Lithium secondary battery and electric device

Abstract

The invention provides a lithium secondary battery and a power utilization device. The lithium secondary battery comprises a positive electrode piece, a negative electrode piece and electrolyte, the negative electrode piece comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side of the negative electrode current collector, the negative electrode film layer comprises a negative electrode active material, and the specific surface area a of the negative electrode active material is greater than or equal to 2m < 2 > / g and less than or equal to 5m < 2 > / g; the electrolyte comprises propylene carbonate, a first additive and a compound with a formula I, R is selected from halogen, n is an integer selected from 1-6, and the first additive comprises at least one of ethylene sulfate, vinylene carbonate, fluoroethylene carbonate, 1, 3-propane sultone, 1, 3-propene sultone, tetravinyl silane or methylene methanedisulfonate. The lithium secondary battery provided by the invention has relatively good low-temperature quick charge performance and also has cycle performance.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a lithium secondary battery and an electrical device. Background Art

[0002] In recent years, batteries have been widely used in energy storage power systems such as hydropower, thermal, wind and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0003] Lithium secondary batteries are widely used due to their high energy density and other advantages. However, in low temperature environments, the charging capacity of lithium secondary batteries decreases significantly. Therefore, improving the low temperature fast charging performance of lithium secondary batteries is an urgent problem to be solved. Summary of the invention

[0004] The present application is made in view of the above-mentioned subject, and aims to solve at least one of the technical problems existing in the prior art. To this end, the present application provides a lithium secondary battery and an electric device.

[0005] A first aspect of the present application provides a lithium secondary battery, the lithium secondary battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte.

[0006] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer includes a negative electrode active material, and the specific surface area a of the negative electrode active material is 2m 2 / g≤a≤5m 2 / g, the electrolyte includes propylene carbonate, a first additive and a compound having the following formula I

[0007]

[0008] R is selected from halogen, n is an integer selected from 1-6,

[0009] The first additive includes at least one of vinyl sulfate, vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, tetravinylsilane or methylene methanedisulfonate.

[0010] By reasonably designing the specific surface area (BET) of the negative electrode active material and the electrolyte composition, the lithium secondary battery of the present application has good low-temperature fast charging performance while taking into account the cycle performance. Specifically, the specific surface area of ​​the negative electrode active material of the present application is 2m 2 / g≤a≤5m 2 / g range, when the specific surface area of ​​the negative electrode active material is within this range, on the one hand, it is beneficial to increase the transmission path of lithium ions and increase the reaction or active sites of lithium ions; on the other hand, it is also beneficial to reduce the content of non-releasable active lithium adsorbed on the surface of the negative electrode active material, thereby reducing the irreversible capacity loss during the first charging process. Therefore, the specific surface area of ​​the above-mentioned negative electrode active material is beneficial to improving the low-temperature fast charging performance of the lithium secondary battery of the present application. At the same time, the electrolyte of the present application includes propylene carbonate. Propylene carbonate has a higher dielectric constant and a lower freezing point, which is beneficial to improve the low-temperature conductivity of the electrolyte, thereby helping to improve the low-temperature fast charging performance of the lithium secondary battery of the present application. However, propylene carbonate is easily embedded in the negative electrode active material (such as graphite) together with lithium ions on the negative electrode side, which leads to the stripping of the negative electrode active material and deteriorates the cycle performance of the secondary battery. In particular, the specific surface area of ​​the negative electrode active material of the present application is large, there are more side reactions, and the cycle performance is more easily affected.

[0011] To this end, the present application adds a compound of formula I to the electrolyte. The compound of formula I contains a large π bond and is easily bonded to the negative electrode active material, such as graphite, silicon-carbon mixture, etc., so that it is enriched on the surface of the negative electrode active material, so that its surface has a certain negative charge, promotes the desolvation of lithium ions solvated by propylene carbonate, is beneficial to inhibit the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, and is beneficial to reduce the degree of stripping of the negative electrode active material, thereby improving the cycle performance of the lithium secondary battery of the present application.

[0012] However, the compound of formula I does not participate in the formation of the negative electrode solid electrolyte interface film (SEI film). The present application introduces a first additive into the electrolyte, which is conducive to the formation of the SEI film on the negative electrode side. The formation of the SEI film is conducive to inhibiting the decomposition of the electrolyte, and is also conducive to reducing the degree of side reactions of the negative electrode active material due to the increased specific surface area, thereby helping to improve the cycle performance of the lithium secondary battery of the present application. In summary, the lithium secondary battery of the present application has good low-temperature fast charging performance, while taking into account good cycle performance.

[0013] In any embodiment, the specific surface area a of the negative electrode active material is 2 m 2 / g≤a≤4m 2 / g.

[0014] When the specific surface area of ​​the negative electrode active material is within the above range, it is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery of the present application while taking into account the cycle performance.

[0015] In any embodiment, the single-side density b of the negative electrode film layer is 65 g / m 2 ≤b≤100g / m 2 .

[0016] When the single-side density of the negative electrode film layer is within the above range, it is beneficial to improve the capacity of the lithium secondary battery of the present application, and is also beneficial to provide more reaction sites for lithium ions, thereby further improving the low-temperature fast charging performance of the lithium secondary battery of the present application.

[0017] In any embodiment, the negative electrode active material includes one or more of graphite or its modified product, and a silicon-carbon mixture.

[0018] The compound of formula I is easier to bond with the above-mentioned negative electrode active material, thereby promoting the desolvation of lithium ions solvated by propylene carbonate, which is beneficial to further inhibit the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, and is beneficial to reduce the degree of stripping of the negative electrode active material, thereby further improving the cycle stability of the lithium secondary battery of the present application.

[0019] In any embodiment, R is selected from fluorine, chlorine or bromine, and n is selected from 1, 4 or 6.

[0020] In any embodiment, the compound of Formula I comprises at least one of 1-fluorobenzene, 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene or hexafluorobenzene.

[0021] Compared to other halogen atoms, fluorine atoms have stronger electron-withdrawing ability. Using fluorine atoms to replace benzene rings can make the center of the benzene ring lack electrons, which is then conducive to enhancing the π-π interaction between the compound of formula I and the negative electrode active material, and is conducive to the compound of formula I enriched on the surface of the negative electrode active material, suppressing the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, thereby further improving the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application. Further, compared to 1-fluorobenzene, when the compound of formula I is selected from tetrafluorobenzene and hexafluorobenzene, as the number of fluorine atoms on the benzene ring increases, the more electrons are lacking in the center of the benzene ring, the stronger the π-π interaction between the benzene ring and the negative electrode active material, which is conducive to further suppressing the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, and further improving the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0022] In any embodiment, based on the total mass of the electrolyte, the mass percentage content W1 of the compound of formula I is 0.5%≤W1≤10%, optionally, based on the total mass of the electrolyte, the mass percentage content W1 of the compound of formula I is 1%≤W1≤7%, more optionally, based on the total mass of the electrolyte, the mass percentage content W1 of the compound of formula I is 2%≤W1≤5%; and / or

[0023] Based on the total mass of the electrolyte, the mass percentage W2 of the propylene carbonate is 2%≤W2≤10%. Optionally, based on the total mass of the electrolyte, the mass percentage W2 of the propylene carbonate is 5%≤W2≤8%.

[0024] When the compound of formula I is within the above range, on the one hand, it is beneficial to maintain an appropriate viscosity of the electrolyte, which is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery of the present application. On the other hand, it is also beneficial to further inhibit the stripping of propylene carbonate from the negative electrode active material, thereby helping to further improve the cycle performance.

[0025] When the mass percentage of propylene carbonate is within the above range, it is beneficial to further improve the low-temperature conductivity of the electrolyte, and is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application. At the same time, it can also reduce the amount of embedded graphite, thereby reducing the degree of graphite stripping, thereby further reducing the degree of deterioration of cycle performance.

