Secondary battery, method for manufacturing secondary battery, battery module, battery pack, and electric device

Abstract

The present application discloses a secondary battery, a method for manufacturing the secondary battery, a battery module, a battery pack, and an electric device. The secondary battery includes an electrolyte and a positive electrode sheet, the positive electrode sheet having a molecular formula of Li a Ni b Co c M1 d M2 e O f A g (wherein M1, M2, A, a, b, c, d, e, f, and g are each as defined herein), the electrolyte comprises lithium difluoro(oxalato)borate, the mass percent content of the lithium difluoro(oxalato)borate being x%, based on the total mass of the electrolyte, and 0

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to Chinese Patent Application No. 202111321034.7, filed on November 9, 2021, entitled "Secondary Battery, Method for Manufacturing a Secondary Battery, Battery Module, Battery Pack, and Electrical Device," which is incorporated herein by reference in its entirety.

[0002] The present application relates to the field of battery technology, and in particular to a secondary battery, a method for manufacturing a secondary battery, a battery module, a battery pack, and an electric device. [Background technology]

[0003] Secondary batteries are charged and discharged by repeatedly inserting and removing lithium ions between the positive and negative electrodes, and have excellent characteristics such as high energy density, long cycle life, no pollution, and no memory effect. Therefore, secondary batteries, as clean energy, have been widely used in many fields such as electronic products, power storage systems such as hydroelectric, thermal, wind and solar power plants, power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc., in order to adapt to the sustainable development strategy of the environment and energy. Cobalt is an important component element of the positive electrode active material of secondary batteries, but the content of cobalt in the earth's crust is relatively low, and it is difficult and expensive to mine, so low-cobalt or cobalt-free positive electrode active materials have become an inevitable development trend. However, cobalt contributes greatly to the lithium ion diffusion rate of the positive electrode active material, and low-cobalt or cobalt-free positive electrode active materials will reduce the lithium ion diffusion rate, affecting the cycle life of secondary batteries. Summary of the Invention

[0004] The present application aims to provide a secondary battery, a method for manufacturing a secondary battery, a battery module, a battery pack, and an electrical device for increasing the lithium ion diffusion rate of a low cobalt or cobalt-free cathode active material and improving the cycle performance of the secondary battery.

[0005] A first aspect of the present application provides a secondary battery including an electrolytic solution and a cathode sheet. The cathode sheet has a layered material with a molecular formula of Li a Ni b Co c M1 d M2 e O f A g (where M1 is one or two selected from Mn and Al, M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W, A is one or more selected from 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, and f + g = 2). The electrolytic solution contains lithium difluoro(oxalato)borate, and based on the total mass of the electrolytic solution, the mass percentage content of the lithium difluoro(oxalato)borate is x%, and 0 < x ≦ 1.0. The secondary battery satisfies c + x / 10 ≧ 0.10.

[0006] After extensive research, the inventors have found that the cobalt content c of the low-cobalt or cobalt-free positive electrode active material is closely related to the mass percent content x% of lithium difluoro(oxalato)borate in the electrolyte. When the cobalt content c of the low-cobalt or cobalt-free positive electrode active material and the mass percent content x% of lithium difluoro(oxalato)borate in the electrolyte satisfy c+x / 10≧0.10, the B atom in the lithium difluoro(oxalato)borate can be fully bonded to the O atom in the positive electrode active material, which can better reduce the diffusion resistance of lithium ions in the Brueck phase of the low-cobalt or cobalt-free positive electrode active material, and can avoid excessive lithium desorption from the surface of the low-cobalt or cobalt-free positive electrode active material, which is favorable to better stabilizing the crystal structure of the low-cobalt or cobalt-free positive electrode active material and improving the lithium ion diffusion rate. Therefore, the secondary battery can have a significantly improved cycle performance and at the same time good high-temperature storage performance.

[0007] When c+x / 10<0.10, the content of lithium difluoro(oxalato)borate in the electrolyte is insufficient to form a protective film with excellent performance and low resistance on the surface of the low-cobalt or cobalt-free positive electrode active material, and lithium difluoro(oxalato)borate cannot effectively reduce the charge transfer resistance of the low-cobalt or cobalt-free positive electrode active material, and cannot effectively reduce the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive electrode active material and inhibit excessive lithium desorption from the surface of the low-cobalt or cobalt-free positive electrode active material, making it difficult for the secondary battery to have significantly improved cycle performance.

[0008] In any embodiment of the present application, the electrolyte further comprises one or more of the following: vinyl fluorocarbonate, lithium fluorosulfonylimide salt.

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

[0010] Optionally, the fluorosulfonylimide lithium salt comprises one or two of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide.

[0011] In any embodiment of the present application, based on the total weight of the electrolyte, the weight percent content of the vinyl fluorocarbonate is y%, and 0≦y≦2.5. <y≦2.0である。

[0012] In any embodiment of the present application, based on the total mass of the electrolyte, the mass percent content of the fluorosulfonylimide lithium salt is z%, and 0≦z≦2.5. <z≦2.0である。

[0013] After adding fluorovinyl carbonate to the electrolyte, the cycle performance of the secondary battery can be effectively improved, and fluorovinyl carbonate can withstand high-pressure oxidation and contribute to the matching of high-voltage positive electrode active materials, which is favorable for improving the energy density of the secondary battery. After adding fluorosulfonylimide lithium salt to the electrolyte, the rate performance and low-temperature performance of the secondary battery can be greatly improved.

[0014] In any embodiment of the present application, the secondary battery further satisfies 0.5≦y / x≦2.0. Optionally, 0.5≦y / x≦1.0. In this case, the synergistic effect of lithium difluoro(oxalato)borate and vinyl fluorocarbonate can be fully exerted, and the gas generation amount of the secondary battery is not increased, and the cycle performance and energy density of the secondary battery can be further improved.

[0015] In any embodiment of the present application, the secondary battery further satisfies 0.5≦x / z≦2.0. Optionally, 0.5≦x / z≦1.5. In this case, the synergistic effect of lithium difluoro(oxalato)borate and lithium fluorosulfonylimide salt can be fully exerted, and the cycle performance of the secondary battery is not deteriorated, and the rate performance and low temperature performance of the secondary battery can be further improved.

[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 simultaneously. Optionally, the secondary battery further satisfies 0.5≦y / x≦2.0, 0.5≦x / z≦2.0, and 0.25≦y / z≦2.0 simultaneously. In this case, the secondary battery has significantly improved cycle performance, storage performance, rate performance, and low temperature performance at the same time.

[0017] A second aspect of the present application is a method for producing a semiconductor device comprising: Step 1: Assembling a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte into a secondary battery, the positive electrode sheet having a molecular formula of Li a Ni b Co c M1 d M2 e O f A g(M1 is one or two selected from Mn and Al, M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W, A is one or more selected from 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.) It contains a layered material, the electrolyte contains lithium difluoro(oxalato)borate, any vinyl fluorocarbonate, and any lithium fluorosulfonylimide salt. Based on the total mass of the electrolyte, the mass percentage content of the lithium difluoro(oxalato)borate is x%, and 0 < x ≦ 1.0. Based on the total mass of the electrolyte, the mass percentage content of the vinyl fluorocarbonate is y%, and 0 ≦ y ≦ 2.5. Based on the total mass of the electrolyte, the mass percentage content of the lithium fluorosulfonylimide salt is z%, and 0 ≦ z ≦ 2.5 step, Step 2: Screening secondary batteries that satisfy c + x / 10 ≧ 0.10 from the secondary batteries obtained in Step 1, A method for manufacturing a secondary battery including is provided.

