Non-aqueous electrolyte secondary battery

Abstract

In a non-aqueous electrolyte secondary battery, the positive electrode active material comprises a lithium transition metal oxide, which has a layered structure containing a Li layer and contains at least specified amounts of Ni, Ca, and Al, respectively. The proportion of metal elements other than Li present in the Li layer is 0.6 mol% or more and 2.0 mol% or less relative to the total molar number of metal elements other than Li contained in the lithium transition metal oxide. The negative electrode active material has a Ca-containing coating on its surface, and the Ca content in the coating is 15 ppm or more and less than 80 ppm by mass relative to the total mass of the positive electrode active material.

Description

Technical Field

[0001] The present disclosure relates to a nonaqueous electrolyte secondary battery. Background Art

[0002] In recent years, lithium transition metal oxides with high Ni content have attracted much attention as positive electrode active materials with high energy density. For example, Patent Document 1 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, which comprises a general formula Li x Ni y Co z M m A lithium transition metal oxide represented by O2 (wherein M is an element selected from Ba, Sr, and B, 0.9≤x≤1.1, 0.5≤y≤0.95, 0.05≤z≤0.5, 0.0005≤m≤0.02), and having a BET specific surface area value of 0.8m 2 / g or less.

[0003] In addition, Patent Document 2 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, which has an α-NaFeO2 structure and contains one or more transition metal elements selected from the group consisting of Mn, Ni and Co, and alkaline earth metals and W are present on the surface of lithium transition metal oxide particles.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2003-100295

[0007] Patent Document 2: Japanese Patent Application Publication No. 2018-129221 Summary of the invention

[0008] When a lithium transition metal oxide with a high Ni content is used as the positive electrode active material of a non-aqueous electrolyte secondary battery, a large amount of Li is extracted during charging, so repeated charge and discharge will cause the layered crystal structure to collapse, resulting in a problem of reduced battery capacity. It should be noted that the technologies disclosed in Patent Documents 1 and 2 still have room for improvement in charge and discharge cycle characteristics.

[0009] A nonaqueous electrolyte secondary battery as one embodiment of the present disclosure comprises: a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte. The positive electrode active material comprises a lithium transition metal oxide, the lithium transition metal oxide having a layered structure including a Li layer in which Li reversibly enters and exits, and at least containing Ni, Ca, and Al, wherein the content of Ni in the lithium transition metal oxide is 80 mol% or more and 95 mol% or less relative to the total amount of metal elements other than Li, the content of Ca is more than 0 mol% and less than 3 mol% relative to the total amount of metal elements other than Li, the content of Al is more than 0 mol% and less than 8 mol% relative to the total amount of metal elements other than Li, the ratio of metal elements other than Li present in the Li layer is 0.6 mol% or more and less than 2.0 mol% relative to the total molar number of metal elements other than Li contained in the lithium transition metal oxide, and the negative electrode active material has a coating containing Ca on its surface, and the ratio of Ca in the coating is 15 mass ppm or more and less than 80 mass ppm relative to the total mass of the positive electrode active material.

[0010] The nonaqueous electrolyte secondary battery of one embodiment of the present disclosure has a high capacity and can suppress the decrease in battery capacity accompanying charge and discharge. The positive electrode active material includes a lithium transition metal oxide with a high Ni content, and can provide a nonaqueous electrolyte secondary battery with excellent charge and discharge cycle characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a cross-sectional view of a nonaqueous electrolyte secondary battery as an example of the embodiment. DETAILED DESCRIPTION

[0012] In the layered structure of the lithium transition metal oxide contained in the positive electrode active material, there are transition metal layers such as Ni, Li layers, and oxygen layers, and the Li ions present in the Li layer reversibly enter and exit, so that the charge and discharge reaction of the battery is carried out. In the case of a lithium transition metal oxide with a large Ni content, a large amount of Li ions are taken out from the Li layer when the battery is charged, so sometimes the layered structure collapses and the battery capacity is reduced. In particular, the surface activity of the lithium transition metal oxide is high, and the layered structure is easy to become more unstable, so the degradation of the layered structure is easy to promote. In addition, similarly to the positive electrode, in the negative electrode, the negative electrode active material also reacts with the electrolyte, and the surface portion of the negative electrode active material sometimes deteriorates.

