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

Abstract

A positive electrode active material for non-aqueous electrolyte secondary batteries according to an example embodiment of the present invention comprises a lithium transition metal composite oxide having a layered structure and containing not less than 75 mol% of Ni with respect to the total molar quantity of elements excluding Li and O. The lithium transition metal composite oxide is of secondary particles obtained by aggregation of primary particles. A compound represented by the general formula AxByOz (where 1≤x≤2, 1≤y≤5, 4≤z≤9, A is at least one element selected from among Ca and Sr, and B is at least one element selected from among W, Mo, Ti, Si, Nb, and Zr) is adhered at at least the interface between primary particles inside the secondary particles.

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 active material. [Background technology]

[0002] The positive electrode active material constituting the positive electrode of a nonaqueous electrolyte secondary battery has a significant effect on battery performance, such as capacity and cycle characteristics, and therefore positive electrode active materials have been extensively studied. For example, Patent Document 1 discloses a positive electrode active material in which an alkaline earth metal and W are present on the surface of secondary particles of a lithium transition metal composite oxide containing one or more transition metal elements selected from Mn, Ni, and Co. Patent Document 2 also discloses a positive electrode active material containing a lithium transition metal composite oxide containing one or more transition metal elements selected from Mn, Ni, and Co, and further containing Ca and W as a solid solution. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-129221 [Patent Document 2] International Publication No. 2012 / 035664 Summary of the Invention [Problem to be solved by the invention]

[0004] Lithium transition metal composite oxides with a high Ni content are known as high-capacity positive electrode active materials, but because the structure of the particle surface layer is unstable, repeated charge and discharge cycles cause erosion from the surface layer, degrading the material and reducing capacity.

[0005] An object of the present disclosure is to suppress the capacity decrease that occurs with charge and discharge and improve the cycle characteristics in a non-aqueous electrolyte secondary battery that uses a positive electrode active material with a high Ni content. [Means for solving the problem]

[0006] The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure includes a lithium transition metal composite oxide having a layered structure and containing 75 mol % or more of Ni relative to the total molar amount of elements excluding Li and O, and the lithium transition metal composite oxide is a secondary particle formed by aggregation of primary particles, and at least inside the secondary particle, at the interface between the primary particles, a compound represented by general formula A x B y O z (wherein 1≦x≦2, 1≦y≦5, 4≦z≦9, A is at least one selected from Ca and Sr, and B is at least one selected from W, Mo, Ti, Si, Nb, and Zr) is fixed to the surface.

[0007] The non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and a non-aqueous electrolyte. [Effects of the Invention]

[0008] The positive electrode active material according to the present disclosure can improve the cycle characteristics of a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery according to the present disclosure exhibits little capacity loss during charge and discharge and has excellent cycle characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0010] The present inventors have found that in a high-capacity lithium transition metal composite oxide with a high Ni content, the above-mentioned A x B y O zThe researchers found that by fixing the compound, it is possible to effectively suppress the capacity decrease that occurs during battery charge and discharge. As mentioned above, lithium transition metal composite oxides with a high Ni content have an unstable structure on the particle surface that is prone to erosion. x B y O z When the compound is present at the interface between primary particles, it is believed that the particle surface is effectively protected and such erosion is suppressed.

[0011] A x B y O z The compound may be present on the surface of the secondary particles, but as in Comparative Example 5 described later, x B y O z If the compound is present only on the surface of the secondary particles and not on the surface of the primary particles inside the secondary particles, the effect of improving the cycle characteristics cannot be obtained. Also, if only one of elements A and B is present, the surface of the primary particles inside the secondary particles cannot be effectively protected, and the effect of improving the cycle characteristics cannot be obtained. In other words, x B y O z Only when the compound is present at the interface between the primary particles inside the secondary particles, the capacity decrease due to charge and discharge is specifically suppressed, and the cycle characteristics are improved.

[0012] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure and a non-aqueous electrolyte secondary battery using the active material will be described in detail. Note that selective combinations of the components of the multiple embodiments and modifications described below are within the scope of the present disclosure.

[0013] In the following, a cylindrical battery in which a wound-type electrode body 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery), or may be an outer can (laminated battery) made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode body is not limited to a wound type and may be a laminated electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0014] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0015] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixed solvents thereof. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The non-aqueous electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the longitudinal direction and the width direction (short direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 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.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and 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, and the outer can 16 serves as the negative electrode terminal.

