Lithium secondary battery, preparation method therefor, and electrical apparatus

Abstract

A lithium secondary battery, a preparation method therefor, and an electrical apparatus. The lithium secondary battery comprises an electrolyte solution, the electrolyte solution comprising a first solvent, a first additive, and a second additive, and the first solvent comprising a carboxylic ester compound. The reduction potential of the carboxylic ester compound relative to Li+ / Li is less than or equal to 1.4 V, the reduction potential of the first additive relative to Li+ / Li is greater than or equal to 1.4 V, and the reduction potential of the second additive relative to Li+ / Li is greater than or equal to 1.0 V and less than 1.4 V. According to the lithium secondary battery, the stability and compactness of an SEI film are improved by means of improving an electrolyte solution, thereby reducing the degree of gas production during cycling and storage of lithium secondary battery and comprehensively improving the performance of the lithium secondary battery.

Description

Lithium secondary battery, preparation method thereof and power-consuming device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202311680333.9, filed on December 8, 2023, entitled “Lithium Secondary Battery, Preparation Method Thereof and Electrical Device”, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present application relates to the technical field of lithium batteries, and in particular to a lithium secondary battery, a preparation method thereof, and an electrical device. Background Art

[0004] In recent years, the application of lithium-ion batteries has become increasingly widespread. They are widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. As lithium-ion batteries have achieved significant development, higher requirements have been placed on their energy density, cycle performance, and safety performance.

[0005] Improving the power performance, cycle and storage life of batteries has always been the industry's goal, but due to the side reactions of the electrolyte at the anode and cathode interfaces, the overall performance of the battery cell is poor.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide a lithium secondary battery and its preparation method, aiming to improve the stability and density of the SEI (Solid Electrolyte Interphase) membrane by improving the electrolyte, reduce the gas production during the cycle and storage of the lithium secondary battery, improve the storage performance and cycle performance of the lithium secondary battery, and comprehensively improve the performance of the lithium secondary battery.

[0008] In order to achieve the above object, the first aspect of the present application provides a lithium secondary battery, the lithium secondary battery comprising an electrolyte, the electrolyte comprising a first solvent, a first additive and a second additive, the first solvent comprising a carboxylic acid ester compound; wherein the carboxylic acid ester compound relative to Li + / Li reduction potential is less than or equal to 1.4V; the first additive relative to Li + / Li reduction potential is greater than or equal to 1.4V; the second additive relative to Li + The reduction potential of Li is greater than or equal to 1.0V and less than 1.4V.

[0009] Carboxylate solvents can improve the ionic conductivity of the electrolyte due to their low viscosity, thereby improving the fast charging performance of the battery. + The reduction potential of carboxylate solvents is higher than that of Li + The high reduction potential of Li / Li facilitates the formation of an SEI film with the first additive over the carboxylate solvent, thereby reducing the side reactions and gas production of the carboxylate solvent. Simultaneously, the introduction of the second additive competes with the carboxylate solvent to form the SEI film. The SEI film formed by the second additive helps improve the overall density and stability of the SEI film, further reducing the risk of the carboxylate solvent passing through the SEI film and inducing side reactions, thereby improving both the storage performance and the cycling performance of the battery.

[0010] In any embodiment, the electrolyte further comprises a third additive, wherein the third additive is + The reduction potential of / Li is less than 1.0V.

[0011] The third additive introduced into the electrolyte can participate in solvation, forming a solvation structure and inhibiting oxidative decomposition and gas production at both high and room temperatures. Furthermore, some third additives continue to participate in film formation after the first and second additives have formed, further enhancing the density and stability of the SEI film.

[0012] In any embodiment, the carboxylic acid ester compound includes a compound of the structure shown in Formula I

[0013] Wherein, R1 and R2 each independently include at least one of a C1-C5 alkyl group and a C1-C5 halogenated alkyl group.

[0014] In any embodiment, the carboxylic acid ester compound includes at least one of ethyl acetate, methyl acetate, propyl acetate, and ethyl formate.

[0015] In any embodiment, the carboxylate compound includes at least one of ethyl acetate, methyl acetate, and propyl acetate.

[0016] When the above-mentioned carboxylic acid ester compounds are used as solvents, they can reduce the viscosity of the electrolyte, increase the conductivity of the electrolyte, and thus improve the fast charging performance of the battery.

[0017] In any embodiment, the first additive includes at least one of a tetrafluoroborate compound, an oxalatoborate compound, a borate ester compound, vinyl sulfite, 1,3-propylene sultone, and methylene methanedisulfonate.

[0018] In any embodiment, the first additive includes at least one of lithium difluorooxalatoborate, lithium bisoxalatoborate, tris(trimethylsilyl)borate, and 1,3-propylene sultone.

[0019] The above-mentioned type of first additive can form an SEI film in preference to the carboxylate solvent, and the generated SEI film has higher electrical conductivity and lower charge transfer impedance, which can reduce the internal resistance of the battery to a certain extent, enhance the stability of the solvent and the anode, reduce the side reactions of the carboxylate solvent, and comprehensively improve the performance of the lithium secondary battery.

[0020] In any embodiment, the second additive includes at least one of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, organic sulfates, sulfonates, and difluorophosphates.

[0021] In any embodiment, the second additive includes at least one of vinylene carbonate, fluoroethylene carbonate, and vinyl sulfate.

[0022] The above-mentioned type of second additive can compete with the carboxylate solvent to form the SEI film, which helps to improve the overall density and stability of the SEI film, further reduce the risk of the carboxylate solvent passing through the SEI film to induce side reactions, and can improve both the storage performance and cycle performance of the battery.

[0023] In any embodiment, the third additive includes at least one of an organic phosphorus compound, an organic fluorine compound, a phosphate compound, a borate compound, and a sulfonate compound.

[0024] In any embodiment, the third additive includes at least one of tris(2,2,2-trifluoroethyl)phosphite and tris(trimethylsilyl)phosphate.

[0025] The third additive of the above type can participate in solvation, form a solvation structure, and further inhibit the oxidative decomposition and gas generation of the battery at high temperature and room temperature.

[0026] In any embodiment, the lithium secondary battery satisfies: 0.003≤W1 / W4≤0.125, wherein W4 is the mass content of the first solvent in the electrolyte, in %; W1 is the mass content of the first additive in the electrolyte, in %.

[0027] In any embodiment, the lithium secondary battery satisfies: 0.025≤W2 / W4≤0.5, wherein W4 is the mass content of the first solvent in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %.

