Non-aqueous electrolyte secondary battery

Abstract

Provided is a non-aqueous electrolyte secondary battery, wherein: a positive electrode active material includes a lithium transition metal composite oxide containing at least Ni, Mn, and B; the proportion of Ni with respect to the total amount of metal elements, excluding Li, in the lithium transition metal composite oxide is in the range of 80 mol%≤Ni≤95 mol%; the proportion of Mn with respect to the total amount of metal elements, excluding Li, in the lithium transition metal composite oxide is in the range of 0 mol%＜Mn≤20 mol%; the proportion of B with respect to the total amount of metal elements, excluding Li, in the lithium transition metal composite oxide is in the range of 0 mol%＜B≤3 mol%; the lithium transition metal composite oxide has B at least on the surface thereof; and a negative electrode mixture layer has B at least on the surface thereof, and contains B in an amount of 30-1000 mass ppm with respect to the total mass of the negative electrode mixture layer.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Lithium transition metal composite oxides are used as positive electrode active materials, and surface modification of the lithium transition metal composite oxides has been studied for the purpose of improving battery characteristics. For example, Patent Document 1 discloses that the initial capacity and cycle characteristics of a battery can be improved by partially covering the surface of the lithium transition metal composite oxide with an inorganic oxide layer such as aluminum oxide.

[0003] Furthermore, Patent Document 2 discloses that by coating the surface of a lithium nickel composite oxide with lithium metaborate and nickel oxide and setting the coverage rate of the lithium metaborate to 85% or more and less than 95%, it is possible to increase the charging voltage of the battery while improving characteristics such as cycle characteristics. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-116111 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-137947 Summary of the Invention

[0005] In recent years, lithium transition metal composite oxides with a high Ni content have attracted attention as positive electrode active materials with high energy density. However, lithium transition metal composite oxides with a high Ni content have a problem in that, when repeatedly charged and discharged at high temperatures, Ni and metal elements other than Ni, such as Mn, are eluted, and the eluted metal elements, such as Ni and Mn, are deposited on the negative electrode, resulting in a decrease in battery capacity. The technologies disclosed in Patent Documents 1 and 2 do not consider cycle characteristics at high temperatures, and there is still room for improvement.

[0006] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode active material includes a lithium transition metal composite oxide containing at least Ni, Mn, and B. The ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 80 mol% ≤ Ni ≤ 95 mol%, the ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% < Mn ≤ 20 mol%, and the ratio of B to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% < B ≤ 3 mol%. B is present at least on the surface of the lithium transition metal composite oxide. The negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. B is present at least on the surface of the negative electrode mixture layer, and the negative electrode mixture layer contains B in an amount of 30 mass ppm or more and 1000 mass ppm or less based on the total mass of the negative electrode mixture layer.

[0007] According to the non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, it is possible to suppress a decrease in battery capacity accompanying charge and discharge at high temperatures while having a high capacity.

Brief Description of the Drawings

[0008] [Figure 1] FIG. 1 is a longitudinal sectional view of a non-aqueous electrolyte secondary battery as an example of an embodiment.

Modes for Carrying Out the Invention

[0009] The layered structure of the lithium transition metal composite oxide contained in the positive electrode active material contains a transition metal layer, a Li layer, and an oxygen layer. The reversible movement of Li ions in the Li layer allows the battery's charge / discharge reactions to proceed. When using a lithium transition metal composite oxide with a high Ni content, many Li ions are extracted from the Li layer during battery charging, which can cause the layered structure to collapse and reduce battery capacity. In particular, when charging / discharging is performed at high temperatures, the activity is high and the layered structure tends to become more unstable. This can lead to the elution of Ni and other elements from the transition metal layer, which can cause the layered structure to deteriorate and reduce battery capacity. Furthermore, Ni and other elements eluted from the positive electrode can precipitate on the negative electrode, reducing battery capacity.

