Lithium composite oxide, power storage device, and method for producing lithium composite oxide

Abstract

To enhance discharge capacity of a power storage device more.SOLUTION: A lithium composite oxide is used for an electrode active material of a power storage device, contains lithium and an element M, has a cubic crystal structure, and has an oxygen defect rate of 5 mass% or less, which is obtained from increase / decrease in an extreme value of weight data in a range of 500°C or lower when temperature is increased in an air atmosphere by thermogravimetric differential thermal analysis (TG-DTA).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present specification discloses a lithium composite oxide, an electricity storage device, and a method for producing the lithium composite oxide. [Background technology]

[0002] LiMO2 (where M = Co, Ni, or Mn) with a layered rock-salt structure has been used as a positive electrode active material for lithium secondary batteries. While the theoretical maximum discharge capacity with this composition is approximately 280 mAh / g, only approximately 70% of this capacity can be extracted due to potential and structural stability considerations. Therefore, proposals have been made to increase the available discharge capacity by incorporating an excess of Li in the composition. In particular, it is known that the active material with an irregular rock-salt structure allows Li to be charged and discharged three-dimensionally, resulting in high discharge capacity (see, for example, Patent Documents 1 and 2). This is because the amorphous structure allows Li to move three-dimensionally, allowing reversible insertion and desorption of Li even in a Li-excess composition. Furthermore, a lithium composite oxide with a composition of (1-α)LiMnO2·αLi3PO4 (α is 0.1, 0.2, or 0.3) has been proposed, which is a mixture of layered LiMnO2 and Li3PO4 by mechanical milling (see Non-Patent Document 1). This lithium composite oxide has a disordered rock salt structure and Mn 3+ / Mn 4+ It is said to exhibit a high capacity exceeding the theoretical capacity based on the oxidation-reduction of [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2018-92958 [Patent Document 2] Patent Publication No. 2017-202954 [Non-patent literature]

[0004] [Non-Patent Document 1] M. Sawamura, et al., ACS Cent. Sci. 2020, 6, 2326-2338 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the composite oxide of Patent Document 1, for example, Li 1.233 Mo 0.467 Cr 0.3 Although the composition of O2 exhibits a discharge capacity exceeding 280 mAh / g, this is still insufficient, and achieving a higher capacity has been an issue. Patent Document 2 also claims that a composite oxide containing lithium and molybdenum in its composition exhibits a high discharge capacity exceeding 300 mAh / g, but discharge is performed under low voltage conditions with a lower voltage limit of 1.0 V, and the actual discharge capacity practically feasible as a full cell is estimated to be approximately 150 mAh / g, leaving further improvement desirable. Furthermore, the lithium composite oxide in Non-Patent Document 1 also claims that a composite oxide containing lithium, manganese, and phosphorus in its composition exhibits a high capacity exceeding 300 mAh / g, but this is still insufficient, and further improvement desirable.

[0006] The present disclosure has been made to solve such problems, and a main object of the present disclosure is to provide a lithium composite oxide, an electricity storage device, and a method for producing a lithium composite oxide that can further increase discharge capacity. [Means for solving the problem]

[0007] In order to achieve the above-mentioned object, the present inventors have conducted extensive research and found that in a cubic lithium composite oxide containing a disordered rock salt structure, the discharge capacity can be increased by further reducing the oxygen vacancy rate, and have thus completed the invention disclosed in this specification.

[0008] That is, the lithium composite oxide disclosed in the present specification is A lithium composite oxide used as an electrode active material for an electricity storage device, containing lithium and element M and having a cubic crystal structure; The oxygen vacancy rate determined from the extreme increase or decrease in weight data in the range of 500°C or less when heated in an air atmosphere by thermogravimetric differential thermal analysis (TG-DTA) is 5% by mass or less.

[0009] The electricity storage device disclosed in this specification comprises: an electrode having the above-described lithium composite oxide as an electrode active material; an ionically conductive medium in contact with the electrode and conducting lithium ions; It is equipped with the following.

[0010] The method for producing a lithium composite oxide disclosed in the present specification includes the steps of: A method for producing a lithium composite oxide used as an electrode active material for an electricity storage device, comprising: a precursor step in which an orthorhombic lithium composite oxide raw material is mixed and pulverized to obtain a precursor containing lithium and the element M and exhibiting a disordered rock salt structure; a defect reduction step of treating the precursor so that the oxygen vacancy rate, which is determined from an increase or decrease in extreme values ​​of weight data in a temperature range of 500°C or less when the produced lithium composite oxide is subjected to thermogravimetric differential thermal analysis (TG-DTA) in an air atmosphere, is 5% by mass or less; It includes: [Effects of the Invention]

