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

Abstract

To provide a non-aqueous electrolyte secondary battery with the ratio of Ni to the total number of moles of metal elements excluding Li is 80 mol% or more capable of suppressing the decrease in battery capacity caused by charging and discharging.SOLUTION: The cathode active material for non-aqueous electrolyte secondary batteries contains Ni with 80 mol% or more of the total number of moles of metal elements excluding Li; and Lithium transition metal composite oxide containing at least Al, which includes a surface modification layer containing at least Sr formed on the surface of the primary particles of the lithium transition metal composite oxide, in which Sr does not form a solid solution in the lithium transition metal composite oxide.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, non-aqueous electrolyte secondary batteries have been widely used as high-power, high-capacity secondary batteries. These batteries include a positive electrode, a negative electrode, and a non-aqueous electrolyte, and are charged and discharged by transferring lithium ions between the positive electrode and the negative electrode. From the viewpoint of reducing the resistance and increasing the capacity of batteries, there is a demand for improving the properties of the positive electrode active material contained in the positive electrode of the battery.

[0003] For example, Patent Document 1 discloses a positive electrode active material that is a lithium transition metal composite oxide having a layered structure and containing Mn, Ni, Co, Sr, and Mo, in which the Mo content is 0.1 mol % to 1.5 mol % and the Mo / Sr content ratio is 0.5 to 2.0 in molar ratio, thereby achieving high capacity and improved charge / discharge cycle characteristics. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5245210 Summary of the Invention

[0005] In order to obtain a high discharge capacity, it is possible to design a lithium transition metal composite oxide containing a high Ni content in the positive electrode active material. However, if the ratio of Ni to the total number of moles of metal elements excluding Li is 80 mol % or more, the layered structure of the lithium transition metal composite oxide becomes unstable, and the battery capacity may decrease with charge and discharge. The technology in Patent Document 1 does not take into consideration the decrease in battery capacity with charge and discharge in batteries with a high Ni content, and there is still room for improvement.

[0006] Therefore, an object of the present disclosure is to provide a positive electrode active material in which the ratio of Ni to the total number of moles of metal elements excluding Li is 80 mol % or more, and which suppresses a decrease in battery capacity due to charge and discharge.

[0007] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a lithium transition metal composite oxide containing at least 80 mol % or more of Ni and Al relative to the total number of moles of metal elements excluding Li, and a surface modification layer formed on the surfaces of primary particles of the lithium transition metal composite oxide and containing at least Sr, wherein the Sr is not solid-dissolved in the lithium transition metal composite oxide.

[0008] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is characterized by including a positive electrode containing the above-described positive electrode active material, 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 in which the decrease in battery capacity due to charging and discharging is suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0011] The layered structure of a lithium transition metal composite oxide includes a transition metal layer (e.g., Ni), a Li layer, and an oxygen layer. The reversible movement of Li ions from the Li layer promotes the charge / discharge reaction of the battery. If the ratio of Ni to the total moles of metal elements excluding Li in the lithium transition metal composite oxide contained in the positive electrode active material is 80 mol % or more, many Li ions are extracted from the Li layer during battery charging, which can destabilize the layered structure. A modified layer forms on the surface of the lithium transition metal composite oxide with an unstable layered structure due to reaction with the electrolyte. Further structural changes in the lithium transition metal composite oxide occur from the modified layer, resulting in a gradual decrease in battery capacity during charge / discharge. However, by including predetermined amounts of Al and Sr, as in the positive electrode active material for a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure, the synergistic effect of Al and Sr suppresses the reaction with the electrolyte at the surface, and further stabilizes the surface structure, thereby suppressing the decrease in battery capacity during charge / discharge. Since Al does not change its oxidation state during charging and discharging, its inclusion in the transition metal layer is thought to stabilize the structure of the transition metal layer. Furthermore, Sr is thought to be able to change the surface state of the lithium transition metal composite oxide through electronic interactions.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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 active material layer 31 that constitutes the positive electrode 11.

