Lithium ion secondary battery, design method thereof, and electric device

Abstract

The application discloses a lithium ion secondary battery, a design method thereof and an electric device, and belongs to the technical field of new energy sources.The lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a diaphragm; the positive electrode sheet comprises a positive electrode current collector, a positive electrode active material layer and a protective film layer; the protective film layer contains a boron compound; the positive electrode active material layer comprises a lithium cobalt-based ternary material; the mass percentage content of the boron compound in the positive electrode active material layer is b, and 0 < b <= 0.05%; the mass percentage content of the boron compound in the electrolyte is a, and 0.1% <= a <= 12%; the mass concentration of boron elements and the mass concentration of cobalt elements in the positive electrode material layer satisfy the relationship: 750 - lambda <= 32.54ln (epsilon) <= 1400 - lambda, the units of lambda and epsilon are ppm, and the lithium ion secondary battery has good structural stability, kinetic performance, cycle performance, storage performance and safety performance by controlling the mass concentration of boron elements and the mass concentration of cobalt elements in the positive electrode material layer to satisfy the relationship: 750 - lambda <= 32.54ln (epsilon) <= 1400 - lambda, the units of lambdaand epsilon are ppm.

Description

Technical Field

[0001] This invention relates to the field of new energy technology, specifically to a lithium-ion secondary battery, its design method, and an electrical device thereof. Background Technology

[0002] With the development of new energy vehicles, people have placed higher demands on driving range and cost, requiring battery materials to possess both high energy density and low cost. Lithium-cobalt-based ternary materials are currently one of the commonly used cathode active materials, boasting advantages such as high energy density. However, the cobalt element they contain is expensive, and reducing the cobalt content helps control the cost of battery materials. But reducing the cobalt content leads to decreased structural stability of the cathode active material and increased oxygen release, resulting in accelerated capacity decay during charge-discharge cycles and storage, as well as increased gas production at high temperatures. Therefore, it is necessary to address the aforementioned problems caused by reduced cobalt content. Summary of the Invention

[0003] Based on the deficiencies of existing technologies, the purpose of this invention is to provide a lithium-ion secondary battery and its design method, which aims to effectively solve the problem of decreased structural stability of cathode materials caused by reduced cobalt content.

[0004] In a first aspect, the present invention provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator; the positive electrode comprises a positive current collector, a positive active material layer disposed on at least one surface of the positive current collector, and a protective film layer formed on the positive active material layer, the positive active material layer and the protective film layer constituting the positive electrode material layer; the protective film layer contains a boron compound; the positive active material layer comprises a positive active material, and the positive active material comprises a lithium cobalt-based ternary material; the positive active material layer comprises a positive active material; the positive active material layer comprises a positive active material; the positive active material layer comprises a positive active material; the positive active material layer comprises a negative electrode, an electrolyte, and a separator ... negative electrode, an electrolyte, and a separator; the positive active material layer comprises a positive active material; the positive active material layer comprises a negative electrode, an electrolyte, and a separator; the positive active material layer comprises a The positive electrode active material layer contains boron compounds, and the mass percentage of boron compounds in the positive electrode active material layer is b, where 0 < b ≤ 0.05% based on the total mass of the positive electrode active material layer; the electrolyte contains boron compounds, and the mass percentage of boron compounds in the electrolyte is a, where 0.1% ≤ a ≤ 12% based on the total mass of the electrolyte; the mass concentration λ of boron and the mass concentration ε of cobalt in the positive electrode material layer satisfy the following relationship: 750 - λ ≤ 32.54 ln(ε) ≤ 1400 - λ, where the units of λ and ε are ppm.

[0005] Furthermore, the electrolyte contains a boron compound, and the mass percentage of the boron compound in the electrolyte is a, which is 0.1% ≤ a ≤ 12% based on the total mass of the electrolyte.

[0006] Furthermore, the mass percentage of boron compounds in the electrolyte is a, where 0.1% ≤ a ≤ 6%.

[0007] Further, the boron compound in the electrolyte includes at least one of lithium tetrafluoroborate, tris(trimethylsilyl) borate, lithium bis(oxalato) borate, and lithium difluoro(oxalato) borate.

[0008] Further, the boron compound in the electrolyte includes lithium tetrafluoroborate, tris(trimethylsilyl) borate, lithium bis(oxalato) borate, and lithium difluoro(oxalato) borate. Based on the total mass of the electrolyte, the mass percentage content of lithium tetrafluoroborate in the electrolyte is a1, 0.1% ≤ a1 ≤ 2.5%; the mass percentage content of tris(trimethylsilyl) borate in the electrolyte is a2, 0.1% ≤ a2 ≤ 4.3%; the mass percentage content of lithium bis(oxalato) borate in the electrolyte is a3, 0.1% ≤ a3 ≤ 6.2%; the mass percentage content of lithium difluoro(oxalato) borate in the electrolyte is a4, 0.1% ≤ a4 ≤ 9.5%.

[0009] Further, the boron compound contained in the positive electrode active material layer includes at least one of boron oxide and boric acid, and the boron oxide and boric acid coat the positive electrode active material.

[0010] Further, the total mass of the positive electrode material layer is N1, and the mass of the boron compound contained in the electrolyte is N2, where N2 / N1 = 0.0002 - 0.3.

[0011] Further, the lithium cobalt-based ternary material includes Li y , Ni a Co b Mn 1-a-b O 2-y A y , -0.1 ≤ x ≤ 0.2, 0 < a < 1, 0 ≤ b < 1, 0 < a + b < 1, 0 ≤ y < 0.2, and A includes at least one of Mg, Ti, Cr, Zr, Al, V, Rb, Fe, Zn, or Ce.

