13 ) O2 , and LiMn2O . 4,} etc.

In this embodiment, the particles of the positive electrode active material have a flat shape in which the c-axis direction in the layered rock salt crystal structure is shorter than the a-axis direction and the b-axis direction. In the layered rock salt crystal structure, layers made of transition metal oxides (e.g., _CoO2 layers or _NiO2 layers) are stacked in the c-axis direction, so in the particles of the positive electrode active material according to this embodiment, the stacking in the c-axis direction of the layers made of transition metal oxides is suppressed, and it can be said that they grow so as to spread in a direction perpendicular to the c-axis direction.

FIG. 3 is a diagram showing X-ray diffraction peak data of a compound having a layered rock salt type crystal structure. More specifically, FIG. 3 is a graph showing the diffraction angle (2θ) and intensity (Intensity) of a peak observed when a powder X-ray diffraction measurement is performed on lithium cobalt oxide using Cu-Kα radiation. In the X-ray diffraction pattern for the main surface of the positive electrode active material layer 212, when the peak intensity of the (003) plane is I ₀₀₃ and the peak intensity of the (110) plane is I ₁₁₀ , I ₁₁₀ /I ₀₀₃ ≧10. Here, the main surface of the positive electrode active material layer 212 refers to the surface of the positive electrode active material layer 212 on the negative electrode 220 side. In addition, the peaks indicating the (003) plane and the (110) plane are identified by crystal structure analysis based on a peak chart obtained by X-ray diffraction. In the example of FIG. 3, peak P appearing near 2θ=18.9° is the peak of the (003) plane, and peak Q appearing near 2θ=66.4° is the peak of the (110) plane.

When the I ₁₁₀ /I ₀₀₃ of the main surface of the positive electrode active material layer 212 is large, it can be said that many (110) faces of the positive electrode active material are distributed on the main surface of the positive electrode active material layer 212. Here, in the particles of the positive electrode active material having a layered rock salt type crystal structure, lithium ions are inserted and removed between the (110) faces perpendicular to the (003) face, and thus the charge and discharge reaction in the positive electrode 210 occurs. Therefore, when I ₁₁₀ /I ₀₀₃ ≧10, many (110) faces are arranged so as to be stacked in a direction perpendicular to the main surface of the positive electrode collector layer 211, so that the positive electrode active material layer 212 can easily insert and remove lithium ions, and the electronic conductivity with the positive electrode collector layer 211 is improved. This can improve the charging load characteristics.

The crystal structure of the positive electrode active material can be measured by X-ray diffraction. Specifically, X-ray diffraction is performed on the main surface of the positive electrode active material layer 212 under the following conditions, and the obtained X-ray diffraction chart is subjected to Rietveld analysis in the space group R-3m to determine whether or not the crystal structure is a layered rock salt type, and the peaks of the (003) and (110) planes are identified to calculate I ₁₁₀ /I ₀₀₃ .
Measurement equipment: Bruker D8 ADVANCE
X-ray tube: Cu-Kα ray Output voltage: 40 kV
Output current: 40mA
Sampling interval: 0.02°
Counting time: 0.5 sec/step

The porosity of the positive electrode active material layer 212 is 10% or less. As a result, the positive electrode active material layer 212 has a small interface that comes into contact with the electrolyte, improving the charging load characteristics.

The porosity of the positive electrode active material layer 212 is a ratio expressed by (ρ- _ρr )/ _ρr , where _ρr is the true density of the positive electrode active material layer 212 and ρ is the density of the positive electrode active material layer 212. That is, the smaller the porosity of the positive electrode active material layer 212, the closer the density of the positive electrode active material layer 212 is to the true density, and therefore the smaller the voids in the positive electrode active material layer 212. The true density _ρr of the positive electrode active material layer 212 is a weighted average obtained by weighting the true densities of the materials constituting the positive electrode active material layer 212 by mass ratio. The density ρ of the positive electrode active material layer 212 is the weight per unit area of the positive electrode active material layer 212 with respect to the thickness of the positive electrode active material layer 212. A method for measuring the thickness of the positive electrode active material layer 212 will be described later.

