Positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

A positive electrode active material for a non-aqueous electrolyte secondary battery, which has a crystal structure belonging to space group R-3m and is represented by the compositional formula Li [alpha] Na [beta] Ni [1-b-c] Mn [b] X [c] O [2-d] (wherein X represents at least one element selected from the group consisting of transition metal elements other than Li, Na, Ni, and Mn and typical elements, 0.80 lt; alpha is less than or equal to 1.20, 0lt; beta is less than or equal to 0.20, 0.80 lt; alpha + beta is less than or equal to 1.20, 0lt; 1-b-c < = 1, 0 < = blt; 1, 0 < = clt; 1, 0lt; and d < = 0.2.

Description

Technical Field

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode active material. Background Art

[0002] In non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, the positive electrode active material significantly influences battery performance, including input / output characteristics, capacity, and durability. Consequently, extensive research has been conducted on positive electrode active materials. Lithium-transition metal composite oxides containing transition metal elements such as Ni and Mn are commonly used as positive electrode active materials. The type and amount of elements contained in lithium-transition metal composite oxides, as well as the crystal structure of the composite oxide, significantly influence battery performance, and even slight changes in their physical properties can prevent the target performance from being achieved.

[0003] For example, Patent Documents 1 to 3 disclose introducing a predetermined amount of oxygen vacancies into a lithium transition metal composite oxide having a specific composition and a layered structure for the purpose of improving battery performance such as increasing capacity.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-063903

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2002-060223

[0008] Patent Document 3: Japanese Patent Application Laid-Open No. 2003-051311 Summary of the Invention

[0009] In recent years, nonaqueous electrolyte secondary batteries such as lithium-ion secondary batteries have been used as power sources for driving vehicles, and further higher capacity is required. The positive electrode active materials of Patent Documents 1 to 3 still have much room for improvement from the perspective of higher capacity.

[0010] The positive electrode active material for a non-aqueous electrolyte secondary battery disclosed herein is characterized in that it is a positive electrode active material for a non-aqueous electrolyte secondary battery having a crystal structure belonging to the space group R-3m, and having a composition formula Li α Na β Ni 1-b-c Mn b X c O 2-dRepresented, wherein X is at least one selected from transition metal elements or typical elements (meaning elements of Group 1, Group 2, and Group 13 to Group 18 of the periodic table) other than Li, Na, Ni, and Mn, 0.80<α≤1.20, 0<β≤0.20, 0.80<α+β≤1.20, 0<1-bc≤1, 0≤b<1, 0≤c<1, and 0<d≤0.2.

[0011] The nonaqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode includes the positive electrode active material described above.

[0012] According to the positive electrode active material according to the present disclosure, it is possible to increase the capacity of a non-aqueous electrolyte secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a longitudinal cross-sectional view of a nonaqueous electrolyte secondary battery as an example of an embodiment. DETAILED DESCRIPTION

[0014] The present inventors conducted intensive research to increase the capacity of non-aqueous electrolyte secondary batteries and discovered that when a positive electrode active material having a crystal structure belonging to the space group R-3m contains a specified amount of Na and introduces a specified amount of oxygen vacancies, the battery's charge and discharge capacity is significantly increased. Furthermore, even when a specified amount of oxygen vacancies is present in the positive electrode active material, if Na is absent, or even when a specified amount of Na is present but oxygen vacancies are absent, no capacity improvement is achieved.

[0015] It is believed that if oxygen defects are introduced into the layered rock salt structure belonging to the space group R-3m, the electronic conductivity of the positive electrode active material is improved. On the other hand, the crystal structure becomes unstable due to the introduction of oxygen defects. It is believed that it is a specified amount of Na that suppresses the destabilization of the structure. That is, by introducing oxygen defects into a composite oxide with a layered rock salt structure containing a specified amount of Na, for example, a positive electrode active material with high electronic conductivity and excellent structural stability can be obtained. According to the positive electrode active material involved in the present disclosure, the charge and discharge capacity of the non-aqueous electrolyte secondary battery can be greatly improved.

[0016] Hereinafter, an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode active material according to the present disclosure will be described in detail with reference to the accompanying drawings. Furthermore, technical configurations formed by selectively combining the various components of the various embodiments and variations described below are also included in the scope of the present disclosure.

[0017] In the embodiment described below, a non-aqueous electrolyte secondary battery 10 is exemplified as a cylindrical battery in which a wound electrode body 14 is housed in an outer can 16 having a bottomed cylindrical shape. However, the outer can of the battery is not limited to a cylindrical outer can. Other embodiments of the non-aqueous electrolyte secondary battery disclosed herein include, for example, a square battery having a square outer can, a coin-shaped battery having a coin-shaped outer can, and a pouch-type battery having an outer can composed of a laminate sheet including a metal layer and a resin layer. In addition, the electrode body is not limited to a wound type, and may also be a stacked type electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with a separator.

