Secondary batteries and electrical equipment

Abstract

This application provides a secondary battery and an electrical device. The secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative current collector and a negative active material layer coated on at least one side of the negative current collector. The electrolyte includes an ammonium salt additive and a lithium salt, wherein the lithium salt includes LiPF6, and the ammonium salt additive includes the ammonium salt cation represented by Formula I. The concentration of LiPF6 in the electrolyte is a mol / L; the mass percentage content of the ammonium salt additive in the electrolyte is b%; the conductivity of the negative active material layer is c S / cm; a, b, and c satisfy the relationship: 0.75 ≤ a × c / b ≤ 750. Based on the secondary battery of this application, the problem of existing lithium-ion batteries being unable to simultaneously achieve fast charging performance, high-temperature cycle performance, and safety performance is solved.

Description

Technical Field

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

[0002] Lithium-ion batteries, as one of the core technologies of new energy, are widely used in various fields, such as electric vehicles, energy storage systems, and portable electronic products.

[0003] In current mainstream lithium-ion battery systems, lithium hexafluorophosphate (LiPF6) is widely used as a lithium salt in the electrolyte due to its excellent ionic conductivity and suitable electrochemical window. However, lithium salt LiPF6 is highly susceptible to thermal decomposition, generating the strong Lewis acid PF5. PF5 can further react with trace amounts of water in the electrolyte to generate HF, leading to the presence of transition metal ions (such as Fe) in the cathode material. 2+ Mn 2+ Co 2+ Ni 2+ Increased dissolution and gas production severely deteriorate the high-temperature cycle performance and safety performance of batteries. To suppress the dissolution of transition metal ions, current technologies often employ the addition of vinylene carbonate (VC) as a film-forming additive to the electrolyte. VC can form a dense solid electrolyte interphase (SEI) film on the negative electrode surface, effectively inhibiting the dissolution of transition metal ions. However, the addition of high VC content significantly increases the negative electrode interfacial impedance, reduces lithium-ion transport kinetics, and leads to a deterioration in the battery's fast-charging performance. Therefore, simultaneously suppressing the dissolution of transition metal ions and reducing the negative electrode interfacial impedance, enabling lithium-ion batteries to balance fast-charging performance, high-temperature cycle performance, and safety performance, has become a key technical challenge in electrolyte development. Summary of the Invention

[0004] The main objective of this application is to provide a secondary battery and electrical device to solve the problem that existing secondary batteries cannot simultaneously achieve fast charging performance, high-temperature cycle performance, and safety performance.

[0005] To address the aforementioned problems, according to a first aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, and an electrolyte; the negative electrode comprises a negative current collector and a negative active material layer coated on at least one side of the negative current collector; the electrolyte comprises an ammonium salt additive and a lithium salt, the lithium salt comprising LiPF6, and the ammonium salt additive comprising an ammonium salt cation represented by Formula I.

[0006]

[0007] In Formula I, R1, R2, R3 and R4 are independently selected from H, straight-chain or branched alkyl groups having 1 to 7 carbon atoms, and benzene rings, and R1, R2, R3 and R4 are not all H at the same time;

[0008] The concentration of LiPF6 in the electrolyte is a mol / L;

[0009] The mass percentage content of ammonium salt additives in the electrolyte is b%;

[0010] The conductivity of the negative electrode active material layer is cS / cm;

[0011] a, b, and c satisfy the following relationship: 0.75 ≤ a × c / b ≤ 750.

[0012] Furthermore, a ranges from 0.5 to 1.5.

[0013] Furthermore, a ranges from 0.8 to 1.2.

[0014] Furthermore, b ranges from 0.05 to 2.

[0015] Furthermore, b ranges from 0.1 to 1.5.

[0016] Furthermore, c ranges from 3 to 25.

[0017] Furthermore, c ranges from 5 to 20.

[0018] Furthermore, a, b, and c satisfy the relation: 2.7 ≤ a × c / b ≤ 240.

[0019] Furthermore, the electrolyte also includes carbonate film-forming additives, which are selected from one or more of vinylene carbonate, fluoroethylene carbonate, and vinyl carbonate; the mass percentage content of the carbonate film-forming additives in the electrolyte is d%, and d satisfies equations (I) to (III) with a, b, and c:

[0020] 0.0003≤m / (n×a)≤133.33(I);

[0021] m=(d×b)(II;

[0022] n=(0.01c / b)(III)。

[0023] Furthermore, m, n, and a satisfy the relation: 0.02≤m / (n×a)≤17.05.

[0024] Furthermore, m ranges from 0.025 to 10.

[0025] Furthermore, n ranges from 0.33 to 20.

[0026] Furthermore, d ranges from 1 to 3.

[0027] Furthermore, the carbonate film-forming additive is vinylene carbonate.

[0028] Furthermore, the ammonium salt additive also contains anions, the anions being selected from BF4. - F - Cl - PF6 - One or more of them.

[0029] Furthermore, the anion is BF4. - .

[0030] Furthermore, R1, R2, R3 and R4 are each independently selected from straight-chain or branched alkyl groups having 1 to 7 carbon atoms.

[0031] Furthermore, R1, R2, R3, and R4 are independently selected from one of n-propyl, n-butyl, n-pentyl, and n-hexyl.

[0032] Furthermore, R1, R2, R3, and R4 are the same.

[0033] Furthermore, the electrolyte also includes a solvent selected from one or more of the following: ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0034] Furthermore, the negative electrode active material layer includes a negative electrode active material, which is selected from one or two of graphite and silicon-based materials.

[0035] Furthermore, the negative electrode active material is graphite, which is selected from one or two of natural graphite and artificial graphite.

[0036] Furthermore, the graphite particle size Dv50 ranges from 4 μm to 20 μm.

[0037] Furthermore, the specific surface area of ​​graphite is 0.6 m². 2 / g to 2.2m 2 / g.

[0038] Furthermore, the graphitization degree of graphite is 90% to 98%.

[0039] Furthermore, the positive electrode sheet includes a positive current collector and a positive active material layer coated on at least one side of the positive current collector. The positive active material layer includes a positive active material selected from one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, and lithium manganese oxide.

[0040] According to a second aspect of this application, an electrical device is provided, including the aforementioned secondary battery.

[0041] By applying the technical solution of this application, by introducing ammonium salt additives into the electrolyte and synergistically proportioning them with LiPF6, and simultaneously controlling the conductivity of the negative electrode active material layer, the concentration of LiPF6 in the electrolyte (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte (b), and the conductivity of the negative electrode active material layer (c S / cm) satisfy 0.75≤a×c / b≤750. This facilitates an optimized balance between the ion conduction capacity in the secondary battery and the inhibition strength of LiPF6 decomposition in the electrolyte. While reducing the negative electrode interface impedance, improving the lithium-ion transport performance in the secondary battery, and enhancing the fast-charging performance of the secondary battery, it can effectively suppress the decomposition of LiPF6 to generate HF under high-temperature cycling conditions, significantly reduce the dissolution of transition metal ions in the positive electrode material and the amount of gas generated in the battery, and improve the high-temperature cycling performance and safety performance of the secondary battery. Detailed Implementation

[0042] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to the embodiments.

[0043] The secondary battery in this application, also known as a rechargeable battery or storage battery, refers to a battery that can be used again after being discharged by recharging to activate the active materials.

[0044] As described in the background section, existing lithium-ion batteries suffer from difficulties in simultaneously achieving fast charging performance, high-temperature cycle performance, and safety performance. To address these issues, according to a first aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, and an electrolyte; the negative electrode includes a negative current collector and a negative active material layer coated on at least one side of the negative current collector; the electrolyte includes an ammonium salt additive and a lithium salt, wherein the lithium salt includes LiPF6, and the ammonium salt additive comprises an ammonium salt cation represented by Formula I;

[0045]

[0046] In Formula I, R1, R2, R3 and R4 are independently selected from H, straight-chain or branched alkyl groups having 1 to 7 carbon atoms, and benzene rings, and R1, R2, R3 and R4 are not all H at the same time;

[0047] The concentration of LiPF6 in the electrolyte is a mol / L;

[0048] The mass percentage content of ammonium salt additives in the electrolyte is b%;

[0049] The conductivity of the negative electrode active material layer is cS / cm;

[0050] a, b, and c satisfy the following relationship: 0.75 ≤ a × c / b ≤ 750.