Here, when the positive electrode active material is lithium cobalt oxide, the true density _ρr is set to 5.05 g/ ^cm3 ; when it is lithium nickel oxide, the true density _ρr is set to 4.8 g/ ^cm3 ; and when it is a Ni-Mn-Co ternary system (e.g., Li(Ni _1/3 Mn _1/3 Co _1/3 )O ₂ ), the true density _ρr is set to 4.6 g/ ^cm3 , and the porosity can be calculated accordingly.

In the calculation of the porosity, the value of the true density of the positive electrode active material employed can be determined by measuring the crystal structure of the positive electrode active material by powder X-ray diffraction or the like and measuring the ratio of the transition metal element contained in the transition metal contained in the transition metal site. When the crystal structure is mainly a layered rock salt crystal structure and the molar ratio of cobalt (Co) to the transition metal contained in the transition metal site is 50% or more, the positive electrode active material can be calculated as lithium cobalt oxide, that is, the true density _ρr is 5.05 g/cm ^3. When the crystal structure is mainly a layered rock salt crystal structure and the molar ratio of cobalt (Co) and the molar ratio of manganese (Mn) to the transition metal contained in the transition metal site are both less than 50%, and the molar ratio of nickel (Ni) is 50% or more, the positive electrode active material can be calculated as lithium nickel oxide, that is, the true density _ρr is 4.8 g/cm ³ . When the crystal structure is mainly a layered rock salt crystal structure, the molar ratio of cobalt (Co) to the transition metal contained in the transition metal site is less than 50%, and the transition metal site contains cobalt (Co), nickel (Ni), and manganese (Mn), the positive electrode active material is a Ni-Mn-Co ternary system, that is, the porosity can be calculated assuming that the true density _ρr is 4.6 g/cm ^3. Note that even if the transition metal site of the positive electrode active material contains elements other than cobalt, nickel, and manganese, if the true density to be adopted can be determined as described above, the porosity is calculated without correcting the true density.

The thickness of the positive electrode active material layer 212 is preferably 20 μm or more. This prevents an oxide film from being formed on the positive electrode current collector layer 211, improving the initial efficiency.

The thickness of the positive electrode active material layer 212 can be measured by the following method.

First, the electrode body 200 is unwound and cut into a rectangle with sides of about 1 cm. The cut electrode body 200 is washed by stirring in a cleaning solvent, dried to volatilize the cleaning solvent, and the cross section is flattened by cross-sectional milling using an argon ion beam to prepare an observation sample. The observation sample can be prepared, for example, under the following conditions.
Washing solvent: 50 mL of dimethyl carbonate
Washing time: 1 minute Number of washes: 2 times Drying temperature: room temperature Drying time: 1 hour Ion milling device: IM4000 (Hitachi High-Tech)

Next, the cross section of the prepared test piece is observed with a SEM (Scanning Electron Microscope). Here, the observation image is acquired so that the positive electrode active material layer 212 is included in an area ratio of 50% or more, the positive electrode active material layer 212 is included in the observation image over the thickness direction, and the lateral direction of the observation image is parallel to the direction along the main surface of the positive electrode current collector layer 211. The observation magnification is merely an example, and it is preferable to make it as large as possible.
SEM: S-4800 (Hitachi High-Tech)
Acceleration voltage: 3 kV
Observation magnification: 1000 times. In the observation image obtained by SEM, the length in the thickness direction of the positive electrode active material layer 212 is measured at multiple points (e.g., five points). As a result, the thickness of the positive electrode active material layer 212 is calculated as the arithmetic average of the lengths of the positive electrode active material layer 212 in the thickness direction.

The negative electrode 220 includes a negative electrode collector layer 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode collector layer 221 is laminated between the negative electrode active material layers 222.

The negative electrode collector layer 221 is a conductor, and for example, copper foil can be used. In the example of FIG. 1, the shape of the negative electrode collector layer 221 is a rectangular sheet with a protrusion on the negative electrode lead 22 side when viewed in a plan view in the thickness direction. The protrusion of the negative electrode collector layer 221 is connected to the negative electrode lead 22.

The negative electrode active material layer 222 is a layer that contains a negative electrode active material. The negative electrode active material layer 222 is not limited to being composed of only a negative electrode active material, and may contain, for example, a conductive additive and a binder.