[0018] Figure 1 FIG is a cross-sectional view of a non-aqueous electrolyte secondary battery 10 as an example of an embodiment. Figure 1 As shown, the non-aqueous electrolyte secondary battery 10 includes a wound electrode body 14, a non-aqueous electrolyte, and an outer can 16 for storing the electrode body 14 and the non-aqueous electrolyte. The non-aqueous electrolyte secondary battery 10 is, for example, a lithium-ion secondary battery. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a winding structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed therebetween. The outer can 16 is a metal container with a bottomed cylindrical shape that is open at one axial end, and the opening of the outer can 16 is blocked by a sealing body 17. In the following, for ease of explanation, the sealing body 17 side of the battery is set as the top, and the bottom side of the outer can 16 is set as the bottom.

[0019] The non-aqueous electrolyte has lithium ion conductivity and may be a liquid electrolyte (electrolyte) or a solid electrolyte.

[0020] The liquid electrolyte (electrolyte) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Non-aqueous solvents include, for example, esters, ethers, nitriles, amides, and mixed solvents of two or more thereof. As an example of a non-aqueous solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents thereof may be cited. The non-aqueous solvent may also contain a halogen substitute (e.g., fluoroethylene carbonate) obtained by replacing at least a portion of the hydrogen of these solvents with a halogen atom such as fluorine. Electrolyte salts include, for example, lithium salts such as LiPF6. Furthermore, the electrolyte is not limited to a liquid electrolyte and may also be a solid electrolyte using a gel-like polymer.

[0021] As a solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As an inorganic solid electrolyte, materials known in all-solid lithium ion secondary batteries, etc. (such as oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte, for example, comprises a lithium salt and a matrix polymer or comprises a non-aqueous solvent, a lithium salt, and a matrix polymer. As a matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. As polymer materials, fluororesins, acrylic resins, polyether resins, etc. can be listed.

[0022] The positive electrode 11, the negative electrode 12, and the separator 13 constituting the electrode body 14 are all strip-shaped elongated bodies, which are alternately stacked in the radial direction of the electrode body 14 by being wound into a spiral. In order to prevent the precipitation of lithium, the negative electrode 12 is formed to be one circle larger than the positive electrode 11. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction (longitudinal direction) and the width direction (transverse direction). The separator 13 is formed to be at least one circle larger than the positive electrode 11, for example, two pieces are arranged in a manner of sandwiching the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0023] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14. Figure 1 In the example shown, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 toward the sealing body 17, while the negative electrode lead 21 extends through the outside of the insulating plate 19 toward the bottom of the outer can 16. The positive electrode lead 20 is connected to the lower surface of an internal terminal plate 23 of the sealing body 17 by welding or the like. The top plate of the sealing body 17, or cap 27, which is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, with the outer can 16 serving as the negative electrode terminal.

[0024] A gasket 28 is provided between the outer can 16 and the sealing member 17 to ensure the airtightness of the battery interior. The outer can 16 has a groove 22 formed on its side, with a portion of the side protruding inward to support the sealing member 17. The groove 22 is preferably formed annularly along the circumference of the outer can 16, and its upper surface supports the sealing member 17. The sealing member 17 is secured to the upper portion of the outer can 16 by the groove 22 and the open end of the outer can 16 riveted to the sealing member 17.

[0025] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26 and a cover 27 are stacked in order from the electrode body 14 side. The components constituting the sealing body 17 have, for example, a disc shape or a ring shape, and the components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and the insulating member 25 is interposed between their respective peripheral portions. If the internal pressure of the battery rises due to abnormal heat, the lower valve body 24 is deformed and breaks in a manner that pushes the upper valve body 26 toward the cover 27 side, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and the gas is discharged from the opening of the cover 27.

[0026] Hereinafter, the positive electrode 11 , the negative electrode 12 , and the separator 13 constituting the electrode assembly 14 , particularly the positive electrode 11 , will be described in detail.