[0013] Therefore, the inventors of the present invention have conducted in-depth research to solve the above-mentioned problems, and found that by adding a specified amount of Ca to a lithium transition metal oxide containing Ni and Al, a part of the Li in the Li layer is replaced by other metal elements, and the charge-discharge cycle characteristics are specifically improved. It can be seen that at this time, by supplying Ca from the positive electrode to the negative electrode, a coating is formed on the surface of the negative electrode active material, and the Ca concentration in the coating is within a specified range. The effect is reflected. It is believed that by stabilizing the structure of the Li layer based on replacement of other metal elements, stabilizing the transition metal layer based on Al replacement, modifying the surface of the positive electrode active material based on the addition of Ca, and forming a high-quality coating containing Ca on the negative electrode active material, the side reaction of the positive electrode active material / negative electrode active material with the electrolyte is suppressed, resulting in a specific synergistic effect, resulting in a significant improvement in the charge-discharge cycle characteristics.

[0014] Hereinafter, an example of an embodiment of the non-aqueous electrolyte secondary battery disclosed in the present invention is described in detail. Hereinafter, a cylindrical battery is obtained by housing a wound electrode body in a cylindrical battery case, but the electrode body is not limited to a wound type, and may also be a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked one by one through a separator. In addition, the battery case is not limited to a cylindrical shape, and may be, for example, a square shape, a coin shape, etc., and may also be a battery case composed of a laminate sheet including a metal layer and a resin layer.

[0015] Figure 1 FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 as an example of an embodiment. Figure 1 As shown in the example, the nonaqueous electrolyte secondary battery 10 includes an electrode body 14, a nonaqueous electrolyte (not shown), and a battery case 15 that accommodates the electrode body 14 and the nonaqueous electrolyte. The electrode body 14 has a winding structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. The battery case 15 is composed of an outer shell can 16 in a cylindrical shape with a bottom, and a sealing body 17 that seals the opening of the outer shell can 16.

[0016] The electrode body 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. In order to prevent the precipitation of lithium, the negative electrode 12 is formed to be one size larger than the positive electrode 11. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction and the width direction (width direction). The two separators 13 are formed to be at least one size larger than the positive electrode 11, and are arranged in a manner of sandwiching the positive electrode 11, for example.

[0017] The nonaqueous electrolyte secondary battery 10 includes insulating plates 18 and 19 disposed above and below the electrode body 14 , respectively. Figure 1In the example shown, the positive electrode tab 20 mounted on the positive electrode 11 passes through the through hole of the insulating plate 18 and extends to the sealing body 17 side, and the negative electrode tab 21 mounted on the negative electrode 12 passes through the outside of the insulating plate 19 and extends to the bottom side of the outer can 16. The positive electrode tab 20 is connected to the lower surface of the bottom plate 23 of the sealing body 17 by welding or the like, and the cover 27 of the sealing body 17 electrically connected to the bottom plate 23 becomes the positive terminal. The negative electrode tab 21 is connected to the bottom inner surface of the outer can 16 by welding or the like, and the outer can 16 becomes the negative terminal.

[0018] The outer shell can 16 is, for example, a metal container in a cylindrical shape with a bottom. A gasket 28 is provided between the outer shell can 16 and the sealing body 17, and the internal space of the battery case 15 is sealed. The outer shell can 16 has, for example, a grooved portion 22 formed by applying pressure to the side surface from the outside to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer shell can 16, and the sealing body 17 is supported by its upper surface.

[0019] The sealing body 17 has a structure in which a bottom plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26 and a cover 27 are stacked in order from the electrode body 14 side. The components constituting the sealing body 17 have, for example, a disc shape or a ring shape, and the components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective central portions, and an insulating member 25 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat release, the lower valve body 24 is deformed and broken in a manner that pushes the upper valve body 26 toward the cover 27 side, and the current path between the lower valve body 24 and the upper valve body 26 is blocked. When the internal pressure rises further, the upper valve body 26 breaks, and the gas is discharged from the opening of the cover 27.