[0018] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are layered. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0021] [Positive electrode] The positive electrode 11 includes a positive electrode core 30 and a positive electrode mixture layer 31 provided on the surface of the positive electrode core 30. The positive electrode core 30 can be made of a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. The positive electrode mixture layer 31 contains a positive electrode active material, a binder, and a conductive agent, and is preferably provided on both sides of the positive electrode core 30. The positive electrode 11 can be produced, for example, by applying a slurry of the positive electrode mixture onto the positive electrode core 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0022] Examples of the binder contained in the positive electrode mixture layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO). The content of the binder is, for example, 0.5 to 2 mass% with respect to the mass of the positive electrode mixture layer 31.

[0023] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, graphite, carbon nanotubes, etc. The content of the conductive agent is, for example, 0.5 to 10 mass % with respect to the mass of the positive electrode mixture layer 31.

[0024] The positive electrode active material contains a lithium transition metal composite oxide containing 75 mol % or more of Ni relative to the total molar amount of elements excluding Li and O. The lithium transition metal composite oxide has a layered crystal structure. Specific examples include a layered structure belonging to the space group R-3m or a layered structure belonging to the space group C2 / m. The lithium transition metal composite oxide is a secondary particle formed by aggregation of a plurality of primary particles. The particle size of the primary particles is, for example, 0.05 μm to 1 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in a particle image observed with a scanning electron microscope (SEM).

[0025] Hereinafter, for the sake of convenience, the lithium transition metal composite oxide will be referred to as "composite oxide (Z)". The positive electrode active material contains the composite oxide (Z) as a main component. Here, the term "main component" refers to the component with the highest mass ratio among the components constituting the positive electrode active material. The positive electrode mixture layer 31 may contain a composite oxide other than the composite oxide (Z) as the positive electrode active material, but the content of the composite oxide (Z) is preferably 50 mass% or more, and may be substantially 100 mass%.

[0026] The volume-based median diameter (D50) of the composite oxide (Z) is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm. Because the composite oxide (Z) is a secondary particle formed by aggregation of primary particles, the D50 of the composite oxide (Z) refers to the D50 of the secondary particles. D50 refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution of the composite oxide (Z) can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0027] The BET specific surface area of ​​the composite oxide (Z) is 0.5 to 3.5 m 2 / g. If the BET specific surface area is within this range, the cycle characteristics can be improved without reducing the discharge capacity. If the BET specific surface area is smaller than this range, the reaction area decreases, which may result in a decrease in the discharge capacity. On the other hand, if the BET specific surface area is larger than this range, the A x B y O z The surface cannot be sufficiently covered by the compound alone, which reduces the effect of improving cycle characteristics. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) specified in JIS R1626.

[0028] As described above, the composite oxide (Z) contains 75 mol% of Ni relative to the total number of moles of elements excluding Li and O. By setting the Ni content to 75 mol% or more, a battery with high energy density can be obtained. The upper limit of the Ni content is preferably 95 mol%. If the Ni content exceeds 95 mol%, it becomes difficult to ensure the stability of the layered structure of the composite oxide (Z), and the cycle characteristics may deteriorate. An example of a suitable range of the Ni content is 80 to 95 mol%, or 85 to 95 mol%.

[0029] In addition to Li, O, and Ni, elements contained in the composite oxide (Z) include Co, Mn, Al, Na, K, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ge, Sn, Pb, Sc, Ti, Si, V, Cr, Fe, Cu, Zn, Ru, Rh, Re, Pd, Ir, Ag, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Mo, and W. Among these, it is preferable to contain element M, which is at least one element selected from Mn, Al, Co, Fe, Ti, Si, Nb, Mo, W, and Zr. The content of element M is preferably 5 to 25 mol % based on the total molar amount of elements excluding Li and O.

[0030] An example of a suitable composite oxide (Z) is a compound represented by the general formula Li a Ni b Mn cCo d Al e M1 f O 2-g (where 0.95 < a < 1.05, 0.75 ≤ b ≤ 0.95, 0 ≤ c ≤ 0.25, 0 ≤ d ≤ 0.15, 0 ≤ e ≤ 0.1, 0 ≤ f < 0.1, 0 ≤ g ≤ 0.05, b + c + d + e + f = 1, and M1 is at least one element other than Li, Ni, Mn, Co, Al, and O) is a composite oxide.