[0011] The present disclosure provides a lithium composite oxide, an electricity storage device, and a method for producing the lithium composite oxide, which can further increase discharge capacity. The reason for this effect is believed to be as follows. For example, unlike the layered positive electrodes that have been used in practice to date, disordered rock-salt lithium composite oxides allow Li to migrate three-dimensionally, potentially enabling the insertion and desorption of more Li ions. Meanwhile, disordered rock-salt lithium composite oxides are easily subject to oxygen vacancies because they are synthesized mechanochemically by mixing and grinding using a planetary ball mill or the like. In the present disclosure, it is believed that the reduction of oxygen vacancies in the cubic-based disordered rock-salt lithium composite oxides allows the electronic state of adjacent Mn to be ideal, allowing for sufficient oxidation and reduction, thereby further improving discharge capacity. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic diagram showing an example of the configuration of an electricity storage device 20. [Figure 2] XRD patterns of Experimental Examples 1 to 5. [Figure 3] TG-DTA measurement results of Experimental Example 4. [Figure 4] TG-DTA measurement results of Experimental Example 1. [Figure 5] First discharge curves for Experimental Examples 1 to 5. [Figure 6] Schematic diagram showing the estimated crystal structure of Experimental Example 4. [Figure 7] Schematic diagram of a method for filling oxygen vacancies. DETAILED DESCRIPTION OF THE INVENTION

[0013] (lithium composite oxide) The lithium composite oxide of the present disclosure is used as an electrode active material for an electricity storage device. This lithium composite oxide may be used as an electrode active material for an electricity storage device. This lithium composite oxide contains lithium and element M and has a cubic crystal structure. This lithium composite oxide may exhibit an XRD spectrum peak pattern of a cubic disordered rock salt structure (also referred to as a random rock salt structure), or may exhibit a peak pattern of a cubic spinel structure. The disordered rock salt structure exhibits a peak pattern in the XRD spectrum with at least four diffraction peaks in the 2θ ranges of 35° to 39°, 42° to 48°, 62° to 68°, and 80° to 84°. Furthermore, due to the disordered structure, the diffraction peaks are relatively broad, with the half-width of the main peak at 2θ = 42° to 48° being 1.5° or more and the half-width of the peak at 2θ = 62° to 68° being 2° or more. In addition, the spinel structure exhibits a peak pattern in the XRD spectrum with diffraction peaks in at least five of the following 2θ ranges: 15° to 22°, 33° to 39°, 42° to 48°, 62° to 68°, and 80° to 84°. The spinel structure also exhibits relatively broad diffraction peaks, with the peak at 2θ=15° to 22° having a half-width of 1.5° or more and the main peak at 2θ=42° to 48° having a half-width of 1.5° or more.

[0014] This lithium composite oxide has an oxygen deficiency rate of 5% by mass or less, determined from the change in the extreme values ​​of weight data in a range of 500°C or less when heated in an air atmosphere by thermogravimetric differential thermal analysis (TG-DTA). The oxygen deficiency rate is preferably lower, preferably 4% by mass or less, more preferably 2% by mass or less, even more preferably 1% by mass or less, and may be 0.5% by mass or less. The oxygen deficiency rate is determined by using alumina as a reference sample in the thermogravimetric differential thermal analysis, heating the sample to 1000°C in an air stream at a scanning rate of 10°C / min, and based on the obtained weight change profile, taking the minimum value in the range of 100°C to 300°C after adsorbed water is desorbed as a reference value, and the maximum value in the range of 300°C to 500°C as the mass of the composite oxide when it is completely oxidized.

[0015] This lithium composite oxide has a basic composition formula of Li a M x O 2-zIt may be as such. However, in this basic composition formula, 0 < a ≤ 2, 0 < x ≤ 2, 0 ≤ z ≤ 0.15, and M is one or more selected from the group consisting of B, C, N, P, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge. The content a of Li in the basic composition formula varies due to the insertion and extraction of lithium ions and can be any value. For example, it may satisfy 1 ≤ a ≤ 1.25 in a state where charge-discharge is not performed. This lithium composite oxide may contain at least one of the above-mentioned transition metal elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) as the element M, and it is preferably at least Mn. In this basic composition formula, the content x of the element M may satisfy 0.8 or more, or 0.85 or more, or 0.90 or more. Also, this content x may satisfy 0.98 or less, or 0.96 or less, or 0.95 or less. The element M may contain at least Mn and may further contain P, or alternatively, the element M may contain at least Mn and not contain P. When the element M contains P, it may be such that oxygen defects are reduced by PO4 ions. At this time, the lithium composite oxide has the basic composition formula Li a P x1 Mn x2 O 2-zIt may be represented by 0 < a < 2, 0 < x1 ≤ 0.09, 0 < x2 < 1, and 0 ≤ z ≤ 0.15. In this lithium composite oxide, the content x1 of P preferably satisfies 0.02 ≤ x ≤ 0.08, and more preferably satisfies 0.03 ≤ x ≤ 0.08. Also, the content x2 of the element M preferably satisfies 0.65 ≤ y < 1, and more preferably satisfies 0.7 ≤ y ≤ 0.95. Further, the oxygen defect z is preferably less, preferably satisfies z ≤ 0.10, more preferably satisfies z ≤ 0.05 or less, and still more preferably satisfies z ≤ 0.03 or less. This lithium composite oxide may be one in which part of the oxygen is substituted with the element A. Examples of the element A include halogen elements, among which F is preferable. When F is included as the element A, the content z of F preferably satisfies 0 < z < 0.5, and may satisfy 0.3 ≤ z < 0.5.

