Negative electrode active material, lithium ion battery, and method for producing negative electrode active material

Abstract

To provide a negative electrode active material with improved resistance to lithium precipitation.SOLUTION: A negative active material for a lithium-ion battery including a carbon material and a carbon film covering the carbon material, the carbon material contains carbon and boron, the boron content in the carbon material is 0.2 atomic % or more and less than 3.5 atomic %, the negative electrode active material has an R value of 0.35 or more and 0.85 or less, and the R value is the ratio of the D band to the G band in the Raman spectrum of the negative electrode active material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode active material, a lithium ion battery, and a method for producing a negative electrode active material. [Background technology]

[0002] Patent Document 1 (JP 2021-165223 A) discloses a carbon material doped with boron element to substitute for carbon element, wherein the content of boron element in the carbon material is 0.005 to 15 mol%, and when the content of boron element doped to substitute for carbon element on the surface of the carbon material is X (mol%) and the content of boron element in the carbon material is Y (mol%), X / Y<0.8. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-165223 Summary of the Invention [Problem to be solved by the invention]

[0004] Carbon materials such as graphite are widely used as negative electrode active materials in lithium ion batteries (hereinafter abbreviated as "batteries"). Carbon materials have electronic conductivity. However, the electronic conductivity of carbon materials alone is insufficient. Therefore, in Patent Document 1, boron is doped into the carbon material to improve the electronic conductivity.

[0005] However, when carbon materials are doped with boron, the supply of electrons becomes excessive compared to the ionic conduction in the electrode, making lithium (Li) more likely to precipitate. Therefore, there is room for improvement in the resistance to lithium precipitation.

[0006] Therefore, an object of the present disclosure is to provide a negative electrode active material with improved resistance to lithium deposition. [Means for solving the problem]

[0007] The technical configuration and effects of the present disclosure will be described below. However, the mechanism of action in this specification includes speculation. The mechanism of action does not limit the technical scope of the present disclosure.

[0008] [1] A negative electrode active material for a lithium ion battery, a carbon material and a carbon film coating the carbon material, the carbon material comprises carbon and boron; the boron content in the carbon material is equal to or greater than 0.2 atomic % and less than 3.5 atomic %; The negative electrode active material has an R value of 0.35 or more and 0.85 or less, The R value is the ratio of the D band to the G band in the Raman spectrum of the negative electrode active material.

[0009] The R value is an index of crystallinity, with a smaller R value indicating lower crystallinity. By covering a boron-containing carbon material with a low-crystalline carbon film, the electronic conductivity of the surface layer of the carbon material is maintained at an appropriate level. As a result, excessive electron supply is suppressed, which is thought to improve lithium deposition resistance.

[0010] On the other hand, if the boron content in the carbon material is too high, the excess boron may inhibit the movement of electrons, resulting in a decrease in electronic conductivity. Therefore, it is considered necessary to keep the boron content within a predetermined range.

[0011] [2] The negative electrode active material according to [1], wherein the carbon material is artificial graphite.

[0012] [3] A lithium ion battery comprising: a negative electrode containing the negative electrode active material according to [1] or [2]; a positive electrode; a separator; and an electrolyte.

[0013] [4] A method for producing a negative electrode active material for a lithium ion battery, comprising: mixing a carbon source and a boron source to obtain a first mixture; calcining the first mixture to produce a carbon material; mixing the carbon material and a precursor of a carbon film to obtain a second mixture; and firing the second mixture to produce a negative electrode active material in which the carbon material is coated with the carbon film, The method for producing a negative electrode active material, wherein the content of the boron source relative to the carbon source is 1.5% by mass or more and 17.5% by mass or less.

[0014] [5] The firing temperature of the first mixture is 1500°C or higher and 3000°C or lower, [4] The method for producing a negative electrode active material according to [4], wherein the baking time of the first mixture is 30 minutes or more and 2 hours or less.

[0015] [6] The average particle size of the carbon source is 2.5 μm or more and 15 μm or less; The method for producing a negative electrode active material according to [4] or [5], wherein the average particle size of the boron source is 0.1 μm or more and 5.0 μm or less. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a conceptual diagram showing the negative electrode active material in this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing an example of the lithium ion battery of this embodiment. [Figure 3] FIG. 3 is a schematic diagram showing an example of the electrode assembly of this embodiment. [Figure 4] FIG. 4 is an example of a schematic flowchart of the method for producing a negative electrode active material in this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "the present embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "the present example") will be described. However, the present embodiment and the example do not limit the technical scope of the present disclosure.

