Secondary battery, method for preparing secondary battery, battery module, battery pack and power-consuming device

Abstract

The present application discloses a secondary battery, a method for preparing the secondary battery, a battery module, a battery pack and an electric device. The secondary battery includes an electrolyte and a positive electrode sheet, wherein the positive electrode sheet includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A g The layered material, M1, M2, A, a, b, c, d, e, f, and g are respectively as defined herein, the electrolyte includes a compound containing a phosphorus-oxygen double bond, and the compound containing a phosphorus-oxygen double bond includes one or more of lithium difluorophosphate, diethyl (2-cyanoethyl) phosphonate, tripropyl phosphoric anhydride, diethyl acetyl phosphate, and triphenyl phosphate. Based on the total mass of the electrolyte, the mass percentage of the compound containing a phosphorus-oxygen double bond is x% and 0<x≤1.0, and the secondary battery satisfies c+x / 10≥0.10. The present application can increase the lithium ion diffusion rate of low-cobalt or cobalt-free positive electrode active materials and improve the cycle performance of secondary batteries.

Description

Technical Field

[0001] The present application belongs to the field of battery technology, and in particular relates to a secondary battery, a method for preparing a secondary battery, a battery module, a battery pack, and an electrical device. Background Art

[0002] Secondary batteries rely on the reciprocating intercalation and deintercalation of lithium ions between the positive and negative electrodes for charging and discharging. They have outstanding characteristics such as high energy density, long cycle life, zero pollution, and no memory effect. Therefore, as a clean energy source, secondary batteries have gradually spread from electronic products to energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields, to adapt to the sustainable development strategy of the environment and energy. Among them, cobalt is a key component of the positive electrode active material of secondary batteries. However, cobalt is present in a small amount in the earth's crust, is difficult to mine, and is expensive. Therefore, low-cobalt or cobalt-free cathode active materials have become an inevitable development trend. However, cobalt contributes significantly to the lithium ion diffusion rate of the positive electrode active material. Low cobalt or cobalt-free cathode active materials will reduce the lithium ion diffusion rate of the positive electrode active material, affecting the cycle life of the secondary battery. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery, a method for preparing a secondary battery, a battery module, a battery pack and an electrical device, aiming to increase the lithium ion diffusion rate of low-cobalt or cobalt-free positive electrode active materials and improve the cycle performance of the secondary battery.

[0004] The first aspect of the present application provides a secondary battery, comprising an electrolyte and a positive electrode. The positive electrode comprises a molecular formula of Li a Ni b Co c M1 d M2 e O f A gA layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b+c+d+e=1, f+g=2. The electrolyte includes a compound containing a phosphorus-oxygen double bond, and the compound containing a phosphorus-oxygen double bond includes one or more of lithium difluorophosphate, diethyl (2-cyanoethyl) phosphonate, tripropyl phosphoric anhydride, diethyl acetyl phosphate, and triphenyl phosphate. Based on the total mass of the electrolyte, the mass percentage of the compound containing a phosphorus-oxygen double bond is x% and 0<x≤1.0. The secondary battery satisfies c+x / 10≥0.10.

[0005] After extensive research, the inventors discovered that the cobalt content (c) of low-cobalt or cobalt-free positive electrode active materials is closely related to the mass percentage (x%) of compounds containing phosphorus-oxygen double bonds in the electrolyte. When the cobalt content (c) of the low-cobalt or cobalt-free positive electrode active material and the mass percentage (x%) of compounds containing phosphorus-oxygen double bonds in the electrolyte satisfy c+x / 10≥0.10, the O atoms in the compounds containing phosphorus-oxygen double bonds can fully bind to the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material, better stabilizing the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material and preventing excessive lithium removal from the surface of the low-cobalt or cobalt-free positive electrode active material. As a result, the secondary battery can have significantly improved cycle performance while also having good high-temperature storage performance.

[0006] When c+x / 10<0.10, the content of the compound containing phosphorus-oxygen double bonds in the electrolyte is insufficient to form a protective film with excellent performance on the surface of the low-cobalt or cobalt-free positive electrode active material, and the compound containing phosphorus-oxygen double bonds cannot fully stabilize the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material and inhibit the lithium ion transfer on the surface of the low-cobalt or cobalt-free positive electrode active material. Therefore, it is difficult for the secondary battery to have significantly improved cycle performance.

[0007] In any embodiment of the present application, the compound containing a phosphorus-oxygen double bond includes at least lithium difluorophosphate. Lithium difluorophosphate has two separate P=O double bonds in its molecular structure. Therefore, it can fully bind to lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material, better stabilize the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material, and prevent excessive lithium desorption from the surface of the low-cobalt or cobalt-free positive electrode active material.

[0008] In any embodiment of the present application, the electrolyte further comprises one or more of fluoroethylene carbonate and fluorosulfonyl imide lithium salt.

[0009] Optionally, the molecular formula of the lithium fluorosulfonyl imide salt is LiN(SO2R1)(SO2R2), R1 and R2 each independently represent F, or C n F 2n+1 , n is an integer from 1 to 10.

[0010] Optionally, the fluorosulfonyl imide lithium salt includes one or both of lithium bisfluorosulfonyl imide and lithium bistrifluoromethanesulfonyl imide.

[0011] In any embodiment of the present application, based on the total mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is y%, and 0≤y≤2.0. Optionally, 0<y≤1.5.

[0012] In any embodiment of the present application, based on the total mass of the electrolyte, the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0≤z≤2.0. Optionally, 0<z≤1.5.

[0013] Adding fluoroethylene carbonate to the electrolyte can effectively improve the cycle performance of secondary batteries. Furthermore, fluoroethylene carbonate is resistant to high-voltage oxidation, making it suitable for high-voltage cathode active materials, thereby increasing the energy density of secondary batteries. Adding fluorosulfonyl imide lithium salt to the electrolyte can significantly improve the rate capability and low-temperature performance of secondary batteries.

[0014] In any embodiment of the present application, the secondary battery further satisfies 0.5≤y / x≤2.0. Alternatively, 0.5≤y / x≤1.0. In this case, the synergistic effect of the compound containing a phosphorus-oxygen double bond and the fluoroethylene carbonate can be fully utilized, not only without increasing the gas production of the secondary battery, but also further improving the cycle performance and energy density of the secondary battery.

[0015] In any embodiment of the present application, the secondary battery further satisfies 0.5 ≤ x / z ≤ 2.0. Alternatively, 0.5 ≤ x / z ≤ 1.5. In this case, the synergistic effect of the phosphorus-oxygen double bond-containing compound and the fluorosulfonyl imide lithium salt can be fully utilized, not only without deteriorating the cycle performance of the secondary battery, but also further improving the rate performance and low-temperature performance of the secondary battery.

[0016] In any embodiment of the present application, the secondary battery further satisfies 0.5≤y / x≤2.0 and 0.5≤x / z≤2.0. Alternatively, the secondary battery further satisfies 0.5≤y / x≤2.0, 0.5≤x / z≤2.0, and 0.25≤y / z≤2.0. In this case, the secondary battery has significantly improved cycle performance, storage performance, rate performance, and low-temperature performance.

