Ethylene carbonate-free electrolyte and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and provides an electrolyte without ethylene carbonate and a lithium ion battery. The electrolyte comprises a first solvent, a second solvent, a third solvent, a first lithium salt and a second lithium salt, the first solvent comprises propylene carbonate; the second solvent comprises ethyl difluoroacetate; the third solvent comprises n-propyl propionate; in the electrolyte, the content of a first solvent is 1%-8%, the content of a second solvent is 20.2%-75.1%, and the content of a third solvent is 5%-35%; the first lithium salt comprises lithium bis (trifluoromethylsulfonyl) imide, and the second lithium salt comprises lithium hexafluorophosphate; the content of the first lithium salt is marked as A%, the content of the second lithium salt is marked as B%, and 0.1 < = A / B < = 2. According to the electrolyte provided by the invention, the fast charging performance of the battery can be improved on the basis of ensuring high-temperature cycling stability and thermal safety performance in a high-voltage cycling system.

Description

Technical Field

[0001] This invention relates to the technical field of lithium-ion batteries, and more specifically to an electrolyte and a lithium-ion battery. Background Technology

[0002] With the rapid development of high-energy-density battery technology, the application requirements of lithium-ion batteries under high voltage (e.g., >4.53V systems) and high temperature (>45℃) conditions are becoming increasingly urgent. However, traditional electrolyte systems suffer from significant interfacial kinetic sluggishness and uncontrolled side reactions under these extreme conditions, severely limiting the cycle life and safety performance of batteries. Ethylene carbonate (EC), as a major solvent in electrolytes, also suffers from insufficient oxidative stability under higher voltage systems. For example, the decomposition voltage of EC is only 4.3V, limiting the application of high-voltage cathodes (e.g., LiCoO2 >4.5V). In silicon-doped anode systems, it reacts with the silicon-doped anode to generate gases such as CO2 and H2, leading to a large battery expansion rate. At high temperatures, the chemical stability of EC decreases, and it easily reacts with lithium salts (e.g., LiPF6) to generate corrosive substances such as HF, accelerating electrode material corrosion and SEI film rupture. With the development needs of smart devices, developing an electrolyte that can balance high voltage tolerance, high-temperature cycle stability, and rapid charge-discharge performance has become a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide an electrolyte and a lithium-ion battery. The electrolyte provided by this invention does not contain EC. By adding ethyl difluoroacetate, propylene carbonate, and n-propyl propionate to the electrolyte, along with lithium salts lithium bis(trifluoromethanesulfonyl)imide and lithium hexafluorophosphate, and further controlling the ratio of lithium bis(trifluoromethanesulfonyl)imide and lithium hexafluorophosphate in the electrolyte, the battery can improve fast-charging performance while ensuring high-temperature cycle stability and thermal safety.

[0004] To achieve the above objectives, a first aspect of the present invention provides an EC-free electrolyte, the electrolyte comprising a first solvent, a second solvent, a third solvent, a first lithium salt, and a second lithium salt; the first solvent comprises propylene carbonate; the second solvent comprises ethyl difluoroacetate; the third solvent comprises n-propyl propionate; based on the total mass of the electrolyte, the first solvent accounts for 1%-8% of the mass, the second solvent accounts for 20.2%-75.1% of the mass, and the third solvent accounts for 5%-35% of the mass; the first lithium salt comprises lithium bis(trifluoromethanesulfonylimide), and the second lithium salt comprises lithium hexafluorophosphate; based on the total mass of the electrolyte, the mass percentage of the first lithium salt is denoted as A%, and the mass percentage of the second lithium salt is denoted as B%; A and B satisfy: 0.1 ≤ A / B ≤ 2.

[0005] A second aspect of the present invention provides a battery comprising a positive electrode, a negative electrode, and an electrolyte provided in the first aspect of the present invention.

[0006] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0007] The electrolyte provided by this invention contains a high content of ethyl difluoroethylene (DFEA) and removes ethyl acetate (EC) from the electrolyte. DFEA exhibits excellent high-voltage antioxidant properties, inhibiting the oxidative decomposition of the electrolyte under high voltage. Simultaneously, it participates in the formation of the positive and negative electrode interfacial films (CEI and SEI films), enhancing the mechanical strength and stability of the interfacial films, improving the insufficient SEI film formation without EC, and reducing side reaction gas generation at high temperatures. By adding DFEA to the electrolyte and removing EC, the high-temperature performance of the battery under high voltage can be effectively improved. Furthermore, by introducing a composite solvent system composed of n-propyl propionate (PP) and a low content of propylene carbonate (PC), this invention can effectively compensate for the lack of lithium salt dissociation ability in EC-free electrolytes. The invention provides a comprehensive solution. PP's low viscosity enhances electrolyte flow and promotes lithium ion migration after dissociation. PP also dilutes the concentration of DFEA in the electrolyte, preventing excessive interfacial film growth due to high DFEA content. PC, with its high dielectric constant, enhances the solvation of lithium salts, further increasing the number of migrating lithium ions in the electrolyte. The synergistic effect of these two solvents avoids the performance imbalance between dielectric constant and viscosity associated with a single solvent and improves the dissociation efficiency of lithium salts in EC-free electrolyte systems, thereby enhancing kinetic performance. Furthermore, the invention incorporates a lithium salt combination of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF6) into the electrolyte. The large-volume anions in LiTFSI... - (i.e., (CF3SO2)2N) - LiTFSI exhibits high charge delocalization and weak interaction with DFEA (low lattice energy), making it more prone to dissociation in mixed solvents where DFEA is present (compared to LiPF6). This can further increase the concentration of free lithium ions in the electrolyte and enhance ionic conductivity. Furthermore, LiTFSI's excellent high-temperature stability also contributes to improved battery performance at high temperatures. However, because the large volume of LiTFSI anions increases electrolyte viscosity at higher concentrations, it can actually reduce lithium ion migration rates. Therefore, the LiTFSI content needs to be carefully controlled. This invention, by combining LiTFSI with LiPF6, provides an additional lithium ion source for the electrolyte, increasing lithium ion concentration, alleviating lithium ion consumption under high voltage or high temperature conditions, and ensuring ion conduction efficiency. Simultaneously, the low cost of LiPF6 balances the overall electrolyte cost. By controlling the ratio of LiTFSI to LiPF6, the kinetic and high-temperature performance of the battery can be further optimized, ultimately achieving a synergistic improvement in fast-charging performance under high voltage, high-temperature cycle stability, and thermal safety performance.

[0008] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to those ranges or values. For numerical ranges, endpoint values ​​of various ranges, endpoint values ​​of various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In this document, unless otherwise specified, data ranges include endpoints. Attached Figure Description

[0009] Figure 1 The diagram shown is a partially enlarged schematic of the recessed and protruding portions on the electrode sheet in one embodiment of the present invention. Detailed Implementation

[0010] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0011] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0012] The first aspect of this invention provides an EC-free electrolyte, the electrolyte comprising a first solvent, a second solvent, a third solvent, a first lithium salt, and a second lithium salt; the first solvent comprises propylene carbonate; the second solvent comprises ethyl difluoroacetate; the third solvent comprises n-propyl propionate; based on the total mass of the electrolyte, the first solvent accounts for 1%-8% of the mass, the second solvent accounts for 20.2%-75.1% of the mass, and the third solvent accounts for 5%-35% of the mass; the first lithium salt comprises lithium bis(trifluoromethanesulfonylimide), and the second lithium salt comprises lithium hexafluorophosphate; based on the total mass of the electrolyte, the mass percentage of the first lithium salt is denoted as A%, and the mass percentage of the second lithium salt is denoted as B%; A and B satisfy: 0.1 ≤ A / B ≤ 2.

