Secondary battery positive electrode and nonaqueous electrolyte secondary battery

Abstract

A secondary battery positive electrode 5 according to the present disclosure comprises a positive electrode current collector 5a and a positive electrode active material layer 5b supported by the positive electrode current collector 5a. The positive electrode active material layer 5b includes: active material particles; and solid particles containing boron and phosphorus. The solid particles may contain boron phosphate. The ratio of the mass of the solid particles to the mass of the active material particles is, for example, in the range of 0.1 to 4 mass%.

Description

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

[0001] The present disclosure relates to a positive electrode for a secondary battery and a non-aqueous electrolyte secondary battery.

[0002] As known to those skilled in the art, conventional battery electrolytes contain various components. For example, Patent Document 1 discloses that a non-aqueous electrolyte secondary battery containing vinylene carbonate as a non-aqueous solvent has good cycle characteristics.

[0003] Patent Document 2 discloses a non-aqueous electrolyte containing boron phosphate.

[0004] JP 2005-268230 A JP 2017-168374 A

[0005] One of the technical issues with non-aqueous electrolyte secondary batteries is the generation of gas during charge and discharge. The present disclosure provides a positive electrode for a secondary battery that can suppress the generation of gas during charge and discharge of the battery while suppressing a decrease in capacity.

[0006] The present disclosure provides a positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer contains active material particles and solid particles containing boron and phosphorus.

[0007] According to the present disclosure, it is possible to suppress the generation of gas that accompanies charging and discharging of a battery while suppressing the decrease in capacity.

[0008] 1 is a cross-sectional view of a positive electrode for a secondary battery according to Embodiment 1. FIG. 2 is a schematic cross-sectional view showing an example of a non-aqueous electrolyte secondary battery according to Embodiment 2.

[0009] (Findings that Form the Basis of the Present Disclosure) The present inventors fabricated and tested nonaqueous electrolyte secondary batteries using colloidal nonaqueous electrolytes containing boron phosphate particles. As a result, they discovered that colloidal nonaqueous electrolytes are effective in suppressing gas generation during charging and discharging. However, colloidal nonaqueous electrolytes accelerate the capacity loss of nonaqueous electrolyte secondary batteries. The present inventors focused on suppressing capacity loss while taking advantage of the benefit of suppressing gas generation, and came up with the technology of the present disclosure.

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0011] (Embodiment 1) Fig. 1 is a cross-sectional view of a positive electrode for a secondary battery in embodiment 1. The positive electrode for a secondary battery 5 includes a positive electrode current collector 5a and a positive electrode active material layer 5b. The positive electrode active material layer 5b is supported by the positive electrode current collector 5a. Hereinafter, the positive electrode for a secondary battery 5 may be simply referred to as "positive electrode 5."

[0012] Positive electrode active material layer 5b contains active material particles and solid particles containing boron and phosphorus. The solid particles containing boron and phosphorus suppress gas generation that accompanies charge and discharge of a secondary battery using positive electrode 5.

[0013] Although the mechanism by which the solid particles containing boron and phosphorus suppress gas generation is not entirely clear, the following mechanism is presumed. When a battery using the positive electrode 5 is operated, the solid particles containing boron and phosphorus contained in the positive electrode 5 form a coating containing boron and / or phosphorus on the surface of the active material particles. This can suppress decomposition of the non-aqueous solvent at the interface between the non-aqueous electrolyte and the positive electrode 5. The presence of the coating caused by the solid particles can sometimes be confirmed by analyzing the components of the surface of the positive electrode 5 after charging and discharging the battery. A method for analyzing the components of the surface of the positive electrode 5 is, for example, ICP (Inductively Coupled Plasma) analysis.

[0014] Specifically, the solid particles include a compound containing boron and phosphorus. The compound containing boron and phosphorus may be an inorganic compound. This configuration reduces the possibility of unintended side reactions occurring. The solid particles may contain the compound containing boron and phosphorus as a main component, or may consist of the compound. "Main component" refers to the component that is contained in the largest amount by mass. "The solid particles consist of a compound containing boron and phosphorus" means that, except for components that are inevitably mixed in, no components other than the compound containing boron and phosphorus are intentionally added.