[0026] In any embodiment, the mass percentage W1 of the compound of formula I and the mass percentage W2 of the propylene carbonate satisfy 0.05≤W1 / W2≤5, optionally 0.35≤W1 / W2≤2.

[0027] When the ratio W1 / W2 of the mass percentage of the compound of formula I to the mass percentage of propylene carbonate satisfies the above range, it is beneficial to improve the low-temperature conductivity of the electrolyte, maintain the appropriate viscosity of the electrolyte, and further inhibit the stripping of propylene carbonate on the negative electrode active material, thereby further improving the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application, and at the same time, it is beneficial to further take into account the cycle performance.

[0028] In any embodiment, the specific surface area a of the negative electrode active material, the single lateral density b of the negative electrode film layer and the mass percentage W1 of the compound of formula I satisfy (a×b / 180)≤W1×100≤(a×b / 70).

[0029] When the specific surface area value a of the negative electrode active material, the single lateral density b of the negative electrode film layer and the mass percentage W1 of the compound of formula I satisfy the above formula, it is beneficial to increase the transmission path of lithium ions, increase the reaction or active sites of lithium ions, and at the same time, the negative electrode active material can be fully bonded with the compound of formula I, which is beneficial to enhance the desolvation of lithium ions at the negative electrode interface and reduce the degree of stripping of the negative electrode active material, thereby helping to further improve the low-temperature fast charging performance and cycle performance of the secondary battery of the present application.

[0030] In any embodiment, based on the total mass of the electrolyte, the mass percentage W3 of the first additive is 0.3%≤W3≤4%.

[0031] When the mass percentage of the first additive in the electrolyte is within the above range, on the one hand, it is conducive to better forming an organic small molecule polymer SEI film, thereby further inhibiting the decomposition of the electrolyte and reducing the degree of side reactions on the surface of the negative electrode active material; on the other hand, it is conducive to forming an organic small molecule polymer SEI film of appropriate thickness, reducing impedance and improving low-temperature fast charging performance. In summary, when W3 meets the above mass percentage, it is conducive to further improving the cycle performance of the lithium secondary battery of the present application while taking into account the low-temperature fast charging performance.

[0032] In any embodiment, the electrolyte includes a second additive, and the second additive includes at least one of lithium difluorophosphate, lithium difluoromonooxalatoborate, lithium difluorobisoxalatoborate, lithium bis(fluorosulfonyl)imide, or lithium fluorosulfonate.

[0033] The second additive is easily decomposed at the negative electrode interface to produce inorganic lithium salts, thereby forming an SEI film rich in inorganic lithium salts. The first additive of the present application mainly generates an organic small molecule polymer SEI film, and the film formation impedance at the interface needs to be further reduced, and the film stability needs to be further improved. The SEI film rich in inorganic lithium salts generated by the second additive has strong rigidity and high thermal stability. The combination of the first additive and the second additive of the present application is conducive to further improving the stability of the negative electrode SEI film and further reducing the interface impedance of the negative electrode, thereby helping to further improve the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0034] In any embodiment, based on the total mass of the electrolyte, the mass percentage content W4 of the second additive is 0.1%≤W4≤3%, optionally 0.1%≤W4≤2%.

[0035] When the mass percentage of the second additive satisfies the above relationship, it is beneficial to further improve the stability of the SEI film on the negative electrode side and further reduce the interfacial impedance on the negative electrode side, and it is also beneficial to maintain an appropriate viscosity of the electrolyte, thereby further improving the cycle performance of the lithium secondary battery of the embodiment of the present application while taking into account the low-temperature fast charging performance.

[0036] In any embodiment, the first additive includes vinylene carbonate, and based on the total mass of the electrolyte, the mass percentage content W3 of vinylene carbonate is 0.4%≤W3≤3%, optionally, based on the total mass of the electrolyte, the mass percentage content W3 of vinylene carbonate is 0.5%≤W3≤1%; and / or,

[0037] The second additive includes lithium difluorophosphate. Based on the total mass of the electrolyte, the mass percentage W4 of the lithium difluorophosphate is 0.25%≤W4≤1%. Optionally, based on the total mass of the electrolyte, the mass percentage W4 of the lithium difluorophosphate is 0.3%≤W3≤0.5%.

[0038] When the first additive is vinylene carbonate and the second additive is lithium difluorophosphate, vinylene carbonate decomposes at the negative electrode to form an outer SEI layer rich in organic polymer components, and the outer SEI layer has excellent flexibility. Lithium difluorophosphate forms an inner layer of SEI film rich in inorganic salt components at the negative electrode interface, and the inner layer of SEI film has excellent rigidity and thermal stability. The first additive and the second additive include the above substances, so that the SEI film can better adapt to the expansion and contraction of the negative electrode active material during charging and discharging, alleviate the mechanical rupture of the SEI film, and further reduce the interface impedance on the negative electrode side, thereby further improving the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the present application. Further, when the mass percentage of vinylene carbonate and lithium difluorophosphate is within the above range, it is more conducive to the synergistic effect of the two, further improving the stability of the SEI film on the negative electrode side and further reducing the interface impedance on the negative electrode side, which is conducive to further improving the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the present application.

[0039] In any embodiment, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy 0.2%≤W3+W4≤6%, optionally 0.6%≤W3+W4≤4.3%.

[0040] When the mass percentage W3 of the first additive and the mass percentage W4 of the second additive satisfy the above relationship, it is more conducive to the synergistic effect of the two, further improving the stability of the SEI film on the negative electrode side and further reducing the interfacial impedance on the negative electrode side, thereby helping to further improve the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0041] In any embodiment, the specific surface area value a of the negative electrode active material, the single lateral density b of the negative electrode film layer, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy a×b / 360≤(W3+W4)×100≤a×b / 160.

[0042] When the specific surface value a of the negative electrode active material, the single-side density b of the negative electrode film layer, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy the above relationship, specifically, when a×b satisfies the above relationship, it is beneficial to increase lithium ion transmission, so that lithium ions are quickly embedded in the negative electrode, which is beneficial to further improve the low-temperature fast charging performance; when W3+W4 satisfies the above relationship, it is beneficial to further synergize the first additive and the second additive, and is beneficial to form a more stable SEI film at the anode interface, thereby more effectively inhibiting the reduction of the electrolyte at the anode, and further improving the battery cycle performance. Therefore, when a, b, W3 and W4 satisfy the above relationship, it is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application while taking into account the cycle performance.

[0043] In any embodiment, the electrolyte includes a solvent, and the solvent includes at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, r-butyrolactone, diethyl carbonate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate or propyl propionate.

[0044] The above-mentioned solvent can be helpful in dissolving the solutes in the electrolyte to form mobile ions, which is helpful in improving the ionic conductivity of the electrolyte, thereby helping to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0045] In any embodiment, the electrolyte includes a lithium salt, and the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium fluorosulfonyl(perfluorobutylsulfonyl)imide, or lithium bis(trifluoromethylsulfonyl)imide.

[0046] The above-mentioned lithium salt is dissolved in the solvent, which is beneficial to improving the ionic conductivity of the electrolyte, thereby helping to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0047] In any embodiment, the concentration of the lithium salt is 0.8 mol / L-1.3 mol / L.

[0048] When the lithium salt concentration is within the above range, it is beneficial to further improve the ionic conductivity of the electrolyte, thereby helping to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0049] In any embodiment, the compaction density of the negative electrode plate is 1.1 g / cm 3 -1.5g / cm 3 .

[0050] When the compaction density of the negative electrode plate is within the above range, it is beneficial to increase the adhesion between the negative electrode film layer and the negative electrode current collector and improve the contact degree of various substances in the negative electrode film layer, which is beneficial to further reduce the internal resistance of the negative electrode plate and reduce the risk of film layer falling off, thereby helping to further improve the low-temperature fast charging performance of the lithium secondary battery in the embodiment of the present application.

