Method for producing negative electrode active material

Abstract

A method for producing a negative electrode active material according to the present disclosure includes firing of a starting material mixture for the negative electrode active material at a temperature higher than 400°C but lower than 1100°C. The negative electrode active material has a composition that is represented by Li3+x+αV1-xMxO4+α / 2, wherein M is at least one element that is selected from among tetravalent metal elements excluding V and tetravalent metalloid elements, and 0.03 < α < 1.0 and 0 ≤ x < 1 are satisfied.

Description

Method for producing negative electrode active material

[0001] The present disclosure relates to a method for producing a negative electrode active material.

[0002] Patent Document 1 discloses a non-aqueous secondary battery using Li3VO4 as the negative electrode active material.

[0003] Patent Document 2 discloses a co-fired all-solid-state battery using a negative electrode active material obtained by doping LiVO with element A and / or element B. Element A is at least one element selected from the group consisting of Mg, Al, Ga, and Zn. Element B is at least one element selected from the group consisting of Zn, Al, Ga, Si, Ge, P, and Ti.

[0004] JP 2008-77847 A International Publication No. 2019 / 044902

[0005] When the lithium vanadium oxides disclosed in Patent Documents 1 and 2 are used as the negative electrode active material, it is difficult to obtain a battery having a sufficient capacity.

[0006] An object of the present disclosure is to provide a method for producing a negative electrode active material that can improve the capacity of a battery.

[0007] A method for producing a negative electrode active material according to one embodiment of the present disclosure includes firing a raw material mixture of the negative electrode active material at a temperature higher than 400°C and lower than 1100°C, and the negative electrode active material is formed by firing the raw material mixture of the negative electrode active material at a temperature higher than 400°C and lower than 1100°C. 3+x+α V 1-x M x O 4+α / 2 M is at least one selected from tetravalent metal elements and tetravalent metalloid elements excluding V, and 0.03<α<1.0 and 0≦x<1 are satisfied.

[0008] In another aspect of the present disclosure, a method for producing a negative electrode active material includes firing a raw material mixture of the negative electrode active material at a temperature greater than 400°C and less than 1100°C, the negative electrode active material including Li, V, M, and O, M being at least one selected from the group consisting of tetravalent metal elements and tetravalent metalloid elements excluding V, and the negative electrode active material including both a β phase and a γ phase as crystalline phases.

[0009] According to the present disclosure, the capacity of the battery can be improved.

[0010] FIG. 1 is a flowchart showing an example of a method for manufacturing a negative electrode active material according to the first embodiment. FIG. 2 is a flowchart showing another example of a method for manufacturing a negative electrode active material according to the first embodiment. FIG. 3 is a schematic cross-sectional view showing a negative electrode material according to a second embodiment. FIG. 4 is a schematic cross-sectional view showing the configuration of a battery according to a third embodiment. FIG. 5 is an X-ray diffraction pattern of the negative electrode active materials of Example 1 and Comparative Example 1. FIG. 6 is a graph showing the discharge characteristics of the battery of Example 1.

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiments.

[0012] 1 is a flowchart showing an example of a method for manufacturing an anode active material according to Embodiment 1. As shown in step S1000 of FIG. 1 , the method for manufacturing an anode active material according to Embodiment 1 includes firing a raw material mixture of the anode active material at a temperature higher than 400° C. and lower than 1100° C.

[0013] An example of a negative electrode active material obtained by the manufacturing method of this embodiment is Li 3+x+α V 1-x M x O 4+α / 2 The negative electrode active material has a composition represented by the following composition formula (1): α and x satisfy 0.03<α<1.0 and 0≦x<1. M is at least one selected from tetravalent metal elements and tetravalent metalloid elements excluding V. The negative electrode active material produced by the example of the production method of the first embodiment is suitable for improving the capacity of a battery.

[0014] Another example of a negative electrode active material obtained by the manufacturing method of this embodiment contains Li, V, M, and O, and includes both the β-phase and the γ-phase as crystalline phases. M is at least one selected from the group consisting of tetravalent metal elements and tetravalent metalloid elements excluding V. The negative electrode active material manufactured by this other example of the manufacturing method of this embodiment is also suitable for improving the capacity of a battery.

[0015] The β-phase is a crystalline phase that can be generated in lithium vanadium oxide and has a crystalline structure belonging to the space group Pmn21. Specifically, the crystalline structure of the β-phase has tetrahedra composed of LiO4 (hereinafter simply referred to as "LiO4 tetrahedra") and tetrahedra composed of VO4 (hereinafter simply referred to as "VO4 tetrahedra"). In the crystalline structure of the β-phase, the LiO4 tetrahedra and the VO4 tetrahedra are oriented in the same direction (the c-axis direction, which is one side of the crystal lattice) with adjacent tetrahedra sharing vertices. The β-phase has a high theoretical capacity as a negative electrode active material.

[0016] The γ phase is a crystalline phase that can be produced in lithium vanadium oxide and has a crystalline structure belonging to the space group Pcmn. Specifically, the crystalline structure of the γ phase, like the crystalline structure of the β phase, has LiO tetrahedra and VO tetrahedra. In the γ phase, the LiO tetrahedra and VO tetrahedra are tetrahedra that are generally oriented in one direction (the +c-axis direction, which is one side of the crystal lattice) and tetrahedra that are generally oriented in a direction opposite to the one direction (the −c-axis direction, which is one side of the crystal lattice), respectively, at approximately half the ratio. In the γ phase crystal structure, LiO tetrahedra share vertices and one side with adjacent LiO tetrahedra, and VO tetrahedra share vertices with adjacent LiO tetrahedra or VO tetrahedra. The γ phase can be stably obtained by partially substituting V (vanadium) sites in the crystal lattice with a tetravalent metal other than V.

[0017] The ratio of the amount of substance of V to the sum of the amount of substance of V and the amount of substance of M (V / (M+V)) is, for example, more than 0% and less than 10%. With this configuration, a mixed phase of β phase and γ phase is likely to be generated. Furthermore, if the firing temperature during production of the negative electrode active material is high, the γ phase is likely to be generated. Furthermore, if a raw material with a low melting point is selected as the raw material for the negative electrode active material, the γ phase is likely to be generated at a lower firing temperature. Therefore, the volume ratio of the β phase to the γ phase can be controlled by appropriately selecting the ratio of the amount of substance of V to the sum of the amount of substance of V and the amount of substance of M, the firing temperature during production of the negative electrode active material, and the melting point of the raw material for the negative electrode active material.

