Non-aqueous electrolyte secondary battery

Abstract

In a nonaqueous electrolyte secondary battery, a positive electrode active material includes a lithium transition metal composite oxide containing at least Ni, Mn, and B, a ratio of Ni is in a range of 80 mol% ≤ Ni ≤ 95 mol% with respect to a total amount of metal elements other than Li in the lithium transition metal composite oxide, a ratio of Mn is in a range of 0 mol% < Mn ≤ 20 mol% with respect to the total amount of the metal elements other than Li in the lithium transition metal composite oxide, a ratio of B is in a range of 0 mol% < B ≤ 3 mol% with respect to the total amount of the metal elements other than Li in the lithium transition metal composite oxide, B is present at least on a surface of the lithium transition metal composite oxide, B is present at least on a surface of a negative electrode mixture layer, and B is contained in an amount of 30 mass ppm or more and 1000 mass ppm or less with respect to a total mass of the negative electrode mixture layer.

Description

Technical Field

[0001] This disclosure relates to non-aqueous electrolyte secondary batteries. Background Technology

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

[0003] In addition, Patent Document 2 discloses that by covering the surface of the lithium-nickel composite oxide with lithium metaborate and nickel oxide, the coverage of lithium metaborate is 85% or more and less than 95%, thereby improving the charging voltage of the battery and improving its cycle characteristics.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2014-116111

[0007] Patent Document 2: Japanese Patent Application Publication No. 2013-137947 Summary of the Invention

[0008] In recent years, lithium transition metal composite oxides with high Ni content have attracted much attention as high-energy-density positive electrode active materials. However, during repeated charge-discharge at high temperatures, Ni and other metal elements such as Mn dissolve from these Ni-rich composite oxides. These dissolved Ni and Mn elements then precipitate on the negative electrode, leading to a decrease in battery capacity. The technologies disclosed in Patent Documents 1 and 2 focus on the cycling characteristics at high temperatures, and there is still room for improvement.

[0009] 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 comprises: 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, wherein the positive electrode active material comprises a lithium-transition metal composite oxide containing at least Ni, Mn and B, wherein based on the total amount of metal elements excluding Li in the lithium-transition metal composite oxide, the proportion of Ni is in the range of 80 mol% ≤ Ni ≤ 95 mol%, based on the total amount of metal elements excluding Li in the lithium-transition metal composite oxide, the proportion of Mn is in the range of 0 mol% < Mn ≤ 20 mol%, based on the total amount of metal elements excluding Li in the lithium-transition metal composite oxide, the proportion of B is in the range of 0 mol% < B ≤ 3 mol%, and B is present at least on the surface of the lithium-transition metal composite oxide; the negative electrode comprises: 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, B is present at least on the surface of the negative electrode mixture layer, and based on the total mass of the negative electrode mixture layer, B is contained in an amount of 30 mass ppm or more and 1000 mass ppm or less.

[0010] According to the non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, the non-aqueous electrolyte secondary battery can have high capacity and suppress reduction in battery capacity accompanying charge and discharge at high temperatures. Brief Description of Drawings

[0011] Figure 1 Fig. 1 is a longitudinal sectional view of an example of a non-aqueous electrolyte secondary battery according to an embodiment. Detailed Description of Embodiments

[0012] In the layered structure of the lithium-transition metal composite oxide contained in the positive electrode active material, a transition metal layer, a Li layer, and an oxygen layer are provided, and the charge and discharge reaction of the battery proceeds by reversible deintercalation / intercalation of Li ions present in the Li layer. When a lithium-transition metal composite oxide with a high Ni content is used, a large amount of Li ions are extracted from the Li layer during battery charging, so the layered structure collapses, which may lead to a reduction in battery capacity. Particularly when charge and discharge are performed at high temperatures, the activity is high and the layered structure tends to become more unstable, so Ni and other elements are eluted from the transition metal layer, which promotes the deterioration of the layered structure and easily leads to a reduction in battery capacity. In addition, the precipitation of Ni and other elements eluted from the positive electrode on the negative electrode also causes a reduction in battery capacity.

