Negative electrode and non-aqueous electrolyte secondary battery

Abstract

A negative electrode 10 according to the present disclosure comprises: a substrate 12; a negative electrode active material layer 14 that is supported by the substrate 12; and a coating film 16 that contains an inorganic material and is provided on the surface of the negative electrode active material layer 14. The coating film 16 has an average thickness of, e.g., 150 nm or greater. The coating film 16 has an average pore diameter of, e.g., 100 nm or less. The coating film 16 contains, e.g., Li, P, F, and O.

Description

Negative electrode and non-aqueous electrolyte secondary battery

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

[0002] Various additives have been proposed to improve battery performance.

[0003] Patent Document 1 discloses that by incorporating particles of lithium sulfonate salt into the negative electrode, a dense SEI (Solid Electrolyte Interphase) coating is formed on the surface of the negative electrode.

[0004] In the positive electrode disclosed in Patent Document 2, a positive electrode mixture layer and an inorganic particle layer are formed in this order on at least one surface of a positive electrode current collector. The inorganic particle layer contains inorganic particles, lithium phosphate, and an aqueous binder.

[0005] International Publication No. 2023 / 119990 Japanese Patent Application Laid-Open No. 2012-49060

[0006] An object of the present disclosure is to provide a negative electrode suitable for improving the cycle characteristics of a battery.

[0007] The present disclosure provides a negative electrode comprising: a substrate; a negative electrode active material layer supported on the substrate; and a coating containing an inorganic material and provided on a surface of the negative electrode active material layer, wherein the coating exhibits a porous structure in a cross section parallel to a thickness direction of the negative electrode.

[0008] The negative electrode of the present disclosure is suitable for improving the cycle characteristics of a battery.

[0009] FIG. 1 is a cross-sectional view showing a schematic configuration of a negative electrode according to Embodiment 1. FIG. 2 is a cross-sectional view showing a schematic configuration of a nonaqueous electrolyte secondary battery according to Embodiment 2. FIG. 3A is a cross-sectional SEM image (magnification: 10,000 times) of a negative electrode taken out of an electrode assembly according to Example 1. FIG. 3B is an enlarged view (magnification: 100,000 times) of a boxed portion A1 in FIG. 3A. FIG. 4 is a cross-sectional SEM image (magnification: 10,000 times) of a negative electrode taken out of an electrode assembly according to Comparative Example 1. FIG. 5A is a cross-sectional SEM image (magnification: 10,000 times) of a negative electrode after reduction treatment according to Example 2. FIG. 5B is an enlarged view (magnification: 100,000 times) of a boxed portion A2 in FIG. 5A. FIG. 6 is a cross-sectional SEM image (magnification: 10,000 times) of a negative electrode after reduction treatment according to Comparative Example 2. FIG. 7 is an XPS spectrum of the negative electrode after reduction treatment according to Example 2. FIG. 8 is an XPS spectrum of the negative electrode after reduction treatment according to Comparative Example 2. FIG. 9 is a graph showing the relationship between the number of charge / discharge cycles and the capacity retention rate of the batteries of Example 3 and Comparative Example 3.

[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 negative electrode 10 according to embodiment 1. The negative electrode 10 includes a negative electrode current collector 12, a negative electrode active material layer 14, and a coating 16. The negative electrode active material layer 14 is supported by the negative electrode current collector 12. The coating 16 contains an inorganic material and is provided on the surface of the negative electrode active material layer 14. Specifically, the surface on which the coating 16 is provided may be the outermost surface of the negative electrode active material layer 14. The coating 16 exhibits a porous structure in a cross section parallel to the thickness direction of the negative electrode 10.

[0012] The coating 16 improves the cycle characteristics of a battery using the negative electrode 10. Although the reason for this is not entirely clear, it is thought that the coating 16 prevents decomposition of the electrolyte while allowing lithium ions to be exchanged between the electrolyte and the negative electrode active material layer 14. However, the protective function of the coating 16 exceeds the protective function of known SEI coatings. As a result, the cycle characteristics of a battery using the negative electrode 10 are improved.

[0013] It is desirable that the coating 16 has lithium ion conductivity. With such a configuration, it is easy to obtain the effect of improving the cycle characteristics while suppressing an increase in the resistance of the battery. The lithium ion conductivity of the coating 16 can be confirmed by an AC impedance method.

[0014] When the lithium ion transfer resistance (charge transfer resistance) to the negative electrode is defined as Rct, the lithium ion transfer resistance Rct is expressed by the following formula (1) using a frequency factor A, an activation energy Ea, a gas constant R, and an absolute temperature T. In order to reduce the lithium ion transfer resistance Rct, it is necessary to increase the frequency factor A and / or reduce the activation energy Ea.

[0015] 1 / Rct=Aexp(-Ea / RT)...(1)

[0016] The activation energy Ea of the electrode can be measured by an AC impedance method. The inventors examined the activation energy of the negative electrode before and after the formation of the coating 16. As a result, the activation energy of the negative electrode before and after the formation of the coating 16 was 47 kJ, and the activation energy of the negative electrode after the formation of the coating 16 was 45 kJ. In other words, the activation energy of the negative electrode was reduced by the coating 16. Therefore, it can be said that the coating 16 has lithium ion conductivity and also has the function of reducing the migration resistance of lithium ions to the negative electrode.