[0028] In any embodiment, the lithium secondary battery satisfies: 0.3g / Ah≤(D4+D2) / A≤2.3g / Ah, wherein D4 is the mass of the first solvent, in g; D2 is the mass of the second additive, in g; and A is the rated capacity of the lithium secondary battery, in Ah.

[0029] In any embodiment, the lithium secondary battery satisfies: 0.5g / Ah≤(D4+D2) / A≤2g / Ah, wherein D4 is the mass of the first solvent, in g; D2 is the mass of the second additive, in g; and A is the rated capacity of the lithium secondary battery, in Ah.

[0030] Compared with carboxylate solvents, the first additive has a + / Li has a high reduction potential and can form a film on the anode surface in preference to carboxylic acid ester solvents; the relative reaction between the second additive and the carboxylic acid ester solvent is relatively large. + The reduction potentials of the two additives and the carboxylate ester compounds are relatively close, allowing them to compete with the carboxylate ester solvents to form the SEI film. Therefore, controlling the ratio of the two additives to the carboxylate ester compound in the electrolyte within a certain range can effectively reduce the side reactions and gas production of the carboxylate ester solvent, while also improving the battery's storage performance and cycle performance.

[0031] In any embodiment, the lithium secondary battery satisfies: 0.016≤W1 / (W1+W2+W3)≤0.455, wherein W1 is the mass content of the first additive in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %; W3 is the mass content of the third additive in the electrolyte, in %.

[0032] In any embodiment, the lithium secondary battery satisfies: 0.333≤W2 / (W1+W2+W3)≤0.833, wherein W1 is the mass content of the first additive in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %; W3 is the mass content of the third additive in the electrolyte, in %.

[0033] Since relative to Li + Because the reduction potentials of the first and third additives are relatively close, there is a certain degree of competition between the second additive and the carboxylate solvent in film formation. Controlling the second additive content relative to the first and third additives helps enhance the competitive film formation effect between the second additive and the carboxylate solvent. Therefore, controlling the content ratio of the first, second, and third additives within a certain range can balance the storage performance of lithium secondary batteries and comprehensively improve battery performance.

[0034] In any embodiment, the lithium secondary battery satisfies: 10%≤W4≤70%, wherein W4 is the mass content of the first solvent in the electrolyte, in %.

[0035] In any embodiment, the lithium secondary battery satisfies: 20%≤W4≤50%, wherein W4 is the mass content of the first solvent in the electrolyte, in %.

[0036] Carboxylate compounds as solvents can improve electrolyte conductivity, thereby enhancing battery fast-charging performance. However, excessive levels of carboxylate solvents can cause oxidative decomposition and gassing, impairing battery performance. Therefore, controlling the carboxylate content in the electrolyte within an appropriate range can balance SEI film formation and electrolyte ionic conductivity, thereby improving the cycling performance of lithium secondary batteries and reducing gassing during storage.

[0037] In any embodiment, the lithium secondary battery satisfies: 0.1%≤W1≤5%, wherein W1 is the mass content of the first additive in the electrolyte, in %.

[0038] In any embodiment, the lithium secondary battery satisfies: 0.5%≤W1≤2%, wherein W1 is the mass content of the first additive in the electrolyte, in %.

[0039] In any embodiment, the lithium secondary battery satisfies: 1%≤W2≤10%, wherein W2 is the mass content of the second additive in the electrolyte, in %.

[0040] In any embodiment, the lithium secondary battery satisfies: 4%≤W2≤8%, wherein W2 is the mass content of the second additive in the electrolyte, in %.

[0041] In any embodiment, the lithium secondary battery satisfies: 0.1%≤W3≤5%, wherein W3 is the mass content of the third additive in the electrolyte, in %.

[0042] In any embodiment, the lithium secondary battery satisfies: 0.5%≤W3≤2%, wherein W3 is the mass content of the third additive in the electrolyte, in %.

[0043] By controlling the contents of the first additive, the second additive, and the third additive within a certain range, the storage performance of the lithium secondary battery can be taken into consideration and the battery performance can be comprehensively improved.

[0044] In any embodiment, the electrolyte further includes a second solvent, the second solvent including at least one of a cyclic carbonate compound and a chain carbonate compound; the cyclic carbonate compound includes at least one of ethylene carbonate, propylene carbonate, and butylene carbonate; the chain carbonate compound includes at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0045] Adding the above carbonate compounds and chain carbonate compounds to the electrolyte can further enhance the stability of the electrolyte and improve the conductivity of the electrolyte.

[0046] In any embodiment, based on the total mass of the electrolyte, the mass content of the second solvent is ≥30%.

[0047] In any embodiment, based on the total mass of the electrolyte, the mass content of the second solvent is 30% to 85%.

[0048] In any embodiment, the lithium secondary battery satisfies: 14.2 mS / cm≤S≤16.8 mS / cm, wherein S is the ionic conductivity of the electrolyte at 25° C., in units of mS / cm.

[0049] The lithium secondary battery provided in the present application has an ionic conductivity within the above range and thus has good fast charging performance.

[0050] In any embodiment, the lithium secondary battery further comprises a positive electrode sheet and a negative electrode sheet; the positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer located on at least one side of the positive electrode current collector, the positive electrode material layer comprises an olivine-structured lithium-containing phosphate, and the lithium-containing phosphate comprises a compound represented by the general formula II, LiFe 1-x-y Mn x M y PO4 Formula II,

[0051] Among them, 0≤x≤1, 0≤y<1, and M includes at least one of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Hf, Ta, Mo, W, Ru, Ag, Sn, and Pb.

[0052] In any embodiment, the compacted density of the positive electrode sheet is 2.4 g / cm 3 ~2.75g / cm 3 .

[0053] The compaction density will affect the wettability of the electrode. Therefore, controlling the compaction density within a certain range can make the battery have better fast charging performance, high temperature expansion rate, and cycle performance.

[0054] In any embodiment, the negative electrode plate includes a negative electrode current collector and a negative electrode material layer located on at least one side of the negative electrode current collector, and the negative electrode material layer includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase microcarbon beads, silicon-based materials and tin-based materials, and optionally includes natural graphite.

[0055] In any embodiment, the compaction density of the negative electrode sheet is 1.5 g / cm 3 ~1.85g / cm 3 .

[0056] The compaction density will affect the wettability of the electrode. Therefore, controlling the compaction density within a certain range can make the battery have better fast charging performance, high temperature expansion rate, and cycle performance.

[0057] In any embodiment, the porosity of the negative electrode sheet is 20% to 50%.