[0018] The volume ratio of the β phase to the γ phase is not limited. Since the volume ratio of the β phase to the γ phase correlates with the results of X-ray diffraction measurement of the negative electrode active material, the results of the X-ray diffraction measurement can serve as a measure of the volume ratio of the β phase to the γ phase. However, the intensity of the diffraction peak represents the height of the diffraction peak, and does not indicate the area or volume of the β phase and the γ phase. For example, in an X-ray diffraction pattern using Cu-Kα radiation as a radiation source, the ratio of the intensity of the diffraction peak attributable to the (101) plane of the β phase to the intensity of the diffraction peak attributable to the (011) plane of the γ phase is 0.1 or more and 10.0 or less. The ratio of the intensity of the diffraction peaks may be 0.1 or more and 7.0 or less. When the ratio of the diffraction peak intensities falls within this range, a battery having good charge / discharge characteristics can be realized when the negative electrode active material is used in a battery.

[0019] The diffraction peak attributable to the (101) plane of the β phase appears at 22.7°±0.2°, taking into account the change in the lattice constant of the β phase due to doping with the element M. Similarly, the diffraction peak attributable to the (011) plane of the γ phase appears at 22.4°±0.2°, taking into account the change in the lattice constant of the γ phase due to doping with the element M. Therefore, instead of the condition regarding the ratio of the diffraction peak intensities, in an X-ray diffraction pattern using Cu-Kα radiation as a radiation source, the ratio of the intensity of the diffraction peak appearing at a diffraction angle 2θ of 22.7°±0.2° to the intensity of the diffraction peak appearing in a diffraction angle 2θ range of 22.4°±0.2° may be 0.1 or more and 10.0 or less. In addition, when two diffraction peaks are present at an overlapping angle "22.5° to 22.6°" of the diffraction angles 2θ of "22.4°±0.2°" and "22.7°±0.2°," the correspondence relationship between the diffraction peaks and the crystal planes is determined taking into account the magnitude relationship between the intensities of other comparable planes of the β phase and the γ phase. Furthermore, the diffraction peak attributable to the (011) plane of the γ phase is likely to appear at 22.4°±0.1°, and the diffraction peak attributable to the (101) plane of the β phase is likely to appear at 22.7°±0.1°. Therefore, in an X-ray diffraction pattern using Cu-Kα radiation as a radiation source, the ratio of the intensity of the diffraction peak appearing at a diffraction angle 2θ of 22.7°±0.1° to the intensity of the diffraction peak appearing in the diffraction angle 2θ range of 22.4°±0.1° may be 0.1 to 10.0. The ratio of the intensities of the diffraction peaks may be 0.1 to 7.0.

[0020] The raw material mixture is prepared to have a target composition. The raw material mixture contains, for example, a Li raw material, a V raw material, and an M raw material. The raw material mixture may be a mixture containing a Li raw material containing Li and O, a V raw material containing V and O, and an M raw material containing M and O.

[0021] Examples of Li raw materials include hydroxides of Li, oxides of Li, carbonates of Li, nitrates of Li, carbonate salts of Li, nitrides of Li, and organic salts of Li. One or more materials selected from these can be used as the Li raw material. The Li raw material may include at least one selected from the group consisting of Li2CO3, Li2O, LiOH, and LiOH hydrate.

[0022] Examples of V raw materials include V hydroxide, V oxide, V carbonate, V nitrate, V carbonate, V nitride, and organic salt of V. One or more materials selected from these can be used as the V raw material. The V raw material may include at least one selected from the group consisting of VO and NHVO.

[0023] Examples of M raw materials include hydroxides of M, oxides of M, carbonates of M, nitrates of M, carbonates of M, nitrides of M, and organic salts of M. One or more materials selected from these can be used as the M raw material. When M is Ti, the M raw material may contain at least one selected from the group consisting of TiO2 and Ti2O3.

[0024] The firing temperature may be changed depending on the type of raw material. When a raw material with a low melting point is used, the firing temperature of the raw material mixture may be lowered.

[0025] The Li contained in the raw material mixture may evaporate and decrease during the firing in step S1000. Therefore, the Li composition ratio in the raw material mixture may not match the Li composition ratio in the negative electrode active material. Therefore, the contents of Li, V, M, and O in the raw material mixture are adjusted so that "x" and "α" in composition formula (1) fall within the desired range. The Li content in the negative electrode active material can be determined by composition analysis using, for example, ICP emission spectroscopy, atomic absorption spectroscopy, EPMA (Electron Probe Micro Analyzer), or the like.

[0026] The temperature at which the raw material mixture is fired may be 450° C. or higher and 1000° C. or lower. According to this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0027] The temperature at which the raw material mixture is fired may be 500° C. or higher and 900° C. or lower. According to this configuration, the negative electrode active material produced by the manufacturing method of the first embodiment can further improve the capacity of the battery.

[0028] The time for firing the raw material mixture may be 5 hours or more while maintaining the firing temperature at more than 400° C. and less than 1100° C. According to this configuration, the raw material mixture can be easily fired appropriately.

[0029] The time for firing the raw material mixture may be 10 hours or more, or may be 15 hours or more. The time for firing the raw material mixture may be 5 hours or less. According to such a configuration, the raw material mixture can be easily fired appropriately.

[0030] The temperature increase rate during firing the raw material mixture is, for example, 1° C. / min to 10° C. / min. The temperature decrease rate during firing the raw material mixture is, for example, 1° C. / min to 10° C. / min.

[0031] The raw material mixture may be fired in air, dry air, vacuum, or an inert gas atmosphere. Examples of inert gases include helium gas, nitrogen gas, and argon gas. The raw material mixture may be fired in a reducing gas atmosphere. Examples of reducing gases include ammonia gas, methane gas, and hydrogen sulfide gas.

[0032] The raw material mixture may be placed in an aluminum crucible and fired.

