Electrolyte and lithium-ion battery

Abstract

The present application relates to an electrolyte and a lithium-ion battery. The electrolyte includes a solvent and a nitrogen-containing additive, wherein the solvent includes a fluorinated organic solvent; in the general structural formula of the nitrogen-containing additive, at least one of R1, R2, R3, R4, and R5 is selected from a cyano group, an isocyanate group, or a nitro group; in the general structural formula of the fluorinated organic solvent, Z1 is selected from a hydrocarbon group containing an oxygen atom or substituted with 1-6 fluorine atoms and having 1-5 carbon atoms, and Z2 is selected from a hydrocarbon group substituted with 1-6 fluorine atoms or unsubstituted with 1-5 carbon atoms, and at least one of Z1 and Z2 contains a fluorine atom. The solution provided in the present application can improve the thermal shock pass rate, high-temperature cycle performance, and safety performance of the battery, and reduce the occurrence of safety accidents such as combustion and explosion of the battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to an electrolyte and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries are widely used in electric vehicles, mobile phones, laptops and other fields due to their advantages such as high energy density, long cycle life and low self-discharge rate.

[0003] Lithium-ion batteries generate heat during the charging and discharging process. If the heat generated exceeds the battery's heat dissipation capacity, the lithium-ion battery will overheat and the battery material will undergo destructive side reactions such as SEI film decomposition, electrolyte decomposition, positive electrode decomposition, reaction between the negative electrode and the electrolyte, and reaction between the negative electrode and the adhesive, which will deteriorate the battery's cycle performance and even cause safety problems.

[0004] Therefore, how to improve the high-temperature cycle performance of lithium-ion batteries has become an urgent problem that needs to be solved. Summary of the Invention

[0005] In order to solve or partially solve the problems existing in the related art, the present application provides an electrolyte and a lithium-ion battery, which can improve the battery's thermal shock pass rate, high-temperature cycle performance and safety performance, and reduce the occurrence of safety accidents such as battery combustion and explosion.

[0006] In a first aspect, the present application provides an electrolyte,

[0007] It includes a solvent and a nitrogen-containing additive, wherein the solvent includes a fluorinated organic solvent; the general structural formula of the nitrogen-containing additive is as follows:

[0008]

[0009] wherein at least one of R1, R2, R3, R4, and R5 is selected from cyano, isocyanate, or nitro;

[0010] The general structural formula of the fluorinated organic solvent is shown below:

[0011]

[0012] wherein Z1 is selected from a hydrocarbon group containing an oxygen atom or substituted by 1 to 6 fluorine atoms and having 1 to 5 carbon atoms, Z2 is selected from a hydrocarbon group substituted by 1 to 6 fluorine atoms or unsubstituted hydrocarbon group having 1 to 5 carbon atoms, and at least one of Z1 and Z2 contains a fluorine atom.

[0013] In the present application, the fluorine atoms in the fluorinated organic solvent have strong electronegativity and weak polarity, which makes the fluorinated organic solvent have higher oxidation resistance and thermal stability. Therefore, when the fluorinated organic solvent is added to the electrolyte, the thermal shock pass rate of the battery can be greatly improved. At the same time, due to the large viscosity of the fluorinated organic solvent, it is not conducive to the formation of a stable SEI (negative electrode electrolyte interface) layer and CEI (positive electrode electrolyte interface) layer on the electrode surface. During the battery cycle, the SEI layer and the CEI layer will continue to break and repair, resulting in continuous consumption of the electrolyte, while reducing the coulombic efficiency of the battery and worsening the cycle performance of the battery. Therefore, after adding the nitrogen-containing compound of the nitrogen-containing additive described in the present application to the electrolyte, the coordination number of the fluorinated organic solvent in the solvation structure can be effectively reduced, thereby reducing its adverse effect on the electrode interface, inhibiting the growth of Li dendrites and reducing the continuous consumption of the electrolyte. At the same time, the LUMO (Lowest Unoccupied Molecular Orbital) -HOMO (Highest Occupied Molecular Orbital) energy level difference of the nitrogen-containing additive is small, so that it is preferentially decomposed at the electrode interface to form an inorganic-rich SEI layer and CEI layer, which can improve the stability of the interface and thus improve the electrochemical performance of the battery. In addition, the nitro, cyano or isocyanate functional groups in the nitrogen-containing additive can capture trace moisture and acidic byproducts such as HF in the electrolyte, reducing the damage of the electrolyte decomposition products to the electrode interface. Therefore, the nitrogen-containing additives described in this application and the fluorinated organic solvents can play a synergistic role when used in combination, significantly improving the thermal shock pass rate of the battery and improving the high-temperature cycle performance of the battery.

