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

Abstract

A positive electrode active material included in this non-aqueous electrolyte secondary battery positive electrode includes a lithium–transition metal composite oxide. The lithium–transition metal composite oxide contains 80–95 mol% of Ni and 0–20 mol% of Mn, and 3–8 mol% of a metal element other than Li is present in a Li layer of the lithium–transition metal composite oxide. The ratio m / n of the half width m of the diffraction peak for the (003) plane to the half width n of the diffraction peak for the (110) plane in an x-ray diffraction pattern obtained by x-ray diffraction of the positive electrode active material satisfies 0.72≤m / n≤0.85. The lithium–transition metal composite oxide is formed from secondary particles that are aggregates of primary particles, the internal porosity of the secondary particles being 1%–5%.

Description

Technical Field

[0001] The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

Background Art

[0002] In recent years, lithium transition metal composite oxides containing Ni have attracted attention as positive electrode active materials with high energy density. For example, in Patent Document 1, a general formula Li , , , , , ,

[0005] , , , , , , , , ,

[0004] , Ni 1-y M y O2 (where 0.9 ≦ x ≦ 1.2, 0 < y ≦ 0.7, M is one or more elements selected from Mn, Co, etc., and α > 0.20), and in powder X-ray diffraction (XRD), the half-value width of the diffraction peak of the (003) plane is 0.14° or less, and the diffraction peak intensity ratio [(104) / (003)] of the (104) plane and the (003) plane is 0. These positive electrode active materials are disclosed. The diffraction peak intensity ratio [(104) / (003)] of the (104) plane and the (003) plane is 0.80 or less.

[0003] Further, in Patent Document 2, a hexagonal crystal system having a layered structure, a LiNiMn composite oxide containing Mg, and lattice constants a and c obtained by X-ray diffraction method are 2.8640 Å ≦ a ≦ 2.8750 Å, 14.195 Å ≦ c ≦ 14.225 Å. These positive electrode active materials are disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in the lithium transition metal composite oxide contained in the positive electrode active material, when the proportion of Ni is 80 mol% or more with respect to the total molar amount of metal elements excluding Li, since the amount of Li extracted during charging is large, there is a problem that the layered crystal structure is broken and the capacity decreases when charging and discharging are repeated. The technology disclosed in Patent Document 1 does not consider lithium transition metal composite oxides with a high Ni content, and there is still room for improvement in the charge-discharge cycle characteristics.

[0006] Therefore, an object of the present disclosure is to provide a positive electrode for a non-aqueous electrolyte secondary battery including a lithium transition metal composite oxide with a high Ni content, which contributes to improving the charge-discharge cycle characteristics of the battery.

Means for Solving the Problems

[0007] The positive electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a positive electrode for a non-aqueous electrolyte secondary battery including a positive electrode active material. The positive electrode active material includes a lithium transition metal composite oxide having a layered structure and containing at least Ni and Mn. The proportion of Ni with respect to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 80 mol% ≤ Ni ≤ 95 mol%. The proportion of Mn with respect to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% < Mn ≤ 20 mol%. The layered structure includes a Li layer in which Li reversibly enters and exits, and the proportion of metal elements other than Li present in the Li layer is 3 mol% or more and 8 mol% or less with respect to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide. 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 (110) plane in the X-ray diffraction pattern by X-ray diffraction is 0.72 ≤ m / n ≤ 0.85. The lithium transition metal composite oxide in the positive electrode for a non-aqueous electrolyte secondary battery is composed of secondary particles formed by aggregation of primary particles, and the porosity inside the particles of the secondary particles is 1 - 5%.

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

Advantages of the Invention

[0009] According to the positive electrode for a non-aqueous electrolyte secondary battery which is 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.

Brief Description of the Drawings

[0010] [Figure 1] It is a cross-sectional view of a non-aqueous electrolyte secondary battery which is an example of an embodiment.

Mode for Carrying Out the Invention

[0011] In the layered structure of the lithium transition metal composite oxide contained in the positive electrode, there are a transition metal layer containing Ni etc., a Li layer, and an oxygen layer. By the reversible entry and exit of Li ions present in the Li layer, the charge and discharge reaction of the battery proceeds. When a lithium transition metal composite oxide with a high Ni content where the ratio of Ni is 80 mol% or more with respect to the total number of moles of metal elements excluding Li is used, many Li ions are extracted from the Li layer during charging of the battery, so the layered structure collapses and this leads to a decrease in battery capacity. Also, a lithium transition metal composite oxide with a high Ni content has high activity near the particle surface, and due to reaction with the electrolyte etc., generation and erosion of a surface deterioration layer easily occur, leading to a decrease in battery capacity.