[0015] The solid particles may contain, for example, boron phosphate. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0016] The ratio of the mass of the solid particles to the mass of the active material particles is, for example, in the range of 0.1% by mass or more and 4% by mass or less, preferably 1% by mass or more and 3% by mass or less. The solid particles containing boron and phosphorus may be materials that do not function as active materials. In other words, the solid particles may be materials that do not have the ability to absorb and release ions such as lithium ions. By adjusting the amount of solid particles within the above range, gas generation can be suppressed while also avoiding a significant decrease in the capacity of the positive electrode 5. The mass of the active material particles and the mass of the solid particles each refer to the mass per unit volume of the positive electrode active material layer 5b.

[0017] The average particle size of the solid particles is, for example, in the range of 0.1 μm to 10 μm, preferably 2 μm to 6 μm. When the average particle size of the solid particles is within this range, the solid particles can be uniformly distributed in the positive electrode active material layer 5 b. As a result, the gas generation suppression effect can be obtained throughout the positive electrode active material layer 5 b.

[0018] The solid particles may be present on the surfaces of the active material particles and between the active material particles. That is, the solid particles may be fixed to the positive electrode active material layer 5 b. With this configuration, the action of the solid particles is limited to the positive electrode 5, so that side reactions caused by the solid particles can be prevented from occurring in the negative electrode.

[0019] The average particle size of the solid particles may be smaller than the average particle size of the active material particles. In this case, the solid particles are more likely to fit between the active material particles and be more likely to be uniformly distributed in the positive electrode active material layer 5b. With this configuration, the effect of suppressing gas generation can be obtained throughout the positive electrode active material layer 5b. The average particle size of the active material particles is, for example, in the range of 4 μm to 20 μm.

[0020] The solid particles may be nanoparticles, primary particles, secondary particles, or a mixture of primary and secondary particles.

[0021] The average particle size of particles such as active material particles and solid particles can be calculated by the following method. A cross section of the positive electrode 5 is observed with a scanning electron microscope, and the area of ​​the particle in the microscopic image is calculated by image processing. The diameter of a circle having an area equal to the calculated area is regarded as the diameter of that particle. The diameters of an arbitrary number of particles (for example, 10) are calculated, and their average value is regarded as the average particle size of the particle. When selecting target particles, fine particles or coarse particles that deviate from the average size may be excluded. Using SEM-EDX, it is possible to examine the distribution of elements in the observation field, and therefore it is possible to distinguish active material particles from solid particles based on the distribution of elements.

[0022] The solid particles may be uniformly distributed throughout the positive electrode active material layer 5b, and this configuration allows the effect of suppressing gas generation to be obtained throughout the positive electrode active material layer 5b.

[0023] The active material particles are particles of a positive electrode active material. The positive electrode active material can be a material capable of absorbing and releasing lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce the manufacturing cost of the battery and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0024] The positive electrode active material may contain lithium nickel oxide having a layered rock salt crystal structure. The proportion of Ni among metal elements other than Li contained in the lithium nickel oxide may be 50 atomic % or more. The lithium nickel oxide may also contain other transition metals. The lithium nickel oxide is useful for achieving a high operating voltage.

[0025] The lithium nickel oxide may be represented by the following composition formula (1): Element M1 is at least one selected from the group consisting of V, Co, and Mn. Element M2 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. Composition formula (1) satisfies 0.9≦α≦1.10, −0.05≦β≦0.05, 0.5≦x1<1, 0≦x2≦0.5, and 0≦1−x1−x2≦0.5.

[0026] Li α Ni x1 M1 x2 M2 (1-x1-x2) O 2+β ...(1)

[0027] The positive electrode active material layer 5b may contain other materials such as a conductive additive and a binder.

[0028] The conductive additive is used to reduce the resistance of the positive electrode 5. Examples of the conductive additive include a carbon material and a conductive polymer compound. Examples of the carbon material include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compound include polyaniline, polypyrrole, and polythiophene.

[0029] The binder is used to improve the binding property of the material constituting the positive electrode 5. As the binder, polymer materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide can be used.