[0020] The positive electrode 11 , the negative electrode 12 , the separator 13 , and the nonaqueous electrolyte constituting the nonaqueous electrolyte secondary battery 10 , particularly the positive electrode active material contained in the positive electrode composite material layer 31 constituting the positive electrode 11 , will be described in detail below.

[0021] [positive electrode]

[0022] The positive electrode 11 has: a positive electrode collector 30, and a positive electrode composite material layer 31 formed on both sides of the positive electrode collector 30. The positive electrode collector 30 can use a foil of a metal such as aluminum or aluminum alloy that is stable within the potential range of the positive electrode 11, a thin film having the metal configured on the surface, etc. The positive electrode composite material layer 31 includes: a positive electrode active material, a conductive material, and a binding material. The thickness of the positive electrode composite material layer 31 is, for example, 10 μm to 150 μm on one side of the positive electrode collector 30. The positive electrode 11 can be made as follows: a positive electrode slurry containing a positive electrode active material, a conductive material, and a binding material is applied to the surface of the positive electrode collector 30, and the coating is dried and then compressed to form a positive electrode composite material layer 31 on both sides of the positive electrode collector 30, thereby making it.

[0023] As the conductive material contained in the positive electrode composite material layer 31, carbon materials such as carbon black, acetylene black, Ketjen black, and graphite can be exemplified. As the binding material contained in the positive electrode composite material layer 31, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, etc. can be exemplified. These resins can be used in combination with carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), etc.

[0024] The positive electrode composite material layer 31 of the positive electrode 11 contains a positive electrode active material. The positive electrode active material includes a lithium transition metal oxide, which has a layered structure including a Li layer in which Li can reversibly enter and exit, and contains at least Ni and Ca. The layered structure of the lithium transition metal oxide can include, for example, a layered structure belonging to the space group R-3m, a layered structure belonging to the space group C2 / m, and the like. Among them, from the perspective of high capacity, stability of the crystal structure, etc., a layered structure belonging to the space group R-3m is preferred. The positive electrode active material can also use lithium transition metal oxide as the main component and is essentially composed only of lithium transition metal oxide. It should be noted that the positive electrode active material may also include composite oxides or other compounds other than lithium transition metal oxides, without prejudice to the purpose of the present disclosure.

[0025] Lithium transition metal oxides are, for example, secondary particles formed by the aggregation of multiple primary particles. The particle size of the primary particles constituting the secondary particles is, for example, 0.05 μm to 1 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle of the particle image observed using a scanning electron microscope (SEM). The volume-based median particle size (D50) of the lithium transition metal oxide is, 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 in the volume-based particle size distribution becomes 50% from the smaller particle size, and is also called the median diameter. The particle size distribution of lithium transition metal oxides can be measured using a laser diffraction particle size distribution measuring device (e.g., MT3000II manufactured by MicrotracBEL Corp.) using water as the dispersion medium.

[0026] In the lithium transition metal oxide, the content of Ni is 80 mol% or more and 95 mol% or less relative to the total amount of metal elements other than Li, and is preferably 85 mol% or more and 92 mol% or less. By making the content of Ni 80 mol% or more, a battery with high energy density can be obtained. On the other hand, if the content of Ni exceeds 95 mol%, the content of Ca and other metal elements is excessively reduced, and the stability of the layered structure of the lithium transition metal oxide cannot be ensured, and the erosion of the particle surface cannot be suppressed.

[0027] In the lithium transition metal oxide, the content of Ca exceeds 0 mol% and is less than 3 mol% relative to the total amount of metal elements other than Li, and is preferably less than 1 mol%. It is believed that Ca has the effect of suppressing the corrosion of the surface of the lithium transition metal oxide caused by the electrolyte by electronic interaction. If Ca is contained, the cycle characteristics of the battery can be improved. The Ca in the lithium transition metal oxide is, for example, Ca or a compound containing Ca. As a compound containing Ca, CaO can be exemplified.

