A lithium ion secondary battery

Abstract

This application discloses a lithium-ion secondary battery, wherein the positive electrode includes a first positive active layer near the positive current collector and a second positive active layer away from the positive current collector. The first positive active layer includes lithium iron phosphate, and the second positive active layer includes a nickel-cobalt-manganese ternary material. The negative electrode includes a first negative active layer near the negative current collector and a second negative active layer away from the negative current collector. The first negative active layer includes silicon-carbon material, and the second negative active layer includes graphite. The mass content m% of iron in the positive active layer and the mass content w% of silicon in the negative active layer satisfy: 0.2≤m≤4, 0.5≤w≤15, and 0.5≤w / m≤20. The electrolyte includes 0.01%-5% sulfur-containing additives. Through the layered design of the positive / negative active layers, the synergistic control of silicon / iron content, and the optimized matching of the electrolyte, a stable voltage platform and a synergistic improvement in fast charging capability and cycle life can be ensured.

Description

Technical Field

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

[0002] As a high-energy storage medium, the continuous improvement of energy density and fast-charging capability of lithium-ion secondary batteries is the core direction for the development of the electric vehicle and consumer electronics industries. The combined application of ternary cathode materials (especially layered lithium nickel cobalt manganese oxide, or NCM) and silicon-carbon anode materials provides a feasible technical path to achieve the dual goals of high energy density and fast-charging performance. Specifically, ternary cathode materials, with their high theoretical specific capacity and operating voltage platform, large lithium-ion diffusion coefficient, and good electronic conductivity, lay a solid foundation for improving battery energy density and fast-charging capability. On the anode side, silicon-based materials (such as silicon-carbon composite materials) have a much higher theoretical capacity than traditional graphite anodes (typically in the range of 1000-2500 mAh / g). Furthermore, the isotropic lithium intercalation characteristics of silicon, combined with the conductive network of the carbon matrix, can support rapid lithium-ion intercalation and deintercalation, making it suitable for high-voltage fast-charging platforms.

[0003] However, the aforementioned battery systems face multiple severe challenges in practical applications. For example, the high lithium intercalation potential of silicon-carbon anodes significantly reduces the battery's operating voltage. This effect is particularly pronounced at low state of charge (SOC≤1%), easily causing the battery voltage to drop below the cutoff voltage prematurely, resulting in a loss of usable capacity or even failure to discharge normally, severely limiting the effective utilization of energy. Furthermore, the poor intrinsic electronic conductivity of silicon materials and their huge volume expansion during cycling continuously deteriorate the conductive network and interface stability of the electrodes. This not only leads to a decline in kinetic performance and restricts the realization of fast charging capabilities, but also causes repeated rupture and proliferation of the solid electrolyte interphase (SEI) film, continuously consuming active lithium and electrolyte, thus leading to a rapid decline in cycle life. Summary of the Invention

[0004] In view of this, the technical problem to be solved by this application is to overcome the shortcomings of existing lithium-ion secondary batteries in that they cannot achieve stable voltage platform and synergistic improvement in fast charging capability and cycle life.

[0005] To achieve the above objectives, this application adopts the following technical solution.

[0006] According to an embodiment of this application, a lithium-ion secondary battery is provided, comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction. The positive active layer includes a positive active material, which includes lithium iron phosphate material and nickel-cobalt-manganese ternary material. The negative electrode includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector in the thickness direction. The negative active layer includes a negative active material, which includes silicon-based material and graphite material. The positive electrode active layer includes a first positive electrode active layer and a second positive electrode active layer stacked together. The first positive electrode active layer is disposed between the positive electrode current collector and the second positive electrode active layer. The lithium iron phosphate material is located in the first positive electrode active layer, and the nickel-cobalt-manganese ternary material is located at least in the second positive electrode active layer. Based on the mass of the positive electrode active layer, the mass content of iron element is m%, satisfying 0.2≤m≤4. The negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer stacked together. The first negative electrode active layer is disposed between the negative electrode current collector and the second negative electrode active layer. The silicon-based material is located in the first negative electrode active layer, and the graphite material is located at least in the second negative electrode active layer. Based on the mass of the negative electrode active layer, the mass content of silicon element is w%, satisfying: 0.5≤w≤15 and 0.5≤w / m≤20. The electrolyte includes a sulfur-containing additive, and the mass content of the sulfur-containing additive is a%, based on the mass of the electrolyte, satisfying 0.01≤a≤5.

[0007] In some optional embodiments, the thickness of the first positive electrode active layer is h1 μm and the thickness of the second positive electrode active layer is h2 μm, satisfying: 15≤h1≤35, 15≤h2≤35, 0.8≤h1 / h2≤2.2.

[0008] In some optional embodiments, the thickness of the first negative electrode active layer is h3 μm and the thickness of the second negative electrode active layer is h4 μm, satisfying: 20≤h3≤40, 20≤h4≤40, and 0.8≤h3 / h4≤1.8.

[0009] In some alternative embodiments, the sulfur-containing additive includes at least one of 1,3-propanesulfonyl lactone, mannitol sulfate, vinyl sulfate, vinyl sulfite, propylene-1,3-sulfonyl lactone, and methanedisulfonate.

[0010] Further, in some optional embodiments, the sulfur-containing additive includes 1,3-propanesulfonyl lactone and mannitol carbonate sulfate, wherein the content of 1,3-propanesulfonyl lactone is a2% and the content of mannitol carbonate sulfate is a1% based on the mass of the electrolyte, satisfying 0.02≤a1+a2≤5, 0.01≤a1≤3, and 0.01≤a2≤3.

[0011] In some optional embodiments, the first positive electrode active layer includes a first positive electrode active material, which is a lithium iron phosphate material and a nickel-cobalt-manganese ternary material.

[0012] In some optional embodiments, the second positive electrode active layer includes a second positive electrode active material, which is a nickel-cobalt-manganese ternary material.

[0013] In some optional embodiments, the nickel-cobalt-manganese ternary material in the first positive electrode active layer has a single crystal structure with an average particle size of 2μm-5μm.

[0014] In some alternative embodiments, the lithium iron phosphate material comprises primary particles with an average particle size of 400 nm to 700 nm.

[0015] In some alternative embodiments, the first negative electrode active layer includes a first negative electrode active material, which is a silicon-based material and a graphite material.

[0016] In some alternative embodiments, the second negative electrode active layer includes a second negative electrode active material, which is a graphite material.

[0017] In some alternative embodiments, the average particle size of the graphite material in the first negative electrode active layer is 10 μm-15 μm.

[0018] In some alternative embodiments, the average particle size of the graphite material in the second negative electrode active layer is 5 μm-10 μm.

[0019] In some alternative embodiments, the average particle size of the silicon-based material is 5 μm-10 μm.

[0020] In some alternative embodiments, the graphite material in the first negative electrode active layer has a carbon coating layer with a thickness of 1 nm to 20 nm.

[0021] In some alternative embodiments, the silicon-based material includes a silicon-carbon material, which comprises porous carbon and silicon particles located within the porous carbon channels.

