Positive electrode sheet, secondary battery, and electronic device

Abstract

The present application discloses a positive electrode sheet, a secondary battery, and an electronic device. The positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector; the positive electrode material layer contains Co and Al; based on the total number of atoms in the positive electrode material layer, the ratio of the atomic percentage of Al to the atomic percentage of Co is T, and 0.008≤T≤0.032; based on the total number of atoms in a positive electrode active material, the ratio of the atomic percentage of Al to the atomic percentage of Co is T', and 0.012≤T'≤0.02; the positive electrode material layer further contains C and O. The secondary battery of the present application has better high-temperature performance and thermal safety performance.

Description

Cathode sheet, secondary battery and electronic device TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemical energy storage, in particular to a cathode sheet, a secondary battery and an electronic device. BACKGROUND

[0002] With the increasing demand for lithium ion electrochemical devices, lithium ion electrochemical devices widely used in portable electronic devices, electric bicycles, electric vehicles and energy storage devices and other fields are constantly pursuing high charge cut-off voltage and high energy density and more diversified use scenarios.

[0003] Lithium ion battery cathode materials are prone to structural instability under high temperature and high pressure conditions, and a large amount of gas is generated at the same time of the occurrence of side reactions, which seriously affects the high temperature use performance of the assembled secondary battery and electrochemical device. In a more serious case, the drop of the secondary battery or electrochemical device under high temperature conditions can easily cause fire and explosion, which poses a serious safety hazard to the user. SUMMARY

[0004] The present application provides a cathode sheet, a secondary battery and an electronic device, which can improve the high temperature performance and thermal safety performance of the secondary battery.

[0005] In a first aspect, the present application provides a cathode sheet, which comprises a cathode current collector and a cathode material layer arranged on at least one surface of the cathode current collector, the cathode material layer containing Co and Al, the ratio of the atomic percentage of Al to the atomic percentage of Co based on the total number of atoms of the cathode material layer being T, 0.008≤T≤0.032, the ratio of the atomic percentage of Al to the atomic percentage of Co based on the total number of atoms of the cathode active material being T', 0.005≤T'≤0.03, preferably, 0.008≤T≤0.025, 0.012≤T'≤0.02. The atomic number refers to the number of atoms, which is measured according to X-ray photoelectron spectroscopy (XPS). The inventors adjust the ratio of the atomic percentage of Co to the atomic percentage of Al in the cathode material layer within the above range, at the same time, the ratio of the atomic percentage of Co to the atomic percentage of Al in the cathode active material is also suitable, in this way, the high temperature cycle performance of the lithium ion battery can be improved, and the amount of interface gas generated by the lithium ion battery at high temperature can also be inhibited.

[0006] The inventors speculate that when the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode material layer is in the above range, and the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode active material is also in the above range, the positive electrode active material can obtain an aluminum-containing coating layer with a suitable proportion, which can play a role in stabilizing the internal structure of the positive electrode active material at high temperature and high pressure corresponding to the proportion of the positive electrode active material, thereby inhibiting the migration of transition metals on the positive electrode active material into the electrolyte, further improving the charge-discharge performance of the assembled battery at 45°C for 500 cycles, reducing the generation of interface side reactions, reducing the generation of interface gas, and even under higher temperature (65°C) conditions, the amount of interface gas can be inhibited.

[0007] In some embodiments, the positive electrode material layer also contains C and O, and the ratio of the atomic number of Co to the sum of the atomic numbers of Co, C, O and Al in the positive electrode material layer is t, 17.5≤t≤25. The present application controls the atomic number proportion of Co (based on the sum of the atomic numbers of Co, C, O and Al) in the positive electrode material layer in the above range, which is beneficial to stabilize the structure of the positive electrode active material, further improve the charge-discharge performance of the lithium ion battery at 45°C for 500 cycles, and better inhibit the generation of interface gas at high temperature.

[0008] In some embodiments, the ratio of the atomic number of Al to the sum of the atomic numbers of Co, C, O and Al in the positive electrode material layer is t', 0.31≤t'≤0.35. The present application further controls the atomic number proportion of Al (based on the sum of the atomic numbers of Co, C, O and Al) in the positive electrode material layer in the above range, which is more beneficial to improve the charge-discharge performance of the lithium ion battery at 45°C for 500 cycles, and further inhibit the generation of interface gas at high temperature.

[0009] In some embodiments, in the longitudinal section of the positive electrode material layer along the thickness direction, the ratio of the apparent concentration of Co to the apparent concentration of Al is C0, 80≤C0≤155. The present application further controls the ratio C0 of the apparent concentration of Co to the apparent concentration of Al in the longitudinal section of the positive electrode material layer along the thickness direction in the above range, which is beneficial to further improve the structural stability of the whole electrode sheet, and make the whole of each substance element on the electrode sheet satisfy a reasonable level, thereby further controlling the thickness expansion of the battery at high temperature, while taking into account the high temperature performance of the lithium ion battery, the low temperature performance of the lithium ion battery can also be improved.

[0010] In some embodiments, the particle size Dv90 of the positive electrode active material is D1 μm, and the specific surface area is D2 m 2 g -1The ratio D of the particle size Dv90 of the positive electrode active material to the value of the specific surface area BET of the positive electrode active material is within the above range, which can improve the flatness and stability of the structure of the material particles on the electrode sheet, and the flatness of the electrode sheet is improved through the appropriate flatness of the particles, so that the movement rate of the electrons on the surface of the material is more reasonable, thereby further reducing the thickness expansion of the battery caused by the different local electron rates, and further improving the low-temperature performance of the lithium ion battery, especially the low-temperature cycle performance.