[0017] From images (cross-sectional SEM images) of the cross section of the coating 16 taken at a plurality of positions using a scanning electron microscope, it can be confirmed that the coating 16 has a porous structure that is three-dimensional and spreads in a network-like pattern.

[0018] The coating 16 has an average thickness of, for example, 150 nm or more. That is, the coating 16 is significantly thicker than known SEI coatings. When the thickness of known SEI coatings reaches 150 nm, resistance increases excessively, causing a decrease in capacity and output. In contrast, the coating 16 is less likely to increase resistance despite having an average thickness of 150 nm or more. This is presumably because the coating 16 is made of a material that is not insulating but has lithium ion conductivity, thereby reducing activation energy. Furthermore, it is presumed that an appropriate amount of electrolyte is retained within the pores of the coating 16, making it less likely for the electrolyte to dry up even after repeated charge and discharge, resulting in low liquid resistance.

[0019] The upper limit of the average thickness of the coating 16 is not particularly limited, and is, for example, 500 nm, preferably 300 nm. That is, the average thickness of the coating 16 may be 150 nm or more and 500 nm or less, or 150 nm or more and 300 nm or less. By appropriately adjusting the average thickness of the coating 16, it is possible to obtain the effect of improving the cycle characteristics while suppressing an increase in the resistance of the battery.

[0020] The average thickness of the coating 16 can be calculated from an SEM image of a cross section parallel to the thickness direction of the negative electrode 10. Specifically, the thickness of the coating 16 is measured at any multiple points (e.g., 10 points) in the cross-sectional SEM image of the negative electrode 10. The average of the measured values ​​is considered to be the average thickness of the coating 16. Here, if there are large recesses on the surface of the negative electrode active material layer 14, the material of the coating 16 tends to accumulate in the recesses, resulting in the formation of a thick coating 16. If there are large protrusions on the surface of the negative electrode active material layer 14, the material of the coating 16 tends to be less likely to accumulate on the protrusions, resulting in the formation of a thin coating 16. Therefore, it is desirable to measure the thickness of the coating 16 at a flat position on the surface of the negative electrode active material layer 14.

[0021] The coating 16 may contain mesopores. The small pores in the coating 16 result in a uniform current distribution in the negative electrode 10. In other words, current is less likely to concentrate in specific areas on the surface of the negative electrode 10, suppressing the formation of lithium dendrites during charge and discharge. As a result, the cycle characteristics of a battery using the negative electrode 10 are improved. Mesopores refer to pores having a diameter of 2 nm to 50 nm. The diameter of the pores in the coating 16 can be calculated from a cross-sectional SEM image of the negative electrode 10. That is, the area of ​​the pores in the cross-sectional SEM image of the negative electrode 10 is calculated by image processing. For example, it is common to count the number of pores by regarding black areas in the cross-sectional SEM image as pores. A binarization process may be performed to extract the black areas. The diameter of a circle having an area equal to the calculated area can be considered the diameter of the pore.

[0022] The coating 16 may further include macropores. Macropores refer to pores having a diameter of more than 50 nm. When the coating 16 has relatively large pores, the thickness of the coating 16 can be easily ensured.

[0023] The coating 16 may have an average pore size of 100 nm or less. This configuration improves the liquid retention of the coating 16. The improved liquid retention of the coating 16 makes it less likely for localized liquid depletion to occur even after repeated charge / discharge cycles, and low liquid resistance is maintained. As a result, deterioration of cycle characteristics due to liquid depletion is suppressed. The lower limit of the average pore size of the coating 16 is not particularly limited, and is, for example, 10 nm.

[0024] The average pore size of the coating 16 may be the average value of the diameters of a plurality of pores (e.g., 10 or more) that appear in a cross-sectional SEM image of the coating 16. Extremely small pores and extremely large pores may be excluded from the sample.

[0025] The porosity of the coating 16 is, for example, 5% or more and 50% or less. When the porosity of the coating 16 is in this range, the lithium ion transfer rate is optimal. The porosity of the coating 16 can be calculated by image analysis of a cross-sectional SEM image of the coating 16. In order to accurately calculate the porosity, the area of ​​the cross-sectional SEM image used to calculate the porosity should be 1 μm 2 It is desirable that this is the case.

[0026] The coating 16 may not contain an organic binder. That is, the coating 16 does not contain a binder that can be used in the negative electrode active material layer 14. With such a configuration, an increase in the resistance of the battery due to the coating 16 can be suppressed. In particular, it is desirable that the coating 16 does not contain a polymer compound.

[0027] In addition to elements contained in the raw materials of the coating 16, the coating 16 may also contain elements derived from the solvent used to form the coating 16. In the examples described below, the solvent used to form the coating 16 is the nonaqueous electrolyte of a nonaqueous electrolyte secondary battery. In this case, the coating 16 may contain decomposition products of the nonaqueous electrolyte, similar to known SEI coatings.