[0018] <Negative electrode active material> The negative electrode active material of this embodiment is for a lithium ion battery, the details of which will be described later.

[0019] 1 is a conceptual diagram showing the negative electrode active material in this embodiment. The negative electrode active material 5 includes a carbon material 1 and a carbon film 2 that covers the carbon material 1.

[0020] The carbon material 1 contains carbon. Examples of the carbon contained in the carbon material 1 include artificial graphite, natural graphite, soft carbon, hard carbon, carbon black (CB), carbon nanotubes (CNT), and vapor-grown carbon fiber (VGCF). From the viewpoints of electrical conductivity and cost, the carbon contained in the carbon material 1 is preferably artificial graphite or natural graphite, and more preferably artificial graphite.

[0021] The carbon material 1 contains boron. When the carbon material 1 contains boron, the electronic conductivity is improved. Furthermore, when the carbon material 1 contains boron, the durability and dispersibility of the carbon material 1 are improved. The carbon material 1 is essentially composed of carbon and boron.

[0022] The boron content in carbon material 1 is 0.2 atomic % or more and less than 3.5 atomic %. If the boron content in carbon material 1 is less than 0.2 atomic %, the effect of improving electronic conductivity due to the inclusion of boron cannot be obtained. If the boron content in carbon material 1 is 3.5 atomic % or more, the excess boron may inhibit the movement of electrons, resulting in a decrease in electronic conductivity. The boron content in carbon material 1 is preferably 0.3 atomic % or more and 3.3 atomic % or less, and more preferably 1.0 atomic % or more and 2.5 atomic % or less.

[0023] The boron content in carbon material 1 is measured by X-ray photoelectron spectroscopy (XPS). In the XPS spectrum of carbon material 1 measured by XPS, the binding energy of the boron 1s orbital (B1s) has a peak top in the range of 185 eV to 197 eV, and the binding energy of the carbon 1s orbital (C1s) has a peak top in the range of 279 eV to 298 eV. The boron content in carbon material 1 is equal to or greater than 0.2 atomic % and less than 3.5 atomic %, where the sum of the peak areas of the B1s spectrum and the C1s spectrum is taken as 100 atomic %.

[0024] The carbon film 2 coats the carbon material 1. By coating the carbon material 1 with the carbon film 2, the electronic conductivity of the surface layer of the carbon material 1 is maintained appropriately. As a result, excessive supply of electrons is suppressed, and it is expected that the resistance to Li deposition will be improved. The carbon film 2 may coat a part of the carbon material 1. The carbon film 2 may coat substantially the entire carbon material 1. In other words, the carbon film 2 coats at least a part of the surface of the carbon material 1.

[0025] The carbon film 2 may have a thickness of, for example, 0.01 μm or more and 0.2 μm or less. If the thickness of the carbon film 2 is less than 0.01 μm, the effect of covering the carbon material 1 with the carbon film 2 may not be exerted. If the thickness of the carbon film 2 exceeds 0.2 μm, the resistance may become high.

[0026] The thickness of the carbon film 2 can be measured, for example, by the following procedure: A sample is prepared by embedding the negative electrode active material 5 in a resin material. The sample is thinned by FIB (Focused Ion Beam) processing. The sample is observed with a STEM (Scanning Transmission Electron Microscope). The thickness of the carbon film 2 may be determined as the arithmetic average of the values ​​measured for a plurality of negative electrode active materials 5.

[0027] The carbon film 2 contains carbon. The carbon film 2 does not contain boron. The carbon film 2 may, for example, consist essentially of carbon. The carbon may contain, for example, pitch carbide. The carbon content in the carbon film 2 relative to the carbon material 1 may be, for example, 1.0 mass % or more and 10 mass % or less. If the carbon content in the carbon film 2 relative to the carbon material 1 is less than 1.0 mass %, the carbon film 2 may not be formed. If the carbon content in the carbon film 2 relative to the carbon material 1 exceeds 10 mass %, the carbon may aggregate, and the carbon film 2 may not be formed.

[0028] The negative electrode active material 5 has an R value of 0.35 or more and 0.85 or less. That is, the carbon film 2 coating the carbon material 1 has a low-crystalline structure. If the R value is less than 0.35, the effect of coating the carbon material 1 with the carbon film 2 may not be achieved. If the R value exceeds 0.85, the crystallinity may be significantly reduced, preventing electron supply between the negative electrode active materials 5, and thus increasing the resistance. The R value is preferably 0.4 or more and 0.8 or less.