[0051] In any embodiment, the Dv50 of the negative electrode active material is greater than or equal to 12 μm and less than or equal to 17 μm.

[0052] Controlling the Dv50 of the negative electrode active material within the above range is conducive to adjusting the length of the lithium ion transmission path within an appropriate range, which is conducive to further improving the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application. The second aspect of the present application also provides an electrical device, which includes the lithium secondary battery described in the first aspect of the present application.

[0053] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 is a schematic diagram of a lithium secondary battery according to one embodiment of the present application.

[0055] Figure 2 yes Figure 1 An exploded view of a lithium secondary battery according to an embodiment of the present application is shown.

[0056] Figure 3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0057] Figure 4 It is a schematic diagram of a battery pack according to one embodiment of the present application.

[0058] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0059] Figure 6 Schematic diagram of an electrical device using a lithium secondary battery according to an embodiment of the present application as a power source.

[0060] Description of reference numerals:

[0061] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 lithium secondary battery; 51 shell; 52 electrode assembly; 53 top cover assembly DETAILED DESCRIPTION

[0062] Below, the embodiments of the lithium secondary battery, battery module, battery pack and electrical device of the present application are specifically disclosed 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 structure 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 descriptions 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.

[0063] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a 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 a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present 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 real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it 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.

[0064] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0065] 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.

[0066] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means 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), which means 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.

[0067] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0068] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": 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).

[0069] Lithium secondary batteries are widely used in various fields, but their fast charging performance decreases significantly in low temperature environments. In order to solve this problem, attempts can be made from the perspectives of structural design and material improvement of the negative and positive electrodes.

[0070] [Secondary battery]

[0071] Based on this, the present application proposes a lithium secondary battery, which includes a positive electrode sheet, a negative electrode sheet and an electrolyte.

[0072] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer includes a negative electrode active material, and the specific surface area a of the negative electrode active material is 2m 2 / g≤a≤5m 2 / g, the electrolyte includes propylene carbonate, a first additive and a compound having the following formula I

[0073]

[0074] R is selected from halogen, n is an integer selected from 1-6,

[0075] The first additive includes at least one of vinyl sulfate, vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, tetravinylsilane or methylene methanedisulfonate.

[0076] As used herein, the term "halogen" refers to an element of Group VIIA of the periodic table of chemical elements. Specifically, halogen includes fluorine, chlorine, bromine, iodine or astatine.

[0077] As used herein, the term "specific surface area" refers to the total area per unit mass of particles.

[0078] The specific surface area a of the negative electrode active material can be measured by methods and equipment known in the art, for example, it can be tested according to the gas adsorption method with reference to GB / T 19587-2004, equipment model: TRISTAR II 3020. The specific example is as follows: disassemble the lithium secondary battery, obtain the negative electrode plate, soak the plate in dimethyl carbonate (DMC) for 2 to 4 hours, place the soaked plate in a muffle furnace, set its temperature to 400°C, calcine for 2 hours in an air atmosphere, brush the negative electrode powder off the current collector with a small brush, and sieve to remove impurities to obtain the negative electrode active material powder. Take the powder as a sample, immerse the sample tube in -196°C liquid nitrogen, measure the adsorption amount of nitrogen on the solid surface at different pressures at a relative pressure of 0.05-0.30, and obtain the sample monolayer adsorption amount based on the BET multilayer adsorption theory and its formula, so as to calculate the specific surface area of ​​the sample.

[0079] In some embodiments, the specific surface area a of the negative electrode active material may be 2 m 2 / g, 2.1m 2 / g, 2.2m 2 / g, 2.3m 2 / g, 2.4m 2 / g, 2.5m 2 / g, 2.6m 2 / g, 2.7m 2 / g, 2.8m 2 / g, 2.9m 2 / g, 3m 2 / g, 3.1m 2 / g, 3.2m 2 / g, 3.3m 2 / g, 3.3m 2 / g, 3.4m 2 / g, 3.5m 2 / g, 3.6m 2 / g, 3.7m 2 / g, 3.8m 2 / g, 3.9m 2 / g, 4.0m 2 / g, 4.1m 2 / g, 4.2m 2 / g, 4.3m 2 / g, 4.3m 2 / g, 4.4m 2 / g, 4.5m 2 / g, 4.6m 2 / g, 4.7m 2 / g, 4.8m 2 / g, 49m 2 / g, 5.0m 2 / g, or a range consisting of any two of the above specific surface areas a, or a value within the range consisting of the above specific surface areas.

[0080] By reasonably designing the specific surface area of ​​the negative electrode active material and the electrolyte composition, the lithium secondary battery of the present application has good low-temperature fast charging performance while taking into account the cycle performance. Specifically, the specific surface area of ​​the negative electrode active material of the present application is 2m 2 / g≤a≤5m 2 / g range, when the specific surface area of ​​the negative electrode active material is within this range, on the one hand, it is beneficial to reduce the transmission path of lithium ions and increase the reaction or active sites of lithium ions; on the other hand, it is also beneficial to reduce the content of non-releasable active lithium adsorbed on the surface of the negative electrode active material, thereby reducing the irreversible capacity loss during the first charging process. Therefore, the specific surface area of ​​the above-mentioned negative electrode active material is beneficial to improving the low-temperature fast charging performance of the lithium secondary battery of the present application. At the same time, the electrolyte of the present application includes propylene carbonate. Propylene carbonate has a higher dielectric constant and a lower freezing point, which is beneficial to improve the low-temperature conductivity of the electrolyte, thereby helping to improve the low-temperature fast charging performance of the lithium secondary battery of the present application. However, propylene carbonate is easily embedded in the negative electrode active material (such as graphite) together with lithium ions on the negative electrode side, thereby causing the stripping of the negative electrode active material and deteriorating the cycle performance of the secondary battery. In particular, the specific surface area of ​​the negative electrode active material of the present application is large, the content of adsorbed lithium ions is high, and the cycle performance is more easily affected.

[0081] To this end, the present application adds a compound of formula I to the electrolyte. The compound of formula I contains a large π bond and is easily bonded to the negative electrode active material, such as graphite, silicon-carbon mixture, etc., so that it is enriched on the surface of the negative electrode active material, so that its surface has a certain negative charge, promotes the desolvation of lithium ions solvated by propylene carbonate, is beneficial to inhibit the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, and is beneficial to reduce the degree of stripping of the negative electrode active material, thereby improving the cycle performance of the lithium secondary battery of the present application.

[0082] However, the compound of formula I does not participate in the formation of the negative electrode solid electrolyte interface film (SEI film). The present application introduces a first additive into the electrolyte, which is conducive to the formation of the SEI film on the negative electrode side. The formation of the SEI film is conducive to inhibiting the decomposition of the electrolyte, and is also conducive to reducing the degree of side reactions of the negative electrode active material due to the increased specific surface area, thereby helping to improve the cycle performance of the lithium secondary battery of the present application. In summary, the lithium secondary battery of the present application has good low-temperature fast charging performance, while taking into account good cycle performance.

[0083] In some embodiments, the specific surface area a of the negative electrode active material is 2 m 2 / g≤a≤4m 2 / g.

[0084] When the specific surface area of ​​the negative electrode active material is within the above range, it is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery of the present application while taking into account the cycle performance.

[0085] In some embodiments, the single-side density b of the negative electrode film layer is 65 g / m 2 ≤b≤100g / m 2 .

[0086] Herein, the term "single-side density of the negative electrode film layer" refers to the mass of the negative electrode active material per unit area of ​​the single-side negative electrode film layer.