[0028] Ca is preferably present on the surface of the particles of lithium transition metal oxide and near it, for example, in the surface vicinity region within 30nm from the particle surface. Lithium transition metal oxide is generally a secondary particle formed by the aggregation of multiple primary particles, so Ca is preferably present in a high concentration on the surface and near the surface of the primary particles containing the surface of the secondary particles compared to the central portion of the primary particles. That is, Ca is unevenly present on the surface and near it of the primary particles of the lithium transition metal oxide, and the content per unit volume of Ca is higher on the surface than inside the primary particles. Ca can be present in the particle surface or layered structure of the lithium transition metal oxide in the form of a compound, for example. The distribution of Ca in the lithium transition metal oxide can be analyzed by TEM-EDX, etc.

[0029] The content of Al in the lithium transition metal oxide is greater than 0 mol% and less than 8 mol% relative to the total amount of metal elements other than Li, and is preferably greater than 2 mol% and less than 6 mol%. It is believed that Al does not undergo an oxidation number change during charge and discharge, so by containing it in the transition metal layer, the structure of the transition metal layer is stabilized. However, if the content of Al exceeds 8 mol%, Al impurities are generated and the battery capacity is reduced. Al can be uniformly dispersed in the layered structure of the lithium transition metal oxide, for example, or it can be present in a part of the layered structure. In addition, Al can coexist with Ca on the surface of the lithium transition metal oxide particles and in the vicinity thereof, for example, in the vicinity of the surface within 30nm from the particle surface. The Al in the lithium transition metal oxide can exist in the form of Al, a compound containing Al, or a compound containing Ca and Al. As a compound containing Al, Al2O3 can be exemplified. In addition, as a compound containing Ca and Al, Ca3Al2O6 can be exemplified.

[0030] An example of an ideal lithium transition metal oxide is a general formula Li a Ni x Al y M z Ca w O 2-b(wherein, 0.95<a<1.05, 0.80≤x≤0.95, 0≤y≤0.08, 0≤z≤0.1, 0<w≤0.03, 0≤b<0.05, x+y+z+w=1, M is at least one element selected from Co, Mn, Fe, Ti, Si, Nb, Zr, Mo and Zn) is a composite oxide. As M, it is preferred to contain at least one of Co and Mn. The lithium transition metal oxide contains at least one of Co and Mn, and may contain at least one element selected from Fe, Ti, Si, Nb, Zr, Mo and Zn. The molar fraction of the metal element contained in the entire particle of the lithium transition metal oxide can be measured by an inductively coupled plasma emission spectrometer (ICP-AES), an electron beam microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), and the like.

[0031] In the layered structure of the lithium transition metal oxide, there are transition metal layers such as Ni, Li layers, and oxygen layers, and the Li ions present in the Li layer reversibly enter and exit, so that the charge and discharge reaction of the battery is carried out. The ratio of the metal elements other than Li present in the Li layer is 0.6 mol% or more and 2.0 mol% or less, preferably 0.7 mol% or more and 1.9 mol% or less, and more preferably 0.8 mol% or more and 1.8 mol% or less relative to the total molar number of the metal elements other than Li contained in the lithium transition metal oxide. If it is within this range, the structure of the Li layer is stabilized and the charge and discharge cycle characteristics are improved when the Li ions are taken out during charging. On the other hand, when the above ratio is less than 0.6 mol% or exceeds 2.0 mol%, the improvement effect of the charge and discharge cycle characteristics cannot be obtained. It is believed that the metal elements other than Li in the Li layer are mainly Ni, but other metal elements may also be included.

[0032] The ratio of metal elements other than Li present in the Li layer is obtained by Rietveld analysis of an X-ray diffraction spectrum obtained by X-ray diffraction measurement of a lithium transition metal oxide. The X-ray diffraction spectrum is obtained, for example, by a powder X-ray diffraction method based on the following conditions using a powder X-ray diffraction apparatus (Rigaku Co., Ltd., trade name "RINT-TTR", radiation source Cu-Kα).

