Secondary battery, battery module, battery pack, and electric device

Abstract

The application provides a secondary battery, a battery module, a battery pack and a power utilization device; the secondary battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte; the negative electrode sheet comprises a negative electrode film layer containing a negative electrode active material; the electrolyte comprises a boron-containing lithium salt configured to form a solid electrolyte interface film on the surface of the negative electrode active material, wherein the discharge capacity of the secondary battery is denoted as A Ah; the mass of the electrolyte is denoted as B g; the mass percentage content of the boron-containing lithium salt relative to the total mass of the electrolyte is denoted as C%, and the secondary battery satisfies: 1≤B / A≤5, 5×10 ‑6 ≤B×C% / A≤0.25. The application can improve the film forming stability of the electrolyte on the negative electrode active material, thereby ensuring the structural stability of the negative electrode active material and improving the cycle performance of the secondary battery; the electrolyte system is relatively stable and is not prone to decomposition, thereby further improving the high-temperature storage performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of batteries, specifically to a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] Secondary batteries have the characteristics of high capacity and long life, so they are widely used in electronic devices such as mobile phones, laptops, electric vehicles, electric cars, electric airplanes, electric ships, electric toy cars, electric toy ships, electric toy airplanes, and power tools.

[0003] As batteries are used more and more widely, the requirements for the performance of rechargeable batteries are becoming increasingly stringent. For example, they are required to have good cycle performance and long high-temperature storage life. Therefore, improving the cycle performance and high-temperature storage performance of rechargeable batteries is an urgent problem to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a secondary battery, a battery module, a battery pack, and an electrical device.

[0005] The first aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, and an electrolyte; the negative electrode includes a negative electrode film layer containing a negative electrode active material; the electrolyte includes a boron-containing lithium salt, the boron-containing lithium salt being configured to form a solid electrolyte interface film on the surface of the negative electrode active material, wherein the discharge capacity of the secondary battery is denoted as A Ah; the mass of the electrolyte is denoted as B g; the mass percentage of the boron-containing lithium salt relative to the total mass of the electrolyte is denoted as C%, and the secondary battery satisfies: 1≤B / A≤5, 5×10 -6 ≤B×C% / A≤0.25; optionally, 1×10 -3 ≤B×C% / A≤0.23.

[0006] Therefore, this application can control 1≤B / A≤5, 5×10 -6 With a ratio of ≤B×C% / A≤0.25, the combination of electrolyte mass, secondary battery discharge capacity, and boron-containing lithium salt mass percentage can improve the film-forming stability of the electrolyte on the negative electrode active material, thereby ensuring the structural stability of the negative electrode active material and improving the cycle performance of the secondary battery. Furthermore, it allows the capacity of the secondary battery to be fully utilized. In addition, when the discharge capacity of the secondary battery and the boron-containing lithium salt mass percentage are combined, the electrolyte system is more stable and less prone to decomposition, thus further improving the high-temperature storage performance of the secondary battery.

[0007] In any embodiment, the secondary battery satisfies at least one of conditions (1) to (3):

[0008] (1) 0.01≤A≤1000; optionally, 50≤A≤550;

[0009] (2) 0.01≤B≤3000; optionally, 0.5≤B≤1500;

[0010] (3) 1×10 -4 % ≤ C% ≤ 5%; optionally, 0.01% ≤ C% ≤ 3%.

[0011] In the above technical solution, the secondary battery can meet the needs of different scenarios; and when the mass percentage of boron-containing lithium salt is within the above range, a stable SEI film can be formed on the surface of the negative electrode active material, which is also beneficial to ensuring the conductivity of the electrolyte.

[0012] In any embodiment, the electrolyte further includes a negative electrode film-forming additive, which includes a first type of additive and / or a second type of additive. The first type of additive includes at least two of carbonate additives, sulfate additives, and sulfite additives; the second type of additive includes fluorooxalic acid or lithium phosphate skeleton and lithium sulfonylimide skeleton. The mass percentage of the negative electrode film-forming additive relative to the total mass of the electrolyte is denoted as D%. The secondary battery also satisfies: 0.1% ≤ (C+D)% ≤ 20%.

[0013] In the above technical solution, the negative electrode film-forming additive and the boron-containing lithium salt together constitute the SEI film. By adjusting the combined amount of the negative electrode film-forming additive and the boron-containing lithium salt, the resulting SEI film has better uniformity and a more stable structure. Furthermore, it can reduce interfacial impedance, thereby improving the battery performance such as cycle life and high-temperature storage of the secondary battery.

[0014] In any embodiment, the secondary battery also satisfies at least one of conditions (4) to (7):

[0015] (4) The mass percentage of carbonate additives relative to the total mass of the electrolyte is denoted as D1%, 10 -4 %≤D1%≤5%;

[0016] (5) The mass percentage of sulfate ester additives and sulfite ester additives relative to the total mass of the electrolyte is denoted as D2%, 10 -4 %≤D2%≤5%;

[0017] (6) The mass percentage of fluorooxalic acid or lithium phosphate backbone relative to the total mass of the electrolyte is denoted as D3%, 10 -4 %≤D3%≤5%;

[0018] (7) The mass percentage of the sulfonylimide skeleton lithium salt relative to the total mass of the electrolyte is denoted as D4%, 10 -4%≤D4%≤5%.

[0019] This application achieves a stable SEI film on the surface of the negative electrode active material by adjusting the mass percentage of each component in the negative electrode film-forming additive. The SEI film exhibits good chemical and thermal stability.

[0020] In any embodiment, the carbonate additives include cyclic carbonate additives and / or linear carbonate additives; optionally, the cyclic carbonate additives include one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), vinyl ethylene carbonate (VEC), and dioctyl carbonate (CC); the linear carbonate additives include one or more of ethyl allyl carbonate (AEC), diphenyl carbonate (DPC), methyl allyl carbonate (MAC), and polycarbonate (PC); and / or the sulfate additives include cyclic sulfonate additives and / or sulfated hydrocarbon ester additives; optionally, the cyclic sulfonate additives include 1,3-propanesulfonate lactone (PS), propylene sulfonate lactone (PES), and 3-fluoro-1,3-propanesulfonate lactone (PES). One or more of propanesulfonate lactones (FPS); and / or sulfate hydrocarbon ester additives including one or more of vinyl sulfate (DTD), diethyl sulfate (DES), and dimethyl sulfate (DMS); and / or sulfite additives including vinyl sulfite (ES) and / or vinyl vinyl sulfite (VES); and / or fluorooxalic acid or phosphate skeleton lithium salts including one or more of lithium tetrafluoro(oxalate)phosphate (LiTFOP), lithium difluorophosphate (LiPO2F2), and lithium difluorodioxalate phosphate (LiBODFP); and / or sulfonylimide skeleton lithium salts including lithium difluorosulfonylimide [LiN(SO2F)2], lithium di(trifluoromethylsulfonyl)imide (LiN(CF3SO2)2), and lithium di(trifluoromethanesulfonyl)imide (LiTFSI).

