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

Abstract

To provide a cathode active material for a non-aqueous electrolyte secondary battery which contains lithium transition metal composite oxide having high Ni content and is a cathode active material which contributes to the improvement of charge and discharge cycle characteristics of a battery.SOLUTION: This cathode active material for nonaqueous electrolyte secondary batteries contains prescribed lithium transition metal composite oxide having a layer structure, and a compound A containing at least either Ca or Sr existing in the surface or a grain boundary of a primary particle of the lithium transition metal composite oxide. The layer structure includes a Li layer in which Li reversibly enters and exits, the ratio of metal elements other than Li present in the Li layer is 0.7 mol% or more and 3.0 mol% or less with respect to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide, and the ratio m / n of the half value width m of the diffraction peak of the (003) plane to the half value width n of the diffraction peak of the (104) plane in an X-ray diffraction pattern by X-ray diffraction is 0.75≤m / n≤1.0.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery, a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, lithium transition metal composite oxides with a high Ni content have been attracting attention as positive electrode active materials with high energy density. For example, Patent Document 1 discloses a positive electrode active material for non-aqueous electrolyte secondary batteries, which comprises a lithium transition metal composite oxide represented by the general formula LixNiyCozMmO2 (wherein M is an element selected from Ba, Sr, and B, and 0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0.5, and 0.0005≦m≦0.02), and which has a BET specific surface area of ​​0.8 m2 / g or less.

[0003] Furthermore, Patent Document 2 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, which has an α-NaFeO structure, contains one or more transition metal elements selected from the group consisting of Mn, Ni, and Co, and has an alkaline earth metal and W present on the particle surfaces of a lithium transition metal composite oxide. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-100295 [Patent Document 2] Japanese Patent Application Publication No. 2018-129221 Summary of the Invention

[0005] When a lithium transition metal composite oxide with a high Ni content is used as the positive electrode active material for a nonaqueous electrolyte secondary battery, the amount of Li extracted during charging is large, which causes the layered crystal structure to break down and the capacity to decrease with repeated charge and discharge. Note that the technologies disclosed in Patent Documents 1 and 2 still have room for improvement in terms of charge and discharge cycle characteristics.

[0006] The positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure has a layered structure and is represented by the general formula LiaNixMnyMzO2-b (where 0.95 < a < 1.05, 0.7 ≤ x ≤ 0.95, 0 < y ≤ 0.3, 0 ≤ z ≤ 0.3, 0 ≤ b < 0.05, x + y + z = 1, and M is at least one element selected from Al, Co, Fe, Ti, Si, Nb, Mo, W, and Zn), a lithium transition metal composite oxide, and a compound A containing at least one of Ca and Sr present on the surface or grain boundaries of the primary particles of the lithium transition metal composite oxide. The layered structure includes a Li layer in which Li reversibly enters and exits, and the ratio of metal elements other than Li present in the Li layer is 0.7 mol% or more and 3.0 mol% or less with respect to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide, and the ratio m / n of the half-value width m of the diffraction peak of the (003) plane to the half-value width n of the diffraction peak of the (104) plane in the X-ray diffraction pattern by X-ray diffraction is 0.75 ≤ m / n ≤ 1.0.

[0007] The method for manufacturing a positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a step of firing a mixture obtained by dry-mixing a transition metal oxide, a Li compound, and at least one of a Ca compound and a Sr compound at 850°C or lower.

[0008] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte.

[0009] According to the positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, it is possible to provide a high-capacity non-aqueous electrolyte secondary battery that suppresses a decrease in battery capacity associated with charge and discharge. The positive electrode active material for a non-aqueous electrolyte secondary battery contains a lithium transition metal composite oxide having a high Ni content and contributes to an improvement in the charge-discharge cycle characteristics of the battery.

