Lithium secondary battery and electric device

Abstract

Provided in the embodiments of the present application are a lithium secondary battery and an electric device. The lithium secondary battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator located between the positive electrode sheet and the negative electrode sheet, wherein the separator has a porosity of 28% to 50%; and the electrolyte comprises propylene carbonate and the compound of formula I, where R is selected from halogen, and n is an integer selected from 1-6. The lithium secondary battery provided in the embodiments of the present application has good low-temperature fast charging performance and cycle life.

Description

Lithium secondary batteries and electrical devices

[0001] Cross-referencing

[0002] This application incorporates Chinese Patent Application No. 202510018261.4, filed on January 6, 2025, entitled "Lithium Secondary Battery and Electrical Device", which is incorporated herein by reference in its entirety. Technical Field

[0003] This application relates to the field of lithium secondary battery technology, and in particular to a lithium secondary battery and an electrical device thereof. Background Technology

[0004] In recent years, lithium-ion batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace.

[0005] With the widespread application of lithium secondary batteries, higher demands are being placed on their low-temperature fast charging performance and cycle life. Summary of the Invention

[0006] This application is made in view of the above-mentioned issues, and its purpose is to provide a lithium secondary battery and an electrical device, wherein the lithium secondary battery has excellent low-temperature fast charging performance and cycle life.

[0007] To achieve the above objectives, a first aspect of this application provides a lithium secondary battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator located between the positive and negative electrodes. The separator has a porosity of 28% to 50%, and the electrolyte comprises propylene carbonate and a compound having the following formula I.

[0008] Where R is selected from halogens, and n is an integer selected from 1 to 6.

[0009] Propylene carbonate has a high dielectric constant and a low freezing point. Including propylene carbonate in the electrolyte is beneficial for improving the low-temperature conductivity of the electrolyte, thereby improving the low-temperature fast-charging performance of the lithium-ion secondary battery in this embodiment. However, propylene carbonate readily intercalates with lithium ions into the negative electrode active material (e.g., graphite) on the negative electrode side, leading to the stripping of the negative electrode active material and deteriorating the cycle stability of the secondary battery. Therefore, this embodiment adds a compound of Formula I to the electrolyte. This compound contains a large π bond, which readily bonds with the negative electrode active material (e.g., graphite, silicon-carbon mixtures, etc.), thereby enriching it on the surface of the negative electrode active material and giving it a certain degree of negative charge. This promotes the desolvation of lithium ions solubilized by propylene carbonate, helps suppress the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, and helps reduce the degree of stripping of the negative electrode active material, thus improving the cycle stability of the lithium-ion secondary battery in this embodiment. Furthermore, the compound of Formula I helps improve the wettability of the electrolyte, further enhancing the battery's cycle stability and low-temperature fast-charging performance.

[0010] The improved electrolyte wettability allows the electrolyte to quickly wet the microporous structure of the separator, facilitating the rapid transport of lithium ions between the positive and negative electrodes. Therefore, compared to conventional electrolyte designs, the electrolyte design in this embodiment reduces the porosity requirements of the separator for lithium secondary batteries. When the electrolyte of this embodiment is paired with a separator within the aforementioned porosity range, it can mitigate the self-discharge and even short-circuit issues caused by excessive porosity, and also improve the low-temperature fast-charging performance degradation caused by excessively low porosity, further enhancing the low-temperature fast-charging performance and cycle stability of lithium secondary batteries.

[0011] The lithium secondary battery of this application embodiment improves low-temperature fast charging performance and cycle stability through the design of reasonable electrolyte composition and the design of separator porosity.

[0012] In any embodiment, the porosity of the separator is 31% to 40%.

[0013] When the porosity of the separator meets the above range, the low-temperature fast charging performance and cycle stability of the lithium secondary battery are further improved.

[0014] In any embodiment, the separator includes a base membrane and a coating disposed on at least a portion of the surface of the base membrane, the coating including inorganic particles and organic particles.

[0015] When the coating on at least part of the surface of the base film in the separator includes inorganic and organic particles, the coating can improve the overall strength of the separator, reduce the risk of puncture, improve the interfacial contact between the electrode and the separator, reduce the risk of separator wrinkling, and the organic particles in the coating help to leave gaps between the electrode and the separator, thereby reducing the expansion of the battery during charging and discharging, and further improving the cycle stability of the battery.

[0016] In any embodiment, the inorganic particles include at least one of boehmite, alumina, silicon dioxide, magnesium oxide, magnesium hydroxide, titanium dioxide, tin dioxide, barium sulfate, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, cerium oxide, magnesium fluoride, and barium titanate.

[0017] The aforementioned inorganic particles are suitable for the preparation of the separator coating in the embodiments of this application, which helps to improve the overall strength of the separator and thus further enhance the cycle stability of the battery.

[0018] In any embodiment, the organic particles include at least one of the following: homopolymers or copolymers of fluoroolefin monomer units, homopolymers or copolymers of olefin monomer units, homopolymers or copolymers of unsaturated nitrile monomer units, homopolymers or copolymers of epoxy alkane monomer units, homopolymers or copolymers of acrylic monomer units, homopolymers or copolymers of acrylate monomer units, homopolymers or copolymers of styrene monomer units, polyurethane compounds, rubber compounds, and modified compounds of the above homopolymers or copolymers.

[0019] The aforementioned organic particles are suitable for the preparation of the separator coating in the embodiments of this application, which is beneficial to improving the contact interface between the separator and the electrode, and helps to leave gaps between the electrode and the separator, thereby further improving the cycle stability of the battery.

[0020] In any embodiment, inorganic particles are disposed on at least a portion of the surface of the base film, and organic particles are disposed on at least a portion of the surface of the inorganic particles and / or at least partially embedded in the inorganic particles.

[0021] When organic particles are partially embedded in inorganic particles, the structure of the separator coating becomes more robust, and the organic particles are less likely to fall off. This helps the organic particles to more stably perform their bonding and gap-reserving functions, which is beneficial to further improving the cycle stability of the battery.

[0022] In any embodiment, the number-average particle size of the inorganic particles is 0.5 μm to 2.5 μm, and / or the number-average particle size of the organic particles is 2 μm to 25 μm, optionally 4 μm to 20 μm.

[0023] When the number-average particle size of the inorganic particles in the separator coating meets the above-mentioned range, it is beneficial for the inorganic particles to be more uniformly and densely distributed on the base film, thereby further improving the strength of the separator, reducing the risk of the separator being punctured, and further improving the cycle stability of the battery. Moreover, when the number-average particle size of the inorganic particles meets the above-mentioned range, the porosity of the separator is within a suitable range, such as 28% to 50%, thereby further improving the cycle stability and low-temperature fast charging performance of the battery.

[0024] When the number-average particle size of the organic particles in the separator coating meets the above-mentioned range, it avoids the problem of excessively large particle size causing the organic particles to form a film and deteriorate lithium-ion conduction, while also preventing the particle size from being too small, which would prevent them from being uniformly dispersed on the base film and thus failing to provide a gap between the electrode and the separator. Meeting the above-mentioned range ensures that the separator provides a good channel for lithium-ion transport, has a good contact interface with the electrode, and that the organic particles help to leave a gap between the electrode and the separator, thereby further improving the battery's low-temperature fast-charging performance and cycle stability.

[0025] When the number-average particle size of inorganic and organic particles meets the above range, it is more conducive to forming a coating morphology in which organic particles are at least partially embedded in inorganic particles, thereby further improving the cycle stability of the battery.

[0026] In any embodiment, the total thickness of the coating is 1 μm to 7 μm, and can be selected as 2 μm to 4 μm.

[0027] When the thickness of the coating in the separator is controlled to meet the above range, it can both improve the contact interface between the separator and the electrode and the mechanical strength of the separator, and ensure the conduction of lithium ions between the positive and negative electrodes, thereby further improving the cycle stability and low-temperature fast charging performance of the battery.

[0028] In any embodiment, the total thickness of the isolation membrane is 6 μm to 20 μm, and can be selected as 9 μm to 14 μm.

[0029] By controlling the thickness of the separator to meet the above range, the risk of the separator being punctured can be further reduced, enabling the battery to have excellent energy density while further improving cycle stability.

[0030] In any embodiment, R is selected from fluorine, chlorine or bromine, and n is selected from 1, 4 or 6.

[0031] In any embodiment, 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.