[0033] Measuring range: 15-120°

[0034] Scanning speed: 4° / min

[0035] Analysis range: 30-120°

[0036] Background: B-spline

[0037] Profile function: segmented simulation of Voigt function

[0038] Binding condition: Li(3a)+Ni(3a)=1

[0039] Ni(3a)+Ni(3b)=y (y is the content ratio of each Ni)

[0040] ICSD No.:98-009-4814

[0041] In the Rietveld analysis of the X-ray diffraction spectrum, PDXL2 (Rigaku Co., Ltd.) as Rietveld analysis software can be used. In addition, it is preferred that there is no peak derived from calcium oxide (CaO) in the X-ray diffraction spectrum of the lithium transition metal oxide. When CaO is contained to the extent detected by X-ray diffraction measurement, a decrease in charge and discharge capacity may sometimes occur.

[0042] Next, an example of a method for producing a positive electrode active material containing a lithium transition metal compound is described.

[0043] The method for producing a positive electrode active material, for example, comprises the following steps: a first step of obtaining a composite oxide containing Ni 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 calcining the mixture. The ratio of metal elements other than Li in the Li layer of the layered structure of the finally obtained positive electrode active material is adjusted by, for example, controlling the mixing ratio of the raw materials in the second step, the calcination temperature and time in the third step, and the like.

[0044] In the first step, for example, a solution of a metal salt containing Ni and any metal element (Al, Co, Mn, Fe, etc.) is stirred while an alkaline solution such as sodium hydroxide is added dropwise, and the pH is adjusted to the alkaline side (e.g., 8.5 to 12.5), so that a composite hydroxide containing Ni and any metal element is precipitated (coprecipitated), and the composite hydroxide is roasted to obtain a composite oxide containing Ni and any metal element. The roasting temperature is not particularly limited, for example, in the range of 300° C. to 600° C.

[0045] In the second step, the composite oxide obtained in the first step is mixed with a lithium compound and a calcium compound to obtain a mixture. Examples of lithium compounds include Li2CO3, LiOH, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. Examples of calcium compounds include Ca(OH)2, CaO, CaCO3, CaSO4, and Ca(NO3)2. For the mixing ratio of the composite oxide obtained in the first step and the lithium compound, from the perspective of facilitating the adjustment of the above parameters to the above-defined ranges, for example, it is preferred that the molar ratio of the metal element other than Li: Li is in the range of 1:0.98 to 1:1.1. In addition, for the mixing ratio of the composite oxide obtained in the first step and the calcium compound, from the perspective of facilitating the adjustment of the above parameters to the above-defined ranges, for example, it is preferred that the molar ratio of the metal element other than Li: Ca is in the range of 1:0.0005 to 1:0.02. In the second step, when the composite oxide obtained in the first step is mixed with the lithium compound and the calcium compound, other metal raw materials may be added as needed. Other metal raw materials are oxides containing metal elements other than the metal elements constituting the composite oxide obtained in the first step.

[0046] In the third step, the mixture obtained in the second step is roasted at a specified temperature and time to obtain the positive electrode active material of the present embodiment. The roasting of the mixture in the third step, for example, has a multi-stage roasting step, which includes the following steps: a first roasting step of roasting to a first set temperature of 450°C to 680°C at a first heating rate in a roasting furnace under an oxygen gas flow; and a second roasting step of roasting the roasted product obtained by the first roasting step in a roasting furnace under an oxygen gas flow at a second heating rate to a second set temperature of more than 680°C and less than 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 and is in the range of 0.1°C / min to 3.5°C / min. By this multi-stage calcination, the proportion of metal elements other than Li present in the Li layer in the positive electrode active material of the present embodiment finally obtained can be adjusted to 0.6 mol% or more and 2.0 mol% or less. It should be noted that if the first heating rate and the second heating rate are within the above-mentioned limited range, multiple can be set in each temperature zone. From the perspective of adjusting the above-mentioned parameters of the lithium transition metal oxide to the above-mentioned limited range, the holding time of the first set temperature in the first calcination process is preferably less than 5 hours, more preferably less than 3 hours. The holding time of the first set temperature refers to the time after reaching the first set temperature and maintaining the first set temperature. From the perspective of adjusting the above-mentioned parameters of the lithium transition metal oxide to the above-mentioned limited range, the holding time of the second set temperature in the second calcination process is preferably 1 hour to 10 hours, more preferably 1 hour to 5 hours. The holding time of the second set temperature refers to the time after reaching the second set temperature and maintaining the second set temperature. When the mixture is calcined, from the perspective of adjusting the above parameters to the above-mentioned limited ranges, for example, it can be carried out in an oxygen gas flow with an oxygen concentration of 60% or more, and the flow rate of the oxygen gas flow is set to 10 cm / s. 3 The calcination furnace is in the range of 0.2 mL / min to 4 mL / min and is 0.3 L / min or more per 1 kg of the mixture.