[0011] In some embodiments, the powder resistivity of the positive electrode material on the positive electrode sheet under a pressure of 12 MPa is 120-430 Ω / cm. When the positive electrode material on the positive electrode sheet satisfies the powder resistivity, the electron movement on the positive electrode sheet and the lithium ion movement between the positive and negative electrodes can have appropriate rates, thereby being less likely to cause local overheating, further improving the thermal safety performance of the battery, better passing the thermal safety test, and improving the high-temperature long-cycle performance of the battery.

[0012] In the second aspect, the application provides a secondary battery, which comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, and the positive electrode sheet is any one of the positive electrode sheets in the first aspect.

[0013] In some embodiments, the average pore size of the separator ranges from 0.02 μm to 0.05 μm, which can enable the separator in the lithium ion battery to cooperate with the above-mentioned positive electrode sheet to more quickly achieve high-temperature closed pores, thereby improving the thermal chamber test pass rate and high-temperature drop test pass rate of the lithium ion battery at high temperature. The electrolyte at high temperature comprises propylene carbonate and a trinitrile compound, and the mass fraction of the propylene carbonate and the trinitrile compound in the total mass of the electrolyte is Q%, and 2≤Q≤5. The above-mentioned pore size of the separator and the electrolyte containing propylene carbonate and a trinitrile compound are combined, which is beneficial to the circulation of the electrolyte, and the electron migration in the positive electrode material is not hindered, thereby improving the thermal safety performance of the lithium ion battery and better passing the thermal safety test. Furthermore, when the electrolyte contains the above-mentioned content of propylene carbonate and a trinitrile compound, a relatively stable and uniform CEI film can be formed on the positive electrode material through synergistic effect through observation by an electrochemical atomic force microscope, thereby improving the high-temperature cycle performance, and at a higher temperature (greater than 100℃), the CEI film can further hinder the electron movement between the positive and negative electrodes by thickening, thereby preventing explosion, fire and other phenomena under thermal abuse conditions, and further improving the thermal safety performance.

[0014] In some embodiments, the isolation film comprises filler particles and a binder, the filler particles comprise organic particles, and the organic particles have an aspect ratio of (1.1-1.4):1. When the aspect ratio of the organic particles on the isolation film meets the range, the circulation of the electrolyte is more conducive, and when the aspect ratio of the organic particles is too large or too small, the lithium ion movement rate in some local areas of the battery is too fast, and the lithium ion movement rate in some other areas is too slow, thereby easily causing local overheating, thereby further improving the thermal safety performance of the battery, better passing the thermal safety test, and improving the high-temperature long-cycle performance of the battery.

[0015] In some embodiments, the electrolyte further comprises lithium 3-(diphenylphosphinyl)benzenesulfonate, and the mass percentage of lithium 3-(diphenylphosphinyl)benzenesulfonate in the total mass of the electrolyte is 1% to 4%. In this way, during the high-temperature charge-discharge process of the battery, gas molecules generated at the positive electrode interface can be captured, thereby reducing the occurrence of interface side reactions, ultimately improving the capacity retention rate of the battery after 500 cycles at high temperature, and improving the high-temperature cycle performance of the battery.

[0016] In a third aspect, the present application provides an electronic device comprising the secondary battery of any one of the second aspect.

[0017] The positive electrode tab provided by the first aspect of the present application has good heat resistance, so that the secondary battery provided by the second aspect of the present application has better high-temperature performance, better thermal safety, and improved low-temperature performance, and the electronic device provided by the third aspect of the present application also has improved high-temperature performance, thermal safety, and low-temperature performance. DETAILED DESCRIPTION

[0018] Embodiments of the present application will be described in detail below. The embodiments of the present application should not be interpreted as limiting the present application.

[0019] As used herein, the terms "comprise", "contain" and "include" are used in their open, non-limiting sense.

[0020] In addition, sometimes the amounts, ratios and other numerical values are presented in range format in this document. It should be understood that such range format is used for convenience and brevity and should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of a range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited.

[0021] In the DETAILED DESCRIPTION and in the claims, a list of items joined by the term "one or more of" can mean any combination of those items. For example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" can mean A alone; B alone; C alone; A and B together; A and C together; B and C together; or A, B, and C together. The item A can include a single element or multiple elements. The item B can include a single element or multiple elements. The item C can include a single element or multiple elements.

[0022] Positive electrode tab

[0023] The first aspect of the embodiments of the present application provides a positive electrode tab, which includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer contains Co and Al. The ratio of the percentage of the number of atoms of Al to the percentage of the number of atoms of Co, based on the total number of atoms of the positive electrode material layer, is T, and 0.008≤T≤0.032. The ratio of the percentage of the number of atoms of Al to the percentage of the number of atoms of Co, based on the total number of atoms of the positive electrode active material, is T', and 0.005≤T'≤0.03. The positive electrode material layer can be disposed on one side or both sides of the positive electrode current collector. In some embodiments, the positive electrode current collector can be an aluminum foil, and of course, other positive electrode current collectors commonly used in the art can also be used. In some embodiments, the thickness of the positive electrode current collector can be 1 μm to 200 μm. In some embodiments, the positive electrode active material layer can be coated only on a partial region of the positive electrode current collector. In some embodiments, the thickness of the positive electrode active material layer can be 10 μm to 500 μm. It should be understood that these are merely exemplary, and other suitable thicknesses can be used.