[0032] Compared to other halogen atoms, fluorine atoms have a stronger electron-withdrawing ability. Replacing the benzene ring with fluorine atoms makes the benzene ring center electron-deficient, which in turn enhances the π-π interaction between the compound of Formula I and the negative electrode active material. This also facilitates the enrichment of the compound of Formula I on the surface of the negative electrode active material, further suppressing the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, further improving the wettability of the electrolyte, and thus further enhancing the battery's cycle stability and low-temperature fast-charging performance. Furthermore, as the number of fluorine-substituted compounds in Formula I increases, the benzene ring center becomes more electron-deficient, and the stronger the π-π interaction between the benzene ring and the negative electrode active material, which further suppresses the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, further improving the wettability of the electrolyte, and thus further enhancing the battery's cycle stability and low-temperature fast-charging performance.

[0033] In any embodiment, based on the total mass of the electrolyte, the mass percentage W1 of the compound of formula I is 0.5% ≤ W1 ≤ 10%, optionally, W1 is 2% ≤ W1 ≤ 7%; and / or

[0034] Based on the total mass of the electrolyte, the mass percentage of propylene carbonate W2 is 2% ≤ W2 ≤ 10%, and optionally, W2 is 5% ≤ W2 ≤ 8%.

[0035] When the mass percentage of the compound of Formula I is within the above range, it suppresses the stripping of the negative electrode active material by propylene carbonate while maintaining a suitable viscosity of the electrolyte. The electrolyte has excellent conductivity, which is beneficial to further improve the cycle stability of the lithium secondary battery in the present application embodiment, while also taking into account excellent low-temperature fast charging performance.

[0036] When the mass percentage of propylene carbonate is within the above range, it further improves the low-temperature conductivity of the electrolyte, while also reducing the amount of propylene carbonate embedded in the negative electrode active material, thereby reducing the degree of stripping of the negative electrode active material. This is beneficial for further improving the low-temperature fast-charging performance of the lithium secondary battery in the embodiments of this application, while also slowing down the deterioration of the battery cycle performance caused by propylene carbonate.

[0037] In any embodiment, the mass percentage content W1 of the compound of Formula I and the mass percentage content W2 of propylene carbonate satisfy 0.05≤W1 / W2≤5, and optionally, 0.35≤W1 / W2≤2.

[0038] When the ratio W1 / W2 of the mass percentage content of compound I and the mass percentage content of propylene carbonate meets the above range, it is beneficial to improve the low-temperature conductivity of the electrolyte, maintain the appropriate viscosity of the electrolyte, and further suppress the stripping of propylene carbonate on the negative electrode active material, thereby further improving the low-temperature fast charging performance and cycle stability of the lithium secondary battery of the present application embodiment.

[0039] In any embodiment, the electrolyte includes a first additive, which includes at least one of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, tetravinylsilane, or methanedisulfonate.

[0040] Since the compound of Formula I does not participate in the formation of the negative electrode solid electrolyte interphase (SEI) film, the electrolyte of the lithium secondary battery in this application includes a first additive, which is beneficial to the formation of an SEI film rich in organic components on the negative electrode surface. The SEI film rich in organic components has excellent flexibility, which is beneficial to suppress the reduction and decomposition of the electrolyte on the negative electrode side and alleviate the mechanical rupture of the SEI film, thereby further improving the cycle stability of the lithium secondary battery in this application.

[0041] In any embodiment, based on the total mass of the electrolyte, the mass percentage of the first additive W3 is 0.1% ≤ W3 ≤ 4%.

[0042] When the mass percentage of the first additive in the electrolyte is within the above range, it is beneficial to better form an SEI film rich in organic components and to maintain a suitable viscosity of the electrolyte, thereby further improving the cycle stability and low-temperature fast charging performance of the lithium secondary battery in the embodiments of this application.

[0043] In any embodiment, the electrolyte includes a second additive, which includes at least one of lithium difluorophosphate, lithium difluoromonoxalate borate, lithium difluorobisoxalate borate, lithium difluorosulfonylimide, or lithium fluorosulfonate.

[0044] Because the organic-rich SEI film formed by the first additive has poor thermal stability, adding a second additive, including the aforementioned substances, to the electrolyte allows these substances to form an inorganic-rich SEI film at the negative electrode. This inorganic-rich SEI film exhibits excellent rigidity and thermal stability. The second additive works synergistically with the first additive to further enhance the stability of the SEI film, which helps reduce the interfacial impedance on the negative electrode side, thereby further improving the battery's cycle stability and low-temperature fast-charging performance. Furthermore, these substances can also form a positive electrode electrolyte interfacial film (CEI film) on the positive electrode surface, which helps suppress the oxidation reaction of the electrolyte on the positive electrode side, thus further improving the battery's cycle stability.

[0045] In any embodiment, based on the total mass of the electrolyte, the mass percentage of the second additive W4 is 0.1% ≤ W4 ≤ 3%.

[0046] When the mass percentage of the second additive is within the above range, it is beneficial to further improve the stability of the SEI film, reduce the interfacial impedance on the negative electrode side, further suppress the oxidation reaction of the electrolyte on the positive electrode side, and make the viscosity of the electrolyte suitable, thereby further improving the cycle stability and low-temperature fast charging performance of the lithium secondary battery in the present application embodiment.

[0047] 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%, and optionally, 0.4% ≤ W3 + W4 ≤ 4.3%.

[0048] When the contents of the first additive and the second additive satisfy the above relationship, it is beneficial to further utilize their synergistic effect, further reduce the interfacial impedance on the negative electrode side, improve the stability of the SEI film, further suppress the decomposition of the electrolyte on the positive and negative electrode sides, and further help maintain the appropriate viscosity of the electrolyte, thereby further improving the low-temperature fast charging performance and cycle life of the battery.

[0049] In any embodiment, the first additive includes vinylene carbonate, and the mass percentage of vinylene carbonate is 0.4% to 3%, optionally 0.5% to 1%, based on the total mass of the electrolyte; and / or the second additive includes lithium difluorophosphate, and the mass percentage of lithium difluorophosphate is 0.25% to 1%, optionally 0.3% to 0.5%, based on the total mass of the electrolyte.

[0050] Lithium difluorophosphate decomposes preferentially over other components in the electrolyte, forming an inner SEI film rich in inorganic salt components at the negative electrode interface. This inner SEI film exhibits excellent rigidity and thermal stability. Subsequently, vinylene carbonate decomposes at the negative electrode, forming an outer SEI film rich in organic polymer components. This outer SEI film exhibits excellent flexibility. The first additive includes vinylene carbonate, and the second additive includes lithium difluorophosphate. This allows the SEI film to better adapt to the expansion and contraction of the negative electrode active material during charging and discharging, alleviates the mechanical rupture of the SEI film, and further reduces the interfacial impedance on the negative electrode side, thereby further improving the cycle stability and low-temperature fast-charging performance of the lithium secondary battery of this application embodiment. Furthermore, when the mass percentages of vinylene carbonate and lithium difluorophosphate are within the above-mentioned range, it is more conducive to their synergistic effect, further improving the stability of the SEI film on the negative electrode side and further reducing the interfacial impedance on the negative electrode side, further inhibiting the decomposition of the electrolyte on both the positive and negative electrode sides, and further helping to maintain a suitable electrolyte viscosity, thereby further improving the cycle stability and low-temperature fast-charging performance of the lithium secondary battery of this application embodiment.

[0051] In any embodiment, the electrolyte includes a solvent, which includes at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, diethyl carbonate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, or propyl propionate.

[0052] The aforementioned solvent is beneficial for dissolving lithium salts and additives in the electrolyte, improving the ionic conductivity of the electrolyte, and thus further enhancing the low-temperature fast-charging performance and cycle stability of the lithium secondary battery in the embodiments of this application.

[0053] In any embodiment, the electrolyte comprises a lithium salt, which includes at least one of lithium hexafluorophosphate, lithium fluorosulfonyl (perfluorobutylsulfonyl)imide, or lithium bis(trifluoromethylsulfonyl)imide.

[0054] The aforementioned lithium salts possess excellent thermal and chemical stability, and are beneficial for further improving the ionic conductivity of the electrolyte, thereby further enhancing the low-temperature fast-charging performance and cycle stability of the lithium secondary batteries in the embodiments of this application.

[0055] In any embodiment, the concentration of lithium salt is 0.8 mol / L to 1.3 mol / L.

[0056] When the lithium salt concentration meets the above range, it is beneficial to balance the ionic conductivity and wettability of the electrolyte, thereby further improving the low-temperature fast charging performance and cycle stability of the lithium secondary battery in this application embodiment.

