Lithium secondary battery and electric device

Abstract

The application provides a lithium secondary battery and an electric device. The lithium secondary battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte. The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector. The positive electrode film layer comprises a positive electrode active material. The positive electrode active material comprises lithium-containing transition metal phosphate particles. The lithium-containing transition metal phosphate particles comprise lithium iron phosphate salt particles. The lithium iron phosphate salt particles comprise a doping element Q. The doping element Q comprises one or more of W, Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br and Zr. The electrolyte comprises propylene carbonate and a compound having a formula I. R is selected from halogen. n is an integer selected from 1-6. The lithium secondary battery has good low-temperature fast-charging performance and excellent cycle stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium batteries, and in particular to a lithium secondary battery and an electric device. BACKGROUND

[0002] In recent years, batteries are widely used in energy storage power systems such as hydroelectric, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc.

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

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

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

[0006] The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material, the positive electrode active material comprising lithium-containing transition metal phosphate particles, the lithium-containing transition metal phosphate particles comprising lithium iron phosphate particles, the lithium iron phosphate particles comprising a doping element Q, the doping element Q comprising one or more of W, Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, Zr;

[0007] The electrolyte comprises propylene carbonate and a compound having the following formula I,

[0008]

[0009] R is selected from halogen, and n is an integer selected from 1-6.

[0010] The lithium secondary battery of the present application adds propylene carbonate in the electrolyte, which has a high ionic conductivity and a low freezing point, and helps to improve the low-temperature lithium ion liquid phase transmission speed of the electrolyte, thereby being conducive to improving the low-temperature fast-charging performance of the lithium secondary battery of the present application. Moreover, due to the large specific surface area of the lithium iron phosphate salt particles, it is easy to have a side reaction with the electrolyte on the positive electrode side, which reduces the cycle stability of the lithium secondary battery. The added propylene carbonate in the present application has good oxidation resistance on the positive electrode side, which is conducive to inhibiting the side reaction between the lithium iron phosphate salt particles and the electrolyte, thereby being conducive to improving the cycle performance of the battery. However, propylene carbonate is easy to be embedded into the negative electrode active material (such as graphite) together with lithium ions on the negative electrode side, thereby causing the peeling of the negative electrode active material and deteriorating the cycle performance of the secondary battery. Moreover, after the propylene carbonate peels off the negative electrode active material, the electrolyte has a reduction reaction on the negative electrode interface to produce reducing gas, which attacks the positive electrode active material, causing the structure of the positive electrode active material to be broken.

[0011] To this end, the present application adds a compound of formula I in the electrolyte. The compound of formula I contains a large Π bond, which is easy to bond with the negative electrode active material, such as graphite, silicon-carbon mixture, etc., thereby enriching on the surface of the negative electrode active material and making the surface have a certain negative electric property, promoting the desolvation of the lithium ions solvated by propylene carbonate, being conducive to inhibiting the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, and being conducive to reducing the peeling degree of the negative electrode active material, thereby further improving the cycle stability of the lithium secondary battery of the present application.

[0012] Further, the positive electrode active material of the present application uses lithium iron phosphate salt particles containing the above-mentioned doping element Q. The doping element Q is conducive to stabilizing the structure of the lithium iron phosphate salt particles, relieving the collapse of the structure of the lithium iron phosphate salt particles caused by the introduction of propylene carbonate, thereby further improving the cycle performance of the secondary battery; and the doping element Q is conducive to improving the electronic conductivity and lithium ion migration speed of the lithium iron phosphate salt particles, making up for the defect of poor conductivity of the lithium iron phosphate salt particles, so that the low-temperature fast-charging performance of the lithium secondary battery is further improved.

[0013] In summary, the lithium secondary battery of the present application reasonably designs the components of the positive electrode active material and the electrolyte, so that the battery has good low-temperature fast-charging performance while also taking into account the cycle stability.

[0014] In any embodiment, the mass percentage content W1 of the doping element Q is 0.05%≤W1≤0.5%, and optionally, 0.1%≤W1≤0.3%, based on the total mass of the lithium iron phosphate salt particles.

[0015] The doping element Q is beneficial to improve the structural stability, electrical conductivity and migration speed of lithium ions of the lithium iron phosphate salt particles, and is beneficial to improve the cycle stability and low-temperature fast-charging performance of the lithium secondary battery. However, the dissolved doping element Q is easy to migrate to the negative electrode side, which causes damage to the SEI film of the negative electrode side, thereby affecting the cycle stability and low-temperature fast-charging performance of the lithium secondary battery to a certain extent. When the mass percentage content W1 of the doping element Q is within the above range based on the total mass of the lithium iron phosphate salt particles, it is beneficial to further improve the low-temperature fast-charging performance and cycle stability of the lithium secondary battery, and is beneficial to improve the energy density of the positive electrode active material and reduce the preparation cost of the positive electrode active material.

[0016] In any embodiment, R is selected from fluorine, chlorine or bromine, and n is selected from 1, 4 or 6.

[0017] 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.

[0018] Compared with other halogen atoms, the electron-withdrawing ability of fluorine atoms is stronger, and the substitution of benzene rings with fluorine atoms can make the center of the benzene ring electron-deficient, thereby being beneficial to enhance the Π-Π interaction between the compound of formula I and the negative electrode active material, and being beneficial to the enrichment of the compound of formula I on the surface of the negative electrode active material, and inhibiting the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, thereby further improving the low-temperature fast-charging performance of the lithium secondary battery of the present application. Further, compared with 1-fluorobenzene, when the compound of formula I is selected from tetrafluorobenzene and hexafluorobenzene, as the number of fluorine atoms on the benzene ring increases, the center of the benzene ring is more electron-deficient, and the Π-Π interaction between the benzene ring and the negative electrode active material is stronger, which is beneficial to further inhibit the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, and further improve the low-temperature fast-charging performance of the lithium secondary battery of the present application.

[0019] In any embodiment, the mass percentage content W2 of the compound of formula I based on the total mass of the electrolyte is 0.5%≤W2≤10%, and optionally 2%≤W2≤7%; and / or

[0020] The mass percentage content W3 of propylene carbonate based on the total mass of the electrolyte is 2%≤W3≤10%, and optionally 2%≤W3≤8%.

[0021] When the mass percentage content of the compound of formula I is within the above range, on the one hand, it is beneficial to maintain the viscosity of the electrolyte appropriate, and the electrolyte has excellent electrical conductivity, which is beneficial to further improve the low-temperature fast-charging performance and cycle stability of the lithium secondary battery of the present application, and on the other hand, it is also beneficial to further inhibit the stripping of propylene carbonate to the negative electrode active material, thereby being beneficial to further improve the cycle performance.

[0022] When the mass percentage content of the propylene carbonate is within the above range, it is beneficial to further balance the improvement of the low-temperature conductivity of the electrolyte, the inhibition of the side reaction between the lithium iron phosphate salt particles and the electrolyte, and the reduction of the amount of propylene carbonate intercalated into the negative electrode active material, thereby being beneficial to improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application while further taking into account more excellent cycle stability.

[0023] In any embodiment, the mass percentage content W2 of the compound of Formula I and the mass percentage content W3 of the propylene carbonate satisfy 0.05≤W2 / W3≤5, and optionally, 0.35≤W2 / W3≤2.

[0024] When the ratio W2 / W3 of the mass percentage content of the compound of Formula I and the mass percentage content of the propylene carbonate satisfies the above range, it is beneficial to further inhibit the stripping of the propylene carbonate to the negative electrode active material while improving the low-temperature conductivity of the electrolyte and maintaining the appropriate viscosity of the electrolyte, thereby being beneficial to improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application while further taking into account more excellent cycle stability.

[0025] In any embodiment, the electrolyte includes a first additive, and the first additive includes at least one of vinyl carbonate, vinyl sulfate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propene sultone, tetraethenylsilane, or methanedisulfonate methylene.

[0026] Since the compound of Formula I does not participate in the formation of the negative electrode solid electrolyte interface film (SEI film), the lithium secondary battery of the present application adds the first additive to the electrolyte, which is beneficial to forming the SEI film rich in organic components on the surface of the negative electrode, and the SEI film rich in organic components has excellent flexibility, which is beneficial to inhibiting the decomposition of the electrolyte on the negative electrode side, relieving the mechanical rupture of the SEI film, thereby being beneficial to further improving the cycle stability of the lithium secondary battery of the present application.

[0027] In any embodiment, the mass percentage content W4 of the first additive is 0.3%≤W4≤4% based on the total mass of the electrolyte.

[0028] When the mass percentage content of the first additive in the electrolyte is within the above range, it is beneficial to better form the SEI film rich in organic components and to maintain the appropriate viscosity of the electrolyte, thereby being beneficial to further improving the cycle stability and the low-temperature fast-charging performance of the lithium secondary battery of the present application.

[0029] In any embodiment, the electrolyte includes a second additive, and the second additive includes at least one of lithium difluorophosphate, lithium difluoromonooxalate borate, lithium difluoribisoxalate borate, lithium difluorosulfimide, or lithium fluorosulfonate.