The negative electrode active material includes materials capable of absorbing and releasing lithium, such as carbon materials, metals, semimetals, silicon alloys or compounds, and tin (Sn) alloys or compounds.

Carbon materials that can be used as the negative electrode active material include, for example, graphite, non-graphitizable carbon, and easily graphitizable carbon. More specifically, carbon materials include, for example, pyrolytic carbons, cokes, glassy carbon fiber, fired organic polymer compounds, activated carbon, and carbon blacks. Cokes include pitch coke, needle coke, and petroleum coke. Here, fired organic polymer compounds are produced by firing polymer compounds such as phenolic resins and furan resins at an appropriate temperature and carbonizing them.

Metals and semimetals that can be used as negative electrode active materials include, for example, tin, lead (Pb), aluminum, indium (In), silicon, zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, silicon, germanium, tin, and lead are preferred. Silicon and tin are more preferred because they have a high ability to absorb and release lithium and can provide a high energy density.

Silicon alloys that can be used as the negative electrode active material include, for example, those containing at least one of the group consisting of tin, nickel, copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc, indium, silver, titanium (Ti), germanium, bismuth, antimony, and chromium (Cr) as a second constituent element other than silicon. Silicon compounds that can be used as the negative electrode active material include, for example, those containing oxygen (O) or carbon (C), and may contain the above-mentioned second constituent element in addition to silicon.

Tin alloys that can be used as the negative electrode active material include, for example, those that contain at least one of the group consisting of silicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin. Tin compounds that can be used as the negative electrode active material include, for example, those that contain oxygen or carbon, and may contain the above-mentioned second constituent element in addition to tin.

The separator 230 is a film that insulates the positive electrode 210 and the negative electrode 220. The separator 230 is provided between the positive electrode 210 and the negative electrode 220 so that the positive electrode 210 and the negative electrode 220 do not come into direct contact with each other. In the example of FIG. 1, the shape of the separator 230 is a rectangular sheet when viewed in a plan view in the thickness direction. The material of the separator 230 is preferably electrically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and has insulating properties. For example, the separator 230 may be a polymer nonwoven fabric, a porous film, or a layer made of glass or ceramic fibers. It is more preferable that the material of the separator 230 includes a porous polyolefin film. The separator 230 may be made of multiple layers, or may be a composite of a porous polyolefin film and a heat-resistant film containing polyimide, glass, or ceramic fibers.

The electrolyte is impregnated into the separator 230. In the example of FIG. 1, the electrolyte fills the space inside the exterior member 30. The electrolyte is a non-aqueous electrolyte that contains an electrolyte salt and a solvent that dissolves the electrolyte salt.

The electrolyte salt includes, for example, lithium salts such as lithium perchlorate ( _LiClO4 ), lithium hexafluorophosphate ( _LiPF6 ), lithium tetrafluoroborate ( _LiBF4 ), lithium bis(trifluoromethanesulfonyl)imide (LiN( _SO2CF3 ) ₂ ), lithium bis( _{pentafluoroethanesulfonyl} )imide (LiN( _SO2C2F5 ) ₂ ), and lithium _{hexafluoroarsenate} ( _{LiAsF6 )} .

The solvent is a non-aqueous solvent including, for example, lactone-based solvents such as gamma-butyrolactone, gamma-valerolactone, delta-valerolactone, and epsilon-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

The electrolyte preferably contains at least one of fluorinated carboxylate, nitrile, and lithium tetrafluoroborate (LiBF ₄ ) as an additive. This promotes the generation of a low-resistance solid electrolyte interphase (SEI), thereby improving the charging load characteristics. Examples of fluorinated carboxylate include fluoroethylene carbonate (FEC). Examples of nitrile include adiponitrile and succinonitrile.

The components of the electrolyte can be measured by mass analysis using GC-MS (Gas Chromatography-Mass Spectrometry). Specifically, the components of the electrolyte can be measured, for example, by the following method. First, in a dry room with a dew point temperature of -40°C or lower, dimethyl carbonate is weighed so that the concentration of the electrolyte in the measurement sample is about 0.4 mass%, and the electrode body 200 is added and left to stand for 24 hours to extract the electrolyte and prepare a measurement sample. Then, 5 μL of the measurement sample is weighed, poured into a vial and sealed, the measurement sample is volatilized using a headspace sampler, and the measurement sample gas is introduced into a GC-MS for mass analysis. Here, the headspace sampler can be, for example, an HS-20 (Shimadzu Corporation), and the GC-MS can be, for example, a QP-2020 (Shimadzu Corporation). This allows the composition of the electrolyte to be investigated from the resulting chromatograph.