[0029] The R value is measured by Raman spectroscopy. -1 1360 cm for the Raman band (G band) near -1 The R value is the ratio of the Raman band (D band) near I D / I G It can also be written as:

[0030] The measurement conditions for Raman spectroscopy are as follows. However, the optimum conditions may differ depending on the device. [Measurement conditions] Laser: Argon (Ar) laser Excitation wavelength: 532 nm Exposure time: 0.5 seconds Number of scans: 10

[0031] The negative electrode active material 5 has an average particle size (D50) of, for example, 1.0 μm or more and 30 μm or less. If the D50 of the negative electrode active material 5 is outside the above range, the Li deposition resistance may decrease. The negative electrode active material 5 preferably has a D50 of 5.0 μm or more and 20 μm or less. Here, D50 refers to the particle size at which the cumulative frequency from the smaller particle size side reaches 50% in the volume-based particle size distribution. D50 can be measured by a laser diffraction method.

[0032] <Lithium-ion battery> FIG. 2 is a schematic diagram showing an example of a lithium-ion battery according to this embodiment. The battery 100 includes a case 90. The case 90 may have any shape. For example, the case 90 may be rectangular or cylindrical. The case 90 may be made of metal, such as a pouch made of an aluminum (Al) laminate film. A positive electrode terminal 91 and a negative electrode terminal 92 may be provided on the case 90.

[0033] The case 90 houses the electrode assembly 50 and an electrolyte. The electrolyte is impregnated into the electrode assembly 50. The electrode assembly 50 is connected to a positive electrode terminal 91 and a negative electrode terminal 92.

[0034] FIG. 3 is a schematic diagram showing an example of an electrode assembly of this embodiment. The electrode assembly 50 is, for example, a wound type. The electrode assembly 50 includes a positive electrode 20, a separator 40, and a negative electrode 30. The positive electrode 20, the separator 40, and the negative electrode 30 are all strip-shaped sheets. The electrode assembly 50 can be formed by stacking the positive electrode 20 and the negative electrode 30 with the separator 40 interposed therebetween and then spirally winding the stack. Two separators 40 may be used. After winding, the electrode assembly 50 may be formed into a flat shape.

[0035] The electrode assembly 50 is, for example, a laminated type. The electrode assembly 50 is formed by laminating a positive electrode 20 and a negative electrode 30 with a separator 40 interposed therebetween. The electrode assembly 50 may have any laminated structure as long as it includes at least one layer of each of a positive electrode 20, a separator 40, and a negative electrode 30. For example, the electrode assembly 50 may be formed by laminating a positive electrode 20, a separator 40, a negative electrode 30, a separator 40, and a positive electrode 20 in this order.

[0036] 《Negative electrode》 The negative electrode 30 may include a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector may include, for example, copper (Cu) foil, nickel (Ni) foil, etc. The negative electrode current collector may have a thickness of, for example, 5 μm or more and 30 μm or less.

[0037] The negative electrode active material layer may have a thickness of, for example, 10 μm or more and 200 μm or less. The negative electrode active material layer contains at least the above-mentioned negative electrode active material. The negative electrode active material layer may, for example, be substantially made of the negative electrode active material.

[0038] The negative electrode active material layer may contain, in addition to the negative electrode active material, for example, a conductive material, a binder, a thickener, etc. The conductive material may contain, for example, a carbon material such as CB (acetylene black (AB), Ketjen black), graphite, CNT, or VGCF. The binder may contain, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), styrene butadiene rubber (SBR), etc. The thickener may contain, for example, carboxymethyl cellulose (CMC), methyl cellulose (MC), etc. The blending amounts of the conductive material, binder, and thickener may be, for example, 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the negative electrode active material.

[0039] 《Positive electrode》 The positive electrode 20 may include a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector may include, for example, aluminum (Al) foil. The positive electrode current collector may have a thickness of, for example, 10 μm or more and 30 μm or less.