[0021] This application employs one or more of carbonate additives, sulfate additives, sulfite additives, and lithium fluorooxalate phosphate, which can form an SEI film on the surface of the negative electrode active material. The SEI film formed has a rich composition and higher structural stability.

[0022] In any embodiment, the coating weight of the negative electrode film is F mg / 1540.25 mm. 2 The secondary battery also meets the following requirements: 2×10 -5 ≤B×(C+D)% / F≤1.

[0023] When the secondary battery of this application satisfies the above formula, it enables the electrolyte to undergo a film-forming reaction with the surface of the negative electrode active material, and uniformly forms an SEI film on the surface of the negative electrode active material, thereby providing good protection for the negative electrode active material; moreover, the electrolyte has a relatively high conductivity, and lithium ions can be smoothly inserted into the negative electrode active material through the electrolyte after being extracted from the positive electrode, thereby ensuring the cycle performance of the secondary battery.

[0024] In any implementation, 50 ≤ F ≤ 500.

[0025] When the coating weight of the negative electrode film in this application is within the above range, the capacity of the secondary battery can be guaranteed.

[0026] In any embodiment, the molecular formula of the boron-containing lithium salt is LiBF. a O b C c P d In the molecular formula, 0≤a≤4, 0≤b≤8, 0≤c≤4, and 0≤d≤4.

[0027] The boron atom in this application can combine with oxygen-containing oxalic acid ligands. The resulting product exhibits excellent thermal stability and readily forms a high-performance SEI film on the surface of the negative electrode active material, thereby ensuring the structural stability of the negative electrode active material and improving the cycle performance of the secondary battery. Because boron-containing lithium salts possess large anionic groups, they promote lithium-ion dissociation, resulting in a higher lithium-ion transference number in the electrolyte, which is beneficial for improving secondary battery polarization and rate performance.

[0028] In any embodiment, the boron-containing lithium salt includes one or more of lithium tetrafluoroborate (LiBF4), lithium dioxaborate (LiBOB), and lithium difluorooxaborate (LiDFOB).

[0029] The combination of lithium tetrafluoroborate (LiBF4), lithium dioxaborate (LiBOB), and lithium difluorooxaborate (LiDFOB) in this application results in a SEI film with a more diverse composition and a more stable structure. Furthermore, while ensuring structural stability, it also guarantees a relatively low impedance value for the SEI film, thus ensuring the low-temperature performance of the secondary battery.

[0030] A second aspect of this application also provides a battery module including a secondary battery as described in any embodiment of the first aspect of this application.

[0031] A third aspect of this application also provides a battery pack including a battery module as described in the second aspect of this application.

[0032] The fourth aspect of this application also provides an electrical device, including a secondary battery as described in any embodiment of the first aspect of this application, a battery module as described in the second aspect of this application, or a battery pack as described in the third aspect of this application. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced 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.

[0034] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0035] Figure 2 yes Figure 1 An exploded view of the implementation method of the secondary battery.

[0036] Figure 3 This is a schematic diagram of one embodiment of the battery module of this application.

[0037] Figure 4 This is a schematic diagram of one embodiment of the battery pack of this application.

[0038] Figure 5 yes Figure 4 An exploded view of an embodiment of the battery pack shown.

[0039] Figure 6 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0040] The accompanying drawings may not be drawn to scale.

[0041] The annotations in the attached figures are explained as follows:

[0042] 1. Battery pack; 2. Upper casing; 3. Lower casing; 4. Battery module;

[0043] 5. Secondary battery; 51. Housing; 52. Electrode assembly;

[0044] 53. Cover plate;

[0045] 6. Electrical appliances. Detailed Implementation

[0046] The following detailed description discloses embodiments of the secondary battery, battery module, battery pack, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0047] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0048] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions. Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0049] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0050] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0051] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0052] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0053] With the application and promotion of secondary batteries, their comprehensive performance has received increasing attention. Non-aqueous electrolytes are one of the key factors affecting the performance of secondary batteries. Currently, the most widely used commercial non-aqueous electrolyte system is a mixed carbonate solution of lithium hexafluorophosphate. However, lithium hexafluorophosphate has poor thermal stability at high temperatures. Therefore, the inventors added boron-containing lithium salts to the electrolyte system. Boron-containing lithium salts have a high thermal decomposition temperature, thus exhibiting excellent performance in secondary batteries. However, after in-depth research, the inventors discovered that although boron-containing lithium salts have a high thermal decomposition temperature, their thermal stability in the electrolyte system is poor, thus deteriorating the high-temperature storage performance of the secondary battery. Furthermore, the stability of the solid electrolyte interphase (SEI) film formed on the surface of the negative electrode active material is insufficient, failing to provide long-term effective protection for the negative electrode active material. This leads to continuous side reactions between the negative electrode active material and the electrolyte in the later stages, further deteriorating the cycle performance of the secondary battery.

[0054] After conducting extensive research, the inventors of this application unexpectedly discovered that when the discharge capacity of the secondary battery, the mass of the electrolyte, and the mass percentage of boron-containing lithium salt are within a specific range, the secondary battery can simultaneously achieve good cycle performance and high-temperature storage performance.

[0055] Secondary batteries

[0056] In a first aspect, this application proposes a secondary battery. The secondary battery includes a positive electrode, a negative electrode, and an electrolyte; the negative electrode includes a negative electrode film layer containing a negative electrode active material; the electrolyte includes a boron-containing lithium salt, which is configured to form a solid electrolyte interface film on the surface of the negative electrode active material. The discharge capacity of the secondary battery is denoted as A Ah; the mass of the electrolyte is denoted as B g; the mass percentage of the boron-containing lithium salt relative to the total mass of the electrolyte is denoted as C%, and the secondary battery satisfies: 1 ≤ B / A ≤ 5, 5 × 10⁻⁶. -6 ≤B×C% / A≤0.25.

[0057] Although the mechanism is not yet clear, the inventors unexpectedly discovered that by adjusting the relationship between A, B and C% to meet the above range, the cycle performance and high-temperature storage performance of secondary batteries can be improved.