[0013] Therefore, the inventors conducted in-depth research to solve the above-mentioned problems and found that by using a positive electrode active material containing lithium transition metal composite oxides such as Ni with B present on its surface, and then supplying B from the positive electrode to the negative electrode, so that B exists on the surface of the negative electrode compound layer, and if the B concentration in the negative electrode compound layer is within a specified range, the charge-discharge cycle characteristics at high temperatures can be specifically improved. Regarding the positive electrode, it is believed that the B present on the surface of the positive electrode active material forms a coating, thereby inhibiting the dissolution of Ni, etc. Regarding the negative electrode, it is believed that the coexistence of precipitated Ni, etc., and B on the surface of the negative electrode compound layer inhibits the degradation caused by precipitated Ni, etc. When an appropriate amount of B is supplied from the positive electrode to the negative electrode, for example, high-temperature aging at a temperature of about 60°C is sufficient. The battery after this high-temperature aging treatment contains a positive electrode active material with a specified composition of B present on its surface and a negative electrode compound layer with a specified amount of B present on its surface; through their combination, a unique synergistic effect is produced, and the high-temperature cycle characteristics are improved.

[0014] Hereinafter, an example of an embodiment of the non-aqueous electrolyte secondary battery of the present disclosure will be described in detail. The example described below is a cylindrical battery obtained by housing a wound electrode body in a cylindrical outer casing. However, the electrode body is not limited to a wound type; it can be a stacked type formed by alternately stacking multiple positive electrodes and multiple negative electrodes one by one with separators in between. Furthermore, the outer casing is not limited to a cylindrical shape; for example, it can be square, coin-shaped, or a battery casing composed of laminates including metal layers and resin layers.

[0015] Figure 1 This is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, as an example of an embodiment. Figure 1 For example, a non-aqueous electrolyte secondary battery 10 includes: an electrode body 14, a non-aqueous electrolyte, and a battery casing 15 for housing the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound together with a separator 13 in between. The battery casing 15 consists of a bottomed cylindrical outer shell 16 and a sealing body 17 that blocks the opening of the outer shell 16.

[0016] The electrode body 14 comprises an elongated positive electrode 11, an elongated negative electrode 12, two elongated spacers 13, a positive electrode tab 20 bonded to the positive electrode 11, and a negative electrode tab 21 bonded to the negative electrode 12. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 to prevent lithium deposition. That is, the negative electrode 12 is longer than the positive electrode 11 in both the length and width directions (short direction). The two spacers 13 are formed to be at least slightly larger than the positive electrode 11, for example, in a manner that clamps the positive electrode 11.

[0017] The non-aqueous electrolyte secondary battery 10 has insulating plates 18 and 19 respectively disposed above and below the electrode body 14. Figure 1In the example shown, the positive electrode tab 20 installed on the positive electrode 11 extends towards the sealing body 17 through the through hole in the insulating plate 18, and the negative electrode tab 21 installed on the negative electrode 12 extends towards the bottom of the outer casing 16 through the outer side of the insulating plate 19. 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, which is electrically connected to the bottom plate 23, becomes the positive terminal. The negative electrode tab 21 is connected to the inner bottom surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative terminal.

[0018] The outer casing 16 is, for example, a bottomed cylindrical metal container. A gasket 28 is provided between the outer casing 16 and the sealing body 17, sealing the internal space of the battery casing 15. The outer casing 16 has, for example, a groove 22 formed from the externally pressurized side to support the sealing body 17. The groove 22 is preferably formed in a ring shape along the circumference of the outer casing 16, and its upper surface supports the sealing body 17.