[0012] Further, the lithium cobalt-based ternary material includes Li 1+x Ni a Co b Mn 1-a-b O 2-y A y , -0.1 ≤ x ≤ 0.2, 0 < a < 1, 0.01 ≤ b < 0.04, 0 < a + b < 1, 0 ≤ y < 0.2, and A includes at least one of Mg, Ti, Cr, Zr, Al, V, Rb, Fe, Zn, or Ce.

[0013] Further, the thickness of the protective film layer is: 5 - 200 nm.

[0014] Secondly, this application provides a design method for a lithium-ion secondary battery, comprising the following steps: (1) preparing a lithium-ion secondary battery, wherein the lithium-ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode, the separator, and the negative electrode are stacked, and the electrolyte is immersed in the positive electrode, the negative electrode, and the separator; the positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; the positive active material layer includes a positive active material, and the positive active material includes a lithium cobalt-based ternary material; the electrolyte contains a boron compound and a... The positive electrode active material layer contains boron compounds. Based on the total mass of the positive electrode active material layer, the mass percentage of boron compounds in the positive electrode active material layer is b, 0 < b ≤ 0.05%; based on the total mass of the electrolyte, the mass percentage of boron compounds in the electrolyte is a, 0.1% ≤ a ≤ 12%; (2) The lithium-ion secondary battery is charged for the first time to form a protective film layer on the positive electrode active material layer. The positive electrode active material layer and the protective film layer constitute the positive electrode material layer; (3) The mass concentration λ of boron and the mass concentration ε of cobalt in the positive electrode material layer are detected. The units of λ and ε are ppm.

[0015] Thirdly, this application provides an electrical device comprising the lithium-ion secondary battery and / or the lithium-ion secondary battery designed by the design method.

[0016] Compared with the prior art, the beneficial effects of this application are as follows:

[0017] (1) This application ensures that the positive electrode active material has good stability during use by controlling the mass concentration λ of boron and the mass concentration ε of cobalt in the positive electrode material layer to satisfy the above relationship, avoiding side reactions between the positive electrode active material and the electrolyte and the dissolution of transition metals in the positive electrode active material, reducing the oxygen release of the positive electrode active material, reducing the gas production, improving the high voltage resistance, first coulombic efficiency and capacity retention during charge-discharge cycles and storage of the positive electrode material, improving the high temperature performance of the battery, reducing the risk of thermal runaway, and enabling the battery to have good kinetic performance, cycle performance, storage performance and safety performance while having a low cost.

[0018] (2) Based on the different Co content of the positive electrode active material, guide the addition of boron content in the electrolyte and positive electrode active material, effectively solve the technical problems caused by the reduction of cobalt content, such as the decrease in structural stability of the positive electrode active material, the increase in oxygen release, the accelerated capacity decay during charge-discharge cycle and storage, and the increase in high-temperature gas production, so as to ensure that the lithium-ion secondary battery has good dynamic performance, cycle performance, storage performance and safety performance while having a low cost. Detailed Implementation

[0019] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments and comparative examples. The purpose of this description is to provide a detailed understanding of the invention, not to limit its scope. All other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this invention. Unless otherwise specified, the experimental reagents and instruments involved in the implementation of this invention are commonly used reagents and instruments.

[0020] According to a first aspect of this application, a lithium-ion secondary battery is provided, including a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode, the separator, and the negative electrode are stacked, and the electrolyte is immersed in the positive electrode, the negative electrode, and the separator.

[0021] The positive electrode includes a positive current collector and a positive electrode material layer;

[0022] The positive electrode material layer includes a positive electrode active material layer and a protective film layer;

[0023] The positive electrode active material layer includes a first surface and a second surface disposed opposite to each other along the stacking direction. The first surface is in contact with the positive electrode current collector. The protective film layer is formed at least on the second surface of the positive electrode active material layer, that is, it can be formed on the second surface or on the side surface adjacent to the second surface.

[0024] The protective film contains boron compounds;

[0025] The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes a lithium-cobalt-based ternary material;

[0026] The positive electrode active material layer contains boron compounds, and based on the total mass of the positive electrode active material layer, the mass percentage of boron compounds in the positive electrode active material layer is b, where 0 < b ≤ 0.05%.

[0027] The electrolyte contains boron compounds, and based on the total mass of the electrolyte, the mass percentage of boron compounds in the electrolyte is a, where 0.1% ≤ a ≤ 12%.

[0028] The mass concentrations of boron (λ) and cobalt (ε) in the cathode material layer satisfy the following relationship: 750-λ≤32.54ln(ε)≤1400-λ, where λ and ε are in ppm.

[0029] In this application, by controlling the mass concentrations λ of boron and ε of cobalt in the cathode material layer to satisfy the above-mentioned relationship, the cathode active material is ensured to have good stability during use, avoiding side reactions between the cathode active material and the electrolyte and the dissolution of transition metals in the cathode active material, reducing the oxygen release of the cathode active material, reducing gas production, improving the high voltage resistance, initial coulombic efficiency, and capacity retention during charge-discharge cycles and storage of the cathode active material, improving the high temperature performance of the battery, reducing the risk of thermal runaway, and enabling the battery to have good kinetic performance, cycle performance, storage performance, and safety performance while having a lower cost.

[0030] The positive electrode material layer can be disposed on only one side of the positive electrode current collector or on both sides of the positive electrode current collector. That is, the positive electrode active material layer and the protective film layer can be disposed on only one side of the positive electrode current collector or on both sides of the positive electrode current collector.