[0033] 2 is a flowchart showing another example of the method for manufacturing a negative electrode active material according to the first embodiment. As shown in FIG. 2, the method for manufacturing a negative electrode active material according to the first embodiment may include step S1100 of mixing raw materials for the negative electrode active material before step S1000 of firing the raw material mixture for the negative electrode active material. Mixing the raw materials for the negative electrode active material includes mixing a Li raw material, a V raw material, and an M raw material to obtain a raw material mixture.

[0034] In the mixing of step S1100, the Li raw material, the V raw material, and the M raw material may be weighed and mixed to a desired molar ratio. The raw materials may be mixed using a known mixing tool. In this case, a powdered raw material mixture may be fired in the firing of step S1000. Examples of the mixing tool include a mortar, a blender, and a ball mill.

[0035] The raw material for the negative electrode active material may be in the form of crystals, chunks, flakes, powder, etc. The powdered raw material mixture obtained in the mixing in step S1100 may be formed into pellets by uniaxial pressing.

[0036] The firing in step S1000 shown in FIGS. 1 and 2 may be performed multiple times. That is, the negative electrode active material obtained by the first firing is pulverized, mixed, and fired again. In this way, performing firing multiple times may improve the crystallinity of the negative electrode active material. For example, step S1000, in which the raw material mixture of the negative electrode active material is fired, may be performed twice. In this case, the firing temperature for the first time may be lower than the firing temperature for the second time.

[0037] M may be Ti. The case where M is Ti will be described below. The raw material mixture fired in step S1000 may be a raw material mixture containing at least one selected from the group consisting of Li2CO3, Li2O, LiOH, and LiOH hydrate, at least one selected from the group consisting of VO5 and NH4VO3, and at least one selected from the group consisting of TiO2 and Ti2O3.

[0038] When M is Ti, Li 3+x+α V 1-x Ti x O 4+α / 2 Assume that x is 0.05 when the raw materials are mixed in a negative electrode active material having a composition represented by the formula (1). First, raw material powder is prepared assuming that α is 0. For example, Li2CO3, VO5, and TiO2 are prepared in a molar ratio of Li2CO3:VO5:TiO2 = (3.05 / 2):(0.95 / 2):0.05. To the raw material powder prepared in this molar ratio, a Li source substance, such as Li2CO3, is further added and mixed, taking into account the value of α in the target composition. The amount of excess Li source to be mixed in excess can be determined appropriately depending on the value of α in the target composition, the material used as the Li source, and the like. The Li source may be prepared in an excess amount ranging from 0.5 wt% to 40 wt%, or from 1 wt% to 30 wt%, relative to the amount of Li source whose molar ratio is determined assuming that α is 0.

[0039] Lithium hydroxide or a hydrate thereof may be used as the Li source material instead of Li2CO3.

[0040] A reaction product is obtained by firing the mixture of raw material powders. The firing atmosphere may be an air atmosphere or an inert gas atmosphere. The inert atmosphere is, for example, an argon atmosphere or a nitrogen atmosphere. The firing atmosphere may be a reducing gas atmosphere. The reducing gas atmosphere is, for example, an ammonia atmosphere, a methane atmosphere, or a hydrogen sulfide atmosphere.

[0041] The negative electrode active material may be obtained by reacting a mixture of raw material powders mechanochemically (by mechanochemical milling) in a mixing device such as a planetary ball mill. 3+x+α V 1-x Ti x O 4+α / 2 Thus, a negative electrode active material having a composition represented by the composition formula (1) is obtained.

[0042] The negative electrode active material obtained by the manufacturing method of the first embodiment has good electronic conductivity and is therefore suitable for improving the capacity of a battery. An example of the battery is a solid-state battery. The solid-state battery may be a primary battery or a secondary battery. The solid-state battery may be an all-solid-state battery.

[0043] In composition formula (1), when 0.03<α<1.0 is satisfied, the capacity of a battery using the negative electrode active material of the first embodiment is improved because insertion and desorption of Li into and from the negative electrode active material of the first embodiment is facilitated.

[0044] In composition formula (1), the amount of Li and O indicated by "α" may be incorporated into the β-phase and / or γ-phase crystalline phase of the negative electrode active material, or may exist as a crystalline phase other than the β-phase and / or γ-phase. This configuration further improves the capacity of a battery using the negative electrode active material. It should be noted that the negative electrode active materials disclosed in Patent Documents 1 and 2 do not contain Li and O equivalent to the amount indicated by "α."

[0045] In composition formula (1), 0.04≦α≦0.95 may be satisfied. With this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0046] In composition formula (1), 0.06≦α≦0.93 may be satisfied. With this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0047] In composition formula (1), 0.1≦α≦0.6 may be satisfied. With this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0048] In composition formula (1), 0.14≦α≦0.57 may be satisfied. With this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0049] The upper and lower limits of the range of α in composition formula (1) may be defined by any combination selected from the following values: greater than 0.03 (i.e., 0.03<x), 0.04, 0.06, 0.1, 0.14, 0.24, 0.25, 0.32, 0.57, 0.6, 0.93, 0.95, and less than 1.0 (i.e., x<1.0).

[0050] In composition formula (1), 0<x≦0.1 may be satisfied. With this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0051] In composition formula (1), 0<x<1.0 may be satisfied. With this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0052] The upper and lower limits of the range of x in composition formula (1) may be defined by any combination selected from values ​​greater than 0 (i.e., 0<x), 0.05, 0.1, and less than 1.0 (i.e., x<1.0).

[0053] In the composition formula (1), when x satisfies the above range, insertion and desorption of Li into and from the negative electrode active material of the first embodiment is further facilitated, and therefore, as described above, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0054] In composition formula (1), 0.07≦α≦0.21 and 0.01≦x≦0.19 may be satisfied. With such a configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0055] As described above, M is at least one selected from tetravalent metal elements and tetravalent metalloid elements excluding V. The tetravalent metal element and tetravalent metalloid element may be at least one selected from the group consisting of Ti, Zr, Si, Ge, and Sn.

[0056] In composition formula (1), M may contain Ti. With this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery. M may be Ti. With this configuration, the negative electrode active material of the first embodiment can further improve the capacity of the battery.