[0027] [positive electrode]

[0028] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer arranged on the positive electrode core. The positive electrode core can be made of a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, aluminum alloy, stainless steel, titanium, etc., and a film of the metal is arranged on the surface. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core. The positive electrode 11 can be manufactured, for example, by coating a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder on the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0029] As the conductive agent contained in the positive electrode mixture layer, carbon black such as acetylene black and Ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powders, conductive whiskers, etc. can be exemplified. One type of conductive agent can be used alone, or multiple types can be used in combination. The content of the conductive agent is not particularly limited, and is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0030] As the binder contained in the positive electrode mixture layer, fluororesins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, ethylene-propylene-butadiene copolymer, polyacrylonitrile (PAN), polyimide, polyamide, acrylic resins such as ethylene-acrylic acid copolymer, etc. can be exemplified. In addition, these resins can also be used in combination with carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), etc. The binder can be used alone or in combination with multiple types. The content of the binder is not particularly limited, and is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0031] The positive electrode active material has a crystal structure belonging to the space group R-3m and has a composition formula of Li α Na β Ni 1-b- c Mn b X c O 2-d A lithium sodium transition metal composite oxide represented by . In this composition formula, X is at least one selected from transition metal elements and typical elements other than Li, Na, Ni, and Mn, 0.80<α≤1.20, 0<β≤0.20, 0.80<α+β≤1.20, 0<1-bc≤1, 0≤b<1, 0≤c<1, 0<d≤0.2. The composite oxide constituting the positive electrode active material contains Li, Na, and Ni as essential elements, and preferably also contains Mn. Furthermore, the contents of Li, Na, Ni, Mn, and X contained in the positive electrode active material can be measured using an ICP emission spectrometer (e.g., CIROS-120 manufactured by SPECTRO).

[0032] The positive electrode active material is characterized in that it has a layered rock salt structure belonging to the space group R-3m, contains a specified amount of Na, and d in the above composition formula is greater than 0, that is, the molar ratio of O (2-d) is less than 2.0. In the above composition formula, when the molar ratio of O (2-d) is less than 2.0, it means that the oxygen in the layered rock salt structure is deficient, and the value of d becomes the oxygen deficiency. By the presence of a specified amount of Na and oxygen deficiency, the charge and discharge capacity can be greatly improved. Furthermore, the oxygen amount of the positive electrode active material can be measured using an oxygen and nitrogen analyzer (for example, EMGA-920 manufactured by Horiba).

[0033] In the composition formula Li α Na β Ni 1-b-c Mn b X c O d In the embodiment, the molar ratio (β) of Na only needs to be greater than 0 and less than 0.20 (0<β≤0.20), but is preferably less than 0.15, and more preferably less than 0.05. When the molar ratio (β) of Na is 0<β≤0.15 or 0<β≤0.05, it is believed that the layered structure of the composite oxide is stable and the improvement effect of the charge and discharge capacity becomes more significant. Furthermore, if the molar ratio (β) of Na exceeds 0.20, Na ions may be extracted at the positive electrode during charging, and the extracted Na ions may be absorbed at the negative electrode. As a result, the reaction with the non-aqueous electrolyte during charging and discharging generates by-products, which will become the main reason for the reduction in the charge and discharge capacity and charge and discharge efficiency of the battery.

[0034] If a small amount of Na is contained in the positive electrode active material, the layered structure is stabilized, which contributes to a high capacity battery. However, the molar ratio (β) of Na is preferably 0.001 or more, more preferably 0.002 or more. An example of a suitable range of the molar ratio (β) of Na is 0.001 ≤ β ≤ 0.15, or 0.002 ≤ β ≤ 0.05.

[0035] In the composition formula Li α Na β Ni 1-b-c Mn b X c O d In the embodiment of the present invention, the total molar ratio (α+β) of Li and Na is greater than 0.80 and less than 1.20 (0.80<α+β≤1.20), but is preferably less than 1.05, more preferably less than 1.00, and may be less than 0.96. In addition, the total molar ratio (α+β) of Li and Na is preferably greater than 0.90, more preferably greater than 0.91, and particularly preferably greater than 0.92. An example of a suitable range of the total molar ratio (α+β) of Li and Na is 0.90≤α+β≤1.05, 0.91≤α+β≤1.00, or 0.92≤α+β≤0.96. In this case, the improvement effect of the charge and discharge capacity becomes more significant.

[0036] In the composition formula Li α Na β Ni 1-b-c Mn b X c O d In the embodiment of the present invention, the molar ratio of Ni (1-bc) can be 1 or less (0 < 1-bc ≤ 1), but is preferably 0.85 or less, and more preferably 0.75 or less. In addition, the molar ratio of Ni (1-bc) is preferably 0.30 or more, and more preferably 0.40 or more. An example of a suitable range of the molar ratio of Ni (1-bc) is 0.35 ≤ 1-bc ≤ 0.85, or 0.40 ≤ 1-bc ≤ 0.75. In this case, the improvement effect of the charge and discharge capacity becomes more significant.

[0037] As mentioned above, the positive electrode active material preferably contains Mn. α Na β Ni 1-b-c Mn b X c O dIn the embodiment of the present invention, the molar ratio (b) of Mn is preferably 0.75 or less, more preferably 0.65 or less. In addition, the molar ratio (b) of Mn is preferably 0.20 or more, more preferably 0.25 or more. An example of a suitable range of the molar ratio (b) of Mn is 0.25 ≤ b ≤ 0.65. In this case, the improvement effect of the charge and discharge capacity becomes more significant.