[0031] In one embodiment of this application, both the electrolyte and the positive electrode active material layer contain boron compounds; that is, both the electrolyte and the positive electrode active material layer contain boron compounds. When both the electrolyte and the positive electrode active material layer contain boron compounds, these boron compounds are preferentially oxidized during the first charge-discharge cycle, forming a boron-containing protective film layer with good ionic conductivity on the surface of the positive electrode active material layer away from the positive electrode current collector. The presence of this protective film layer can suppress the oxidative decomposition of the electrolyte and the destruction of the lithium-cobalt-based ternary material structure during subsequent cycles. However, the electronic conductivity of this protective film layer is poor. To ensure good cycle stability of the lithium-ion secondary battery during charge-discharge cycles and storage, and to effectively reduce high-temperature gas generation, the mass concentrations of boron and cobalt in the positive electrode material layer must satisfy the above-mentioned relationship.

[0032] In one embodiment of this application, the electrolyte contains a boron compound, and the mass percentage of the boron compound in the electrolyte is 'a', where 0.1% ≤ a ≤ 12%. Optionally, 'a' is any value or a range of any two values ​​selected from 0.1%, 0.5%, 1.5%, 2.5%, 3%, 3.5%, 4.5%, 5.5%, 6%, 6.5%, 7.5%, 8.5%, 9.5%, 10.5%, and 11.5%. Optionally, 'a' is between 0.1% and 3% or between 0.1% and 6%. As an additive in the electrolyte, the boron compound content cannot be too high, otherwise it will lead to high impedance. To ensure that the electrolyte has low impedance, it is preferable that the mass percentage of the boron compound additive in the electrolyte is in the range of 0.1% to 6%, and more preferably in the range of 0.1% to 3%.

[0033] There are no specific limitations on the choice of boron compound in the electrolyte. In some specific embodiments, the boron compound in the electrolyte includes at least one of lithium tetrafluoroborate (LiBF4), tris(trimethylsilane)borate (TMSB), lithium dioxalate borate (LiBOB), and lithium difluorooxalate borate (LiODFB). However, the choice of boron compound in the electrolyte is not limited to these.

[0034] Optionally, the boron compound in the electrolyte includes lithium tetrafluoroborate, tris(trimethylsilane)borate, lithium dioxalate borate, and lithium difluorooxalate borate, wherein, based on the total mass of the electrolyte, the mass percentage of lithium tetrafluoroborate in the electrolyte is a1, 0.1% ≤ a1 ≤ 2.5%, for example, a1 can be any value or a range of any two of 0.15%, 0.60%, 0.85%, 1.2%, 1.8%, 2.1%, and 2.4%. Optionally, a1 is 0.15%–1.2%, or 1.8%–2.1%; the mass percentage of tris(trimethylsilane)borate in the electrolyte is a2, 0.1% ≤ a2 ≤ 4.3%, for example, a2 can be any value or a range of any two values ​​from 0.5%, 1.0%, 1.5%, 2.2%, 3.0%, 3.5%, and 4.0%. Optionally, a2 is 0.5%–1.5%, or 2.2%–4.0%; the mass percentage of lithium dioxalateborate in the electrolyte is a3, 0.1% ≤ a3 ≤ 6.2%, for example, a3 can be any value or a range of any two values ​​from 0.5%, 1.0%, 1.5%, 2.2%, 3.0%, 3.5%, 4.0%, 5.0%, 5.5%, and 6.0%. Optionally, a3 is 0.5% to 3.0%, or 3.5% to 6.0%; the mass percentage of lithium difluorooxalate borate in the electrolyte is a4, 0.1% ≤ a4 ≤ 9.5%. For example, a4 can be any value or a range of any two values ​​from 0.5%, 1.0%, 1.5%, 2.2%, 3.0%, 3.5%, 4.0%, 5.0%, 5.5%, 6.0%, 7.0%, 8.0%, and 9.1%. Optionally, a4 is 1.0% to 3.5%, or 4.0% to 9.1%.

[0035] Optionally, in some specific embodiments, the total mass of the positive electrode material layer is N1, and the mass of the boron compound contained in the electrolyte is N2, wherein N2 / N1 = 0.0002-0.3. For example, N2 / N1 can be any value or a range of any two values ​​from 0.0003, 0.0008, 0.1, 0.15, 0.2, 0.25, and 0.3. Optionally, N2 / N1 is 0.0003 to 0.1, or 0.15 to 0.25. The boron compounds in the protective film mainly originate from the electrolyte. Based on the total mass of the positive electrode material layer, since the positive electrode active material accounts for more than 90% of the entire positive electrode material layer, the amount of boron compounds added to the electrolyte can be adjusted based on the total mass of the positive electrode material layer, that is, indirectly based on the total mass of cobalt in the positive electrode active material. This can effectively regulate the content of boron compounds in the protective film layer, so that the mass concentration λ of boron and the mass concentration ε of cobalt in the positive electrode material layer better satisfy the following relationship: 750-λ≤32.54ln(ε)≤1400-λ.

[0036] When the electrolyte contains boron compounds, simply mix the boron compounds with the other components of the electrolyte until homogeneous. There are no specific restrictions on the order in which the components are added.