[0028] The coating 16 may be a deposited layer of a plurality of particles. The coating 16 can be formed by depositing a plurality of particles on the surface of the negative electrode active material layer 14.

[0029] The plurality of particles may be particles of an inorganic material contained in the coating 16 .

[0030] The coating 16 may include a skeleton formed by a plurality of interconnected particles. The skeleton may be a portion of the coating 16 other than the pores. The skeleton of the coating 16 is presumed to improve the durability of the coating 16 against repeated charge and discharge. As a result, the protective function of the coating 16 continues to be exerted for a long period of time. Therefore, the skeleton of the coating 16 may contribute to improving the cycle characteristics of the battery.

[0031] The plurality of particles may have an average particle diameter of 5 nm or more and 40 nm or less. That is, nano-sized particles can be suitably used as the material for the coating 16. With this configuration, the nano-sized particles can adhere to the negative electrode active material layer 14 by intermolecular forces. As a result, the coating 16 having a sufficient thickness can be formed.

[0032] The average particle size of the plurality of particles can be the average value of the diameters of a plurality of particles (e.g., 10 or more) that appear in a cross-sectional SEM image of the coating 16. That is, the area of ​​the particles in the cross-sectional SEM image of the negative electrode 10 is calculated by image processing. The diameter of a circle having an area equal to the calculated area can be considered to be the diameter of the particle.

[0033] When multiple particles are connected, it is difficult to identify the diameter of each individual particle. In this case, particles whose outlines are clearly recognizable in a cross-sectional SEM image of the coating 16 may be selected, and the average particle size may be calculated from their diameters.

[0034] In the negative electrode 10 of this embodiment, the negative electrode active material layer 14 may have pores. In this case, the negative electrode 10 may further include a particle deposition layer provided on the inner surface of the pores of the negative electrode active material layer 14. The pores of the negative electrode active material layer 14 refer to spaces surrounded by particles of the negative electrode active material contained in the negative electrode active material layer 14. For example, when forming the coating 16 on the surface of the negative electrode active material layer 14, a plurality of particles used as raw materials penetrate into the pores of the negative electrode active material layer 14 to form a particle deposition layer. Like the coating 16, the particle deposition layer can also function to protect the negative electrode active material layer 14. In other words, the particle deposition layer can also protect the inner surface of the pores of the negative electrode active material layer 14.

[0035] The particle deposition layer may be porous, similar to the coating 16. The descriptions regarding the skeleton of the coating 16, the pores of the coating 16, the materials contained in the coating 16, and the properties of the coating 16 can be applied to the particle deposition layer inside the negative electrode active material layer 14.

[0036] The average thickness of the coating 16 on the surface of the negative electrode active material layer 14 may be greater than the average thickness of the particle deposition layer on the inner surface of the pores of the negative electrode active material layer 14. With this configuration, the coating 16 is present on the surface of the negative electrode active material layer 14, which is an area where an interfacial reaction between the electrolyte and the active material is likely to occur, thereby suppressing excessive current concentration. Because the particle deposition layer is thin, lithium ions can be smoothly inserted into and extracted from the negative electrode active material layer 14. The average thickness of the particle deposition layer can be calculated using the same method as for the average thickness of the coating 16. The average thickness of the particle deposition layer is, for example, 10 nm or more and 30 nm or less.

[0037] The ratio of the average thickness of the particle deposition layer on the inner surface of the pores of the anode active material layer 14 to the average thickness of the coating 16 on the surface of the anode active material layer 14 may be in the range of 0.01 to 0.3, or in the range of 0.1 to 0.2. With this configuration, it is possible to achieve both the effect of suppressing current concentration on the surface of the anode active material layer 14 and the effect of facilitating the insertion and desorption of lithium ions into and from the anode active material layer 14.

[0038] The coating 16 may include a known SEI coating. For example, the SEI coating may be present on the inner surfaces of the pores of the coating 16, or the SEI coating may be present at the interface between the negative electrode active material layer 14 and the coating 16.

[0039] The inorganic material contained in the coating 16 may include a phosphorus-containing compound, such as lithium phosphate.

[0040] The main component of the coating 16 may be an inorganic material. The term "main component" refers to the component that is contained in the largest amount by mass.

[0041] The coating 16 may contain Li, P, F, and O. By containing these elements, the coating 16 is endowed with excellent reduction resistance and can also exhibit lithium ion conductivity.

[0042] The coating 16 is Li a P.O. b F c (2≦a≦4, 3≦b≦5, 0<c≦4). a P.O. b F c By including the compound (I), excellent reduction resistance is imparted to the coating 16, and the coating 16 can exhibit lithium ion conductivity.

[0043] The type of element contained in the coating 16 can be confirmed by X-ray photoelectron spectroscopy (XPS) of the coating 16. In this embodiment, the XPS spectrum of the coating 16 may have a peak in the binding energy range of 132 eV to 134 eV. This peak is due to Li x P.O. y F zThis peak is attributed to the 2p orbital of phosphorus contained in the lithium ion transfer resistance (charge transfer resistance).

[0044] As described above, the phosphorus (P) contained in the coating 16 is in various bonding states, that is, the coating 16 contains various phosphorus-containing compounds.