[0057] The shape of the negative electrode active material of the first embodiment is not limited. Examples of the shape include an acicular shape, a spherical shape, and an oval spherical shape. The negative electrode active material of the first embodiment may be in the form of particles. The negative electrode active material of the first embodiment may be formed into the shape of a pellet or a plate.

[0058] When the negative electrode active material of the first embodiment is particulate (e.g., spherical), the particles of the negative electrode active material may have a median diameter of 0.1 μm or more and 100 μm or less, or may have a median diameter of 0.5 μm or more and 10 μm or less. This allows the negative electrode active material of the first embodiment and other materials, such as a solid electrolyte, to be dispersed well.

[0059] The median particle size refers to the particle size (d50) corresponding to 50% cumulative volume in the volume-based particle size distribution. The volume-based particle size distribution can be measured using a laser diffraction measurement device or an image analysis device.

[0060] Second Embodiment A second embodiment will be described below, and the matters described in the first embodiment will be omitted as appropriate.

[0061] 3 shows an anode material 100 according to a second embodiment. The anode material 100 includes an anode active material 111 and a solid electrolyte 112. The anode material 100 may include a conductive additive 113 for the purpose of enhancing electronic conductivity. The anode active material 111, the solid electrolyte 112, and the conductive additive 113 are, for example, each in a particulate form. The anode active material 111 includes the anode active material of the first embodiment. The anode active material of the first embodiment is suitable for improving the capacity of a battery.

[0062] The negative electrode active material 111 may contain the negative electrode active material of the first embodiment as a main component. "Containing the negative electrode active material of the first embodiment as a main component" means that the component contained in the largest amount by mass is the negative electrode active material of the first embodiment. The negative electrode active material 111 may consist solely of the negative electrode active material of the first embodiment.

[0063] The conductive additive 113 may be provided so as to cover at least a part of the negative electrode active material 111. This can increase the contact area between the conductive additive 113 and the negative electrode active material 111. As a result, when the negative electrode material 100 is used in a battery, the resistance of the battery decreases, and the output of the battery can be increased.

[0064] The ratio of the volume of the conductive additive 113 to the total volume of the negative electrode active material 111 and the conductive additive 113 may be 0.01 or more and 0.4 or less.

[0065] The negative electrode material 100 according to the second embodiment contains the negative electrode active material of the first embodiment, and therefore, by using the negative electrode material 100 according to the second embodiment, it is possible to obtain a negative electrode suitable for improving the capacity of a battery.

[0066] Third Embodiment A third embodiment will be described below. The matters described in the first and second embodiments will be omitted as appropriate.

[0067] 4 shows a battery 1000 as an example of the third embodiment. The battery 1000 includes a positive electrode 101, an electrolyte layer 102, and a negative electrode 103. The electrolyte layer 102 is disposed between the positive electrode 101 and the negative electrode 103.

[0068] The positive electrode 101 includes a positive electrode active material and a solid electrolyte.

[0069] The electrolyte layer 102 includes an electrolyte material, such as a solid electrolyte material.

[0070] The negative electrode 103 includes a negative electrode active material 111 and a solid electrolyte 112 .

[0071] The negative electrode active material 111 is a particle containing the negative electrode active material of the first embodiment. The negative electrode active material 111 may be a particle containing the negative electrode active material of the first embodiment as a main component. "Particles containing the negative electrode active material of the first embodiment as a main component" means particles in which the component contained most abundantly by mass is the negative electrode active material of the first embodiment. The negative electrode active material 111 may be a particle consisting only of the negative electrode active material of the first embodiment.

[0072] The negative electrode active material 111 may have a median diameter of 0.1 μm or more and 100 μm or less. When the negative electrode active material 111 has a median diameter of 0.1 μm or more, the negative electrode active material 111 and the solid electrolyte 112 can be well dispersed in the negative electrode 103. This improves the charge / discharge characteristics of the battery 1000. When the negative electrode active material 111 has a median diameter of 100 μm or less, the lithium diffusion rate within the negative electrode active material 111 improves. This allows the battery 1000 to operate at high power.

[0073] The negative electrode active material 111 may have a larger median diameter than the solid electrolyte 112. This allows the negative electrode active material 111 and the solid electrolyte 112 to be dispersed well.

[0074] In order to improve the energy density and output of the battery 1000, in the negative electrode 103, the ratio of the volume of the negative electrode active material 111 to the total volume of the negative electrode active material 111 and the solid electrolyte 112 may be 0.30 or more and 0.95 or less.

[0075] To improve the energy density and output of the battery 1000, the negative electrode 103 may have a thickness of 10 μm or more and 500 μm or less.

[0076] The solid electrolyte 112 contained in the negative electrode 103 may be a sulfide solid electrolyte, a halide solid electrolyte, an oxide solid electrolyte, or a polymer solid electrolyte.

[0077] In the present disclosure, a "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur. An "oxide solid electrolyte" refers to a solid electrolyte containing oxygen. The oxide solid electrolyte may contain anions other than oxygen (excluding sulfur anions and halogen anions). A "halide solid electrolyte" refers to a solid electrolyte containing a halogen element but not sulfur. The halide solid electrolyte may contain not only a halogen element but also oxygen.

[0078] Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-SiS2, Li2S-B2S3, Li2S-GeS2, and Li 3.25 Ge 0.25 P 0.75 S4 or Li 10 GeP2S 12 is.

[0079] Examples of halide solid electrolytes include Li a Me b Y c It is a compound having a composition represented by the composition formula X6, where a + mb + 3c = 6 and c > 0 are satisfied. Me is at least one element selected from the group consisting of metal elements other than Li and Y and metalloid elements. X is at least one element selected from the group consisting of F, Cl, Br, and I. The value of m represents the valence of Me.

[0080] The "metalloid elements" are B, Si, Ge, As, Sb, and Te. The "metal elements" are all elements included in Groups 1 to 12 of the periodic table (excluding H) and all elements included in Groups 13 to 16 of the periodic table (excluding B, Si, Ge, As, Sb, Te, C, N, P, O, S, and Se).

[0081] In order to increase the ionic conductivity of the halide solid electrolyte, Me may be at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Sc, Al, Ga, Bi, Zr, Hf, Ti, Sn, Ta, and Nb.