The battery according to the first embodiment has been described above, but the secondary battery according to the first embodiment is not limited to the one shown in FIG. 1. Other examples will be described below using the drawings, but the same components as those in FIG. 1 and FIG. 2 will be denoted by reference symbols and will not be described.

FIG. 4 is a cutaway view showing a different example of the secondary battery according to the first embodiment. FIG. 5 is a schematic view of a cross section taken along line V-V in FIG. 4. The secondary battery 1A shown in FIGS. 4 and 5 differs from the example shown in FIG. 1 in that the electrode body 200 has a structure in which the electrode body 200 is wound around the positive electrode lead 21A and the negative electrode lead 22A. As shown in FIG. 4, the secondary battery 1A includes a battery element 20A, an exterior member 30, and an adhesive 32.

As shown in FIG. 5, the battery element 20A includes an electrode body 200A, a positive electrode lead 21A, a negative electrode lead 22A, and a protective material 23. The positive electrode lead 21A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the positive electrode lead 21A is provided near the center of the battery element 20A. The negative electrode lead 22A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the negative electrode lead 22A is provided near the center of the battery element 20A. The protective material 23 is a member that protects the outside of the battery element 20A. The protective material 23A is provided so as to be wrapped around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

5, the electrode body 200A includes a positive electrode 210A including a positive electrode collector layer 211A and a positive electrode active material layer 212A, a negative electrode 220A including a negative electrode collector layer 221A and a negative electrode active material layer 222A, and a separator 230A. The electrode body 200A has a structure wound around the positive electrode lead 21A and the negative electrode lead 22A, and is laminated in the following order from the outside, i.e., from the protective material 23 side: the negative electrode collector layer 221A, the negative electrode active material layer 222A, the separator 230A, the positive electrode active material layer 212A, the positive electrode collector layer 211A, the positive electrode active material layer 212A, the separator 230A, and the negative electrode active material layer 222A. In the electrode body 200A, no layers other than the negative electrode collector layer 221A, the separator 230A, and the positive electrode collector layer 211A are provided near the positive electrode lead 21A and the negative electrode lead 22A. With this structure, the positive electrode collector layer 211A is connected to the positive electrode lead 21A, and the negative electrode collector layer 221A is connected to the negative electrode lead 22A.

The above describes an example of the secondary battery according to this embodiment, but the battery according to this embodiment is not limited to the example shown in FIG. 1 and FIG. 4. For example, when the electrolyte is not filled in the space surrounded by the exterior member 30 and is held as a gel, the electrode body may further include an electrolyte layer between the positive electrode active material layer or the negative electrode active material layer and the separator, which serves as the electrolyte of the secondary battery. In this case, the electrolyte layer is a gel-like layer made of a polymer compound that holds the electrolyte. The polymer compound that constitutes the gel of the electrolyte layer can be any polymer compound that absorbs the solvent of the electrolyte solution and gels. Examples of the polymer compound that constitutes the gel of the electrolyte layer include fluorine-based polymer compounds such as copolymers of polyvinylidene fluoride or vinylidene fluoride and hexafluoropropylene, ether-based polymer compounds such as polyethylene oxide or a crosslinked body containing polyethylene oxide, and polymer compounds containing polyacrylonitrile, polypropylene oxide, or polymethyl methacrylate as a monomer. The polymer compound constituting the gel of the electrolyte layer is preferably a fluorine-based polymer compound from the viewpoint of stability against redox reactions, and more preferably a copolymer containing vinylidene fluoride and hexafluoropropylene as monomers. The copolymer constituting the gel of the electrolyte layer may further contain, as components, a monoester of an unsaturated dibasic acid such as monomethyl maleate ester, an ethylene halide such as trifluorochloroethylene, a cyclic carbonate ester of an unsaturated compound such as vinylene carbonate, and an epoxy group-containing acrylic vinyl monomer. This allows high cycle characteristics to be obtained.