[0045] Other configurations of the negative electrode 10 will be described.

[0046] The negative electrode current collector 12 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 12 as a conductive auxiliary material.

[0047] The negative electrode active material layer 14 includes a negative electrode active material. The negative electrode active material may 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.

[0048] The negative electrode active material layer 14 may contain at least one selected from the group consisting of graphite and silicon as the negative electrode active material. Graphite may be the only negative electrode active material contained in the negative electrode active material layer 14. 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 battery.

[0049] The negative electrode active material layer 14 may contain other materials such as a conductive additive and a binder.

[0050] Examples of the conductive additive include carbon materials and conductive polymers. 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 include polyaniline, polypyrrole, and polythiophene. At least one selected from these conductive additives can be used.

[0051] The binder is used to improve the binding properties of the materials constituting the negative electrode 10. Examples of the binder that can be used include polymeric 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.

[0052] Next, a method for manufacturing the negative electrode 10 will be described.

[0053] The negative electrode 10 can be manufactured by bringing a dispersion containing particles of an inorganic material into contact with the surface of the negative electrode active material layer 14. For example, the negative electrode active material layer 14 is formed on the negative electrode current collector 12 by a dry or wet process. Then, the coating 16 is formed on the surface of the negative electrode active material layer 14 by the method described above.

[0054] Examples of inorganic materials are as described above. The inorganic material particles are preferably particles having lithium ion conductivity.

[0055] The solvent in the dispersion is water or a non-aqueous solvent. Considering that the negative electrode 10 is used in a non-aqueous electrolyte secondary battery, it is desirable that the inorganic material particles be insoluble in the non-aqueous solvent.

[0056] There are no particular limitations on the method for bringing the dispersion liquid containing inorganic material particles into contact with the surface of the negative electrode active material layer 14. The dispersion liquid may be applied to the surface of the negative electrode active material layer 14, or the negative electrode active material layer 14 may be immersed in the dispersion liquid.

[0057] The dispersion containing inorganic material particles may be a nonaqueous solvent used in nonaqueous electrolyte secondary batteries, or may be a nonaqueous electrolyte for nonaqueous electrolyte secondary batteries. That is, the dispersion containing inorganic material particles may contain a nonaqueous solvent and inorganic material particles. Desirable nonaqueous solvents include dimethyl carbonate, dimethyl ether, and mixtures thereof. Furthermore, when the nonaqueous electrolyte also serves as a dispersion containing inorganic material particles, the dispersion may contain a nonaqueous solvent, an electrolyte salt, and inorganic material particles. In this case, by injecting the nonaqueous electrolyte into the battery container, the negative electrode is impregnated with the nonaqueous electrolyte, and the inorganic material particles adhere to the surface of the negative electrode active material layer 14, forming a coating 16. If the negative electrode active material layer 14 has pores, the nonaqueous electrolyte may penetrate into the pores. This may result in the formation of a particle deposition layer.

[0058] Fluorine contained in the non-aqueous solvent and the electrolyte salt may contribute to the formation of the coating 16. In this respect, fluorine may be contained in at least one of the non-aqueous solvent and the electrolyte salt. As a solvent for the dispersion, for example, fluoroethylene carbonate or the like may be used.

[0059] The average thickness of the coating 16 can be controlled by adjusting the content of inorganic particles in the dispersion liquid. The content of particles in the dispersion liquid is, for example, 0.1 vol% to 30 vol%, or alternatively, 0.1 vol% to 20 vol%, or alternatively, 0.5 vol% to 10 vol%, or alternatively, 1 vol% to 8 vol%, or alternatively, 1 vol% to 4 vol%.

[0060] (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 10, and a pair of separators 7. The electrode group 4 is impregnated with the 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 sealing plate 2. The negative electrode 10 includes a negative electrode current collector 12 and a negative electrode active material layer 14. One end of a negative electrode lead 10c is connected to the negative electrode 10. The other end of the negative electrode lead 10c 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.

[0061] By using the negative electrode 10 described in the first embodiment, the cycle characteristics of the nonaqueous electrolyte secondary battery 100 can be improved.

[0062] The non-aqueous electrolyte secondary battery 100 may be a lithium secondary battery.

[0063] The positive electrode current collector 5a is a foil made of a metal material such as aluminum, an aluminum alloy, stainless steel, titanium, or a titanium alloy.

[0064] The positive electrode active material layer 5b is supported on the positive electrode current collector 5a. The positive electrode active material layer 5b includes a positive electrode active material. 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.

[0065] The positive electrode active material layer 5b may contain other materials such as a conductive additive, a binder, etc. As the conductive additive and the binder, materials that can be used for the negative electrode active material layer 14 can also be used for the positive electrode active material layer 5b.

[0066] The electrolyte is a non-aqueous electrolyte impregnated into the positive electrode 5, the negative electrode 10, 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 10.

[0067] The electrolyte solution contains a non-aqueous solvent and a lithium salt.

[0068] Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, nitriles, amides, etc. One selected from these solvents may be used, or two or more may be used in combination.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] The electrode group 4 may be wound into a cylindrical shape or an oval shape.