[0017] A second aspect of the present application provides a method for preparing a secondary battery, comprising at least the steps of:

[0018] S10, assembling a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte into a secondary battery, wherein the positive electrode sheet includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A g A layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b+c+d+e=1, f+g=2, the electrolyte comprises a compound containing a phosphorus-oxygen double bond, an optional fluoroethylene carbonate, and A selected fluorosulfonyl imide lithium salt, wherein the compound containing a phosphorus-oxygen double bond comprises one or more of lithium difluorophosphate, diethyl (2-cyanoethyl) phosphonate, tripropyl phosphoric anhydride, diethyl acetyl phosphate, and triphenyl phosphate, wherein the mass percentage of the phosphorus-oxygen double bond-containing compound is x% and 0<x≤1.0 based on the total mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is y% and 0≤y≤2.0 based on the total mass of the electrolyte, and the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0≤z≤2.0 based on the total mass of the electrolyte;

[0019] S20, screening out secondary batteries satisfying c+x / 10≥0.10 from the secondary batteries obtained in step 1.

[0020] When the secondary battery satisfies c+x / 10≥0.10, the O atoms in the compound containing the phosphorus-oxygen double bond can fully combine with the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material, better stabilizing the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material and preventing the surface of the low-cobalt or cobalt-free positive electrode active material from excessive lithium removal. Therefore, the secondary batteries obtained by the preparation method of the present application can have significantly improved cycle performance and good high-temperature storage performance.

[0021] In any embodiment of the present application, the method further comprises the step of selecting secondary batteries that satisfy 0.5≤y / x≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary batteries have good storage performance and significantly improved cycle performance and energy density.

[0022] In any embodiment of the present application, the method further comprises the step of selecting secondary batteries satisfying 0.5≤x / z≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary batteries have good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0023] In any embodiment of the present application, the method further comprises the step of selecting secondary batteries that simultaneously satisfy 0.5≤y / x≤2.0 and 0.5≤x / z≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary batteries have good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0024] In any embodiment of the present application, the method further comprises the step of selecting secondary batteries that simultaneously satisfy 0.5≤y / x≤2.0, 0.5≤x / z≤2.0, and 0.25≤y / z≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary batteries have good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0025] A third aspect of the present application provides a battery module, which includes one of the secondary battery according to the first aspect of the present application and the secondary battery prepared according to the method according to the second aspect of the present application.

[0026] A fourth aspect of the present application provides a battery pack, which includes one of the secondary battery of the first aspect of the present application, the secondary battery prepared according to the method of the second aspect of the present application, and the battery module of the third aspect of the present application.

[0027] The fifth aspect of the present application provides an electrical device, which includes at least one of the secondary battery of the first aspect of the present application, the secondary battery prepared according to the method of the second aspect of the present application, the battery module of the third aspect of the present application, and the battery pack of the fourth aspect of the present application.

[0028] The battery module, battery pack and electric device of the present application include the secondary battery provided by the present application, and thus have at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on the drawings without inventive effort.

[0030] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of the present application.

[0031] Figure 2 yes Figure 1 An exploded schematic diagram of an embodiment of a secondary battery.

[0032] Figure 3 It is a schematic diagram of an embodiment of a battery module of the present application.

[0033] Figure 4 It is a schematic diagram of an embodiment of the battery pack of the present application.

[0034] Figure 5 yes Figure 4 An exploded view of an embodiment of a battery pack is shown.

[0035] Figure 6 This is a schematic diagram of an embodiment of an electric device including the secondary battery of the present application as a power source. DETAILED DESCRIPTION

[0036] Below, the secondary battery, method for preparing a secondary battery, battery module, battery pack and embodiment of 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.

[0037] " 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.

[0038] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0039] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of this application.

[0040] 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.

[0041] 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.

[0042] 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).

[0043] Secondary batteries, also known as rechargeable batteries or storage batteries, are batteries that can be recharged after discharge to activate the active materials and continue to be used. Typically, a secondary battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. During the charge and discharge process of a secondary battery, lithium ions are inserted and removed back and forth between the positive and negative electrodes. The separator is placed between the positive and negative electrodes, primarily to prevent a short circuit between the positive and negative electrodes while allowing lithium ions to pass through. The electrolyte conducts lithium ions between the positive and negative electrodes.

[0044] When a secondary battery is charged, lithium ions are preferentially released from the surface of the positive electrode active material, and then the lithium ions in the bulk of the positive electrode active material are replenished to the surface in a timely manner. When the cobalt content of the positive electrode active material is high, the lithium ions in the bulk of the positive electrode active material can be replenished to the surface of the positive electrode active material in a timely manner. However, when the cobalt content of the positive electrode active material is low, the lithium ions in the bulk of the positive electrode active material do not have time to be replenished to the surface of the positive electrode active material, and the lithium ions on the surface have already been released, which will cause excessive lithium removal from the surface of the positive electrode active material, thereby affecting the crystal structure of the positive electrode active material (for example, irreversible distortion and increased lattice defects of the positive electrode active material), and reducing the cycle performance of the secondary battery. Therefore, improving the lithium ion diffusion rate of low-cobalt or cobalt-free positive electrode active materials has important practical significance.

[0045] After extensive research, the inventors have proposed a low-cobalt or cobalt-free secondary battery with significantly improved lithium ion diffusion rate and cycle performance.

[0046] The first aspect of the embodiment of the present application provides a secondary battery, which includes an electrolyte and a positive electrode. The positive electrode includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A g A layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b+c+d+e=1, f+g=2. The electrolyte includes a compound containing a phosphorus-oxygen double bond, and the compound containing a phosphorus-oxygen double bond includes one or more of lithium difluorophosphate, diethyl (2-cyanoethyl) phosphonate, tripropyl phosphoric anhydride, diethyl acetyl phosphate, and triphenyl phosphate. Based on the total mass of the electrolyte, the mass percentage of the compound containing a phosphorus-oxygen double bond is x% and 0<x≤1.0. The secondary battery satisfies c+x / 10≥0.10.

[0047] Researchers have been working to improve the lithium ion diffusion rate of low-cobalt or cobalt-free positive electrode active materials, but no good solution has been found yet.

[0048] The inventors of this application unexpectedly discovered that after adding a compound containing a phosphorus-oxygen double bond (e.g., one or more of lithium difluorophosphate, diethyl (2-cyanoethyl) phosphonate, tripropyl phosphoric anhydride, diethyl acetyl phosphate, and triphenyl phosphate) to the electrolyte, the compound containing a phosphorus-oxygen double bond can form a protective film on the surface of the positive electrode active material. Furthermore, the compound containing a phosphorus-oxygen double bond has a P=O structure, and the lone electron pair on the O has a low binding energy, which allows it to bind to lithium ions on the surface of the positive electrode active material, thereby stabilizing the lithium ions on the surface of the positive electrode active material. During charging of a secondary battery, most of the lithium ions on the surface of the positive electrode active material are fixed to the surface of the positive electrode active material by the P=O structure of the phosphorus-oxygen double bond-containing compound, making them less likely to escape. This allows lithium ions in the bulk of the positive electrode active material sufficient time to diffuse to the surface of the positive electrode active material and replenish the lithium ions on the surface of the positive electrode active material in a timely manner, thus preventing the occurrence of structural, chemical, or electrochemical instabilities in the positive electrode active material due to excessive delithiation from the surface of the positive electrode active material, such as irreversible distortion and increased lattice defects in the positive electrode active material. Therefore, after adding a compound containing a phosphorus-oxygen double bond to the electrolyte, the surface of the low-cobalt or cobalt-free positive electrode active material can be stabilized, preventing the surface of the low-cobalt or cobalt-free positive electrode active material from excessive lithium removal and preventing the crystal structure of the low-cobalt or cobalt-free positive electrode active material from being destroyed, so that the secondary battery can have good cycle performance.