[0012] Therefore, the inventors diligently studied to solve the above problems and found that the structure of the lithium transition metal composite oxide can be stabilized by including a predetermined amount of Mn in the transition metal layer of the lithium transition metal composite oxide contained in the positive electrode, which does not undergo an oxidation state change during charging and discharging, while including a predetermined amount of metal elements other than Li in the Li layer, and further creating a layered structure with appropriate strain in the planar direction such that the ratio of the full width at half maximum m of the (003) plane and the full width at half maximum n of the (110) plane of the X-ray diffraction pattern is within a predetermined range. Furthermore, they found that by keeping the porosity of the lithium transition metal composite oxide in the positive electrode within a predetermined range, the reaction with the electrolyte is controlled, the formation of a surface degradation layer and erosion are suppressed, and the cycle characteristics are improved. It is believed that the improvement in cycle characteristics was due to the unique synergistic effect of the structural stabilization of the lithium transition metal composite oxide and the suppression of the formation of a surface degradation layer and erosion by controlling the porosity of the lithium transition metal composite oxide in the positive electrode.

[0013] In this specification, the notation "value (A) ~ value (B)" means that the values ​​are greater than or equal to value (A) and less than or equal to value (B).

[0014] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery using the positive electrode for a non-aqueous electrolyte secondary battery according to this disclosure will be described in detail. In the following, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16 will be exemplified, but the outer casing is not limited to a cylindrical outer casing, and may be, for example, a rectangular outer casing, or an outer casing composed of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode body may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with separators in between.

[0015] Figure 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As illustrated in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte, and an outer casing 16 that houses the electrode body 14 and the electrolyte. The electrode body 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 wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. In the following, for convenience of explanation, the side of the battery with the sealing body 17 will be referred to as the top, and the bottom side of the outer casing 16 as the bottom.

[0016] A non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of lithium salts such as LiPF6 are used as the electrolyte salt. Note that the electrolyte is not limited to a liquid electrolyte; it may also be a solid electrolyte using a gel-like polymer or the like.

[0017] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all elongated strip-shaped bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the longitudinal direction and the width direction (short direction) than the positive electrode 11. The two separators 13 are formed to be at least slightly larger in dimensions than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0018] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends outside the insulating plate 19 towards the bottom of the outer can 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes 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 becomes the negative electrode terminal.

[0019] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery. The outer casing 16 has a grooved portion 22 formed on its side surface, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0020] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except 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, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0021] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the non-aqueous electrolyte secondary battery 10, with particular attention paid to the positive electrode 11.

[0022] [Positive electrode] The positive electrode 11 comprises a positive electrode current collector and a positive electrode composite layer formed on both sides of the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The positive electrode composite layer contains a positive electrode active material, a conductive material, and a binder. The positive electrode 11 can be manufactured by applying a positive electrode slurry containing the positive electrode active material, conductive material, and binder to the surface of the positive electrode current collector (coating step), drying the coating (drying step), and rolling to form the positive electrode composite layer on both sides of the positive electrode current collector (rolling step).

[0023] Examples of conductive materials included in the positive electrode composite layer include carbon materials such as carbon black, acetylene black, Ketjenblack, carbon nanotubes (CNTs), graphene, and graphite. Examples of binders included in the positive electrode composite layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. These resins may also be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0024] The positive electrode active material contained in the positive electrode composite layer has a layered structure and includes a lithium transition metal composite oxide containing at least Ni and Mn.

[0025] Examples of layered structures of lithium transition metal composite oxides include layered structures belonging to space group R-3m and layered structures belonging to space group C2 / m. Among these, a layered structure belonging to space group R-3m is preferred in terms of high capacity and crystal structure stability. The layered structure of lithium transition metal composite oxide includes a transition metal layer, a Li layer, and an oxygen layer. The Li layer is a layer in which Li reversibly enters and exits.

[0026] The ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 80 mol% ≤ Ni ≤ 95 mol%, preferably 83 mol% or more, and more preferably 85 mol% or more.