[0013] The EC-free electrolyte provided by this invention utilizes a high content of ethyl difluoroethylene (DFEA) to inhibit the oxidative decomposition of the electrolyte under high voltage by leveraging DFEA's high-voltage antioxidant properties. Simultaneously, DFEA participates in the formation of the positive and negative electrode interfacial films, enhancing the mechanical strength and stability of the interfacial films, reducing high-temperature side reaction gas generation, and improving the high-temperature performance of the battery under high voltage. A composite solvent system is formed by introducing n-propyl propionate (PP) and a low content of propylene carbonate (PC). PP enhances the electrolyte's fluidity with its low viscosity, promotes lithium-ion migration, and dilutes the DFEA concentration to prevent excessive interfacial film growth. PC, with its high dielectric constant... This invention enhances the solubility of lithium salts, with both components synergistically compensating for the insufficient dissociation capacity of lithium salts in EC-free systems, thereby improving lithium salt dissociation efficiency and kinetic performance. Further, by combining lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF6), LiTFSI, with its large-volume anion and high delocalization, is more easily dissociated in mixed solvents containing DFEA, increasing free lithium ion concentration and ionic conductivity, while exhibiting excellent high-temperature stability. The combination with LiPF6, and by controlling the ratio of their contents, balances the viscosity issue at high LiTFSI concentrations, further optimizing kinetic and high-temperature performance. Through the synergistic effect of multiple components, this invention overcomes the performance bottlenecks of existing electrolyte systems in high-temperature and high-voltage scenarios, providing core material support for the commercial application of next-generation high-energy-density power batteries (such as silicon-based anodes and lithium-rich manganese-based cathodes).

[0014] For example, the mass percentage of the first solvent can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or any value within the range of the above two-to-one combinations. If the percentage is less than 1%, the improvement on the dielectric constant of the electrolyte is limited, the dissociation efficiency of the lithium salt decreases, and the concentration of free lithium ions is insufficient, which will lead to the deterioration of the battery's kinetic performance. If the percentage is greater than 8%, PC molecules are prone to intercalation reactions with the negative electrode graphite material (solvent co-intercalation between graphite layers), which can easily lead to graphite layer peeling, damage to the negative electrode structure, and rapid capacity decay during battery cycling. At the same time, excessive PC will also increase the viscosity of the electrolyte, offsetting its effect on improving the dielectric constant, thus hindering lithium ion migration, and may also aggravate side reactions at high temperatures (such as reacting with lithium salt to generate corrosive substances), reducing battery safety.

[0015] In some embodiments, the propylene carbonate accounts for 1%-8% of the total mass of the electrolyte.

[0016] For example, the mass percentage of the second solvent can be 20.2%, 25%, 30%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 50%, 60%, 65%, 70%, 75.1%, or any value within the range of the above pairs, preferably 25%-70%, and more preferably 30%-65%. If the percentage is less than 20.2%, the oxidation resistance of DFEA cannot be fully utilized, the electrolyte is easily oxidized and decomposed under high voltage, resulting in an unstable CEI film and increased dissolution of transition metals; at the same time, the mechanical strength and expansion resistance of the formed SEI film decrease, the side reaction gas production increases at high temperatures, the battery expansion rate increases, and the high-temperature performance decreases. If the DFEA content is higher than 75.1%, the intermolecular hydrogen bonding of DFEA molecules will be enhanced, thereby increasing the overall viscosity of the electrolyte, resulting in decreased fluidity, increased resistance to lithium ion migration, and deterioration of kinetic performance. In addition, high concentrations of DFEA will participate excessively in the formation of the interfacial film, resulting in an excessively thick film layer, which further hinders lithium ions from crossing the interface. At the same time, it may also trigger the aggravation of local side reactions, which will reduce the high-temperature cycling stability of the battery.

[0017] In some embodiments, the ethyl difluoroacetate accounts for 20.2%-75.1% of the total mass of the electrolyte, preferably 25%-70%, and more preferably 30%-65%.

[0018] It should be noted that ethyl difluoroacetate refers to compounds formed by replacing two hydrogen atoms at any position in the ethyl acetate molecule with fluorine atoms, including but not limited to 2,2-difluoroethyl acetate and ethyl 2,2-difluoroacetate.

[0019] In some embodiments, the ethyl difluoroacetate includes at least one of 2,2-difluoroethyl acetate and ethyl 2,2-difluoroacetate.

[0020] For example, the mass percentage of the third solvent can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, or any value within the range of the above two-to-one combinations, preferably 15%-30%. If the percentage is less than 5%, the dilution effect of PP on DFEA is limited, and the viscosity problem and the problem of excessively thick interfacial film caused by excessively high DFEA concentration cannot be alleviated. The electrolyte has poor fluidity, lithium ion migration is hindered, and fast charging performance and high-temperature cycling stability will both decrease. If the percentage is too high, excessive PP will over-dilute DFEA, weakening the high-voltage oxidation resistance and the role of DFEA in forming a stable interfacial film. In addition, PP has a low dielectric constant, and excessively high content will reduce the overall dielectric constant of the electrolyte, affecting the lithium salt dissociation efficiency, resulting in insufficient free lithium ion concentration and decreased kinetic performance.

[0021] In some embodiments, the mass percentage of n-propyl propionate is 5%-35% based on the total mass of the electrolyte, preferably 15%-30%.

[0022] In some embodiments, the mass percentage of the second solvent is greater than that of the third solvent. DFEA exhibits excellent high-voltage oxidation resistance, suppressing electrolyte oxidation and decomposition and side reaction gas generation under high voltage. PP mainly plays an auxiliary role, compensating for the insufficient lithium salt dissociation capability of electrolytes without EC. By further controlling the mass percentage of the second solvent to be greater than that of the third solvent, excessive dilution of DFEA by PP can be avoided. The appropriate ratio of the two can reduce side reactions while maintaining an efficient ion transport pathway, thereby improving the high-temperature and kinetic performance of the battery.

[0023] For example, the value of A / B can be 0.1, 0.3, 0.5, 0.6, 0.7, 1, 1.5, 2, or any value within the range of the above pairs, preferably 0.2 ≤ A / B ≤ 1.5. If A / B is too high, the proportion of LiTFSI will be too large, resulting in a large volume of TFSI. - Excessive anions increase electrolyte viscosity, which in turn reduces lithium-ion migration rate and deteriorates kinetic performance. If the A / B ratio is too low, the proportion of LiPF6 is too large. However, LiPF6 has a low degree of dissociation in the EC-free solvent system of this invention, which leads to insufficient free lithium-ion concentration and decreased ionic conductivity, failing to meet the requirements of high-rate charge and discharge. In addition, LiPF6 is easily hydrolyzed at high temperatures to generate HF. Excessive HF will aggravate the corrosion of positive and negative electrode materials, damage the interfacial film stability, and reduce the high-temperature cycle performance and safety of the battery.

[0024] In some implementations, A and B satisfy: 10 ≤ (A+B) ≤ 25, where (A+B) can be, for example, 10, 12, 14, 16, 18, 20, 22, 24, 25, or any value within the range of the two values ​​mentioned above. If (A+B) is too high, the excessively high total lithium salt concentration will significantly increase the electrolyte viscosity, hindering lithium ion migration and leading to a decrease in kinetic performance. Simultaneously, excessive lithium salt may lead to excessive deposition on the electrode surface, increasing the risk of dendrite growth, damaging the integrity of the SEI film, exacerbating side reaction gas production, and reducing battery cycle life and safety performance. If (A+B) is too low, insufficient total lithium salt concentration will result in a reduced number of free lithium ions, failing to meet the lithium intercalation requirements of both positive and negative electrodes, leading to a decrease in battery capacity and energy density.

[0025] In some embodiments, the mass percentage (A) of the first lithium salt, based on the total mass of the electrolyte, is 0.5%-12%, for example, it can be 0.5%, 1%, 2%, 4%, 6%, 8%, 10%, 12%, or any value within the range of the above pairs, preferably 1%-10%. This invention further limits the mass percentage of the first lithium salt to the above range, which can both improve the ionic conductivity and high-temperature stability of the electrolyte by utilizing its large-volume anionic characteristics, and avoid increasing the electrolyte viscosity due to an excessively high percentage, or failing to fully utilize its performance due to an excessively low percentage, thus ensuring a balance between electrolyte kinetics and high-temperature performance.

[0026] In some embodiments, the mass percentage of the second lithium salt, based on the total mass of the electrolyte, is 5%-20%, for example, it can be 5%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, or any value within the range of any combination of the above values. This invention further limits the mass percentage of the second lithium salt to the above range, which can provide a sufficient lithium ion source for the electrolyte to ensure ion conduction efficiency, while utilizing its low-cost characteristics to balance the cost of the electrolyte, and avoiding the following: an excessively high percentage will exacerbate high-temperature hydrolysis and gas generation corrosion of the electrode structure, while an excessively low percentage will lead to insufficient free lithium ions, affecting battery performance.