[0016] (Method for producing lithium composite oxide) The method for producing the lithium composite oxide of the present disclosure is the method for producing the above-described lithium composite oxide used as an electrode active material of an electric storage device. This production method includes a precursor step of obtaining a precursor and a defect reduction step of treating the precursor. Note that materials, mixing ratios, compositions, etc. can be appropriately selected from those exemplified for the above-described lithium composite oxide. For this reason, for the sake of convenience of explanation, some detailed descriptions thereof are omitted.

[0017] (Precursor step) In the precursor step, a treatment is performed to obtain a precursor containing lithium and the element M and having a disordered rock salt structure by mixing and pulverizing an orthorhombic lithium composite oxide raw material. The precursor may be a lithium composite oxide having a disordered rock salt structure before reduction of oxygen defects. In this step, a lithium composite oxide raw material containing the element M (where M is one or more of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge) and having an orthorhombic XRD spectrum is used. As the raw material, a compound represented by LiMO2 is preferably used, and LiMnO2 is more preferably used. The lithium composite oxide of the precursor has, for example, a basic composition formula of Lia M x O 2-z Alternatively, the precursor may have the basic formula Li a P x1 Mn x2 O 2-z The coefficients a, x, and z of this basic composition formula may be suitably selected from the ranges explained for the lithium composite oxides described above. Furthermore, the element M of this basic composition formula may be suitably selected from the ranges explained for the lithium composite oxides described above.

[0018] In this step, the raw materials are preferably mixed and pulverized by a mechanochemical method. The mixed and pulverized process may be performed for a period of 2 to 40 hours. The pulverization can be performed using, for example, a ball mill or a planetary ball mill, with a planetary ball mill being particularly preferred. The rotation speed of the planetary ball mill can be appropriately set depending on the volume of the raw materials to be accommodated. For example, a range of 400 to 1000 rpm is preferred, a range of 500 to 800 rpm is more preferred, and a range of 550 to 650 rpm is even more preferred. Prior to the mixed and pulverized process, a raw material preparation process may be performed to produce an orthorhombic lithium composite oxide. In this raw material preparation process, an orthorhombic lithium composite oxide is prepared by adjusting the calcination conditions. In this raw material preparation process, orthorhombic LiMnO2 may be produced. In the raw material preparation process, orthorhombic Li2MnO3 is obtained by first calcining the raw material at a temperature of 800 to 1100°C for a period of 6 to 15 hours. Next, Li2MnO3 and MnO are mixed in equimolar amounts and fired in an inert atmosphere (e.g., in Ar) at a temperature of 900°C to 1100°C for 6 hours to 24 hours, thereby obtaining orthorhombic LiMnO2.

[0019] (Defect reduction process) In this step, the precursor is treated so that the oxygen vacancy rate of the lithium composite oxide after preparation is 5% by mass or less. The oxygen vacancy rate is determined by performing thermogravimetric differential thermal analysis (TG-DTA) on the lithium composite oxide after preparation and measuring the increase or decrease in the extreme values ​​of weight data in the range of 500°C or less when the temperature is increased in an air atmosphere. This oxygen vacancy rate is preferably as low as possible. In this defect reduction step, the precursor may be subjected to heat treatment in an oxidizing atmosphere at 250°C to 500°C. This treatment is preferable because it can reduce oxygen vacancies without incorporating other impurity elements. Furthermore, this heat treatment is more preferably performed under conditions that minimize the transition from a disordered rock salt structure to a spinel structure or that minimize crystallization. In the heat treatment, oxygen vacancies can be further reduced by oxidizing the precursor. This heat treatment is preferably performed in an oxygen atmosphere, but may also be performed in an air atmosphere. Furthermore, this heat treatment may be performed at a temperature range of 275°C or higher or 300°C or higher, or at a temperature range of 400°C or lower, 350°C or lower, or 325°C or lower. The heat treatment time may be 10 minutes or more, 20 minutes or more, or 30 minutes or more, or may be 2 hours or less, 1 hour or less, or 45 minutes or less.

