Lithium secondary battery and electric device

Abstract

The invention provides a lithium secondary battery and a power utilization device. The lithium secondary battery comprises an electrolyte and a negative pole piece, the electrolyte comprises chain carboxylic ester and cyclic carbonate, and the negative pole piece comprises a negative current collector and a negative film layer located on at least one side of the negative current collector; wherein based on the total mass of the electrolyte, the mass content W1 of the chain carbonate is 5%-75%, and the mass content W2 of the cyclic carbonate is 2%-40%; the electrolyte further comprises a lithium salt, the lithium salt comprises lithium bis (fluorosulfonyl) imide LiFSI, and the molar concentration C2 of the lithium bis (fluorosulfonyl) imide LiFSI is 0.3 mol / L to 1 mol / L. The lithium secondary battery has excellent quick charge performance and cycle performance.

Description

[0001] This application is a divisional application based on the invention with application number 202310883089.X, application date July 18, 2023, applicant being Contemporary Amperex Technology Co., Ltd., and invention name being “Lithium Secondary Batteries and Electrical Devices”. Technical Field

[0002] The present application relates to the technical field of lithium batteries, and in particular to a lithium secondary battery and an electrical device. Background Art

[0003] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields. The requirements for the electrical performance of secondary batteries are constantly increasing, and there are also increasingly high expectations for the charging rate of secondary batteries.

[0004] The performance of the electrolyte has a critical impact on the performance of secondary batteries. An electrolyte with the right composition can improve the electrical performance or charging rate of secondary batteries, meeting the different performance requirements of batteries in different fields. Therefore, there is still a need for the research and development of electrolytes to meet the application needs of the next generation of electrochemical systems. Summary of the Invention

[0005] The present application is made in view of the above-mentioned problems, and its purpose is to provide a lithium secondary battery having excellent fast charging performance and cycle performance.

[0006] To achieve the above-mentioned object, the first aspect of the present application provides a lithium secondary battery, comprising an electrolyte and a negative electrode plate, wherein the electrolyte comprises a chain carboxylate and a cyclic carbonate, and the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector;

[0007] Among them, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylate and the mass content W2 of the cyclic carbonate satisfy: 0.1≤W2 / W1≤2, and the mass content W1 of the chain carboxylate and the thickness H of the negative electrode film layer satisfy: 0.2≤H / 100W1≤20, where the unit of H is μm.

[0008] The chain carboxylate introduced into the electrolyte can significantly reduce the viscosity of the electrolyte and improve the wettability of the electrolyte to the negative electrode, thereby improving the liquid phase transmission rate of lithium ions and reducing the migration resistance of lithium ions, significantly improving the fast charging performance of the lithium secondary battery; at the same time, the mass content W1 of the chain carboxylate is controlled to satisfy the above-mentioned relationship with the mass content W2 of the cyclic carbonate and the thickness H of the negative electrode film layer, so as to reduce the side reaction between the metallic lithium and the chain carboxylate on the negative electrode side during the cycle of the lithium secondary battery, and reduce the degree of lithium plating on the negative electrode due to insufficient electrolyte infiltration, so that the lithium secondary battery can have both fast charging performance and cycle performance.

[0009] In any embodiment, the electrolyte further includes a sulfate compound, and based on the total mass of the electrolyte, the mass content W1 of the chain carboxylate and the mass content W3 of the sulfate compound satisfy: 0.005≤W3 / W1≤0.4.

[0010] Sulfate ester compounds can undergo oxidation reaction on the positive electrode side of the lithium secondary battery before the chain carboxylate esters. The oxides are deposited at the interface of the positive electrode to form a CEI film, which can reduce the degree of oxidation reaction of the chain carboxylate ester under high pressure on the positive electrode side, reduce the loss and gas production of the chain carboxylate ester, and further improve the fast charging performance and storage performance of the lithium secondary battery.

[0011] In any embodiment, the chain carboxylic acid ester includes one or more of ethyl formate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, propyl propionate, propyl acetate, butyl formate, and methyl butyrate, and optionally includes one or more of ethyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0012] The above-mentioned chain carboxylate has a lower viscosity due to its main chain carbon number being less than 5, which can significantly reduce the viscosity of the electrolyte, increase the liquid phase transmission rate of lithium ions, and thus increase the conductivity of the electrolyte, which is beneficial to improving the fast charging performance of lithium secondary batteries.

[0013] In any embodiment, the cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, and optionally includes one or more of ethylene carbonate and fluoroethylene carbonate.

[0014] The above-mentioned cyclic carbonates can all be reduced on the negative electrode side to form a passivation layer, thereby reducing the side reaction between the chain carboxylate and metallic lithium and improving the cycle performance and storage performance of the lithium secondary battery.

[0015] In any embodiment, the sulfate ester compound includes the structural compound shown in formula I, formula II, diethyl sulfate, dimethyl sulfate, dihydro-1,3,2-dimethyl sulfate, One or more of thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide,

[0016]

[0017] Wherein, R1 and R2 each independently include hydrogen atoms, C 1-6 The alkyl group, R3 includes C 1-6 Alternatively, the sulfate compounds include 4,4'-disulfate, vinyl sulfate, diethyl sulfate, dimethyl sulfate, dihydro-1,3,2-disulfate, One or more of thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide.

[0018] The above-mentioned sulfate compounds can all participate in the formation of the CEI film on the positive electrode side, reduce the degree of oxidation reaction of the chain carboxylic acid ester, and improve the fast charging performance and storage performance of the battery.

[0019] In any embodiment, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylic acid ester is 5% to 75%, and can be optionally 20% to 75%.

[0020] Controlling the mass content of the chain carboxylate within an appropriate range can not only significantly reduce the viscosity of the electrolyte, increase the liquid phase transmission rate of lithium ions, and thus improve the conductivity of the electrolyte, which is beneficial to improving the fast charging performance of lithium secondary batteries, but also reduce the aggravation of the side reaction between the chain carboxylate and the metallic lithium on the negative electrode side.

[0021] In any embodiment, based on the total mass of the electrolyte, the mass content W2 of the cyclic carbonate is 5% to 40%, and can be optionally 10% to 30%.

[0022] Controlling the mass content of cyclic carbonate within an appropriate range can not only provide sufficient cyclic carbonate to dissociate the lithium salt in the electrolyte and participate in the formation of a passivation layer on the surface of the negative electrode, thereby improving the conductivity of the electrolyte, but also reduce the effect of introducing too much cyclic carbonate on the viscosity of the electrolyte, thereby comprehensively improving the fast charging performance of the lithium secondary battery.