[0019] [Positive electrode] The positive electrode 11 has a positive electrode current collector 30 and a positive electrode active material 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 active material layer 31 contains a positive electrode active material, a conductive material, and a binder. The thickness of the positive electrode active material 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 a positive electrode active material, a conductive material, a binder, etc. to the surface of the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode active material layer 31 on both sides of the positive electrode current collector 30.

[0020] Examples of the conductive material contained in the positive electrode active material layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode active material 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.

[0021] The positive electrode active material includes a lithium transition metal composite oxide and a surface modification layer formed on the surface of primary particles of the lithium transition metal composite oxide. The lithium transition metal composite oxide contains at least 80 mol % or more of Ni and Al relative to the total number of moles of metal elements excluding Li. By ensuring that the proportion of Ni relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide is 80 mol % or more, a high-capacity battery can be obtained.

[0022] The lithium transition metal composite oxide has a layered structure. 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.

[0023] The ratio of Ni to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide is preferably 90 mol % or more, which allows for a battery with a higher capacity to be obtained.

[0024] Lithium transition metal composite oxides have the general formula Li a Ni x Al y Co z M w O 2-b(where 0.95 < a < 1.05, 0.8 ≤ x ≤ 0.96, 0 < y ≤ 0.10, 0 ≤ z ≤ 0.15, 0 ≤ w ≤ 0.1, 0 ≤ b < 0.05, x + y + z + w = 1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn). The composite oxide can be represented as such. 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 element contained in the whole particles of the lithium transition metal composite oxide is measured by inductively coupled plasma (ICP) emission spectroscopy.

[0025] It is preferable that a, which indicates the ratio of Li in the lithium transition metal composite oxide, satisfies 0.95 ≤ a < 1.05, and more preferably satisfies 0.97 ≤ a ≤ 1.03. When a is less than 0.95, the battery capacity may decrease compared with the case where a satisfies the above range. When a is 1.05 or more, since more Li compounds need to be added compared with the case where a satisfies the above range, it may not be economical from the viewpoint of production cost.

[0026] It is preferable that y, which indicates the ratio of Al to the total molar number of the metal elements excluding Li in the lithium transition metal composite oxide, satisfies 0 < y ≤ 0.10, and more preferably satisfies 0.03 ≤ y ≤ 0.07. Since the oxidation number of Al 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 > 0.10, Al impurities are generated and the battery capacity decreases. Al may be uniformly dispersed in the layered structure of the lithium transition metal composite oxide, or may be present in a part of the layered structure.

[0027] Co and M (M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn) are optional components. z and w, which represent the ratios of Co and M to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, preferably satisfy the following conditions: 0≦z≦0.15, 0≦w≦0.1, respectively. Co is expensive, so from the viewpoint of production costs, it is preferable to keep the Co content low.

[0028] The lithium transition metal composite oxide is, for example, a secondary particle formed by agglomeration of a plurality of primary particles. The particle size of the primary particles constituting the secondary particle is, for example, 0.05 μm to 1 μm. The particle size of the primary particle is measured as the diameter of the circumscribed circle in a particle image observed with a scanning electron microscope (SEM). The surface modification layer exists on the surface of the primary particle. In other words, the surface modification layer exists on the surface of the secondary particle of the lithium transition metal composite oxide or at the interface where the primary particles contact each other.

[0029] The lithium transition metal composite oxide is a particle 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 refers to the particle size at which the cumulative frequency of particles in the volume-based particle size distribution is 50% from the smallest particle size, 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 particle size distribution analyzer (e.g., MT3000II manufactured by Microtrack Bell Corporation) using water as the dispersion medium.

[0030] The lithium transition metal composite oxide has a surface layer extending from the surface to the interior thereof and a main body extending inside the surface layer. The thickness of the surface layer is, for example, 1 nm to 5 nm.