Brief Description of the Drawings

[0010] [Figure 1]FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. [Figure 2] FIG. 2 shows the X-ray diffraction patterns of Examples 2 and 3, SrO, and CaO. DETAILED DESCRIPTION OF THE INVENTION

[0011] The layered structure of the lithium transition metal composite oxide contained in the positive electrode active material contains a transition metal layer containing Ni and other metals, a Li layer, and an oxygen layer. The reversible movement of Li ions in the Li layer allows the battery's charge / discharge reactions to proceed. When using a lithium transition metal composite oxide with a high Ni content, many Li ions are extracted from the Li layer during battery charging, causing the layered structure to collapse and leading to a decrease in battery capacity. Furthermore, lithium transition metal composite oxides with a high Ni content have high activity near the particle surface, making the structure prone to instability. This makes them susceptible to the formation of a surface degradation layer or erosion due to reactions with the electrolyte, leading to a decrease in battery capacity.

[0012] To solve the above problems, the inventors conducted extensive research and found that by first incorporating a predetermined amount of Mn, which does not undergo oxidation state change during charge and discharge, into the transition metal layer, incorporating a predetermined amount of a metal element other than Li into the Li layer, and then creating a layered structure with appropriate distortion in the plane direction so that the ratio of the half-width m of the (003) plane to the half-width n of the (104) plane in the X-ray diffraction pattern falls within a predetermined range, it is possible to increase battery capacity while maintaining the structure of the lithium transition metal composite oxide. Furthermore, the inventors discovered that protecting the surface of the lithium transition metal composite oxide with a compound containing at least one of Ca and Sr can suppress erosion of the structurally deteriorated layer. For lithium transition metal composite oxides with a high Ni content, strengthening the layered structure framework or protecting the surface alone is not sufficient. However, applying both of these features synergistically improves charge and discharge cycle performance.

[0013] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly is housed in a cylindrical battery case will be exemplified. However, the electrode assembly is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are alternately stacked one by one with separators interposed therebetween. Furthermore, the battery case is not limited to a cylindrical shape and may be, for example, a prismatic or coin-shaped battery case, or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0014] Fig. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As illustrated in Fig. 1, the nonaqueous electrolyte secondary battery 10 includes an electrode assembly 14, a nonaqueous electrolyte (not shown), and a battery case 15 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. The battery case 15 includes a cylindrical outer can 16 with a bottom, and a sealing member 17 that closes the opening of the outer can 16.

[0015] The electrode assembly 14 is composed of a long positive electrode 11, a long negative electrode 12, two long separators 13, a positive electrode tab 20 joined to the positive electrode 11, and a negative electrode tab 21 joined to the negative electrode 12. 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 the longitudinal direction and width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged to sandwich the positive electrode 11, for example.

[0016] The nonaqueous electrolyte secondary battery 10 includes insulating plates 18 and 19 disposed above and below the electrode assembly 14. In the example shown in Fig. 1 , a positive electrode tab 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode tab 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the outer can 16. The positive electrode tab 20 is connected to the underside of a bottom plate 23 of the sealing body 17 by welding or the like, and a cap 27 of the sealing body 17 electrically connected to the bottom plate 23 serves as the positive electrode terminal. The negative electrode tab 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.

[0017] The outer can 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the outer can 16 and the sealing body 17, sealing the internal space of the battery case 15. The outer can 16 has a grooved portion 22 that supports the sealing body 17, formed, for example, by pressing the side surface from the outside. 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 upper surface.

[0018] The sealing body 17 has a structure in which, in order from the electrode body 14 side, a bottom plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. 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 each other. The lower valve body 24 and the upper valve body 26 are connected to each other 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.

[0019] The positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below, particularly the positive electrode active material contained in the positive electrode mixture layer 31 that constitutes the positive electrode 11.

[0020] [Positive electrode] The positive electrode 11 has a positive electrode current collector 30 and a positive electrode composite layer 31 formed on both sides of the positive electrode current collector 30. The positive electrode current collector 30 can be 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 its surface. The positive electrode composite layer 31 contains a positive electrode active material, a conductive material, and a binder. The thickness of the positive electrode composite layer 31 is, for example, 10 μm to 150 μm on one side of the positive electrode current collector 30. The positive electrode 11 can be produced by applying a positive electrode slurry containing the positive electrode active material, the conductive material, the binder, etc. to the surface of the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode composite layer 31 on both sides of the positive electrode current collector 30.