[0019] The sealing body 17 has a structure in which a base plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cover 27 are stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 1 has, for example, a circular or annular shape, and all 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 the insulating member 25 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat dissipation, the lower valve body 24 deforms and breaks in a manner that pushes the upper valve body 26 toward the cover 27 side, thus blocking the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cover 27.

[0020] The following details the positive electrode active material contained in the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte, particularly the positive electrode compound layer 31 constituting the positive electrode 11, which constitute the non-aqueous electrolyte secondary battery 10.

[0021] [positive electrode]

[0022] The positive electrode 11 has a positive current collector 30 and a positive electrode additive layer 31 formed on the surface of the positive current collector 30. The positive current collector 30 may use a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a thin film of the metal disposed on its surface. The positive electrode additive layer 31 contains a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 is manufactured by coating the surface of the positive current collector 30 with a positive electrode additive slurry containing a positive electrode active material, a conductive agent, and a binder, drying the coating, and then compressing it to form the positive electrode additive layer 31 on both sides of the positive current collector 30.

[0023] Examples of conductive agents contained in the positive electrode binder layer 31 include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. Examples of binders contained in the positive electrode binder layer 31 include fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. These resins can also be used in combination with carboxymethyl cellulose (CMC) or its salts, polyethylene oxide 19 (PEO), etc.

[0024] The positive electrode active material comprises a lithium transition metal composite oxide. The lithium transition metal composite oxide has a layered structure comprising Li layers with reversible insertion / extraction of Li. Examples of layered structures include those belonging to space group R-3m and those belonging to space group C2 / m. For high capacity and crystal structure stability, a layered structure belonging to space group R-3m is preferred for the lithium transition metal composite oxide. The positive electrode active material can be composed primarily of the lithium transition metal composite oxide, essentially consisting only of it. It should be noted that, without prejudice to the purpose of this disclosure, the positive electrode active material may contain composite oxides other than lithium transition metal composite oxides, or other compounds.

[0025] Lithium transition metal composite 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 determined by measuring the diameter of the circumcircle in a particle image observed using a scanning electron microscope (SEM). The lithium transition metal composite oxide has a median particle size (D50) in volumetric terms, 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 in the volumetric particle size distribution where the cumulative frequency from the smallest particle size is 50%, also known as the median diameter. The particle size distribution of lithium transition metal composite oxides can be measured using a laser diffraction-type particle size distribution measuring device (e.g., MicrotracBEL Co., Ltd., MT3000II) with water as the dispersion medium.

[0026] Lithium transition metal complex oxides contain at least Ni, Mn, and B. That is, Ni, Mn, and B are essential components in lithium transition metal complex oxides.

[0027] Relative to the total amount of metal elements other than Li in the lithium-transition metal composite oxide, the ratio of Ni is in the range of 80 mol% ≤ Ni ≤ 95 mol%, and preferably in the range of 80 mol% ≤ Ni ≤ 90 mol%. A battery with high energy density can be obtained by setting the Ni content to 80 mol% or more. On the other hand, if the Ni content exceeds 95 mol%, the content of other metal elements is too low, the stability of the layered structure of the lithium-transition metal composite oxide cannot be ensured, and the erosion on the particle surface cannot be suppressed.

[0028] Relative to the total amount of metal elements other than Li in the lithium-transition metal composite oxide, the ratio of Mn is in the range of 0 mol% < Mn ≤ 20 mol%, and preferably in the range of 3 mol% ≤ Mn ≤ 10 mol%. Mn does not undergo oxidation number change during charge and discharge, so it is considered that the inclusion of Mn in the transition metal layer stabilizes the structure of the transition metal layer.

[0029] Relative to the total amount of metal elements other than Li in the lithium-transition metal composite oxide, the ratio of B is in the range of 0 mol% < B ≤ 3 mol%, and preferably in the range of 0.1 mol% ≤ B ≤ 2 mol%. Furthermore, B is present at least on the surface of the lithium-transition metal composite oxide. Thereby, B existing on the surface of the positive electrode active material forms a coating film, which can suppress the elution of Ni and the like. In addition, as described later, B moves from the positive electrode to the negative electrode, so that Ni and the like precipitated on the surface of the negative electrode mixture layer coexist with B, thereby suppressing the deterioration caused by the precipitated Ni and the like.

