A secondary battery and its application

Abstract

This application belongs to the field of lithium-ion battery technology and provides a secondary battery and its application. The secondary battery includes an electrolyte; the electrolyte includes additives and a solvent, wherein the content of the additives in the electrolyte is 'a'; the solvent includes a carboxylic acid ester compound, wherein the content of the carboxylic acid ester compound in the solvent is 'b'; the ratio of the negative electrode interface impedance to the total impedance of the secondary battery is 'c'; n = a × c / b, where 0.0000068 ≤ n ≤ 0.036. The secondary battery of this invention precisely matches the electrolyte components with the internal impedance characteristics of the battery, effectively improving lithium-ion transport efficiency at low temperatures and simultaneously suppressing the deposition of metallic lithium on the negative electrode surface. Furthermore, the film-forming additives enhance the stability of the negative electrode interface, ultimately achieving a synergistic improvement in the low-temperature fast-charging performance, safety, and cycle life of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a secondary battery and its application. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and no memory effect, have been widely used in new energy vehicles, energy storage systems, and portable electronic devices. As application scenarios continue to expand, the demand for their use in low-temperature conditions such as high-altitude and cold regions, low-temperature industrial environments, and outdoor mobile devices is becoming increasingly urgent.

[0003] In low-temperature environments, lithium-ion batteries inevitably experience lithium plating on the negative electrode surface. Current solutions primarily focus on optimizing electrolyte formulations (e.g., adding carboxylic acid esters to reduce electrolyte viscosity) and modifying the negative electrode material structure (e.g., surface coating to improve ion transport performance). However, these methods often fail to simultaneously mitigate low-temperature lithium plating while maintaining the battery's overall electrochemical performance, negatively impacting cycle performance and rate capability, and failing to achieve a synergistic improvement in both low-temperature lithium plating suppression and core battery performance.

[0004] Therefore, there is an urgent need in this field to develop a secondary battery with excellent low-temperature performance that can effectively suppress lithium plating at low temperatures while ensuring the overall electrochemical performance of the battery, so as to accurately meet the actual application needs of lithium-ion batteries in low-temperature environments. Summary of the Invention

[0005] This application provides a secondary battery that, by constructing the relationship between multiple parameters n=a×c / b and limiting the range of n, solves the problem of lithium plating in secondary batteries under low-temperature conditions and optimizes their low-temperature fast-charging performance, thus providing the industry with a secondary battery with excellent low-temperature fast-charging performance.

[0006] This application also provides an electrical device, including the aforementioned secondary battery, which is suitable for diverse application scenarios.

[0007] In a first aspect, embodiments of this application provide a secondary battery, the secondary battery comprising an electrolyte;

[0008] The electrolyte includes additives and solvents, and the content of the additives in the electrolyte is a;

[0009] The solvent includes a carboxylic acid ester compound, and the content of the carboxylic acid ester compound in the solvent is b;

[0010] The ratio of the negative electrode interface impedance of the secondary battery to the total impedance of the secondary battery is c.

[0011] n = a × c / b, where 0.0000068 ≤ n ≤ 0.036.

[0012] Secondly, embodiments of this application provide an electrical device including the aforementioned secondary battery.

[0013] The secondary battery provided in this application embodiment achieves a synergistic improvement in lithium-ion transport efficiency and interface stability under low-temperature conditions by constructing a relationship between the content of film-forming additives in the electrolyte (a), the content of carboxylic acid ester compounds in the solvent (b), and the ratio of the negative electrode interface impedance to the total impedance (c) of the secondary battery, with the condition 0.0000068≤n≤0.036. This significantly improves the low-temperature fast-charging performance, cycle life, and safety of the secondary battery. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0015] As mentioned earlier, lithium-ion batteries are prone to lithium deposition on the negative electrode surface at low temperatures. This is because low temperatures significantly increase electrolyte viscosity, leading to a sharp drop in lithium ion transport rate within the electrolyte. This results in significant concentration polarization on the negative electrode surface, ultimately inducing lithium deposition. Existing technologies for mitigating this low-temperature lithium deposition problem often negatively impact battery cycle performance and rate performance, making it difficult to achieve a synergistic improvement in both low-temperature lithium deposition suppression and core battery performance. The inventors discovered that this contradiction may stem from a mismatch between the charge transport capacity of the electrolyte in current secondary batteries and the lithium ion reception capacity of the negative electrode. Specifically, while optimizing the ion transport rate within the secondary battery at low temperatures, the ions cannot quickly embed after migrating to the negative electrode, accumulating on the surface and forming metallic lithium deposition. Therefore, this invention establishes quantitative correlations between key parameters, facilitating multi-parameter synergistic control of the secondary battery. This not only suppresses low-temperature lithium deposition at the negative electrode but also improves its low-temperature ion transport kinetics.

[0016] Based on this, the present invention provides a secondary battery, the secondary battery including an electrolyte;

[0017] The electrolyte includes film-forming additives and solvents, and the content of film-forming additives in the electrolyte is a;

[0018] The solvent includes carboxylic acid ester compounds, and the content of carboxylic acid ester compounds in the solvent is b;

[0019] The ratio of the negative electrode interface impedance to the total impedance of the secondary battery is c.

[0020] n = a × c / b, where 0.0000068 ≤ n ≤ 0.036.

[0021] The secondary battery of the present invention establishes a quantitative relationship n=a×c / b by using the content of film-forming additives a, the content of carboxylic acid ester compound solvent b, and the ratio of negative electrode interface impedance to total impedance c in the electrolyte, thereby achieving dynamic balance of electrolyte-negative electrode interface impedance at low temperature.

[0022] Specifically, by adding a film-forming additive with a content of 'a' to the electrolyte, a uniform and dense SEI film with abundant ion conduction channels can be induced to form on the surface of the negative electrode active material. This film structure is stable, and the ion transport channels do not shrink at low temperatures. It can significantly reduce the interfacial impedance between the electrolyte and the negative electrode, promote the rapid penetration of lithium ions through the film layer and form a uniform lithium ion flow, and avoid the local enrichment and deposition of lithium ions and concentration polarization on the negative electrode surface. Based on inducing rapid lithium ion insertion, it fundamentally inhibits the precipitation of metallic lithium. At the same time, adding the film-forming additive can reduce the interfacial side reactions between the negative electrode active material and the electrolyte, inhibit the irreversible decomposition of the electrolyte, avoid repeated rupture and reconstruction of the SEI film, and effectively ensure the continuity of lithium ion transport. The above-mentioned effects of the film-forming additive not only optimize the lithium ion insertion rate and effectively prevent the precipitation of metallic lithium on the negative electrode surface, thus optimizing the capacity performance of the secondary battery, but also reduce the formation of lithium dendrites and the risk of them piercing the separator, effectively improving the fast-charging performance and cycle life of the secondary battery in low-temperature environments.