[0031] The ratio of Al to the total number of moles of metal elements excluding Li in the surface layer is 1.3 times or more the ratio of Al to the total number of moles of metal elements excluding Li in the main body. This makes the structure of the surface layer more stable than that of the main body, and the synergistic effect with the surface modification layer described below can suppress the decrease in battery capacity associated with charge and discharge. Note that the ratio of Al to the total number of moles of metal elements excluding Li in the surface layer can be, for example, 4 times or less the ratio of Al to the total number of moles of metal elements excluding Li in the main body.

[0032] The surface modification layer contains at least Sr. The surface modification layer may contain, for example, Sr or a compound containing Sr. An example of the compound containing Sr is SrO2. The surface modification layer may further contain at least one selected from Al or a compound containing Al, and a compound containing Sr and Al. An example of the compound containing Al is Al2O3. An example of the compound containing Sr and Al is SrAlO4. The surface modification layer may further contain Li. Li present on the surface of the lithium transition metal composite oxide described below may be incorporated into the surface modification layer.

[0033] The ratio of Al to the total number of moles of metal elements excluding Li in the surface modification layer can be greater than the ratio of Al to the total number of moles of metal elements excluding Li in the main body of the lithium transition metal composite oxide.

[0034] The ratio of Al to the total number of moles of metal elements excluding Li in the surface modification layer is preferably 1.9 times or more the ratio of Al to the total number of moles of metal elements excluding Li in the main body of the lithium transition metal composite oxide.

[0035] The proportion of Sr in the surface modification layer can be 0.05 mol % to 0.25 mol % relative to the total number of moles of metal elements excluding Li in the surface modification layer. Within this range, the surface state of the lithium transition metal composite oxide can be changed by electronic interaction.

[0036] The thickness of the surface modification layer is, for example, 0.1 nm to 2 nm. Within this range, the reaction between the surface of the lithium transition metal composite oxide and the electrolyte is suppressed, and the synergistic effect with the surface layer can suppress the decrease in battery capacity due to charge and discharge.

[0037] The amount of Li remaining on the surface of the lithium transition metal composite oxide (hereinafter sometimes referred to as the residual Li amount) can be 0.03 wt% to 0.08 wt%. The Li present on the surface of the lithium transition metal composite oxide includes Li contained in the surface modification layer and Li present on the surface modification layer in the form of a Li compound, for example, but not contained in the surface modification layer.

[0038] The amount of residual Li can be determined by dispersing and dissolving the positive electrode active material in water and then titrating it. The specific measurement method is as follows. (1) 1 g of the positive electrode active material is added to 30 ml of pure water and stirred to prepare a suspension in which the active material is dispersed in water. (2) The suspension is filtered, and purified water is added to make up to 70 ml, to obtain a filtrate containing Li dissolved from the active material. (3) While measuring the pH of the filtrate, add hydrochloric acid dropwise to the filtrate, and calculate the amount of Li dissolved in the filtrate from the amount of hydrochloric acid (titration amount) consumed up to the first inflection point (near pH 8) and the second inflection point (near pH 4) on the pH curve.

[0039] The content of the lithium transition metal composite oxide in the positive electrode active material is preferably 90 mass% or more, and more preferably 99 mass% or more, relative to the total mass of the positive electrode active material, for example, from the viewpoint of improving the battery capacity and effectively suppressing deterioration of the charge-discharge cycle characteristics.

[0040] The positive electrode active material of this embodiment may contain, in addition to the lithium transition metal composite oxide of this embodiment, other lithium transition metal composite oxides, such as lithium transition metal composite oxides having a Ni content of 0 mol % or more but less than 85 mol %.

[0041] Next, an example of a method for producing a positive electrode active material including a lithium transition metal composite oxide and a surface modification layer will be described.