[0020] Alternatively, the defect reduction step may involve an addition process in which a phosphorus- and oxygen-containing compound is mixed and ground with a precursor and the compound is introduced into the precursor. In this addition process, oxygen defects can be reduced by introducing the phosphorus- and oxygen-containing compound into the structure. In this addition process, Li3PO4 is preferably used as the compound. The amount of Li3PO4 added may be appropriately determined depending on the oxygen deficiency rate of the precursor. For example, Li3PO4 may be added to the precursor in a range of 2% by mass to 25% by mass. This amount is more preferably 2.5% by mass or more, and may be 3% by mass or more or 4% by mass or more. Furthermore, this amount may be 20% by mass or less, 10% by mass or less, or 7.5% by mass or less. In this addition process, it is more preferable to perform mixing and grinding using a mechanochemical method. Mixing and grinding can be performed, for example, using a ball mill or a planetary ball mill, with a planetary ball mill being particularly preferred. The rotation speed of the planetary ball mill may be appropriately set depending on the volume of the material to be treated, but is preferably in the range of 400 rpm to 1000 rpm, more preferably 500 rpm to 800 rpm, and even more preferably 550 rpm to 650 rpm. By performing these steps, the above-mentioned tetragonal lithium composite oxide having an oxygen deficiency rate of 5 mass% or less can be produced.

[0021] (Electricity storage device) The electricity storage device of the present disclosure includes an electrode having the above-described lithium composite oxide as an electrode active material and an ion-conductive medium in contact with the electrode and conducting lithium ions. This electricity storage device may include a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an ion-conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions. This electricity storage device may also include the above-described lithium composite oxide as the positive electrode active material. Furthermore, this electricity storage device may be a lithium secondary battery using metallic lithium or a lithium alloy as the negative electrode active material, a lithium ion secondary battery having a negative electrode active material that absorbs and releases lithium ions, or a hybrid capacitor having a negative electrode active material that adsorbs and desorbs ions.

[0022] The positive electrode may be formed by mixing a positive electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like positive electrode mixture, applying it to the surface of a current collector, drying it, and compressing it to increase electrode density as needed. The conductive material contained in the positive electrode is not particularly limited as long as it is an electronically conductive material that does not adversely affect the battery performance of the positive electrode. For example, a mixture of one or more of graphite such as natural graphite (scale graphite, flake graphite) or artificial graphite, acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.) can be used. Among these, carbon black and acetylene black are preferred as the conductive material from the viewpoints of electronic conductivity and coatability. The binder contained in the positive electrode serves to bind the active material particles and conductive material particles together. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber; thermoplastic resins such as polypropylene and polyethylene; ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR), either alone or in combination. Water-based binders such as cellulose-based binders and aqueous dispersions of styrene butadiene rubber (SBR) can also be used. Examples of solvents that can be used to disperse the positive electrode active material, conductive material, and binder include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. Alternatively, dispersants, thickeners, and the like can be added to water to form a slurry of the active material with a latex such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more kinds. Application methods include, for example, roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc. Any of these can be used to obtain a desired thickness and shape.Examples of current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymers, and conductive glass, as well as aluminum or copper whose surfaces have been treated with carbon, nickel, titanium, or silver to improve adhesion, conductivity, and oxidation resistance. These surfaces can also be subjected to oxidation treatment. Current collectors may be in the form of foils, films, sheets, nets, punched or expanded materials, laths, porous materials, foams, or fiber aggregates. The thickness of the current collector is, for example, 1 to 500 μm.

[0023] The negative electrode may be formed by closely adhering a negative electrode active material to a current collector. Alternatively, the negative electrode active material may be mixed with a binder and, if necessary, a conductive material, and an appropriate solvent added to form a paste of the negative electrode material. This paste is then applied to the surface of a current collector, dried, and, if necessary, compressed to increase electrode density. Examples of negative electrode active materials include inorganic compounds such as lithium, lithium alloys, and tin compounds; carbonaceous materials capable of absorbing and releasing lithium ions; composite oxides containing multiple elements; and conductive polymers. Examples of carbonaceous materials include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Among these, graphites such as artificial graphite and natural graphite are preferred because they have an operating potential close to that of metallic lithium, allow for charging and discharging at high operating voltages, suppress self-discharge when using a lithium salt as a supporting electrolyte, and reduce irreversible capacity during charging. Examples of composite oxides include lithium-titanium composite oxide and lithium-vanadium composite oxide. Of these, carbonaceous materials are preferred as negative electrode active materials from the standpoint of safety. The conductive materials, binders, solvents, etc. used in the negative electrode can be the same as those exemplified for the positive electrode. For the negative electrode current collector, copper, nickel, stainless steel, titanium, aluminum, baked carbon, conductive polymers, conductive glass, Al-Cd alloys, etc., as well as copper whose surface has been treated with carbon, nickel, titanium, silver, etc., can also be used to improve adhesion, conductivity, and reduction resistance. These surfaces can also be oxidized. The shape of the current collector can be the same as that of the positive electrode.