[0030] The surface of the lithium-transition metal composite oxide refers to the particle surface of the lithium-transition metal composite oxide and the vicinity thereof, for example, a region near the surface within 30 nm from the particle surface. Lithium-transition metal composite oxides are generally secondary particles formed by aggregation of a plurality of primary particles, therefore, B is preferably present at high concentrations on the surface of the secondary particles and in the vicinity of the surface of the secondary particles. The distribution of B in the lithium-transition metal composite oxide can be analyzed by TEM-EDX or the like.

[0031] B can exist on the surface of the lithium transition metal composite oxide in the form of a boron compound. 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 dispersed and exist on the surface of the lithium transition metal composite oxide. When dispersed 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 primary particles constituting the lithium transition metal composite oxide. It should be noted that particles of the boron compound can be confirmed by SEM. Preferably, the boron compound is attached to a part of the surface of secondary particles constituting the lithium transition metal composite oxide in a wide range, instead of being unevenly present on a part of the surface of secondary particles constituting the lithium transition metal composite oxide. In addition, 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.

[0032] The lithium transition metal composite oxide further contains M1 (M1 is at least one element selected from elements of Groups 4 to 6), and based on the total amount of metal elements other than Li in the lithium transition metal composite oxide, the ratio of M1 may be in a range of 0 mol% < M1 ≤ 5 mol%. M1 may be solid-dissolved in the crystal of the lithium transition metal composite oxide, or may be precipitated on the particle surface or at grain boundaries, or both. By containing M1 in the lithium transition metal composite oxide, the durability and safety of a battery can be improved.

[0033] An example of a suitable lithium transition metal composite oxide is represented by the general formula Li a Ni x Mn y 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 or more elements selected from the group consisting of Zr, Ti, Mo, Ta, Nb and W, M3 is at least one or more elements selected from the group consisting of Mg, Ca, Sr, Al and Si). The molar fraction of 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 spectroscopy (EDX), or the like.

[0034] Next, an example of a method for producing a positive electrode active material containing the lithium transition metal composite oxide will be described.

[0035] The method for manufacturing a positive electrode active material includes, for example, the following steps: a first step, obtaining a composite oxide containing Ni, Mn and any metal element; a second step, mixing the composite oxide obtained in the first step with a lithium compound to obtain a mixture; a third step, calcining the mixture to obtain a calcined product; and a fourth step, adding a B compound to the calcined product to obtain a positive electrode active material having B present on its surface.

[0036] In the first step, for example, while stirring a solution containing Ni, Mn, and any metal element (such as Co), an alkaline solution such as sodium hydroxide is added dropwise to adjust the pH to the alkaline side (e.g., 8.5–12.5), causing a composite hydroxide containing Ni, Mn, and any metal element to precipitate (co-precipitate). This composite hydroxide is then calcined to obtain a composite oxide containing Ni, Mn, and any metal element. The calcination temperature is not particularly limited, but is, for example, in the range of 300°C to 600°C.

[0037] 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. When mixing the composite oxide obtained in the first step with the lithium compound in the second step, other metal raw materials may be added as needed. These other metal raw materials are oxides containing metal elements other than those constituting the composite oxide obtained in the first step, such as Nb₂O₅.

[0038] 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, under an oxygen flow at 450°C to 800°C, or it can be carried out in multiple stages. The obtained fired product can be washed with a liquid such as water and dried.

[0039] In step 4, the calcined material obtained in step 3 is dry-mixed with raw material B, heat-treated, and then pulverized to obtain a positive electrode active material with B present on the particle surface. The heat treatment in step 4 can be carried out, for example, at 200°C to 500°C under atmospheric conditions. Examples of raw material B include boric acid (H3BO3), metaboric acid (HBO2), and tetraboric acid (H2B4O7).