[0057] In any embodiment, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes one or more of graphite or its modified form, and silicon-carbon mixture.

[0058] The compounds of Formula I are more likely to bond with the above-mentioned negative electrode active materials, thereby promoting the desolvation of lithium ions solubilized by propylene carbonate, which is beneficial to further suppress the co-intercalation 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 embodiment.

[0059] A second aspect of this application provides an electrical device, characterized in that it includes a lithium secondary battery according to any embodiment of the first aspect of this application. Attached Figure Description

[0060] Figure 1 is a schematic diagram of a lithium secondary battery according to an embodiment of this application;

[0061] Figure 2 is an exploded view of a lithium secondary battery according to an embodiment of this application shown in Figure 1;

[0062] Figure 3 is a schematic diagram of a battery module according to an embodiment of this application;

[0063] Figure 4 is a schematic diagram of a battery pack according to an embodiment of this application;

[0064] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4;

[0065] Figure 6 is a schematic diagram of an electrical device using a lithium secondary battery as a power source according to an embodiment of this application.

[0066] Explanation of reference numerals in the attached diagram: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Lithium secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0067] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the lithium secondary battery and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0068] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0069] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0070] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0071] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0072] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0073] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: 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).

[0074] Lithium-ion batteries are widely used in various fields; however, due to the influence of components such as electrolytes and separators, their low-temperature fast-charging performance and cycle life are relatively poor. This application provides a lithium-ion battery with excellent low-temperature fast-charging performance and cycle life.

[0075] [Lithium-ion rechargeable battery]

[0076] This application provides a lithium secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator between the positive and negative electrodes. The separator has a porosity of 28% to 50%, and the electrolyte includes propylene carbonate and a compound having the following formula I.

[0077] Where R is selected from halogens, and n is an integer selected from 1 to 6.

[0078] In this document, the term "porosity" has its well-known meaning in the art, referring to the proportion of the volume of pores in a separator to the total volume of the separator. Higher porosity in a separator means a more abundant pore structure, better electrolyte permeability, and a higher lithium-ion transport rate.

[0079] In some implementations, the porosity of the separator is the porosity of the separator in the initial state of the lithium secondary battery.

[0080] The porosity of the separator in this application can be measured using methods known in the art. For example, a separator sample from a battery cell in its initial state can be immersed in a dimethyl carbonate (DMC) solution for 8 hours, removed, and dried. The porosity can then be tested according to standard GB / T 24586-2009. Specifically, a separator sample is added to a container of known volume, the container is sealed, and a certain amount of helium gas is introduced. The pressure inside the container is measured using a pressure sensor. Then, the gas inside the container is diffused to another container with known pressure and volume. The equilibrium pressure between the two connected containers is measured using a pressure sensor. The true volume V1 of the separator sample is obtained according to Bohr's law. The apparent volume V2 of the separator sample is obtained by testing the thickness and area of ​​the separator. The porosity P is then P = (V2 - V1) / V2 × 100%. The method for testing the thickness of the separator is to take a separator sample from a battery cell in its initial state, immerse it in a DMC solution for 8 hours, remove, and dry it. The thickness of the separator is randomly measured at 10 different locations using a thickness gauge, and the average value is recorded as the thickness of the separator.

[0081] In some embodiments, the porosity of the separator can be selected as 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, or other unlisted values ​​in the range of 28% to 50%.

[0082] In this article, the term "halogen" refers to elements in Group VIIA of the periodic table of chemical elements. Specifically, halogens include elements such as fluorine, chlorine, bromine, iodine, and astatine.

[0083] The components of the electrolyte can be detected using methods known to those skilled in the art. For example, the composition of the electrolyte can be determined by liquid chromatography, ultraviolet spectrophotometry, or ultraviolet-visible spectrophotometry. Exemplarily, the free electrolyte is diluted 3 to 10 times with acetonitrile to obtain a diluted electrolyte solution to be tested. Using a GC-MS 3100 organic component gas chromatograph, the diluted electrolyte solution is placed in the instrument for full-scan qualitative analysis. The injection port temperature is 250°C, and the scan range is 35 μm to 270 μm. After the test, a total ion chromatogram of each organic compound is obtained, and the corresponding organic compound type is determined by comparing the peak positions in the chromatogram. In some embodiments, in the compounds of Formula I, n is 1, 2, 3, 4, 5, or 6.

[0084] Propylene carbonate has a high dielectric constant and a low freezing point. Including propylene carbonate in the electrolyte is beneficial for improving the low-temperature conductivity of the electrolyte, thereby improving the low-temperature fast-charging performance of the lithium-ion secondary battery in this embodiment. However, propylene carbonate readily intercalates with lithium ions into the negative electrode active material (e.g., graphite) on the negative electrode side, leading to the stripping of the negative electrode active material and deteriorating the cycle stability of the secondary battery. Therefore, this embodiment adds a compound of Formula I to the electrolyte. This compound contains a large π bond, which readily bonds with the negative electrode active material (e.g., graphite, silicon-carbon mixtures, etc.), thereby enriching it on the surface of the negative electrode active material and giving it a certain degree of negative charge. This promotes the desolvation of lithium ions solubilized by propylene carbonate, helps suppress the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, and helps reduce the degree of stripping of the negative electrode active material, thus improving the cycle stability of the lithium-ion secondary battery in this embodiment. Furthermore, the compound of Formula I helps improve the wettability of the electrolyte, further enhancing the battery's cycle stability and low-temperature fast-charging performance.

[0085] The improved electrolyte wettability allows the electrolyte to quickly wet the microporous structure of the separator, which is beneficial for lithium ions to quickly pass through the microporous structure of the separator to and from the positive and negative electrodes. Therefore, compared with conventional electrolyte designs, the electrolyte design of this application embodiment reduces the requirement for separator porosity in lithium secondary batteries. When the electrolyte of this application embodiment is paired with a separator within the above-mentioned porosity range, it can improve the situation where excessive porosity causes self-discharge or even short circuits in the battery, and it can also improve the situation where excessive porosity causes deterioration of low-temperature fast charging performance, further improving the low-temperature fast charging performance and cycle stability of lithium secondary batteries.

[0086] The lithium secondary battery of this application embodiment improves low-temperature fast charging performance and cycle stability through the design of reasonable electrolyte composition and the design of separator porosity.

[0087] [Isolation membrane]

[0088] In some embodiments, the porosity of the separator is 31% to 40%.

[0089] When the porosity of the separator meets the above range, the low-temperature fast charging performance and cycle stability of the lithium secondary battery are further improved.

[0090] In some embodiments, the separator includes a base membrane and a coating disposed on at least a portion of the surface of the base membrane, the coating including inorganic particles and organic particles.

[0091] When the coating on at least part of the surface of the base film in the separator includes inorganic and organic particles, the coating can improve the overall strength of the separator, reduce the risk of puncture, improve the interfacial contact between the electrode and the separator, reduce the risk of separator wrinkling, and the organic particles in the coating help to leave gaps between the electrode and the separator, thereby reducing the expansion of the battery during charging and discharging, and further improving the cycle stability of the battery.

[0092] In some embodiments, the inorganic particles include at least one of boehmite, alumina, silicon dioxide, magnesium oxide, magnesium hydroxide, titanium dioxide, tin dioxide, barium sulfate, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, cerium oxide, magnesium fluoride, and barium titanate.

[0093] In this article, "boehmite," also known as diaspore, has the chemical formula γ-AlOOH (hydrated aluminum oxide) and is one of the main components of bauxite.

[0094] The aforementioned inorganic particles are suitable for the preparation of the separator coating in the embodiments of this application, which helps to improve the overall strength of the separator and thus further enhance the cycle stability of the battery.

[0095] In some embodiments, the organic particles include at least one of the following: homopolymers or copolymers of fluoroolefin monomer units, homopolymers or copolymers of olefin monomer units, homopolymers or copolymers of unsaturated nitrile monomer units, homopolymers or copolymers of epoxy alkane monomer units, homopolymers or copolymers of acrylic monomer units, homopolymers or copolymers of acrylate monomer units, homopolymers or copolymers of styrene monomer units, polyurethane compounds, rubber compounds, and modified compounds of the above homopolymers or copolymers.

[0096] In some embodiments, the organic particles include butyl methacrylate-isooctyl methacrylate copolymer.

[0097] In some embodiments, the organic particles include a vinylidene fluoride-trifluoroethylene copolymer.