[0087] The single-sided density b of the negative electrode film layer can be measured by methods and equipment known in the art. An example is as follows: take a negative electrode sheet with a negative electrode film layer on both sides of a regular size, weigh it, and record its mass as m1. Measure its length and width, and calculate its area as S1; soak the negative electrode sheet in deionized water and perform ultrasonic cleaning. After the negative electrode film layer is completely removed from the current collector, take out the current collector, dry it at 120°C for 2h, and weigh the mass of the current collector, which is recorded as m2; the mass of the double-sided film layer M=m1-m2, the mass of the single-sided film layer is M / 2, and the single-sided density of the negative electrode film layer is: M / (2×S1).

[0088] In some embodiments, the single-side density b of the negative electrode film layer can be 65 g / m 2 , 68g / m 2 , 70g / m 2 , 73g / m 2 , 75g / m 2 , 78g / m 2 , 80g / m 2 , 83g / m 2 , 85g / m 2 , 88g / m 2 , 90g / m 2, 93g / m 2 , 95g / m 2 , 98g / m 2 , 100g / m 2 , or a value within any two of the above-mentioned single-side density composition ranges or composition ranges.

[0089] When the single-side density of the negative electrode film layer is within the above range, it is beneficial to improve the capacity of the lithium secondary battery of the embodiment of the present application, and is also beneficial to provide more reaction sites for lithium ions, thereby further improving the low-temperature fast charging performance of the lithium secondary battery of the present application.

[0090] In some embodiments, the negative electrode active material includes one or more of graphite or its modified product, and a silicon-carbon mixture.

[0091] In some embodiments, the graphite includes natural graphite or artificial graphite.

[0092] In some embodiments, the modification of graphite includes a doped modification or a coated modification. In some embodiments, the silicon-carbon mixture includes a mixture of graphite and a silicon-based material. In some embodiments, the mass ratio of graphite to silicon-based material is 98:2-60:40.

[0093] In some embodiments, the silicon-based material includes at least one of elemental silicon, silicon-oxygen compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.

[0094] In this article, the term "silicon carbon mixture" generally refers to a mixture of silicon (Si) and carbon (C), where silicon is used as an active material and carbon is used as a conductive agent or binder. This mixture can be a physical mixture of silicon particles and carbon particles, or a composite material in which silicon is embedded in a carbon matrix.

[0095] The compound of formula I is easier to bond with the above-mentioned negative electrode active material, thereby promoting the desolvation of lithium ions solvated by propylene carbonate, which is beneficial to further inhibit the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, and is beneficial to reduce the degree of stripping of the negative electrode active material, thereby further improving the cycle stability of the lithium secondary battery of the present application.

[0096] [Electrolyte]

[0097] In some embodiments, R is selected from fluorine, chlorine or bromine, and n is selected from 1, 4 or 6.

[0098] In some embodiments, R is fluorine.

[0099] In some embodiments, n is selected from 1, 2, 3, 4, 5 or 6.

[0100] In some embodiments, the compound of Formula I includes at least one of 1-fluorobenzene, 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene, or hexafluorobenzene.

[0101] Compared to other halogen atoms, fluorine atoms have stronger electron-withdrawing ability. Using fluorine atoms to replace benzene rings can make the center of the benzene ring lack electrons, which is then conducive to enhancing the π-π interaction between the compound of formula I and the negative electrode active material, and is conducive to the compound of formula I enriched on the surface of the negative electrode active material, suppressing the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, thereby further improving the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application. Further, compared to 1-fluorobenzene, when the compound of formula I is selected from tetrafluorobenzene and hexafluorobenzene, as the number of fluorine atoms on the benzene ring increases, the more electrons are lacking in the center of the benzene ring, the stronger the π-π interaction between the benzene ring and the negative electrode active material, which is conducive to further suppressing the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, and further improving the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0102] In some embodiments, based on the total mass of the electrolyte, the mass percentage W1 of the compound of Formula I is 0.5%≤W1≤10%.

[0103] In some embodiments, based on the total mass of the electrolyte, the mass percentage W1 of the compound of Formula I is 1%≤W1≤7%.

[0104] In some embodiments, based on the total mass of the electrolyte, the mass percentage W1 of the compound of Formula I is 2%≤W1≤5%.

[0105] In some embodiments, based on the total mass of the electrolyte, the mass percentage W1 of the compound of Formula I can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or a range composed of any two of the above W1 or a numerical value in the range of composition.

[0106] In some embodiments, the mass percentage content W1 of the compound of formula I is the mass percentage content of the compound of formula I in the electrolyte of the lithium secondary battery in the initial state. In some embodiments, the mass percentage content W1 of the compound of formula I is the mass percentage content of the compound of formula I in the electrolyte of the lithium secondary battery in the working state.

[0107] In some embodiments, based on the total mass of the electrolyte, the mass percentage content W2 of propylene carbonate is 2%≤W2≤10%.

[0108] In some embodiments, based on the total mass of the electrolyte, the mass percentage content W2 of propylene carbonate is 5%≤W2≤8%.

[0109] In some embodiments, based on the total mass of the electrolyte, the mass percentage W2 of propylene carbonate can be 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or a range composed of any two of the above W2 or a numerical value in the range of the composition.

[0110] In some embodiments, the mass percentage content W2 of propylene carbonate is the mass percentage content of propylene carbonate in the electrolyte of the lithium secondary battery in the initial state. In some embodiments, the mass percentage content W2 of propylene carbonate is the mass percentage content of propylene carbonate in the electrolyte of the lithium secondary battery in the working state.

[0111] When the compound of formula I is within the above range, on the one hand, it is beneficial to maintain the appropriate viscosity of the electrolyte, which is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application. On the other hand, it is also beneficial to further inhibit the stripping of propylene carbonate from the negative electrode active material, thereby helping to further improve the cycle performance.

[0112] When the mass percentage of propylene carbonate is within the above range, it is beneficial to further improve the low-temperature conductivity of the electrolyte, and is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application. At the same time, it can also reduce the amount of embedded graphite and reduce the degree of graphite stripping, thereby further reducing the degree of deterioration of the cycle performance.

[0113] In some embodiments, the mass percentage W1 of the compound of Formula I and the mass percentage W2 of propylene carbonate satisfy 0.05≤W1 / W2≤5.

[0114] In some embodiments, the mass percentage W1 of the compound of Formula I and the mass percentage W2 of propylene carbonate satisfy 0.35≤W1 / W2≤2.

[0115] In some embodiments, the ratio W1 / W2 of the mass percentage W1 of the compound of formula I to the mass percentage W2 of propylene carbonate can be 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, or a range composed of any two of the above values ​​or a value in a range composed of.

[0116] When the ratio W1 / W2 of the mass percentage of the compound of formula I to the mass percentage of propylene carbonate satisfies the above range, it is beneficial to improve the low-temperature conductivity of the electrolyte, maintain the appropriate viscosity of the electrolyte, and further inhibit the stripping of propylene carbonate on the negative electrode active material, thereby further improving the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application, and at the same time, it is beneficial to further take into account the cycle performance.

[0117] In any embodiment, the specific surface area a of the negative electrode active material, the single lateral density b of the negative electrode film layer and the mass percentage W1 of the compound of formula I satisfy (a×b / 180)≤W1×100≤(a×b / 70).

[0118] When the specific surface area value a of the negative electrode active material, the single lateral density b of the negative electrode film layer and the mass percentage W1 of the compound of formula I satisfy the above formula, it is beneficial to increase the transmission path of lithium ions, increase the reaction or active sites of lithium ions, and at the same time, the negative electrode active material can be fully bonded with the compound of formula I, which is beneficial to enhance the desolvation of lithium ions at the negative electrode interface and reduce the degree of stripping of the negative electrode active material, thereby helping to further improve the low-temperature fast charging performance and cycle performance of the secondary battery of the present application.