[0030] The positive electrode current collector 5a can be a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. Aluminum and its alloys are suitable materials for the positive electrode current collector 5a because they are inexpensive and easy to form into thin films. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the positive electrode current collector 5a as a conductive auxiliary material.

[0031] The positive electrode 5 of this embodiment can be produced by the following method. Active material particles, solid particles, a conductive additive, a binder, and a solvent are mixed to prepare a positive electrode mixture. The positive electrode mixture is applied to the positive electrode current collector 5a to form a coating film. The coating film is dried to remove the solvent, thereby obtaining the positive electrode 5.

[0032] (Embodiment 2) FIG. 2 is a schematic cross-sectional view showing an example of a nonaqueous electrolyte secondary battery according to Embodiment 2. The secondary battery 100 includes a container 1, an electrode group 4, and an electrolyte (not shown). The electrode group 4 has a wound structure. The electrode group 4 is housed in the container 1. The electrode group 4 includes a positive electrode 5, a negative electrode 6, and a pair of separators 7. The electrode group 4 is impregnated with an electrolyte. The opening of the container 1 is closed with a sealing plate 2. The positive electrode 5 includes a positive electrode current collector 5a and a positive electrode active material layer 5b. One end of a positive electrode lead 5c is connected to the positive electrode 5. The other end of the positive electrode lead 5c is connected to the back surface of the sealing plate 2. An insulating packing 3 is disposed around the periphery of the sealing plate 2. The negative electrode 6 includes a negative electrode current collector 6a and a negative electrode active material layer 6b. One end of a negative electrode lead 6c is connected to the negative electrode 6. The other end of the negative electrode lead 6c is connected to the bottom surface of the container 1. An insulating ring 8 is disposed on each of the upper and lower surfaces of the electrode group 4 .

[0033] By using the positive electrode 5 described in the first embodiment, it is possible to suppress the generation of gas that accompanies the charge and discharge of the secondary battery 100 while suppressing the decrease in capacity.

[0034] The negative electrode current collector 6a may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the negative electrode current collector 6a as a conductive auxiliary material.

[0035] The negative electrode active material layer 6b includes a negative electrode active material. The negative electrode active material can be a material capable of absorbing and releasing lithium ions. The negative electrode active material includes, for example, at least one selected from the group consisting of carbon materials and materials capable of forming an alloy with lithium. Examples of carbon materials include graphite. Examples of materials capable of forming an alloy with lithium include silicon, silicon-containing oxides, tin, zinc alloys, bismuth, and germanium. One type selected from these negative electrode active materials may be used, or two or more types may be used in combination.

[0036] The negative electrode active material layer 6b may contain at least one selected from the group consisting of graphite and silicon as the negative electrode active material. The negative electrode active material layer 6b may contain only graphite as the negative electrode active material. Graphite is recommended because it is resistant to deterioration even when repeatedly charged and discharged at a deep depth. Carbon materials other than graphite may also be used as the negative electrode active material. Silicon has a larger capacity than graphite and is therefore advantageous for increasing the capacity of the secondary battery 100.

[0037] The negative electrode active material layer 6b may contain other materials such as a conductive additive, a binder, etc. Materials that can be used as the conductive additive and binder for the positive electrode active material layer 5b can also be used for the negative electrode active material layer 6b.

[0038] The negative electrode 6 (specifically, the negative electrode active material layer 6 b) may not contain solid particles containing boron and phosphorus. With this configuration, side reactions caused by the solid particles can be prevented from occurring in the negative electrode 6.

[0039] The electrolyte is a non-aqueous electrolyte impregnated into the positive electrode 5, the negative electrode 6, and the separator 7. The electrolyte may fill the internal space of the container 1. The electrolyte allows lithium ions to move between the positive electrode 5 and the negative electrode 6.

[0040] The electrolyte solution includes a non-aqueous solvent and a lithium salt.

[0041] Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, nitriles, amides, etc. One of these solvents may be used alone, or two or more of them may be used in combination.

[0042] In the nonaqueous electrolyte secondary battery 100, solid particle components containing boron and phosphorus may dissolve slightly in the nonaqueous electrolyte. Even in this case, the mass of the solid particles present in the positive electrode active material layer 5b per unit volume is greater than the mass of the solid particle components dissolved in the nonaqueous electrolyte per unit volume. This configuration can suppress gas generation while avoiding side reactions caused by the solid particles at the negative electrode 6.

