Electrolyte, secondary battery, and electronic apparatus

Abstract

Disclosed in the present application are an electrolyte, a secondary battery and an electronic apparatus. The electrolyte comprises ethylene sulfate and vinylene carbonate, wherein on the basis of the mass of the electrolyte, the mass percentage content of the ethylene sulfate is A% and the mass percentage content of the vinylene carbonate is B%, 0.1≤A / B≤4; the electrolyte at least comprises ethyl propionate and propylene carbonate; the electrolyte further comprises a first substance, wherein the first substance is at least two selected from lithium difluorophosphate, lithium difluoro(oxalato)borate, lithium bis(trifluoromethanesulfonyl)imide or lithium hexafluorophosphate, and at least contains lithium difluorophosphate, and on the basis of the total mass of the electrolyte, the mass percentage content of the first substance is 0.3 wt%-1.2 wt%. In the process of charge and discharge of the secondary battery of the present application, the low-temperature cycle performance and the overcharge prevention capacity under a large current condition of the secondary battery and an electrochemical apparatus can be improved.

Description

Electrolyte, secondary battery, and electronic device

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on March 7, 2024, with application number 202410260553.4 and application name “Electrolyte, 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 energy storage, and in particular to an electrolyte, a secondary battery, and an electronic device. Background Art

[0003] Lithium-ion batteries, with their high energy storage density, high nominal voltage, and compact size and light weight, are widely used in portable electronic devices, electric bicycles, electric vehicles, and energy storage devices. With the advancement of technology and improvements in quality of life, the use of lithium-ion batteries in daily life has become more diverse, and people's demand for lithium-ion battery performance has also continued to increase. Summary of the Invention

[0004] In view of this, the present application provides an electrolyte, a secondary battery, and an electronic device, which can further improve the performance of lithium-ion batteries.

[0005] In a first aspect, the present application provides an electrolyte, comprising vinyl sulfate (DTD) and vinylene carbonate (VC), wherein the mass percentage of the vinyl sulfate is A%, the mass percentage of the vinylene carbonate is B%, 0.1≤A / B≤4, the electrolyte comprising at least ethyl propionate and propylene carbonate, and the electrolyte further comprising a first substance, the first substance being selected from at least two of lithium difluorophosphate, lithium difluorooxalatoborate, lithium bis(trifluoromethanesulfonylimide), or lithium hexafluorophosphate, and containing at least lithium difluorophosphate, wherein the mass percentage of the first substance is 0.3wt% to 1.2wt% based on the mass of the electrolyte. The electrolyte meeting the above conditions can form a stable and uniform mixed solid electrolyte interface film (SEI film) at the negative electrode interface during charge and discharge, reduce the generation of other side reactions at the interface, reduce gas generation at the interface, improve the low-temperature cycle performance of the assembled secondary battery and electrochemical device, and improve its overcharge prevention capability under high current conditions. Preferably, the electrolyte satisfies: 0.2≤A / B≤3.

[0006] In some embodiments, the electrolyte satisfies at least one of the following conditions: (1) 0.2≤A≤0.5; (2) 0.5≤B≤1. Under the premise of 0.1≤A / B≤4, further ensuring that the range of A or B conforms to the above range can further form a more stable hybrid solid electrolyte interface film (SEI film), reduce the generation of side reactions, thereby further reducing interfacial gas generation, and improve the low-temperature cycle performance of the assembled secondary battery and electrochemical device, and enhance its overcharge prevention capability under high current conditions.

[0007] In some embodiments, based on the mass of the electrolyte, the mass percentage of ethyl propionate (EP) is C%, the mass percentage of ethyl propionate and vinyl sulfate is W%, and 8.6≤W≤15.5. This further improves the overcharge resistance of the assembled secondary battery and electrochemical device under high current conditions.

[0008] In some embodiments, based on the mass of the electrolyte, the mass percentage of propylene carbonate (PC) is D%, the mass percentage of propylene carbonate and vinylene carbonate is W'%, and 3.7 ≤ W' ≤ 12.5. Electrolytes meeting this range can further improve the low-temperature cycling performance of assembled secondary batteries and electrochemical devices, and enhance their overcharge protection capabilities under high current conditions.

[0009] In some embodiments, based on the mass of the electrolyte, the mass percentage of the first substance is 0.4 wt % to 2 wt %, and the electrolyte further includes diethyl carbonate and propyl propionate, wherein the mass ratio of ethyl propionate, propylene carbonate, diethyl carbonate, and propyl propionate is (4-8):(10-14):(40-44):(38-42). In this way, while improving the low-temperature cycle performance of the assembled secondary battery and electrochemical device, it is possible to further improve the hot box test of the secondary battery, thereby improving the thermal safety performance of the secondary battery and the electrochemical device.