[0037] In one embodiment of this application, the positive electrode active material layer contains a boron compound, and the mass percentage of the boron compound in the positive electrode active material layer is b, where 0 ≤ b ≤ 0.05%. For example, b can be any value or a range of any two values ​​selected from 0.0001%, 0.001%, 0.005%, 0.01%, 0.015%, 0.02%, 0.025%, 0.028%, 0.03%, 0.035%, 0.038%, 0.04%, and 0.045%. Optionally, b can be 0.01% to 0.04%, or 0.02% to 0.03%. By adding a boron compound of a certain mass percentage to the positive electrode active material layer, when the electrolyte is injected into the battery, the electrolyte wets the positive electrode active material layer. During the first charge and discharge of the battery, the boron-containing compound forms a boron-containing protective film layer on the positive electrode active material layer. This protective film layer, with a suitable thickness and size, can effectively protect the positive electrode active material layer and improve its structural stability. At the same time, the boron content in the formed protective film layer will not be excessive, avoiding high impedance that would affect the battery's dynamic performance.

[0038] There are no specific limitations on the choice of boron compound in the positive electrode active material layer. In some specific embodiments, the boron compound in the positive electrode active material layer includes at least one of boron oxide and boric acid, which are coated on the surface of the positive electrode active material. However, the choice of boron compound in the positive electrode active material layer is not limited to this.

[0039] When the positive electrode active material layer contains a boron compound, as an example of the preparation method of the positive electrode sheet, the boron compound and the positive electrode active material are dispersed in a solvent and mixed evenly to obtain a positive electrode slurry. The obtained positive electrode slurry is coated on the positive electrode current collector, and after processes such as drying and cold pressing, a positive electrode sheet is obtained, wherein the boron compound is uniformly dispersed in the positive electrode active material layer. For example, the prepared positive electrode sheet is divided into multiple parts of the same size, and the mass concentration content of boron element in each part of the positive electrode sheet is obtained, and the difference in the mass concentration content of boron element between any two parts of the positive electrode sheet is within 0 - 1.5 ppm.

[0040] Further, based on the design of the content of the boron compound added in the electrolyte and the content of the boron compound in the positive electrode active material layer, the thickness of the protective film layer is: 5 - 200 nm. For example, the thickness of the protective film layer can also be: one of 10 nm, 30 nm, 60 nm, 90 nm, 120 nm, 150 nm, 180 nm or the range formed between any two of them. Designing the thickness of the protective film layer within the range of 5 - 200 nm can well regulate the stability of the positive electrode active material, and at the same time avoid too high thickness, resulting in large impedance and affecting the conductivity of the battery.

[0041] Exemplarily, the lithium cobalt-based ternary material includes Li 1+x Ni a Co b Mn 1-a-b O 2-y A y , -0.1 ≤ x ≤ 0.2, 0 < a < 1, 0 ≤ b < 1, 0 < a + b < 1, 0 ≤ y < 0.2, and A includes at least one of Mg, Ti, Cr, Zr, Al, V, Rb, Fe, Zn and Ce.

[0042] Optionally, x can also be optionally, x can also be any value in -0.08, -0.04, -0.008, -0.004, -0.05, 0, 0.004, 0.008, 0.04, 0.08, 0.12, 0.2, 0.3, 0.4, 0.45, 0.5 or the range composed of any two of these values. For example, x can be -0.05 to 0.1 or 0 to 0.5.

[0043] Optionally, a can also be any value in 0.1, 0.15, 0.25, 0.3, 0.35, 0.45, 0.5, 0.55, 0.65, 0.7, 0.75, 0.85, 0.9, 0.95 or the range composed of any two of these values. For example, a can be 0.25 to 0.85, or 0.45 to 0.95.

[0044] Optionally, b can also be any value from 0.01, 0.05, 0.1, 0.15, 0.3, 0.4 or a range of any two values ​​therein. For example, b can be 0.01-0.1 or 0.15-0.3.

[0045] Optionally, y can also be any value or a range of any two values ​​from 0, 0.11, 0.1, 0.12, 0.13, 0.15, 0.16, 0.17, 0.18, and 0.19. y can also be 0.10 to 0.18, or 0.12 to 0.16.

[0046] As an example, x = 0, y = 0. Reducing the value of b can lower material costs; preferably, b is below 0.2, or below 0.1, or below 0.05. Increasing the value of a can increase the Ni content in the material, resulting in higher specific capacity and better rate performance; preferably, a is above 0.6, or above 0.8, or above 0.9.

[0047] In some specific embodiments, the mass fraction of the positive electrode active material in the positive electrode active material layer is 75% to 99%. Optionally, the mass fraction of the positive electrode active material in the positive electrode active material layer can also be 78% to 96%, or 82% to 92%, or 86% to 88%. Optionally, the mass fraction of the positive electrode active material in the positive electrode active material layer can also be one of 80%, 85%, 90%, and 95%.

[0048] In one embodiment of this application, the positive electrode active material layer further comprises at least one of a conductive agent, a binder, etc. There are no specific limitations on the selection of the conductive agent and the binder. As an example, the conductive agent includes at least one of conductive carbon black and carbon nanotubes; the binder includes at least one of polyvinylidene fluoride (PVDF), styrene-butadiene latex (SBR), sodium carboxymethyl cellulose (CMC-Na), and sodium alginate (SA). However, the selection of the conductive agent and the binder in the positive electrode active material layer is not limited to these. In some specific embodiments, the mass fraction of the conductive agent in the positive electrode active material layer is 0.1% to 3%. Optionally, the mass fraction of the conductive agent in the positive electrode active material layer may also be 0.5% to 2.5%, or 1% to 2%. Optionally, the mass fraction of the conductive agent in the positive electrode active material layer may also be one of 0.3%, 0.8%, 1.2%, 1.5%, 2.2%, and 2.8%. In some specific embodiments, the mass fraction of the binder in the positive electrode active material layer is 1% to 9%. Optionally, the mass fraction of the binder in the positive electrode active material layer can be 3% to 7%, or 4% to 5%. Optionally, the mass fraction of the binder in the positive electrode active material layer can be one of 2%, 3.5%, 5.5%, 7.5%, or 8.5%.