[0098] The aforementioned organic particles are suitable for the preparation of the separator coating in the embodiments of this application, which is beneficial to improving the contact interface between the separator and the electrode, and helps to leave gaps between the electrode and the separator, thereby further improving the cycle stability of the battery.

[0099] In some embodiments, inorganic particles are disposed on at least a portion of the surface of the base film, and organic particles are disposed on at least a portion of the surface of the inorganic particles and / or at least partially embedded in the inorganic particles.

[0100] In this application, the inorganic particles, organic particles, and their distribution on the separator membrane can be tested using the following methods. Specifically:

[0101] A sample of the separator membrane from the battery cell in its initial state was immersed in dimethyl carbonate (DMC) solution for 8 hours, dried, and then subjected to ion-polished cross-section SEM analysis to observe the relative positions of organic and inorganic particles in the separator membrane cross-section. First, the morphology or EDS was used to determine whether the particles in the separator membrane cross-section were organic or inorganic. Organic particles were irregularly shaped and dispersed, while inorganic particles were angular polygonal particles connected in a continuous mass. Organic particles did not contain metal elements or metalloid elements, and were mainly composed of carbon, while inorganic particles contained metal elements or metalloid elements. Organic particles were observed to be embedded within or located above inorganic particles.

[0102] When organic particles are partially embedded in inorganic particles, the structure of the separator coating becomes more robust, and the organic particles are less likely to fall off. This helps the organic particles to more stably perform their bonding and gap-reserving functions, which is beneficial to further improving the cycle stability of the battery.

[0103] In some embodiments, the number-average particle size of the inorganic particles is 0.5 μm to 2.5 μm.

[0104] In this document, the term "number-average particle size of inorganic particles" refers to the arithmetic mean of the particle size of inorganic particles in the separator coating, which is calculated based on the number of inorganic particles. The particle size of inorganic particles refers to the longest distance between two points on the edge of an inorganic particle.

[0105] In this application, the number-average particle size of inorganic and organic particles can be tested using methods known in the art. Specifically: a sample of the separator membrane in the battery cell in its initial state is taken, immersed in DMC solution for 8 hours, and then dried. The sample is then placed under an SEM, magnified to 1Kx, and 20 inorganic particles and 20 organic particles are selected under random field of view. The particle size of each particle is measured using the major diameter statistical method. The longest distance connecting two points on the edge of the particle is recorded as the particle size of a single particle. The number-average particle size of inorganic particles is the sum of the statistically analyzed inorganic particle sizes divided by the number of statistically analyzed inorganic particles, and the number-average particle size of organic particles is the sum of the statistically analyzed organic particle sizes divided by the number of statistically analyzed organic particles.

[0106] In some embodiments, the number-average particle size of the inorganic particles is 0.5 μm, 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, or other unlisted values ​​in the range of 0.5 μm to 2.5 μm.

[0107] In some embodiments, the number-average particle size of the organic particles is 2 μm to 25 μm.

[0108] In some embodiments, the number-average particle size of the organic particles is 4 μm to 20 μm.

[0109] In some embodiments, the number-average particle size of the organic particles is 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm, 25 μm, or other unlisted values ​​in the range of 2 μm to 25 μm.

[0110] When the number-average particle size of the inorganic particles in the separator coating meets the above-mentioned range, it is beneficial for the inorganic particles to be more uniformly and densely distributed on the base film, thereby further improving the strength of the separator, reducing the risk of the separator being punctured, and further improving the cycle stability of the battery. Moreover, when the number-average particle size of the inorganic particles meets the above-mentioned range, the porosity of the separator is within a suitable range, such as 28% to 50%, thereby further improving the cycle stability and low-temperature fast charging performance of the battery.

[0111] When the number-average particle size of the organic particles in the separator coating meets the above-mentioned range, it avoids the problem of excessively large particle size causing the organic particles to form a film and deteriorate lithium-ion conduction, while also preventing the particle size from being too small, which would prevent them from being uniformly dispersed on the base film and thus failing to provide a gap between the electrode and the separator. Meeting the above-mentioned range ensures that the separator provides a good channel for lithium-ion transport, has a good contact interface with the electrode, and that the organic particles help to leave a gap between the electrode and the separator, thereby further improving the battery's low-temperature fast-charging performance and cycle stability.

[0112] When the number-average particle size of inorganic and organic particles meets the above range, it is more conducive to forming a coating morphology in which organic particles are at least partially embedded in inorganic particles, thereby further improving the cycle stability of the battery.

[0113] In some embodiments, the total thickness of the coating is 1 μm to 7 μm.

[0114] In some embodiments, the total thickness of the coating is 2 μm to 4 μm.

[0115] In this article, "total coating thickness" refers to the total thickness of the coating on the surface of the separator. If both surfaces of the separator have coatings, then the total coating thickness refers to the sum of the thicknesses of the coatings on the two surfaces.

[0116] In this application, the total thickness of the coating can be tested using methods known in the art. As an example, a sample of the separator in the battery cell in its initial state is immersed in DMC solution for 8 hours, then removed and dried. The thickness of the separator is randomly measured at 10 different locations using a thickness gauge, and the average value is recorded as the total thickness A0 of the separator. Then, the separator is immersed in N-methylpyrrolidone (NMP) solution and ultrasonically treated for 10 minutes. After drying, the thickness of the separator is measured at 10 different locations using a thickness gauge, and the average value is recorded as the thickness A1 of the base film. Therefore, the total thickness of the coating = A0 - A1.

[0117] In some embodiments, the total thickness of the coating is 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, or other unlisted values ​​in the range of 1 μm to 7 μm.

[0118] When the thickness of the coating in the separator is controlled to meet the above range, it can both improve the contact interface between the separator and the electrode and the mechanical strength of the separator, and ensure the conduction of lithium ions between the positive and negative electrodes, thereby further improving the cycle stability and low-temperature fast charging performance of the battery.

[0119] In some embodiments, the total thickness of the separator is 6 μm to 20 μm.

[0120] In some embodiments, the total thickness of the separator is 9 μm to 14 μm.

[0121] In this application, the total thickness of the separator can be tested using methods known in the art. For example, see the above-mentioned test method for "total coating thickness".

[0122] In some embodiments, the total thickness of the separator is 6 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 16 μm, 18 μm, 20 μm, or other unlisted values ​​in the range of 6 μm to 20 μm.

[0123] By controlling the thickness of the separator to meet the above range, the risk of the separator being punctured can be further reduced, enabling the battery to have excellent energy density while further improving cycle stability.

[0124] Electrolyte

[0125] In some embodiments, R is selected from fluorine, chlorine or bromine, and n is selected from 1, 4 or 6.

[0126] 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.

[0127] Compared to other halogen atoms, fluorine atoms have a stronger electron-withdrawing ability. Replacing the benzene ring with fluorine atoms makes the benzene ring center electron-deficient, which in turn enhances the π-π interaction between the compound of Formula I and the negative electrode active material. This also facilitates the enrichment of the compound of Formula I on the surface of the negative electrode active material, further suppressing the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, further improving the wettability of the electrolyte, and thus further enhancing the battery's cycle stability and low-temperature fast-charging performance. Furthermore, as the number of fluorine-substituted compounds in Formula I increases, the benzene ring center becomes more electron-deficient, and the stronger the π-π interaction between the benzene ring and the negative electrode active material, which further suppresses the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, further improving the wettability of the electrolyte, and thus further enhancing the battery's cycle stability and low-temperature fast-charging performance.

[0128] In some embodiments, the mass percentage W1 of the compound of Formula I is 0.5% ≤ W1 ≤ 10% based on the total mass of the electrolyte.

[0129] In some embodiments, the mass percentage W1 of the compound of Formula I is 2% ≤ W1 ≤ 7% based on the total mass of the electrolyte.

[0130] In some embodiments, the mass percentage W1 of the compound of Formula I, based on the total mass of the electrolyte, is 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 10%, or other unlisted values ​​in the range of 0.5% to 10%.

[0131] In some embodiments, the mass percentage W1 of the compound of formula I is the mass percentage of the compound of formula I in the electrolyte of the lithium secondary battery in the initial state. In some embodiments, the mass percentage W1 of the compound of formula I is the mass percentage of the compound of formula I in the electrolyte of the lithium secondary battery in the operating state.

[0132] When the mass percentage of the compound of Formula I is within the above range, it suppresses the stripping of the negative electrode active material by propylene carbonate while maintaining a suitable viscosity of the electrolyte. The electrolyte has excellent conductivity, which is beneficial to further improve the cycle stability of the lithium secondary battery in the present application embodiment, while also taking into account excellent low-temperature fast charging performance.