[0043] The mass of the solid particles present in a unit volume of the positive electrode active material layer can be determined by the following method. A positive electrode of any size is cut out from a nonaqueous electrolyte secondary battery, and its volume is calculated from the thickness and area of ​​the positive electrode active material layer. A positive electrode composite constituting the positive electrode active material layer is sampled and washed with an organic solvent, and its mass is then calculated. Quantitative analysis of the elements contained in the positive electrode composite is performed using a chemical analysis method such as ICP analysis. By focusing on elements (e.g., boron) contained only in the solid particles, the content ratio of the solid particles in the positive electrode composite can be calculated. In other words, the mass of the solid particles present in a unit volume of the positive electrode active material layer can be calculated.

[0044] The mass of the solid particle components dissolved in a unit volume of nonaqueous electrolyte can be determined by the following method: A unit volume of nonaqueous electrolyte is collected from a nonaqueous electrolyte secondary battery. Quantitative analysis of the elements contained in the nonaqueous electrolyte is performed using a chemical analysis method such as ICP analysis. By focusing on elements derived from the solid particles (e.g., boron), the mass of the solid particle components dissolved in a unit volume of nonaqueous electrolyte can be calculated.

[0045] Examples of lithium salts include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One selected from these lithium salts may be used, or two or more may be used in combination.

[0046] The separator 7 has lithium ion conductivity. The material of the separator 7 is not particularly limited as long as it allows lithium ions to pass through. The material of the separator 7 can be at least one selected from the group consisting of a gel electrolyte, an ion exchange resin membrane, a semipermeable membrane, and a porous membrane. Making the separator 7 from these materials can sufficiently ensure the safety of the nonaqueous electrolyte secondary battery 100. Examples of gel electrolytes include gel electrolytes containing fluororesins such as PVdF. Examples of ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Examples of porous membranes include porous membranes made of polyolefin resins and porous membranes containing glass paper obtained by weaving glass fibers into nonwoven fabric.

[0047] The container 1 is made of a metal such as aluminum or stainless steel, and may have a cylindrical shape or a rectangular tube shape.

[0048] The electrode group 4 may be wound into a cylindrical shape or an oval shape.

[0049] The shape of the secondary battery 100 is not limited to a cylindrical shape, and various shapes such as a coin shape, a square shape, a sheet shape, a button shape, a flat shape, and a laminated shape can be adopted as the shape of the secondary battery 100.

[0050] The technology of the present disclosure can be applied to various types of secondary batteries, such as sodium secondary batteries and magnesium secondary batteries, in addition to lithium secondary batteries.

[0051] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0052] (Technology 1) A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer contains active material particles and solid particles containing boron and phosphorus.

[0053] According to the present disclosure, it is possible to suppress the generation of gas that accompanies charging and discharging of a battery while suppressing the decrease in capacity.

[0054] (Technology 2) The positive electrode for a secondary battery according to Technology 1, wherein the solid particles include an inorganic compound containing boron and phosphorus. With this configuration, it is possible to reduce the possibility of an unintended side reaction occurring.

[0055] (Technology 3) The positive electrode for a secondary battery according to Technology 1 or 2, wherein the solid particles contain boron phosphate. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0056] (Technology 4) The positive electrode for a secondary battery according to any one of Technologies 1 to 3, wherein the ratio of the mass of the solid particles to the mass of the active material particles is in the range of 0.1 mass % to 4 mass %. By adjusting the amount of the solid particles to the above range, gas generation can be suppressed while also avoiding a significant decrease in the capacity of the positive electrode.

[0057] (Technology 5) The positive electrode for a secondary battery according to any one of Technologies 1 to 4, wherein the solid particles have an average particle size in the range of 0.1 μm to 10 μm. When the average particle size of the solid particles is in this range, the solid particles can be uniformly distributed in the positive electrode active material layer.