[0024] In some embodiments, the positive electrode active material can include at least one of lithium cobaltate, lithium nickel cobalt manganese oxide, or lithium nickel cobalt aluminum oxide. The positive electrode active material can be subjected to a doping and / or coating treatment. When the positive electrode material is lithium nickel cobalt aluminum oxide, the ratio of aluminum to cobalt on the positive electrode tab / positive electrode material is the ratio of the total amount of cobalt atoms and aluminum atoms of the positive electrode material matrix and the coating layer.

[0025] In some embodiments, the positive electrode material surface can further be provided with a covering layer containing lithium phosphate and lithium niobate, the mass ratio of lithium phosphate to lithium niobate being 1:5 to 1:1, and the thickness of the covering layer being 2 to 3 μm.

[0026] In some embodiments, the positive electrode active material layer further includes a positive electrode binder and a positive electrode conductive agent. In some embodiments, the positive electrode binder can include at least one of polyvinylidene fluoride, vinylidene-hexafluoropropylene copolymer, styrene-acrylate copolymer, styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, or polyhexafluoropropylene. In some embodiments, the positive electrode conductive agent can include at least one of conductive carbon black, acetylene black, ketjen black, graphene, carbon nanotube, or carbon fiber.

[0027] The inventors have found that by adjusting the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode material layer to be within the above range, while also adjusting the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode active material to be within the above range, the high-temperature cycle performance of the lithium ion battery can be improved, and the amount of interface gas generated at high temperature can also be inhibited. The inventors have found that when the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode material layer is within the above range, and the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode active material is also within the above range, the positive electrode active material can obtain an aluminum-containing coating layer with a suitable proportion, which can play a role in stabilizing the internal structure of the positive electrode active material at high temperature and high pressure, thereby inhibiting the transition metal on the positive electrode active material from migrating into the electrolyte, further improving the charge-discharge performance of the assembled battery at 45°C for 500 cycles, reducing the generation of interface side reactions, reducing the generation of interface gas, and even under conditions of a higher temperature (65°C), the amount of interface gas generated can be inhibited.

[0028] Specifically, in some embodiments, 0.008 < T < 0.03. In some embodiments, 0.008 < T < 0.028. In some embodiments, 0.008 < T < 0.025. In some embodiments, 0.01 < T < 0.02. In some embodiments, 0.012 < T < 0.016.

[0029] Specifically, in some embodiments, 0.008 < T' < 0.028. In some embodiments, 0.01 < T' < 0.025. In some embodiments, 0.012 < T' < 0.02. In some embodiments, 0.015 < T' < 0.018.

[0030] In some embodiments, the atomic percentage of Co is greater than or equal to 12 atomic percent and less than or equal to 30 atomic percent, based on the total number of atoms of the positive electrode material layer.

[0031] In some embodiments, the atomic percentage of Co is greater than or equal to 15 atomic percent, based on the total number of atoms of the positive electrode active material.

[0032] In some embodiments, the positive electrode material layer also contains C and O, and the ratio of the number of atoms of Co to the sum of the number of atoms of Co, C, O and Al in the positive electrode material layer is t, 17.5≤t≤25. Controlling the proportion of the number of atoms of Co in the positive electrode material layer within the above range is conducive to stabilizing the structure of the positive electrode active material, further improving the charge-discharge performance of the lithium ion battery at 45°C after 500 cycles, and better inhibiting the generation of interface gas at high temperature. Specifically, in some embodiments, 17.5≤t≤24. In some embodiments, 17.5≤t≤22. In some embodiments, 17.5≤t≤21.8.

[0033] In some embodiments, the ratio of the number of atoms of Al to the sum of the number of atoms of Co, C, O and Al in the positive electrode material layer is t', 0.31≤t'≤0.35. Specifically, in some embodiments, 0.32≤t'≤0.35. In some embodiments, 0.33≤t'≤0.35. In some embodiments, 0.34≤t'≤0.35.

[0034] In some embodiments, in the longitudinal section of the positive electrode material layer along the thickness direction, the ratio of the apparent concentrations of Co and Al is C0, 80≤C0≤155. At this time, it is conducive to further improving the structural stability of the whole electrode sheet and making the whole of each substance element on the electrode sheet meet a reasonable level, thereby further controlling the thickness expansion of the battery at high temperature, while taking into account the high-temperature performance of the lithium ion battery, it can also improve the low-temperature performance of the lithium ion battery. Specifically, in some embodiments, 85≤C0≤140. In some embodiments, 90≤C0≤130. In some embodiments, 100≤C0≤120. In some embodiments, 105≤C0≤115.

[0035] For example, the ratio C0 of the apparent concentrations of Co and Al is 80, 90, 100, 105, 115, 125, 135, 145, 155 or a range consisting of any two of the above values.

[0036] In some embodiments, in the longitudinal section of the positive electrode material layer along the thickness direction, the apparent concentration of Al is 0.89-1.8. For example, the apparent concentration of Al is 0.89, 0.9, 1.1, 1.3, 1.5, 1.6, 1.8 or a range consisting of any two of the above values.