[0023] Carboxylic acid ester compounds possess inherently low cohesive energy and high molecular flexibility, and do not exhibit significant molecular aggregation even at low temperatures. By adding a solvent containing carboxylic acid ester compounds (where the content of carboxylic acid ester compounds is b) to the electrolyte, the overall viscosity of the electrolyte can be significantly reduced, thereby increasing the ionic conductivity and lithium-ion transference number of the electrolyte. This increases the rate of lithium-ion migration from the positive electrode to the negative electrode, achieving a match between the lithium-ion transport rate and charge transfer rate at low temperatures, and reducing concentration polarization at the negative electrode surface. Simultaneously, carboxylic acid ester compounds can promote the formation of a thinner, more easily dissociable lithium-ion solvation sheath, significantly reducing the desolvation energy barrier when lithium-ions migrate to the negative electrode surface, increasing the cross-interface transfer rate of lithium-ions, and reducing metallic lithium deposition caused by interfacial reaction lag. Furthermore, carboxylic acid ester compounds have good surface wettability, which can promote the electrolyte to fully fill the voids inside the secondary battery, forming continuous lithium-ion transport channels. This facilitates uniform lithium-ion transport within the electrode, avoiding localized lithium deposition caused by impaired lithium-ion transport in certain areas, and improving the uniformity and stability of lithium-ion transport during fast charging. The low-temperature fast-charging performance of secondary batteries is significantly improved by the action of carboxylic acid ester compounds.

[0024] By adjusting the ratio *c* of the negative electrode interface impedance to the total impedance of the secondary battery, optimizations can be achieved in areas such as polarization suppression, ion-electron transport matching, and interface stability. Controlling this ratio *c* can effectively improve the cross-interface transport efficiency of lithium ions under low-temperature conditions, eliminate lithium ion enrichment on the negative electrode surface, and fundamentally suppress the initiation and growth of lithium dendrites. Simultaneously, it can reduce activation polarization and concentration polarization during battery cycling, decrease localized thickening and cracking of the SEI film, and ensure long-term stability of the negative electrode interface.

[0025] In summary, by considering the content of film-forming additives (a) in the electrolyte, the content of carboxylic acid ester compounds in the solvent (b), and the ratio of the negative electrode interface impedance to the total impedance (c), a dynamic equilibrium system is established between these parameters: n = a × c / b, satisfying 0.0000068 ≤ n ≤ 0.036. Specifically, increasing the content of film-forming additives (a) promotes the formation of the SEI film; adjusting the content of carboxylic acid ester compounds in the solvent (b) precisely controls the electrolyte viscosity, optimizing lithium-ion transport kinetics to balance the ion transport resistance caused by an excessively thick SEI film; adjusting the ratio of the negative electrode interface impedance to the total impedance (c) promotes rapid lithium-ion transport. Based on the dynamic balance control between a, b, and c, not only can the deposition of metallic lithium on the negative electrode surface of the secondary battery be effectively suppressed at low temperatures, but the migration rate of lithium ions can also be optimized simultaneously. Thus, under low-temperature conditions, the lithium-ion transport rate is synergistically improved while effectively suppressing the deposition of metallic lithium on the negative electrode surface, thereby improving the low-temperature performance of the secondary battery. In a preferred embodiment, 0.00011 ≤ n ≤ 0.0009, resulting in superior overall performance of the secondary battery. It should be noted that parameter n is dimensionless and has no physical unit.

[0026] It is understood that the electrolyte also includes lithium salts. The present invention does not limit the specific type of lithium salts, but exemplary, it includes, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0027] It is understood that the solvent of the present invention includes carbonate compounds in addition to carboxylic acid ester compounds. The present invention does not limit the specific type of carbonate compound, but includes, but is not limited to, at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, and trifluoroethyl methyl carbonate. The content of carbonate compounds in the solvent is 30% to 95%.

[0028] The secondary battery of this invention integrates the content of film-forming additives (a) in the electrolyte, the content of carboxylic acid ester compounds (b) in the solvent, and the ratio of the negative electrode interface impedance to the total impedance (c) to construct a control relationship n = a × c / b, and adjusts it to 0.0000068 ≤ n ≤ 0.036, so that the parameters achieve a dynamic adaptive balance. This invention precisely matches the electrolyte components with the internal impedance characteristics of the battery, which not only effectively improves the lithium-ion transport efficiency at low temperatures and simultaneously suppresses the deposition of metallic lithium on the negative electrode surface, but also enhances the stability of the negative electrode interface under the action of film-forming additives, ultimately achieving a synergistic improvement in the low-temperature fast-charging performance, safety, and cycle life of the secondary battery.

[0029] To further optimize the overall performance of the secondary battery in a low-temperature environment, in one specific embodiment, the content 'a' of the film-forming additive satisfies 0.2% ≤ a ≤ 4%.

[0030] By controlling the content 'a' of the film-forming additive to meet the requirement of 0.2% ≤ a ≤ 4%, a more uniform and dense SEI film can be induced to form on the negative electrode surface, making the film layer more effective in suppressing interfacial side reactions between the electrolyte and the negative electrode. Simultaneously, the SEI film thickness can be precisely optimized, significantly reducing interfacial impedance at low temperatures, thereby simultaneously optimizing the cycle life and low-temperature fast-charging performance of the secondary battery. In a preferred embodiment, with 1.5% ≤ a ≤ 3%, the fast-charging and cycle performance of the secondary battery reaches an even better level.

[0031] To further optimize the ion transport performance of the electrolyte at low temperatures, in one specific embodiment, the content b of the carboxylic acid ester compound satisfies 5% ≤ b ≤ 70%.

[0032] By adjusting the content of carboxylic acid ester compounds (b) in the solvent to meet the requirement of 5% ≤ b ≤ 70%, the overall viscosity of the electrolyte can be further reduced, thereby increasing the lithium-ion transport rate in the liquid phase and further optimizing the low-temperature fast-charging performance of the secondary battery. In a preferred embodiment, with 30% ≤ b ≤ 70%, the fast-charging performance of the secondary battery reaches an even better level.

[0033] In one specific implementation, 0.2% ≤ c ≤ 5%.