[0018] When the secondary battery satisfies c + x / 10 ≧ 0.10, the B atom in lithium difluoro(oxalato)borate can be sufficiently bonded to the O atom in the cathode active material, better reducing the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free cathode active material, and avoiding excessive lithium desorption on the surface of the low-cobalt or cobalt-free cathode active material. Thus, it can be advantageous for better stabilization of the crystal structure of the low-cobalt or cobalt-free cathode active material and improvement of the lithium ion diffusion rate. Therefore, the secondary batteries obtained by the manufacturing method according to the present application can all have significantly improved cycle performance and good high-temperature storage performance.

[0019] In any embodiment of the present application, the method further includes a step of screening secondary batteries that satisfy 0.5≦y / x≦2.0 from the secondary batteries obtained in step 2. In this case, the manufactured secondary battery has good storage performance and significantly improved cycle performance and energy density at the same time.

[0020] In any embodiment of the present application, the method further includes a step of screening secondary batteries that satisfy 0.5≦x / z≦2.0 from the secondary batteries obtained in step 2. In this case, the manufactured secondary battery has good storage performance and significantly improved cycle performance, rate performance and low temperature performance at the same time.

[0021] In any embodiment of the present application, the method further includes a step of screening 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 manufactured secondary battery simultaneously has good storage performance and significantly improved cycle performance, rate performance, and low temperature performance.

[0022] In any embodiment of the present application, the method further includes a step of screening 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 manufactured secondary battery simultaneously has good storage performance and significantly improved cycle performance, rate performance, and low temperature performance.

[0023] A third aspect of the present application provides a battery module including one of the secondary batteries according to the first aspect of the present application and the secondary batteries manufactured by the method according to the second aspect of the present application.

[0024] A fourth aspect of the present application provides a battery pack including one of the secondary battery of the first aspect of the present application, the secondary battery manufactured by the method of the second aspect of the present application, and the battery module of the third aspect of the present application.

[0025] A fifth aspect of the present application provides an electric device including at least one of the secondary battery of the first aspect of the present application, the secondary battery manufactured by 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.

[0026] The battery module, battery pack, and electric device according to the present application include the secondary battery provided by the present application, and therefore have at least the same advantages as the secondary battery. [Brief description of the drawings]

[0027] In order to more clearly describe the technical solutions of the embodiments of the present application, the drawings that need to be used in the embodiments of the present application are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application, and those skilled in the art can obtain other drawings from the drawings without creative efforts.

[0028] [Figure 1] 1 is a schematic diagram of an embodiment of a secondary battery according to the present application. [Diagram 2] 2 is an exploded schematic view of the embodiment of the secondary battery shown in FIG. 1. [Diagram 3] 1 is a schematic diagram of an embodiment of a battery module according to the present application. [Figure 4] 1 is a schematic diagram of one embodiment of a battery pack according to the present application. [Diagram 5] 5 is an exploded schematic view of the embodiment of the battery pack shown in FIG. 4. [Figure 6] 1 is a schematic diagram of an embodiment of an electrical device including a secondary battery according to the present application as a power source. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Hereinafter, with appropriate reference to the drawings, specifically disclosed embodiments of a secondary battery, a method for manufacturing a secondary battery, a battery module, a battery pack, and an electric device according to the present invention will be described in detail. However, matters that do not require detailed explanation may be omitted. For example, detailed explanations of matters that are already well known, or duplicate explanations of substantially the same configurations may be omitted. This is to avoid the following explanation becoming unnecessarily redundant and to facilitate understanding by those skilled in the art. Note that the drawings and the following explanation are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims.

[0030] The "ranges" disclosed herein are defined in the form of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Such defined ranges may or may not include the end values, and may be arbitrarily combined, i.e., any lower limit may be combined with any other upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, the ranges of 60-110 and 80-120 are also expected to be understood. Also, if the minimum range values ​​listed are 1 and 2, and the maximum range values ​​listed are 3, 4, and 5, then ranges such as 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5 are all expected. In this specification, unless otherwise stated, the numerical range "a-b" represents an abbreviation for any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed herein, and "0-5" is simply shorthand for combinations of these numbers. Also, when a parameter is expressed as an integer ≧2, this is equivalent to disclosing that the parameter is an integer, e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0031] Unless otherwise specified, all the embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution, and such technical solution is deemed to be included in the disclosure content of the present application.

[0032] Unless otherwise specified, all technical features and optional technical features of the present application can be combined with each other to form a new technical solution, and such a technical solution is deemed to be included in the disclosure content of the present application.

[0033] Unless otherwise stated, all steps in the present application may be performed in sequence or randomly, preferably in sequence. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed in sequence, but may also include steps (b) and (a) performed in sequence. For example, the method mentioned above may further include step (c), which means that step (c) may be added to the method in any order, for example, the method includes steps (a), (b) and (c), the method includes steps (a), (c) and (b), or the method includes steps (c), (a) and (b), etc.

[0034] Unless otherwise specified, the terms "comprise" and "contain" referred to in this application mean to be open-ended and may be closed-ended. For example, the terms "comprise" and "contain" mean that other components not listed may also be included or contained, and may only include or contain the listed components.

[0035] Unless otherwise specified, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." Specifically, any of the following conditions also satisfy the condition "A or B": A is true (or exists) and B is false (or does not exist), A is false (or does not exist) and B is true (or exists), or A and B are true (or exist).

[0036] A secondary battery, also called a rechargeable battery or storage battery, refers to a battery that can be used continuously after being discharged by recharging it to activate the active material. A secondary battery usually includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the charge and discharge process of a secondary battery, lithium ions are repeatedly inserted and removed between the positive electrode sheet and the negative electrode sheet. The separator is placed between the positive electrode sheet and the negative electrode sheet, and mainly serves to prevent short circuits between the positive and negative electrodes and allows the passage of ions. The electrolyte serves to conduct ions between the positive and negative electrode sheets.

[0037] When the secondary battery is charged, lithium ions escape preferentially from the surface of the positive electrode active material, and the lithium ions in the bulk phase of the positive electrode active material are timely captured on the surface. When the cobalt content of the positive electrode active material is relatively high, the lithium ions in the bulk phase of the positive electrode active material can be timely captured on the surface of the positive electrode active material. However, when the cobalt content of the positive electrode active material is relatively low, the lithium ions in the bulk phase of the positive electrode active material cannot be captured on the surface of the positive electrode active material in time, and the lithium ions on the surface also escape, resulting in excessive lithium desorption on the surface of the positive electrode active material, which affects the crystal structure of the positive electrode active material (for example, the irreversible distortion and increase in lattice defects of the positive electrode active material), and reduces the cycle performance of the secondary battery. Therefore, increasing the lithium ion diffusion rate of low-cobalt or cobalt-free positive electrode active materials has important practical significance.

[0038] As a result of extensive research, the inventors present a low-cobalt or cobalt-free secondary battery with significantly improved lithium ion diffusion rate and cycle performance.

[0039] A first aspect of the present embodiment provides a secondary battery including an electrolyte and a positive electrode sheet. The positive electrode sheet has a molecular formula of Li a Ni b Co c M1 d M2 e O f A g(M1 is one or two selected from Mn and Al, M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is one or more selected from 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.) It contains a layered material. The electrolytic solution contains lithium difluoro(oxalato)borate (LiDFOB), and based on the total mass of the electrolytic solution, the mass percentage content of the lithium difluoro(oxalato)borate is x%, and 0 < x ≦ 1.0. The secondary battery satisfies c + x / 10 ≧ 0.10.)