[0049] Phosphorus-oxygen double bond-containing compounds have poor solubility in electrolytes. Excessive addition of these compounds can worsen the viscosity of the electrolyte, thereby reducing the ionic conductivity of the electrolyte to a certain extent. Therefore, the amount of these compounds added should be moderate.

[0050] After extensive research, the inventors found that the cobalt content c of the low-cobalt or cobalt-free positive electrode active material is closely related to the mass percentage x% of the compound containing phosphorus-oxygen double bonds in the electrolyte. When the cobalt content c of the low-cobalt or cobalt-free positive electrode active material and the mass percentage x% of the compound containing phosphorus-oxygen double bonds in the electrolyte meet c+x / 10≥0.10, the O atoms in the compound containing phosphorus-oxygen double bonds can be fully combined with the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material, better stabilizing the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material and avoiding excessive lithium removal from the surface of the low-cobalt or cobalt-free positive electrode active material. Therefore, the secondary battery can have significantly improved cycle performance while also having good high-temperature storage performance. In some embodiments, c+x / 10 may be ≥0.10, ≥0.11, ≥0.12, ≥0.13, ≥0.14, ≥0.15, ≥0.16, ≥0.17, ≥0.18, or ≥0.19.

[0051] When c+x / 10<0.10, the content of the compound containing phosphorus-oxygen double bonds in the electrolyte is insufficient to form a protective film with excellent performance on the surface of the low-cobalt or cobalt-free positive electrode active material, and the compound containing phosphorus-oxygen double bonds cannot fully stabilize the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material and inhibit the lithium ion transfer on the surface of the low-cobalt or cobalt-free positive electrode active material. Therefore, it is difficult for the secondary battery to have significantly improved cycle performance.

[0052] In some embodiments, the compound containing a phosphorus-oxygen double bond includes at least lithium difluorophosphate. Lithium difluorophosphate has two separate P=O double bonds in its molecular structure, and therefore can fully bind to lithium ions on the surface of a low-cobalt or cobalt-free positive electrode active material, thereby better stabilizing the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material and preventing excessive delithiation of the surface of the low-cobalt or cobalt-free positive electrode active material.

[0053] In some embodiments, the molecular formula is Li a Ni b Co c M1 d M2 e O f A g The layered material can be optionally modified by doping with M2 cations, doping with A anions, or doping with both M2 cations and A anions. The crystal structure of the layered material obtained after doping is more stable, which can further improve the electrochemical performance of the secondary battery, such as cycle performance, rate performance, etc.

[0054] In some embodiments, A is selected from F. After F doping modification, Li a Ni b Co c M1 d M2 e O f A g The structure is more stable, which can make the secondary battery have better cycle performance and rate performance.

[0055] In some embodiments, M1 is selected from Mn.

[0056] In some embodiments, M1 is selected from Al.

[0057] In some embodiments, M1 is selected from a combination of Mn and Al. The molar ratio of Mn to Al is not particularly limited and can be selected according to actual needs.

[0058] In some embodiments, 0.50≤b<0.98. Alternatively, 0.55≤b<0.98, 0.60≤b<0.98, 0.65≤b<0.98, 0.70≤b<0.98, 0.75≤b<0.98, 0.80≤b<0.98.

[0059] In some embodiments, c=0.

[0060] In some embodiments, 0<c<0.1. Alternatively, 0<c≤0.09, 0<c≤0.08, 0<c≤0.07, 0<c≤0.06, 0<c≤0.05, 0<c≤0.04, 0<c≤0.03, 0<c≤0.02, or 0<c≤0.01.

[0061] In some embodiments, 0<d≤0.45. Alternatively, 0<d≤0.40, 0<d≤0.35, 0<d≤0.30, 0<d≤0.25, 0<d≤0.20, 0<d≤0.15, or 0<d≤0.10.

[0062] In some embodiments, e=0.

[0063] In some embodiments, 0<e≤0.5. Alternatively, 0<e≤0.45, 0<e≤0.40, 0<e≤0.35, 0<e≤0.30, 0<e≤0.25, 0<e≤0.20, 0<e≤0.15, 0<e≤0.10, or 0<e≤0.05.

[0064] In some embodiments, f=2, g=0.

[0065] In some embodiments, f=0, g=2.

[0066] In some embodiments, 0<f<2, 0<g<2, and f+g=2.

[0067] As an example, the molecular formula is Li a Ni b Co c M1 d M2 e O f A g Layered materials include but are not limited to LiNi 0.7 Mn 0.3 O2、LiNi 0.69 Co 0.01 Mn 0.3 O2、LiNi 0.68 Co 0.02 Mn 0.3 O2、LiNi 0.65 Co 0.05 Mn 0.3 O2、LiNi 0.63 Co 0.07 Mn 0.3 O2、LiNi 0.61 Co 0.09 Mn 0.3One or more of O2.

[0068] Li a Ni b Co c M1 d M2 e O f A g It can be prepared according to conventional methods in the art. An exemplary preparation method is as follows: a lithium source, a nickel source, a cobalt source, an M1 element precursor, an optional M2 element precursor, and an optional A element precursor are mixed and sintered to obtain the obtained product. The sintering atmosphere may be an oxygen-containing atmosphere, for example, an air atmosphere or an oxygen atmosphere. The O2 concentration of the sintering atmosphere is, for example, 70% to 100%. The sintering temperature and sintering time may be adjusted according to actual conditions. As an example, the lithium source includes but is not limited to one or more of lithium oxide (Li2O), lithium phosphate (Li3PO4), lithium dihydrogen phosphate (LiH2PO4), lithium acetate (CH3COOLi), lithium hydroxide (LiOH), lithium carbonate (Li2CO3) and lithium nitrate (LiNO3). As an example, the nickel source includes but is not limited to one or more of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate and nickel acetate. As an example, the cobalt source includes but is not limited to one or more of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate and cobalt acetate. As an example, the precursor of the M1 element includes but is not limited to one or more of the oxide, nitrate, carbonate, hydroxide and acetic acid compounds of the M1 element. As an example, the precursor of the M2 element includes but is not limited to one or more of the oxide, nitrate, carbonate, hydroxide and acetic acid compounds of the M2 element. As an example, the precursor of the A element includes but is not limited to one or more of ammonium fluoride, lithium fluoride, hydrogen fluoride, ammonium chloride, lithium chloride, hydrogen chloride, ammonium nitrate, ammonium nitrite, ammonium carbonate, ammonium bicarbonate, ammonium phosphate, phosphoric acid, ammonium sulfate, ammonium bisulfite, ammonium sulfite, ammonium bisulfide, hydrogen sulfide, lithium sulfide, ammonium sulfide and elemental sulfur.

[0069] In some embodiments, the electrolyte further comprises fluoroethylene carbonate (FEC). Based on the total mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is y% and 0≤y≤2.0. For example, y is 0, 0.10, 0.20, 0.50, 0.75, 1.0, 1.25, 1.50, 1.75, 2.0 or a range consisting of any of the above values. Optionally, 0<y≤2.0, 0<y≤1.75, 0<y≤1.5, 0<y≤1.25, 0<y≤1.0, 0<y≤0.75, or 0<y≤0.5.