[0034] By adjusting c to satisfy 0.2%≤c≤5%, the lithium-ion insertion rate on the negative electrode surface can be significantly improved, effectively alleviating the problem of lithium-ion enrichment on the negative electrode surface, reducing the deposition of metallic lithium on the negative electrode surface, and further suppressing the phenomenon of low-temperature lithium deposition. This, in turn, improves the capacity performance, cycle life, and safety performance of the secondary battery in low-temperature environments. In a preferred embodiment, 0.4%≤c≤1% achieves even better cycle performance of the secondary battery.

[0035] It should be noted that parameter c can be controlled by adjusting the content 'a' of film-forming additives in the electrolyte. The smaller 'a' is, the smaller the negative electrode interface impedance is, and therefore the smaller 'c' is.

[0036] To further optimize the performance of the electrolyte at low temperatures, in one specific embodiment, the carboxylic acid ester compound includes 3 to 6 carbon atoms.

[0037] By adjusting the number of carbon atoms in carboxylic acid ester compounds to 3-6, the solubility, viscosity characteristics, and low-temperature stability of carboxylic acid ester compounds can be synergistically optimized, allowing each core intrinsic property to precisely adapt to the application requirements of low-temperature electrolytes. Specifically, the number of carbon atoms in a carboxylic acid ester compound plays a decisive regulatory role in its intrinsic intermolecular forces, molecular flexibility, and polarity, thus directly affecting its solubility and viscosity characteristics for lithium ions. The alkyl chains of carboxylic acid ester compounds are nonpolar hydrophobic chains. By adjusting the number of carbon atoms within them, the intermolecular contact area can be effectively optimized, and the intermolecular cohesive energy can be reduced simultaneously. At the same time, because the short alkyl chains in this range have less steric hindrance, they endow the compounds with higher molecular flexibility, so that their molecular mobility does not significantly decrease at low temperatures, and they are less prone to molecular aggregation, resulting in lower intrinsic viscosity. Furthermore, the steric hindrance effect of the short alkyl chains on the carbonyl group is weaker, making it easier for lithium ions to coordinate with the carbonyl group, which can effectively optimize the solubility performance of carboxylic acid ester compounds for lithium ions.

[0038] In one specific embodiment, the film-forming additive includes ester-based film-forming additives.

[0039] Specifically, ester-based film-forming additives contain highly reducing active groups such as ester groups and carbonyl groups, which enable the ester groups to selectively react with the negative electrode surface, precisely occupying the film-forming active sites on the negative electrode surface, thereby constructing a more uniform, dense, and highly ionicly conductive SEI film layer, further enhancing interface stability.

[0040] In one specific embodiment, the ester film-forming additive includes at least one of vinylene carbonate (VC), ethylene ethylene carbonate (VEC), ethylene carbonate (EC), fluoroethyleneene carbonate (VFEC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), 1,3-propanesulfonyl lactone (PS), 1,3-propenesulfonyl lactone (PES), trimethyl phosphate (TMP), triethyl phosphate (TEP), methanedisulfonate methylene (MMDS), triphenyl phosphite (TPP), vinyl sulfate (DTD), and propylene sulfite (PS).

[0041] When ester-based film-forming additives include the types mentioned above, the SEI film they induce possesses both a more uniform and dense structure and superior ionic conductivity. For example, the synergistic effect of VC and VFEC can form a fluorinated SEI film, whose fluoride component (such as LiF) exhibits high ionic conductivity and low electronic conductivity, effectively suppressing lithium dendrite growth; PS, relying on its own cyclic structure, can form a dense multilayer film structure at the negative electrode interface, further enhancing the stability of the negative electrode interface.

[0042] In one specific embodiment, the carboxylic acid ester compound includes at least one selected from methyl acetate (MA), ethyl acetate (EA), ethyl propionate (EP), methyl butyrate (MB), γ-butyrolactone (GBL), and δ-valerolactone (DVL).

[0043] When carboxylic acid ester compounds include the types mentioned above, the overall viscosity of the electrolyte can be further optimized, the transport rate of lithium ions in the electrolyte can be promoted, and the low-temperature fast charging performance of the secondary battery can be improved.

[0044] To further optimize the low-temperature performance of secondary batteries, in one specific embodiment, the viscosity of the electrolyte is 1.0~5 mPa·s. By adjusting the viscosity of the electrolyte within this range, the resistance to lithium-ion liquid phase transport is reduced, thereby further improving the migration rate of lithium ions at low temperatures and enhancing the low-temperature fast-charging performance of the secondary battery.

[0045] To further optimize the lithium-ion transport rate, in one specific embodiment, the ionic conductivity of the electrolyte is 8~20 mS / cm.

[0046] The overall optimization of the electrolyte viscosity to 1.0~5 mPa·s and ionic conductivity to 8~20 mS / cm can effectively reduce the liquid phase transport resistance of lithium ions in the electrolyte, improve the lithium ion migration rate and transport efficiency, fundamentally improve the transport kinetics of lithium ions at low temperatures, and thus synergistically improve the overall low-temperature performance of secondary batteries, such as low-temperature charge and discharge, cycle stability and anti-lithium plating.

[0047] To further optimize the low-temperature performance of secondary batteries, in one specific embodiment, the negative electrode interface impedance is 0.00035~0.005Ω.

[0048] By adjusting the negative electrode interface impedance to 0.00035~0.005Ω, the energy barrier for lithium ions to penetrate the SEI film and embed into the negative electrode can be optimized, which significantly improves the charge transfer rate between the electrolyte and the negative electrode interface, reduces the accumulation of lithium ions on the negative electrode surface, thereby reducing the deposition of metallic lithium, and further optimizing the low-temperature capacity performance, cycle life and safety of the secondary battery.

[0049] In one specific implementation, the total impedance is 0.05~0.2Ω.

[0050] By adjusting the total impedance of the secondary battery to 0.05~0.2Ω, the cross-interface transport efficiency and liquid phase diffusion rate of lithium ions can be significantly improved, precisely matching the kinetic requirements of fast charging and achieving efficient synergistic matching between lithium ion transport rate and electron transport rate.

[0051] In one specific embodiment, the secondary battery further includes a negative electrode sheet, which includes a negative electrode material layer, which includes a negative electrode material, and the negative electrode material includes a negative electrode active material.

[0052] The negative electrode active material includes a negative electrode active material matrix and a carbon coating layer disposed on at least a portion of the outer surface of the negative electrode active material matrix;

[0053] The carbon coating layer includes carbon materials.