[0040] Researchers have worked to increase the lithium ion diffusion rate of low-cobalt or cobalt-free cathode active materials, but there has been no good solution so far.

[0041] The inventors of the present application unexpectedly discovered that after adding lithium difluoro(oxalato)borate to the electrolyte, lithium difluoro(oxalato)borate forms a low-resistance protective film on the surface of the positive electrode active material, and the B atom in lithium difluoro(oxalato)borate is easily bonded with the O atom in the positive electrode active material, thereby reducing the charge transfer resistance of the positive electrode active material and reducing the diffusion resistance of lithium ions in the Bulk phase of the positive electrode active material. Therefore, after adding lithium difluoro(oxalato)borate to the electrolyte, the low-cobalt or cobalt-free positive electrode active material has a greatly improved lithium ion diffusion rate, and the lithium ions in the bulk phase of the low-cobalt or cobalt-free positive electrode active material can be timely captured on the surface, and the crystal structure of the low-cobalt or cobalt-free positive electrode active material can be stabilized by avoiding excessive lithium desorption on the surface of the low-cobalt or cobalt-free positive electrode active material. Since the crystal structure of the low-cobalt or cobalt-free positive electrode active material of the present invention is more stable, the probability of problems such as irreversible distortion of the positive electrode active material or an increase in lattice defects due to excessive lithium desorption from the surface of the low-cobalt or cobalt-free positive electrode active material causing instability in the structural, chemical or electrochemical properties of the positive electrode active material can be significantly reduced.

[0042] Lithium difluoro(oxalato)borate itself does not resist oxidation, and if too much is added, it deteriorates the storage performance of the secondary battery, especially in a high-temperature environment. Therefore, the amount of lithium difluoro(oxalato)borate added must be appropriate.

[0043] After extensive research, the inventors have found that the cobalt content c of the low-cobalt or cobalt-free positive electrode active material is closely related to the mass percent content x% of lithium difluoro(oxalato)borate in the electrolyte. When the cobalt content c of the low-cobalt or cobalt-free positive electrode active material and the mass percent content x% of lithium difluoro(oxalato)borate in the electrolyte satisfy c+x / 10≧0.10, the B atom in the lithium difluoro(oxalato)borate can be fully bonded to the O atom in the positive electrode active material, which can better reduce the diffusion resistance of lithium ions in the Brueck phase of the low-cobalt or cobalt-free positive electrode active material, and can avoid excessive lithium desorption from the surface of the low-cobalt or cobalt-free positive electrode active material, which is favorable to better stabilizing the crystal structure of the low-cobalt or cobalt-free positive electrode active material and improving the lithium ion diffusion rate. Thus, the secondary battery can have good high temperature storage performance while having significantly improved cycle performance. In some embodiments, c+x / 10 can be ≧0.10, ≧0.11, ≧0.12, ≧0.13, ≧0.14, ≧0.15, ≧0.16, ≧0.17, ≧0.18, or ≧0.19.

[0044] When c+x / 10<0.10, the content of lithium difluoro(oxalato)borate in the electrolyte is insufficient to form a protective film with excellent performance and low resistance on the surface of the low-cobalt or cobalt-free positive electrode active material, and lithium difluoro(oxalato)borate cannot effectively reduce the charge transfer resistance of the low-cobalt or cobalt-free positive electrode active material, and cannot effectively reduce the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive electrode active material and inhibit excessive lithium desorption from the surface of the low-cobalt or cobalt-free positive electrode active material, making it difficult for the secondary battery to have significantly improved cycle performance.

[0045] In some embodiments, the molecular formula is Li a Ni b Co c M1 d M2e O f A g The layered material, wherein X is an integer of 1 to 5, can be optionally modified by doping with M2 cations, by doping with A anions, or by simultaneously doping with M2 cations and A anions, and the doped layered material has a more stable crystal structure and can further improve the electrochemical performance, such as cycle performance, rate performance, etc., of the secondary battery.

[0046] In some embodiments, A is selected from F. After being modified by F doping, Li a Ni b Co c M1 d M2 e O f The Ag structure is more stable, allowing the secondary battery to have better cycle performance and rate performance.

[0047] In some embodiments, M1 is selected from Mn.

[0048] In some embodiments, M1 is selected from Al.

[0049] In some embodiments, M1 is selected from the combination of Mn and Al. The molar ratio of Mn and Al is not particularly limited and may be selected according to actual needs.

[0050] In some embodiments, 0.50≦b<0.98. Optionally, 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.

[0051] In some embodiments, c=0.

[0052] In some embodiments, 0 < c < 0.1. Optionally, 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.

[0053] In some embodiments, 0 < d ≤ 0.45. Optionally, 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.

[0054] In some embodiments, e = 0.

[0055] In some embodiments, 0 < e ≤ 0.5. Optionally, 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.

[0056] In some embodiments, f = 2, g = 0.

[0057] In some embodiments, f = 0, g = 2.

[0058] In some embodiments, 0 < f < 2, 0 < g < 2, and f + g = 2.

[0059] As an example, the layered material with the molecular formula Li a Ni b Co c M1 d M2 e O f A g is 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 Mn0.3 O2, LiNi 0.63 Co 0.07 Mn 0.3 O2, LiNi 0.61 Co 0.09 Mn 0.3 O2.

[0060] Li a Ni b Co c M1 d M2 e O f A gmay be prepared according to a method commonly used in the art. An exemplary preparation method is as follows, that is, 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 calcined to obtain the above-mentioned. The calcination atmosphere may be an oxygen-containing atmosphere, for example, an air atmosphere or an oxygen atmosphere. The O2 concentration of the calcination atmosphere is, for example, 70% to 100%. The calcination temperature and the calcination time may be adjusted according to the actual situation. For example, the lithium source may include, but is not limited to, one or more of lithium oxide (Li2O), lithium phosphate (Li3PO4), lithium dihydrogen phosphate (LiH2PO4), lithium acetate (CH3COOLi), lithium hydride (LiOH), lithium carbonate (Li2CO3), and lithium nitrate (LiNO3). For example, the nickel source may include, but is not limited to, one or more of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate. By way of 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. By way of example, the M1 element precursor includes, but is not limited to, one or more of an oxide, a nitrate compound, a carbonate compound, a hydroxide, and an acetate compound of the M1 element. By way of example, the M2 element precursor includes, but is not limited to, one or more of an oxide, a nitrate compound, a carbonate compound, a hydroxide, and an acetate compound of the M2 element. By way of example, the A element precursor 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 hydrogen sulfate, ammonium hydrogen sulfite, ammonium sulfite, ammonium hydrogen sulfide, hydrogen sulfide, lithium sulfide, ammonium sulfide, and elemental sulfur.

[0061] In some embodiments, the electrolytic solution further contains vinyl fluorocarbonate (FEC). Based on the total mass of the electrolytic solution, the mass percentage content of the vinyl fluorocarbonate is y%, and 0 ≤ y ≤ 2.5. For example, y is 0, 0.10, 0.20, 0.50, 0.75, 1.0, 1.25, 1.50, 1.75, 2.0, 2.25, 2.50 or a range formed by any of the above numerical values. Optionally, 0 < y ≤ 2.5, 0 < y ≤ 2.25, 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.

[0062] In the case of a secondary battery, vinyl fluorocarbonate undergoes a reduction decomposition reaction at a relatively high potential and forms a solid electrolyte interface film (also abbreviated as SEI film) with a certain degree of flexibility on the surface of the negative electrode active material, and can suppress the reduction decomposition of the organic solvent at a relatively low potential and the embedding of the organic solvent into the negative electrode active material. Therefore, after adding vinyl fluorocarbonate to the electrolytic solution, the cycle performance of the secondary battery can be effectively improved. In addition, since vinyl fluorocarbonate can withstand high-voltage oxidation and is advantageous for matching with a high-voltage positive electrode active material, it contributes to the improvement of the energy density of the secondary battery.