[0119] In some embodiments, based on the total mass of the electrolyte, the mass percentage content W3 of the first additive is 0.3%≤W3≤4%.

[0120] In some embodiments, based on the total mass of the electrolyte, the mass percentage W3 of the first additive can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4%, or a range composed of any two of the above W3 or a value in the range of composition.

[0121] It should be understood that since the first additive in the electrolyte will be consumed during the formation and circulation process to generate relevant components in the SEI film, the mass percentage of the first additive in the electrolyte may be lower than the initial added mass percentage of the first additive in the electrolyte.

[0122] When the mass percentage of the first additive in the electrolyte is within the above range, on the one hand, it is conducive to better forming an organic small molecule polymer SEI film, thereby further inhibiting the decomposition of the electrolyte and reducing the degree of side reactions on the surface of the negative electrode active material; on the other hand, it is conducive to forming an organic small molecule polymer SEI film of appropriate thickness, reducing impedance and improving low-temperature fast charging performance. In summary, when W3 meets the above mass percentage, it is conducive to further improving the cycle performance of the lithium secondary battery of the embodiment of the present application while taking into account the low-temperature fast charging performance.

[0123] In some embodiments, the electrolyte includes a second additive, and the second additive includes at least one of lithium difluorophosphate, lithium difluoromonooxalatoborate, lithium difluorobisoxalatoborate, lithium bis(fluorosulfonyl)imide, or lithium fluorosulfonate.

[0124] The second additive is easily decomposed at the negative electrode interface to produce inorganic lithium salts, thereby forming an SEI film rich in inorganic lithium salts. The first additive of the present application mainly generates an organic small molecule polymer SEI film, and the film formation impedance at the interface needs to be further reduced, and the film stability needs to be further improved. The SEI film rich in inorganic lithium salts generated by the second additive has strong rigidity and high thermal stability. The combination of the first additive and the second additive of the present application is conducive to further improving the stability of the negative electrode SEI film and further reducing the interface impedance of the negative electrode, thereby helping to further improve the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0125] In some embodiments, based on the total mass of the electrolyte, the mass percentage content W4 of the second additive is 0.1%≤W4≤3%.

[0126] In some embodiments, based on the total mass of the electrolyte, the mass percentage content W4 of the second additive is 0.1%≤W4≤2%.

[0127] In some embodiments, based on the total mass of the electrolyte, the mass percentage W4 of the second additive can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, or a range composed of any two of the above W4 or a value in the range of composition.

[0128] It should be understood that since the second additive in the electrolyte will be consumed during the formation and circulation process to generate relevant components in the SEI film, the mass percentage of the second additive in the electrolyte may be lower than the initial added mass percentage of the second additive in the electrolyte.

[0129] When the mass percentage of the second additive satisfies the above relationship, it is beneficial to further improve the stability of the SEI film on the negative electrode side and further reduce the interfacial impedance on the negative electrode side, and it is also beneficial to maintain an appropriate viscosity of the electrolyte, thereby further improving the cycle performance of the lithium secondary battery of the embodiment of the present application while taking into account the low-temperature fast charging performance.

[0130] In some embodiments, the first additive includes vinylene carbonate, and based on the total mass of the electrolyte, the mass percentage W3 of the vinylene carbonate is 0.4%≤W3≤3%.

[0131] In some embodiments, based on the total mass of the electrolyte, the mass percentage content W3 of vinylene carbonate is 0.5%≤W3≤1%

[0132] In some embodiments, based on the total mass of the electrolyte, the mass percentage W3 of vinylene carbonate can be 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, or a range composed of any two of the above W3 or a value in the range of composition.

[0133] In some embodiments, the second additive includes lithium difluorophosphate, and based on the total mass of the electrolyte, the mass percentage W4 of the lithium difluorophosphate is 0.25%≤W4≤1%.

[0134] In some embodiments, based on the total mass of the electrolyte, the mass percentage content W4 of lithium difluorophosphate is 0.3%≤W3≤0.5%.

[0135] In some embodiments, based on the total mass of the electrolyte, the mass percentage W4 of lithium difluorophosphate can be 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.95%, 1%, or a range composed of any two of the above W4 or a value in the range of composition.

[0136] When the first additive is vinylene carbonate and the second additive is lithium difluorophosphate, vinylene carbonate decomposes at the negative electrode to form an outer SEI layer rich in organic polymer components, and the outer SEI layer has excellent flexibility. Lithium difluorophosphate forms an inner SEI film rich in inorganic salt components at the negative electrode interface, and the inner SEI film has excellent rigidity and thermal stability. The first additive and the second additive include the above substances, so that the SEI film can better adapt to the expansion and contraction of the negative electrode active material during charging and discharging, alleviate the mechanical rupture of the SEI film, and further reduce the interface impedance on the negative electrode side, thereby further improving the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the present application. Further, when the mass percentage of vinylene carbonate and lithium difluorophosphate is within the above range, it is more conducive to the synergistic effect of the two, further improving the stability of the SEI film on the negative electrode side and further reducing the interface impedance on the negative electrode side, which is conducive to further improving the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0137] In some embodiments, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy 0.2%≤W3+W4≤6%.

[0138] In some embodiments, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy 0.6%≤W3+W4≤4.3%.

[0139] In some embodiments, the sum W3+W4 of the mass percentage W3 of the first additive and the mass percentage W4 of the second additive can be 0.2%, 0.4%, 0.6%, 0.8%, 0.9%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, 4.2%, 4.4%, 4.6%, 4.8%, 5%, 5.2%, 5.4%, 5.6%, 5.8%, 6%, or a range consisting of any two of the above values ​​or a value in the range.

[0140] When the mass percentage W3 of the first additive and the mass percentage W4 of the second additive satisfy the above relationship, it is more conducive to the synergistic effect of the two, further improving the stability of the SEI film on the negative electrode side and further reducing the interfacial impedance on the negative electrode side, which is beneficial to further improve the cycle performance and low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0141] In some embodiments, the specific surface area a of the negative electrode active material, the single lateral density b of the negative electrode film layer, the mass percentage W3 of the first additive and the mass percentage W4 of the second additive satisfy a×b / 360≤(W3+W4)×100≤a×b / 160.

[0142] When the specific surface value a of the negative electrode active material, the single-side density b of the negative electrode film layer, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy the above relationship, specifically, when a×b satisfies the above relationship, it is beneficial to increase lithium ion transmission, so that lithium ions are quickly embedded in the negative electrode, which is beneficial to further improve the low-temperature fast charging performance; when W3+W4 satisfies the above relationship, it is beneficial to further synergize the first additive and the second additive, and is beneficial to form a more stable SEI film at the anode interface, thereby more effectively inhibiting the reduction of the electrolyte at the anode, and further improving the battery cycle performance. Therefore, when a, b, W3 and W4 satisfy the above relationship, it is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application while taking into account the cycle performance.

[0143] In some embodiments, the electrolyte includes a solvent, and the solvent includes at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, r-butyrolactone, diethyl carbonate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, or propyl propionate.

[0144] The above-mentioned solvent can be helpful in dissolving the solutes in the electrolyte to form mobile ions, which is helpful in improving the ionic conductivity of the electrolyte, thereby helping to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0145] In some embodiments, the electrolyte includes a lithium salt including at least one of lithium hexafluorophosphate, lithium fluorosulfonyl (perfluorobutylsulfonyl) imide, or lithium bis (trifluoromethylsulfonyl) imide.

[0146] The above-mentioned lithium salt is dissolved in the solvent, which is beneficial to improving the ionic conductivity of the electrolyte, thereby helping to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0147] In some embodiments, the concentration of the lithium salt is 0.8 mol / L-1.3 mol / L.

[0148] In some embodiments, the concentration of the lithium salt may be 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, or a range consisting of any two of the above concentrations or a value within the range.