[0047] [negative electrode]

[0048] The negative electrode 12 has: a negative electrode collector 40, and a negative electrode composite material layer 41 formed on both sides of the negative electrode collector 40. The negative electrode collector 40 may be made of a foil of a metal such as copper or a copper alloy that is stable within the potential range of the negative electrode 12, or a thin film having the metal disposed on the surface. The negative electrode composite material layer 41 includes: a negative electrode active material and a binding material. The thickness of the negative electrode composite material layer 41 is, for example, 10 μm to 150 μm on one side of the negative electrode collector 40. The negative electrode 12 can be made as follows: a negative electrode composite material slurry containing a negative electrode active material, a binding material, etc. is applied to the surface of the negative electrode collector 40, and the coating is dried and then rolled to form a negative electrode composite material layer 41 on both sides of the negative electrode collector 40, thereby making it.

[0049] The negative electrode active material contained in the negative electrode composite material layer 41 is not particularly limited as long as it can reversibly store and release lithium ions, and includes carbon materials such as graphite. The graphite can be natural graphite such as flaky graphite, block graphite, and earthy graphite, block artificial graphite, and artificial graphite such as graphitized mesophase carbon microbeads.

[0050] The negative electrode active material has a coating containing Ca on its surface. The ratio of Ca in the coating is 15 mass ppm or more and less than 80 mass ppm relative to the total mass of the positive electrode active material. Within this range, the charge-discharge cycle characteristics are improved.

[0051] The negative electrode active material may further contain Ni in the coating. Ni, for example, gradually moves from the positive electrode and is supplied to the negative electrode. It should be noted that Ni may be contained in the negative electrode active material in advance. The molar ratio of the Ca content in the coating to the Ni content is preferably 0.5≤Ca / Ni≤1.0. If it is within this range, the charge and discharge cycle characteristics are further improved. The contents of Ca and Ni in the coating of the negative electrode active material can be analyzed using TEM-EDX or the like.

[0052] The negative electrode active material may include metals such as Si and Sn alloyed with Li, metal compounds containing Si, Sn, etc., lithium-titanium composite oxides, etc. In addition, those provided with a carbon film may also be used. For example, SiO x The Si-containing compound represented by (0.5≤x≤1.6) or Si fine particles are dispersed in Li 2y SiO (2+y) A Si-containing compound formed in a lithium silicate phase represented by (0<y<2) or the like is used in combination with graphite.

[0053] The binder contained in the negative electrode composite material layer 41 may be a fluorine-containing resin such as PTFE and PVdF, PAN, polyimide, acrylic resin, polyolefin, etc., as in the case of the positive electrode 11, but preferably styrene-butadiene rubber (SBR) is used. In addition, the negative electrode composite material layer 41 may include CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc.

[0054] [Separator]

[0055] For example, a porous sheet having ion permeability and insulation is used as the separator 13. Specific examples of the porous sheet include microporous films, woven fabrics, nonwoven fabrics, etc. As the material of the separator 13, polyolefins such as polyethylene and polypropylene, cellulose, etc. are ideal. The separator 13 can have a single-layer structure or a laminated structure. In addition, a resin layer with high heat resistance such as aramid resin or a filler layer containing a filler of an inorganic compound can be provided on the surface of the separator 13.

[0056] [Non-aqueous electrolyte]

[0057] 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 include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more thereof. The non-aqueous solvent may contain a halogen-substituted body formed by replacing at least a portion of the hydrogen of these solvents with a halogen atom such as fluorine. As halogen-substituted bodies, fluorinated cyclic carbonates such as fluorinated ethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylates such as methyl fluoropropionate (FMP), and the like may be cited.