[0037] In some embodiments, the particle size Dv90 of the positive electrode active material is D1 μm, and the specific surface area is D2 m 2 g -1, D = D1 / D2, 3≤D≤8, 20≤D1≤27. In this way, the flatness and stability of the structure of the material particles on the pole piece can be improved, thereby further reducing the thickness expansion of the battery caused by the difference in local electron rate, and further improving the low-temperature performance of the lithium ion battery, especially the low-temperature cycle performance. For example, the value of D is 3, 4, 5, 6, 7, 8, or a range formed by any two of the above values. For example, the particle size Dv90 of the positive electrode material is 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, or a range formed by any two of the above values.

[0038] In some embodiments, the specific surface area of the positive electrode active material is 0.11-0.15 m 2 g -1 . For example, the specific surface area of the positive electrode active material is 0.11 m 2 g -1 , 0.12 m 2 g -1 , 0.13 m 2 g -1 , 0.14 m 2 g -1 , 0.15 m 2 g -1 , or a range formed by any two of the above values.

[0039] In some embodiments, the powder resistivity of the positive electrode material on the positive electrode pole piece under a pressure of 12 MPa is 120-430 Ω / cm. For example, the powder resistivity of the positive electrode material on the positive electrode pole piece under a pressure of 12 MPa can be 120 Ω / cm, 150 Ω / cm, 200 Ω / cm, 250 Ω / cm, 300 Ω / cm, 350 Ω / cm, 400 Ω / cm, 430 Ω / cm, or a range formed by any two of the above values.

[0040] Secondary battery

[0041] The secondary battery of the present application includes a positive electrode pole piece, a negative electrode pole piece, a separator, and an electrolyte, the separator is disposed between the positive electrode pole piece and the negative electrode pole piece, and the positive electrode pole piece is any one of the above positive electrode pole pieces.

[0042] The separator includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene, or ultrahigh molecular weight polyethylene. In particular, polyethylene and polypropylene have good effects on preventing short circuits and can improve the safety of the battery through the shutdown effect. In some embodiments, the thickness of the separator is in the range of about 3 μm to 480 μm.

[0043] In some embodiments, the separator film has a porous layer, and the binder of the porous layer is selected from at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, or polyhexafluoropropylene. The porous layer on the surface of the separator film can improve the heat resistance, oxidation resistance, and electrolyte infiltration performance of the separator film, and enhance the adhesion between the separator film and the electrode sheet. The separator film can also include one of a high-melting-point crystalline polymer or a high-temperature-resistant amorphous polymer, and the high-temperature-resistant resin includes at least one of polypropylene, poly-4-methylpentene, polytetrafluoroethylene, polyvinylidene fluoride, and a cyclic olefin-based copolymer. The high-melting-point crystalline polymer includes at least one of polypropylene, poly-4-methylpentene, polytetrafluoroethylene, or polyvinylidene fluoride, and the high-temperature-resistant amorphous polymer includes a cyclic olefin-based copolymer. The mass percentage content z of the high-temperature-resistant resin is 2% to 10% based on the mass of the polyolefin porous substrate. For example, the mass percentage content z of the high-temperature-resistant resin is 2%, 3%, 5%, 7%, 8%, 10%, or a range between any two of the values. When the high-temperature-resistant resin of the above type is added to the polyolefin porous substrate and the mass percentage content of the high-temperature-resistant resin is controlled within the above range, the melting temperature of the separator film is improved, and the strength and high-temperature performance of the electrochemical device are improved.

[0044] In some embodiments, the average pore size of the separator film is in the range of 0.02 μm to 0.05 μm. The separator film with the above pore size is combined with an electrolyte containing propylene carbonate and a trinitrile compound, which facilitates the circulation of the electrolyte and does not hinder the electron migration inside the positive electrode material, thereby improving the thermal safety performance of the lithium ion battery and better passing the thermal safety test. For example, the average pore size of the separator film is in the range of 0.02 μm, 0.025 μm, 0.03 μm, 0.035 μm, 0.04 μm, 0.045 μm, 0.05 μm, or a range between any two of the values.

[0045] In some embodiments, the separator film includes filler particles and a binder, and the filler particles include organic particles, and the aspect ratio of the organic particles is (1.1-1.4): 1. For example, the aspect ratio of the organic particles can be 1.1: 1, 1.2: 1, 1.3: 1, 1.4: 1.

[0046] The electrolyte includes a lithium salt and a non-aqueous solvent. The lithium salt is not particularly limited as long as the object of the present application is achieved. For example, the lithium salt can include, but is not limited to, at least one of LiPF6, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. The content of the lithium salt in the electrolyte is not particularly limited as long as the object of the present application is achieved. For example, the mass percentage of the lithium salt based on the mass of the electrolyte is 5% to 23%, for example, the concentration of the lithium salt in the electrolyte can be 5%, 8%, 12%, 16%, 20%, 23%, or a range between any two of the above values. The non-aqueous solvent is not particularly limited as long as the object of the present application is achieved. For example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylic acid ester compound, an ether compound, or another organic solvent. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The chain carbonate compound can 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 methyl ethyl carbonate (MEC). The cyclic carbonate compound can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC), except for PC. The fluorinated carbonate compound can 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-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, or trifluoromethyl ethylene carbonate. The carboxylic acid ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can 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 other organic solvent can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.The content of the non-aqueous solvent in the electrolyte is not particularly limited in the present application, as long as the purpose of the present application can be achieved. Among them, the content of the non-aqueous solvent = 100% - the content of the lithium salt.