[0063] In some embodiments, the mass percentage content x% of lithium difluoro(oxalato)borate and the mass percentage content y% of vinyl fluorocarbonate further satisfy 0.5 ≤ y / x ≤ 2.0. Optionally, 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.

[0064] After adding vinyl fluorocarbonate to the electrolyte, the cycle performance of the secondary battery can be effectively improved, but when vinyl fluorocarbonate is decomposed, HF is formed, which destroys the structural stability of the positive electrode active material, increases the gas generation amount of the secondary battery, and deteriorates the storage performance of the secondary battery. Lithium difluoro(oxalato)borate is a stabilizer for the positive electrode active material, and its B atom also has the function of interacting with the O atom on the surface of the positive electrode active material, thereby suppressing the destruction of the structure of the positive electrode active material caused by HF. The combined use of lithium difluoro(oxalato)borate and vinyl fluorocarbonate can fully exert the improving effect of vinyl fluorocarbonate on the cycle performance and energy density of the secondary battery. In addition, by appropriately controlling the relationship between the mass percent content x% of lithium difluoro(oxalato)borate and the mass percent content y% of vinyl fluorocarbonate to satisfy 0.5≦y / x≦2.0, the synergistic effect of lithium difluoro(oxalato)borate and vinyl fluorocarbonate can be fully exerted, not only not increasing the gas generation amount of the secondary battery, but also further improving the cycle performance and energy density of the secondary battery.

[0065] In some embodiments, the electrolyte further comprises a fluorosulfonylimide lithium salt. Optionally, the fluorosulfonylimide lithium salt has the molecular formula LiN(SO2R1)(SO2R2) (wherein R1 and R2 are each independently F or C). n F 2n+1 and n is an integer of 1 to 10.) As an example, the fluorosulfonylimide lithium salt includes one or two of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0066] Based on the total mass of the electrolyte, the mass percentage content of the lithium fluorosulfonylimide salt is z%, and 0 ≦ z ≦ 2.5. For example, z is 0, 0.10, 0.20, 0.50, 0.75, 1.0, 1.25, 1.50, 1.75, 2.0, 2.25, 2.50 or a range formed by any of the above numerical values. Optionally, 0 < z ≦ 2.5, 0 < z ≦ 2.25, 0 < z ≦ 2.0, 0 < z ≦ 1.75, 0 < z ≦ 1.50, 0 < z ≦ 1.25, 0 < z ≦ 1.0, 0 < z ≦ 0.75, or 0 < z ≦ 0.50.

[0067] The fluorosulfonylimide anion is a weakly coordinating anion centered on N that contains -F or -C having a strong electron-withdrawing property with a conjugated group. n F 2n+1 Since the anion charge is highly delocalized, the interaction force between the anion and lithium ions is weakened. Therefore, the lithium fluorosulfonylimide salt has a relatively low lattice energy, is easy to dissociate, can increase the ionic conductivity of the electrolyte, decrease the viscosity of the electrolyte, and improve the rate performance and low-temperature performance of the secondary battery. In addition, the lithium fluorosulfonylimide salt further has relatively 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 reaction between the negative electrode active material and the electrolyte. Therefore, after adding the lithium fluorosulfonylimide salt to the electrolyte, the rate performance and low-temperature performance of the secondary battery can be significantly improved.

[0068] In some embodiments, the weight percent content x% of lithium difluoro(oxalato)borate and the weight percent content z% of the lithium fluorosulfonylimide salt further satisfy 0.5≦x / z≦2.0, or optionally 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.

[0069] After adding fluorosulfonylimide lithium salt to the electrolyte, the rate performance and low temperature performance of the secondary battery can be improved, but fluorosulfonylimide lithium salt cannot withstand high pressure, corrodes the positive electrode current collector (e.g., aluminum foil) at a relatively high potential, increases the side reaction between the positive electrode active material and the electrolyte, and has a poor effect of forming a film on the surface of the positive electrode active material, which is likely to affect the cycle performance of the secondary battery. When lithium difluoro(oxalato)borate and fluorosulfonylimide lithium salt are used in combination, lithium difluoro(oxalato)borate can act as a stabilizer for the positive electrode active material to form a protective film with excellent performance and low resistance 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 combined use of lithium difluoro(oxalato)borate and fluorosulfonylimide lithium salt is advantageous in fully exerting the effect of fluorosulfonylimide lithium salt in improving the rate performance and low temperature performance of the secondary battery. In addition, by appropriately controlling the relationship between the mass percent content x% of lithium difluoro(oxalato)borate and the mass percent content z% of the lithium fluorosulfonylimide salt so as to satisfy 0.5≦x / z≦2.0, the synergistic effect of lithium difluoro(oxalato)borate and the lithium fluorosulfonylimide salt can be fully exerted, and not only the cycle performance of the secondary battery is not deteriorated, but also the rate performance and low temperature performance of the secondary battery can be further improved.

[0070] In some embodiments, the electrolyte further comprises simultaneously fluorovinyl carbonate and a fluorosulfonylimide lithium salt.

[0071] In some embodiments, the secondary battery further simultaneously satisfies 0.5≦y / x≦2.0 and 0.5≦x / z≦2.0. Furthermore, the secondary battery simultaneously 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 simultaneously has significantly improved cycle performance, storage performance, rate performance and low temperature performance.

[0072] Fluorovinyl carbonate can effectively improve the cycle performance of secondary batteries, fluorosulfonylimide lithium salt can improve the rate performance and low-temperature performance of secondary batteries, and difluoro(oxalato)lithium borate can form a protective film with excellent performance and low resistance on the surface of the positive electrode active material as a stabilizer of the positive electrode active material, greatly improve the lithium ion diffusion rate of low-cobalt or cobalt-free positive electrode active material, inhibit side reactions between the positive electrode active material and the electrolyte, and inhibit the destruction of the positive electrode active material structure by HF. Therefore, appropriately controlling the relationship between the contents of fluorovinyl carbonate, fluorosulfonylimide lithium salt, and difluoro(oxalato)lithium borate is favorable to exerting the synergistic effect between the three, and fully inhibiting the disadvantages of using each component alone.

[0073] The electrolyte of the secondary battery according to the present application is advantageous in matching with the high voltage positive electrode active material, and thus further improves 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 ≧4.1 V. When the charge capacity of the positive electrode sheet is 100% of the measured capacity of the negative electrode sheet, the positive electrode charge voltage is ≧4.2 V.

[0074] As an example, the positive electrode charging voltage is measured according to the following steps.

[0075] (1) Vinyl carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and LiPF6 was uniformly dissolved in the above solution to obtain an electrolyte solution, with a LiPF6 concentration of 1 mol / L. The negative electrode sheet was cut into a small disk with a unit area, and a metallic lithium sheet was used as the counter electrode and a polyethylene film was used as the separator, and the disk was assembled into a CR2430 button battery in an argon-protected glove box. After leaving the obtained button battery for 12 hours, it was discharged to 0.005 V at a constant current of 0.1 mA at 25°C, and then charged to 2 V at a constant current of 0.1 mA, and the charging capacity of the button battery was recorded as the capacity per unit area of ​​the negative electrode sheet.