[0023] In any embodiment, based on the total mass of the electrolyte, the mass content W3 of the sulfate compound is 0.1% to 2%, and can be optionally 0.1% to 1%.

[0024] Controlling the mass content of sulfate ester compounds within an appropriate range can not only provide sufficient sulfate ester compounds to participate in the formation of CEI film on the positive electrode side, inhibit the oxidation reaction of carboxylic acid esters, and improve the fast charging performance of lithium secondary batteries, but also prevent the introduction of excessive sulfate ester compounds from increasing the production cost of lithium secondary batteries.

[0025] In any embodiment, the thickness of the negative electrode film layer is 10 μm to 100 μm, and can be optionally 20 μm to 70 μm.

[0026] By controlling the thickness of the negative electrode film layer within an appropriate range, both the fast charging performance and energy density of the battery can be taken into account.

[0027] In any embodiment, the electrolyte further includes a lithium salt, and the lithium salt includes LiPF6 and LiFSI. In any embodiment, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy the following relationship: 0.8 mol / L ≤ C1 + C2 ≤ 1.3 mol / L, and optionally 0.8 mol / L ≤ C1 + C2 ≤ 1.2 mol / L.

[0028] In any embodiment, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy: 1≤C1 / C2≤5, and can be optionally 1≤C1 / C2≤3.

[0029] LiFSI has a high degree of dissociation in the electrolyte, which can significantly improve the room-temperature conductivity of the electrolyte. However, at the same time, LiFSI has the risk of corroding the positive electrode current collector aluminum foil. The dissociation degree of LiPF6 in the electrolyte is not as good as that of LiFSI, but LiPF6 has the effect of passivating the aluminum foil and inhibiting its corrosion. When the molar concentration of LiFSI and the molar concentration of LiPF6 are controlled to meet a certain relationship, the conductivity of the electrolyte can be improved, which is beneficial to improving the fast charging performance of lithium secondary batteries, and the risk of aluminum foil corrosion can be reduced, thereby improving the safety of lithium secondary batteries.

[0030] In any embodiment, the molar concentration C1 of LiPF6 is 0.2 mol / L to 1.2 mol / L, and can be optionally 0.2 mol / L to 1.1 mol / L; in any embodiment, the molar concentration C2 of LiFSI is 0.1 mol / L to 1 mol / L, and can be optionally 0.2 mol / L to 1 mol / L.

[0031] Controlling the molar concentrations of LiFSI and LiPF6 within appropriate ranges helps to balance the fast charging performance and safety of lithium secondary batteries.

[0032] In any embodiment, the electrolyte further includes an additive, and the additive includes one or more of lithium difluorophosphate, lithium fluorosulfonate, trimethylphosphite, vinylene carbonate, lithium tetrafluoroborate, lithium difluorobisoxalatophosphate, tris(trimethylsilyloxy)borate, trimethylfluorosilane, adiponitrile, and succinonitrile, and optionally includes one or more of lithium difluorophosphate, lithium fluorosulfonate, lithium tetrafluoroborate, and lithium difluorooxalatoborate.

[0033] All of the above additives can participate in the formation of the CEI film on the positive electrode side or the SEI film on the negative electrode side, reducing direct contact between the chain carboxylic acid ester and the positive electrode or metallic lithium, thereby reducing the consumption of active lithium and the reductive decomposition of the electrolyte. Lithium difluorophosphate, lithium fluorosulfonate, trimethylphosphite, vinylene carbonate, lithium tetrafluoroborate, or lithium difluorobis(oxalatophosphate) participate in the formation of the CEI film on the positive electrode side, while tris(trimethylsilyl)borate, trimethylfluorosilane, adiponitrile, or succinonitrile participate in the formation of the SEI film on the negative electrode side.

[0034] In any embodiment, the positive electrode sheet in the lithium secondary battery includes a positive electrode active material, and the positive electrode active material includes Li d [Ni x Co y X1 z M1 1-x-y-z ]O 2-S , one or more of LiMn2O4, Li2MnO3·(1-a)LiAO2, LiM2X2O4,

[0035] Wherein, 0.1≤d≤1, 0≤S≤2, X1 includes Mn and / or Al, M1 includes one or more of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Zr, Sr, V, Ti, 0≤x<1, 0≤y≤1, 0≤z≤1, x+y+z≤1; A includes one or more of Ni, Co, Mn, 0<a<1; M2 includes one or more of Fe, Mn, Ni, Co, X2O4 h- Where X2 includes one or more of S, P, As, V, Mo, and W, and h=2 or 3.

[0036] The above-mentioned positive electrode active materials can all be used in combination with an electrolyte to realize a lithium secondary battery with excellent fast charging performance and cycle performance.

[0037] In any embodiment, the negative electrode film layer includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase microcarbon beads, silicon-based materials, and tin-based materials, and optionally includes one or more of artificial graphite and natural graphite.

[0038] The negative electrode film layer having the above-mentioned negative electrode active material can be used in combination with an electrolyte to realize a lithium secondary battery with excellent fast charging performance and cycle performance.

[0039] In any embodiment, the compaction density of the negative electrode film layer is 0.9 g / cm 3 ~1.6g / cm 3 , optional 1.1g / cm 3 ~1.6g / cm 3 .

[0040] Controlling the compaction density of the negative electrode film layer within an appropriate range can not only meet the requirement of sufficient contact between the negative electrode active materials so that the battery has excellent electron transport performance and energy density, but also reduce the risk of the pores between the negative electrode active materials being crushed during the preparation process, which is not conducive to lithium ion insertion and extraction.

[0041] A second aspect of the present application provides an electrical device comprising the lithium secondary battery of the first aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a schematic diagram of a lithium secondary battery according to one embodiment of the present application.

[0043] Figure 2 yes Figure 1 FIG. 1 is an exploded view of a lithium secondary battery according to an embodiment of the present application.

[0044] Figure 3 Schematic diagram of a battery module according to one embodiment of the present application.

[0045] Figure 4 Schematic diagram of a battery pack according to one embodiment of the present application.

[0046] Figure 5 yes Figure 5 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0047] Figure 6 Schematic diagram of an electrical device using a lithium secondary battery as a power source according to one embodiment of the present application.