[0149] When the lithium salt concentration is within the above range, it is beneficial to improve the ionic conductivity of the electrolyte, thereby helping to further improve the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application.

[0150] [Negative electrode]

[0151] In some embodiments, the compaction density of the negative electrode sheet is 1.1 g / cm 3 -1.5g / cm 3 .

[0152] The compaction density of the negative electrode sheet can be measured by methods and instruments known in the art, and the specific examples are as follows: when the negative electrode sheet is coated on one side, the compaction density of the film layer on one side of the negative electrode sheet = m / (V1-V2); when the negative electrode sheet is coated on both sides, the compaction density of the film layer on one side of the negative electrode sheet = m / [2×(V1-V2)], where m represents the weight of the film layer, V1 represents the volume of the negative electrode sheet, and V2 represents the volume of the negative electrode collector. m can be obtained by subtracting the weight of the negative electrode collector from the weight of the negative electrode sheet, the product of the surface area of ​​the negative electrode sheet and the thickness of the negative electrode sheet is the volume V1 of the negative electrode sheet, the product of the surface area of ​​the negative electrode sheet and the thickness of the negative electrode collector is V2, and the thickness of the negative electrode collector and the thickness of the negative electrode sheet are obtained by measuring the thickness of the empty foil in the tab area with a micrometer.

[0153] In some embodiments, the compaction density of the negative electrode sheet can be 1.1 g / 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 , or a range consisting of any two of the above-mentioned compacted densities or a value within the range of the composition.

[0154] When the compaction density of the negative electrode plate is within the above range, it is beneficial to improve the adhesion between the negative electrode film layer and the negative electrode current collector and to improve the contact degree of various substances in the negative electrode film layer, which is beneficial to further reduce the internal resistance of the negative electrode plate and reduce the risk of film layer falling off, thereby helping to further improve the low-temperature fast charging performance of the lithium secondary battery in the embodiment of the present application.

[0155] In some embodiments, the Dv50 of the negative electrode active material is greater than or equal to 12 μm and less than or equal to 17 μm.

[0156] As used herein, the term "Dv50" refers to the particle size at which the volume cumulative particle size distribution percentage in the particles reaches 50%.

[0157] The Dv50 of the negative electrode active material can be measured by methods and equipment known in the art. For example, referring to GB / T19077.1-2016, the Dv50 value of the negative electrode active material can be determined using a laser particle size analyzer (Malvern Master Size 3000).

[0158] In some embodiments, the Dv50 of the negative electrode active material can be 12μm, 12.5μm, 13μm, 13.5μm, 14μm, 14.5μm, 15μm, 15.5μm, 16μm, 16.5μm, 17μm, or a range composed of any two of the above Dv50 or a value in the range composed of.

[0159] Controlling the Dv50 of the negative electrode active material within the above range is conducive to adjusting the transmission path of lithium ions within an appropriate range, which is conducive to further improving the low-temperature fast charging performance of the lithium secondary battery of the embodiment of the present application. In some embodiments, the negative electrode current collector has two surfaces opposite to each other in its own 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.

[0160] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. 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 material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0161] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from 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).

[0162] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0163] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0164] In some embodiments, the negative electrode sheet can be prepared in the following manner: 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 collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0165] [Positive electrode]

[0166] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0167] As an example, the positive electrode current collector has two surfaces facing each other in its own thickness direction.

[0168] 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.).

[0169] In some embodiments, the positive electrode active material may adopt a positive electrode active material for a battery that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: a lithium-containing phosphate with an olivine structure, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 )、LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 )、LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and its modified compounds, etc. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0170] In some embodiments, the positive electrode plate may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0171] In some embodiments, the positive electrode plate may further include a conductive agent. As an 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.

[0172] In some embodiments, the positive electrode sheet can be prepared in the following manner: 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 collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0173] [Isolation film]

[0174] In some embodiments, the lithium secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous structure separator with good chemical stability and mechanical stability can be selected.

[0175] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0176] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0177] In some embodiments, the lithium secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0178] 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 package, such as a bag-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate can be listed.

[0179] The present application has no particular limitation on the shape of the lithium secondary battery, which may be cylindrical, square or any other shape. Figure 1 The lithium secondary battery 5 is a square structure as an example.

[0180] In some embodiments, reference Figure 2 , the outer packaging may include a shell 51 and a cover plate 53. Among them, 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 receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving 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 receiving cavity. The electrolyte is infiltrated 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.

[0181] 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, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0182] Figure 3 4 is an example of a battery module. Figure 3 In the battery module 4, the plurality of lithium secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of lithium secondary batteries 5 may be fixed by fasteners.

[0183] Optionally, the battery module 4 may further include a housing having a housing space, and the plurality of lithium secondary batteries 5 are housed in the housing space.

[0184] 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 according to the application and capacity of the battery pack.

[0185] Figure 4 and Figure 5 1 is a battery pack 1 as an example. Figure 4 and Figure 5The battery pack 1 may include a battery box and a plurality of 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 a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0186] [Electrical devices]

[0187] In addition, the present application also provides an electrical device, which includes the lithium secondary battery of the present application.

[0188] In some embodiments, the power-consuming device of the embodiment of the present application may also include at least one of a battery module or a battery pack. The lithium secondary battery, battery module, or battery pack may be used as a power source for the power-consuming device, or may be used as an energy storage unit for the power-consuming device. The power-consuming 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 are not limited thereto.

[0189] As the electrical device, a lithium secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

[0190] Figure 6 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the power consumption device's requirements for high power and high energy density of lithium secondary batteries, a battery pack or a battery module can be used.

[0191] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a lithium secondary battery may be used as a power source.

[0192] Example

[0193] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0194] 1. Preparation method

[0195] Embodiment 1:

[0196] 1) Preparation of electrolyte

[0197] In a glove box filled with argon (water content <10ppm, oxygen content <1ppm), ethylene carbonate (EC) / ethyl methyl carbonate (EMC) are uniformly mixed in a volume ratio of 3 / 7, and lithium salt LiPF6 is slowly added to the mixed solvent. After the lithium salt is completely dissolved, the concentration of LiPF6 in the electrolyte is 1 mol / L. 1-Fluorobenzene, propylene carbonate, the first additive vinyl carbonate and the second additive lithium difluorophosphate are added in sequence, stirred evenly, and an electrolyte is obtained, wherein, based on the total mass of the electrolyte, the mass percentage content W1 of 1-fluorobenzene is 2.5%, the mass percentage content of propylene carbonate is 5%, the mass percentage content of vinyl carbonate is 0.8%, and the mass percentage content of lithium difluorophosphate is 0.3%.

[0198] 2) Preparation of negative electrode sheet

[0199] The negative electrode active material artificial graphite, conductive agent carbon black, thickener carboxymethyl cellulose, and binder styrene butadiene rubber were added to deionized water in a mass ratio of 80:15:3:2 and stirred evenly to obtain a negative electrode slurry. The specific surface area of ​​graphite is 0.65 g / cm 2 , the Dv50 of graphite is 15.7 μm, and the solid content of the negative electrode slurry is 30 wt%;

[0200] The negative electrode slurry was prepared at 98.7 g / m 2 The single-sided coating surface density is coated on the copper foil, dried at 85° C., and then cold pressed, slit, and dried at 120° C. under vacuum conditions for 12 hours to obtain the negative electrode sheet.

[0201] Among them, the compaction density of the negative electrode sheet is 1.38g / cm 3 .