[0010] In some embodiments, the first substance further comprises at least lithium bis(trifluoromethanesulfonyl imide), the sum of the mass percentages of the lithium difluorophosphate and the lithium bis(trifluoromethanesulfonyl imide ranges from 0.4 wt% to 0.5 wt%, the mass ratio of the lithium difluorophosphate to the lithium difluorooxalatoborate is 2:5 to 1, and the mass ratio of the lithium difluorophosphate to lithium bis(trifluoromethanesulfonyl imide is 1:5 to 1. Preferably, the mass ratio of the lithium difluorophosphate to lithium bis(trifluoromethanesulfonyl imide is 0.6 to 1.

[0011] In a second aspect, the present application provides a secondary battery, which includes the above-mentioned electrolyte.

[0012] In some embodiments, the secondary battery further includes a positive electrode sheet, the positive electrode sheet including 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 including at least one of first material particles, second material particles, or inorganic ceramic particles, and at least including the first material particles. The first material particles are lithium transition metal phosphate particles having an olivine-type crystal structure, and the second material particles are lithium manganese composite oxide particles having a layered crystal structure.

[0013] In some embodiments, the positive electrode material layer also includes a conductive agent and a binder, and the first material particles include lithium iron phosphate. Based on the mass of the positive electrode material layer, the mass percentage of the lithium iron phosphate is 75wt% to 88wt%, the mass percentage of the binder is 6wt% to 15wt%, and the mass percentage of the conductive agent is 2wt% to 10wt%. In this way, the stability of the positive electrode material can be further improved, and the rate performance of the secondary battery at high temperature can be improved. In addition, the selection of a positive electrode material that meets this range in combination with the electrolyte mentioned above in this application can further reduce the floating thickness expansion rate of the electrochemical device assembled into a secondary battery and improve its floating capacity retention rate. Preferably, the mass percentage of the lithium iron phosphate is 80wt% to 84wt%, the mass percentage of the binder is 6wt% to 12wt%, and the mass percentage of the conductive agent is 8wt% to 10wt%.

[0014] In some embodiments, the compaction density of the positive electrode material layer is Pg / cm 3 , 2.8≤P≤4.0. In this case, the stability of the positive electrode material can be further improved, thereby enhancing the rate performance of the secondary battery at high temperature, and further reducing the float charge thickness expansion rate of the electrochemical device assembled from the secondary battery, thereby improving its float charge capacity retention rate.

[0015] In some embodiments, the secondary battery further comprises a separator, comprising a base membrane and a functional coating disposed on at least one surface of the base membrane, the functional coating containing polymer particles, the polymer particles having a swelling degree of less than 45% in the electrolyte at 90°C for 20 hours. Secondary batteries meeting this range can have a better fit between the separator and the electrolyte. At high temperatures (80-100°C), the pores of the separator have good permeability, which can reduce the impedance of the secondary battery, thereby improving the high-temperature rate performance of the battery and reducing the float charge thickness expansion rate of the electrochemical device assembled with the secondary battery, further improving its float charge capacity retention rate.

[0016] In a third aspect, the present application provides an electronic device, which includes the above-mentioned secondary battery.

[0017] The secondary battery described in this application can improve the low-temperature cycling performance of the assembled secondary battery and electrochemical device during the charge and discharge process, as well as its overcharge resistance under high current conditions. Combining this electrolyte with the positive electrode material meeting the requirements of this application can improve the secondary battery's rate performance at high temperatures, while also reducing the secondary battery's float charge thickness expansion rate and increasing its float charge capacity retention rate. DETAILED DESCRIPTION

[0018] The 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. As used in this application, the terms "comprise", "contain" and "comprising" are used in an open, non-restrictive sense.

[0019] In addition, amounts, ratios, and other numerical values ​​are sometimes presented herein in a range format. It should be understood that such a range format is used for convenience and brevity and should be interpreted flexibly to include not only the values ​​explicitly specified as limits of the range, but also all individual values ​​or sub-ranges encompassed within the range, as if each value and sub-range were explicitly specified.

[0020] In the detailed description and claims, a list of items connected by the terms "one or more of," "one or more of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A or B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, or C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0021] electrolyte

[0022] The electrolyte includes vinyl sulfate and vinylene carbonate. Based on the mass of the electrolyte, the mass percentage of vinyl sulfate is A%, and the mass percentage of vinylene carbonate is B%, and 0.1≤A / B≤4. The electrolyte includes at least ethyl propionate and propylene carbonate. The electrolyte also includes a first substance, the first substance being selected from at least two of lithium difluorophosphate, lithium difluorooxalatoborate, lithium bistrifluoromethanesulfonyl imide, or lithium hexafluorophosphate, and at least containing lithium difluorophosphate. The mass percentage of the first substance is 0.3wt% to 1.2wt% based on the total mass of the electrolyte. Exemplarily, the A / B ratio is 0.1, 0.2, 0.4, 0.6, 0.8, 1, 1.3, 1.5, 1.8, 2, 2.4, 2.6, 2.8, 3, 3.5, 4, or a range consisting of any two of the foregoing values. Illustratively, the mass percentage of the first substance is 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt% or a range consisting of any two of the above values.