[0040] [negative electrode]

[0041] The negative electrode 12 has a negative electrode current collector 40 and a negative electrode binder 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 stable within the potential range of the negative electrode 12, such as copper or a copper alloy, or a thin film of the metal disposed on its surface. The negative electrode binder layer 41 contains a negative electrode active material and a binder. The negative electrode 12 is manufactured by coating the surface of the negative electrode current collector 40 with a negative electrode binder slurry containing a negative electrode active material and a binder, drying the coating, and then calendering it to form the negative electrode binder layer 41 on both sides of the negative electrode current collector 40.

[0042] The negative electrode active material contained in the negative electrode compound layer 41 is not particularly limited as long as it can reversibly absorb, store, and release lithium ions; for example, carbon materials such as graphite can be used. Graphite can be any of the following: natural graphite such as flake graphite, block graphite, and amorphous graphite; block artificial graphite; and artificial graphite such as graphitized mesophase carbon microspheres. Furthermore, as the negative electrode active material, a Si-based active material composed of at least one of Si and Si-containing compounds can be used, or a combination of carbon-based and Si-based active materials can be used.

[0043] Similar to the case of the positive electrode 11, the binder contained in the negative electrode binder layer 41 can be fluorinated resins such as PTFE and PVdF, PAN, polyimide, acrylic resins, polyolefins, etc., with styrene-butadiene rubber (SBR) being preferred. Furthermore, the negative electrode binder layer 41 may contain CMC or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc.

[0044] The negative electrode mixture layer 41 contains at least B on its surface, with a concentration of B between 30 ppm and 1000 ppm relative to the total mass of the negative electrode mixture layer 41. Therefore, an appropriate amount of B can coexist with precipitated Ni and other substances on the surface of the negative electrode mixture layer 41, thus suppressing degradation caused by Ni and other substances. The presence of B on the surface of the negative electrode mixture layer 41 can be confirmed by analysis using SEM, TEM, EPMA, etc.

[0045] The content of B in the negative electrode mixture layer 41 relative to the total mass of the negative electrode mixture layer 41, i.e., the content rate of B in the negative electrode mixture layer 41, can be calculated by the following method.

[0046] (1) Add ion-exchange water to the negative electrode 12 to detach the negative electrode mixture layer 41.

[0047] (2) Cut out the negative electrode mixture layer 41 and measure its weight.

[0048] (3) Add aqua regia and hydrofluoric acid to the cut negative electrode mixture layer 41 and heat to dissolve. Filter out insoluble components such as carbon to prepare an aqueous solution. Make up the volume of the aqueous solution with deionized water. The result obtained by measuring the concentration of B by ICP-AES is taken as the content of B in the negative electrode mixture layer 41.

[0049] The content of B in the negative electrode mixture layer 41 determined in (4)(3) is divided by the weight of the negative electrode mixture layer 41 determined in (2) to obtain the content rate of B in the negative electrode mixture layer 41.

[0050] After fabricating a non-aqueous electrolyte secondary battery 10 comprising a positive electrode 11, a negative electrode 12, and a non-aqueous electrolyte (described later), a high-temperature aging treatment is performed. This allows B to move from the positive electrode 11, ensuring that B is present on the surface of the negative electrode flux layer 41 and that the negative electrode flux layer 41 contains an appropriate amount of B. The high-temperature aging treatment is performed by charging the secondary battery 10 and maintaining it at a high temperature, thereby causing B to move from the positive electrode 11 to the negative electrode 12. The temperature and time of the high-temperature aging treatment are not particularly limited; for example, it can be maintained at 60°C for 9 hours.

[0051] The negative electrode mixture layer 41 further contains Ni on its surface. In the negative electrode mixture layer 41, the molar ratio of Ni to B can be 0.05 ≤ Ni / B ≤ 1.0. Ni dissolved from the positive electrode 11 easily precipitates 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 way as the B content, and the molar ratio of Ni to B in the negative electrode mixture layer 41 (Ni / B) can be calculated from the ratio of Ni to B content.