[0014] At the same time, the selection of the Z1 and Z2 groups of the fluorinated organic solvent ensures that the chain length of the fluorinated organic solvent is not too long, so that the overall viscosity of the fluorinated organic solvent and the electrolyte system can be controlled, which is beneficial to the transmission of lithium ions in the electrolyte system and can improve the comprehensive electrochemical properties of the battery, such as the discharge performance and cycle performance.

[0015] In some embodiments of the present application,

[0016] The nitrogen-containing additive is selected from the following compounds:

[0017]

[0018] And / or, the fluorinated organic solvent is selected from the following compounds:

[0019]

[0020] The nitrogen-containing additives in this application, whose R1, R2, R3, R4, and R5 can all be selected from cyano, isocyanate or nitro groups, are preferably substituted with a para-structure. The para-structure has a trifluoroacetic acid group that is relatively far away from the nitrogen-containing functional group relative to the ortho-structure, and the interaction force between the steric hindrance small molecules is large, which is more conducive to the reaction. The trifluoroacetic acid group has a strong electron-withdrawing induction effect and lipophilicity, and it contains a stable fluorocarbon structure, which is more conducive to the formation of the electrolyte interface layer CEI layer and SEI layer at the electrode interface; at the same time, the compound of the para-structure has a lower melting and boiling point than the ortho-structure, a smaller viscosity, and better battery kinetics, which is conducive to the transmission of lithium ions in the electrolyte system. The fluorine atoms in the fluorinated organic solvent are distributed in Z1 or Z2, which can avoid the fluorinated organic solvent and the electrolyte system from having too high a viscosity, so that the battery has a good thermal shock pass rate and can avoid seriously deteriorating the cycle performance of the battery.

[0021] In some embodiments of the present application, the nitrogen-containing additive accounts for W1% by weight of the electrolyte, with 0.1≤W1≤5, and the fluorinated organic solvent accounts for W2% by weight of the electrolyte, with 5≤W2≤30. When the nitrogen-containing additive and the fluorinated organic solvent are used together, if the content of the nitrogen-containing additive or the fluorinated organic solvent is too low, they cannot play their role in the electrolyte. For example, when the content of the fluorinated organic solvent is too low, the thermal shock pass rate of the battery is difficult to improve. When the content of the nitrogen-containing additive is too low, it only plays the role of coordinating the coordination number of the fluorinated solvent and cannot play the role of reducing the adverse effects of the fluorinated organic solvent on the interface. When the content is too high, the overall viscosity of the electrolyte is increased, resulting in greater resistance to the movement of lithium ions in the electrolyte, reduced diffusion capacity, and affecting the discharge rate performance of the lithium-ion battery. When the content of the nitrogen-containing additive is too high, it will also increase the electrode surface impedance and deteriorate the battery cycle performance. Therefore, limiting the content of the fluorinated organic solvent and the nitrogen-containing additive to the above range can effectively improve the overall performance of the battery while controlling production costs.

[0022] In some embodiments of the present application, the mass ratio of the nitrogen-containing additive to the fluorinated organic solvent in the electrolyte is 0.2≤W1 / W2≤1. When the nitrogen-containing additive and the fluorinated organic solvent are used in combination, the mass ratio of the nitrogen-containing additive to the fluorinated organic solvent needs to be within a certain range so that the nitrogen-containing additive can better coordinate the coordination number of the fluorinated solvent, reduce the adverse effects of the fluorinated organic solvent on the interface, and thus significantly improve the cycle performance of the battery.

[0023] In some embodiments of the present application, the solvent further comprises at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, and tetrahydrofuran. The electrolyte solvent system comprising a fluorinated organic solvent and at least one of the aforementioned solvents has superior properties to a single solvent, can overcome the temperature limitation of lithium-ion batteries, and can maintain good cycle performance even under high-temperature conditions.

[0024] The second aspect of the present application provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises a positive electrode collector and a positive electrode material layer arranged on the surface of the positive electrode collector, the negative electrode sheet comprises a negative electrode collector and a negative electrode material layer arranged on the surface of the negative electrode collector, and the electrolyte is the electrolyte described in the first aspect of the present application.