[0042] The method for producing a positive electrode active material includes, for example, a first step of obtaining a composite oxide containing Ni, Al, and an arbitrary metal element, a second step of mixing the composite oxide obtained in the first step with a lithium compound to obtain a mixture, and a third step of firing the mixture. Parameters such as the composition and thickness of the surface layer and surface modification layer in the finally obtained positive electrode active material can be adjusted by controlling, for example, the mixing ratio of the raw materials in the second step and the firing temperature and time in the third step.

[0043] 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, Al, and an optional metal element (Co, Mn, Fe, etc.) to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (co-precipitating) a composite hydroxide containing Ni, Al, and the optional metal element, and the composite hydroxide is calcined to obtain a composite oxide containing Ni, Al, 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.

[0044] In the second step, the composite oxide obtained in the first step is mixed with a lithium compound and a strontium compound to obtain a mixture. Examples of lithium compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. Examples of strontium compounds include Sr(OH)2, Sr(OH)2·8H2O, SrO, SrCo3, SrSO4, and Sr(NO3)2. The mixing ratio of the composite oxide obtained in the first step and the lithium 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 parameters described above to the specified ranges. Furthermore, the mixing ratio of the composite oxide obtained in the first step and the strontium compound is preferably, for example, such that the molar ratio of metal elements excluding Li to Sr is in the range of 1:0.0005 to 1:0.0018, in order to facilitate adjustment of each of the above parameters within the ranges specified above. In the second step, when the composite oxide obtained in the first step is mixed with the lithium compound and the strontium 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 composite oxide obtained in the first step, etc.

[0045] In the third step, the mixture obtained in the second step is fired at a predetermined temperature for a predetermined time to obtain a cathode active material according to this embodiment. The firing of the mixture in the third step includes a multi-stage firing process, for example, 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 more than 680°C to 800°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. By performing such multi-stage firing, the composition and thickness parameters of the surface layer and surface modification layer in the finally obtained cathode active material according to this embodiment can be adjusted to the above-specified ranges. 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 0 to 5 hours, more preferably 0 to 3 hours, in order to adjust each of the above 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 which the first set temperature is maintained after the first set temperature is reached. The holding time of the second set temperature in the second firing step is preferably 1 to 10 hours, more preferably 1 to 5 hours, in order to adjust each of the above 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 which the second set temperature is maintained after the second set temperature is reached. When firing the mixture, in order to adjust each of the above 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.

[0046] 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 Al y Coz M w Sr α O 2-b (where 0.95 < a < 1.05, 0.8 ≤ x ≤ 0.96, 0 < y ≤ 0.10, 0 ≤ z ≤ 0.15, 0 ≤ w ≤ 0.1, 0.05 ≤ α ≤ 0.18, 0 ≤ b < 0.05, x + y + z + w = 1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn). Note that Sr is not dissolved in the lithium transition metal composite oxide but is contained in the surface modification layer present on the surface of the lithium transition metal composite oxide. Also, a part of Al may be contained in the surface modification layer.

[0047] In addition, the composition of the interior and surface layer of the lithium transition metal composite oxide in the positive electrode active material, as well as the composition of the surface modification layer, can be measured for the ratio of Ni, Co, Al, and M by analyzing each location in the cross-section of the primary particles of the positive electrode active material using energy-dispersive X-ray spectroscopy (TEM-EDX). Since the surface modification layer is thinner than the spot diameter of the electron beam irradiated, the composition of the surface modification layer is affected by the composition of the adjacent surface layer. Even if Ni, Co, and Mn are detected in the measurement results of the surface layer, it is considered that Ni, Co, and M do not actually exist in the surface layer. Also, since the addition amount of Sr is small like α above, the presence or absence can be confirmed but it cannot be quantitatively measured.

[0048] [Negative Electrode] The negative electrode 12 has a negative electrode current collector 40 and negative electrode active material layers 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, or a film having such a metal disposed on the surface layer can be used. The negative electrode active material layer 41 contains a negative electrode active material and a binder. The thickness of the negative electrode active material 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 produced by applying a negative electrode 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 active material layers 41 on both surfaces of the negative electrode current collector 40.