[0023] In some embodiments, 0.2≤A≤0.5. For example, the mass percentage A of the vinyl sulfate is 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, 0.5 wt%, or a range consisting of any two of the above values.

[0024] In some embodiments, 0.5≤B≤1. The mass percentage B of the vinylene carbonate is 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt% or a range consisting of any two of the above values.

[0025] In some embodiments, based on the mass of the electrolyte, the mass percentage of the ethyl propionate is C%, the mass percentage of the ethyl propionate and the vinyl sulfate is W%, and 8.6≤W≤15.5. Exemplarily, the mass percentage W of the ethyl propionate and the vinyl sulfate is 8.6wt%, 8.8wt%, 9wt%, 9.5wt%, 10wt%, 10.5wt%, 11wt%, 11.5wt%, 12wt%, 12.5wt%, 13wt%, 13.5wt%, 14wt%, 14.5wt%, 15wt%, 15.5wt%, or a range consisting of any two of the above values.

[0026] In some embodiments, based on the mass of the electrolyte, the mass percentage of the propylene carbonate is D%, the mass percentage of the propylene carbonate and the vinylene carbonate is W′%, and 3.7≤W′≤12.5. Exemplarily, the mass percentage of the propylene carbonate and the vinylene carbonate, W′, is 3.7, 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, or a range consisting of any two of the above values.

[0027] In some embodiments, the first substance further comprises at least lithium bis(trifluoromethanesulfonyl imide), the sum of the mass percentages of the lithium difluorophosphate and the lithium bis(trifluoromethanesulfonyl imide is in the range of 0.4wt% to 0.5wt%, the mass ratio of the lithium difluorophosphate to the lithium difluorooxalatoborate is 2:5 to 1, and the mass ratio of the lithium difluorophosphate to the lithium bis(trifluoromethanesulfonyl imide is 1:5 to 1. Exemplarily, the mass ratio of the lithium difluorophosphate to the lithium difluorooxalatoborate is 2:5, 2.5:5, 3:5, 3.5:5, 4:5, 4.5:5, 5:5, or a range consisting of any two of the above values. Exemplarily, the mass ratio of the lithium difluorophosphate to the lithium bis(trifluoromethanesulfonyl imide is 1:5, 1.5:5, 2:5, 2.5:5, 3:5, 3.5:5, 4:5, 4.5:5, 5:5, or a range consisting of any two of the above values.

[0028] In particular, the present application has no particular restrictions on the concentration of the first substance in the electrolyte, and is preferably 0.5 mol / L or more, more preferably 0.8 mol / L or more, and further preferably 1.0 mol / L or more. In addition, it is preferably 3 mol / L or less, more preferably 2 mol / L or less, and further preferably 1.7 mol / L or less. If the concentration of the first substance is too low, it may result in an insufficient number of mobile lithium ions in the electrolyte. On the other hand, if the concentration of the first substance is too high, it may cause the viscosity of the electrolyte to increase, resulting in an increase in the impedance of the electrolyte, which may lead to a decrease in the performance of the electrochemical device.

[0029] In some embodiments, the electrolyte further comprises other additives, the other additives comprising at least one of diethyl carbonate (DEC), propyl propionate (PP), 4-methylethylene sulfate (MDTD), 1,3-propane sultone (PS), 1,4-butane sultone (BS), or 1,3-propene sultone (PST), succinonitrile (SN), glutaronitrile (GN), adiponitrile (ADN), 2-methyleneglutaronitrile, dipropylmalononitrile, 1,3,6-hexanetrinitrile (HTCN), 1,2,6-hexanetrinitrile, 1,3,5-pentanetrinitrile, or 1,2-bis(cyanoethoxy)ethane (EDPN). Based on the mass of the electrolyte, the ratio of the other additives to the sum of the mass percentages of the vinyl sulfate and the vinylene carbonate is 8 to 16.

[0030] In some embodiments, the electrolyte further comprises at least one of fluoroether, fluoroethylene carbonate, or ether nitrile.

[0031] In some embodiments, the electrolyte may further include a non-aqueous solvent selected from a carbonate compound, a carboxylate compound, an ether compound, other organic solvents or a combination thereof. Wherein, the carbonate compound may be a chain carbonate compound, a cyclic carbonate compound, a fluorinated carbonate compound or a combination thereof. The chain carbonate compound is selected from ethyl propionate (EP), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC) or a combination thereof. The cyclic carbonate compound is selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC) or a combination thereof. The fluorocarbonate compound is selected from fluoroethylene carbonate (FEC), 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, trifluoromethylethylene carbonate, or a combination thereof. The carboxylate compound is selected from methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerolactone, mevalonolactone, caprolactone, methyl formate, or a combination thereof. The ether compound is selected from dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or a combination thereof. The other organic solvent is selected from dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, a phosphate ester, or a combination thereof.