[0058] A second aspect of the present application provides an electrical device comprising the lithium secondary battery of the first aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0060] FIG2 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG1 ;

[0061] 3 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application;

[0062] Description of reference numerals: 5 secondary battery; 51 housing; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0063] Below, the embodiments of the lithium secondary battery, its preparation method and the electric device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0064] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0065] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0066] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0067] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0068] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0069] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0070] Carboxylate compounds as solvents can reduce electrolyte viscosity, increase electrolyte conductivity, and thus improve battery fast-charging performance. However, carboxylate electrolytes are incompatible with anode graphite and can react with graphite to produce gas. Furthermore, carboxylate solvents have smaller molecular structures and easily penetrate the SEI membrane, further inducing side reactions and deteriorating battery performance. Therefore, higher requirements are placed on the stability and density of the SEI membrane.

[0071] [Electrolyte]

[0072] In order to achieve the above object, the present application provides a lithium secondary battery, the lithium secondary battery comprising an electrolyte, the electrolyte comprising a first solvent, a first additive and a second additive, the first solvent comprising a carboxylic acid ester compound; wherein the carboxylic acid ester compound relative to Li + / Li reduction potential is less than or equal to 1.4V; the first additive relative to Li + / Li reduction potential is greater than or equal to 1.4V; the second additive relative to Li + The reduction potential of Li is greater than or equal to 1.0V and less than 1.4V.

[0073] In this paper, “reduction potential” and “relative to Li + "Reduction potential of / Li" refers to the redox potential of the solvent or additive itself relative to Li ions, and "reaction potential" and "film formation reaction potential in the full cell" refer to the potential at which film formation begins during the formation stage in a full cell with a graphite anode. Reaction potential = 3.4 V - relative to Li + / Li reduction potential.

[0074] Due to their low viscosity, carboxylate solvents can improve the ionic conductivity of the electrolyte, thereby enhancing the battery's fast-charging performance. However, if too much carboxylate solvent participates in film formation, the resulting SEI film is less effective, resulting in significant gas production during circulation and storage, reducing battery life. Therefore, adding additives with a high reduction potential to the electrolyte can inhibit the excessive reaction of carboxylate solvent with graphite to form a film, reducing gas production during circulation and storage, thereby improving the storage and cycling performance of lithium secondary batteries and comprehensively improving the performance of lithium secondary batteries.

[0075] The first additive has a ratio of carboxylic acid ester solvent to Li+ The high reduction potential of Li / Li allows it to form an SEI film in preference to carboxylate solvents. The SEI film formed by the first additive has high conductivity and low charge transfer resistance, which can, to a certain extent, reduce the battery's internal resistance, enhance the stability of the solvent and anode, and reduce the side reactions of the carboxylate solvent, thereby comprehensively improving the performance of lithium secondary batteries.

[0076] At the same time, the main function of introducing the second additive is to compete with the carboxylic acid ester solvent to form a SEI film after the first additive forms a film, continue to enhance the film density and thickness, further enhance the stability of the solvent and the anode, and help to improve the overall density and stability of the SEI film, further reduce the risk of carboxylic acid ester solvents passing through the SEI film to induce side reactions, and can take into account the improvement of the storage performance and cycle performance of the battery.

[0077] In some embodiments, the electrolyte further includes a third additive, wherein the third additive is + The reduction potential of / Li is less than 1.0V.

[0078] The third additive introduced into the electrolyte participates in solvation, forming a solvation structure and inhibiting oxidative decomposition and gas production at both high and room temperatures. Furthermore, some of the third additive continues to participate in film formation after the first and second additives have formed, further enhancing the density and stability of the SEI film.

[0079] In some embodiments, the carboxylate compound includes a compound having a structure shown in Formula I,

[0080] Wherein, R1 and R2 each independently include at least one of C1-C5 alkyl and C1-C5 haloalkyl. In some embodiments, R1 and R2 each independently include at least one of C1-C5 alkyl and C1-C5 haloalkyl.

[0081] As used herein, the term "C1-C5 alkyl" refers to a straight or branched hydrocarbon chain group consisting solely of carbon and hydrogen atoms, with no unsaturated bonds present, having from one to five carbon atoms, and connected to the remainder of the molecule by a single bond. Exemplary C1-C5 alkyl groups include methyl, ethyl, and propyl.

[0082] "Haloalkyl" refers to an alkyl group substituted with a halogen, where halogen refers to an element from Group VIIA of the Periodic System, including fluorine, chlorine, bromine, and iodine.

[0083] In some embodiments, the carboxylate compound includes at least one of ethyl acetate, methyl acetate, propyl acetate, and ethyl formate.

[0084] In some embodiments, the carboxylate compound includes at least one of ethyl acetate, methyl acetate, and propyl acetate.

[0085] The above-mentioned carboxylic acid ester compounds as solvents can reduce the viscosity of the electrolyte, increase the conductivity of the electrolyte, and thus improve the fast charging performance of the battery.

[0086] In some embodiments, the first additive includes at least one of a tetrafluoroborate compound, an oxalate borate compound, a borate ester compound, vinyl ethylene sulfite, 1,3-propylene sultone, and methylene methanedisulfonate.

[0087] In some embodiments, the first additive includes at least one of lithium difluorooxalatoborate, lithium bisoxalatoborate, tris(trimethylsilyl)borate, and 1,3-propylene sultone.

[0088] The above-mentioned type of first additive can participate in the formation of SEI film in preference to carboxylate solvents, and the generated SEI film has higher electrical conductivity and lower charge transfer impedance, which can reduce the internal resistance of the battery to a certain extent, enhance the stability of the solvent and the anode, reduce the side reactions of the carboxylate solvent, and comprehensively improve the performance of the lithium secondary battery.

[0089] In some embodiments, the second additive includes at least one of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, organic sulfates, sulfonates, and difluorophosphates.

[0090] In some embodiments, the second additive includes at least one of vinylene carbonate, fluoroethylene carbonate, and vinyl sulfate.

[0091] The above-mentioned type of second additive can compete with the carboxylate solvent to form the SEI film, which helps to improve the overall density and stability of the SEI film, further reduce the risk of the carboxylate solvent passing through the SEI film to induce side reactions, and can improve both the storage performance and cycle performance of the battery.

[0092] In some embodiments, the third additive includes at least one of an organic phosphorus compound, an organic fluorine compound, a phosphate compound, a borate compound, and a sulfonate compound.

[0093] In some embodiments, the third additive includes at least one of tris(2,2,2-trifluoroethyl)phosphite and tris(trimethylsilyl)phosphate.

[0094] The third additive of the above type can participate in solvation, form a solvation structure, and further inhibit the oxidative decomposition and gas generation of the battery at high temperature and room temperature.