[0133] In some embodiments, the mass percentage of propylene carbonate W2, based on the total mass of the electrolyte, is 2% ≤ W2 ≤ 10%.

[0134] In some embodiments, the mass percentage of propylene carbonate W2, based on the total mass of the electrolyte, is 5% ≤ W2 ≤ 8%.

[0135] In some embodiments, the mass percentage of propylene carbonate W2, based on the total mass of the electrolyte, can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or other unlisted values ​​within the range of 2% to 10%.

[0136] In some embodiments, the mass percentage W2 of propylene carbonate is the mass percentage of propylene carbonate in the electrolyte of the lithium secondary battery in its initial state. In some embodiments, the mass percentage W2 of propylene carbonate is the mass percentage of propylene carbonate in the electrolyte of the lithium secondary battery in its operating state.

[0137] When the mass percentage of propylene carbonate is within the above range, it further improves the low-temperature conductivity of the electrolyte, while also reducing the amount of propylene carbonate embedded in the negative electrode active material, thereby reducing the degree of stripping of the negative electrode active material. This is beneficial for further improving the low-temperature fast-charging performance of the lithium secondary battery in the embodiments of this application, while also slowing down the deterioration of the battery cycle performance caused by propylene carbonate.

[0138] In some embodiments, the mass percentage of the compound of formula I, W1, and the mass percentage of propylene carbonate, W2, satisfy 0.05 ≤ W1 / W2 ≤ 5.

[0139] In some embodiments, the mass percentage of the compound of formula I, W1, and the mass percentage of propylene carbonate, W2, satisfy 0.35 ≤ W1 / W2 ≤ 2.

[0140] In some embodiments, the ratio W1 / W2 of the mass percentage of the compound of Formula I, W1, to the mass percentage of propylene carbonate, W2, can be 0.05, 0.10, 0.5, 0.8, 1.0, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, or other unlisted values ​​in the range of 0.05 to 5.

[0141] When the ratio W1 / W2 of the mass percentage content of compound I and the mass percentage content of propylene carbonate meets the above range, it is beneficial to improve the low-temperature conductivity of the electrolyte, maintain the appropriate viscosity of the electrolyte, and further suppress the stripping of propylene carbonate on the negative electrode active material, thereby further improving the low-temperature fast charging performance and cycle stability of the lithium secondary battery of the present application embodiment.

[0142] In some embodiments, the electrolyte includes a first additive, which includes at least one of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, tetravinylsilane, or methanedisulfonate.

[0143] Since the compound of Formula I does not participate in the formation of the negative electrode solid electrolyte interphase (SEI) film, the electrolyte of the lithium secondary battery in this application includes a first additive, which is beneficial to the formation of an SEI film rich in organic components on the negative electrode surface. The SEI film rich in organic components has excellent flexibility, which is beneficial to suppress the reduction and decomposition of the electrolyte on the negative electrode side and alleviate the mechanical rupture of the SEI film, thereby further improving the cycle stability of the lithium secondary battery in this application.

[0144] In some embodiments, the mass percentage of the first additive W3 is 0.1% ≤ W3 ≤ 4% based on the total mass of the electrolyte.

[0145] In some embodiments, the mass percentage W3 of the first additive, based on the total mass of the electrolyte, can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or other unlisted values ​​within the range of 0.1% to 4%.

[0146] It should be understood that, due to the consumption of the first additive in the electrolyte during formation and cycling, and the generation of related components in the SEI film, the mass percentage of the first additive in the electrolyte may be lower than the initial mass percentage of the first additive added to the electrolyte.

[0147] When the mass percentage of the first additive in the electrolyte is within the above range, it is beneficial to better form an SEI film rich in organic components and to maintain a suitable viscosity of the electrolyte, thereby further improving the cycle stability and low-temperature fast charging performance of the lithium secondary battery in the embodiments of this application.

[0148] In some embodiments, the electrolyte includes a second additive, which includes at least one of lithium difluorophosphate, lithium difluoromonoxalate borate, lithium difluorobisoxalate borate, lithium difluorosulfonylimide, or lithium fluorosulfonate.

[0149] The types of additives in the electrolyte can be determined by methods known to those skilled in the art. For example, the composition of the electrolyte can be determined by liquid chromatography, ultraviolet spectrophotometry, or ultraviolet-visible spectrophotometry. For instance, using an ion chromatograph (IC) to test the inorganic content in the electrolyte, a quantitative amount of electrolyte (with a dilution concentration at the midpoint of the standard curve) is weighed and diluted to 100 mL with ultrapure water. The ion chromatogram is automatically injected to detect the inorganic ions. The chromatogram peak positions are compared to identify the corresponding inorganic ions, and the percentage of the corresponding inorganic ion content is calculated based on the peak area.

[0150] Because the organic-rich SEI film formed by the first additive has high impedance and poor thermal stability, adding a second additive containing the aforementioned substances to the electrolyte allows these substances to form an inorganic-rich SEI film at the negative electrode. This inorganic-rich SEI film exhibits excellent rigidity and thermal stability. The second additive works synergistically with the first additive to further enhance the stability of the SEI film and reduce the interfacial impedance on the negative electrode side, thereby further improving the battery's cycle stability and low-temperature fast-charging performance. Furthermore, these substances can also form a positive electrode electrolyte interfacial film (CEI film) on the positive electrode surface, which helps suppress the oxidation reaction of the electrolyte on the positive electrode side, thus further improving the battery's cycle stability.

[0151] In some embodiments, the mass percentage of the second additive W4 is 0.1% ≤ W4 ≤ 3% based on the total mass of the electrolyte.

[0152] In some embodiments, the mass percentage W4 of the second additive, based on the total mass of the electrolyte, can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or other unlisted values ​​within the range of 0.1% to 3%.

[0153] It should be understood that, since the second additive in the electrolyte will be consumed during the formation and cycling process, and related components in the SEI membrane and CEI membrane will be generated, the mass percentage of the second additive in the electrolyte may be lower than the initial mass percentage of the second additive added to the electrolyte.

[0154] When the mass percentage of the second additive is within the above range, it is beneficial to further improve the stability of the SEI film, reduce the interfacial impedance on the negative electrode side, further suppress the oxidation reaction of the electrolyte on the positive electrode side, and make the viscosity of the electrolyte suitable, thereby further improving the cycle stability and low-temperature fast charging performance of the lithium secondary battery in the present application embodiment.

[0155] 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%.

[0156] In some embodiments, the mass percentage content W3 of the first additive and the mass percentage content W4 of the second additive satisfy 0.4% ≤ W3 + W4 ≤ 4.3%.

[0157] In some embodiments, the sum of the mass percentage of the first additive W3 and the mass percentage of the second additive W4 (W3+W4) can be 0.2%, 0.4%, 0.8%, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 6.0%, or other unlisted values ​​in the range of 0.2% to 6%.

[0158] When the contents of the first additive and the second additive satisfy the above relationship, it is beneficial to further utilize their synergistic effect, further reduce the interfacial impedance on the negative electrode side, improve the stability of the SEI film, further suppress the decomposition of the electrolyte on the positive and negative electrode sides, and further help maintain the appropriate viscosity of the electrolyte, thereby further improving the low-temperature fast charging performance and cycle life of the battery.

[0159] In some embodiments, the first additive includes vinylene carbonate, and the mass percentage of vinylene carbonate is 0.4% to 3% based on the total mass of the electrolyte.

[0160] In some embodiments, the mass percentage of vinylene carbonate is 0.5% to 1% based on the total mass of the electrolyte.

[0161] In some embodiments, the mass percentage of vinylene carbonate, based on the total mass of the electrolyte, may be 0.4%, 0.5%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, or other unlisted values ​​within the range of 0.4% to 3%.

[0162] In some embodiments, the second additive includes lithium difluorophosphate, with the lithium difluorophosphate content being 0.25% to 1% by mass based on the total mass of the electrolyte.

[0163] In some embodiments, the mass percentage of lithium difluorophosphate is 0.3% to 0.5% based on the total mass of the electrolyte.

[0164] In some embodiments, the mass percentage of lithium difluorophosphate may be 0.25%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or other unlisted values ​​in the range of 0.25% to 1%, based on the total mass of the electrolyte.