[0032] secondary batteries

[0033] The secondary battery includes the above-mentioned electrolyte, a positive electrode sheet, a separator, and a negative electrode sheet, wherein the positive electrode sheet and the negative electrode sheet are separated by the separator disposed therebetween.

[0034] Positive electrode

[0035] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode material layer includes at least one of first material particles, second material particles, or inorganic ceramic particles, and at least contains first material particles. The first material particles are lithium transition metal phosphate particles having an olivine-type crystal structure, and the second material particles are lithium manganese composite oxide particles having a layered crystal structure.

[0036] In some embodiments, the positive electrode material layer further includes a conductive agent and a binder, and the first material particles include lithium iron phosphate. Based on the mass of the positive electrode material layer, the mass percentage of the lithium iron phosphate is 75wt% to 88wt%, the mass percentage of the binder is 6wt% to 15wt%, and the mass percentage of the conductive agent is 2wt% to 10wt%. Exemplarily, the mass percentage of the lithium iron phosphate is 75wt%, 77wt%, 79wt%, 80wt%, 82wt%, 84wt%, 86wt%, 88wt%, or a range consisting of any two of the above values. Exemplarily, the mass percentage of the binder is 6wt%, 8wt%, 10wt%, 11wt%, 12wt%, 13wt%, 15wt%, or a range consisting of any two of the above values. Exemplarily, the mass percentage of the conductive agent is 2wt%, 4wt%, 5wt%, 6wt%, 8wt%, 10wt%, or a range consisting of any two of the above values.

[0037] In some embodiments, the compaction density of the positive electrode material layer is P g / cm 3 , 2.8≤P≤4.0. Exemplarily, the compaction density P of the positive electrode material layer is 2.8 g / cm 3 , 3.0g / cm 3 、3.2g / cm 3 、3.4g / cm 3 、3.6g / cm 3 、3.8g / cm 3 , 4.0g / cm 3 Or a range consisting of any two of the above values.

[0038] In some embodiments, the positive electrode current collector may be aluminum foil, although other common positive electrode current collectors in the art may also be used. The thickness of the positive electrode current collector may be 1 μm to 200 μm. The positive electrode active material layer may be coated only on a portion of the positive electrode current collector. The thickness of the positive electrode active material layer may be 10 μm to 500 μm. It should be understood that these are merely exemplary, and other suitable thicknesses may be used.

[0039] In some embodiments, the binder includes at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, styrene-acrylate copolymer, styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinyl pyrrolidone, polyvinyl ether, polytetrafluoroethylene or polyhexafluoropropylene.

[0040] In some embodiments, the conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes, or carbon fibers.

[0041] The positive electrode sheet of the present application can be prepared according to conventional methods in the art. For example, a positive electrode slurry comprising first material particles, second material particles, inorganic ceramic particles, a conductive agent, and a binder is first applied to at least one surface of a positive electrode current collector to form a positive electrode active material coating. The positive electrode sheet is then obtained through drying, cold pressing, and other processes.

[0042] Isolation film

[0043] In some embodiments, the separator includes a base film and a functional coating disposed on at least one surface of the base film, wherein the functional coating contains polymer particles, wherein the polymer particles have a swelling degree of less than 45% when placed in the electrolyte at 90° C. for 20 hours. The polymer particles include polymer particles based on propylene, such as polypropylene (PP).

[0044] The base film of the separator is selected from at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene may include at least one of high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. Polyethylene and polypropylene are particularly effective in preventing short circuits and can improve battery safety through a shutdown effect. The separator has a thickness ranging from approximately 3 μm to 500 μm.

[0045] In some embodiments, a porous layer is further provided between the base film and the functional coating, the porous layer comprising inorganic particles and binder particles. The inorganic particles are selected from at least one of aluminum oxide (Al2O3), silicon oxide (SiO2), magnesium oxide (MgO), titanium oxide (TiO2), hafnium dioxide (HfO2), tin oxide (SnO2), cerium dioxide (CeO2), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO2), yttrium oxide (Y2O3), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The binder particles are selected from at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyethylene ether, polytetrafluoroethylene or polyhexafluoropropylene. The pores of the isolation membrane have a diameter in the range of about 0.01 μm to 1 μm. The porous layer on the surface of the isolation membrane can improve the heat resistance, oxidation resistance and electrolyte wetting performance of the isolation membrane, and enhance the adhesion between the isolation membrane and the electrode.