[0024] The ion-conducting medium may be a non-aqueous electrolyte solution containing a supporting salt containing lithium and a non-aqueous solvent. Examples of the solvent for the non-aqueous electrolyte solution include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, and these can be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Of these, a combination of a cyclic carbonate and a chain carbonate is preferred.

[0025] Examples of supporting salts include LiPF, LiBF, LiAsF, LiCF, SO, LiN(CF, SO), LiC(CF, SO), LiSbF, LiSiF, LiAlF, LiSCN, LiClO, LiCl, LiF, LiBr, LiI, and LiAlCl. From the viewpoint of electrical properties, it is preferable to use a combination of one or more salts selected from the group consisting of inorganic salts such as LiPF, LiBF, LiAsF, and LiClO, and organic salts such as LiCF, SO, LiN(CF, SO), and LiC(CF, SO). The concentration of this supporting salt in the nonaqueous electrolyte is preferably 0.1 mol / L to 5 mol / L, and more preferably 0.5 mol / L to 2 mol / L. When the concentration of the dissolving supporting electrolyte is 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolyte can be made more stable.

[0026] The power storage device of the present disclosure may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the power storage device, but examples thereof include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0027] The power storage device of the present disclosure preferably has an initial discharge capacity of the positive electrode greater than 300 mAh / g when charge / discharge measurements are performed at a temperature of 20°C, a potential range of 4.8 to 1.5 V relative to lithium, and a current value of 20 mA / g. This initial discharge capacity is preferably greater, for example, preferably 310 mAh / g or greater, more preferably 320 mAh / g or greater, even more preferably 330 mAh / g or greater, and most preferably 340 mAh / g or greater. This initial discharge capacity can be adjusted depending on the oxygen vacancy rate, the content x of element M, and the like. Furthermore, when similar charge / discharge measurements are performed on this power storage device, the discharge capacity at the 10th cycle is preferably 200 mAh / g or greater. This discharge capacity at the 10th cycle is preferably greater, for example, preferably 250 mAh / g or greater, more preferably 270 mAh / g or greater, and even more preferably 300 mAh / g or greater.

[0028] The shape of the electricity storage device of the present disclosure is not particularly limited, and examples thereof include coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flat, and rectangular shapes. Furthermore, a plurality of such electricity storage devices may be connected in series to form large devices for use in electric vehicles and the like. FIG. 1 is a schematic diagram showing an example of an electricity storage device 20 of this embodiment. This electricity storage device 20 includes a positive electrode sheet 23 in which a positive electrode composite 22 is formed on a current collector 21, a negative electrode sheet 26 in which a negative electrode composite 25 is formed on the surface of a current collector 24, a separator 28 provided between the positive electrode sheet 23 and the negative electrode sheet 26, and a nonaqueous electrolyte solution 29 filling the space between the positive electrode sheet 23 and the negative electrode sheet 26. This electricity storage device 20 is formed by sandwiching a separator 28 between a positive electrode sheet 23 and a negative electrode sheet 26, winding them together, and inserting them into a cylindrical case 32, and arranging a positive electrode terminal 34 connected to the positive electrode sheet 23 and a negative electrode terminal 36 connected to the negative electrode sheet 26. The positive electrode mixture 22 contains, as a positive electrode active material, the above-mentioned lithium composite oxide having an oxygen deficiency rate of 5 mass% or less.

[0029] The lithium composite oxide, its manufacturing method, and the energy storage device of the present embodiment described above can further increase the discharge capacity. The reason for this effect is presumed to be as follows. For example, unlike the layered positive electrodes that have been used so far, disordered rock-salt lithium composite oxides allow Li to migrate three-dimensionally, potentially enabling the insertion and desorption of more Li ions. On the other hand, disordered rock-salt lithium composite oxides are easily subject to oxygen defects because they are synthesized by pulverization using a planetary ball mill or the like. In the present disclosure, it is presumed that reducing oxygen defects in a cubic-based disordered rock-salt lithium composite oxide makes the electronic state of adjacent Mn ideal, allowing for sufficient oxidation and reduction, thereby further improving the discharge capacity. Furthermore, in the method for manufacturing a lithium composite oxide using an addition treatment, a disordered rock-salt lithium composite oxide is prepared by co-milling, and then a compound containing phosphorus and oxygen is added and further co-milled. This allows for a sufficient reduction in oxygen defects compared to preparing a disordered rock-salt lithium composite oxide in a state where a compound containing phosphorus and oxygen is added.

[0030] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure.

[0031] For example, in the defect reduction step of the above-described manufacturing method, the raw material may further include a compound containing a halogen element, element A. Examples of the A source include LiF, LiCl, LiBr, LiI, and LiS, and among these, LiF is preferred.

