Patent Publication Number: US-2023137455-A1

Title: Positive electrode active material for lithium-ion battery, production method therefor, and lithium-ion battery

Description:
FIELD 
     The present disclosure relates to a positive electrode active material for a lithium-ion battery, a production method therefor, and a lithium-ion battery. 
     BACKGROUND 
     Patent Literature 1 discloses a positive electrode active material for a lithium-ion battery having a layered rock salt structure belonging to space group R-3m, and having a composition represented by Li 1+x Ti y V z D a O 2  (where D is a doping element, 0≤x&lt;1, 0&lt;y&lt;0.5, 0.3≤z&lt;1, and 0≤a≤0.2). Furthermore, Non-Patent Literature 1 discloses an electrode active material having a layered rock salt structure and having a composition represented by (1-x)LiVO 2 .xLi 2 TiO 3  (where 0≤x≤0.6). 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP 2019-091580 A 
     Non-Patent Literature 
     [NPL 1] Journal of Power Sources, 174, 2007, pp. 1007-1011 
     SUMMARY 
     Technical Problem 
     According to the findings of the present inventors, positive electrode active materials having a rock salt structure have significant volume changes during charging and discharging. When applied to a lithium-ion battery, this can be a factor in deteriorating the cycle characteristics of the battery. 
     Solution to Problem 
     As one means for solving the problem described above, the present disclosure provides: 
     a positive electrode active material for a lithium-ion battery, 
     having a disordered rock salt structure belonging to space group Fm-3m, and 
     having a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85). 
     As one means for solving the problem described above, the present disclosure provides: 
     a lithium-ion battery comprising the positive electrode active material of the present disclosure. 
     The positive electrode active material for a lithium-ion battery of the present disclosure can be produced by, for example, the following method. Specifically, the production method of the present disclosure may comprise the steps of: 
     producing an intermediate substance having a layered rock salt structure and having a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85), and 
     subjecting the intermediate substance to dry mechanical milling to obtain a positive electrode active material having a disordered rock salt structure belonging to space group Fm-3m, and having a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85). 
     Effects 
     The positive electrode active material of the present disclosure has small volume changes during charging and discharging. By applying such a positive electrode active material to a lithium-ion battery, the cycle characteristics of the battery can easily be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows an example of a method for producing a positive electrode active material for a lithium-ion battery. 
         FIG.  2    schematically shows an example of the structure of a positive electrode and a lithium-ion battery. 
         FIG.  3    is a comparison of the X-ray diffraction peaks of an intermediate substance before dry mechanical milling and a positive electrode active material after dry mechanical milling. 
         FIG.  4    shows powder X-ray diffraction peaks of a positive electrode active material according to an Example. 
         FIG.  5    shows the shift change of X-ray diffraction peaks due to charging and discharging of a positive electrode active material according to an Example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     1. Positive Electrode Active Material for a Lithium-Ion Battery 
     The positive electrode active material for a lithium-ion battery of the present disclosure has a disordered rock salt structure belonging to space group Fm-3m, and has a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85). 
     1.1 Crystal Structure 
     Regarding conventionally known positive electrode active materials having a layered rock salt structure, reversible transition metal migration during charging and discharging processes is difficult, and the volume tends to expand or contract greatly in one dimension along with the insertion or desorption of Li. Conversely, the positive electrode active material of the present disclosure has a disordered rock salt structure belonging to space group Fm-3m. According to such a positive electrode active material, volume changes are easily suppressed due to the movement of V. For example, a volume change, if any, is likely to be isotropic expansion and contraction, and a one-dimensional volume change is unlikely to occur. In the positive electrode active material of the present disclosure, change in the lattice constant of the crystal structure during charging and discharging is suppressed to, for example, 1% or less. Thus, by applying the positive electrode active material, which has small volume change due to charging and discharging, to a lithium-ion battery, the cycle characteristics of the battery can easily be improved. 
     1.2 Composition 
     The positive electrode active material of the present disclosure has the composition represented by Li 1+x Ti y V z O 2 . Here, the relationships 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85 are satisfied. When such a composition is provided, the disordered rock salt structure described above is easily maintained, and the effects described above are easily exhibited. Furthermore, when such a composition is provided, high charge-discharge capacity can easily be obtained. 
     x is greater than 0, and may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, 0.10 or more, or 0.11 or more. Furthermore, x is 0.20 or less, and may be 0.19 or less, 0.18 or less, 0.17 or less, 0.16 or less, 0.15 or less, or 0.14 or less. 
     y is greater than 0, and may be 0.01 or more, 0.03 or more, 0.05 or more, 0.07 or more, 0.09 or more, 0.10 or more, 0.11 or more, 0.13 or more, 0.15 or more, 0.17 or more, 0.19 or more, 0.21 or more, or 0.22 or more. Furthermore, y is 0.40 or less, and may be 0.39 or less, 0.38 or less, 0.37 or less, 0.36 or less, 0.35 or less, 0.34 or less, 0.33 or less, 0.32 or less, 0.31 or less, 0.30 or less, or 0.29 or less. 
     z is 0.40 or more, and may be 0.42 or more, 0.44 or more, 0.46 or more, 0.48 or more, 0.50 or more, 0.52 or more, 0.54 or more, 0.56 or more, or 0.57 or more. Furthermore, z is 0.85 or less, and may be 0.83 or less, 0.81 or less, 0.79 or less, 0.77 or less, 0.75 or less, 0.73 or less, 0.71 or less, 0.69 or less, or 0.67 or less. 
     1.3 Others 
     The positive electrode active material of the present disclosure is only required to have the crystal structure and composition described above and is not particularly limited by the other constituent features. 
     1.3.1 Shape 