[0070] For secondary batteries, fluoroethylene carbonate can undergo a reduction decomposition reaction at a higher potential and form a solid electrolyte interface film (SEI film) with a certain degree of flexibility on the surface of the negative electrode active material. At the same time, it can also inhibit the reduction decomposition of organic solvents at lower potentials and inhibit the embedding of organic solvents into the negative electrode active material. Therefore, after adding fluoroethylene carbonate to the electrolyte, the cycle performance of the secondary battery can be effectively improved. In addition, fluoroethylene carbonate is resistant to high-voltage oxidation, which is conducive to matching high-voltage positive electrode active materials, thereby helping to improve the energy density of the secondary battery.

[0071] In some embodiments, the mass percentage x% of the compound containing a phosphorus-oxygen double bond and the mass percentage y% of the fluoroethylene carbonate further satisfy 0.5≤y / x≤2.0. Alternatively, 0.5≤y / x≤1.9, 0.5≤y / x≤1.8, 0.5≤y / x≤1.7, 0.5≤y / x≤1.6, 0.5≤y / x≤1.5, 0.5≤y / x≤1.4, 0.5≤y / x≤1.3, 0.5≤y / x≤1.2, 0.5≤y / x≤1.1, or 0.5≤y / x≤1.0.

[0072] Adding fluoroethylene carbonate to the electrolyte can effectively improve the cycle performance of secondary batteries. However, fluoroethylene carbonate decomposes to form HF, which can destabilize the positive electrode active material, increase gas production, and deteriorate the battery's storage performance. Phosphorus-oxygen double-bond-containing compounds, as stabilizers for positive electrode active materials, can inhibit HF from damaging the structure of the positive electrode active material. Combining these compounds with fluoroethylene carbonate can fully leverage the improved cycling performance and energy density of secondary batteries. Furthermore, rationally controlling the relationship between the mass percentage of the phosphorus-oxygen double-bond-containing compound (x%) and the mass percentage of the fluoroethylene carbonate (y%), such that 0.5 ≤ y / x ≤ 2.0, can fully leverage the synergistic effect of the phosphorus-oxygen double-bond-containing compound and fluoroethylene carbonate, not only preventing an increase in gas production but also further improving the battery's cycle performance and energy density.

[0073] In some embodiments, the electrolyte further comprises a lithium fluorinated sulfonyl imide salt. Optionally, the molecular formula of the lithium fluorinated sulfonyl imide salt is LiN(SO2R1)(SO2R2), where R1 and R2 each independently represent F, or C. n F 2n+1 , n is an integer of 1 to 10. As an example, the fluorosulfonyl imide lithium salt includes one or both of lithium bisfluorosulfonyl imide (LiFSI) and lithium bistrifluoromethanesulfonyl imide (LiTFSI).

[0074] Based on the total mass of the electrolyte, the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0≤z≤2.0. For example, z is 0, 0.10, 0.20, 0.50, 0.75, 1.0, 1.25, 1.50, 1.75, 2.0 or any range thereof. Alternatively, 0<z≤2.0, 0<z≤1.75, 0<z≤1.5, 0<z≤1.25, 0<z≤1.0, 0<z≤0.75, or 0<z≤0.5.

[0075] Fluorosulfonyl imide anion is a weakly coordinated anion centered on nitrogen, containing a conjugated group and a strongly electron-withdrawing -F or -C n F 2n+1 , the anion charge is highly delocalized, and the interaction force between the anion and the lithium ion is weakened. Therefore, the fluorosulfonyl imide lithium salt has a low lattice energy and is easy to dissociate, thereby improving the ionic conductivity of the electrolyte, reducing the viscosity of the electrolyte, and improving the rate performance and low-temperature performance of the secondary battery. At the same time, the fluorosulfonyl imide lithium salt also has high thermal stability and a wider electrochemical window, and can form a LiF-rich SEI film on the surface of the negative electrode active material. The LiF-rich SEI film is thinner, has lower impedance and higher thermal stability, and can reduce the side reactions between the negative electrode active material and the electrolyte. Therefore, after adding the fluorosulfonyl imide lithium salt to the electrolyte, the rate performance and low-temperature performance of the secondary battery can be significantly improved.

[0076] In some embodiments, the mass percentage x% of the compound containing a phosphorus-oxygen double bond and the mass percentage z% of the lithium fluorosulfonyl imide salt further satisfy 0.5≤x / z≤2.0. Alternatively, 0.5≤x / z≤1.9, 0.5≤x / z≤1.8, 0.5≤x / z≤1.7, 0.5≤x / z≤1.6, 0.5≤x / z≤1.5, 0.5≤x / z≤1.4, 0.5≤x / z≤1.3, 0.5≤x / z≤1.2, 0.5≤x / z≤1.1, or 0.5≤x / z≤1.0.

[0077] After adding fluorosulfonyl imide lithium salt to the electrolyte, the rate performance and low-temperature performance of the secondary battery can be improved. However, fluorosulfonyl imide lithium salt is not resistant to high voltage and will corrode the positive electrode current collector (for example, aluminum foil) at a higher potential, increase the side reaction between the positive electrode active material and the electrolyte, and its film-forming effect on the surface of the positive electrode active material is poor, which easily affects the cycle performance of the secondary battery. The compound containing phosphorus and oxygen double bonds is used in combination with fluorosulfonyl imide lithium salt. The compound containing phosphorus and oxygen double bonds is used as a stabilizer for the positive electrode active material, which can form a protective film with excellent performance on the surface of the positive electrode active material and inhibit the side reaction between the positive electrode active material and the electrolyte. Therefore, the compound containing phosphorus and oxygen double bonds is used in combination with fluorosulfonyl imide lithium salt, which is conducive to giving full play to the improvement effect of fluorosulfonyl imide lithium salt on the rate performance and low-temperature performance of the secondary battery. In addition, by rationally controlling the relationship between the mass percentage x% of the compound containing a phosphorus-oxygen double bond and the mass percentage z% of the fluorosulfonyl imide lithium salt so as to satisfy 0.5≤x / z≤2.0, the synergistic effect of the compound containing a phosphorus-oxygen double bond and the fluorosulfonyl imide lithium salt can be fully exerted, which will not only not deteriorate the cycle performance of the secondary battery, but also further improve the rate performance and low-temperature performance of the secondary battery.

[0078] In some embodiments, the electrolyte further includes fluoroethylene carbonate and fluorosulfonyl imide lithium salt.

[0079] In some embodiments, the secondary battery also satisfies 0.5≤y / x≤2.0 and 0.5≤x / z≤2.0. Furthermore, the secondary battery also satisfies 0.5≤y / x≤2.0, 0.5≤x / z≤2.0, and 0.25≤y / z≤2.0. In this case, the secondary battery has significantly improved cycle performance, storage performance, rate performance, and low-temperature performance.

[0080] Fluorinated ethylene carbonate can effectively improve the cycle performance of secondary batteries, while fluorosulfonyl imide lithium salts can improve the rate performance and low-temperature performance of secondary batteries. Phosphorus-oxygen double-bond-containing compounds, as stabilizers for positive electrode active materials, can form a high-performance protective film on the surface of the positive electrode active material, stabilizing lithium ions on the surface of low-cobalt or cobalt-free positive electrode active materials and preventing lithium over-desorption from the surface of low-cobalt or cobalt-free positive electrode active materials. Simultaneously, they inhibit side reactions between the positive electrode active material and the electrolyte, and suppress HF damage to the structure of the positive electrode active material. Therefore, rationally controlling the relationship between the content of fluoroethylene carbonate, fluorosulfonyl imide lithium salts, and phosphorus-oxygen double-bond-containing compounds is beneficial for fully leveraging the synergistic effect among the three and fully suppressing the defects of using each component alone.