[0049] This application does not impose specific limitations on the selection of the positive electrode current collector. As an example, the positive electrode current collector may be selected from metal foil or alloy foil, wherein the metal foil includes aluminum foil, but the selection of the positive electrode current collector is not limited to this.

[0050] This application does not impose specific limitations on the selection of the negative electrode sheet. As an example, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one side of the negative current collector. The negative active material layer includes a negative active material. The negative current collector is selected from metal foil or alloy foil, and the metal foil includes copper. The negative active material is selected from natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, elemental silicon, silicon-carbon composites, SiO, Li-Sn alloys, Li-Sn-O alloys, elemental Sn, SnO, SnO2, and spinel-structured lithium titanate Li4Ti5O. 12 At least one of Li-Al alloys and metallic lithium. However, the choice of negative electrode current collector and negative electrode active material is not limited to these.

[0051] In some specific embodiments, the mass fraction of the negative electrode active material in the negative electrode active material layer is 75% to 99%. Optionally, the mass fraction of the negative electrode active material in the negative electrode active material layer may also be one of 78% to 96%, 82% to 92%, or 86% to 88%. Optionally, the mass fraction of the negative electrode active material in the negative electrode active material layer may also be one of 80%, 85%, 90%, or 95%.

[0052] In one embodiment of this application, the negative electrode active material layer further comprises at least one of a conductive agent, a binder, etc. There are no specific limitations on the selection of the conductive agent and the binder. As an example, the conductive agent includes at least one of conductive carbon black and carbon nanotubes; the binder includes at least one of polyvinylidene fluoride (PVDF), styrene-butadiene latex (SBR), sodium carboxymethyl cellulose (CMC-Na), and sodium alginate (SA). However, the selection of the conductive agent and the binder in the negative electrode active material layer is not limited to these. In some specific embodiments, the mass fraction of the conductive agent in the negative electrode active material layer is 0.1% to 3%. Optionally, the mass fraction of the conductive agent in the negative electrode active material layer may also be 0.5% to 2.5%, or 1% to 2%. Optionally, the mass fraction of the conductive agent in the negative electrode active material layer may also be one of 0.3%, 0.8%, 1.2%, 1.5%, 2.2%, and 2.8%. In some specific embodiments, the mass fraction of the binder in the negative electrode active material layer is 1% to 9%. Optionally, the mass fraction of the binder in the negative electrode active material layer can be 3% to 7%, or 4% to 5%. Optionally, the mass fraction of the binder in the negative electrode active material layer can be one of 1.5%, 2%, 3.5%, 5.5%, 7.5%, or 8.5%.

[0053] The electrolyte contains lithium salts and solvents, among which there are no specific restrictions on the selection of lithium salts and solvents. As an example, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium dioxalate borate, lithium difluorooxalate borate, lithium difluorodioxalate phosphate, lithium perchlorate, lithium difluorophosphate, and lithium pentafluoroethyl trifluoroborate; the solvent is selected from at least one of fluoroethylene carbonate, ethylene carbonate, propylene carbonate, butene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, diethyl carbonate, methyl propyl carbonate, diphenyl carbonate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, γ-butyrolactone, acetonitrile, and sulfolane. However, the selection of lithium salts and solvents in the electrolyte is not limited to these.

[0054] The electrolyte may or may not contain boron-free additives. As an example, the boron-free additive may be selected from at least one of the following: vinylene carbonate (VC), ethylene ethylene carbonate (VEC), ethylene sulfate (DTD), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), succinic anionyl (SN), adiponitrile (ADN), glutaronitrile (GLN), hexanetrionitrile (HTN), sulfonate cyclic quaternary ammonium salts, tris(trimethylsilane) phosphate (TMSP), and tris(trimethylsilane) borate (TMSB), to further improve the battery's safety, cycle performance, or other properties.

[0055] This application does not impose specific limitations on the selection of the separator; it can be any type of separator commonly used in the field of lithium-ion batteries. As an example, the separator is selected from at least one of polypropylene separators, polyethylene separators, etc., but is not limited to these.

[0056] According to a second aspect of this application, a method for designing a lithium-ion secondary battery is provided, comprising the following steps:

[0057] (1) A lithium-ion secondary battery is prepared, wherein the lithium-ion secondary battery includes a positive electrode, a negative electrode, an electrolyte and a separator, wherein the positive electrode, the separator and the negative electrode are stacked, and the electrolyte is immersed in the positive electrode, the negative electrode and the separator;

[0058] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector;

[0059] The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes a lithium-cobalt-based ternary material;

[0060] The electrolyte contains boron compounds and the positive electrode active material layer contains boron compounds. Based on the total mass of the positive electrode active material layer, the mass percentage of boron compounds in the positive electrode active material layer is b, where 0 < b ≤ 0.05%.

[0061] Based on the total mass of the electrolyte, the mass percentage of boron compounds in the electrolyte is a, where 0.1% ≤ a ≤ 12%.