[0022] In some optional embodiments, the surface of the second negative electrode active layer away from the negative electrode current collector has a recess, satisfying at least one of the following conditions: (1) The opening width of the recess is 0.05mm-0.2mm; (2) The distance between adjacent recesses is 0.5mm-5mm; (3) The dimension of the recess along the thickness direction of the negative electrode sheet is T1, and the thickness of the negative electrode active layer on the surface of the negative electrode current collector is T2, 0.2≤T1 / T2≤0.5; (4) The total volume of the recess is T3, and the volume of the negative electrode active layer on the surface of the negative electrode current collector is T4, 0.005≤T3 / T4≤0.03; (5) The negative electrode sheet also includes a negative electrode tab, which extends from the negative electrode current collector and the extension direction of the negative electrode tab is perpendicular to the thickness direction of the negative electrode sheet; the recess is a groove, and the length direction of the groove is parallel to the extension direction of the negative electrode tab.

[0023] In some optional embodiments, the electrolyte further includes a carbonate solvent and a lithium salt, wherein the lithium salt has a mass content of 8%-18% based on the mass of the electrolyte; the carbonate solvent includes at least one of dimethyl carbonate, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, and propylene carbonate.

[0024] In some optional embodiments, the carbonate solvent includes dimethyl carbonate and ethylene carbonate, wherein the mass content of dimethyl carbonate is C1 and the mass content of ethylene carbonate is C2, based on the mass of the electrolyte, satisfying: 5≤C1≤60, 5≤C2≤40, 0.2≤C1 / C2≤10.

[0025] In some alternative embodiments, the diaphragm includes a base membrane, a ceramic layer, and an adhesive layer, wherein the ceramic layer is disposed on at least one surface along the thickness direction of the base membrane, and the adhesive layer is disposed on the side surface of the ceramic layer away from the base membrane, wherein the coverage of the adhesive layer on one side is 5%-95%.

[0026] In some alternative embodiments, the adhesive layer is disposed on the surface of the base film.

[0027] In some alternative embodiments, when the ceramic layer is disposed on at least one side surface in the thickness direction of the base film, the ceramic layer faces the positive electrode, and the thickness of the ceramic layer is 0.5 μm-2 μm.

[0028] In some alternative embodiments, the porosity of the diaphragm is 30%-60%.

[0029] The technical solution of this application has the following advantages: The lithium-ion secondary battery provided in this application includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector along its thickness direction. The positive active layer includes a positive active material, which includes lithium iron phosphate material and nickel-cobalt-manganese ternary material. The negative electrode includes a negative current collector and a negative active layer disposed on at least one surface of the negative current collector along its thickness direction. The negative active layer includes a negative active material, which includes silicon-based material and graphite material. The positive active layer includes a first positive active layer and a second positive active layer stacked together. The first positive active layer is disposed between the positive current collector and the second positive active layer, and the lithium iron phosphate material is located in the first positive active layer. In the positive electrode active layer, the nickel-cobalt-manganese ternary material is located at least in the second positive electrode active layer; based on the mass of the positive electrode active layer, the mass content of iron is m%, satisfying 0.2≤m≤4; the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer stacked together, the first negative electrode active layer is disposed between the negative electrode current collector and the second negative electrode active layer, the silicon-based material is located in the first negative electrode active layer, and the graphite material is located at least in the second negative electrode active layer; based on the mass of the negative electrode active layer, the mass content of silicon is w%, satisfying: 0.5≤w≤15 and 0.5≤w / m≤20; the electrolyte includes a sulfur-containing additive, based on the mass of the electrolyte, the mass content of the sulfur-containing additive is a%, satisfying 0.01≤a≤5.

[0030] This application achieves a balance between the positive and negative electrode potentials through a layered design of the positive and negative electrode active layers, synergistic regulation of silicon and iron content, and optimized matching of the electrolyte. This stabilizes the voltage platform, increases the battery's operating voltage range, improves energy output efficiency, and enhances the stability of the positive electrode interface. As a result, the battery can achieve a synergistic improvement in voltage platform stability, fast charging capability, and cycle life.

[0031] Additional aspects and advantages of the embodiments of this application will be described and shown in part in the following description, or illustrated by practice of the embodiments of this application. Detailed Implementation

[0032] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0033] It should be noted in the description of this application that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the technical features involved in the different embodiments of this application described below may be combined with each other as long as they do not conflict with each other.

[0034] In battery systems employing ternary cathodes and silicon-doped anodes, the anode side suffers from problems such as poor kinetic performance of silicon-carbon materials, reduced cycle life, and high lithium intercalation potential leading to insufficient low-SOC voltage. On the cathode side, existing technologies disclose a composite cathode material composed of a specific amount of NCM and lithium iron phosphate (LFP), which can increase the battery's operating voltage range, specific capacity, and cycle life. However, this study found significant differences in the lithium-ion diffusion coefficients and electronic conductivity between NCM and LFP, and their voltage plateaus are severely misaligned, resulting in asynchronous charging and discharging. This leads to a sharp increase in battery polarization, reduced fast-charging capability, and deterioration of overall kinetic performance.

[0035] In order to address the shortcomings of lithium-ion secondary batteries in related technologies that cannot achieve stable voltage platform and synergistic improvement in fast charging capability and cycle life, this application proposes the following solution.

[0036] According to an embodiment of this application, a lithium-ion secondary battery is provided, comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction. The positive active layer includes a positive active material, which includes lithium iron phosphate material and nickel-cobalt-manganese ternary material. The negative electrode includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector in the thickness direction. The negative active layer includes a negative active material, which includes silicon-based material and graphite material. The positive electrode active layer includes a first positive electrode active layer and a second positive electrode active layer stacked together. The first positive electrode active layer is disposed between the positive electrode current collector and the second positive electrode active layer. The lithium iron phosphate material is located in the first positive electrode active layer, and the nickel-cobalt-manganese ternary material is located at least in the second positive electrode active layer. Based on the mass of the positive electrode active layer, the mass content of iron element is m%, satisfying 0.2≤m≤4. The negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer stacked together. The first negative electrode active layer is disposed between the negative electrode current collector and the second negative electrode active layer. The silicon-based material is located in the first negative electrode active layer, and the graphite material is located at least in the second negative electrode active layer. Based on the mass of the negative electrode active layer, the mass content of silicon element is w%, satisfying: 0.5≤w≤15 and 0.5≤w / m≤20. The electrolyte includes a sulfur-containing additive, and the mass content of the sulfur-containing additive is a%, based on the mass of the electrolyte, satisfying 0.01≤a≤5.

[0037] The negative electrode of this application is constructed by layering silicon-based materials and graphite materials, wherein the silicon-based material is located in the inner layer (i.e., the first negative electrode active layer), while the graphite material is located in at least the outer layer (i.e., the second negative electrode active layer). In this way, on the one hand, the graphite material can be in direct contact with the electrolyte, giving full play to the excellent kinetic properties of the graphite material and ensuring the rapid response capability of the battery under high-rate charge and discharge conditions; on the other hand, controlling the mass content w% of silicon element in the negative electrode active layer between 0.5% and 15% not only helps to improve the specific capacity of the negative electrode, but also effectively buffers the damage to the electrode structure caused by the volume expansion of silicon-based material during cycling by confining the relatively small amount of silicon-based material in the inner layer, reducing the consumption of active lithium and ensuring the cycle stability of the battery.

