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

Abstract

The application discloses a secondary battery, a method for preparing the secondary battery, a battery module, a battery pack and a power utilization device. The secondary battery comprises an electrolyte and a positive electrode sheet, the positive electrode sheet comprises a layered material with 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, g are respectively defined herein, the electrolyte comprises lithium difluoro(oxalato)borate, the mass percentage of the lithium difluoro(oxalato)borate in the total mass of the electrolyte is x% and 0 The application can improve the lithium ion diffusion rate of low-cobalt or cobalt-free positive electrode active material and improve the cycle performance of the secondary battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries, and particularly relates to a secondary battery, a method for preparing a secondary battery, a battery module, a battery pack and a power utilization device. BACKGROUND

[0002] The secondary battery charges and discharges by means of lithium ions reciprocatingly deintercalating between the positive electrode and the negative electrode, and has the outstanding characteristics of high energy density, long cycle life, no pollution and no memory effect. Therefore, as a clean energy, the secondary battery has gradually been popularized from electronic products to energy storage power systems such as hydroelectric, thermal, wind and solar power stations, and to many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc. Among them, cobalt is an important element of the positive electrode active material of the secondary battery. However, cobalt is less in the earth's crust, difficult to mine and expensive in price, so low-cobalt or cobalt-free has become an inevitable development trend of the positive electrode active material. However, cobalt contributes greatly to the lithium ion diffusion rate of the positive electrode active material, and low-cobalt or cobalt-free will reduce the lithium ion diffusion rate of the positive electrode active material, affecting the cycle life of the secondary battery. SUMMARY

[0003] The application aims to provide a secondary battery, a method for preparing a secondary battery, a battery module, a battery pack and a power utilization device, and aims to improve the lithium ion diffusion rate of the low-cobalt or cobalt-free positive electrode active material and improve the cycle performance of the secondary battery.

[0004] The first aspect of the application provides a secondary battery, comprising an electrolyte and a positive electrode sheet. The positive electrode sheet comprises a layered material with a molecular formula of Li a Ni b Co c M1 d M2 e O f A g , wherein M1 is selected from one or both 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 The electrolyte comprises lithium difluoro(oxalato)borate, and the mass percentage of the lithium difluoro(oxalato)borate in the total mass of the electrolyte is x% and 0

[0005] The inventors have found through a large number of studies that the cobalt content c of the low-cobalt or cobalt-free positive electrode active material is closely related to the mass percentage content x% of lithium difluorooxalate borate in the electrolyte. When the cobalt content c of the low-cobalt or cobalt-free positive electrode active material and the mass percentage content x% of lithium difluorooxalate borate in the electrolyte satisfy c+x / 10≥0.10, the B atoms in lithium difluorooxalate borate can be fully combined with the O atoms in the positive electrode active material, better reducing the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive electrode active material and avoiding over-delithiation of the surface of the low-cobalt or cobalt-free positive electrode active material, thereby favorably 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 significantly improved cycle performance while also having good high-temperature storage performance.

[0006] When c+x / 10<0.10, the content of lithium difluorooxalate borate in the electrolyte is insufficient to form a low-impedance protective film with good performance on the surface of the low-cobalt or cobalt-free positive electrode active material, and lithium difluorooxalate borate cannot effectively reduce the charge transfer resistance of the low-cobalt or cobalt-free positive electrode active material, 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 cannot inhibit over-delithiation of the surface of the low-cobalt or cobalt-free positive electrode active material. Therefore, the secondary battery is difficult to have significantly improved cycle performance.

[0007] In any embodiment of the present application, the electrolyte further comprises one or more of fluoroethylene carbonate and lithium fluorosulfonylimide salt.

[0008] Optionally, the lithium fluorosulfonylimide salt has a molecular formula of LiN(SO2R1)(SO2R2), and R1 and R2 each independently represent F or C n F 2n+1 n is an integer of 1-10.

[0009] Optionally, the lithium fluorosulfonylimide salt comprises one or both of lithium bisfluorosulfonylimide and lithium bistrifluoromethylsulfonylimide.

[0010] In any embodiment of the present application, the mass percentage content of the fluoroethylene carbonate is y% and 0≤y≤2.5 based on the total mass of the electrolyte. Optionally, 0

[0011] In any embodiment of the present application, the mass percentage content of the lithium fluorosulfonylimide salt is z% and 0≤z≤2.5 based on the total mass of the electrolyte. Optionally, 0

[0012] The addition of fluoroethylene carbonate in the electrolyte can effectively improve the cycle performance of the secondary battery; the fluoroethylene carbonate is resistant to high-pressure oxidation, which is conducive to matching high-voltage positive active materials, thereby being conducive to improving the energy density of the secondary battery. The addition of lithium fluorosulfonylimide salt in the electrolyte can significantly improve the rate performance and low-temperature performance of the secondary battery.

[0013] In any embodiment of the present application, the secondary battery also satisfies 0.5≤y / x≤2.0. Optionally, 0.5≤y / x≤1.0. At this time, the synergistic effect of lithium difluorophosphate and fluoroethylene carbonate can be fully exerted, which not only does not increase the gas production of the secondary battery, but also further improves the cycle performance and energy density of the secondary battery.

[0014] In any embodiment of the present application, the secondary battery also satisfies 0.5≤x / z≤2.0. Optionally, 0.5≤x / z≤1.5. At this time, the synergistic effect of lithium difluorophosphate and lithium fluorosulfonylimide salt can be fully exerted, which not only does not deteriorate the cycle performance of the secondary battery, but also further improves the rate performance and low-temperature performance of the secondary battery.

[0015] In any embodiment of the present application, the secondary battery also simultaneously satisfies 0.5≤y / x≤2.0 and 0.5≤x / z≤2.0. Optionally, the secondary battery also simultaneously satisfies 0.5≤y / x≤2.0, 0.5≤x / z≤2.0 and 0.25≤y / z≤2.0. At this time, the secondary battery simultaneously has significantly improved cycle performance, storage performance, rate performance and low-temperature performance.

[0016] The second aspect of the present application provides a method for preparing a secondary battery, at least comprising the steps of:

[0017] Step 1, assembling a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte into a secondary battery, wherein the positive electrode sheet comprises a molecular formula of Li a Ni b Co c M1 d M2 e O f A ga layered material of formula (I) wherein M1 is selected from one or both of Mn, 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

[0018] Step 2: selecting the secondary battery satisfying c+x / 10≥0.10 from the secondary battery obtained in step 1.