[0076] (2) Vinyl carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and LiPF6 was uniformly dissolved in the above solution to obtain an electrolyte solution, with a LiPF6 concentration of 1 mol / L. The positive electrode sheet was cut into a small disk of unit area, a metallic lithium sheet was used as the counter electrode, and a polyethylene film was used as the separator, and the resulting CR2430 button battery was assembled in an argon-protected glove box. The resulting button battery was left for 12 hours, and then charged at a constant current of 0.1 mA at 25°C. The voltages corresponding to when the button battery charging capacity reached 90% and 100% of the capacity per unit area of ​​the negative electrode sheet obtained in step (1) were recorded as the positive electrode charging voltage.

[0077] In some embodiments, the secondary battery includes an electrolyte and a positive electrode sheet. The positive electrode sheet has a molecular formula of Li a Ni b Co c M1 d M2 e O f A g(M1 is one or two selected from Mn and Al, M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is one or more selected from 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.) It contains a layered material. The electrolytic solution contains lithium difluoro(oxalato)borate and vinyl fluorocarbonate. Based on the total mass of the electrolytic solution, the mass percentage content of the lithium difluoro(oxalato)borate is x%, and 0 < x ≦ 1.0, and the mass percentage content of the vinyl fluorocarbonate is y%, and 0 ≦ y ≦ 2.5. The secondary battery satisfies c + x / 10 ≧ 0.10 and 0.5 ≦ y / x ≦ 2.0.

[0078] In some embodiments, the secondary battery includes an electrolytic solution and a positive electrode sheet. The positive electrode sheet has a molecular formula of Li a Ni b Co c M1 d M2 e O f A g (M1 is one or two selected from Mn and Al, M2 is one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is one or more selected from 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.) It contains a layered material. The electrolytic solution contains lithium difluoro(oxalato)borate and lithium fluorosulfonylimide salt. Based on the total mass of the electrolytic solution, the mass percentage content of the lithium difluoro(oxalato)borate is x%, and 0 < x ≦ 1.0, and the mass percentage content of the lithium fluorosulfonylimide salt is z%, and 0 ≦ z ≦ 2.5. The secondary battery satisfies c + x / 10 ≧ 0.10 and 0.5 ≦ x / z ≦ 2.0.

[0079] In some embodiments, the secondary battery includes an electrolytic solution and a positive electrode sheet. The positive electrode sheet has a molecular formula of Li a Ni b Co c M1 d M2 e O f A g (where M1 is one or two selected from Mn and Al, M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W, A is one or more selected from 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, and f + g = 2). The electrolytic solution includes lithium difluoro(oxalato)borate, vinyl fluorocarbonate, and a lithium fluorosulfonylimide salt, and the lithium fluorosulfonylimide salt includes one or two of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide. Based on the total mass of the electrolytic solution, the mass percentage content of the lithium difluoro(oxalato)borate is x%, and 0 < x ≦ 1.0, the mass percentage content of the vinyl fluorocarbonate is y%, and 0 ≦ y ≦ 2.5, and the mass percentage content of the lithium fluorosulfonylimide salt is z%, and 0 ≦ z ≦ 2.5. The secondary battery satisfies c + x / 10 ≧ 0.10, 0.5 ≦ y / x ≦ 2.0, and 0.5 ≦ x / z ≦ 2.0.

[0080] In some embodiments, the secondary battery includes an electrolytic solution and a positive electrode sheet. The positive electrode sheet has a molecular formula of Li a Ni b Co c M1 d M2 e O f A g(M1 is one or two selected from Mn and Al, M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te and W, A is one or more selected from 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.) It contains a layered material. The electrolyte contains lithium difluoro(oxalato)borate, vinyl fluorocarbonate and lithium fluorosulfonylimide salt, and the lithium fluorosulfonylimide salt contains one or two of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide. Based on the total mass of the electrolyte, the mass percentage content of the lithium difluoro(oxalato)borate is x%, and 0 < x ≦ 1.0, the mass percentage content of the vinyl fluorocarbonate is y%, and 0 ≦ y ≦ 2.5, and the mass percentage content of the lithium fluorosulfonylimide salt is z%, and 0 ≦ z ≦ 2.5. 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.)

[0081] In some embodiments, the electrolyte further contains an electrolyte salt and an organic solvent. The types of the electrolyte salt and the organic solvent are not specifically limited and can be selected according to actual needs.

[0082] For example, the electrolyte salt may contain one or more of lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium perchlorate LiClO4, lithium hexafluoroarsenate LiAsF6, lithium trifluoromethanesulfonate LiTFS, lithium bis(oxalato)borate LiBOB, lithium difluorophosphate LiPO2F2, lithium difluoro(bisoxalato)phosphate LiDFOP and lithium tetrafluoro(oxalato)phosphate LiTFOP. Optionally, the electrolyte salt contains LiPF6.

[0083] By way of example, the organic solvent may include one or more of vinyl carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl ester 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), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0084] In the embodiment of the secondary battery of the present application, the electrolyte does not exclude components other than the above components. In some examples, the electrolyte may further include other additives, such as, for example, an additive for improving the overcharge performance of the battery, an additive for improving the high temperature performance of the battery, and an additive for improving the low temperature output performance of the battery.

[0085] The electrolyte can be prepared by a method commonly used in this field. For example, an electrolyte can be obtained by uniformly mixing an organic solvent, an electrolyte salt, lithium difluoro(oxalato)borate, any vinyl fluorocarbonate, and any lithium fluorosulfonyl imide salt. The order of addition of each material is not particularly limited. For example, an electrolyte salt, lithium difluoro(oxalato)borate, any vinyl fluorocarbonate, and any lithium fluorosulfonyl imide salt are added to an organic solvent and uniformly mixed to obtain an electrolyte, or an electrolyte salt is first added to an organic solvent, and then lithium difluoro(oxalato)borate, any vinyl fluorocarbonate, and any lithium fluorosulfonyl imide salt are uniformly mixed in the organic solvent to obtain an electrolyte.

[0086] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector and including 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.

[0087] The positive electrode current collector may be a metal foil sheet or a composite current collector. An example of the metal foil sheet may be aluminum foil. The composite current collector may include a polymer base layer and a metal material layer formed on at least one surface of the polymer base layer. For example, the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. For example, the polymer base layer may include polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and the like.

[0088] The positive electrode membrane layer typically includes a positive electrode active material, an optional binder, and an optional conductive agent. The positive electrode membrane layer is typically formed by applying a positive electrode slurry to a positive electrode current collector, drying, and cold rolling. The positive electrode slurry is typically 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 uniformly. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP). For example, the binder used in the positive electrode membrane 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. By way of example, the conductive agent used in 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.

[0089] In some embodiments, the positive electrode active material has the molecular formula Li a Ni b Co c M1 d M2 e O f A g The present invention includes a layered material in which

[0090] In some embodiments, Li a Ni b Co c M1 d M2 e O f A g The surface of the positive electrode active material may have a coating layer such as a carbon coating layer. The carbon coating layer is advantageous in stabilizing the surface of the positive electrode active material, further reducing the charge transfer resistance of the positive electrode active material, and reducing the diffusion resistance of lithium ions in the bulk phase of the positive electrode active material. Optionally, the carbon coating layer is amorphous carbon, such as soft carbon or hard carbon.

[0091] In some embodiments, the positive electrode active material includes Li a Ni b Co c M1 d M2 e O f A g Other components other than the above are not excluded, for example, the positive electrode active material may further include one or more of lithium-containing phosphates with an olivine structure and their modified compounds. For example, the lithium-containing phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, lithium iron phosphate and carbon composite material, lithium manganese phosphate, lithium manganese phosphate and carbon composite material, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composite material, and their modified compounds. The present application is not limited to these materials, and other known traditional materials used as secondary battery positive electrode active materials may be used. These positive electrode active materials may be used alone or in combination of two or more.