[0058] The discharge capacity (Ah) of a secondary battery refers to the capacity value measured at room temperature using a constant current charge-discharge tester with the corresponding current and voltage set. Taking ternary materials as an example, the test method for the discharge capacity of a secondary battery is as follows: The secondary battery is placed on a constant current charge-discharge tester at a test temperature of 25℃. The test procedure is as follows: First, charge the battery at a certain rate of 0.05C (0.05C is the current value at which all the charge in the secondary battery is discharged in 20 hours) to the upper limit voltage of the finished battery (the upper limit voltage of ternary materials is generally 4.2-4.5V). After standing for 30 minutes, discharge the battery at a rate of 0.05C to the lower limit voltage (the lower limit voltage of ternary materials is generally 2.5V). The measured discharge capacity is the discharge capacity (Ah) of the secondary battery.

[0059] The mass of electrolyte, B g, is the mass of electrolyte in the finished secondary battery. The determination method is as follows: An exemplary method for obtaining electrolyte from a secondary battery includes the following steps: Discharge the secondary battery to the discharge cutoff voltage (for safety reasons, the battery is generally in a fully discharged state) and then centrifuge it. First, pour out and collect the free electrolyte from the filling hole. Then, centrifuge the remaining bare cell parts, such as the electrode sheet and separator, at a centrifugation speed greater than 10,000 rpm. Repeat the centrifugation and collection of electrolyte until no obvious liquid is present after centrifugation. The total amount of collected electrolyte is the mass of electrolyte in the finished cell, B g.

[0060] Electrolyte is a crucial component of lithium-ion batteries, playing a vital role in transporting lithium ions between the positive and negative electrodes in a secondary battery. Boron-containing lithium salts, as lithium salts with boron (B) atoms as the central atom, can coordinate with alkoxy groups, ortho- and ortho-hydroxy groups, and carboxylic acids to form anionic complexes. These anionic complexes primarily exhibit large π-conjugated structures, with a relatively dispersed negative charge distribution of the central ion, resulting in delocalized charge. Furthermore, the large radius of the anion makes it difficult for it to form strong ion pairs with lithium ions in organic solvents. Therefore, it can increase the lithium-ion transference number in the electrolyte, which is beneficial for improving polarization and the rate performance of the secondary battery. In the early stages, boron-containing lithium salts can form a high-performance solid electrolyte interface (SEI) film on the surface of the negative electrode active material. The SEI film is insoluble in organic solvents and can exist stably in organic electrolytes. This effectively reduces the continuous reduction and decomposition of solvent molecules in the electrolyte at the negative electrode interface, preventing the formation of unstable SEI and ensuring the structural stability of the negative electrode active material, thereby improving the early cycle performance of the secondary battery.

[0061] The determination of the mass percentage (C%) of boron-containing lithium salts can be performed according to methods known in the art. For example, it can be determined by gas chromatography-mass spectrometry (GC-MS), ion chromatography (IC), liquid chromatography (LC), nuclear magnetic resonance spectroscopy (NMR), etc.

[0062] It should be noted that when testing the parameters of the electrolyte, it can be obtained from the secondary battery. An exemplary method for obtaining electrolyte from a secondary battery includes the following steps: discharging the secondary battery to the discharge cutoff voltage (for safety, the battery is generally left fully discharged), centrifuging it, and then taking an appropriate amount of the centrifuged liquid, which is the electrolyte. Alternatively, the electrolyte can be obtained directly from the secondary battery's filling port.

[0063] This application can control 1≤B / A≤5, 5×10 -6 ≤B×C% / A≤0.23, the combination of electrolyte mass, secondary battery discharge capacity, and boron-containing lithium salt mass percentage can improve the film-forming stability of the electrolyte on the negative electrode active material, thereby ensuring the structural stability of the negative electrode active material and improving the cycle performance of the secondary battery; it also allows the capacity of the secondary battery to be fully utilized; furthermore, when the discharge capacity of the secondary battery and the boron-containing lithium salt mass percentage are combined, the electrolyte system is more stable and less prone to decomposition, thereby further improving the high-temperature storage performance of the secondary battery. Optionally, 10 -4 ≤B×C% / A≤0.2; for example, 5×10 -6 ≤B×C% / A≤0.25、5×10 -5≤B×C% / A≤0.25、5×10 -4 ≤B×C% / A≤0.25、3×10 -4 ≤B×C% / A≤0.25、1×10 -4 ≤B×C% / A≤0.25、1×10 -4 ≤B×C% / A≤0.2、5×10 -3 ≤B×C% / A≤0.25、5×10 -2 ≤B×C% / A≤0.25, 0.5≤B×C% / A≤0.25, 5×10 -6 ≤B×C% / A≤0.2、5×10 -6 ≤B×C% / A≤0.1、5×10 -6 ≤B×C% / A≤0.01 or 5×10 -6 ≤B×C% / A≤2×10 -3 wait.

[0064] In some implementations, 0.01 ≤ A ≤ 1000; when the discharge capacity of the secondary battery is within the above range, it can meet the needs of different scenarios. Optionally, 50 ≤ A ≤ 550; for example, the discharge capacity A Ah of the secondary battery can be 0.01Ah, 0.1Ah, 0.2Ah, 0.5Ah, 1Ah, 2Ah, 5Ah, 10Ah, 15Ah, 20Ah, 50Ah, 80Ah, 100Ah, 200Ah, 300Ah, 500Ah, 600Ah, 700Ah, 800Ah, or 1000Ah; or a range consisting of any two of the above values.

[0065] In some implementations, 0.01 ≤ B ≤ 3000; when the mass of the electrolyte is within the above range, it can meet the needs of different industrial applications. Optionally, 0.5 ≤ B ≤ 1500; for example, the mass B g of the electrolyte can be 0.01g, 0.05g, 0.1g, 0.2g, 0.5g, 0.6g, 0.8g, 1g, 2g, 5g, 8g, 10g, 15g, 20g, 30g, 50g, 80g, 100g, 200g, 300g, 500g, 700g, 800g, 1000g, 1500g, 2000g, 2500g, 2800g, or 3000g; or a range consisting of any two of the above values.

[0066] In some implementations, 1×10 -4 When the mass percentage of boron-containing lithium salt is within the above range, a stable SEI film can be formed on the surface of the negative electrode active material, which is also beneficial to ensuring the conductivity of the electrolyte. Optionally, 0.1% ≤ C ≤ 3%; for example, the mass percentage C% of boron-containing lithium salt can be 1 × 10⁻⁶. -4%, 1.5×10 -4 %, 2×10 -4 %, 5×10 -4 %, 1×10 -3 %, 1.5×10 -3 %, 2×10 -3 %, 5×10 -3 %, 1×10 -2 %, 5×10 -2 %, 0.1%, 0.5%, 0.8%, 1%, 1.5%, 2%, 3%, 4% or 5%; or a range consisting of any two of the above values.

[0067] Electrolyte

[0068] The electrolyte acts as a conductor of metal ions between the positive and negative electrodes. The electrolyte used in this application can be any electrolyte known in the art for secondary batteries. The electrolyte includes lithium salts and organic solvents.