[0046] Negative electrode

[0047] The negative electrode plate includes a negative electrode current collector and a negative electrode material layer provided on at least one side surface of the negative electrode current collector, wherein the negative electrode material layer includes a negative electrode active material. In some embodiments, the negative electrode current collector may be at least one of copper foil, nickel foil, or a carbon-based current collector. In some embodiments, the thickness of the negative electrode current collector may be 1 μm to 200 μm. In some embodiments, the negative electrode material layer may be coated only on a partial area of ​​the negative electrode current collector. In some embodiments, the thickness of the negative electrode material layer may be 10 μm to 500 μm. It should be understood that these are merely exemplary and other suitable thicknesses may be used.

[0048] In some embodiments, the negative electrode active material includes at least one of natural graphite, artificial graphite, or a silicon-based material. In some embodiments, the silicon-based material includes at least one of silicon, a silicon oxide, a silicon carbon compound, or a silicon alloy.

[0049] In some embodiments, the negative electrode material layer may further include a negative electrode conductive agent and / or a negative electrode binder. The negative electrode conductive agent may include at least one of carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the negative electrode binder may include at least one of sodium carboxymethyl cellulose, polyacrylic acid, polyacrylate, polyacrylate, polyvinyl pyrrolidone, polyimide, polysiloxane, or styrene-butadiene rubber. It should be understood that the materials disclosed above are merely exemplary, and the negative electrode active material layer may employ any other suitable material.

[0050] In some embodiments, the mass ratio of the negative electrode active material, the negative electrode conductor and the negative electrode binder in the negative electrode active material layer can be (80-99):(0.5-10):(0.5-10). It should be understood that this is only exemplary and not intended to limit the present application.

[0051] The negative electrode sheet can be prepared according to conventional methods in the art. For example, the negative electrode active material and optional conductive agent and binder are dispersed in a solvent, which can be N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is then coated on the negative electrode current collector, and the negative electrode sheet is obtained through processes such as drying and cold pressing.

[0052] electronic devices

[0053] The electronic devices of the embodiments of the present application are not particularly limited and can be any electronic devices known in the art. In some embodiments, the electronic devices may include, but are not limited to, laptop computers, pen-type computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors.

[0054] The following examples and comparative examples are given to illustrate the embodiments of the present application in more detail. Unless otherwise stated, the parts, percentages and ratios listed below are all based on weight, and the raw materials used are commercially available or synthesized according to conventional methods.

[0055] Example 1-1

[0056] (1) Preparation of lithium-ion batteries

[0057] <Preparation of positive electrode sheet>

[0058] The first material particles of lithium iron phosphate, the positive electrode conductive agent acetylene black, and the positive electrode binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 85:7:8, and N-methylpyrrolidone (NMP) was added as a solvent and stirred evenly to prepare a positive electrode slurry with a solid content of 75wt%. The positive electrode slurry was evenly coated on one surface of a 13μm thick positive electrode current collector aluminum foil and dried at 85°C to obtain a positive electrode sheet coated with a positive electrode mixture layer on one side. The above steps were repeated on the other side of the positive electrode current collector aluminum foil to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides. After cold pressing, cutting, and slitting, the sheet was dried under vacuum conditions at 85°C for 4 hours to obtain a positive electrode sheet with a specification of 74mm×867mm.

[0059] <Preparation of negative electrode sheet>

[0060] A negative electrode active material (artificial graphite), a conductive agent (conductive carbon black), a binder (styrene-butadiene rubber (SBR), and a thickener (sodium carboxymethyl cellulose)) were mixed in a weight ratio of 96.5:1.5:1:1, deionized water was added, and the mixture was stirred uniformly in a vacuum mixer to obtain a negative electrode slurry having a solid content of 75 wt%. The negative electrode slurry was evenly coated on one surface of a 13 μm negative electrode current collector copper foil and dried at 120°C to obtain a negative electrode sheet coated with a 90 μm thick negative electrode active material layer on one side. The above steps were repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet coated with a negative electrode active material layer on both sides. The negative electrode sheet was then cold pressed, cut, and slit to obtain a negative electrode sheet with a size of 76 mm × 851 mm.

[0061] <Preparation of Electrolyte>

[0062] In an argon-filled glove box, 0.2% of LiPO2F2 (lithium difluorophosphate) and 0.5% of LiDFOB (lithium difluorooxalatoborate) were added to the total mass of the electrolyte, and the electrolyte solvent was weighed in a mass ratio of 6:12:42:40 of EP (ethyl propionate): PC (propylene carbonate): DEC (diethyl carbonate): PP (propyl propionate). In addition, an additive of DTD:VC was weighed in a mass ratio of 1:10, so that in the final electrolyte, the mass proportion of DTD (vinyl sulfate) was 0.2% and the mass proportion of VC (vinylene carbonate) was 0.5%.

[0063] <Isolation Film>

[0064] A 10 μm thick polyethylene (PE) microporous membrane was selected as the separator.