[0048] Description of reference numerals:

[0049] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 lithium secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly DETAILED DESCRIPTION

[0050] Below, the embodiments of the lithium secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0051] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0052] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0053] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0054] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0055] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0056] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0057] The conductivity of the electrolyte seriously affects the battery performance. For example, for lithium batteries, lower conductivity can lead to adverse phenomena such as reduced fast charging performance of lithium batteries. By introducing low-viscosity solvents into the electrolyte to reduce the viscosity of the electrolyte system, the lithium ion transfer rate is increased, and thus the conductivity of the electrolyte is improved. However, the introduction of low-viscosity solvents will bring other adverse effects. For example, the side reaction between the low-viscosity solvent chain carboxylate and the precipitated metallic lithium will consume part of the metallic lithium and generate gas, which seriously affects the battery's cycle performance and storage performance. Therefore, it is necessary to design a new battery that can take into account both fast charging performance and cycle performance.

[0058] [Lithium secondary battery]

[0059] Based on this, the present application proposes a lithium secondary battery, which includes an electrolyte and a negative electrode plate, wherein the electrolyte includes a chain carboxylate and a cyclic carbonate, and the negative electrode plate includes a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector;

[0060] Among them, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylic acid ester and the mass content W2 of the cyclic carbonate satisfy: 0.1≤W2 / W1≤2, and the mass content W1 of the chain carboxylic acid ester and the thickness H (μm) of the negative electrode film layer satisfy: 0.2≤H / 100W1≤20, where the unit of H is μm.

[0061] As used herein, the term "chain carboxylate" refers to a carboxylate having a main chain carbon number of less than 5. Generally speaking, the longest carbon chain in the chain organic compound is the main chain. Herein, exemplary chain carboxylate includes one or more of ethyl formate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, propyl propionate, propyl acetate, butyl formate, and methyl butyrate, and optionally includes one or more of ethyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0062] Herein, "W2 / W1" refers to the ratio between W2 and W1.

[0063] Herein, “H / 100W1” refers to the ratio of the thickness H to 100 times W1, where the unit of H is μm.

[0064] In some embodiments, the cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, and optionally includes one or more of ethylene carbonate and fluoroethylene carbonate.

[0065] In some embodiments, based on the total mass of the electrolyte, the ratio of the mass content W1 of the linear carboxylate to the mass content W2 of the cyclic carbonate can be selected to be 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 1.1, 1.2, 1.4, 1.5, 1.6, 1.8, 2, or a value in the range consisting of any two of the above points.

[0066] In some embodiments, the mass content W1 of the chain carboxylic acid ester in the electrolyte and the thickness H of the negative electrode film layer, that is, H / 100W1 can be selected as 0.2, 0.4, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a value in the range consisting of any two of the above points.

[0067] Chain carboxylates have low viscosity and low melting point. Introducing chain carboxylates into the electrolyte can significantly reduce its viscosity, which is beneficial to improving the liquid phase transmission rate of lithium ions, thereby improving the electrical conductivity of the electrolyte, and is beneficial to improving the fast charging performance of lithium secondary batteries; however, chain carboxylates are also prone to side reactions with metallic lithium precipitated on the negative electrode side, which not only consumes chain carboxylates and metallic lithium, affecting the cycle performance of the battery, but also produces gas, causing flatulence of the lithium secondary battery, affecting the transmission of lithium ions and the storage performance of the lithium secondary battery.

[0068] The cyclic carbonate added to the electrolyte can be reduced to form a passivation layer on the surface of the negative electrode, reducing the extent of side reactions between the metallic lithium and the chain carboxylate on the negative electrode side, thereby improving the cycling and storage performance of the lithium secondary battery. Controlling the amount of chain carboxylate and cyclic carbonate not only improves the conductivity of the electrolyte but also helps suppress side reactions between the chain carboxylate and metallic lithium, enabling the lithium secondary battery to achieve excellent fast charging performance, cycling performance, and storage performance.

[0069] In addition, in order to improve the energy density of lithium secondary batteries, the coating thickness of the active material of the negative electrode gradually becomes thicker, resulting in a gradual deterioration in the wettability of the electrolyte. Poor electrolyte wetting can lead to a "broken bridge" phenomenon in the transmission of lithium ions. During charging, the areas where the negative electrode plates are not wetted are invalid areas, and lithium deposition may occur in the areas where the negative electrode plates are wetted, resulting in a sharp deterioration in the battery cycle performance and safety performance. However, after the introduction of chain carboxylates into the electrolyte, the reduction in the viscosity of the electrolyte is conducive to improving the wettability of the electrolyte to the negative electrode plates, thereby reducing the migration resistance of lithium ions and improving the fast charging performance of lithium secondary batteries. The thickness of the negative electrode film layer needs to be controlled within an appropriate range to take into account the requirements of energy density and fast charging performance. By controlling the relationship between the mass content of the chain carboxylic acid ester and the thickness of the negative electrode film layer in the negative electrode plate to satisfy the above-mentioned relationship, on the one hand, the electrolyte can quickly infiltrate the negative electrode film layer and improve the migration rate of lithium ions, thereby meeting the fast charging performance requirements of the lithium secondary battery. At the same time, it can reduce the degree of lithium plating on the negative electrode due to insufficient electrolyte infiltration, and reduce the impact of lithium dendrites on the cycle performance and storage performance of the lithium secondary battery. On the other hand, it can also reduce the degree of aggravation of side reactions caused by excessive chain carboxylic acid esters or meet the energy density requirements of the lithium secondary battery.

[0070] In this article, the term "fast charging performance" refers to the capacity retention rate of a lithium secondary battery under high-rate charging conditions.

[0071] In some embodiments, the electrolyte further comprises a sulfate compound, and based on the total mass of the electrolyte, the mass content W1 of the chain carboxylate and the mass content W3 of the sulfate compound satisfy the relationship: 0.005≤W3 / W1≤0.4. In some embodiments, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylate and the mass content W3 of the sulfate compound satisfy W3 / W1, which can be 0.005, 0.006, 0.008, 0.01, 0.02, 0.04, 0.05, 0.06, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, or a value in a range consisting of any two of the foregoing.

[0072] In some embodiments, sulfate compounds include compounds of formula I, formula II, compounds of the structures shown, diethyl sulfate, dimethyl sulfate, dihydro-1,3,2-dimethyl sulfate, One or more of thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide,

[0073]

[0074] Wherein, R1 and R2 each independently include hydrogen atoms, C 1-6 The alkyl group, R3 includes C1-6 The alkyl group, the C 1-6 The alkyl group includes one or more of methyl, ethyl, propyl, isopropyl and n-butyl.