[0031] The ratio of Li in the composite oxide (Z), a, more preferably satisfies 0.95 ≤ a < 1.05, and particularly preferably satisfies 0.97 ≤ a ≤ 1.03. When a is less than 0.95, the capacity may decrease compared to when a satisfies the above range. When a is 1.05 or more, more Li compounds need to be added compared to when a satisfies the above range, which may not be economical from the perspective of manufacturing cost. Also, since Co is expensive, it is preferable to suppress the Co content in consideration of the manufacturing cost.

[0032] When the composite oxide (Z) contains Al, the ratio of Al, e, more preferably satisfies 0.02 ≤ e ≤ 0.07. Since Al does not cause a change in the oxidation number even during charge and discharge, it is considered that the structure of the transition metal layer is stabilized by being contained in the transition metal layer. On the other hand, if the Al content is too high, it will lead to a decrease in capacity. Al may be uniformly dispersed within the layered structure of the lithium transition metal composite oxide, or may be present in a part within the layered structure.

[0033] At the interface between primary particles inside the secondary particles of the composite oxide (Z), there is adhered a compound represented by the general formula A x B y O z (where 1 ≤ x ≤ 2, 1 ≤ y ≤ 5, 4 ≤ z ≤ 9, A is at least one selected from Ca and Sr, and B is at least one selected from W, Mo, Ti, Si, Nb, and Zr) (hereinafter referred to as the "A x B y O z compound"). x B y Oz When the compound is present on the surface of the primary particles inside the secondary particles of the composite oxide (Z), the erosion and deterioration of the composite oxide (Z) are effectively suppressed, and the cycle characteristics of the battery are specifically improved.

[0034] A x B y O z The presence of the compound can be confirmed by measuring the cross section of the secondary particles using TEM-EDX (Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy). x B y O z The compound may be, for example, scattered on the surface of the primary particle, or may be present in the form of a layer that widely covers the surface of the primary particle.

[0035] A x B y O z The compound may further be present on the surface of the secondary particles of the composite oxide (Z). x B y O z The compound is widely present on the surface of the primary particles, both inside and on the surface of the secondary particles. The secondary particles of the composite oxide (Z) are formed by agglomeration of, for example, five or more primary particles, and the surface area of ​​the primary particles is larger inside than on the surface of the secondary particles. x B y O z The compound is contained in a larger amount inside the secondary particles than on the surface.

[0036] A x B y O z Specific examples of the compound include CaWO4, CaMoO3, CaMoO4, CaTiO3, Ca2TiO4, CaSiO3, Ca2SiO4, CaNbO3, CaNb2O6, CaZrO3, CaZr4O9, SrWO4, SrMoO3, SrMoO4, SrTiO3, Sr2TiO4, SrSiO3, Sr2SiO4, SrNbO3, SrNb2O6, SrZrO3, and SrZr4O9.

[0037] A x By O z The content of element A in the compound is calculated by dividing the compound oxide (Z) and A x B y O z It is preferable that the amount of the element in the compound is 3 mol % or less relative to the total molar amount of elements excluding Li and O. In this case, problems such as an increase in resistance do not occur, and cycle characteristics can be efficiently improved. x B y O z Similarly, the content of element B in the compound is also determined by the complex oxide (Z) and A. x B y O z It is preferably 3 mol % or less of the total molar amount of elements excluding Li and O in the compound. x B y O z The content of elements A and B in the compound is, for example, 0.1 mol % or more. x B y O z A part of the elements A and B added to form the compound may be dissolved in the composite oxide (Z).

[0038] Below, A is the interface between primary particles. x B y O z An example of a method for producing the compound-adhered composite oxide (Z) will be described.

[0039] The process for producing the composite oxide (Z) includes, for example, a first step of obtaining a composite oxide containing Ni and the like, a second step of mixing the composite oxide with a lithium compound to obtain a mixture, a third step of firing the mixture, and a fourth step of washing the fired product with water and drying it by heating. x B y O z The compounds can be fixed to the interfaces between primary particles inside the secondary particles of the composite oxide (Z) by adding a compound containing element A and a compound containing element B during the production process of the composite oxide (Z). The compound containing element B is added in the second or fourth step. The compound containing element A may also be added in the fourth step, but is preferably added in the second step.