[0095] In some embodiments, the lithium secondary battery satisfies the following: 0.003 ≤ W1 / W4 ≤ 0.125. In some embodiments, the ratio of W1 / W4 is 0.005, 0.01, 0.02, 0.025, 0.05, 0.1, 0.125, or any range or value therebetween. Wherein, W4 is the mass content of the first solvent in the electrolyte, expressed in %, and W1 is the mass content of the first additive in the electrolyte, expressed in %.

[0096] In some embodiments, the lithium secondary battery satisfies the following: 0.025 ≤ W2 / W4 ≤ 0.5. In some embodiments, the ratio of W2 / W4 is 0.025, 0.07, 0.1, 0.125, 0.2, 0.25, 0.3, 0.4, 0.5, or any range or value therebetween. Wherein, W4 is the mass content of the first solvent in the electrolyte, expressed in %, and W2 is the mass content of the second additive in the electrolyte, expressed in %.

[0097] In some embodiments, the lithium secondary battery satisfies the following: 0.3 g / Ah ≤ (D4 + D2) / A ≤ 2.3 g / Ah. In some embodiments, the ratio (D4 + D2) / A is 0.3, 0.5, 0.8, 1.0, 1.5, 1.8, 2.0, 2.2, 2.3, or any range or value therebetween. Wherein, D4 is the mass of the first solvent in g; D2 is the mass of the second additive in g; and A is the rated capacity of the lithium secondary battery in Ah.

[0098] In some embodiments, the lithium secondary battery satisfies: 0.5g / Ah≤(D4+D2) / A≤2g / Ah, wherein D4 is the mass of the first solvent, in g; D2 is the mass of the second additive, in g; and A is the rated capacity of the lithium secondary battery, in Ah.

[0099] In this article, "rated capacity" refers to the amount of electricity discharged by the battery under certain conditions (discharge rate, temperature, termination voltage, etc.), that is, the capacity of the battery, usually in ampere-hours. The determination method is: at 25°C, first discharge the fresh battery at a constant current rate of 0.33C to 2.0V, let it stand for 30 minutes, then charge it at a constant current rate of 0.33C to 3.8V, then charge it at a constant voltage rate of 0.05C to 3.8V, let it stand for 30 minutes, and discharge it at a constant current rate of 0.33C to 2.0V. The discharge capacity at this time is recorded as the rated capacity of the battery.

[0100] Compared with carboxylate solvents, the first additive has a + / Li has a high reduction potential and can form a film on the anode surface in preference to carboxylic acid ester solvents; the relative reaction between the second additive and the carboxylic acid ester solvent is relatively large. + The reduction potentials of the two additives and the carboxylate ester compounds are relatively close, allowing them to compete with the carboxylate ester solvents to form the SEI film. Therefore, controlling the ratio of the two additives to the carboxylate ester compound in the electrolyte within a certain range can effectively reduce the side reactions and gas production of the carboxylate ester solvent, while also improving the battery's storage performance and cycle performance.

[0101] In some embodiments, the lithium secondary battery satisfies: 0.016≤W1 / (W1+W2+W3)≤0.455. In some embodiments, the ratio of W1 / (W1+W2+W3) is 0.016, 0.07, 0.09, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or a range or value between any two of the above. Wherein, W1 is the mass content of the first additive in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %; W3 is the mass content of the third additive in the electrolyte, in %.

[0102] In some embodiments, the lithium secondary battery satisfies: 0.333≤W2 / (W1+W2+W3)≤0.833. In some embodiments, the ratio of W2 / (W1+W2+W3) is 0.333, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, or a range or value between any two of the above. Wherein, W1 is the mass content of the first additive in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %; W3 is the mass content of the third additive in the electrolyte, in %.

[0103] Compared with carboxylate solvents, the first additive has a + / Li has a high reduction potential and can form a film on the anode surface in preference to carboxylic acid ester solvents. The SEI film it forms has the advantages of high conductivity and low charge transfer resistance. + / Li reduction potential is relatively close, and it can compete with carboxylic acid ester solvents to form SEI film. The SEI film formed has the advantages of good stability and high strength, and can also play a role in stabilizing cycle performance. At the same time, it is precisely because of the relative +Because the reduction potentials of lithium ions and lithium ions are relatively close, there is a certain degree of competition between the second additive and the carboxylate solvent in film formation. Therefore, the content of the second additive must be controlled to be higher than that of the first and third additives to ensure that the second additive competes with the carboxylate solvent in film formation. Therefore, controlling the content ratio of the first, second, and third additives within a certain range balances the storage performance of lithium secondary batteries and comprehensively improves battery performance.

[0104] In some embodiments, the lithium secondary battery satisfies: 10%≤W4≤70%, wherein W4 is the mass content of the first solvent in the electrolyte, in %. In some embodiments, W4 is 10%, 20%, 40%, 50%, 70%, or a range or value between any two of the foregoing.

[0105] In some embodiments, the lithium secondary battery satisfies: 20%≤W4≤50%, wherein W4 is the mass content of the first solvent in the electrolyte, in %.

[0106] Carboxylate compounds, when used as solvents, can improve electrolyte conductivity, thereby enhancing battery fast-charging performance. However, excessive levels of carboxylate solvents can cause oxidative decomposition and gassing, which can impair battery performance. Therefore, controlling the carboxylate content in the electrolyte within an appropriate range can balance SEI film formation and electrolyte ionic conductivity, thereby improving the cycling performance of lithium secondary batteries and reducing gassing during storage.

[0107] In some embodiments, the lithium secondary battery satisfies: 0.1%≤W1≤5%, wherein W1 is the mass content of the first additive in the electrolyte, in %. In some embodiments, W1 is 0.5%, 1%, 2%, 5%, or a range or value between any two of the foregoing.

[0108] In some embodiments, the lithium secondary battery satisfies: 0.5%≤W1≤2%, wherein W1 is the mass content of the first additive in the electrolyte, in %.

[0109] In some embodiments, the lithium secondary battery satisfies: 1%≤W2≤10%, wherein W2 is the mass content of the second additive in the electrolyte, in %. In some embodiments, W2 is 1%, 4%, 5%, 8%, 10%, or a range or value between any two of the foregoing.

[0110] In some embodiments, the lithium secondary battery satisfies: 4%≤W2≤8%, wherein W2 is the mass content of the second additive in the electrolyte, in %.

[0111] In some embodiments, the lithium secondary battery satisfies: 0.1%≤W3≤5%, wherein W3 is the mass content of the third additive in the electrolyte, in %. In some embodiments, W3 is 0.1%, 0.5%, 2%, 5%, or a range or value between any two of the foregoing.