[0081] The electrolyte of the secondary battery of this application is advantageously compatible with high-voltage positive electrode active materials, thereby further improving the energy density of the secondary battery. In some embodiments, when the charge capacity per unit area of ​​the positive electrode sheet is 90% of the capacity per unit area of ​​the negative electrode sheet, the positive electrode charge voltage is ≥ 4.1V. When the charge capacity of the positive electrode sheet is 100% of the tested capacity of the negative electrode sheet, the positive electrode charge voltage is ≥ 4.2V.

[0082] As an example, the positive electrode charging voltage can be tested according to the following steps.

[0083] (1) Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the above solution to obtain an electrolyte, wherein the concentration of LiPF6 was 1 mol / L; the negative electrode was cut into small discs per unit area, and a metal lithium sheet was used as the counter electrode and a polyethylene film was used as the separator. The CR2430 button cell was assembled in an argon-protected glove box. After the obtained button cell was allowed to stand for 12 hours, it was discharged at a constant current of 0.1 mA to 0.005 V at 25°C, and then charged at a constant current of 0.1 mA to 2 V. The charging capacity of the button cell was recorded as the negative electrode capacity per unit area.

[0084] (2) Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the above solution to obtain an electrolyte, wherein the concentration of LiPF6 was 1 mol / L; the positive electrode plate was cut into small discs per unit area, and a metal lithium plate was used as a counter electrode and a polyethylene film was used as an isolation membrane. The CR2430 button cell was assembled in an argon-protected glove box. After the obtained button cell was allowed to stand for 12 hours, it was charged at a constant current of 0.1 mA at 25°C, and the voltage corresponding to the charge capacity of the button cell reaching 90% and 100% of the capacity per unit area of ​​the negative electrode plate obtained in step (1) was recorded as the positive electrode charging voltage.

[0085] In some embodiments, the secondary battery includes an electrolyte and a positive electrode. The positive electrode includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A gA layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b+c+d+e=1, f+g=2. The electrolyte comprises a compound containing a phosphorus-oxygen double bond and fluoroethylene carbonate, and the compound containing a phosphorus-oxygen double bond comprises lithium difluorophosphate. Based on the total mass of the electrolyte, the mass percentage of the compound containing a phosphorus-oxygen double bond is x% and 0<x≤1.0, and the mass percentage of the fluoroethylene carbonate is y% and 0≤y≤2.0. The secondary battery satisfies c+x / 10≧0.10 and 0.5≦y / x≦2.0.

[0086] In some embodiments, the secondary battery includes an electrolyte and a positive electrode. The positive electrode includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A g A layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b+c+d+e=1, f+g=2. The electrolyte includes a compound containing a phosphorus-oxygen double bond and a fluorosulfonyl imide lithium salt, the compound containing a phosphorus-oxygen double bond includes lithium difluorophosphate, and the fluorosulfonyl imide lithium salt includes one or two of lithium bisfluorosulfonyl imide and lithium bistrifluoromethanesulfonyl imide. Based on the total mass of the electrolyte, the mass percentage of the phosphorus-oxygen double bond-containing compound is x% and 0<x≤1.0, the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0≤z≤2.0. The secondary battery satisfies c+x / 10≥0.10 and 0.5≤x / z≤2.0.

[0087] In some embodiments, the secondary battery includes an electrolyte and a positive electrode. The positive electrode includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A gA layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b+c+d+e=1, f+g=2. The electrolyte includes a compound containing a phosphorus-oxygen double bond, fluoroethylene carbonate and a fluorosulfonyl imide lithium salt, the compound containing a phosphorus-oxygen double bond includes lithium difluorophosphate, and the fluorosulfonyl imide lithium salt includes one or two of lithium bisfluorosulfonyl imide and lithium bistrifluoromethanesulfonyl imide. Based on the total mass of the electrolyte, the mass percentage of the compound containing a phosphorus-oxygen double bond is x% and 0<x≤1.0, the mass percentage of the fluoroethylene carbonate is y% and 0≤y≤2.0, and the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0≤z≤2.0. The secondary battery satisfies c+x / 10≥0.10, 0.5≤y / x≤2.0, and 0.5≤x / z≤2.0.

[0088] In some embodiments, the secondary battery includes an electrolyte and a positive electrode. The positive electrode includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A g A layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b+c+d+e=1, f+g=2. The electrolyte includes a compound containing a phosphorus-oxygen double bond, fluoroethylene carbonate and a fluorosulfonyl imide lithium salt, the compound containing a phosphorus-oxygen double bond includes lithium difluorophosphate, and the fluorosulfonyl imide lithium salt includes one or two of lithium bisfluorosulfonyl imide and lithium bistrifluoromethanesulfonyl imide. Based on the total mass of the electrolyte, the mass percentage of the compound containing a phosphorus-oxygen double bond is x% and 0<x≤1.0, the mass percentage of the fluoroethylene carbonate is y% and 0≤y≤2.0, and the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0≤z≤2.0. The secondary battery satisfies c+x / 10≥0.10, 0.5≤y / x≤2.0, 0.5≤x / z≤2.0, and 0.25≤y / z≤2.0.

[0089] In some embodiments, the electrolyte solution further comprises an electrolyte salt and an organic solvent. The types of the electrolyte salt and the organic solvent are not particularly limited and can be selected according to actual needs.

[0090] As an example, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium trifluoromethanesulfonate (LiTFS), lithium bis(oxalatoborate) (LiBOB), lithium difluorobis(oxalatophosphate) (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP). Alternatively, the electrolyte salt includes LiPF6.

[0091] As an example, the organic solvent may include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0092] In the embodiments of the secondary battery of the present application, the electrolyte does not exclude other components in addition to the above components. In some embodiments, the electrolyte may also optionally include other additives, such as additives to improve the battery's overcharge performance, additives to improve the battery's high-temperature performance, additives to improve the battery's low-temperature power performance, etc.

[0093] The electrolyte can be prepared according to conventional methods in the art. For example, an organic solvent, an electrolyte salt, a compound containing a phosphorus-oxygen double bond, optionally fluoroethylene carbonate, and optionally a fluorosulfonyl imide lithium salt can be mixed uniformly to obtain an electrolyte. The order of adding the materials is not particularly limited. For example, the electrolyte salt, the compound containing a phosphorus-oxygen double bond, optionally fluoroethylene carbonate, and optionally a fluorosulfonyl imide lithium salt are added to an organic solvent and mixed uniformly to obtain an electrolyte; alternatively, the electrolyte salt is first added to an organic solvent, and then the compound containing a phosphorus-oxygen double bond, optionally fluoroethylene carbonate, and optionally a fluorosulfonyl imide lithium salt are added to the organic solvent and mixed uniformly to obtain an electrolyte.

[0094] In some embodiments, 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 and comprising a positive electrode active material. For example, the positive electrode current collector has two opposing surfaces in the thickness direction of the positive electrode current collector, and the positive electrode film layer is disposed on either or both of the two opposing surfaces of the positive electrode current collector.