[0040] In the first step, for example, an alkaline solution such as sodium hydroxide is added dropwise to a stirred solution of a metal salt containing Ni and the element M, and the pH is adjusted to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (co-precipitating) a composite hydroxide containing Ni and the element M. The composite hydroxide is calcined to obtain a composite oxide containing Ni and the element M. The calcination temperature is not particularly limited, but is, for example, 300°C to 600°C.

[0041] In the second step, for example, the composite oxide obtained in the first step is mixed with a lithium compound, a compound containing element A, and a compound containing element B. The compound containing element B may be added in the fourth step, as described above. Examples of lithium compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. The composite oxide and the lithium compound are preferably mixed in such a ratio that the molar ratio of the total amount of Ni and element M to Li is 1:0.98 to 1:1.12.

[0042] Examples of compounds containing element A include Ca(OH)2, CaO, CaCO3, CaSO4, Ca(NO3)2, Sr(OH)2, Sr(OH)2·8H2O, Sr(OH)2·H2O, SrO, SrCO3, SrSO4, and Sr(NO3)2. These compounds may be dried and dehydrated before use to reduce the amount of moisture generated during firing. These compounds may also be ground to a particle size of 0.1 to 20 μm. Compounds containing element B include hydroxides, oxides, carbonates, sulfates, and nitrates of element B. These compounds may also be dried and dehydrated before use to reduce the amount of moisture generated during firing. These compounds may also be ground to a particle size of 0.1 to 20 μm.

[0043] The compound containing the composite oxide and the element A is preferably mixed in a ratio such that the molar ratio of the total amount of Ni and element M to element A is 1:0.0005 to 1:0.03. When using a plurality of compounds containing element A, they are mixed so that the total amount of element A contained in the compounds satisfies the above ratio. The suitable mixing ratio with the composite oxide is also the same for the compound containing element B. Furthermore, elements A and B are preferably mixed in the ratio of A described in paragraph 0036. x B y O z It is preferable to mix the compounds in accordance with the stoichiometric ratio.

[0044] The firing step of the mixture in the third step is a multi-stage firing step that includes at least a first firing step in which the mixture is fired at 450°C to 680°C under an oxygen stream, and a second firing step in which the fired product obtained in the first firing step is fired at a temperature exceeding 680°C under an oxygen stream. In the first firing step, the temperature is raised to a first set temperature of 680°C or lower at a first heating rate of 0.2°C / min to 5.5°C / min. In the second firing step, the temperature is raised to a second set temperature of 900°C or lower at a second heating rate of 0.1°C / min to 3.5°C / min that is slower than the first heating rate. Note that the first and second heating rates may be set multiple times for each predetermined temperature range within the above range.

[0045] The holding time of the first set temperature in the first firing step is preferably 5 hours or less, more preferably 3 hours or less. The holding time of the first set temperature is the time for which the first set temperature is maintained after reaching the first set temperature, and the holding time may be zero. The holding time of the second set temperature in the second firing step is preferably 1 hour to 10 hours, more preferably 1 hour to 5 hours. The holding time of the second set temperature is the time for which the second set temperature is maintained after reaching the second set temperature. The firing of the mixture is carried out, for example, in an oxygen stream with an oxygen concentration of 60% or more, and the flow rate of the oxygen stream is set to 10 cm / min. 3 The flow rate shall be 0.2 mL / min to 4 mL / min per kg of the mixture and 0.3 L / min or more per kg of the mixture.

[0046] In the fourth step, the fired product obtained in the third step is washed with water to remove impurities, and the washed fired product is then heated and dried. If necessary, the fired product is pulverized, classified, or the like, to adjust the D50 of the positive electrode active material to a desired range. If a compound containing element B is not added in the second step, for example, a compound containing element B is added to the washed fired product and mixed. The washed fired product may be dried at a temperature below 100°C. However, if a compound containing element B is added in the fourth step, it is preferable to heat the mixture of the compound containing element B and the fired product to a temperature of 100°C or higher. An example of a suitable temperature range in this case is 150°C to 250°C. The drying process may be performed either under vacuum or in air. An example of the drying time is 1 to 5 hours.