[0025] The lithium-ion battery of the present application uses an electrolyte composed of nitrogen-containing additives (nitrogen-containing compounds) and fluorinated organic solvents. The fluorinated organic solvents help improve the battery's thermal shock pass rate, improve the battery's high-temperature cycle performance and safety performance, and the nitrogen-containing additives can effectively reduce the coordination number of the fluorinated organic solvent in the solvent structure, thereby reducing the adverse effects of the fluorinated organic solvent on the positive and negative electrode interfaces. At the same time, it can reduce the damage to the electrode interface caused by electrolyte decomposition products, improve the stability of the interface, and improve the high-temperature cycle performance of the battery.

[0026] In some embodiments of the present application, the positive electrode material layer includes a first material layer and a second material layer, wherein the first material layer is disposed between the positive electrode current collector and the second material layer. The thickness of the first material layer is D1 μm, and the thickness of the second material layer is D2 μm, wherein 5 ≤ D1 ≤ 30, and 20 ≤ W2 / (D1 / D2) ≤ 50. The coordinated arrangement of the first and second material layers can synergistically alleviate electrode expansion stress with the electrolyte, inhibit collapse of the positive electrode material layer structure, and prevent exposure of the positive electrode active material, thereby imparting good thermal stability to the electrode and improving the battery's thermal shock resistance and high-temperature storage performance. Furthermore, the arrangement can effectively inhibit the continued decomposition of the electrolyte, resulting in excellent safety and dynamic performance of the electrode plate.

[0027] The first material layer of the positive electrode plate acts like a safety primer, effectively improving the safety performance of the battery. However, if the thickness of the first material layer is too thick, there is a risk of increased manufacturing costs and low battery cell capacity. Therefore, limiting the thickness of the first material layer to the above range can not only improve the safety performance of the battery, but also enable the battery to maintain an appropriate battery cell capacity. When the fluorinated organic solvent and the first material layer are present at the same time, the thermal shock and high-temperature cycle performance of the battery can be significantly improved. However, as the thickness of the first material layer increases, not only will the manufacturing cost of the battery increase, but the battery capacity will also be reduced. Therefore, when the thickness of the fluorinated organic solvent and the first and second material layers is limited to the above ratio range, the overall performance of the battery is optimal.

[0028] In some embodiments of the present application, the first material layer contains a first active material, and the second material layer contains a second active material, the first active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate or lithium titanate, and the second active material includes at least one of lithium cobaltate, lithium iron phosphate, lithium manganese oxide, lithium nickelate, lithium nickel cobalt manganese oxide or lithium-rich manganese-based material.

[0029] In some embodiments of the present application, the aluminum content of the second active material is X ppm, where 4000 ≤ X ≤ 12000, and 0.1 ≤ W1*1000 / X ≤ 0.2. The aluminum in the second active material can increase the specific surface area and porosity of the positive electrode, promote the diffusion of lithium ions, stabilize the structural stability of the positive electrode, and thus improve the energy storage efficiency, thermal shock, and high-temperature cycle performance of the battery. However, excessive aluminum content will increase the cost of battery manufacturing, while causing low cell capacity, and is not conducive to the structure and performance of the CEI layer on the surface of the positive electrode, thereby worsening the cycle performance of the battery. Therefore, limiting the aluminum content in the second active material within the above-mentioned ratio range can not only save material costs, but also improve the thermal shock and high-temperature cycle performance of the battery and improve the safety performance of the battery; and nitrogen-containing additives can preferentially decompose at the positive electrode interface to form a CEI layer rich in inorganic substances, thereby improving the stability of the positive electrode interface and effectively compensating for the adverse effects of the aluminum content on the CEI layer of the positive electrode sheet; at the same time, when the additive content is too high, it will also increase the positive electrode surface impedance. Therefore, when the content of nitrogen-containing additives in the electrolyte and the aluminum content in the second active material are limited within the above-mentioned ratio range, the comprehensive performance of the battery is optimal.

[0030] In some embodiments of the present application, the total nitrogen content in the positive and negative electrode sheets is no less than 100 ppm. The nitrogen-containing additives (nitrogen-containing compounds) in the electrolyte can form an inorganic-rich electrolyte interface layer on the surfaces of both the positive and negative electrode sheets after battery formation, thereby increasing the stability of the interface and thus improving the electrochemical performance of the battery.

[0031] The technical solution provided in this application may include the following beneficial effects: through the combined use of nitrogen-containing additives and fluorinated organic solvents in the electrolyte, the good thermal stability and oxidation resistance of the fluorinated solvent can effectively improve the thermal shock pass rate of the battery and improve the safety performance of the battery; and nitrogen-containing compounds as nitrogen-containing additives can effectively reduce the coordination number of the fluorinated organic solvent in the solvent, thereby reducing the adverse effects of the fluorinated organic solvent on the electrode interface; and the nitrogen-containing additives can preferentially decompose at the electrode interface to form an electrolyte interface layer rich in inorganic substances, thereby effectively improving the interface stability and improving the high-temperature cycle performance of the battery.

