Secondary battery and electronic device

Abstract

A secondary battery, the secondary battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet comprises a positive current collector, a base coating layer, and a positive electrode active material layer, the base coating layer being arranged between the positive current collector and the positive electrode active material layer. The base coating layer comprises an inorganic metal oxide, the element M in the inorganic metal oxide comprises at least one of Al, Ti, Sn, Sb, or Mg, and the inorganic metal oxide has a mass percentage content W1 of 55% to 99%. A first compound in the electrolyte comprises at least one of fluoroethylene carbonate, difluoroethylene carbonate, formula (I), formula (II), or formula (III), and the mass percentage content of the first compound is a%, 0.15 ≤ a ≤ 21. By regulating and controlling the type of element included in the inorganic metal oxide, W1, and the type and content of the first compound in the electrolyte to be within the ranges above, the safety performance of the secondary battery can be improved while the cyclic stability of the secondary battery is ensured.

Description

Secondary battery and electronic device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on October 25, 2023, with application number 202311394244.8 and invention name “A Secondary Battery and Electronic Device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of electrochemical technology, and in particular to a secondary battery and an electronic device. Background Art

[0003] Secondary batteries (such as lithium-ion batteries) are widely used due to their high energy density and long cycle life. Currently, the additives in the electrolyte of lithium-ion batteries that have better improvement in cycle performance include carbonate type, sulfate type, acid anhydride type, phosphate type, sulfonate type, borate type and silicate type.

[0004] Among them, when the additives are of carbonate type, sulfate type or anhydride type, some of the above additives are easy to react with trace water in the electrolyte to generate acidic substances, especially in the high temperature process (for example, the temperature is greater than or equal to 45°C). The acidic substances generated are easy to corrode the aluminum foil, resulting in reduced toughness of the aluminum foil. At the same time, the acidic substances will corrode the surface layer of the positive electrode active material, causing the positive electrode active material particles to undergo phase change to produce greater volume expansion, and cause serious deformation of the aluminum foil. The reduced toughness or deformation of the aluminum foil will lead to reduced safety performance of the lithium-ion battery.

[0005] Summary of the Invention

[0006] The purpose of this application is to provide a secondary battery and an electronic device that improves the safety performance of the secondary battery while ensuring the cycle stability of the secondary battery. The specific technical solution is as follows:

[0007] The first aspect of the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet comprises a positive current collector, an undercoat layer disposed on at least one surface of the positive current collector, and a positive electrode active material layer, wherein the undercoat layer is disposed between the positive current collector and the positive electrode active material layer. The undercoat layer comprises an inorganic metal oxide, wherein the inorganic metal oxide comprises an element M, wherein the element M comprises at least one of Al, Ti, Sn, Sb, or Mg, and preferably, the element M comprises at least one of Ti, Sn, or Sb. The mass percentage content of the inorganic metal oxide, based on the mass of the undercoat layer, is W1, 55%≤W1≤99%, and preferably, 60%≤W1≤85%.

[0008] The electrolyte includes a first compound, wherein the first compound includes at least one of fluoroethylene carbonate, difluoroethylene carbonate, a compound represented by formula (I), a compound represented by formula (II), or a compound represented by formula (III).

[0009] wherein n and m are independently selected from integers of 1-3, and R0, Rn and Rm are each independently selected from hydrogen, fluorine, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl; when substituted, the substituent of each group is halogen;

[0010] Based on the mass of the electrolyte, the mass percentage of the first compound is a%, 0.15≤a≤21, preferably, 0.5≤a≤12. By regulating the content of the inorganic metal oxide in the bottom coating of the positive electrode sheet, the mass percentage of the inorganic metal oxide, the types of elements included in the inorganic metal oxide, and the type and mass percentage of the first compound in the electrolyte within the scope of this application, the safety performance of the secondary battery can be improved while ensuring the cycling stability of the secondary battery.

[0011] In one embodiment of the present application, the thickness of the primer layer is A μm, 2.1 ≤ A ≤ 15.5, preferably, 2.5 ≤ A ≤ 9.5. By adjusting the value of A within the scope of the present application, the primer layer can have a suitable thickness, effectively inhibiting the corrosion of the positive electrode current collector by acidic substances, and maintaining the energy density of the secondary battery while improving the safety performance of the secondary battery.

[0012] In one embodiment of the present application, 0.02 ≤ a / A ≤ 5.1. By adjusting the value of a / A within the range of the present application, the mass percentage of the first compound can be matched with the thickness of the primer layer, effectively inhibiting the corrosion of the positive electrode current collector by acidic substances, and improving the safety performance of the secondary battery while ensuring the cycle stability of the secondary battery.

[0013] In one embodiment of the present application, the compound represented by formula (I) includes at least one of succinic anhydride, glutaric anhydride or adipic anhydride, the compound represented by formula (II) includes at least one of maleic anhydride, citraconic anhydride, trifluoromethylmaleic anhydride, dimethylmaleic anhydride, 3-fluorofuran-2,5-dione or 3,4-difluoromaleic anhydride, and the compound represented by formula (III) includes at least one of vinyl sulfate, propylene sulfate, butylene sulfate, 1,3,2-dioxathiane-2,2-dioxide or 2-methyl-1,3-propanedisulfite. By selecting the compound represented by formula (I), the compound represented by formula (II) and the compound represented by formula (III), the safety performance of the secondary battery can be improved while ensuring the cycle stability of the secondary battery.

[0014] In one embodiment of the present application, the inorganic metal oxide includes at least one of Al2O3, TiO2, SnO2, SnO, Sb2O3, Sb2O4, Sb2O5, or MgO. By selecting the above inorganic metal oxides, the inorganic metal oxides can react with acidic substances, reducing the content of the acidic substances and reducing the corrosion of the acidic substances on the positive electrode current collector and the positive electrode active material, thereby ensuring the cycle stability of the secondary battery while improving the safety performance of the secondary battery.