[0069] In some embodiments, the lithium salt includes a boron-containing lithium salt; the boron-containing lithium salt has the molecular formula LiBF. a O b C c P d In the molecular formula, 0≤a≤4, 0≤b≤8, 0≤c≤4, and 0≤d≤4.

[0070] Boolean (B) atoms can combine with oxygen-containing oxalic acid ligands, resulting in a product with excellent thermal stability. This product readily forms a high-performance SEI film on the surface of the negative electrode active material, ensuring its structural stability and improving the cycle performance of the secondary battery. Boolean atoms can also combine with halogen atoms, especially fluorine atoms. Fluorine atoms have a strong electron-withdrawing inductive effect, high thermal and chemical stability, and due to their large anionic groups, they promote lithium-ion dissociation, resulting in a higher lithium-ion transference number in the electrolyte. This is beneficial for improving secondary battery polarization and rate performance.

[0071] As an example, boron-containing lithium salts include one or more of lithium tetrafluoroborate (LiBF4), lithium bis(C2O4)2 (LiBOB), and lithium bis(fluorooxalate)borate (LiBC2O4F2 (LiDFOB)). Further, boron-containing lithium salts include compositions of lithium tetrafluoroborate LiBF4, lithium bis(C2O4)2 (LiBOB), and lithium bis(fluorooxalate)borate (LiBC2O4F2 (LiDFOB)).

[0072] When lithium tetrafluoroborate (LiBF4) is used in conjunction with organic solvents such as carbonates or additives in the electrolyte, the system formed by LiBF4 has a relatively low viscosity, which is beneficial for lithium-ion release and thus improves the conductivity of the electrolyte. The SEI film formed by LiBF4 has a more uniform thickness and better kinetic activity, resulting in lower charge transfer resistance in the secondary battery, thereby significantly improving the low-temperature performance of the secondary battery. The SEI film is not prone to thermal decomposition and its performance is more stable at high temperatures, thus significantly improving the high-temperature performance of the secondary battery.

[0073] Either lithium bis(oxalato)borate (LiBOB) or lithium difluorooxalato)borate (LiDFOB) can passivate the positive current collector in the positive electrode sheet, reducing the risk of side reactions that corrode the positive current collector and improving the structural stability of the positive electrode sheet. Furthermore, electrolytes containing either LiBOB or LiDFOB are less likely to generate acidic substances, further reducing the risk of corrosion of the positive current collector. LiBOB and LiDFOB exhibit good compatibility with the positive electrode active material, facilitating lithium ion migration; they can also form an effective SEI film on the surface of the negative electrode active material, improving its protective performance.

[0074] The combined use of lithium tetrafluoroborate (LiBF4), lithium dioxaborate (LiBOB), and lithium dioxaborate (LiDFOB) results in a SEI film with a more diverse composition and greater structural stability. Furthermore, while ensuring structural stability, it also guarantees a relatively low impedance value for the SEI film, thus ensuring the low-temperature performance of the secondary battery.

[0075] When the secondary battery of this application contains the above-mentioned boron-containing lithium salt, a structurally stable SEI film with a relatively low impedance value can be formed on the surface of the negative electrode active material, thereby improving the high and low temperature performance of the secondary battery.

[0076] In some embodiments, the lithium salt may further include one or more combinations of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO2F2), lithium difluorodioxophosphate (LiDFOP), and lithium tetrafluorooxophosphate (LiTFOP). The use of the above lithium salts in combination with boron-containing lithium salts can improve the conductivity of the electrolyte.

[0077] As an example, lithium salts may include one or more combinations selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bistrifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalateborate (LiDFOB), and lithium dioxalateborate (LiBOB).

[0078] In some embodiments, the organic solvent may also include one or more combinations selected from 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), butyl ester carbonate (BC), 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).

[0079] In some embodiments, the electrolyte may further include film-forming additives, such as negative electrode film-forming additives; the negative electrode film-forming additives include a first type of additives and / or a second type of additives, the first type of additives including at least two of carbonate additives, sulfate additives and sulfite additives; the second type of additives includes fluorooxalic acid or lithium phosphate skeleton salts and / or lithium sulfonylimide skeleton salts.

[0080] One or more of carbonate additives, sulfate additives, sulfite additives, and lithium fluorooxalate phosphate salts are included. Anode film-forming additives can form an SEI film on the surface of the anode active material. The resulting SEI film has a rich composition and higher structural stability.

[0081] The mass percentage of the negative electrode film-forming additive relative to the total mass of the electrolyte is denoted as D%; the mass percentage of the boron-containing lithium salt relative to the total mass of the electrolyte is denoted as C%, wherein the secondary battery satisfies: 0.01% ≤ (C+D)% ≤ 20%.

[0082] Negative electrode film-forming additives and boron-containing lithium salts together constitute the SEI film. By adjusting the combined amount of negative electrode film-forming additives and boron-containing lithium salts, the resulting SEI film exhibits better uniformity and a more stable structure. Furthermore, it can reduce interfacial impedance, thereby improving battery performance such as cycle life and high-temperature storage. Optionally, 0.1% ≤ (C+D)% ≤ 12%; exemplaryly, (C+D)% can be 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 16, 18, or 20; or a range consisting of any two of the above values.

[0083] As an example, carbonate additives include cyclic carbonate additives and / or linear carbonate additives. Further, cyclic carbonate additives include one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), vinyl ethylene carbonate (VEC), and dioctyl carbonate (CC). Linear carbonate additives include one or more of ethyl allyl carbonate (AEC), diphenyl carbonate (DPC), methyl allyl carbonate (MAC), and polycarbonate (PC). The mass percentage of carbonate additives relative to the total mass of the electrolyte is denoted as D1%.

[0084] The following requirement can be met by adjusting the mass percentage (D1%) of carbonate additives: 10 -4 The content of D1% ≤ 5% ensures the formation of a stable SEI film on the surface of the negative electrode active material, exhibiting good chemical and thermal stability. For example, the mass percentage of carbonate additives, D1%, can be 1 × 10⁻⁶. -4 %, 1.5×10 -4 %, 2×10 -4 %, 5×10 -4 %, 1×10 -3 %, 1.5×10 -3 %, 2×10 -3 %, 5×10 -3 %, 1×10 -2 %, 5×10 -2 %, 0.1%, 0.5%, 0.8%, 1%, 1.5%, 2%, 3%, 4% or 5%; or a range consisting of any two of the above values.

[0085] As an example, sulfate ester additives include cyclic sulfonate additives and / or sulfate hydrocarbon ester additives. Further, cyclic sulfonate additives include one or more of 1,3-propanesulfonate lactone (PS), propenesulfonate lactone (PES), and 3-fluoro-1,3-propanesulfonate lactone (FPS). Sulfate hydrocarbon ester additives include one or more of vinyl sulfate (DTD), diethyl sulfate (DES), and dimethyl sulfate (DMS).