[0065] Inorganic boehmite particles with a Dv50 of 1.5 μm were mixed with polyacrylate in a mass ratio of 90:10 and dissolved in deionized water to form an inorganic coating slurry with a solid content of 50%. The resulting inorganic coating slurry was then evenly coated on both sides of the PE base film using a micro-concave coating method to obtain a heat-resistant layer, which was then dried in an oven. The thickness of the inorganic coating was 1.5 μm.

[0066] Preparation of functional coatings

[0067] Polymer particles polypropylene PP (weight average molecular weight of 10,000-14,000), sodium carboxymethyl cellulose and wetting agent dimethyl siloxane are added to the blender in sequence and stirred evenly. Deionized water is then added and stirred to adjust the viscosity of the slurry to 40 mPa·s and the solid content to 5% to obtain an organic coating slurry. The organic coating slurry is evenly coated on the inorganic coating on both sides of the base film and dried in an oven to obtain a first coating. The thickness of the first coating is 3 μm. The mass ratio of polymer particles, sodium carboxymethyl cellulose and dimethyl siloxane is 95:0.5:4.5. The polymer particles are spherical or quasi-spherical in the isolation membrane, and the sphericity R of the polymer satisfies 0.7≤R≤1.0.

[0068] <Preparation of lithium-ion batteries>

[0069] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed between the positive and negative electrode sheets to serve as an isolation. The electrode assembly is wound and the electrode tabs are welded. The electrode assembly is placed in an outer packaging aluminum-plastic film. After dehydration at 80°C, the above-mentioned electrolyte is injected. After vacuum packaging, standing, formation, shaping, capacity testing and other processes, a lithium-ion battery is obtained.

[0070] (2) Test method

[0071] (1) Low temperature performance test:

[0072] The prepared battery was charged and discharged twice at 0.2C, then charged to 4.0V at 0.5C. The battery was placed in a -15°C environment and discharged to 2.5V at a rate of 0.5C for 500 cycles. The ratio of the discharge capacity at -15°C in the 500th cycle to the discharge capacity at 0.5C at room temperature was the low-temperature capacity retention rate of the material at -15°C.

[0073] (2) 3C 5.5V overcharge test:

[0074] The battery was discharged at 0.5C to 2.5V at 25°C, then charged at a constant current of 3C to 5V, and then charged at a constant voltage for 3 hours. The surface temperature change of the battery cell was monitored (the passing standard was that the battery cell did not catch fire, burn, or explode). Ten batteries were selected for testing in each example, and the number of batteries that passed was calculated.

[0075] (3) Hot box test:

[0076] The lithium-ion batteries in each example and comparative example were charged at room temperature at a constant current rate of 0.5C to a full charge voltage of 3.8V. Charging was continued at a constant voltage of 3.80V to a cutoff current of 0.05C, bringing them to a fully charged state. The appearance was inspected to ensure that the lithium-ion batteries were in normal usable condition. The fully charged batteries were placed in an oven and heated at a rate of 5°C / min until reaching the designated hot box test temperature of 140°C. The temperature was maintained for one hour, during which the battery condition was observed.

[0077] Judgment standard: The battery does not catch fire or explode.

[0078] Hot box test pass rate = number of hot box tests passed / total number.

[0079] (4) 45℃ rate performance test:

[0080] Place the lithium-ion battery in a 45°C constant temperature box and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature.

[0081] The battery charge and discharge performance test is carried out according to the following steps:

[0082] a) When the battery is charged to 3.8V at a constant current of 0.1C (1C = 0.38mA), switch to constant voltage charging and stop charging when the charging current drops to 0.05C rate current;

[0083] b) The battery is discharged at a constant current rate of 0.1C until the voltage reaches 2.0V and then stops discharging;

[0084] c) Count the charge and discharge capacity of the battery.

[0085] Battery rate performance test, respectively test the first discharge capacity at 1C / 2C / 3C / 5C rate. Taking 3C rate as an example, follow the steps below:

[0086] a) When the battery is charged at a constant current rate of 3C to 3.8V, switch to constant voltage charging and stop charging when the charging current drops to a current rate of 0.05C;

[0087] b) The battery is discharged at a constant current rate of 3C until the voltage reaches 2.0V and then stops discharging;

[0088] c) Calculate the percentage of the battery's initial discharge capacity to its initial charge capacity.

[0089] (5) Float charge performance test:

[0090] Place the lithium-ion battery in a 25°C constant temperature box and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. Charge at a constant current of 1C to a voltage of 3.8V, charge at a constant voltage to a current of 0.05C, and then discharge at a constant current of 1C to a voltage of 2.0V. Record the discharge capacity as the initial capacity of the lithium-ion battery. Then charge at a constant current of 0.5C to a voltage of 3.8V, charge at a constant voltage to a current of 0.05C, and use a micrometer to test and record the thickness of the battery as the initial thickness. Transfer the test lithium-ion battery to a 45°C constant temperature box and charge at a constant voltage of 3.8V for 60 days. After 60 days, transfer the battery to a 25°C constant temperature box, let it stand for 60 minutes, and discharge at a constant current of 1C to a voltage of 2.0V. Record the discharge capacity as the discharge capacity of the lithium-ion battery after storage. Then charge with a constant current of 1C to a voltage of 3.8V, charge with a constant voltage to a current of 0.05C, and then discharge with a constant current of 1C to 2.0V. Record the discharge capacity as the recoverable capacity of the lithium-ion battery, and measure the thickness of the lithium-ion battery as the thickness after float charge.