[0032] Furthermore, by combining nitrogen-containing additives in the electrolyte and aluminum elements in the positive electrode active material, and combining the first material layer of the positive electrode and a fluorinated organic solvent, the problems of low cell capacity and increased material costs caused by the aluminum element and the first material layer can be effectively improved, thereby improving the comprehensive electrochemical performance of the battery and saving manufacturing costs. DETAILED DESCRIPTION

[0033] To facilitate understanding of the present invention, the present invention will be described in detail below. However, before describing the present invention in detail, it should be understood that the present invention is not limited to the specific embodiments described. It should also be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to be limiting.

[0034] Where a range of values ​​is provided, it is understood that each intervening value between the upper and lower limits of the range and any other specified or intervening values ​​in the specified range is encompassed within the present invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the present invention, subject to any express exclusions in the specified range. Where a specified range includes one or both limits, ranges excluding either or both of those included limits are also encompassed within the present invention.

[0035] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although any methods and materials or equivalents to those described herein may also be used in the practice or testing of the present invention, preferred methods and materials are now described.

[0036] Lithium-ion batteries generate heat during charging and discharging. If the heat generated exceeds the battery's heat dissipation capacity, the battery will overheat, and the battery materials will undergo destructive side reactions such as SEI film decomposition, electrolyte decomposition, positive electrode decomposition, negative electrode-electrolyte reaction, and negative electrode-binder reaction, which will deteriorate the battery's cycle performance and even cause safety issues. This application uses nitrogen-containing additives and fluorinated organic solvents in combination to significantly improve the battery's thermal shock resistance and high-temperature cycle performance.

[0037] In a first aspect, the present application provides an electrolyte solution comprising a solvent and a nitrogen-containing additive, wherein the solvent comprises a fluorinated organic solvent; and the general structural formula of the nitrogen-containing additive is as follows:

[0038]

[0039] wherein at least one of R1, R2, R3, R4, and R5 is selected from cyano, isocyanate, or nitro;

[0040] The general structural formula of fluorinated organic solvents is shown below:

[0041]

[0042] Wherein Z1 is selected from an alkane group or an alkene group having 1 to 5 carbon atoms and containing an oxygen atom or substituted by 1 to 6 fluorine atoms, Z2 is selected from an alkane group or an alkene group having 1 to 5 carbon atoms and substituted by 1 to 6 fluorine atoms or unsubstituted, and at least one of Z1 and Z2 contains a fluorine atom.

[0043] In some preferred embodiments, the nitrogen-containing additive is selected from the following compounds:

[0044]

[0045] In some preferred embodiments, the fluorinated organic solvent is selected from the following compounds:

[0046]

[0047] In some optional embodiments, the mass proportion of nitrogen-containing additives in the electrolyte is W1%, 0.01≤W1≤10, for example, it can be 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 8%, 10%, etc., or it can be any other value within the above range.

[0048] In some preferred embodiments, the mass proportion of the nitrogen-containing additive in the electrolyte is W1% to 0.1% to 5%.

[0049] In some optional embodiments, the mass proportion of the fluorinated organic solvent in the electrolyte is W2%, 1≤W2≤40, for example, it can be 1%, 5%, 10%, 15%, 20%, 25%, 30%, 40%, 5%, etc., or it can be any other value within the above range.

[0050] In some preferred embodiments, the mass proportion W2% of the fluorinated organic solvent in the electrolyte is 5% to 30%.

[0051] In some preferred embodiments, the mass ratio of the nitrogen-containing additive to the fluorinated organic solvent in the electrolyte is 0.2≤W1 / W2≤1.

[0052] In some optional embodiments, the solvent further comprises at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, and tetrahydrofuran.

[0053] In some optional embodiments, the electrolyte further includes a lithium salt.

[0054] In some optional embodiments, the concentration of the lithium salt in the electrolyte is 0.5 mol / L to 2 mol / L, for example, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.5 mol / L, or 2 mol / L, or any other value within the above range. When the lithium salt concentration is too low, the conductivity of the electrolyte is low, which affects the rate and cycle performance of the entire battery system. When the lithium salt concentration is too high, the viscosity of the electrolyte is too high, which is also not conducive to improving the rate performance of the entire battery system.