[0040] The positive electrode active material layer may have a thickness of, for example, 10 μm or more and 200 μm or less. The positive electrode active material layer contains at least a positive electrode active material. The positive electrode active material layer may, for example, be substantially made of a positive electrode active material. The positive electrode active material may be, for example, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, etc. (for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 The positive electrode active material may contain at least one selected from the group consisting of lithium niobate (LiNbO), lithium nickel cobalt aluminate, and lithium iron phosphate. The positive electrode active material may be subjected to a surface treatment. A buffer layer may be formed on the surface of the positive electrode active material by the surface treatment. The buffer layer may contain, for example, lithium niobate (LiNbO).

[0041] The positive electrode active material layer may contain, in addition to the positive electrode active material, for example, a conductive material, a binder, etc. The conductive material may contain, for example, a carbon material such as CB (acetylene black (AB), Ketjen black), graphite, CNT, or VGCF. The binder may contain, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), or styrene butadiene rubber (SBR). The blending amounts of the conductive material and binder may be, for example, 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the positive electrode active material.

[0042] <Separator> The separator 40 is interposed between the positive electrode 20 and the negative electrode 30. The separator 40 spatially separates the positive electrode 20 and the negative electrode 30. The separator 40 blocks electronic conduction between the positive electrode 20 and the negative electrode 30. The separator 40 is porous. The separator 40 may be made of, for example, polyolefin. The separator 40 may have, for example, a single-layer structure. The separator 40 may be made of, for example, a polyethylene (PE) layer. The separator 40 may have, for example, a multi-layer structure. The separator 40 may have, for example, a three-layer structure. The separator 40 may include, for example, a polypropylene (PP) layer, a PE layer, and a PP layer. The PP layer, the PE layer, and the PP layer may be laminated in this order. The separator 40 may have a thickness of, for example, 5 μm to 40 μm. The separator 40 may have a porosity of, for example, 30% to 60%. For example, a heat-resistant layer may be formed on the surface of the separator 40. The heat-resistant layer may include a heat-resistant material such as boehmite or alumina.

[0043] 《Electrolyte》 The electrolyte solution contains a non-aqueous solvent and a supporting salt. For example, a non-aqueous solvent such as an organic solvent containing a supporting salt can be used. The non-aqueous solvent can be, for example, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), or the like. One type of solvent can be used alone, or two or more types of solvents can be used in combination.

[0044] The supporting salt is dissolved in a non-aqueous solvent. The supporting salt may be, for example, a lithium salt (LiPF6, LiBF4, LiAsF6, LiClO4, LiCF3SO3, etc.). One type of supporting salt may be used alone, or two or more types of supporting salts may be used in combination. The supporting salt may have a molar concentration of, for example, 0.5 mol / L or more and 2 mol / L or less.

[0045] The electrolyte may further contain an optional additive. For example, the additive may be contained in a mass fraction of 0.1% to 5%. The additive may be, for example, vinylene carbonate (VC), lithium difluorophosphate (LiPO2F2), lithium fluorosulfonate (FSO3Li), lithium bis(oxalato)borate (LiBOB), or the like. One type of additive may be used alone, or two or more types of additives may be used in combination.

[0046] <Method of manufacturing negative electrode active material> FIG. 4 is a schematic flowchart of an example of a method for producing a negative electrode active material in this embodiment. The method for producing a negative electrode active material includes "(a) preparation of a first mixture," "(b) production of a carbon material," "(c) preparation of a second mixture," and "(d) production of a negative electrode active material." Examples of methods for producing a negative electrode active material include a gas phase method, a liquid phase method, and a solid phase method. Note that the method for producing a negative electrode active material is an example and is not limited to these.

[0047] (a) Preparation of the first mixture A method for producing a negative electrode active material includes preparing a first mixture by mixing a carbon source and a boron source.

[0048] Carbon sources include inorganic carbon sources and organic carbon sources. Inorganic carbon sources include artificial graphite, natural graphite, soft carbon, hard carbon, CB, CNT, VGCF, etc. The organic carbon source is not particularly limited as long as it is an organic carbon raw material that is carbonized to carbon particles after firing as described below, and examples include pitch, coke, lignite, biomass, phenolic resins, polyimide resins, polyamide resins, etc. Among these, pitch and coke, which are also used as raw materials for artificial graphite, are preferred.

[0049] The D50 of the carbon source may be, for example, 2.5 μm or more and 15 μm or less, or 5.0 μm or more and 10 μm or less.