[0112] In some embodiments, the lithium secondary battery satisfies: 0.5%≤W3≤2%, wherein W3 is the mass content of the third additive in the electrolyte, in %.

[0113] By controlling the contents of the first additive, the second additive, and the third additive within a certain range, the storage performance of the lithium secondary battery can be taken into consideration and the battery performance can be comprehensively improved.

[0114] In some embodiments, the electrolyte further includes a second solvent, the second solvent including at least one of a cyclic carbonate compound and a chain carbonate compound; the cyclic carbonate compound includes at least one of ethylene carbonate, propylene carbonate, and butylene carbonate; the chain carbonate compound includes at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0115] In some embodiments, based on the total mass of the electrolyte, the mass content of the second solvent is ≥30%.

[0116] In some embodiments, the mass content of the second solvent is 30% to 85% based on the total mass of the electrolyte. In some embodiments, the mass content of the second solvent is 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or any range therebetween or a value within the range, based on the total mass of the electrolyte.

[0117] In some embodiments, the lithium secondary battery satisfies the following: 14.2 mS / cm ≤ S ≤ 16.8 mS / cm. In some embodiments, S is 14.5 mS, 15 mS, 15.5 mS, 16 mS, 16.5 mS, 16.8 mS, or any range therebetween. Wherein, S is the ionic conductivity of the electrolyte at 25°C, in mS / cm.

[0118] In this article, “ionic conductivity” refers to the conductivity of the electrolyte to ions, which can reflect the conductivity of the electrolyte to ions.

[0119] The lithium secondary battery provided in the present application has an ionic conductivity within the above range and thus has good fast charging performance.

[0120] The present application also provides a method for preparing a lithium secondary battery.

[0121] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0122] In some embodiments, the lithium secondary battery further includes a positive electrode sheet and a negative electrode sheet.

[0123] [Positive electrode]

[0124] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer located on at least one side of the positive electrode current collector.

[0125] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode material layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0126] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer located on at least one side of the positive electrode current collector, wherein the positive electrode material layer includes an olivine-structured lithium-containing phosphate, and the lithium-containing phosphate includes a compound represented by the general formula II, LiFe 1-x-y Mn x M y PO4 Formula II,

[0127] Among them, 0≤x≤1, 0≤y<1, and M includes at least one of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Hf, Ta, Mo, W, Ru, Ag, Sn, and Pb.

[0128] In some embodiments, the compacted density of the positive electrode sheet is 2.4 g / cm 3 ~2.75g / cm 3 .

[0129] In this article, "compacted density" means: compacted density = surface density / (thickness of the electrode after rolling - thickness of the current collector). The surface density test method is: take a 2*2mm surface electrode, use the electrode weight m1 to weigh the electrode, calculate the weight m2 of the 2.2mm substrate area based on the area, thickness and density of the electrode substrate, and calculate the surface density A as (m1-m2) / 4. Then, use a micrometer to measure the electrode thickness d1 and the electrode thickness d2, and calculate the compacted density based on the surface density as A / (d1-d2).

[0130] The compaction density will affect the wettability of the electrode. Therefore, controlling the compaction density within a certain range can make the battery have better fast charging performance, high temperature expansion rate, and cycle performance.

[0131] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0132] In some embodiments, the positive electrode material layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0133] In some embodiments, the positive electrode material layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0134] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode material layer, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0135] [Negative electrode]

[0136] The negative electrode plate includes a negative electrode current collector and a negative electrode material layer located on at least one side of the negative electrode current collector.

[0137] As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode material layer is disposed on either or both of the two facing surfaces of the negative electrode current collector.

[0138] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode material layer located on at least one side of the negative electrode current collector, and the negative electrode material layer includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase microcarbon beads, silicon-based materials and tin-based materials, and optionally includes natural graphite.

[0139] In some embodiments, in a lithium secondary battery using natural graphite as the negative electrode active material, the film-forming reaction potential of the first solvent in the whole cell is greater than or equal to 2.1 V, the film-forming reaction potential of the first additive in the whole cell is less than or equal to 2.0 V, the film-forming reaction potential of the second additive in the whole cell is greater than 2.0 V and less than 2.4 V, and the film-forming reaction potential of the third additive in the whole cell is greater than or equal to 2.4 V.

[0140] In some embodiments, the compaction density of the negative electrode sheet is 1.5 g / cm 3 ~1.85g / cm 3 .

[0141] The compaction density will affect the wettability of the electrode. Therefore, controlling the compaction density within a certain range can make the battery have better fast charging performance, high temperature expansion rate, and cycle performance.

[0142] In some embodiments, the porosity of the negative electrode sheet is 20% to 50%.

[0143] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0144] In some embodiments, the negative electrode material layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0145] In some embodiments, the negative electrode material layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0146] In some embodiments, the negative electrode material layer may optionally further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0147] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode material layer, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0148] [Isolation film]

[0149] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0150] 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, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0151] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0152] [Secondary battery]

[0153] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0154] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0155] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG1 shows a secondary battery 5 with a square structure as an example.

[0156] In some embodiments, referring to Figure 2, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0157] [Electrical devices]

[0158] In addition, the present application also provides an electrical device, which includes the lithium secondary battery provided by the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0159] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0160] Figure 3 shows an example of an electric device. This device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0161] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0162] Example

[0163] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0164] 1. Preparation method

[0165] Example 1

[0166] 1) Electrolyte

[0167] In an argon atmosphere glove box (H2O content <10ppm, O2 content <1ppm), the first solvent vinylene carbonate and the second solvent ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC to EMC mass ratio is 1:1) are mixed, and 15wt% of lithium hexafluorophosphate (LiPF6) is added and stirred until it is completely dissolved. After returning to room temperature, the first additive lithium difluorooxalatoborate, the second additive vinylene carbonate, and the third additive tris(trimethylsilane) phosphate are added and stirred evenly to prepare an electrolyte. Based on the total mass of the electrolyte, the mass content W4 of the first solvent is 40%, the mass content W1 of the first additive lithium difluorooxalatoborate is 1%, the mass content W2 of the second additive vinylene carbonate is 5%, and the mass content W3 of the third additive tris(trimethylsilane) phosphate is 1%.

[0168] 2) Preparation of positive electrode sheet

[0169] The positive electrode active material LiFePO4, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were thoroughly stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 97:1:2 to obtain a positive electrode slurry with a solid content of 66%. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, dried, and cold pressed to obtain a positive electrode sheet. The single-side coating weight of the positive electrode sheet is 420mg / 1540.25mm 2 , compacted density is 2.55g / cm 3 .