[0030] Due to the large specific surface area of the lithium iron phosphate salt particles, the water content thereof is high. The crystallization water in the lithium iron phosphate salt particles is easy to enter the electrolyte, which can cause decomposition of the electrolyte to produce by-products such as hydrofluoric acid. Hydrofluoric acid can easily corrode the positive electrode interface, and then cause the dissolution of the doped elements in the lithium iron phosphate salt particles. Further, the dissolved doped elements are easy to migrate to the negative electrode to damage the SEI film of the negative electrode, and then deteriorate the cycle stability of the battery. When a second additive including the above-mentioned substance is added to the electrolyte, the second additive can form a positive electrode electrolyte interface film (CEI film) on the surface of the positive electrode, thereby being beneficial to inhibit the release of crystallization water and the dissolution of doped elements, inhibit the oxidative decomposition of the electrolyte on the positive electrode side, and further inhibit the side reaction between the lithium iron phosphate salt particles and the electrolyte on the positive electrode side, thereby further improving the cycle stability of the battery. At the same time, since the SEI film rich in organic components formed by the first additive has poor thermal stability, and the above-mentioned second additive can also form an SEI film rich in inorganic components on the negative electrode, the SEI film rich in inorganic components has excellent rigidity and thermal stability. Therefore, the second additive and the first additive synergistically act to further improve the stability of the SEI film, which is beneficial to reduce the interface impedance on the negative electrode side, thereby being beneficial to further improve the cycle stability and low-temperature charging performance of the battery.

[0031] In any embodiment, the mass percentage content W5 of the second additive based on the total mass of the electrolyte is 0.1%≤W5≤3%, and optionally, 0.1%≤W5≤2%.

[0032] When the mass percentage content of the second additive is in the above-mentioned range, it is beneficial to inhibit the release of crystallization water and the dissolution of doped elements, inhibit the oxidative decomposition of the electrolyte on the positive electrode side and the side reaction between the lithium iron phosphate salt particles and the electrolyte on the positive electrode side, improve the stability of the SEI film, reduce the interface impedance on the negative electrode side, and maintain the viscosity of the electrolyte appropriate, thereby being beneficial to further improve the cycle stability and low-temperature charging performance of the lithium secondary battery of the embodiments of the present application.

[0033] In any embodiment, the first additive includes vinylene carbonate, and the mass percentage content W4 of the vinylene carbonate based on the total mass of the electrolyte is 0.4%≤W4≤3%, and optionally, 0.5%≤W4≤1%; and / or,

[0034] The second additive includes lithium difluorophosphate, and the mass percentage content W5 of the lithium difluorophosphate based on the total mass of the electrolyte is 0.25%≤W5≤1%, and optionally, 0.3%≤W5≤0.5%.

[0035] The lithium difluorophosphate can be preferentially decomposed from other components in the electrolyte, forming an inner layer of the SEI film rich in inorganic salt components at the negative electrode interface, which has excellent rigidity and thermal stability; then the vinylene carbonate decomposes at the negative electrode to form an outer layer of the SEI film rich in organic polymer components, which has excellent flexibility. The first additive and the second additive include the above-mentioned substances, so that the SEI film can better adapt to the expansion and contraction of the negative active material during the charging and discharging process, alleviate the mechanical rupture of the SEI film, further reduce the interface impedance on the negative side, and thus further improve the cycle stability and low-temperature charging performance of the lithium secondary battery. Further, when the mass percentage content of the vinylene carbonate and the lithium difluorophosphate is in the above-mentioned range, the synergistic effect of the two is more favorable, further improving the stability of the SEI film on the negative side and further reducing the interface impedance on the negative side, further inhibiting the release of crystallization water and the dissolution of doped elements, inhibiting the decomposition of the electrolyte on the positive side and the side reaction with the lithium iron phosphate salt particles, and further helping to maintain the appropriate viscosity of the electrolyte, thereby further improving the cycle stability and low-temperature charging performance of the lithium secondary battery.

[0036] In any embodiment, the mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive satisfy 0.4%≤W4+W5≤6%, and optionally, 0.5%≤W4+W5≤4.3%.

[0037] When the mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive satisfy the above-mentioned relationship, the synergistic effect of the two is more favorable, further improving the stability of the SEI film on the negative side and further reducing the interface impedance on the negative side, and further helping to maintain the appropriate viscosity of the electrolyte, thereby further improving the cycle stability and low-temperature charging performance of the lithium secondary battery of the present application.

[0038] In any embodiment, the total amount of substance of the doped element Q in the positive electrode film layer is M1 mol, the total amount of substance of the second additive in the electrolyte is M2 mol, and M1 and M2 satisfy 0.04≤M1 / M2≤12, and optionally, 0.1≤M1 / M2≤1.4.

[0039] When the total amount of substance M1 of the doped element Q in the positive electrode film layer and the total amount of substance M2 of the second additive in the electrolyte satisfy the above-mentioned relationship, the lithium secondary battery of the present application has more excellent low-temperature fast charging performance, while also further taking into account the cycle stability. Specifically, when M1 and M2 satisfy the above-mentioned relationship, it is beneficial for the doped element Q to improve the cycle stability and low-temperature fast charging performance of the lithium secondary battery, while reducing the influence of its dissolution on the cycle performance of the lithium secondary battery by adding appropriate second additives.

[0040] In any embodiment, the electrolyte comprises a solvent, and the solvent comprises at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, r-butyrolactone, diethyl carbonate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, or propyl propionate.

[0041] The above solvent can be beneficial to dissolve solute in the electrolyte, form movable ions, and be beneficial to improve ionic conductivity of the electrolyte, thereby being beneficial to further improve the low-temperature fast-charging performance of the lithium secondary battery of the embodiment of the application.

[0042] In any embodiment, the electrolyte comprises a lithium salt, and the lithium salt comprises at least one of lithium hexafluorophosphate, lithium fluorosulfonyl (perfluorobutylsulfonyl) imide, or lithium bis(trifluoromethylsulfonyl) imide.

[0043] The above lithium salt is dissolved in the solvent, which is beneficial to improve ionic conductivity of the electrolyte, thereby being beneficial to further improve the low-temperature fast-charging performance of the lithium secondary battery of the embodiment of the application.

[0044] In any embodiment, the concentration of the lithium salt is 0.8 mol / L-1.3 mol / L.

[0045] When the concentration of the lithium salt is in the above range, it is beneficial to further improve ionic conductivity of the electrolyte, thereby being beneficial to further improve the low-temperature fast-charging performance of the lithium secondary battery of the embodiment of the application.

[0046] In any embodiment, the volume particle size distribution particle size Dv50 of the positive electrode active material is 60 μm-100 μm; and / or

[0047] The volume particle size distribution particle size Dv90 of the positive electrode active material is 60 μm-250 μm.

[0048] When the Dv50 and Dv90 of the positive electrode active material are in the above range, it is beneficial to balance the relationship between shortening the lithium ion diffusion path of the positive electrode active material and relieving the increase in side reactions caused by the excessively large specific surface area of the positive electrode active material, thereby being beneficial to further improve the low-temperature fast-charging performance of the lithium secondary battery of the embodiment of the application while further taking into account the cycle stability.

[0049] In any embodiment, the BET specific surface area of the positive electrode active material is 11 m 2 / g-25 m 2 / g.

[0050] When the BET specific surface area of the positive electrode active material is in the appropriate range, it is beneficial to further improve the low-temperature fast-charging performance of the lithium secondary battery of the embodiment of the application while further taking into account the cycle stability.

[0051] In any embodiment, the compacted density of the positive electrode tab is 1.1 g / cm 3 -1.5 g / cm 3 .

[0052] When the compacted density of the positive electrode tab is within the above range, the positive electrode tab has a suitable porosity, which is beneficial for the infiltration of electrolyte, and is beneficial for further improving the low-temperature fast-charging performance and cycle stability of the lithium secondary battery of the present application while taking into account excellent energy density.

[0053] In any embodiment, the lithium iron phosphate salt particles are a component having the following general formula:

[0054] Li m Fe x P y O j Q q wherein Q comprises one or more of W, Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, Zr, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0

[0055] In any embodiment, the positive electrode tab comprises a coating layer, the coating layer is arranged on at least one side of the positive electrode current collector, the positive electrode film layer is arranged on at least part of the surface of the side of the coating layer away from the positive electrode current collector, and the coating layer comprises a conductive agent and a binder.

[0056] The above-mentioned coating layer is arranged between the positive electrode current collector and the positive electrode film layer, which is beneficial for improving the conductivity of the positive electrode tab and the adhesion between the positive electrode film layer and the positive electrode current collector, thereby being beneficial for further improving the low-temperature fast-charging performance and cycle stability of the lithium secondary battery of the present application.

[0057] In any embodiment, the thickness of the coating layer is 1 μm-5 μm.

[0058] Controlling the thickness of the coating layer within the above range is beneficial for further improving the low-temperature fast-charging performance of the lithium secondary battery of the present application.

[0059] In any embodiment, the mass percentage content of the conductive agent is greater than or equal to 20% and less than or equal to 50%, based on the total mass of the coating layer; and / or

[0060] The mass percentage content of the binder is greater than or equal to 50% and less than or equal to 80%.

[0061] The control of the conductive agent within a reasonable range is conducive to further improving the conductivity of the positive electrode sheet; the control of the binder within a reasonable range is conducive to further improving the adhesion of the coating to the positive electrode current collector and the positive electrode film layer, conducive to electron conduction and the structural stability of the positive electrode sheet. In summary, the control of the mass percentage content of the conductive agent and the binder within the above range is conducive to further improving the low-temperature fast-charging performance and cycle stability of the lithium secondary battery of the present application.

[0062] In any embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer including a negative electrode active material, the negative electrode active material including one or more of graphite or a modification thereof, a silicon-carbon mixture.

[0063] The compound of formula I is more easily bonded to the above-mentioned negative electrode active material, thereby promoting the desolvation of lithium ions solvated by propylene carbonate, and is conducive to further inhibiting the co-intercalation of propylene carbonate and lithium ions on the side of the negative electrode active material, and is conducive to reducing the degree of peeling of the negative electrode active material, thereby further improving the cycle stability of the lithium secondary battery of the present application.