[0165] Lithium difluorophosphate decomposes preferentially over other components in the electrolyte, forming an inner SEI film rich in inorganic salt components at the negative electrode interface. This inner SEI film exhibits excellent rigidity and thermal stability. Subsequently, vinylene carbonate decomposes at the negative electrode, forming an outer SEI film rich in organic polymer components. This outer SEI film exhibits excellent flexibility. The first additive includes vinylene carbonate, and the second additive includes lithium difluorophosphate. This allows the SEI film to better adapt to the expansion and contraction of the negative electrode active material during charging and discharging, alleviates the mechanical rupture of the SEI film, and further reduces the interfacial impedance on the negative electrode side, thereby further improving the cycle stability and low-temperature fast-charging performance of the lithium secondary battery of this application embodiment. Furthermore, when the mass percentages of vinylene carbonate and lithium difluorophosphate are within the above-mentioned range, it is more conducive to their synergistic effect, further improving the stability of the SEI film on the negative electrode side and further reducing the interfacial impedance on the negative electrode side, further inhibiting the decomposition of the electrolyte on both the positive and negative electrode sides, and further helping to maintain a suitable electrolyte viscosity, thereby further improving the cycle stability and low-temperature fast-charging performance of the lithium secondary battery of this application embodiment.

[0166] In some embodiments, the electrolyte includes a solvent, which includes at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, diethyl carbonate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, or propyl propionate.

[0167] The aforementioned solvent is beneficial for dissolving lithium salts and additives in the electrolyte, improving the ionic conductivity of the electrolyte, and thus further enhancing the low-temperature fast-charging performance and cycle stability of the lithium secondary battery in the embodiments of this application.

[0168] In some embodiments, the electrolyte includes a lithium salt, which includes at least one of lithium hexafluorophosphate, lithium fluorosulfonyl (perfluorobutylsulfonyl)imide, or lithium bis(trifluoromethylsulfonyl)imide.

[0169] The aforementioned lithium salts possess excellent thermal and chemical stability, and are beneficial for further improving the ionic conductivity of the electrolyte, thereby further enhancing the low-temperature fast-charging performance and cycle stability of the lithium secondary batteries in the embodiments of this application.

[0170] In some embodiments, the concentration of lithium salt is 0.8 mol / L to 1.3 mol / L.

[0171] In some embodiments, the concentration of the lithium salt can be 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, or other unlisted values ​​in the range of 0.8 mol / L to 1.3 mol / L.

[0172] When the lithium salt concentration meets the above range, it is beneficial to balance the ionic conductivity and wettability of the electrolyte, thereby further improving the low-temperature fast charging performance and cycle stability of the lithium secondary battery in this application embodiment.

[0173] [Positive electrode plate]

[0174] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0175] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0176] In some embodiments, the positive 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 substrate and a metal layer formed on at least one surface of the polymer substrate. 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0177] In some embodiments, the positive electrode active material may be a known positive electrode active material for lithium secondary batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM211), LiNi 0.6 Co 0.2Mn 0.2 O2 (also known as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), and lithium manganese phosphate and carbon composites.

[0178] In some embodiments, the positive electrode may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a terpolymer of PVDF-tetrafluoroethylene-propylene, a terpolymer of PVDF-hexafluoropropylene-tetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorinated acrylate resin.

[0179] In some embodiments, the positive electrode may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0180] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0181] [Negative electrode plate]

[0182] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0183] As an example, the negative electrode current collector has two surfaces opposite 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.

[0184] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0185] In some embodiments, the negative electrode active material includes one or more of graphite or its modified forms, and silicon-carbon mixtures.

[0186] In some implementations, graphite includes natural graphite or synthetic graphite.

[0187] In some embodiments, the modified graphite includes doped or coated modified graphite. In some embodiments, the modified graphite includes a mixture of graphite and silicon-based materials. In some embodiments, the mass ratio of graphite to silicon-based materials is 98:2-60:40.

[0188] In some embodiments, the silicon-based material includes at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.

[0189] In this document, the term "silicon-carbon mixture" generally refers to a mixture of silicon (Si) and carbon (C), in which silicon acts as the active material and carbon acts as a conductive agent or binder. Such a 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.

[0190] The compounds of Formula I are more likely to bond with the above-mentioned negative electrode active materials, thereby promoting the desolvation of lithium ions solubilized by propylene carbonate, which is beneficial to further suppress the co-intercalation 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 embodiment.

[0191] In some embodiments, the negative electrode film layer may optionally 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).

[0192] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0193] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0194] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0195] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0196] In some embodiments, the lithium secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0197] In some implementations, the outer packaging of the lithium secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the individual battery cells can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0198] This application does not impose any particular limitation on the shape of the lithium secondary battery, which can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square-structured lithium secondary battery 5 as an example.

[0199] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The lithium secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0200] In some implementations, lithium secondary batteries can be assembled into battery modules, and the number of lithium secondary batteries contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0201] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple lithium secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple lithium secondary batteries 5 can be fixed in place using fasteners.

[0202] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of lithium secondary batteries 5 are received.

[0203] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0204] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0205] [Electrical appliances]

[0206] Furthermore, a second aspect of this application also provides an electrical device, which includes a lithium secondary battery provided in any embodiment of the first aspect of this application. The lithium secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0207] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0208] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the individual battery cells, a battery pack or battery module can be used.

[0209] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0210] Example

[0211] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0212] I. Preparation Method

[0213] Example 1

[0214] 1) Preparation of electrolyte

[0215] In an argon-filled glove box (water content <10ppm, oxygen content <1ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 3:7 to obtain an electrolyte solvent. Then, fully dried lithium salt LiPF6, the above electrolyte solvent, 1-fluorobenzene, propylene carbonate, the first additive vinylene carbonate, and the second additive lithium difluorophosphate were mixed to prepare an electrolyte with a lithium salt concentration of 1 mol / L. Based on the total mass of the electrolyte, the mass percentage of compound I, 1-fluorobenzene, was 3% (W1), the mass percentage of propylene carbonate was 5% (W2), the mass percentage of the first additive was 0.8% (W3), and the mass percentage of the second additive was 0.3% (W4).

[0216] 2) Preparation of the separating membrane

[0217] Preparation of coating slurry: Inorganic boehmite particles, organic butyl methacrylate-isooctyl acrylate-styrene copolymer particles, sodium carboxymethyl cellulose (CMC-Na) dispersant, and organosilicon-modified polyether wetting agent were mixed evenly in an appropriate amount of deionized water at a dry weight ratio of 70:28:1:1 to obtain the coating slurry. The number average particle size of the inorganic boehmite particles was 1.5 μm, and the number average particle size of the organic particles was 10 μm.

[0218] A 7μm thick polyethylene (PE) substrate was used as the base film layer. The aforementioned slurry was coated onto both surfaces of the PE substrate using a coating machine. The finished separator film was then produced through drying and slitting processes. Testing showed that the total thickness of the separator coating was 2μm, the total thickness of the separator film was 9μm, and the porosity of the separator film was 35%.

[0219] 3) Preparation of positive electrode sheet

[0220] The positive electrode active material lithium iron phosphate, the binder polyvinylidene fluoride, and the conductive agent acetylene black are mixed in a weight ratio of 97:2:1 and dissolved in the solvent N-methylpyrrolidone (NMP) to prepare a positive electrode slurry. The slurry is then coated onto the current collector aluminum foil, dried, and then cold-pressed, trimmed, cut, and slit to produce the positive electrode sheet.

[0221] 4) Preparation of negative electrode sheet

[0222] The negative electrode active material graphite, binder styrene-butadiene rubber (SBR), thickener sodium carboxymethyl cellulose (CMC-Na), and conductive agent carbon black (Super P) are mixed thoroughly in an appropriate amount of deionized water at a mass ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry. The slurry is then coated onto the current collector copper foil, dried, and then cold-pressed, trimmed, cut, and slit to obtain the negative electrode sheet.

[0223] 5) Assembly of lithium secondary batteries:

[0224] The positive electrode, separator, and negative electrode are stacked in sequence. The separator must be able to isolate the anode and cathode. The bare cell is obtained by winding. The bare cell is placed in the outer packaging, electrolyte is injected, and after processes such as encapsulation, formation, and degassing, a lithium secondary battery is finally obtained.

[0225] Examples 2-5

[0226] The lithium secondary batteries in Examples 2-5 are prepared in a manner similar to that in Example 1, except that the types of inorganic and organic particles and their number-average particle size are adjusted, thereby changing the porosity of the separator, as detailed in Table 1.

[0227] Examples 6-7

[0228] The lithium secondary batteries of Examples 6-7 are prepared in a manner similar to that of Example 1, except that the total thickness of the coating and / or the total thickness of the separator are adjusted.