[0058] (Technology 6) The positive electrode for a secondary battery according to any one of Technologies 1 to 5, wherein the solid particles are present on the surfaces of the active material particles and between the active material particles. With this configuration, the action of the solid particles is limited to the positive electrode, so that side reactions caused by the solid particles can be prevented from occurring in the negative electrode.

[0059] (Technology 7) The positive electrode for a secondary battery according to any one of Technologies 1 to 6, wherein the solid particles are uniformly distributed throughout the positive electrode active material layer. With this configuration, the effect of suppressing gas generation can be obtained throughout the entire positive electrode active material layer.

[0060] (Technology 8) A nonaqueous electrolyte secondary battery including a negative electrode and the positive electrode for a secondary battery according to any one of Technologies 1 to 7. According to the present disclosure, it is possible to suppress gas generation accompanying charge and discharge of the battery while suppressing a decrease in capacity.

[0061] (Technology 9) A nonaqueous electrolyte secondary battery according to Technology 8, wherein the negative electrode does not contain the solid particles. With this configuration, it is possible to prevent side reactions caused by the solid particles from occurring in the negative electrode.

[0062] (Technology 10) The nonaqueous electrolyte secondary battery according to Technology 8 or 9, further comprising a nonaqueous electrolyte, wherein the mass of the solid particles present in the positive electrode active material layer per unit volume is greater than the mass of the solid particle components dissolved in the nonaqueous electrolyte per unit volume. With this configuration, it is possible to suppress gas generation while avoiding side reactions caused by the solid particles from occurring at the negative electrode.

[0063] (Example 1) LiNi 0.8 Mn 0.2 A positive electrode slurry was prepared by mixing and stirring positive electrode active material particles having a composition of O2, boron phosphate particles having a median diameter of 5 μm, acetylene black (AB), polyvinylidene fluoride (PVDF), and N-methyl-pyrrolidone (NMP). The mass ratio of the positive electrode active material particles, AB, and PVDF was positive electrode active material:AB:PVDF=96:2:2. The mass ratio of the boron phosphate particles to the positive electrode active material particles was 2.0 mass%. The positive electrode slurry was applied to the surface of aluminum foil to form a coating film, which was dried and then rolled. This produced the positive electrode of Example 1.

[0064] A non-aqueous solvent was prepared by mixing ethylene carbonate (EC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:FEC:DMC:EMC = 10:10:75:5. LiPF was dissolved in the resulting non-aqueous solvent to a concentration of 1.3 mol / L to obtain a non-aqueous electrolyte solution.

[0065] A laminate half cell was fabricated using a positive electrode, a Li metal foil (2 cm×2 cm, 200 μm thick) as a counter electrode, a separator (Celgard, #2320), and a non-aqueous electrolyte.

[0066] In this manner, an evaluation cell for Example 1 was obtained.

[0067] Comparative Example 1 An evaluation cell for Comparative Example 1 was prepared in the same manner as in Example 1, except that boron phosphate particles were not used.

[0068] Comparative Example 2 A positive electrode of Comparative Example 2 was obtained in the same manner as in Example 1, except that boron phosphate particles were not used.

[0069] Ethylene carbonate (EC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of EC:FEC:DMC:EMC = 10:10:75:5 to prepare a non-aqueous solvent. Boron phosphate particles were dispersed in the resulting non-aqueous solvent to obtain a dispersion. LiPF6 was dissolved in the dispersion to a concentration of 1.3 mol / L to obtain a non-aqueous electrolyte solution of Comparative Example 2. The non-aqueous electrolyte solution of Comparative Example 2 was cloudy. The content of boron phosphate in the non-aqueous electrolyte solution was 0.5% by mass. The mass ratio of boron phosphate to the positive electrode active material was 2.0% by mass.

[0070] An evaluation cell for Comparative Example 2 was fabricated in the same manner as in Example 1 using the positive electrode and nonaqueous electrolyte of Comparative Example 2.

[0071] [Measurement of Amount of Gas Generated and Discharge Capacity] The evaluation cells of Example 1, Comparative Example 1, and Comparative Example 2 were charged and discharged in the following manner, and the amount of gas generated during charge and discharge and the discharge capacity were measured. The charge and discharge were performed in an ambient atmosphere at 55°C.