[0038] Meanwhile, the positive electrode of this application also adopts a double-layer design. By placing LFP in the inner layer (i.e., the first positive electrode active layer) and NCM in the outer layer (i.e., the second positive electrode active layer), the outer NCM can preferentially undergo lithium delithiation during battery charging due to its high voltage and good kinetics. The LFP has a lower voltage plateau, and when the voltage rises to the charging voltage of the LFP, the lithium ions in the LFP will be delithiated. During discharge, the NCM mainly plays a role in the medium and high voltage range. The polarization voltage generated by the outer NCM during discharge will not affect the LFP. When the voltage drops to the low voltage range, the LFP will discharge at the bottom. Therefore, by layering NCM and LFP, this application allows them to work in parallel across voltage ranges. This ensures that NCM fully utilizes its advantages of high energy density, power performance, and fast charging capability. Furthermore, by controlling the iron content (m%) in the positive electrode active layer within the range of 0.2%-4% and the w / m value within the range of 0.5-20, it not only utilizes the stable voltage platform of LFP at low SOC to compensate for the voltage drop caused by the high lithium intercalation potential of silicon-based materials, thus stabilizing the voltage platform and widening the battery's usable discharge window, but also ensures that the fast charging dynamics of the battery within the 10%-90% SOC range remain unaffected. In other words, the solution presented in this application perfectly solves the problem of voltage asynchrony between NCM and LFP, effectively alleviates polarization during high-rate charging and discharging, and improves the overall dynamic performance of the battery.

[0039] Based on this, the electrolyte of this application also contains 0.01%-5% sulfur-containing additives. These additives can participate in the formation of a stable solid electrolyte interface (CEI) film on the positive electrode surface, thereby enhancing the stability of the positive electrode interface, effectively inhibiting the degradation of the positive electrode structure and the decomposition of the electrolyte at high temperatures, and improving the high-temperature cycle life of the battery.

[0040] In summary, this application achieves a balance between the potentials of the positive and negative electrodes through the layered design of the positive and negative electrode active layers, the synergistic regulation of silicon / iron element content, and the optimized matching of the electrolyte. This stabilizes the voltage platform, increases the battery's operating voltage range, improves energy output efficiency, and enhances the stability of the positive electrode interface, thereby ensuring that the battery can achieve a synergistic improvement in voltage platform stability, fast charging capability, and cycle life.

[0041] It is understood that in this application, the nickel-cobalt-manganese ternary material (NCM) can be of the chemical formula LiNi. x1 Co y1 Mn z1 The substance O2, wherein 0 < x1 < 1, 0 < y1 < 1, 0 < z1 < 1, and x1 + y1 + z1 = 1, can also be a material modified by elemental doping and / or coating. The doping elements include W, Zr, V, and Al, and the coating material includes at least one of Al2O3 and MgO. Similarly, lithium iron phosphate (LFP) materials can be substances with the chemical formula LiFePO4, or materials modified by elemental doping and / or coating. The doping elements include at least one of Ti, Sc, and B, and the coating material includes amorphous carbon, which can be formed by sintering acetylene onto the surface of lithium iron phosphate. The aforementioned modification methods for NCM and LFP are common modification strategies in the prior art and will not be elaborated further here.

[0042] If the iron content in the positive electrode active layer is too low (less than 0.2%), it means that the amount of lithium iron phosphate material added in the first positive electrode active layer is insufficient. It is difficult to continue to provide capacity when the battery is discharged to the low voltage range. It cannot effectively compensate for the voltage drop caused by the high lithium intercalation potential of silicon-based materials. As a result, the battery is still prone to prematurely reaching the discharge cutoff voltage under low charge state, causing a loss of usable capacity. This weakens the synergistic effect of the nickel-cobalt-manganese ternary material and the lithium iron phosphate material working together in the voltage range in the positive electrode double-layer structure.

[0043] If the iron content in the positive electrode active layer is too high (more than 4%), the proportion of lithium iron phosphate material in the first positive electrode active layer will be too high. On the one hand, this will reduce the overall energy density of the positive electrode active layer. On the other hand, due to the low intrinsic ionic conductivity of lithium iron phosphate, excessive addition will hinder the rapid delithiation and lithium insertion process of the outer nickel-cobalt-manganese ternary material in the 10%-90% SOC range. This will affect the full utilization of the high energy density, excellent power performance and fast charging capability of the nickel-cobalt-manganese ternary material, thereby disrupting the balance of their working relationship between voltage ranges and weakening the fast charging dynamics of the battery in the medium and high voltage range.

[0044] It should be noted that the test method for the iron content in the positive electrode active layer includes: after the battery is discharged, the positive electrode sheet is removed, dried, and scraped to obtain a positive electrode powder sample; 0.05g-0.2g of sample is accurately weighed and placed in the polytetrafluoroethylene inner container of a microwave digestion vessel, concentrated nitric acid and hydrogen peroxide are added sequentially, the container is tightened and placed in a microwave digestion instrument, and completely digested according to the preset gradient temperature program to decompose all nickel-cobalt-manganese ternary materials, lithium iron phosphate materials, conductive agents and binders in the sample; after digestion, the vessel is cooled to room temperature, the digestion solution is completely transferred to a volumetric flask, the inner container is washed multiple times with ultrapure water and the washing solutions are combined, the volume is adjusted to the mark, and the solution is shaken well for later use; an inductively coupled plasma atomic emission spectrometer is used to measure a series of standard solutions using the sensitive analytical spectral lines of iron to obtain the emission light intensity and plot a standard curve, and the emission light intensity of the sample solution is compared with the standard curve to calculate the iron content in the positive electrode active layer.

[0045] For example, the mass content of iron in the positive electrode active layer may be 0.20%, 0.25%, 0.31%, 0.38%, 0.47%, 0.58%, 0.72%, 0.89%, 1.10%, 1.36%, 1.68%, 2.07%, 2.56%, 3.16%, 4.00%, or a value within the range of any two of the above values.

[0046] If the mass content of silicon in the negative electrode active layer is too low (less than 0.5%), the amount of silicon-based material added in the first negative electrode active layer will be insufficient, making it difficult to fully utilize its high specific capacity advantage. The specific capacity improvement of the negative electrode sheet will be limited, and it will not be able to effectively meet the design requirements of high energy density. At this time, the negative electrode mainly relies on graphite material to provide capacity. Although the negative electrode impedance is relatively small, this will weaken the synergistic effect of silicon-based material and graphite material in energy density and kinetic performance in the negative electrode double-layer structure.