[0055] In some preferred embodiments, the concentration of lithium salt in the electrolyte is 0.8 mol / L to 2 mol / L; more preferably, it is 0.9 mol / L to 1.3 mol / L.

[0056] In some optional embodiments, the lithium salt can be selected from one or more of organic electrolyte salts and inorganic electrolyte salts, such as LiPF6, LiBF4, LiSbF6, LiAsF6, LiTaF6, LiAlCl4, Li2B 10 Cl 10 、Li2B 10 F 10 , LiClO4, LiCF3SO3, etc.; it can also be a lithium salt of chelated orthoborate and chelated orthophosphate, such as lithium bis(oxalatoborate) [LiB(C2O4) 2 ]、Lithium bis(malonate borate)[LiB(O2CCH2CO2) 2 ]、Lithium bis(difluoromalonate)borate [LiB(O2CCF2CO2) 2 ]、lithium (malonate oxalate) borate [LiB(C2O4)(O2CCH2CO2)]、lithium (difluoromalonate oxalate) borate [LiB(C2O4)(O2CCF2CO2)]、lithium trioxalophosphate [LiP(C2O4) 3 ] and lithium tris(difluoromalonate)phosphate [LiP(O2CCF2CO2) 3 ].

[0057] The lithium salt in the electrolyte of the present application can be selected from any one, or a combination of any two or more of the above.

[0058] In some optional embodiments, the electrolyte also includes other additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain performance of the battery, such as additives that improve high temperature performance, additives that improve low temperature performance of the battery, and additives that improve overcharge performance of the battery.

[0059] In some optional embodiments, negative electrode film-forming additives, i.e., other additives that promote the formation of SEI film, include but are not limited to vinylene carbonate and its derivatives, ethylene carbonate derivatives having non-conjugated unsaturated bonds in their side chains, cyclic carbonates substituted by halogens, and salts of chelated orthoborates and chelated orthophosphates.

[0060] In some optional embodiments, the other additives include one or more of vinylene carbonate, vinyl ethylene carbonate, methylene vinyl carbonate, fluoroethylene carbonate, trifluoromethylethylene carbonate, and bisfluoroethylene carbonate.

[0061] The second aspect of the present application provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the electrolyte is the electrolyte described in the first aspect of the present application. During the battery's charge and discharge process, active ions are intercalated and released between the positive and negative electrode sheets. The separator is disposed between the positive and negative electrode sheets to provide isolation, while the electrolyte conducts ions between the positive and negative electrode sheets.

[0062] In some optional embodiments, the positive electrode plate includes a positive electrode current collector and a positive electrode material layer disposed on the surface of the positive electrode current collector. The positive electrode current collector can be a conventional metal foil or a composite current collector, such as aluminum foil.

[0063] In some optional embodiments, the positive electrode material layer includes a first material layer and a second material layer, the first material layer is arranged between the positive electrode current collector and the second material layer, the thickness of the first material layer is D1 μm, and the thickness of the second material layer is D2 μm, wherein 5≤D1≤30, 20≤W2 / (D1 / D2)≤50.

[0064] In some optional embodiments, the first material layer includes a first active material, the second material layer includes a second active material, and the first active material and the second active material constitute the positive electrode active material of the positive electrode material layer. The first active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, or lithium titanate, and the second active material includes at least one of lithium cobaltate, lithium iron phosphate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, or a lithium-rich manganese-based material.

[0065] In some preferred embodiments, the aluminum content in the second active material is X ppm, wherein 4000≤X≤12000, and 0.1≤W1*1000 / X≤0.2.

[0066] In some optional embodiments, the positive electrode material may further include a binder, a conductive agent or other optional auxiliary agents. The specific types of the binder, conductive agent and other optional auxiliary agents in this application are not limited, and the binder, conductive agent or other optional auxiliary agents known in the art that can be used for the positive electrode material can be used. In some specific embodiments, as an example, the binder can be selected from at least one of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB); the conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, SuperP (SP), graphene and carbon nanofibers.

[0067] In some embodiments, the preparation of the positive electrode sheet includes the following steps:

[0068] (1) mixing a first active material, a conductive agent, and a binder, and then adding a solvent, stirring and mixing to form a first positive electrode slurry, coating the first positive electrode slurry on a positive electrode current collector, and drying to form a first material layer, which serves as a carrier for a second material layer;

[0069] (2) Mixing the second positive electrode active material, the conductive agent, and the binder, and then adding a solvent, stirring and mixing to form a second positive electrode slurry, and coating the second positive electrode slurry on the first material layer to form a second material layer; after drying, cold pressing, cutting, slitting and other processes, the positive electrode sheet is obtained.