[0082] Another example of a halide solid electrolyte is Li α M β O γ X δ The compound has a composition represented by the following composition formula: where α, β, γ, and δ are all greater than 0. M is at least one element selected from the group consisting of metal elements and metalloid elements other than Li. X is at least one element selected from the group consisting of Cl, Br, and I. The composition of the compound satisfies 0.9≦α≦1.2, β=1.0, 1.0≦γ≦1.3, and 3.6≦δ≦4.0.

[0083] Examples of oxide solid electrolytes include: (i) NASICON-type solid electrolytes such as LiTi2(PO4)3 or its elemental substitution products; (ii) perovskite-type solid electrolytes such as (LaLi)TiO3; (iii) Li 14 ZnGeO 16 LISICON-type solid electrolytes such as LiSiO, LiGeO or elemental substitutions thereof; (iv) LiLaZrO 12 or an element-substituted product thereof, or (v) Li3PO4 or an N-substituted product thereof.

[0084] An example of a polymer solid electrolyte is a compound of a polymer compound and a lithium salt. The polymer compound may have an ethylene oxide structure. A polymer compound having an ethylene oxide structure can contain a large amount of lithium salt, and therefore has higher ionic conductivity. The polymer solid electrolyte may be a composite compound of polyethylene oxide and a lithium salt. An example of such a polymer solid electrolyte is lithium bis(trifluoromethanesulfonyl)imide.

[0085] Examples of lithium salts include LiPF, LiBF, LiSbF, LiAsF, LiSOCF, LiN(SOCF), LiN(SOCF), LiN(SOCF)(SOCF), or LiC(SOCF). One lithium salt selected from these may be used alone. Alternatively, a mixture of two or more lithium salts selected from these may be used.

[0086] The positive electrode 101 includes a material capable of absorbing and releasing metal ions such as lithium ions, and includes, for example, a positive electrode active material (for example, particles of a positive electrode active material).

[0087] Examples of the positive electrode active material include a lithium-containing transition metal oxide, a transition metal fluoride, a polyanionic material, a fluorinated polyanionic material, a transition metal sulfide, a transition metal oxysulfide, or a transition metal oxynitride. Examples of the lithium-containing transition metal oxide include Li(Ni,Co,Al)O, Li(Ni,Co,Mn)O, or LiCoO.

[0088] In the present disclosure, "(A, B, C)" means "at least one selected from the group consisting of A, B, and C."

[0089] From the viewpoint of cost and safety of the battery 1000, lithium phosphate may be used as the positive electrode active material.

[0090] The positive electrode active material may have a median diameter of 0.1 μm or more and 100 μm or less. When the positive electrode active material has a median diameter of 0.1 μm or more, the positive electrode active material and the solid electrolyte can be well dispersed in the positive electrode 101. This improves the charge / discharge characteristics of the battery 1000. When the positive electrode active material has a median diameter of 100 μm or less, the lithium diffusion rate within the positive electrode active material improves. This allows the battery 1000 to operate at high power.

[0091] The positive electrode active material may have a larger median diameter than the solid electrolyte, which allows the positive electrode active material and the solid electrolyte to be dispersed well.

[0092] In order to improve the energy density and output of the battery 1000, the ratio of the volume of the positive electrode active material to the total volume of the positive electrode active material and the solid electrolyte in the positive electrode 101 may be 0.30 or more and 0.95 or less.

[0093] A coating layer may be formed on the surface of the positive electrode active material. This can suppress an increase in the reaction overvoltage of the battery. Examples of coating materials contained in the coating layer include a sulfide solid electrolyte, an oxide solid electrolyte, a polymer solid electrolyte, or a halide solid electrolyte.

[0094] The coating material may be a halide solid electrolyte material or an oxide solid electrolyte material. The halide solid electrolyte material may contain F. This improves the stability of the coating material at high potentials. As a result, the battery 1000 has high charge / discharge efficiency. The oxide solid electrolyte may be lithium niobate or a polyanion material that is stable even at high potentials. With this configuration, the battery 1000 has high charge / discharge efficiency.

[0095] To improve the energy density and output of the battery 1000, the positive electrode 101 may have a thickness of 10 μm or more and 500 μm or less.

[0096] The solid electrolyte contained in the positive electrode 101 may be a sulfide solid electrolyte, a halide solid electrolyte, an oxide solid electrolyte, a polymer solid electrolyte, or an organic polymer solid electrolyte.

[0097] The electrolyte layer 102 includes an electrolyte material. The electrolyte material is, for example, a solid electrolyte material. The electrolyte layer 102 may be an electrolyte layer configured from a solid electrolyte layer. The solid electrolyte material included in the electrolyte layer 102 may be a sulfide solid electrolyte, a halide solid electrolyte, or a polymer solid electrolyte.

[0098] The electrolyte layer 102 may have a thickness of 1 μm or more and 100 μm or less. When the electrolyte layer 102 has a thickness of 1 μm or more, the positive electrode 101 and the negative electrode 103 are less likely to short-circuit. When the electrolyte layer 102 has a thickness of 100 μm or less, the battery 1000 can operate at high power.

[0099] At least one selected from the group consisting of the positive electrode 101, the electrolyte layer 102, and the negative electrode 103 may contain a non-aqueous electrolyte solution, a gel electrolyte, or an ionic liquid for the purpose of facilitating the exchange of lithium ions and improving the output characteristics of the battery.

[0100] The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent are cyclic carbonate ester solvents, chain carbonate ester solvents, cyclic ether solvents, chain ether solvents, cyclic ester solvents, chain ester solvents, and fluorine solvents. Examples of the cyclic carbonate ester solvent are ethylene carbonate, propylene carbonate, and butylene carbonate. Examples of the chain carbonate ester solvent are dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Examples of the cyclic ether solvent are tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. Examples of the chain ether solvent are 1,2-dimethoxyethane and 1,2-diethoxyethane. An example of the cyclic ester solvent is γ-butyrolactone. An example of the chain ester solvent is methyl acetate.

[0101] Examples of fluorine-containing solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate. One non-aqueous solvent selected from these may be used alone. Alternatively, a mixture of two or more non-aqueous solvents selected from these may be used.