[0027] The ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% < Mn ≤ 20 mol%, preferably in the range of 2 mol% ≤ Mn ≤ 17 mol%, and more preferably in the range of 5 mol% ≤ Mn ≤ 15 mol%. Since the oxidation number of Mn does not change 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. Mn may be uniformly dispersed in the layered structure of the lithium transition metal composite oxide, may be present in a part of the layered structure, or may be distributed in a state with a concentration gradient in the layered structure.

[0028] The lithium transition metal composite oxide may further contain M (M is at least one element selected from B, Al, Ca, Co, Fe, Ti, Si, Sr, Nb, Mo, W, Zr, and Zn). The ratio of M to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is preferably in the range of 0 mol% ≤ M ≤ 7 mol%, more preferably in the range of 0.01 mol% ≤ M ≤ 5 mol%, and particularly preferably in the range of 0.05 mol% ≤ M ≤ 3 mol%. By the lithium transition metal composite oxide containing M, the charge-discharge capacity and initial efficiency of the battery are improved.

[0029] Also, the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is preferably in the range of 0 mol% ≤ Co ≤ 5 mol%, more preferably in the range of 0 mol% ≤ Co ≤ 3 mol%, and particularly preferably substantially free of Co in the range of 0 mol% ≤ Co ≤ 0.1 mol%. "Substantially free of Co" means the case where Co is not contained at all and the case where Co is mixed as an impurity (the case where Co is mixed in an amount that cannot be accurately quantified).

[0030] The lithium transition metal composite oxide has the general formula Li x Ni y Mn z M α O 2-β(where 0.95 ≦ x ≦ 1.05, 0.80 ≦ y ≦ 0.95, 0 < z ≦ 0.2, 0 ≦ α ≦ 0.07, 0 ≦ β ≦ 0.05, y + z + α = 1, and M is at least one element selected from B, Al, Ca, Co, Fe, Ti, Si, Sr, Nb, Mo, W, Zr, and Zn) may be represented.

[0031] The positive electrode active material may contain a lithium transition metal composite oxide other than those 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 element 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.

[0032] The lithium transition metal composite oxide is 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 reaches 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 Corporation) with water as the dispersion medium.

[0033] 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, from 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).

[0034] Lithium transition metal composite oxides may have a surface modification layer on the surface of secondary particles or at the interface where primary particles come into contact. The surface modification layer may also contain at least one element selected from Co, Ca, Sr, W, B, Nb, Mo, Al, and Zr (hereinafter referred to as the surface modification element), and it is particularly preferable that it contains at least one of Ca, Sr, B, and W. The surface modification layer suppresses side reactions between the lithium transition metal composite oxide and the electrolyte, thereby suppressing battery degradation.

[0035] The content of surface-modifying elements in the surface-modifying layer can be, for example, 0.05 mol% to 0.50 mol% relative to the lithium transition metal composite oxide. Within this range, the surface state of the lithium transition metal composite oxide can be stabilized by electronic interactions. The presence of surface-modifying elements in the surface-modifying layer can be confirmed by energy-dispersive X-ray spectroscopy (TEM-EDX). Furthermore, the content of surface-modifying elements in the surface-modifying layer can be measured by inductively coupled plasma (ICP) emission spectroscopy of a solution obtained by dissolving the lithium transition metal composite oxide in a mixed solution of aqua regia and hydrofluoric acid.

[0036] The thickness of the surface modification layer is, for example, 0.1 nm or more. This suppresses the reaction with the electrolyte on the surface of the lithium transition metal composite oxide. Alternatively, the thickness of the surface modification layer may be, for example, 5 nm or less.

[0037] In the layered structure of the lithium transition metal composite oxide, the proportion of metal elements other than Li present in the Li layer is 3 mol% to 8 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 3 mol%, the stability of the layered structure when Li ions are extracted from the Li layer decreases, the structure breaks down, and this leads to a decrease in battery capacity. Furthermore, if the proportion of metal elements other than Li in the Li layer exceeds 8 mol%, the diffusivity of Li ions in the Li layer decreases, leading to a decrease in battery capacity and an increase in the battery's reaction resistance. The metal elements present in the Li layer are mainly Ni, but other metal elements may also be included. The proportion of metal elements other than Li present in the Li layer can be adjusted, for example, by the amount of Li compound mixed when manufacturing the lithium transition metal composite oxide, the set temperature and holding time during multi-stage firing of the mixture of the transition metal oxide and the Li compound, and the heating rate.