[0027] In some embodiments, the electrolyte includes a first additive, which includes fluoroethylene carbonate (FEC). The mass percentage of the first additive, based on the total mass of the electrolyte, is denoted as m1%, where m1 satisfies 5 ≤ ​​m1 ≤ 30. That is, the mass percentage of the first additive can be, for example, 5%, 8%, 10%, 15%, 20%, 25%, 30%, or any value within the range of any of the above pairs, preferably 8 ≤ m1 ≤ 20. FEC undergoes reduction and decomposition on the negative electrode surface, participating in the formation of the SEI film. The fluorinated groups introduced by FEC, along with inorganic components such as LiF, further enhance the mechanical strength and chemical stability of the SEI film, suppressing continuous side reactions between the electrolyte and the negative electrode active material (such as silicon or graphite), and reducing active lithium loss. Simultaneously, FEC can mitigate the damage to the SEI film caused by the volume expansion of the silicon-based negative electrode, reducing the risk of gas generation at high temperatures, thereby improving the cycle stability and storage performance of the battery. If the content is higher than 30%, the viscosity of FEC itself is high, and excessive amounts will significantly increase the overall viscosity of the electrolyte, reduce the lithium-ion migration rate, and lead to deterioration of kinetic performance. Excessive participation in SEI film formation will result in an excessively thick film layer, increasing interfacial impedance and hindering lithium-ion crossing the interface. If the content is lower than 5%, it cannot further enhance the mechanical strength of the SEI film and has little effect on improving the volume expansion of silicon-based anodes. The inhibition effect on the side reactions between the anode and the electrolyte is weakened, gas production increases at high temperatures, the battery expansion rate increases, and the safety performance decreases.

[0028] In some embodiments, the electrolyte includes a second additive, which includes vinylene carbonate (VC); the mass percentage of the second additive based on the total mass of the electrolyte is denoted as m2%; the m2 satisfies 0.01≤m2≤3, that is, the mass percentage of the second additive can be, for example, 0.01%, 0.02%, 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or any value within the range of the above pairs, preferably 0.1≤m2≤2. VC forms a dense organic polymer layer by undergoing free radical polymerization on the surface of the negative electrode (such as a silicon-based negative electrode). This inhibits negative electrode expansion, improves the flexibility of the SEI film, and further controls the content of the second additive in the electrolyte. This can prevent the content from exceeding 3%. Excessive VC will lead to over-polymerization, resulting in an excessively thick organic polymer layer or excessive cross-linking, which will increase interfacial impedance and reduce the lithium-ion migration rate. Unreacted VC may also undergo side reactions with lithium salts (such as LiPF6) at high temperatures, damaging the stability of the interfacial film and even causing gas expansion.

[0029] In some embodiments, the electrolyte includes both fluoroethylene carbonate and vinylene carbonate. When FEC and VC are present in the electrolyte, VC, as the top layer of the SEI film, can synergistically work with the LiF-rich inorganic underlayer formed by FEC. On the one hand, the organic polymer layer can improve the flexibility of the SEI film and adapt to the volume expansion of the silicon-based anode; on the other hand, its dense structure can further block the contact between the electrolyte and the anode, reduce side reactions, and improve the cycle stability of the battery.

[0030] In some embodiments, the electrolyte includes a third additive, which includes adiponitrile (ADN) and 1,3,6-hexanetrionitrile (HTCN). The mass percentage of the third additive, based on the total mass of the electrolyte, is denoted as m3%, where m3 satisfies 0.5 ≤ m3 ≤ 5. That is, the mass percentage of the third additive can be, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any value within the range of any of the above pairs. Further addition of ADN and HTCN can improve the safety of the battery under high-temperature voltage. The CN group of ADN can neutralize free HF and H2O in the electrolyte, reducing acid corrosion of the positive electrode material. HTCN can reduce the reactivity of the electrolyte with the high-voltage positive electrode by adsorbing metal ions. Further controlling the proportion of the third additive in the electrolyte can prevent the content from exceeding 5%. Excessive complexation of transition metal ions may reduce the number of positive electrode active sites and decrease battery capacity. Unreacted nitrile compounds may decompose at high temperatures, producing toxic gases or corrosive substances, reducing battery safety.

[0031] In some embodiments, the electrolyte includes a sulfur-containing additive, with the mass percentage of the sulfur-containing additive denoted as m4% based on the total mass of the electrolyte. m4 satisfies 1 ≤ m4 ≤ 6, meaning the mass percentage of the sulfur-containing additive can be, for example, 1%, 2%, 3%, 4%, 5%, 6%, or any value within the range of any of the above pairs. The sulfur-containing additive can be oxidized and decomposed at the positive and / or negative electrodes to generate an inorganic-organic composite interfacial film rich in sulfides (such as Li2SO3, Li2S) and LiF. The formed composite interfacial film maintains stability under high temperature and high voltage, and its high thermal decomposition temperature effectively prevents high-temperature gas expansion problems. Further controlling the mass percentage of the sulfur-containing additive in the electrolyte can prevent an excessively high sulfur content (above 6%) from leading to an excessively high proportion of sulfur-based components in the interfacial film, which may reduce the ionic conductivity of the film, increase interfacial impedance, and deteriorate cycle performance.

[0032] In some embodiments, the sulfur-containing additives include vinyl sulfate (DTD), 1,3-propanesulfonyl lactone (PS), and butanesulfonyl lactone (BS).

[0033] One or more of them.

[0034] In some embodiments, the electrolyte includes a third lithium salt, which includes at least one of lithium difluorophosphate (LiPO2F2), lithium tetrafluoroborate (LiBF4), and lithium difluorooxalate borate. The third lithium salt accounts for 0.01%-3% of the total mass of the electrolyte, for example, 0.01%, 0.02%, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, or any value within the range of any two of the above values, preferably 0.1%-2%. The third lithium salt serves as an auxiliary lithium salt, wherein LiPO2F2 preferentially decomposes on the electrode surface, participating in the formation of a LiF and Li-rich electrolyte. x PO y A thin SEI film of F2 reduces interfacial impedance and inhibits continuous electrolyte decomposition; the anion of LiBF4 (BF4) - ) and Li + With weak coordination ability, it can still maintain high ion mobility at low temperature and reduce charge transfer impedance; the anions of lithium difluorooxalate borate simultaneously form CEI film containing BO bonds and SEI film containing LiF at the positive and negative electrodes, which enhances the interface stability.

[0035] A second aspect of the present invention provides a battery comprising a positive electrode, a negative electrode, and an electrolyte provided in the first aspect of the present invention; the negative electrode comprises a negative current collector and a negative active material layer disposed on at least one side surface of the negative current collector; the positive electrode comprises a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector. The lithium-ion battery including the electrolyte provided by the present invention can balance high temperature and dynamic performance under high voltage conditions.

[0036] In some embodiments, the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer located between the first negative electrode active material layer and the negative electrode current collector; the first negative electrode active material layer includes a silicon-based material, and the second negative electrode active material layer includes a carbon-based material; the silicon-based material is spherical or near-spherical. In this invention, the negative electrode sheet can be coated in a double-layer manner to form a first negative electrode active material layer (top layer) including spherical or near-spherical silicon-based material. The high sphericity of the silicon-based material in the first negative electrode active material layer can optimize the particle packing method, giving the top layer higher porosity, improving electrolyte wettability, and reducing the volume expansion of the silicon-based material itself, reducing side reactions during charging and discharging, and the silicon-based material itself can effectively improve the battery energy density; the second negative electrode active material layer (bottom layer) is mainly composed of carbon-based material, utilizing the high conductivity and layered structure of the carbon-based material to ensure low-impedance contact with the negative electrode current collector, providing a fast path for electron transport. The two work together to ensure good battery dynamic performance while effectively improving battery energy density.

[0037] In some embodiments, the first negative electrode active material layer further includes a carbon-based material.

[0038] In some embodiments, the silicon-based material includes at least one of silicon-carbon materials or silicon-oxygen materials.

[0039] In some embodiments, the carbon-based material includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

[0040] In some embodiments, the compaction density of the second negative electrode active material layer is higher than that of the first negative electrode active material layer.