[0051] In this application, the electrolyte plays the role of conducting ions between the positive and negative electrodes. LiPF6 serves as the salt source for lithium ion conduction, and its content can increase the ionic conductivity of the electrolyte, thereby improving the fast-charging performance of the secondary battery. However, if the content of LiPF6 is too high, it will accelerate the thermal decomposition of LiPF6 to generate the strong Lewis acid PF5 (LiPF6→LiF+PF5), which then reacts with trace amounts of water to generate HF (PF5+H2O→HF+POF3). This leads to a large amount of Fe, Mn and other transition metal ions in the positive electrode material dissolving and increasing the amount of gas produced in the battery, causing damage to the structure of the positive electrode material and deteriorating the high-temperature cycle performance and safety performance of the secondary battery.

[0052] The introduction of ammonium salt cations interferes with Li + The solvated sheath layer reduces PF6 in the electrolyte through electrostatic shielding. - With Li + The coordination strength is increased, thereby inhibiting the decomposition reaction of LiPF6 and reducing the formation of strong Lewis acid PF5, thus blocking the HF formation pathway at the source, reducing the dissolution of transition metal ions in the cathode material and the amount of gas produced by the battery, and improving the high-temperature cycle performance and safety performance of the secondary battery; at the same time, the dissolution of transition metal ions such as Fe and Mn in the cathode material is effectively suppressed, avoiding the reduction and deposition of transition metal ions on the negative electrode surface and the continuous regeneration of the SEI film, thereby reducing the interfacial impedance, improving the lithium ion transport efficiency at the interface, and improving the fast charging performance of the secondary battery.

[0053] When the ammonium salt cation is an organic ammonium salt cation with a hydrophobic group (at least one of R1, R2, R3, and R4 is an alkyl or benzene ring), it can effectively displace Li in the electrolyte. + The surrounding solvent molecules interfere with Li through steric hindrance. + Solvation of the sheath layer reduces PF6 in LiPF6. - With Li + Direct contact inhibits the thermal decomposition reaction of LiPF6 and reduces the formation of the strong Lewis acid PF5. Simultaneously, the organic ammonium salt cation forms an isolation barrier in the electrolyte, physically hindering the contact between PF5 and water molecules, reducing the reactivity of the strong Lewis acid PF5, thereby blocking the HF formation pathway at its source and significantly reducing the dissolution of transition metal ions in the cathode material and the amount of gas produced by the battery.

[0054] Increasing the content of ammonium salt cations can improve the high-temperature cycle performance and safety performance of batteries. However, if the content of ammonium salt cations is too high, it is easy to adsorb and form a high-resistance insulating layer on the surface of the negative electrode, which increases the interface impedance of the negative electrode, resulting in the obstruction of lithium ion transport at the interface and deteriorating the fast charging performance of the secondary battery.

[0055] The conductivity of the negative electrode active material layer is cS / cm. Increasing c improves the conductivity of the negative electrode, which helps reduce the interface impedance and enhances lithium-ion transport performance at the interface, thus improving fast-charging performance. However, excessively high c, while providing advantages for fast charging, also significantly exacerbates interfacial side reactions and the thermal decomposition of LiPF6, leading to increased HF generation. This, in turn, causes the dissolution of transition metal ions in the positive electrode material and increases battery gas production. Simultaneously, the reduction and deposition of transition metal ions on the negative electrode surface damages the SEI film structure, triggering a vicious cycle of SEI film rupture-regeneration-thickening, resulting in deterioration of the high-temperature cycle stability and safety performance of the secondary battery.

[0056] The concentration of LiPF6 in the electrolyte (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte (b%), and the conductivity of the negative electrode active material layer (c S / cm) satisfy 0.75 ≤ a × c / b ≤ 750. In these conditions, the secondary battery exhibits high fast-charging performance, high high-temperature cycle performance, and good safety performance. If a × c / b is less than 0.75, the fast-charging performance of the secondary battery is poor; if a × c / b is greater than 750, the high-temperature cycle performance and safety performance of the secondary battery are poor.

[0057] In summary, based on the secondary battery of this application, by introducing ammonium salt additives into the electrolyte and synergistically proportioning them with LiPF6, and simultaneously controlling the conductivity of the negative electrode active material layer, the concentration of LiPF6 in the electrolyte (a mol / L), the mass percentage content of the ammonium salt additive in the electrolyte (b), and the conductivity of the negative electrode active material layer (c S / cm) satisfy 0.75≤a×c / b≤750. This facilitates an optimized balance between the ion conduction capacity of the secondary battery and the inhibition strength of LiPF6 decomposition in the electrolyte. While reducing the negative electrode interface impedance, improving the lithium-ion transport performance of the secondary battery, and enhancing the fast-charging performance of the secondary battery, it can effectively suppress the decomposition of LiPF6 to generate HF under high-temperature cycling conditions, significantly reduce the dissolution of transition metal ions in the positive electrode material and the amount of gas generated in the battery, and improve the high-temperature cycling performance and safety performance of the secondary battery.

[0058] In the above embodiments, a×c / b can specifically be a range of 0.75, 10, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750 or any two of them, and is not limited here.

[0059] In some embodiments, 'a' is 0.5 to 1.5. A LiPF6 concentration 'a' in the electrolyte, ranging from 0.5 mol / L to 1.5 mol / L, is beneficial for improving the lithium-ion transport performance of the electrolyte while ensuring lower HF generation, transition metal ion dissolution, and gas production, thus obtaining a rechargeable battery that combines fast-charging performance, high-temperature cycling performance, and safety performance. 'a' can specifically be a range of 0.5, 0.8, 1, 1.2, 1.5, or any combination thereof, and is not limited herein.

[0060] In some embodiments, a is 0.8 to 1.2. The concentration of LiPF6 in the electrolyte, a mol / L, is 0.8 mol / L to 1.2 mol / L, providing a more sufficient Li mol / L concentration. + The concentration is adjusted to maintain a high ionic conductivity in the electrolyte. At the same time, LiPF6 has a low high-temperature thermal decomposition rate, which keeps the amount of PF5 generated at a low level and reduces the HF generation pathway. This improves the ion transport efficiency of the electrolyte under high-rate charge and discharge conditions, while effectively maintaining chemical stability under high-temperature conditions. This ensures high-temperature cycle performance and safety performance while achieving high-rate charging.

[0061] In some embodiments, b is 0.05 to 2. When the mass percentage content b% of the ammonium salt additive in the electrolyte is 0.05% to 2%, it is beneficial to block the formation pathway of HF, reduce the dissolution of transition metal ions in the cathode material and the amount of gas generated by the battery, and facilitate the formation of a dense, low-resistance, and stable solid electrolyte interface film, thereby improving the high-temperature cycle stability, safety performance, and fast-charging performance of the secondary battery. b can specifically be a range of 0.05, 0.1, 0.2, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, or any two of these ranges, and is not limited here.

[0062] In some embodiments, b is 0.1 to 1.5. A mass percentage content of 0.1% to 1.5% for the ammonium salt additive in the electrolyte is more conducive to obtaining a secondary battery that combines fast-charging performance, high-temperature cycling performance, and safety performance.

[0063] In some embodiments, c is 3 to 25. The conductivity of the negative electrode active material layer is 3 S / cm to 25 S / cm. The negative electrode has high electron conductivity, which allows lithium ions to quickly embed between graphite layers during charging, reducing polarization voltage and improving rate performance. At the same time, it helps to reduce the excessive reduction reaction of the electrolyte caused by excessive electron density and reduces the thickening and loosening of the SEI film, forming a denser, low-resistance, and stable SEI film, thereby improving the high-temperature cycle stability, safety performance, and fast-charging performance of the secondary battery. c can specifically be a range of 3, 5, 10, 15, 20, 25, or any two of them, and is not limited here.