[0038] The proportion of metal elements other than Li in the Li layer can be obtained from the Rietveld analysis results of the X-ray diffraction pattern obtained by X-ray diffraction measurement of lithium transition metal composite oxides. For Rietveld analysis of X-ray diffraction patterns, for example, Rietveld analysis software such as PDXL2 (Rigaku Corporation) can be used.

[0039] The X-ray diffraction pattern was obtained by powder X-ray diffraction using a powder X-ray diffractometer (manufactured by Rigaku Corporation, product name "RINT-TTR", source Cu-Kα) under the following conditions. Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background: B-spline Profile function: Split type pseudo-Voigt function Constraint condition: Li(3a)+Ni(3a)=1 Ni(3a) + Ni(3b) = α (where α is the proportion of Ni in each) ICSD No.: 98-009-4814

[0040] For lithium transition metal composite oxides, the ratio m / n of the full width at half maximum (FMAX) of the diffraction peak of the (003) plane to the full width at half maximum (FMAX) of the diffraction peak of the (110) plane in the X-ray diffraction pattern obtained by the above-mentioned X-ray diffraction is 0.72 ≤ m / n ≤ 0.85. Within this range, the layered structure can be made to have an appropriate strain in the planar direction, so a battery with high capacity and improved charge-discharge cycle characteristics can be obtained. If m / n is less than 0.72, the strain of the layered structure is too large and the layered structure becomes brittle. Also, if m / n is greater than 0.85, the battery capacity decreases.

[0041] In the positive electrode 11, the lithium transition metal composite oxide is composed of secondary particles formed by the aggregation of multiple primary particles. In the positive electrode 11, the porosity of the secondary particles of the lithium transition metal composite oxide is preferably 1 to 5%, and more preferably 1 to 3%. Furthermore, by keeping the porosity within the above predetermined range, the reaction with the electrolyte is controlled, the formation of a surface degradation layer and erosion are suppressed, and the cycle characteristics are improved. If the porosity of the secondary particles of the lithium transition metal composite oxide in the positive electrode 11 is less than 1%, the reaction efficiency between the positive electrode and the electrolyte is likely to decrease. Also, if the porosity exceeds 5%, there will be more side reactions on the surface of the lithium transition metal composite oxide, and the degradation of the positive electrode will progress easily. The porosity of the secondary particles of the lithium transition metal composite oxide in the positive electrode 11 can be adjusted by the rolling process during positive electrode manufacturing and by the heat treatment temperature in the first step and the firing temperature in the third step in the positive electrode active material manufacturing method described later.

[0042] Next, an example of a method for producing a positive electrode active material containing a lithium transition metal composite oxide will be described.

[0043] A method for producing a positive electrode active material includes, for example, a first step of obtaining a transition metal oxide containing Ni and Mn, a second step of mixing the transition metal oxide with a Li compound to obtain a mixture, and a third step of calcining the mixture to obtain a lithium transition metal composite oxide.

[0044] In the first step, for example, an alkaline solution such as sodium hydroxide is added dropwise while stirring a solution of metal salts containing Ni and Mn to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (coprecipitation) a transition metal hydroxide containing Ni and Mn. The transition metal hydroxide is then heat-treated to obtain a transition metal oxide containing Ni and Mn. The heat treatment temperature is, for example, in the range of 300°C to 600°C. Note that the transition metal oxide may also contain metal elements other than Ni and Mn, such as Co.

[0045] In the second step, the transition metal oxide obtained in the first step and the Li compound are first dry-mixed to obtain a mixture. Examples of Li compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. 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. Furthermore, when mixing the transition metal oxide and the Li compound, compounds containing metal elements other than Li, Ni, and Mn may be added. Examples of compounds containing metal elements other than Li, Ni, and Mn include Ca compounds such as Ca(OH)2, Sr compounds such as Sr(OH)2, Al compounds such as Al(OH)3, Nb compounds such as Nb2O5, and W compounds such as WO3.