[0038] In the composition formula Li α Na β Ni 1-b-c Mn b X c O d In the embodiment, X can be at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Sn, Ti, Si, V, Cr, Fe, Cu, Zn, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Ta, Mo, W, Co, and Al. When a small amount of X is added, the improvement in charge and discharge capacity becomes more significant. The molar ratio (c) of X is preferably 0.05 or less (0 ≤ c ≤ 0.05), and more preferably 0.03 or less (0 ≤ c ≤ 0.03).

[0039] In the composition formula Li α Na β Ni 1-b-c Mn b X c O d In , X is preferably at least one selected from Al, Co, and Zr, and particularly preferably Al or Co. For example, when the positive electrode active material contains Al, an example of a suitable range of the molar ratio (c) of Al is 0.005≤c≤0.020.

[0040] The positive electrode active material has a composite oxide (hereinafter referred to as "Li-Na composite oxide") having a composition represented by the above composition formula and having oxygen defects in the crystal structure as the main component. Here, the so-called main component means the component with the highest mass ratio among the constituent components of the positive electrode active material. In the mixture layer of the positive electrode 11, a composite oxide other than the Li-Na composite oxide can be used as the positive electrode active material, but the content of the Li-Na composite oxide is preferably 50% by mass or more, and can also be substantially 100% by mass.

[0041] Li-Na composite oxide is, for example, a secondary particle formed by the aggregation of multiple primary particles. An example of the volume-based median particle size (D50) of the Li-Na composite oxide is 1 μm or more and 30 μm or more and 20 μm or less. The D50 of the composite oxide is the particle size when the volume cumulative value becomes 50% in the particle size distribution measured by the laser diffraction scattering method. The BET specific surface area of ​​the Li-Na composite oxide is, for example, 0.1 m 210 m or more and 10 m or less 2 0.5 m or less, or 0.5 m 2 5 m or more and 5 m or less 2 The BET specific surface area of the composite oxide was measured according to the BET method (nitrogen adsorption method) described in JIS R 1626. If D50 and the BET specific surface area are within this range, high capacity can be easily achieved.

[0042] The Li-Na composite oxide, as described above, is a composite oxide with oxygen defects in the layered rock salt structure belonging to the space group R-3m. The Li-Na composite oxide is represented by the composition formula Li α Na β Ni 1-b-c Mn b X c O 2-d where 0 < d ≤ 0.2. It is considered that if oxygen defects are introduced into the layered rock salt structure, the electronic conductivity of the composite oxide increases. On the other hand, the crystal structure becomes unstable. However, by adding a specified amount of Na, the destabilization of the structure is suppressed, and a positive electrode active material with high capacity can be obtained. Furthermore, as a method for introducing oxygen defects, for example, long-term firing during the synthesis of the positive electrode active material can be cited, but if the conventional general synthesis method is used, a positive electrode active material with high capacity cannot be obtained.

[0043] The Li-Na composite oxide only needs to contain oxygen defects in its crystal structure, but in the composition formula Li α Na β Ni 1-b- c Mn b X c O 2-d the molar ratio (2 - d) of O is preferably 1.98 or less, more preferably 1.97 or less, and particularly preferably 1.96 or less. That is, the amount of oxygen defects (d) is preferably 0.02 or more, more preferably 0.03 or more, and particularly preferably 0.04 or more. The lower limit of the molar ratio (2 - d) of O is 1.80, but preferably 1.81, more preferably 1.83, and particularly preferably 1.85. That is, the amount of oxygen defects (d) is preferably 0.19 or less, more preferably 0.17 or less, and particularly preferably 0.15 or less. Furthermore, if the molar ratio (2 - d) of O is less than 1.80, the crystal structure is greatly destabilized, and stability cannot be ensured even by adding Na.

[0044] In the composition formula Li α Na β Ni 1-b-c Mn b X c O 2-dIn the embodiment, the preferred range of the molar ratio of O (2-d) varies slightly depending on the type and amount of the added element X, but is 1.81 ≤ 2-d ≤ 1.98, 1.83 ≤ 2-d ≤ 1.97, or 1.85 ≤ 2-d ≤ 1.96. Specifically, the preferred range of the oxygen deficiency (d) is 0.02 ≤ d ≤ 0.19, 0.03 ≤ d ≤ 0.17, or 0.04 ≤ d ≤ 0.15. In this case, the improvement in charge and discharge capacity becomes more significant.