[0064] In some embodiments, c is 5 to 20. The conductivity of the negative electrode active material layer is 5 S / cm to 20 S / cm, which is beneficial for obtaining a secondary battery that combines fast charging performance, high-temperature cycling performance and safety performance; preferably, c is 15 to 18.

[0065] In some embodiments, a, b, and c satisfy the relationship: 2.7 ≤ a × c / b ≤ 240. When the concentration of LiPF6 in the electrolyte is a mol / L, the mass percentage content of ammonium salt additive in the electrolyte is b%, and the conductivity of the negative electrode active material layer is c S / cm, and these conditions satisfy 2.7 ≤ a × c / b ≤ 240, the system possesses a more sufficient ion-electron synergistic driving force, which is beneficial for low-polarity fast charging. At the same time, it has a higher suppression strength for PF5 formation, so that the dissolution of transition metal ions in the positive electrode material and the battery gas production are maintained at a low level. This achieves synergistic optimization between the lithium-ion conduction driving force and the PF5 suppression strength in the electrolyte, improving the fast charging performance of the secondary battery while maintaining its high-temperature cycle performance and safety performance.

[0066] In some embodiments, the electrolyte further includes a carbonate film-forming additive selected from one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), and vinyl carbonate (VEC); the mass percentage content of the carbonate film-forming additive in the electrolyte is d%, and d satisfies equations (I) to (III) with respect to a, b, and c:

[0067] 0.0003≤m / (n×a)≤133.33(I);

[0068] m=(d×b)(II;

[0069] n=(0.01c / b)(III)。

[0070] Based on the technical solution of this application embodiment, LiPF6 is the salt source for lithium-ion conduction, but it is also the source of PF5 and HF. The concentration of LiPF6 in the electrolyte is a mol / L. Increasing a can improve the lithium-ion transport efficiency of the electrolyte, but it will also aggravate the thermal decomposition of LiPF6 and the generation of HF. This will aggravate the dissolution of transition metal ions in the positive electrode material and the increase of battery gas production caused by HF generation. Transition metal ions are reduced and deposited on the surface of the negative electrode, destroying the SEI film structure and deteriorating the high-temperature cycle performance and safety performance of the secondary battery.

[0071] The conductivity of the negative electrode active material layer is c S / cm. As c increases, the conductivity of the negative electrode is improved, which helps to reduce the interface impedance of the negative electrode and improve the lithium ion transport performance at the interface, thereby improving the fast charging performance. However, the high electron density of the negative electrode will accelerate the reduction of electrolyte, the thermal decomposition of LiPF6 and the generation of HF, which will damage the SEI film structure.

[0072] The ammonium salt additive in the electrolyte has a mass percentage content of b%. The ammonium salt cations inhibit the thermal decomposition of LiPF6 and the generation of HF through electrostatic shielding, steric hindrance, and physical barriers. Increasing b helps reduce the risk of SEI film structure damage, but if its content is too high, the lithium-ion transport efficiency is low, thus reducing fast-charging performance.

[0073] n (0.01c / b) can be used to characterize the risk of SEI film structure damage caused by the thermal decomposition of LiPF6 in the electrolyte system. A lower n (0.01c / b) is beneficial in reducing the risk of SEI film structure damage and improving the high-temperature cycle performance and safety performance of the secondary battery. However, when n is too low, the lithium-ion transport efficiency is low, thus reducing fast-charging performance.

[0074] By introducing carbonate-based film-forming additives into the electrolyte, a primary SEI film rich in ROCO2Li and LiF is generated on the negative electrode surface during the first charge, providing mechanical integrity. The mass percentage content of carbonate-based film-forming additives in the electrolyte is d%. Increasing the mass percentage (d) promotes the formation of a stable and dense primary SEI film, improving the high-temperature cycle performance and safety of the secondary battery. Conversely, an excessively large mass percentage (d) increases the risk of continuous thickening and structural loosening of the primary SEI film, reducing its stability and deteriorating the high-temperature cycle performance and safety of the secondary battery.

[0075] Ammonium salt additives inhibit the thermal decomposition of LiPF6, block HF generation, provide a clean interface for the SEI film, and reduce the dissolution of transition metal ions that damage the SEI film, thus promoting the formation of a stable and dense initial SEI film. The mass percentage content of ammonium salt additives in the electrolyte is b%. A higher b value promotes the formation of a stable and dense initial SEI film; however, if the b value is too high, ammonium salt cations are more likely to adsorb on the negative electrode surface, forming a high-resistivity insulating layer, increasing the negative electrode interface impedance, and thus reducing the fast-charging performance of the secondary battery.

[0076] The value of m (d×b) can be used to characterize the stability of the initial SEI film in the electrolyte system. Increasing m promotes the formation of a stable and dense initial SEI film, improving the high-temperature cycle performance and safety of the secondary battery. However, if m is too large, d or b will also be large. An excessively large d value can easily lead to the continuous thickening and loosening of the initial SEI film, reducing its stability and impairing the high-temperature cycle performance and safety of the secondary battery. Conversely, an excessively large b value can cause ammonium salt cations to adsorb onto the negative electrode surface, forming a high-resistance insulating layer, increasing the negative electrode interface impedance and thus reducing the fast-charging performance of the secondary battery. Conversely, if m is too small, d or b will be too small, hindering the formation of a stable and dense initial SEI film, thereby reducing the high-temperature cycle performance and safety of the secondary battery.

[0077] The value of m / (n×a) can be used to characterize the SEI film performance of electrolyte systems containing carbonate-based film-forming additives. Increasing m / (n×a) is beneficial for forming a stable and dense initial SEI film, reducing the risk of SEI film structure damage caused by LiPF6 thermal decomposition, and improving the high-temperature cycle performance and safety performance of secondary batteries. When m / (n×a) is too large, m is too large, or n is too large, or a is too small, which reduces fast charging performance.

[0078] In summary, the concentration of LiPF6 in the electrolyte (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte (b), and the conductivity of the negative electrode active material layer (c S / cm) satisfy 0.75 ≤ a × c / b ≤ 750, and 0.0003 ≤ m / (n × a) ≤ 133.33, where m = (d × b) and n = (0.01c / b). This is beneficial for obtaining a structurally stable and dense initial SEI film, reducing the risk of SEI film structure damage due to LiPF6 thermal decomposition, improving the high-temperature cycle performance and safety performance of the secondary battery, while maintaining its high fast-charging performance.

[0079] In some embodiments, m, n, and a satisfy the relationship: 0.02≤m / (n×a)≤17.05. 0.02≤m / (n×a)≤17.05 is beneficial for achieving an optimized balance between the SEI film performance and fast-charging performance of electrolyte systems containing carbonate-based film-forming additives, improving the high-temperature cycle performance and safety performance of secondary batteries, while maintaining their high fast-charging performance.

[0080] In some embodiments, m is 0.025 to 10. A m of 0.025 to 10 is beneficial for forming a structurally stable and dense initial SEI film, improving the high-temperature cycle performance and safety performance of the secondary battery, while maintaining its high fast-charging performance; preferably, m is 0.5 to 4.5.

[0081] In some embodiments, n is 0.33 to 20. An n value of 0.33 to 20 is beneficial in reducing the risk of SEI film structure damage due to the thermal decomposition of LiPF6 in the electrolyte system containing carbonate-based film-forming additives, thereby improving the high-temperature cycle performance and safety performance of the secondary battery while maintaining its high fast-charging performance.

[0082] In some embodiments, d is 1 to 3. The mass percentage content of carbonate film-forming additives in the electrolyte is d% to 3%, which is beneficial for forming a stable and dense initial SEI film and improving the high-temperature cycle performance and safety performance of the secondary battery.