[0010] Therefore, the inventors conducted extensive research to solve the above-mentioned problems and found that, by using a positive electrode active material in which B is present on the surface of a lithium transition metal composite oxide containing Ni or the like, and further supplying B from the positive electrode to the negative electrode to cause B to be present on the surface of the negative electrode mixture layer, when the B concentration in the negative electrode mixture layer is within a predetermined range, the high-temperature charge-discharge cycle characteristics are specifically improved. It is believed that, for the positive electrode, the B present on the surface of the positive electrode active material forms a coating, thereby suppressing the elution of Ni or the like. For the negative electrode, it is believed that the coexistence of precipitated Ni or the like with B on the surface of the negative electrode mixture layer suppresses deterioration due to the precipitated Ni or the like. To supply an appropriate amount of B from the positive electrode to the negative electrode, high-temperature aging can be performed, for example, at a temperature of approximately 60°C. The battery after this high-temperature aging treatment contains a positive electrode active material of a predetermined composition with B present on its surface and a negative electrode mixture layer with a predetermined amount of B present on its surface, and this combination creates a unique synergistic effect, improving the high-temperature cycle characteristics.

[0011] 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 body is housed in a cylindrical exterior body will be exemplified. However, the electrode body 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 exterior body is not limited to a cylindrical shape, and may be, for example, a prismatic or coin-shaped body, or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0012] 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, 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 body 17 that closes the opening of the outer can 16.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] The positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below, particularly the positive electrode active material contained in the positive electrode mixture layer 31 that constitutes the positive electrode 11.

[0018] [Positive electrode] The positive electrode 11 has a positive electrode current collector 30 and a positive electrode mixture layer 31 formed on the surface 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 the surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, 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 mixture layer 31 on both sides of the positive electrode current collector 30.

[0019] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide 19 (PEO), or the like.

[0020] The positive electrode active material includes a lithium transition metal composite oxide. The lithium transition metal composite oxide has a layered structure including a Li layer through which Li reversibly enters and exits. Examples of the layered structure include a layered structure belonging to the space group R-3m and a layered structure belonging to the space group C2 / m. From the viewpoints of high capacity and stable crystal structure, the lithium transition metal composite oxide preferably has a layered structure belonging to the space group R-3m. The positive electrode active material may be composed essentially of the lithium transition metal composite oxide as a main component. Note that the positive electrode active material may also include a composite oxide other than the lithium transition metal composite oxide or other compounds, as long as the object of the present disclosure is not impaired.

[0021] The lithium transition metal composite oxide is, for example, a secondary particle formed by the aggregation of multiple 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 particles is measured as the diameter of the circumscribed circle in a particle image observed with a scanning electron microscope (SEM). 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 the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution of the 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 a dispersion medium.

[0022] The lithium transition metal composite oxide contains at least Ni, Mn, and B. That is, in the lithium transition metal composite oxide, Ni, Mn, and B are essential components.

[0023] The ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 80 mol% ≤ Ni ≤ 95 mol%, and the range of 80 mol% ≤ Ni ≤ 90 mol% is preferred. By setting the content of Ni to 80 mol% or more, a battery with a high energy density can be obtained. On the other hand, when the content of Ni exceeds 95 mol%, the content of other metal elements becomes too low to ensure the stability of the layered structure of the lithium transition metal composite oxide, and erosion of the particle surface cannot be suppressed.

[0024] The ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% < Mn ≤ 20 mol%, and the range of 3 mol% ≤ Mn ≤ 10 mol% is preferred. Since the oxidation number of Mn does not change during charge and discharge, it is considered that the structure of the transition metal layer is stabilized by being contained in the transition metal layer.

[0025] The ratio of B to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol% < B ≤ 3 mol%, and the range of 0.1 mol% ≤ B ≤ 2 mol% is preferred. Further, B is present at least on the surface of the lithium transition metal composite oxide. Thereby, B present on the surface of the positive electrode active material forms a film, and elution of Ni etc. can be suppressed. Also, as will be described later, B moves from the positive electrode to the negative electrode, and by coexisting B with Ni etc. deposited on the surface of the negative electrode binder layer, deterioration caused by the deposited Ni etc. can be suppressed.

[0026] The surface of the lithium transition metal composite oxide means the particle surface of the lithium transition metal composite oxide and its vicinity, for example, the surface vicinity region within 30 nm from the particle surface. Since the lithium transition metal composite oxide is generally a secondary particle formed by aggregation of a plurality of primary particles, it is preferable that B is present at a high concentration on the surface of the secondary particle and in the vicinity of the surface of the secondary particle. The distribution of B in the lithium transition metal composite oxide can be analyzed by TEM-EDX or the like.