[0045] Li-Na composite oxide is produced by the following steps: (1) a step of mixing and calcining a sodium raw material and a nickel raw material to synthesize a sodium composite oxide; and (2) a step of reacting the sodium composite oxide with a lithium compound to exchange a portion of the Na in the sodium composite oxide with Li. In step (1), it is preferred to also add a manganese raw material, and a raw material containing element X may also be added. In the synthesis step of the Li-Na composite oxide, the synthesis conditions are controlled in such a way that a predetermined amount of oxygen deficiency is introduced, and in step (2), control is performed so that a predetermined amount of Na remains. In controlling the amount of oxygen deficiency, for example, the calcination conditions of step (1) are important. Furthermore, the exchange conditions of step (2) may also be used to control the oxygen deficiency.

[0046] The sodium raw material is at least one selected from metallic sodium and sodium compounds. The sodium compound is not particularly limited as long as it contains Na. Examples thereof include acetates such as CH3COONa and CH3COONa·3H2O, nitrates such as NaNO3, sulfates such as Na2SO4, carbonates such as Na2CO3, bicarbonates such as NaHCO3, hydroxides such as NaOH, and oxides such as Na2O and Na2O2. Among these, Na2CO3, NaHCO3, NaOH, and NaNO3 are preferred.

[0047] The nickel raw material is at least one selected from metallic nickel and nickel compounds. The nickel compound is not particularly limited as long as it contains nickel, and examples thereof include oxides such as NiO, hydroxides such as NiOH, Ni(OH)2, and NiOOH, nitrates such as NiNO3, carbonates such as NiCO3 and Ni4CO3(OH)6(H2O)4, and sulfates such as NiSO4. Among these, Ni(OH)2 is preferred.

[0048] The manganese raw material is at least one selected from metallic manganese and manganese compounds. The manganese compound is not particularly limited as long as it contains Mn. Examples thereof include oxides such as MnO, Mn2O3, Mn3O4, and MnO2; hydroxides such as Mn(OH)2 and MnOOH; carbonates such as MnCO3; nitrates such as Mn(NO3)2; and sulfates such as MnSO4. Among these, Mn(OH)2 is preferred.

[0049] The raw material containing element X is at least one selected from element X and a compound of element X. The compound containing element X is not particularly limited as long as it contains X, and examples thereof include oxides, hydroxides, carbonates, nitrates, sulfates, and the like. Furthermore, as the raw material for the sodium composite oxide, a compound containing Ni and Mn, a compound containing Ni and X, a compound containing Mn and X, or a compound containing Ni, Mn, and X can be used.

[0050] The mixing ratio of the raw materials of the sodium composite oxide is not particularly limited as long as it is appropriately set. For example, when the molar ratio of Na in the mixture of the raw materials of the sodium composite oxide is represented by a, the molar ratio of Ni is represented by 1-bc, the molar ratio of Mn is represented by b, and the molar ratio of the element X is represented by c, it is preferably set so that 0.95≤a≤1.05, 0.25<b≤0.65, 0≤c≤0.05, and 0.4≤1-bc<0.75. In addition, the mixing method of the raw materials is not particularly limited as long as it can uniformly mix the raw materials. Mixing using a known mixer such as a stirring mixer can be exemplified.

[0051] The mixture of the above raw materials is fired in the atmosphere or in an oxygen flow using a firing furnace. The firing temperature is preferably 700°C or higher and 900°C or lower, more preferably 750°C or higher and 850°C or lower. The heating rate is preferably slow, for example, 0.3°C / minute or higher and 5.0°C / minute or lower, or 0.5°C / minute or higher and 3.0°C / minute or lower. The firing time is preferably 20 hours or more when the firing temperature is 750°C or higher and 850°C or lower. Here, the so-called firing time means the time from when the temperature of the firing furnace reaches the firing temperature until the firing is completed and cooling begins. The fired product is rapidly cooled in the atmosphere, for example, by being taken out of the firing furnace.

[0052] By extending the firing time in step (1), oxygen vacancies can be introduced into the finally obtained Li-Na composite oxide. As described above, the oxygen vacancies ((d) in the above composition formula) are preferably 0.04 to 0.15. However, to control the oxygen vacancies within this range, the firing time is preferably set to 10 to 50 hours, or 20 to 30 hours, at a firing temperature of 750°C to 850°C. The sodium composite oxide can be obtained, for example, by rapidly cooling the fired product in the atmosphere and then pulverizing it using a known method.

[0053] In step (2), a portion of the Na in the sodium composite oxide is exchanged for Li. That is, the Li exchange needs to be performed so that a predetermined amount of Na remains. A suitable method for exchanging Na for Li is to add a molten salt bed of a lithium salt to the sodium composite oxide and heat it. The lithium salt is, for example, at least one selected from lithium nitrate, lithium sulfate, lithium chloride, lithium carbonate, lithium hydroxide, lithium iodide, and lithium bromide.