[0021] Examples of the conductive material contained in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. 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 carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0022] The positive electrode active material contained in the positive electrode composite layer 31 includes a lithium transition metal composite oxide having a layered structure, and a compound A containing at least one of Ca and Sr, which is present on the surfaces of primary particles, including the surfaces of secondary particles, or on the grain boundaries of the lithium transition metal composite oxide.

[0023] Examples of the layered structure of the lithium transition metal composite oxide include a layered structure belonging to the space group R-3m and a layered structure belonging to the space group C2 / m. Among these, a layered structure belonging to the space group R-3m is preferred in terms of high capacity, stability of the crystal structure, etc. The layered structure of the lithium transition metal composite oxide includes a transition metal layer, a Li layer, and an oxygen layer.

[0024] Lithium transition metal composite oxides have the general formula Li a Ni x Mn y Mz O 2-b (where 0.95 < a < 1.05, 0.7 ≤ x ≤ 0.95, 0 < y ≤ 0.3, 0 ≤ z ≤ 0.3, 0 ≤ b < 0.05, x + y + z = 1, and M is at least one element selected from Al, Co, Fe, Ti, Si, Nb, Mo, W, and Zn). In addition, the positive electrode active material may contain a lithium transition metal composite oxide other than that represented by the above general formula, or other compounds, as long as the object of the present disclosure is not impaired. The molar fraction of the metal elements contained in the lithium transition metal composite oxide can be measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0025] a, which represents the ratio of Li in the lithium transition metal composite oxide, satisfies 0.95 ≤ a < 1.05, and preferably satisfies 0.97 ≤ a ≤ 1.03. When a is less than 0.95, the battery capacity may decrease as compared with the case where a satisfies the above range. When a is 1.05 or more, it may lead to a decrease in charge-discharge cycle characteristics as compared with the case where a satisfies the above range.

[0026] x, which represents the ratio of Ni to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide, satisfies 0.7 ≤ x ≤ 0.95, and preferably satisfies 0.8 ≤ x ≤ 0.95. By setting x to 0.7 or more, a high-capacity battery can be obtained. In addition, when x is 0.8 or more, the effect of improving cycle characteristics due to the stabilization of the structure of the lithium transition metal composite oxide is easily obtained. When x exceeds 0.95, a sufficient amount of Mn and M cannot be contained, so the layered structure of the lithium transition metal composite oxide becomes unstable.

[0027] The content ratio y of Mn to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide preferably satisfies 0 < y ≤ 0.3, and more preferably satisfies 0.01 ≤ y ≤ 0.15. Since the oxidation number of Mn does not change 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, when y exceeds 0.3, the content of Ni decreases and the battery capacity decreases. Mn may be uniformly dispersed, for example, within the layered structure of the lithium transition metal composite oxide, or may be present in a part within the layered structure.

[0028] M (M is at least one element selected from Al, Co, Fe, Ti, Si, Nb, Mo, W, and Zn) is an optional component. The content ratio z of M to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide preferably satisfies 0 ≤ z ≤ 0.3.

[0029] The lithium transition metal composite oxide is, for example, particles having a volume-based median diameter (D50) of, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm, and particularly preferably 7 μm to 15 μm. D50 means the particle diameter at which the cumulative frequency in the volume-based particle size distribution becomes 50% from the smaller particle diameter side, and is also called the median diameter. The particle size distribution of the lithium transition metal composite oxide can be measured using a laser diffraction type particle size distribution measuring device (for example, MT3000II manufactured by Microtrac Bell Co., Ltd.) with water as a dispersion medium.