[0064] The second aspect of the present application also provides a power utilization device, which includes the lithium secondary battery of the first aspect of the present application.

[0065] The above description is only a summary of the technical solutions of the present application. In order to more clearly understand the technical means of the present application, the specific embodiments of the present application can be implemented in accordance with the content of the description, and in order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 is a schematic diagram of a lithium secondary battery of an embodiment of the present application.

[0067] Figure 2 is a schematic diagram of a lithium secondary battery of an embodiment of the present application. Figure 1

[0068] Figure 3 is a schematic diagram of a battery module of an embodiment of the present application.

[0069] Figure 4 is a schematic diagram of a battery pack of an embodiment of the present application.

[0070] Figure 5 is a schematic diagram of a battery pack of an embodiment of the present application. Figure 4

[0071] Figure 6 is a schematic diagram of a power utilization device using the lithium secondary battery of an embodiment of the present application as a power source. ​​

[0072] Reference Signs List:

[0073] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 lithium secondary battery; 51 case; 52 electrode assembly; 53 top cap assembly DETAILED DESCRIPTION

[0074] Hereinafter, specific embodiments of the lithium secondary battery and the electric device according to the present application will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well, repeated description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0075] The "ranges" disclosed in the present application are defined in the form of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if the ranges 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges 60-110 and 80-120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise specified, a numerical range "a-b" represents a shorthand manner of describing the arbitrary combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed herein, and "0-5" is just a shorthand notation for these numerical combinations. In addition, when it is stated that a certain parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0076] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0077] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0078] If not specified otherwise, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0079] If not specified otherwise, the terms "comprising" and "including" as used in the present application are meant to be interpreted in an open way, i.e. also in a closed way. For example, the terms "comprising" and "including" can mean that further components, which are not listed, can also be included or comprised, or that only the listed components are included or comprised.

[0080] If not specified otherwise, the term "or" in the present application is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following satisfy the condition "A or B": A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present).

[0081] Lithium secondary batteries are widely used in various fields, but their fast charging performance at low temperature environment decreases significantly. In order to solve this problem, modification of positive active material and improvement of electrolyte can be considered.

[0082] [lithium secondary battery]

[0083] Based on this, the present application provides a lithium secondary battery, the lithium secondary battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte,

[0084] The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprising a positive active material, the positive active material comprising lithium-containing transition metal phosphate particles, the lithium-containing transition metal phosphate particles comprising lithium iron phosphate particles, the lithium iron phosphate particles comprising a doping element Q, the doping element Q comprising one or more of W, Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, Zr.

[0085] The electrolyte comprises propylene carbonate and a compound having the following formula I,

[0086]

[0087] R is selected from halogen, n is an integer selected from 1-6.

[0088] The doping elements in the lithium iron phosphate salt particles can be measured by methods and devices known in the art, for example, determined by inductively coupled plasma (ICP) spectrometry analysis, for example, refer to the standards YS / T1006.2-2014, GB / T23367.2-2009 or YS / T1028.5-2015.

[0089] In some embodiments, the doping element Q comprises one or more of V, Mg, B, Zr, Ti.

[0090] In some embodiments, the doping element Q comprises V.

[0091] In this paper, the term "halogen" refers to the elements in Group VIIA of the periodic table of chemical elements, specifically, halogen includes fluorine, chlorine, bromine, iodine or astatine and the like.

[0092] In some embodiments, the compound of formula I, n is 1, 2, 3, 4, 5 or 6.

[0093] The lithium secondary battery of the present application adds propylene carbonate in the electrolyte, which has a high ionic conductivity and a low freezing point, which helps to improve the low temperature lithium ion liquid phase transmission speed of the electrolyte, thereby facilitating the improvement of the low temperature fast charging performance of the lithium secondary battery of the present application. And because the specific surface area of the lithium iron phosphate salt particles is large, it is easy to have a side reaction with the electrolyte on the positive electrode side, reducing the cycle stability of the lithium secondary battery. The added propylene carbonate in the present application has good oxidation resistance on the positive electrode side, which is conducive to inhibiting the side reaction between the lithium iron phosphate salt particles and the electrolyte, thereby facilitating the improvement of the cycle performance of the battery. However, propylene carbonate is easy to be embedded into the negative electrode active material (such as graphite) together with lithium ions on the negative electrode side, thereby causing the stripping of the negative electrode active material and deteriorating the cycle performance of the secondary battery. And after the propylene carbonate strips the negative electrode active material, the electrolyte undergoes a reduction reaction at the negative electrode interface, producing reducing gas, which attacks the positive electrode active material, causing the structure of the positive electrode active material to be broken.

[0094] To this end, the compound of formula I is added to the electrolyte, the compound of formula I contains a large Π bond, which is easy to bond with the negative electrode active material, such as graphite, silicon-carbon mixture, etc., thereby enriching on the surface of the negative electrode active material, making the surface have a certain negative charge, promoting the desolvation of the lithium ion solvated by propylene carbonate, which is conducive to inhibiting the co-embedding of propylene carbonate and lithium ions on the negative electrode active material side, and is conducive to reducing 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.

[0095] Furthermore, the positive electrode active material of this application uses lithium iron phosphate particles including the aforementioned dopant element Q. The dopant element Q is beneficial to stabilizing the structure of the lithium iron phosphate particles and alleviating the collapse of the lithium iron phosphate particle structure caused by the introduction of propylene carbonate, thereby further improving the cycle performance of the secondary battery. Moreover, the dopant element Q is beneficial to improving the electronic conductivity and lithium ion migration speed of the lithium iron phosphate particles, making up for the defect of poor conductivity of lithium iron phosphate particles, and further improving the fast charging performance of the lithium secondary battery at low temperature.

[0096] In summary, the lithium secondary battery of this application, through the rational design of the positive electrode active material and electrolyte composition, enables the battery to have good low-temperature fast charging performance while also taking into account cycle stability.

[0097] In some implementations, based on the total mass of lithium iron phosphate salt particles, the mass percentage content W1 of dopant element Q is 0.05% ≤ W1 ≤ 0.5%.

[0098] In some implementations, the mass percentage of dopant element Q, W1, is 0.1% ≤ W1 ≤ 0.3% based on the total mass of the lithium iron phosphate salt particles.

[0099] Based on the total mass of lithium iron phosphate salt particles, the mass percentage content W1 of dopant element Q can be measured by methods and equipment known in the art, for example, by inductively coupled plasma (ICP) spectroscopy analysis, for example, with reference to standards YS / T1006.2-2014, GB / T23367.2-2009 or YS / T1028.5-2015.

[0100] In some embodiments, based on the total mass of lithium iron phosphate salt particles, the mass percentage content W1 of dopant element Q can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, or a range of any two of the above W1s, or a value within that range.

[0101] In some embodiments, based on the total mass of lithium iron phosphate particles, the mass percentage content W1 of dopant element Q is the mass percentage content in the initial state. In some embodiments, based on the total mass of lithium iron phosphate particles, the mass percentage content W1 of dopant element Q is the mass percentage content after operation.

[0102] The doping element Q is beneficial to improving the structural stability, electrical conductivity and migration speed of lithium ions of the lithium iron phosphate salt particles, and is beneficial to improving the cycle stability and low-temperature fast-charging performance of the lithium secondary battery. However, the dissolved doping element Q is easy to migrate to the negative electrode side, causing damage to the SEI film of the negative electrode side, thereby affecting the cycle stability and low-temperature fast-charging performance of the lithium secondary battery to a certain extent. When the mass percentage content W1 of the doping element Q is in the above range based on the total mass of the lithium iron phosphate salt particles, the lithium secondary battery is beneficial to further improving the low-temperature fast-charging performance and cycle stability, and is beneficial to improving the energy density of the positive electrode active material and reducing the preparation cost of the positive electrode active material.

[0103] [Electrolyte]

[0104] In some embodiments, R is selected from fluorine, chlorine, or bromine, and n is selected from 1, 4, or 6.

[0105] In some embodiments, R is fluorine.

[0106] 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.

[0107] Compared with other halogen atoms, the electron-withdrawing ability of fluorine atoms is stronger, and the adoption of fluorine atoms to substitute benzene rings can make the center of the benzene ring electron-deficient, thereby being beneficial to enhancing the Π-Π interaction between the compound of Formula I and the negative electrode active material, being beneficial to the enrichment of the compound of Formula I on the surface of the negative electrode active material, and inhibiting the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, thereby further improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application. Further, compared with 1-fluorobenzene, when the compound of Formula I is selected from tetrafluorobenzene and hexafluorobenzene, with the increase in the number of fluorine atoms on the benzene ring, the center of the benzene ring is more electron-deficient, and the Π-Π interaction between the benzene ring and the negative electrode active material is stronger, which is beneficial to further inhibiting the co-intercalation of propylene carbonate and lithium ions on the negative electrode active material side, and further improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application.

[0108] In some embodiments, the mass percentage content W2 of the compound of Formula I based on the total mass of the electrolyte is 0.5%≤W2≤10%.

[0109] In some embodiments, the mass percentage content W2 of the compound of Formula I based on the total mass of the electrolyte is 2%≤W2≤7%.

[0110] In some embodiments, the mass percentage content W2 of the compound of Formula I can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or a range or a value in the range consisting of any two of the above W2.

[0111] In some embodiments, the mass percentage content W2 of the compound of Formula I is the mass percentage content of the compound of Formula I in the electrolyte of the lithium secondary battery in the initial state. In some embodiments, the mass percentage content W2 of the compound of Formula I is the mass percentage content of the compound of Formula I in the electrolyte of the lithium secondary battery in the working state.