[0050] The boron source is not particularly limited, and examples thereof include boron carbide, boron oxide, boron nitride, metal borides, boron oxoacids, boranes, etc. For example, boron carbide includes BC and B 12 Examples of boron oxides include BCO2 and B2O2, examples of boron nitrides include BN, examples of metal borides include AlB2, CoB, and FeB, examples of boron oxoacids include orthoboric acid and metaboric acid, and examples of boranes include monoborane and diborane.

[0051] The D50 of the boron source may be, for example, 0.1 μm or more and 5.0 μm or less, or 0.2 μm or more and 2.0 μm or less. If the D50 of the boron source is less than 0.1 μm, it may be difficult to uniformly disperse the carbon source and the boron source, and the desired carbon material may not be obtained. If the D50 of the boron source is more than 5.0 μm, the contact frequency between the carbon source and the boron source may be low, and the desired carbon material may not be obtained.

[0052] The content of the boron source relative to the carbon source may be adjusted to the boron content of the carbon material described above. The amount of the boron source added relative to the carbon source is, for example, 1.5% by mass to 17.5% by mass, preferably 2.0% by mass to 15% by mass, and more preferably 5.0% by mass to 12.5% ​​by mass.

[0053] The method for mixing the carbon source and the boron source is not particularly limited, and may be dry mixing or wet mixing.

[0054] (b) Production of carbon materials The method for producing a negative electrode active material includes producing a carbon material by firing the first mixture. "(a) Preparation of the first mixture" and "(b) Production of the carbon material" may be performed simultaneously.

[0055] The firing conditions for the first mixture vary depending on the type and amount of the carbon source and boron source used. The firing temperature may be, for example, 1500°C to 3000°C, or 2000°C to 3000°C. The firing time may be, for example, 30 minutes to 120 minutes, or 60 minutes to 90 minutes. The firing atmosphere may be air, or may be an inert atmosphere such as nitrogen or Ar. Furthermore, firing may be performed in a single stage at constant temperature, time, and atmosphere, or in multiple stages with different temperature, time, and atmosphere.

[0056] (c) Preparation of the second mixture The method for producing the negative electrode active material includes mixing a carbon material and a precursor of the carbon film to prepare a second mixture.

[0057] Examples of the carbon film precursor include the same carbon source as described above. The content of the carbon film precursor may be such that the carbon film can be produced to have the above-described R value. The amount of the carbon film precursor added to the first mixture may be, for example, 1.0 mass % or more and 15 mass % or less, or 5.0 mass % or more and 10 mass % or less.

[0058] (d) Production of negative electrode active material The method for producing a negative electrode active material includes firing the second mixture to produce a negative electrode active material in which the carbon material is coated with a carbon film. "(c) Preparation of the second mixture" and "(d) Production of the negative electrode active material" may be performed simultaneously.

[0059] The firing conditions for the second mixture vary depending on the type and amount of the carbon film precursor used. The firing temperature may be, for example, 500°C to 2000°C, or 1000°C to 1500°C. The firing time may be, for example, 30 minutes to 120 minutes, or 60 minutes to 90 minutes. The firing atmosphere may be air, or may be an inert atmosphere such as nitrogen or Ar. Furthermore, firing may be performed in a single stage with the temperature, time, and atmosphere kept constant, or in multiple stages with the temperature, time, and atmosphere changed. [Example]

[0060] The present embodiment will be described below using examples, but the present embodiment is not limited to these.

[0061] Example 1 (Negative electrode active material) Coke (D50: 7 μm) was prepared as a carbon source, and boron carbide (BC) (D50: 0.5 μm) was prepared as a boron source. Coke and BC were mixed so that the mass of BC was 5.0 mass% relative to the coke to obtain a first mixture. The first mixture was fired at 2900°C for 1 hour to produce a carbon material. The boron content of the carbon material was measured by XPS. The results are shown in Table 1. In Examples 2 to 4 and Comparative Examples 3 to 6 described below, the boron content of the carbon material was measured in the same manner.

[0062] Coal tar pitch was prepared as a precursor for the carbon film. The carbon material and coal tar pitch were mixed so that the mass of the coal tar pitch was 5.0 mass % relative to the carbon material to obtain a second mixture. The second mixture was fired at 1000°C for 1 hour to produce a negative electrode active material in which a carbon film was formed on the carbon material. The R value was measured using the above-mentioned measurement method and measurement conditions. The D50 of the negative electrode active material was also measured. The results are shown in Table 1. The R value and D50 will be measured in the same manner in Examples 2 to 4 and Comparative Examples 1 to 6 described below.