[0091] Float charge thickness expansion rate = (float charge thickness - initial thickness) / initial thickness × 100%

[0092] Float capacity retention rate = (initial discharge capacity - recoverable capacity) / initial discharge capacity × 100%.

[0093] (6) Polymer swelling test:

[0094] A polymer was added to water to obtain an emulsion with a solid content of 30 wt%. The emulsion was coated on a glass substrate and dried at 85°C to obtain a polymer film. A polymer film with a mass of m1 was immersed in a test electrolyte at 90°C for 20 hours. The mass of the polymer film at this time was recorded as m2. The polymer swelling degree = (m2 - m1) / m1 × 100%. Each example or comparative example was tested three times, and the average value was taken as the final polymer swelling degree. The test electrolyte was the same as the electrolyte used to prepare the battery.

[0095] (7) Compaction density test of positive electrode material layer:

[0096] The battery discharged to a voltage of 2.0V was disassembled, and the positive electrode sheet was taken out. The positive electrode sheet was placed in a dimethyl carbonate solvent and soaked for 30 minutes to remove the electrolyte and by-products on the surface of the positive electrode sheet. Then, it was dried in a fume hood for 4 hours. The dried positive electrode sheet was taken out, and 5 positive electrode sheets of 5 cm × 5 cm were selected. The thickness of the positive electrode sheet was measured by a micrometer, which was recorded as d0. The positive electrode active material layer in the positive electrode sheet was scraped off with a scraper, and the weight of the positive electrode active material layer was weighed by a balance, which was recorded as m1. The thickness of the current collector after removing the active material was measured by a micrometer, which was recorded as d1. The compaction density of the positive electrode active material layer was calculated according to the following formula:

[0097] Compacted density P = m1 / [5cm×5cm×(d0-d1)], unit: g / cm 3 .

[0098] The compaction density of the positive electrode active material layer is the average value of 5 positive electrode sheets.

[0099] Example 1-2 to Example 1-19

[0100] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0101] Comparative Example 1-1 to Comparative Example 1-6

[0102] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0103] Comparative Examples 1-7

[0104] Except that vinyl sulfate and vinylene carbonate were not added, the rest was the same as Example 1-1.

[0105] Example 2-1 to Example 2-12

[0106] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1.

[0107] Example 3-1 to Example 3-15

[0108] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as Example 1-1.

[0109] Table 1

[0110] Combined with Table 1, compared with Example 1-1, the ratio of vinyl sulfate and vinylene carbonate in the electrolyte in Comparative Examples 1-1 to Comparative Examples 1-7 is not appropriate, A / B, which results in that vinyl sulfate and vinylene carbonate cannot play a synergistic role with other components, and cannot form an effective mixed solid electrolyte interface film at the negative electrode interface that can reduce the generation of other side reactions on the interface and gas production on the interface. Therefore, it is not conducive to improving the low-temperature cycle performance of the lithium-ion battery and its anti-overcharge capability under high current conditions.

[0111] In particular, when an appropriate amount of vinyl sulfate and an appropriate amount of vinylene carbonate are added to the electrolyte described in the present application (i.e., an electrolyte containing at least ethyl propionate, propylene carbonate and a first substance), vinyl sulfate, vinylene carbonate and other components can better play a synergistic role, forming an effective mixed solid electrolyte interface film at the negative electrode interface that can reduce the generation of other side reactions on the interface and gas production on the interface, further improving the low-temperature cycle performance of the lithium-ion battery and its anti-overcharge capability under high current conditions.

[0112] In particular, when an appropriate amount of ethyl propionate is added to the electrolyte, it helps to further enhance the overcharge prevention capability of the lithium-ion battery under high current conditions.

[0113] In particular, when an appropriate amount of propylene carbonate is added to the electrolyte, the low-temperature cycle performance of the lithium-ion battery and its ability to prevent overcharge under high current conditions can be further improved.

[0114] In particular, when the contents of vinyl sulfate, vinylene carbonate, ethyl propionate and propylene carbonate in the electrolyte are all within the preferred range, the effect of improving the low-temperature cycle performance of the lithium-ion battery and its anti-overcharge capability under high current conditions is better.