[0083] In some embodiments, the carbonate-based film-forming additive is VC, which is beneficial for forming a stable and dense initial SEI film, reducing gas production, and improving the high-temperature cycle performance and safety performance of the secondary battery.

[0084] In some embodiments, the ammonium salt additive further comprises an anion selected from BF4. - F - Cl - PF6 - One or more of them.

[0085] In the technical solution of this application embodiment, the anion BF4 - It can coordinate with the strong Lewis acid PF5 produced by the decomposition of LiPF6 at high temperature to form a stable [BF4] - The PF5] complex, via ammonium salt cations and BF4 - The synergistic effect of anions effectively inhibits the reaction of PF5 with trace amounts of moisture to generate highly corrosive HF, fundamentally blocking the dissolution pathway of transition metal ions from the cathode material and reducing gas production, thereby improving the high-temperature cycle performance and safety performance of the secondary battery. Meanwhile, BF4... - This is beneficial for forming SEI films rich in LiF and boron fluoride, thus improving the ionic conductivity and structural stability of the SEI film; the anion is F. - or Cl - It can participate in the construction of SEI membranes, which is beneficial for forming SEI membranes rich in LiF or LiCl, thereby improving the ionic conductivity and structural stability of the SEI membrane; the anion is PF6. - This is beneficial for improving the overall ionic conductivity of the electrolyte and enhancing the fast-charging performance of the secondary battery; the preferred anion is BF4. - This is beneficial for improving the high-temperature cycle performance, safety performance, and fast-charging performance of secondary batteries. As an example, the cation in the boron-containing ammonium salt additive can be one or more of tetramethylammonium, tetraethylammonium, tetra-n-propylammonium, tetraisopropylammonium, tetra-n-butylammonium, tetraisobutylammonium, tetra-sec-butylammonium, tetra-tert-butylammonium, tetra-n-pentylammonium, tetra-n-hexylammonium, tetra-n-heptylammonium, and tetraphenylammonium. As an example, the ammonium salt additive can be one or more of tetramethylammonium fluoroborate, tetraethylammonium fluoroborate, tetra-n-butylammonium fluoroborate, and tetraphenylammonium fluoroborate.

[0086] In some embodiments, R1, R2, R3 and R4 are each independently selected from straight-chain or branched alkyl groups having 1 to 7 carbon atoms.

[0087] In the technical solution of this application embodiment, R1, R2, R3, and R4 in the ammonium salt cation are all defined as alkyl groups, so that the ammonium salt additive exhibits a highly uniform all-alkyl substitution configuration, thereby enhancing its steric stability and consistency. This is beneficial for effectively improving the control precision of the solvation shell by the ammonium salt cation, blocking the HF generation pathway from the source, and stabilizing and suppressing the dissolution behavior of the cathode material and the gas production of the battery during high-temperature cycling, thus improving the high-temperature cycling performance, safety performance, and fast-charging performance of the secondary battery. As an example, R1, R2, R3, and R4 can be independently selected from one of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, and n-heptyl, including but not limited to the alkyl groups listed above, and their isomers are all included within the scope of protection of this application.

[0088] In some embodiments, R1, R2, R3, and R4 are independently selected from one of n-propyl, n-butyl, n-pentyl, and n-hexyl, which is beneficial for further improving the high-temperature cycle performance, safety performance, and fast charging performance of the secondary battery.

[0089] In some embodiments, R1, R2, R3, and R4 are the same, which is beneficial for further improving the high-temperature cycle performance, safety performance, and fast charging performance of the secondary battery. As an example, R1, R2, R3, and R4 can all be n-propyl, n-butyl, n-pentyl, or n-hexyl.

[0090] In some embodiments, the electrolyte further includes a solvent selected from 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), butyl 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).

[0091] In the technical solutions of this application embodiment, esters such as ethylene carbonate, methyl ethyl carbonate, dimethyl carbonate, sulfolane, ethyl acetate, and 1,4-butyrolactone are used as electrolyte solvents, which helps to reduce the viscosity of the system while maintaining high ionic conductivity, synergistically improve lithium-ion migration kinetics, and enhance the fast-charging performance of the secondary battery; sulfones have high thermal stability and antioxidant properties, which helps to suppress electrolyte decomposition, cathode metal dissolution, and battery gas production under high temperature conditions, thereby improving the high-temperature cycle stability and safety performance of the secondary battery.

[0092] In some embodiments, the lithium salt further includes one or more of lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0093] In some embodiments, the negative electrode active material layer includes a negative electrode active material, which is selected from one or two of graphite and silicon-based materials. This application does not specifically limit the type of silicon-based material, which can be silicon-carbon and / or silicon-oxygen materials. As an example, the silicon-based material can be one or more of silicon-carbon composite negative electrode materials, silicon suboxide negative electrode materials, modified silicon suboxide negative electrode materials, and nano-silicon materials; preferably, the negative electrode active material is graphite, selected from one or two of natural graphite and artificial graphite. Using graphite as the negative electrode active material can fully utilize its high electrical conductivity and, through synergistic effects with LiPF6 and ammonium salt additives, significantly improve its fast-charging performance while ensuring high battery high-temperature cycle stability. This allows the electrolyte to achieve transition metal ion dissolution suppression and interface kinetic optimization in battery systems using graphite as the negative electrode.

[0094] In some embodiments, the graphite particle size Dv50 is from 4 μm to 20 μm.

[0095] In this application, particle size Dv50 refers to the particle diameter corresponding to a cumulative volume percentage of 50% in the particle size volume distribution curve.

[0096] In the technical solution of this application embodiment, the negative electrode active material is graphite, and the particle size Dv50 is controlled to be 4 μm to 20 μm. This helps to optimize and improve the coating uniformity and electrode density of the negative electrode slurry, reduce the non-uniformity of the ion diffusion path, and thus improve the insertion and extraction kinetic efficiency of lithium ions in the negative electrode. At the same time, the above-mentioned particle size range is conducive to reducing the volume stress concentration of the electrode during cycling, reducing particle breakage and the continuous reconstruction problem of the SEI film, reducing side reaction gas generation and interface impedance, and improving the high-temperature cycle performance, safety performance and fast charging performance of the secondary battery.

[0097] In some embodiments, the specific surface area of ​​graphite is 0.6 m². 2 / g to 2.2 m 2 / g. The negative electrode active material is graphite, and its specific surface area is within the above range, which is beneficial to provide sufficient sites and reduce the problem of SEI film thickening caused by side reactions. This is conducive to the formation of a uniform, dense and stable SEI film on the negative electrode surface, thereby improving the fast charging performance, high temperature cycle performance and safety performance of the secondary battery.

[0098] In some embodiments, the graphite degree of the graphite is 90% to 98%. The negative electrode active material is graphite with a graphite degree of 90% to 98%. Graphite possesses excellent lattice order, significantly improving electron transport efficiency and helping to ensure high fast-charging performance of the secondary battery. Simultaneously, it retains appropriate grain boundaries and edge defects, providing mild reducing activity for the film-forming agent, promoting the formation of a structurally stable, LiF-rich SEI film, enhancing interface stability, and contributing to obtaining a secondary battery with high fast-charging performance, high-temperature cycling performance, and safety performance.

[0099] In some embodiments, the negative electrode active material layer further comprises a negative electrode conductive agent and a negative electrode binder.

[0100] This application does not specifically limit the type of negative electrode conductive agent. In some embodiments, as an example, the negative electrode conductive agent can be one or more of conventional negative electrode conductive agents such as acetylene black and carbon nanotubes.

[0101] This application does not impose specific limitations on the type of negative electrode binder. In some embodiments, as examples, the negative electrode binder may be one or more of conventional negative electrode binders such as styrene-butadiene rubber latex (SBR), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), and sodium carboxymethyl cellulose (CMC).

[0102] This application does not specifically limit the type of negative electrode current collector. In some embodiments, as an example, the negative electrode current collector can be one of the conventional negative electrode current collectors such as copper foil.