[0075] Optionally, the sulfate compounds include 4,4'-disulfate, vinyl sulfate, diethyl sulfate, dimethyl sulfate, dihydro-1,3,2-disulfate, One or more of thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide.

[0076] In this article, dihydro-1,3,2-dihydro The CAS number of thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide is 496-45-7.

[0077] In this context, when R1 in Formula I is selected from a hydrogen atom, R2 is selected from The obtained substance is 4,4'-vinyl disulfate.

[0078] It is understandable that when a lithium secondary battery is charging, under high voltage, the chain carboxylate is prone to undergo oxidation reaction at the positive electrode plate interface to produce gas, which is the "oxidation gas" phenomenon. This is because the transition metal ions in the positive electrode plate have a catalytic effect, and the chain carboxylate is prone to oxidation reaction under the catalytic action of the transition metal ions when in contact with the positive electrode plate. However, the introduction of sulfate ester compounds can cause sulfate ester compounds to take precedence over chain carboxylate in undergoing oxidation reaction at the positive electrode plate interface. The oxidation products are deposited at the positive electrode plate interface, forming a CEI film. The formation of the CEI film can isolate the chain carboxylate from direct contact with the positive electrode plate, which can reduce the degree of oxidation reaction of the chain carboxylate on the positive electrode plate side, reduce the loss of chain carboxylate and the degree of gas production, and further improve the fast charging performance and storage performance of the lithium secondary battery.

[0079] In some embodiments, the mass content W1 of the linear carboxylate is 5% to 75%, and can be 20% to 75%, based on the total mass of the electrolyte. In some embodiments, the mass content W1 of the linear carboxylate is 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or a value in a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0080] Controlling the mass content of the chain carboxylate within an appropriate range can not only significantly reduce the viscosity of the electrolyte, increase the liquid phase transmission rate of lithium ions, and thus improve the conductivity of the electrolyte, which is beneficial to improving the fast charging performance of lithium secondary batteries, but also reduce the aggravation of the side reaction between the chain carboxylate and the metallic lithium on the negative electrode side.

[0081] In some embodiments, based on the total mass of the electrolyte, the mass content W2 of the cyclic carbonate is 5% to 40%, optionally 10% to 30%. In some embodiments, based on the total mass of the electrolyte, the mass content W2 of the cyclic carbonate can be optionally 5%, 8%, 10%, 12%, 15%, 17%, 20%, 25%, 28%, 30%, 33%, 35%, 38%, 40%, or a value in the range consisting of any two of the above points.

[0082] Controlling the mass content of cyclic carbonate within an appropriate range can not only provide sufficient cyclic carbonate to dissociate the lithium salt in the electrolyte and participate in the formation of a passivation layer on the surface of the negative electrode, thereby improving the conductivity of the electrolyte, but also reduce the effect of introducing too much cyclic carbonate on the viscosity of the electrolyte, thereby comprehensively improving the fast charging performance of the lithium secondary battery.

[0083] In some embodiments, the mass content W3 of the sulfate ester compound is 0.1% to 2%, or optionally 0.1% to 1%, based on the total mass of the electrolyte. In some embodiments, the mass content W3 of the sulfate ester compound is 0.1%, 0.2%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2%, or a value in a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0084] Controlling the mass content of sulfate ester compounds within an appropriate range can not only provide sufficient sulfate ester compounds to participate in the formation of CEI film on the positive electrode side, inhibit the oxidation reaction of carboxylic acid esters, and improve the fast charging performance of lithium secondary batteries, but also prevent the introduction of excessive sulfate ester compounds from increasing the production cost of lithium secondary batteries.

[0085] In some embodiments, the thickness of the negative electrode film layer is 10 μm to 100 μm, and can be 20 μm to 70 μm. In some embodiments, the thickness of the negative electrode film layer can be 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, or a value in a range consisting of any two of the above points.

[0086] By controlling the thickness of the negative electrode film layer within an appropriate range, both the fast charging performance and energy density of the battery can be taken into account.

[0087] In some embodiments, the electrolyte further includes linear carbonate, and the linear carbonate includes at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

[0088] In some embodiments, the electrolyte further includes a lithium salt, and the lithium salt includes LiPF6 and / or LiFSI.

[0089] In some embodiments, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy: 0.8 mol / L≤C1+C2≤1.3 mol / L, optionally 0.8 mol / L≤C1+C2≤1.2 mol / L.

[0090] In some embodiments, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy C1+C2 and can be 0.8 mol / L, 0.85 mol / L, 0.9 mol / L, 0.95 mol / L, 1.0 mol / L, 1.05 mol / L, 1.1 mol / L, 1.15 mol / L, 1.2 mol / L, 1.25 mol / L, 1.3 mol / L, or a value in the range of any two of the above points.

[0091] In some embodiments, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy: 1≤C1 / C2≤5, which can be 1≤C1 / C2≤3. In some embodiments, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy C1 / C2, which can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, or a value in a range consisting of any two of the above points.

[0092] LiFSI has a high degree of dissociation in the electrolyte, which can significantly improve the room-temperature conductivity of the electrolyte. However, LiFSI carries the risk of corroding aluminum foil. The dissociation degree of LiPF6 in the electrolyte is not as high as that of LiFSI, but LiPF6 has the effect of passivating aluminum foil and inhibiting its corrosion. When the molar concentration of LiFSI and the molar concentration of LiPF6 are controlled to meet a certain relationship, the conductivity of the electrolyte can be improved, which is beneficial for improving the fast-charging performance of lithium secondary batteries, while also reducing the risk of aluminum foil corrosion and improving the safety of lithium secondary batteries.

[0093] In some embodiments, the molar concentration C1 of LiPF6 is 0.2 mol / L to 1.2 mol / L, optionally 0.2 mol / L to 1.1 mol / L, and the molar concentration C2 of LiFSI is 0.1 mol / L to 1 mol / L, optionally 0.2 mol / L to 1 mol / L. In some embodiments, the molar concentration C1 of LiPF6 can be selected as 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, or a value in the range consisting of any two of the above points, and the molar concentration C2 of LiFSI can be selected as 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, or a value in the range consisting of any two of the above points.

[0094] The molar concentrations of LiFSI and LiPF6 are controlled within appropriate ranges, taking into account both the fast charging performance and safety of the lithium secondary battery.