[0047] If the mass content of silicon in the negative electrode active layer is too high (more than 15%), the proportion of silicon-based material in the first negative electrode active layer will be too high. Since the intrinsic kinetic properties of silicon-based materials are poor, too much silicon-based material will deteriorate the battery's fast charging capability. At the same time, the severe volume expansion effect of silicon-based materials during cycling will be significantly aggravated, which will easily lead to the destruction of the electrode structure and repeated rupture and reconstruction of the solid electrolyte interface film. This will result in a large consumption of active lithium and electrolyte, causing a rapid decline in battery cycle life. In addition, too much silicon-based material will lead to an excessive voltage drop at low SOC, which will affect the battery's capacity performance.

[0048] It should be noted that the specific test method for the silicon content in the negative electrode active layer includes: disassembling the lithium-ion battery and removing the negative electrode sheet, soaking and rinsing it with dimethyl carbonate and drying it, then treating it at 400℃ for 2 hours in an inert atmosphere to peel the negative electrode active material layer off the current collector and collect it; taking 5mg-15mg of sample, using a thermogravimetric analyzer in an air or oxygen atmosphere, heating it from room temperature to 900℃ at a rate of 10℃ / min and holding it at that temperature for 40 minutes, allowing the non-silicon components to fully combust and oxidize and volatilize, while simultaneously oxidizing the silicon completely to silicon dioxide. The residual weight percentage of the sample at the end of the test is the ash content of the negative electrode active layer (this ash content corresponds to the mass of silicon dioxide); dividing the ash value by the molar mass of silicon dioxide to obtain the molar amount of silicon dioxide, and then multiplying it by the molar mass of silicon, gives the mass content of silicon in the negative electrode active layer.

[0049] For example, the mass content of silicon in the negative electrode active layer may be 0.50%, 0.64%, 0.82%, 1.04%, 1.33%, 1.70%, 2.17%, 2.77%, 3.54%, 4.52%, 5.77%, 7.37%, 9.41%, 12.02%, 15.00%, or a value within the range of any two of the above values.

[0050] If the ratio w / m between the mass content of silicon and the mass content of iron in the positive electrode active layer is too small (less than 0.5), it means that the silicon content is relatively low or the iron content is relatively high, which will reduce the energy density of the battery. To compensate for the risk of reduced energy density, the areal density of the electrode needs to be increased, which will lead to a decrease in the battery's fast charging capability and high-temperature cycle performance. If the ratio w / m between the mass content of silicon and the mass content of iron in the positive electrode active layer is too large (greater than 20), it means that the silicon content is relatively high or the iron content is relatively low. Since silicon-based materials have a high lithium intercalation potential and lithium iron phosphate has insufficient voltage compensation capability, the battery still faces the problem of low discharge voltage and increased DC internal resistance under low charge state. At the same time, the high internal resistance will further aggravate charge and discharge polarization, thus adversely affecting cycle life.

[0051] For example, the value of w / m can be 0.50, 0.65, 0.84, 1.09, 1.42, 1.84, 2.39, 3.11, 4.04, 5.25, 6.82, 8.86, 11.51, 14.95, 20.00, or a value within the range of any two of the above values.

[0052] If the mass content of sulfur-containing additives is too low (less than 0.01%), it will be difficult to form a continuous and dense solid electrolyte interface film on the positive electrode surface. This will result in insufficient protection of the positive electrode structure at high temperatures, and the decomposition of the electrolyte will not be effectively suppressed. Consequently, the high-temperature cycle life and thermal stability of the battery will be difficult to improve effectively.

[0053] If the mass content of sulfur-containing additives is too high (more than 5%), the excess additives may form an excessively thick or uneven interfacial film on the positive electrode surface, increasing the lithium-ion transport impedance and affecting the battery's rate performance and fast charging capability. At the same time, some additives that do not participate in film formation may undergo side reactions in the electrolyte, which may adversely affect the battery's cycle stability and high-temperature performance.

[0054] It should be noted that the mass content of sulfur-containing additives can be obtained by gas chromatography, gas chromatography-mass spectrometry, and ion chromatography. For example, the mass content of sulfur-containing additives can be 0.01, 0.016, 0.024, 0.038, 0.059, 0.092, 0.14, 0.22, 0.35, 0.54, 0.85, 1.32, 2.06, 3.21, 5.00, or values ​​within any range of two of the above values.

[0055] In some embodiments, by controlling the thickness h1 of the first positive electrode active layer between 15 μm and 35 μm, the positive electrode sheet can have suitable porosity, facilitating electrolyte wetting and ensuring that lithium iron phosphate plays a role in stabilizing voltage. If the thickness h1 of the first positive electrode active layer is too thin (less than 15 μm), the compaction density of the positive electrode sheet increases accordingly under the same areal density, resulting in increased resistance to lithium ion transport in the first positive electrode active layer. This makes it difficult for lithium iron phosphate material to fully exert its voltage compensation function at low SOC, and the battery still faces the risk of prematurely reaching the discharge cutoff voltage. At the same time, the larger compaction density of the positive electrode sheet is not conducive to electrolyte wetting, leading to a decrease in the battery's fast charging performance and high-temperature cycle stability. If the thickness h1 of the first positive electrode active layer is too thick (greater than 35 μm), the side reactions between the active material and the electrolyte increase at high temperatures, leading to deterioration in cycle performance.

[0056] Furthermore, by controlling the thickness h2 of the second positive electrode active layer between 15μm and 35μm, the positive electrode sheet can have suitable porosity, facilitating electrolyte wetting and ensuring high cycle stability and excellent fast-charging performance within the 10%-90% SOC range. If the thickness h2 of the second positive electrode active layer is too thin (less than 15μm), the compaction density of the positive electrode sheet increases accordingly under the same areal density, leading to increased lithium-ion transport resistance in the second positive electrode active layer and decreased fast-charging and high-temperature cycle performance. If the thickness h2 of the second positive electrode active layer is too thick (greater than 35μm), the side reactions between the active material and the electrolyte increase at high temperatures, thus adversely affecting the high-temperature cycle performance of the battery.

[0057] Meanwhile, by controlling the ratio h1 / h2 between the thickness h1 of the first positive electrode active layer and the thickness h2 of the second positive electrode active layer to be between 0.8 and 2.2, it is possible to ensure that lithium iron phosphate can fully exert its voltage compensation function under low SOC, while maintaining the battery's fast charging capability and high-temperature cycle life in the high charge range. If the total thickness of the positive electrode active layer remains consistent, and the value of h1 / h2 is too small (less than 0.8), the first positive electrode active layer will be relatively too thin, increasing the difficulty of contact between the electrolyte and lithium iron phosphate. This will lead to a decrease in the voltage compensation capability of lithium iron phosphate and an increase in DC internal resistance, and will also worsen the battery's high-temperature cycle stability. Alternatively, it may mean that the second positive electrode active layer is relatively too thick, which not only leads to poor wettability of the electrolyte to lithium iron phosphate, making it difficult to exert its voltage compensation function under low SOC, but also increases the side reactions between the active material and the electrolyte at high temperatures, thus adversely affecting the battery's high-temperature cycle performance.