[0047] In addition, inside the secondary particles of the composite oxide (Z), A exists at the interface between the primary particles. x B y O z In order to fix the compound, it is necessary to add a compound containing element A and a compound containing element B. In other words, even if a compound containing both elements A and B is used, it is difficult to fix the compound A on the surface of the primary particle inside the secondary particle. x B y O z Compound A cannot exist. x B y O z The compound is thought to be formed when element A melts and incorporates element B. Unless heat treatment is carried out at a temperature of at least 100°C in the presence of a compound containing element A and a compound containing element B, A will not form on the surface of the primary particle inside the secondary particle. x B y O z The compound cannot exist.

[0048] [Negative electrode] The negative electrode 12 includes a negative electrode core 40 and a negative electrode mixture layer 41 provided on the surface of the negative electrode core 40. The negative electrode core 40 can be made of a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The negative electrode mixture layer 41 contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core 40. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core 40, drying the coating, and then compressing it to form the negative electrode mixture layer 41 on both sides of the negative electrode core 40. The negative electrode mixture layer 41 may contain a conductive agent similar to that used in the positive electrode 11.

[0049] The negative electrode mixture layer 41 contains, as the negative electrode active material, for example, a carbon material that reversibly absorbs and releases lithium ions. Suitable examples of the carbon material include natural graphite, such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite, such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). Alternatively, the negative electrode active material may include an active material containing at least one of an element that alloys with Li, such as Si or Sn, and a compound containing such an element. A suitable example of such an active material is a silicon material in which Si particles are dispersed in a silicon oxide phase or a silicate phase, such as lithium silicate. For example, a carbon material, such as graphite, and a silicon material are used in combination as the negative electrode active material.

[0050] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer 41 can be, for example, a fluororesin, PAN, polyimide, acrylic resin, or polyolefin. However, it is preferable to use styrene-butadiene rubber (SBR). It is also preferable that the negative electrode mixture layer 41 further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use a combination of SBR with CMC or a salt thereof, or PAA or a salt thereof. The negative electrode mixture layer 41 may also contain a conductive agent.

[0051] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefins, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as an aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 13. [Example]

[0052] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0053] Example 1 [Synthesis of positive electrode active material] [Ni 0.8 Mn 0.1 Co 0.1 A composite hydroxide represented by the formula [(OH)2] was calcined at 500°C for 8 hours to obtain a composite oxide. The composite oxide was mixed with lithium hydroxide, calcium hydroxide, molybdenum oxide, and tungsten oxide so that the molar ratio of the total amount of Li, Ni, Mn, and Co to Ca, Mo, and W was 1.05:1.00:0.01:0.01:0.01. The mixture was heated under an oxygen stream (10 cm) with an oxygen concentration of 95%. 3 The mixture was fired by heating from room temperature to 650°C at a rate of 2.0°C / min with a flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture, and then heated to 750°C at a rate of 0.5°C / min. The fired product was washed with water to remove impurities and vacuum dried at 180°C for 2 hours to obtain a positive electrode active material.

[0054] The resulting positive electrode active material was measured using an ICP optical emission spectrometer (Thermo Fisher Scientific, iCAP6300), and the elements listed in Table 1 below were confirmed, excluding Li, O, and impurity elements. Furthermore, synchrotron X-ray diffraction measurements were used to identify the compounds present in the positive electrode active material, confirming the presence of CaMoO3 and CaWO4. TEM-EDX also confirmed that Ca, Mo, and W were present at the interfaces between primary particles within the secondary particles.

[0055] [Preparation of positive electrode] The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 91:7:2, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core. In addition, an exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode core.

[0056] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 3:3:4 (25°C). LiPF6 was dissolved in the mixed solvent to a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte solution.

[0057] [Test cell construction] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the lithium metal foil as the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte solution was poured into it. The opening of the exterior body was then sealed to obtain a test cell Al.

[0058] <Example 2> Test cell A2 was produced in the same manner as in Example 1, except that strontium hydroxide and titanium hydroxide were added instead of calcium hydroxide, molybdenum oxide, and tungsten oxide in synthesizing the positive electrode active material.

[0059] Example 3 In the synthesis of the positive electrode active material, [Ni 0.86 Mn 0.14 A composite hydroxide represented by the formula [(OH)] was used, and a composite oxide obtained by calcining the composite hydroxide was mixed with lithium hydroxide, calcium hydroxide, and tungsten oxide so that the molar ratio of the total amount of Li, Ni, and Mn to Ca and W was 1.03:1.00:0.005:0.005. The mixture was heated under an oxygen stream (10 cm) with an oxygen concentration of 95%. 3 The mixture was fired by heating from room temperature to 650°C at a temperature increase rate of 3.0°C / min with a flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture, and then fired again at a temperature increase rate of 1.0°C / min to 780°C. Test cell A3 was produced in the same manner as in Example 1, except that the fired product was washed with water to remove impurities and vacuum dried at 200°C for 2 hours to obtain a positive electrode active material.