[0054] The heating temperature in the Li exchange process is preferably 200°C or more and 400°C or less, more preferably 250°C or more and 350°C or less. If the heating temperature exceeds 400°C, the reaction proceeds rapidly, and an uneven reaction may occur. On the other hand, if the heating temperature is less than 200°C, the reaction does not proceed fully, and Na tends to remain excessively. The heating treatment time is set to 3 hours or more and 10 hours or less, for example, after heating at a heating rate of 3.0°C / min or more and 8.0°C / min or less to reach the target heating treatment temperature. After the heating treatment, cooling is performed. Furthermore, the cooling method is not particularly limited, and may be, for example, natural cooling (natural cooling in the furnace). After cooling, the obtained product is fully washed with water, ethanol or methanol and dried to obtain a Li-Na composite oxide. The drying atmosphere after washing is carried out in the atmosphere or in a vacuum, and there is no particular limitation. In addition, the heating treatment or washing treatment may be performed again after washing.

[0055] [negative electrode]

[0056] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer arranged on the negative electrode core. The negative electrode core can be made of a foil of a metal that is stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, nickel alloy, or a film having the metal arranged on the surface. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be produced, for example, by coating a negative electrode mixture slurry containing a negative electrode active material and a binder on the negative electrode core, drying the coating, and then compressing the coating to form negative electrode mixture layers on both sides of the negative electrode core. Furthermore, the negative electrode mixture layer may also contain a conductive agent such as CNT.

[0057] There are no particular restrictions on the negative electrode active material as long as it can reversibly absorb and release lithium ions. Carbon materials such as graphite are generally used. In addition, elements such as Si and Sn that alloy with Li, materials containing such elements, etc. can also be used as the negative electrode active material. Among them, silicon-containing materials containing Si are preferred. In addition, lithium titanate, etc., which have a higher charge and discharge potential relative to metallic lithium than carbon materials, etc., can also be used as the negative electrode active material. One type of negative electrode active material can be used alone, or multiple types can be used in combination.

[0058] The carbon material that functions as the negative electrode active material is, for example, at least one selected from natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, artificial graphite such as blocky artificial graphite (MAG) and graphitized mesocarbon microbeads (MCMB), natural graphite such as flaky graphite, blocky graphite, and earthy graphite, or a mixture thereof, is preferably used. Silicon-containing materials that function as negative electrode active materials include, for example, silicon alloys, silicon compounds, and composite materials containing Si. Very suitable silicon-containing materials are composite particles comprising an ion-conducting phase and a Si phase dispersed in the ion-conducting phase.

[0059] The binder contained in the negative electrode mixture layer can also use fluororesins, olefin resins, PAN, polyimide, polyamide, acrylic resins, etc. in the same way as in the case of the positive electrode 11, but polyvinyl acetate, styrene-butadiene rubber (SBR), etc. can also be used. Among them, SBR is preferably used. The binder can be used alone or in combination of multiple types. In addition, the negative electrode mixture layer preferably contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. They act as thickeners in the negative electrode mixture slurry. The content of the binder is not particularly limited, and is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the negative electrode mixture layer.

[0060] [Diaphragm]

[0061] The separator 13 is a porous sheet having ion permeability and insulation. Specific examples of the porous sheet include microporous films, woven fabrics, non-woven fabrics, and the like. The separator 13 is preferably made of polyolefins such as polyethylene and polypropylene, or cellulose. The separator 13 may be a single-layer structure or a multi-layer structure. Alternatively, a heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0062] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of the inorganic filler include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0063] Example

[0064] The present disclosure is further illustrated below by way of examples, but the present disclosure is not limited by these examples.

[0065] <Example 1>

[0066] [Preparation of positive electrode active material]

[0067] A hydroxide containing Ni and Mn in a 1:1 molar ratio was mixed with sodium carbonate in a molar ratio of Ni:Mn:Na = 0.5:0.5:1.05. The resulting mixture was heated at a heating rate of 1°C / minute, calcined in air at 800°C for 24 hours, and then rapidly cooled in air to produce a Na-containing composite oxide. A Li-containing molten salt was prepared by mixing lithium nitrate and lithium chloride in a molar ratio of 88:12. The Li-containing molten salt and the Na-containing composite oxide were mixed in a molar ratio of Li:Na = 30:1. The resulting mixture was heated at a heating rate of 5°C / minute and heated in air at 280°C for 5 hours. The product was then cooled at a cooling rate of 2°C / minute, washed with a sufficient amount of water, and then heated in a vacuum at 160°C for 4 hours to produce a Li-Na composite oxide.