[0032] The present disclosure may be any of the following [1] to

[10] . [1] A lithium composite oxide used as an electrode active material for an electricity storage device, containing lithium and element M and having a cubic crystal structure; The oxygen vacancy rate determined from the extreme increase or decrease in weight data in the range of 500°C or less when heated in an air atmosphere by thermogravimetric differential thermal analysis (TG-DTA) is 5% by mass or less. Lithium composite oxide. [2] The lithium composite oxide according to [1], wherein the oxygen defect rate is 1% by mass or less. [3] The basic composition formula is Li a M x O2 (where 0 < a ≤ 2, 0 < x ≤ 2, and M is one or more selected from the group consisting of B, C, N, P, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge), the lithium composite oxide according to [1] or [2]. [4] The lithium composite oxide according to [3], wherein 0.8 ≤ x ≤ 1.0. [5] The lithium composite oxide according to any one of [1] to [4], wherein the element M contains at least Mn and may further contain P. [6] The lithium composite oxide according to any one of [1] to [4], wherein the element M contains at least Mn and does not contain P. [7] An electrode having the lithium composite oxide according to any one of [1] to [6] as an electrode active material, An ion conduction medium that contacts the electrode and conducts lithium ions, A power storage device comprising the above. [8] A method for producing a lithium composite oxide used as an electrode active material of a power storage device, A precursor step of obtaining a precursor containing lithium and element M and showing a disordered rock salt structure by mixing and pulverizing a rhombohedral lithium composite oxide raw material, A defect reduction step of treating the precursor so that the oxygen defect rate obtained from the increase and decrease of the extreme value of the weight data in the range of 500 °C or less when performing thermogravimetric differential thermal analysis (TG-DTA) on the produced lithium composite oxide and heating in an air atmosphere is 5% by mass or less, A method for producing a lithium composite oxide comprising the above. [9] The method for producing a lithium composite oxide according to [8], wherein in the defect reduction step, the precursor is heat-treated by heating at 250 °C or higher and 500 °C or lower in an oxidizing atmosphere.

[10] The method for producing a lithium composite oxide according to [8], wherein the defect reduction step includes mixing and grinding a compound containing phosphorus and oxygen with the precursor, and introducing the compound into the precursor. [Example]

[0033] Specific examples of fabricating the lithium composite oxide and electricity storage device of the present disclosure will be described below as experimental examples, with Experimental Examples 1 to 3 and 5 corresponding to working examples, and Experimental Example 4 corresponding to a comparative example.

[0034] [Synthesis of positive electrode active material] (Experimental Examples 1 and 4) Orthorhombic LiMnO2 was placed in a 500 mL zirconia pot together with 5 mm diameter zirconia balls under an Ar atmosphere and ground in a planetary ball mill at 560 rpm for 28 hours to obtain LiMnO2, a composite oxide with a disordered rock-salt phase (Experimental Example 4). The obtained disordered rock-salt LiMnO2 was calcined in a tubular calcination furnace under an air atmosphere at 300 °C for 30 minutes to obtain tetragonal LiMnO2, which was designated Experimental Example 1.

[0035] (Experimental Examples 2, 3, and 5) The disordered rock-salt-type LiMnO2 of Experimental Example 4 obtained as raw materials and Li3PO4 were mixed in an agate bowl so that the Li3PO4 concentrations were 2.5 mass%, 5 mass%, and 20 mass%, respectively. The mixture was then sealed in a zirconia pot together with zirconia balls and ground in a planetary ball mill at 600 rpm for 36 hours to obtain composite oxides having a disordered rock-salt-type phase, which were designated Experimental Examples 5, 2, and 3, respectively.

[0036] [X-ray diffraction (XRD)] The obtained positive electrode active material was flatly formed on a glass sample plate and measured under atmospheric pressure using an X-ray diffractometer (Rigaku, Ultima IV). Figure 2 shows the XRD patterns for Experimental Examples 1 to 5. The phase classifications determined by XRD are summarized in Table 1.

[0037] [Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)] The compositions of the composite oxides in Experimental Examples 1 to 5 were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, PS3520UVDDIIII manufactured by Hitachi High-Tech Science). The components of the composite oxides in Experimental Examples 1 to 5 were analyzed by ICP, and the composition formulas without considering oxygen defects were determined.

[0038] [Calorimetric differential thermal analysis (TG-DTA)] TG-DTA measurements were performed using a Rigaku 8320 series analyzer to quantify the oxygen vacancy rate of the positive electrode active material. A reference sample was prepared by placing 13 mg of Al2O3 on a platinum pan. 13 mg of the measurement sample was weighed onto the platinum pan and heated to 1000 °C in an air stream at a scanning rate of 10 °C / min. Based on the weight change profile obtained from this TG-DTA measurement, the minimum value of the weight data in the range from 100 °C to 300 °C after desorption of adsorbed water was defined as the reference value, and the maximum value in the range from 300 °C to 500 °C was defined as the mass of the fully oxidized composite oxide. The oxygen vacancy rate was calculated by calculating the difference between the reference value and the weight data. Figure 3 shows the TG-DTA measurement results for Experimental Example 4. Figure 4 shows the TG-DTA measurement results for Experimental Example 1.