[0058] If the value of h1 / h2 is too large (greater than 2.2), the first positive electrode active layer is relatively too thick, which prolongs the transport path of lithium ions in the first positive electrode active layer. This not only affects the voltage compensation function of lithium iron phosphate at low SOC, but also leads to a decrease in fast charging capability. In addition, a thicker first positive electrode active layer will also lead to an increase in side reactions between the active material and the electrolyte at high temperatures, thus adversely affecting the high-temperature cycle performance of the battery. Alternatively, if the second positive electrode active layer is relatively too thin, the wetting resistance of the electrolyte will increase, which will adversely affect the fast charging performance, cycle stability and DC internal resistance of the battery.

[0059] Furthermore, the total thickness of the first and second positive electrode active layers needs to be controlled between 30μm and 75μm to balance various electrochemical performance characteristics of the battery. When the total thickness of the first and second positive electrode active layers is small, the compaction density of the positive electrode increases, the ion transport channels are compressed, resulting in a decrease in fast charging capability and an increase in DC internal resistance. Conversely, when the total thickness of the first and second positive electrode active layers is large, lithium ion transport is smoother, which improves the battery's fast charging performance and reduces DC internal resistance. However, an excessively thick positive electrode active layer will exacerbate the side reactions between the active material and the electrolyte, leading to poor high-temperature cycle stability.

[0060] It should be noted that the testing methods for the thicknesses h1 and h2 of the first and second positive electrode active layers include: observing the cross-section of the positive electrode sheet along the thickness direction using a scanning electron microscope and taking energy dispersive spectroscopy (EDS) images of the cross-section. The location of the iron element at the farthest end from the positive electrode current collector is used as the boundary between the first and second positive electrode active layers. The end containing the iron element is the first positive electrode active layer, and the end without the iron element is the second positive electrode active layer. Cross-sections at five different locations are observed, and the maximum thickness value obtained is taken as the thickness value of that layer. For example, the thickness of the first positive electrode active layer can be, for example, 15.0 μm, 16.4 μm, 17.9 μm, 19.3 μm, 20.7 μm, 22.1 μm, 23.6 μm, 25.0 μm, 26.4 μm, 27.9 μm, 29.3 μm, 30.7 μm, 32.1 μm, 33.6 μm, 35.0 μm, etc., or values ​​within the range of any two of the above values; the thickness of the second positive electrode active layer can be, for example, 15.0 μm, 16.4 μm, 17.9 μm, 19.3 μm, 20.7 μm, 25.0 μm, 26.4 μm, 27.9 μm, 29.3 μm, 30.7 μm, 32.1 μm, 33.6 μm, 35.0 μm, etc. The values ​​are 2.1μm, 23.6μm, 25.0μm, 26.4μm, 27.9μm, 29.3μm, 30.7μm, 32.1μm, 33.6μm, 35.0μm, etc., or values ​​within the range of any two of the above values; the values ​​of h1 / h2 can be, for example, 0.80, 0.90, 1.00, 1.10, 1.20, 1.30, 1.40, 1.50, 1.60, 1.70, 1.80, 1.90, 2.00, 2.10, 2.20, etc., or values ​​within the range of any two of the above values.

[0061] In some embodiments, by controlling the thickness h3 of the first negative electrode active layer between 20 μm and 40 μm, the negative electrode sheet can have suitable porosity, facilitating electrolyte wetting and allowing the silicon-based material to fully utilize its high specific capacity advantage. Simultaneously, the silicon-based material is confined within the first negative electrode active layer near the negative electrode current collector, effectively buffering the damage to the electrode structure caused by volume expansion during cycling, reducing the consumption of active lithium, and thus ensuring the cycle stability of the battery. If the thickness h3 of the first negative electrode active layer is too large (greater than 40 μm), the side reactions between the silicon-based material and the electrolyte increase, not only deteriorating the high-temperature cycle performance of the battery but also causing repeated SEI rupture and regeneration. A thicker SEI film leads to increased internal resistance and decreased fast-charging capability. If the thickness h3 of the first negative electrode active layer is too small (less than 20 μm), the compaction density of the negative electrode sheet increases, the porosity decreases, and the lithium-ion transport channels are blocked, resulting in increased DC internal resistance and decreased fast-charging performance.

[0062] Furthermore, by controlling the thickness h4 of the second negative electrode active layer between 20μm and 40μm, the negative electrode sheet can have suitable porosity, facilitating electrolyte wetting and allowing the graphite material to exert its excellent fast-charging performance. This ensures the battery's rapid response capability under high-rate charge and discharge conditions. Simultaneously, the second negative electrode active layer, in direct contact with the separator, can respond quickly during fast charging, establishing a lithium-ion concentration gradient and driving the first negative electrode active layer to participate smoothly in the reaction. If the thickness h4 of the second negative electrode active layer is too small (less than 20μm), the electrolyte's wettability to the second negative electrode active layer will deteriorate, reducing the battery's fast-charging performance. If the thickness h4 of the second negative electrode active layer is too large (greater than 40μm), the diffusion path of lithium ions during charge and discharge will be prolonged, and the path for lithium ions to reach the first negative electrode active layer will be longer, potentially increasing electrode polarization and affecting the battery's kinetic performance at high rates.

[0063] Meanwhile, by controlling the ratio h3 / h4 between the thickness h3 of the first negative electrode active layer and the thickness h4 of the second negative electrode active layer to be between 0.8 and 1.8, the high capacity advantage of the silicon-based material in the first negative electrode active layer and the fast charging performance of the graphite material in the second negative electrode active layer can be well matched, which not only ensures the improvement of the battery's energy density, but also maintains excellent fast charging capability, thus achieving a balance and optimization of energy density and fast charging performance. If the h3 / h4 value is too small (less than 0.8), the first negative electrode active layer is relatively too thin, obstructing the lithium-ion transport channel, leading to increased DC internal resistance and decreased fast-charging performance. Alternatively, if the second negative electrode active layer is relatively too thick, the diffusion path of lithium ions during charging and discharging is prolonged, resulting in poorer kinetic performance of the battery at high rates. If the h3 / h4 value is too large (greater than 1.8), the first negative electrode active layer is relatively too thick, increasing side reactions between the silicon-based material and the electrolyte, resulting in poor high-temperature cycle performance of the battery, and also causing the SEI film to thicken, reducing the battery's fast-charging capability. Alternatively, if the second negative electrode active layer is relatively too thin, the wettability of the electrolyte to the second negative electrode active layer is poor, resulting in decreased kinetic performance of the battery at high rates.

[0064] Furthermore, the total thickness of the first and second negative electrode active layers needs to be controlled between 40μm and 80μm to balance various electrochemical performance characteristics of the battery. When the total thickness of the first and second negative electrode active layers is thinner, the compaction density of the negative electrode sheet increases accordingly, the porosity decreases, and the lithium-ion transport channels are blocked, resulting in increased DC internal resistance and decreased fast-charging performance. Conversely, when the total thickness of the first and second negative electrode active layers is thicker, ion transport is smoother, which can reduce the DC internal resistance of the battery and improve fast-charging performance.