[0019] When the secondary battery satisfies c+x / 10≥0.10, the B atoms in the lithium difluoro(oxalato)borate can be fully combined with the O atoms in the positive active material, which can better reduce the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive active material, avoid over-delithiation of the surface of the low-cobalt or cobalt-free positive active material, thereby favorably stabilizing the crystal structure of the low-cobalt or cobalt-free positive active material and improving the lithium ion diffusion rate. Therefore, the secondary battery prepared by the method of the present application can have significantly improved cycle performance and good high-temperature storage performance.

[0020] In any embodiment of the present application, the method further comprises the step of: selecting the secondary battery satisfying 0.5≤y / x≤2.0 from the secondary battery obtained in step 2. At this time, the prepared secondary battery has good storage performance and significantly improved cycle performance and energy density.

[0021] In any embodiment of the present application, the method further comprises the step of: selecting the secondary battery satisfying 0.5≤x / z≤2.0 from the secondary battery obtained in step 2. At this time, the prepared secondary battery 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 comprises the step of: selecting the secondary battery satisfying both 0.5≤y / x≤2.0 and 0.5≤x / z≤2.0 from the secondary battery obtained in step 2. At this time, the prepared secondary battery has good storage performance and significantly improved cycle performance, rate performance and low-temperature performance.

[0023] In any embodiment of this application, the method further includes the step of: selecting secondary batteries from the secondary batteries obtained in step 2 that simultaneously satisfy 0.5≤y / x≤2.0, 0.5≤x / z≤2.0, and 0.25≤y / z≤2.0. In this case, the prepared secondary battery simultaneously exhibits good storage performance as well as significantly improved cycle performance, rate performance, and low-temperature performance.

[0024] A third aspect of this application provides a battery module, which includes one of the secondary battery of the first aspect of this application and a secondary battery prepared according to the method of the second aspect of this application.

[0025] The fourth aspect of this application provides a battery pack, which includes a secondary battery according to the first aspect of this application, a secondary battery prepared according to the method of the second aspect of this application, and a battery module according to the third aspect of this application.

[0026] The fifth aspect of this application provides an electrical device comprising at least one of the secondary battery of the first aspect of this application, a secondary battery prepared according to the method of the second aspect of this application, a battery module of the third aspect of this application, and a battery pack of the fourth aspect of this application.

[0027] The battery module, battery pack, and power device of this application include the secondary battery provided in this application, and therefore have at least the same advantages as the secondary battery. Attached Figure Description

[0028] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0030] Figure 2 yes Figure 1 An exploded view of an embodiment of the secondary battery shown.

[0031] Figure 3 This is a schematic diagram of one embodiment of the battery module of this application.

[0032] Figure 4 This is a schematic diagram of one embodiment of the battery pack of this application.

[0033] Figure 5 yes Figure 4 An exploded view of an embodiment of the battery pack shown.

[0034] Figure 6 is a schematic view of an embodiment of an electric device including the secondary battery of the present application as a power source. DETAILED DESCRIPTION

[0035] Hereinafter, embodiments of the secondary battery, the method for manufacturing the secondary battery, the battery module, the battery pack, and the electric device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed explanations are omitted. For example, there are cases where detailed explanations of matters known well, repeated explanations of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0036] The "ranges" disclosed in the present application are defined in the form of lower and upper limits, and a given range is defined by selecting one lower limit and one upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if the ranges 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges 60-110 and 80-120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise specified, the numerical range "a-b" represents a shorthand notation for any integer combination of the real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is just a shorthand notation for these numerical combinations. In addition, when it is stated that a certain parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0037] If not specifically stated, 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.

[0038] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0039] If not specified otherwise, all of the steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0040] If not specified otherwise, the terms "comprising" and "including" mentioned in the present application mean open-ended, and can also mean closed-ended. For example, the terms "comprising" and "including" can mean that other components not listed can also be included, or can mean that only the listed components are included.

[0041] If not specified otherwise, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following satisfy the condition "A or B": A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present).

[0042] Secondary batteries, also known as rechargeable batteries or accumulators, are batteries that can be activated by charging after discharging and continue to be used. Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the charging and discharging process of the secondary battery, lithium ions are inserted into and extracted from between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, and mainly functions to prevent short circuiting between the positive and negative electrodes, while allowing the passage of lithium ions. The electrolyte functions to conduct lithium ions between the positive electrode sheet and the negative electrode sheet.

[0043] When the secondary battery is charged, lithium ions are preferentially extracted from the surface of the positive electrode active material, and then lithium ions in the bulk phase of the positive electrode active material are timely supplemented to the surface. When the cobalt content of the positive electrode active material is high, the lithium ions in the bulk phase of the positive electrode active material can be timely supplemented to the surface of the positive electrode active material. However, when the cobalt content of the positive electrode active material is low, the lithium ions in the bulk phase of the positive electrode active material cannot be supplemented to the surface in time, and the lithium ions on the surface have already been extracted, which leads to over-extraction of lithium on the surface of the positive electrode active material, thereby affecting the crystal structure of the positive electrode active material (e.g., irreversible distortion and increased lattice defects of the positive electrode active material), and reducing the cycle performance of the secondary battery. Therefore, it is of great practical significance to improve the lithium ion diffusion rate of low-cobalt or cobalt-free positive electrode active materials.

[0044] After extensive research, the inventors proposed a low-cobalt or cobalt-free secondary battery with significantly improved lithium-ion diffusion rate and cycle performance.

[0045] A first aspect of this application provides a secondary battery comprising an electrolyte and a positive electrode. The positive electrode comprises a material with the molecular formula Li. a Ni b Co c M1 d M2 e O f A g The layered material comprises M1 selected from one or two of Mn and Al, M2 selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W, and A selected from one or more of F, N, P, and S. The following conditions apply: 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 lithium difluorooxalate borate (LiDFOB), and the mass percentage of lithium difluorooxalate borate is x% based on the total mass of the electrolyte, with 0 < x ≤ 1.0. The secondary battery satisfies c + x / 10 ≥ 0.10.

[0046] Researchers have been working to improve the lithium-ion diffusion rate of low-cobalt or cobalt-free cathode active materials, but there is still no good solution.