[0039] [Half-cell fabrication] A dry composite was prepared by mixing 70% by mass of any of the composite oxides of Experimental Examples 1 to 5, 25% by mass of Ketjen Black (ECP-600, manufactured by Mitsubishi Chemical), and 5% by mass of polytetrafluoroethylene (F-104, manufactured by Daikin Industries, Ltd.) and pulverizing them in a mixer. The obtained dry composite was formed into pellets with a diameter of 10 mm, which were placed on an aluminum expand metal and pressed to form a positive electrode. The weight of the dry composite was 12 to 18 mg / cm. 2 The concentration was adjusted to be within the range of 1. A half-cell was fabricated by placing metallic Li as the negative electrode, a separator (Tonen Chemical Co., Ltd., E20MMS), and an electrolyte between the two electrodes. The electrolyte was prepared by dissolving LiPF6 at a concentration of 1M in a mixed solvent containing 30% by volume of ethylene carbonate (EC), 40% by volume of dimethyl carbonate (DMC), and 30% by volume of ethyl methyl carbonate (EMC).

[0040] [Charge / discharge test] Using the obtained half cell, 4.8 to 1.5V vs. Li + Ten cycles of constant-current charge / discharge measurements were performed at a current value of 20 mA / g in the potential range of 0.1% / Li. The measurements were performed in a temperature environment of 20°C. Figure 5 shows the initial discharge curves for Experimental Examples 1 to 5, and Figures 5A to 5E show the discharge curves for Experimental Examples 1 to 5, respectively. The current value and capacity were expressed as values ​​per weight of the positive electrode active material.

[0041] [Results and Discussion] The composite oxides of Experimental Examples 1 to 5 (Li a M x Table 1 summarizes the M element species and variable x of O2), the composition formula calculated by ICP-OES without considering the oxygen vacancy rate, the phase classification by XRD, the crystallite size (nm), the oxygen vacancy rate (mass%) obtained by TG-DTA, and the initial discharge capacity (mAh / g). In the XRD measurements shown in Figure 2, Example 1 exhibited a pattern derived from a cubic spinel structure, while all other materials exhibited XRD patterns derived from a disordered cubic rock-salt structure. All of the synthesized composite oxides possessed a cubic structure, suggesting the presence of oxygen and oxygen vacancy sites at the anion sites present in the lattice. The ratio of oxygen vacancy sites to the total anion sites (oxygen vacancy rate) was estimated using differential thermal analysis (TG-DTA) measurements shown in Figures 3 and 4. For the untreated Example 4, weight changes due to the desorption of adsorbed water below 200°C, oxidation below 500°C, and thermal decomposition below 1000°C were observed. The weight increase from the time of desorption of the adsorbed material to the end of the oxidation reaction is considered to be the introduction of oxygen (oxidation) into oxygen vacancies present in the composite oxide, and the oxygen vacancy rate was calculated from this amount.

[0042] The composition of the positive electrode determined by ICP-OES analysis was Li a M x O2, the composition taking into account the oxygen vacancy rate is Li a αM xThe oxygen vacancy rate was calculated using the following formula (3): From the TG-DTA results shown in Figure 3, the oxygen vacancy rate for Experimental Example 4 was estimated to be 9.1% by mass. On the other hand, the oxygen vacancy rate for Experimental Example 1 shown in Figure 4 was 0.1% by mass. Experimental Example 1 was a powder of the cathode material from Experimental Example 4 calcined at 300°C in an air stream. Therefore, it is believed that the oxygen vacancies disappeared due to oxidation. Similarly, the oxygen vacancy rates of Experimental Examples 2, 3, and 5 were examined and found to be 0.6 mass%, 0.3 mass%, and 3.9 mass%, respectively. These results demonstrate that the oxygen vacancy rate can be further reduced in composite oxides exhibiting a cubic crystal structure by heat treatment and the addition of phosphoric acid. Li a αM x αO2α+(1-α)O2→ Li a αM x αO2…Equation (1) α=(1-Waf / Wbef)×M / 2×MO+1…Equation (2) Oxygen defect rate (mass%)=(1-α)×100...Equation (3)

[0043] As shown in Figure 5, in the results of a constant-current charge-discharge test using a half cell, the initial discharge capacity of Experimental Example 4, in which the oxygen vacancy rate was 9.1% by mass, was 227 mAh / g. On the other hand, the initial discharge capacity of Experimental Example 5, in which the oxygen vacancy rate was 3.9% by mass, was 310 mAh / g, demonstrating that composite oxides with reduced oxygen vacancy rates can further improve discharge capacity. Furthermore, Experimental Examples 1 to 3, in which the oxygen vacancy rate was less than 1% by mass, exhibited high discharge capacities of 322 mAh / g, 346 mAh / g, and 323 mAh / g, respectively.