[0027] B may exist in the state of a boron compound on the surface of the lithium transition metal composite oxide. Examples of the boron compound include boric acid (H3BO3), boron oxide (B2O3), and lithium borate (LiBO2, Li2B4O7). The boron compound may be formed so as to cover the entire surface of the lithium transition metal composite oxide, or may be scattered on the surface of the lithium transition metal composite oxide. When it is scattered on the surface of the lithium transition metal composite oxide, the particle size of the boron compound is generally smaller than the particle size of the primary particles constituting the lithium transition metal composite oxide. The particles of the boron compound can be confirmed by SEM. It is preferable that the boron compound is attached over a wide range without being unevenly distributed on a part of the surface of the secondary particles constituting the lithium transition metal composite oxide. Further, the thickness of the boron compound on the surface of the lithium transition metal composite oxide may be, for example, 10 nm to 100 nm.

[0028] The lithium transition metal composite oxide further contains M1 (M1 is at least one element selected from the elements of Groups 4 to 6), and the ratio of M1 to the total amount of metal elements excluding Li in the lithium transition metal composite oxide may be in the range of 0 mol% < M1 ≤ 5 mol%. M1 may be solid-dissolved in the crystal of the lithium transition metal composite oxide, may be precipitated on the particle surface or grain boundaries, or may be both. By the lithium transition metal composite oxide containing M1, the durability and safety of the battery can be improved.

[0029] An example of a suitable lithium transition metal composite oxide is the general formula Li a Ni x Mny Co z M2 w M3 v B u O 2-b (0.8 < a < 1.2, 0.80 ≤ x ≤ 0.95, 0 < y ≤ 0.20, 0 ≤ z < 0.20, 0 ≤ w ≤ 0.05, 0 ≤ v ≤ 0.05, 0 < u ≤ 0.03, 0 ≤ b < 0.05, x + y + z + w + v + u = 1, M2 is at least one element selected from Zr, Ti, Mo, Ta, Nb, and W, M3 is at least one element selected from Mg, Ca, Sr, Al, and Si) is a composite oxide. The molar fraction of the metal elements contained in the whole particles of the lithium transition metal composite oxide can be measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), etc.

[0030] Next, an example of a method for producing a positive electrode active material containing a lithium transition metal composite oxide will be described.

[0031] The method for producing a positive electrode active material includes, for example, a first step of obtaining a composite oxide containing Ni, Mn, 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, a third step of firing the mixture to obtain a fired product, and a fourth step of adding a B compound to the fired product to obtain a positive electrode active material having B present on the surface.

[0032] In the first step, for example, while stirring a solution of a metal salt containing Ni, Mn, and an arbitrary metal element (such as Co), an alkaline solution such as sodium hydroxide is dropped, and the pH is adjusted to the alkaline side (for example, 8.5 to 12.5) to precipitate (co-precipitate) a composite hydroxide containing Ni, Mn, and an arbitrary metal element, and by firing the composite hydroxide, a composite oxide containing Ni, Mn, and an arbitrary metal element is obtained. The firing temperature is not particularly limited, but is, for example, in the range of 300°C to 600°C.

[0033] In the second step, the composite oxide obtained in the first step is mixed with a lithium compound to obtain a mixture. Examples of lithium compounds include Li2CO3, LiOH, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. In the second step, when the composite oxide obtained in the first step is mixed with the lithium compound, other metal raw materials may be added as needed. The other metal raw materials are oxides containing metal elements other than the metal elements constituting the composite oxide obtained in the first step, such as Nb2O5.

[0034] In the third step, the mixture obtained in the second step is fired to obtain a fired product. The firing of the mixture in the third step can be carried out, for example, in an oxygen stream at 450°C to 800°C, and may be carried out in multiple stages. The fired product obtained may be washed with a liquid such as water and dried.

[0035] In the fourth step, the fired product obtained in the third step is dry-mixed with a B raw material, followed by heat treatment and pulverization to obtain a positive electrode active material having B present on the particle surface. The heat treatment in the fourth step can be carried out, for example, in an air atmosphere at 200°C to 500°C. Examples of the B raw material include boric acid (H3BO3), metaboric acid (HBO2), and tetraboric acid (H2B4O7).

[0036] [Negative electrode] The negative electrode 12 has a negative electrode current collector 40 and a negative electrode mixture layer 41 formed on the surface of the negative electrode current collector 40. The negative electrode current collector 40 can be a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on its surface. The negative electrode mixture layer 41 contains a negative electrode active material and a binder. The negative electrode 12 can be produced by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc. to the surface of the negative electrode current collector 40, drying the coating, and then rolling the coating to form the negative electrode mixture layer 41 on both sides of the negative electrode current collector 40.