[0047] The inventors of this application unexpectedly discovered that, upon adding lithium difluorooxalate borate to the electrolyte, lithium difluorooxalate borate can form a low-resistance protective film on the surface of the positive electrode active material. Furthermore, the boron atoms in lithium difluorooxalate borate readily combine with the oxygen atoms in the positive electrode active material, reducing the charge transfer impedance of the positive electrode active material and thus lowering the diffusion resistance of lithium ions within the bulk phase of the positive electrode active material. Therefore, after adding lithium difluorooxalate borate to the electrolyte, low-cobalt or cobalt-free positive electrode active materials exhibit a significantly improved lithium-ion diffusion rate. Lithium ions within the bulk phase of the low-cobalt or cobalt-free positive electrode active material can be promptly replenished to the surface, preventing excessive delithiation from the surface of the low-cobalt or cobalt-free positive electrode active material, thereby stabilizing the crystal structure of the low-cobalt or cobalt-free positive electrode active material. Since the crystal structure of the low-cobalt or cobalt-free cathode active material of this application is more stable, it can greatly reduce the probability of problems such as the instability of the structural, chemical or electrochemical properties of the cathode active material due to over-delithiation on the surface of the low-cobalt or cobalt-free cathode active material, such as the problem of irreversible distortion and increased lattice defects in the cathode active material.

[0048] The lithium difluorophosphate itself is not resistant to oxidation, and when the amount of the lithium difluorophosphate is too much, the storage performance of the secondary battery, especially the storage performance in a high-temperature environment, is deteriorated. Therefore, the amount of the lithium difluorophosphate should be moderate.

[0049] The inventors have found through a large number of studies that the cobalt content c of the low-cobalt or cobalt-free positive electrode active material is closely related to the mass percentage content x% of the lithium difluorophosphate in the electrolyte. When the cobalt content c of the low-cobalt or cobalt-free positive electrode active material and the mass percentage content x% of the lithium difluorophosphate in the electrolyte satisfy c+x / 10≥0.10, the B atoms in the lithium difluorophosphate can be fully combined with the O atoms in the positive electrode active material, better reducing the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive electrode active material and avoiding over-delithiation of the surface of the low-cobalt or cobalt-free positive electrode active material, thereby being conducive 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 significantly improved cycle performance while also having good high-temperature storage 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.

[0050] When c+x / 10<0.10, the content of the lithium difluorophosphate in the electrolyte is insufficient to form a low-resistance protective film with good performance on the surface of the low-cobalt or cobalt-free positive electrode active material, and the lithium difluorophosphate cannot effectively reduce the charge transfer resistance of the low-cobalt or cobalt-free positive electrode active material, 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 cannot inhibit over-delithiation of the surface of the low-cobalt or cobalt-free positive electrode active material. Therefore, the secondary battery is difficult to have significantly improved cycle performance.

[0051] In some embodiments, the molecular formula of the layered material is Li a Ni b Co c M1 d M2 e O f A g The layered material can be optionally doped with M2 cations, doped with A anions, or doped with both M2 cations and A anions. After doping, the crystal structure of the layered material is more stable, and the electrochemical performance of the secondary battery, such as cycle performance and rate performance, can be further improved.

[0052] In some embodiments, A is selected from F. After doping with F, the Li a Ni b Co c M1 d M2 e Of A g The structure of the compound is more stable, and can make the secondary battery have better cycle performance and rate performance.

[0053] In some embodiments, M1 is selected from Mn.

[0054] In some embodiments, M1 is selected from Al.

[0055] In some embodiments, M1 is selected from a combination of Mn and Al. The molar ratio of Mn and Al is not particularly limited and can be selected according to actual needs.

[0056] 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.

[0057] In some embodiments, c=0.

[0058] In some embodiments, 0

[0059] In some embodiments, 0

[0060] In some embodiments, e=0.

[0061] In some embodiments, 0

[0062] In some embodiments, f=2, g=0.

[0063] In some embodiments, f=0, g=2.

[0064] In some embodiments, 0

[0065] As an example, the molecular formula is Li a Ni b Co c M1d M2 e O f A g 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 0.3 O2.

[0066] Li a Ni b Co c M1 d M2 e O f A gThe lithium source, the nickel source, the cobalt source, the M1 element precursor, the optional M2 element precursor, and the optional A element precursor can be mixed and sintered to obtain the positive electrode active material. The sintering atmosphere can be an oxygen-containing atmosphere, for example, an air atmosphere or an oxygen atmosphere. The O2 concentration of the sintering atmosphere can be, for example, 70% to 100%. The sintering temperature and the sintering time can 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 M1 element precursor includes, but is not limited to, one or more of an oxide, a nitrate compound, a carbonate compound, a hydroxide compound, and an acetate compound of the M1 element. As an 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 compound, and an acetate compound of the M2 element. As an 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 bisulfate, ammonium bisulfite, ammonium sulfite, ammonium hydrosulfide, hydrogen sulfide, lithium sulfide, ammonium sulfide, and elemental sulfur.

[0067] In some embodiments, the electrolyte further comprises fluoroethylene carbonate (FEC). The mass percentage of the fluoroethylene carbonate is y% and 0≤y≤2.5 based on the total mass of the electrolyte. 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 between any of these values. Alternatively, 0

[0068] For secondary batteries, fluoroethylene carbonate can occur at a higher potential reduction decomposition reaction, and form a solid-state electrolyte interface film (SEI film) with a certain flexibility on the surface of the negative active material, while also inhibiting the reduction decomposition of organic solvents at a lower potential and inhibiting the intercalation of organic solvents into the negative 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 pressure oxidation, which is conducive to matching high voltage positive active materials, thereby facilitating the improvement of the energy density of the secondary battery.

[0069] In some embodiments, the mass percentage content x% of lithium difluoro(oxalato)borate and the mass percentage content y% of fluoroethylene carbonate also 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.

[0070] After adding fluoroethylene carbonate to the electrolyte, the cycle performance of the secondary battery can be effectively improved, but when fluoroethylene carbonate decomposes, HF will be formed, which will destroy the structural stability of the positive active material, increase the gas production of the secondary battery, and deteriorate the storage performance of the secondary battery. Lithium difluoro(oxalato)borate as a positive active material stabilizer, the B atom therein also has the function of interacting with the O atom on the surface of the positive active material, inhibiting the destruction of the structure of the positive active material by HF. The combination of lithium difluoro(oxalato)borate and fluoroethylene carbonate is conducive to fully exerting the improvement effect of fluoroethylene carbonate on the cycle performance and energy density of the secondary battery. In addition, by reasonably controlling the relationship between the mass percentage content x% of lithium difluoro(oxalato)borate and the mass percentage content y% of fluoroethylene carbonate, so that it satisfies 0.5≤y / x≤2.0, the synergistic effect of lithium difluoro(oxalato)borate and fluoroethylene carbonate can be fully exerted, which not only will not increase the gas production of the secondary battery, but also will further improve the cycle performance and energy density of the secondary battery.