[0062] (2) The lithium-ion secondary battery is charged for the first time to form a protective film layer on the positive electrode active material layer, wherein the positive electrode active material layer and the protective film layer constitute the positive electrode material layer;

[0063] (3) Detect the mass concentration λ of boron and the mass concentration ε of cobalt in the positive electrode material layer, where λ and ε are in ppm;

[0064] (4) Determine whether λ and ε satisfy 750-λ≤32.54ln(ε)≤1400-λ. If they satisfy, the target lithium-ion secondary battery is obtained. If they do not satisfy, adjust the mass concentration of boron compound in the electrolyte and the mass concentration of boron compound in the positive electrode active material layer, and repeat steps (1) to (4) until they satisfy.

[0065] The above design method guides the addition of boron in the electrolyte and positive electrode active material based on the different Co content of the positive electrode material layer, avoiding excessive or insufficient boron. It effectively solves the technical problems caused by reduced cobalt content, such as decreased structural stability of the positive electrode material, increased oxygen release, accelerated capacity decay during charge-discharge cycles, and increased gas production at high temperatures. This ensures that lithium-ion secondary batteries have good kinetic performance, cycle performance, storage performance, and safety performance while having a low cost.

[0066] As an example, the specific steps for detecting the mass concentrations λ of boron and ε of cobalt in the positive electrode material layer include: disassembling the lithium-ion secondary battery to obtain the positive electrode sheet, cleaning it with a cleaning agent to remove the electrolyte from the surface of the positive electrode sheet, drying it, weighing it, dissolving it with acid to obtain the dissolved solution, measuring the mass concentrations of boron and cobalt in the dissolved solution, and calculating the mass concentrations λ of boron and ε of cobalt in the positive electrode material layer. The cleaning agent is selected from at least one of dimethyl carbonate (DMC), and the acid used for dissolving is selected from at least one of 65%–68% nitric acid, 36%–38% hydrochloric acid, etc. However, the selection of the cleaning agent and the acid used is not limited to these.

[0067] As an example, when detecting the mass concentration λ of boron and the mass concentration ε of cobalt in the cathode material layer, inductively coupled plasma spectrometry or X-ray energy dispersive spectroscopy is used for measurement.

[0068] According to a third aspect of this application, an electrical device is provided, comprising the aforementioned lithium-ion secondary battery or a lithium-ion secondary battery designed using the aforementioned design method. The type of electrical device is not specifically limited, and may include, for example, an electric vehicle, a computer, a drone, a lighting appliance, a toy, etc., but is not limited thereto.

[0069] The present application will be further illustrated below through specific embodiments.

[0070] Examples and Comparative Examples

[0071] Each embodiment and comparative example provides a lithium-ion secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode, separator, and negative electrode are stacked, and the electrolyte is immersed in the positive electrode, negative electrode, and separator.

[0072] Protective film layer;

[0073] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector;

[0074] The positive electrode active material layer includes the positive electrode active material.

[0075] The protective films in each embodiment and Comparative Examples 2-5 contain boron compounds, while the protective films in the comparative examples do not contain boron compounds. Table 1 shows the positive electrode active material, the additives in the electrolyte and their mass fractions, the boron compounds and their mass fractions in the positive electrode active material layer, and the mass concentrations λ and ε of boron and cobalt in the positive electrode active material layer in each embodiment and comparative example.

[0076] The preparation methods of the lithium-ion secondary batteries in each embodiment and comparative example are as follows:

[0077] Preparation of positive electrode sheet: The positive active material, conductive carbon black SP, binder PVDF and boron compound are added to the solvent NMP and mixed evenly to obtain a positive slurry. The obtained positive slurry is evenly coated on both sides of the positive current collector aluminum foil. After drying, cold pressing, slitting and cutting, the positive electrode sheet is obtained. The mass ratio of positive active material, conductive carbon black SP and binder PVDF is 94:3:3.

[0078] Preparation of negative electrode sheet: The negative electrode active material graphite and the conductive agent acetylene black (SuperP) are mixed evenly in a mixing tank, and then the binder SBR and deionized water are added and stirred evenly to obtain a black slurry. The obtained black slurry is coated on both sides of copper foil, and after baking, rolling and cutting, the negative electrode sheet is obtained. The mass ratio of negative electrode active material graphite, conductive agent acetylene black and binder SBR is 96:1:3.

[0079] Preparation of electrolyte: Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed evenly in a mass ratio of 30:30:40 to obtain a mixed solvent. Lithium salt LiPF6, or lithium salt LiPF6 and additives, are then dissolved in the obtained mixed solvent and mixed evenly to obtain the electrolyte. The concentration of LiPF6 in the electrolyte is 1 mol / L, and the mass percentage of LiODFB is 0.3%.

[0080] Preparation of lithium-ion secondary batteries: The positive electrode, negative electrode and separator obtained above are stacked in the order of positive electrode, separator and negative electrode, wound, hot-pressed and shaped, and the tabs are welded to obtain bare cells. They are then encapsulated with aluminum-plastic film. After that, the cells are placed in an oven at 85±10℃ and baked for 24h±12h to ensure that the water content of the electrode is qualified. Then, electrolyte is injected. After depressurization encapsulation, standing, formation (i.e., first charge and discharge), shaping and other processes, lithium-ion secondary batteries are made.

[0081] The method for determining the mass concentration of boron and cobalt in the cathode material layer is as follows:

[0082] 1) Disassemble the lithium-ion secondary battery to obtain the positive electrode sheet, clean it with DMC solvent to remove the electrolyte on the surface, dry it at room temperature, and then cut the positive electrode sheet into 18mm diameter round pieces and weigh it, or the weight of the positive electrode sheet M1.