[0037] The negative electrode active material contained in the negative electrode mixture layer 41 is not particularly limited as long as it can reversibly absorb and release lithium ions, and for example, a carbon material such as graphite may be used. The graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads. Furthermore, as the negative electrode active material, a Si-based active material composed of at least one of Si and a Si-containing compound may be used, or a carbon-based active material and a Si-based active material may be used in combination.

[0038] The binder contained in the negative electrode mixture layer 41 may be a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, or polyolefin, as in the case of the positive electrode 11, but is preferably styrene-butadiene rubber (SBR). The negative electrode mixture layer 41 may also contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

[0039] The negative electrode mixture layer 41 has B present at least on its surface, and contains B in an amount of 30 ppm by mass to 1000 ppm by mass with respect to the total mass of the negative electrode mixture layer 41. This allows an appropriate amount of B to coexist with precipitated Ni, etc. on the surface of the negative electrode mixture layer 41, thereby suppressing deterioration due to Ni, etc. The presence of B on the surface of the negative electrode mixture layer 41 can be confirmed by analysis using SEM, TEM, EPMA, etc.

[0040] The content of B in the negative electrode mixture layer 41 relative to the total mass of the negative electrode mixture layer 41, that is, the content of B in the negative electrode mixture layer 41, can be calculated by the following method. (1) Ion-exchanged water is added to the negative electrode 12 to remove the negative electrode mixture layer 41. (2) The negative electrode mixture layer 41 is cut out and its weight is measured. (3) Aqua regia and hydrofluoric acid are added to the cut-out negative electrode mixture layer 41, and the mixture is heated and dissolved. Insoluble matters such as carbon are filtered off to prepare an aqueous solution. The aqueous solution is adjusted to a constant volume with ion-exchanged water, and the B concentration is measured by ICP-AES. The result is defined as the B content in the negative electrode mixture layer 41. (4) The content of B in the negative electrode mixture layer 41 measured in (3) was divided by the weight of the negative electrode mixture layer 41 measured in (2) to obtain the content of B in the negative electrode mixture layer 41.

[0041] After fabricating a nonaqueous electrolyte secondary battery 10 including a positive electrode 11, a negative electrode 12, and a nonaqueous electrolyte described below, high-temperature aging treatment is performed to migrate B from the positive electrode 11, causing B to be present on the surface of the negative electrode mixture layer 41 and allowing an appropriate amount of B to be contained in the negative electrode mixture layer 41. The high-temperature aging treatment is a treatment in which the secondary battery 10 is maintained at a high temperature after charging, thereby migrating B from the positive electrode 11 to the negative electrode 12. The temperature and time of the high-temperature aging treatment are not particularly limited, but may be, for example, maintained at 60°C for 9 hours.

[0042] The negative electrode mixture layer 41 may further contain Ni on its surface, and the molar ratio of Ni to B in the negative electrode mixture layer 41 may be 0.05≦Ni / B≦1.0. Ni eluted from the positive electrode 11 is likely to deposit on the surface of the negative electrode mixture layer 41. The presence of Ni on the surface of the negative electrode mixture layer 41 can be confirmed by SEM. The Ni content in the negative electrode mixture layer 41 can be calculated in the same manner as the B content described above, and the molar ratio of Ni to B (Ni / B) in the negative electrode mixture layer 41 can be calculated from the ratio of the Ni and B contents.

[0043] When the positive electrode active material contains M1, the negative electrode mixture layer 41 may further contain M1 on its surface. M1 eluted from the positive electrode 11 is likely to deposit on the surface of the negative electrode mixture layer 41. The presence of M1 on the surface of the negative electrode mixture layer 41 can be confirmed by SEM. Furthermore, the molar ratio of M1 to B in the negative electrode mixture layer 41 may be 0.05≦M1 / B≦1.0. The content of M1 in the negative electrode mixture layer 41 can be calculated in the same manner as the content of B described above, and the molar ratio of M1 to B in the negative electrode mixture layer 41 (M1 / B) can be calculated from the ratio of the contents of M1 and B.

[0044] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a laminated structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin, or a filler layer containing an inorganic compound filler may be provided on the surface of the separator 13.