[0054] Specifically, by depositing a carbon coating layer containing carbon materials on the surface of the negative electrode active material matrix, the surface activity of the negative electrode material can be effectively reduced. Furthermore, the physical isolation effect reduces direct contact between the negative electrode active material matrix and the electrolyte, improving the stability of the negative electrode interface. Simultaneously, the porous structure of the carbon coating layer provides a continuous and rapid transport channel for lithium ions, significantly reducing the resistance to lithium ion insertion-extraction at low temperatures, lowering the negative electrode interface impedance, and suppressing uneven lithium ion deposition on the negative electrode surface. This alleviates the problem of metallic lithium deposition caused by lithium ion enrichment on the negative electrode surface, thereby optimizing the capacity performance, cycle life, and battery safety of the secondary battery under low-temperature conditions. In addition, the carbon coating layer is a conductive phase, forming a continuous conductive network, improving the overall electronic conductivity of the negative electrode material, reducing contact resistance during electron transport, and allowing for a better match between lithium ion and electron transport. This further reduces polarization during charging and discharging, optimizing the energy conversion efficiency of the secondary battery.

[0055] It should be noted that the porous conductive structure coated with carbon layer is not only suitable for low temperature, but can also provide a fast transport channel for lithium ions at room temperature and medium and high temperature. At the same time, it improves the surface electronic conductivity of the negative electrode material, reduces the transport resistance between lithium ions and electrons, and makes the battery less polarized and has a higher capacity retention rate when charged and discharged at high rates, thus meeting the requirements of high current conditions.

[0056] This invention does not limit the preparation steps of the negative electrode active material; by example, it may include the following steps:

[0057] 1) The graphite-based precursor is pre-calcined under an inert atmosphere at a temperature of 900-1200℃ for 2-4 hours, followed by pulverization and classification to a Dv50 of 5-12μm, resulting in pre-calcined powder with controllable particle size. A pulverizer can be used for pulverization at a speed of 1500-3500 rpm, preferably 1800-2800 rpm.

[0058] 2) Mix the pre-calcined powder with 5~20wt% asphalt compound, heat and knead at 200~300℃ to melt the asphalt compound and uniformly coat the surface of the pre-calcined powder, and then granulate to prepare precursor particles with D50 of 10~20μm.

[0059] 4) The precursor particles are graphitized at a temperature of 2500~3000℃ under an inert atmosphere to obtain the negative electrode active material matrix.

[0060] 5) The negative electrode active material matrix, carbon source, and deionized water are uniformly mixed, and then spray-dried to obtain a dried mixture. The dried mixture is kept at 800~1200℃ for 12~18 hours and then sieved and graded to obtain the negative electrode active material. A cyclone classifier can be used for sieving and grading, with the classifying wheel rotating at 2000~2800 rpm, preferably 2200~2600 rpm.

[0061] Graphite-based precursors include, but are not limited to, at least one of petroleum coke, needle coke, pitch coke, calcined coke, mesophase pitch microspheres, and anthracite. Carbon sources include, but are not limited to, at least one of pitch, resin, or glucose.

[0062] The negative electrode material also includes binders and conductive agents. This invention does not limit the specific type of binder, but includes, but is not limited to, at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate; this invention does not limit the specific type of conductive agent, but includes, but is not limited to, at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene.

[0063] It is understood that the negative electrode sheet also includes a negative electrode current collector. The present invention does not limit the specific type of the negative electrode current collector, including but not limited to at least one of copper foil, nickel foam, and copper foam.

[0064] In one specific implementation, the mass of the carbon coating layer is 0.5% to 15% based on the mass of the negative electrode active material.

[0065] By adjusting the mass of the carbon coating layer to 0.5-15% of the mass of the negative electrode active material, the uniformity and continuity of the carbon coating layer can be optimized, thereby improving the matching of electron transport and ion transport under fast charging conditions and optimizing their synergistic and efficient conduction.

[0066] In one specific embodiment, the thickness of the carbon coating layer is 1~20 nm.

[0067] By adjusting the thickness of the carbon coating layer to 1~20nm, the barrier effect between the electrolyte and the negative electrode active material matrix can be achieved, as well as the synergistic optimization of the mechanical properties of the carbon coating layer, thereby further improving the cycle life of the secondary battery.

[0068] To further optimize the process, in one specific implementation, the coating rate of the negative electrode active material is 50% to 99%.

[0069] By adjusting the coating rate of the negative electrode active material to 50%–99%, the physical barrier effect of the coated carbon layer can be optimized, effectively suppressing side reactions between the electrolyte and the negative electrode active material matrix, reducing irreversible loss of active lithium during cycling, and thus enhancing the low-temperature cycling stability of the secondary battery, thereby improving the cycle life and capacity performance of the secondary battery. It should be noted that the concentration (c) can be controlled by adjusting the coating rate of the carbon layer on the negative electrode surface; the higher the coating rate, the lower the concentration (c).

[0070] In one specific implementation, the BET of the negative electrode material is 0.5~5m. 2 / g.

[0071] By adjusting the BET of the negative electrode material to 0.5~5m 2 / g, by regulating the number of reactive sites on the surface of the negative electrode material, the lithium ions are more fully and uniformly inserted into the negative electrode lattice, thereby significantly improving the lithium ion capacity of the negative electrode and effectively enhancing the capacity performance of the secondary battery.

[0072] This invention does not specifically limit the type of negative electrode active material matrix. In one specific embodiment, the negative electrode active material matrix includes at least one of petroleum coke, needle coke, mesophase carbon microspheres, graphitized carbon fibers, flake graphite, amorphous graphite, natural graphite / artificial graphite composites, and graphite-soft carbon / hard carbon composites. Because these materials intrinsically possess a layered crystal structure, they exhibit high lithium intercalation capacity, low lithium intercalation / deintercalation potential, and good structural stability, while also possessing excellent electronic conductivity. When the negative electrode active material matrix includes the above-mentioned types, the battery energy density and charge transport kinetics of the secondary battery can be significantly optimized.

[0073] This invention does not specifically limit the type of carbon material. In one specific embodiment, the carbon material includes at least one of coal tar, coal pitch, petroleum pitch, phenolic resin, epoxy resin, furan resin, furfuryl alcohol resin, polyacrylonitrile, and sugar organic compounds.

[0074] In one specific implementation, the graphitization degree of the negative electrode material is 88%~96%.

[0075] By adjusting the graphitization degree of the anode material to 88-96%, the low-temperature performance, high-rate fast-charging performance, and cycle life of the secondary battery can be significantly improved. Specifically, the anode at this graphitization level retains some crystal defects and micropore sites, forming a three-dimensional lithium-ion transport network, effectively reducing the lithium-ion intercalation barrier at low temperatures and alleviating low-temperature polarization; simultaneously, it optimizes the high-spec ratio. 2 The hybrid carbon structure and ordered graphite layered structure optimize the transport of electrons and lithium ions. In addition, it can optimize the distribution of active sites on the surface of the anode material, induce the formation of a more uniform and dense SEI film, reduce the irreversible decomposition of the electrolyte and the consumption of active lithium, and maintain the long-term stability of the anode interface.