[0030] The lithium transition metal composite oxide is, for example, secondary particles formed by aggregation of a plurality of primary particles. The particle diameter of the primary particles constituting the secondary particles is, for example, 0.05 μm to 1 μm. The particle diameter of the primary particles is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM).

[0031] Compound A is present on the surface or grain boundaries of primary particles of the lithium transition metal composite oxide. This can suppress the formation and erosion of a structurally degraded layer on the surface of the lithium transition metal composite oxide due to, for example, a reaction with the electrolyte. Here, the surface of the primary particles includes the surface of the secondary particles. Furthermore, the grain boundaries of the primary particles refer to the interfaces between primary particles. Compound A being present on the surface or grain boundaries of the primary particles means that it is in contact with the surface or grain boundaries of the primary particles, or is within a range of 10 nm or less from the surface or grain boundaries of the primary particles. Compound A may be, for example, uniformly dispersed over the entire surface and interface of the lithium transition metal composite oxide, or may be present only in part.

[0032] Compound A contains at least one of Ca and Sr. Compound A may contain a Ca compound or a Sr compound. Examples of the Ca compound include CaO, Ca(OH)2, and CaCO3. Examples of the Sr compound include SrO, Sr(OH)2, and SrCO3.

[0033] The total amount of Ca and Sr in Compound A may be 1 mol % or less relative to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide, thereby further improving the charge-discharge cycle characteristics.

[0034] The layered structure of the lithium transition metal composite oxide includes a Li layer through which Li reversibly enters and exits, and the proportion of metal elements other than Li present in the Li layer is 0.7 mol % to 3.0 mol % relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide. If the proportion of metal elements other than Li in the Li layer is less than 0.7 mol %, the stability of the layered structure decreases when Li ions in the Li layer are extracted, causing the structure to break down and leading to a decrease in battery capacity. Furthermore, if the proportion of metal elements other than Li in the Li layer exceeds 3.0 mol %, the diffusibility of Li ions in the Li layer decreases, resulting in a decrease in battery capacity and an increase in battery reaction resistance. The metal element present in the Li layer is mainly Ni, but may also contain other metal elements.

[0035] The proportion of metal elements other than Li in the Li layer can be obtained from the results of Rietveld analysis of the X-ray diffraction pattern obtained by X-ray diffraction measurement of the positive electrode active material. Rietveld analysis of the X-ray diffraction pattern can be performed using, for example, Rietveld analysis software PDXL2 (Rigaku Corporation).

[0036] The X-ray diffraction pattern is obtained by powder X-ray diffraction using a powder X-ray diffractometer (manufactured by Rigaku Corporation, trade name "RINT-TTR", radiation source Cu-Kα) under the following conditions. Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background: B-splines Profile function: Split pseudo-Voigt function Constraint condition: Li(3a)+Ni(3a)=1 Ni(3a) + Ni(3b) = α (α is the Ni content in each) ICSD No.:98-009-4814 The positive electrode active material preferably has a crystallite size s calculated by the Scherrer equation from the half-width n of the diffraction peak of the (104) plane in the X-ray diffraction pattern obtained by the X-ray diffraction, in the range of 400 Å≦s≦800 Å. If the crystallite size s of the lithium transition metal composite oxide is smaller than 400 Å, the crystallinity may decrease, leading to a decrease in battery capacity. If the crystallite size s of the lithium transition metal composite oxide exceeds 800 Å, the diffusibility of Li may decrease, leading to a decrease in battery output characteristics. The Scherrer equation is expressed as follows:

[0037] s=Kλ / Bcosθ In the above formula, s is the crystallite size, λ is the wavelength of the X-ray, B is the half-width of the diffraction peak of the (104) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this embodiment, K is set to 0.9.

[0038] The positive electrode active material has an X-ray diffraction pattern in which the ratio m / n of the half-width m of the diffraction peak of the (003) plane to the half-width n of the diffraction peak of the (104) plane is 0.75≦m / n≦1.0. Within this range, the layered structure can be appropriately distorted in the plane direction, resulting in a battery with high capacity and improved charge / discharge cycle characteristics. If m / n is less than 0.75, the layered structure will be too distorted, making it brittle. Furthermore, if m / n is greater than 1.0, the battery capacity will decrease.