[0015] In one embodiment of the present application, the primer layer further includes a binder and a conductive agent. Based on the mass of the primer layer, the binder has a mass percentage of W2, and the conductive agent has a mass percentage of W3, 0.5%≤W2≤25%, and 0.5%≤W3≤20%. By regulating the values ​​of W2 and W3 within the scope of this application, the binder mass percentage and the conductive agent mass percentage can be adjusted to be within an appropriate range, thereby providing the inorganic metal oxide with an appropriate mass percentage, which is beneficial for effectively protecting the positive electrode current collector; and the inorganic metal oxide can be better bonded to the positive electrode current collector, resulting in the positive electrode sheet having a higher electron conductivity, thereby ensuring the cycle stability of the secondary battery while improving the safety performance of the secondary battery.

[0016] In one embodiment of the present application, the binder satisfies at least one of the following characteristics:

[0017] (1) The binder comprises a polymer formed by at least one monomer selected from the group consisting of acrylic acid, acrylamide, lithium acrylate, sodium acrylate, acrylonitrile, methyl acrylate, ethyl acrylate, methyl 2-methacrylate, and ethyl 2-methacrylate;

[0018] (2) The binder comprises at least one of sodium carboxymethyl cellulose, potassium carboxymethyl cellulose or nitrile rubber;

[0019] (3) The weight average molecular weight of the binder is 150,000 to 1.95 million.

[0020] In one embodiment of the present application, the conductive agent includes at least one of graphene, graphite fiber, carbon nanotubes, or conductive carbon black. By selecting the above conductive agent, the positive electrode plate can have a higher electron conductivity, which is conducive to improving the cycle performance of the secondary battery.

[0021] In one embodiment of the present application, the negative electrode plate includes a negative electrode active material, wherein the negative electrode active material includes at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, or a silicon-based composite material. The negative electrode plate including the above-mentioned negative electrode active material provides a secondary battery with high cycle stability and good safety performance.

[0022] The second aspect of the present application provides an electronic device, which includes the secondary battery according to any one of the aforementioned embodiments. Therefore, the electronic device provided by the present application has good performance.

[0023] Beneficial effects of this application:

[0024] The present application provides a secondary battery and an electronic device, the secondary battery comprising a positive electrode plate, a negative electrode plate and an electrolyte, the positive electrode plate comprising a positive electrode collector and an undercoat layer and a positive electrode active material layer arranged on at least one surface of the positive electrode collector, the undercoat layer being arranged between the positive electrode collector and the positive electrode active material layer; the undercoat layer comprising an inorganic metal oxide, the inorganic metal oxide comprising an element M, the element M comprising at least one of Al, Ti, Sn, Sb or Mg, and the mass percentage of the inorganic metal oxide based on the mass of the undercoat layer is W1, 55%≤W1≤99%; the electrolyte comprising a first compound, the first compound comprising at least one of fluoroethylene carbonate, difluoroethylene carbonate, a compound represented by formula (I), a compound represented by formula (II) or a compound represented by formula (III), and the mass percentage of the first compound based on the mass of the electrolyte is a%, 0.15≤a≤21. By regulating the bottom coating of the positive electrode plate including inorganic metal oxide, the mass percentage of the inorganic metal oxide, the types of elements included in the inorganic metal oxide, and the type and mass percentage of the first compound in the electrolyte within the scope of this application, the safety performance of the secondary battery can be improved while ensuring the cycle stability of the secondary battery.

[0025] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0027] FIG1 is a schematic diagram of a positive electrode sheet according to an embodiment of the present application. DETAILED DESCRIPTION

[0028] The following will be combined with the accompanying drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0029] It should be noted that, in the specific embodiments of the present application, lithium-ion batteries are used as an example of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0030] The present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet comprises a positive current collector, an undercoat layer disposed on at least one surface of the positive current collector, and a positive electrode active material layer, wherein the undercoat layer is disposed between the positive current collector and the positive electrode active material layer. As shown in FIG1 , a positive electrode sheet 10 comprises a positive current collector 11, with an undercoat layer 12 and a positive electrode active material layer 13 disposed sequentially on both surfaces of the positive current collector 11. The undercoat layer comprises an inorganic metal oxide, the inorganic metal oxide comprising an element M, the element M comprising at least one of Al, Ti, Sn, Sb, or Mg, and preferably, the element M comprises at least one of Ti, Sn, or Sb. Based on the mass of the base coating, the mass percentage of the inorganic metal oxide is W1, 55%≤W1≤99%, preferably, 60%≤W1≤85%, and illustratively, W1 can be 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99% or a range consisting of any two of the above values.

[0031] The electrolyte includes a first compound, the first compound includes at least one of fluoroethylene carbonate, difluoroethylene carbonate, a compound represented by formula (I), a compound represented by formula (II), or a compound represented by formula (III),

[0032] wherein n and m are independently selected from integers of 1-3, and R0, Rn and Rm are each independently selected from hydrogen, fluorine, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl; when substituted, the substituent of each group is halogen;

[0033] Based on the mass of the electrolyte, the mass percentage of the first compound is a%, 0.15≤a≤21, preferably, 0.5≤a≤12, and illustratively, a can be 0.15, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, or a range consisting of any two of the above values. In the present application, the first compound is an additive.

[0034] In the present application, when the first compound includes two or more types, there is no particular limitation on the mass percentage of each type, as long as the sum of the mass percentages of the first compounds meets the scope of the present application.