[0046] In the third step, the mixture obtained in the second step is fired at 950 °C or lower for a predetermined time to obtain a lithium transition metal composite oxide. When fired at a temperature exceeding 950 °C, the porosity of the lithium transition metal composite oxide in the positive electrode 11 may fall outside the predetermined range during the rolling process in the production of the positive electrode, and sufficient effects may not be obtained. The firing of the mixture in the third step includes, for example, a first firing step of firing in a firing furnace under an oxygen stream to a first set temperature of 450 °C to 680 °C at a first heating rate, and firing the fired product obtained by the first firing step in the firing furnace under an oxygen stream to a second set temperature exceeding 680 °C and 850 °C or lower at a second heating rate, and includes a multi-step firing process. 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 and is in the range of 0.1 °C / min to 3.5 °C / min. In addition, the first heating rate and the second heating rate may be set in plural for each temperature region 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. 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. 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 terms of adjusting each of the above parameters within the above-specified range, for example, it is performed in an oxygen stream with an oxygen concentration of 60% or more, and the flow rate of the oxygen stream can be set within the range of 0.2 mL / min to 4 mL / min per 10 cm of the firing furnace and 0.3 L / min or more per 1 kg of the mixture. 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 (110) plane in the X-ray diffraction pattern can be adjusted by the mixing ratio of the compounds in the second step and the firing temperature and firing time in the third step. Further, the lithium transition metal composite oxide obtained in the third step may be washed with water. 3 per, it can be set within the range of 0.2 mL / min to 4 mL / min and 0.3 L / min or more per 1 kg of the mixture. 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 (110) plane in the X-ray diffraction pattern can be adjusted by the mixing ratio of the compounds in the second step and the firing temperature and firing time in the third step.

[0047] [Negative electrode] The negative electrode 12 has a negative electrode current collector and a negative electrode composite layer formed on both surfaces of the negative electrode current collector. For the negative electrode current collector, 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, or the like can be used. The negative electrode composite layer contains a negative electrode active material and a binder. The negative electrode 12 can be produced 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, drying the coating film, and then rolling to form the negative electrode composite layer on both surfaces of the negative electrode current collector.

[0048] The negative electrode active material contained in the negative electrode composite layer 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 flaky graphite, massive graphite, and earthy graphite, artificial massive graphite, and artificial graphite such as graphitized mesophase carbon microbeads. Further, 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, or the like 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) may be used in combination with graphite.

[0049] For the binder contained in the negative electrode composite layer, similar to the case of the positive electrode 11, a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, polyolefin, etc. may be used, but preferably styrene-butadiene rubber (SBR) is used. Further, the negative electrode composite layer may contain CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), or the like.

[0050] [Separator] For example, a porous sheet having ion permeability and insulating properties can be used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a laminated structure. Furthermore, the surface of the separator 13 may be provided with a highly heat-resistant resin layer such as aramid resin, and a filler layer containing an inorganic compound filler.

[0051] [Non-aqueous electrolytes] Non-aqueous electrolytes include, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0052] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters 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 linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0053] Examples of the above ethers include cyclic ethers such as 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, crown ether, etc., and chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl 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, tetraethylene glycol dimethyl ether, etc.

[0054] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiMnCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), 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), etc., LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1Examples include imide salts such as SO2){l,m are integers greater than or equal to 0}. Lithium salts may be used individually or in mixtures of multiple types. Of these, LiPF6 is preferred from the viewpoint of ionic conductivity and electrochemical stability. The concentration of the lithium salt is, for example, 0.8 moles to 1.8 moles per liter of non-aqueous solvent. Furthermore, vinylene carbonate or propane-sultone additives may be added. [Examples]

[0055] The present disclosure will be further explained below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0056] [Fabrication of positive electrode active material] <Example 1> [Ni obtained by coprecipitation method] 0.91 Mn 0.09 The composite hydroxide represented by ](OH)2 is calcined at 500°C for 8 hours, and the composite oxide (Ni 0.91 Mn 0.09 Step 1: O2 was obtained. A mixture was obtained by mixing lithium hydroxide (LiOH) with the total amount of Ni and Mn from the above composite oxide so that the molar ratio of Li was 1:1.01 (Step 2). The mixture was calcined from room temperature to 650°C at a heating rate of 2°C / min under an oxygen stream with an oxygen concentration of 95% (flow rate of 5 L / min per 1 kg of mixture), and then calcined from 650°C to 750°C at a heating rate of 1°C / min. Impurities were removed from the calcined product by washing with water to obtain the lithium transition metal composite oxide as the positive electrode active material of Example 1 (Step 3). The results of the analysis of the lithium transition metal composite oxide by ICP-AES are shown in Table 1. X-ray diffraction measurements were 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 5.1 mol%. The ratio m / n of the full width at half maximum (FMAX) of the diffraction peaks of the (003) plane to the full width at half maximum (FMAX) of the diffraction peaks of the (110) plane in the X-ray diffraction pattern obtained by X-ray diffraction was 0.79.