     Regarding the shape of the positive electrode active material, an appropriate shape such as particulate or thin film may be selected in accordance with the shape of the battery. When the positive electrode active material is particulate, the particles may be solid particles or hollow particles. The particles of the positive electrode active material may be primary particles or secondary particles in which a plurality of primary particles are aggregated. The average particle diameter (D50) of the particles of the positive electrode active material may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. The average particle diameter D50 referred to herein is the particle diameter (median diameter) at the integrated value of 50% in a volume-based particle size distribution determined by the laser diffraction/scattering method. 
     1.3.2 Protective Layer 
     A protective layer containing a Li-ion conductive oxide may be formed on the surface of the positive electrode active material of the present disclosure. As a result, reactions between the positive electrode active material and a sulfide (for example, a sulfide solid electrolyte, which is described later) can easily be suppressed. Examples of the Li-ion conductive oxide include Li 3 BO 3 , LiBO 2 , Li 2 CO 3 , LiAlO 2 , Li 4 SiO 4 , Li 2 SiO 3 , Li 3 PO 4 , Li 2 SO 4 , Li 2 TiO 3 , Li 4 Ti 5 O 12 , Li 2 Ti 2 O 5 , Li 2 ZrO 3 , LiNbO 3 , Li 2 MoO 4 , and Li 2 WO 4 . An element of the Li-ion conductive oxide may be partially substituted with a doping element such as P or B. The coverage (area ratio) of the protective layer may be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer may be, for example, 0.1 nm or more, or 1 nm or more, and may be 100 nm or less, or 20 nm or less. 
     2. Method for Production of Positive Electrode Active Material for Lithium-Ion Battery 
     The positive electrode active material of the present disclosure can be produced by, for example, the following method. As shown in  FIG.  1   , the production method according to an embodiment may comprise: 
     step S1: producing an intermediate substance having a layered rock salt structure and having a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85), and 
     step S2: subjecting the intermediate substance to dry mechanical milling to obtain a positive electrode active material having a disordered rock salt structure belonging to space group Fm-3m, and having a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85). 
     2.1 Step S1 
     In step S1, an intermediate substance having a layered rock salt structure (belonging to space group R-3m) and having a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85) is produced. The intermediate substance can be produced by, for example, firing (calcining, baking, sintering) a mixture of a Li source, a Ti source, and a V source, after optionally molding the mixture. 
     2.1.1 Raw Materials 
     Examples of the Li source constituting the mixture include Li 2 CO 3 . Examples of the Ti source include TiO 2 . Further, examples of the V source include V 2 O 3 . By adopting Li 2 CO 3 , TiO 2 , and V 2 O 3  as the Li source, the Ti source, and the V source, respectively, the desired layered rock salt structure is easily formed in the intermediate substance, and the desired disordered rock salt structure is easily formed in the ultimate positive electrode active material. Alternatively, raw materials other than the those described above may be used as the raw materials constituting the mixture, and further, a compound (such as a composite oxide) which serves as at least two of the Li source, the Ti source, and the V source may be used. The composition ratio of Li, Ti, and V contained in the mixture can be appropriately determined in accordance with the composition ratio of the ultimate positive electrode active material. The mixture may be adjusted so that Li is included in excess. As a result, even if Li is volatilized during the firing in step S1, or if Li is consumed by a side reaction in step S2, which will be described later, the Li shortfall can be compensated for. That is, the desired composition is easily obtained in the ultimate positive electrode active material. 
     2.1.2 Mixing Means 
     The method for mixing the Li source, the Ti source, and the V source is not particularly limited. The Li source, the Ti source, and the V source can be uniformly mixed by wet mechanical milling using a solvent. For example, an organic solvent such as ethanol may be used as the solvent. The wet mechanical milling can be performed by mechanical mixing means such as, for example, a planetary ball mill. The mixing conditions (mixing time, rotation speed, number of repetitions, etc.) of the wet mechanical milling are not particularly limited, and the conditions may be such that the Li source, the Ti source, and the V source are uniformly mixed to the extent that the desired layered rock salt structure can be formed after firing, which is described later. 
     2.1.3 Molding 
     The mixture described above may be molded into pellets prior to firing. The sizes and shapes of the molded bodies are not particularly limited. 
     2.1.4 Firing 
     By firing the mixture or molded bodies described above, an intermediate substance having a layered rock salt structure and the composition described above is obtained. The firing atmosphere is not particularly limited, and may be, for example, an oxygen-containing atmosphere such as the ambient atmosphere or an air atmosphere, or an inert gas atmosphere such as an Ar atmosphere. Particularly when it is an inert gas atmosphere, the desired intermediate substance can easily be obtained. The firing temperature is not particularly limited as long as a layered rock salt structure can be obtained. For example, it may be 800° C. or higher, or 850° C. or higher, and may be 1000° C. or lower, or 950° C. or lower. The firing time (retention time at the firing temperature) is not particularly limited either, and may be, for example, 5 hours or more, 7 hours or more, 10 hours or more, or 12 hours or more, and may be 100 hours or less, 50 hours or less, or 20 hours or less. 
     2.2 Step S2 
     In step S2, by subjecting the intermediate substance described above to dry mechanical milling, a positive electrode active material having a disordered rock salt structure belonging to space group Fm-3m, and having a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85) is obtained. Specifically, by subjecting the intermediate substance described above to dry mechanical milling, in which a solvent is not substantially used, the layered rock salt structure of the intermediate substance is changed, whereby a positive electrode active material having a disordered rock salt structure is obtained. Dry mechanical milling can be performed by mechanical mixing means such as, for example, a planetary ball mill. The dry mechanical milling conditions (mixing time, rotation speed, number of repetitions, etc.) are not particularly limited. For example, when using a planetary ball mill, the rotation speed may be 500 to 700 rpm, the rotation time may be 10 to 20 minutes, and the rest time may be 1 to 5 minutes. The rotation and the rest may be repeated multiple times. Further, a set of rotation and rest may be repeated multiple times. By adjusting the dry mechanical milling conditions, for example, the crystallite size of the positive electrode active material can be controlled. 