[0095] The positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may include polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0096] The positive electrode film layer generally comprises a positive electrode active material and an optional binder and an optional conductive agent. The positive electrode film layer is generally formed by coating the positive electrode slurry on the positive electrode current collector, drying and cold pressing. The positive electrode slurry is generally formed by dispersing the positive electrode active material, the optional conductive agent, the optional binder and any other components in a solvent and stirring them evenly. The solvent may be N-methylpyrrolidone (NMP), but is not limited to this. As an example, the binder for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin. As an example, the conductive agent for the positive electrode film layer may include one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0097] In some embodiments, the positive electrode active material includes the above-mentioned molecular formula Li a Ni b Co c M1 d M2 e O f A g layered materials.

[0098] In some embodiments, Li a Ni b Co c M1 d M2 e O f A gThe surface of the cathode active material may further include a coating, such as a carbon coating. The carbon coating further stabilizes lithium ions on the surface of the low-cobalt or cobalt-free cathode active material, preventing excessive delithiation of the surface of the low-cobalt or cobalt-free cathode active material. Optionally, the carbon coating is amorphous carbon, such as soft carbon or hard carbon.

[0099] In some embodiments, the positive electrode active material does not exclude Li a Ni b Co c M1 d M2 e O f A g In addition to other components, for example, the positive electrode active material also includes one or more of the lithium-containing phosphates with an olivine structure and modified compounds thereof. As an example, the lithium-containing phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, a composite material of lithium iron manganese phosphate and carbon, and their respective modified compounds. The present application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for secondary batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more.

[0100] In some embodiments, based on the total mass of the positive electrode film layer, the molecular formula is Li a Ni b Co c M1 d M2 e O f A g The mass percentage of the layered material is 80% to 99%. a Ni b Co c M1 d M2 e O f A g The mass percentage of the layered material is 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or any range consisting of the above values. a Ni b Co c M1 d M2 e O f A gThe mass percentage of the layered material is 85% to 99%, 90% to 99%, 95% to 99%, 80% to 98%, 85% to 98%, 90% to 98%, 95% to 98%, 80% to 97%, 85% to 97%, 90% to 97%, or 95% to 97%.

[0101] The positive electrode sheet does not exclude other additional functional layers in addition to the positive electrode film layer. For example, in some embodiments, the positive electrode sheet described herein further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the positive electrode current collector and the positive electrode film layer and disposed on the surface of the positive electrode current collector. In other embodiments, the positive electrode sheet described herein further includes a protective layer covering the surface of the positive electrode film layer.

[0102] The secondary battery according to the present application also includes a negative electrode plate. In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two opposing surfaces in the thickness direction of the negative electrode current collector, and the negative electrode film layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector.

[0103] The negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may include polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0104] The negative electrode film layer generally comprises a negative electrode active material, an optional binder, an optional conductive agent and other optional additives. The negative electrode film layer is generally formed by coating the negative electrode slurry on the negative electrode current collector, drying and cold pressing. The negative electrode slurry is generally formed by dispersing the negative electrode active material, the optional conductive agent, the optional binder and other optional additives in a solvent and stirring them evenly. The solvent may be N-methylpyrrolidone (NMP) or deionized water, but is not limited thereto. As an example, the binder for the negative electrode film layer may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). As an example, the conductive agent used for the negative electrode film layer may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Other optional additives may include thickeners (e.g., sodium carboxymethyl cellulose CMC-Na), PTC thermistor materials, etc.

[0105] The negative electrode active material may be a negative electrode active material for a secondary battery that is well known in the art. As an example, the negative electrode active material may include one or more of natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material may include one or more of elemental tin, tin oxide, and tin alloy material. The present application is not limited to these materials, and other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0106] The negative electrode sheet does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode sheet described herein further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet described herein further includes a protective layer covering the surface of the negative electrode film layer.

[0107] The secondary battery according to the present application also includes a separator. This separator is disposed between the positive and negative electrode sheets to provide isolation. The present application does not specifically limit the type of separator; any known porous separator with good chemical and mechanical stability can be used.

[0108] In some embodiments, the material of the isolation membrane may include one or more of fiberglass, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The isolation membrane may be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer may be the same or different.

[0109] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet can be made into an electrode assembly through a winding process or a lamination process.

[0110] In some embodiments, the secondary battery may include an outer packaging. The outer packaging may be used to encapsulate the electrode assembly and electrolyte. The outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0111] The present application has no particular restrictions on the shape of the secondary battery, which can be cylindrical, square or any other shape. Figure 1 The secondary battery 5 is a square structure as an example.

[0112] In some embodiments, as Figure 2 As shown, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover 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 impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be adjusted according to demand.

[0113] Method for preparing a secondary battery

[0114] A second aspect of the embodiments of the present application provides a method for preparing a secondary battery, the method comprising at least step 1 and step 2.

[0115] Step 1: Assemble the positive electrode sheet, separator, negative electrode sheet, and electrolyte into a secondary battery.

[0116] The positive electrode plate includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A gA layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b+c+d+e=1, f+g=2.

[0117] The electrolyte includes a compound containing a phosphorus-oxygen double bond, optional fluoroethylene carbonate, and optional fluorosulfonyl imide lithium salt, and the compound containing a phosphorus-oxygen double bond includes one or more of lithium difluorophosphate, diethyl (2-cyanoethyl) phosphonate, tripropyl phosphoric anhydride, acetyl diethyl phosphate, and triphenyl phosphate.

[0118] Based on the total mass of the electrolyte, the mass percentage of the compound containing phosphorus-oxygen double bonds is x% and 0<x≤1.0, based on the total mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is y% and 0≤y≤2.0, and based on the total mass of the electrolyte, the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0≤z≤2.0.

[0119] Step 2: Screen out secondary batteries that satisfy c+x / 10≥0.10 from the secondary batteries obtained in step 1.

[0120] When the secondary battery satisfies c+x / 10≥0.10, the O atoms in the compound containing the phosphorus-oxygen double bond can fully combine with the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material, better stabilizing the lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material and preventing the surface of the low-cobalt or cobalt-free positive electrode active material from excessive lithium removal. Therefore, the secondary batteries obtained by the preparation method of the present application can have significantly improved cycle performance and good high-temperature storage performance.

[0121] In some embodiments, the compound containing a phosphorus-oxygen double bond includes at least lithium difluorophosphate.

[0122] In some embodiments, the molecular formula of the lithium fluorosulfonyl imide salt is LiN(SO2R1)(SO2R2), R1 and R2 each independently represent F, or C n F 2n+1 , n is an integer of 1 to 10. Optionally, the fluorosulfonyl imide lithium salt includes one or both of lithium bisfluorosulfonyl imide (LiFSI) and lithium bistrifluoromethanesulfonyl imide (LiTFSI).

[0123] In some embodiments, the method further comprises the step of selecting secondary batteries that satisfy 0.5≤y / x≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary batteries have good storage performance and significantly improved cycle performance and energy density.