[0071] In some embodiments, the electrolyte further comprises a lithium salt of fluoro-sulfonyl imide. Optionally, the lithium salt of fluoro-sulfonyl imide has a molecular formula of LiN(SO2R1)(SO2R2), R1, R2 each independently represents F, or C n F 2n+1 n is an integer from 1 to 10. As an example, the lithium salt of fluoro-sulfonyl imide includes one or both of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0072] The mass percentage of the lithium fluorosulfonylimide salt is z% based on the total mass of the electrolyte, 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 any range formed by any of these values. Alternatively, 0

[0073] The fluorosulfonylimide anion is a weakly coordinated anion centered on N, containing a conjugated group and a strong electron-withdrawing -F or -C n F 2n+1 The anion charge is highly delocalized, and the interaction between the anion and lithium ion is weakened. Therefore, the lithium fluorosulfonylimide salt has a lower lattice energy, is easily dissociated, and thus can improve the ionic conductivity of the electrolyte, reduce the viscosity of the electrolyte, and improve the rate performance and low-temperature performance of the secondary battery. At the same time, the lithium fluorosulfonylimide salt also has high thermal stability and a wider electrochemical window, can form a SEI film rich in LiF on the surface of the negative active material, and the SEI film rich in LiF is thinner, has lower impedance, and has higher thermal stability, and can reduce the side reaction between the negative 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.

[0074] In some embodiments, the mass percentage x% of the lithium difluoro(oxalato)borate and the mass percentage z% of the lithium fluorosulfonylimide salt also 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.

[0075] The lithium fluorosulfonylimide salt can improve the rate performance and low-temperature performance of the secondary battery after being added into the electrolyte. However, the lithium fluorosulfonylimide salt is not resistant to high pressure, and can corrode the positive current collector (for example, an aluminum foil) at a higher potential, increase the side reaction between the positive active material and the electrolyte, and has a poor film forming effect on the surface of the positive active material, which can easily affect the cycle performance of the secondary battery. The lithium difluoro(oxalato)borate, as a stabilizer for the positive active material, can form a low-impedance protective film with excellent performance on the surface of the positive active material, and inhibit the side reaction between the positive active material and the electrolyte. Therefore, the combination of the lithium fluorosulfonylimide salt and the lithium difluoro(oxalato)borate can fully exert the improvement effect of the lithium fluorosulfonylimide salt on the rate performance and low-temperature performance of the secondary battery. In addition, the relationship between the mass percentage content x% of the lithium difluoro(oxalato)borate and the mass percentage content z% of the lithium fluorosulfonylimide salt is reasonably controlled to satisfy 0.5≤x / z≤2.0, so that the synergistic effect of the lithium difluoro(oxalato)borate and the lithium fluorosulfonylimide salt can be fully exerted, and the cycle performance of the secondary battery is not only not deteriorated, but also the rate performance and low-temperature performance of the secondary battery are further improved.

[0076] In some embodiments, the electrolyte further simultaneously comprises fluoroethylene carbonate and lithium fluorosulfonylimide salt.

[0077] In some embodiments, the secondary battery further simultaneously satisfies 0.5≤y / x≤2.0 and 0.5≤x / z≤2.0. Further, the secondary battery further simultaneously satisfies 0.5≤y / x≤2.0, 0.5≤x / z≤2.0 and 0.25≤y / z≤2.0. At this time, the secondary battery has significantly improved cycle performance, storage performance, rate performance and low-temperature performance.

[0078] The fluoroethylene carbonate can effectively improve the cycle performance of the secondary battery, the lithium fluorosulfonylimide salt can improve the rate performance and low-temperature performance of the secondary battery, and the lithium difluoro(oxalato)borate, as a stabilizer for the positive active material, can form a low-impedance protective film with excellent performance on the surface of the positive active material, significantly improve the lithium ion diffusion rate of the low-cobalt or cobalt-free positive active material, and simultaneously inhibit the side reaction between the positive active material and the electrolyte and the destruction of the structure of the positive active material by HF. Therefore, the relationship between the contents of the fluoroethylene carbonate, the lithium fluorosulfonylimide salt and the lithium difluoro(oxalato)borate is reasonably controlled, which is beneficial to fully exert the synergistic effect among the three components and fully inhibit the defects of each component used alone.

[0079] The electrolyte of the secondary battery of the present application is beneficial to match high-voltage positive electrode active material, thereby further improving the energy density of the secondary battery. In some embodiments, when the charging capacity of the unit area positive electrode sheet is 90% of the unit area negative electrode sheet capacity, the positive electrode charging voltage ≥ 4.1V. When the charging capacity of the positive electrode sheet is 100% of the negative electrode sheet test capacity, the positive electrode charging voltage ≥ 4.2V.

[0080] As an example, the positive electrode charging voltage is tested according to the following steps.

[0081] (1) Mix ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1, then uniformly dissolve LiPF6 in the above solution to obtain an electrolyte, wherein the concentration of LiPF6 is 1 mol / L; cut the negative electrode sheet into small round pieces with unit area, use a metal lithium sheet as the counter electrode, and use a polyethylene film as the separator film to assemble a CR2430 type button cell in an argon-protected glove box. After the obtained button cell is left for 12 hours, discharge it at 25°C with a constant current of 0.1 mA to 0.005V, then charge it with a constant current of 0.1 mA to 2V, record the charging capacity of the button cell as the unit area negative electrode sheet capacity.

[0082] (1) Mix ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1, then uniformly dissolve LiPF6 in the above solution to obtain an electrolyte, wherein the concentration of LiPF6 is 1 mol / L; cut the negative electrode sheet into small round pieces with unit area, use a metal lithium sheet as the counter electrode, and use a polyethylene film as the separator film to assemble a CR2430 type button cell in an argon-protected glove box. After the obtained button cell is left for 12 hours, discharge it at 25°C with a constant current of 0.1 mA to 0.005V, then charge it with a constant current of 0.1 mA to 2V, record the charging capacity of the button cell as the unit area negative electrode sheet capacity.

[0083] In some embodiments, the secondary battery comprises an electrolyte and a positive electrode sheet. The positive electrode sheet comprises a molecule of formula Li a Ni b Co c M1 d M2 e O f A gThe layered material comprises, 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, and A is selected from one or more of F, N, P and S, with the following properties: 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 lithium difluorooxalate borate and fluoroethylene carbonate. Based on the total mass of the electrolyte, the mass percentage of lithium difluorooxalate borate is x% and 0<x≤1.0, and the mass percentage of fluoroethylene carbonate is y% and 0≤y≤2.5. The secondary battery satisfies c+x / 10≥0.10 and 0.5≤y / x≤2.0.