[0095] In some embodiments, the electrolyte further includes an additive, and the additive includes one or more of lithium difluorophosphate, lithium fluorosulfonate, trimethylphosphite, vinylene carbonate, lithium tetrafluoroborate, lithium difluorobisoxalatophosphate, tris(trimethylsilyloxy)borate, trimethylfluorosilane, adiponitrile, and succinonitrile, and optionally includes one or more of lithium difluorophosphate, lithium fluorosulfonate, lithium tetrafluoroborate, and lithium difluorooxalatoborate.

[0096] All of the above additives can participate in the formation of a CEI film on the positive electrode side or a SEI film on the negative electrode side, reducing direct contact between the chain carboxylic acid ester and the positive electrode or metallic lithium, thereby reducing active lithium consumption and the reductive decomposition of the electrolyte. Lithium difluorophosphate, lithium fluorosulfonate, trimethylphosphite, vinylene carbonate, lithium tetrafluoroborate, or lithium difluorobis(oxalatophosphate) participate in the formation of a CEI film on the positive electrode side, which helps reduce the oxidative decomposition of the electrolyte on the positive electrode side, thereby reducing the cathode interface impedance, facilitating the rapid release of lithium ions, and improving fast charging performance. The reduction in the degree of oxidative decomposition can also reduce battery cell flatulence and active lithium consumption, thereby improving battery cell life. Tris(trimethylsilyl)borate, trimethylfluorosilane, adiponitrile, or succinonitrile participate in the formation of an SEI film on the negative electrode side, which helps reduce the reductive decomposition of the electrolyte on the negative electrode side, reducing gas production, and improving the electrical performance of the battery.

[0097] In some embodiments, the conductivity of the electrolyte is 9 mS·cm to 15 mS·cm at 25° C. The conductivity of the electrolyte can be obtained by a detection method known in the art.

[0098] When the electrolyte conductivity is within an appropriate range, both the fast-charging performance and the storage performance of the lithium secondary battery can be taken into account. If the electrolyte conductivity is too low, the fast-charging performance of the lithium secondary battery will be seriously affected; if the electrolyte conductivity is too high, the gas generation rate will increase, which is not conducive to the storage performance of the lithium secondary battery.

[0099] In some embodiments, the filling coefficient of the lithium secondary battery is 1.5 g / Ah to 4.0 g / Ah. In some embodiments, the filling coefficient of the lithium secondary battery can be 1.5 g / Ah, 2 g / Ah, 2.5 g / Ah, 3 g / Ah, 3.5 g / Ah, 4.0 g / Ah, or a value in a range consisting of any two of the above points.

[0100] In this article, the term "filling factor" is the ratio of electrolyte mass (g) to cell capacity (Ah).

[0101] Controlling the filling coefficient of lithium secondary batteries within an appropriate range can not only provide enough electrolyte to meet the needs of lithium secondary batteries, but also release a certain amount of space to accommodate more positive electrode active materials / negative electrode active materials, thereby improving the energy density of lithium secondary batteries, or release a certain amount of space for gas transfer, thereby reducing the internal pressure of the lithium secondary battery shell and improving the service life and safety of lithium secondary batteries.

[0102] [Positive electrode]

[0103] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0104] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0105] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0106] In some embodiments, the positive electrode active material may be a battery positive electrode active material known in the art. For example, the positive electrode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and modified compounds thereof. However, this application is not limited to these materials; other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used singly or in combination.

[0107] In some embodiments, the positive electrode active material includes Li d [Ni x Co y X1 z M1 1-x-y-z ]O 2-S , one or more of LiMn2O4, Li2MnO3·(1-a)LiAO2, LiM2X2O4,

[0108] Wherein, 0.1≤d≤1, 0≤S≤2, X1 includes Mn and / or Al, M1 includes one or more of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Zr, Sr, V, Ti, 0≤x<1, 0≤y≤1, 0≤z≤1, x+y+z≤1; A includes one or more of Ni, Co, Mn, 0<a<1; M2 includes one or more of Fe, Mn, Ni, Co, X2O4 h- Where X2 includes one or more of S, P, As, V, Mo, and W, and h=2 or 3.

[0109] The above-mentioned positive electrode active materials can all be used in combination with an electrolyte to realize a lithium secondary battery with excellent fast charging performance and cycle performance.

[0110] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0111] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0112] In some embodiments, the compaction density of the positive electrode film layer of the positive electrode sheet is 3.3 g / cm 3 ~3.7g / cm 3 .

[0113] In this document, the compaction density of the positive electrode film layer can be measured using any known testing method. For example, the compaction density of the positive electrode film layer can be calculated by taking a positive electrode sheet with a unit area S, weighed as M1, the mass of the aluminum foil per unit area S as M2, measuring the thickness of the positive electrode sheet as H1, and the thickness of the aluminum foil as H2. The compaction density of the positive electrode film layer is then calculated as (M1-M2) / ((H1-H2)×S).

[0114] The compaction density of the positive electrode film layer is within an appropriate range, which can take into account both the DC impedance and energy density of the lithium secondary battery.

[0115] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0116] [Negative electrode]

[0117] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector and containing at least a negative electrode active material.

[0118] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0119] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0120] In some embodiments, the negative electrode film layer includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase microcarbon beads, silicon-based materials, and tin-based materials, and optionally includes one or more of artificial graphite and natural graphite.

[0121] Negative electrode films containing the aforementioned negative electrode active materials can be used in conjunction with an electrolyte to achieve lithium secondary batteries with excellent fast-charging and cycling performance. Furthermore, artificial graphite or natural graphite offers significant advantages as negative electrode active materials due to their large specific capacity, stable graphite structure, and minimal expansion and contraction when lithium is inserted and removed.

[0122] In some embodiments, the compaction density of the negative electrode film layer is 0.9 g / cm 3 ~1.6g / cm 3 , optional 1.1g / cm 3 ~1.6g / cm 3 In some embodiments, the compaction density of the negative electrode film layer can be selected to be 0.9 g / cm 3 , 0.95g / cm 3 , 1.0g / cm 3 , 1.05g / cm 3 , 1.1g / cm 3 , 1.15g / cm 3 , 1.2g / cm 3 , 1.25g / cm 3 , 1.3g / cm 3 , 1.35g / cm 3 , 1.4g / cm 3 , 1.45g / cm 3 , 1.5g / cm 3 , 1.55g / cm 3 , 1.6g / cm 3 , or a value in the range formed by any two of the above points.