[0083] 2) Use NMP or alcohol or other solvents to wipe the positive electrode material layer off the positive electrode sheet, then cut 10 current collector discs with a diameter of 18mm, weigh them and take the average to obtain the mass M2 of the positive electrode current collector, and further obtain the mass of the positive electrode material layer as M3 = M1 - M2.

[0084] 3) Add 37% nitric acid for acid hydrolysis, filter, dilute the resulting solution and quantitatively test the content of Co and B elements by ICP (inductively coupled plasma optical emission spectrometer), and further calculate the mass concentration λ of boron and the mass concentration ε of cobalt in the cathode material layer by combining with M3.

[0085] Table 1

[0086]

[0087]

[0088] The performance of the lithium-ion secondary batteries in each embodiment and comparative example was tested, and the specific methods are as follows:

[0089] Room temperature cycling: The battery was subjected to charge-discharge cycle tests at 25℃ with a charge / discharge rate of 1C / 1C within the range of 2.8 to 4.3V. The initial discharge capacity and the discharge capacity after each cycle were recorded. After 1000 cycles, the capacity retention rate was calculated using the following formula.

[0090] Capacity retention rate after n cycles = (Discharge capacity in the nth cycle / Initial discharge capacity) * 100%

[0091] The capacity retention rate after n cycles is recorded in the following format: x%@ncls, which means the capacity retention rate after n cycles is x%.

[0092] High-temperature cycling: The battery was placed in a 45℃ chamber for 120 minutes, and then charged and discharged in a 45℃ constant temperature chamber at a charge / discharge rate of 1C / 1C within the range of 2.8 to 4.3V. The initial discharge capacity and the discharge capacity after each cycle were recorded. After 800 cycles, the capacity retention rate was calculated and recorded. The formula for calculating the capacity retention rate and the format for recording the results are the same as above.

[0093] High-temperature storage: Fully charge the battery to obtain a fresh battery, and store it in a 60℃ constant temperature chamber. Take it out every 10 days, fully charge it again, and continue storing it in the 60℃ constant temperature chamber. After 90 or 120 days, take it out and test its recoverable capacity. The recoverable capacity test procedure is as follows:

[0094] 1. Discharge at a constant current of 1C to 2.8V, then let stand for 10 minutes;

[0095] 2. Charge the battery with 1C CC-CV to 4.3V, with a cutoff current of 0.05C, and let it stand for 10 minutes.

[0096] 3. Discharge at a constant current of 1C to 2.8V, and the discharged capacity is recorded as the recoverable capacity.

[0097] Calculate the storage capacity retention rate using the following formula.

[0098] The capacity retention rate after m days of storage = (Recoverable capacity after m days of storage at 60℃) / (Recoverable capacity of a fresh battery) * 100%, where the test method for the recoverable capacity of a fresh battery is the same as the test method for the recoverable capacity after storage.

[0099] The storage capacity retention rate after m days is recorded in the following format: y%@mD, which means the storage capacity retention rate after m days is y%.

[0100] Volume expansion experiment: Charge the battery to 4.3V at 1C and test the volume by water displacement method. Record the initial volume and the volume after 7 days of storage at 85℃. Volume expansion rate = (volume after 7 days of storage at 85℃ - initial volume) / initial volume * 100%.

[0101] The test results are shown in Table 2.

[0102] Table 2

[0103]

[0104]

[0105] As shown in Table 1, compared with Comparative Examples 2-3, Examples 1-3 all controlled the content of B2O3 in the positive electrode active material layer to be 0.01%, and at the same time set different mass percentages of boron compounds in the electrolyte so that the mass concentrations λ of boron and ε of cobalt in the positive electrode material layer satisfy 750-λ≤32.54ln(ε)≤1400-λ, which significantly improved the room temperature cycle stability, high temperature cycle stability and high temperature storage stability of the battery, and significantly reduced the high temperature gas production.

[0106] Compared to Comparative Examples 4-5, Examples 7-10, by controlling the mass concentration of boron in the positive electrode active material layer and simultaneously setting different mass percentages of boron compounds in the electrolyte, ensured that the mass concentrations λ of boron and ε of cobalt in the positive electrode material layer satisfied 750-λ≤32.54ln(ε)≤1400-λ. This also significantly improved the room temperature cycle stability, high temperature cycle stability, and high temperature storage stability of the battery, while significantly reducing the high temperature gas production.

[0107] Compared with Example 1, Examples 11-13 adjusted the type of positive electrode active material, with different nickel and cobalt contents. At the same time, boron compounds with different mass percentages were added to the electrolyte. This also shows that by controlling the mass concentrations λ of boron and ε of cobalt in the positive electrode material layer to satisfy 750-λ≤32.54ln(ε)≤1400-λ, the room temperature cycle stability, high temperature cycle stability, and high temperature storage stability of the battery are significantly improved, while the high temperature gas production is significantly reduced.

[0108] Based on the data from Comparative Example 1, the addition of an appropriate amount of boron-containing compound to the electrolyte and an excessive amount of boron-containing compound to the positive electrode active material layer resulted in poor room temperature cycle stability, relatively poor high temperature cycle stability, and relatively poor high temperature storage stability of the battery, while significantly increasing high temperature gas production. In Comparative Examples 2-3, boron-containing compounds were added only to the electrolyte; and in Comparative Examples 4-5, boron-containing compounds were added only to the positive electrode active material. These results in poor room temperature cycle stability, relatively poor high temperature cycle stability, and relatively poor high temperature storage stability of the battery, while significantly increasing high temperature gas production.