[0103] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, negative electrode conductive agent, negative electrode binder and any other components, in a solvent (e.g., water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and after drying, rolling, cutting and other processes, the negative electrode sheet can be obtained.

[0104] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer coated on at least one side of the positive current collector. The positive active material layer includes a positive active material selected from one or more of lithium iron phosphate (LiFePO4), lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, and lithium manganese oxide. By using the above-mentioned materials as the positive active material in the positive active material layer, and by introducing ammonium salt additives with specific structures into the electrolyte, the formation pathway of HF can be effectively blocked, the dissolution of transition metal ions in lithium iron phosphate can be inhibited, and the structural integrity and capacity retention of the secondary battery under high-temperature cycling conditions can be improved.

[0105] In some embodiments, the positive electrode active material layer may also include a positive electrode conductive agent and a positive electrode binder.

[0106] In this application, no particular restrictions are placed on the type of positive electrode conductive agent, as long as it has suitable electronic conductivity and does not cause adverse chemical changes in the secondary battery. As an example, the positive electrode conductive agent can be at least one of carbon nanotubes (CNTs), carbon black (e.g., Super P), and graphene.

[0107] In this application, the positive electrode binder is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. This application does not have any particular limitation on the type of positive electrode binder, and any conventional choice in the field of secondary batteries can be used. As an example, the binder can be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, sodium carboxymethyl cellulose (CMC), or sodium alginate.

[0108] This application does not impose any particular limitations on the positive electrode current collector, as long as it is conductive and will not cause adverse chemical changes in the secondary battery. Materials used include, for example, stainless steel, aluminum, nickel, titanium, sintered carbon; or aluminum or stainless steel that has undergone a surface treatment with one of carbon, nickel, titanium, silver, etc. In some embodiments, the positive electrode sheet can be prepared by dispersing the components used to prepare the positive electrode sheet, such as the positive electrode active material, positive electrode conductive agent, positive electrode binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector; and then performing processes such as drying, rolling, and cutting to obtain the positive electrode sheet.

[0109] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive electrode active material, positive electrode conductive agent, positive electrode binder and any other components, in a solvent (e.g., water) to form a positive electrode slurry; coating the positive electrode slurry onto a positive electrode current collector, and obtaining the positive electrode sheet after drying, rolling, cutting and other processes.

[0110] In some embodiments, the secondary battery further includes an outer casing and a separator located between the positive and negative electrodes. The positive electrode, negative electrode, electrolyte, and separator are assembled within the outer casing. During the charging and discharging process of the secondary battery, active ions (e.g., lithium ions) repeatedly insert and extract between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily serves to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. The electrolyte, located between the positive and negative electrodes, primarily functions to conduct active ions.

[0111] This application does not impose any particular restriction on the type of diaphragm; any well-known porous diaphragm with good chemical and mechanical stability can be selected. As an example, the diaphragm can be one of PP, PE, or PP / PF. Alternatively, the diaphragm can have a coating on the surface of a base membrane, wherein the base membrane coating can be one of PP, PE, or PP / PF, and the coating can be an inorganic coating and / or an organic coating. The inorganic coating can be selected from alumina ceramic layers, osmium silicate, etc., and the organic coating can be selected from PVDF, etc.

[0112] As an example, the preparation process of a secondary battery is as follows: the positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, the electrodes are wound or stacked to obtain an electrode assembly. The electrode assembly is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0113] According to a second aspect of this application, an electrical device is provided, including the aforementioned secondary battery.

[0114] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0115] Example 1

[0116] A method for preparing a secondary battery includes the following steps:

[0117] Step 1, Preparation of the positive electrode sheet: LiFePO4, conductive agent carbon black (Super P), and binder PVDF are mixed at a mass ratio of 96:1.5:2.5, and then dispersed in the solvent NMP to obtain a positive electrode slurry. The positive electrode slurry is coated onto one side of a positive electrode current collector aluminum foil (15 μm thick). The positive electrode current collector coated with the positive electrode slurry is placed in a vacuum furnace and dried at 100°C for 12 hours. The dried positive electrode slurry forms a positive electrode active material layer, resulting in an aluminum foil coated with the positive electrode active material layer. The aluminum foil coated with the positive electrode active material layer is then rolled and slit to obtain the positive electrode sheet. The areal density of the positive electrode sheet is 400 g / m³. 2 The compacted density is 2.65 g / cm³. 3 .

[0118] Step 2, Preparation of the negative electrode: Artificial graphite (particle size Dv50 of 10 μm, specific surface area of ​​1.5 m²) is used. 2 The negative electrode slurry is prepared by mixing a graphitized copper foil (95% graphitization), conductive agent CNT, and binder CMC in a mass ratio of 96:1.5:2.5 and dispersing them in deionized water. The negative electrode slurry is then uniformly coated onto a 120 μm thick copper foil current collector. The coated copper foil is transferred to a vacuum drying oven and dried at 100°C for 12 hours. The dried negative electrode slurry forms a negative electrode active material layer, resulting in a copper foil coated with this layer. This coated copper foil is then rolled and slit to obtain the negative electrode sheet. The areal density of the negative electrode sheet is 180 g / m³. 2 The compacted density is 1.65 g / cm³. 3 The conductivity of the negative electrode active material layer is 18 S / cm.

[0119] Step 3, Electrolyte Preparation: EC and EMC are mixed in a weight ratio of 3:7 to obtain a mixed organic solvent. The dried lithium salt LiPF6, film-forming additives, and negative electrode preparation materials are dissolved in the mixed organic solvent to prepare the electrolyte. The concentration of LiPF6 in the electrolyte is 1.15 mol / L (a mol / L), the mass percentage content of ammonium salt additives is 0.3% (b%), and the mass percentage content of carbonate film-forming additives is 2% (d%). The values ​​of a×c / b, m, n, and m / (n×a) are shown in Table 1. The carbonate film-forming additive is VC. R1, R2, R3, and R4 in the ammonium salt additives are all n-butyl, and the anion is BF4. - .

[0120] Step 4, Preparation of the secondary battery: The above-mentioned positive electrode sheet, polyethylene separator, and negative electrode sheet are stacked to obtain a bare cell; the bare cell is placed in an outer packaging shell, dried, and then the above-mentioned electrolyte is injected into the bare cell, followed by vacuum sealing. After sealing, the cell is left to stand at 45°C for 24 hours, and then charged at a constant current of 0.05C to 10% SOC of the theoretical capacity (X% SOC is the percentage of the battery's current remaining usable capacity relative to the theoretical capacity, where 0≤X≤100), and then charged at... The battery is charged at 0.33C to 70% SOC of its theoretical capacity for formation. After formation, it is encapsulated and degassed again at 60°C and a vacuum of less than or equal to 50Pa. Then it is left to stand at 45°C for 12 hours. At 25°C, it is charged at a constant current of 0.33C to 3.65V. After charging, it is left to stand for 30 minutes. Then it is discharged at a constant current of 0.33C to 2.5V. After discharging, it is left to stand for 30 minutes. The above charge-discharge cycle is repeated twice to obtain a secondary battery.

[0121] Example 2

[0122] The only difference between it and Example 1 is that:

[0123] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.09:1.41:2.5; the conductivity of the prepared negative electrode active material layer is 17.00 S / cm.

[0124] In step three, the concentration of LiPF6 in the electrolyte is 1.20 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 0.25% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 2.10% (d%).

[0125] Example 3

[0126] The only difference between it and Example 1 is that:

[0127] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.17:1.33:2.5; the conductivity of the prepared negative electrode active material layer is 16.0 S / cm.

[0128] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 1.10 mol / L, and the mass percentage content of carbonate film-forming additives in the electrolyte, d%, is 2.20%.

[0129] Example 4

[0130] The only difference between it and Example 1 is that:

[0131] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.09:1.41:2.5; the conductivity of the prepared negative electrode active material layer is 17.00 S / cm.