[0102] Examples of lithium salts include LiPF, LiBF, LiSbF, LiAsF, LiSOCF, LiN(SOCF), LiN(SOCF), LiN(SOCF)(SOCF), or LiC(SOCF). One lithium salt selected from these may be used alone. Alternatively, a mixture of two or more lithium salts selected from these may be used. The concentration of the lithium salt is, for example, in the range of 0.5 mol / L to 2 mol / L.

[0103] The gel electrolyte may be a polymer material impregnated with a non-aqueous electrolyte, such as polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, or a polymer having an ethylene oxide bond.

[0104] Examples of cations contained in the ionic liquid are: (i) aliphatic chain quaternary salts such as tetraalkylammonium or tetraalkylphosphonium; (ii) aliphatic cyclic ammoniums such as pyrrolidiniums, morpholiniums, imidazoliniums, tetrahydropyrimidiniums, piperaziniums, or piperidiniums; or (iii) nitrogen-containing heterocyclic aromatic cations such as pyridiniums or imidazoliums.

[0105] An example of an anion contained in an ionic liquid is PF6 - , BF4 - , SbF6 - , AsF6 - , SO3CF3 - , N(SO2CF3)2 - , N(SO2C2F5)2 - , N(SO2CF3)(SO2C4F9) - , or C(SO2CF3)3 - is.

[0106] The ionic liquid may include a lithium salt.

[0107] At least one selected from the group consisting of the positive electrode 101, the electrolyte layer 102, and the negative electrode 103 may contain a binder for the purpose of improving adhesion between particles.

[0108] Examples of binders include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polymethyl ester of acrylic acid, polyethyl ester of acrylic acid, polyhexyl ester of acrylic acid, polymethacrylic acid, polymethyl ester of methacrylic acid, polyethyl ester of methacrylic acid, polyhexyl ester of methacrylic acid, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene-butadiene rubber, and carboxymethyl cellulose. Copolymers may also be used as binders. Examples of such binders include copolymers of two or more materials selected from the group consisting of tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene. A mixture of two or more of the above materials may also be used as binders.

[0109] The negative electrode 103 may contain a conductive additive 113 for the purpose of increasing electronic conductivity. Examples of the conductive additive 113 include: (i) graphites such as natural graphite or artificial graphite, (ii) carbon blacks such as acetylene black or ketjen black, (iii) conductive fibers such as carbon fiber or metal fiber, (iv) carbon fluoride, (v) metal powders such as aluminum, (vi) conductive whiskers such as zinc oxide or potassium titanate, (vii) conductive metal oxides such as titanium oxide, or (viii) conductive polymer compounds such as polyaniline, polypyrrole, or polythiophene. To reduce costs, the conductive additives (i) or (ii) may be used.

[0110] The positive electrode 101 may contain not only a positive electrode active material but also a conductive additive. Examples of the conductive additive material are as described above.

[0111] In the negative electrode 103, the conductive additive 113 may cover at least a portion of the surface of the negative electrode active material 111. This increases the contact area between the conductive additive 113 and the negative electrode active material 111. As a result, the battery resistance decreases and the output can be increased. In the positive electrode 101, the conductive additive may also cover at least a portion of the surface of the positive electrode active material.

[0112] In the negative electrode 103, the ratio of the volume of the conductive additive 113 to the total volume of the negative electrode active material 111 and the conductive additive 113 may be 0.01 or more and 0.4 or less. In the positive electrode 101, the ratio of the volume of the conductive additive to the total volume of the positive electrode active material and the conductive additive may be 0.01 or more and 0.4 or less.

[0113] 3 may be included in the negative electrode 103. A coating layer may be formed on the surface of the negative electrode active material 111 to prevent the solid electrolyte 112 from reacting with the negative electrode active material 111. With this configuration, the battery 1000 has high charge / discharge efficiency.

[0114] Examples of the coating material contained in the coating layer include a sulfide solid electrolyte, an oxide solid electrolyte, a polymer solid electrolyte, or a halide solid electrolyte.

[0115] An example of a sulfide solid electrolyte is Li2S-P2S5. An example of an oxide solid electrolyte is trilithium phosphate. An example of a polymer solid electrolyte is a composite compound of polyethylene oxide and a lithium salt. An example of such a polymer solid electrolyte is lithium bis(trifluoromethanesulfonyl)imide.

[0116] Examples of the shape of the battery according to the third embodiment include a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, and a laminate type.

[0117] The battery according to the third embodiment may be manufactured, for example, by preparing a material for forming a positive electrode, a material for forming an electrolyte layer, and a material for forming a negative electrode, and by a known method, fabricating a laminate in which the positive electrode, the electrolyte layer, and the negative electrode are arranged in this order.

[0118] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0119] (Technology 1) A method for producing a negative electrode active material, comprising firing a raw material mixture of the negative electrode active material at a temperature higher than 400°C and lower than 1100°C, wherein the negative electrode active material is a mixture of Li 3+x+α V 1-x M x O 4+α / 2 wherein M is at least one selected from the group consisting of tetravalent metal elements and tetravalent metalloid elements excluding V, and 0.03<α<1.0 and 0≦x<1 are satisfied.

[0120] According to the present disclosure, when the negative electrode active material obtained by the production method is used in a battery, the capacity of the battery is improved.

[0121] (Technology 2) A method for producing a negative electrode active material according to Technology 1, which satisfies 0.07≦α≦0.21 and 0.01≦x≦0.19. With this configuration, when the negative electrode active material obtained by this production method is used in a battery, the capacity of the battery is further improved.

[0122] (Technology 3) A method for producing a negative electrode active material, comprising: firing a raw material mixture of the negative electrode active material at a temperature higher than 400°C and lower than 1100°C, wherein the negative electrode active material contains Li, V, M, and O, M is at least one selected from the group consisting of tetravalent metallic elements and tetravalent metalloid elements excluding V, and the negative electrode active material contains both a β phase and a γ phase as crystalline phases.

[0123] According to the present disclosure, when the negative electrode active material obtained by the manufacturing method is used in a battery, the battery capacity is improved. Furthermore, since the negative electrode active material obtained by the manufacturing method contains both the β phase and the γ phase, when the negative electrode active material obtained by the manufacturing method is used in a battery, both the capacity and durability of the battery can be achieved.