[0086] As an example, sulfite additives include vinyl sulfite ES and / or vinyl vinyl sulfite VES.

[0087] The mass percentage of the sulfate or sulfite additive relative to the total mass of the electrolyte is denoted as D2%.

[0088] The desired result can be achieved by adjusting the mass percentage (D2%) of sulfate or sulfite additives to: 10 -4 The content of D2% ≤ 5% ensures the formation of a stable SEI film on the surface of the negative electrode active material, exhibiting good chemical and thermal stability. For example, the mass percentage D2% of the sulfate or sulfite additive can be 1 × 10⁻⁶. -4 %, 1.5×10 -4 %, 2×10 -4 %, 5×10 -4 %, 1×10 -3 %, 1.5×10 -3 %, 2×10 -3 %, 5×10 -3 %, 1×10 -2 %, 5×10 -2 %, 0.1%, 0.5%, 0.8%, 1%, 1.5%, 2%, 3%, 4% or 5%; or a range consisting of any two of the above values.

[0089] As an example, the fluorooxalic acid or phosphate backbone lithium salt includes one or more of lithium tetrafluoro(oxalate)phosphate (LiTFOP), lithium difluorophosphate (LiPO2F2), and lithium difluorodioxalate phosphate (LiBODFP). The mass percentage of the fluorooxalic acid or phosphate backbone lithium salt relative to the total mass of the electrolyte is denoted as D3%.

[0090] The following requirement can be met by adjusting the mass percentage (D3%) of fluorooxalic acid or lithium phosphate backbone: 10- 4 The content of D3% ≤ 5% ensures the formation of a stable SEI film on the surface of the negative electrode active material, exhibiting good chemical and thermal stability. For example, the mass percentage D3% of the fluorooxalic acid or lithium phosphate backbone can be 1 × 10⁻⁶. -4 %, 1.5×10 -4 %, 2×10 -4 %, 5×10 -4 %, 1×10 -3 %, 1.5×10 -3 %, 2×10 -3 %, 5×10 -3 %, 1×10 -2%, 5×10 -2 %, 0.1%, 0.5%, 0.8%, 1%, 1.5%, 2%, 3%, 4% or 5%; or a range consisting of any two of the above values.

[0091] As an example, the sulfonylimide framework lithium salt includes one or more of lithium bis(fluorosulfonylimide)[LiN(SO2F)2], lithium bis(trifluoromethylsulfonyl)imideLiN(CF3SO2)2, and lithium bis(trifluoromethanesulfonyl)imideLiTFSI. The mass percentage of the sulfonylimide framework lithium salt relative to the total mass of the electrolyte is denoted as D4%.

[0092] The following condition can be satisfied by adjusting the mass percentage D4% of the lithium salt with sulfonamide skeleton: 10 -4 The mass percentage (D4%) is ≤5%, which allows it to form a stable SEI film on the surface of the negative electrode active material. This SEI film exhibits good chemical and thermal stability. For example, the mass percentage (D4%) of the sulfonylimide backbone lithium salt can be 1 × 10⁻⁶. -4 %, 1.5×10 -4 %, 2×10 -4 %, 5×10 -4 %, 1×10 -3 %, 1.5×10 -3 %, 2×10 -3 %, 5×10 -3 %, 1×10 -2 %, 5×10 -2 %, 0.1%, 0.5%, 0.8%, 1%, 1.5%, 2%, 3%, 4% or 5%; or a range consisting of any two of the above values.

[0093] The electrolyte of this application can be prepared using methods conventional in the art. For example, additives, solvents, electrolyte salts, etc., can be mixed evenly to obtain the electrolyte. There are no particular restrictions on the order in which the materials are added; for example, the additives, electrolyte salts, etc., can be added to the non-aqueous solvent and mixed evenly to obtain a non-aqueous electrolyte.

[0094] In this application, the components and their contents in the electrolyte can be determined according to methods known in the art. For example, they can be determined by gas chromatography-mass spectrometry (GC-MS), ion chromatography (IC), liquid chromatography (LC), nuclear magnetic resonance spectroscopy (NMR), etc.

[0095] It should be noted that during the electrolyte testing of this application, freshly prepared electrolyte can be used directly, or electrolyte can be obtained from a secondary battery. An exemplary method for obtaining electrolyte from a secondary battery includes the following steps: discharging the secondary battery to the discharge cutoff voltage (for safety, the battery is generally left fully discharged), followed by centrifugation. A suitable amount of the centrifuged liquid is then taken as the non-aqueous electrolyte. Alternatively, the non-aqueous electrolyte can be obtained directly from the secondary battery's filling port.

[0096] [Negative electrode plate]

[0097] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0098] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0099] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, and lithium-aluminum alloys, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0100] In some embodiments, the coating weight of the negative electrode film is F mg / 1540.25 mm. 2 The secondary battery also meets the following requirements: 2×10 -5 ≤B×(C+D)% / F≤1. In this paper, the coating weight of the negative electrode film refers to the coating weight of the negative electrode film layer disposed on one surface of the negative electrode current collector.

[0101] When a secondary battery satisfies the above formula, a film-forming reaction can occur between the electrolyte and the surface of the negative electrode active material, resulting in a uniform SEI film on the surface of the negative electrode active material, thus providing good protection for it. Furthermore, the electrolyte has relatively high conductivity, allowing lithium ions to easily pass through the electrolyte and embed into the negative electrode active material after being extracted from the positive electrode, thereby ensuring the cycle performance of the secondary battery. Optionally, 5×10 -5 ≤B×(C+D)% / F≤0.5; for example, B×(C+D)% / F can be 2×10 -55×10 -5 1×10 -4 2×10 -4 2×10 -3 5×10 -2 0.1 or 1; or a range consisting of any two of the above values.

[0102] The coating weight of the negative electrode film is a well-known concept in the art and can be determined using the differential gravity method. The negative electrode film quality testing method is as follows: the obtained negative electrode sheet of the secondary battery is vacuum dried at 105°C for 48 hours, using a coating with an area of ​​1540.25 mm². 2 Several small discs were stamped using a circular stamping machine, and their weights were recorded to obtain an average value of m0 mg. Then, the single-sided negative electrode active material was completely removed by wiping with deionized water until no obvious black marks remained on the negative electrode current collector. After drying at 105℃ for 5 hours, several small discs were stamped again using the same stamping machine, and their weights were recorded to obtain an average value of m1 mg. The coating weight of the negative electrode film was (m0-m1) mg / 1540.25 mm. 2 The values ​​of m0-m1 correspond to F in this paper.