[0132] In step three, the concentration of LiPF6 in the electrolyte is 1.10 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 1.10% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 2.80% (d%).

[0133] Example 5

[0134] The only difference between it and Example 1 is that:

[0135] In step three, the concentration of LiPF6 in the electrolyte is 1.20 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 0.06% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 1.10% (d%).

[0136] Example 6

[0137] The only difference between it and Example 1 is that:

[0138] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.26:1.24:2.5; the conductivity of the prepared negative electrode active material layer is 15.00 S / cm.

[0139] In step three, the concentration of LiPF6 in the electrolyte is 1.18 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 0.38% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 4% (d%).

[0140] Example 7

[0141] The only difference between it and Example 1 is that:

[0142] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.09:1.41:2.5; the conductivity of the prepared negative electrode active material layer is 17 S / cm.

[0143] In step three, the concentration of LiPF6 in the electrolyte is 1.1 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 1.55% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 3% (d%).

[0144] Example 8

[0145] The only difference between it and Example 1 is that:

[0146] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.09:1.41:2.5; the conductivity of the prepared negative electrode active material layer is 17 S / cm.

[0147] In step three, the concentration of LiPF6 in the electrolyte is 1.1 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 1.1% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 3.2% (d%).

[0148] Example 9

[0149] The only difference between it and Example 1 is that:

[0150] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.17:1.33:2.5; the conductivity of the prepared negative electrode active material layer is 16 S / cm.

[0151] In step three, the concentration of LiPF6 in the electrolyte is 0.9 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 2% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 1.5% (d%).

[0152] Example 10

[0153] The only difference between it and Example 1 is that:

[0154] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 0.5 mol / L.

[0155] Example 11

[0156] The only difference between it and Example 1 is that:

[0157] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 1.5 mol / L.

[0158] Example 12

[0159] The only difference between it and Example 1 is that:

[0160] In step three, the mass percentage (b%) of the ammonium salt additive in the electrolyte is 0.05%.

[0161] Example 13

[0162] The only difference between it and Example 1 is that:

[0163] In step three, the mass percentage (b%) of the ammonium salt additive in the electrolyte is 2%.

[0164] Example 14

[0165] The only difference between it and Example 1 is that:

[0166] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 97.30:0.20:2.5; the conductivity of the prepared negative electrode active material layer is 3 S / cm.

[0167] Example 15

[0168] The only difference between it and Example 1 is that:

[0169] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 97.13:0.37:2.5; the conductivity of the prepared negative electrode active material layer is 5 S / cm.

[0170] Example 16

[0171] The only difference between it and Example 1 is that:

[0172] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 95.83:1.67:2.5; the conductivity of the prepared negative electrode active material layer is 20 S / cm.

[0173] Example 17

[0174] The only difference between it and Example 1 is that:

[0175] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 95.39:2.11:2.5; the conductivity of the prepared negative electrode active material layer is 25 S / cm.

[0176] Example 18

[0177] The only difference between it and Example 1 is that:

[0178] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 97.30:0.20:2.5; the conductivity of the prepared negative electrode active material layer is 3 S / cm.

[0179] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 0.5 mol / L, and the mass percentage content of ammonium salt additive in the electrolyte, b%, is 2%.

[0180] Example 19

[0181] The only difference between it and Example 1 is that:

[0182] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed at a mass ratio of 95.39:S2.11:2.5; the conductivity of the prepared negative electrode active material layer is 25 S / cm.

[0183] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 1.5 mol / L, and the mass percentage content of ammonium salt additive in the electrolyte, b%, is 0.05%.

[0184] Example 20

[0185] The only difference between it and Example 1 is that:

[0186] In step three, the mass percentage (d%) of the carbonate film-forming additives in the electrolyte is 0.02%.

[0187] Example 21

[0188] The only difference between it and Example 1 is that:

[0189] In step three, the mass percentage (d%) of the carbonate film-forming additives in the electrolyte is 40%.

[0190] Example 22

[0191] The only difference between it and Example 1 is that:

[0192] In step three, R1, R2, R3 and R4 in the ammonium salt additives are all n-hexyl groups.

[0193] Example 23

[0194] The only difference between it and Example 1 is that:

[0195] In step three, R1, R2, R3 and R4 in the ammonium salt additive are n-propyl, n-butyl, n-pentyl and n-butyl, respectively.

[0196] Example 24

[0197] The only difference between it and Example 1 is that:

[0198] In step three, R1, R2, R3 and R4 in the ammonium salt additive are n-propyl, n-butyl, n-pentyl and hydrogen, respectively.

[0199] Example 25

[0200] The only difference between it and Example 1 is that:

[0201] In step three, R1, R2, R3 and R4 in the ammonium salt additive are n-propyl, n-butyl, n-pentyl and phenyl, respectively.

[0202] Example 26

[0203] The only difference between it and Example 1 is that:

[0204] In step three, the carbonate film-forming additive VC is replaced with FEC.

[0205] S3. Preparation of electrolyte

[0206] Same as Example 1

[0207] S4. Preparation of secondary batteries

[0208] Same as Example 1

[0209] Example 27

[0210] A method for preparing a secondary battery includes the following steps:

[0211] Step 1, Preparation of the positive electrode: LiMn 0.6 Fe 0.4 PO4, conductive agent carbon black (Super P), and binder PVDF were mixed at a mass ratio of 96:1.5:2.5 and then dispersed in solvent NMP to obtain a positive electrode slurry. The positive electrode slurry was coated onto one side of a 15μm thick aluminum foil used as a positive electrode current collector. The current collector coated with the slurry was placed in a vacuum furnace and dried at 100℃ for 12 hours. The dried slurry formed a positive electrode active material layer, resulting in an aluminum foil coated with the positive electrode active material layer. This aluminum foil was then rolled and slit to obtain a positive electrode sheet. The areal density of the positive electrode sheet was 400 g / m³. 2 The compacted density is 2.4 g / cm³. 3 .

[0212] Step 2, Preparation of the negative electrode: Silicon-carbon composite negative electrode material (particle size Dv50 of 8 μm, specific surface area of ​​15 m²) is prepared. 2The negative electrode slurry is prepared by mixing CNT (conductive agent), CMC (binder), and CNT in a mass ratio of 96:1.5:2.5 and dispersing them in deionized water. The negative electrode slurry is then uniformly coated onto a copper foil (120 mm thick) used as a negative electrode current collector. The coated current collector is then transferred to a vacuum drying oven and dried at 100°C for 12 hours. The dried slurry forms a negative electrode active material layer, resulting in a copper foil coated with this layer. This copper foil is then rolled and slit to obtain the negative electrode sheet. The areal density of the negative electrode sheet is 170 g / m³. 2 The compacted density is 1.55 g / cm³. 3 The conductivity of the negative electrode active material layer is 14 S / cm.

[0213] Step 3, preparation of electrolyte: same as in Example 1.

[0214] Step 4, preparation of the secondary battery: same as in Example 1.

[0215] Comparative Example 1

[0216] The only difference between it and Example 1 is that:

[0217] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.09:1.41:2.5; the conductivity of the prepared negative electrode active material layer is 17 S / cm.

[0218] In step three, the concentration of LiPF6 in the electrolyte is 1.1 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 2.3% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 0.01% (d%).

[0219] Comparative Example 2

[0220] The only difference between it and Example 1 is that:

[0221] In step three, the concentration of LiPF6 in the electrolyte is 0.9 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 0.02% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 3.5% (d%).

[0222] Comparative Example 3

[0223] The only difference between it and Example 1 is that:

[0224] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 97.39:0.11:2.5; the conductivity of the prepared negative electrode active material layer is 2 S / cm.

[0225] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 1 mol / L, and the mass percentage content of ammonium salt additive in the electrolyte, b%, is 0.35%.

[0226] Comparative Example 4

[0227] The only difference between it and Example 1 is that:

[0228] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 0.01 mol / L.

[0229] Comparative Example 5

[0230] The only difference between it and Example 1 is that:

[0231] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 13 mol / L.