[0035] The inventors have discovered that when carbonate, sulfate, or anhydride additives are used in the electrolyte of secondary batteries, some of these additives react with trace amounts of water in the electrolyte at high temperatures or during cycling to produce acidic substances. These acidic substances can easily corrode the positive electrode current collector and cause the positive active material to swell, resulting in reduced safety performance of the secondary battery. A primer layer is provided on the positive electrode current collector. The primer layer includes an inorganic metal oxide. The inorganic metal oxide can react with the acidic substance, exerting a self-sacrificing effect, reducing the content of the acidic substance, reducing the corrosion of the acidic substance on the positive electrode current collector and the positive active material, and alleviating the deformation of the positive electrode current collector caused by the expansion of the positive active material and the reduction of the toughness of the positive electrode current collector caused by the corrosion of the positive electrode current collector. At the same time, providing a primer layer on the positive electrode current collector can also reduce direct contact between the electrolyte and the positive electrode current collector, playing a protective role, and improving the safety performance of the secondary battery while ensuring the cycling stability of the secondary battery. By regulating the bottom coating of the positive electrode plate including inorganic metal oxide, the mass percentage of the inorganic metal oxide, the types of elements included in the inorganic metal oxide, and the type and mass percentage of the first compound in the electrolyte within the scope of this application, the safety performance of the secondary battery can be improved while ensuring the cycle stability of the secondary battery.

[0036] In one embodiment of the present application, as shown in Figure 1, the thickness of the primer layer 12 is Aμm, 2.1≤A≤15.5, preferably, 2.5≤A≤9.5. For example, A can be 2.1, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5 or a range consisting of any two of the above values. By regulating the value of A within the scope of this application, the primer layer can have a suitable thickness, can effectively inhibit the corrosion of the positive electrode current collector by the acidic substance, and can also ensure the energy density of the secondary battery while improving the safety performance of the secondary battery.

[0037] In one embodiment of the present application, 0.02≤a / A≤5.1. For example, a / A can be 0.02, 0.05, 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.1, or a range consisting of any two of the above values. By regulating the value of a / A within the scope of this application, the mass percentage of the first compound can be matched with the thickness of the primer layer, effectively inhibiting the corrosion of the positive electrode current collector by the acidic substance, and improving the safety performance of the secondary battery while ensuring the cycle stability of the secondary battery.

[0038] In one embodiment of the present application, the compound represented by formula (I) includes at least one of succinic anhydride, glutaric anhydride or adipic anhydride, the compound represented by formula (II) includes at least one of maleic anhydride, citraconic anhydride, trifluoromethylmaleic anhydride, dimethylmaleic anhydride, 3-fluorofuran-2,5-dione or 3,4-difluoromaleic anhydride, and the compound represented by formula (III) includes at least one of vinyl sulfate, propylene sulfate, butylene sulfate, 1,3,2-dioxathiane-2,2-dioxide or 2-methyl-1,3-propanedisulfite. By selecting the compound represented by formula (I), the compound represented by formula (II) and the compound represented by formula (III), the safety performance of the secondary battery can be improved while ensuring the cycle stability of the secondary battery.

[0039] In one embodiment of the present application, the inorganic metal oxide includes at least one of Al2O3, TiO2, SnO2, SnO, Sb2O3, Sb2O4, Sb2O5, or MgO. By selecting such inorganic metal oxides, the inorganic metal oxide can react with acidic substances, reducing the content of the acidic substances and reducing the corrosion of the acidic substances on the positive electrode current collector and the positive electrode active material. This can ensure the cycle stability of the secondary battery while improving the safety performance of the secondary battery.

[0040] In one embodiment of the present application, the primer layer further includes a binder and a conductive agent. Based on the mass of the primer layer, the mass percentage of the binder is W2, and the mass percentage of the conductive agent is W3. 0.5% ≤ W2 ≤ 25%. For example, W2 can be 0.5%, 1%, 3%, 5%, 7%, 9%, 10%, 11%, 13%, 15%, 17%, 19%, 20%, 21%, 23%, 25%, or a range consisting of any two of the above values; 0.5% ≤ W3 ≤ 20%. For example, W3 can be 0.5%, 1%, 3%, 5%, 7%, 9%, 10%, 11%, 13%, 15%, 17%, 19%, 20%, or a range consisting of any two of the above values. By regulating the values ​​of W2 and W3 within the scope of this application, the mass percentage of the binder and the mass percentage of the conductive agent can be made appropriate, so that the inorganic metal oxide has a suitable mass percentage, which is beneficial to effectively protect the positive electrode current collector; and the inorganic metal oxide can be better bonded to the positive electrode current collector, so that the positive electrode sheet has a higher electron conductivity, thereby ensuring the cycle stability of the secondary battery while improving the safety performance of the secondary battery.

[0041] In one embodiment of the present application, the binder comprises a polymer formed from at least one of acrylic acid, acrylamide, lithium acrylate, sodium acrylate, acrylonitrile, methyl acrylate, ethyl acrylate, methyl 2-methacrylate, or ethyl 2-methacrylate. The use of such a binder allows for better bonding between the inorganic metal oxide and the positive electrode current collector, resulting in a higher electron conductivity of the positive electrode sheet, which is beneficial for improving the cycle performance of the secondary battery.

[0042] In one embodiment of the present application, the binder includes at least one of sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, or nitrile rubber. The use of such a binder allows for better bonding between the inorganic metal oxide and the positive electrode current collector, resulting in a higher electron conductivity of the positive electrode sheet, which is beneficial for improving the cycle performance of the secondary battery.

[0043] In one embodiment of the present application, the weight average molecular weight of the binder is 150,000 to 1.95 million. Exemplarily, the weight average molecular weight of the binder can be 150,000, 160,000, 350,000, 550,000, 750,000, 950,000, 1.15 million, 1.35 million, 1.55 million, 1.75 million, 1.95 million or a range consisting of any two of the above values. By regulating the weight average molecular weight of the binder within the scope of this application, it is possible to take into account the film forming property and adhesion of the primer layer, improve the uniformity and stability of the primer layer, effectively inhibit the corrosion of the positive electrode current collector by acidic substances, and improve the safety performance of the secondary battery while ensuring the cycle stability of the secondary battery.

[0044] In one embodiment of the present application, the conductive agent includes at least one of graphene, graphite fiber, carbon nanotubes, or conductive carbon black. By selecting the above conductive agent, the positive electrode plate can have a higher electron conductivity, which is conducive to improving the cycle performance of the secondary battery.

[0045] In the present application, the carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes; the conductive carbon black may include but is not limited to at least one of acetylene black or Ketjen black.