As described above, the battery 1 according to the first embodiment includes a positive electrode 210, a negative electrode 220, a separator 230, and an electrolyte, the positive electrode 210 includes a positive electrode current collector layer 211, and a positive electrode active material layer 212, the positive electrode active material layer 212 is a sintered body of a positive electrode active material having a layered rock salt crystal structure, and in an X-ray diffraction pattern for a main surface of the positive electrode active material layer 212, when the peak intensity of the (003) plane is I ₀₀₃ and the peak intensity of the (110) plane is I ₁₁₀ , I ₁₁₀ /I ₀₀₃ ≧10, and the porosity of the positive electrode active material layer 212 is 10% or less. This makes it possible to improve the initial efficiency and the load characteristics.

The electrolyte contains at least one of fluorinated carboxylate, nitrile, and lithium tetrafluoroborate. This promotes the formation of SEI, improving the charging load characteristics.

In a preferred embodiment, the thickness of the positive electrode active material layer 212 is 20 μm or more. This improves the initial efficiency.

The method for producing the positive electrode according to the first embodiment will be described below. The method for synthesizing the positive electrode according to the first embodiment includes a primary firing process, a crushing process, a pressurizing process, a secondary firing process, a grinding process, and a joining process. Note that the method for producing the positive electrode described below is merely an example, and is not limited to this.

The primary firing step is a step of firing a mixture of raw material powders of the positive electrode active material layer. Specifically, when the positive electrode active material layer is a sintered body of lithium cobalt oxide, in the primary firing step, a mixture of cobalt oxide (Co ₃ O ₄ ) powder and lithium carbonate (Li ₂ CO ₃ ) powder is fired to synthesize lithium cobalt oxide powder. In this case, the primary firing condition is preferably 900° C. for 16 hours or more. In this case, the grain growth of lithium cobalt oxide particles is promoted, so that the crystallinity of the positive electrode active material can be increased.

The crushing process is a process in which the primary fired product obtained in the primary firing process is crushed. The primary fired product is crushed, for example, in a pot mill. At this time, it is preferable to adjust the crushing time of the pot mill, etc., so that the particle size (median diameter) of 50% of the cumulative value of the powder of the primary fired product is 10 μm or less. In this case, the porosity can be reduced, thereby improving the charging load characteristics.

The pressing process is a process in which the primary fired material is compressed into a plate shape after the crushing process. This allows the positive electrode active material particles to be made into plate-like particles, and increases the directional dependency of the crystal planes.

The secondary firing process is a process in which the primary fired product is fired after the pressurizing process. This allows a pellet-shaped secondary fired product to be obtained. This orients the (110) faces of the positive electrode active material particles so that they extend in a direction that intersects with the thickness direction of the pellet.

The grinding process is a process in which the side surface of the secondary fired product, i.e. the surface perpendicular to the thickness direction of the pellet, is ground to a thickness appropriate for the thickness of the positive electrode active material layer, with the side surface being the main surface of the positive electrode active material layer. This produces the positive electrode active material layer.

The bonding step is a step of bonding the prepared positive electrode active material layer to the positive electrode current collector layer. Specifically, the main surface of the positive electrode active material layer, i.e., the side surface of the secondary fired product, is bonded to the main surface of the positive electrode current collector layer. This causes many (003) planes to spread along the main surface of the positive electrode current collector layer, making it possible to increase I ₁₁₀ /I ₀₀₃ and improve the charging load characteristics.

The above steps allow the production of the positive electrode according to the first embodiment.

(Example)
Examples will be described below. Note that the present invention is not limited to these examples. In the following description, the term " _D50 particle size" refers to the particle size (median size) at an integrated value of 50%.

(Example 1-1)
The positive electrode according to Example 1-1 was prepared by the following method.

In the primary firing step, a mixture of cobalt oxide (Co ₃ O ₄ ) powder and lithium carbonate (Li ₂ CO ₃ ) powder was fired at 900° C. for 20 hours to synthesize lithium cobalt oxide powder as a primary fired product.

In the pulverization step, the lithium cobalt oxide powder was pulverized in a pot mill while adjusting the pulverization time so that the _D50 particle size was 2 μm.