[0092] In some embodiments, the molecular formula is Li a Ni b Co c M1 d M2 e O f A g The mass percent content of the layered material is 80% to 99%. For example, the molecular formula is Li a Ni b Co c M1 d M2 e O f A g The mass percent content of the layered material having the molecular formula Li is 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or a range formed by any of the preceding numerical values. a Ni b Co c M1 d M2 e O f A g The mass percent content of the layered material is 85%-99%, 90%-99%, 95%-99%, 80%-98%, 85%-98%, 90%-98%, 95%-98%, 80%-97%, 85%-97%, 90%-97%, or 95%-97%.

[0093] The positive electrode sheet does not exclude any additional functional layers other than the positive electrode film layer. For example, in some embodiments, the positive electrode sheet according to the present application further includes a conductive undercoat layer (e.g., made of a conductive agent and a binder) sandwiched between the positive electrode collector and the positive electrode film layer and provided on the positive electrode collector surface. In other embodiments, the positive electrode sheet according to the present application further includes a protective layer covering the surface of the positive electrode film layer.

[0094] The secondary battery according to the present application further includes a negative electrode sheet. In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two surfaces facing each other in the thickness direction of the negative electrode current collector, and the negative electrode film layer is disposed on either one or both of the two facing surfaces of the negative electrode current collector.

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

[0096] The negative electrode membrane layer typically includes a negative electrode active material, an optional binder, an optional conductive agent, and any other optional auxiliary agents. The negative electrode membrane layer is typically formed by applying a negative electrode slurry to a negative electrode current collector, drying, and cold rolling. The negative electrode slurry is typically formed by dispersing the negative electrode active material, the optional conductive agent, the optional binder, and any other optional auxiliary agents in a solvent and uniformly stirring the mixture. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP) or deionized water. As an example, the binder used in the negative electrode membrane 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 chitosan methyl carboxylate (CMCS). For example, the conductive agent used in 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 auxiliary agents may include thickeners (e.g., sodium carboxymethylcellulose CMC-Na), PTC thermistor materials, etc.

[0097] The negative electrode active material may adopt a negative electrode active material used in a secondary battery known in the art. For example, the negative electrode active material may include one or more of natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based material, tin-based material, 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. 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 known traditional materials used as negative electrode active materials in secondary batteries may be used. These negative electrode active materials may be used alone or in combination of two or more.

[0098] The negative electrode sheet does not exclude additional functional layers other than the negative electrode film layer. For example, in some embodiments, the negative electrode sheet according to the present application further includes a conductive undercoat layer (e.g., made of a conductive agent and a binder) sandwiched between the negative electrode collector and the negative electrode film layer and provided on the surface of the negative electrode collector. In other embodiments, the negative electrode sheet according to the present application further includes a protective layer covering the surface of the negative electrode film layer.

[0099] The secondary battery according to the present application further includes a separator. The separator is provided between the positive electrode sheet and the negative electrode sheet and serves to isolate the positive electrode sheet and the negative electrode sheet. In the present application, the type of the separator is not particularly limited, and any known separator having a porous structure and excellent chemical and mechanical stability may be selected and used.

[0100] In some embodiments, the separator may be made of one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a monolayer film, but may also be a multilayer composite film. When the separator is a multilayer composite film, the materials of each layer may be the same or different.

[0101] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet can be formed into an electrode assembly by a winding process or a stacking process.

[0102] In some embodiments, the secondary battery may include an exterior. The exterior is used to seal the electrode assembly and the electrolyte. The exterior of the secondary battery may be a hard case, such as a hard plastic case, an aluminum case, or a steel case. The exterior of the secondary battery may be a soft bag, such as a pouch-type soft bag. The material of the soft bag may be one or more of plastics, such as polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0103] In the present application, the shape of the secondary battery is not particularly limited and may be cylindrical, square, or any other arbitrary shape. For example, FIG. 1 shows a secondary battery 5 having a square structure as an example.

[0104] In some embodiments, as shown in FIG. 2, the exterior may include a case 51 and a cover plate 53. The case 51 includes a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates may enclose to form an accommodation cavity. The case 51 has an opening communicating with the accommodation cavity, and the cover plate 53 is used to cover the opening so as to close the accommodation cavity. The positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly 52 by a winding process or a lamination process. The electrode assembly 52 is sealed within the accommodation cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 included in the secondary battery 5 may be one or more and can be adjusted according to actual needs.

[0105] Method for manufacturing a secondary battery A second aspect of the embodiments of the present application provides a method for manufacturing a secondary battery including at least step 1 and step 2.

[0106] Step 1: Assembling a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte into a secondary battery The positive electrode sheet has a molecular formula of Li a Ni b Co c M1 d M2 e O f A g (where M1 is one or two selected from Mn and Al, M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W, A is one or more selected from 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, and f + g = 2.) and includes a layered material.

[0107] The electrolytic solution contains lithium difluoro(oxalato)borate, an arbitrary vinyl fluorocarbonate, and an arbitrary lithium fluorosulfonylimide salt. Based on the total mass of the electrolytic solution, the mass percentage content of the lithium difluoro(oxalato)borate is x%, and 0 < x ≤ 1.0. Based on the total mass of the electrolytic solution, the mass percentage content of the vinyl fluorocarbonate is y%, and 0 ≤ y ≤ 2.5. Based on the total mass of the electrolytic solution, the mass percentage content of the lithium fluorosulfonylimide salt is z%, and 0 ≤ z ≤ 2.5.

[0108] Step 2: Screening the secondary battery obtained in Step 1 to obtain a secondary battery that satisfies c + x / 10 ≥ 0.10 When the secondary battery satisfies c + x / 10 ≥ 0.10, the B atom in the lithium difluoro(oxalato)borate is sufficiently bonded to the O atom in the positive electrode active material, which can better reduce the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive electrode active material, and avoid excessive lithium desorption on the surface of the low-cobalt or cobalt-free positive electrode active material, thereby being beneficial to better stabilizing the crystal structure of the low-cobalt or cobalt-free positive electrode active material and improving the lithium ion diffusion rate. Therefore, the secondary batteries obtained by the manufacturing method according to the present application can all have significantly improved cycle performance and good high-temperature storage performance.

[0109] In some embodiments, the molecular formula of the lithium fluorosulfonylimide salt is LiN(SO2R1)(SO2R2) (where R1 and R2 each independently represent F or C n F 2n+1 and n is an integer from 1 to 10). Optionally, the lithium fluorosulfonylimide salt contains one or two of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0110] In some embodiments, the method further includes screening secondary batteries that satisfy 0.5≦y / x≦2.0 from the secondary batteries obtained in step 2. In this case, the manufactured secondary battery has good storage performance and significantly improved cycle performance and energy density at the same time.

[0111] In some embodiments, the method further includes screening secondary batteries that satisfy 0.5≦x / z≦2.0 from the secondary batteries obtained in step 2. In this case, the manufactured secondary battery has good storage performance and significantly improved cycle performance, rate performance and low temperature performance at the same time.

[0112] In some embodiments, the method further includes a step of screening 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 manufactured secondary battery simultaneously has good storage performance, and significantly improved cycle performance, rate performance and low temperature performance.

[0113] In some embodiments, the method further includes a step of screening 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 manufactured secondary battery simultaneously has good storage performance, and significantly improved cycle performance, rate performance, and low temperature performance.