[0049] The negative electrode active material contained in the negative electrode active material layer 41 is not particularly limited as long as it can reversibly occlude and release lithium ions, and generally carbon materials such as graphite are 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. Further, as the negative electrode active material, metals that alloy with Li such as Si and Sn, metal compounds containing Si, Sn, etc., lithium titanium composite oxides, 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) may be used in combination with graphite.

[0050] As the binder contained in the negative electrode active material layer 41, fluorine-containing resins such as PTFE and PVdF, PAN, polyimide, acrylic resin, polyolefin, etc. may be used as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferably used. Further, the negative electrode active material layer 41 may contain CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc.

[0051] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulation is used. Specific examples of the porous sheet include microporous thin films, woven fabrics, non-woven fabrics, etc. As the material of the separator, polyolefins such as polyethylene and polypropylene, cellulose, etc. are suitable. The separator 13 may have a single-layer structure or a laminated structure. Further, a resin layer with high heat resistance such as an aramid resin, or a filler layer containing a filler of an inorganic compound may be provided on the surface of the separator 13.

[0052] [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).

[0053] 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).

[0054] 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.; 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.

[0055] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, 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 of the lithium salt include imide salts such as PF6 (LiPF2) (where l and m are integers of 0 or more). The lithium salt may be used alone or in combination. Among these, LiPF6 is preferred from the viewpoints of ionic conductivity, electrochemical stability, etc. The concentration of the lithium salt is, for example, 0.8 mol to 1.8 mol per liter of the non-aqueous solvent. Furthermore, vinylene carbonate or a propane sultone-based additive may be added. [Example]

[0056] Hereinafter, the present disclosure will be further described with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0057] [Preparation of positive electrode active material] Example 1 [Ni 0.82 Al 0.05 Co 0.13 The composite hydroxide represented by ](OH)2 was calcined at 500°C for 8 hours to form the composite oxide (Ni 0.82 Al 0.05 Co 0.13 LiOH, Sr(OH)2, and the above composite oxide were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr was 1.03:1:0.0005 to obtain a mixture. The mixture was heated under an oxygen flow (10 cm) with an oxygen concentration of 95%. 3 The mixture was fired at a flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture at a temperature increase rate of 2.0°C / min from room temperature to 650°C, and then at a temperature increase rate of 0.5°C / min from 650°C to 780°C. Impurities were removed by washing the fired product with water to obtain a positive electrode active material. The composition of the obtained positive electrode active material was measured using an ICP optical emission spectrometer (manufactured by Thermo Fisher Scientific, product name "iCAP6300") and found to be LiNi 0.82 Al 0.05 Co 0.13 Sr 0.0005 This was used as the positive electrode active material of Example 1.

[0058] <Comparative Example 1> LiOH and composite oxide (Ni 0.82 Al 0.05 Co 0.13 A positive electrode active material was obtained in the same manner as in Example 1, except that Li and O2) were mixed so that the molar ratio of Li to the total amount of Ni, Al, and Co was 1.03:1. 0.82 Al 0.05 Co 0.13 This was used as the positive electrode active material of Comparative Example 1.

[0059] <Example 2> [Ni 0.87 Al 0.04 Co 0.09 ](OH)2 was used to obtain the composite oxide (Ni 0.87 Al 0.04 Co 0.09 A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, Sr(OH)2, and the above composite oxide were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr was 1.03:1:0.001. The composition of the obtained positive electrode active material was LiNi 0.87 Al 0.04 Co 0.09 Sr 0.001 This was used as the positive electrode active material of Example 2.

[0060] <Comparative Example 2> LiOH and composite oxide (Ni 0.87 Al 0.04 Co 0.09 A positive electrode active material was obtained in the same manner as in Example 2, except that Li and O2 were mixed so that the molar ratio of Li to the total amount of Ni, Al, and Co was 1.03:1. The composition of the obtained positive electrode active material was LiNi 0.87 Al 0.04 Co 0.09 This was used as the positive electrode active material of Comparative Example 2.