[0232] Comparative Example 6

[0233] The only difference between it and Example 1 is that:

[0234] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 97.21:0.29:2.5; the conductivity of the prepared negative electrode active material layer is 4 S / cm.

[0235] In step three, the concentration of LiPF6 in the electrolyte is 0.35 mol / L (a mol / L), the mass percentage content of ammonium salt additives in the electrolyte is 2% (b%), and the mass percentage content of carbonate film-forming additives in the electrolyte is 1% (d%).

[0236] Comparative Example 7

[0237] The only difference between it and Example 1 is that:

[0238] In step three, the mass percentage (b%) of the ammonium salt additive in the electrolyte is 0.025%.

[0239] Comparative Example 8

[0240] The only difference between it and Example 1 is that:

[0241] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 99.99:0.01:2.5; the conductivity of the prepared negative electrode active material layer is 0.15 cS / cm.

[0242] In step three, the mass percentage (b%) of the ammonium salt additive in the electrolyte is 0.3%.

[0243] Comparative Example 9

[0244] The only difference between it and Example 1 is that:

[0245] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 95.83:1.67:2.5; the conductivity of the prepared negative electrode active material layer is 20 cS / cm.

[0246] In step three, the mass percentage (b%) of the ammonium salt additive in the electrolyte is 0.03%.

[0247] Comparative Example 10

[0248] The only difference between it and Example 1 is that:

[0249] In step two, artificial graphite, conductive agent CNT, and binder CMC are mixed in a mass ratio of 96.35:1.15:2.5; the conductivity of the prepared negative electrode active material layer is 14 cS / cm.

[0250] In step three, the concentration of LiPF6 in the electrolyte, a mol / L, is 1.1 mol / L, and the mass percentage content of ammonium salt additive in the electrolyte, b%, is 0.02%.

[0251] Performance testing

[0252] 1. Conductivity test of the negative electrode active material layer

[0253] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V, resulting in a fully discharged secondary battery. The secondary battery was disassembled, and the negative electrode was removed. The negative electrode was soaked in dimethyl carbonate solution for 2 hours, then removed and dried. The negative electrode active material layer was then scraped to remove powder, which was poured into the mold cavity. The upper pressure head was placed in, and the surface of the powder was leveled by shaking. The mold was placed on the pressure stage of a four-probe conductivity meter, and the four-probe tester was connected. The target pressure was set to 20MPa, and the instrument was pressurized to the set value and maintained for 5 minutes. The sample resistance was then measured, and after the resistance value stabilized, the resistance R was recorded. Simultaneously, the sample thickness h was recorded. The conductivity σ was calculated using the following formula:

[0254] σ = h / (R×S)

[0255] Where σ is the conductivity (in S / cm); h is the sample thickness (in cm); R is the measured resistance (in Ω); and S is the cross-sectional area of ​​the mold (in cm²). 2 S = π × (d / 2) 2 (d is the mold diameter, in cm).

[0256] 2. Fast charging performance test

[0257] The stacked three-electrode testing method is adopted, and the specific steps are as follows:

[0258] (1) Disassemble the secondary battery to be tested, take out the positive electrode and the negative electrode, use DMC solvent for ultrasonic cleaning for 72 hours to remove residual electrolyte, and then place it in a vacuum oven and dry it at 80°C for 12 hours; use the positive electrode as the working electrode, the negative electrode as the counter electrode, the 0.5mm diameter copper wire as the reference electrode, the electrolyte as the original secondary battery electrolyte, and the separator as Celgard 2400, assemble the three electrodes in an argon atmosphere glove box to test the secondary battery;

[0259] (2) The following tests were performed at 25°C using a battery testing system (LAND 900): constant current charging at 0.33C to the cutoff voltage of 3.65V, constant voltage charging until the current drops to 0.05C, and then constant current discharging at 0.33C to the cutoff voltage of 2.5V. The above charge-discharge cycle was repeated twice, and the charging capacity of the second cycle was used as the baseline capacity for subsequent SOC calculation; charging at 0.33C to 10% SOC, recording time t1, immediately switching to 4C constant current charging, and stopping charging when the negative electrode reference potential is less than or equal to 0mV or the voltage reaches 3.65V; after reaching the cutoff condition, charging at 0.2C stepwise until the current drops to 0.05C to cut off charging, and recording the time t2 when the charging capacity reaches 80% SOC. The time taken from t1 to t2 (i.e., the charging time from 10% SOC to 80% SOC) is the fast charging time (in minutes), recorded as the fast charging time at 25°C. The results are shown in Table 1.

[0260] 3. High-Temperature Cyclic Performance Test Method

[0261] The secondary battery under test was placed in a high-temperature test chamber and left to stand at 60℃ for 4 hours to ensure internal thermal equilibrium. Then, it was subjected to 1000 charge-discharge cycles. The discharge capacity of the first cycle and the discharge capacity of the 1000th cycle were recorded. The capacity retention rate after 1000 cycles at 60℃ was calculated and recorded as the capacity retention rate after 1000 cycles at 60℃. The results are shown in Table 1. The charge-discharge conditions were as follows: constant current charging at 1C to 3.65V, constant voltage charging until the current drops to 0.05C, standing for 10 minutes, and then constant current discharging at 1C to 2.5V, and standing for 10 minutes. The capacity retention rate after 1000 cycles at 60℃ = (discharge capacity of the 1000th cycle) / discharge capacity of the first cycle × 100%, where the discharge capacity of the first cycle is the discharge capacity of the second cycle.

[0262] 4. Gas production test

[0263] The secondary battery under test was charged to 100% SOC at room temperature using a constant current and constant voltage of 0.33C (charge and discharge conditions: constant current at 0.33C to cutoff voltage 3.65V, constant voltage charging until current drops to 0.05C). The initial secondary battery volume V0 (in mL) was measured using the water displacement method. The secondary battery was then placed in a 60℃ constant temperature chamber, and the charging was repeated every 7 days (charging conditions: constant current at 0.33C to cutoff voltage 3.65V, constant voltage charging until current drops to 0.05C). After each charging, the battery was allowed to stand for 24 hours, and the secondary battery volume was measured using the water displacement method. The secondary battery volume V1 (in mL) after 56 days was recorded, and the gas production at 60℃ (in mL / Ah) was calculated. The results are shown in Table 1.

[0264] Wherein, the gas production at 60℃ = (V1-V0) / nominal rated capacity of the secondary battery (in Ah);

[0265] The specific steps for testing the volume of a secondary battery using the water displacement method are as follows: (i) Add an appropriate amount of pure water to the container and test its density ρliquid using a hydrometer and record the result; (ii) Adjust the balance to a horizontal level and tare the diaphragm (tare the diaphragm before testing each secondary battery); (iii) Submerge the secondary battery body along with its tabs in the solution, ensuring that the secondary battery does not contact the container wall. After stabilization, read the value and record the mass M of the displaced pure water; (iv) Turn off the balance and seal the container to prevent reagent evaporation. The formula for calculating the volume of a secondary battery is M / ρliquid. Before and after storage, tests are performed to obtain the masses M0 and M1 of the displaced pure water. The difference between V1 and V0, V1-V0, is calculated as follows: V1-V0 = M1 / ρliquid - M0 / ρliquid.

[0266] Table 1

[0267]

[0268] As shown in Table 1, the secondary batteries prepared in Examples 1 to 27 have excellent fast charging performance and high-temperature cycle stability, while also having low gas production. The total amount of gas generated by side reactions such as electrolyte decomposition, positive electrode metal dissolution, and continuous SEI film regeneration is low, and the secondary batteries have good safety performance.

[0269] Compared to Comparative Examples 1 to 9, in Example 1, the concentration of LiPF6 a mol / L was 0.5 mol / L to 1.5 mol / L, the mass percentage content of ammonium salt additive b% was 0.05% to 2%, and the conductivity c S / cm of the negative electrode active material layer was 3 S / cm to 25 S / cm. Furthermore, a, b, and c satisfy 0.75≤a×c / b≤750. The fast charging performance and high-temperature cycle stability of the prepared secondary battery were significantly improved, and the gas production was significantly reduced.