[0073] The shape of the nonaqueous electrolyte 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 nonaqueous electrolyte secondary battery 100.

[0074] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0075] (Technology 1) A negative electrode comprising: a substrate; a negative electrode active material layer supported on the substrate; and a coating film containing an inorganic material and provided on a surface of the negative electrode active material layer, wherein the coating film exhibits a porous structure in a cross section parallel to a thickness direction of the negative electrode.

[0076] The negative electrode of the present disclosure is suitable for improving the cycle characteristics of a battery.

[0077] (Technology 2) The negative electrode according to Technology 1, wherein the coating has an average thickness of 150 nm or more. With this configuration, it is possible to obtain the effect of improving cycle characteristics while suppressing an increase in the resistance of the battery.

[0078] (Technology 3) The negative electrode according to Technology 1 or 2, wherein the coating contains mesopores. With this configuration, the current distribution becomes uniform, current concentration on the surface of the negative electrode is less likely to occur, and cycle characteristics are improved.

[0079] (Technology 4) The negative electrode according to Technology 3, wherein the coating further includes macropores. When the coating has relatively large pores, the thickness of the coating is easily ensured.

[0080] (Technology 5) The negative electrode according to any one of Technologies 1 to 4, wherein the coating has an average pore size of 100 nm or less. With this configuration, deterioration of cycle characteristics due to liquid drying up can be suppressed.

[0081] (Technology 6) The negative electrode according to any one of Technologies 1 to 5, wherein the coating does not contain an organic binder. With this configuration, it is possible to suppress an increase in the resistance of the battery due to the coating.

[0082] (Technology 7) The negative electrode according to any one of Technologies 1 to 6, wherein the coating is a deposited layer of a plurality of particles. The coating can be formed by depositing a plurality of particles on the surface of the negative electrode active material layer.

[0083] (Technology 8) The negative electrode according to any one of Technologies 1 to 7, wherein the coating includes a skeleton formed by a plurality of connected particles. It is presumed that the skeleton structure of the coating improves the durability of the coating against repeated charge and discharge.

[0084] (Technology 9) The negative electrode according to Technology 7 or 8, wherein the plurality of particles have an average particle size of 5 nm or more and 40 nm or less. With this configuration, the nano-sized particles can be attached to the negative electrode active material layer by intermolecular forces.

[0085] (Technology 10) The negative electrode according to any one of Technologies 1 to 9, wherein the negative electrode active material layer includes pores, and the negative electrode further includes a particle deposition layer provided on the inner surface of the pores of the negative electrode active material layer. Like a coating, the particle deposition layer can also function to protect the negative electrode active material layer.

[0086] (Technology 11) The negative electrode according to Technology 10, wherein the coating has an average thickness greater than the average thickness of the particle deposition layer. With this configuration, lithium ions can be smoothly inserted and extracted.

[0087] (Technology 12) The negative electrode according to Technology 10 or 11, wherein the ratio of the average thickness of the particle deposition layer to the average thickness of the coating is in the range of 0.01 to 0.3. With this configuration, it is possible to achieve both the effect of suppressing current concentration on the surface of the negative electrode active material layer and the effect of facilitating the insertion and desorption of lithium ions into and from the negative electrode active material layer.

[0088] (Technology 13) The negative electrode according to any one of Techniques 1 to 12, wherein the coating contains Li, P, F, and O. By containing these elements, the coating is endowed with excellent reduction resistance and can exhibit lithium ion conductivity.

[0089] (Technique 14) The film is made of Li a P.O. b F c 14. The negative electrode according to any one of claims 1 to 13, comprising: (2≦a≦4, 3≦b≦5, 0<c≦4). a P.O. b F c By including the compound (I), excellent reduction resistance is imparted to the coating film, and the coating film can exhibit lithium ion conductivity.

[0090] (Technology 15) The negative electrode according to any one of Technologies 1 to 14, wherein an XPS spectrum of the coating has a peak in a binding energy range of 132 eV to 134 eV. With this configuration, it is easy to obtain the effect of improving cycle characteristics while suppressing an increase in battery resistance.

[0091] (Technology 16) A nonaqueous electrolyte secondary battery comprising: a positive electrode; the negative electrode according to any one of Technologies 1 to 15; and a separator disposed between the positive electrode and the negative electrode.

[0092] (Preparation of positive electrode) LiNi 0.8 Co 0.15 Al 0.05 A positive electrode slurry was prepared by mixing and stirring a positive electrode active material (NCA) having a composition of O2, acetylene black (AB), polyvinylidene fluoride (PVDF), and N-methyl-pyrrolidone (NMP). The mass ratio of NCM, AB, and PVDF was NCM:AB:PVDF = 96:2:2. The positive electrode slurry was applied to the surface of aluminum foil to form a coating film, which was then dried and rolled. This resulted in a positive electrode common to Examples 1 to 3 and Comparative Examples 1 to 3.