[0103] In some implementations, 50 ≤ F ≤ 500.

[0104] When the coating weight of the negative electrode film is within the above range, the capacity of the secondary battery can be guaranteed. For example, F can be 50, 60, 80, 100, 120, 150, 160, 180, 200, 220, 250, 280, 300, 350, 380, 400, 420, 450, 480 or 500.

[0105] In some embodiments, the negative electrode film layer may optionally include a negative electrode binder. This application does not impose any particular limitation on the type of negative electrode binder. As an example, the negative electrode binder may include one or more combinations selected from styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the mass percentage of the negative electrode binder is less than 5% based on the total mass of the negative electrode film layer.

[0106] In some embodiments, the negative electrode film layer may optionally include a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent. As an example, the negative electrode conductive agent may include one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the negative electrode conductive agent is less than 5% based on the total mass of the negative electrode film layer.

[0107] In some embodiments, the negative electrode film may optionally include other additives. As an example, other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC-Na), PTC thermistor materials, etc. In some embodiments, the mass percentage of other additives is less than 2% based on the total mass of the negative electrode film.

[0108] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. Examples of metal foils include copper foil or copper alloy foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. Examples of the metal material include one or more combinations selected from copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer substrate may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0109] The negative electrode film is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is usually formed by dispersing the negative electrode active material, optional conductive agent, optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these.

[0110] The negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet also includes a protective layer covering the surface of the negative electrode film layer.

[0111] [Positive electrode plate]

[0112] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. For example, the positive current collector has two surfaces opposite each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0113] The positive electrode film layer includes a positive electrode active material, which may be a positive electrode active material known in the art for use in secondary batteries. For example, the positive electrode active material may include at least one of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Examples of lithium transition metal oxides may include at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds. Examples of lithium-containing phosphates with an olivine structure may include at least one of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds. This application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for secondary batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more.

[0114] In some embodiments, the positive electrode active material includes LiNi x Co y Mn z M 1-x-y-z O2 or LiNi a Co b Al c N 1-a-b-c O2, wherein M and N are each independently selected from any one of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti, and 0≤y≤1, 0≤x<1, 0≤z≤1, x+y+z≤1, 0≤a≤1, 0≤b≤1, 0≤c≤1, a+b+c≤1. When the positive electrode active material is used in conjunction with boron-containing lithium salts, the B atoms in the boron-containing lithium salts readily combine with the O atoms 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 within the bulk phase of the positive electrode active material. Therefore, when the non-aqueous electrolyte contains appropriate amounts of lithium tetrafluoroborate and lithium difluorooxalate borate, low-cobalt or cobalt-free cathode active materials exhibit significantly improved lithium-ion diffusion rates. Lithium ions within the bulk phase of the low-cobalt or cobalt-free cathode active material can be promptly replenished to the surface, preventing excessive delithiation and thus stabilizing the crystal structure. Because the crystal structure of low-cobalt or cobalt-free cathode active materials is more stable, the probability of instability in the structural, chemical, or electrochemical properties of the cathode active material due to excessive delithiation on its surface is greatly reduced. For example, this can lead to irreversible distortion and increased lattice defects.

[0115] LiNi x Coy Mn z M 1-x-y-z O2 or LiNi a Co b Al c N 1-a-b-c O2 can be prepared according to conventional methods in the art. An exemplary preparation method is as follows: a lithium source, a nickel source, a cobalt source, a manganese source, an aluminum source, an M-element precursor, and an N-element precursor are mixed and then sintered. The sintering atmosphere can be an oxygen-containing atmosphere, such as an air atmosphere or an oxygen atmosphere. The O2 concentration in the sintering atmosphere is, for example, 70% to 100%. The sintering temperature and sintering time can be adjusted according to actual conditions.

[0116] As examples, lithium sources include, but are not limited to, at least one 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 examples, nickel sources include, but are not limited to, at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate. As examples, cobalt sources include, but are not limited to, at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate. As examples, manganese sources include, but are not limited to, at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate. As examples, aluminum sources include, but are not limited to, at least one of aluminum sulfate, aluminum nitrate, aluminum chloride, aluminum oxalate, and aluminum acetate. As examples, precursors of element M include, but are not limited to, at least one of oxides, nitrate compounds, carbonate compounds, hydroxides, and acetate compounds of element M. As an example, precursors of nitrogen include, but are not limited to, at least one 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 hydrogen sulfide, hydrogen sulfide, lithium sulfide, ammonium sulfide, and elemental sulfur.

[0117] In some embodiments, based on the total mass of the positive electrode film, the molecular formula is LiNi. x Co y Mn z M 1-x-y-z O2 or LiNi a Co b Al c N 1-a-b-c The layered material containing O2 has a mass percentage of 80% to 99%. For example, the molecular formula LiNi x Co y Mn z M 1-x-y-z O2 or LiNi a Co b Alc N 1-a-b-c The mass percentage of the O2 layered material can be any range consisting of 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or higher. Optionally, the molecular formula is LiNi. x Co y Mn z M 1-x-y-z O2 or LiNi a Co b Al c N 1-a-b-c The mass percentage of layered O2 is 85% to 99%, 90% to 99%, 95% to 99%, 80% to 98%, 85% to 98%, 90% to 98%, 95% to 98%, 80% to 97%, 85% to 97%, 90% to 97%, or 95% to 97%.

[0118] In some embodiments, the positive electrode film may optionally include a positive electrode conductive agent. This application does not impose any particular limitation on the type of positive electrode conductive agent. As an example, the positive electrode conductive agent includes one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the positive electrode conductive agent is less than 5% based on the total mass of the positive electrode film.

[0119] In some embodiments, the positive electrode film layer may optionally include a positive electrode binder. This application does not impose any particular limitation on the type of positive electrode binder. As an example, the positive electrode binder may include one or more combinations selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. In some embodiments, the mass percentage of the positive electrode binder is less than 5% based on the total mass of the positive electrode film layer.

[0120] In some embodiments, the positive current collector may be a metal foil or a composite current collector. Examples of metal foils include aluminum foil or aluminum alloy foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. Examples of the metal material include one or more combinations selected from aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer substrate may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0121] The positive electrode film is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is usually formed by dispersing the positive electrode active material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to it.

[0122] [Isolation membrane]

[0123] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0124] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0125] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0126] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0127] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0128] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0129] In some embodiments, such as Figure 1 and Figure 2As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. Positive electrode sheets, negative electrode sheets, and a separator may be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0130] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is then placed in an outer packaging, dried, and injected with an electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0131] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. 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.

[0132] Figure 3 This is a schematic diagram of battery module 4 as an example. Figure 3 As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0133] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0134] In some embodiments, the battery modules described above 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.