[0202] 3) Preparation of positive electrode

[0203] The positive electrode active material lithium iron phosphate (LiFePO4), the conductive agent conductive carbon black, and the binder polyvinylidene fluoride are added to the solvent N-methylpyrrolidone in a mass ratio of 8:1:1, and stirred evenly to obtain a positive electrode slurry, wherein the solid content in the positive electrode slurry is 50wt%;

[0204] Then the positive electrode slurry was adjusted to 197.4 g / m 2 The double-sided coating surface density is coated on the aluminum foil, dried at 85°C and then cold pressed, and then trimmed, cut into pieces, and slit, and then dried at 85°C under vacuum conditions for 4 hours to make the positive electrode sheet.

[0205] 4) Isolation film

[0206] A 16 μm polyethylene (PE) film was used as the isolation membrane.

[0207] 5) Preparation of batteries

[0208] The prepared positive electrode sheet, isolation film, and negative electrode sheet are stacked in order, so that the isolation film is placed between the positive and negative electrode sheets to isolate the positive and negative electrodes, and the bare battery cell is wound, and the pole ears are welded to the bare battery cell. The bare battery cell is placed in an aluminum shell and baked in a vacuum oven at 100°C for 8h, and the prepared electrolyte is injected into the dried battery cell. After packaging, standing, formation, shaping, and capacity testing, the lithium secondary battery of Example 1 is obtained.

[0209] Example 2-3

[0210] The preparation method of the secondary battery of Example 2-3 is similar to that of the secondary battery of Example 1, except that the specific surface area a of the negative electrode active material is adjusted, as shown in Table 1-2.

[0211] Embodiment 4-5

[0212] The preparation method of the secondary battery of Example 4-5 is similar to that of the secondary battery of Example 1, except that the single-side coating surface density b of the negative electrode active material is adjusted, as shown in Table 1-2.

[0213] Embodiment 6-13

[0214] The preparation methods of the secondary batteries of Examples 6-13 are similar to those of the secondary battery of Example 1, except that the content W1 of the compound of Formula I and / or the content W2 of propylene carbonate are adjusted, as shown in Table 1-2.

[0215] Examples 14-21

[0216] The preparation methods of the secondary batteries of Examples 14-21 are similar to those of the secondary battery of Example 1, and specifically the mass percentage content W3 of the first additive and / or the mass percentage content W4 of the second additive are adjusted, as shown in Table 1-2.

[0217] Embodiment 22

[0218] The preparation method of the secondary battery of Example 22 is similar to that of the secondary battery of Example 1, and the types of the first additive and the second additive are specifically adjusted, as shown in Table 1-2.

[0219] Embodiment 23

[0220] The preparation method of the secondary battery of Example 23 is similar to that of the secondary battery of Example 1, except that the type of the compound of Formula I is adjusted, as shown in Table 1-2.

[0221] Comparative Example 1

[0222] The preparation method of the secondary battery of Comparative Example 1 is similar to that of the secondary battery of Example 1, except that the compound of Formula I, propylene carbonate, the first additive and the second additive are not added to the electrolyte, as shown in Table 1-2.

[0223] Comparative Examples 2-3

[0224] The preparation method of the secondary battery of Comparative Example 2-3 is similar to that of the secondary battery of Example 1, except that the specific surface area of ​​the negative electrode active material is different and the second additive is not added to the electrolyte, as shown in Table 1-2.

[0225] Comparative Example 4

[0226] The preparation method of the secondary battery of Comparative Example 4 is similar to that of the secondary battery of Example 1, except that the first additive and the second additive are not added to the electrolyte, as shown in Table 1-2.

[0227] 2. Battery performance test

[0228] 1. Cycle performance test of secondary batteries

[0229] Place the battery in an environment of 25°C for 24 hours, then charge the cell at a constant current of 1C to 3.65V, then charge it at a constant voltage to 0.05C, let it stand for 10 minutes, and then discharge it at a constant current of 1C to 2V. Record the discharge capacity as C0. Perform 500 cycles according to the above charge and discharge process. The discharge capacity of the 500th discharge is C1. The cycle capacity retention rate of the cell = C1 / C0×100%

[0230] 2. Low temperature fast charging performance test of secondary batteries

[0231] Place the battery in an environment of 25°C for 24 hours, then charge the battery cell at a constant current of 1C to 3.65V, then charge it at a constant voltage to 0.05C, let it stand for 10 minutes, then discharge it at a constant current of 1C to 2.5V, and record the discharge capacity as D0. Then place the above battery in an environment of -10°C and charge it at a current of 0.5D0 until the cut-off voltage is 3.65V, record the charge capacity as D1, then discharge it at a current of D0 to 2.0V, let it stand for 30 minutes, then charge it at a current of 2D0 to 3.65V, record the charge capacity as D2, and the low-temperature charge capacity retention rate = D2 / D1. The low-temperature fast charging performance of the secondary battery is characterized by the low-temperature charge capacity retention rate. III. Analysis of test results of various embodiments and comparative examples

[0232] The batteries of the embodiments and comparative examples were prepared according to the above methods, and various performance parameters were measured. The secondary battery preparation parameters are shown in Table 1 and Table 2, and the secondary battery performance test results are shown in Table 3.

[0233] Table 1

[0234]

[0235]

[0236] Table 2

[0237]

[0238]

[0239] Table 3

[0240]

[0241] As can be seen from Table 1 and Table 2, Examples 1-23 provide a lithium secondary battery, the lithium secondary battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer comprises a negative electrode active material, and the specific surface area a of the negative electrode active material is 2 m 2 / g≤a≤5m 2 / g, the electrolyte includes propylene carbonate, a first additive and a compound having the following formula I

[0242]

[0243] R is selected from halogen, n is selected from an integer of 1 to 6,

[0244] The first additive includes at least one of vinyl sulfate, vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, tetravinylsilane or methylene methanedisulfonate.

[0245] It can be seen from Examples 1-23 and Comparative Examples 1-4 that the lithium secondary battery of the present application has good low-temperature fast charging performance while taking into account cycle performance.

[0246] It can be seen from Examples 1-3, Example 21 and Comparative Examples 2-3 that when the specific surface area a of the negative electrode active material is 2 m 2 / g≤a≤5m 2 / g, which is beneficial to improving the low-temperature fast charging performance of the lithium secondary battery of the present application and taking into account the cycle performance. It can be seen from Example 21 and Comparative Example 4 that when the electrolyte includes the first additive of the present application, it is beneficial to improve the cycle performance of the lithium secondary battery.

[0247] It can be seen from Examples 1, 4-5 that when the single-side density b of the negative electrode film layer is 65 g / m 2 ≤b≤100g / m 2 It is beneficial to further improve the low-temperature fast charging performance of lithium secondary batteries.

[0248] It can be seen from Examples 1 and 6-9 that when the mass percentage content W1 of the compound of formula I is 0.5% ≤ W1 ≤ 10%, it is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery while taking into account the cycle performance. By comparing Examples 1, 7-8 with Examples 6 and 9, it can be seen that when the mass percentage content W1 of the compound of formula I is 1% ≤ W1 ≤ 7%, it is beneficial to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery. By comparing Examples 1 and 7 with Examples 6, 8-9, it can be seen that when the mass percentage content W1 of the compound of formula I is 2% ≤ W1 ≤ 5%, it is beneficial to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery.

[0249] By comparing Examples 1 and 7 with Examples 6, 8-9, it can be seen that when the specific surface area a of the negative electrode active material, the single lateral density b of the negative electrode film layer and the mass percentage W1 of the compound of Formula I satisfy a×b / 180≤100×W1≤a×b / 70, it is beneficial to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery.

[0250] It can be seen from Examples 1 and 10-12 that when the mass percentage content W2 of propylene carbonate is 2%≤W2≤10%, it is beneficial to further improve the low-temperature fast charging performance of the secondary battery while taking into account the cycle performance. By comparing Examples 1 and 11 with Examples 10 and 12, it can be seen that when the mass percentage content W2 of propylene carbonate is 5%≤W2≤8%, it is beneficial to further improve the low-temperature fast charging performance of the lithium secondary battery while taking into account the cycle performance.