In the pressing process, the crushed lithium cobalt oxide powder was compressed into a plate shape so that the porosity was 0.0%.

In the secondary firing process, the plate-shaped lithium cobalt oxide powder was fired at 1100°C for 60 hours to obtain pellets, which were the secondary fired product.

In the grinding process, the side surface of the secondary fired pellet was used as the main surface of the positive electrode active material layer, and grinding was performed so that the thickness of the positive electrode active material layer was 50 μm. In this way, the positive electrode active material layer was produced.

In the bonding process, the main surface of the positive electrode active material layer was bonded to the main surface of the aluminum foil serving as the positive electrode current collector layer. This produced the positive electrode.

The negative electrode for Example 1-1 was made by punching lithium foil into a circle with a diameter of 16 mm.

The electrolyte solution of Example 1-1 was prepared by adding 1 part by mass of FEC as an additive to 100 parts by mass of a solution in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved as an electrolyte salt to a concentration of 1 mol/L in a solvent containing a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 3:7.

The battery of Example 1-1 was fabricated by injecting the above electrolyte into a laminate of the above positive and negative electrodes sandwiching a separator punched into a circle with a diameter of 17 mm.

For the positive electrode according to Example 1-1, the _D50 particle size of the positive electrode active material was measured by SEM.

The SEM observation was performed under the following conditions: Five observation images were obtained so that the positive electrode active material layer accounted for 50% or more in terms of area ratio, the positive electrode active material layer was included in the observation image throughout the thickness direction, and the lateral direction of the observation image was parallel to the direction along the main surface of the positive electrode current collector layer.
SEM: S-4800 (Hitachi High-Tech)
Acceleration voltage: 3 kV
Observation magnification: 1000x

Based on the SEM observation image, the _D50 particle size of the positive electrode active material was calculated by the following method. First, the range in which the positive electrode active material layer was included in the thickness direction was extracted from five observation images obtained by SEM. Here, the range to be extracted was a square region with a side length of 40 μm. Next, from the extracted range, image processing was performed using image processing software, and the grain boundaries of the positive electrode active material particles were binarized and extracted. Here, the image processing software used was GIMP (version: 2.6.11). Then, 20 positive electrode active material particles were selected from the extracted range, and the maximum length of the region surrounded by the grain boundaries of the positive electrode active material particles was measured as the particle size of the positive electrode active material particles. As a result, the _D50 particle size of the positive electrode active material was calculated as the median value of the particle size of a total of 100 positive electrode active material particles measured in the five observation images.

For the positive electrode active material layer according to Example 1-1, I ₁₁₀ /I ₀₀₃ was calculated by X-ray diffraction measurement. The X-ray diffraction measurement was performed on the main surface of the positive electrode active material layer under the following conditions. The X-ray diffraction chart obtained was subjected to Rietveld analysis in space group R-3m to identify the peaks of the (003) plane and the (110) plane, and I ₁₁₀ /I ₀₀₃ was calculated.
Measurement equipment: Bruker D8 ADVANCE
X-ray tube: Cu-Kα ray Output voltage: 40 kV
Output current: 40mA
Sampling interval: 0.02°
Counting time: 0.5 sec/step

The battery according to Example 1-1 was measured for charge load characteristics under the following conditions. The charge load characteristics are the ratio of the discharge capacity to that when charging and discharging at 0.1 C. Here, the magnitude of the charge and discharge current at the C rate was calculated assuming the capacity of the positive electrode active material to be 160 mAh/g.
Charging rate: 2C
Charging method: CC
Charge control voltage: 4.4V
Discharge rate: 0.1C
Discharge method: CC
Discharge end voltage: 3.0V

The initial efficiency of the battery according to Example 1-1 was measured under the following conditions: The initial efficiency is the charge/discharge efficiency in the first charge/discharge, that is, the ratio of the discharge capacity to the charge capacity in the first charge.
Charging rate: 0.1C
Charging method: CCCV
Charge control voltage: 4.4V
Charging cutoff current: 0.065mA
Discharge rate: 0.1C
Discharge method: CC
Discharge end voltage: 3.0V

(Examples 1-2 and 1-3)
In Examples 1-2 and 1-3, as shown in Table 1, batteries were produced and measured in the same manner as in Example 1-1, except that the conditions for the primary firing were changed.