     2.3 Supplement 
     Note that in the production method of the present disclosure from the mixture through the intermediate substance to the positive electrode active material, the composition ratio of Li, Ti, and V may vary or may be substantially the same without variation. Furthermore, though the intermediate substance having a layered rock salt structure is produced by a solid phase reaction method in the explanation above, the intermediate substance production method is not limited thereto. Furthermore, though the intermediate substance having a layered rock salt structure is subjected to dry mechanical milling in the above explanation, there is room for the adoption of other methods for obtaining the positive electrode active material having a disordered rock salt structure. However, as far as the present inventors have confirmed, after producing an intermediate substance having a layered rock salt structure, by subjecting the intermediate substance to dry mechanical milling, the target disordered rock salt structure can be obtained more stably and easily. 
     3. Positive Electrode for Lithium-Ion Battery 
     The technology of the present disclosure also has an aspect as a positive electrode for a lithium-ion battery. Specifically, the positive electrode of the present disclosure comprises the positive electrode active material described above. As shown in  FIG.  2   , the positive electrode  10  according to an embodiment may comprise a positive electrode active material layer  11  and a positive electrode current collector  12 , and in this case, the positive electrode active material layer  11  may comprise the positive electrode active material described above. 
     3.1 Positive Electrode Active Material Layer 
     The positive electrode active material layer  11  comprises at least the positive electrode active material described above, and may optionally comprise an electrolyte, a conductive agent, and a binder. The content of each of the positive electrode active material, the electrolyte, the conductive agent, and the binder in the positive electrode active material layer  11  may be appropriately determined in accordance with the target battery performance. For example, when the entire positive electrode active material layer  11  (total solid content) is 100% by mass, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, and may be 100% by mass or less, or 90% by mass or less. The shape of the positive electrode active material layer  11  is not particularly limited, and may be, for example, a sheet-like positive electrode active material layer  11  having a substantially flat surface. The thickness of the positive electrode active material layer  11  is not particularly limited, and may be, for example, 0.1 μm or more, or 1 μm or more, and may be 2 mm or less, or 1 mm or less. 
     3.1.1 Positive Electrode Active Material 
     The positive electrode active material layer  11  may comprise only the positive electrode active material having a disordered rock salt structure described above as the positive electrode active material. Alternatively, the positive electrode active material layer  11  may comprises a different type of positive electrode active material (another positive electrode active material) in addition to the positive electrode active material described above. From the viewpoint of further enhancing the effect of the technology of the present disclosure, the content of other positive electrode active material in the positive electrode active material layer  11  may be small. For example, the positive electrode active material having a disordered rock salt structure described above may occupy 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or 99% by mass or more of the total positive electrode active material contained in the positive electrode active material layer  11 . 
     3.1.2 Electrolyte 
     The electrolyte may be a solid electrolyte, or may be a liquid electrolyte (electrolytic solution). When the positive electrode  10  is a positive electrode for a solid-state battery (a battery containing a solid electrolyte), the positive electrode active material layer  11  may comprise a solid electrolyte as the electrolyte. Furthermore, when the positive electrode  10  is a positive electrode for a liquid electrolyte battery, the positive electrode active material layer  11  may contain an electrolytic solution as an electrolyte, or the positive electrode active material layer  11  is in contact with an electrolytic solution. When the positive electrode  10  is a positive electrode for a liquid electrolyte battery, it is sufficient that the positive electrode active material layer  11  and the electrolytic solution be in contact at least after the battery is constructed. The same is true for the negative electrode  30 , which will be described later. 
     The solid electrolyte may be any known solid electrolyte for batteries. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes have higher ionic conductivity than organic polymer electrolytes. Furthermore, inorganic solid electrolytes have excellent heat resistance as compared with organic polymer electrolytes. As the inorganic solid electrolyte, for example, oxide solid electrolytes such as lithium lanthanum zirconate, LiPON, Li 1+X Al X Ge 2-X (PO 4 ) 3 , Li-SiO-based glasses, and Li-Al-S-O-based glasses; and sulfide solid electrolytes such as Li 2 S—P 2 S 5 , Li 2 S—SiS 2 , LiI—Si 2 S—P 2 S 5 , Li 2 S—P 2 S 5 —LiI—LiBr, LiI—Li 2 S—P 2 S 5 , LiI—Li 2 S—P 2 O 5 , LiI—Li 3 PO 4 —P 2 S 5 , and Li 2 S—P 2 S 5 —GeS 2  can be exemplified. In particular, sulfide solid electrolytes, and thereamong, sulfide solid electrolytes containing Li 2 S—P 2 S 5 , have high performance. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may be, for example, particulate. Only one type of solid electrolyte may be used alone, or two or more types may be used in combination. 
     The electrolytic solution may contain, for example, lithium ions as carrier ions. The electrolytic solution may be an aqueous electrolytic solution or a non-aqueous electrolytic solution. The composition of the electrolytic solution may be the same as that of the composition of known lithium-ion battery electrolytic solutions. For example, as the electrolytic solution, a solution obtained by dissolving a lithium salt at a predetermined concentration in a carbonate-based solvent can be used. Examples of the carbonate-based solvent include fluoroethylene carbonate (FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC). Examples of the lithium salt include LiPF 6 . 
     3.1.3 Conductive Agent 