[0109] The above explanation shows that by designing the cobalt content of the positive electrode and the boron content of the electrolyte and the positive electrode, the mass concentration λ of boron and the mass concentration ε of cobalt in the positive electrode material layer can satisfy the above formula, so that the lithium-ion secondary battery can simultaneously achieve good kinetic performance, cycle performance, storage performance and long cycle life.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, Includes positive electrode, negative electrode, electrolyte, and separator; The positive electrode sheet includes a positive current collector, a positive active material layer disposed on at least one surface of the positive current collector, and a protective film layer formed on the positive active material layer, wherein the positive active material layer and the protective film layer constitute the positive material layer; The protective film contains boron compounds; The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes a lithium-cobalt-based ternary material; The positive electrode active material layer contains a boron compound, which includes boric acid. Based on the total mass of the positive electrode active material layer, the mass percentage of the boron compound in the positive electrode active material layer is b, where 0 < b ≤ 0.05%. The electrolyte contains boron compounds, and based on the total mass of the electrolyte, the mass percentage of boron compounds in the electrolyte is a, where 0.1% ≤ a ≤ 12%. The mass concentrations λ of boron and ε of cobalt in the cathode material layer satisfy the following condition: 750-λ≤32.54ln(ε)≤1400-λ, where λ and ε are in ppm.

2. The lithium-ion secondary battery as described in claim 1, characterized in that, The mass percentage of boron compounds in the electrolyte is a, where 0.1% ≤ a ≤ 6%.

3. The lithium-ion secondary battery as described in claim 1, characterized in that, The boron compound in the electrolyte includes at least one of lithium tetrafluoroborate, tris(trimethylsilane)borate, lithium dioxalate borate, and lithium difluorooxalate borate.

4. The lithium-ion secondary battery as described in claim 3, characterized in that, The boron compounds in the electrolyte include lithium tetrafluoroborate, tris(trimethylsilane)borate, lithium dioxalate borate, and lithium difluorooxalate borate. Based on the total mass of the electrolyte, the mass percentage of lithium tetrafluoroborate in the electrolyte is a1, 0.1% ≤ a1 ≤ 2.5%; the mass percentage of tris(trimethylsilane)borate in the electrolyte is a2, 0.1% ≤ a2 ≤ 4.3%; the mass percentage of lithium dioxalate borate in the electrolyte is a3, 0.1% ≤ a3 ≤ 6.2%; and the mass percentage of lithium difluorooxalate borate in the electrolyte is a4, 0.1% ≤ a4 ≤ 9.5%.

5. The lithium-ion secondary battery as described in claim 1, characterized in that, The boric acid is coated on the surface of the positive electrode active material.

6. The lithium-ion secondary battery as described in claim 1, characterized in that, The total mass of the positive electrode material layer is N1, and the mass of the boron compound contained in the electrolyte is N2, wherein N2 / N1 = 0.0002-0.

3.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, characterized in that, The lithium cobalt-based ternary material includes Li 1+x Ni a Co b Mn 1-a-b O 2-y A y , -0.1 ≤ x ≤ 0.2, 0 < a < 1, 0 ≤ b < 1, 0 < a + b < 1, 0 ≤ y < 0.2, and A includes at least one of Mg, Ti, Cr, Zr, Al, V, Rb, Fe, Zn, or Ce.

8. The lithium-ion secondary battery as described in claim 7, characterized in that, The lithium cobalt-based ternary material includes Li 1+ x Ni a Co b Mn 1-a-b O 2-y A y , -0.1 ≤ x ≤ 0.2, 0 < a < 1, 0.01 ≤ b < 0.04, 0 < a + b < 1, 0 ≤ y < 0.2, and A includes at least one of Mg, Ti, Cr, Zr, Al, V, Rb, Fe, Zn, or Ce.

9. The lithium-ion secondary battery as described in claim 8, characterized in that, The thickness of the protective film is 5-200 nm.

10. A design method for a lithium-ion secondary battery, characterized in that, Includes the following steps: (1) A lithium-ion secondary battery is prepared, wherein the lithium-ion secondary battery includes a positive electrode, a negative electrode, an electrolyte and a separator, wherein the positive electrode, the separator and the negative electrode are stacked, and the electrolyte is immersed in the positive electrode, the negative electrode and the separator; The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes a lithium-cobalt-based ternary material; The electrolyte contains boron compounds and the positive electrode active material layer contains boron compounds. Based on the total mass of the positive electrode active material layer, the mass percentage of boron compounds in the positive electrode active material layer is b, where 0 < b ≤ 0.05%; based on the total mass of the electrolyte, the mass percentage of boron compounds in the electrolyte is a, where 0.1% ≤ a ≤ 12%; the boron compounds in the positive electrode active material layer include boric acid. (2) The lithium-ion secondary battery is charged for the first time to form a protective film layer on the positive electrode active material layer, wherein the positive electrode active material layer and the protective film layer constitute the positive electrode material layer; (3) Detect the mass concentration λ of boron and the mass concentration ε of cobalt in the positive electrode material layer, where λ and ε are in ppm; (4) Determine whether λ and ε satisfy 750-λ≤ 32.54ln(ε) ≤1400-λ. If they satisfy, the target lithium-ion secondary battery is obtained. If they do not satisfy, adjust the mass concentration of boron compound in the electrolyte and the mass concentration of boron compound in the positive electrode active material layer, and repeat steps (1) to (4) until they satisfy.

11. An electrical appliance, characterized in that, The lithium-ion secondary battery comprises any one of the lithium-ion secondary batteries as described in any one of claims 1-9 or designed by the design method as described in claim 10.

Images