[0058] Examples of the above-mentioned 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 carboxylates such as γ-butyrolactone (GBL) and γ-valerolactone (GVL), and chain carboxylates such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0059] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether. , dihexyl ether, ethyl vinyl ether, butyl vinyl ether, 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 and other chain ethers, etc.

[0060] The electrolyte salt is preferably a lithium salt. Examples of lithium salts 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, Li2B4O7, Li(B(C2O4)F2) and other borates, LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2) {l, m is an integer greater than 0} and other imide salts. The lithium salt can be used alone or in combination. Among them, LiPF6 is preferably used from the viewpoint of ion conductivity, electrochemical stability, etc. The concentration of the lithium salt is, for example, 0.8 mol to 1.8 mol per 1L of non-aqueous solvent. In addition, vinylene carbonate and propane sultone additives can be further added.

[0061] <Example>

[0062] Hereinafter, the present disclosure will be further described by way of Examples and Comparative Examples, but the present disclosure is not limited to the following Examples.

[0063] [Preparation of positive electrode active material]

[0064] <Example 1>

[0065] Relative to the general formula Ni0.91 Co 0.05 Al 0.04 The metal composite oxide represented by O2 is mixed with calcium hydroxide (Ca(OH)2) in such a way that the total amount of Ni, Co and Al and the content of Ca are 0.1 mol%, and lithium hydroxide monohydrate (LiOH·H2O) is further mixed in such a way that the molar ratio of the total amount of Ni, Co, Al and Ca to Li is 1:1.02. The mixture is calcined from room temperature to 650°C at a heating rate of 2°C / min under an oxygen gas flow with an oxygen concentration of 95% (a flow rate of 10 L / min per 1 kg of the mixture), and then calcined from 650°C to 720°C at a heating rate of 1°C / min. The calcined product is washed with water to remove impurities, thereby obtaining the positive electrode active material of Example 1. The composition of the positive electrode active material of Example 1 was analyzed according to ICP-AES, and the result was Li 0.99 Ni 0.909 Co 0.05 Al 0.04 Ca 0.001 O2.

[0066] [Production of positive electrode]

[0067] The positive electrode active material is mixed in a ratio of 95 parts by mass, acetylene black as a conductive material is mixed in a ratio of 3 parts by mass, and polyvinylidene fluoride as a binding material is mixed in a ratio of 2 parts by mass, and mixed with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. Then, the slurry is applied to a positive electrode collector formed by an aluminum foil having a thickness of 15 μm, and after the coating is dried, the coating is rolled by a rolling roller and cut into a specified electrode size to obtain a positive electrode having a positive electrode composite material layer formed on both sides of the positive electrode core. It should be noted that a portion of the positive electrode is provided with an exposed portion exposed on the surface of the positive electrode core. The positive electrode is also made in the same manner in other embodiments and comparative examples.

[0068] [Production of negative electrode]

[0069] Natural graphite is used as the negative electrode active material. The negative electrode active material is mixed with sodium carboxymethyl cellulose (CMC-Na) and styrene-butadiene rubber (SBR) at a solid content mass ratio of 100:1:1 in an aqueous solution to prepare a negative electrode composite material slurry. The negative electrode composite material slurry is applied to both sides of a negative electrode core formed by copper foil, and after the coating film is dried, the coating film is rolled with a rolling roller and cut into a specified electrode size to obtain a negative electrode having a negative electrode composite material layer formed on both sides of the negative electrode core. It should be noted that an exposed portion where the surface of the negative electrode core is exposed is provided on a part of the negative electrode.

[0070] [Preparation of non-aqueous electrolyte]

[0071] Ethylene carbonate (EC), ethyl methyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed at 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 non-aqueous electrolyte.

[0072] [Production of test battery cells]

[0073] An aluminum lead is installed on the exposed portion of the positive electrode, and a nickel lead is installed on the exposed portion of the negative electrode. The positive electrode and the negative electrode are spirally wound with a polyolefin separator interposed therebetween, and then press-formed in the radial direction to produce a flat wound electrode body. The electrode body is housed in an outer shell, and after the non-aqueous electrolyte is injected, the opening of the outer shell is sealed to obtain a test battery cell.