[0115] Table 2

[0116] As can be seen from Table 2, when an appropriate amount of the first substance is added to the electrolyte, and the type and content of the first substance are both within the aforementioned ranges, it is possible to improve the low-temperature cycling performance of the lithium-ion battery while further improving its thermal safety performance. In particular, when the type and content of the first substance are within the preferred range, the effect of improving the low-temperature cycling performance and thermal safety performance of the lithium-ion battery is even greater.

[0117] Table 3

[0118] As can be seen from Table 3, when the positive electrode active material layer contains lithium iron phosphate with an olivine crystal structure and the contents of the lithium iron phosphate, conductive agent, and binder are all within suitable ranges, the rate performance of the lithium-ion battery at high temperatures can be further improved, and the float charge thickness expansion rate of the lithium-ion battery can be reduced, thereby improving its float charge performance. In particular, when the contents of the lithium iron phosphate, conductive agent, and binder are all within the preferred range, the effect of improving the performance of the lithium-ion battery is even greater.

[0119] 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 and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. An electrolyte, characterized in that: The electrolyte comprises vinyl sulfate and vinylene carbonate, wherein the mass percentage of the vinyl sulfate is A%, and the mass percentage of the vinylene carbonate is B%, based on the mass of the electrolyte, and 0.1≤A / B≤4; The electrolyte comprises at least ethyl propionate and propylene carbonate; The electrolyte further includes a first substance, wherein the first substance is selected from at least two of lithium difluorophosphate, lithium difluorooxalatoborate, lithium bis(trifluoromethanesulfonylimide) or lithium hexafluorophosphate, and at least includes lithium difluorophosphate; Based on the mass of the electrolyte, the mass percentage of the first substance is 0.3 wt % to 1.2 wt %.

2. The electrolyte according to claim 1, characterized in that The electrolyte satisfies: 0.2≤A / B≤3.

3. The electrolyte according to claim 1 or 2, characterized in that The electrolyte satisfies at least one of the following conditions: (1) 0.2≤A≤0.5; (2) 0.5≤B≤1.

4. The electrolyte according to claim 1 or 2, characterized in that Based on the mass of the electrolyte, the mass percentage of the ethyl propionate is C%, the sum of the mass percentages of the ethyl propionate and the vinyl sulfate is W%, and 8.6≤W≤15.

5.

5. The electrolyte according to claim 1 or 2, characterized in that Based on the mass of the electrolyte, the mass percentage of the propylene carbonate is D%, the sum of the mass percentages of the propylene carbonate and the vinylene carbonate is W′%, and 3.7≤W′≤12.

5.

6. The electrolyte according to claim 1 or 2, characterized in that Based on the mass of the electrolyte, the mass percentage of the first substance is 0.4wt% to 2wt%; The electrolyte also includes diethyl carbonate and propyl propionate, and the mass ratio of ethyl propionate, propylene carbonate, diethyl carbonate and propyl propionate is (4-8):(10-14):(40-44):(38-42).

7. The electrolyte according to claim 1, characterized in that The first substance at least includes lithium bis(trifluoromethanesulfonyl)imide, and the sum of the mass percentages of the lithium difluorophosphate and the lithium bis(trifluoromethanesulfonyl)imide is in the range of 0.4 wt % to 0.5 wt %; and / or, The mass ratio of the lithium difluorophosphate to the lithium bis(trifluoromethanesulfonyl)imide is 1:5-1.

8. A secondary battery, characterized in that: The secondary battery comprises the electrolyte according to any one of claims 1 to 7.

9. The secondary battery according to claim 8, characterized in that The secondary battery further comprises a positive electrode plate, wherein the positive electrode plate comprises a positive electrode current collector and a positive electrode material layer disposed on at least one side surface of the positive electrode current collector; The positive electrode material layer includes at least one of first material particles, second material particles or inorganic ceramic particles, and at least contains the first material particles; The first material particles are lithium transition metal phosphate particles having an olivine-type crystal structure, and the second material particles are lithium manganese composite oxide particles having a layered crystal structure.

10. The secondary battery according to claim 9, wherein The positive electrode material layer further includes a conductive agent and a binder; The first material particles include lithium iron phosphate; Based on the mass of the positive electrode material layer, the mass percentage of the lithium iron phosphate is 75wt% to 88wt%, the mass percentage of the binder is 6wt% to 15wt%, and the mass percentage of the conductive agent is 2wt% to 10wt%.

11. The secondary battery according to claim 9, wherein The compaction density of the positive electrode material layer is P g / cm 3 , 2.8≤P≤4.

0.

12. The secondary battery according to claim 8, wherein The secondary battery further includes a separator; The isolation film includes a base film and a functional coating provided on at least one side surface of the base film; The functional coating contains polymer particles, and the swelling degree of the polymer particles after being placed in the electrolyte at 90° C. for 20 hours is less than 45%.

13. An electronic device, characterized in that: The electronic device includes the secondary battery according to any one of claims 8 to 12.