[0076] To further improve the overall performance of the secondary battery, in one specific embodiment, the secondary battery further includes a separator, the separator including a separator substrate and a separator functional layer disposed on at least one surface of the separator substrate;

[0077] The thickness ratio of the membrane substrate to the membrane functional layer is (0.3~30):1.

[0078] It should be noted that the membrane substrate provides structural support for the membrane as a whole, while the membrane functional layer endows the membrane with higher ion transport performance through its characteristic structure. By adjusting the thickness ratio of the membrane substrate to the membrane functional layer to (0.3~30):1, the lithium ion transport rate and interface stability of the membrane can be optimized simultaneously.

[0079] This invention does not limit the specific material type of the membrane substrate, but includes, but is not limited to, at least one of PE, PP, PP / PE / PP, PET, and PI. This invention also does not limit the specific composition of the membrane functional layer, and in one specific embodiment, includes, but is not limited to, at least one of alumina, boehmite, LATP, SiO2, PVDF, PVDF-HFP, aramid, PBO, PI, and wood fiber.

[0080] The present invention does not limit the specific preparation method of the diaphragm. For example, it can be: using PE as the diaphragm substrate, applying alumina as coating 1 on both sides or one side with a micro-concave roller, and then spraying polyvinylidene fluoride as coating 2 on the base film or coating 1 to obtain the diaphragm.

[0081] To further optimize the structural support performance of the membrane substrate, in one specific embodiment, the thickness of the membrane substrate is 3~15μm. By adjusting the thickness of the membrane substrate within this range, the mechanical strength of the membrane substrate and the overall ion transport resistance of the membrane can be synergistically optimized, thereby effectively improving the cycle life and safety of the secondary battery.

[0082] In one specific embodiment, the thickness of the membrane functional layer is 0.5~10μm. By adjusting the thickness of the membrane functional layer within this range, the overall lithium-ion transport rate of the membrane can be optimized, and the distribution of the electrolyte inside the membrane can be effectively controlled to be more uniform, thereby significantly optimizing the rate performance of the secondary battery.

[0083] To further optimize the ion transport performance of the separator, in one specific embodiment, the porosity of the separator is 30% to 80%. By controlling the porosity of the separator within this range, a more continuous and uniform three-dimensional pore network can be formed inside the separator, effectively optimizing the lithium ion transport path and reducing its transport resistance, thereby significantly improving the overall ion transport efficiency of the separator. At the same time, relying on the uniformly distributed pore structure within this porosity range, the distribution state of lithium ion flux can be precisely controlled, thereby optimizing the problem of ion transport rate lagging behind electron transport rate under low-temperature conditions, reducing charge transfer impedance, and achieving a significant improvement in the low-temperature fast charging performance of the secondary battery.

[0084] Secondly, the present invention also provides an electrical device comprising the aforementioned secondary battery. This electrical device broadly refers to various devices or systems that rely on electrical energy for operation, particularly including mobile or stationary equipment that uses electrical energy as a power source for all or part of its operation. Exemplarily, the device can be a transportation vehicle, such as a conventional internal combustion engine vehicle, a gas-powered vehicle, or a vehicle driven by new energy sources, wherein new energy vehicles can further include pure electric vehicles, hybrid vehicles, range-extended electric vehicles, etc.

[0085] Besides vehicles, this electrical equipment can also encompass other terminal products that rely on energy storage devices for operation, such as communication terminals (e.g., mobile phones), portable computing devices (e.g., tablets and laptops), consumer electronics (e.g., digital media players, electric toys), work tools (e.g., power tools), and various vehicles (e.g., electric ships, drones). Furthermore, in the aerospace field, this equipment can also include aircraft, launch vehicles, spacecraft, and other devices that require high-energy-density power systems.

[0086] By using the secondary battery provided by this invention, the above-mentioned electrical equipment can achieve significant improvements in low-temperature fast charging performance, service life, and safety, which is conducive to improving the overall performance and reliability of the equipment and is suitable for diversified application scenarios with high requirements for power systems.

[0087] The secondary battery provided in this application achieves a synergistic improvement in lithium-ion transport efficiency and interface stability under low-temperature conditions by constructing a relationship between the content of film-forming additives in the electrolyte (a), the content of carboxylic acid ester compounds in the solvent (b), and the ratio of the negative electrode interface impedance to the total impedance (c) of the secondary battery, with the condition 0.0000068≤n≤0.036. This significantly improves the low-temperature fast-charging performance, cycle life, and safety of the secondary battery.

[0088] The technical solution of this application and how the technical solution of this application solves the above-mentioned technical problems are described in detail below with specific embodiments. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments.

[0089] Example 1

[0090] The secondary battery of the present invention is prepared by the following steps:

[0091] 1) Electrolyte preparation: Methyl acetate (80g), ethylene carbonate (10g), and ethyl methyl carbonate (10g) are mixed to obtain an organic solvent. Then, fully dried lithium salt LiPF6 (14g) is dissolved in the mixed organic solvent, and vinylene ester (2.4g) film-forming additive is added to prepare an electrolyte with a viscosity of 3mPa·s (ionic conductivity of 12mS / cm).

[0092] 2) Preparation of negative electrode sheet: Petroleum coke was pre-calcined at 1000℃ for 3 hours in an inert atmosphere, followed by pulverization and classification to obtain pre-calcined powder. The pre-calcined powder was mixed with asphalt at a mass ratio of 95:5, kneaded at 250℃, and granulated to obtain precursor particles. The precursor powder was pre-carbonized in an inert atmosphere at 1100℃ to obtain pre-carbonized calcined needle coke. The calcined needle coke was graphitized at 3000℃ for 8 hours, and after natural cooling to room temperature, it was dispersed to obtain the negative electrode active material matrix. The negative electrode active material matrix (92g), asphalt (6g), and water were uniformly mixed, followed by spray drying and holding at 950℃ for 3 hours. Finally, after deagglomeration and sieving, artificial graphite (with a carbon layer thickness of 10nm and a coating rate of 70%) was obtained as the negative electrode active material.