[0074] [Evaluation of capacity maintenance rate]

[0075] The test battery cell was subjected to the following cycle test. The discharge capacity at the first cycle and the discharge capacity at the 30th cycle of the cycle test were obtained, and the capacity retention rate was calculated according to the following formula.

[0076] Capacity retention rate (%) = (discharge capacity at the 30th cycle ÷ discharge capacity at the 1st cycle) × 100

[0077] <Cyclic test>

[0078] The test battery cell was charged at a constant current of 0.2 It until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 1 / 100 It. Thereafter, the battery cell was discharged at a constant current of 0.2 It until the battery voltage reached 2.5 V. This charge and discharge cycle was repeated 30 times.

[0079] <Examples 2 to 6, Comparative Examples 1 to 7>

[0080] The raw materials used, the raw material mixing ratio and the manufacturing conditions (the molar ratio of Li relative to the total amount of metal elements other than Li, the calcination temperature of the second stage) were changed to synthesize lithium transition metal oxides (positive electrode active materials) having the composition shown in Table 1. Except for this, test battery cells were prepared and evaluated in the same manner as in Example 1.

[0081] Table 1 shows the evaluation results of the capacity retention rate. Table 1 also shows the ratio of the metal elements other than Li present in the Li layer to the total molar number of metal elements other than Li, the ratio of Ca contained in the negative electrode active material to the total mass of the positive electrode active material, and the molar ratio of the Ca content in the coating of the negative electrode active material to the Ni content. It should be noted that for the lithium transition metal oxides of the embodiments and comparative examples, powder X-ray diffraction measurement was carried out under known conditions to obtain X-ray diffraction spectra. From all the X-ray diffraction spectra of the embodiments and comparative examples, diffraction lines showing a layered structure were confirmed, and no CaO peak was confirmed.

[0082] [Table 1]

[0083]

[0084] As shown in Table 1, the test battery cells of Examples 1 to 6 had a higher capacity retention rate than any of the test battery cells of Comparative Examples 1 to 7.

[0085] Description of Reference Numerals

[0086] 10 Non-aqueous electrolyte secondary battery

[0087] 11 Positive electrode

[0088] 12 Negative electrode

[0089] 13 Divider

[0090] 14 Electrode body

[0091] 15 Battery housing

[0092] 16 Shell tank

[0093] 17 Sealing body

[0094] 18, 19 Insulation board

[0095] 20 positive electrode tab

[0096] 21. Negative electrode tab

[0097] 22 groove part

[0098] 23 Base Plate

[0099] 24 Lower valve body

[0100] 25 Insulation components

[0101] 26 Upper valve body

[0102] 27 Cover

[0103] 28 Gasket

[0104] 30 Positive electrode collector

[0105] 31. Positive electrode composite material layer

[0106] 40 Negative electrode collector

[0107] 41 negative electrode composite material layer

Claims

1. A non-aqueous electrolyte secondary battery comprising: a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte, The positive electrode active material includes a lithium transition metal oxide having a layered structure including a Li layer in which Li can be reversibly introduced and withdrawn, and containing at least Ni, Ca, and Al. The lithium transition metal oxide is of the general formula Li a Ni x Al y M z Ca w O 2-b The composite oxide shown in the formula, wherein 0.95<a<1.05, 0.80≤x≤0.95, 0≤y≤0.08, 0≤z≤0.1, 0<w≤0.03, 0≤b<0.05, x+y+z+w=1, M is at least one element selected from Co, Mn, Fe, Ti, Si, Nb, Zr, Mo and Zn, The ratio of the metal element other than Li present in the Li layer is 0.6 mol% or more and 2.0 mol% or less relative to the total number of moles of the metal element other than Li contained in the lithium transition metal oxide, The negative electrode active material has a coating containing Ca on its surface. The ratio of Ca in the coating is 15 mass ppm or more and less than 80 mass ppm with respect to the total mass of the positive electrode active material.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein Regarding the negative electrode active material, the coating further contains Ni.

3. The nonaqueous electrolyte secondary battery according to claim 2, wherein: The molar ratio of the Ca content to the Ni content in the coating is 0.5≤Ca / Ni≤1.0.

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