     Examples of the conductive agent include carbon materials such as vapor grown carbon fiber (VGCF), acetylene black (AB), Ketj en black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metal materials such as nickel, aluminum, and stainless steel. The conductive agent may be, for example, particulate or fibrous, and the size thereof is not particularly limited. One type of conductive agent may be used alone, or two or more types may be used in combination. 
     3.1.4 Binder 
     Examples of the binder include butadiene rubber (BR) based binder, butylene rubber (IIR) based binder, acrylate butadiene rubber (ABR) based binder, styrene-butadiene rubber (SBR) based binder, polyvinylidene fluoride (PVdF) based binder, polytetrafluoroethylene (PTFE) based binder, and polyimide (PI) based binder. Only one type of binder may be used alone, or two or more types may be used in combination. 
     3.2 Positive Electrode Current Collector 
     As shown in  FIG.  2   , the positive electrode  10  may comprise a positive electrode current collector  12  which is in contact with the positive electrode active material layer  11  described above. Any general positive electrode current collector for batteries can be used as the positive electrode current collector  12 . Furthermore, the positive electrode current collector  12  may be in the form of a foil, plate, mesh, punched metal, or foam. The positive electrode current collector  12  may be composed of a metal foil or a metal mesh. In particular, metal foil is excellent in handleability. The positive electrode current collector  12  may be composed of a plurality of sheets of foil. Examples of the metal constituting the positive electrode current collector  12  include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, from the viewpoint of ensuring oxidation resistance, the positive electrode current collector  12  may contain Al. The positive electrode current collector  12  may have some type of coating layer on the surface thereof for the purpose of, for example, adjusting resistance. Furthermore, the positive electrode current collector  12  may be a metal foil or a base material plated or vapor-deposited with the metal described above. When the positive electrode current collector  12  is composed of a plurality of metal foils, it may have some type of layer between the plurality of metal foils. The thickness of the positive electrode current collector  12  is not particularly limited. For example, it may be 0.1 μm or more, or 1 μm or more, and may be 1 mm or less, or 100 μm or less. 
     3.3 Others 
     In addition to the constituents described above, the positive electrode  10  may have any of constituents which are conventional for positive electrodes for batteries. Examples thereof include tabs and terminals. The positive electrode  10  can be produced by a known method except that the positive electrode active material having the disordered rock salt structure described above is used. For example, the positive electrode active material layer  11  can be easily molded by dry or wet molding of a positive electrode mixture containing the various components described above. The positive electrode active material layer  11  may be molded together with the positive electrode current collector  12  or may be molded separately from the positive electrode current collector  12 . 
     4. Lithium-Ion Battery 
     The technology of the present disclosure also includes an aspect as a lithium-ion battery. Specifically, the lithium-ion battery of the present disclosure comprises the positive electrode active material of the present disclosure. As described above, the positive electrode active material of the present disclosure has small volume changes during charging and discharging, and thus, when applied to a lithium-ion battery, the cycle characteristics of the battery can easily be improved. The constituents of the lithium-ion battery of the present disclosure are not particularly limited as long as the positive electrode active material of the present disclosure is contained. For example, as shown in  FIG.  2   , a lithium-ion battery  100  according to an embodiment may comprise a positive electrode  10 , an electrolyte layer  20 , and a negative electrode  30 . The positive electrode  10  is as described above. 
     4.1 Electrolyte Layer 
     The electrolyte layer  20  comprises at least an electrolyte. When the lithium-ion battery  100  is a solid-state battery, the electrolyte layer  20  comprises a solid electrolyte and optionally a binder. In this case, the contents of the solid electrolyte and the binder of the electrolyte layer  20  are not particularly limited. On the other hand, when the lithium-ion battery  100  is a liquid electrolyte battery, the electrolyte layer  20  may contain an electrolytic solution, and it may comprise a separator for containing an electrolytic solution and preventing contact between the positive electrode active material layer  11  and the negative electrode active material layer  31 . The thickness of the electrolyte layer  20  is not particularly limited, and may be, for example, 0.1 μm or more, or 1 μm or more, and may be 2 mm or less or 1 mm or less. 
     The solid electrolyte, the electrolytic solution, and the binder are as described above. The separator may be a separator which is commonly used in lithium-ion batteries, and examples thereof include the one composed of a resin such as polyethylene (PE), polypropylene (PP), polyester, or polyamide. The separator may have a single-layer structure or a multi-layer structure. Examples of the separator having a multilayer structure include a separator having a two-layer PE/PP structure and a separator having a three-layer PP/PE/PP or PE/PP/PE structure. The separator may be composed of a nonwoven fabric such as a cellulose nonwoven fabric, a resin nonwoven fabric, or a glass fiber nonwoven fabric. 
     4.2 Negative Electrode 
     As shown in  FIG.  2   , the negative electrode  30  may comprise a negative electrode active material layer  31  and a negative electrode current collector  32 . 
     4.2.1 Negative Electrode Active Material Layer 
     The negative electrode active material layer  31  contains at least a negative electrode active material, and may optionally contain an electrolyte, a conductive agent, and a binder. The content of each of the negative electrode active material, the electrolyte, the conductive agent, and the binder in the negative electrode active material layer  31  may be appropriately determined according to the desired battery performance. For example, when the entire negative electrode active material layer  31  (total solid content) is 100% by mass, the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, and may be 100% by mass or less or 90% by mass or less. The shape of the negative electrode active material layer  31  is not particularly limited, and may be, for example, a sheet-like negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer  31  is not particularly limited, and may be, for example, 0.1 μm or more, or 1 μm or more, and may be 2 mm or less or 1 mm or less. 