[0251] It can be seen from Examples 1 and 6-13 that when the mass percentage content W1 of the compound of Formula I and the mass percentage content W2 of the propylene carbonate satisfy 0.05≤W1 / W2≤5, it is beneficial to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery. By comparing Examples 1, 7-11 and Examples 6, 12-13, it can be seen that when the mass percentage content W1 of the compound of Formula I and the mass percentage content W2 of the propylene carbonate satisfy 0.35≤W1 / W2≤2, it is beneficial to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery.

[0252] From the comparison between Examples 1, 14-20 and Example 21, it can be seen that when the electrolyte includes the second additive, it is beneficial to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery. From Examples 1, 3, 17-20, it can be seen that when the electrolyte includes the second additive in a percentage content W3 of 0.1% ≤ W4 ≤ 3%, further, 0.25% ≤ W4 ≤ 1%, and further, 0.3% ≤ W3 ≤ 0.5%, it is helpful to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery.

[0253] It can be seen from Examples 1, 3, 14-16, and 19-20 that when the content W3 of the first additive is 0.1%≤W3≤4%, further, 0.4%≤W3≤3%, and further, 0.5%≤W3≤1%, it helps to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery.

[0254] It can be seen from Examples 1 and 14-20 that when the mass percentage W3 of the first additive and the mass percentage W4 of the second additive satisfy 0.2%≤W3+W4≤6%, and further satisfy 0.6%≤W3+W4≤4.3%, it is beneficial to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery.

[0255] By comparing Examples 1-5, 16, 19 with Examples 14-15, 17-18 and Example 20, it can be seen that when the specific surface area value a of the negative electrode active material, the single lateral density b of the negative electrode film layer, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy a×b / 360≤(W3+W4)×100≤a×b / 160, it is beneficial to further improve the low-temperature fast charging performance and cycle performance of the lithium secondary battery.

[0256] It can be seen from Example 1 and Example 22 that the types of the first additive and the second additive are applicable to the various compounds listed in this application.

[0257] It can be seen from Example 1 and Example 23 that when the compound of Formula I includes hexafluorobenzene, the low-temperature fast charging performance and cycle performance of the lithium secondary battery are further improved.

[0258] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining 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 lithium secondary battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer includes a negative electrode active material, and the specific surface area a of the negative electrode active material is 2m 2 / g≤a≤5m 2 / g, the electrolyte includes propylene carbonate and a first additive and a compound having the following formula I R is selected from halogen, n is an integer selected from 1-6, The first additive includes at least one of vinyl sulfate, vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, tetravinylsilane or methylene methanedisulfonate.

2. The lithium secondary battery according to claim 1, characterized in that: The specific surface area a of the negative electrode active material is 2 m 2 / g≤a≤4m 2 / g.

3. The lithium secondary battery according to claim 1 or 2, characterized in that: The single side density b of the negative electrode film layer is 65 g / m 2 ≤b≤100g / m 2 .

4. The lithium secondary battery according to claims 1 to 3, characterized in that: The negative electrode active material includes one or more of graphite or its modified product, and a silicon-carbon mixture.

5. The lithium secondary battery according to any one of claims 1 to 4, characterized in that: R is selected from fluorine, chlorine or bromine, and n is selected from 1, 4 or 6.

6. The lithium secondary battery according to any one of claims 1 to 5, characterized in that: The compound of formula I includes at least one of 1-fluorobenzene, 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene or hexafluorobenzene.

7. The lithium secondary battery according to any one of claims 1 to 6, characterized in that: Based on the total mass of the electrolyte, the mass percentage content W1 of the compound of formula I is 0.5%≤W1≤10%, optionally, based on the total mass of the electrolyte, the mass percentage content W1 of the compound of formula I is 1%≤W1≤7%, more optionally, based on the total mass of the electrolyte, the mass percentage content W1 of the compound of formula I is 2%≤W1≤5%; and / or Based on the total mass of the electrolyte, the mass percentage W2 of the propylene carbonate is 2%≤W2≤10%. Optionally, based on the total mass of the electrolyte, the mass percentage W2 of the propylene carbonate is 5%≤W2≤8%.

8. The lithium secondary battery according to any one of claims 1 to 7, characterized in that: The mass percentage W1 of the compound of formula I and the mass percentage W2 of the propylene carbonate satisfy 0.05≤W1 / W2≤5, optionally 0.35≤W1 / W2≤2.

9. The lithium secondary battery according to any one of claims 3 to 8, characterized in that: The specific surface value a of the negative electrode active material, the single lateral density b of the negative electrode film layer and the mass percentage content W1 of the compound of formula I satisfy (a×b / 180)≤W1×100≤(a×b / 70).

10. The lithium secondary battery according to any one of claims 1 to 9, characterized in that: Based on the total mass of the electrolyte, the mass percentage content W3 of the first additive is 0.3%≤W3≤4%.

11. The lithium secondary battery according to any one of claims 1 to 10, characterized in that: The electrolyte includes a second additive, and the second additive includes at least one of lithium difluorophosphate, lithium difluoromonooxalatoborate, lithium difluorobisoxalatoborate, lithium bis(fluorosulfonyl)imide, or lithium fluorosulfonate.

12. The lithium secondary battery according to claim 11, characterized in that: Based on the total mass of the electrolyte, the mass percentage content W4 of the second additive is 0.1%≤W4≤3%, and optionally W4 is 0.1%≤W4≤2%.

13. The lithium secondary battery according to claim 11 or 12, characterized in that: The first additive includes vinylene carbonate, and based on the total mass of the electrolyte, the mass percentage content W3 of vinylene carbonate is 0.4%≤W3≤3%, optionally, based on the total mass of the electrolyte, the mass percentage content W3 of vinylene carbonate is 0.5%≤W3≤1%; and / or, The second additive includes lithium difluorophosphate. Based on the total mass of the electrolyte, the mass percentage W4 of the lithium difluorophosphate is 0.25%≤W4≤1%. Optionally, based on the total mass of the electrolyte, the mass percentage W4 of the lithium difluorophosphate is 0.3%≤W3≤0.5%.

14. The lithium secondary battery according to any one of claims 11 to 13, characterized in that: The mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy 0.2%≤W3+W4≤6%, optionally 0.6%≤W3+W4≤4.3%.

15. The lithium secondary battery according to any one of claims 11 to 14, characterized in that: The specific surface value a of the negative electrode active material, the single lateral density b of the negative electrode film layer, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy a×b / 360≤(W3+W4)×100≤a×b / 160.

16. The lithium secondary battery according to any one of claims 1 to 15, characterized in that: The electrolyte includes a solvent, and the solvent includes at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, r-butyrolactone, diethyl carbonate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate or propyl propionate.

17. The lithium secondary battery according to any one of claims 1 to 16, characterized in that: The electrolyte includes a lithium salt, and the lithium salt includes at least one of lithium hexafluorophosphate, lithium fluorosulfonyl (perfluorobutylsulfonyl) imide, or lithium bis (trifluoromethylsulfonyl) imide.

18. The lithium secondary battery according to claim 17, characterized in that: The concentration of the lithium salt is 0.8 mol / L-1.3 mol / L.

19. The lithium secondary battery according to any one of claims 1 to 18, characterized in that: The compaction density of the negative electrode sheet is 1.1 g / cm 3 -1.5g / cm 3 .

20. The lithium secondary battery according to any one of claims 1 to 19, characterized in that: The Dv50 of the negative electrode active material is greater than or equal to 12 μm and less than or equal to 17 μm.

21. An electrical device, characterized in that: The electric device comprises a lithium secondary battery according to any one of claims 1 to 20.

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