[0039] It is preferable that the X-ray diffraction pattern of the positive electrode active material obtained by the X-ray diffraction measurement does not include peaks derived from CaO and SrO. If CaO and SrO are contained in an amount detectable by X-ray diffraction measurement, a decrease in battery capacity may occur.

[0040] Next, an example of a method for producing a positive electrode active material containing a lithium transition metal composite oxide and compound A will be described.

[0041] The method for producing a positive electrode active material includes, for example, a first step of obtaining a transition metal oxide containing Ni, Mn, and an arbitrary metal element, a second step of mixing the transition metal oxide obtained in the first step with a Li compound to obtain a mixture, and a third step of firing the mixture.

[0042] 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, Mn, and an optional metal element (Co, Al, Nb, etc.) to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (co-precipitating) a transition metal hydroxide containing Ni, Mn, and the optional metal element, and the transition metal hydroxide is then calcined to obtain a transition metal oxide containing Ni, Mn, and the optional metal element. The calcination temperature is not particularly limited, but is, for example, in the range of 300°C to 600°C.

[0043] In the second step, the transition metal oxide obtained in the first step is dry-mixed with a Li compound and at least one of a Ca compound and a Sr compound to obtain a mixture. Examples of Li compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. Examples of Ca compounds include Ca(OH)2, CaO, CaCO3, CaSO4, and Ca(NO3)2. Examples of Sr compounds include Sr(OH)2, Sr(OH)2·8H2O, SrO, SrCO3, SrSO4, and Sr(NO3)2. The mixing ratio of the transition metal oxide obtained in the first step and the Li compound is preferably such that the molar ratio of metal elements other than Li to Li is in the range of 1:0.98 to 1:1.1, in order to easily adjust each of the above parameters within the specified ranges. Furthermore, the mixing ratio of the transition metal oxide obtained in the first step with the Ca compound or Sr compound is preferably set so that the molar ratio of metal elements excluding Li to Ca and Sr is in the range of 1:0.0003 to 1:0.03, in order to facilitate adjustment of each of the above parameters within the ranges specified above. In the second step, when mixing the transition metal oxide obtained in the first step with the Li compound and the Ca compound or Sr compound, other metal raw materials may be added as necessary. The other metal raw materials are oxides containing metal elements other than the metal elements constituting the transition metal oxide obtained in the first step, etc.

[0044] In the third step, the mixture obtained in the second step is fired at 850°C or below for a predetermined time to obtain the cathode active material according to this embodiment. If the firing temperature exceeds 850°C, the compound A containing at least one of Ca and Sr may aggregate in specific areas, resulting in insufficient effects. The firing of the mixture in the third step may involve a multi-stage firing process, including a first firing step in which the mixture is fired in a firing furnace under an oxygen stream at a first heating rate to a first set temperature of 450°C to 680°C, and a second firing step in which the fired product obtained in the first firing step is fired in a firing furnace under an oxygen stream at a second heating rate to a second set temperature of greater than 680°C but not greater than 850°C. Here, the first heating rate is in the range of 1.5°C / min to 5.5°C / min, and the second heating rate is slower than the first heating rate, in the range of 0.1°C / min to 3.5°C / min. The first temperature increase rate and the second temperature increase rate may be set in plural for each temperature range as long as they are within the above-specified range. 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, from the viewpoint of adjusting each of the above-specified parameters of the lithium transition metal composite oxide to the above-specified range. The holding time of the first set temperature is the time for maintaining the first set temperature after reaching the first set temperature. 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, from the viewpoint of adjusting each of the above-specified parameters of the lithium transition metal composite oxide to the above-specified range. The holding time of the second set temperature is the time for maintaining the second set temperature after reaching the second set temperature. When firing the mixture, in order to adjust each of the above-specified parameters to the above-specified range, for example, the firing is carried out 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 can be in the range of 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.