[0070] In some optional embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on the surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent. The specific types of the negative electrode current collector, negative electrode active material, negative electrode binder, and negative electrode conductive agent in this application are not limited and can be any material known in the art for negative electrode materials without limitation.

[0071] As an example, the negative electrode current collector can be selected from a metal foil or a composite current collector, for example, it can be selected from copper foil; the negative electrode active material can be selected from graphite and / or silicon, such as natural graphite, artificial graphite, mesophase microcarbon beads (abbreviated as MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structured lithiated TiO2-Li4Ti5O12 , Li-Al alloy; the negative electrode binder can be selected from at least one of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB); the negative electrode conductive agent can be selected from at least one of conductive carbon black, acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers, and superconducting carbon.

[0072] In some optional embodiments, the total content of nitrogen in the positive electrode sheet and the negative electrode sheet is not less than 100 ppm.

[0073] In some embodiments, the separator of the present application can be selected from any known porous separator with good chemical and mechanical stability. The separator can be made of at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0074] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0075] The batteries disclosed in the embodiments of this application can be used in electrical devices that use batteries as power sources or in various energy storage systems that use batteries as energy storage elements. Electrical devices include, but are not limited to, mobile phones, tablets, computers, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, and the like.

[0076] To make the present invention easier to understand, the present invention will be further described in detail below with reference to the following examples. These examples are merely illustrative and do not limit the scope of application of the present invention. Unless otherwise specified, the raw materials or components used in the present invention can be obtained through commercial channels or conventional methods.

[0077] Example 1

[0078] (1) Preparation of electrolyte

[0079] Ethylene carbonate EC, propylene carbonate PC, diethyl carbonate DEC, and propyl propionate PP were mixed in a mass ratio of 1:1:1:1 to serve as an organic solvent; additives and a fluorinated organic solvent were added to the organic solvent in the mass percentages shown in Example 1 in Table 1, and after uniform mixing, LiPF6 was added to obtain an electrolyte with a LiPF6 concentration of 1.1 mol / L.

[0080] (2) Preparation of positive electrode sheet

[0081] First material layer: Mix the first active material, lithium iron phosphate (LiFePO4), the conductive agent, carbon nanotubes (CNT), and the binder, polyvinylidene fluoride, in a mass ratio of 95:2:3. Add N-methylpyrrolidone (NMP) and stir in a vacuum mixer until the system forms a uniform positive electrode slurry. Then, evenly coat the positive electrode slurry on the positive electrode current collector aluminum foil and dry at 85°C to form the first material layer, which serves as a carrier for the second material layer. (Thickness of the first material layer D1 = 5μm)

[0082] Second material layer: The second active material lithium cobalt oxide LiCoO2 (aluminum content is 8000ppm), the conductive agent carbon nanotubes (CNT), and the binder polyvinylidene fluoride are mixed in a mass ratio of 95:2:3, and N-methylpyrrolidone (NMP) is added. The mixture is stirred under the action of a vacuum mixer until the system becomes a uniform positive electrode slurry, and then the positive electrode slurry is evenly coated on the first material layer to form a second material layer (the thickness of the second material layer D2 = 100μm); after drying at 85°C, it is cold pressed, cut into pieces, and slit, and then dried under vacuum conditions at 85°C for 4h to obtain a positive electrode sheet.

[0083] (3) Preparation of negative electrode sheet

[0084] Graphite (negative electrode active material), acetylene black (conductive agent), styrene-butadiene rubber (binder), and sodium carboxymethyl cellulose (thickener) were thoroughly mixed in a suitable amount of deionized water at a mass ratio of 96:1.2:1.5:1.3 to form a uniform negative electrode slurry. This negative electrode slurry was then applied to the negative electrode current collector copper foil, dried, and cold pressed to obtain a negative electrode sheet.

[0085] (4) Preparation of lithium-ion batteries

[0086] PE porous polymer film is used as the separator.

[0087] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrodes to provide isolation. The stacked electrode sheets and separator are then wound to form a core. The core is placed in a pre-punched aluminum-plastic film bag. The prepared electrolyte is then injected into the baked and dried battery cells. After vacuum packaging, resting, and formation, the lithium-ion battery is complete.

[0088] Examples 2 to 18 and Comparative Examples 1 to 5 use the same method as Example 1, the difference being the types and contents of the additives and organic solvents in the electrolyte. Specific differences can be found in Table 1.