[0046] In one embodiment of the present application, the negative electrode plate includes a negative electrode active material, and the negative electrode active material includes at least one of natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, soft carbon, silicon, or a silicon-based composite material. The negative electrode plate includes the above-mentioned negative electrode active material, and the secondary battery has high cycle stability and good safety performance. In the present application, the silicon-based composite material may include SiO.

[0047] The present application has no particular restrictions on the method for preparing the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the method for preparing the positive electrode sheet may include the following steps: mixing an inorganic metal oxide, a binder and a conductive agent in a certain proportion, adding a solvent, and mixing evenly to obtain an inorganic slurry; uniformly coating the inorganic slurry on one side of the positive electrode current collector, drying, and obtaining a positive electrode sheet coated with a primer layer on one side; mixing a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent in a certain proportion, adding a solvent, and mixing evenly to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on the primer layer, dried to obtain a positive electrode sheet coated with a primer layer and a positive electrode active material layer on one side, and then the above steps are repeated on the other side of the positive electrode current collector to obtain a positive electrode sheet. The present application has no particular restrictions on the solvent, as long as the purpose of the present application can be achieved. For example, the solvent can be N-methylpyrrolidone (NMP). The present application has no particular limitation on the solid content of the inorganic slurry, as long as the purpose of the present application can be achieved. For example, the solid content of the inorganic slurry can be 20 wt % to 50 wt %. The present application has no particular limitation on the solid content of the positive electrode slurry, as long as the purpose of the present application can be achieved. For example, the solid content of the positive electrode slurry can be 60 wt % to 80 wt %.

[0048] In the present application, the positive electrode sheet includes a positive electrode current collector and an undercoat layer and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector. The above-mentioned "undercoat layer and positive electrode active material layer arranged on at least one surface of the positive electrode current collector" means that the undercoat layer and the positive electrode active material layer can be arranged on one surface of the positive electrode current collector along the thickness direction of itself, or can be arranged on two surfaces of the positive electrode current collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of ​​the positive electrode current collector or a partial area of ​​the positive electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the positive electrode current collector, as long as the purpose of this application can be achieved. For example, it can include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector).

[0049] The positive electrode active material layer of the present application includes a positive electrode active material. The present application has no particular restrictions on the positive electrode active material, as long as the purpose of the present application can be achieved. For example, the positive electrode active material may include lithium nickel cobalt manganese oxide (such as common NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium iron manganese phosphate or lithium titanate. The positive electrode active material layer of the present application also includes a positive electrode conductor and a positive electrode binder. The present application has no particular restrictions on the type of positive electrode conductor, as long as the purpose of the present application can be achieved. For example, the positive electrode conductor may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers. The above-mentioned conductive carbon black may include but is not limited to at least one of acetylene black or Ketjen black. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fiber may include but is not limited to vapor grown carbon fiber (VGCF) and / or nano carbon fiber. The above-mentioned metal material may include but is not limited to metal powder and / or metal fiber. Specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymer may include but is not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. The present application has no particular restrictions on the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode binder may include but is not limited to polyacrylate, polyimide, polyamide, polyamide-imide, polyvinylidene fluoride (PVDF), polystyrene butadiene copolymer (styrene-butadiene rubber), sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium hydroxymethyl cellulose or potassium hydroxymethyl cellulose. The present application has no particular restrictions on the mass ratio of the positive electrode active material, positive electrode conductor and positive electrode binder in the positive electrode active material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved.

[0050] This application does not particularly limit the thickness of the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm. This application does not particularly limit the thickness of the positive electrode active material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode active material layer before cold pressing is 30μm to 250μm, and the thickness of the positive electrode active material layer after cold pressing is 15μm to 150μm. This application does not particularly limit the thickness of the positive electrode sheet, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode sheet is 50μm to 500μm.

[0051] In the present application, the electrolyte also includes a lithium salt and a non-aqueous solvent. The lithium salt may include various lithium salts commonly used in the art, such as at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalatoborate) (LiBOB) or lithium difluoroborate. The present application has no particular restrictions on the non-aqueous solvent, as long as the purpose of the present application can be achieved, for example, it may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvent. The above-mentioned carbonate compound may include but is not limited to at least one of a chain carbonate compound, a cyclic carbonate compound or a fluorocarbonate compound. The above-mentioned chain carbonate compound may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC) or ethyl methyl carbonate (EMC). The cyclic carbonate compound may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound may include but is not limited to at least one of 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate or trifluoromethylethylene carbonate. The carboxylate compound may include but is not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone or caprolactone. The above-mentioned ether compound may include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The above-mentioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The present application has no particular restrictions on the mass percentages of the lithium salt and the non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, based on the mass of the electrolyte, the mass percentage of the lithium salt may be 10% to 30%, and the mass percentage of the non-aqueous solvent may be 49% to 89%.

[0052] In the present application, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector. The above-mentioned "negative electrode active material layer arranged on at least one surface of the negative electrode current collector" means that the negative electrode active material layer can be arranged on one surface of the negative electrode current collector along the thickness direction of itself, or can be arranged on two surfaces of the negative electrode current collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a partial area of ​​the negative electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the negative electrode current collector, as long as the purpose of this application can be achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or a composite current collector, etc.

[0053] The negative electrode active material layer of the present application includes a negative electrode active material. The negative electrode active material layer of the present application also includes a negative electrode binder. The present application has no special restrictions on the negative electrode binder, as long as the purpose of the present application can be achieved. For example, the negative electrode binder can be at least one of the above-mentioned positive electrode binders. The negative electrode active material layer of the present application also includes a negative electrode conductive agent. The present application has no special restrictions on the negative electrode conductive agent, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent can be at least one of the above-mentioned positive electrode conductive agents. The present application has no special restrictions on the mass ratio of the negative electrode active material, the negative electrode binder, and the negative electrode conductive agent in the negative electrode active material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved.