[0093] Artificial graphite, conductive agent Super P, binder CMC, and binder SBR were mixed in a mass ratio of 96.4:0.6:0.6:2.4 to obtain a negative electrode material (BET of 1.5 and graphitization degree of 93%), which was then dispersed in water to obtain a negative electrode slurry. Subsequently, the negative electrode slurry was uniformly coated on both sides of a copper foil, dried, rolled, and slit to obtain a negative electrode sheet.

[0094] 3) Separator Preparation: PE was used as the separator substrate (7μm thick). High-purity alumina nanoparticles were mixed and ground with a dispersion medium and additives to form a uniform slurry. The slurry was then uniformly coated onto the surface of the base membrane using a microgravure coating method as coating layer 1. After drying and curing, a 2μm thick ceramic coating was formed. Finally, an adhesive layer was prepared by spraying. A suitable adhesive was selected and mixed with solvent to form a uniform adhesive solution, which was then uniformly sprayed onto the surfaces of coating layer 1 and the PE base membrane, with a controlled thickness of 3μm. After drying and curing to remove residual solvent, a complete separator was obtained.

[0095] 4) Preparation of positive electrode sheet: The positive electrode active material lithium iron phosphate, conductive agent Super P and binder PVDF are mixed in a mass ratio of 96:2:2, NMP solvent is added, and the mixture is stirred under vacuum in a double planetary slurry reactor until the system is homogeneous to obtain positive electrode slurry; the positive electrode slurry is evenly coated on both sides of aluminum foil, dried in an oven, and then cold-pressed and slit to obtain positive electrode sheet.

[0096] 5) Assemble the secondary battery: The positive electrode, separator, and negative electrode are stacked in sequence so that the separator is placed between the positive and negative electrodes to isolate them. Then, the cells are wound to obtain the bare cells. The bare cells are placed in the outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, and shaping, the secondary battery 1 is obtained.

[0097] Example 2

[0098] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (27g), ethylene carbonate (40g), ethyl methyl carbonate (33g), and vinylene oxide film-forming additive in step 1) is adjusted to 2.16g. This embodiment yields secondary battery 2.

[0099] Example 3

[0100] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (40g), ethylene carbonate (30g), ethyl methyl carbonate (30g), and vinylene film-forming additive in step 1) is adjusted to 2.88g. This embodiment yields secondary battery 3.

[0101] Example 4

[0102] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (67.5g), ethylene carbonate (17.5g), ethyl methyl carbonate (15g), and vinylene film-forming additive in step 1) is adjusted to 3.24g. This embodiment yields secondary battery 4.

[0103] Example 5

[0104] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (81.8g), ethylene carbonate (10g), ethyl methyl carbonate (8.2g), and vinylene film-forming additive in step 1) is adjusted to 1.8g. This embodiment yields secondary battery 5.

[0105] Example 6

[0106] This embodiment is basically the same as Embodiment 2, except that the amounts of methyl acetate (65.5g), ethylene carbonate (20g), and methyl ethyl carbonate (14.6g) in step 1) are adjusted. The amount of carbon material pitch (10g) in step 2) is also adjusted to achieve a coating rate of 80%. This embodiment yields a secondary battery 6.

[0107] Example 7

[0108] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (26.7g), ethylene carbonate (30g), and ethyl methyl carbonate (43.3g) in step 2) is adjusted, and the mass of vinylene film-forming additive is 3.6g. The carbon material pitch (12g) in step 2) is also adjusted to achieve a coating rate of 85%. This embodiment produces a secondary battery 7.

[0109] Example 8

[0110] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (25.5g), ethylene carbonate (24.5g), ethyl methyl carbonate (50g), and vinylene film-forming additive in step 1) is adjusted to 4.08g. This embodiment yields secondary battery 8.

[0111] Example 9

[0112] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (13.3g), ethylene carbonate (50g), ethyl methyl carbonate (36.7g), and vinylene film-forming additive in step 1) is adjusted to 4.8g. This embodiment yields a secondary battery 9.

[0113] Example 10

[0114] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (8.3g), ethylene carbonate (60g), ethyl methyl carbonate (31.7g), and vinylene film-forming additive in step 1) is adjusted to 6g. This embodiment yields a secondary battery 10.

[0115] Example 11

[0116] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (70.6g), ethylene carbonate (19.4g), and ethyl methyl carbonate (10g) in step 1) is adjusted, and the mass of vinylene film-forming additive is 0.24g. The carbon material pitch (12g) in step 2) is adjusted to achieve a coating rate of 85%. This embodiment prepares a secondary battery 11.

[0117] Example 12

[0118] This embodiment is basically the same as Embodiment 1, except that the mass of methyl acetate (5.6g), ethylene carbonate (50g), ethyl methyl carbonate (44.4g), and vinylene film-forming additive in step 1) is adjusted to 4.8g. This embodiment yields a secondary battery 12.

[0119] Comparative Example 1

[0120] This comparative example is basically the same as Example 1, except that the mass of methyl acetate (5g), ethylene carbonate (50g), ethyl methyl carbonate (45g), and vinylene film-forming additive in step 1) is adjusted to 4.8g. This comparative example yields a secondary battery 13.

[0121] Comparative Example 2

[0122] This comparative example is basically the same as Comparative Example 1, except that the amounts of methyl acetate (3g), ethylene carbonate (50g), and methyl ethyl carbonate (47g) in step 1) are adjusted. This comparative example yields secondary battery 14.

[0123] Comparative Example 3

[0124] This comparative example is basically the same as Comparative Example 2, except that the mass of methyl acetate (66.7g), ethylene carbonate (13.3g), ethyl methyl carbonate (20g), and vinylene film-forming additive in step 1) is adjusted to 0.24g. The carbon material pitch (8g) in step 2) is also adjusted to achieve a coating rate of 75%. This comparative example yields a secondary battery 15.

[0125] Comparative Example 4

[0126] This comparative example is basically the same as Example 1, except that methyl acetate (96g), ethylene carbonate (2g), and ethyl methyl carbonate (2g) are replaced in step 1), and the mass of the vinylene film-forming additive is 0.144g. This comparative example yields a secondary battery 16.

[0127] Experimental Example 1

[0128] The following tests were performed on the negative electrode materials of all embodiments and comparative examples.

[0129] 1. Carbon coating thickness test

[0130] The thickness of the carbon coating layer was measured using high-resolution transmission electron microscopy (HRTEM). The anode material powder to be tested was dispersed in anhydrous ethanol and sonicated for 10 min. Two drops of the dispersion were then placed on the surface of a microgrid copper mesh and vacuum dried at room temperature (25℃, 1 h). The sample was placed on the HRTEM stage, and the high-resolution transmission mode was used with an accelerating voltage of 200 kV. The interface boundary between the coating layer and the anode substrate was clearly focused. Using the built-in scale of the instrument, five test points were selected at different locations on the same particle. Ten different particles from the same batch of samples were selected for measurement. The average value of all test points was calculated, which is the thickness of the carbon coating layer of the anode material, in nm.