[0065] It should be noted that the testing methods for the thicknesses h3 and h4 of the first and second negative electrode active layers include: observing the cross-section of the negative electrode sheet along the thickness direction using a scanning electron microscope and taking energy dispersive spectroscopy (EDS) images of the cross-section. The location of the silicon element at the farthest point from the negative electrode current collector is taken as the boundary between the first and second negative electrode active layers. The end containing silicon is the first negative electrode active layer, and the end without silicon is the second negative electrode active layer. Cross-sections at five different locations are observed, and the maximum thickness value obtained is taken as the thickness value of that layer. For example, the thickness of the first negative electrode active layer can be, for example, 20.0 μm, 21.4 μm, 22.9 μm, 24.3 μm, 25.7 μm, 27.1 μm, 28.6 μm, 30.0 μm, 31.4 μm, 32.9 μm, 34.3 μm, 35.7 μm, 37.1 μm, 38.6 μm, 40.0 μm, or a value within the range of any two of the above values; the thickness of the second negative electrode active layer can be, for example, 20.0 μm, 21.4 μm, 22.9 μm, 24.3 μm, 25.7 μm, 27.1 μm, 28.6 μm, 30.0 μm, 31.4 μm, 32.9 μm, 34.3 μm, 35.7 μm, 37.1 μm, 38.6 μm, 40.0 μm, etc. 7.1μm, 28.6μm, 30.0μm, 31.4μm, 32.9μm, 34.3μm, 35.7μm, 37.1μm, 38.6μm, 40.0μm, etc., or values ​​within the range of any two of the above values; the value of h3 / h4 can be, for example, 0.80, 0.87, 0.94, 1.01, 1.08, 1.15, 1.22, 1.29, 1.36, 1.43, 1.50, 1.57, 1.64, 1.71, 1.8, etc., or values ​​within the range of any two of the above values.

[0066] For example, the sulfur-containing additive includes at least one of 1,3-propanesulfonyl lactone, mannitol sulfate, vinyl sulfate, vinyl sulfite, propylene-1,3-sulfonyl lactone, and methanedisulfonate. To suppress the oxidative decomposition and gas generation of the electrolyte on the positive electrode surface and to improve the high-temperature performance of the battery, this application adds the above-mentioned sulfur-containing additive to the electrolyte to enhance interface stability. Among them, sulfonate additives such as 1,3-propanesulfonyl lactone, propylene-1,3-sulfonyl lactone, and methanedisulfonate methylene ester can form a stable solid electrolyte interface film rich in sulfur on the surface of the nickel-cobalt-manganese ternary cathode material, effectively inhibiting the oxidative decomposition of the electrolyte at high temperatures, reducing the dissolution of transition metal ions, thereby stabilizing the cathode structure and enhancing the thermal stability and cycle life of the battery; sulfate additives such as vinyl sulfate and vinylene sulfate can form an interface film with good flexibility and ion conductivity on the anode surface, which can adapt to the volume expansion of silicon-based materials during cycling, reduce the repeated rupture and reconstruction of the interface film, thereby reducing the consumption of active lithium and electrolyte; mannitol carbonate sulfate, as a novel sulfur-containing additive, can participate in film formation at both the positive and anode, forming an interface film with excellent thermal stability, further inhibiting side reactions during high-temperature cycling.

[0067] In one implementation, the sulfur-containing additive includes 1,3-propanesulfonyl lactone and mannitol carbonate sulfate, with the content of 1,3-propanesulfonyl lactone being a2 and the content of mannitol carbonate sulfate being a1, satisfying 0.02≤a1+a2≤5, 0.01≤a1≤3, and 0.01≤a2≤3. Both additives have sulfur ring structures. After the positive electrode undergoes oxidative decomposition, the sulfur ring structure can partially cover the catalytic active sites on the positive electrode surface, inhibiting the generation of gas from the oxidized electrolyte and exhibiting good film-forming ability. Through the synergistic effect of these two additives, 1,3-propanesulfonyl lactone is not excessively consumed in the early stage of cycling, and there is enough 1,3-propanesulfonyl lactone to continue to form a film at the broken positive electrode interface in the later stage of cycling, which inhibits gas generation in the later stage of cycling. At the same time, mannitol carbonate sulfate has a unique molecular structure with ethylene sulfate on both sides and ethylene carbonate in the middle, which allows the molecule to form a film on both the positive and negative electrode surfaces, thereby preventing the reduction and decomposition of the electrolyte and further improving the high-temperature cycle capacity retention rate of the battery.

[0068] In some embodiments, the first positive electrode active layer includes a first positive electrode active material, which is a lithium iron phosphate material and a nickel-cobalt-manganese ternary material.

[0069] In some other embodiments, the first positive electrode active layer may further include a third positive electrode active material, which includes at least one of lithium cobalt oxide, lithium-rich manganese-based material, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium vanadium phosphate.

[0070] In some embodiments, the second positive electrode active layer includes a second positive electrode active material, which is a nickel-cobalt-manganese ternary material.

[0071] In some other embodiments, the second positive electrode active layer may further include a fourth positive electrode active material, which includes at least one of lithium cobalt oxide, lithium-rich manganese-based material, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, nickel cobalt manganese aluminum material, and lithium vanadium phosphate material.

[0072] In some embodiments, the nickel-cobalt-manganese ternary material in the first positive electrode active layer has a single-crystal structure. By controlling its average particle size within the range of 2μm-5μm, the advantage of the good structural stability of the single-crystal material can be fully utilized, effectively suppressing particle cracking and interfacial side reactions during cycling. At the same time, a suitable particle size is conducive to ensuring the rapid diffusion of lithium ions, thereby improving the kinetic performance and cycling stability of the electrode. If the average particle size of the nickel-cobalt-manganese ternary material is too small (less than 2μm), the specific surface area of ​​the particles is too large, which easily increases the active sites for side reactions with the electrolyte; if the average particle size of the nickel-cobalt-manganese ternary material is too large (greater than 5μm), the diffusion path of lithium ions inside the particles is prolonged, resulting in increased polarization and decreased fast charging capability.

[0073] The term "single crystal" refers to a material composed of a single grain with a consistent internal lattice orientation and no grain boundaries. Compared to polycrystalline materials, it has higher mechanical strength and structural stability and can effectively suppress particle cracking caused by volume changes during cycling.

[0074] It should be noted that the method for testing the average particle size of the nickel-cobalt-manganese ternary material single crystal structure includes: observing the cross-section of the positive electrode sheet along the thickness direction using a scanning electron microscope, determining the position of the nickel-cobalt-manganese ternary material using energy dispersive spectroscopy (EDS), randomly selecting 50 nickel-cobalt-manganese ternary material particles from the cross-sectional image, measuring their particle size, and calculating the average value as the average particle size at that position; repeating the above test at 5 different positions, and using the arithmetic mean of the average particle sizes at the 5 positions as the final average particle size. For example, the average particle size of the nickel-cobalt-manganese ternary material single crystal structure can be 2.0 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.9 μm, 3.1 μm, 3.3 μm, 3.5 μm, 3.7 μm, 3.9 μm, 4.1 μm, 4.4 μm, 4.6 μm, 4.8 μm, 5.0 μm, or a value within any two of the above ranges.