[0057] [Fabrication of the positive electrode] A positive electrode slurry was prepared by mixing 95 parts by mass of the above 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). Next, the slurry was applied to a positive electrode current collector made of 15 μm thick aluminum foil, and after the coating film was dried, the coating film was rolled out using a rolling mill and cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode composite layer was formed on both sides of the positive electrode core. An exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode core was exposed.

[0058] [Preparation of non-aqueous electrolytes] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF6) was dissolved in this mixed solvent to a concentration of 1.2 mol / liter to prepare a non-aqueous electrolyte.

[0059] [Preparation of test cells] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to a lithium metal foil as the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a polyolefin separator, and then press-formed radially to create a flat, wound electrode body. This electrode body was housed in an outer casing made of aluminum laminate sheet, the non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to obtain a test cell.

[0060] [Measurement of porosity of lithium transition metal composite oxide in the positive electrode] The porosity of the lithium transition metal composite oxide in the positive electrode of the above test cell was measured. The method for measuring the porosity was as follows: First, the fabricated positive electrode was processed using a cross-section polisher (CP) method, and the polished surface was observed using a Scanning Ion Microscopy (SIM) at a magnification of 1,000 to 10,000 times. From the obtained SIM image, the porosity of the lithium transition metal composite oxide contained in the positive electrode was calculated using Image-Pro PLUS analysis software. Specifically, the color of the vacant portion of the secondary particles of the lithium transition metal composite oxide and the color of the non-vacant portion of the secondary particles of the lithium transition metal composite oxide were divided into white and black or black and white portions, the area of ​​each portion was determined, and the porosity was calculated from the ratio of these areas.

[0061] [Evaluation of initial efficiency] The above test cell was charged at a constant current of 0.2It at a temperature of 25°C until the battery voltage reached 4.3V, and then charged at a constant voltage until the current value was reduced to 1 / 100It at 4.3V. After that, it was discharged at a constant current of 0.2It until the battery voltage reached 2.5V. The initial efficiency was calculated from the charge and discharge capacities measured during the above charge and discharge using the following formula. Initial efficiency (%) = Discharge capacity / Charge capacity × 100

[0062] [Evaluation of capacity retention rate] The following cycle test was performed on the above test cell. The discharge capacity after the first cycle and the discharge capacity after the 30th cycle were determined, and the capacity retention rate was calculated using the following formula. Capacity retention rate (%) = (Discharge capacity at 30th cycle ÷ Discharge capacity at 1st cycle) × 100 <Cycle Testing> The test cell was charged with a constant current of 0.2It at a temperature of 25°C until the battery voltage reached 4.3V, and then charged with a constant voltage until the current value was 1 / 20It at 4.3V. After that, it was discharged with a constant current of 0.2It until the battery voltage reached 2.5V. This charge-discharge cycle was repeated 30 times.

[0063] <Example 2> In the preparation of positive electrode active material, the general formula is Ni 0.92 Mn 0.08 A test cell was prepared in the same manner as in Example 1, except that a transition metal oxide and Ca(OH)2 were mixed so that the Ca content was 1 mol% relative to the total amount of Ni and Mn in the transition metal oxide represented by O2, and lithium hydroxide (LiOH) was mixed so that the molar ratio of Li to the total amount of Ni, Mn, and Ca was 1:1.01. Measurement and evaluation were then performed.

[0064] <Example 3> In the preparation of positive electrode active material, the general formula is Ni 0.915 Mn 0.085 A test cell was prepared in the same manner as in Example 1, except that a transition metal oxide and Sr(OH)2 were mixed so that the Sr content was 0.5 mol% relative to the total amount of Ni and Mn in the transition metal oxide represented by O2, and lithium hydroxide (LiOH) was mixed so that the molar ratio of the total amount of Ni, Mn, and Sr to Li was 1:1.01. Measurement and evaluation were then performed.

[0065] <Example 4> In the preparation of positive electrode active material, the general formula is Ni 0.93 Mn 0.07 A test cell was prepared in the same manner as in Example 1, except that a transition metal oxide was mixed with Al(OH)3 so that the Al content was 2 mol% relative to the total amount of Ni and Mn represented by O2, and lithium hydroxide (LiOH) was mixed so that the molar ratio of Li to the total amount of Ni, Mn, and Al was 1:1.03. Measurement and evaluation were then performed.