[0112] In some embodiments, the mass percentage content W3 of propylene carbonate is 2%≤W3≤10% based on the total mass of the electrolyte.

[0113] In some embodiments, the mass percentage content W3 of propylene carbonate is 2%≤W3≤8% based on the total mass of the electrolyte.

[0114] In some embodiments, the mass percentage content W3 of propylene carbonate can be 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or a range or a value in the range consisting of any two of the above W3 based on the total mass of the electrolyte.

[0115] In some embodiments, the mass percentage content W3 of propylene carbonate is the mass percentage content of propylene carbonate in the electrolyte of the lithium secondary battery in the initial state. In some embodiments, the mass percentage content W3 of propylene carbonate is the mass percentage content of propylene carbonate in the electrolyte of the lithium secondary battery in the working state.

[0116] When the mass percentage content of the compound of Formula I is within the above range, on the one hand, it is conducive to maintaining the viscosity of the electrolyte to be appropriate, the electrolyte has excellent electrical conductivity, and it is conducive to further improving the low-temperature fast-charging performance and cycle stability of the lithium secondary battery of the present application, on the other hand, it is also conducive to further inhibiting the stripping of propylene carbonate to the negative active material, thereby being conducive to further improving the cycle performance.

[0117] When the mass percentage content of the propylene carbonate is within the above range, it is beneficial to further balance the improvement of the low-temperature conductivity of the electrolyte, inhibit the side reaction between the lithium iron phosphate salt particles and the electrolyte, and reduce the amount of propylene carbonate intercalated into the negative active material, thereby being beneficial to improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application while further taking into account more excellent cycle stability.

[0118] In some embodiments, the mass percentage content W2 of the compound of formula I and the mass percentage content W3 of the propylene carbonate satisfy 0.05≤W2 / W3≤5.

[0119] In some embodiments, the mass percentage content W2 of the compound of formula I and the mass percentage content W3 of the propylene carbonate satisfy 0.35≤W2 / W3≤2.

[0120] In some embodiments, the ratio W2 / W3 of the mass percentage content W2 of the compound of formula I and the mass percentage content W3 of the propylene carbonate can be 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, or a range or a value in the range composed of any two of the above values.

[0121] When the ratio W2 / W3 of the mass percentage content of the compound of formula I and the mass percentage content of the propylene carbonate satisfies the above range, it is beneficial to further inhibit the stripping of the propylene carbonate to the negative active material while improving the low-temperature conductivity of the electrolyte and maintaining the appropriate viscosity of the electrolyte, thereby being beneficial to improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application while further taking into account more excellent cycle stability.

[0122] In some embodiments, the electrolyte comprises a first additive, and the first additive comprises at least one of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propene sultone, tetra-vinyl silane, or methanedisulfonate methylene.

[0123] Since the compound of formula I does not participate in the formation of the negative electrode solid electrolyte interface film (SEI film), the lithium secondary battery of the present application adds the first additive to the electrolyte, which is beneficial to the formation of the SEI film rich in organic components on the surface of the negative electrode, the SEI film rich in organic components has excellent flexibility, which is beneficial to inhibit the decomposition of the electrolyte on the negative electrode side, relieve the mechanical rupture of the SEI film, thereby further improving the cycle stability of the lithium secondary battery of the present application.

[0124] In some embodiments, the mass percentage content W4 of the first additive based on the total mass of the electrolyte is 0.3%≤W4≤4%.

[0125] In some embodiments, the mass percentage content W4 of the first additive based on the total mass of the electrolyte can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4%, or a range or a value in the range composed of any two of the above W4.

[0126] It should be understood that since the first additive in the electrolyte will be consumed in the formation and cycling process, the relevant components in the SEI film are generated, and the mass percentage content of the first additive in the electrolyte may be lower than the initial added mass percentage content of the first additive in the electrolyte.

[0127] When the mass percentage content of the first additive in the electrolyte is within the above range, it is beneficial to better form the SEI film rich in organic components, and it is also beneficial to maintain the viscosity of the electrolyte suitable, thereby further improving the low-temperature fast-charging performance of the cycle stability of the lithium secondary battery of the present application.

[0128] In some embodiments, the electrolyte comprises a second additive, and the second additive comprises at least one of lithium difluorophosphate, lithium difluoromonooxalate borate, lithium difluoribisoxalate borate, lithium difluorosulfonylimide, or lithium fluorosulfonate.

[0129] Due to the large specific surface area of the lithium iron phosphate salt particles, the water content thereof is high. The crystallization water in the lithium iron phosphate salt particles is easy to enter the electrolyte, which can cause decomposition of the electrolyte and produce by-products such as hydrofluoric acid. Hydrofluoric acid can easily corrode the positive electrode interface, and then cause the dissolution of the doped elements in the lithium iron phosphate salt particles. Further, the dissolved doped elements are easy to migrate to the negative electrode and damage the SEI film of the negative electrode, and then deteriorate the cycle stability of the battery. When a second additive including the above-mentioned substance is added to the electrolyte, the second additive can form a positive electrode electrolyte interface film (CEI film) on the surface of the positive electrode, thereby being beneficial to inhibit the release of crystallization water and the dissolution of doped elements, inhibit the oxidative decomposition of the electrolyte on the positive electrode side, and at the same time, be beneficial to further inhibit the side reaction between the lithium iron phosphate salt particles and the electrolyte on the positive electrode side, thereby further improving the cycle stability of the battery. At the same time, since the SEI film rich in organic components formed by the first additive has poor thermal stability, and the above-mentioned second additive can also form an SEI film rich in inorganic components on the negative electrode, the SEI film rich in inorganic components has excellent rigidity and thermal stability, therefore the second additive and the first additive synergistically act to further improve the stability of the SEI film, which is beneficial to reduce the interface impedance on the negative electrode side, thereby being beneficial to further improve the cycle stability and low-temperature charging performance of the battery.

[0130] In some embodiments, the mass percentage content W5 of the second additive based on the total mass of the electrolyte is 0.1%≤W5≤3%.

[0131] In some embodiments, the mass percentage content W5 of the second additive based on the total mass of the electrolyte is 0.1%≤W5≤2%.

[0132] In some embodiments, the mass percentage content W5 of the second additive based on the total mass of the electrolyte can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, or a range or a value in the range composed of any two of the above W5.

[0133] It should be understood that, due to the consumption of the second additive in the electrolyte during formation and cycling, the mass percentage content of the second additive in the electrolyte can be lower than the initial added mass percentage content of the second additive in the electrolyte

[0134] When the mass percentage content of the second additive is in the above range, it is not only beneficial to further inhibit the release of crystal water and the dissolution of doped elements, inhibit the oxidative decomposition of the electrolyte on the positive electrode side and the side reaction with the lithium iron phosphate salt particles, but also beneficial to further improve the stability of the SEI film, reduce the interfacial impedance on the negative electrode side, and maintain the viscosity of the electrolyte appropriate, thereby further improving the cycle stability and low-temperature charging performance of the lithium secondary battery of the embodiments of the present application.

[0135] In some embodiments, the first additive includes vinylene carbonate, and the mass percentage content W4 of the vinylene carbonate based on the total mass of the electrolyte is 0.4%≤W4≤3%.

[0136] The second additive includes lithium difluorophosphate, and the mass percentage content W5 of the lithium difluorophosphate based on the total mass of the electrolyte is 0.25%≤W5≤1%.

[0137] In some embodiments, the mass percentage content W4 of the vinylene carbonate based on the total mass of the electrolyte is 0.5%≤W4≤1%.

[0138] In some embodiments, the mass percentage content W4 of the vinylene carbonate based on the total mass of the electrolyte can be 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, or a range composed of any two of the above W4 or a value in the composed range.

[0139] In some embodiments, the mass percentage content W5 of the lithium difluorophosphate based on the total mass of the electrolyte is 0.3%≤W5≤0.5%.

[0140] In some embodiments, the mass percentage content W5 of the lithium difluorophosphate based on the total mass of the electrolyte can be 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.95%, 1%, or a range composed of any two of the above W5 or a value in the composed range.

[0141] The lithium difluorophosphate can be preferentially decomposed from other components in the electrolyte, forming an inner layer of the SEI film rich in inorganic salt components at the negative electrode interface, which has excellent rigidity and thermal stability; then the vinylene carbonate decomposes at the negative electrode to form an outer layer of the SEI film rich in organic polymer components, which has excellent flexibility. The first additive and the second additive include the above-mentioned substances, so that the SEI film can better adapt to the expansion and contraction of the negative active material during the charging and discharging process, alleviate the mechanical rupture of the SEI film, further reduce the interface impedance on the negative side, and further help to maintain the appropriate viscosity of the electrolyte, thereby further improving the cycle stability and low-temperature charging performance of the lithium secondary battery. Further, when the mass percentage content of the vinylene carbonate and the lithium difluorophosphate is in the above-mentioned range, the synergistic effect of the two is more beneficial, further improving the stability of the SEI film on the negative side and further reducing the interface impedance on the negative side, further inhibiting the release of crystallization water and the dissolution of doped elements, inhibiting the decomposition of the electrolyte on the positive side and the side reaction with the lithium iron phosphate salt particles, and further helping to maintain the appropriate viscosity of the electrolyte, thereby further improving the cycle stability and low-temperature charging performance of the lithium secondary battery.

[0142] In some embodiments, the mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive satisfy 0.2%≤W4+W5≤6%.

[0143] In some embodiments, the mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive satisfy 0.5%≤W4+W5≤4.3%.