[0068] The elements and their contents in the obtained Li-Na composite oxide were measured using an ICP emission spectrometer (CIROS-120 manufactured by SPECTRO). Furthermore, the oxygen content was measured using an EMGA-920 manufactured by Horiba, Ltd. The composition of the Li-Na composite oxide was determined by combining the ICP emission spectrometer analysis results and the oxygen content measurement results.

[0069] [Production of positive electrode]

[0070] The above-mentioned Li-Na composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a solid content mass ratio of 92:5:3, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode mixture slurry. This positive electrode slurry was applied to a positive electrode core composed of aluminum foil. After the coating film was dried, it was rolled using a calendering roller to obtain a positive electrode with a positive electrode mixture layer formed on the positive electrode core.

[0071] [Preparation of non-aqueous electrolyte]

[0072] A non-aqueous electrolyte solution was prepared by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent obtained by mixing fluoroethylene carbonate (FEC) and methyl propionate (FMP) in a volume ratio of 1:3 to obtain a mixed solvent.

[0073] [Preparation of test cell]

[0074] Lithium metal foil was used as the negative electrode. The positive and negative electrodes were arranged facing each other with a separator interposed therebetween to form an electrode assembly. This electrode assembly and the non-aqueous electrolyte were placed in a coin-shaped outer can. The opening of the outer can was sealed with a gasket and a sealing member to produce a test cell (non-aqueous electrolyte secondary battery).

[0075] <Example 2>

[0076] A positive electrode active material and a test cell were produced in the same manner as in Example 1, except that in the synthesis step of the Na-containing composite oxide, a hydroxide containing Ni and Mn in a molar ratio of 1:1, aluminum nitrate nonahydrate, and sodium carbonate were mixed in a molar ratio of Ni:Mn:Al:Na=0.4975:0.4975:0.005:1.050.

[0077] <Example 3>

[0078] A Na-containing composite oxide synthesized by the same method as in Example 1 and lithium hydroxide were mixed in a molar ratio of Na:Li=1:2, and the mixture thus obtained was heated at a heating rate of 5°C / min, heated at a temperature of 280°C in air for 5 hours, and then cooled at a cooling rate of 2°C / min to obtain a Li-Na mixture. Except for this, the positive electrode active material and the test unit were produced in the same manner as in Example 1.

[0079] <Comparative Example 1>

[0080] A hydroxide containing Ni and Mn at a molar ratio of 1:1 was mixed with lithium hydroxide at a molar ratio of Ni:Mn:Li = 0.5:0.5:1.05. The resulting mixture was heated at a heating rate of 5°C / minute, calcined in air at 900°C for 10 hours, and then cooled at a cooling rate of 10°C / minute to produce a lithium transition metal composite oxide. A test cell was produced in the same manner as in Example 1, except that this composite oxide was used as the positive electrode active material.

[0081] <Comparative Example 2>

[0082] A positive electrode active material and a test cell were prepared in the same manner as in Comparative Example 1, except that the calcination time of the mixture was changed from 10 hours to 100 hours.

[0083] [Evaluation of discharge capacity]

[0084] Each test cell of the Example and the Comparative Example was charged at a constant current of 0.2C at 25°C until the battery voltage reached 4.5V, and then charged at a constant voltage of 4.5V until the current value reached 0.02C. After a rest of 20 minutes, the battery was discharged at a constant current of 0.2C until the battery voltage reached 2.5V, and the discharge capacity was determined. The discharge capacity measurement results are shown in Table 1 together with the composition of the positive electrode active material.

[0085] Table 1

[0086]

[0087] As shown in Table 1, the test cells of the Examples all had higher capacities than the test cells of the Comparative Examples. Specifically, when the positive electrode active material contains a specified amount of Na and has a specified amount of oxygen vacancies, the battery capacity can be significantly improved. When the positive electrode active material of Comparative Example 1 contains no Na and has no oxygen vacancies, the same high capacity as when the positive electrode active material of the Examples is used cannot be achieved.

[0088] In addition, even if oxygen defects are present as in the positive electrode active material of Comparative Example 2, high capacity cannot be achieved in the absence of Na, and the capacity is reduced. The positive electrode active material of the embodiment introduces a predetermined amount of oxygen defects by extending the firing time during the synthesis of the Na-containing composite oxide, which greatly contributes to the high capacity of the battery. However, in the synthesis method of the positive electrode active material of the comparative example, the long firing time leads to a decrease in capacity. It can also be seen that in the positive electrode active material of the embodiment, further high capacity can be achieved by adding Al, etc., but in the test unit of the comparative example, even if Al, etc. is added, there is almost no effect, or the capacity is reduced.

[0089] The present disclosure is further illustrated by the following embodiments.