[0131] 2. Coverage rate test

[0132] The coating efficiency of the negative electrode active material was tested using transmission electron microscopy (TEM). The specific steps were as follows: the negative electrode active material was ultrasonically dispersed in anhydrous ethanol, dropped onto a microgrid copper mesh, and then vacuum dried. A TEM accelerating voltage of 200 kV was set, and the particle surface was observed in high-resolution mode to clearly distinguish the interface boundary between the coating layer and the substrate. TEM images of 15 particles were selected, and the area percentage of the coated region was statistically analyzed using image analysis software. The average value was taken as the coating efficiency (in %).

[0133] 3. BET test

[0134] The sample was pretreated at 100℃ and degassed under vacuum for 3 hours to completely remove the moisture and impurities adsorbed on the sample surface. The test temperature was the liquid nitrogen temperature (-196℃), with nitrogen as the adsorbate and the adsorption pressure range controlled between 0.05 and 0.30 P / P0.

[0135] Operating procedure: Place the pretreated sample into a sample tube, immerse it in liquid nitrogen, purge with nitrogen gas and gradually adjust the pressure, record the adsorption amount at different pressures, plot the adsorption-desorption isotherms, and calculate the BET specific surface area of ​​the sample by fitting the BET equation using software. (Unit: m²) 2 / g.

[0136] 4. Graphitization degree test

[0137] The degree of graphitization of the negative electrode material was detected by X-ray diffraction (XRD). Key temperature parameters for testing: sample pretreatment temperature 80℃, constant temperature drying for 2h to remove adsorbed moisture and residual organic matter on the sample surface to avoid interference with diffraction signals; the entire testing process was conducted at room temperature (25℃).

[0138] Operating procedure: The pretreated negative electrode sample is pressed into a uniform thin sheet and placed on the sample stage of an XRD diffractometer. Cu-Kα rays (wavelength 0.15406 nm) are selected, with a scanning range of 2θ = 10°~80° and a scanning speed of 5° / min. The diffraction pattern is analyzed using software, focusing on the diffraction peaks of the graphite (002) crystal plane, and the degree of graphitization (G) is calculated.

[0139] Experimental Example 2

[0140] The following tests were performed on the secondary batteries of all embodiments and comparative examples.

[0141] 1. Viscosity test

[0142] The viscosity of the electrolyte was measured at 25°C using a digital viscometer, with the unit being mPa·s.

[0143] 2. Ionic conductivity test

[0144] The ionic conductivity of the electrolyte was measured using a conductivity cell. The unit is mS·cm. -1 .

[0145] A conductivity electrode (platinum black electrode) and an electrochemical workstation were used. Before testing, the electrode constant was calibrated with standard KCl solution, and the electrode must be dry and clean. Electrolyte was injected into the electrode test cell, air bubbles were removed, and the cell was placed in a constant temperature bath at 25℃±0.1℃ for 30 minutes to ensure temperature stability. Test parameter settings: frequency range 10... 5 Hz~10 -1 Hz, AC excitation voltage 5mV, impedance spectrum was collected and solution resistance was obtained using the high-frequency intercept. Conductivity was calculated using the formula κ=K cell / R calculation, where K cell Let be the electrode constant, and R be the measured resistance.

[0146] 3. Negative electrode interface impedance ratio test (c)

[0147] Single-cell or pouch cells assembled using positive electrode, negative electrode, separator, and electrolyte were kept at a constant temperature of 25℃±0.1℃ and open-circuit potential (OCV) for 1 hour to ensure system stability; the electrochemical workstation frequency range was 10. 5 The frequency range is Hz to 0.01Hz, with an AC excitation amplitude of 5mV. First, the EIS impedance of the two electrodes of the full cell is measured. Then, the impedance data is imported into the DRT analysis software, and regularization parameters are set to decompose and obtain the impedance peaks corresponding to different relaxation times. The high-frequency peak corresponds to the electrolyte ohmic impedance, the mid-frequency peak corresponds to the SEI film impedance, and the mid-to-low frequency peak corresponds to the charge transfer impedance.

[0148] Calculation method: The total impedance R of the full cell is obtained by decomposing the peak area using DRT and integrating the peak areas. total Negative electrode SEI film impedance RSEI And negative electrode charge transfer impedance Rct, negative electrode interface total impedance R n =R SEI +Rct, final calculation of proportion: negative electrode interface impedance proportion = (R n / R total ()×100%. The test needs to be performed in triplicate, with a relative deviation of ≤3%. During DRT decomposition, the regularization parameters need to be optimized to avoid peak overlap and ensure accurate decomposition results. At the same time, good electrode contact and the absence of air bubbles should be ensured to reduce test errors. The test results are shown in Table 1.

[0149] 4. Thickness test of diaphragm substrate and diaphragm functional layer

[0150] The thickness of the membrane substrate and functional layer was measured using the SEM cross-sectional method. First, a small section of the membrane sample was cut and precisely etched using focused ion beam (FIB) technology to obtain a deformation-free cross-section. After preparation, the sample surface was sputter-coated with gold.

[0151] Test parameter settings: Place the sample into a scanning electron microscope, adjust the accelerating voltage to 10kV, control the working distance at 5mm, and use secondary electron imaging mode to acquire high-resolution cross-sectional images. The porous continuous structure in the image is the membrane substrate, and the dense thin layer attached to the surface is the functional layer.

[0152] During measurement, image analysis software was used to read the distance between the upper and lower interfaces of the functional layer (functional layer thickness) and the distance between the upper and lower interfaces of the substrate layer (substrate thickness). Ten points were randomly measured in different regions of the sample, and the arithmetic mean was taken as the final thickness data.

[0153] 5. Diaphragm porosity test

[0154] The membrane porosity was tested using the liquid absorption method. The specific steps were as follows: the membrane was cut and its apparent volume V was measured. It was then thoroughly dried, and its dry weight m was measured. The dried membrane was then immersed in n-butanol under vacuum for 1 hour. After wiping away any residual liquid droplets, its wet weight m1 was measured. The membrane porosity was calculated using the following formula:

[0155] Diaphragm porosity = (m1 - m) × 100% / (ρliquid × apparent volume)

[0156] Where ρliquid is the density of n-butanol, and the membrane pore size is expressed in units of %.