[0144] In some embodiments, the sum W4+W5 of the mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive can be 0.2%, 0.4%, 0.6%, 0.8%, 0.9%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, 4.2%, 4.4%, 4.6%, 4.8%, 5%, 5.2%, 5.4%, 5.6%, 5.8%, 6%, or a range composed of any two of the above values or a value in the composed range.

[0145] When the mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive satisfy the above relationship, the synergistic effect of the two is more beneficial, further improving the stability of the SEI film on the negative side and further reducing the interface impedance on the negative side, and further helping to maintain the appropriate viscosity of the electrolyte, thereby further improving the cycle stability and low-temperature charging performance of the lithium secondary battery of the embodiments of the present application.

[0146] In some embodiments, the total amount of substance of the doping element Q in the positive electrode film layer is M1 mol, the total amount of substance of the second additive in the electrolyte is M2 mol, and M1 and M2 satisfy 0.04≤M1 / M2≤12.

[0147] In some embodiments, the total amount of substance of the doping element Q in the positive electrode film layer is M1 mol, and the total amount of substance of the second additive in the electrolyte is M2 mol, and M1 and M2 satisfy 0.1≤M1 / M2≤1.4.

[0148] In this article, the term "amount of substance" is a physical quantity that represents a collection containing a certain number of particles. The unit of amount of substance is mole, abbreviated as mol, and the symbol is mol. Internationally, 1 mol is defined as the amount of substance of a system that contains exactly 6.02214076×10 23 atoms or molecules, etc.

[0149] In some embodiments, the ratio M1 / M2 of the total amount of substance of the doping element Q in the positive electrode film layer M1 and the total amount of substance of the second additive in the electrolyte M2 can be 0.04, 0.07, 0.1, 0.2, 0.4, 0.6, 0.8, 1, 4, 7, 10, 12, or a range composed of any two of the above values or a value in the composed range.

[0150] When the total amount of substance of the doping element Q in the positive electrode film layer M1 and the total amount of substance of the second additive in the electrolyte M2 satisfy the above relationship, the lithium secondary battery of the embodiments of the present application has more excellent low-temperature fast-charging performance, and further takes into account the cycle stability. Specifically, when M1 and M2 satisfy the above relationship, it is beneficial for the doping element Q to improve the cycle stability and low-temperature fast-charging performance of the lithium secondary battery, and to reduce the influence of its dissolution on the cycle performance of the lithium secondary battery by adding appropriate second additives.

[0151] In some embodiments, the electrolyte includes a solvent, and the solvent includes at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, r-butyrolactone, diethyl carbonate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, or propyl propionate.

[0152] The above-mentioned solvent can be beneficial to dissolve the solute in the electrolyte, form movable ions, and be beneficial to improve the ionic conductivity of the electrolyte, thereby being beneficial to further improve the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application.

[0153] In some embodiments, the electrolyte includes a lithium salt, and the lithium salt includes at least one of lithium hexafluorophosphate, lithium fluorosulfonyl (perfluorobutylsulfonyl) imide, or bis(trifluoromethylsulfonyl) imide lithium.

[0154] The lithium salt is dissolved in the solvent, which is conducive to improving the ionic conductivity of the electrolyte, thereby further improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiment.

[0155] In some embodiments, the concentration of the lithium salt is 0.8 mol / L-1.3 mol / L.

[0156] In some embodiments, the concentration of the lithium salt can be 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, or a range composed of any two of the above or a value in the composed range.

[0157] When the concentration of the lithium salt is in the above range, it is conducive to further improving the ionic conductivity of the electrolyte, thereby further improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiment.

[0158] [Positive electrode sheet]

[0159] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer including a positive electrode active material.

[0160] As an example, the positive electrode current collector has two opposite surfaces in the thickness direction of itself, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0161] In some embodiments, the volume particle size distribution particle size Dv50 of the positive electrode active material is 60-100 pm.

[0162] In some embodiments, the volume particle size distribution particle size Dv50 of the positive electrode active material can be 60 pm, 65 pm, 70 pm, 75 pm, 80 pm, 85 pm, 90 pm, 95 pm, 100 pm, or a range composed of any two of the above or a value in the composed range.

[0163] In some embodiments, the volume particle size distribution particle size Dv90 of the positive electrode active material is 150-250 pm.

[0164] In some embodiments, the volume particle size distribution particle size Dv90 of the positive electrode active material can be 150 pm, 160 pm, 170 pm, 180 pm, 190 pm, 200 pm, 210 pm, 220 pm, 230 pm, 240 pm, 250 pm, or a range composed of any two of the above or a value in the composed range.

[0165] In the present text, the terms "Dv50", "Dv90" refer to the particle size corresponding to the 50%, 90% of the cumulative volume distribution of the particles.

[0166] In the present text, the Dv50, Dv90 of the positive electrode active material can be measured by methods and devices known in the art. For example, disassemble the lithium secondary battery, obtain the positive electrode sheet, peel off the positive electrode film layer, wash thoroughly with acetone to remove the binder; dry treatment to obtain powder for subsequent characterization test. Take the appropriate amount of powder to be tested, refer to GB / T19077-2016, and use a laser particle size analyzer to measure. The test instrument can be a Mastersizer 3000 laser particle size analyzer of Malvern Instruments Ltd., UK.

[0167] When the Dv50, Dv90 of the positive electrode active material is in the above range, it is beneficial to balance the shortening of the lithium ion diffusion path of the positive electrode active material and the alleviation of the increase in side reactions caused by the excessive specific surface area of the positive electrode active material, thereby facilitating further improvement in the low-temperature fast-charging performance of the lithium secondary battery of the present application while further taking into account the cycle stability.

[0168] In some embodiments, the BET specific surface area of the positive electrode active material is 11 m 2 / g-25 m 2 / g.

[0169] In the present text, the term "specific surface area" or "BET" refers to the total area possessed by unit mass of material.

[0170] The specific surface area of the positive electrode active material can be measured by methods and devices known in the present application. For example, the specific surface area of the positive electrode active material can be tested by gas adsorption method according to GB / T19587-2017 test standard method, as follows: disassemble the lithium secondary battery, obtain the positive electrode sheet, peel off the positive electrode film layer, wash thoroughly with acetone to remove the binder; dry treatment to obtain powder. Take the powder as sample, immerse the sample tube in liquid nitrogen at-196℃, measure the adsorption amount of nitrogen on the solid surface under different pressures under 0.05-0.30 relative pressure, calculate the specific surface area of the sample based on the BET multilayer adsorption theory and its formula.

[0171] In some embodiments, the BET specific surface area of the positive electrode active material is 11 m 2 / g, 12 m 2 / g, 13 m 2 / g, 14 m 2 / g, 15 m 2 / g, 16 m 2 / g, 17 m 2 / g, 18 m2 / g, 19m 2 / g, 20m 2 / g, 21m 2 / g, 22m 2 / g, 23m 2 / g, 24m 2 / g, 25m 2 / g, or a range consisting of or consisting essentially of any two of the above specific surface areas, or a value within the range.

[0172] When the BET specific surface area of the positive electrode active material is within the appropriate range, it is beneficial to further improve the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application while further taking into account the cycle stability.

[0173] In some embodiments, the positive electrode tab has a compacted density of 1.1 g / cm 3 -1.5 g / cm 3 .

[0174] The compacted density of the positive electrode tab can be measured by methods and instruments known in the art, and specific examples are as follows:

[0175] When the positive electrode tab is single-sided coated, the compacted density of the film layer on one side of the positive electrode tab = m / (V1-V2); when the positive electrode tab is double-sided coated, the compacted density of the film layer on one side of the positive electrode tab = m / [2x(V1-V2)], m represents the weight of the film layer, V1 represents the volume of the positive electrode tab, and V2 represents the volume of the positive electrode current collector. m can be obtained by subtracting the weight of the positive electrode current collector from the weight of the positive electrode tab, and the product of the surface area of the positive electrode tab and the thickness of the positive electrode tab is the volume V1 of the positive electrode tab, and the product of the surface area of the positive electrode tab and the thickness of the positive electrode current collector is V2, and the thickness of the positive electrode current collector and the thickness of the positive electrode tab are obtained by measuring the thickness of the tab area empty foil with a micrometer.

[0176] In some embodiments, the compacted density of the positive electrode tab can be 1.1 g / cm 3 , 1.15 g / cm 3 , 1.2 g / cm 3 , 1.25 g / cm 3 , 1.3 g / cm 3 , 1.35 g / cm 3 , 1.4 g / cm 3 , 1.45 g / cm 3 , 1.5 g / cm 3 , or a range consisting of or consisting essentially of any two of the above compacted densities, or a value within the range.

[0177] When the compaction density of the positive electrode sheet is within the above range, the positive electrode sheet has a suitable porosity, which is beneficial to the infiltration of electrolyte, and is beneficial to further improving the low-temperature fast-charging performance and cycle stability of the lithium secondary battery of the embodiments of the present application, while taking into account the excellent energy density.

[0178] In some embodiments, the lithium iron phosphate salt particles are a component having the following general formula:

[0179] Li m Fe x P y O j Q q wherein Q comprises one or more of W, Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, Zr, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0

[0180] As used herein, "the lithium iron phosphate salt particles include a component having the following general formula" is not only limited to the substances represented by the molecular formula, but also includes other substances formed by further appropriate modification on the basis of the molecular formula, which is not limited herein. "General formula" is used only for convenience of description, and is not intended to limit the present application. It can be understood that new materials or new substances obtained by appropriate modification on the basis of the listed positive electrode active materials are also within the scope of the positive electrode active materials, and the aforementioned appropriate modification refers to acceptable modification of the positive electrode active material, and non-limiting examples include doping modification and coating modification.