[0084] In some embodiments, the secondary battery includes an electrolyte and a positive electrode. The positive electrode comprises a material with the molecular formula Li. a Ni b Co c M1 d M2 e O f A g The layered material comprises, 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, and A is selected from one or more of F, N, P and S, with the following properties: 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 lithium difluorooxalatoborate and lithium fluorosulfonylimide, wherein the lithium fluorosulfonylimide comprises one or two of lithium bisfluorosulfonylimide and lithium bistrifluoromethanesulfonylimide. Based on the total mass of the electrolyte, the mass percentage of lithium difluorooxalatoborate is x% and 0 < x ≤ 1.0, and the mass percentage of lithium fluorosulfonylimide is z% and 0 ≤ z ≤ 2.5. The secondary battery satisfies c + x / 10 ≥ 0.10 and 0.5 ≤ x / z ≤ 2.0.

[0085] In some embodiments, the secondary battery includes an electrolyte and a positive electrode. The positive electrode comprises a material with the molecular formula Li. a Ni b Co c M1 d M2 e O f A gThe layered material comprises M1 selected from one or two of Mn and Al, M2 selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W, and A selected from one or more of F, N, P, and S, with the following properties: 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 lithium difluorooxalatoborate, fluoroethylene carbonate, and lithium fluorosulfonylimide, wherein the lithium fluorosulfonylimide comprises one or two of lithium bisfluorosulfonylimide and lithium bistrifluoromethanesulfonylimide. Based on the total mass of the electrolyte, the mass percentage of lithium difluorooxalatoborate is x% and 0 < x ≤ 1.0, the mass percentage of fluoroethylene carbonate is y% and 0 ≤ y ≤ 2.5, and the mass percentage of lithium fluorosulfonylimide 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.

[0086] In some embodiments, the secondary battery includes an electrolyte and a positive electrode. The positive electrode comprises a material with the molecular formula Li. a Ni b Co c M1 d M2 e O f A g The layered material comprises M1 selected from one or two of Mn and Al, M2 selected from one or more of Si, Ti, Mo, V, Ge, Se, Zr, Nb, Ru, Pd, Sb, Ce, Te, and W, and A selected from one or more of F, N, P, and S, with the following properties: 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 lithium difluorooxalatoborate, fluoroethylene carbonate, and lithium fluorosulfonylimide, wherein the lithium fluorosulfonylimide comprises one or two of lithium bisfluorosulfonylimide and lithium bistrifluoromethanesulfonylimide. Based on the total mass of the electrolyte, the mass percentage of lithium difluorooxalatoborate is x% and 0 < x ≤ 1.0, the mass percentage of fluoroethylene carbonate is y% and 0 ≤ y ≤ 2.5, and the mass percentage of lithium fluorosulfonylimide 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.

[0087] In some embodiments, the electrolyte further comprises an electrolyte salt and an organic solvent. The kind of electrolyte salt and organic solvent is not particularly limited and can be selected according to actual needs.

[0088] As an example, the electrolyte salt can include 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 difluorophosphate bis(oxalate) LiDFOP, and lithium tetrafluoro-phosphate oxalate LiTFOP. Alternatively, the electrolyte salt includes LiPF6.

[0089] As an example, the organic solvent can include one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl 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), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0090] 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 can further include other additives, for example, an additive for improving overcharge performance of the battery, an additive for improving high-temperature performance of the battery, an additive for improving low-temperature power performance of the battery, and the like.

[0091] The electrolyte can be prepared according to a method conventional in the art. For example, the organic solvent, the electrolyte salt, the lithium difluoro(oxalato)borate, the optional fluoroethylene carbonate, and the optional lithium fluoro-sulfonimide salt can be mixed uniformly to obtain the electrolyte. The order of addition of the materials is not particularly limited, for example, the electrolyte salt, the lithium difluoro(oxalato)borate, the optional fluoroethylene carbonate, and the optional lithium fluoro-sulfonimide salt can be added to the organic solvent and mixed uniformly to obtain the electrolyte; or the electrolyte salt can be added to the organic solvent first, and then the lithium difluoro(oxalato)borate, the optional fluoroethylene carbonate, and the optional lithium fluoro-sulfonimide salt can be added to the organic solvent and mixed uniformly to obtain the electrolyte.

[0092] In some embodiments, the positive electrode tab 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 including a positive electrode active material. For example, the positive electrode current collector has two surfaces opposite in the thickness direction of the positive electrode current collector, and the positive electrode film layer is disposed on either one or both of the two opposite surfaces of the positive electrode current collector.

[0093] The positive electrode current collector can employ a metal foil or a composite current collector. As an example of the metal foil, an aluminum foil can be employed. The composite current collector can 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 can include one or more of aluminum, an aluminum alloy, nickel, a nickel alloy, titanium, a titanium alloy, silver, and a silver alloy. As an example, the polymer material base layer can include polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), or the like.

[0094] The positive electrode film layer generally contains a positive electrode active material and, optionally, a binder and, optionally, a conductive agent. The positive electrode film layer is generally formed by coating a positive electrode slurry on a positive electrode current collector, drying, and cold-pressing. The positive electrode slurry is generally formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring until uniform. The solvent can be N-methylpyrrolidone (NMP), but is not limited thereto. As an example, the binder used in the positive electrode film layer can include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin. As an example, the conductive agent used in the positive electrode film layer can include one or more of super P, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0095] In some embodiments, the positive electrode active material includes a layered material of the above formula Li a Ni b Co c M1 d M2 e O f A g .

[0096] In some embodiments, Li a Ni b Co c M1 d M2 e O f A gThe surface of the positive electrode active material can also have a coating layer, such as a carbon coating layer. The carbon coating layer is beneficial to stabilize the surface of the positive electrode active material and further reduce the charge transfer impedance of the positive electrode active material and 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, hard carbon, or the like.

[0097] In some embodiments, the positive electrode active material does not exclude other components in addition to Li a Ni b Co c M1 d M2 e O f A g For example, the olivine-structured lithium-containing phosphates can include, but are not limited to, one or more of lithium iron phosphate, a composite of lithium iron phosphate and carbon, lithium manganese phosphate, a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon, and respective modified compounds thereof. The present application is not limited to these materials, and other conventionally known materials that can be used as secondary battery positive electrode active materials can also be used. These positive electrode active materials can be used alone or in combination of two or more.

[0098] In some embodiments, the positive electrode film layer has a total mass, and the mass percentage of the layered material having the formula Li a Ni b Co c M1 d M2 e O f A g For example, the positive electrode film layer has a total mass, and the mass percentage of the layered material having the formula Li a Ni b Co c M1 d M2 e O f A g For example, the positive electrode film layer has a total mass, and the mass percentage of the layered material having the formula Li a Ni b Co c M1 d M2 e O f A gThe layer material has a mass percentage content of 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%.