[0047] In some embodiments, the electrolyte comprises propylene carbonate and a trinitrile compound, the mass ratio of propylene carbonate to the trinitrile compound is Q%, 2≤Q≤5, based on the total mass of the electrolyte. Preferably, 3≤Q≤5. The content of propylene carbonate is 8.18%-8.31%, and the content of the trinitrile compound is 1.66%-2.73%, based on the total mass of the electrolyte, the trinitrile compound comprising 1,3,6-hexanetricarbonitrile and / or 1,2,3-tris(2-cyanoethoxy)propane.

[0048] In some embodiments, the electrolyte further comprises lithium 3-(diphenylphosphoryl)benzenesulfonate, the mass percentage of lithium 3-(diphenylphosphoryl)benzenesulfonate is 1%-4%, based on the total mass of the electrolyte. When the electrolyte contains propylene carbonate and a trinitrile compound in the above-mentioned content, a relatively stable and uniform CEI film can be formed on the positive electrode material by synergistic effect through the observation of electrochemical atomic force microscopy, thereby improving the high-temperature cycle performance, further improving the thermal safety performance and the anti-dropping performance at high temperature. Illustratively, the mass percentage of lithium 3-(diphenylphosphoryl)benzenesulfonate is 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or a range consisting of any two of the above values.

[0049] The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The thickness of the negative electrode active layer is not particularly limited as long as the object of the present application can be achieved, for example, the thickness of the negative electrode active layer is 30 μm to 120 μm. In some embodiments, the negative electrode active material can include at least one of a carbon material or a silicon-based material. In some embodiments, the carbon material includes, but is not limited to, at least one of natural graphite, artificial graphite, meso-phase carbon microbeads, hard carbon, or soft carbon. In some embodiments, the silicon-based material includes, but is not limited to, at least one of silicon, silicon-oxygen composite material, or silicon-carbon composite material. The negative electrode current collector is not particularly limited as long as the object of the present application can be achieved, for example, can include a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector (for example, a composite current collector in which a metal layer is disposed on the surface of a polymer layer), etc. The thickness of the negative electrode current collector is not particularly limited as long as the object of the present application can be achieved, for example, the thickness of the negative electrode current collector is 5 μm to 12 μm. The negative electrode active layer can further include a binder and a thickening agent, the kind of the binder and the thickening agent is not particularly limited as long as the object of the present application can be achieved. For example, the binder can include, but is not limited to, at least one of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, styrene butadiene rubber, or acrylated styrene butadiene rubber; the thickening agent can include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. The negative electrode active layer can further include a conductive agent, the kind of the conductive agent is not particularly limited as long as the object of the present application can be achieved. For example, the conductive agent can include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, ketjen black, graphene, a metal material, or a conductive polymer. The mass ratio of the negative electrode active material, the conductive agent, the binder, and the thickening agent in the negative electrode active layer is not particularly limited, a person skilled in the art can select according to actual needs as long as the object of the present application can be achieved. Optionally, the negative electrode sheet can further include a conductive layer, the conductive layer is located between the negative electrode current collector and the negative electrode active layer. The composition of the conductive layer is not particularly limited, can be a commonly used conductive layer in the art. For example, the conductive layer includes a conductive agent and a binder. The conductive agent and the binder in the conductive layer are not particularly limited, for example, can be at least one of the conductive agent and the binder in the negative electrode active layer described above.

[0050] The secondary battery can be prepared according to a conventional method in the art. Illustratively, the positive electrode sheet, the separator, and the negative electrode sheet described above are stacked in order, with the separator between the positive electrode sheet and the negative electrode sheet to play a role of separation, to obtain an electrode assembly, which can also be obtained after being wound; the electrode assembly is placed in a packaging shell, an electrolyte is injected and sealed to obtain a secondary battery.

[0051] The structure of the lithium battery is not particularly limited, and a coin-type battery, a cylindrical battery, a prismatic battery, or a pouch-type battery having a single layer or multiple layers of a separator can be used. The use of the lithium ion battery of the present application is not particularly limited, and it can be used for any electronic device known in the art. In some embodiments, the lithium ion battery of the present application can be used for, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio player, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a timepiece, a power tool, a flash, a camera, a home-use large storage battery, and a lithium ion capacitor, etc.

[0052] Electronic device

[0053] The electronic device of the present application includes any one of the secondary batteries described above. The electronic device of the present application includes, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio player, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a timepiece, a power tool, a flash, a camera, a home-use large storage battery, and a lithium ion capacitor, etc.

[0054] Hereinafter, embodiments and comparative examples are presented to more specifically describe the embodiments of the present application. Unless otherwise stated, the parts, percentages, and ratios listed below are based on weight, and the raw materials used are commercially available or synthesized according to conventional methods.

[0055] Example 1-1

[0056] (I) Preparation of lithium ion battery

[0057] <Preparation of positive electrode sheet>

[0058] Preparation of positive active material

[0059] The isopropyl alcohol aluminum was selected as the aluminum source in a mass ratio of Al2O3:LiCoO2=1:62.5, the amount of isopropyl alcohol aluminum was calculated according to the amount of the required alumina, 100 g of lithium cobaltate, 6.4 g of isopropyl alcohol aluminum were placed in a rotating furnace and heated to 140°C, so that the isopropyl alcohol aluminum therein was vaporized, and then the temperature was kept for 2 h, then water vapor (about 450 ml) was slowly introduced, and then rotated for 1 h, and then the temperature was increased to 300°C and kept for 3 h, and then naturally cooled to room temperature, to obtain the product of alumina-coated LiCoO2, which was recorded as the first product.