[0181] As used herein, "the lithium iron phosphate salt particles include a component having the following general formula", the subscripts m, x, y, j, q of each element in the general formula represent the molar ratio.

[0182] In some embodiments, m can be 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, or a range composed of any two of the above m or a value in the range composed of the above m.

[0183] In some embodiments, x can be 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or a range composed of any two of the above x or a value in the range composed of the above x.

[0184] In some embodiments, y can be 0.95, 0.98, 1.0, or a range composed of any two of the above y or a value in the range composed of the above y.

[0185] In some embodiments, j can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, or a range consisting of any two of the foregoing or a number in the range.

[0186] In some embodiments, q can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a range consisting of any two of the foregoing or a number in the range.

[0187] In some embodiments, the positive electrode tab comprises a coating layer, the coating layer is disposed on at least one side of the positive current collector, the positive film layer is disposed on at least part of the surface of the side of the coating layer away from the positive current collector, and the coating layer comprises a conductive agent and a binder.

[0188] In some embodiments, the conductive agent comprises at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0189] In some embodiments, the binder comprises at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylic ester resin.

[0190] The coating layer is disposed between the positive current collector and the positive film layer, which is conducive to improving the conductivity of the positive electrode tab and the adhesion between the positive film layer and the positive current collector, thereby further improving the low-temperature fast-charging performance and cycle stability of the lithium secondary battery of the embodiments of the present application.

[0191] In some embodiments, the thickness of the coating layer is 1-5 μm.

[0192] The thickness of the coating layer in the positive electrode tab can be measured by methods and devices known in the art, and a specific example is as follows: ion polishing section element analysis is performed according to the method of GB / T17359-2012.

[0193] In some embodiments, the thickness of the coating layer can be 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or a range consisting of any two of the foregoing or a number in the range.

[0194] Controlling the thickness of the coating layer within the above range is conducive to further improving the low-temperature fast-charging performance of the lithium secondary battery of the embodiments of the present application.

[0195] In some embodiments, the mass percentage of the conductive agent is greater than or equal to 20% and less than or equal to 50% based on the total mass of the coating.

[0196] In some embodiments, the mass percentage of the conductive agent can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range between any two of the aforementioned mass percentages or a value within the range.

[0197] In some embodiments, the mass percentage of the binder is greater than or equal to 50% and less than or equal to 80% based on the total mass of the coating.

[0198] In some embodiments, the mass percentage of the binder can be 50%, 55%, 60%, 65%, 70%, 75%, 80%, or a range between any two of the aforementioned values or a value within the range.

[0199] Controlling the mass percentage of the conductive agent within a reasonable range is conducive to further improving the conductivity of the positive electrode sheet; controlling the mass percentage of the binder within a reasonable range is conducive to further improving the adhesion of the coating to the positive electrode current collector and the positive electrode film layer, conducive to electron conduction and the structural stability of the positive electrode sheet. In summary, controlling the mass percentage of the conductive agent and the binder within the aforementioned ranges is conducive to further improving the low-temperature fast-charging performance and cycle stability of the lithium secondary battery of the present application.

[0200] In some embodiments, the positive electrode current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a high polymer material base layer and a metal layer formed on at least one surface of the high polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a high polymer material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0201] In some embodiments, the positive electrode film layer can also optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0202] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent can include at least one of super-P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0203] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and then drying, cold-pressing, or the like to obtain the positive electrode tab.

[0204] [Negative electrode tab]

[0205] The negative electrode tab includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0206] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is disposed on either one or both of the two surfaces of the negative electrode current collector.

[0207] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. As a metal foil, for example, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0208] In some embodiments, the negative electrode active material includes one or more of graphite or a modified material thereof, a silicon-carbon mixture.

[0209] In some embodiments, the graphite includes natural graphite or artificial graphite.

[0210] In some embodiments, the modified material of graphite includes a doping modified material or a coating modified material. In some embodiments, the modified material of graphite includes a mixture of graphite and a silicon-based material. In some embodiments, the mass ratio of graphite and the silicon-based material is 98:2-60:40.

[0211] In some embodiments, the silicon-based material includes at least one of elemental silicon, a silicon oxide compound, a silicon-carbon composite, a silicon-nitrogen composite, and a silicon alloy.

[0212] In the present disclosure, the term "silicon-carbon mixture" generally refers to a mixture of silicon (Si) and carbon (C), in which silicon serves as an active material and carbon serves 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.

[0213] The compound of Formula I is more easily bonded to the above-mentioned negative electrode active material, thereby facilitating the desolvation of lithium ions solvated by propylene carbonate, favoring further inhibition of the co-intercalation of propylene carbonate and lithium ions on the side of the negative electrode active material, and favoring the reduction of the degree of peeling of the negative electrode active material, thereby further improving the cycle stability of the lithium secondary battery of the present application.

[0214] In some embodiments, the negative electrode film layer further optionally comprises a binder. The binder can 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).

[0215] In some embodiments, the negative electrode film layer further optionally comprises a conductive agent. The conductive agent can be selected from at least one of super-conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0216] In some embodiments, the negative electrode film layer further optionally comprises other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0217] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and after drying, cold pressing, and the like, the negative electrode sheet can be obtained.

[0218] [Separator]

[0219] In some embodiments, the lithium secondary battery further comprises a separator. The type of separator is not particularly limited in the present application, and any known porous structure separator with good chemical stability and mechanical stability can be used.

[0220] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0221] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator film can be made into an electrode assembly through a winding process or a stacking process.

[0222] In some embodiments, the lithium secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0223] In some embodiments, the outer package of the lithium secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the lithium secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0224] The shape of the lithium secondary battery according to the present application is not particularly limited, and it can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure lithium secondary battery 5 as an example.

[0225] In some embodiments, referring to Figure 2 , the outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator film can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the lithium secondary battery 5 can be one or more, and the person skilled in the art can select according to the specific actual needs.

[0226] In some embodiments, the lithium secondary battery can be assembled into a battery module, and the number of lithium secondary batteries contained in the battery module can be one or more, and the specific number can be selected by the person skilled in the art according to the application and capacity of the battery module.

[0227] Figure 3 is a battery module 4 as an example. Referring to Figure 3 , in the battery module 4, a plurality of lithium secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, it can also be arranged in any other way. Further, the plurality of lithium secondary batteries 5 can be fixed by fasteners.

[0228] Optionally, the battery module 4 can also include a housing having a receiving space, and the plurality of lithium secondary batteries 5 are received in the receiving space.

[0229] In some embodiments, the above battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0230] Figure 4 and Figure 5 is a battery pack 1 as an example. Referring to Figure 4 and Figure 5 In the battery pack 1, a battery box and a plurality of battery modules 4 arranged in the battery box can be included. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0231] [Electric device]

[0232] In addition, the present application also provides an electric device, which includes the lithium secondary battery of the present application.

[0233] In some embodiments, the electric device of the present application can also include at least one of a battery module or a battery pack. The lithium secondary battery, the battery module, or the battery pack can be used as a power supply of the electric device, and can also be used as an energy storage unit of the electric device. The electric device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0234] As the electric device, the lithium secondary battery, the battery module or the battery pack can be selected according to the use requirements thereof.

[0235] Figure 6 is an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the electric device for the lithium secondary battery, a battery pack or a battery module can be used.

[0236] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and a lithium secondary battery can be used as a power supply.

[0237] Embodiments

[0238] Hereinafter, the examples of the present application will be described. The examples described below are illustrative and are intended to explain the present application, and are not to be understood as limiting the present application. In the examples, unless a specific technique or condition is mentioned, the technique or condition described in the literature in the art or according to the product manual is used. The reagents or instruments used, unless the manufacturer is mentioned, are all conventional products that can be obtained commercially.

[0239] I. Preparation method

[0240] Example 1:

[0241] 1) Preparation of positive electrode active material

[0242] 0.984 g of iron vanadate (FeVO4), 37.6 g of ferrous oxalate (Fe(C2O4)2·2H2O), 18.6 g of diammonium hydrogen phosphate ((NH4)2HPO4), 11.6 g of lithium carbonate (Li2CO3), and 1 g of acetylene black were mixed, sand-milled, and spray-dried, and then calcined under inert gas protection, with a sintering temperature of 780°C and a sintering time of 12 h, to prepare a positive electrode active material of vanadium-doped (mass percentage W1 of 0.15%) lithium iron phosphate salt particles. The volume particle size distribution particle size Dv50 of the positive electrode active material was 87 μm, Dv90 was 210 μm, the specific surface area was 13 m 2 / g.

[0243] 2) Preparation of positive electrode sheet

[0244] The prepared positive electrode active material, conductive agent conductive carbon black, and binder polyvinylidene fluoride were added to the solvent N-methyl pyrrolidone at a mass ratio of 92:4:4, stirred uniformly to obtain a positive electrode slurry, and the solid content in the positive electrode slurry was 50 wt%; then the positive electrode slurry was coated on an aluminum foil at a double-sided coating area density of 410.3 g / m 2 , dried at 85°C, and then cold-pressed, followed by edge cutting, sheet cutting, and strip separation, and then dried at 85°C under vacuum for 4 h to prepare the positive electrode sheet, and the tap density of the positive electrode sheet was 1.38 g / cm 3 .