[0061] Example 3 [Ni 0.92 Al 0.05 Co 0.01 Mn 0.02](OH)2 was used to obtain the composite oxide (Ni 0.92 Al 0.05 Co 0.01 Mn 0.02 A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, Sr(OH)2, and the above composite oxide were mixed so that the molar ratio of the total amount of Li, Ni, Al, Co, and Mn to Sr was 1.03:1:0.0007. The composition of the obtained positive electrode active material was LiNi 0.92 Al 0.05 Co 0.01 Mn 0.02 Sr 0.0007 This was used as the positive electrode active material of Example 3.

[0062] <Comparative Example 3> LiOH and composite oxide (Ni 0.92 Al 0.05 Co 0.01 Mn 0.02 A positive electrode active material was obtained in the same manner as in Example 3, except that Li and O2) were mixed so that the molar ratio of Li to the total amount of Ni, Al, Co, and Mn was 1.03:1. 0.92 Al 0.05 Co 0.01 Mn 0.02 This was used as the positive electrode active material of Comparative Example 3.

[0063] Example 4 [Ni 0.91 Al 0.05 Mn 0.04 ](OH)2 was used to obtain the composite oxide (Ni 0.91 Al 0.05 Mn 0.04 A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, Sr(OH)2, and the above composite oxide were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Mn to Sr was 1.03:1:0.0015. The composition of the obtained positive electrode active material was LiNi 0.91 Al 0.05 Mn 0.04 Sr 0.0015This was used as the positive electrode active material of Example 4.

[0064] <Comparative Example 4> LiOH and composite oxide (Ni 0.91 Al 0.05 Mn 0.04 A positive electrode active material was obtained in the same manner as in Example 4, except that Li and O2) were mixed so that the molar ratio of Li to the total amount of Ni, Al, and Mn was 1.03:1. 0.91 Al 0.05 Mn 0.04 This was used as the positive electrode active material of Comparative Example 4.

[0065] <Example 5> [Ni 0.92 Al 0.06 Mn 0.02 ](OH)2 was used to obtain the composite oxide (Ni 0.92 Al 0.06 Mn 0.02 A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, Sr(OH)2, and the above composite oxide were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Mn to Sr was 1.03:1:0.0018. The composition of the obtained positive electrode active material was LiNi 0.92 Al 0.06 Mn 0.02 Sr 0.0018 This was used as the positive electrode active material of Example 5.

[0066] <Comparative Example 5> LiOH and composite oxide (Ni 0.92 Al 0.06 Mn 0.02 A positive electrode active material was obtained in the same manner as in Example 5, except that Li and O2 were mixed so that the molar ratio of Li to the total amount of Ni, Al, and Mn was 1.03:1. 0.92 Al 0.06 Mn 0.02 This was used as the positive electrode active material of Comparative Example 5.

[0067] <Comparative Example 6> [Ni 0.60 Co 0.21 Mn 0.19 ](OH)2 was used to obtain the composite oxide (Ni 0.60 Co 0.21 Mn 0.19 A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, Sr(OH)2, and the above composite oxide were mixed so that the molar ratio of the total amount of Li, Ni, Co, and Mn to Sr was 1.03:1:0.001. The composition of the obtained positive electrode active material was LiNi 0.60 Co 0.21 Mn 0.19 Sr 0.001 This was used as the positive electrode active material of Comparative Example 6.

[0068] <Comparative Example 7> LiOH and composite oxide (Ni 0.60 Co 0.21 Mn 0.19 A positive electrode active material was obtained in the same manner as in Comparative Example 6, except that Li and O2) were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.03:1. 0.60 Co 0.21 Mn 0.19 This was used as the positive electrode active material of Comparative Example 7.