[0060] The first product (Dv90 of 20 μm, BET of 0.15 m 2 g -1 ), conductive carbon black and binder polyvinylidene fluoride (PVDF) were stirred into a uniform slurry in a mass ratio of 82:9:9, and then the slurry was obtained after stirring, coating, drying, rolling, and spot welding of the tab, to obtain the battery positive electrode.

[0061] The amount of isopropyl alcohol aluminum can also be obtained by the above calculation method in the remaining examples, and the others are the same as in Example 1-1.

[0062] <Preparation of negative electrode tab>

[0063] The negative active material (artificial graphite with a particle size of 12 μm) 100 g, conductive agent (carbon black) 1 g, and binder (styrene-butadiene rubber, SBR) 4 g were added to 3 g of N-methyl pyrrolidone and 110 g of water to form a negative electrode slurry, and the solid content of the negative electrode slurry was 80 wt%, and the negative electrode slurry was obtained after stirring, coating, drying, rolling, and spot welding of the tab, to obtain the battery negative electrode.

[0064] <Preparation of electrolyte>

[0065] In an argon-filled glove box, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, and propylene carbonate were mixed in a mass ratio of 1:2:1:2, 3-(diphenylphosphino)benzenesulfonic acid lithium accounted for 1% of the total mass of the electrolyte, and a certain amount of 1,3,6-hexanetricarbonitrile was added, and a certain amount of lithium salt was added to make the final concentration of lithium salt in the electrolyte 8%, and the mass ratio of propylene carbonate and 1,3,6-hexanetricarbonitrile in the total mass of the electrolyte was 2.

[0066] <Separator>

[0067] The boehmite with a volume average particle size Dv90 of 1.8 μm and the binder polyacrylate were dispersed in deionized water in a mass ratio of 86:14 to form an inorganic coating slurry with a solid content of 48%, and the predetermined slurry was coated on the two surfaces of the negative electrode ring at a speed of 0.6 m / min by gravure roll coating, and then dried by blowing hot air at 120°C at a wind speed of 0.5 m / sec, to form an inorganic coating layer with a thickness of 3 μm adhered to the surface of the PE substrate.

[0068] The first polymer with a melt index of 5 g / 10 min, the second polymer with a melt index of 15 g / 10 min, and the carboxymethyl cellulose sodium were added into a stirrer and stirred evenly; the wetting agent dimethyl silicone was added into the stirrer, and then deionized water was added to stir and adjust the viscosity of the slurry to 42 mPa·s and the solid content to 5%, to obtain a first coating slurry.

[0069] The first coating slurry was uniformly coated on the inorganic coating layer, and after drying in an oven, a first coating layer was obtained. The coating weight of the first coating slurry was 1.8 mg / 5000 mm 2 , and the thickness of the first coating layer was 2 μm. The mass ratio of the first polymer, the second polymer, the carboxymethyl cellulose sodium, and the dimethyl silicone was 75:20:0.7:4.3.

[0070] Regarding the regulation of the average pore size of the separation film, the mass ratio of boehmite and the binder or the solid content of the inorganic coating layer can be adjusted, or the viscosity and the solid content of the slurry can be adjusted. The above methods can all achieve the regulation of the average pore size of the separation film. The specific operation can be referred to the conventional means of the prior art, and is specifically selected according to the actual situation. The present application is not limited.

[0071] <Preparation of a lithium ion battery>

[0072] The positive electrode sheet, the separation film, and the negative electrode sheet were sequentially stacked to make the separation film between the positive electrode sheet and the negative electrode sheet to play a role of separation, and then wound into a bare battery cell. The bare battery cell was placed in an aluminum plastic film, and after removing the water at 80℃, electrolyte was injected and sealed. Then, after the processes of standing, formation, shaping, etc., a lithium ion battery was obtained. The formation process was as follows: at 45℃, the first cycle of charging and discharging was carried out, and the process was as follows: first, constant current charging at 0.1C rate for 10 min, then constant current charging at 0.5C rate to a specified voltage Q=4.6V, then constant voltage charging until the current was less than or equal to 0.05C, and then constant current discharging at 0.5C rate to 2.5V.

[0073] Examples 1-2 to 1-7

[0074] Except for adjusting the relevant preparation parameters according to Table 2, the rest was the same as Example 1-1.

[0075] Comparative Examples 1-1 to 1-3

[0076] Except for adjusting the relevant preparation parameters according to Table 2, the rest was the same as Example 1-1.

[0077] Examples 2-1 to 2-12

[0078] The rest was the same as Example 1-1 except that the relevant preparation parameters were adjusted according to Table 1 and Table 3.

[0079] Table 1

[0080] Examples 3-1 to 3-17

[0081] The rest was the same as Example 1-1 except that the relevant preparation parameters were adjusted according to Table 4.

[0082] Examples 4-1 to 4-6

[0083] The rest was the same as Example 1-1 except that the relevant preparation parameters were adjusted according to Table 5.

[0084] (II) Test method

[0085] (1) Low temperature (-25℃) performance test

[0086] The lithium ion battery of each example and comparative example was repeatedly charged and discharged by the following steps, and the discharge capacity retention rate of the lithium ion battery was calculated.

[0087] In an environment of -25℃, the first charge and discharge was carried out, constant current charging to full charge voltage 4.2V (4.5V for nickel cobalt manganese lithium material or nickel cobalt aluminum lithium material) was carried out at a charge current of 2C, then constant voltage charging was carried out at the maximum voltage until the current was 0.02C, then constant current discharge was carried out at a discharge current of 0.5C until the final voltage was 3V, and the discharge capacity of the first cycle was recorded; then the above steps were repeated for 400 cycles of charge and discharge, and the discharge capacity of the 600th cycle was recorded.