[0075] In some embodiments, the lithium iron phosphate material comprises primary particles. By controlling the average particle size between 400 nm and 700 nm, the solid-phase diffusion path of lithium ions can be effectively shortened, enabling the lithium iron phosphate material to respond quickly under low charge conditions. This effectively compensates for the voltage deficiency caused by poor potential matching between nickel-cobalt-manganese ternary materials and silicon-based materials, ensuring that the battery maintains sufficient discharge voltage across the full charge range. If the average particle size of the primary particles of the lithium iron phosphate material is too small (less than 400 nm), the particle specific surface area is too large, which easily exacerbates the side reactions with the electrolyte, leading to a decrease in high-temperature cycle life, but it is beneficial to ensure the voltage compensation effect under low charge. If the average particle size of the primary particles of the lithium iron phosphate material is too large (greater than 700 nm), the diffusion path of lithium ions inside the particles is prolonged, the voltage compensation capability decreases, and the DC internal resistance of the battery increases.

[0076] The term "primary particle" refers to the original particles formed during the material preparation process. They are the smallest solid units that can exist independently without agglomeration.

[0077] It should be noted that the test method for the average particle size of primary lithium iron phosphate material includes: observing the cross-section of the positive electrode sheet along the thickness direction using a scanning electron microscope, and determining the location of the lithium iron phosphate material by energy dispersive spectroscopy (EDS) (based on the distribution of iron or phosphorus elements). Ten primary lithium iron phosphate particles are randomly selected from the cross-sectional image, their particle size is measured, and the average value is calculated as the average particle size at that location. The above test is repeated at five different locations, and the arithmetic mean of the average particle sizes at the five locations is taken as the final average particle size. For example, the average particle size of the primary particles of lithium iron phosphate material can be, for example, 400.0 nm, 421.4 nm, 442.9 nm, 464.3 nm, 485.7 nm, 507.1 nm, 528.6 nm, 550.0 nm, 571.4 nm, 592.9 nm, 614.3 nm, 635.7 nm, 657.1 nm, 678.6 nm, 700.0 nm, etc., or a value within the range of any two of the above values.

[0078] In some embodiments, the first negative electrode active layer includes a first negative electrode active material, which is a silicon-based material and a graphite material.

[0079] In some other embodiments, the first negative electrode active layer may further include a third negative electrode active material, which includes at least one of hard carbon, soft carbon, mesophase carbon microspheres, lithium titanate, and nano-carbon.

[0080] In some embodiments, the second negative electrode active layer includes a second negative electrode active material, which is a graphite material.

[0081] In some other embodiments, the second negative electrode active layer may further include a fourth negative electrode active material, which includes at least one of hard carbon, soft carbon, mesophase carbon microspheres, lithium titanate, and nano-carbon materials.

[0082] In some embodiments, by controlling the average particle size of the graphite material in the first negative electrode active layer to be within the range of 10μm-15μm, a larger particle size can be used to provide higher tap density and lithium storage space, thereby improving the energy density of the negative electrode active layer. Simultaneously, the large-particle-size graphite, placed together with the silicon-based material in the first negative electrode active layer, helps to buffer the impact of the volume expansion of the silicon-based material on the electrode structure. If the average particle size of the graphite material in the first negative electrode active layer is too large (greater than 15μm), the diffusion path of lithium ions within the graphite material is prolonged, leading to a decrease in lithium intercalation kinetics and affecting the fast-charging performance of the battery. If the average particle size of the graphite material in the first negative electrode active layer is too small (less than 10μm), the material packing density and tap density decrease, making it difficult to provide sufficient lithium storage space, thus limiting the improvement in energy density. Furthermore, the increased specific surface area of ​​the small-particle-size material can exacerbate side reactions with the electrolyte.

[0083] In some implementations, by controlling the average particle size of the graphite material in the second negative electrode active layer to be between 5 μm and 10 μm, the smaller material size can shorten the diffusion path of lithium ions in the solid phase. This allows the second negative electrode active layer to respond quickly during charging and discharging, establishing a lithium ion concentration gradient first and driving the first negative electrode active layer to participate smoothly in the reaction, thereby ensuring the battery's fast-charging kinetics performance under high-rate conditions. If the average particle size of the graphite material in the second negative electrode active layer is too large (greater than 10 μm), the lithium ion diffusion path is prolonged, making rapid insertion and extraction difficult and reducing fast-charging performance. If the average particle size of the graphite material in the second negative electrode active layer is too small (less than 5 μm), the material's specific surface area is too large, easily exacerbating side reactions with the electrolyte. At the same time, the porosity between materials increases, and the compaction density decreases, affecting the overall energy density and cycle stability of the negative electrode active layer.

[0084] It should be noted that the test method for the average particle size of graphite material in the first and second negative electrode active layers includes: observing the cross-section of the negative electrode sheet along the thickness direction using a scanning electron microscope, and determining the position of the graphite material using energy dispersive spectroscopy (EDS) (the location is determined by the distribution of carbon elements, and the interference of silicon-based materials in the first negative electrode active layer needs to be excluded). 50 graphite material particles are randomly selected from the cross-sectional image, their particle size is measured and the average value is calculated as the average particle size at that position. The above test is repeated at 5 different positions, and the arithmetic mean of the average particle size at the 5 positions is taken as the final average particle size. For example, the average particle size of the graphite material in the first negative electrode active layer may be, for example, 10.0 μm, 10.4 μm, 10.7 μm, 11.1 μm, 11.4 μm, 11.8 μm, 12.1 μm, 12.5 μm, 12.9 μm, 13.2 μm, 13.6 μm, 13.9 μm, 14.3 μm, 14.6 μm, 15.0 μm, or any two of the above values. The values ​​are within the range of; for example, the average particle size of the graphite material in the second negative electrode active layer may be, for example, 5.0 μm, 5.4 μm, 5.7 μm, 6.1 μm, 6.4 μm, 6.8 μm, 7.1 μm, 7.5 μm, 7.9 μm, 8.2 μm, 8.6 μm, 8.9 μm, 9.3 μm, 9.6 μm, 10.0 μm, etc., or values ​​within the range of any two of the above values.

[0085] In some implementations, by controlling the average particle size of the silicon-based material within the range of 5μm-10μm, the solid-phase diffusion path of lithium ions in the silicon-based material can be shortened, diffusion resistance can be reduced to improve the rate performance of the electrode, and stress concentration caused by volume expansion can be alleviated to some extent, reducing damage to the electrode structure, thereby improving cycle stability. If the average particle size of the silicon-based material is too small (less than 5μm), the specific surface area of ​​the silicon-based material increases significantly, which easily aggravates the side reactions with the electrolyte, leading to increased consumption of active lithium. At the same time, the dispersibility deteriorates, making slurry preparation difficult and affecting the processing performance of the electrode. If the average particle size of the silicon-based material is too large (greater than 10μm), the diffusion path of lithium ions inside the silicon-based material is lengthened, the lithium intercalation kinetics decrease, the rate performance decreases, and the stress concentration caused by the volume expansion of large-sized materials during cycling is more likely to cause silicon-based material cracking, accelerating electrode structure failure and deterioration of interface stability.