[0124] In some embodiments, the method further comprises the step of selecting secondary batteries that satisfy 0.5≤x / z≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary batteries have good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0125] In some embodiments, the method further comprises the step of selecting secondary batteries that simultaneously satisfy 0.5≤y / x≤2.0 and 0.5≤x / z≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary batteries have good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0126] In some embodiments, the method further comprises the step of selecting secondary batteries that simultaneously satisfy 0.5≤y / x≤2.0, 0.5≤x / z≤2.0, and 0.25≤y / z≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary batteries have good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0127] Battery modules and battery packs

[0128] In some embodiments of the present application, the secondary batteries according to the present application can be assembled into a battery module. The battery module can contain multiple secondary batteries, and the specific number can be adjusted according to the application and capacity of the battery module.

[0129] Figure 3 FIG. 4 is a schematic diagram of a battery module 4 as an example. Figure 3 As shown, in the battery module 4, the plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other manner. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0130] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0131] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0132] Figure 4 and Figure 5FIG. 1 is a schematic diagram of a battery pack 1 as an example. Figure 4 and Figure 5 As shown, a battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 is used to cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0133] Electrical devices

[0134] The embodiments of the present application also provide an electric device, which includes at least one of the secondary battery, battery module, or battery pack of the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device can be, but is not limited to, a mobile device (such as a mobile phone, a laptop 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, a satellite, an energy storage system, etc.

[0135] The electrical device may select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0136] Figure 6 The diagram is a schematic diagram of an exemplary electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of this device, a battery pack or battery module can be used.

[0137] As another example, the electric device may be a mobile phone, a tablet computer, a laptop computer, etc. Such an electric device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0138] Example

[0139] The following examples describe the present disclosure in more detail and are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise indicated, all parts, percentages, and ratios reported in the following examples are by weight, and all reagents used in the examples are commercially available or synthesized according to conventional methods and used directly without further processing, and all instruments used in the examples are commercially available.

[0140] Example 1

[0141] Preparation of positive electrode

[0142] The positive electrode active material LiNi 0.65 Co 0.05 Mn 0.3 O2, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in an appropriate amount of solvent NMP in a weight ratio of 97.5:1.4:1.1 to form a uniform positive electrode slurry; the positive electrode slurry is evenly coated on the surface of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet is obtained.

[0143] Preparation of negative electrode sheet

[0144] The negative electrode active material graphite, binder styrene-butadiene rubber (SBR), thickener sodium carboxymethyl cellulose (CMC-Na), and conductive agent carbon black (Super P) are fully stirred and mixed in an appropriate amount of solvent deionized water in a weight ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry; the negative electrode slurry is evenly coated on the surface of the negative electrode current collector copper foil, and after drying and cold pressing, a negative electrode sheet is obtained.

[0145] Isolation film

[0146] A porous polyethylene (PE) film is used as the separator.

[0147] Preparation of electrolyte

[0148] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 and lithium difluorophosphate are uniformly dissolved in the organic solvent to obtain an electrolyte, wherein the concentration of LiPF6 is 1 mol / L and the mass percentage of lithium difluorophosphate based on the total mass of the electrolyte is 0.5%.

[0149] Preparation of secondary batteries

[0150] The positive electrode sheet, separator, and negative electrode sheet are stacked and wound in order to obtain an electrode assembly; the electrode assembly is placed in an outer package, the above-mentioned electrolyte is added, and after packaging, standing, formation, aging and other processes, a secondary battery is obtained.

[0151] Examples 2 to 29 and Comparative Examples 1 to 3

[0152] The secondary battery preparation method is similar to that of Example 1, except that the type of positive electrode active material and the electrolyte preparation parameters are adjusted. Specific parameters are detailed in Table 1. Fluorinated ethylene carbonate was also added to the electrolytes of Examples 12-17, lithium bis(fluorosulfonyl)imide was also added to the electrolytes of Examples 18-23, and both fluoroethylene carbonate and lithium bis(fluorosulfonyl)imide were added to the electrolytes of Examples 24-29. In Table 1, x% represents the mass percentage of the compound containing a phosphorus-oxygen double bond based on the total mass of the electrolyte; y% represents the mass percentage of fluoroethylene carbonate based on the total mass of the electrolyte; and z% represents the mass percentage of lithium bis(fluorosulfonyl)imide based on the total mass of the electrolyte.

[0153] Table 1

[0154]

[0155]

[0156] Test section

[0157] (1) Secondary battery room temperature cycle performance test

[0158] At 25°C, charge the secondary battery at a constant current of 1C to 4.3V. Continue constant voltage charging until the current reaches 0.05C. At this point, the secondary battery is fully charged. Record the charge capacity at this point, which is the charge capacity of the first cycle. After the secondary battery rests for 5 minutes, discharge it at a constant current of 1C to 2.8V. This constitutes one cycle of charge and discharge. Record the discharge capacity at this point, which is the discharge capacity of the first cycle. Perform cyclic charge and discharge tests on the secondary battery according to the above method, and record the discharge capacity after each cycle.

[0159] Capacity retention rate (%) of the secondary battery after 600 cycles at 25° C. = discharge capacity after 600 cycles / discharge capacity at the first cycle×100%.

[0160] (2) Secondary battery high temperature cycle performance test

[0161] At 45°C, charge the secondary battery at a constant current of 1C to 4.3V. Continue constant voltage charging until the current reaches 0.05C. At this point, the secondary battery is fully charged. Record the charge capacity at this point, which is the charge capacity of the first cycle. After the secondary battery rests for 5 minutes, discharge it at a constant current of 1C to 2.8V. This constitutes one cycle of charge and discharge. Record the discharge capacity at this point, which is the discharge capacity of the first cycle. Perform cyclic charge and discharge tests on the secondary battery according to the above method, and record the discharge capacity after each cycle.

[0162] Capacity retention rate (%) of the secondary battery after 600 cycles at 45° C. = discharge capacity after 600 cycles / discharge capacity at the first cycle×100%.

[0163] (3) Secondary battery initial DC internal resistance test

[0164] At 25°C, the secondary battery is charged to 4.3V at a constant current of 1C, and then continued to be charged at a constant voltage until the current reaches 0.05C. At this time, the secondary battery is in a fully charged state. The secondary battery is discharged at a constant current of 0.5C and adjusted to 50% SOC. At this time, the voltage of the secondary battery is recorded as U1. The secondary battery is discharged at a constant current of 4C for 30 seconds, with a sampling point of 0.1 seconds. The voltage at the end of discharge is recorded as U2.

[0165] The initial DC internal resistance of the secondary battery is represented by the discharge DC internal resistance of the secondary battery at 50% SOC. The initial DC internal resistance of the secondary battery (Ω) = (U1-U2) / 4C.

[0166] (4) Secondary battery high temperature storage performance test

[0167] At 60°C, charge the secondary battery at a constant current of 1C to 4.3V, and continue to charge at a constant voltage until the current reaches 0.05C. At this time, the volume of the secondary battery is tested by the water displacement method and recorded as V0. Place the secondary battery in a constant temperature box at 60°C and take it out after storage for 30 days. At this time, the volume of the secondary battery is tested by the water displacement method and recorded as V1.

[0168] The volume expansion ratio (%) of the secondary battery after storage at 60° C. for 30 days=[(V1−V0) / V0]×100%.

[0169] Table 2 shows the performance test results of Examples 1 to 29 and Comparative Examples 1 to 3.