[0054] This application does not particularly limit the thickness of the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode current collector is 5μm to 15μm. This application does not particularly limit the thickness of the negative electrode active material layer, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode active material layer before cold pressing is 30μm to 250μm, and the thickness of the negative electrode active material layer after cold pressing is 15μm to 150μm. This application does not particularly limit the thickness of the negative electrode pole piece, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode pole piece is 50μm to 500μm.

[0055] In the present application, the secondary battery also includes a diaphragm. The diaphragm is used to separate the positive electrode plate and the negative electrode plate, prevent internal short circuit of the secondary battery, allow electrolyte ions to pass freely, and do not affect the electrochemical charge and discharge process. The present application has no special restrictions on the diaphragm, as long as the purpose of the present application can be achieved. For example, the material of the diaphragm may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid; the type of diaphragm may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane.

[0056] In the present application, the separator may include a substrate and a surface treatment layer. The substrate may be a non-woven fabric or a composite film having a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic material. For example, the inorganic layer includes inorganic particles and a separator binder. The present application has no particular restrictions on the inorganic particles, and may include, for example, at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application has no particular restrictions on the separator binder, and may, for example, be at least one of the aforementioned positive electrode binders. The polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinyl pyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0057] The secondary battery of this application also includes a packaging bag for containing the positive electrode sheet, separator, negative electrode sheet, and electrolyte, as well as other components of the secondary battery known in the art. This application does not limit these other components. This application does not specifically limit the packaging bag and can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0058] In the present application, the secondary battery may include, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery, etc.

[0059] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and is not particularly limited in the present application. For example, it may include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, overcurrent protection elements, guide plates, etc. may also be placed in the packaging bag as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging. Among them, the packaging bag is a packaging bag known in the art, and the present application does not limit this.

[0060] The second aspect of the present application provides an electronic device, which includes the secondary battery according to any one of the aforementioned embodiments. Therefore, the electronic device provided by the present application has good performance.

[0061] The present application does not particularly limit the type of electronic device, and the electronic device may be any electronic device known in the art. In some embodiments, the electronic device may include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0062] Example

[0063] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0064] Test methods and equipment:

[0065] Type and content of elements, mass percentage of inorganic metal oxides test:

[0066] Take the lithium-ion battery and disassemble it, separate the positive electrode plate, use tape to peel off the positive electrode active material layer on the surface of the positive electrode plate to expose the primer layer, and use a knife to scrape the primer layer on the positive electrode plate to obtain a solid powder sample. Use an inductively coupled plasma optical emission spectrometer (ICP-OES) to detect the type and content of elements in the solid powder sample. Weigh 0.2g of the above solid powder sample and dissolve it in a nitric acid solution with a concentration of 42%. Then, the nitric acid solution in which the solid powder sample was dissolved is tested to obtain the type and content of elements in the solid powder sample. The mass percentage of the corresponding inorganic metal oxide is calculated based on the type and content of the metal element.

[0067] Test of mass percentage of the first compound:

[0068] The mass percentage of the first compound was tested using gas chromatography. After the lithium-ion battery was discharged to 3V, the packaging bag of the lithium-ion battery was cut open at the tab, and the lithium-ion battery was inserted into a centrifuge tube with the opening downward and centrifuged to obtain an electrolyte. The mass percentage of the first compound in the electrolyte obtained by centrifugation was tested using gas chromatography.

[0069] Weight average molecular weight test:

[0070] The weight average molecular weight of the binder was determined using a gel permeation chromatography (GPC) instrument. 0.01 g of the binder was dissolved in 5 mL of a solvent (N-methylpyrrolidone) to obtain a solution. After the solution was completely dissolved, impurities in the solution were filtered using a filter. The weight average molecular weight of the binder was then determined using a gel permeation chromatography instrument (PL-GPC220).

[0071] Cyclic performance test:

[0072] At 45°C, charge the lithium-ion battery to 4.3V at a constant current of 0.5C, leave it for 30 minutes, and then discharge it to 3.0V at a constant current of 0.5C. This is a charge and discharge cycle process. The first discharge capacity is recorded as C1. Repeat the above method for 200 charge and discharge cycles. The discharge capacity after the 200th cycle is recorded as C 200 .

[0073] The cycle capacity retention rate of lithium-ion batteries = (C 200 / C1)×100%.

[0074] Drop test:

[0075] Take the lithium-ion battery after 200 cycles at 45°C (discharged to 3.0V) and charge it at 0.5C constant current to 4.45V at 25°C. Then, charge it at 4.45V with a constant voltage to a current of 0.025C. In a test environment of 20±5°C, drop it from a height of 2m (based on the concrete drop floor) onto a concrete drop floor. Drop it once along three sides, once along three edges, and once along eight corners. This is a total of one round of testing. The order of drops is: once on each of the three sides → once on each of the three edges → once on each of the eight corners. The lithium-ion battery passes the test if there is no smoke, leakage, fire, or explosion. 50 lithium-ion batteries are tested in each group. The lithium-ion battery is a soft-pack lithium-ion battery, which is a rectangular parallelepiped, including six faces: top, bottom, left, right, front, and back. The above three faces refer to one face each of the top, bottom, left, right, and front; and twelve edges, namely, four lengths, four widths, and four heights. The above three edges refer to one edge each of the length, width, and height.

[0076] Example 1-1

[0077] <Preparation of positive electrode sheet>

[0078] An inorganic metal oxide TiO2, a binder of nitrile rubber, and a conductive agent of carbon nanotubes were mixed in a mass ratio of 58:22:20, and then N-methylpyrrolidone (NMP) was added as a solvent. The mixture was stirred evenly in a vacuum mixer to obtain an inorganic slurry with a solid content of 30 wt%. The inorganic slurry was evenly coated on one side of a 12 μm thick positive electrode current collector aluminum foil. The aluminum foil was dried at 90°C for 1 hour to obtain a single-sided primed positive electrode sheet with a thickness of 2.15 μm. The primed layer comprised the inorganic metal oxide TiO2, which included the element M, wherein the element M was Ti. The positive electrode active material, lithium cobalt oxide (molecular formula: LiCoO2), the positive electrode binder, polyvinylidene fluoride (PVDF), and the positive electrode conductive agent, conductive carbon black (Super P), were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added as a solvent and stirred evenly in a vacuum mixer to produce a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was evenly coated on the surface of the primer layer and dried at 120°C for 1 hour to produce a positive electrode sheet coated on one side with a primer layer and a positive electrode active material layer (230 μm thick). The above steps were repeated on the other side of the aluminum foil to produce a positive electrode sheet coated on both sides with a primer layer and a positive electrode active material layer. After cold pressing and slitting, the resulting positive electrode sheet had a size of 74 mm x 867 mm. The thickness of the positive electrode active material layer after cold pressing was 102 μm.