[0063] (Negative electrode) A Cu foil (thickness: 10 μm) was prepared as the negative electrode current collector, SBR as the binder, CMC as the thickener, and N-methyl-2-pyrrolidone (NMP) as the dispersion medium. The negative electrode active material, binder, thickener, and dispersion medium were mixed to prepare a negative electrode slurry. The mixing ratio (mass ratio) of the negative electrode active material, binder, and thickener was 98:1:1. The negative electrode slurry was applied to the surface of the negative electrode current collector and dried to form a negative electrode active material layer. The negative electrode active material layer was compressed to produce a negative electrode.

[0064] (positive electrode) The positive electrode current collector was made of Al foil (thickness: 15 μm), and the positive electrode active material was Li(Ni 1 / 3 Co 1 / 3 Mn 1 / 3 )O2, AB as a conductive material, PVdF as a binder, and NMP as a dispersion medium were prepared. A positive electrode slurry was prepared by mixing the positive electrode active material, the conductive material, the binder, and the dispersion medium. The mixing ratio (mass ratio) of the positive electrode active material, the conductive material, and the binder was 92:5:3. The positive electrode slurry was applied to the surface of a positive electrode current collector and dried to form a positive electrode active material layer. The positive electrode active material layer was compressed to produce a positive electrode.

[0065] (separator) A separator (porous membrane) with a thickness of 24 μm was prepared. This separator has a three-layer structure. The three-layer structure is made up of a PP porous layer, a PE porous layer, and a PP porous layer stacked in this order. Alumina (thickness: 4 μm) is applied to one side of the separator as a heat-resistant material.

[0066] (electrolyte) A mixed solvent was prepared by mixing EC, DMC, and EMC. The volume ratio of EC, DMC, and EMC was 3:3:4. An electrolyte solution was prepared by dissolving 1.0 mol / L of LiPF6 in the solvent.

[0067] (Lithium-ion battery) An electrode body was formed by stacking a positive electrode, a separator, and a negative electrode in this order. The separator was stacked so that the alumina-coated surface faced the positive electrode. A pouch made of laminated film was prepared as a case. The electrode body was housed in the case. An electrolyte solution was poured into the case. After the electrolyte solution was poured, the case was sealed. After an interval of at least 3 hours, an initial charge was performed, followed by aging for 20 hours in an environment at 60°C, thereby producing the battery of Example 1. Note that "aging" refers to storing the battery for a predetermined period of time in an environment at a temperature above room temperature.

[0068] Example 2 A negative electrode active material was produced in the same manner as in Example 1, except that the coke and B4C were mixed so that the mass of B4C relative to the mass of the coke was 7.0 mass%. Thereafter, a battery of Example 2 was produced in the same manner as in Example 1.

[0069] Example 3 A negative electrode active material was produced in the same manner as in Example 1, except that coke and B4C were mixed so that the mass of B4C relative to the mass of the coke was 10 mass %. Thereafter, a battery of Example 3 was produced in the same manner as in Example 1.

[0070] Example 4 A negative electrode active material was produced in the same manner as in Example 1, except that coke and B4C were mixed so that the mass of B4C relative to the mass of the coke was 15 mass %. Thereafter, a battery of Example 4 was produced in the same manner as in Example 1.

[0071] Example 5 A negative electrode active material was produced in the same manner as in Example 1, except that BC (D50: 1.5 μm) was used as the boron source and that the coke and BC were mixed so that the mass of BC relative to the coke was 10 mass %. Thereafter, a battery of Example 5 was produced in the same manner as in Example 1.

[0072] Example 6 An anode active material was produced in the same manner as in Example 1, except that the firing temperature of the first mixture was set to 2300°C and that the coke and B4C were mixed so that the mass of B4C relative to the coke was 7.0 mass%. Thereafter, a battery of Example 6 was produced in the same manner as in Example 1.

[0073] Comparative Example 1 Coke (D50: 7 μm) was prepared as a carbon source. The coke was baked at 2900° C. for 1 hour to produce artificial graphite, which is the negative electrode active material. Then, a battery of Comparative Example 1 was produced in the same manner as in Example 1.

[0074] Comparative Example 2 A negative electrode active material was prepared in the same manner as in Example 1, except that the artificial graphite prepared in Comparative Example 1 was used as the carbon material. Then, a battery of Comparative Example 2 was prepared in the same manner as in Example 1.