[0041] In some embodiments, the compaction density of the first negative electrode active material layer is 1.5 g / cm³. 3 -1.9g / cm 3 For example, it could be 1.5g / cm³. 3 1.6g / cm 3 1.66 g / cm 3 1.67 g / cm 3 1.68g / cm 31.69 g / cm 3 1.7g / cm 3 1.71g / cm 3 1.72g / cm 3 1.8g / cm 3 1.9g / cm 3 Or any value within the range formed by the pairwise values ​​mentioned above.

[0042] In some embodiments, the compaction density of the second negative electrode active material layer is 1.52 g / cm³. 3 -1.94g / cm 3 For example, it can be 1.52 g / cm³. 3 1.6g / cm 3 1.7g / cm 3 1.71g / cm 3 1.72g / cm 3 1.73g / cm 3 1.74 g / cm 3 1.8g / cm 3 1.94 g / cm 3 Or any value within the range formed by the pairwise values ​​mentioned above.

[0043] The high density of the lower layer, combined with the high conductivity and layered structure of the carbon-based material, provides a stable lithium intercalation substrate, while optimized interlayer spacing accelerates Li-… + Diffusion improves battery dynamics performance.

[0044] In some embodiments, the sphericity of the silicon-based material is 0.85-0.99, for example, it can be 0.85, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, or any value within the range of any pairwise values ​​mentioned above. High sphericity silicon-carbon materials possess highly consistent ion / electron transport paths, and when combined with a high DFEA content, significantly increase the number of lithium ions entering and exiting the silicon-carbon material simultaneously, thereby enhancing the battery's fast-charging performance.

[0045] In this invention, the sphericity of silicon-based materials can be tested using a scanning electron microscope (SEM). The testing method may include the following steps: analyzing the image of each silicon-based material particle in an SEM image at a certain magnification using image processing software (such as Image Pro Plus) to obtain the perimeter and area of ​​each particle; calculating the perimeter equivalent radius r1 and area equivalent radius r2 of each particle; then the sphericity of each particle S = r2 / r1; and finally, performing a number-weighted average of the sphericity of each particle (for example, selecting 50 particles) to obtain the sphericity of the silicon-based material.

[0046] In some embodiments, the particle size Dv50 of the silicon-based material is 5μm-15μm, for example, it can be 5μm, 9μm, 11μm, 13μm, 15μm, or any value within the range of any two of the above values. If the particle size of the silicon-based material is too small, the specific surface area will increase, exacerbating side reactions; if it is too large, the lithium-ion diffusion distance will increase, the kinetics will decrease, and the expansion stress will concentrate.

[0047] In some embodiments, the particle size Dv50 of the carbon-based material is 2μm-25μm, for example, it can be 2μm, 5μm, 10μm, 15μm, 20μm, 25μm, or any value within the range of any two of the above values. If the particle size of the carbon-based material is too small, it results in low packing density and increased electronic conductivity; if it is too large, the contact area with the current collector is reduced, hindering electron transport.

[0048] In this invention, the particle size Dv50 of silicon-based materials and carbon-based materials can be tested using conventional testing methods in the art, such as laser particle size analysis. For example, a Malvern particle size analyzer can be used for measurement.

[0049] In some implementations, the mass percentage of silicon, based on the total mass of the negative electrode active material layer, is 1.5%-50%, for example, it can be 1.5%, 2%, 5%, 10%, 20%, 30%, 40%, 50%, or any value within the range of any combination of the above values. If the mass percentage of silicon is too small, the improvement in battery energy density will be limited; if it is too large, the volume expansion of the silicon-based material will be aggravated, easily damaging the SEI film and increasing side reactions.

[0050] In this invention, the mass percentage of silicon in the total mass of the negative electrode active coating can be determined using conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and soaked in dimethyl carbonate (DMC) solvent for 12 hours, then rinsed with DMC solvent to remove lithium salts adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active coating can then be peeled off from the negative electrode current collector, and the negative electrode active coating is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature (25°C) to 900°C at a rate of 10°C / min, and held at 900°C for 40 minutes. This allows the non-silicon components in the negative electrode active coating to volatilize while silicon is fully oxidized to silicon dioxide. The remaining substance is the ash content of the negative electrode active coating. The mass content of silicon in the negative electrode active material layer can be calculated based on the mass of ash. The calculation formula is as follows: Based on the total mass of the negative electrode active material layer, the mass percentage of silicon = 7 × mass of ash / (15 × mass of test sample).

[0051] In some embodiments, the thickness of the first negative electrode active material layer is 5μm-50μm, for example, it can be 5μm, 10μm, 20μm, 30μm, 40μm, 50μm or any value within the range of the above two-point values.

[0052] In some embodiments, the thickness of the second negative electrode active material layer is 5μm-50μm, for example, it can be 5μm, 10μm, 20μm, 30μm, 40μm, 50μm or any value within the range of the above two points.

[0053] In some embodiments, the negative electrode active material layer includes a fibrous material. The fibrous network formed by the fibrous material can further improve electronic conductivity. When the negative electrode sheet is a double-layer coating (including a first negative electrode active material layer and a second negative electrode active material layer), the fibrous material in the first negative electrode active material layer can work synergistically with the highly spherical silicon-based material to buffer and absorb the volume expansion stress of the negative electrode, thereby improving battery safety.

[0054] In some embodiments, the fiber material includes at least one of polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, viscose-based carbon fiber, carbon nanofiber, graphene fiber, carbon nanotube, and vapor-grown carbon fiber.

[0055] In some embodiments, the fiber material accounts for 0.3%-10% of the total mass of the negative electrode active material layer, for example, it can be 0.3%, 0.5%, 1%, 2%, 4%, 6%, 8%, 10% or any value within the range of the above pairs of values, preferably 0.5%-2%.

[0056] In some embodiments, the mass percentage of fiber material in the first negative electrode active material layer is greater than the mass percentage of fiber material in the second negative electrode active material layer.

[0057] In some embodiments, the second negative electrode active material layer does not include fibrous material.

[0058] In some embodiments, the aspect ratio of the fiber material is 10-500, for example, it can be 10, 50, 100, 200, 300, 400, 500 or any value within the range of the above pairs, preferably 15-350. By further limiting the aspect ratio of the fiber material, it is possible to ensure that the fiber material effectively bridges the gaps between the negative electrode active material particles, mitigating the volume expansion and contraction during battery charging and discharging.

[0059] In some embodiments, the average length of the fiber material is 2μm-100μm, for example, it can be 2μm, 3μm, 5μm, 10μm, 50μm, 85μm, 100μm or any value in the range of the above two-point values, preferably 3μm-85μm.

[0060] In some embodiments, the average diameter of the fiber material is 0.02μm-60μm, for example, it can be 0.02μm, 0.05μm, 0.1μm, 5μm, 10μm, 20μm, 30μm, 40μm, 45μm, 50μm, 60μm or any value within the range of the above two-point values, preferably 0.05μm-50μm.

[0061] The average length and average diameter of the fiber material, within the aforementioned range, can further improve and mitigate volume expansion and contraction during battery charging and discharging, thereby improving battery cycle and thermal safety performance.

[0062] In this invention, the average diameter and average length of the fiber material are measured using a scanning electron microscope (SEM). For example, the steps could involve randomly selecting a certain number of fiber materials (e.g., 20) under a scanning electron microscope, calculating their diameter and length separately, and then averaging the results to obtain the average diameter and average length of the fiber material.

[0063] In some embodiments, the negative electrode active material layer is provided with a plurality of grooves, the depth of which is less than the thickness of the negative electrode active material layer.

[0064] In some embodiments, the groove satisfies at least one of the following conditions:

[0065] (1) The distance between adjacent grooves is 0.5mm-10mm;

[0066] (2) The width of the groove is 20μm-500μm;

[0067] (3) The depth of the groove is 5μm-35μm.

[0068] In this invention, grooves formed on the negative electrode active material layer are embedded in the surface of the active material layer to construct low-impedance electron transport channels, effectively improving the electronic conductivity of silicon-based materials and alleviating polarization caused by the poor intrinsic conductivity of silicon. At the same time, the grooves can serve as temporary reservoirs for electrolyte, continuously replenishing the active material layer with electrolyte during fast charging, avoiding interruption of lithium-ion transport due to local electrolyte drying. In addition, the micron-scale groove structure accelerates electrolyte penetration through capillary action, significantly shortening the wetting time, and especially improving the problem of poor electrolyte wetting in high-voltage solid electrode sheets.