[0170] 3) Preparation of negative electrode sheet

[0171] The negative electrode active material graphite, the conductive agent acetylene black, the thickener sodium carboxymethyl cellulose (CMC-Na), and the binder styrene butadiene rubber (SBR) were mixed uniformly in deionized water at a mass ratio of 96:1:1:2 to prepare a negative electrode slurry with a solid content of 53%. The negative electrode slurry was coated on the current collector copper foil; after drying and cold pressing, the negative electrode sheet was obtained. The single-sided coating weight of the negative electrode sheet was 190mg / 1540.25mm 2 , compacted density is 1.6g / cm 3 , the porosity is 30%.

[0172] 4) Isolation film

[0173] A polyethylene (PE) film coated with nano-aluminum oxide was used as the separator.

[0174] 5) Preparation of batteries

[0175] The positive electrode sheet, separator, and negative electrode sheet are stacked and wound in sequence to form a wound electrode assembly. The electrode assembly is placed in a square aluminum shell for outer packaging, dried, and then injected with electrolyte. The lithium secondary battery is produced through packaging, static standing, formation, aging, secondary packaging, and capacity measurement. The lithium secondary battery has a liquid retention coefficient of 3.0g / Ah.

[0176] The secondary batteries of Examples 2 to 23 and the secondary batteries of Comparative Examples 1 to 4 were prepared in a similar manner to the secondary battery of Example 1, except that the mass contents and substance types of the first solvent, first additive, second additive, and third additive in the lithium secondary battery were adjusted. The different preparation parameters are detailed in Tables 1 and 2.

[0177] 2. Performance Testing

[0178] 1. Potential test

[0179] 1) The first solvent, the first additive, the second additive, and the third additive relative to Li + / Test method for the reduction potential of Li

[0180] Place the electrolyte in a clean 500ml wide-mouth bottle, seal it with a rubber stopper, insert different electrodes into the electrolyte and take them out from the rubber stopper. Use a platinum electrode as the indicator electrode and connect it to the positive electrode of the cyclic voltammetry test equipment. Use a saturated calomel electrode as the reference electrode and connect it to the negative electrode of the cyclic voltammetry test equipment. Turn on the cyclic voltammetry test equipment and scan at a speed of 0.5mV / s for 3 weeks in the range of -3V to 3V. After the scan is completed, determine the relative Li + / Li reduction potential.

[0181] 2) Test method for film-forming reaction potential in a full cell of the first solvent, the first additive, the second additive, and the third additive

[0182] Film formation reaction potential in full cell = 3.4 V relative to Li + / Li reduction potential.

[0183] 2. Electrolyte

[0184] 1) Liquid retention coefficient

[0185] Liquid retention coefficient = total weight of electrolyte in the battery / nominal capacity of the battery.

[0186] 2) Ionic conductivity

[0187] Use a dry, clean, corrosion-resistant sample bottle to take approximately 100 mL of electrolyte sample, seal it in a constant temperature water bath, and shake the sample occasionally to maintain a constant temperature of 25°C (with a deviation of ±0.5°C). After the sample temperature stabilizes, test its conductivity using a commercially available conductivity meter. After wiping the conductivity meter clean with calibration fluid, place it vertically into the liquid to be tested, click Start Test, and wait for the data to stabilize for at least 10 seconds before recording the test results.

[0188] 3) Performance parameters of SEI film

[0189] There is no clear means to measure the properties of the SEI film, but the gas production during battery storage can be used to provide indirect feedback on the quality of the SEI film. This is because gas production during battery storage is mainly due to the reaction between the solvent and the anode. Therefore, the better the strength and density of the SEI film, the lower the gas production of the battery during storage.

[0190] The test method for gas production during storage at 60°C is as follows: At 25°C, the lithium secondary battery is charged at a constant current rate of 1C to 3.8V, then charged at a constant voltage of 0.05C. This state is marked as fully charged, and the battery volume V0 is measured at this time. The fully charged battery is then placed in a 60°C constant temperature chamber and charged at a constant current rate of 1C to 3.8V, then charged at a constant voltage of 0.05C. After that, it is left for 30 minutes and discharged at a constant current rate of 1C to 2.8V. After 300 cycles, the battery is charged at a constant current rate of 1C to 3.8V at 25°C, then charged at a constant voltage of 0.05C. This state is marked as fully charged, and the battery volume V1 is measured at this time. The value of V1-V0 is used to characterize the film formation quality of the SEI film.

[0191] 3. Battery

[0192] 1) Cycle performance

[0193] At 60°C, charge the lithium secondary battery at a constant current rate of 1C to 3.8V, then charge it at a constant voltage of 0.05C, then let it sit for 30 minutes, and then discharge it at a constant current rate of 1C to 2.8V. Record the discharge capacity as C0. Cycle according to the above charge and discharge process, and record the number of cycles when the cycle fading reaches 80% SOH. The battery discharge capacity is Cn, and the battery cycle capacity retention rate = C1 / Cn×100%.

[0194] 2) Storage performance

[0195] At 25°C, charge the lithium secondary battery at a constant current rate of 0.33C to 3.8V, then charge it at a constant voltage of 0.05C. After standing for 30 minutes, discharge it at a constant current rate of 0.33C to 2.0V. Record the discharge capacity as D0. Then, store the lithium secondary battery at 60°C for 90 days. Remove the lithium secondary battery and return it to 25°C. Discharge it at a constant current rate of 0.33C to 2.0V. After standing for 30 minutes, charge it at 0.33C to 3.8V. Then charge it at a constant voltage of 0.05C. After standing for 30 minutes, discharge it at 0.33C to 2.0V. Record the discharge capacity as D1. Discharge capacity retention = D1 / D0 × 100%.

[0196] 3) Low temperature capacity retention rate

[0197] At 25°C, let it stand for 2 hours, charge the lithium secondary battery to 3.8V at a constant current rate of 0.33C, then charge it at a constant voltage of 0.05C, then let it stand for 2 hours, and then discharge it to 2.0V at a constant current rate of 1C, and record the discharge capacity as E0; at 25°C, let it stand for 2h, charge the lithium secondary battery to 3.8V at a constant current rate of 0.33C, then charge it at a constant voltage of 0.05C, then let it stand at -20°C for 2 hours, and then discharge it to 2.0V at a constant current rate of 1C, and record the discharge capacity as E1. The capacity retention rate of the battery at -20°C = E1 / E0×100%.

[0198] 3. Analysis of test results of various embodiments and comparative examples

[0199] Batteries of various embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 3 below.