[0135] Figure 4 and Figure 5 This is a schematic diagram of battery pack 1 as an example. Figure 4 and Figure 5 As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0136] Electrical appliances

[0137] Secondly, this application provides an electrical device, which includes at least one of the secondary battery, battery module, and battery pack described in this application. The secondary battery, battery module, and battery pack can be used as the power source for the electrical device or as the energy storage unit of the electrical device. The electrical device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0138] Electrical devices can be equipped with secondary batteries, battery modules, or battery packs depending on their usage requirements.

[0139] Figure 6 This is a schematic diagram of an example electrical device. The electrical device 6 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this electrical device, a battery pack 1 or a battery module can be used.

[0140] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0141] Example

[0142] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0143] Example 1

[0144] 1. Preparation of positive electrode sheet

[0145] Aluminum foil with a thickness of 12μm was used as the positive electrode current collector.

[0146] LiNi, the positive electrode active material 0.6 Co 0.2 Mn 0.2O2, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) are mixed thoroughly in an appropriate amount of solvent NMP at a weight ratio of 97.5:1.4:1.1 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, the positive electrode sheet is obtained.

[0147] 2. Preparation of negative electrode sheet

[0148] A copper foil with a thickness of 8μm was used as the negative electrode current collector.

[0149] The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na), and the conductive agent carbon black (Super P) are mixed thoroughly in an appropriate amount of deionized water at a weight ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry. The negative electrode slurry is then uniformly coated onto the surface of the negative electrode current collector copper foil. After drying and cold pressing, the negative electrode sheet is obtained.

[0150] 3. Separating membrane

[0151] Porous polyethylene (PE) membrane is used as the separator.

[0152] 4. Preparation of electrolyte

[0153] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate (EC) and diethyl carbonate (DMC) are mixed at a volume ratio of 1:1 to obtain an electrolyte solvent. Subsequently, lithium salt, the mixed solvent, and additives are mixed to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0154] 5. Preparation of secondary batteries

[0155] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound up. The electrode assembly is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0156] Example 2

[0157] Examples 2-1 to 2-10

[0158] The secondary batteries of Examples 2-1 to 2-10 were prepared in a similar manner to those of Example 1. The difference from Example 1 is that the mass percentage C of boron-containing lithium salt was adjusted in Examples 2-1 to 2-10.

[0159] Examples 2-11 and 2-12

[0160] The secondary batteries of Examples 2-11 to 2-12 were prepared in a similar manner to those of Example 1. The difference from Example 1 is that the types of boron-containing lithium salts were adjusted in Examples 2-11 to 2-12.

[0161] Comparative Example 1

[0162] The secondary battery of Comparative Example 1 was prepared in a similar manner to that of Example 1, except that no boron-containing lithium salt was added, and only LiPF6 was added.

[0163] Comparative Example 2 and Comparative Example 3

[0164] The secondary batteries of Comparative Examples 2 and 3 were prepared in a similar manner to those of Example 1, except that the mass percentage C of boron-containing lithium salt was adjusted in Comparative Examples 2 and 3.

[0165] Example 3

[0166] The secondary batteries of Examples 3-1 to 3-7 were prepared in a similar manner to those of Example 1. The difference from Example 1 is that the mass of the electrolyte Bg and the discharge capacity AAh of the secondary battery were adjusted in Examples 3-1 to 3-7.

[0167] Example 4

[0168] The secondary batteries of Examples 4-1 to 4-9 were prepared in a similar manner to that of Example 1. The difference from Example 1 is that the type and content of the negative electrode film-forming additive were adjusted in Example 4.

[0169] Example 5

[0170] The secondary batteries in Examples 5-1 to 5-4 were prepared using a method similar to that in Example 1. In Example 5, the coating weight F mg / 1540.25 mm of the negative electrode film was adjusted. 2 .

[0171] Data from Examples 1 to 5 and the comparative examples are shown in Tables 1 and 2:

[0172] Table 1

[0173]

[0174]

[0175] Table 2

[0176]

[0177]

[0178] Test section

[0179] (1) Cyclic performance test of secondary batteries at room temperature

[0180] At 25℃, the secondary battery was charged at a constant current of 0.5C to 4.4V, and then charged at a constant voltage until the current reached 0.05C. At this point, the secondary battery was fully charged, and the charging capacity was recorded, which is the first charge capacity. After the secondary battery was left to stand for 5 minutes, it was discharged at a constant current of 1C to 2.8V. This completes one charge-discharge cycle, and the discharge capacity was recorded, which is the first discharge capacity. The secondary battery was subjected to cyclic charge-discharge tests using the above method, and the discharge capacity after each cycle was recorded. The capacity retention rate (%) of the secondary battery after 600 cycles at 25℃ = discharge capacity after 600 cycles / discharge capacity of the first cycle × 100%.

[0181] (2) High-temperature cycle performance test of secondary batteries

[0182] At 45℃, the secondary battery was charged at a constant current of 0.5C to 4.4V, and then charged at a constant voltage until the current reached 0.05C. At this point, the secondary battery was fully charged, and the charging capacity was recorded, which is the first charge capacity. After the secondary battery was left to stand for 5 minutes, it was discharged at a constant current of 1C to 2.8V. This completes one charge-discharge cycle, and the discharge capacity was recorded, which is the first discharge capacity. The secondary battery was subjected to cyclic charge-discharge tests using the above method, and the discharge capacity after each cycle was recorded. The capacity retention rate (%) of the secondary battery after 600 cycles at 45℃ = discharge capacity after 600 cycles / discharge capacity of the first cycle × 100%.

[0183] (3) High-temperature storage performance test of secondary batteries

[0184] The battery was left to stand at 60℃ for 6 hours. It was then charged to 4.4V at 0.33C, followed by constant voltage charging to 0.05C. After standing for 5 minutes, it was discharged to 2.8V at 0.33C, with an initial discharge capacity of A0 Ah. The battery was then charged to 4.4V at 0.5C, followed by constant voltage charging to 0.05C. The fully charged battery was then placed in a 60℃ high-temperature furnace for 100 days. It was then discharged to 2.8V at 1C, with a final discharge capacity of A1 Ah. The capacity retention rate (%) of the secondary battery after 100 days of storage at 60℃ is calculated as [(A1-A0) / A0]×100%.

[0185] Storage volume test method: After the battery has been left to stand at 60℃ for 6 hours, charge it at a constant current of 1C to 4.3V, then continue charging at a constant voltage until the current reaches 0.05C. At this point, measure the volume of the secondary battery using the water displacement method and record it as V0. Place the secondary battery in a 60℃ constant temperature chamber and store it for 30 days. Remove it and measure the volume of the secondary battery again using the water displacement method, recording it as V1. The volume expansion rate (%) of the secondary battery after 30 days of storage at 60℃ = [(V1-V0) / V0] × 100%.