[0060] Example 4 Test cell A4 was produced in the same manner as in Example 3, except that zirconium oxide was added instead of tungsten oxide in the synthesis of the positive electrode active material.

[0061] <Example 5> In the synthesis of the positive electrode active material, [Ni 0.91 Mn 0.09 Test cell A5 was produced in the same manner as in Example 1, except that a composite hydroxide represented by the formula [Chemical Formula 1](OH)2 was used, and a composite oxide obtained by calcining the composite hydroxide was mixed with lithium hydroxide, strontium hydroxide, and titanium hydroxide so that the molar ratio of the total amount of Li, Ni, and Mn to Sr and Ti was 1.05:1.00:0.025:0.025.

[0062] Example 6 Test cell A6 was produced in the same manner as in Example 5, except that calcium hydroxide and molybdenum oxide were added instead of strontium hydroxide and titanium hydroxide in synthesizing the positive electrode active material.

[0063] Example 7 Test cell A7 was produced in the same manner as in Example 5, except that calcium hydroxide was added instead of strontium hydroxide in the synthesis of the positive electrode active material.

[0064] Example 8 Test cell A8 was produced in the same manner as in Example 5, except that calcium hydroxide and tungsten oxide were added instead of strontium hydroxide and titanium hydroxide in synthesizing the positive electrode active material.

[0065] Example 9 [Ni 0.9 Co 0.05 Al 0.05 A composite hydroxide represented by the formula [(OH)2] was calcined at 500°C for 8 hours to obtain a composite oxide. The composite oxide, lithium hydroxide, and calcium hydroxide were mixed so that the molar ratio of the total amount of Li, Ni, Co, and Al to Ca was 1.03:1.00:0.005. The mixture was heated under an oxygen stream (10 cm) with an oxygen concentration of 95%. 3 The mixture was fired by heating from room temperature to 650°C at a rate of 3.0°C / min with a flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture, and then heated to 730°C at a rate of 0.5°C / min. The fired product was washed with water to remove impurities, and a predetermined amount of tungsten oxide was added. The product was then vacuum-dried at 180°C for 2 hours to obtain a positive electrode active material. Test cell A9 was produced in the same manner as in Example 1, except that the positive electrode active material was used for the positive electrode.

[0066] Example 10 In the synthesis of the positive electrode active material, [Ni 0.93 Mn 0.07A test cell A10 was produced in the same manner as in Example 1, except that a composite hydroxide represented by the formula [Chemical Formula 1](OH)2 was used, and a composite oxide obtained by calcining the composite hydroxide was mixed with lithium hydroxide, strontium hydroxide, and molybdenum oxide so that the molar ratio of the total amount of Li, Ni, and Mn to Sr and Mo was 1.05:1.00:0.005:0.005.

[0067] Example 11 Test cell A11 was produced in the same manner as in Example 10, except that calcium hydroxide and zirconium oxide were added instead of strontium hydroxide and molybdenum oxide in synthesizing the positive electrode active material.

[0068] <Comparative Example 1> Test cell B1 was produced in the same manner as in Example 1, except that calcium hydroxide, molybdenum oxide, and tungsten oxide were not added in the synthesis of the positive electrode active material.

[0069] <Comparative Example 2> Test cell B2 was produced in the same manner as in Example 3, except that calcium hydroxide and tungsten oxide were not added in the synthesis of the positive electrode active material.

[0070] <Comparative Example 3> Test cell B3 was produced in the same manner as in Example 5, except that strontium hydroxide and titanium hydroxide were not added in the synthesis of the positive electrode active material.

[0071] <Comparative Example 4> In the synthesis of positive electrode active materials ,acid Test cell B4 was prepared in the same manner as in Example 9, except that molybdenum chloride was not added.

[0072] <Comparative Example 5> Test cell B5 was produced in the same manner as in Example 3, except that in the synthesis of the positive electrode active material, the timing of adding tungsten oxide was changed to after the fired product was washed with water, and drying after washing was performed under atmospheric pressure at 80° C. In the positive electrode active material of Comparative Example 5, CaWO4 was present only on the surfaces of the secondary particles, and CaWO4 was not observed at the interfaces between primary particles inside the secondary particles.