[0200] Table 1 Preparation parameters

[0201] Table 2 Preparation parameters

[0202] Table 3 Performance parameters

[0203] The electrolytes in Examples 1 to 23 all include a first solvent, a first additive, a second additive, and a third additive. The lithium secondary batteries containing the electrolytes all have excellent low-temperature capacity retention, storage performance, and cycle performance, and have low storage gas production.

[0204] The electrolyte of Comparative Example 1 does not contain the first additive, the second additive, and the third additive. From the comparison of Examples 1 to 23 with Comparative Example 1, it can be seen that adding the first additive, the second additive, and the third additive to the electrolyte can effectively improve the low-temperature capacity retention rate, storage performance, and cycle performance of the lithium secondary battery, and can significantly reduce the storage gas production of the lithium secondary battery.

[0205] The electrolytes of Comparative Examples 2 to 4 contain only any one of the first additive, the second additive, and the third additive: the electrolyte of Comparative Example 2 contains only the second additive, the electrolyte of Comparative Example 3 contains only the first additive, and the electrolyte of Comparative Example 4 contains only the third additive.

[0206] From the comparison between Comparative Example 1 and Comparative Examples 2 to 4, it can be seen that when any one of the three additives is added to the electrolyte, the different performances of the lithium secondary battery can be improved to a certain extent: when only the second additive is added (Comparative Example 2), the low-temperature capacity retention rate, storage performance, and cycle performance of the battery can be improved, and the storage gas production of the lithium secondary battery can be reduced to a certain extent; when only the first additive is added (Comparative Example 3) or when only the third additive is added (Comparative Example 4), the low-temperature capacity retention rate, storage performance, and cycle performance of the battery can be improved to a certain extent.

[0207] From the comparison of Examples 1 to 18 with Comparative Examples 2 to 4, it can be seen that, compared with adding only one additive, adding three additives to the electrolyte at the same time can further improve the low-temperature capacity retention rate, storage performance and cycle performance of the lithium secondary battery, and can significantly reduce the storage gas production of the lithium secondary battery.

[0208] As can be seen from Examples 1 to 5, when the mass content W4 of the first solvent (ethyl acetate) is controlled to be 10% to 70%, the lithium secondary battery can have a higher low-temperature capacity retention rate, storage performance and cycle performance, and a lower storage gas production. Among them, when W4 is 70%, due to the high content of the first solvent, the electrolyte reaction activity is too strong, and its ability to improve storage performance, cycle performance, and storage gas production is relatively weak; when W4 is 10%, due to the low content of the first solvent, its storage gas production is low, but the lower conductivity makes its improvement in the battery charging capacity relatively weak. Therefore, when W4 is 20% to 50%, the comprehensive performance of the lithium secondary battery is better.

[0209] As can be seen from Examples 6 to 9, when the mass content W1 of the first additive (lithium difluorooxalatoborate) is controlled to be 0.1% to 5%, the lithium secondary battery can have a higher low-temperature capacity retention rate, storage performance and cycle performance, and has a lower storage gas production. Among them, when W1 is 5%, the content of the first additive is higher, and when W1 is 0.1%, the content of the first additive is lower. In these two cases, its ability to improve the cycle performance is relatively weak. Therefore, when W1 is 0.5% to 2%, the comprehensive performance of the lithium secondary battery is better.

[0210] As can be seen from Examples 10 to 13, when the mass content W2 of the second additive (vinylene carbonate) is controlled to be 1% to 10%, the lithium secondary battery can have a higher low-temperature capacity retention rate, storage performance and cycle performance, and a lower storage gas production. Among them, when W2 is 10%, the content of the second additive is high, resulting in an overly dense SEI film, and its ability to improve the battery's charging capacity is relatively weak; when W2 is 1%, the content of the second additive is low, the SEI film quality is poor, and because the second additive participates in the film formation during the cycle, its ability to improve storage performance, cycle performance, and storage gas production is relatively weak. Therefore, when W2 is 4% to 8%, the comprehensive performance of the lithium secondary battery is better.

[0211] As can be seen from Examples 14 to 17, when the mass content W3 of the third additive (tris(trimethylsilyl)phosphate) is controlled to be 0.1% to 5%, the lithium secondary battery can have a higher low-temperature capacity retention rate, storage performance and cycle performance, and has a lower storage gas production. Among them, since the third additive can increase the thermal stability of the electrolyte, when W3 is 0.1%, the content of the third additive is low, so its ability to improve the cycle performance is relatively weak; when W3 is 5%, the content of the third additive is high, so its ability to improve the cycle performance also decreases to a certain extent. Therefore, when W3 is 0.5% to 2%, the comprehensive performance of the lithium secondary battery is better.

[0212] It can be seen from Examples 1 and 18 to 23 that the use of a variety of different types of first solvents (e.g., ethyl acetate, methyl acetate, propyl acetate), or the use of a variety of different types of first additives (e.g., lithium difluorooxalatoborate, lithium bisoxalatoborate), or the use of a variety of different types of second additives (e.g., vinylene carbonate, fluoroethylene carbonate, vinyl sulfate), or the use of a variety of different types of third additives (e.g., tris(trimethylsilyl)phosphate, tris(2,2,2-trifluoroethyl)phosphite) can all enable the lithium secondary battery to have excellent low-temperature capacity retention, storage performance, and cycle performance, and have lower storage gas production.

[0213] The first additive is relative to Li + The reduction potential of Li / Li is greater than or equal to 1.4V, for example, lithium difluorooxalatoborate is 1.9V (Li / Li + ), lithium bis(oxalatoborate) is 1.8V(Li / Li + ), tris(trimethylsilyl)borate is 1.8V(Li / Li + ), 1,3-propene sultone is 2.6V(Li / Li + ).

[0214] The reduction potential of the second additive relative to Li+ / Li is greater than or equal to 1.0 V and less than 1.4 V. For example, vinylene carbonate is 1.0 V (Li / Li + ), fluoroethylene carbonate is 1.2V (Li / Li + ), vinyl sulfate is 1.05V (Li / Li + ).

[0215] The third additive relative to Li + The reduction potential of Li / Li is less than 1.0V, for example, tris(2,2,2-trifluoroethyl)phosphite is 0.8V (Li / Li + ), tris(trimethylsilyl)phosphate is 0.8V(Li / Li + ), tris(trimethylsilyl)borate is 0.7V(Li / Li + ).