[0066] <Example 5> In the preparation of positive electrode active material, the general formula is Ni 0.91 Mn 0.09 A test cell was prepared in the same manner as in Example 1, except that a transition metal oxide and Nb2O5 were mixed so that the Nb content was 1 mol% relative to the total amount of Ni and Mn in the transition metal oxide represented by O2, and lithium hydroxide (LiOH) was mixed so that the molar ratio of the total amount of Ni, Mn, and Nb to Li was 1:1.02. Measurement and evaluation were then performed.

[0067] <Example 6> In the preparation of positive electrode active material, the general formula is Ni 0.915 Mn 0.085 A test cell was prepared in the same manner as in Example 1, except that a transition metal oxide and WO3 were mixed so that the W content was 0.5 mol% relative to the total amount of Ni and Mn in the transition metal oxide represented by O2, and lithium hydroxide (LiOH) was mixed so that the molar ratio of the total amount of Ni, Mn, and W to Li was 1:1.02. Measurement and evaluation were then performed.

[0068] <Example 7> In the preparation of the positive electrode active material, [Ni obtained by coprecipitation method] 0.93 Mn 0.07 The composite hydroxide represented by ](OH)2 is calcined at 500°C for 8 hours, and the composite oxide (Ni 0.93 Mn 0.07 O2) was obtained. Furthermore, in the third step, a test cell was prepared in the same manner as in Example 1, except that it was fired from room temperature to 650°C at a heating rate of 2°C / min, and then fired from 650°C to 700°C at a heating rate of 1°C / min, and then measured and evaluated.

[0069] <Example 8> In the preparation of the positive electrode active material, [Ni obtained by coprecipitation method] 0.85 Mn 0.15 The composite hydroxide represented by ](OH)2 is calcined at 500°C for 8 hours, and the composite oxide (Ni 0.85 Mn 0.15 O2) was obtained. A transition metal oxide and H3BO3 were mixed so that the B content was 2 mol% relative to the total amount of Ni and Mn in the above composite oxide. Furthermore, lithium hydroxide (LiOH) was mixed so that the molar ratio of Ni, Mn, B and Li was 1:1.05 to obtain a mixture (second step). Furthermore, in the third step, a test cell was prepared in the same manner as in Example 1, except that it was fired from room temperature to 650°C at a heating rate of 2°C / min, and then fired from 650°C to 800°C at a heating rate of 1°C / min, and then measured and evaluated.

[0070] <Comparative Example 1> In the third step of preparing the positive electrode active material, a test cell was prepared in the same manner as in Example 1, except that it was fired from room temperature to 650°C at a heating rate of 2°C / min, and then fired from 650°C to 780°C at a heating rate of 1°C / min. The cell was then measured and evaluated.

[0071] <Comparative Example 2> In the second step of preparing the positive electrode active material, a test cell was prepared in the same manner as in Example 1, except that lithium hydroxide (LiOH) was mixed with the total amount of Ni and Mn of the composite oxide so that the molar ratio of Li was 1:1.05 to obtain a mixture, and then measurements and evaluations were performed.

[0072] <Comparative Example 3> In the third step of preparing the positive electrode active material, a test cell was prepared in the same manner as in Example 1, except that it was fired from room temperature to 650°C at a heating rate of 2°C / min, and then fired from 650°C to 730°C at a heating rate of 1°C / min. The cell was then measured and evaluated.

[0073] <Comparative Example 4> In the second step of preparing the positive electrode active material, a test cell was prepared in the same manner as in Example 1, except that lithium hydroxide (LiOH) was mixed with the total amount of Ni and Mn of the composite oxide so that the molar ratio of Li was 1:0.98. Measurement and evaluation were then performed.

[0074] <Comparative Example 5> In the third step of preparing the positive electrode active material, a test cell was prepared in the same manner as in Example 1, except that it was fired from room temperature to 650°C at a heating rate of 1°C / min, and then fired from 650°C to 730°C at a heating rate of 0.3°C / min. The cell was then measured and evaluated.

[0075] <Comparative Example 6> In the third step of preparing the positive electrode active material, a test cell was prepared in the same manner as in Example 1, except that it was fired from room temperature to 650°C at a heating rate of 3°C / min, and then fired from 650°C to 750°C at a heating rate of 1°C / min. The cell was then measured and evaluated.