[0052] When the positive electrode active material contains M1, the negative electrode mixture layer 41 may also contain M1 on its surface. M1 dissolved from the positive electrode 11 easily precipitates onto 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, in the negative electrode mixture layer 41, the molar ratio of M1 to B can be 0.05 ≤ M1 / B ≤ 1.0. The content of M1 in the negative electrode mixture layer 41 can be calculated in the same way as the content of B, 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 to B.

[0053] [Separator]

[0054] For example, a porous sheet with ion-permeable and insulating properties can be used in the separator 13. Specific examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 can be a single-layer structure or a multilayer structure. In addition, a resin layer with high heat resistance, such as an aromatic polyamide resin, or a filler layer containing inorganic compounds can be provided on the surface of the separator 13.

[0055] [Non-aqueous electrolytes]

[0056] The non-aqueous electrolyte may comprise, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these solvents. The non-aqueous solvent may contain a halogen substitute obtained by replacing at least a portion of the hydrogen atoms of these solvents with halogen atoms such as fluorine. Examples of electrolyte salts include lithium salts such as LiPF6. A single lithium salt may be used, or multiple lithium salts may be used in combination. The concentration of the lithium salt relative to 1 L of the non-aqueous solvent may be set to, for example, 0.8 mol to 1.8 mol. Furthermore, vinylene carbonate (VC) and propanesulfonate lactone additives may be added.

[0057] <Example>

[0058] The present disclosure will be further described below with reference to embodiments and comparative examples, but the present disclosure is not limited to the following embodiments.

[0059] <Example 1>

[0060] [Preparation of positive electrode active material]

[0061] [Ni] obtained by co-precipitation method 0.85 Co 0.08 Mn 0.07 The composite hydroxide shown in [(OH)2] was calcined at 500°C for 8 hours to obtain the composite oxide (Ni). 0.85 Co 0.08 Mn 0.07(Step 1) Next, lithium hydroxide (LiOH) and the above-mentioned composite oxide were mixed at a molar ratio of Li to Ni, Co, and Mn of 1.02:1 (Step 2). The mixture was calcined from room temperature to 720°C under an oxygen flow, and the resulting calcined product was washed with water and dried (Step 3). The washed and dried calcined product was dry-mixed with boric acid (H3BO3) at a molar ratio of Ni, Co, and Mn to B in H3BO3 of 1:0.01. After calcination at 300°C for 3 hours in atmospheric conditions, the mixture was pulverized to obtain the positive electrode active material of Example 1 with B present on the particle surface (Step 4). The ICP-AES analysis results of the composition of the positive electrode active material of Example 1 were LiNi 0.842 Co 0.079 Mn 0.069 B 0.01 O2.

[0062] [The production of the positive electrode]

[0063] The above-mentioned positive electrode active material was mixed with acetylene black and polyvinylidene fluoride (PVdF) at a solid component mass ratio of 96.3:2.5:1.2. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added, and the mixture was then kneaded to prepare a positive electrode slurry. This positive electrode slurry was coated onto a positive electrode current collector formed of aluminum foil. After the coating dried, it was calendered using calendering rollers and cut into specified electrode sizes, resulting in a positive electrode with positive electrode slurry layers formed on both sides of the positive electrode current collector. It should be noted that a portion of the positive electrode has an exposed portion on the surface of the positive electrode current collector.

[0064] [Making the negative electrode]

[0065] Natural graphite was used as the negative electrode active material. The negative electrode active material was mixed with sodium carboxymethyl cellulose (CMC-Na) and styrene-butadiene rubber (SBR) in an aqueous solution at a solid component mass ratio of 100:1:1 to prepare a negative electrode slurry. This negative electrode slurry was coated onto both sides of a negative electrode current collector formed from copper foil. After the coating dried, it was calendered using calendering rollers and cut into specified electrode sizes, resulting in a negative electrode with a negative electrode slurry layer formed on both sides of the negative electrode current collector. It should be noted that a portion of the negative electrode has an exposed portion on the surface of the negative electrode current collector.