(Comparative Example 1-1)
In Comparative Example 1-1, as shown in Table 1, except that the conditions for the primary firing were changed, a battery was produced in the same manner as in Example 1-1, and measurements were performed.

(Comparative Example 1-2)
In Comparative Example 1-2, a battery was produced and measured in the same manner as in Example 1-1, except that the method for producing the positive electrode active material layer was changed. The method for producing the positive electrode active material layer according to Comparative Example 1-2 will be described below.

In the primary firing step, cobalt oxide (Co ₃ O ₄ ) powder and lithium carbonate (Li ₂ CO ₃ ) powder were mixed and fired at 800° C. for 5 hours to synthesize lithium cobalt oxide powder as a primary fired product.

In the pulverization step, the lithium cobalt oxide powder was pulverized in a pot mill while adjusting the pulverization time so that the _D50 particle size was 2 μm.

Next, 100 parts by mass of the precursor powder, 100 parts by mass of the dispersion medium, 10 parts by mass of the binder, 4 parts by mass of the plasticizer, and 2 parts by mass of the dispersant were mixed. The precursor powder was a mixed powder of 10 parts by mass of lithium cobalt oxide powder and 90 parts by mass of cobalt oxide (Co ₃ O ₄ ) powder with a D ₅₀ particle size of 0.3 μm as matrix particles. The dispersion medium was a solvent in which toluene and isopropanol were mixed at a volume ratio of 1:1. The binder was polyvinyl butyral. The plasticizer was bis(2-ethylhexyl) phthalate. The dispersant was Rheodol (registered trademark) SP-O30. The mixture was defoamed by stirring under reduced pressure, and the viscosity was adjusted to 4000 mPa·s or more and 10000 mPa·s or less to prepare a positive electrode slurry. Here, the viscosity was measured with an LVT type viscometer (Brookfield). The prepared positive electrode slurry was formed into a sheet on a polyethylene terephthalate (PET) film by a doctor blade method at a forming speed of 100 m/h so as to have a thickness of 40 μm after drying, thereby obtaining a green sheet.

Then, the PET film was peeled off from the green sheet, placed on a zirconia setter, and fired at 900° C. for 5 hours to obtain a sintered plate. Then, both sides of the sintered plate were placed on a zirconia setter, the zirconia setter was placed in a 90 mm square alumina sheath, and the plate was held at 800° C. for 5 hours in air, after which the Co ₃ O ₄ sintered plate was sandwiched between lithium sheets, placed on the zirconia setter, and fired at 900° C. for 20 hours. Here, the thickness of the lithium sheets was adjusted so that the molar ratio of Li to Co in the fired product was 1:1.

In the grinding process, the main surface of the secondary fired product was used as the main surface of the positive electrode active material layer, and the layer was ground so that the thickness of the positive electrode active material layer was 50 μm.

The above steps were used to prepare the positive electrode active material for Comparative Example 1-2.

(Example 2-1) to (Example 2-3)
In Examples 2-1 to 2-3, as shown in Table 1, batteries were fabricated and measured in the same manner as in Example 1-1, except that the active material particle size was changed to change the porosity of the positive electrode active material layer.

(Comparative Example 2-1)
In Comparative Example 2-1, as shown in Table 1, a battery was produced in the same manner as in Example 1-1, except that the active material particle size was changed to change the porosity of the positive electrode active material layer, and measurements were performed.

(Example 3-1)
In Example 3-1, as shown in Table 1, a battery was produced in the same manner as in Example 1-1, except that the additive in the electrolyte was changed to adiponitrile, and measurements were performed.

(Example 3-2)
In Example 3-2, as shown in Table 1, a battery was produced in the same manner as in Example 1-1, except that the additive in the electrolyte was changed to succinonitrile, and measurements were performed.

(Example 3-3)
In Example 3-3, as shown in Table 1, a battery was produced in the same manner as in Example 1-1, except that the additive of the electrolyte was changed to _LiBF4 , and measurements were performed.

(Example 3-4)
In Example 3-4, as shown in Table 1, a battery was produced and measured in the same manner as in Example 1-1, except that no additive was added to the electrolyte.