[0157] Table 1 Parameters of secondary batteries

[0158]

[0159] Experimental Example 3

[0160] The electrochemical performance of all secondary batteries from the examples and comparative examples was tested as follows, and the results are shown in Table 2.

[0161] 1. Low-temperature fast charging performance test

[0162] The fast-charging performance of the battery prepared above was tested, and the test process is as follows:

[0163] At 25℃, the battery was charged at a constant current of 0.02C for 300 minutes, then left to stand for 10 minutes. It was then charged at a constant current of 0.1C for 300 minutes, left to stand for 1 hour to complete formation, and finally charged at a constant current and constant voltage of 0.33C to a full charge of 3.8V. It was then discharged at 0.33C to 2V, charged at a constant current of 0.33C to 3.8V, charged at a constant voltage to the cutoff current of 0.05C, and then discharged at a constant current of 0.33C to 2V. The discharge capacity was recorded as the standard capacity Q0 at 0.33C.

[0164] At 0℃, the battery was charged at a constant current of 2C to a negative parameter potential of 0mV, then charged at a constant current of 1.6C to a negative parameter potential of 0mV, and then charged at a constant current of 1.2C to a negative parameter potential of 0mV. This process was repeated with decreasing currents to 0.8C, 0.6C, 0.4C, 0.3C, 0.2C, and 0.1C until a negative parameter potential of 0mV was reached. The charging time was recorded as the fast charging time T. The battery was then discharged at 0.33C to a terminal voltage of 2.0V, and the discharge capacity was recorded as the standard capacity Q1 at 0.33C. This test was repeated three times. Unit: hours.

[0165] The fast charging capacity retention rate was obtained using the formula: (Q1 / Q0)×100%. The test was repeated 3 times. (Unit: ).

[0166] 2. Low-temperature cycling performance test

[0167] The batteries prepared above were subjected to a cycle performance test at 0°C. The test procedure was as follows: The batteries after being capacitated were cycled according to the fast charging strategy of the above low-temperature fast charging test until the capacity decayed to 80%. The number of cycles when the capacity decayed to 80% was recorded. The test was repeated 3 times. The unit is cycles.

[0168] Table 2 Electrochemical performance of secondary batteries

[0169]

[0170] As shown in Table 2, the secondary battery of the present invention, by establishing the parameter relationship n=a×c / b and limiting the value of n to satisfy 0.0000068≤n≤0.036, can effectively shorten the fast charging time of the battery under low temperature conditions, improve the low temperature fast charging capacity retention rate and low temperature cycle performance, and achieve simultaneous optimization of the battery's low temperature fast charging performance and cycle life.

[0171] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A secondary battery, characterized in that, The secondary battery includes an electrolyte; The electrolyte includes film-forming additives and solvents, and the content of the film-forming additives in the electrolyte is a; The solvent includes a carboxylic acid ester compound, and the content of the carboxylic acid ester compound in the solvent is b; The ratio of the negative electrode interface impedance of the secondary battery to the total impedance of the secondary battery is c. n = a × c / b, where 0.0000068 ≤ n ≤ 0.

036.

2. The secondary battery according to claim 1, characterized in that, The condition 'a' satisfies 0.2% ≤ a ≤ 4%.

3. The secondary battery according to claim 1, characterized in that, The condition b satisfies 5% ≤ b ≤ 70%.

4. The secondary battery according to claim 1, characterized in that, The condition c satisfies 0.2% ≤ c ≤ 5%.

5. The secondary battery according to claim 1, characterized in that, The carboxylic acid ester compound comprises 3 to 6 carbon atoms; and / or, The film-forming additives include ester-based film-forming additives.

6. The secondary battery according to claim 5, characterized in that, The ester film-forming additives include at least one of vinylene carbonate, ethylene ethylene carbonate, ethylene carbonate, fluoroethyleneene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, trimethyl phosphate, triethyl phosphate, methanedisulfonate, triphenyl phosphite, vinyl sulfate, and propylene sulfite; and / or, The carboxylic acid ester compound includes at least one of methyl acetate, ethyl acetate, ethyl propionate, methyl butyrate, γ-butyrolactone, and δ-valerolactone.

7. The secondary battery according to claim 1, characterized in that, The viscosity of the electrolyte is 1.0~5 mPa·s; and / or, The electrolyte has an ionic conductivity of 8~20 mS / cm.

8. The secondary battery according to claim 1, characterized in that, The negative electrode interface impedance is 0.00035~0.005Ω; and / or, The total impedance is 0.05~0.2Ω.

9. The secondary battery according to any one of claims 1-8, characterized in that, The secondary battery further includes a negative electrode sheet, the negative electrode sheet includes a negative electrode material layer, the negative electrode material layer includes a negative electrode material, and the negative electrode material includes a negative electrode active material; The negative electrode active material includes a negative electrode active material matrix and a carbon coating layer disposed on at least a portion of the outer surface of the negative electrode active material matrix; The carbon coating layer comprises carbon materials.

10. The secondary battery according to claim 9, characterized in that, The mass of the carbon coating layer is 0.5% to 15% based on the mass of the negative electrode active material.

11. The secondary battery according to claim 9, characterized in that, The thickness of the carbon coating layer is 1~20nm.

12. The secondary battery according to claim 9, characterized in that, The coating rate of the negative electrode active material is 50%~99%.

13. The secondary battery according to claim 9, characterized in that, The BET of the negative electrode material is 0.5~5m. 2 / g.

14. The secondary battery according to claim 9, characterized in that, The negative electrode active material matrix includes at least one of petroleum coke, needle coke, mesophase carbon microspheres, graphitized carbon fibers, flake graphite, amorphous graphite, natural graphite / artificial graphite composite, and graphite-soft carbon / hard carbon composite; and / or, The carbon material includes at least one of coal tar, coal pitch, petroleum pitch, phenolic resin, epoxy resin, furan resin, furfuryl alcohol resin, polyacrylonitrile, and carbohydrate organic compounds.

15. The secondary battery according to claim 9, characterized in that, The graphitization degree of the negative electrode material is 88%~96%.

16. The secondary battery according to any one of claims 1-8 and 10-15, characterized in that, The secondary battery further includes a separator, which includes a separator substrate and a separator functional layer disposed on at least one surface of the separator substrate; The thickness ratio of the diaphragm substrate to the diaphragm functional layer is (0.3~30):

1.

17. The secondary battery according to claim 16, characterized in that, The thickness of the diaphragm substrate is 3~15μm; and / or, The thickness of the diaphragm functional layer is 0.5~10μm.

18. The secondary battery according to claim 16, characterized in that, The porosity of the diaphragm is 30% to 80%.

19. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1-18.

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