[0069] It should be noted that when the negative electrode sheet is single-layer coated, that is, there is only one layer of negative electrode active material on one side of the negative electrode current collector, the groove is located on the surface of the negative electrode active material layer away from the negative electrode current collector; when the negative electrode sheet is double-layer coated, the groove is located on the surface of the first negative electrode active material layer away from the negative electrode current collector.

[0070] In this invention, the groove can be formed using conventional techniques in the field, such as laser drilling or wire bonding.

[0071] In this invention, the shape of the groove is not specifically limited; for example, it can be a linear groove.

[0072] For example, the spacing between adjacent grooves can be 0.5mm, 1mm, 2mm, 4mm, 6mm, 8mm, 10mm, or any value within the range of the above pairs. The spacing between adjacent grooves can be understood as the shortest straight distance on the surface of the negative electrode sheet between two adjacent nearest grooves along the length of the negative electrode sheet. If the groove spacing is too small, the grooves will be too dense, reducing the effective carrying area of ​​the active material, lowering the battery energy density, and potentially causing a decrease in the mechanical strength of the electrode sheet due to the dense mesh structure. If the groove spacing is too large, the spacing will be too wide, resulting in insufficient coverage of the electron transport channels, failing to effectively reduce the overall impedance, and reducing the electrolyte storage and permeation paths, thus not significantly improving the problem of local electrolyte drying during fast charging.

[0073] For example, the width of the groove can be 20μm, 50μm, 100μm, 200μm, 300μm, 400μm, 500μm, or any value within the range of any two of the above values. The width of the groove can be understood as the "diameter of the groove," that is, the maximum straight-line distance between the two inner edges of the groove in the direction parallel to the negative electrode. If the groove width is too small, it will limit the electrolyte storage capacity, resulting in insufficient replenishment capacity during fast charging; if the groove width is too large, it will occupy too much space for active material, reducing energy density, while simultaneously weakening capillary action, slowing down the electrolyte penetration rate, and decreasing wetting efficiency.

[0074] For example, the depth of the groove can be 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, or any value within the range of any two of the above values. The depth of the groove can be understood as the vertical distance from the surface of the negative electrode active material layer to the bottom of the groove, and the groove does not penetrate the negative electrode current collector. If the groove depth is too shallow, the electrolyte storage capacity will be insufficient, failing to meet the fast charging requirements, and the electron transport channel embedding depth will be insufficient, resulting in limited polarization improvement. If the groove depth is too deep, it will weaken the structural stability of the negative electrode active material layer, and during charge and discharge cycles, interlayer cracking is easily caused by the volume expansion of the silicon-based material, affecting the integrity of the electrode sheet.

[0075] In this invention, the spacing between adjacent grooves, the width of the grooves, and the depth of the grooves can be observed and tested using conventional methods in the art, such as using a scanning electron microscope. A certain number of grooves (e.g., 10) are randomly selected under the scanning electron microscope, and the average value is taken.

[0076] In some embodiments, the positive electrode sheet includes a first surface and a second surface disposed opposite to each other in the thickness direction. The first surface has a plurality of protrusions spaced apart, and the second surface has a plurality of corresponding recesses. The uneven texture formed on the positive electrode sheet can increase the electron conduction path, reduce the interfacial contact resistance, and improve the charge transport efficiency. At the same time, it increases the specific surface area of ​​the positive electrode sheet, expands the electrolyte contact area, increases the wetting speed, reduces local polarization, and, combined with a higher DFEA content in the electrolyte, can further improve the battery dynamic performance.

[0077] In some implementations, the protrusions or recesses are regular or irregular shapes, such as circles, squares, hexagons, other spliced ​​shapes, or irregular shapes.

[0078] In some embodiments, the protrusions and recesses can be arranged at uniform intervals, or the arrangement can be adjusted according to actual needs, such as partially arranged at uniform intervals and partially arranged at non-uniform intervals.

[0079] In some embodiments, the protrusions and / or recesses satisfy at least one of the following conditions:

[0080] (a) The average spacing between adjacent protrusions is 0.5 mm to 16 mm; and / or, the average spacing between adjacent recesses is 0.5 mm to 16 mm;

[0081] (b) The height of the protrusion is 1 μm-40 μm; and / or, the depth of the recess is 1 μm-40 μm;

[0082] (c) The diameter of the protrusion is 0.2 mm to 8 mm; and / or the diameter of the recess is 0.2 mm to 8 mm.

[0083] For example, the average spacing between adjacent protrusions and / or the average spacing between adjacent recesses can be, for example, 0.5 mm, 1 mm, 2 mm, 4 mm, 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, or any value within the range of the above pairwise values. The average spacing between adjacent protrusions and the average spacing between adjacent recesses can be the same or different. The spacing between adjacent protrusions / the spacing between adjacent recesses can be understood as the shortest straight distance on the surface of the positive electrode sheet between two adjacent nearest protrusions / recesses along the length direction of the positive electrode sheet, denoted as the spacing between adjacent protrusions or the spacing between adjacent recesses. Figure 1 As shown in D1 and D2. When the spacing between the protrusions and the recesses meets the above range, the arrangement density of the protrusions / recesses is within a suitable range. Controlling the number of protrusions / recesses within a certain area can effectively improve the fast charging performance of the battery while avoiding the problem of unstable positive electrode structure caused by excessively small spacing.

[0084] For example, the height of the protrusion and / or the depth of the recess can be, for example, 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, or any value within the range of any pair of values ​​mentioned above. The height of the protrusion and the depth of the recess can be the same or different. The height of the protrusion can be understood as the maximum vertical distance from the top of the protrusion away from the positive current collector to the first surface of the positive electrode plate, such as... Figure 1 As shown in H1; the depth of the recess can be understood as the maximum vertical distance from the bottom of the first recess near the positive current collector to the second surface of the positive electrode plate, as shown in H1. Figure 1 As shown in H2. When the height of the protrusion and the depth of the recess meet the above range, it can avoid the situation where the height of the protrusion is too high or the depth of the recess is too deep, resulting in a fragile protrusion structure that is easily broken by mechanical extrusion and wear; it can also avoid the situation where the height of the protrusion is too low or the depth of the recess is too shallow, so that the electrolyte cannot properly wet the inside of the positive electrode, and the improvement of the battery's fast charging performance is not significant.

[0085] For example, the diameter of the protrusion and / or the diameter of the recess can be, for example, 0.2 mm, 0.5 mm, 1 mm, 2 mm, 4 mm, 6 mm, 8 mm, or any value within the range of any pair of values ​​mentioned above. The diameter of the protrusion and the diameter of the recess can be the same or different. The diameter of the protrusion / diameter of the recess can be understood as the maximum straight-line distance between the two edges of the protrusion / recess in a direction parallel to the positive electrode plate, such as... Figure 1 As shown in R1 and R2. When the diameters of the protrusions and recesses meet the above-mentioned ranges, the area of ​​the protrusions and recesses can be controlled, avoiding excessively large diameters. This can lead to deformation during battery cycling, causing the positive electrode active material to detach from the current collector, resulting in rapid capacity decay. Furthermore, the detached positive electrode material may form conductive paths inside the battery, causing safety issues such as short circuits and thermal runaway. Conversely, excessively small diameters can also be avoided, as the electrolyte cannot adequately wet the inside of the electrode, resulting in minimal improvement in battery fast-charging performance.

[0086] In this invention, the diameter, height / depth, and average spacing of the protrusions and depressions can be observed and tested using conventional methods in the art, such as using a scanning electron microscope. A certain number of protrusions / depressions (e.g., 10) are randomly selected under the scanning electron microscope, and the average value is obtained.

[0087] In some embodiments, the positive electrode active material layer comprises a positive electrode active material, which comprises a layered lithium composite oxide with the general chemical formula Li. (1+x) Ni y Co z M (1-y-z) O2, where -0.1≤x≤1; 0≤y≤1, 0≤z≤1, and 0≤y+z≤1; M is one or more of Mg, Zn, Ga, Ba, Al, Fe, Cr, Sn, V, Mn, Sc, Ti, Nb, Mo, and Zr.

[0088] In some embodiments, the positive electrode active material includes aluminum-doped lithium cobalt oxide.