[0079] <Preparation of negative electrode sheet>

[0080] The negative electrode active material, negative electrode binder sodium carboxymethyl cellulose (CMC-Na), and negative electrode binder styrene-butadiene rubber were mixed in a mass ratio of 85:2:13. Deionized water was added and mixed thoroughly in a vacuum mixer to obtain a negative electrode slurry with a solid content of 30 wt%. The negative electrode slurry was evenly coated on one side of a 12 μm thick negative electrode current collector copper foil and dried at 120°C for 1 hour to obtain a negative electrode sheet with a single-sided negative electrode active material layer and a coating thickness of 143 μm. The above steps were repeated on the other side of the copper foil to obtain a negative electrode sheet with a double-sided negative electrode active material layer. The sheet was then cold pressed and slit to obtain a negative electrode sheet with a size of 78 mm x 875 mm. The negative electrode active material was a mixture of SiO and artificial graphite. The mass ratio of SiO to artificial graphite was 20:80 based on the mass of the negative electrode active material. The thickness of the negative electrode active material layer on one side after cold pressing was 80 μm.

[0081] <Preparation of Separator>

[0082] A porous polypropylene film with a thickness of 12 μm (provided by Celgard Company) was used.

[0083] <Preparation of Electrolyte>

[0084] In an argon atmosphere glove box with a water content of less than 10 ppm, ethylene carbonate and diethyl carbonate were mixed in a mass ratio of 3:7 to obtain an organic solvent. A lithium salt, LiPF6, and a first compound, fluoroethylene carbonate, were then added to the organic solvent and stirred to obtain an electrolyte. Based on the total mass of the electrolyte, the mass percentage a% of the first compound was 0.15%, the mass percentage of the lithium salt was 13%, and the balance was the organic solvent.

[0085] <Preparation of lithium-ion batteries>

[0086] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrode sheets to separate them. The electrodes are then wound, with the positive tabs connected to the positive and negative tabs connected to the negative electrode sheets, to form an electrode assembly. The electrode assembly is placed inside an outer packaging, which is an aluminum foil bag. The positive and negative tabs are then led out of the inner space of the outer packaging to the outer space. After baking at 80°C for 10 hours to remove moisture, an electrolyte is injected into the inner space of the outer packaging. After vacuum packaging, standing, formation, and shaping, a lithium-ion battery is obtained.

[0087] Example 1-2 to Example 1-26

[0088] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. Specifically, as the mass percentage of the inorganic metal oxide W1 changes, the sum of the mass percentages of the binder and the conductive agent also changes, while the mass ratio of the binder to the conductive agent remains unchanged. When the mass percentage of the added first compound changes, the mass percentage of the organic solvent also changes, while the mass ratio of ethylene carbonate to diethyl carbonate and the mass percentage of the lithium salt remain unchanged.

[0089] Examples 1-27

[0090] Except that the negative electrode active material in <Preparation of Negative Electrode Sheet> is artificial graphite, the rest is the same as Example 1-1.

[0091] Example 2-1 to Example 2-12

[0092] The preparation parameters were the same as in Examples 1-5 except that they were adjusted according to Table 2. When the mass percentage of the first compound added changed, the mass percentage of the organic solvent also changed, while the mass ratio of ethylene carbonate to diethyl carbonate and the mass percentage of the lithium salt remained unchanged.

[0093] Example 3-1 to Example 3-20

[0094] The preparation parameters were the same as those in Example 2-3 except that they were adjusted according to Table 3. The inorganic metal oxide content W1 also changed with the change in the binder content W2 and the conductive agent content W3.

[0095] Comparative Example 1

[0096] Except for preparing the positive electrode sheet according to the following method and adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0097] <Preparation of positive electrode sheet>

[0098] The positive electrode active material, lithium cobalt oxide (molecular formula: LiCoO2), the positive electrode binder, polyvinylidene fluoride (PVDF), and the positive electrode conductive agent, conductive carbon black (Super P), were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added as a solvent and stirred evenly in a vacuum mixer to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was evenly coated on one side of a 12 μm thick positive electrode current collector aluminum foil and dried at 120°C for 1 hour to obtain a positive electrode sheet with a single-sided positive electrode active material layer coated with a coating thickness of 227.8 μm. The above steps were repeated on the other side of the aluminum foil to obtain a positive electrode sheet with a double-sided positive electrode active material layer. The sheet was then cold pressed and slit to obtain a positive electrode sheet with a size of 74 mm × 867 mm. The thickness of the positive electrode active material layer after cold pressing was 102 μm.

[0099] Comparative Examples 2 to 5

[0100] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. Here, as the mass percentage of the inorganic metal oxide W1 changes, the sum of the mass percentages of the binder and the conductive agent also changes, while the mass ratio of the binder to the conductive agent remains unchanged. Here, when the mass percentage of the added first compound changes, the mass percentage of the organic solvent also changes, while the mass ratio of ethylene carbonate to diethyl carbonate and the mass percentage of the lithium salt remain unchanged.