[0123] Controlling the compaction density of the negative electrode film layer within an appropriate range can not only meet the requirement of sufficient contact between the negative electrode active materials so that the battery has excellent electron transport performance and energy density, but also reduce the risk of the pores between the negative electrode active materials being crushed during the preparation process, which is not conducive to lithium ion insertion and extraction.

[0124] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder may include at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0125] In some embodiments, the negative electrode film layer may further include a conductive agent, which may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0126] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0127] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0128] [Isolation film]

[0129] In some embodiments, the lithium secondary battery further includes a separator, which can be any known porous separator with good chemical and mechanical stability.

[0130] In some embodiments, the material of the separator can be selected from 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. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0131] 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.

[0132] In some embodiments, the lithium secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0133] To alleviate the problem of secondary battery cell expansion force, expansion space can be reserved for the cells in the design. Specifically, the group margin of the lithium secondary battery is controlled to be 82% to 95%. The group margin is the percentage of the cell thickness to the inner thickness of the battery shell.

[0134] In some embodiments, the group margin of the lithium secondary battery can be selected as 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, or a value in a range consisting of any two of the above points.

[0135] Controlling the group margin of the lithium secondary battery within an appropriate range, that is, providing enough space to accommodate the electrode assembly and a certain space for gas transfer, reduces the internal pressure of the lithium secondary battery shell, and improves the service life and safety of the lithium secondary battery.

[0136] In some embodiments, the outer packaging of the lithium secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the lithium secondary battery can also be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0137] The shape of the lithium secondary battery in this application can be cylindrical, square or any other shape. For example, Figure 1 The lithium secondary battery 5 is a square structure as an example.

[0138] In some embodiments, reference Figure 2 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a accommodating cavity. The shell 51 has an opening connected to the accommodating cavity, and the cover plate 53 can be covered on the opening to close the accommodating cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the accommodating cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the lithium secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0139] In some embodiments, lithium secondary batteries can be assembled into a battery module. The number of lithium secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0140] Figure 3 4 is an example of a battery module. Figure 3 In the battery module 4, the plurality of lithium secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other manner. The plurality of lithium secondary batteries 5 can further be fixed by fasteners.

[0141] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of lithium secondary batteries 5 are received in the receiving space.

[0142] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0143] Figure 4 and Figure 5 The battery pack 1 is used as an example. Figure 4 and Figure 5The battery pack 1 may include a battery box and multiple battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0144] In addition, the present application also provides an electric device, which includes at least one of the lithium secondary battery, battery module, or battery pack provided in the present application. The lithium secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0145] As the electrical device, a lithium secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0146] Figure 6 This is an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of lithium secondary batteries in this device, a battery pack or battery module can be used.

[0147] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a lithium secondary battery as a power source.

[0148] Example

[0149] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0150] 1. Preparation method

[0151] Example 1

[0152] 1) Electrolyte

[0153] In an argon atmosphere glove box (H2O content <10ppm, O2 content <1ppm), 5g of ethyl formate, 10g of ethylene carbonate, and 71.13g of ethyl methyl carbonate were mixed to obtain a mixed solvent. Lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were then added to the mixed solvent to a molar concentration of 0.7mol / L (8.75g) and 0.3mol / L (4.62g), respectively. The mixture was stirred thoroughly, followed by the addition of 0.5g of 4,4'-ethylene disulfate and the stirring continued. This resulted in an electrolyte solution.

[0154] 2) Preparation of positive electrode sheet

[0155] The positive electrode active material LiNi 0.5 Co 0.2 Mn 0.3 O2, conductive agent Super P, and binder polyvinylidene fluoride (PVDF) are prepared into positive electrode slurry in N-methylpyrrolidone (NMP); the solid content in the positive electrode slurry is 50wt%, and the solid content of LiNi 0.5 Co 0.2 Mn 0.3 The mass ratio of O2, Super P, and PVDF is 8:1:1. The positive electrode slurry is coated on the negative electrode current collector aluminum foil and dried at 85°C before cold pressing. After trimming, cutting, and slitting, it is dried under vacuum conditions at 85°C for 4 hours to produce the positive electrode sheet.

[0156] 3) Preparation of negative electrode sheet

[0157] Graphite, the negative electrode active material, Super P, a conductive agent, carboxymethyl cellulose (CMC), a thickener, and styrene-butadiene rubber (SBR) were mixed uniformly in deionized water to create a negative electrode slurry. The solids content of the negative electrode slurry was 30 wt%, and the mass ratio of graphite, Super P, CMC, and SBR was 80:15:3:2. The negative electrode slurry was coated onto a negative electrode current collector copper foil and dried at 85°C. The resulting sheet was then cold-pressed, trimmed, cut, and slit, and then dried at 120°C under vacuum for 12 hours to produce a 50 μm thick negative electrode film.

[0158] 4) Isolation film

[0159] A 16 μm polyethylene film (PE) was used as the separator.

[0160] 5) Battery Preparation

[0161] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is placed in the middle of the positive and negative electrode sheets to isolate the positive and negative electrode sheets, and the bare battery cell is wound to obtain the bare battery cell. The tabs are welded and the bare battery cell is placed in an outer package. The electrolyte prepared above is injected into the dried battery cell. After packaging, standing, formation, shaping, capacity testing and other processes, the lithium secondary battery product of Example 1 is obtained.

[0162] The secondary batteries of Examples 2 to 29 and the secondary batteries of Comparative Examples 1 to 6 were prepared in a similar manner to the secondary battery of Example 1, except that the composition and product parameters of the battery electrodes were adjusted. The different product parameters are detailed in Tables 1 and 2.

[0163] The sizes of W1, W2, and W3, as well as the mass content of the additives, can be adjusted by adjusting the mass content of ethyl methyl carbonate in the electrolyte.

[0164] 2. Performance Testing

[0165] 1. Compaction density of negative electrode film layer

[0166] The compaction density of the film layer of the negative electrode sheet is obtained by calculation. Take a negative electrode sheet with a unit area of s, weigh it as m1, the mass of the copper foil under the unit area s is m2, measure the thickness of the negative electrode sheet h1, the thickness of the copper foil is h2, and the compaction density of the film layer of the negative electrode sheet = (m1-m2) / ((h1-h2)×s).

[0167] 2. Battery fast charging performance

[0168] The prepared battery was discharged to 2.8 V at 1 C, then allowed to stand for 5 min, charged to 4.25 V at 0.5 C, and the charge capacity C0 was recorded. Then, it was discharged to 2.8 V at 1 C and allowed to stand for 5 min, and then charged to 4.25 V at 2 C, and the charge capacity C1 was recorded. The battery charge capacity retention rate = C1 / C0×100%.