[0089] Aluminum-doped lithium cobalt oxide materials are suitable for high-voltage cycling systems. 3+ Al2O3 partially replaces cobalt sites, forming strong Al-O bonds (bond energy higher than Co-O), which enhances the lattice stability of lithium cobalt oxide materials, delays the phase transition initiation voltage, and reduces cobalt dissolution during cycling. Furthermore, the formation of an Al2O3 coating layer on the surface of lithium cobalt oxide can isolate the direct contact between the electrolyte and lithium cobalt oxide, inhibit the interfacial corrosion of electrolyte decomposition products, reduce the surface impedance of the positive electrode, and thus improve the cycle capacity retention rate of the battery under high voltage and high temperature conditions.

[0090] In some embodiments, the aluminum content, based on the total mass of the positive electrode active material, is greater than or equal to 7000 ppm. Preferably, the aluminum content is between 7000 ppm and 15000 ppm, for example, it can be 7000 ppm, 8000 ppm, 10000 ppm, 11000 ppm, 13000 ppm, 15000 ppm, or any value within the range of any combination of the above values. If the aluminum content is less than 7000 ppm, its stabilizing effect on the lithium cobalt oxide material layer structure is insufficient. Under high voltage, the lithium cobalt oxide material is prone to phase transition (transformation from layered to spinel phase), leading to increased dissolution of transition metal ions and increased release of lattice oxygen, which can easily trigger electrolyte decomposition and thermal runaway risks. If the aluminum content is greater than 15000 ppm, excessive Al... 3+ It will occupy Co 3+ The lattice sites reduce the theoretical capacity of lithium cobalt oxide materials.

[0091] In this invention, the aluminum content, based on the total mass of the positive electrode active material, can be tested by ICP-MS (inductively coupled plasma mass spectrometry).

[0092] In some embodiments, the charging cutoff voltage of the battery is greater than or equal to 4.53V. The battery of the present invention is adapted to a high-voltage charging system, and can balance high-temperature performance and dynamic performance under high-voltage conditions.

[0093] In some embodiments, the battery includes a separator, the separator comprising a substrate layer and a functional coating located on at least one surface of the substrate layer; the functional coating comprises nitrogen-containing particles, the nitrogen-containing particles comprising melamine-based compounds. The nitrogen-containing groups can combine with free acidic substances such as PF5 and HF in the electrolyte, reducing chemical erosion of the positive electrode active material, delaying interface degradation, improving the positive electrode interface, and thus enhancing high-temperature cycling stability. Furthermore, after removing EC and adding a large amount of DFEA to the electrolyte, the amount of gas generated under high temperature and pressure is reduced. Combined with the high decomposition temperature and rigid molecular framework of melamine-based compounds, this improves the stability of the separator under high temperature and pressure, further suppressing the generation of gases (such as CO2 and H2) from electrolyte oxidation and decomposition during high-temperature cycling, and reducing the risk of interfacial thermal runaway.

[0094] In some embodiments, the melamine compounds include at least one selected from melamine, melamine cyanurate, melamine polyphosphate, melamine thiocyanate, melamine formaldehyde resin, 1,3,5-triazine-2,4,6-triamine, 2,4,6-tris(aminohexanoic acid)-1,3,5-triazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazine-2-yl)guanidine, 2,4-diamino-6-dimethylamino-1,3,5-triazine, cyanuric chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-methoxy-1,3,5-triazine.

[0095] In some embodiments, the functional coating is located on the side of the separator closest to the positive electrode. This functional coating directly faces the positive electrode active material and can capture HF acid and H2O residues in the electrolyte, reducing the corrosion of the positive electrode active material by byproducts.

[0096] In some embodiments, the mass percentage of nitrogen element is 10%-60% based on the total mass of the functional coating, for example, it can be 10%, 20%, 30%, 40%, 50%, 60% or any value within the range of the above two-to-one values.

[0097] In this invention, the nitrogen content, based on the total mass of the functional coating, can be tested using the EDS (energy-dispersive X-ray spectroscopy) method.

[0098] In some embodiments, the functional coating further includes an adhesive. The present invention does not specifically limit the type of adhesive; conventional adhesives in the art can be used.

[0099] In some embodiments, the thickness of the functional coating is 0.3 μm-5 μm, for example, it can be 0.3 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, or any value within the range of any two of the above values. By controlling the thickness range of the functional coating, it is possible to avoid a thickness below 0.3 μm, which would have limited improvement on the high-temperature resistance and other properties of the separator; and to avoid a thickness greater than 5 μm, which would increase the overall thickness of the separator, leading to a decrease in the volumetric energy density of the battery; at the same time, an excessively thick functional coating may increase the lithium-ion transport resistance, reduce the electrolyte ionic conductivity, and deteriorate the battery kinetic performance.

[0100] In some embodiments, the substrate layer comprises a polyolefin material. Exemplarily, the polyolefin material includes at least one of polypropylene and polyethylene.

[0101] In some embodiments, the separator optionally includes a polymer coating located on the outermost side of the separator facing the positive and / or negative electrode. The polymer coating includes at least one of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), PVDF-hexafluoropropylene modified polyvinylidene fluoride and its copolymers, polyacrylonitrile, polymethyl methacrylate (PMMA), polyacrylic acid, styrene-butadiene rubber (SBR), polyvinyl alcohol and its copolymerized polyvinyl alcohol, polyvinyl acetate, polyacrylamide, phenolic resin, epoxy resin, waterborne polyurethane, ethylene-vinyl acetate copolymer, multi-component acrylic copolymer, lithium polystyrene sulfonate, pure styrene latex, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene, polyvinylpyrrolidone, polyethylene oxide, cellulose acetate, butyl acetate, propyl acetate, cyanoethyl amylopectin, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, and cyanoethyl sucrose. The polymer coating has good flexibility and wettability, which can enhance the adhesion of the separator to the positive and negative electrodes and reduce interfacial resistance. At the same time, its chemical stability can prevent excessive reaction between the electrolyte and the substrate layer and protect the integrity of the separator structure.

[0102] It should be noted that the optional polymer coating located on the outermost side of the separator facing the positive and / or negative electrode means that the separator may or may not include a polymer coating. When the separator includes a polymer coating, there are three cases: (1) both sides of the separator substrate layer have polymer coatings, located on the outermost side of the separator, facing the positive and negative electrode respectively; (2) one side of the separator substrate layer has a polymer coating, located on the outermost side of the separator, facing the positive electrode; (3) one side of the separator substrate layer has a polymer coating, located on the outermost side of the separator, facing the negative electrode.

[0103] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0104] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0105] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0106] Example 1-1

[0107] (1) Preparation of positive electrode sheet

[0108] Lithium cobalt oxide (LiCoO2), polyvinylidene fluoride (PVDF), conductive carbon black (SP, super P), and carbon nanotubes (CNT) were mixed in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone (NMP) was added, and the mixture was stirred under vacuum until it formed a uniform and fluid positive electrode slurry. The positive electrode slurry was uniformly coated on both surfaces of an aluminum foil (12μm). The coated aluminum foil was dried, and then rolled and slit to obtain the desired positive electrode sheet. The tab grooves on the surface of the positive electrode sheet were then laser-cleaned, and positive tabs were welded into the tab grooves. Tab adhesive protective paper was then attached to the surface of the positive tabs.

[0109] (2) Preparation of negative electrode sheet

[0110] Artificial graphite, silicon carbide, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber, conductive carbon black (SP), single-walled carbon nanotubes (SWCNTs), and fiber materials were mixed in a mass ratio of 69.5:24:2.5:1.5:1:0.5:1. Deionized water was added, and the mixture was stirred under vacuum to obtain a negative electrode active slurry. The negative electrode active slurry was uniformly coated onto both surfaces of a 6 μm copper foil. The coated copper foil was air-dried at room temperature, then transferred to an 80°C oven for 10 hours. After cold pressing and slitting, the negative electrode sheet was obtained. The fiber material had an average length of 50 μm, an average diameter of 0.2 μm, and an aspect ratio of 250.

[0111] (3) Preparation of electrolyte

[0112] In an argon-filled glove box with a water content of <0.1 ppm and an oxygen content of <0.1 ppm, the first solvent (PC), the second solvent (2,2-difluoroethyl acetate), and the third solvent (PP) were mixed evenly. Then, the first additive (FEC), the second additive (VC), the third additive (ADN and HTCN in a 1:1 ratio), the sulfur-containing additive (1,3-propanesulfonyl lactone), the first lithium salt (LITFSI), the second lithium salt (LiPF6), and the third lithium salt (lithium difluorophosphate, lithium tetrafluoroborate, and lithium difluorooxalate borate in a 1:1:1 ratio) were added to form a homogeneous solution. After passing the physical property test, the electrolyte was obtained. The specific mass percentages of each component in the electrolyte are shown in Table 1.