(Example 4-1) and (Example 4-3)
In Examples 4-1 to 4-3, as shown in Table 1, batteries were produced and measured in the same manner as in Example 1-1, except that the thickness of the positive electrode active material layer was changed.

The results are shown in Table 1.

As shown in Table 1, in Examples 1-1 to 1-3, I ₁₁₀ /I ₀₀₃ was 10 or more, and therefore, compared with Comparative Example 1-1 in which I ₁₁₀ /I ₀₀₃ was less than 10, the charging load characteristics were improved.

As shown in Table 1, in Example 1-1, the positive electrode active material layer was fabricated so that the (003) plane extended over the main surface thereof, and therefore, compared with Comparative Example 1-2 in which the positive electrode active material layer was fabricated so that the (110) plane extended over the main surface thereof, it was possible to make I ₁₁₀ /I ₀₀₃ 10 or more, and therefore it was possible to improve the charging load characteristics.

As shown in Table 1, in Examples 1-1 and 2-1 to 2-3, by setting the porosity of the positive electrode active material layer to 10% or less, the initial efficiency could be improved compared to Comparative Example 2-1, in which the porosity of the positive electrode active material layer was greater than 9.0%.

As shown in Table 1, in Examples 1-1 and 3-1 to 3-3, the additives in the electrolyte were fluorinated carboxylate, nitrile, and lithium tetrafluoroborate, and the charging load characteristics were improved compared to Example 3-4, in which no additives were added.

As shown in Table 1, in Examples 1-1, 4-1, and 4-2, by making the thickness of the positive electrode active material layer 20 μm or more, the charging load characteristics were improved compared to Example 4-3, in which the thickness of the positive electrode active material layer was less than 20 μm.

The above-described embodiment is intended to facilitate understanding of the present invention, and is not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit of the present invention, and equivalents thereof are also included in the present invention.

The present invention may take the following forms.
(1)
The battery includes a positive electrode, a negative electrode, a separator, and an electrolyte solution.
The positive electrode includes a positive electrode current collector layer and a positive electrode active material layer,
the positive electrode active material layer contains particles of a positive electrode active material having a layered rock salt type crystal structure,
The particles of the positive electrode active material have a plate-like shape,
In the X-ray diffraction pattern for the main surface of the positive electrode active material layer, when the peak intensity of the (003) plane is I _{003 and} the peak intensity of the (110) plane is I ₁₁₀ /I ₀₀₃ ≧10,
The positive electrode active material layer has a porosity of 10% or less.
(2)
The secondary battery according to (1), wherein the electrolyte solution contains at least one of a fluorinated carboxylate, a nitrile, and lithium tetrafluoroborate.
(3)
The secondary battery according to (1) or (2), wherein the positive electrode active material layer has a thickness of 20 μm or more.

REFERENCE SIGNS LIST 1, 1A secondary battery 20, 20A battery element 21, 21A positive electrode lead 22, 22A negative electrode lead 23 protective material 30 exterior member 30a, 30b exterior sheet 31 recess 32 adhesive material 200, 200A electrode body 210, 210A positive electrode 211, 211A positive electrode current collector layer 212, 212A positive electrode active material layer 220, 220A negative electrode 221, 221A negative electrode current collector layer 222, 222A negative electrode active material layer 230, 230A separator P, Q peak

Claims (3)

The battery includes a positive electrode, a negative electrode, a separator, and an electrolyte solution.
The positive electrode includes a positive electrode current collector layer and a positive electrode active material layer,
the positive electrode active material layer contains particles of a positive electrode active material having a layered rock salt type crystal structure,
The particles of the positive electrode active material have a plate-like shape,
In the X-ray diffraction pattern for the main surface of the positive electrode active material layer, when the peak intensity of the (003) plane is I _{003 and} the peak intensity of the (110) plane is I ₁₁₀ /I ₀₀₃ ≧10,
The positive electrode active material layer has a porosity of 10% or less. The secondary battery according to claim 1, wherein the electrolyte contains at least one of fluorinated carboxylate, nitrile, and lithium tetrafluoroborate. The secondary battery according to claim 1 or 2, wherein the thickness of the positive electrode active material layer is 20 μm or more.

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