[0229] Examples 8-9

[0230] The lithium secondary batteries of Examples 8-9 are prepared in a manner similar to that of Example 1, except that the compound of Formula I in the electrolyte is replaced with other types, as detailed in Table 2.

[0231] Examples 10-13

[0232] The lithium secondary batteries of Examples 10-13 are prepared in a manner similar to that of Example 1, except that the amount of compound I in the electrolyte, W1, is changed, as detailed in Table 2.

[0233] Examples 14-16

[0234] The lithium secondary batteries in Examples 14-16 are prepared in a manner similar to that in Example 1, except that the amount of propylene carbonate added, W2, in the electrolyte is changed, as detailed in Table 2.

[0235] Examples 17-18

[0236] The lithium secondary batteries of Examples 17-18 are prepared in a manner similar to that of Example 1, except that the amount of the first additive W3 added to the electrolyte is changed, as detailed in Table 2.

[0237] Examples 19-20

[0238] The lithium secondary batteries of Examples 19-20 are prepared in a manner similar to that of Example 1, except that the amount of the second additive W4 added to the electrolyte is changed, as detailed in Table 2.

[0239] Example 21

[0240] The lithium secondary battery of Example 21 is prepared in a similar manner to that of Example 1, except that the amount of the first additive W3 and the amount of the second additive W4 added to the electrolyte are changed, as detailed in Table 2.

[0241] Example 22

[0242] The preparation method of the lithium secondary battery in Example 22 is basically similar to that in Example 1, except that the types of the first additive and the second additive in the electrolyte are changed, as shown in Table 2.

[0243] Comparative Example 1

[0244] The preparation methods of Comparative Example 1 and Example 1 are basically similar, except that a separating membrane with a porosity in the range of 28% to 50% is not used, and 1-fluorobenzene, propylene carbonate, the first additive and the second additive are not added to the electrolyte, as detailed in Tables 1 and 2.

[0245] Comparative Example 2

[0246] The preparation method of Comparative Example 2 is basically similar to that of Example 1, except that a separation membrane with a porosity in the range of 28% to 50% is not used, as detailed in Tables 1 and 2.

[0247] Comparative Example 3

[0248] The preparation method of Comparative Example 3 is basically similar to that of Example 1, except that 1-fluorobenzene and propylene carbonate are not added to the electrolyte, as detailed in Tables 1 and 2.

[0249] Comparative Example 4

[0250] The preparation method of Comparative Example 4 is basically similar to that of Example 1, except that propylene carbonate is not added to the electrolyte, as detailed in Tables 1 and 2.

[0251] Comparative Example 5

[0252] The preparation method of Comparative Example 5 is basically similar to that of Example 1, except that 1-fluorobenzene is not added to the electrolyte, as detailed in Tables 1 and 2.

[0253] II. Performance Testing

[0254] 1. Cell cycle life test method

[0255] At 25℃, the battery cell was charged at 1C to 3.65V, then charged at a constant voltage to 0.05C, left to stand for 10 minutes, and then discharged at 1C to 2V. The first discharge capacity was recorded as C0. The charge-discharge cycle test was performed according to the above procedure, and the capacity retention rate after each cycle was calculated. The capacity retention rate after the 25℃ cycle is: Capacity retention rate after the nth cycle = (Discharge capacity after the nth cycle / Discharge capacity C0 of the first cycle) * 100%. The capacity retention rate after 500 cycles was recorded.

[0256] 2. Low-temperature fast charging performance test of battery cells

[0257] At 25℃, the battery cell is charged at 1C to 3.65V, then charged at a constant voltage to 0.05C, and then left to stand for 10 minutes. It is then discharged at 1C to 2V, and the discharge capacity is recorded as D0. The battery is then placed at -10℃ for charging. It is charged at 0.5D0 to 3.65V, and the charging capacity is recorded as D1. It is then discharged at 1D0 to 2.0V, left to stand for 30 minutes, and then charged at 2D0 to 3.65V, and the charging capacity is recorded as D2. Therefore, the low-temperature fast charging capacity retention rate = D2 / D1.

[0258] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0259] Lithium secondary batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The results are shown in the table below.

[0260] Table 1

[0261] Table 2

[0262] Table 3

[0263] As shown in Tables 1-3, Examples 1-22 provide a lithium secondary battery, including a positive electrode, a negative electrode, an electrolyte, and a separator between the positive and negative electrodes. The separator has a porosity of 28%–50%, and the electrolyte includes propylene carbonate and a compound having Formula I.

[0264] Where R is selected from halogens, and n is an integer selected from 1 to 6.

[0265] The lithium secondary battery of this application embodiment has excellent low-temperature fast charging performance and cycle performance.

[0266] As can be seen from the comparison between Examples 1-22 and Comparative Examples 1-5, the low-temperature fast charging performance and cycle performance of the lithium secondary batteries in the embodiments of this application are further improved.

[0267] Comparative Example 1's lithium secondary battery's electrolyte does not contain propylene carbonate, and the porosity of the separator is too small, resulting in poor low-temperature fast charging performance and cycle stability of the lithium secondary battery.

[0268] In contrast, the electrolyte of the lithium secondary battery in Comparative Example 2 includes propylene carbonate and a compound of Formula I, which increases the low-temperature conductivity of the electrolyte. However, due to the small porosity of the separator and the low lithium-ion shuttle efficiency of the separator, the low-temperature fast-charging performance and cycle stability of the lithium secondary battery are still poor.

[0269] Even though the separator porosity of the lithium secondary battery in Comparative Example 3 is in the range of 28% to 50%, the electrolyte lacks propylene carbonate and compounds of Formula I, resulting in low-temperature conductivity and poor low-temperature fast-charging performance. Furthermore, due to the poor wettability of the electrolyte to the separator, the lithium-ion shuttle efficiency is also low when the separator porosity is in the range of 28% to 50%, further deteriorating the cycle stability of the lithium secondary battery.

[0270] The electrolyte of the lithium secondary battery in Comparative Example 4 does not include propylene carbonate. Even if it includes the compound of Formula I and the permeability of the separator is within a suitable range, it cannot improve the low-temperature conductivity of the electrolyte. The low-temperature fast charging performance of the lithium secondary battery is poor, which is also detrimental to the cycle stability of the lithium secondary battery.

[0271] The electrolyte of the lithium secondary battery in Comparative Example 5 includes propylene carbonate, but does not contain the compound of Formula I. This results in severe stripping of the negative electrode active material by propylene carbonate, which deteriorates the cycle stability of the battery. Furthermore, due to the poor wettability of the electrolyte to the separator at this time, the lithium-ion shuttle efficiency of the separator is also low when the porosity is in the range of 28% to 50%, which is also detrimental to the low-temperature fast charging performance of the battery.

[0272] As can be seen from Examples 1-5, when the porosity of the separator is 28%–50%, the lithium secondary battery exhibits excellent low-temperature fast charging performance and cycle performance. A comparison of Examples 1, 3, and 4 with Examples 2 and 5 shows that when the porosity of the separator is 31%–40%, the low-temperature fast charging performance and cycle performance of the lithium secondary battery are further improved.

[0273] As can be seen from Examples 1-22, the separator includes a base film and a coating disposed on at least a portion of the surface of the base film. The coating includes inorganic particles and organic particles, and the battery has excellent cycle stability.

[0274] As can be seen from Examples 1-5, the number-average particle size of inorganic particles is 0.5μm to 2.5μm, and the lithium secondary battery has excellent low-temperature fast charging performance and cycle performance.

[0275] As can be seen from Examples 1-5, when the number-average particle size of the organic particles is 2μm to 25μm, the lithium secondary battery exhibits excellent low-temperature fast-charging performance and cycle performance. A comparison of Examples 1-3 and 5 with Example 2 shows that when the number-average particle size of the organic particles is 4μm to 20μm, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved.

[0276] As can be seen from Examples 1, 6, and 7, when the total coating thickness is 2μm to 4μm, the lithium secondary battery exhibits excellent low-temperature fast charging performance and cycle performance.

[0277] As can be seen from Examples 1, 6, and 7, when the total thickness of the separator is 9μm to 14μm, the lithium secondary battery has excellent low-temperature fast charging performance and cycle performance.

[0278] As can be seen from Examples 1, 8, and 9, when R is selected from fluorine, chlorine, or bromine, and n is selected from 1, 4, or 6, the lithium secondary battery exhibits excellent low-temperature fast charging performance and cycle performance. When n is selected from 4 or 6, the low-temperature fast charging performance and cycle performance of the lithium secondary battery are further improved.