     Various substances having a lower potential (charge/discharge potential) at which lithium ions are inserted or desorbed than the positive electrode active material described above can be used as the negative electrode active material. For example, silicon-based active materials such as Si, Si alloys, and silicon oxide; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; and metallic lithium and lithium alloys can be used. Only one type of the negative electrode active material may be used alone, or two or more types may be used in combination. 
     The shape of the negative electrode active material may be a conventional shape as a negative electrode active material for batteries. For example, the negative electrode active material may be particulate. The negative electrode active material particles may be primary particles or secondary particles in which a plurality of primary particles are aggregated. The average particle diameter (D50) of the negative electrode active material particles may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. Alternatively, the negative electrode active material may be in the form of a sheet (foil, film) such as metallic lithium foil. That is, the negative electrode active material layer  31  may be composed of a negative electrode active material sheet. 
     Examples of the electrolyte which can be included in the negative electrode active material layer  31  include the solid electrolytes and electrolytic solutions described above. Examples of the conductive agent which can be included in the negative electrode active material layer  31  include the carbon materials and the metal materials described above. The binder which can be included in the negative electrode active material layer  31  may be appropriately selected from, for example, those exemplified as binders which can be included in the positive electrode active material layer  11  described above. 
     4.2.2 Negative Electrode Current Collector 
     As shown in  FIG.  2   , the negative electrode  30  may comprise a negative electrode current collector  32  which is in contact with the negative electrode active material layer  31  described above. Any conventional negative electrode current collector for batteries can be used as the negative electrode current collector  32 . Furthermore, the negative electrode current collector  32  may be in the form of a foil, plate, mesh, punched metal, or foam. The negative electrode current collector  32  may be a metal foil or a metal mesh, or may be a carbon sheet. In particular, metal foil is excellent in handleability. The negative electrode current collector  32  may be composed of a plurality of foils or sheets. Examples of the metal constituting the negative electrode current collector  32  include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, the negative electrode current collector  32  may comprise at least one metal selected from Cu, Ni, and stainless steel from the viewpoint of ensuring reduction resistance and preventing alloying with lithium. The negative electrode current collector  32  may have some type of coating layer on the surface thereof for the purpose of, for example, adjusting resistance. Furthermore, the negative electrode current collector  32  may be a metal foil or a base material plated or vapor-deposited with the metal described above. When the negative electrode current collector  32  is composed of a plurality of metal foils, it may have some type of layer between the plurality of metal foils. The thickness of the negative electrode current collector  32  is not particularly limited. For example, it may be 0.1 μm or more, or 1 μm or more, and may be 1 mm or less, or 100 μm or less. In addition to the constituents described above, the negative electrode  30  may have any constituents which are conventional as a negative electrode for batteries. Examples thereof include tabs and terminals. 
     4.3 Others 
     The lithium-ion battery  100  may comprise each of the constituents described above housed inside an exterior body. As the exterior body, any known exterior body for batteries can be used. A plurality of batteries  100  may be electrically connected and stacked to form an assembled battery. In this case, the assembled battery may be housed inside a known battery case. The lithium-ion battery  100  may also have obvious constituents such as necessary terminals. Examples of the shape of the lithium-ion battery  100  include a coin shape, a laminate shape, a cylindrical shape, and a rectangular shape. 
     5. Lithium-Ion Battery Production Method 
     The lithium-ion battery  100  can be produced by adopting a known method. For example, the method for producing the lithium-ion battery  100  includes laminating the positive electrode  10 , the electrolyte layer  20 , and the negative electrode  30  described above. A specific example of the method for producing the lithium-ion battery  100  is shown below. However, the method for producing the lithium-ion battery  100  is not limited to the method described below, and each layer may be formed by, for example, dry molding.
     (1) The positive electrode active material constituting the positive electrode active material layer is dispersed in a solvent to obtain a slurry for the positive electrode layer. The solvent used in this case is not particularly limited, and water and various organic solvents can be used, and N-methylpyrrolidone (NMP) may be used. The slurry is applied to the surface of a positive electrode current collector using a doctor blade and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector, thereby obtaining a positive electrode.   (2) The negative electrode active material constituting the negative electrode active material layer is dispersed in a solvent to obtain a slurry for the negative electrode layer. The solvent used in this case is not particularly limited, and water and various organic solvents can be used, and N-methylpyrrolidone (NMP) may be used. The slurry is applied to the surface of a negative electrode current collector using a doctor blade and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector, thereby obtaining a negative electrode. Alternatively, a metal foil which is a negative electrode active material may be used as-is as a negative electrode.   (3) Each layer is laminated so that the negative electrode and the positive electrode interpose the electrolyte layer (solid electrolyte layer or separator), whereby a laminate having a negative electrode current collector, a negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector in this order is obtained. Other members such as terminals are attached to the laminate as necessary.   (4) A secondary battery is formed by housing the laminate in a battery case, in the case of a liquid electrolyte battery, filling the battery case with an electrolytic solution, immersing the laminate in the electrolytic solution, and sealing the laminate in the battery case. In the case of a liquid electrolyte battery, the electrolytic solution may be included in the negative electrode active material layer, the separator, and the positive electrode active material layer in stage (3) described above.   