[0093] [Example 1] (Preparation of Negative Electrode) An appropriate amount of water was added to the negative electrode mixture and mixed to obtain a negative electrode slurry. The negative electrode mixture was a mixture of a negative electrode active material (graphite), a binder, and a conductive agent. The binders used were sodium polyacrylate (PAA-Na), a sodium salt of CMC (CMC-Na), and styrene butadiene rubber (SBR). The contents of PAA-Na, CMC-Na, and SBR in the negative electrode mixture were each 1% by mass. Next, the negative electrode slurry was applied to the surface of copper foil, the coating was dried, and then rolled to form a negative electrode mixture layer, obtaining a negative electrode.

[0094] (Preparation of Pretreatment Solution) Ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:FEC:DMC:EMC = 10:5:10:75 to prepare a nonaqueous solvent. LiPF was dissolved in the nonaqueous solvent to a concentration of 1.3 mol / L to obtain a solution. 15 g of the obtained solution, 1.2 g of LiPO particles (average particle size 10 μm), and 70 g of ZrO balls (average particle size 0.5 mm) as mixing media were placed in a ball mill and mixed at 300 rpm for 2 hours. The average particle size of the LiPO particles after mixing was 25 nm. The supernatant of the obtained mixture was removed by sucking up with a dropper, and the ZrO balls were obtained, obtaining the pretreatment solution of Example 1.

[0095] (Preparation of Electrode Assembly of Example 1) A current collecting lead was attached to each of the positive electrode and the negative electrode. An electrode assembly was prepared by combining the positive electrode, a separator (Celgard, #2320), and a negative electrode. The electrode assembly was placed in a glass container, and a pretreatment solution was poured into the glass container. This gave the electrode assembly of Example 1.

[0096] (Observation of Cross Section of Negative Electrode) The electrode assembly of Example 1 was left for 24 hours from the time of production. Thereafter, the electrode assembly was disassembled and the negative electrode was removed. The removed negative electrode was washed with DMC. Then, a cross section of the negative electrode was formed using a broad ion beam. The cross section of the negative electrode was observed using a scanning electron microscope. The results are shown in FIGS. 3A and 3B.

[0097] [Comparative Example 1] An electrode body of Comparative Example 1 was produced by the same method as in Example 1, except that the pretreatment solution did not contain lithium phosphate. The negative electrode was removed from the electrode body of Comparative Example 1 by the same method as in Example 1, and a cross section of the removed negative electrode was formed and observed. The results are shown in Figure 4.

[0098] (Discussion) Fig. 3A is a cross-sectional SEM image (magnification: 10,000 times) of the negative electrode taken out from the electrode assembly of Example 1. Fig. 3B is an enlarged view (magnification: 100,000 times) of the boxed area A1 in Fig. 3A. Fig. 4 is a cross-sectional SEM image (magnification: 10,000 times) of the negative electrode taken out from the electrode assembly of Comparative Example 1.

[0099] 3A and 3B, a porous coating was formed on the surface of the negative electrode active material layer in the negative electrode removed from the electrode assembly of Example 1. That is, even though no charge / discharge was performed, a porous coating was formed on the surface of the negative electrode active material layer according to Example 1. This fact indicates that charge / discharge is not essential for the formation of a porous coating.

[0100] The negative electrode removed from the electrode assembly was washed with DMC. The porous coating was a stable coating that was not removed by washing, even though it did not contain a binder.

[0101] As shown in Figure 3A, the porous coating had a thickness of 250 nm to 500 nm. The average thickness was 300 nm. As shown in Figure 3B, the porous coating had a skeleton of multiple interconnected particles. The pores in the porous coating included mesopores with diameters of 2 nm to 50 nm and macropores with diameters greater than 50 nm.

[0102] As shown in the boxed area B1 in FIG. 3A , in the negative electrode of Example 1, a particle deposition layer was formed on the inner surface of the pores of the negative electrode active material layer. It is presumed that this particle deposition layer was formed by the reaction of lithium phosphate particles, the raw material, with components in the electrolyte. The thickness of the particle deposition layer was less than the thickness of the porous coating on the surface of the negative electrode active material layer. In the example shown in FIG. 3A , the particle deposition layer had an island-like structure. In other words, the particle deposition layer may be present locally on the inner surface of the pores of the negative electrode active material layer.

[0103] As shown in Fig. 4, no porous coating was formed on the surface of the negative electrode active material layer in the negative electrode of Comparative Example 1. The coating on the surface of the negative electrode active material layer in Fig. 4 is a protective film for protecting the object to be observed from the electron beam.

[0104] Example 2 An electrode assembly of Example 2 was produced by the same method as in Example 1.

[0105] (Reduction Treatment of Negative Electrode) A constant current of 0.2 C was applied to the electrode body of Example 2 to reduce the negative electrode until the potential difference between the negative electrode and the positive electrode reached 4.3 V. The reduction of the negative electrode was carried out at an ambient temperature of 25°C.

[0106] (Cross-section observation of negative electrode) After the reduction treatment of the negative electrode, the electrode assembly of Example 2 was disassembled and the negative electrode was removed. The removed negative electrode was washed with DMC. The cross section of the negative electrode of Example 2 after the reduction treatment was observed with a scanning electron microscope using the same method as in Example 1. The results are shown in Figures 5A and 5B.