[0099] The positive electrode tab does not exclude other additional functional layers besides the positive electrode film layer. For example, in some embodiments, the positive electrode tab described in the present application 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, disposed on the surface of the positive electrode current collector. In some other embodiments, the positive electrode tab described in the present application further includes a protective layer covering the surface of the positive electrode film layer.

[0100] The secondary battery according to the present application also includes a negative electrode tab. In some embodiments, the negative electrode tab includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two opposite surfaces in the thickness direction of itself, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0101] The negative electrode current collector can be a metal foil or a composite current collector. As an example of the metal foil, a copper foil can be used. The composite current collector can 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 can 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 can include polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0102] The negative electrode film layer generally includes 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 a negative electrode slurry on a negative electrode current collector, drying, and cold-pressing. The negative electrode slurry is generally formed by dispersing and uniformly stirring the negative electrode active material, the optional conductive agent, the optional binder, and other optional additives in a solvent. The solvent can be N-methyl pyrrolidone (NMP) or deionized water, but is not limited thereto. As an example, the binder used in the negative electrode film layer can include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethylacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), carboxymethyl chitosan (CMCS). As an example, the conductive agent used in the negative electrode film layer can include one or more of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers. Other optional additives can include a thickening agent (e.g., sodium carboxymethyl cellulose CMC-Na), a PTC thermistor material, etc.

[0103] The negative electrode active material can employ a negative electrode active material commonly known in the art for use in a secondary battery. As an example, the negative electrode active material can include one or more of natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based material, tin-based material, lithium titanate. The silicon-based material can include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, silicon alloy material. The tin-based material can include one or more of elemental tin, tin oxide, tin alloy material. The present application is not limited to these materials, and other conventionally known materials that can be used as a negative electrode active material for a secondary battery can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0104] The negative electrode tab does not exclude other additional functional layers other than the negative electrode film layer. For example, in some embodiments, the negative electrode tab described in the present application further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) interposed between the negative electrode current collector and the negative electrode film layer, disposed on the surface of the negative electrode current collector. In some other embodiments, the negative electrode tab described in the present application further includes a protective layer covering the surface of the negative electrode film layer.

[0105] The secondary battery according to the present application further includes a separator. The separator is disposed between the positive electrode tab and the negative electrode tab, and serves to separate. The present application does not particularly limit the type of the separator, and any conventionally known porous structure separator having good chemical stability and mechanical stability can be used.

[0106] In some embodiments, the material of the separator film can include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single-layer film or a multi-layer composite film. When the separator film is a multi-layer composite film, the materials of the layers can be the same or different.

[0107] In some embodiments, the positive electrode sheet, the separator film, and the negative electrode sheet can be used to form an electrode assembly through a winding process or a stacking process.

[0108] In some embodiments, the secondary battery can include an outer package. The outer package can be used to encapsulate the electrode assembly and the electrolyte as described above. The outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), or the like.

[0109] The shape of the secondary battery is not particularly limited in the present application, and the secondary battery can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure as an example.

[0110] In some embodiments, as shown in Figure 2 The outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate form an accommodation cavity. The shell 51 has an opening communicating with the accommodation cavity, and the cover plate 53 is used to cover the opening to close the accommodation cavity. The positive electrode sheet, the negative electrode sheet, and the separator film can be used to form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the accommodation 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 requirements.

[0111] Method for preparing secondary battery

[0112] The second aspect of the embodiments of the present application provides a method for preparing a secondary battery, which includes at least steps 1 and 2.

[0113] Step 1: Assembling a positive electrode sheet, a separator film, a negative electrode sheet, and an electrolyte into a secondary battery.

[0114] The positive electrode sheet includes a molecule of Li a Ni b Co c M1 d M2 e O f A gA layer material of formula (I) wherein M1 is selected from one or both of Mn, 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

[0115] The electrolyte comprises lithium difluoro(oxalato)borate, optionally fluoroethylene carbonate, optionally a lithium salt of a fluoro-sulfonimide, the mass percentage of the lithium difluoro(oxalato)borate is x% and 0

[0116] Step 2: selecting the secondary battery satisfying c+x / 10≥0.10 from the secondary battery obtained in step 1.

[0117] When the secondary battery satisfies c+x / 10≥0.10, the B atoms in the lithium difluoro(oxalato)borate can be fully combined with the O atoms in the positive active material, which can better reduce the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive active material, avoid over-delithiation of the surface of the low-cobalt or cobalt-free positive active material, thereby favorably stabilizing the crystal structure of the low-cobalt or cobalt-free positive active material and improving the lithium ion diffusion rate. Therefore, the secondary battery prepared by the method of the present application can have significantly improved cycle performance and good high-temperature storage performance.

[0118] In some embodiments, the lithium salt of a fluoro-sulfonimide has a molecular formula of LiN(SO2R1)(SO2R2), R1 and R2 each independently represent F, or C n F 2n+1 n is an integer of 1-10. Optionally, the lithium salt of a fluoro-sulfonimide comprises one or both of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0119] In some embodiments, the method further comprises the step of: selecting the secondary battery satisfying 0.5≤y / x≤2.0 from the secondary battery obtained in step 2. At this time, the prepared secondary battery has good storage performance and significantly improved cycle performance and energy density.

[0120] In some embodiments, the method further comprises the step of selecting the secondary battery satisfying 0.5≤x / z≤2.0 from the secondary batteries obtained in step 2. In this case, the prepared secondary battery has both good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0121] In some embodiments, the method further comprises the step of selecting the secondary battery satisfying both 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 battery has both good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0122] In some embodiments, the method further comprises the step of selecting the secondary battery satisfying both 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 battery has both good storage performance and significantly improved cycle performance, rate performance, and low-temperature performance.

[0123] Battery module and battery pack

[0124] In some embodiments of the present application, the secondary battery according to the present application can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0125] Figure 3 is a schematic view of a battery module 4 as an example. As shown in Figure 3 In the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements can also be used. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0126] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0127] In some embodiments, the above battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0128] Figure 4 and Figure 5 is a schematic view of a battery pack 1 as an example. As shown in Figure 4 and Figure 5As shown, the battery pack 1 can include a battery case and a plurality of battery modules 4 disposed in the battery case. The battery case includes an upper case 2 for capping a lower case 3 and forms an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0129] Electric device

[0130] The present application also provides an electric device including at least one of the secondary battery, the battery module, or the battery pack of the present application. The secondary battery, the battery module, or the battery pack can be used as a power source of the electric device, or can be used as an energy storage unit of the electric device. The electric device can be, but is not limited to, a mobile device (e.g., a cell phone, a notebook computer, 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 kart, etc.), an electric train, a ship and a satellite, an energy storage system, etc.