[0169] 3. Battery cycle performance

[0170] At 25°C, the prepared battery was charged at a constant current of 1C to 4.25V, then charged at a constant voltage of 4.25V until the current dropped to 0.05C. After standing for 10 minutes, the battery was discharged at a constant current of 1C to 2.8V. This was the first charge / discharge cycle of the battery, and the discharge capacity at this time was recorded as the discharge capacity of the battery in the first cycle (C0). The above steps were repeated for the same battery, and the process capacity of the battery after the 300th cycle was (C1). The capacity retention rate after 300 cycles = C1 / C0 × 100%. The test procedures for the comparative example and other examples were the same as above.

[0171] 4. Battery storage and gas production performance

[0172] At 25°C, the prepared full battery was allowed to stand for 30 minutes, and then charged to 4.2V at a constant current rate of 0.1C, and then charged to 0.05C at a constant voltage at 4.2V, allowed to stand for 5 minutes, and then the volume V1 of the battery cell was tested by the drainage method. After storage at 60°C for 30 days, the battery cell was taken out, cooled to 25°C, charged to 4.2V at 0.1C, and then charged to 0.05C at a constant voltage. The volume V2 of the battery was tested by the drainage method, and the volume expansion rate of the battery = (V2-V1) / V1×100%.

[0173] 3. Analysis of test results of various embodiments and comparative examples

[0174] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in Tables 1 and 2 below.

[0175] Table 1

[0176]

[0177]

[0178] Table 2

[0179]

[0180]

[0181] The lithium secondary batteries of Examples 1 to 29 all include an electrolyte and a negative electrode plate, the electrolyte includes a chain carboxylate and a cyclic carbonate, and the negative electrode plate includes a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector; wherein, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylate and the mass content W2 of the cyclic carbonate satisfy: 0.1≤W2 / W1≤2, the mass content W1 of the chain carboxylate and the thickness H of the negative electrode film layer satisfy: 0.2≤H / 100W1≤20, and the unit of H is μm.

[0182] From the comparison of Examples 1 to 7, Examples 22 to 24 and Comparative Examples 1 to 4, it can be seen that, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylate and the mass content W2 of the cyclic carbonate are controlled to satisfy: 0.1≤W2 / W1≤2, and the mass content W1 of the chain carboxylate and the thickness H of the negative electrode film layer satisfy: 0.2≤H / 100W1≤20, which is beneficial to improving the fast charging performance and cycle performance of the lithium secondary battery, as well as reducing the storage gas production rate of the lithium secondary battery, thereby improving its storage performance.

[0183] From the comparison of Examples 2 to 6, 23 to 24 with Examples 1, 7, and 23, it can be seen that, based on the total mass of the electrolyte, further controlling the mass content W1 of the chain carboxylic acid ester and the mass content W2 of the cyclic carbonate to satisfy: 0.1≤W2 / W1≤1.5 is beneficial to further improve the storage performance of the battery.

[0184] As can be seen from Examples 1 to 4, the mass content W1 of the chain carboxylate is controlled to be 5% to 75% based on the total mass of the electrolyte, so that the lithium secondary battery has excellent fast charging performance, cycle performance, and storage performance. As can be seen from the comparison of Examples 2 to 4 with Example 1, further controlling the mass content W1 of the chain carboxylate to be 20% to 75% based on the total mass of the electrolyte is beneficial to further improve the fast charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0185] As can be seen from Examples 2, 5 to 7, the mass content W2 of the cyclic carbonate is controlled to be 5% to 40% based on the total mass of the electrolyte, so that the lithium secondary battery has excellent fast charging performance, cycle performance, and storage performance. As can be seen from the comparison of Examples 2 and 6 with Examples 5 and 7, further controlling the mass content W2 of the cyclic carbonate to be 5% to 30% based on the total mass of the electrolyte is beneficial to further improve the fast charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0186] It can be seen from Examples 2 and 22 to 24 that the thickness of the negative electrode film layer is controlled to be 10 μm to 100 μm, so that the lithium secondary battery has excellent fast charging performance, cycle performance and storage performance.

[0187] As can be seen from Examples 2, 8 to 11, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI are controlled to satisfy: 1≤C1 / C2≤5, so that the lithium secondary battery has excellent fast charging performance, cycle performance, and storage performance. As can be seen from the comparison of Examples 2, 8, 10 to 11 with Example 9, further controlling the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI to satisfy: 1≤C1 / C2≤3 is conducive to further improving the fast charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0188] As can be seen from Examples 2, 5-7, and 12-14, the chain carboxylate can be ethyl formate or methyl acetate, the cyclic carbonate can be ethylene carbonate or fluoroethylene carbonate, and the sulfate compound can be 4,4'-ethylene disulfate or a substance represented by the structure of Formula I (R1 = b, R2 = b, R3 = methyl), all of which can enable the lithium secondary battery to have excellent fast charging performance, cycle performance, and storage performance. As can be seen from Examples 2, 5-7, and 12-14, based on the total mass of the electrolyte, controlling the mass content W2 of the cyclic carbonate to 5% to 40% can take into account the fast charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0189] As can be seen from Examples 12 to 18, the chain carboxylic acid ester can be selected as methyl acetate, ethyl acetate, methyl propionate, ethyl propionate or propyl acetate, the cyclic carbonate can be selected as ethylene carbonate or fluoroethylene carbonate, and the sulfate compound can be selected as the substance shown in the structure of Formula I (R1=b, R2=b, R3=methyl), which can also make the lithium secondary battery have excellent fast charging performance, cycle performance and storage performance.

[0190] As can be seen from Examples 1 to 4 and 19 to 21, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylic acid ester and the mass content W3 of the sulfate compound are controlled to satisfy the following relationship: 0.005≤W3 / W1≤0.4, which can make the lithium secondary battery have excellent fast charging performance, cycle performance and storage performance. As can be seen from the comparison between Examples 1 to 4 and 20 to 21 and Example 19, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylic acid ester and the mass content W3 of the sulfate compound are further controlled to satisfy the following relationship: 0.005≤W3 / W1≤0.25, which is beneficial to improving the fast charging performance, cycle performance and storage performance of the lithium secondary battery.

[0191] It can be seen from Examples 1 to 4 and 19 to 21 that, based on the total mass of the electrolyte, controlling the mass content W3 of the sulfate ester compound to 0.1% to 2% can enable the lithium secondary battery to have excellent fast charging performance, cycle performance and storage performance.