[0124] (Technology 4) The method for producing an anode active material according to any one of Technologies 1 to 3, wherein the temperature for firing the raw material mixture is 450° C. or higher and 1000° C. or lower. With this configuration, when the anode active material obtained by this production method is used in a battery, the capacity of the battery is further improved.

[0125] (Technology 5) The method for producing an anode active material according to any one of Technologies 1 to 4, wherein the temperature for firing the raw material mixture is 500° C. or higher and 900° C. or lower. According to this configuration, when the anode active material obtained by this production method is used in a battery, the capacity of the battery is further improved.

[0126] (Technology 6) The method for producing a negative electrode active material according to any one of Technologies 1 to 5, wherein the raw material mixture is a mixture containing a Li raw material containing Li and O, a V raw material containing V and O, and an M raw material containing M and O. According to this configuration, the raw material mixture can be obtained cheaply and easily.

[0127] (Technology 7) The method for producing a negative electrode active material according to any one of Technologies 1 to 6, wherein M includes Ti. According to this configuration, when the negative electrode active material obtained by the production method is used in a battery, the capacity of the battery is further improved.

[0128] (Technology 8) The method for producing a negative electrode active material according to any one of Technologies 1 to 7, wherein the raw material mixture is fired for 5 hours or more. With this configuration, the raw material mixture is more likely to be fired appropriately.

[0129] Hereinafter, the present disclosure will be described in detail using examples and comparative examples. Note that the method for producing a negative electrode active material and the method for producing a battery according to the present disclosure are not limited to the following examples.

[0130] Example 1 The negative electrode active material of Example 1 was prepared as follows. First, Li2CO3 (manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.9%), VO5 (manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.9%), and TiO2 (manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.9%) were prepared in a molar ratio of Li2CO3:VO5:TiO2 = 1.525:0.475:0.05. Li2CO3 was added in an excess of 3 wt% relative to the weight of Li2CO3 in the above molar ratio. These raw material powders were mixed in a mortar to obtain a mixed powder. The obtained mixed powder was pre-fired in air at 600°C for 3 hours. The pre-fired powder was then fired in air at 900°C for 15 hours. In this manner, the negative electrode active material of Example 1 was prepared. Here, the temperature was increased at an average rate of 5°C / min, and decreased at an average rate of 3°C / min.

[0131] Example 2 Li2CO3, VO, and TiO2 were prepared in a molar ratio of Li2CO3:VO:TiO2 = 1.525:0.475:0.05. Li2CO3 was added in an amount 10 wt% in excess of the weight of Li2CO3 in the molar ratio. The firing conditions were the same as in Example 1, except that the firing temperature was changed to 940°C. In this manner, the negative electrode active material of Example 2 was produced.

[0132] Example 3 A negative electrode active material of Example 3 was produced in the same manner as in Example 2, except that the temperature and time of the main baking were changed to 985° C. and 5 hours, respectively.

[0133] Example 4 A negative electrode active material of Example 4 was produced in the same manner as in Example 2, except that the temperature of the main baking was changed to 800° C. and the excess amount of Li 2 CO 3 was changed to 3 wt %.

[0134] Example 5 A negative electrode active material of Example 5 was produced in the same manner as in Example 2, except that the temperature of the main baking was changed to 700° C. and the excess amount of Li 2 CO 3 was changed to 3 wt %.

[0135] Example 6 A negative electrode active material of Example 6 was produced in the same manner as in Example 2, except that the temperature of the main baking was changed to 600° C. and the excess amount of Li 2 CO 3 was changed to 3 wt %.

[0136] Example 7: The Li raw material was replaced with LiOH instead of LiCO, and LiOH, VO, and TiO were prepared in a molar ratio of LiOH:VO:TiO = 3.05:0.475:0.05. The LiOH raw material powder was added in an amount 3 wt% in excess of the weight of LiOH in the molar ratio. The firing conditions were the same as in Example 1, except that the firing temperature was changed to 500°C. In this manner, the negative electrode active material of Example 7 was produced.

[0137] Comparative Example 1 A negative electrode active material of Comparative Example 1 was produced in the same manner as in Example 7, except that the temperature for main baking was changed to 400°C.

[0138] Comparative Example 2 A negative electrode active material of Comparative Example 2 was produced in the same manner as in Example 2, except that the temperature of the main firing was changed to 1100° C. The negative electrode active material of Comparative Example 2 was strongly bonded to the crucible upon melting and could not be removed from the crucible, and therefore, evaluations such as composition analysis could not be performed.

[0139] Comparative Example 3: Li2CO3, VO, and TiO2 were prepared in a molar ratio of Li2CO3:VO:TiO2 = 1.525:0.475:0.05. However, no excess Li2CO3 was added. The firing conditions were the same as in Example 1, except that the firing temperature was changed to 800°C. In this manner, the negative electrode active material of Comparative Example 3 was prepared.

[0140] [Composition Analysis] In the negative electrode active materials of Examples 1 to 7 and Comparative Examples 1 and 3, Li 3+x+α V 1-x M x O 4+α / 2"x" (Ti content) in composition formula (1) represented by the formula was measured by ICP atomic emission spectroscopy (PS3520VDDII manufactured by Hitachi High-Tech Science Corporation), and "α" (Li content) was measured by atomic absorption spectroscopy (Z-2300 manufactured by Hitachi High-Tech Norges Corporation). "x" and "α" for Examples 1 to 7 and Comparative Examples 1 and 3 are shown in Table 1.

[0141] [X-ray Diffraction Measurement] Fig. 5 shows the X-ray diffraction patterns of the negative electrode active materials of Example 1 and Comparative Example 1. The X-ray diffraction patterns were measured by the following method.

[0142] The X-ray diffraction patterns of the negative electrode active materials of Example 1 and Comparative Example 1 were measured using an X-ray diffractometer (RIGAKU Corporation, MiniFlex 600) in a dry atmosphere having a dew point of −45° C. or less. Cu-Kα radiation (wavelengths 1.5405 Å and 1.5444 Å) was used as the X-ray source.