[0066] [Preparation of non-aqueous electrolytes]

[0067] Lithium hexafluoride phosphate (LiPF6) was dissolved at a concentration of 1.1 mol / L in a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4. A non-aqueous electrolyte containing ethylene carbonate (VC) was then prepared at a concentration of 2.0% by mass relative to the aforementioned mixed solvent.

[0068] [Fabrication of the Experimental Battery Cell]

[0069] Aluminum leads are installed on the exposed portion of the positive electrode, and nickel leads are installed on the exposed portion of the negative electrode. The positive and negative electrodes are then wound in a spiral shape with a polyolefin separator in between, and then pressed radially to form a flat, wound electrode body. This electrode body is then housed in a casing, and the non-aqueous electrolyte is injected. The opening of the casing is then sealed to obtain the test battery cell.

[0070] [High-temperature curing treatment]

[0071] For the aforementioned test battery cell, it was charged at a constant current of 0.1C at 25°C until the battery voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current reached 0.05C. Afterwards, it was discharged at a constant current of 0.1C until the battery voltage reached 2.5V. Then, it was charged at a constant current of 0.1C until the battery voltage reached 3.7V, and then charged at a constant voltage of 3.7V until the current reached 0.05C. This charged test battery cell was then maintained at 60°C for 9 hours. Afterwards, it was charged at a constant current of 0.1C at 25°C until the battery voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current reached 0.05C. Finally, it was discharged at a constant current of 0.1C until the battery voltage reached 2.5V. This high-temperature curing test battery cell was used as the initial test battery cell.

[0072] [Evaluation of Capacity Maintenance Rate]

[0073] For the initial test battery cells described above, the following high-temperature cycling test was conducted. The discharge capacity of the first cycle and the discharge capacity of the 300th cycle of the high-temperature cycling test were determined, and the capacity retention rate was calculated according to the following formula.

[0074] Capacity retention (%) = (Discharge capacity at 300th cycle ÷ Discharge capacity at 1st cycle) × 100

[0075] High Temperature Cyclic Test

[0076] For the test battery cells, the cells were charged at a constant current of 0.5C at 45°C until the battery voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current reached 0.02C. Afterwards, they were discharged at a constant current of 0.5C until the battery voltage reached 2.5V. This charge-discharge cycle was repeated 300 times. After the high-temperature cycling test, the boron 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. It should be noted that during the high-temperature curing treatment, B migrated from the positive electrode to the negative electrode.

[0077] <Example 2>

[0078] In the second step of preparing the positive electrode active material, LiOH, the aforementioned composite oxide, and Nb₂O₅ were mixed in a molar ratio of Li to Ni, Co, and Mn to Nb of 1.02:1:0.01. Otherwise, test battery cells were prepared and evaluated in the same manner as in Example 1. It should be noted that the ICP-AES analysis results of the positive electrode active material composition in Example 2 showed LiNi 0.833 Co 0.078 Mn 0.069 Nb 0.01 B 0.01 O2. In addition, after the high-temperature cycling test, the content of B in the negative electrode mixture layer was 114 ppm, the molar ratio of Ni to B in the negative electrode mixture layer (Ni / B) was 0.10, and the molar ratio of Nb to B in the negative electrode mixture layer (Nb / B) was 0.23.

[0079] <Comparative Example 1>

[0080] In the fabrication of the positive electrode active material, step 4 is omitted. Otherwise, the test battery cells are fabricated and evaluated in the same manner as in Example 1. It should be noted that the ICP-AES analysis results of the composition of the positive electrode active material in Comparative Example 1 are LiNi. 0.85 Co 0.08 Mn 0.07 O2.