[0113] (4) The diaphragm is an 8μm thick polyethylene diaphragm (provided by Asahi Kasei Corporation).

[0114] (5) Battery fabrication

[0115] The prepared positive electrode, separator, and negative electrode are stacked in sequence, ensuring that the separator is between the positive and negative electrodes to provide isolation. Then, the unfilled bare cell is obtained by winding. The bare cell is placed in an outer packaging foil, and the prepared electrolyte is injected into the dried bare cell. After vacuum sealing, settling, formation, shaping, and sorting, the desired lithium-ion battery is obtained.

[0116] Battery test

[0117] (i) Cyclic performance test

[0118] The lithium-ion batteries obtained in the above examples and comparative examples were subjected to charge-discharge cycles at 45°C and a discharge rate of 2C within the charge-discharge cutoff voltage range (4.55V). The discharge capacity Q1 of the first cycle and the discharge capacity Q600 of the 600th cycle were recorded. The capacity retention rate and thickness expansion rate after 600 cycles at 45°C were calculated by Q600 / Q1×100%.

[0119] After the fully charged battery is left to stand for 24 hours, the initial thickness T0 is measured. Charge and discharge cycles are performed according to the above standard. The thickness T600 is measured in the 600th cycle. The thickness expansion rate is calculated as [(Tn-T0) / T0]×100%.

[0120] (ii) Hot Box Safety Test

[0121] The lithium-ion batteries obtained in the above embodiments and comparative examples were discharged at 0.2C to the lower limit voltage of 3.0V at 25°C. After standing for 10 minutes, they were charged at 1C to the upper limit voltage of 4.55V with a cutoff current of 0.05C. At this point, the lithium-ion batteries were fully charged. The fully charged lithium-ion batteries were then placed in an oven, and the temperature inside the oven was increased at a rate of 5°C / min. When the temperature inside the oven reached 132°C, the temperature was kept constant for 60 minutes. The lithium-ion batteries were observed to see if they caught fire or exploded. If they did not catch fire or explode, the test was considered passed. A total of 15 tests were conducted, and a passing score of 10 or more was considered qualified.

[0122] (iii) Fast charging performance test

[0123] At 25℃±3℃, a fully charged battery is discharged at 0.5C to the lower limit voltage of 3.0V, left to stand for 10 minutes, and the initial discharge capacity Q0 is recorded. The battery is then fully charged at a constant current of 3C, with a cutoff current of 0.02C, and discharged at a constant current of 0.5C, and the discharge capacity Q1 is recorded. The discharge capacity retention rate at 3C is then calculated as Q1 / Q0 × 100%.

[0124] Examples 1-2, Examples 2 through 6, and Comparative Examples 1 through 7 were all performed according to Example 1-1. The differences from Example 1-1 were as follows: in Example 1-2, ethyl 2,2-difluoroacetate was replaced in equal amounts with ethyl 2,2-difluoroacetate in Example 1-1; in Example 2-1, the content of ethyl 2,2-difluoroacetate was 20.2% and the content of PP was 39.8%; in Example 2-2, the content of ethyl 2,2-difluoroacetate was 25% and the content of PP was 35%; in Example 2-3, the content of ethyl 2,2-difluoroacetate was 30% and the content of PP was 30%; in Example 2-4, the content of PC was 1%, the content of ethyl 2,2-difluoroacetate was 65%, the content of PP was 5%, the content of LiTFSI was 4%, and the content of LiPF6 was... 6%; In Examples 2-5, the PC content was 1%, the 2,2-difluoroethyl acetate content was 70%, the PP content was 5%, the LiTFSI content was 4%, the LiPF6 content was 6%, the first additive content was 11%, the third additive content was 1.5%, and the sulfur-containing additive content was 1%; In Examples 2-6, the PC content was 1%, the 2,2-difluoroethyl acetate content was 75.1%, the PP content was 5%, the LiTFSI content was 4%, the LiPF6 content was 6%, the third lithium salt content was 0.1%, the first additive content was 6.5%, the third additive content was 0.9%, and the sulfur-containing additive content was 1.2%; In Examples 2-7, the PC content is 8% and the PP content is 14%; in Example 3-1, the LiTFSI content is 2% and the LiPF6 content is 15%; in Example 3-2, the LiTFSI content is 3% and the LiPF6 content is 14%; in Example 3-3, the LiTFSI content is 10.2% and the LiPF6 content is 6.8%; in Example 3-4, the LiTFSI content is 11.3% and the LiPF6 content is 5.7%; in Example 4-1, the first additive content is 13.7% and the second additive content is 0%; in Example 4-2, the PP content is 16.2% and the second additive content is 2%; In Example 4-3, the PP content was 17.5%, the first additive content was 11.2%, and the second additive content was 3%; in Example 4-4, the PP content was 16.5%, the first additive content was 11.2%, and the second additive content was 4%; in Example 4-5, the PP content was 11.5%, and the first additive content was 20%; in Example 4-6, the PP content was 6.5%, and the first additive content was 25%; in Example 5-1, the PP content was 19.5%, and the sulfur-containing additive content was 0.5%; in Example 5-2, the PP content was 14%, and the sulfur-containing additive content was 6%; in Example 6-1, the LiPF6 content was 11%.The electrolyte contained 3% PC and 0% lithium salt; in Comparative Example 1, PC content was 8%, 2,2-difluoroethyl acetate content was 15%, and PP content was 41%; in Comparative Example 2, PC content was 0% and PP content was 22%; in Comparative Example 3, PC content was 10% and PP content was 12%; in Comparative Example 4, 2,2-difluoroethyl acetate content was 60% and PP content was 0%; in Comparative Example 5, an equal amount of ethylene carbonate (EC) replaced 2,2-difluoroethyl acetate in the electrolyte; in Comparative Example 6, LiTFSI content was 13% and LiPF6 content was 4%; in Comparative Example 7, LiTFSI content was 0% and LiPF6 content was 17%. All component contents are based on the total mass of the electrolyte. Specific data are shown in Table 1.

[0125] Table 1

[0126]

[0127] Based on the data in Table 1, the electrolyte provided by this invention does not contain EC. By adding high amounts of ethyl difluoroacetate, propylene carbonate, and n-propyl propionate to the electrolyte, combined with lithium salts lithium bis(trifluoromethanesulfonyl)imide and lithium hexafluorophosphate, and further adjusting the ratio of lithium bis(trifluoromethanesulfonyl)imide and lithium hexafluorophosphate in the electrolyte, the battery can meet the fast charging performance while simultaneously improving the high-temperature cycle stability and thermal safety performance.

[0128] Example 7 was carried out in accordance with Example 1-1. Example 7 further selected a double-layer coating for the negative electrode, that is, the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer located between the first negative electrode active material layer and the negative electrode current collector. In Example 7, the preparation method and composition of the first negative electrode active material layer are the same as those in Example 1-1. The active material in the second negative electrode active material layer is artificial graphite, excluding silicon carbon materials and fibrous materials. The mass ratio of artificial graphite, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber, conductive carbon black (SP), and single-walled carbon nanotubes (SWCNTs) in the second negative electrode active material layer is 94.5:2.5:1.5:1:0.5. The Dv50 of artificial graphite in the first and second negative electrode active material layers is the same. In Examples 1-1, the first negative electrode active material layer is a single layer with a thickness of 50 μm. In Examples 7-1 to 7-4, the thickness of both the first and second negative electrode active material layers is 25 μm. In Example 7-5, both the first and second negative electrode active material layers are 50 μm thick. In Example 7-6, both the first and second negative electrode active material layers are 5 μm thick. The total proportion of silicon in the negative electrode is adjusted according to the amount of silicon-carbon added, as shown in Table 2.

[0129] Table 2

[0130]

[0131] Note: The units for both silicon-based material Dv50 and carbon-based material Dv50 are μm.

[0132] Based on the data in Table 2, the battery provided by the present invention, by employing a double-layer coating method on the negative electrode, including a first negative electrode active material layer (top layer) of highly spherical silicon-based material and a second negative electrode active material layer (bottom layer) with carbon-based active material as the main active material, can further improve the battery's fast charging performance, high-temperature cycle stability and thermal safety performance.