[0045] The molar fraction of the metal element contained in the positive electrode active material obtained above was measured by inductively coupled plasma (ICP) emission spectroscopy, and the molar fraction of the metal element contained in the positive electrode active material was determined by the general formula Li a Ni x Mn y M z Ca α Sr β O2-b (where 0.95 < a < 1.05, 0.7 ≤ x ≤ 0.95, 0 < y ≤ 0.3, 0 ≤ z ≤ 0.3, α + β > 0, 0 ≤ b < 0.05, x + y + z = 1, and M is at least one element selected from Al, Co, Fe, Ti, Si, Nb, Mo, W, and Zn). Note that Ca and Sr are contained in Compound A present on the surface of the lithium transition metal composite oxide.

[0046] [Negative Electrode] The negative electrode 12 has a negative electrode current collector 40 and a negative electrode composite layer 41 formed on both surfaces of the negative electrode current collector 40. For the negative electrode current collector 40, a foil of a metal stable within the potential range of the negative electrode 12 such as copper or a copper alloy, a film having such a metal disposed on the surface layer, etc. can be used. The negative electrode composite layer 41 contains a negative electrode active material and a binder. The thickness of the negative electrode composite layer 41 is, for example, 10 μm to 150 μm on one side of the negative electrode current collector 40. The negative electrode 12 can be manufactured by applying a negative electrode composite slurry containing a negative electrode active material, a binder, etc. to the surface of the negative electrode current collector 40, drying the coating film, and then rolling to form the negative electrode composite layer 41 on both surfaces of the negative electrode current collector 40.

[0047] The negative electrode active material contained in the negative electrode composite layer 41 is not particularly limited as long as it can reversibly occlude and release lithium ions, and generally a carbon material such as graphite is used. The graphite may be any of natural graphite such as flake graphite, massive graphite, and earthy graphite, artificial massive graphite, and artificial graphite such as graphitized mesophase carbon microbeads. Also, as the negative electrode active material, a metal that alloys with Li such as Si or Sn, a metal compound containing Si or Sn, a lithium titanium composite oxide, etc. may be used. Further, those provided with a carbon coating may be used. For example, a Si-containing compound represented by SiO x (0.5 ≤ x ≤ 1.6), or Li 2y SiO (2+y) A Si-containing compound in which fine particles of Si are dispersed in a lithium silicate phase represented by (0 < y < 2), etc. may be used in combination with graphite.

[0048] The binder contained in negative electrode mixture layer 41 may be a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, or polyolefin, as in the case of positive electrode 11, but is preferably styrene-butadiene rubber (SBR). Negative electrode mixture layer 41 may also contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

[0049] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. 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 and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a laminated structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin, or a filler layer containing an inorganic compound filler may be provided on the surface of the separator 13.

[0050] [Non-aqueous electrolyte] The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, 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 halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0051] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0052] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0053] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF, LiClO, LiPF, LiAsF, LiSbF, LiMnCl, LiSCN, LiCF, SO, LiCF, CO, Li(P(C), O)F, and LiPF.6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more}, and imide salts such as these may be mentioned. The lithium salt may be used individually by these 1 type, or may be used in mixture of multiple types. Among these, from viewpoints, such as ionic conductivity and electrochemical stability, it is preferable to use LiPF6. The concentration of the lithium salt is, for example, 0.8 mol to 1.8 mol per 1 L of nonaqueous solvent. Further, vinylene carbonate or a propane sultone type additive may be added.

Example

[0054] Hereinafter, the present disclosure will be further described by examples and comparative examples, but the present disclosure is not limited to the following examples.