[0279] As can be seen from Examples 1, 8, and 9, when 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, the lithium secondary battery exhibits excellent low-temperature fast-charging performance and cycle performance. When the compound of Formula I includes at least one of 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene, or hexafluorobenzene, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved.

[0280] As can be seen from Examples 1 and 10-13, when the mass percentage W1 of the compound of Formula I is 0.5% ≤ W1 ≤ 10% based on the total mass of the electrolyte, the lithium secondary battery exhibits excellent low-temperature fast charging performance and cycle performance. A comparison of Examples 1, 11, and 12 with Examples 10 and 13 shows that when W1 is 2% ≤ W1 ≤ 7%, the low-temperature fast charging performance and cycle performance of the lithium secondary battery are further improved.

[0281] As can be seen from Examples 1 and 14-16, based on the total mass of the electrolyte, when the mass percentage of propylene carbonate W2 is 2% ≤ W2 ≤ 10%, the lithium secondary battery exhibits excellent low-temperature fast-charging performance and cycle performance. When W2 is 5% ≤ W2 ≤ 8%, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved.

[0282] As can be seen from Examples 1 and 10-16, when the mass percentage content W1 of the compound of Formula I and the mass percentage content W2 of propylene carbonate satisfy 0.05 ≤ W1 / W2 ≤ 5, the lithium secondary battery exhibits excellent low-temperature fast charging performance and cycle performance. When W1 and W2 satisfy 0.35 ≤ W1 / W2 ≤ 2, the low-temperature fast charging performance and cycle performance of the lithium secondary battery are further improved.

[0283] As can be seen from Examples 1, 17, 18, and 21, when the mass percentage content W3 of the first additive is 0.1% ≤ W3 ≤ 4% based on the total mass of the electrolyte, the lithium secondary battery exhibits excellent low-temperature fast-charging performance and cycle performance. A comparison of Examples 1 and 21 with Examples 17 and 18 shows that when W3 is 0.5%-1%, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved.

[0284] As can be seen from Examples 1, 19, 20, and 21, when the mass percentage content W4 of the second additive is 0.1% ≤ W4 ≤ 3% based on the total mass of the electrolyte, the lithium secondary battery exhibits excellent low-temperature fast-charging performance and cycle performance. A comparison of Examples 1 and 21 with Examples 18 and 19 shows that when W4 is 0.3%-0.5%, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved.

[0285] As can be seen from Examples 1 and 17-21, when the mass percentage content of the first additive W3 and the mass percentage content of the second additive W4 satisfy 0.4% ≤ W3 + W4 ≤ 4.3%, the lithium secondary battery has excellent low-temperature fast charging performance and cycle performance.

[0286] As can be seen from the comparison between Example 1 and Example 22, when the first additive includes vinylene carbonate and the second additive includes lithium difluorophosphate, the low-temperature fast charging performance and cycle performance of the lithium secondary battery are further improved.

[0287] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium secondary battery, the lithium secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator located between the positive electrode and the negative electrode, characterized in that, The porosity of the separator is 28%–50%, and the electrolyte comprises propylene carbonate and a compound having the following formula I. Where R is selected from halogens and n is selected from integers from 1 to 6.

2. The lithium secondary battery according to claim 1, characterized in that, The porosity of the isolation membrane is 31% to 40%.

3. The lithium secondary battery according to claim 1 or 2, characterized in that, The isolation membrane includes a base membrane and a coating disposed on at least a portion of the surface of the base membrane, the coating comprising inorganic particles and organic particles.

4. The lithium secondary battery according to claim 3, characterized in that, The inorganic particles include at least one of boehmite, alumina, silicon dioxide, magnesium oxide, magnesium hydroxide, titanium dioxide, tin dioxide, barium sulfate, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, cerium oxide, magnesium fluoride, and barium titanate.

5. The lithium secondary battery according to claim 3 or 4, characterized in that, The organic particles include at least one of the following: homopolymers or copolymers of fluoroolefin monomer units, homopolymers or copolymers of olefin monomer units, homopolymers or copolymers of unsaturated nitrile monomer units, homopolymers or copolymers of epoxy alkane monomer units, homopolymers or copolymers of acrylic monomer units, homopolymers or copolymers of acrylate monomer units, homopolymers or copolymers of styrene monomer units, polyurethane compounds, rubber compounds, and modified compounds of the above homopolymers or copolymers.

6. The lithium secondary battery according to any one of claims 3 to 5, characterized in that, The inorganic particles are disposed on at least a portion of the surface of the base film, and the organic particles are disposed on at least a portion of the surface of the inorganic particles and / or at least partially embedded in the inorganic particles.

7. The lithium secondary battery according to any one of claims 3 to 6, characterized in that, The number-average particle size of the inorganic particles is 0.5 μm to 2.5 μm; and / or The number-average particle size of the organic particles is 2μm to 25μm, and can be selected as 4μm to 20μm.

8. The lithium secondary battery according to any one of claims 3 to 7, characterized in that, The total thickness of the coating is 1μm to 7μm, and can be selected as 2μm to 4μm.

9. The lithium secondary battery according to any one of claims 1 to 8, characterized in that, The total thickness of the isolation membrane is 6μm to 20μm, and can be selected as 9μm to 14μm.

10. The lithium secondary battery according to any one of claims 1 to 9, characterized in that, R is selected from fluorine, chlorine, or bromine, and n is selected from 1, 4, or 6.

11. The lithium secondary battery according to any one of claims 1 to 10, characterized in that, The compounds of Formula I include at least one of 1-fluorobenzene, 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene, or hexafluorobenzene.

12. The lithium secondary battery according to any one of claims 1 to 11, characterized in that, Based on the total mass of the electrolyte, the mass percentage W1 of the compound of Formula I is 0.5% ≤ W1 ≤ 10%, optionally, 2% ≤ W1 ≤ 7%; and / or Based on the total mass of the electrolyte, the mass percentage of propylene carbonate W2 is 2% ≤ W2 ≤ 10%, and optionally, 5% ≤ W1 ≤ 8%.

13. The lithium secondary battery according to claim 12, characterized in that, 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, and optionally, 0.35 ≤ W1 / W2 ≤ 2.

14. The lithium secondary battery according to any one of claims 1 to 13, characterized in that, The electrolyte includes a first additive, which includes at least one of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, tetravinylsilane, or methanedisulfonate.

15. The lithium secondary battery according to claim 14, characterized in that, Based on the total mass of the electrolyte, the mass percentage of the first additive, W3, is 0.1% ≤ W3 ≤ 4%.

16. The lithium secondary battery according to any one of claims 1 to 15, characterized in that, The electrolyte includes a second additive, which includes at least one of lithium difluorophosphate, lithium difluoromonoxalate borate, lithium difluorobisoxalate borate, lithium difluorosulfonylimide, or lithium fluorosulfonate.

17. The lithium secondary battery according to claim 16, characterized in that, Based on the total mass of the electrolyte, the mass percentage of the second additive, W4, is 0.1% ≤ W4 ≤ 3%.

18. The lithium secondary battery according to claim 17, 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%, and optionally, 0.4% ≤ W3 + W4 ≤ 4.3%.

19. The lithium secondary battery according to any one of claims 16 to 18, characterized in that, The first additive comprises vinylene carbonate, and the mass percentage of vinylene carbonate, based on the total mass of the electrolyte, is 0.4% to 3%, optionally 0.5% to 1%; and / or The second additive includes lithium difluorophosphate, and the mass percentage of lithium difluorophosphate is 0.25% to 1%, optionally 0.3% to 0.5%, based on the total mass of the electrolyte.

20. The lithium secondary battery according to any one of claims 1 to 19, characterized in that, The electrolyte includes a solvent, which includes at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, diethyl carbonate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, or propyl propionate.

21. The lithium secondary battery according to any one of claims 1 to 20, characterized in that, The electrolyte includes a lithium salt, which includes at least one of lithium hexafluorophosphate, lithium fluorosulfonyl (perfluorobutylsulfonyl)imide, or lithium bis(trifluoromethylsulfonyl)imide.

22. The lithium secondary battery according to claim 21, characterized in that, The concentration of lithium salt in the electrolyte is 0.8 mol / L to 1.3 mol / L.

23. The lithium secondary battery according to any one of claims 1 to 22, characterized in that, The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes one or more of graphite or its modified form, and silicon-carbon mixture.

24. An electrical appliance, characterized in that, The electrical device includes a lithium secondary battery according to any one of claims 1 to 23.

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