[0216] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium secondary battery, wherein: The lithium secondary battery includes an electrolyte, the electrolyte includes a first solvent, a first additive and a second additive, and the first solvent includes a carboxylic acid ester compound; Wherein, the carboxylic acid ester compound is relative to Li + / The reduction potential of Li is less than or equal to 1.4V; The first additive is relative to Li + / The reduction potential of Li is greater than or equal to 1.4V; The second additive is relative to Li + The reduction potential of / Li is greater than or equal to 1.0V and less than 1.4V.

2. The lithium secondary battery according to claim 1, wherein The carboxylate compounds include compounds with the structure shown in Formula I, Wherein, R1 and R2 each independently include at least one of a C1-C5 alkyl group and a C1-C5 halogenated alkyl group.

3. The lithium secondary battery according to claim 1 or 2, wherein: The carboxylate compound includes at least one of ethyl acetate, methyl acetate, propyl acetate and ethyl formate.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein The first additive includes at least one of a tetrafluoroborate compound, an oxalate borate compound, a borate ester compound, vinyl sulfite, 1,3-propylene sultone, and methylene methane disulfonate.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein The first additive includes at least one of lithium difluorooxalatoborate, lithium bisoxalatoborate, tris(trimethylsilyl)borate, and 1,3-propylene sultone.

6. The lithium secondary battery according to any one of claims 1 to 5, wherein The second additive includes at least one of vinylene carbonate, fluoroethylene carbonate, vinyl vinyl carbonate, organic sulfates, sulfonates, and difluorophosphates.

7. The lithium secondary battery according to any one of claims 1 to 6, wherein The second additive includes at least one of vinylene carbonate, fluoroethylene carbonate, and vinyl sulfate.

8. The lithium secondary battery according to any one of claims 1 to 7, wherein The electrolyte further includes a third additive, wherein the third additive is + / The reduction potential of Li is less than 1.0V.

9. The lithium secondary battery according to claim 8, wherein: The third additive includes at least one of an organic phosphide, an organic fluorine compound, a phosphate compound, a borate compound, and a sulfonate compound.

10. The lithium secondary battery according to claim 8 or 9, wherein: The third additive includes at least one of tris(2,2,2-trifluoroethyl)phosphite and tris(trimethylsilyl)phosphate.

11. The lithium secondary battery according to any one of claims 1 to 10, wherein The lithium secondary battery satisfies: 0.003≤W1 / W4≤0.125; and / or 0.025≤W2 / W4≤0.5; Wherein, W4 is the mass content of the first solvent in the electrolyte, in %; W1 is the mass content of the first additive in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %.

12. The lithium secondary battery according to any one of claims 1 to 11, wherein The lithium secondary battery satisfies: 0.3g / Ah≤(D4+D2) / A≤2.3g / Ah; Wherein, D4 is the mass of the first solvent, in g; D2 is the mass of the second additive, in g; and A is the rated capacity of the lithium secondary battery, in Ah.

13. The lithium secondary battery according to any one of claims 1 to 12, wherein: The lithium secondary battery meets the following conditions: 0.5g / Ah≤(D4+D2) / A≤2g / Ah; Wherein, D4 is the mass of the first solvent, in g; D2 is the mass of the second additive, in g; and A is the rated capacity of the lithium secondary battery, in Ah.

14. The lithium secondary battery according to any one of claims 8 to 13, wherein The lithium secondary battery satisfies: 0.016≤W1 / (W1+W2+W3)≤0.455; and / or 0.333≤W2 / (W1+W2+W3)≤0.833; Wherein, W1 is the mass content of the first additive in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %; W3 is the mass content of the third additive in the electrolyte, in %.

15. The lithium secondary battery according to any one of claims 8 to 14, wherein The lithium secondary battery satisfies: 10%≤W4≤70%; 0.1%≤W1≤5%; 1%≤W2≤10%; and / or 0.1%≤W3≤5%; Among them, W4 is the mass content of the first solvent in the electrolyte, in %; W1 is the mass content of the first additive in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %; W3 is the mass content of the third additive in the electrolyte, in %.

16. The lithium secondary battery according to any one of claims 8 to 15, wherein The lithium secondary battery satisfies: 20%≤W4≤50%; 0.5%≤W1≤2%; 4%≤W2≤8%; and / or 0.5%≤W3≤2%; Among them, W4 is the mass content of the first solvent in the electrolyte, in %; W1 is the mass content of the first additive in the electrolyte, in %; W2 is the mass content of the second additive in the electrolyte, in %; W3 is the mass content of the third additive in the electrolyte, in %.

17. The lithium secondary battery according to any one of claims 1 to 16, wherein: The electrolyte further includes a second solvent, wherein the second solvent includes at least one of a cyclic carbonate compound and a chain carbonate compound; The cyclic carbonate compounds include ethylene carbonate, propylene carbonate, butyl carbonate, at least one of the enesters; The chain carbonate compound includes at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate.

18. The lithium secondary battery according to claim 17, wherein: Based on the total mass of the electrolyte, the mass content of the second solvent is ≥30%.

19. The lithium secondary battery according to claim 17 or 18, wherein: Based on the total mass of the electrolyte, the mass content of the second solvent is 30% to 85%.

20. The lithium secondary battery according to claims 1 to 19, wherein: The lithium secondary battery meets the following requirements: 14.2mS / cm≤S≤16.8mS / cm; Wherein, S is the ionic conductivity of the electrolyte at 25° C., in units of mS / cm.

21. The lithium secondary battery according to any one of claims 1 to 20, wherein The lithium secondary battery also includes a positive electrode sheet and a negative electrode sheet; The positive electrode plate includes a positive electrode current collector and a positive electrode material layer located on at least one side of the positive electrode current collector, the positive electrode material layer includes a lithium-containing phosphate with an olivine structure, and the lithium-containing phosphate includes a compound with a general formula as shown in Formula II, LiFe 1-x-y Mn x M y PO4 of formula II, Among them, 0≤x≤1, 0≤y<1, and M includes at least one of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Hf, Ta, Mo, W, Ru, Ag, Sn, and Pb.

22. The lithium secondary battery according to any one of claims 1 to 21, wherein The negative electrode plate includes a negative electrode current collector and a negative electrode material layer located on at least one side of the negative electrode current collector, and the negative electrode material layer includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, intermediate phase micro carbon beads, silicon-based materials and tin-based materials.

23. The lithium secondary battery according to claim 21 or 22, wherein: The compaction density of the positive electrode sheet is 2.4 g / cm 3 ~2.75g / cm 3 The compaction density of the negative electrode sheet is 1.5 g / cm 3 ~1.85g / cm 3 The porosity of the negative electrode sheet is 20% to 50%.

24. An electrical device, wherein: A lithium secondary battery comprising the lithium secondary battery according to any one of claims 1 to 23.

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