[0073] <Comparative Example 6> Test cell B6 was produced in the same manner as in Example 3, except that calcium hydroxide was not added in the synthesis of the positive electrode active material.

[0074] <Comparative Example 7> Test cell B7 was produced in the same manner as in Example 9, except that molybdenum oxide was not added in the synthesis of the positive electrode active material.

[0075] [Capacity retention rate evaluation] A cycle test was carried out on each test cell of the examples and comparative examples. The discharge capacity at the first cycle and the discharge capacity at the 30th cycle of the cycle test were determined, and the capacity retention rate was calculated by the following formula. Capacity retention rate (%) = (30th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 <Cycle test> Test cells A1-4, B1, 2, 5, and 6 were charged at a constant current of 0.2 It at a temperature of 25°C until the battery voltage reached 4.4 V, and then charged at a constant voltage until the current reached 1 / 100 It at 4.4 V. They were then discharged at a constant current of 0.2 It until the battery voltage reached 2.5 V. This charge-discharge cycle was repeated 30 times. Test cells A5-9, B3, 4, and 7 were charged in the same manner as test cells A1-4, B1, 2, 5, and 6, except that they were charged at a constant current of 0.2 It until the battery voltage reached 4.3 V, and then charged at a constant voltage until the current reached 1 / 100 It at 4.3 V.

[0076] The calculated capacity retention rates are shown in Tables 1 to 4. The capacity retention rates shown in Table 1 are relative values ​​when the capacity retention rate of test cell B1 of Comparative Example 1 is set to 100. The capacity retention rates shown in Table 2 are relative values ​​when the capacity retention rate of test cell B2 of Comparative Example 2 is set to 100. The capacity retention rates shown in Table 3 are relative values ​​when the capacity retention rate of test cell B3 of Comparative Example 3 is set to 100. The capacity retention rates shown in Table 4 are relative values ​​when the capacity retention rate of test cell B4 of Comparative Example 4 is set to 100.

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] [Table 4]

[0081] As shown in Tables 1 to 4, the test cells of the examples all have higher capacity retention rates after cycle testing and superior cycle characteristics than the test cells of the corresponding comparative examples. In the synthesis of the positive electrode active material, the elements A (Ca, Sr) and B (W, Mo, Ti, Si, Nb, Zr) were not added (Comparative Examples 1 to 4), only one of the elements A and B was added (Comparative Examples 6 and 7), and the elements A and B were added in the same manner. x B y O z When the compound was present only on the surface of the secondary particles (Comparative Example 5), no effect of improving the cycle characteristics was obtained. [Explanation of symbols]

[0082] 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 plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode core body, 31 positive electrode mixture layer, 40 negative electrode core body, 41 negative electrode mixture layer

Claims

1. It contains a lithium transition metal composite oxide having a layered structure and containing 75 mol % or more of Ni relative to the total molar amount of elements excluding Li and O, The lithium transition metal composite oxide is a secondary particle formed by aggregation of primary particles, and at least in the interior of the secondary particle, at the interface between the primary particles, a compound represented by the general formula A ｘ B ｙ O ｚ (wherein 1≦x≦2, 1≦y≦5, 4≦z≦9, A is at least one selected from Ca and Sr, and B is at least one selected from W, Mo, Ti, and Zr) is fixed to the surface of the substrate, a total content of element A and a total content of element B are each 0.1 mol % or more and 3 mol % or less relative to the total molar amounts of elements excluding Li and O in the lithium transition metal composite oxide and the compound.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the compound is at least one selected from CaWO4, CaMooO3, Ca2TiO4, CaZrO3, SrMoO3, and Sr2TiO4.

3. The lithium transition metal composite oxide contains an element M, which is at least one element selected from Mn, Al, Co, Fe, Ti, Si, Nb, Mo, W, and Zr, 3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of element M is 5 to 25 mol % based on the total molar amount of elements excluding Li and O.

4. The lithium transition metal composite oxide has a BET specific surface area of ​​0.5 to 3.5 m ２ The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the SiO2 content is 1 / g.

5. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material according to any one of claims 1 to 4, a negative electrode, and a non-aqueous electrolyte.

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