[0270] Compared to Comparative Example 10, the mass percentage content b% of ammonium salt additives in Examples 1 to 3 was 0.05% to 2%, and the relationship between a, b and c satisfied 0.75≤a×c / b≤750. The fast charging performance and high temperature cycle stability of the prepared secondary batteries were significantly improved, and the gas production was significantly reduced.

[0271] Compared to Examples 4 and 5, in Examples 1 to 3, m / (n×a) further satisfies 0.02≤m / (n×a)≤17.05, resulting in improved fast-charging performance and high-temperature cycle stability of the prepared secondary batteries, and reduced gas production.

[0272] Compared to Examples 6 and 8, Examples 1 to 3 further satisfy d% to 3%, resulting in improved fast-charging performance and high-temperature cycle stability of the prepared secondary batteries, and reduced gas production.

[0273] Compared to Example 7, Examples 1 to 3 further satisfy m to be 0.5 to 4.5, n to be 0.33 to 20, and 0.02≤m / (n×a)≤17.05, resulting in improved fast-charging performance and high-temperature cycle stability of the prepared secondary batteries, and reduced gas production.

[0274] Compared to Example 9, Examples 1 to 3 further satisfy b% to be 0.1% to 1.5%, which improves the fast charging performance and high-temperature cycle stability of the prepared secondary batteries and reduces the gas production.

[0275] Compared to Examples 10 and 11, Example 1 further satisfies the requirement that a mol / L is 0.8 mol / L to 1.2 mol / L, resulting in improved fast-charging performance and high-temperature cycling stability of the prepared secondary battery, while reducing gas production.

[0276] Compared to Examples 12 and 13, Example 1 further satisfies b% as 0.1% to 1.5%, resulting in improved fast-charging performance and high-temperature cycling stability of the prepared secondary battery, while reducing gas production.

[0277] Compared to Examples 14 to 17, Examples 1 to 3 further satisfy the requirement that c S / cm is 15 S / cm to 18 S / cm, and the fast charging performance and high-temperature cycle stability of the prepared secondary battery are further improved, while the gas production is further reduced.

[0278] Compared to Examples 20 and 21, Examples 1 to 3 further satisfy d% to 3%, resulting in improved fast-charging performance and high-temperature cycle stability of the prepared secondary batteries, and reduced gas production.

[0279] Compared to Example 18, Examples 1 to 3 further satisfy the following conditions: a mol / L is 0.8 mol / L to 1.2 mol / L, b% is 0.1% to 1.5%, c S / cm is 5 S / cm to 20 S / cm, and 2.7≤a×c / b≤240. The fast charging performance of the prepared secondary batteries is significantly improved.

[0280] Compared to Example 19, Examples 1 to 3 further satisfy the following conditions: a mol / L is 0.8 mol / L to 1.2 mol / L, b% is 0.1% to 1.5%, c S / cm is 5 S / cm to 20 S / cm, and 2.7≤a×c / b≤240. The high-temperature cycling stability of the prepared secondary battery is significantly improved, and the gas production is significantly reduced.

[0281] Compared to Examples 24 and 25, when R1, R2, R3 and R4 in Examples 1, 22 and 23 are all alkyl groups, the fast-charging performance and high-temperature cycle stability of the prepared secondary batteries are improved, while the gas production is reduced.

[0282] Compared to the use of FEC as a film-forming additive in Example 26, the use of VC as a film-forming additive in Example 1 resulted in improved fast-charging performance and high-temperature cycle stability of the prepared secondary battery, while reducing gas production.

[0283] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and an electrolyte; the negative electrode includes a negative current collector and a negative active material layer coated on at least one side of the negative current collector; the electrolyte includes an ammonium salt additive and a lithium salt, the lithium salt including LiPF6, and the ammonium salt additive containing an ammonium salt cation represented by Formula I; In Formula I, R1, R2, R3 and R4 are independently selected from H, straight-chain or branched alkyl groups having 1 to 7 carbon atoms, and benzene rings, and R1, R2, R3 and R4 are not all H at the same time; The concentration of LiPF6 in the electrolyte is a mol / L; The mass percentage content of the ammonium salt additive in the electrolyte is b%; The conductivity of the negative electrode active material layer is c S / cm; a, b, and c satisfy the following relationship: 0.75 ≤ a × c / b ≤ 750.

2. The secondary battery according to claim 1, characterized in that, The value of a is between 0.5 and 1.

5.

3. The secondary battery according to claim 2, characterized in that, The value of a is between 0.8 and 1.

2.

4. The secondary battery according to claim 1, characterized in that, The value of b is between 0.05 and 2.

5. The secondary battery according to claim 4, characterized in that, The value of b is between 0.1 and 1.

5.

6. The secondary battery according to claim 1, characterized in that, The value of c is between 3 and 25.

7. The secondary battery according to claim 6, characterized in that, The value of c is between 5 and 20.

8. The secondary battery according to claim 1, characterized in that, The terms a, b, and c satisfy the following relationship: 2.7 ≤ a × c / b ≤ 240.

9. The secondary battery according to any one of claims 1 to 8, characterized in that, The electrolyte further includes a carbonate film-forming additive, which is selected from one or more of vinylene carbonate, fluoroethylene carbonate, and vinyl carbonate; the mass percentage content of the carbonate film-forming additive in the electrolyte is d%, and d satisfies equations (I) to (III) with a, b, and c: 0.0003≤m / (n×a)≤133.33(I); m=(d×b)(II; n=(0.01c / b)(III)。 10. The secondary battery according to claim 9, characterized in that, m, n, and a satisfy the relation: 0.02≤m / (n×a)≤17.

05.

11. The secondary battery according to claim 9, characterized in that, m ranges from 0.025 to 10.

12. The secondary battery according to claim 9, characterized in that, n ranges from 0.33 to 20.

13. The secondary battery according to claim 9, characterized in that, d ranges from 1 to 3.

14. The secondary battery according to claim 9, characterized in that, The carbonate film-forming additive is vinylene carbonate.

15. The secondary battery according to any one of claims 1 to 8, characterized in that, The ammonium salt additive also contains anion, which is selected from BF4. - F - Cl - PF6 - One or more of them.

16. The secondary battery according to claim 15, characterized in that, The anion is BF4. - .

17. The secondary battery according to any one of claims 1 to 8, characterized in that, R1, R2, R3 and R4 are each independently selected from straight-chain or branched alkyl groups having 1 to 7 carbon atoms.

18. The secondary battery according to claim 17, characterized in that, R1, R2, R3 and R4 are independently selected from one of n-propyl, n-butyl, n-pentyl and n-hexyl.

19. The secondary battery according to claim 17, characterized in that, R1, R2, R3 and R4 are the same.

20. The secondary battery according to any one of claims 1 to 8, characterized in that, The electrolyte further includes a solvent selected from one or more of the following: ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

21. The secondary battery according to any one of claims 1 to 8, characterized in that, The negative electrode active material layer includes a negative electrode active material, which is selected from one or two of graphite and silicon-based materials.

22. The secondary battery according to claim 21, characterized in that, The negative electrode active material is graphite, which is selected from one or two of natural graphite and artificial graphite.

23. The secondary battery according to claim 21, characterized in that, The graphite has a particle size Dv50 of 4 μm to 20 μm.

24. The secondary battery according to claim 21, characterized in that, The specific surface area of ​​the graphite is 0.6 m². 2 / g to 2.2m 2 / g.

25. The secondary battery according to claim 21, characterized in that, The graphite has a graphitization degree of 90% to 98%.

26. The secondary battery according to any one of claims 1 to 8, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer coated on at least one side of the positive current collector. The positive active material layer includes a positive active material selected from one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, and lithium manganese oxide.

27. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1 to 26.

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