[0186] Test Results

[0187] The effects of this application on improving the cycle performance and storage performance of secondary batteries are shown in Table 3.

[0188] Table 3

[0189]

[0190]

[0191] As shown in the table, when no boron-containing lithium salt was added in Comparative Example 1, the negative electrode active material could not be protected, which made the structure of the negative electrode active material easily damaged during the secondary battery cycle, resulting in poor cycle performance and storage performance of the secondary battery.

[0192] In the embodiments of this application and Comparative Examples 2 and 3, boron-containing lithium salts were added to the electrolyte, which provided good protection for the negative electrode active material, thereby improving the cycle performance and storage performance of the secondary battery. However, Comparative Examples 2 and 3 still could not significantly improve the performance of the secondary battery. The reason for this may be that excessive amounts of boron-containing lithium salts were added, and the SEI film formed by boron-containing lithium salts alone on the negative electrode active material is not stable enough to provide long-term effective protection for the negative electrode active material. In Examples 1 to 2-12, by adjusting the amount of boron-containing lithium salts added to satisfy a certain relationship, 1≤B / A≤5, 5×10 -6 ≤B×C% / A≤0.25; especially satisfying 1×10 -3 When ≤B×C% / A≤0.23, boron-containing lithium salts can be combined with other parameters. Not only can boron-containing lithium salts form a stable and dense SEI film on the surface of the negative electrode active material, but they can also provide good long-term protection for the negative electrode active material.

[0193] Examples 3-1 to 3-7, by controlling the amount of electrolyte injected and the discharge capacity A Ah of the secondary battery, especially the electrolyte mass B satisfying: 0.01≤B≤3000; optionally, 0.5≤B≤1500, can adjust the thickness of the SEI film formed by boron-containing lithium salt on the surface of the negative electrode active material, thereby improving the cycle performance and storage performance of the secondary battery.

[0194] Examples 4-1 to 4-7 show that by controlling the type and content of the negative electrode film-forming additives, the effective components in the electrolyte can form a stable and richly composed SEI film on the surface of the negative electrode active material, which can further improve the protection of the negative electrode active material, thereby further improving the cycle performance and storage performance of the secondary battery.

[0195] Examples 5-1 to 5-4 demonstrate how adjusting the coating weight of the negative electrode film allows the electrolyte to fully protect the surface of the negative electrode active material, resulting in the uniform formation of an SEI film on the surface of the negative electrode active material, thereby improving the cycle performance and storage performance of the secondary battery.

[0196] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A secondary battery, comprising: Positive electrode sheet; A negative electrode sheet, which includes a negative electrode film layer containing a negative electrode active material; The electrolyte includes a boron-containing lithium salt configured to form a solid electrolyte interface film on the surface of the negative electrode active material. The discharge capacity of the secondary battery is denoted as A Ah; 2≤A≤5; The mass of the electrolyte is denoted as B g; 2 ≤ B ≤ 5; The boron-containing lithium salt includes lithium tetrafluoroborate and lithium difluorooxalate borate, and its mass percentage relative to the total mass of the electrolyte is denoted as C%, where 0.1 ≤ C ≤ 0.

2. The secondary battery satisfies: 1≤B / A≤5, 1×10 -3 ≤B×C% / A≤0.23; The coating weight of the negative electrode film is F mg / 1540.25 mm. 2 , 180≤F≤200; The electrolyte also includes a negative electrode film-forming additive, which includes a first type of additive and a second type of additive. The first type of additive includes carbonate additives, and the first type of additive also includes at least one of sulfate additives and sulfite additives. The second type of additive includes fluorooxalic acid or lithium phosphate skeleton and lithium sulfonylimide skeleton. The mass percentage of the carbonate additive relative to the total mass of the electrolyte is denoted as D1%, where 0.01% ≤ D1% ≤ 0.05%. The mass percentage of the sulfate ester additive and the sulfite ester additive relative to the total mass of the electrolyte is denoted as D2%, where 0.01% ≤ D2% ≤ 0.05%. The mass percentage of the fluorooxalic acid or lithium phosphate backbone relative to the total mass of the electrolyte is denoted as D3%, where 0.8% ≤ D3% ≤ 1%. The mass percentage of the sulfonamide skeleton lithium salt relative to the total mass of the electrolyte is denoted as D4%, where 0.001% ≤ D4% ≤ 0.0015%.

2. The secondary battery according to claim 1, wherein, The mass percentage of the negative electrode film-forming additive relative to the total mass of the electrolyte is denoted as D, and the secondary battery also satisfies: 0.1% ≤ (C+D)% ≤ 4.4%.

3. The secondary battery according to claim 1, wherein, The carbonate additives include cyclic carbonate additives and / or linear carbonate additives; and / or The sulfate ester additives include cyclic sulfonate additives and / or sulfate hydrocarbon ester additives; and / or The sulfite additives include vinyl sulfite ES and / or vinyl vinyl sulfite VES; and / or The fluorooxalic acid or phosphate backbone lithium salt includes one or more of lithium tetrafluoro(oxalate) phosphate LiTFOP, lithium difluorophosphate LiPO2F2, and lithium difluorodioxalate phosphate LiBODFP; and / or The sulfonyl imide skeleton lithium salt includes one or more of lithium bis(fluorosulfonyl)imide [LiN(SO2F)2], lithium bis(trifluoromethylsulfonyl)imide LiN(CF3SO2)2, and lithium bis(trifluoromethanesulfonyl)imide LiTFSI.

4. The secondary battery according to claim 3, wherein, The cyclic carbonate additives include one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), vinyl ethylene carbonate (VEC), and dioctyl carbonate (CC); the linear carbonate additives include one or more of ethyl allyl carbonate (AEC), diphenyl carbonate (DPC), methyl allyl carbonate (MAC), and polycarbonate (VA); and / or The cyclic sulfonate additives include one or more of 1,3-propanesulfonate lactone (PS), propenesulfonate lactone (PES), and 3-fluoro-1,3-propanesulfonate lactone (FPS); and / or the sulfate hydrocarbon ester additives include one or more of vinyl sulfate (DTD), diethyl sulfate (DES), and dimethyl sulfate (DMS).

5. The secondary battery according to claim 1, wherein, The secondary battery also meets the following requirement: 2×10 -5 ≤B×(C+D)% / F≤1.

6. A battery module comprising a secondary battery as described in any one of claims 1 to 5.

7. A battery pack comprising the battery module as claimed in claim 6.

8. An electrical device comprising a secondary battery as described in any one of claims 1 to 5, a battery module as described in claim 6, or a battery pack as described in claim 7.

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