[0101] The preparation parameters and electrical performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0102] Table 1

[0103] Note: (1) In Table 1, “ / ” indicates no relevant preparation parameters; (2) Taking Example 1-16 as an example, the first compound in Table 1 is “fluoroethylene carbonate + vinyl sulfate”, and the mass percentage a% of the first compound is “5% + 1%”, that is, the first compound added is two substances, fluoroethylene carbonate and vinyl sulfate. Based on the mass of the electrolyte, the mass percentage of fluoroethylene carbonate is 5%, and the mass percentage of vinyl sulfate is 1%. The same applies to other examples and comparative examples; (3) Taking Example 1-21 as an example, the inorganic metal oxide in Table 1 is “TiO2 + Al2O3”, the element M is “Ti + Al”, and the mass percentage of the inorganic metal oxide is The content W1 is "32%+35%", that is, two inorganic metal oxides TiO2 and Al2O3 are added to the primer layer, the element M includes Ti and Al, and based on the mass of the primer layer, the mass percentage of the inorganic metal oxide TiO2 is 32%, and the mass percentage of the inorganic metal oxide Al2O3 is 35%. The same applies to other embodiments and comparative examples; (4) Taking Example 1-1 as an example, the drop test pass number "15 / 50" in Table 1 means that 50 lithium-ion batteries were drop tested, of which 15 lithium-ion batteries did not smoke, leak, catch fire, or explode, and 15 lithium-ion batteries passed the drop test. The same applies to other embodiments and comparative examples.

[0104] Referring to Table 1, Examples 1-1 to 1-27 and Comparative Examples 1 to 5 show that when the undercoat layer of the positive electrode sheet includes an inorganic metal oxide, the mass percentage of the inorganic metal oxide, the types of elements included in the inorganic metal oxide, and the type and mass percentage of the first compound in the electrolyte are within the range of this application, the lithium-ion battery has a high cycle capacity retention rate and a high number of drop test pass rates, indicating that the lithium-ion battery has good cycle stability and good safety performance. In contrast, in Comparative Examples 1 to 5, the lithium-ion battery has a low cycle capacity retention rate and a low number of drop test pass rates, indicating that the lithium-ion battery has poor cycle stability and poor safety performance.

[0105] In Examples 1-1 to 1-8, by adjusting the mass percentage of the first compound, the cycle stability of the lithium-ion battery can be improved within a certain range. When the mass percentage of the first compound is low, the number of pass points in the drop test is high, and the safety performance of the lithium-ion battery is significantly improved. As the mass percentage of the first compound increases, the improvement in the safety performance of the lithium-ion battery decreases. When the mass percentage of the first compound is 0.5% to 12%, a relatively stable solid electrolyte interface film can be generated on the surface of the negative electrode plate, and less acidic substances are generated, which has less impact on the aluminum foil, thereby achieving higher cycle stability of the lithium-ion battery and better safety performance.

[0106] In Examples 1-9 to 1-15, the mass percentage of the inorganic metal oxide in the undercoat layer is within the range of this application, and a high number of drop test passes are achieved, which can improve the safety performance of the lithium-ion battery. Increasing the mass percentage of the inorganic metal oxide in the undercoat layer can, on the one hand, reduce direct contact between the electrolyte and the aluminum foil, and on the other hand, the inorganic metal oxide can fully react with the acidic substance generated by the first compound, thereby increasing the strength of the aluminum foil and enabling the lithium-ion battery to maintain strong impact resistance during the drop process. When the mass percentage of the inorganic metal oxide is 60% to 85%, the safety performance improvement is more significant.

[0107] In Examples 1-16 to 1-25, by adding multiple types of first compounds or adding multiple inorganic metal oxides to the undercoat layer, the lithium-ion battery achieved a high cycle capacity retention rate and a high number of drop test passes, demonstrating that both the cycle stability and safety performance of the lithium-ion battery can be improved. In Comparative Example 1, the positive electrode sheet does not include an undercoat layer. Due to the presence of the first compound, the acidic substances generated during the cycle of the lithium-ion battery easily corrode the aluminum foil and the positive electrode active material, resulting in poor safety performance of the lithium-ion battery. Comparative Examples 2 and 5 show that if the mass percentage of the inorganic metal oxide in the undercoat layer is too low, the safety performance of the lithium-ion battery will be poor; if the mass percentage of the inorganic metal oxide in the undercoat layer is too high, the safety performance of the lithium-ion battery cannot be further improved. It can be seen from Comparative Examples 3 and 4 that if the mass percentage of the first compound is too low, the cycle stability of the lithium-ion battery will be poor. This is because if the content of the first compound is too little, it will be difficult to form a stable solid electrolyte interface film on the surface of the negative electrode plate, and the instability of the negative electrode plate will be detrimental to the cycle stability of the lithium-ion battery; if the mass percentage of the first compound is too high, the cycle stability of the lithium-ion battery cannot be further improved, and the safety performance of the lithium-ion battery will be poor.

[0108] Table 2

[0109] The thickness A of the primer layer usually affects the cycle stability, safety performance and energy density of the lithium-ion battery. It can be seen from Examples 1-5 and 2-1 to 2-8 that when the thickness of the primer layer is increased, the number of passes in the drop test can be increased within a certain range, which is beneficial to improving the safety performance of the lithium-ion battery. When the thickness of the primer layer is 2.5μm to 9.5μm, the lithium-ion battery has good cycle stability and good safety performance. It can be seen from Examples 1-5 and 2-1 to 2-2 that the thickness of the primer layer is relatively small, and the effect of inhibiting the corrosion of the aluminum foil by the first compound is relatively limited, and the improvement of the safety performance of the lithium-ion battery is limited, but the thickness of the primer layer is within the scope of this application, and the lithium-ion battery has relatively good safety performance. It can be seen from Examples 2-6 to 2-8 that the thickness of the primer layer is relatively large, which cannot further improve the cycle performance of the lithium-ion battery and will also affect the energy density of the lithium-ion battery.

[0110] The a / A value generally affects the cycling stability and safety performance of lithium-ion batteries. As shown in Examples 2-1 to 2-12, by adjusting the a / A value within the range of this application, the lithium-ion battery has a higher cycling capacity retention rate and a higher number of drop test passes, indicating that the lithium-ion battery has higher cycling stability and better safety performance.