[0114] Battery module and battery pack In some embodiments of the present application, the secondary battery according to the present application may be assembled into a battery module, and the number of secondary batteries included in the battery module may be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0115] Fig. 3 is a schematic diagram of an example battery module 4. As shown in Fig. 3, in the battery module 4, the multiple secondary batteries 5 may be arranged in order along the longitudinal direction of the battery module 4. Of course, they may be arranged in any other manner. Furthermore, the multiple secondary batteries 5 may be fixed by fasteners.

[0116] Optionally, the battery module 4 may further include a housing having an accommodation space for accommodating a plurality of secondary batteries 5 .

[0117] In some embodiments, the battery modules may be assembled into a battery pack, and the number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0118] 4 and 5 are schematic diagrams of an example battery pack 1. As shown in FIG. 4 and FIG. 5, the battery pack 1 may include a battery housing and a plurality of battery modules 4 disposed in the battery housing. The battery housing includes an upper housing 2 and a lower housing 3, and the upper housing 2 is capable of being covered by the lower housing 3 and forms an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in any manner within the battery housing.

[0119] Electrical equipment The embodiment of the present application further provides an electric device including at least one of the secondary battery, battery module, or battery pack according to the present application. The secondary battery, battery module, or battery pack may be used as a power source for the electric device, or may be used as an energy storage unit for the electric device. The electric device may be, but is not limited to, a mobile device (e.g., a mobile phone, a laptop, etc.), an electric vehicle (e.g., 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.), a train, a ship, a satellite, an energy storage system, etc.

[0120] The electric device can select a secondary battery, a battery module or a battery pack according to its usage needs.

[0121] 6 is a schematic diagram of an example of an electric device, such as a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. A battery pack or battery module can be used to meet the high power and high energy density needs of the electric device.

[0122] Other examples of the electric device may be a mobile phone, a tablet, a notebook computer, etc. Such electric devices are usually required to be thin and can use a secondary battery as a power source.

[0123] Working Example The following examples further illustrate the contents disclosed in this application. These examples are used only to illustrate the present application, and various modifications and variations within the scope of the contents disclosed in this application are obvious to those skilled in the art. Unless otherwise specified, the parts, percentages and ratios described in the following examples are all based on weight, and all reagents used in the examples are commercially available or can be synthesized by conventional methods, and can be used as they are without further treatment, and the equipment used in the examples are all commercially available.

[0124] Example 1 Manufacture of positive electrode sheets Cathode active material LiNi 0.65 Co 0.05 Mn 0.3 O2, carbon black as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 97.5:1.4:1.1 with sufficient stirring in an appropriate amount of NMP solvent to form a uniform positive electrode slurry. The positive electrode slurry was then evenly applied to the surface of aluminum foil as a positive electrode current collector, dried, and cold pressed to obtain a positive electrode sheet.

[0125] Manufacture of negative electrode sheets The negative electrode active material graphite, the binder styrene butadiene rubber (SBR), the thickener sodium carboxymethylcellulose (CMC-Na), and the conductive agent carbon black (Super P) were mixed in a weight ratio of 96.2:1.8:1.2:0.8 with sufficient stirring in an appropriate amount of deionized water as a solvent to form a uniform negative electrode slurry. The negative electrode slurry was then applied to the surface of copper foil as a negative electrode current collector, dried, and cold-rolled to obtain a negative electrode sheet.

[0126] Separator A porous polyethylene (PE) membrane was used as the separator.

[0127] Electrolyte production Vinyl carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 and lithium difluoro(oxalato)borate were uniformly dissolved in the organic solvent to obtain an electrolyte, in which the concentration of LiPF6 was 1 mol / L and the mass percent content of lithium difluoro(oxalato)borate was 0.5% based on the total mass of the electrolyte.

[0128] Secondary battery manufacturing The positive electrode sheet, the separator, and the negative electrode sheet were stacked in this order and wound up to obtain an electrode assembly. The electrode assembly was then placed in an outer casing, the above-mentioned electrolyte solution was added, and the secondary battery was obtained through processes such as sealing, leaving, chemical formation, and aging.

[0129] Examples 2 to 25 and Comparative Examples 1 to 3 The manufacturing method of the secondary battery was similar to that of Example 1, except that the type of positive electrode active material and the manufacturing parameters of the electrolyte were adjusted. The specific parameters are shown in Table 1. Vinyl fluorocarbonate was further added to the electrolyte of Examples 8 to 13, lithium bis(fluorosulfonyl)imide was further added to the electrolyte of Examples 14 to 19, and vinyl fluorocarbonate and lithium bis(fluorosulfonyl)imide were further added simultaneously to the electrolyte of Examples 20 to 25. In Table 1, x% is the mass percent content of lithium difluoro(oxalato)borate based on the total mass of the electrolyte, y% is the mass percent content of vinyl fluorocarbonate based on the total mass of the electrolyte, and z% is the mass percent content of lithium bis(fluorosulfonyl)imide based on the total mass of the electrolyte.

[0130] [Table 1-1]

[0131] [Table 1-2]

[0132] Measuring part (1) Measurement of secondary battery room temperature cycle performance At 25°C, the secondary battery was charged to 4.3V at a constant current of 1C, and then charged to a current of 0.05C at a constant voltage, at which point the secondary battery was fully charged and the charge capacity at this point was recorded and defined as the charge capacity of the first cycle, and after leaving the secondary battery for 5 minutes, it was discharged to 2.8V at a constant current of 1C, which was one cycle of charge and discharge process, and the discharge capacity at this point was recorded and defined as the discharge capacity of the first cycle. The secondary battery was subjected to cycle charge and discharge measurements using the method described above, and the discharge capacity after each cycle was recorded.

[0133] Capacity retention (%) after 600 cycles of the secondary battery at 25°C = discharge capacity after 600 cycles / discharge capacity of the first cycle × 100%.

[0134] (2) High-temperature cycle performance measurement of secondary batteries The secondary battery was charged at 45°C at a constant current of 1C up to 4.3V, and then continuously charged at a constant voltage down to a current of 0.05C, at which point the secondary battery was in a fully charged state, and the charge capacity at this point was recorded and designated as the charge capacity of the first cycle, and the secondary battery was left for 5 minutes and then discharged at a constant current of 1C up to 2.8V, which was one cycle of charge and discharge process, and the discharge capacity at this point was recorded and designated as the discharge capacity of the first cycle. The secondary battery was subjected to cycle charge and discharge measurements using the method described above, and the discharge capacity after each cycle was recorded.

[0135] Capacity retention (%) after 600 cycles of the secondary battery at 45°C = discharge capacity after 600 cycles / discharge capacity of the first cycle × 100%.

[0136] (3) Secondary battery initial DC internal resistance measurement At 25°C, the secondary battery was charged to 4.3V at a constant current of 1C, and then charged to a current of 0.05C at a constant voltage. At this time, the secondary battery was in a fully charged state. The secondary battery was discharged at a constant current of 0.5C and adjusted to 50% SOC. The voltage of the secondary battery at this time was recorded as U1. The secondary battery was discharged for 30 seconds at a constant current of 4C, sampling was performed at 0.1 second intervals, and the end-of-discharge voltage was recorded as U2.

[0137] The discharge DC internal resistance of the secondary battery at 50% SOC was defined as the initial DC internal resistance of the secondary battery, and the initial DC internal resistance of the secondary battery (Ω) = (U1-U2) / 4C.

[0138] (4) High-temperature storage performance measurement of secondary batteries The secondary battery was charged at 60°C at a constant current of 1C up to 4.3V, and then continuously charged at a constant voltage down to a current of 0.05C. At this time, the volume of the secondary battery was measured by a drainage method and recorded as V0. The secondary battery was placed in a thermostat at 60°C and stored for 30 days, after which it was taken out. At this time, the volume of the secondary battery was measured by a drainage method and recorded as V1.