[0107] (X-ray photoelectron spectroscopy analysis of negative electrode surface) X-ray photoelectron spectroscopy (XPS) was performed on the surface of the negative electrode after the reduction treatment of Example 2. Phosphorus (P) was selected as the element to be measured, and the binding energy scan range was set to a binding energy range of 128 eV to 142 eV. In the analysis of the XPS spectrum, peak separation was performed using a pseudo-Voigt function. The results are shown in FIG.

[0108] [Comparative Example 2] An electrode body of Comparative Example 2 was produced by the same method as in Example 2, except that the pretreatment solution did not contain lithium phosphate. The negative electrode of the electrode body of Comparative Example 2 was subjected to a reduction treatment by the same method as in Example 2. Thereafter, the electrode body of Comparative Example 2 was disassembled, and the negative electrode was removed. The removed negative electrode was washed with DMC. The cross section of the negative electrode of Comparative Example 2 after the reduction treatment was observed with a scanning electron microscope by the same method as in Example 1. The results are shown in FIG. 6.

[0109] X-ray photoelectron spectroscopy analysis was performed on the surface of the negative electrode after reduction treatment in Comparative Example 2 using the same method as in Example 2. The results are shown in FIG.

[0110] (Discussion) Fig. 5A is a cross-sectional SEM image (magnification: 10,000 times) of the negative electrode after reduction treatment in Example 2. Fig. 5B is an enlarged view (magnification: 100,000 times) of the boxed portion A2 in Fig. 5A. Fig. 6 is a cross-sectional SEM image (magnification: 10,000 times) of the negative electrode after reduction treatment in Comparative Example 2.

[0111] 5A and 5B , a porous coating was formed on the surface of the negative electrode active material layer in the negative electrode of Example 2. That is, the porous coating was maintained even after the reduction treatment. The porous coating in the negative electrode of Example 2 had a thickness approximately equal to that of the porous coating in the negative electrode removed from the electrode body of Example 1 (which was only immersed in the pretreatment solution).

[0112] As shown in the boxed area B2 in Figure 5A, in the negative electrode of Example 2, a particle deposition layer was formed on the inner surface of the pores of the negative electrode active material layer. This particle deposition layer is presumed to have been formed by the reaction of lithium phosphate particles, the raw material, with components in the electrolyte. The thickness of the particle deposition layer was less than the thickness of the porous coating on the surface of the negative electrode active material layer. The average thickness of the particle deposition layer was 15 nm.

[0113] The negative electrode of Example 2 removed from the electrode assembly was washed with DMC. The porous coating of Example 2 was a stable coating that was not removed by washing, even though it did not contain a binder.

[0114] 6 , no porous coating was formed on the surface of the negative electrode active material layer in the negative electrode of Comparative Example 2. Further, upon detailed analysis of the negative electrode of Comparative Example 2, it was found that a dense SEI coating having a thickness of about 30 nm was formed on the surface of the negative electrode active material layer in the negative electrode of Comparative Example 2.

[0115] 7 shows the XPS spectrum of the anode after reduction treatment in Example 2. FIG. 8 shows the XPS spectrum of the anode after reduction treatment in Comparative Example 2. As shown in FIG. 8, the XPS spectrum of the anode of Comparative Example 2 contains LiPF6, Li α PF β , Li α P.O. β F γ The peaks included those attributable to phosphorus (P) in each of the bonding states. Phosphorus and fluorine are elements derived from the electrolyte salt and non-aqueous solvent. α, β, and γ are positive real numbers.

[0116] In contrast, as shown in FIG. 7, the XPS spectrum of the negative electrode of Example 2 shows that Li α PF β The peaks assigned to phosphorus (P) in the bond state of Li α P.O. β F γ In addition to the peaks attributable to phosphorus (P) in the bond state, a new peak was included. x P.O. y F zThis peak is attributed to phosphorus (P) in the bonding state of Li, and is the peak with the greatest intensity among the peaks attributed to the 2p orbital of phosphorus. x P.O. y F z The ratio of phosphorus atoms in the bond state of Li was the largest. x, y, and z are positive real numbers. Energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) showed that the porous coating in the negative electrode of Example 2 had a composition of Li, O, P, and F. a P.O. b F c It was confirmed that a, b, and c satisfied the following conditions: 2≦a≦4, 3≦b≦5, and 0<c≦4, respectively.

[0117] The porosity and average area of ​​the pores in the porous coating were calculated by image analysis of a cross-sectional SEM image of the negative electrode taken out of the electrode assembly of Example 1. Similarly, the porosity and average area of ​​the pores in the porous coating were calculated by image analysis of a cross-sectional SEM image of the negative electrode after the reduction treatment of Example 2. The results are shown in Table 1.

[0118]

[0119] As shown in Table 1, no significant change was observed in the porosity of the porous coating before and after the reduction treatment of the negative electrode. The average pore diameter (equivalent diameter) calculated from the average area of ​​the pores was 33.0 nm in Example 1 and 25.6 nm in Example 2.

[0120] [Example 3] An electrode assembly was produced in the same manner as in Example 1. The electrode assembly was placed in a glass container, and a pretreatment solution was poured into the glass container. In this way, the electrode assembly of Example 3 was obtained.