[0131] The electric device can select the secondary battery, the battery module, or the battery pack according to its use requirement.

[0132] Figure 6 is a schematic view of an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of high power and high energy density of the electric device, a battery pack or a battery module can be used.

[0133] An electric device as another example can be a cell phone, a tablet computer, a notebook computer, etc. The electric device usually requires thinness and lightness, and a secondary battery can be used as a power source.

[0134] Embodiment

[0135] The following examples more specifically describe the present disclosure, which are merely illustrative and not limiting, as various modifications and changes can be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported herein are based on weight, and all reagents used in the examples are commercially available or synthesized according to conventional methods and used without further purification, and the instruments used in the examples are commercially available.

[0136] Example 1

[0137] Preparation of positive electrode sheet

[0138] The positive active material LiNi0.65 Co 0.05 Mn 0.3 O2, conductive agent carbon black, binder polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 97.5:1.4:1.1 in a proper amount of solvent NMP to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the surface of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet is obtained.

[0139] Preparation of negative electrode sheet

[0140] The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR), the thickening agent sodium carboxymethyl cellulose (CMC-Na), and the conductive agent carbon black (Super P) are mixed in a weight ratio of 96.2:1.8:1.2:0.8 in a proper amount of solvent deionized water to form a uniform negative electrode slurry; the negative electrode slurry is uniformly coated on the surface of the negative electrode current collector copper foil, and after drying and cold pressing, a negative electrode sheet is obtained.

[0141] Separator film

[0142] A porous polyethylene (PE) film is used as the separator film.

[0143] Preparation of electrolyte

[0144] Ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 and lithium difluoro(oxalato)borate are uniformly dissolved in the above organic solvent to obtain an electrolyte, wherein the concentration of LiPF6 is 1 mol / L, and the mass percentage of lithium difluoro(oxalato)borate based on the total mass of the electrolyte is 0.5%.

[0145] Preparation of secondary battery

[0146] The positive electrode sheet, the separator film, and the negative electrode sheet are stacked and wound in sequence to obtain an electrode assembly; the electrode assembly is placed in an outer package, the above electrolyte is added, and after processes such as packaging, standing, formation, and aging, a secondary battery is obtained.

[0147] Examples 2-25 and Comparative Examples 1-3

[0148] The preparation method of the secondary battery is similar to that of Example 1, except that the type of the positive active material and the preparation parameters of the electrolyte are adjusted, and the specific parameters are shown in Table 1. In the electrolytes of Examples 8-13, fluoroethylene carbonate is further added. In the electrolytes of Examples 14-19, lithium bisfluorosulfonylimide is further added. In the electrolytes of Examples 20-25, both fluoroethylene carbonate and lithium bisfluorosulfonylimide are further added. In Table 1, x% is the mass percentage content of lithium difluoro(oxalato)borate based on the total mass of the electrolyte; y% is the mass percentage content of fluoroethylene carbonate based on the total mass of the electrolyte; and z% is the mass percentage content of lithium bisfluorosulfonylimide based on the total mass of the electrolyte.

[0149] Table 1

[0150]

[0151]

[0152] Test section

[0153] (1) Secondary battery normal temperature cycle performance test

[0154] At 25°C, the secondary battery is charged at 1C constant current to 4.3V, and then charged at constant voltage until the current is 0.05C. At this time, the secondary battery is fully charged, and the charge capacity at this time is recorded, which is the charge capacity of the 1st cycle. After the secondary battery is left for 5 min, it is discharged at 1C constant current to 2.8V, which is one cycle of charge and discharge. The discharge capacity at this time is recorded, which is the discharge capacity of the 1st cycle. The secondary battery is tested by the above method for cycle charge and discharge, and the discharge capacity after each cycle is recorded.

[0155] The capacity retention rate of the secondary battery after 600 cycles at 25°C (%) = the discharge capacity after 600 cycles / the discharge capacity of the 1st cycle x 100%.

[0156] (2) Secondary battery high temperature cycle performance test

[0157] At 45°C, the secondary battery is charged at 1C constant current to 4.3V, and then charged at constant voltage until the current is 0.05C. At this time, the secondary battery is fully charged, and the charge capacity at this time is recorded, which is the charge capacity of the 1st cycle. After the secondary battery is left for 5 min, it is discharged at 1C constant current to 2.8V, which is one cycle of charge and discharge. The discharge capacity at this time is recorded, which is the discharge capacity of the 1st cycle. The secondary battery is tested by the above method for cycle charge and discharge, and the discharge capacity after each cycle is recorded.

[0158] The capacity retention rate of the secondary battery after 600 cycles at 45°C (%) = the discharge capacity after 600 cycles / the discharge capacity of the 1st cycle x 100%.

[0159] (3) Initial DC internal resistance test of secondary battery

[0160] At 25°C, the secondary battery was charged at 1C constant current to 4.3V, and continued to be charged at constant voltage until the current was 0.05C, at which time the secondary battery was fully charged; the secondary battery was discharged at 0.5C constant current and adjusted to 50% SOC, at which time the voltage of the secondary battery was recorded as U1; the secondary battery was discharged at 4C constant current for 30 seconds, with a 0.1 second sampling point, and the end-of-discharge voltage was recorded as U2.

[0161] The discharge DC internal resistance of the secondary battery at 50% SOC represented the initial DC internal resistance of the secondary battery, and the initial DC internal resistance (Ω) of the secondary battery = (U1-U2) / 4C.

[0162] (4) High-temperature storage performance test of secondary battery

[0163] At 60°C, the secondary battery was charged at 1C constant current to 4.3V, and continued to be charged at constant voltage until the current was 0.05C, at which time the volume of the secondary battery was tested by the drainage method and recorded as V0; the secondary battery was placed in a constant-temperature oven at 60°C, and after 30 days of storage, it was taken out, at which time the volume of the secondary battery was tested by the drainage method and recorded as V1.

[0164] The volume expansion rate (%) of the secondary battery after 30 days of storage at 60°C = [(V1-V0) / V0]x100%.

[0165] Table 2 gives the performance test results of Examples 1-25 and Comparative Examples 1-3.