[0055] [Preparation of positive electrode active material] <Example 1> General formula Ni 0.82 Mn 0.03 Co 0.15A transition metal oxide was mixed with Sr(OH)2 and Ca(OH)2 so that the Sr and Ca contents were 1.0 mol% and 0.1 mol%, respectively, relative to the total amount of Ni, Mn, and Co in the transition metal oxide (represented by O2). Lithium hydroxide monohydrate (LiOH·HO) was then added so that the molar ratio of the total amount of Ni, Mn, Co, Sr, and Ca to Li was 1:1.03. The mixture was then fired under an oxygen stream with an oxygen concentration of 95% (flow rate of 5 L / min per kg of mixture) at a heating rate of 2°C / min from room temperature to 650°C, and then at a heating rate of 1°C / min from 650°C to 800°C. Impurities were removed by washing the fired material, yielding the positive electrode active material of Example 1. Analysis of the composition of the positive electrode active material of Example 1 using ICP-AES revealed that the Li 0.99 Ni 0.82 Mn 0.03 Co 0.15 Sr 0.01 Ca 0.001 O2. X-ray diffraction measurement was also performed on the positive electrode active material of Example 1. The ratio of metal elements other than Li present in the Li layer to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide was 0.87 mol %. The ratio m / n of the half-width m of the diffraction peak of the (003) plane to the half-width n of the diffraction peak of the (104) plane in the X-ray diffraction pattern obtained by X-ray diffraction was 0.978.

[0056] [Preparation of positive electrode] A positive electrode slurry was prepared by mixing 95 parts by mass of the positive electrode active material, 3 parts by mass of acetylene black as a conductive material, and 2 parts by mass of polyvinylidene fluoride as a binder, and then mixing this with N-methyl-2-pyrrolidone (NMP). The slurry was then applied to a positive electrode current collector made of aluminum foil with a thickness of 15 μm. After drying the coating, the coating was rolled with a rolling roller and cut to a predetermined electrode size to obtain a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core. An exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode core.

[0057] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluorophosphate (LiPF6) was dissolved in the mixed solvent to a concentration of 1.2 mol / L to prepare a nonaqueous electrolyte.

[0058] [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.

[0059] [Capacity retention rate evaluation] The test cell was subjected to the following cycle test. 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 using the following formula.

[0060] Capacity retention rate (%) = (30th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 <Cycle test> The test cell was charged at a constant current of 0.2 It at a temperature of 25°C until the battery voltage reached 4.3 V, and then charged at a constant voltage until the current value reached 1 / 100 It at 4.3 V. It was 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.

[0061] <Examples 2 and 4> Test cells were prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was synthesized by changing the raw materials, raw material compounding ratio, the molar ratio of the total amount of metal elements other than Li to Li to 1:1.05, and the second-stage firing temperature to 750°C.

[0062] Example 3 Test cells were prepared and evaluated in the same manner as in Example 4, except that the raw material composition ratio was changed and the positive electrode active material was synthesized under an oxygen flow with an oxygen concentration of 95% (flow rate of 10 L / min per 1 kg of mixture).

[0063] <Example 5> Test cells were fabricated and evaluated in the same manner as in Example 2, except for the raw materials used, the raw material blending ratio, and the fact that the positive electrode active material was synthesized by firing from room temperature to 650°C at a heating rate of 5°C / min, and then firing from 650°C to 750°C at a heating rate of 3°C / min.

[0064] <Examples 6 to 8> Test cells were fabricated and evaluated in the same manner as in Example 1, except that the raw materials, raw material blending ratio, and second-stage firing temperature were changed to 730°C to synthesize the positive electrode active material.

[0065] <Comparative Example 1> Test cells were fabricated and evaluated in the same manner as in Example 1, except that the positive electrode active material was synthesized by changing the raw material blend ratio.

[0066] <Comparative Example 2> Test cells were fabricated and evaluated in the same manner as in Example 2, except that the raw materials and the raw material blending ratios used were changed to synthesize the positive electrode active materials.

[0067] <Comparative Example 3> Test cells were prepared in the same manner as in Example 2, except that the raw material compounding ratio, the molar ratio of the total amount of metal elements other than Li to Li was changed to 1:0.95, and the second-stage firing temperature was changed to 850°C to synthesize the positive electrode active material, and then the test cells were evaluated.