[0121] A constant current of 0.2 C was applied to the electrode body of Example 3 to reduce the negative electrode until the potential difference between the negative electrode and the positive electrode reached 4.3 V. After a 20-minute pause, the electrode body of Example 3 was discharged at a current value of 0.2 C to oxidize the negative electrode until the potential difference between the negative electrode and the positive electrode reached 2.5 V. The reduction and oxidation of the negative electrode were carried out at an ambient temperature of 25°C.

[0122] After the reduction and oxidation of the negative electrode, the electrode assembly of Example 3 was disassembled to remove the negative electrode. The removed negative electrode was washed with DMC. Thus, the negative electrode of Example 3 was obtained.

[0123] (Preparation of non-aqueous electrolyte) Ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:FEC:DMC:EMC = 10:5:10:75 to prepare a non-aqueous solvent. LiPF was dissolved in the non-aqueous solvent to a concentration of 1.3 mol / L to obtain a non-aqueous electrolyte.

[0124] (Fabrication of Battery) A current collecting lead was attached to each of the positive electrode and the negative electrode. The positive electrode, a separator (Celgard, #2320), and the negative electrode of Example 3 were combined and placed in a container made of laminate film. A nonaqueous electrolyte was poured into the container, which was then sealed. This produced a battery of Example 3.

[0125] [Comparative Example 3] A negative electrode of Comparative Example 3 was produced by the same method as in Example 1, except that the pretreatment solution did not contain lithium phosphate. A battery of Comparative Example 3 was produced using the negative electrode of Comparative Example 3 by the same method as in Example 3.

[0126] (Cycle Test) A cycle test was performed on the batteries of Example 3 and Comparative Example 3 as follows. The batteries were placed in a thermostatic chamber at 25°C. Constant-current charging was performed at a current value of 0.2 C until the voltage reached 4.3 V, followed by constant-voltage charging 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 0.2 C until the voltage reached 2.5 V. A 20-minute rest period was provided between charging and discharging. This charge / discharge cycle was repeated 100 times. However, the discharge current was set to 0.05 C at the first, 50th, and 100th cycles, and the discharge capacity was measured. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle was calculated as the capacity retention rate (%). The results are shown in Table 2 and FIG. 9.

[0127]

[0128] 9 is a graph showing the relationship between the number of charge / discharge cycles and the capacity retention rate of the batteries of Example 3 and Comparative Example 3. As shown in Table 2 and FIG. 9, the capacity retention rate of the battery of Example 3 was 94.0%. As shown in Table 2 and FIG. 9, the capacity retention rate of the battery of Comparative Example 3 was 86.8%. The capacity retention rate of the battery of Example 3 was significantly higher than that of the battery of Comparative Example 3. This result indicates that the porous coating provided on the surface of the negative electrode active material layer is effective in improving the cycle characteristics of the battery of Example 3.

[0129] In Example 3, in order to remove excess Li3PO4 particles contained in the nonaqueous electrolyte, an electrochemical current was passed through the pretreatment solution, and a battery was assembled using the negative electrode after the current was passed through.

[0130] The technology of the present disclosure is useful for non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries.

Claims

1. A negative electrode comprising: a substrate; a negative electrode active material layer supported on the substrate; and a coating containing an inorganic material and provided on a surface of the negative electrode active material layer, wherein the coating exhibits a porous structure in a cross section parallel to the thickness direction of the negative electrode.

2. The negative electrode according to claim 1, wherein the coating has an average thickness of 150 nm or more.

3. The negative electrode of claim 1, wherein the coating contains mesopores.

4. The negative electrode of claim 3, wherein the coating further comprises macropores.

5. The negative electrode according to claim 1, wherein the coating has an average pore size of 100 nm or less.

6. The negative electrode according to claim 1, wherein the coating does not contain an organic binder.

7. The negative electrode according to claim 1, wherein the coating is a deposited layer of a plurality of particles.

8. The negative electrode according to claim 1, wherein the coating comprises a framework of a plurality of interconnected particles.

9. The negative electrode according to claim 7, wherein the plurality of particles have an average particle diameter of 5 nm or more and 40 nm or less.

10. The negative electrode according to claim 1, wherein the negative electrode active material layer includes pores, and the negative electrode further comprises a particle deposition layer provided on the inner surfaces of the pores of the negative electrode active material layer.

11. The negative electrode according to claim 10, wherein the average thickness of the coating is greater than the average thickness of the particle deposition layer.

12. The negative electrode according to claim 10, wherein the ratio of the average thickness of the particle deposition layer to the average thickness of the coating is in the range of 0.01 to 0.

3.

13. The negative electrode of claim 1, wherein the coating comprises Li, P, F, and O.

14. The coating is Li a P.O. b F c The negative electrode of claim 1 , comprising: (2≦a≦4, 3≦b≦5, 0<c≦4).

15. The negative electrode according to claim 1, wherein the XPS spectrum of the coating has a peak in the binding energy range of 132 eV or more and 134 eV or less.

16. A non-aqueous electrolyte secondary battery comprising: a positive electrode; the negative electrode according to claim 1; and a separator disposed between the positive electrode and the negative electrode.

Images