[0090] Technical composition 1: A positive electrode active material for a non-aqueous electrolyte secondary battery, which is a positive electrode active material for a non-aqueous electrolyte secondary battery having a crystal structure belonging to the space group R-3m, with a composition formula Li α Na β Ni 1-b-c Mn b X c O 2-d In the formula, X is at least one selected from transition metal elements and typical elements other than Li, Na, Ni, and Mn, 0.80<α≤1.20, 0<β≤0.20, 0.80<α+β≤1.20, 0<1-bc≤1, 0≤b<1, 0≤c<1, 0 <d≤0.2。

[0091] Technical configuration 2: The positive electrode active material for the non-aqueous electrolyte secondary battery according to technical configuration 1, wherein the composition formula is Li α Na β Ni 1-b-c Mn b X c O 2-d The molar ratio (β) of Na is 0<β≤0.20, and the total molar ratio (α+β) of Li and Na is 0.80<α+β≤1.20.

[0092] Technical configuration 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to technical configuration 1 or 2, wherein the composition formula is Li α Na β Ni 1-b-c Mn b X c O 2-d In the above embodiment, the molar ratio of O (2-d) is 1.85≤2-d≤1.96.

[0093] Technical Configuration 4: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Technical Configurations 1 to 3, wherein the composition formula is Li α Na β Ni 1-b-c Mn b X c O d In the embodiment, the molar ratio of Ni (1-bc) is 0.3≤1-bc≤0.9.

[0094] Technical Configuration 5: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Technical Configurations 1 to 4, wherein the composition formula is Li α Na β Ni 1-b-c Mn b X c O d In the embodiment, the molar ratio of Ni (1-bc) is 0.40≤1-bc≤0.75, and the molar ratio of Mn (b) is 0.25≤b≤0.65.

[0095] Technical Configuration 6: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Technical Configurations 1 to 5, wherein the composition formula is Li α Na β Ni 1-b-c Mn b X c O d wherein X is at least one selected from Mg, Ca, Sr, Ba, Sn, Ti, Si, V, Cr, Fe, Cu, Zn, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Ta, Mo, W, Co, and Al.

[0096] Technical configuration 7: The positive electrode active material for the non-aqueous electrolyte secondary battery according to technical configuration 6, wherein the composition formula is Li α Na β Ni 1-b-c Mn b X c O d In the embodiment, X is at least one selected from the group consisting of Al, Co and Zr.

[0097] Technical Configuration 8: A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode comprises the positive electrode active material according to any one of Technical Configurations 1 to 7.

[0098] Description of Reference Numerals

[0099] 10: Non-aqueous electrolyte secondary battery; 11: Positive electrode; 12: Negative electrode; 13: Separator; 14: Electrode body; 16: Outer can; 17: Sealing body; 18, 19: Insulating plates; 20: Positive electrode lead; 21: Negative electrode lead; 22: Slotted portion; 23: Internal terminal plate; 24: Lower valve body; 25: Insulating member; 26: Upper valve body; 27: Cover; 28: Gasket.

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, the positive electrode active material having a crystal structure belonging to the space group R-3m, With the composition formula Li α Na β Ni 1-b-c Mn b X c O 2-d express, Wherein, X is at least one selected from transition metal elements and typical elements other than Li, Na, Ni, and Mn, 0.80<α≤1.20, 0<β≤0.20, 0.80<α+β≤1.20, 0<1-bc≤1, 0≤b<1, 0≤c<1, 0 <d≤0.2。 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, In the composition formula Li α Na β Ni 1-b-c Mn b X c O 2-d In the embodiment, the molar ratio β of Na is 0<β≤0.20, and the total molar ratio α+β of Li and Na is 0.80<α+β≤1.

20.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, In the composition formula Li α Na β Ni 1-b-c Mn b X c O 2-d In the above embodiment, the molar ratio of O 2-d is 1.85≤2-d≤1.

96.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 3, In the composition formula Li α Na β Ni 1-b-c Mn b X c O d In the embodiment, the molar ratio of Ni (1-bc) is 0.3≤1-bc≤0.

9.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 4, In the composition formula Li α Na β Ni 1-b-c Mn b X c O d In the embodiment, the molar ratio 1-bc of Ni is 0.40≤1-bc≤0.75, and the molar ratio b of Mn is 0.25≤b≤0.

65.

6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 4, In the composition formula Li α Na β Ni 1-b-c Mn b X c O d wherein X is at least one selected from Mg, Ca, Sr, Ba, Sn, Ti, Si, V, Cr, Fe, Cu, Zn, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Ta, Mo, W, Co, and Al.

7. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 6, In the composition formula Li α Na β Ni 1-b-c Mn b X c O d In the embodiment, X is at least one selected from the group consisting of Al, Co and Zr. 8 . A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode comprises the positive electrode active material according to claim 1 .

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