[0068] <Comparative Example 4> Test cells were prepared and evaluated in the same manner as in Example 2, except that the raw materials used, the raw material blending ratio, and the oxygen concentration were changed to 95% and the positive electrode active material was synthesized under an oxygen flow (flow rate of 0.1 L / min per 1 kg of mixture).

[0069] <Comparative Example 5> Test cells were fabricated and evaluated in the same manner as in Example 6, except that the raw materials and the compounding ratio of the raw materials used were changed to synthesize the positive electrode active material.

[0070] <Comparative Example 6> Test cells were fabricated and evaluated in the same manner as in Example 1, except that the raw materials, raw material blending ratio, and second-stage firing temperature were changed to 730°C to synthesize the positive electrode active material.

[0071] <Comparative Example 7> Test cells were prepared and evaluated in the same manner as in Example 7, except that the raw material compounding ratio and the molar ratio of the total amount of metal elements other than Li to Li were changed to 1:1.1 to synthesize the positive electrode active material.

[0072] The capacity retention rates of the examples and comparative examples are shown in Tables 1 to 3. The evaluation results of the capacity retention rates shown in Tables 1 to 3 are expressed relative to the capacity retention rates of the test cells of Comparative Examples 1, 2, and 5, which are set to 100%. Tables 1 to 3 also show the ratio m / n of the half-width m of the diffraction peak of the (003) plane to the half-width n of the diffraction peak of the (104) plane in the X-ray diffraction pattern obtained by X-ray diffraction, and the proportion of metal elements other than Li present in the Li layer relative to the total number of moles of metal elements other than Li.

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3] As shown in Tables 1 to 3, Examples 1 to 8 had higher capacity retention rates than Comparative Examples 1 to 7. In addition, in none of the Examples did the X-ray diffraction pattern have peaks derived from SrO or CaO. As an example, the X-ray diffraction patterns of Examples 2 and 3 and SrO and CaO are shown in Figure 2. [Explanation of symbols]

[0076] 10 Nonaqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery case 16 outer can 17 Sealing body 18,19 Insulating plate 20 Positive electrode tab 21 Negative electrode tab 22 Grooved part 23 Bottom plate 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket 30 Positive electrode current collector 31 Positive electrode mixture layer 40 Negative electrode current collector 41 Negative electrode composite layer

Claims

1. a lithium transition metal composite oxide having a layered structure, containing lithium, nickel, and manganese, wherein the Ni content is 70 mol% or more and the Mn content is more than 0 mol% and less than 30 mol%, relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide; a compound A containing at least one of Ca and Sr, which is present on the surface or grain boundary of the primary particles of the lithium transition metal composite oxide; the layered structure includes a Li layer through which Li reversibly enters and leaves, and the ratio of metal elements other than Li present in the Li layer is 0.7 mol % or more and 3.0 mol % or less with respect to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide; the ratio m / n of the half-width m of the diffraction peak of the (003) plane to the half-width n of the diffraction peak of the (104) plane in the X-ray diffraction pattern obtained by X-ray diffraction is 0.75≦m / n≦1.0; A positive electrode active material for a non-aqueous electrolyte secondary battery, which has an X-ray diffraction pattern obtained by X-ray diffraction measurement that does not contain peaks derived from CaO and SrO.

2. A positive electrode active material for a non-aqueous electrolyte secondary battery as described in claim 1, wherein the total amount of Ca and Sr in compound A is 1.1 mol% or less relative to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide.

3. 3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the crystallite size s calculated from the half-width n of the diffraction peak of the (104) plane in an X-ray diffraction pattern by Scherrer's equation is in the range of 400 Å≦s≦800 Å.

4. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, comprising: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, the method comprising: firing a dry-mixture of a transition metal oxide, a Li compound, and at least one of a Ca compound and a Sr compound at 850°C or less.

5. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, a negative electrode, and a non-aqueous electrolyte.

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