[0075] Comparative Example 3 The carbon material produced in Example 1 was prepared as the negative electrode active material. Then, a battery of Comparative Example 3 was produced in the same manner as in Example 1.

[0076] Comparative Example 4 The carbon material produced in Example 2 was prepared as the negative electrode active material. Then, the battery of Comparative Example 4 was produced in the same manner as in Example 1.

[0077] Comparative Example 5 A negative electrode active material was produced in the same manner as in Example 1, except that the coke and B4C were mixed so that the mass of B4C relative to the mass of the coke was 1.0 mass %. Thereafter, a battery of Comparative Example 5 was produced in the same manner as in Example 1.

[0078] Comparative Example 6 A negative electrode active material was produced in the same manner as in Example 1, except that the coke and B4C were mixed so that the mass of B4C relative to the mass of the coke was 18 mass %. Then, a battery of Comparative Example 5 was produced in the same manner as in Example 1.

[0079] <Evaluation> Each battery was charged and discharged at -10°C, a state of charge (SOC) of 60%, and a rate of 15C, and the capacity was measured after one charge / discharge cycle and after 500 cycles. A 10-minute rest period was given for each cycle. The capacity retention rate was calculated by dividing the capacity after 500 cycles by the capacity after one cycle. A higher capacity retention rate indicates a higher resistance to Li precipitation. The results of this capacity retention rate are shown in Table 1 as a relative value of the capacity retention rate obtained for the battery of each Example or Comparative Example, with the capacity retention rate obtained for the battery of Comparative Example 1 as the reference. Note that "C" is the unit of current rate. "1C" indicates the current rate at which the SOC reaches 100% from 0% in one hour of charging.

[0080] [Table 1]

[0081] <Result> Examples 1 to 6 had higher capacity retention rates and improved resistance to Li precipitation than Comparative Example 1.

[0082] On the other hand, the Li precipitation resistance decreased in Comparative Example 2, which used a carbon material not containing boron, and in Comparative Examples 3 and 4, which used carbon materials not coated with a carbon film. It is believed that simply coating a boron-free carbon material with a carbon film or using only a boron-containing carbon material does not improve the Li precipitation resistance.

[0083] Furthermore, the Li precipitation resistance also decreased in Comparative Examples 5 and 6, in which the boron content in the carbon material was 0.1 atomic % and 3.5 atomic %, respectively. It is believed that there is a suitable range for the boron content in the carbon material.

[0084] The present embodiment and examples are illustrative in all respects. The present embodiment and examples are not limiting. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the claims. For example, it is also intended from the beginning that any configuration may be extracted from the present embodiment and examples and that they may be combined in any desired manner. [Explanation of symbols]

[0085] 1 carbon material, 2 carbon film, 5 negative electrode active material, 20 positive electrode, 30 negative electrode, 40 separator, 50 electrode body, 90 case, 91 positive electrode terminal, 92 negative electrode terminal, 100 lithium ion battery.

Claims

1. A negative electrode active material for a lithium ion battery, comprising: a carbon material and a carbon film coating the carbon material, the carbon material comprises carbon and boron; the boron content in the carbon material is equal to or greater than 0.2 atomic % and less than 3.5 atomic %; the negative electrode active material has an R value of 0.35 or more and 0.85 or less, The R value is a ratio of the D band to the G band in the Raman spectrum of the negative electrode active material.

2. The negative electrode active material according to claim 1 , wherein the carbon material includes artificial graphite.

3. A lithium ion battery comprising: a negative electrode containing the negative electrode active material according to claim 1 or 2; a positive electrode; a separator; and an electrolyte.

4. A method for producing a negative electrode active material for a lithium ion battery, comprising: mixing a carbon source and a boron source to obtain a first mixture; calcining the first mixture to produce a carbon material; mixing the carbon material and a precursor of a carbon film to obtain a second mixture; and firing the second mixture to produce a negative electrode active material in which the carbon material is coated with the carbon film, a content of the boron source relative to the carbon source of 1.5% by mass to 17.5% by mass,

5. the firing temperature of the first mixture is 1500°C or higher and 3000°C or lower; The method for producing a negative electrode active material according to claim 4 , wherein the first mixture is baked for 30 minutes or more and 2 hours or less.

6. the average particle size of the carbon source is 2.5 μm or more and 15 μm or less; 6. The method for producing a negative electrode active material according to claim 4, wherein the boron source has an average particle size of 0.1 μm or more and 5.0 μm or less.

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