[0088] Cycle capacity retention rate = (discharge capacity of the 600th cycle / discharge capacity of the first cycle) * 100%.

[0089] (2) Drop test at 45℃

[0090] The assembled battery was fixed in the drop test fixture with double-sided tape, the six sides of the fixture were numbered 1, 2, 3, 4, 5, 6 in turn, and the four corners of the fixture were numbered C1, C2, C3, C4 in turn.

[0091] At 45℃, the fixture was placed on a test platform 1.5m high, and the lithium ion secondary battery was dropped in turn according to the order of numbers 1-6, and then the lithium ion secondary battery was dropped in turn according to the order of numbers C1-C4, and the cycle was repeated 6 times to complete the drop test. After standing for 1h, it was observed whether the packaging shell of the lithium ion secondary battery was damaged or the top seal was broken;

[0092] A. Disassemble the lithium ion secondary battery to observe whether the tab of the cell is broken;

[0093] B. Disassemble the lithium ion secondary battery to observe whether the two side separators in the width direction of the cell are displaced or wrinkled;

[0094] C. Disassemble the lithium ion secondary battery to observe whether the positive plate and the negative plate have internal short circuit contact;

[0095] If the above conditions do not occur, it is passed. 15 lithium ion secondary batteries are tested in each group, and the pass rate of the lithium ion secondary battery drop test is recorded.

[0096] (3) Thickness expansion test of the battery after 500 cycles at 50°C

[0097] The lithium ion batteries of each example and comparative example are repeatedly charged and discharged by the following steps, and the discharge capacity retention rate of the lithium ion battery is calculated.

[0098] In an environment of 40°C, the first charge and discharge is carried out, constant current charging is carried out to the full charge voltage of 4.2V (or 4.5V for nickel-cobalt-manganese lithium material or nickel-cobalt-aluminum lithium material) at a charge current of 2C, then constant voltage charging is carried out at the maximum voltage until the current is 0.02C, then constant current discharging is carried out at a discharge current of 0.5C until the final voltage is 3.0V, and the discharge capacity of the first cycle is recorded; then the above steps are repeated for 500 cycles of charge and discharge, and the discharge capacity of the 500th cycle is recorded.

[0099] Cycle capacity retention rate = (discharge capacity of the 500th cycle / discharge capacity of the first cycle) * 100%.

[0100] (4) 50°C cycle performance test

[0101] In an environment of 50°C, the first charge and discharge is carried out, constant current charging is carried out to the full charge voltage of 4.2V (or 4.5V for nickel-cobalt-manganese lithium material or nickel-cobalt-aluminum lithium material) at a charge current of 2C, then constant voltage charging is carried out at the maximum voltage until the current is 0.02C, then constant current discharging is carried out at a discharge current of 0.5C until the final voltage is 3.0V, and the discharge capacity of the first cycle is recorded; then the above steps are repeated for 500 cycles of charge and discharge, and the discharge capacity of the 500th cycle is recorded.

[0102] Cycle capacity retention rate = (discharge capacity of the 500th cycle / discharge capacity of the first cycle) * 100%.

[0103] (5) Gas production amount test of the battery at 65°C

[0104] After constant current charging at 0.2 C at 65°C until reaching 4.2 V (or 4.5 V for nickel cobalt manganese lithium material or nickel cobalt aluminum lithium material), constant voltage charging at 4.2 V is performed for 1 hour. Then, the charged battery is stored in a constant temperature chamber at 65°C. After 200 hours, the battery is taken out of the constant temperature chamber, and after returning to room temperature, the gas generation amount of each battery is measured, and the 4.2 V storage characteristics of the battery are evaluated by this method. The gas generation amount is measured using the following Archimedes method: the test battery is placed in a container filled with ultrapure water, and the volume of the single-layer laminated battery is measured from the change in weight before and after. As a device for measuring the volume from the change in weight, a densimeter MDS-300 manufactured by Alpha Mirage Corp. is used.

[0105] (6) 145°C hot box test

[0106] Step 1: The prepared battery is charged to a cut-off voltage of 4.2 V at a constant current of 0.5 C, and then charged to a cut-off current of 200 mA at a constant voltage, and then left for 5 minutes.

[0107] Step 2: The fully charged battery is attached with a temperature-sensitive line between the two pole ears, and the voltage is monitored by connecting the two pole ears, and the cell is vertically hung in the box.

[0108] Step 3: The hot box is heated to 145°C at a rate of 5°C and kept for 60 min.

[0109] Evaluation criteria: the battery does not catch fire or explode, which is passed. The number of batteries that pass the hot box test is calculated.

[0110] (7) Positive electrode material powder resistivity test

[0111] The powder volume resistivity p of the positive electrode active material under a pressure of 12 MPa can be tested by a powder volume resistivity test method. For example, a four-probe method is used for testing. The test method includes: taking 2 g of positive electrode active material powder into a sample table, applying a pressure of 12 MPa to the powder by a pressure machine, and after the pressure is stable, the powder volume resistivity p of the positive electrode active material under a pressure of 12 MPa is read by a resistivity meter.