[0139] Volume expansion rate (%) after storing a secondary battery at 60°C for 30 days = [(V1-V0) / V0] × 100%.

[0140] Table 2 shows the performance measurement results of Examples 1 to 25 and Comparative Examples 1 to 3.

[0141] [Table 2]

[0142] As can be seen from the measurement results in Table 2, when lithium difluoro(oxalato)borate 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 content x% of lithium difluoro(oxalato)borate in the electrolyte satisfy c+x / 10≧0.10, the secondary battery has significantly improved cycle performance and good high-temperature storage performance. In Comparative Examples 1 to 3, lithium difluoro(oxalato)borate is not added to the electrolyte, or the content of lithium difluoro(oxalato)borate added is insufficient. In this case, lithium difluoro(oxalato)borate cannot form a protective film with excellent performance and low resistance on the surface of the low-cobalt or cobalt-free positive electrode active material, and lithium difluoro(oxalato)borate cannot effectively reduce the charge transfer resistance of the low-cobalt or cobalt-free positive electrode active material, and cannot effectively reduce the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive electrode active material and suppress excessive lithium desorption from the surface of the low-cobalt or cobalt-free positive electrode active material, making it difficult for the secondary battery to have significantly improved cycle performance.

[0143] In addition, as can be seen from the measurement results in Table 2, by further appropriately controlling the relationship between the mass percent content x% of lithium difluoro(oxalato)borate and the mass percent content y% of vinyl fluorocarbonate so as to satisfy 0.5≦y / x≦2.0, the synergistic effect of lithium difluoro(oxalato)borate and vinyl fluorocarbonate can be fully exerted, and not only the amount of gas generated by the secondary battery is not increased, but also the cycle performance of the secondary battery can be further improved.

[0144] In addition, as can be seen from the measurement results in Table 2, by further appropriately controlling the relationship between the mass percent content x% of lithium difluoro(oxalato)borate and the mass percent content z% of lithium bis(fluorosulfonyl)imide to satisfy 0.5≦x / z≦2.0, the synergistic effect of lithium difluoro(oxalato)borate and lithium bis(fluorosulfonyl)imide can be fully exerted, and not only can the cycle performance of the secondary battery not be deteriorated, but the initial DC internal resistance of the secondary battery can be further reduced and the rate performance of the secondary battery can be improved.

[0145] In addition, as can be seen from the measurement results in Table 2, the relationship between the mass percent content x% of lithium difluoro(oxalato)borate, the mass percent content y% of vinyl fluorocarbonate, and the mass percent content z% of lithium bis(fluorosulfonyl)imide was further appropriately controlled to satisfy 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 good high-temperature storage performance, and further has greatly improved cycle performance and rate performance.

[0146] The present application is not limited to the above-mentioned embodiment. The above-mentioned embodiment is merely an example, and any embodiment that has substantially the same configuration as the technical idea and has the same effect within the scope of the configuration of the present application is included in the technical scope of the present application. In addition, various modifications that a person skilled in the art can conceive of to the embodiment and other forms constructed by combining some of the components in the embodiment are also included in the scope of the present application, as long as they do not deviate from the gist of the present application.

Claims

1. A secondary battery including an electrolyte and a positive electrode sheet, The positive electrode sheet has a molecular formula of Li ａ Ni ｂ Co ｃ M1 ｄ M2 ｅ O ｆ A ｇ (M1 is one or two selected from Mn and Al; M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W; A is one or more selected from F, N, P, and S; and 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, and f+g=2), the electrolyte comprises lithium difluoro(oxalato)borate, the mass percent content of the lithium difluoro(oxalato)borate being x %, based on the total mass of the electrolyte, and 0<x≦1.0; c + x / 10 ≧ 0.10 is satisfied; Secondary battery.

2. The electrolyte solution further contains one or more of the following: vinyl fluorocarbonate, lithium fluorosulfonylimide salt; Optionally, the molecular formula of the lithium fluorosulfonylimide salt is LiN(SO ２ R １ ) (SO ２ R ２ ) (R １ , R ２ are each independently F or C ｎ F ２ｎ＋１ and n is an integer from 1 to 10; Optionally, the fluorosulfonylimide lithium salt comprises one or two of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide; The secondary battery according to claim 1 .

3. and / or the weight percent content of the vinyl fluorocarbonate is y %, based on the total weight of the electrolyte, and 0≦y≦2.5, optionally 0<y≦2.0; and / or Based on the total mass of the electrolyte, the mass percent content of the fluorosulfonylimide lithium salt is z%, and 0≦z≦2.5, optionally 0<z≦2.0; The secondary battery according to claim 2 .

4. The secondary battery is (1) 0.5≦y / x≦2.0, and optionally 0.5≦y / x≦1.0; (2) 0.5≦x / z≦2.0, and optionally 0.5≦x / z≦1.5; 4. The secondary battery according to claim 3, further satisfying one or two of the following relationship formulas:

5. The secondary battery according to claim 4 , further satisfying 0.5≦y / x≦2.0, 0.5≦x / z≦2.0, and 0.25≦y / z≦2.0 simultaneously.

6. Step 1: Assembling a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte into a secondary battery, The positive electrode sheet has a molecular formula of Li ａ Ni ｂ Co ｃ M1 ｄ M2 ｅ O ｆ A ｇ (M1 is one or two selected from Mn and Al; M2 is one or more selected from Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W; A is one or more selected from F, N, P, and S; and 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, and f+g=2), The electrolyte comprises lithium difluoro(oxalato)borate, an optional fluorovinyl carbonate, and an optional lithium fluorosulfonylimide salt; Based on the total mass of the electrolyte solution, the mass percent content of the lithium difluoro(oxalato)borate is x%, and 0<x≦1.0, based on the total mass of the electrolyte solution, the mass percent content of the vinyl fluorocarbonate is y%, and 0≦y≦2.5, based on the total mass of the electrolyte solution, the mass percent content of the lithium fluorosulfonylimide salt is z%, and 0≦z≦2.5; Step 2: screening the secondary batteries obtained in step 1 for those that satisfy c+x / 10≧0.10; A method for manufacturing a secondary battery comprising at least the steps of:

7. The method according to claim 6, further comprising the step of screening the secondary batteries obtained in step 2 for secondary batteries that satisfy 0.5≦y / x≦2.

0.

8. The method according to claim 6, further comprising the step of screening the secondary batteries obtained in step 2 for secondary batteries that satisfy 0.5≦x / z≦2.

0.

9. The method according to claim 6, further comprising the step of screening the secondary batteries obtained in step 2 for secondary batteries which simultaneously satisfy 0.5≦y / x≦2.0 and 0.5≦x / z≦2.

0.

10. The method according to claim 6, further comprising the step of screening the secondary batteries obtained in step 2 for those which simultaneously satisfy 0.5≦y / x≦2.0, 0.5≦x / z≦2.0 and 0.25≦y / z≦2.

0.

11. A battery module comprising one of the secondary batteries according to any one of claims 1 to 5 and the secondary batteries obtained by the method according to any one of claims 6 to 10.

12. A battery pack comprising one of the following: a secondary battery according to any one of claims 1 to 5; a secondary battery obtained by the method according to any one of claims 6 to 10; and a battery module according to claim 11.

13. An electric device comprising at least one of the following: the secondary battery according to any one of claims 1 to 5; a secondary battery obtained by the method according to any one of claims 6 to 10; the battery module according to claim 11; or the battery pack according to claim 12.

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