[0072] (First cycle) Constant current charging was performed at a current value of 0.2 C until the voltage reached 4.3 V, and constant voltage charging was performed at a voltage of 4.3 V until the current value reached 0.02 C. After a rest period of 20 minutes, constant current discharging was performed at a current value of 0.2 C until the voltage reached 2.5 V. After the rest period of 20 minutes, the initial volume of the evaluation cell was measured by the Archimedes method in an ambient atmosphere at 20°C.

[0073] (2nd to 50th cycles) Constant current charging was performed at a current value of 0.3 C until the voltage reached 4.3 V, and constant voltage charging was performed at a voltage of 4.3 V until the current value reached 0.02 C. Subsequently, constant current discharging was performed at a current value of 0.5 C until the voltage reached 2.5 V. A 20-minute rest period was provided between charging and discharging. This charge / discharge process was repeated 49 times, for a total of 50 charge / discharge cycles. The volume of the evaluation cell was then measured by the Archimedes method in an ambient atmosphere at 20°C.

[0074] The amount of gas generated at 50 cycles was calculated by subtracting the initial volume from the volume of the evaluation cell at 50 cycles. The results are shown in Table 1.

[0075] The capacity decrease rate was calculated using the following formula (2). The results are shown in Table 1.

[0076] (Capacity reduction rate) = 100 x (C1 - C 25 ) / C1 (2) C1: Discharge capacity at the first cycle C 25 : Discharge capacity at 25th cycle

[0077]

[0078] In Table 1, the "gas generation amount (%)" column indicates a relative value to that of Comparative Example 1.

[0079] As shown in Table 1, the gas generation amounts in the evaluation cells of Example 1 and Comparative Example 2 were significantly lower than the gas generation amount in Comparative Example 1. This result indicates that gas generation was significantly suppressed by the boron phosphate particles in the evaluation cell.

[0080] The rate of capacity decline of the evaluation cell of Example 1 was approximately equal to the rate of capacity decline of the evaluation cell of Comparative Example 1. In other words, in Example 1, no acceleration of capacity decline due to the boron phosphate particles was observed.

[0081] In contrast, the capacity loss rate of the evaluation cell of Comparative Example 2 was significantly higher than that of the evaluation cells of Example 1 and Comparative Example 1. When the evaluation cell of Comparative Example 2 was disassembled and observed after charging and discharging, the surface of the positive electrode was found to have slightly discolored white. This is thought to be due to the decomposition of boron phosphate in the cloudy electrolyte at the negative electrode, resulting in the degradation of the surface of the positive electrode by the decomposition products. This is thought to have resulted in a significant acceleration of the capacity loss.

[0082] The technology of the present disclosure is useful for, for example, lithium ion secondary batteries.

Claims

1. A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer contains active material particles and solid particles containing boron and phosphorus.

2. The positive electrode for a secondary battery according to claim 1, wherein the solid particles include an inorganic compound containing boron and phosphorus.

3. The positive electrode for a secondary battery according to claim 1, wherein the solid particles contain boron phosphate.

4. The positive electrode for a secondary battery according to claim 1, wherein the ratio of the mass of said solid particles to the mass of said active material particles is in the range of 0.1 mass % or more and 4 mass % or less.

5. The positive electrode for a secondary battery according to claim 1, wherein the solid particles have an average particle size in the range of 0.1 μm or more and 10 μm or less.

6. The positive electrode for a secondary battery according to claim 1, wherein the solid particles are present on the surfaces of the active material particles and between the active material particles.

7. The positive electrode for a secondary battery according to claim 1, wherein the solid particles are uniformly distributed throughout the positive electrode active material layer.

8. A non-aqueous electrolyte secondary battery comprising: a negative electrode; and the positive electrode for a secondary battery according to claim 1.

9. The nonaqueous electrolyte secondary battery according to claim 8, wherein the negative electrode does not contain the solid particles.

10. The nonaqueous electrolyte secondary battery according to claim 8, further comprising a nonaqueous electrolyte solution, wherein the mass of the solid particles present in a unit volume of the positive electrode active material layer is greater than the mass of the components of the solid particles dissolved in a unit volume of the nonaqueous electrolyte solution.

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