     EXAMPLES 
     The technology of the present disclosure will be described in further detailed below while showing the Examples, but the technology of the present disclosure is not limited to the following Examples. 
     1. Positive Electrode Active Material Synthesis 
     1.1 Examples 1 to 6 
     The positive electrode active material having a disordered rock salt structure was synthesized by the following method. 
     1.1.1 Production of Intermediate Substance 
     Li 2 CO 3  (excess of 3%), TiO 2 , and V 2 O 3  as raw materials were each weighed so as to achieve a predetermined ratio, and mixed by wet ball milling (wet BM), which is one type of wet mechanical milling. The wet BM conditions are as shown in Table 1 below. Thereafter, the mixture was pelletized. The molded pellets were placed on a boat-shaped aluminum board, wrapped with Cu foil, and fired at 900° C. for 12 hours in an Ar atmosphere to obtain the intermediate substance. By changing the composition of the raw materials, the values of x, y, and z were variously changed, and a plurality of types of intermediate substances were obtained. From the X-ray diffraction peaks and elemental analysis, it was confirmed that the plurality of types of intermediate substances all had a layered rock salt structure and had a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85). 
     1.1.2 Dry Mechanical Milling 
     Positive electrode active materials were obtained by subjecting the respective intermediate substances to dry ball milling (dry BM), which is one type of dry mechanical milling. The dry BM conditions are as shown in Table 1 below. It was confirmed that the positive electrode active materials had a disordered rock salt structure belonging to space group Fm-3m, and had a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤z≤0.85).  FIG.  3    shows an example of X-ray diffraction peaks of an intermediate substance before dry mechanical milling and an example of X-ray diffraction peaks of a positive electrode active material after dry mechanical milling. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Set no. 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Rotation 
                 Rotation 
                 Rest 
                 Repetition 
                 Set 
                 Zirconia balls 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Container 
                 speed 
                 time 
                 time 
                 no. 
                 no. 
                 10 mm 
                 5 mm 
                 1 mm 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Wet BM 
                 45 mL 
                 300 rpm 
                 15 min 
                 3 min 
                 17 
                 1 
                 5 pieces 
                 10 pieces 
                 4 g 
               