[0081] <Comparative Example 2>

[0082] In the fabrication of the positive electrode active material, step 4 is omitted. Otherwise, the test battery cells are fabricated and evaluated in the same manner as in Example 2. It should be noted that the ICP-AES analysis results of the composition of the positive electrode active material in Comparative Example 2 are LiNi. 0.842 Co 0.079 Mn 0.069 Nb 0.01 O2.

[0083] Table 1 shows the evaluation results of capacity retention. Table 1 also shows the composition of the positive electrode active material, the content of B in the negative electrode compound layer, the molar ratio of Ni to B in the negative electrode compound layer (Ni / B), and the molar ratio of Nb to B in the negative electrode compound layer (Nb / B).

[0084] [Table 1]

[0085]

[0086] As shown in Table 1, the test battery cells of Examples 1 and 2 have higher capacity retention rates after high-temperature cycling compared with the test battery cells of Comparative Examples 1 and 2.

[0087] Explanation of reference numerals in the attached figures

[0088] 10 Non-aqueous electrolyte secondary batteries

[0089] 11 positive electrode

[0090] 12 negative electrode

[0091] 13 dividers

[0092] 14 electrode bodies

[0093] 15 Battery casing

[0094] 16-shell can

[0095] 17 sealing bodies

[0096] 18, 19 Insulation Boards

[0097] 20 positive electrode tabs

[0098] 21 negative electrode tabs

[0099] 22 Groove section

[0100] 23 base plate

[0101] 24 Lower Valve Body

[0102] 25 Insulating Components

[0103] 26 Upper Valve Body

[0104] 27 covers

[0105] 28 gasket

[0106] 30 Positive Current Collector

[0107] 31 Positive Electrode Mixture Layer

[0108] 40 negative current collector

[0109] 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, said positive electrode comprises: 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, said positive electrode active material comprises a lithium transition metal composite oxide containing at least Ni, Mn and B, with respect to a total amount of metal elements other than Li in the lithium transition metal composite oxide, a ratio of Ni is in a range of 80 mol% ≤ Ni ≤ 95 mol%, with respect to the total amount of metal elements other than Li in the lithium transition metal composite oxide, a ratio of Mn is in a range of 0 mol% < Mn ≤ 20 mol%, with respect to the total amount of metal elements other than Li in the lithium transition metal composite oxide, a ratio of B is in a range of 0 mol% < B ≤ 3 mol%, said lithium transition metal composite oxide further contains M1, and M1 is Nb, with respect to the total amount of metal elements other than Li in the lithium transition metal composite oxide, a ratio of M1 is in a range of 0 mol% < M1 ≤ 5 mol%; said lithium transition metal composite oxide is obtained by adding a B compound to a burned product containing Ni, Mn, Li and M1, B is present at least on a surface of the lithium transition metal composite oxide, and B present on the surface of the positive electrode active material forms a coating film, said negative electrode comprises: 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, B is present at least on a surface of the negative electrode mixture layer, and B is contained in an amount of 30 mass ppm or more and 1000 mass ppm or less based on a total mass of the negative electrode mixture layer, M1 is further contained on a surface of the negative electrode mixture layer, and in the negative electrode mixture layer, a molar ratio of M1 to B satisfies 0.05 ≤ M1 / B ≤ 1.0, B and M1 are supplied from the positive electrode to the negative electrode, so that B and M1 are present on the surface of the negative electrode mixture layer.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein Ni is further present on a surface of the negative electrode mixture layer, in the negative electrode mixture layer, a molar ratio of Ni to B satisfies 0.05 ≤ Ni / B ≤ 1.

0.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The lithium-transition metal composite oxide is represented by the general formula Li a Ni x Mn y Co z M2 w M3 v B u O 2-b , wherein 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 Nb, and M3 is at least one element selected from the group consisting of Mg, Ca, Sr, Al and Si.

Images