[0069] The positive electrode active materials of Examples 1 to 5 and Comparative Examples 1 to 7 were subjected to TEM-EDX measurement, and composition analysis was performed on the interior and surface layers of the lithium transition metal composite oxides, as well as on the surface modification layers. Since Sr could not be quantified, its presence or absence was analyzed based on the presence or absence of a peak. Furthermore, the amount of residual Li in the positive electrode active materials of Examples 1 to 5 and Comparative Examples 1 to 7 was measured. The results are shown in Table 1.

[0070] [Table 1]

[0071] In Examples 1 to 5, the Al content in the surface modification layer was higher than the Al content in the main body portion and the Al content in the surface layer. On the other hand, in Comparative Examples 1 to 5, such a tendency was not observed. In Examples 1 to 5, the Al content in the surface modification layer was higher than that in the surface layer, indicating the presence of Al in the surface modification layer. However, in Comparative Examples 1 to 5, the Al content in the surface modification layer and the surface layer was similar, suggesting that Al was affected by the composition of the adjacent surface layer and therefore no Al was present in the surface modification layer. Furthermore, in Examples 1 to 5 and Comparative Example 6, in which Sr was added, Sr was detected only in the surface modification layer and not in the surface layer or the main body portion. It is considered that the composition of the surface modification layer was affected by the composition of the adjacent surface layer in all samples, and Ni, Co, and Mn were not present in the surface modification layer. Furthermore, residual Li was detected in all samples.

[0072] Next, test cells were fabricated using the positive electrode active materials of Examples 1 to 5 and Comparative Examples 1 to 7 as follows.

[0073] [Preparation of positive electrode] A positive electrode slurry was prepared by mixing 91 parts by mass of the positive electrode active material of Examples 1 to 5 and Comparative Examples 1 to 7, 7 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. The coating film was dried, and then rolled with a rolling roller and cut to a predetermined electrode size, resulting in 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 in part of the positive electrode, exposing the surface of the positive electrode core. Positive electrodes were also prepared in the same manner in the other Examples and Comparative Examples.

[0074] [Preparation of negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in an aqueous solution at a solids mass ratio of 100:1:1 to prepare a negative electrode composite slurry. The negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried. The coating was then rolled using a rolling roller and cut to a predetermined electrode size to obtain a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core. An exposed portion was provided in part of the negative electrode, exposing the surface of the negative electrode core.

[0075] [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.

[0076] [Test cell construction] An aluminum lead was attached to the exposed portion of the positive electrode containing the positive electrode active material of Examples 1 to 5 and Comparative Examples 1 to 7, and a nickel lead was attached to the exposed portion of the negative electrode, and the positive electrode and negative electrode were spirally wound with a polyolefin separator interposed therebetween to prepare a wound electrode assembly. This electrode assembly was placed in an outer casing, and the nonaqueous electrolyte solution was poured into it. The opening of the outer casing was then sealed to obtain a test cell.

[0077] [Capacity retention rate evaluation] The following cycle test was carried out on batteries fabricated by incorporating positive electrodes containing the positive electrode active materials of Examples 1 to 5 and Comparative Examples 1 to 7. The discharge capacity at the first cycle and the discharge capacity at the 100th cycle of the cycle test were determined, and the capacity retention rate was calculated using the following formula.

[0078] Capacity retention rate (%) = (100th cycle discharge capacity ÷ 1st cycle discharge capacity amount)×100 <Cycle test> The test cell was charged at a constant current of 0.5 It in a 45°C environment until the battery voltage reached 4.2 V, and then charged at a constant voltage until the current value reached 1 / 50 It at 4.2 V. It was then discharged at a constant current of 0.5 It until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 100 times.

[0079] The capacity retention rates of Examples 1 to 5 and Comparative Examples 1 to 7 are shown in Tables 2 to 7. The amount of residual Li is also shown in Tables 2 to 7. The capacity retention rate of the test cell of Example 1 shown in Table 2 is expressed relative to the capacity retention rate of the test cell of Comparative Example 1, which is set to 100%.