               
                 Dry BM 
                 45 mL 
                 600 rpm 
                 15 min 
                 3 min 
                 40 
                 3 
                 3 pieces 
                 10 pieces 
                 2 g 
               
               
                   
               
            
           
         
       
     
     1.2 Comparative Examples 1 to 3 
     Various positive electrode active materials (LiNi 1/3 Co 1/3 Mn 1/3 O 2 , LiNi 0.85 Co 0.10 Al 0.05 O 2 , and LiNi 0.5 Mn 0.5 O 2 ) having a layered rock salt structure were prepared. 
     1.3 Comparative Examples 4 and 5 
     LiVO 2  as a positive electrode active material which does not contain Ti was prepared. Furthermore, Li 2 TiO 3  as a positive electrode active material which does not contain V was prepared. The specific methods for synthesizing LiVO 2  and Li 2 TiO 3  conform to the method for synthesizing the Li-Ti-VO-based positive electrode active materials according to Examples 1 to 6 described above. 
     2. XRD Measurement and Results 
     2.1 Powder XRD 
     Powder X-ray diffraction measurement was performed on the positive electrode active materials according to the Examples. Specifically, X-ray diffraction analysis (using CuKα as a radiation source) was performed in SmartLab using a non-reflective sample plate.  FIG.  4    shows the X-ray diffraction peaks of the positive electrode active material for x=0.14, y=0.29, and z=0.57. 
     As shown in  FIG.  4   , peaks which can be attributed to a disordered rock salt structure can be confirmed. All of the positive electrode active materials according to Examples 1 to 6 had a disordered rock salt structure belonging to space group Fm-3m. 
     2.2 Operando XRD 
     Changes in crystal structure during charging and discharging were observed by synchrotron radiation X-ray diffraction measurement. Table 2 below shows the maximum volume change rate along the c-axis during charging and discharging for Example 1 and Comparative Examples 1 to 3. Further,  FIG.  5    shows an example of the shift change of the X-ray diffraction peaks due to charging and discharging of a positive electrode active material according to an Example. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Maximum volume  
               