[0112] Table 2

[0113] In combination with Table 2, the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode material layer of Comparative Examples 1-1 to 1-3 and the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode active material thereof are not within the scope of the present application, and the capacity retention rate of 500 cycles at 45°C thereof is not higher than 70%, and the gas production at 65°C thereof is higher than 3.5 ml. The ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode material layer of Example 1-1 and the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode active material thereof are both within the appropriate range, and the capacity retention rate of 500 cycles at 45°C is significantly improved, and the gas production at 65°C is significantly reduced.

[0114] In particular, when the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode material layer is further adjusted to be within the preferred range, and at the same time, the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode active material is also within the preferred range, the effect of improving the high-temperature cycle performance and high-temperature gas production of the lithium ion battery is better.

[0115] In particular, when the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode material layer is further adjusted to be within the preferred range, and at the same time, the ratio of the atomic percentage of Co to the atomic percentage of Al in the positive electrode active material is also within the preferred range, the effect of improving the high-temperature cycle performance and high-temperature gas production of the lithium ion battery is better.

[0116] Table 3

[0117] In combination with Table 3, when the ratio of the apparent concentration of cobalt atoms to the apparent concentration of aluminum atoms in the longitudinal section of the positive electrode material layer along the thickness direction is further adjusted to be within the appropriate range, and at the same time, the ratio of the particle size Dv90 of the positive electrode active material to the specific surface area thereof is also appropriate, the high-temperature performance and low-temperature performance of the lithium ion battery can be improved.

[0118] Table 4

[0119] It can be seen from Table 4 that further adjusting the average pore size of the separator film to meet the range of the present application can improve the pass rate of the lithium ion battery in the 145°C hot box test and the pass rate of the drop performance test at 45°C, and improve the thermal safety performance and drop resistance performance at high temperature of the lithium ion battery and electronic device. When the content of propylene carbonate is caused to meet the range of the present application, the thermal safety performance and drop resistance performance at high temperature of the electrochemical device and electronic device can be improved. Further, by limiting the type of trinitrile compound and the type meeting the range of the present application, the pass rate of the electrochemical device in the 145°C hot box test and the pass rate of the drop performance test at 45°C can be improved, and the thermal safety performance and drop resistance performance at high temperature of the electrochemical device and electronic device can be improved. When the mass ratio of propylene carbonate to trinitrile compound is limited to meet the range of the present application, the pass rate of the electrochemical device in the 145°C hot box test and the pass rate of the drop performance test at 45°C can also be significantly improved. When 1% to 4% of lithium 3-(diphenylphosphino)benzenesulfonate is added thereto, the thermal safety performance and drop resistance performance at high temperature of the assembled lithium ion battery can be significantly improved.

[0120] Table 5

[0121] It can be seen from Table 5 that when the aspect ratio of the organic particles on the separator film meets 1.1:1 to 1.4:1 of the present application, the thermal safety performance and long cycle performance at high temperature of the lithium ion battery can be significantly improved.

[0122] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A positive electrode plate, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector, wherein the positive electrode material layer contains Co and Al; Based on the total number of atoms in the positive electrode material layer, the ratio of the atomic percentage of Al to the atomic percentage of Co is T, 0.008≤T≤0.032; The ratio of the atomic percentage of Al to the atomic percentage of Co based on the total number of atoms of the positive electrode active material is T′, 0.012≤T′≤0.02; The positive electrode material layer also contains C and O.

2. The positive electrode sheet according to claim 1, characterized in that: 0.008≤T≤0.025。 3. The positive electrode sheet according to claim 1 or 2, characterized in that: The ratio of the number of Co atoms in the positive electrode material layer to the sum of the number of Co, C, O, and Al atoms is t, 17.5≤t≤25; and / or, The ratio of the number of Al atoms in the positive electrode material layer to the sum of the total number of Co, C, O, and Al atoms is t', and 0.32≤t'≤0.

35.

4. The positive electrode sheet according to claim 1 or 2, characterized in that: In a longitudinal section of the positive electrode material layer along the thickness direction, the apparent concentration ratio of Co to Al is C0,80≤C0≤155.

5. The positive electrode sheet according to claim 4, characterized in that: The particle size Dv90 of the positive electrode active material is D1 μm, and its specific surface area is D2 m 2 g -1 , D=D1 / D2, 130≤D≤250, 20≤D1≤27.

6. The positive electrode sheet according to claim 1 or 2, characterized in that: The powder resistivity of the positive electrode material on the positive electrode plate is 120-430Ω / cm under a pressure of 12MPa.

7. A secondary battery, characterized in that: The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; The positive electrode sheet is the positive electrode sheet according to any one of claims 1 to 6.

8. The secondary battery according to claim 7, wherein: The average pore size of the isolation membrane ranges from 0.02 μm to 0.05 μm.

9. The secondary battery according to claim 7, wherein The isolation film includes filler particles and a binder; The filler particles include organic particles, and the aspect ratio of the organic particles is (1.1-1.4):

1.

10. The secondary battery according to any one of claims 7 to 9, characterized in that The electrolyte comprises propylene carbonate and a trinitrile compound, and the mass ratio of the propylene carbonate to the trinitrile compound is Q%, and 2≤Q≤5.

11. The secondary battery according to claim 10, wherein: The electrolyte further comprises lithium 3-(diphenylphosphino)benzenesulfonate, and the mass percentage of the lithium 3-(diphenylphosphino)benzenesulfonate is 1% to 4% based on the total mass of the electrolyte.

12. An electronic device, characterized in that: The electronic device includes the secondary battery according to any one of claims 7 to 11.

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