[0045] [Non-aqueous electrolyte] The non-aqueous electrolyte may contain, 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, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may be, for example, a lithium salt such as LiPF6. One type of lithium salt may be used alone, or multiple types may be used in combination. The concentration of the lithium salt may be, for example, 0.8 mol to 1.8 mol per liter of non-aqueous solvent. Furthermore, vinylene carbonate (VC) or a propane sultone-based additive may be added.

[0046] <Example> 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.

[0047] Example 1 [Preparation of positive electrode active material] [Ni 0.85 Co 0.08 Mn 0.07 The composite hydroxide represented by ](OH)2 was calcined at 500°C for 8 hours to form the composite oxide (Ni 0.85 Co 0.08 Mn 0.07O2) was obtained (Step 1). Next, lithium hydroxide (LiOH) and the above composite oxide were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.02:1 (Step 2). This mixture was fired under an oxygen stream from room temperature to 720°C, and the resulting fired product was washed with water and dried (Step 3). This washed and dried fired product was dry-mixed with boric acid (H3BO3) so that the molar ratio of the total amount of Ni, Co, and Mn to B in H3BO3 was 1:0.01, and the mixture was fired in air at 300°C for 3 hours, followed by pulverization, to obtain a positive electrode active material of Example 1 in which B was present on the particle surface (Step 4). The composition of the positive electrode active material of Example 1 was determined by ICP-AES analysis to be LiNi 0.842 Co 0.079 Mn 0.069 B 0.01 It was O2.

[0048] [Preparation of positive electrode] The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 96.3:2.5:1.2, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added. The mixture was then kneaded to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to a positive electrode current collector made of aluminum foil, the coating was dried, and the coating was rolled using a rolling roller and cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode current collector. An exposed portion was provided on a portion of the positive electrode where the surface of the positive electrode current collector was exposed.

[0049] [Preparation of negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, carboxymethylcellulose 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 mixture slurry. The negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried. The coating was then rolled using a rolling roller and cut to the specified electrode size to obtain a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode current collector. An exposed portion was provided on a portion of the negative electrode where the surface of the negative electrode current collector was exposed.

[0050] [Preparation of non-aqueous electrolyte] Lithium hexafluorophosphate (LiPF6) was dissolved at a concentration of 1.1 mol / L in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4. Furthermore, vinylene carbonate (VC) was dissolved in the mixed solvent at a concentration of 2.0 mass% to prepare a nonaqueous electrolyte.

[0051] [Test cell construction] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode, and the positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an outer casing, and the nonaqueous electrolyte was poured into it. The opening of the outer casing was then sealed to obtain a test cell.

[0052] [High temperature aging treatment] The test cell was subjected to constant current charging at 0.1C in a 25°C environment until the battery voltage reached 4.2V, followed by constant voltage charging at 4.2V until the current reached 0.05C. It was then discharged at a constant current of 0.1C until the battery voltage reached 2.5V. It was then charged at a constant current of 0.1C until the battery voltage reached 3.7V, followed by constant voltage charging at 3.7V until the current reached 0.05C. The test cell in this charged state was then held in a 60°C environment for 9 hours. It was then charged at a constant current of 0.1C in a 25°C environment until the battery voltage reached 4.2V, followed by constant voltage charging at 4.2V until the current reached 0.05C, followed by constant current discharging at 0.1C until the battery voltage reached 2.5V. The test cell after this high-temperature aging treatment was designated the initial test cell.

[0053] [Capacity retention rate evaluation] The initial test cell was subjected to the following high-temperature cycle test. The discharge capacity at the first cycle and the discharge capacity at the 300th cycle of the high-temperature cycle test were determined, and the capacity retention rate was calculated using the following formula.

[0054] Capacity retention rate (%) = (300th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 <High temperature cycle test> The test cell was charged at a constant current of 0.5 C in a 45°C environment until the battery voltage reached 4.2 V, and then charged at a constant voltage of 0.02 C at 4.2 V. It was then discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This charge-discharge cycle was repeated 300 times. After the high-temperature cycle test, the B content in the negative electrode mixture layer was 221 ppm, and the molar ratio of Ni to B (Ni / B) in the negative electrode mixture layer was 0.19. Note that migration of B from the positive electrode to the negative electrode occurs during high-temperature aging.