               
                   
                   
                 Composition 
                 change rate (C-axis) 
               
               
                   
                   
               
             
            
               
                   
                 Ex 1 
                 Li 8/7 Ti 2/7 V 4/7 O 2   
                 0.46% 
               
               
                   
                 Comp Ex1 
                 LiNi 1/3 Co 1/3 Mn 1/3 O 2   
                  2.0% 
               
               
                   
                 Comp Ex2 
                 LiNi 0.85 Co 0.10 Al 0.05 O 2   
                  2.5% 
               
               
                   
                 Comp Ex3 
                 LiNi 0.5 Mn 0.5 O 2   
                  1.4% 
               
               
                   
                   
               
            
           
         
       
     
     As is clear from the results shown in Table 2 and  FIG.  5   , very small volume changes of less than 1% occurred in the positive electrode active material according to the Example during charging and discharging. Conversely, it was found that positive electrode active materials having a layered rock salt structure, as in Comparative Examples 1 to 3, undergo large volume changes of more than 1% during charging and discharging. From the above results, when the positive electrode active material according to an Example is applied to a lithium-ion battery, mechanical deterioration caused by volume changes of the active material during charging and discharging processes can be suppressed, whereby the cycle characteristics of the battery can easily be improved. 
     3. Production and Evaluation of Lithium-Ion Battery 
     The positive electrode active material described above, acetylene black as a conductive agent, and PVDF as a binder were weighed at a mass ratio of 76.5:13.5:10.0, mixed and dispersed in N-methylpyrrolidone to obtain a positive electrode mixture slurry. A positive electrode was obtained by applying the positive electrode mixture slurry onto an Al current collector foil and vacuum-drying at 120° C. overnight. A coin cell (CR2032) was produced using the positive electrode, a non-aqueous electrolytic solution (electrolyte: 1 M LiPF 6 , solvent:ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed at 30:70 vol %), and a metallic lithium foil as a negative electrode. The charge/discharge characteristics of the produced coin cell were evaluated in a constant temperature bath maintained at room temperature at a voltage range of 1.2 to 4.3 V and 10 mA/g. Table 3 below shows the discharge capacities of the positive electrodes of the Examples and Comparative Examples. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Composition 
                 Discharge capacity (mAh/g) 
               
               
                   
                   
               
             
            
               
                   
                 Ex 2 
                 Li 1.05 Ti 0.10 V 0.85 O 2   
                 211 
               
               
                   
                 Ex 3 
                 Li 1.11 Ti 0.22 V 0.67 O 2   
                 245 
               
               
                   
                 Ex 4 
                 Li 1.14 Ti 0.29 V 0.57 O 2   
                 270 
               
               
                   
                 Ex 5 
                 Li 1.17 Ti 0.33 V 0.50 O 2   
                 213 
               
               
                   
                 Ex 6 
                 Li 1.20 Ti 0.40 V 0.40 O 2   
                 181 
               
               
                   
                 Comp Ex4 
                 LiVO 2   
                 156 
               
               
                   
                 Comp Ex5 
                 Li 2 TiO 3   
                  0 
               
               
                   
                   
               
            
           
         
       
     
     As shown in  FIG.  3   , it can be understood that as compared to the positive electrode active materials represented by LiVO 2  and Li 2 TiO 3 , the positive electrode active materials having a disordered rock salt structure and a composition represented by Li 1+x Ti y V z O 2  could ensure sufficient discharge capacity when applied to the positive electrode of a lithium-ion battery. 
     As described above, positive electrode active materials which satisfy the following requirements (1) and (2) have small volume changes during charging and discharging, and can improve the cycle characteristics of a lithium-ion battery when used, for example, in a wide SOC range with a reversible capacity of 250 mAh/g or more. Furthermore, a positive electrode having a sufficient discharge capacity can be obtained. 
     (1) Having a disordered rock salt structure belonging to space group Fm-3m. (2) Having a composition represented by Li 1+x Ti y V z O 2  (where 0&lt;x≤0.20, 0&lt;y≤0.40, and 0.40≤x≤0.85). 
     REFERENCE SIGNS LIST 
     
         
           10  positive electrode 
           11  positive electrode active material layer 
           12  positive electrode current collector 
           20  electrolyte layer 
           30  negative electrode 
           31  negative electrode active material layer 
           32  negative electrode current collector 
           100  lithium-ion battery