[0245] 3) Preparation of electrolyte

[0246] In an argon-filled glove box (water content <10 ppm, oxygen content <1 ppm), ethylene carbonate (EC) / methyl ethyl carbonate (EMC) were uniformly mixed in a volume ratio of 3 / 7, and lithium salt LiPF6 was slowly added into the above mixed solvent. After the lithium salt was completely dissolved, the concentration of LiPF6 in the electrolyte was 1 mol / L. 1-fluorobenzene, propylene carbonate, the first additive vinylene carbonate and the second additive lithium difluorophosphate were sequentially added and stirred uniformly to obtain the electrolyte. The mass percentage of 1-fluorobenzene W2 was 3%, the mass percentage of propylene carbonate W3 was 5%, the mass percentage of vinylene carbonate W4 was 0.8%, and the mass percentage of lithium difluorophosphate was 0.3% based on the total mass of the electrolyte.

[0247] 4) Preparation of the negative electrode sheet

[0248] The negative electrode active material artificial graphite, the conductive agent carbon black, the thickening agent carboxymethyl cellulose and the binder styrene-butadiene rubber were added into deionized water in a mass ratio of 80:15:3:2, and stirred uniformly to obtain a negative electrode slurry. The solid content of the negative electrode slurry was 30 wt%. The negative electrode slurry was coated on a copper foil in a double-sided coating density of 197.4 g / m 2 , dried at 85°C, then cold-pressed, cut, and dried at 120°C under vacuum for 12 h to obtain the negative electrode sheet.

[0249] 5) Isolation film

[0250] A 16 μm polyethylene (PE) film was used as the isolation film.

[0251] 6) Preparation of the battery

[0252] The prepared positive electrode sheet, the isolation film and the negative electrode sheet were stacked in order, with the isolation film between the positive electrode sheet and the negative electrode sheet to separate the positive electrode and the negative electrode. The bare cell was obtained by winding. The bare cell was welded with tabs, and was put into an aluminum shell and baked in a vacuum oven at 100°C for 8 h. The electrolyte prepared above was injected into the dried cell. After packaging, standing, formation, shaping and capacity testing, the lithium secondary battery of Example 1 was obtained. The total amount of substance M1 of the doped elements in the positive electrode film layer of the lithium secondary battery was 0.127 mol, and the total amount of substance M2 of the second additive in the electrolyte was 0.051 mol.

[0253] Examples 2-5

[0254] The lithium secondary batteries of Examples 2-5 were prepared in a similar manner to the lithium secondary battery of Example 1, except that the mass percentage W1 of the doped elements Q in the lithium iron phosphate salt particles was adjusted. See Tables 1 and 2 for details.

[0255] Examples 6-8

[0256] The lithium secondary batteries of Examples 6-8 were prepared in a similar manner to the lithium secondary battery of Example 1, except that the amount of the second additive in the electrolyte was adjusted, as shown in Tables 1 and 2.

[0257] Examples 9-12

[0258] The lithium secondary batteries of Examples 9-12 were prepared in a similar manner to the lithium secondary battery of Example 1, except that the mass percentage content W1 of the doping element Q in the lithium iron phosphate salt particles and the amount of the second additive in the electrolyte were adjusted, as shown in Tables 1 and 2.

[0259] Examples 13-15

[0260] The lithium secondary batteries of Examples 13-15 were prepared in a similar manner to the lithium secondary battery of Example 1, except that the amount of the first additive in the electrolyte was adjusted, as shown in Tables 1 and 2.

[0261] Examples 16-19

[0262] The lithium secondary batteries of Examples 16-19 were prepared in a similar manner to the lithium secondary battery of Example 1, except that the amount of the compound of Formula I in the electrolyte was adjusted, as shown in Tables 1 and 2.

[0263] Examples 20-22

[0264] The lithium secondary batteries of Examples 20-22 were prepared in a similar manner to the lithium secondary battery of Example 1, except that the amount of propylene carbonate in the electrolyte was adjusted, as shown in Tables 1 and 2.

[0265] Example 23

[0266] The lithium secondary battery of Example 23 was prepared in a similar manner to the lithium secondary battery of Example 1, except that the amounts of the compound of Formula I and propylene carbonate in the electrolyte were adjusted, as shown in Tables 1 and 2.

[0267] Example 24

[0268] The lithium secondary battery of Example 24 was prepared in a similar manner to the lithium secondary battery of Example 1, except that the type of the doping element Q, the type of the first additive, and the type of the second additive were adjusted, as shown in Tables 1 and 2.

[0269] Example 25

[0270] The lithium secondary battery of Example 25 was prepared in a similar manner to the lithium secondary battery of Example 1, except that the type of the compound of Formula I was adjusted, as shown in Tables 1 and 2.

[0271] Example 26

[0272] The lithium secondary battery of Example 26 is prepared in a similar way to the lithium secondary battery of Example 1, except that the first additive and the second additive are not added to the electrolyte, as shown in Tables 1 and 2.

[0273] Example 27

[0274] The lithium secondary battery of Example 27 is prepared in a similar manner to the lithium secondary battery of Example 1, except that there is a coating between the positive electrode current collector and the positive electrode film. The coating is prepared as follows:

[0275] Conductive carbon black (conductive agent) and polyvinylidene fluoride (PVDF) (binder) were added to deionized water at a mass ratio of 5:5 and stirred until the slurry viscosity reached 200 cP. The slurry was then coated onto the surface of the positive electrode current collector, with a single-sided coating density of 0.01 mg / mm². 2 The coating thickness is 2μm, and the positive electrode film layer can be prepared after drying at 100℃.

[0276] Comparative Example 1

[0277] The lithium secondary battery of Comparative Example 1 is prepared in a similar manner to the lithium secondary battery of Example 26, except that the lithium iron phosphate salt particles do not contain the dopant element Q, and the electrolyte does not contain the compound of Formula I or propylene carbonate, as shown in Tables 1 and 2.

[0278] Comparative Example 2

[0279] The lithium secondary battery of Comparative Example 2 is prepared in a similar manner to the lithium secondary battery of Example 26, except that the lithium iron phosphate salt particles do not contain the doping element Q, as shown in Tables 1 and 2.

[0280] Comparative Example 3

[0281] The lithium secondary battery of Comparative Example 3 was prepared in a similar manner to the lithium secondary battery of Example 26, except that the compound of Formula I and propylene carbonate were not added to the electrolyte, as shown in Tables 1 and 2.

[0282] Comparative Example 4

[0283] The lithium secondary battery of Comparative Example 4 was prepared in a similar manner to the lithium secondary battery of Example 26, except that propylene carbonate was not added to the electrolyte, as shown in Tables 1 and 2.

[0284] Comparative Example 5

[0285] The lithium secondary battery of Comparative Example 5 was prepared in a similar manner to the lithium secondary battery of Example 26, except that no compound of Formula I was added to the electrolyte, as shown in Tables 1 and 2.

[0286] II. Battery Performance Testing

[0287] 1. Cycle performance test

[0288] The battery was placed in an environment of 25℃ for 24h, then the battery cell was charged at a constant current of 1C to 3.65V, then charged at a constant voltage of 0.05C, and then discharged at a constant current of 1C to 2V after 10min of rest, and the discharge capacity was recorded as C0. The above charging and discharging process was repeated for 500 cycles, and the discharge capacity of the 500th cycle was recorded as C1. The cycle capacity retention rate of the battery cell was C1 / C0 x 100%.

[0289] 2. Low temperature fast charging performance test

[0290] The battery was placed in an environment of 25℃ for 24h, then the battery cell was charged at a constant current of 1C to 3.65V, then charged at a constant voltage of 0.05C, and then discharged at a constant current of 1C to 2.5V after 5min of rest, and the discharge capacity was recorded as D0. Then the above battery was placed in an environment of -10℃ and charged at a current of 0.5D0 to a cut-off voltage of 3.65V, and the charge capacity was recorded as D1. Then the battery was discharged at a current of D0 to 2.0V, and then rested for 30min. Then the battery was charged at a current of 2D0 to 3.65V, and the charge capacity was recorded as D2. The low temperature charge capacity retention rate was D2 / D1, and the low temperature fast charging performance of the lithium secondary battery was characterized by the low temperature charge capacity retention rate.

[0291] III. Analysis of test results of each example and comparative example

[0292] The lithium secondary batteries of each example and comparative example were prepared according to the above method, and each performance parameter was measured. The results are shown in the following table.

[0293] Table 1

[0294]

[0295] Table 2

[0296]

[0297]

[0298] Table 3

[0299]

[0300]

[0301] As can be seen from Tables 1-3, examples 1-27 provide a lithium secondary battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises lithium-containing transition metal phosphate particles, the lithium-containing transition metal phosphate particles comprise lithium iron phosphate salt particles, the lithium iron phosphate salt particles comprise a doping element Q, the doping element Q comprises one or more of W, Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, Zr; the electrolyte comprises propylene carbonate and a compound having formula I, R is selected from halogen, n is selected from an integer from 1 to 6.

[0302] As can be seen from the comparison of examples 1-27 and comparative examples 1-5, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery of the present application are further improved.

[0303] As can be seen from examples 1-5, when the mass percentage content W1 of the doping element Q is 0.05%≤W1≤0.5% based on the total mass of the lithium iron phosphate salt particles, it is beneficial to further improve the low-temperature fast-charging performance and cycle performance of the lithium secondary battery. As can be seen from the comparison of examples 1, 4, 5 and examples 2, 3, when 0.1%≤W1≤0.3%, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved.

[0304] As can be seen from examples 1, 6-9, when the mass percentage content W5 of the second additive is 0.1%≤W5≤3% based on the total mass of the electrolyte, it is beneficial to further improve the low-temperature fast-charging performance and cycle performance of the lithium secondary battery. As can be seen from the comparison of examples 2, 9 and 12, when the mass percentage content W5 of the second additive is 0.1%≤W5≤3%, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved.