[0192] From the comparison between Examples 25 to 26 and Example 2, it can be seen that the introduction of the additive lithium difluorophosphate or lithium fluorosulfonate into the electrolyte is beneficial to improving the fast charging performance, cycle performance and storage performance of the battery.

[0193] From Examples 2, 27 to 29, it can be seen that the compaction density of the negative electrode film is controlled to be 0.9 g / cm 3 ~1.6g / cm 3 , so that the lithium secondary battery has excellent fast charging performance, cycle performance and storage performance. From the comparison of Examples 2, 28 to 29 with Example 27, it can be seen that the compaction density of the negative electrode film layer is further controlled to be 1.1g / cm 3 ~1.6g / cm3 , which will help further improve the battery's fast charging performance, cycle performance and storage performance.

[0194] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium secondary battery, characterized in that: The invention comprises an electrolyte and a negative electrode plate, wherein the electrolyte comprises a chain carboxylate and a cyclic carbonate, and the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector; Wherein, based on the total mass of the electrolyte, the mass content W1 of the chain carbonate is 5% to 75%, and the mass content W2 of the cyclic carbonate is 2% to 40%; The electrolyte further includes a lithium salt, wherein the lithium salt includes lithium bis(fluorosulfonyl)imide LiFSI, and the molar concentration C2 of lithium bis(fluorosulfonyl)imide LiFSI is 0.3 mol / L to 1 mol / L.

2. The lithium secondary battery according to claim 1, wherein Based on the total mass of the electrolyte, the mass content W1 of the chain carboxylate and the mass content W2 of the cyclic carbonate satisfy the following relationship: 0.1≤W2 / W1≤2.

3. The lithium secondary battery according to claim 1 or 2, characterized in that The mass content W1 of the chain carboxylic acid ester and the thickness H of the negative electrode film layer satisfy the following relationship: 0.2≤H / 100W1≤20, where the unit of H is μm.

4. The lithium secondary battery according to any one of claims 1 to 3, characterized in that The lithium salt further comprises lithium hexafluorophosphate LiPF6, and the molar concentration C1 of lithium hexafluorophosphate LiPF6 is 0.2 mol / L to 1.2 mol / L, and can be optionally 0.2 mol / L to 1.1 mol / L.

5. The lithium secondary battery according to claim 4, characterized in that The molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy the following conditions: 0.8 mol / L≤C1+C2≤1.3 mol / L, which may be 0.8 mol / L≤C1+C2≤1.2 mol / L; and / or, The molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy: 1≤C1 / C2≤5, which can be optionally 1≤C1 / C2≤3.

6. The lithium secondary battery according to any one of claims 1 to 5, characterized in that The electrolyte further includes a sulfate compound. Based on the total mass of the electrolyte, the mass content W1 of the chain carboxylate and the mass content W3 of the sulfate compound satisfy the following relationship: 0.005≤W3 / W1≤0.

4.

7. The lithium secondary battery according to claim 6, characterized in that The chain carboxylic acid ester includes one or more of ethyl formate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, propyl propionate, propyl acetate, butyl formate, and methyl butyrate, and optionally includes one or more of ethyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and propyl propionate; and / or The cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, and optionally includes one or more of ethylene carbonate and fluoroethylene carbonate; and / or The sulfate compounds include compounds represented by formula I and formula II, diethyl sulfate, dimethyl sulfate, dihydro-1,3,2-dimethyl sulfate, One or more of thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide, Wherein, R1 and R2 each independently include hydrogen atoms, C 1-6 The alkyl group, R3 includes C 1-6 The alkyl group, Optionally, the sulfate compounds include 4,4'-disulfate, vinyl sulfate, diethyl sulfate, dimethyl sulfate, dihydro-1,3,2-disulfate, One or more of thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide.

8. The lithium secondary battery according to claim 6 or 7, characterized in that Based on the total mass of the electrolyte, the mass content W1 of the chain carboxylic acid ester is 20% to 75%; Based on the total mass of the electrolyte, the mass content W2 of the cyclic carbonate is 5% to 40%, optionally 10% to 30%; and / or Based on the total mass of the electrolyte, the mass content W3 of the sulfate compound is 0.1% to 2%, and can be optionally 0.1% to 1%.

9. The lithium secondary battery according to any one of claims 1 to 8, characterized in that The thickness H of the negative electrode film layer is 10 μm to 100 μm, and can be optionally 20 μm to 70 μm.

10. The lithium secondary battery according to any one of claims 1 to 9, characterized in that The electrolyte also includes additives, which include one or more of lithium difluorophosphate, lithium fluorosulfonate, trimethylphosphite, vinylene carbonate, lithium tetrafluoroborate, lithium difluorobisoxalatophosphate, tris(trimethylsilyloxy)borate, trimethylfluorosilane, adiponitrile, and succinonitrile, and optionally include one or more of lithium difluorophosphate, lithium fluorosulfonate, lithium tetrafluoroborate, and lithium difluorooxalatoborate.

11. The lithium secondary battery according to any one of claims 1 to 10, characterized in that The positive electrode sheet in the lithium secondary battery includes a positive electrode active material, and the positive electrode active material includes Li d [Ni x Co y X1 z M1 1-x-y-z ]O 2-S , one or more of LiMn2O4, Li2MnO3·(1-a)LiAO2, LiM2X2O4, Wherein, 0.1≤d≤1, 0≤S≤2, X1 includes Mn and / or Al, M1 includes one or more of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Zr, Sr, V, Ti, 0≤x<1, 0≤y≤1, 0≤z≤1, x+y+z≤1; A includes one or more of Ni, Co, Mn, 0<a<1; M2 includes one or more of Fe, Mn, Ni, Co, X2O4 h- Where X2 includes one or more of S, P, As, V, Mo, and W, and h=2 or 3.

12. The lithium secondary battery according to any one of claims 1 to 11, characterized in that The negative electrode film layer includes a negative electrode active material, which includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase microcarbon beads, silicon-based materials, and tin-based materials, and optionally includes one or more of artificial graphite and natural graphite.

13. The lithium secondary battery according to any one of claims 1 to 12, characterized in that The compaction density of the negative electrode film layer is 0.9 g / cm 3 ~1.6g / cm 3 , optional 1.1g / cm 3 ~1.6g / cm 3 .

14. An electrical device, characterized in that: A lithium secondary battery comprising the lithium secondary battery according to any one of claims 1 to 13.

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