[0080] The capacity retention rates of the test cells of Example 2 shown in Table 3 are expressed relative to the capacity retention rate of the test cells of Comparative Example 2, which is set at 100%.

[0081] The capacity retention rates of the test cells of Example 3 shown in Table 4 are expressed relative to the capacity retention rate of the test cells of Comparative Example 3, which is set at 100%.

[0082] The capacity retention rates of the test cells of Example 4 shown in Table 5 are expressed relative to the capacity retention rate of the test cells of Comparative Example 4, which is set at 100%.

[0083] The capacity retention rates of the test cells of Example 5 shown in Table 6 are expressed relative to the capacity retention rate of the test cell of Comparative Example 5, which is set at 100%.

[0084] The capacity retention rate of the test cell of Comparative Example 7 shown in Table 7 is expressed relative to the capacity retention rate of the test cell of Comparative Example 6, which is set to 100%.

[0085] [Table 2]

[0086] [Table 3]

[0087] [Table 4]

[0088] [Table 5]

[0089] [Table 6]

[0090] [Table 7]

[0091] In all of Tables 2 to 6, the examples in which Sr was contained in the surface modification layer had a higher capacity retention rate than the comparative examples in which Sr was not contained in the surface modification layer. It is presumed that the surface layer contained more Al than the main body, and that a portion of the Al was contained in the surface modification layer. In Table 7, the lithium transition metal composite oxides in both comparative examples 6 and 7 did not contain Al, so there was no difference in the capacity retention rate whether Sr was added or not. [Explanation of symbols]

[0092] 10 non-aqueous 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 portion, 23 bottom plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode current collector, 31 positive electrode active material layer, 40 negative electrode current collector, 41 negative electrode active material layer

Claims

1. a lithium transition metal composite oxide containing at least 80 mol % or more of Ni and Al relative to the total number of moles of metal elements excluding Li; a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide and containing at least Sr; Sr is not solid-solved in the lithium transition metal composite oxide, the lithium transition metal composite oxide has a surface layer present from the surface to the interior thereof and a main body present on the interior side of the surface layer, a ratio of Al to the total number of moles of metal elements excluding Li in the surface modification layer is greater than a ratio of Al to the total number of moles of metal elements excluding Li in the main body.

2. a lithium transition metal composite oxide containing at least 80 mol % or more of Ni and Al relative to the total number of moles of metal elements excluding Li; a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide, the surface modification layer containing at least Sr and Al; The lithium transition metal composite oxide has the general formula Li ａ Ni ｘ Al ｙ Co ｚ M ｗ O ２－ｂ (wherein 0.95<a<1.05, 0.8≦x≦0.96, 0<y≦0.10, 0≦z≦0.15, 0≦b<0.05, x+y+z+w=1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn).

3. 2. The positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of Al to a total number of moles of metal elements excluding Li in the surface layer is 1.3 times or more a ratio of Al to a total number of moles of metal elements excluding Li in the main body portion.

4. A lithium transition metal composite oxide containing at least 80 mol% or more of Ni and Al relative to the total number of moles of metal elements excluding Li; a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide and containing at least Sr; Sr is not solid-solved in the lithium transition metal composite oxide, The positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the proportion of Sr in the surface modification layer is 0.05 mol % to 0.25 mol % with respect to the total number of moles of metal elements excluding Li in the surface modification layer.

5. 5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio of Ni to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide is 90 mol % or more.

6. A lithium transition metal composite oxide containing at least 80 mol% or more of Ni and Al relative to the total number of moles of metal elements excluding Li; a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide and containing at least Sr; Sr is not solid-solved in the lithium transition metal composite oxide, The amount of Li remaining on the surface of the lithium transition metal composite oxide is 0.03 wt % to 0.08 wt %.

7. 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 6, a negative electrode, and a non-aqueous electrolyte.

Images