[0055] <Example 2> In the second step of preparing the positive electrode active material, LiOH, the composite oxide, and Nb2O5 were mixed so that the molar ratio of Li, Ni, Co, and Mn to Nb was 1.02:1:0.01. The composition of the positive electrode active material in Example 2 was determined to be LiNi 0.833 Co 0.078 Mn 0.069 Nb 0.01 B 0.01 After the high-temperature cycle test, the B content in the negative electrode mixture layer was 114 ppm, the molar ratio of Ni to B (Ni / B) in the negative electrode mixture layer was 0.10, and the molar ratio of Nb to B (Nb / B) in the negative electrode mixture layer was 0.23.

[0056] <Comparative Example 1> A test cell was prepared and evaluated in the same manner as in Example 1, except that the fourth step was not performed in the preparation of the positive electrode active material. As a result of ICP-AES analysis, the composition of the positive electrode active material of Comparative Example 1 was LiNi 0.85 Co 0.08 Mn 0.07 It was O2.

[0057] <Comparative Example 2> A test cell was prepared and evaluated in the same manner as in Example 2, except that the fourth step was not performed in the preparation of the positive electrode active material. As a result of ICP-AES analysis, the composition of the positive electrode active material of Comparative Example 2 was LiNi 0.842 Co 0.079 Mn 0.069 Nb 0.01 It was O2.

[0058] The evaluation results of the capacity retention rate are shown in Table 1. Table 1 also shows the composition of the positive electrode active material, the content of B in the negative electrode mixture layer, the molar ratio of Ni to B in the negative electrode mixture layer (Ni / B), and the molar ratio of Nb to B in the negative electrode mixture layer (Nb / B).

[0059] [Table 1]

[0060] As shown in Table 1, the test cells of Examples 1 and 2 had higher capacity retention rates after high-temperature cycling than the test cells of Comparative Examples 1 and 2. [Explanation of symbols]

[0061] 10 Nonaqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery case 16 outer can 17 Sealing body 18,19 Insulating plate 20 Positive electrode tab 21 Negative electrode tab 22 Grooved part 23 Bottom plate 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket 30 Positive electrode current collector 31 Positive electrode mixture layer 40 Negative electrode current collector 41 Negative electrode mixture layer

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on a surface of the positive electrode current collector and containing a positive electrode active material, the positive electrode active material includes a lithium transition metal composite oxide containing at least Ni, Mn, and B; a ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 80 mol%≦Ni≦95 mol%, a ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol%<Mn≦20 mol%, a ratio of B to the total amount of metal elements excluding Li in the lithium transition metal composite oxide is in the range of 0 mol%<B≦3 mol%, The lithium transition metal composite oxide has B at least on the surface thereof, the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector and containing a negative electrode active material, the negative electrode mixture layer has B present at least on its surface, and contains B in an amount of 30 ppm by mass to 1000 ppm by mass relative to the total mass of the negative electrode mixture layer.

2. The negative electrode mixture layer further has Ni present on the surface thereof, 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the molar ratio of Ni to B in the negative electrode mixture layer is 0.05≦Ni / B≦1.

0.

3. The lithium transition metal composite oxide further contains M1 (M1 is at least one element selected from the elements of Groups 4 to 6), 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of M1 to the total amount of metal elements excluding Li in said lithium transition metal composite oxide is in the range of 0 mol % < M1 ≦ 5 mol %.

4. The nonaqueous electrolyte secondary battery according to claim 3 , wherein the negative electrode mixture layer further contains M1 on the surface thereof.

5. 5. The nonaqueous electrolyte secondary battery in accordance with claim 4, wherein in the negative electrode mixture layer, the molar ratio of M1 to B satisfies 0.05≦M1 / B≦1.

0.

6. The lithium transition metal composite oxide has the general formula Li ａ Ni ｘ Mn ｙ Co ｚ M2 ｗ M3 ｖ B ｕ O ２－ｂ 6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the formula is: (0.8<a<1.2, 0.80≦x≦0.95, 0<y≦0.20, 0≦z<0.20, 0≦w≦0.05, 0≦v≦0.05, 0<u≦0.03, 0≦b<0.05, x+y+z+w+v+u=1, M2 is at least one element selected from Zr, Ti, Mo, Ta, Nb, and W, and M3 is at least one element selected from Mg, Ca, Sr, Al, and Si).

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