[0089] Battery testing and analysis

[0090] (1) 45℃ cycle test:

[0091] The test method is as follows: In a constant temperature chamber at 45°C ± 2°C, the lithium-ion battery is charged at a constant current and constant voltage of 1C to 4.5V, the cutoff current is 0.05C, and then discharged at 1C to 3V. Multiple charge and discharge cycles are performed under these conditions. The capacity retention rate of each battery after 800 cycles is calculated. Each group of five batteries has an average capacity retention rate after each cycle and is recorded in Table 1.

[0092] The calculation formula is: Capacity retention rate (%) = discharge capacity corresponding to the number of cycles (mAh) / discharge capacity of the third cycle (mAh) * 100%

[0093] (2) Thermal shock test:

[0094] Test method: Ten batteries were discharged at 1C to 3.0V at room temperature, then charged at 1C constant current and constant voltage to 4.5V, with a cutoff of 0.05C. The fully charged batteries were placed in an oven, and the temperature was raised to 150°C±2°C at a rate of (5°C±2°C) / min and maintained at that temperature for 60 minutes. The batteries passed the test if they did not catch fire or explode. The test results are recorded in Table 1.

[0095] Table 1

[0096]

[0097]

[0098]

[0099]

[0100] Note: “ / ” means not added.

[0101] From the comparison of the data of Comparative Example 1 and Example 1, it can be seen that when the nitrogen-containing additives and fluorinated organic solvents described in this application are added to the electrolyte at the same time, the thermal shock pass rate of the battery increases from 10% to 90%, and the capacity retention rate after 800 cycles at 45°C increases from 52.5% to 65.5%. It can be seen that by adding the nitrogen-containing additives and fluorinated organic solvents described in this application to the electrolyte at the same time, the thermal shock pass rate and high-temperature cycle performance of the lithium-ion battery can be improved.

[0102] From the comparison of the data of Comparative Examples 2-5 and Examples 1-7, it can be seen that after using the nitrogen-containing additives and fluorinated organic solvents described in this application, the thermal shock pass rate of the battery can be maintained at around 90%, and the capacity retention rate after 800 cycles at 45°C is maintained at around 66%. When the nitrogen-containing additives or fluorinated organic solvents described in this application are added alone, or when a combination of the additives and organic solvents described in the comparative examples is used, the thermal shock pass rate is slightly improved compared to Comparative Example 1, and the capacity retention rate after 800 cycles at 45°C is slightly improved. However, both the thermal shock pass rate and the high-temperature cycle capacity retention rate are far inferior to those of Examples 1-7 of this application. It can be seen that the addition of the nitrogen-containing additives and fluorinated organic solvents described in this application to the electrolyte, and the combined use of the two can play a synergistic role, greatly improving the thermal shock pass rate and high-temperature cycle performance of lithium-ion batteries.

[0103] From the data comparison of Example 1 and Examples 8-13, it can be seen that the content of nitrogen-containing additives and fluorinated organic solvents in the electrolyte needs to reach a certain range during the combined use period in order to significantly improve the thermal shock pass rate and high-temperature cycle performance of the battery. When the content of nitrogen-containing additives or fluorinated organic solvents is too low, it is difficult for them to play the role of improving battery performance. When the content of nitrogen-containing additives is too high, it is easy to increase the surface impedance of the electrode. When the content of fluorinated organic solvents is too high, it will increase the viscosity of the electrolyte. Therefore, controlling the mass proportion of nitrogen-containing additives in the electrolyte to 0.1% to 5% and the mass proportion of fluorinated organic solvents in the electrolyte to 5% to 30% has a better effect on improving the thermal shock pass rate and high-temperature cycle performance of the battery.

[0104] Furthermore, based on the data comparison of Examples 14-18, it can be seen that when the mass ratio of the nitrogen-containing additive to the fluorinated organic solvent is within a certain range, preferably 0.2 to 1, the improvement effect of the battery's thermal shock pass rate and high-temperature cycle performance is optimal.

[0105] Samples 1-11

[0106] Eleven lithium-ion battery samples were prepared using the same preparation method as in Example 1. The differences between the samples were that the thickness D1 of the first material layer of the positive electrode, the aluminum content X in the second positive electrode material, and the content of the fluorinated organic solvent were adjusted respectively. The negative electrode active material used was a mixture of graphite and Si, and the Si content in the negative electrode active material and the composition and content of the binder in the negative electrode material layer were adjusted. The specific parameter adjustments can be found in Table 2.