[0111] Table 3

[0112] Note: (1) Taking Example 3-19 as an example, the binder in Table 3 is "nitrile rubber + polyacrylamide", and the mass percentage content W2 of the binder is "4% + 10%", that is, the added binder is two substances, nitrile rubber and polyacrylamide. Based on the mass of the base coating, the mass percentage content of nitrile rubber is 4%, and the mass percentage content of polyacrylamide is 10%; the conductive agent in Table 3 is "graphene + acetylene black", and the mass percentage content W3 of the conductive agent is "5% + 5%", that is, the added conductive agent is two substances, graphene and acetylene black. Based on the mass of the base coating, the mass percentage content of graphene is 5%, and the mass percentage content of acetylene black is 5%. The same applies to other examples; (2) Taking Example 3-17 as an example, the binder in Table 3 is "nitrile rubber + polyacrylamide", and the weight average molecular weight of the binder is "160,000 + 190,000", that is, the weight average molecular weight of the binder nitrile rubber is 160,000, and the weight average molecular weight of the binder polyacrylamide is 190,000. The same applies to other examples.

[0113] The mass percentage of the binder W2 and the mass percentage of the conductive agent W3 usually affect the cycle stability and safety performance of the lithium-ion battery. It can be seen from Examples 3-1 to 3-16 that by regulating the mass percentage of the binder W2 and the mass percentage of the conductive agent W3 within the scope of this application, the cycle capacity retention rate of the lithium-ion battery is high and the number of drop test passes is high, indicating that the lithium-ion battery has good cycle stability and safety performance. When the mass percentage of the conductive agent is relatively low, it will affect the electron conductivity of the positive electrode sheet, thereby affecting the cycle stability of the lithium-ion battery, but the mass percentage of the conductive agent within the scope of this application has good cycle stability.

[0114] The type and weight-average molecular weight of the binder and the type of conductive agent generally affect the cycling stability and safety performance of lithium-ion batteries. As shown in Examples 3-1 to 3-20, the use of the aforementioned binders and the adjustment of their weight-average molecular weight within the scope of this application, as well as the use of the aforementioned conductive agents, resulted in a higher cycling capacity retention rate and a higher number of drop test passes, indicating that the lithium-ion batteries have good cycling stability and safety performance.

[0115] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive current collector and an undercoat layer and a positive active material layer disposed on at least one surface of the positive current collector, wherein the undercoat layer is disposed between the positive current collector and the positive active material layer; the undercoat layer comprises an inorganic metal oxide, wherein the inorganic metal oxide comprises an element M, wherein the element M comprises at least one of Al, Ti, Sn, Sb or Mg, and wherein the mass percentage of the inorganic metal oxide is W1, 55%≤W1≤99% based on the mass of the undercoat layer; The electrolyte includes a first compound, wherein the first compound includes at least one of fluoroethylene carbonate, difluoroethylene carbonate, a compound represented by formula (I), a compound represented by formula (II), or a compound represented by formula (III), in, n and m are independently selected from integers of 1-3, R0, Rn and Rm are each independently selected from hydrogen, fluorine, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl; when substituted, the substituent of each group is halogen; Based on the mass of the electrolyte, the mass percentage of the first compound is a%, and 0.15≤a≤21.

2. The secondary battery according to claim 1, wherein The element M includes at least one of Ti, Sn or Sb.

3. The secondary battery according to claim 1, wherein The secondary battery satisfies at least one of the following characteristics: (1)0.5≤a≤12； (2)60％≤W1≤85％。 4. The secondary battery according to claim 1, wherein The thickness of the primer layer is A μm, 2.1≤A≤15.

5.

5. The secondary battery according to claim 1, wherein The thickness of the primer layer is A μm, 2.5≤A≤9.

5.

6. The secondary battery according to claim 4, wherein 0.02≤a / A≤5.

1.

7. The secondary battery according to claim 1, wherein The compound represented by formula (I) includes at least one of succinic anhydride, glutaric anhydride or adipic anhydride, the compound represented by formula (II) includes at least one of maleic anhydride, citraconic anhydride, trifluoromethylmaleic anhydride, dimethylmaleic anhydride, 3-fluorofuran-2,5-dione or 3,4-difluoromaleic anhydride, and the compound represented by formula (III) includes at least one of vinyl sulfate, propylene sulfate, butyl sulfate, 1,3,2-dioxathiazole-2,2-dioxide or 2-methyl-1,3-propanedisulfite.

8. The secondary battery according to claim 1, wherein The inorganic metal oxide includes at least one of Al2O3, TiO2, SnO2, SnO, Sb2O3, Sb2O4, Sb2O5 or MgO.

9. The secondary battery according to claim 1, wherein The primer layer also includes a binder and a conductive agent. Based on the mass of the primer layer, the mass percentage of the binder is W2, the mass percentage of the conductive agent is W3, 0.5%≤W2≤25%, 0.5%≤W3≤20%.

10. The secondary battery according to claim 9, wherein The adhesive satisfies at least one of the following characteristics: (1) The binder comprises a polymer formed by at least one monomer selected from the group consisting of acrylic acid, acrylamide, lithium acrylate, sodium acrylate, acrylonitrile, methyl acrylate, ethyl acrylate, methyl 2-methacrylate or ethyl 2-methacrylate; (2) The binder comprises at least one of sodium carboxymethyl cellulose, potassium carboxymethyl cellulose or nitrile rubber; (3) The weight average molecular weight of the binder is 150,000 to 1.95 million.

11. The secondary battery according to claim 9, wherein The conductive agent includes at least one of graphene, graphite fiber, carbon nanotube or conductive carbon black.

12. The secondary battery according to claim 1, wherein The negative electrode plate includes a negative electrode active material, and the negative electrode active material includes at least one of natural graphite, artificial graphite, mesophase micro carbon beads, hard carbon, soft carbon, silicon or silicon-based composite materials. 13 . An electronic device comprising the secondary battery according to claim 1 .

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