[0143] As shown in Fig. 5, the X-ray diffraction pattern obtained from the negative electrode active material of Example 1 had both a diffraction peak attributable to the crystal planes of the β phase and a diffraction peak attributable to the crystal planes of the γ phase. That is, the negative electrode active material of Example 1 contained both the β phase and the γ phase as crystalline phases. As shown in Fig. 5, the X-ray diffraction pattern obtained from the negative electrode active material of Comparative Example 1 had a diffraction peak attributable to the crystal planes of the β phase and a diffraction peak not attributable to either the crystal planes of the β phase or the γ phase. That is, the negative electrode active material of Example 1 contained, as crystalline phases, the β phase and impurities not belonging to the β phase.

[0144] Similar to Example 1 and Comparative Example 1, the X-ray diffraction patterns of the negative electrode active materials of Examples 2 to 7 and Comparative Example 3 were measured. The negative electrode active materials of Examples 2 to 5 and Example 7 each contained a β phase and a γ phase as crystalline phases. The negative electrode active material of Example 6 contained only a β phase as crystalline phase. The negative electrode active material of Comparative Example 3 contained only a β phase as crystalline phase.

[0145] [Battery Fabrication] The negative electrode active materials of Examples 1 to 7 and Comparative Examples 1 and 3 and the solid electrolyte Li3PS4 were prepared in an argon atmosphere having a dew point of -60°C or lower, at a volume ratio of negative electrode active material:solid electrolyte = 60:40. These materials were mixed in an agate mortar. In this way, the negative electrode mixtures of Examples 1 to 7 and Comparative Examples 1 and 3 were obtained.

[0146] A solid electrolyte Li3PS4 (80 mg) and a negative electrode mixture (6.5 mg) were stacked in an insulating cylinder with an inner diameter of 9.5 mm to obtain a laminate. A pressure of 360 MPa was applied to this laminate to form a solid electrolyte layer and a negative electrode. The solid electrolyte layer had a thickness of 500 μm.

[0147] Next, Li (thickness: 300 μm) was laminated on the solid electrolyte layer, and a pressure of 80 MPa was applied to this laminate to form a positive electrode.

[0148] Next, stainless steel current collectors were attached to the positive and negative electrodes, and current collecting leads were attached to the current collectors.

[0149] Finally, the inside of the insulating tube was isolated from the outside atmosphere using an insulating ferrule, and the inside of the tube was sealed.

[0150] In this manner, batteries of Examples 1 to 7 and Comparative Examples 1 and 3 were fabricated.

[0151] [Charge / Discharge Test] Fig. 6 is a graph showing the initial discharge characteristics of the battery of Example 1. The horizontal axis represents the discharge capacity, and the vertical axis represents the voltage. The results shown in Fig. 6 were measured by the following method.

[0152] The battery of Example 1 was placed in a thermostatic chamber maintained at 25°C.

[0153] The battery of Example 1 was discharged at a current value corresponding to a 0.1 C rate (10-hour rate) relative to the theoretical capacity of the battery until the voltage reached 0.3 V. Next, the battery of Example 1 was charged at a current value corresponding to a 0.05 C rate until the voltage reached 2.5 V.

[0154] Charge-discharge tests were also carried out on the batteries of Examples 2 to 7 and Comparative Examples 1 and 3 in the same manner as the battery of Example 1. The discharge capacities measured by the charge-discharge tests on the batteries of Examples 1 to 7 and Comparative Examples 1 and 3 are shown in Table 1.

[0155]

[0156] (Discussion) As shown by the results of Examples 1 to 7, the batteries using the negative electrode active material produced through the firing temperature of more than 400° C. and less than 1100° C. in the negative electrode exhibited high discharge capacity.

[0157] The battery using the negative electrode active material of Comparative Example 1, which was manufactured through a baking temperature of 400°C, exhibited a low discharge capacity. As shown by the results of Comparative Example 2, when the baking temperature reached 1100°C, the negative electrode active material melted. The negative electrode active materials of Examples 1 to 5 and 7 had a mixed phase of β phase and γ phase. When a negative electrode active material having a mixed phase of β phase and γ phase was used, the battery tended to exhibit a high discharge capacity.

[0158] As described above, the negative electrode active material produced by the method of the present disclosure exhibits a high discharge capacity, and a battery containing the negative electrode active material in the negative electrode is suitable for improving capacity.

[0159] The method for producing a negative electrode active material according to the present disclosure can be used, for example, in lithium ion secondary batteries.

[0160] REFERENCE SIGNS LIST 100 Negative electrode material 101 Positive electrode 102 Electrolyte layer 103 Negative electrode 111 Negative electrode active material 112 Solid electrolyte 113 Conductive additive 1000 Battery

Claims

1. A method for producing a negative electrode active material, comprising firing a raw material mixture of the negative electrode active material at a temperature of more than 400° C. and less than 1100° C., wherein the negative electrode active material is Li 3+x+α V 1-x M x O 4+α / 2 M is at least one selected from the group consisting of tetravalent metal elements and tetravalent metalloid elements other than V, and 0.03<α<1.0 and 0≦x<1 are satisfied.

2. The method for producing a negative electrode active material according to claim 1, wherein 0.07≦α≦0.21 and 0.01≦x≦0.19 are satisfied.

3. A method for producing a negative electrode active material, comprising: firing a raw material mixture of the negative electrode active material at a temperature higher than 400°C and lower than 1100°C, the negative electrode active material containing Li, V, M, and O, M being at least one selected from the group consisting of tetravalent metallic elements and tetravalent metalloid elements excluding V, and the negative electrode active material containing both a β phase and a γ phase as crystal phases.

4. The method for producing a negative electrode active material according to claim 1 or 3, wherein the temperature for firing the raw material mixture is 450° C. or higher and 1000° C. or lower.

5. The method for producing a negative electrode active material according to claim 1 or 3, wherein the temperature for firing the raw material mixture is 500°C or higher and 900°C or lower.

6. The method for producing a negative electrode active material according to claim 1 or 3, wherein the raw material mixture is a mixture containing a Li raw material containing Li and O, a V raw material containing V and O, and an M raw material containing M and O.

7. The method for producing a negative electrode active material according to claim 1 or 3, wherein M contains Ti.

8. The method for producing a negative electrode active material according to claim 1 or 3, wherein the raw material mixture is fired for 5 hours or more.

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