[0107] Samples 1-11 were subjected to a 45°C cycle test and a thermal shock test. The specific test methods can be found in Example 1, and the test results can be found in Table 2.

[0108] Table 2

[0109]

[0110]

[0111] The data of samples 1-6 show that as the aluminum content in the second active material increases, the thermal shock pass rate and high-temperature cycle performance of the battery are improved, but too high an aluminum content will affect the capacity of the battery, increase the material cost, and affect the structure and performance of the positive electrode CEI layer. The improvement in the thermal shock pass rate of the battery is no longer significant and may even worsen the battery's cycle performance. Nitrogen-containing additives can form a stable CEI film at the positive electrode and can effectively make up for this defect. Therefore, when the nitrogen-containing additive content and the aluminum content in the second active material are within a certain proportion range (0.1≤W1*1000 / X≤0.2), the overall performance improvement effect of the battery is best.

[0112] The data of samples 7-11 show that as the thickness of the first material layer increases, the thermal shock pass rate and high-temperature cycle performance of the battery are improved, but the increase in the thickness of the first material layer will increase the manufacturing cost of the battery and cause the battery capacity to be low. When the thickness of the first material layer continues to increase and reaches a certain upper limit, the improvement in high-temperature cycle is no longer significant. Therefore, when the thickness of the fluorinated solvent and the first material layer is within a certain ratio range (20≤W2 / (D1 / D2)≤50), the overall performance improvement of the battery is best.

[0113] It should be noted that the embodiments described above are only used to explain the present application and do not constitute any limitation to the present application. The present application has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words, rather than restrictive words. The present application may be modified as specified within the scope of the claims of the present application, and the present application may be revised without departing from the scope and spirit of the present application. Although the present application described therein relates to specific methods, materials and embodiments, it does not mean that the present application is limited to the specific examples disclosed therein. On the contrary, the present application can be extended to all other methods and applications with the same function.

Claims

1. An electrolyte, characterized in that: It includes a solvent and a nitrogen-containing additive, wherein the solvent includes a fluorinated organic solvent; the general structural formula of the nitrogen-containing additive is as follows: ； wherein at least one of R1, R2, R3, R4, and R5 is selected from cyano, isocyanate, or nitro; The general structural formula of the fluorinated organic solvent is shown below: ； wherein Z1 is selected from a hydrocarbon group containing an oxygen atom or substituted by 1 to 6 fluorine atoms and having 1 to 5 carbon atoms, Z2 is selected from a hydrocarbon group substituted by 1 to 6 fluorine atoms or unsubstituted hydrocarbon group having 1 to 5 carbon atoms, and at least one of Z1 and Z2 contains a fluorine atom; The mass proportion of the nitrogen-containing additive in the electrolyte is W1%, 0.1≤W1≤5, and the mass proportion of the fluorinated organic solvent in the electrolyte is W2%, 5≤W2≤30; the mass ratio of the nitrogen-containing additive to the fluorinated organic solvent in the electrolyte is 0.2≤W1 / W2≤1.

2. The electrolyte according to claim 1, wherein: The nitrogen-containing additive is selected from one of the following compounds: ； And / or, the fluorinated organic solvent is selected from one of the following compounds: 。 3. The electrolyte according to claim 1, wherein: The solvent further comprises at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, and tetrahydrofuran.

4. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer disposed on the surface of the positive electrode current collector, and the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer disposed on the surface of the negative electrode current collector, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 3.

5. The lithium-ion battery according to claim 4, characterized in that The positive electrode material layer includes a first material layer and a second material layer. The first material layer is arranged between the positive electrode current collector and the second material layer. The thickness of the first material layer is D1 μm, and the thickness of the second material layer is D2 μm. Among them, 5≤D1≤30, 20≤W2 / (D1 / D2)≤50.

6. The lithium-ion battery according to claim 5, characterized in that The first material layer contains a first active material, and the second material layer contains a second active material. The first active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate or lithium titanate, and the second active material includes at least one of lithium cobaltate, lithium iron phosphate, lithium manganese oxide, lithium nickelate, lithium nickel cobalt manganese oxide or lithium-rich manganese-based material.

7. The lithium-ion battery according to claim 6, characterized in that The aluminum content in the second active material is X ppm, wherein 4000≤X≤12000, and 0.1≤W1*1000 / X≤0.

2.

8. The lithium-ion battery according to any one of claims 4 to 7, characterized in that The total content of nitrogen in the positive electrode sheet and the negative electrode sheet is not less than 100 ppm.

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