[0170] Table 2

[0171]

[0172]

[0173] The test results in Table 2 show that when a phosphorus-oxygen double-bond-containing compound is added to the electrolyte and the cobalt content (c) of the low-cobalt or cobalt-free positive electrode active material and the mass percentage (x%) of the phosphorus-oxygen double-bond-containing compound in the electrolyte satisfy c + x / 10 ≥ 0.10, the secondary battery exhibits significantly improved cycle performance and good high-temperature storage performance. Comparative Examples 1 to 3 do not add a phosphorus-oxygen double-bond-containing compound to the electrolyte, or the amount of the added phosphorus-oxygen double-bond-containing compound is insufficient. In these cases, the phosphorus-oxygen double-bond-containing compound is unable to form a high-performance protective film on the surface of the low-cobalt or cobalt-free positive electrode active material. Furthermore, the phosphorus-oxygen double-bond-containing compound is unable to fully stabilize lithium ions on the surface of the low-cobalt or cobalt-free positive electrode active material and inhibit lithium desorption from the surface of the low-cobalt or cobalt-free positive electrode active material. Consequently, the secondary battery is unlikely to exhibit significantly improved cycle performance.

[0174] It can also be seen from the test results in Table 2 that further reasonable control of the relationship between the mass percentage x% of the compound containing a phosphorus-oxygen double bond and the mass percentage y% of fluoroethylene carbonate so as to satisfy 0.5≤y / x≤2.0 can fully exert the synergistic effect of the compound containing a phosphorus-oxygen double bond and fluoroethylene carbonate, not only will not increase the gas production of the secondary battery, but also further improve the cycle performance of the secondary battery.

[0175] It can also be seen from the test results in Table 2 that further reasonable control of the relationship between the mass percentage x% of the compound containing a phosphorus-oxygen double bond and the mass percentage z% of the lithium bis(fluorosulfonyl)imide so as to satisfy 0.5≤x / z≤2.0 can fully exert the synergistic effect of the compound containing a phosphorus-oxygen double bond and the lithium bis(fluorosulfonyl)imide, not only does not deteriorate the cycle performance of the secondary battery, but also can further reduce the initial DC internal resistance of the secondary battery and improve the rate performance of the secondary battery.

[0176] The test results in Table 2 also show that by further rationally controlling the relationship between the mass percentage x% of the phosphorus-oxygen double-bond-containing compound, the mass percentage y% of fluoroethylene carbonate, and the mass percentage z% of lithium bis(fluorosulfonyl)imide to satisfy 0.5 ≤ y / x ≤ 2.0, 0.5 ≤ x / z ≤ 2.0, and 0.25 ≤ y / z ≤ 2.0, the secondary battery can achieve significantly improved cycle performance and rate capability in addition to good high-temperature storage performance.

[0177] 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 methods constructed by applying various modifications that can be imagined by those skilled in the art to the embodiments and combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery comprising an electrolyte and a positive electrode plate, characterized in that: The positive electrode plate includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A g A layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b + c + d + e = 1, f + g = 2, The electrolyte includes a compound containing a phosphorus-oxygen double bond, fluoroethylene carbonate and a lithium fluorosulfonyl imide salt, wherein the compound containing a phosphorus-oxygen double bond includes one or more of lithium difluorophosphate, diethyl (2-cyanoethyl) phosphonate, tripropyl phosphoric anhydride, diethyl acetyl phosphate and triphenyl phosphate. Based on the total mass of the electrolyte, the mass percentage of the compound containing a phosphorus-oxygen double bond is x% and 0<x≤1.0, based on the total mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is y% and 0<y≤2.0, based on the total mass of the electrolyte, the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0<z≤2.0, The secondary battery satisfies c + x / 10 ≥ 0.

10.

2. The secondary battery according to claim 1, wherein The compound containing phosphorus-oxygen double bonds includes at least lithium difluorophosphate.

3. The secondary battery according to claim 1, wherein The molecular formula of lithium fluorosulfonyl imide salt is LiN(SO2R1)(SO2R2), R1 and R2 each independently represent F, or C n F 2n+1 , n is an integer from 1 to 10.

4. The secondary battery according to claim 1, wherein The lithium fluorosulfonyl imide salt includes one or both of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide.

5. The secondary battery according to claim 1, wherein 0<y≤1.5, and / or, 0＜z≤1.5。 6. The secondary battery according to claim 1, wherein The secondary battery also satisfies one or both of the following relations (1)-(2): (1) 0.5≤y / x≤2.0, (2) 0.5≤x / z≤2.

0.

7. The secondary battery according to claim 6, characterized in that The secondary battery also satisfies one or both of the following relations (1)-(2): (1) 0.5≤y / x≤1.0, (2) 0.5≤x / z≤1.

5.

8. The secondary battery according to claim 1, wherein The secondary battery also satisfies 0.5≤y / x≤2.0, 0.5≤x / z≤2.0, and 0.25≤y / z≤2.0 simultaneously.

9. A method for preparing a secondary battery, comprising the steps of: S10, assembling the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte into a secondary battery, The positive electrode plate includes a molecular formula of Li a Ni b Co c M1 d M2 e O f A g A layered material, wherein M1 is selected from one or two of Mn and Al, M2 is selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is selected from one or more of F, N, P and S, 0.8≤a≤1.2, 0<b<0.98, 0≤c<0.1, 0<d<0.5, 0≤e≤0.5, 0≤f≤2, 0≤g≤2, b + c + d + e = 1, f + g = 2, The electrolyte includes a compound containing a phosphorus-oxygen double bond, fluoroethylene carbonate and a lithium fluorosulfonyl imide salt, wherein the compound containing a phosphorus-oxygen double bond includes one or more of lithium difluorophosphate, diethyl (2-cyanoethyl) phosphonate, tripropyl phosphoric anhydride, diethyl acetyl phosphate and triphenyl phosphate. Based on the total mass of the electrolyte, the mass percentage of the compound containing a phosphorus-oxygen double bond is x% and 0<x≤1.0, based on the total mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is y% and 0<y≤2.0, based on the total mass of the electrolyte, the mass percentage of the fluorosulfonyl imide lithium salt is z% and 0<z≤2.0; S20 , screening out secondary batteries satisfying c + x / 10 ≥ 0.10 from the secondary batteries obtained in step S10 .

10. The method according to claim 9, characterized in that The method further includes the step of screening out secondary batteries satisfying 0.5≤y / x≤2.0 from the secondary batteries obtained in step S20.

11. The method according to claim 9, characterized in that The method further includes the step of screening out secondary batteries satisfying 0.5≤x / z≤2.0 from the secondary batteries obtained in step S20.

12. The method according to claim 9, characterized in that The method further includes the step of selecting secondary batteries that simultaneously satisfy 0.5≤y / x≤2.0 and 0.5≤x / z≤2.0 from the secondary batteries obtained in step S20.

13. The method according to claim 9, characterized in that The method further includes the step of selecting secondary batteries that simultaneously satisfy 0.5≤y / x≤2.0, 0.5≤x / z≤2.0, and 0.25≤y / z≤2.0 from the secondary batteries obtained in step S20.

14. A battery module comprising the secondary battery according to any one of claims 1 to 8 or the secondary battery obtained by the method according to any one of claims 9 to 13. 15 . A battery pack comprising the secondary battery according to claim 1 , the secondary battery obtained by the method according to claim 9 , or the battery module according to claim 14 .

16. An electrical device comprising at least one of the secondary battery according to any one of claims 1 to 8, the secondary battery obtained by the method according to any one of claims 9 to 13, the battery module according to claim 14, and the battery pack according to claim 15.

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