[0305] As can be seen from the comparison of examples 1 and example 24, when the second additive comprises lithium difluorophosphate, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved. As can be seen from the comparison of examples 1, 8 and examples 6, 7, when the second additive comprises lithium difluorophosphate and W5 is 0.3%≤W4≤0.5%, the low-temperature fast-charging performance and cycle performance of the lithium secondary battery are further improved.

[0306] As can be seen from the comparison of Examples 1-10 and Examples 11 and 12, when the total amount of substance M1 of the doping element Q in the positive electrode film layer and the total amount of substance M2 of the second additive in the electrolyte satisfy 0.04≤M1 / M2≤12, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved. As can be seen from the comparison of Examples 1, 2, 4, 5, 7, and 8 and Examples 9 and 10, when 0.1≤M1 / M2≤1.4, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved.

[0307] As can be seen from Examples 1, 13-15, when the mass percentage content W4 of the first additive based on the total mass of the electrolyte is 0.3%≤W4≤4%, it is beneficial to further improve the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery.

[0308] As can be seen from the comparison of Example 1 and Example 24, when the first additive includes vinylene carbonate, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved. As can be seen from the comparison of Examples 1 and 15 and Examples 13 and 14, when the first additive includes vinylene carbonate, and the mass percentage content W4 of the vinylene carbonate based on the total mass of the electrolyte is 0.5%≤W4≤1%, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved.

[0309] As can be seen from Examples 1, 6-15, when the mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive satisfy 0.5%≤W4+W5≤4.3%, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved.

[0310] As can be seen from Examples 1, 16-19, when the mass percentage content W2 of the compound of Formula I based on the total mass of the electrolyte is 0.5%≤W2≤10%, it is beneficial to further improve the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery. As can be seen from the comparison of Examples 1, 17 and 18 and Examples 16 and 19, when 2%≤W2≤7%, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved.

[0311] As can be seen from Examples 1, 20-22, when the mass percentage content W3 of the propylene carbonate based on the total mass of the electrolyte is 2%≤W3≤10%, it is beneficial to further improve the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery. As can be seen from the comparison of Examples 1, 20 and 21 and Example 22, when 2%≤W3≤8%, the lithium secondary battery has excellent low-temperature fast-charging performance, and the cycle performance is further improved.

[0312] As can be seen from the embodiment 1 and the embodiments 16-23, when 0.05≤W2 / W3≤5, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved. As can be seen from the comparison between the embodiment 1 and the embodiments 17-21 and the comparison between the embodiment 16 and the embodiments 22 and 23, when 0.35≤W2 / W3≤2, the lithium secondary battery has excellent low-temperature fast-charging performance and the cycle performance is further improved.

[0313] As can be seen from the embodiment 1 and the embodiment 25, when the compound of formula I comprises at least one of 1-fluorobenzene, 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene or hexafluorobenzene, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved. As can be seen from the comparison between the embodiment 25 and the embodiment 1, when the compound of formula I comprises hexafluorobenzene, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved.

[0314] As can be seen from the comparison between the embodiment 26 and the embodiment 1, when the positive electrode sheet comprises a coating, the low-temperature fast-charging performance and the cycle performance of the lithium secondary battery are further improved.

[0315] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are only examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea within the scope of the technical solutions of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications that can be thought of by those skilled in the art are applied to the embodiments, and other modes constructed by combining part of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium secondary battery, characterized in that, The lithium secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive active material, which includes lithium-containing transition metal phosphate particles. The lithium-containing transition metal phosphate particles include lithium iron phosphate particles. The lithium iron phosphate particles include a dopant element Q, which includes one or more of Al, V, and Ti. The electrolyte comprises propylene carbonate and a compound having the following formula I. Formula I R is selected from halogens, and n is selected from integers from 1 to 6; Based on the total mass of the lithium iron phosphate salt particles, the mass percentage content W1 of the dopant element Q is 0.05% ≤ W1 ≤ 0.5%; The electrolyte includes a second additive, which includes lithium difluorophosphate. Based on the total mass of the electrolyte, the mass percentage of lithium difluorophosphate, W5, is 0.25% ≤ W5 ≤ 1%. The BET specific surface area of ​​the positive electrode active material is 11m². 2 / g-25m 2 / g.

2. The lithium secondary battery according to claim 1, characterized in that, Based on the total mass of the lithium iron phosphate salt particles, the mass percentage content W1 of the dopant element Q is 0.1% ≤ W1 ≤ 0.3%.

3. The lithium secondary battery according to claim 1, characterized in that, R is selected from fluorine, chlorine, or bromine, and n is selected from 1, 4, or 6.

4. The lithium secondary battery according to claim 1, 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.

5. The lithium secondary battery according to claim 1, characterized in that, Based on the total mass of the electrolyte, the mass percentage W2 of the compound of formula I is 0.5% ≤ W2 ≤ 10%; and / or Based on the total mass of the electrolyte, the mass percentage of propylene carbonate W3 is 2% ≤ W3 ≤ 10%.

6. The lithium secondary battery according to claim 5, characterized in that, Based on the total mass of the electrolyte, the mass percentage W2 of the compound of formula I is 2% ≤ W2 ≤ 7%; and / or Based on the total mass of the electrolyte, the mass percentage of propylene carbonate W3 is 2% ≤ W3 ≤ 8%.

7. The lithium secondary battery according to claim 5, characterized in that, The mass percentage content W2 of the compound of Formula I and the mass percentage content W3 of the propylene carbonate satisfy 0.05 ≤ W2 / W3 ≤ 5.

8. The lithium secondary battery according to claim 7, characterized in that, The mass percentage of the compound of Formula I, W2, and the mass percentage of the propylene carbonate, W3, satisfy 0.35 ≤ W2 / W3 ≤ 2.

9. The lithium secondary battery according to claim 1, 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.

10. The lithium secondary battery according to claim 9, characterized in that, Based on the total mass of the electrolyte, the mass percentage of the first additive, W4, is 0.3% ≤ W4 ≤ 4%.

11. The lithium secondary battery according to claim 1, characterized in that, The second additive also includes at least one of lithium difluoromonoxalate borate, lithium difluorobisoxalate borate, lithium bisfluorosulfonylimide, or lithium fluorosulfonate.

12. The lithium secondary battery according to claim 9, characterized in that, The first additive includes vinylene carbonate, and based on the total mass of the electrolyte, the mass percentage of vinylene carbonate W4 is 0.4% ≤ W4 ≤ 3%; and / or, Based on the total mass of the electrolyte, the mass percentage of lithium difluorophosphate, W5, is 0.3% ≤ W5 ≤ 0.5%.

13. The lithium secondary battery according to claim 12, characterized in that, The first additive includes vinylene carbonate, and the mass percentage of vinylene carbonate W4 is 0.5% ≤ W4 ≤ 1% based on the total mass of the electrolyte.

14. The lithium secondary battery according to claim 9, characterized in that, The mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive satisfy 0.4%≤W4+W5≤6%.

15. The lithium secondary battery according to claim 14, characterized in that, The mass percentage content W4 of the first additive and the mass percentage content W5 of the second additive satisfy 0.5%≤W4+W5≤4.3%.

16. The lithium secondary battery according to claim 1, characterized in that, The total amount of doped element Q in the positive electrode film is M1 mol, and the total amount of the second additive in the electrolyte is M2 mol. M1 and M2 satisfy 0.04≤M1 / M2≤12.

17. The lithium secondary battery according to claim 16, characterized in that, The total amount of doped element Q in the positive electrode film is M1 mol, and the total amount of the second additive in the electrolyte is M2 mol, wherein M1 and M2 satisfy 0.1≤M1 / M2≤1.

4.

18. The lithium secondary battery according to any one of claims 1 to 17, 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.

19. The lithium secondary battery according to any one of claims 1 to 17, 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.

20. The lithium secondary battery according to claim 19, characterized in that, The concentration of the lithium salt is 0.8 mol / L to 1.3 mol / L.

21. The lithium secondary battery according to any one of claims 1 to 17, characterized in that, The volumetric particle size distribution (Dv50) of the positive electrode active material is 60 μm-100 μm; and / or The volumetric particle size distribution (Dv90) of the positive electrode active material is 150 μm-250 μm.

22. The lithium secondary battery according to any one of claims 1 to 17, characterized in that, The compaction density of the positive electrode sheet is 1.1 g / cm³. 3 -1.5g / cm 3 .

23. The lithium secondary battery according to any one of claims 1 to 17, characterized in that, The lithium iron phosphate salt particles are components having the following general formula: The m Fe x P y O j Q q , Wherein, Q includes one or more of Al, V, and Ti, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.

1.

24. The lithium secondary battery according to any one of claims 1 to 17, characterized in that, The positive electrode sheet includes a coating, the coating being disposed on at least one side of the positive current collector, and the positive electrode film being disposed on at least a portion of the surface of the coating away from the positive current collector. The coating includes a conductive agent and a binder.

25. The lithium secondary battery according to claim 24, characterized in that, The thickness of the coating is 1μm-5μm.

26. The lithium secondary battery according to claim 24, characterized in that, Based on the total mass of the coating, the mass percentage of the conductive agent is greater than or equal to 20% and less than or equal to 50%; and / or The adhesive has a mass percentage of 50% or more and 80% or less.

27. The lithium secondary battery according to any one of claims 1 to 17, 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.

28. An electrical appliance, characterized in that, The electrical device includes a lithium secondary battery according to any one of claims 1 to 27.

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