[0133] Example 8 was conducted in accordance with Example 7-1, but the diaphragm was improved by using a diaphragm with a functional coating containing melamine compounds. Specific data are shown in Table 3. The diaphragm of Example 7-1 was the same as that of Example 1-1.

[0134] Table 3

[0135]

[0136]

[0137] Note: The functional coating also includes an adhesive, which is selected from polyacrylate.

[0138] Based on the data in Table 3, this invention can further improve the battery's fast charging performance, high-temperature cycle stability, and thermal safety performance by further selecting a separator containing melamine compounds in the functional coating and controlling the thickness and nitrogen content of the functional coating.

[0139] Example 9 was carried out in accordance with Example 8-1. In Example 9, aluminum-doped lithium cobalt oxide was used as the positive electrode active material. The specific data are shown in Table 4.

[0140] Table 4

[0141] Example Aluminum doping level (ppm) Capacity retention Thickness expansion rate % Hot box throughput Example 9-1 7500 91.34 7.21 15 / 15 Example 9-2 15000 90.81 7.78 15 / 15

[0142] Based on the data in Table 4, this invention further improves the battery's fast charging performance, high-temperature cycle stability, and thermal safety performance by further selecting aluminum-doped lithium cobalt oxide as the positive electrode active material and controlling the amount of aluminum doping.

[0143] Example 10 was performed in accordance with Example 9-1. In Example 10, the negative electrode sheet underwent laser wire bonding treatment, and the specific data are shown in Table 5. Example 9-1 was the same as Example 1-1, except that the negative electrode sheet was not subjected to wire bonding treatment.

[0144] Table 5

[0145]

[0146] Based on the data in Table 5, this invention can further improve the battery's fast charging performance, high-temperature cycle stability, and thermal safety performance by further wire bonding of the negative electrode sheet and controlling the spacing, width, and depth of the grooves within a suitable range.

[0147] Example 11-1 was performed in accordance with Example 10-1, with the positive electrode further subjected to embossing and rolling treatment. Specific data are shown in Table 6. Example 10-1 was the same as Example 1-1, except that the positive electrode sheet was not rolled.

[0148] Table 6

[0149]

[0150] Note: The depth, spacing, and diameter of the embossing in the table are measured based on the indentations formed by roller pressing.

[0151] Based on the data in Table 6, the present invention further performs a rolling process on the positive electrode sheet and controls the embossing depth, spacing, and diameter formed by the rolling process within a suitable range, which can further improve the battery's fast charging performance, high-temperature cycle stability, and thermal safety performance.

[0152] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. Furthermore, it should be noted that the scope of the methods and apparatuses in the embodiments of the present invention is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

[0153] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A vinyl carbonate-free electrolyte, characterized in that, The electrolyte comprises a first solvent, a second solvent, a third solvent, a first lithium salt, and a second lithium salt; the first solvent is propylene carbonate; the second solvent is ethyl difluoroacetate; the third solvent is n-propyl propionate; based on the total mass of the electrolyte, the first solvent accounts for 1%-8% of the mass, the second solvent accounts for 20.2%-75.1% of the mass, and the third solvent accounts for 5%-35% of the mass. The first lithium salt is lithium bis(trifluoromethanesulfonyl)imide, and the second lithium salt is lithium hexafluorophosphate. The mass percentage of the first lithium salt is denoted as A% based on the total mass of the electrolyte, and the mass percentage of the second lithium salt is denoted as B%. A and B satisfy: 0.1≤A / B≤2.

2. The electrolyte according to claim 1, characterized in that, The second solvent accounts for 25%-70% of the total mass of the electrolyte, preferably 30%-65%; And / or, the ethyl difluoroacetate includes at least one of 2,2-difluoroethyl acetate and ethyl 2,2-difluoroacetate; And / or, the mass percentage of the second solvent is greater than the mass percentage of the third solvent; And / or, A and B satisfy: 0.2 ≤ A / B ≤ 1.5; And / or, A and B satisfy: 10≤(A+B)≤25.

3. The electrolyte according to claim 1 or 2, characterized in that, The electrolyte includes a first additive, which includes fluoroethylene carbonate; the mass percentage of the first additive based on the total mass of the electrolyte is denoted as m1%, where m1 satisfies: 5 ≤ m1 ≤ 30, preferably 8 ≤ m1 ≤ 20. And / or, the electrolyte includes a second additive, the second additive including vinylene carbonate; the mass percentage of the second additive based on the total mass of the electrolyte is denoted as m2%; the m2 satisfies: 0.01≤m2≤3, preferably 0.1≤m2≤2.

4. The electrolyte according to claim 1 or 2, characterized in that, The electrolyte includes a third additive, which includes adiponitrile and 1,3,6-hexanetrionitrile; the mass percentage of the third additive based on the total mass of the electrolyte is denoted as m3%, where m3 satisfies: 0.5≤m3≤5; And / or, the electrolyte includes a sulfur-containing additive, and the mass percentage of the sulfur-containing additive based on the total mass of the electrolyte is denoted as m4%, where m4 satisfies: 1≤m4≤6; And / or, the electrolyte includes a third lithium salt, which includes at least one of lithium difluorophosphate, lithium tetrafluoroborate, and lithium difluorooxalate borate; the mass percentage of the third lithium salt is 0.01%-3% based on the total mass of the electrolyte, preferably 0.1%-2%.

5. A battery, characterized in that, Includes a positive electrode, a negative electrode, and the electrolyte according to any one of claims 1-4; The negative electrode includes a negative current collector and a negative active material layer disposed on at least one side surface of the negative current collector; the positive electrode includes a positive current collector and a positive active material layer located on at least one side surface of the positive current collector.

6. The battery according to claim 5, characterized in that, The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer located between the first negative electrode active material layer and the negative electrode current collector; the first negative electrode active material layer includes a silicon-based material, and the second negative electrode active material layer includes a carbon-based material; the silicon-based material is spherical or near-spherical. Preferably, the mass percentage of silicon element is 1.5%-50% based on the total mass of the negative electrode active material layer; Preferably, the sphericity of the silicon-based material is 0.85-0.99; Preferably, the particle size Dv50 of the silicon-based material is 5μm-15μm, and / or the particle size Dv50 of the carbon-based material is 2μm-25μm; Preferably, the thickness of the first negative electrode active material layer is 5μm-50μm, and / or the thickness of the second negative electrode active material layer is 5μm-50μm.

7. The battery according to claim 5, characterized in that, The negative electrode active material layer is provided with a plurality of grooves, the depth of which is less than the thickness of the negative electrode active material layer. Preferably, the groove satisfies at least one of the following conditions: (1) The distance between adjacent grooves is 0.5mm-10mm; (2) The width of the groove is 20μm-500μm; (3) The depth of the groove is 5μm-35μm.

8. The battery according to claim 5, characterized in that, In the thickness direction of the positive electrode sheet, the positive electrode sheet includes a first surface and a second surface disposed opposite to each other, the first surface is provided with a plurality of protrusions at intervals, and the second surface is provided with a plurality of recesses corresponding to each other. Preferably, the protrusion and / or recess satisfy at least one of the following conditions: (a) The average spacing between adjacent protrusions is 0.5 mm to 16 mm; and / or, the average spacing between adjacent recesses is 0.5 mm to 16 mm; (b) The height of the protrusion is 1 μm-40 μm; and / or, the depth of the recess is 1 μm-40 μm; (c) The diameter of the protrusion is 0.2 mm to 8 mm; and / or the diameter of the recess is 0.2 mm to 8 mm.

9. The battery according to claim 5, characterized in that, The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes an aluminum-doped lithium cobalt oxide material. Preferably, the aluminum content is greater than or equal to 7000 ppm based on the total mass of the positive electrode active material; And / or, the charging cut-off voltage of the battery is greater than or equal to 4.53V.

10. The battery according to any one of claims 5-9, characterized in that, The battery includes a separator, the separator including a substrate layer and a functional coating located on at least one side surface of the substrate layer; The functional coating includes nitrogen-containing particles, which include melamine-based compounds. Preferably, the mass percentage of nitrogen element is 10%-60% based on the total mass of the functional coating; Preferably, the thickness of the functional coating is 0.3μm-5μm.

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