[0166] Table 2

[0167]

[0168] From the test results in Table 2, it can be seen that when the cobalt content c of the low-cobalt or cobalt-free positive electrode active material and the mass percentage content x% of the lithium difluoroborate in the electrolyte satisfy c+x / 10≥0.10, the secondary battery has significantly improved cycle performance, and also has good high-temperature storage performance. In Comparative Examples 1-3, no lithium difluoroborate was added to the electrolyte, or the content of the added lithium difluoroborate was insufficient, at which time the lithium difluoroborate could not form a low-impedance protective film with good performance on the surface of the low-cobalt or cobalt-free positive electrode active material, and the lithium difluoroborate could not effectively reduce the charge transfer impedance of the low-cobalt or cobalt-free positive electrode active material, could not effectively reduce the diffusion resistance of lithium ions in the bulk phase of the low-cobalt or cobalt-free positive electrode active material, and could not inhibit the over-delithiation of the surface of the low-cobalt or cobalt-free positive electrode active material, so the secondary battery was difficult to have significantly improved cycle performance.

[0169] The test results in Table 2 also show that further reasonable control of the mass percentage of lithium difluorooxalate borate (x%) and the mass percentage of fluoroethylene carbonate (y%), so that they satisfy 0.5≤y / x≤2.0, can give full play to the synergistic effect of lithium difluorooxalate borate and fluoroethylene carbonate. This will not only not increase the gas production of the secondary battery, but will also further improve the cycle performance of the secondary battery.

[0170] The test results in Table 2 also show that further reasonable control of the mass percentage of lithium difluorooxalate borate (x%) and lithium difluorosulfonyl imide (z%), ensuring that 0.5 ≤ x / z ≤ 2.0, can fully leverage the synergistic effect of lithium difluorooxalate borate and lithium difluorosulfonyl imide. This will not only prevent the degradation of the secondary battery's cycle performance but also further reduce the initial DC internal resistance and improve the rate performance of the secondary battery.

[0171] The test results in Table 2 also show that further reasonable control of the mass percentages of lithium difluorooxalatoborate (x%), fluoroethylene carbonate (y%), and lithium difluorosulfonylimide (z%) is needed to satisfy 0.5 ≤ y / x ≤ 2.0, 0.5 ≤ x / z ≤ 2.0, and 0.25 ≤ y / z ≤ 2.0. At this point, the secondary battery can exhibit significantly improved cycle performance and rate performance in addition to good high-temperature storage performance.

[0172] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery comprising an electrolyte and a positive electrode sheet, characterized in that, The positive electrode plate comprises a layered material with a molecular formula of Li a Ni b Co c M1 d M2 e O f A g , wherein M1 is selected from one or both 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 the electrolyte comprises lithium difluoro(oxalato)borate, the mass percentage of the lithium difluoro(oxalato)borate is x% based on the total mass of the electrolyte, and 0 < x ≤ 1.0, the secondary battery satisfies c + x / 10 ≥ 0.10, the electrolyte comprises fluoroethylene carbonate, the mass percentage of the fluoroethylene carbonate is y% based on the total mass of the electrolyte, and 0 ≤ y ≤ 2.5, the electrolyte comprises a lithium salt of a fluorosulfonylimide, the mass percentage of the lithium salt of the fluorosulfonylimide is z% based on the total mass of the electrolyte, and 0 ≤ z ≤ 2.5, the secondary battery also simultaneously satisfies 0.5 ≤ y / x ≤ 2.0, 0.5 ≤ x / z ≤ 2.0, and 0.25 ≤ y / z ≤ 1.

0.

2. The secondary battery according to claim 1, characterized by The lithium salt of fluorosulfimide has a molecular formula of LiN(SO2R1)(SO2R2), R1 and R2 each independently represent F or C n F 2n+1 , and n is an integer of 1 to 10.

3. The secondary battery according to claim 2, characterized by the lithium salt of the fluorosulfonylimide comprises one or both of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide.

4. The secondary battery according to claim 1, characterized by y satisfies 0 < y ≤ 2.0 and / or z satisfies 0 < z ≤ 2.

0.

5. The secondary battery according to claim 1, characterized by the secondary battery also satisfies one or both of the following relationships (1) and (2): (1) 0.5 ≤ y / x ≤ 1.0, (2) 0.5 ≤ x / z ≤ 1.

5.

6. A method for producing a secondary battery, characterized by, at least comprising steps of: Step 1, assembling a secondary battery from a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte, The positive electrode plate comprises a layered material with a molecular formula of Li a Ni b Co c M1 d M2 e O f A g , wherein M1 is selected from one or both 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 the electrolyte comprises lithium difluoro(oxalato)borate, fluoroethylene carbonate, and a lithium salt of a fluorosulfonylimide, the mass percentage of the lithium difluoro(oxalato)borate is x% based on the total mass of the electrolyte, and 0 < x ≤ 1.0, the mass percentage of the fluoroethylene carbonate is y% based on the total mass of the electrolyte, and 0 ≤ y ≤ 2.5, and the mass percentage of the lithium salt of the fluorosulfonylimide is z% based on the total mass of the electrolyte, and 0 ≤ z ≤ 2.5; Step 2, screening the secondary battery obtained in Step 1 to obtain a secondary battery satisfying c + x / 10 ≥ 0.10; Step 3, screening the secondary battery obtained in Step 2 to obtain a secondary battery simultaneously satisfying 0.5 ≤ y / x ≤ 2.0, 0.5 ≤ x / z ≤ 2.0, and 0.25 ≤ y / z ≤ 1.

0.

7. The method of claim 6, wherein, further comprising a step of screening the secondary battery obtained in Step 3 to obtain a secondary battery satisfying 0.5 ≤ y / x ≤ 1.

0.

8. The method of claim 6, wherein, further comprising a step of screening the secondary battery obtained in Step 3 to obtain a secondary battery satisfying 0.5 ≤ x / z ≤ 1.

5.

9. The method of claim 6, wherein, further comprising a step of screening the secondary battery obtained in Step 3 to obtain a secondary battery simultaneously satisfying 0.5 ≤ y / x ≤ 1.0 and 0.5 ≤ x / z ≤ 1.

5. 10.A battery module comprising one of the secondary battery according to any one of claims 1-5 and the secondary battery obtained by the method according to any one of claims 6-9. 11.A battery pack comprising one of the secondary battery according to any one of claims 1-5, the secondary battery obtained by the method according to any one of claims 6-9, and the battery module according to claim 10.

12. An electric device comprising at least one of the secondary battery according to any one of claims 1 to 5, the secondary battery obtained by the method according to any one of claims 6 to 9, the battery module according to claim 10, and the battery pack according to claim 11.

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