[0044] Figure 6 is a schematic diagram showing the estimated crystal structure of the composite oxide of Experimental Example 4. Figure 7 is a schematic diagram of a method for filling oxygen vacancies, showing the estimated crystal structure after heat treatment (Experimental Example 1) and the estimated crystal structure after phosphoric acid addition treatment (Experimental Example 2). Experimental Example 4 has a structure attributed to a cubic crystal, with Li, Mn, and vacant cation sites randomly arranged at the cation sites, and oxygen and oxygen vacant sites randomly present at the anion sites. In Experimental Example 4, the oxygen vacancy rate was estimated to be 9.1 mass% based on TG-DTA analysis. It was presumed that the presence of many oxygen vacancies changes the electronic state of adjacent Mn from the ideal state, preventing sufficient oxidation and reduction, resulting in a decrease in the available charge / discharge capacity. In particular, it was presumed that the presence of oxygen vacancies is likely to be a factor in the insufficient capacity of composite oxides with a disordered rock-salt structure.

[0045] On the other hand, as shown in Figure 7, in the composite oxide of Experimental Example 1, in which the oxygen vacancy rate was reduced to 0.1% by heat treatment, most of the Mn was able to form bonds with O, and it was presumed that high capacity could be achieved by the deintercalation and insertion of Li associated with the oxidation-reduction of Mn and O. Furthermore, Experimental Examples 2 and 3 were materials in which Li3PO4 was introduced into the composite oxide of Experimental Example 4, and it was presumed that the oxygen vacancy rate was reduced by the scheme shown in Figure 7. As in Experimental Examples 2 and 3, it was presumed that the oxygen vacancy rate could be reduced by introducing phosphoric acid (Li3PO4) into the lattice in addition to the heat treatment under air flow as in Experimental Example 1. Note that under the conditions of Experimental Example 5, the amount of Li3PO4 introduced was small, so the reduction rate of oxygen vacancies was small at 3.9%, but the discharge capacity improved to 310 mAh / g.

[0046] [Table 1]

[0047] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure. [Industrial Applicability]

[0048] The lithium composite oxide, the electricity storage device, and the method for producing the lithium composite oxide disclosed in this specification can be used in the technical field of secondary batteries. [Explanation of symbols]

[0049] 20 Energy storage device, 21 Current collector, 22 Positive electrode composite, 23 Positive electrode sheet, 24 Current collector, 25 Negative electrode composite, 26 Negative electrode sheet, 28 Separator, 29 Non-aqueous electrolyte, 32 Cylindrical case, 34 Positive electrode terminal, 36 Negative electrode terminal.

Claims

1. A lithium composite oxide used as an electrode active material for an electricity storage device, The present invention relates to a crystalline structure of a cubic crystal, which contains lithium and an element M containing at least Mn and P, and which exhibits an XRD pattern with a peak pattern having diffraction peaks in at least four of the following ranges of 2θ: 35° or more and 39° or less, 42° or more and 48° or less, 62° or more and 68° or less, and 80° or more and 84° or less; the oxygen deficiency rate is 1% by mass or less, as determined by thermogravimetric differential thermal analysis (TG-DTA) from the extreme increase or decrease in weight data in a temperature range of 500°C or less in an air atmosphere; A lithium composite oxide having a basic composition formula of Li a M x O 2-z (wherein 0<a<2, 0.8≦x≦1.0, 0≦z≦0.15, and M may contain, in addition to Mn and P, one or more elements selected from the group consisting of B, C, N, Al, Si, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, and Ge).

2. An electrode having the lithium composite oxide according to claim 1 as an electrode active material; an ionically conductive medium in contact with the electrode and conducting lithium ions; An electricity storage device comprising:

3. A method for producing a lithium composite oxide used as an electrode active material for an electricity storage device, comprising: a precursor step of obtaining a precursor containing lithium and an element M containing at least Mn and exhibiting a disordered rock salt structure by mixing and grinding an orthorhombic lithium composite oxide raw material; a defect reduction step of treating the precursor so that the oxygen vacancy rate, which is determined from an increase or decrease in extreme values ​​of weight data in a temperature range of 500°C or less when the produced lithium composite oxide is subjected to thermogravimetric differential thermal analysis (TG-DTA) in an air atmosphere, is 5% by mass or less; A method for producing a lithium composite oxide comprising:

4. The method for producing a lithium composite oxide according to claim 3 , wherein the defect reduction step comprises a heat treatment in which the precursor is heated at 250° C. or higher and 500° C. or lower in an oxidizing atmosphere.

5. 4. The method for producing a lithium composite oxide according to claim 3, wherein the defect reduction step includes an addition process in which a compound containing phosphorus and oxygen is mixed and pulverized with the precursor, and the compound is introduced into the precursor.

Images