[0076] <Comparative Example 7> In the third step of preparing the positive electrode active material, the test cell was prepared in the same manner as in Example 7, except that it was fired from room temperature to 650°C at a heating rate of 3°C / min, and then fired from 650°C to 720°C at a heating rate of 2°C / min. The cell was then measured and evaluated.

[0077] <Comparative Example 8> In the second step of preparing the positive electrode active material, a test cell was prepared in the same manner as in Example 8, except that lithium hydroxide (LiOH) was mixed with the total amount of Ni and Mn of the composite oxide so that the molar ratio of Li was 1:1.05 to obtain a mixture, and then measurements and evaluations were performed.

[0078] Tables 1-3 show the initial efficiency and capacity retention rates of the examples and comparative examples. The capacity retention rate evaluation results shown in Tables 1-3 are expressed relatively, with the initial efficiency and capacity retention rates of the test cells of Comparative Examples 1, 7, and 8 set to 100%. In addition, Tables 1-3 also show the ratio of metal elements other than Li present in the Li layer to the total number of moles of metal elements excluding Li, as well as the full width at half maximum (FMAX) n of the diffraction peak of the (110) plane, the FMAX m of the diffraction peak of the (003) plane, and the m / n ratio in the X-ray diffraction pattern obtained by X-ray diffraction, and the porosity inside the lithium transition metal composite oxide particles in the positive electrode.

[0079] [Table 1]

[0080] [Table 2]

[0081] [Table 3]

[0082] As shown in Tables 1-3, Examples 1-8 have higher initial efficiency and capacity retention rates than Comparative Examples 1-8. Furthermore, a comparison between Example 1 and Examples 2-6 shows that the lithium transition metal composite oxide contained in the positive electrode active material has a higher capacity retention rate when it contains metal elements such as Ca, Sr, Al, Nb, and W in addition to Li, Ni, and Mn. [Explanation of Symbols]

[0083] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating material, 26 Upper valve body, 27 Cap, 28 Gasket

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery containing a positive electrode active material, The positive electrode active material has a layered structure and includes a lithium transition metal composite oxide containing at least Ni and Mn. The ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 80 mol% ≤ Ni ≤ 95 mol%. The ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% < Mn ≤ 20 mol%, The ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% ≤ Co ≤ 0.1 mol%. The layered structure includes a Li layer in which Li reversibly enters and exits, and the proportion of metal elements other than Li present in the Li layer is 3 mol% or more and 8 mol% or less relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide. The ratio m / n of the full width at half maximum (FMAX) of the diffraction peak of the (003) plane to the full width at half maximum (FMAX) of the diffraction peak of the (110) plane in the X-ray diffraction pattern obtained by X-ray diffraction is 0.72 ≤ m / n ≤ 0.

85. The lithium transition metal composite oxide in the positive electrode for a non-aqueous electrolyte secondary battery is composed of secondary particles formed by the aggregation of primary particles, and the porosity of the internal structure of the secondary particles is 1 to 3%.

2. The lithium transition metal composite oxide further contains M (where M is at least one element selected from Al, Co, Ca, Fe, Ti, Sr, Nb, Mo, W, Zr, and Zn), The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio of M to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% ≤ M ≤ 7 mol%.

3. The lithium transition metal composite oxide has a surface modification layer on the surface of the secondary particles or at the interface where the primary particles come into contact with each other. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the surface modification layer contains at least one element selected from Ca, Sr, W, B, Nb, Mo, Al, and Zr.

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 3, wherein the surface modification layer contains at least one element of Ca, Sr, B, and W.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 4, wherein the surface modification layer contains Ca.

6. The lithium transition metal composite oxide has the general formula Li ｘ Ni ｙ Mn ｚ M α O ２－β A positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, represented by the formula (wherein 0.95 ≤ x ≤ 1.05, 0.80 ≤ y ≤ 0.95, 0 < z ≤ 0.2, 0 ≤ α ≤ 0.07, 0 ≤ β ≤ 0.05, y + z + α = 1, and M is at least one element selected from Al, Ca, Co, Fe, Ti, Sr, Nb, Mo, W, Zr, and Zn).

7. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the porosity inside the secondary particles is 1 to 1.8%.

8. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the porosity inside the secondary particles is 1.4 to 3%.

9. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the porosity inside the secondary particles is 1.4 to 1.8%.

10. A non-aqueous electrolyte secondary battery comprising a positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 9, a negative electrode, and a non-aqueous electrolyte.

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