[0086] It should be noted that the method for testing the average particle size of silicon-based materials includes: observing the cross-section of the negative electrode sheet along its thickness direction using a scanning electron microscope, determining the location of the silicon-based material using energy dispersive spectroscopy (EDS) (based on the distribution of silicon elements), randomly selecting 10 silicon-based material particles from the cross-sectional image, measuring their particle size, and calculating the average value as the average particle size at that location; repeating the above test at 5 different locations, and using the arithmetic mean of the average particle sizes at the 5 locations as the final average particle size. For example, the average particle size of the silicon-based material can be 5.00 μm, 5.36 μm, 5.71 μm, 6.07 μm, 6.43 μm, 6.79 μm, 7.14 μm, 7.50 μm, 7.86 μm, 8.21 μm, 8.57 μm, 8.93 μm, 9.29 μm, 9.64 μm, 10.00 μm, or a value within any two of the above ranges.

[0087] In some embodiments, the graphite material in the first negative electrode active layer has a carbon coating layer. By controlling the thickness of the carbon coating layer to be 1nm-20nm, a continuous and dense conductive network can be formed on the graphite surface, significantly improving the electronic contact between graphite materials. At the same time, a moderate coating thickness helps to shorten the lithium-ion transport path, thereby further improving the fast-charging kinetics of the electrode. If the thickness of the carbon coating layer is too thin (less than 1nm), it is difficult to form a continuous and complete coating layer on the graphite surface, resulting in limited improvement in electronic conductivity and no significant improvement in fast-charging performance. If the thickness of the carbon coating layer is too thick (greater than 20nm), it will increase the diffusion resistance of lithium ions in the coating layer, leading to a decrease in lithium-ion insertion and extraction kinetics. At the same time, an excessively thick coating layer will reduce the proportion of active material, adversely affecting the energy density of the battery.

[0088] It should be noted that the thickness of the carbon coating layer can be obtained by transmission electron microscopy. For example, the thickness of the carbon coating layer can be 1.0 nm, 2.4 nm, 3.7 nm, 5.1 nm, 6.4 nm, 7.8 nm, 9.1 nm, 10.5 nm, 11.9 nm, 13.2 nm, 14.6 nm, 15.9 nm, 17.3 nm, 18.6 nm, 20.0 nm, or a value within any two of the above ranges.

[0089] In some embodiments, the silicon-based material includes a silicon-carbon material, which comprises porous carbon and silicon particles located within the pores of the porous carbon. By confining the silicon particles within the pore structure of the porous carbon, the mechanical strength and pore space of the porous carbon matrix can effectively buffer the volume expansion of the silicon particles during lithium intercalation, preventing the silicon particles from cracking or detaching from the electrode structure due to drastic volume changes. Simultaneously, the porous carbon matrix possesses excellent electronic conductivity and ion transport channels, which can compensate for the low intrinsic conductivity of silicon materials and shorten the lithium-ion transport path, thereby significantly improving battery energy density while effectively enhancing the cycle stability and rate performance of the electrode.

[0090] Furthermore, in some embodiments, the porous carbon is derived from the pyrolysis products of resin materials such as phenolic resin, furan resin, and melamine resin, and therefore can be called resin-based porous carbon. Compared with coal-based, biomass-based, and pitch-based porous carbon, resin-based porous carbon has strong structural controllability, is easy to modify with heteroatoms in situ, and has good compatibility with electrolytes, making it very suitable for electrochemical applications.

[0091] In some embodiments, the surface of the second negative electrode active layer away from the negative electrode current collector has a recess, satisfying at least one of the following conditions: (1) The opening width of the recess is 0.05mm-0.2mm; (2) The distance between adjacent recesses is 0.5mm-5mm; (3) The dimension of the recess along the thickness direction of the negative electrode sheet is T1, and the thickness of the negative electrode active layer on the surface of the negative electrode current collector is T2, 0.2≤T1 / T2≤0.5; (4) The total volume of the recess is T3, and the volume of the negative electrode active layer on the surface of the negative electrode current collector is T4, 0.005≤T3 / T4≤0.03; (5) The negative electrode sheet also includes a negative electrode tab, which extends from the negative electrode current collector and the extension direction of the negative electrode tab is perpendicular to the thickness direction of the negative electrode sheet; the recess is a groove, and the length direction of the groove is parallel to the extension direction of the negative electrode tab.

[0092] This application creates a recess on the surface of the second negative electrode active layer away from the negative electrode current collector, and controls the opening width of the recess between 0.05mm and 0.2mm. This allows the recess to act as a local "reservoir" during battery electrolyte injection and formation, rapidly wetting the negative electrode sheet and ensuring the battery is rich in electrolyte during formation, thereby forming a stable and uniform solid electrolyte interface film. Simultaneously, the recess provides a buffer space for the volume expansion of the silicon-based material during cycling, reducing compression of the internal space of the particles and lowering the risk of electrolyte being squeezed out. If the opening width is too small (less than 0.05mm), the recess volume is insufficient, limiting its electrolyte storage and buffering capacity, making it difficult to function effectively. If the opening width is too large (greater than 0.2mm), it causes excessive damage to the second negative electrode active layer, potentially sacrificing energy density and affecting the integrity of the electrode structure.

[0093] By controlling the distance between adjacent recesses to be between 0.5mm and 5mm, a uniformly distributed conductive network can be formed on the surface of the second negative electrode active layer, ensuring that the electrolyte can quickly wet the entire negative electrode active layer and maintain uniform ion transport during cycling. If the distance between adjacent recesses is too small (less than 0.5mm), the recesses are too dense, resulting in significant cumulative damage to the second negative electrode active layer and a noticeable loss of energy density. If the distance between adjacent recesses is too large (greater than 5mm), the distance between the recesses is too far, resulting in insufficient lateral replenishment of the electrolyte and difficulty in providing electrolyte to distant areas in a timely manner during volume expansion.

[0094] By controlling the ratio T1 / T2 between the dimension T1 of the recess along the thickness direction of the negative electrode sheet and the thickness T2 of the negative electrode active layer on the surface of the negative electrode current collector, which is between 0.2 and 0.5, the recess can have sufficient depth to store electrolyte and provide expansion buffer space. If T1 / T2 is less than 0.2, the recess depth is insufficient, the electrolyte storage and buffering effect is limited, and it is difficult to effectively improve electrolyte distribution and alleviate volume expansion. If T1 / T2 is greater than 0.5, the recess is too deep, which may cut off the conductive network on the surface of the current collector, causing a sharp increase in the local resistance of the electrode sheet, and lithium metal deposition may occur at the edge of the deep processing area due to uneven current distribution, increasing safety risks.

[0095] By controlling the ratio T3 / T4 between the total volume T3 of the recess and the volume T4 of the negative electrode active layer on the surface of the negative electrode current collector to be between 0.005 and 0.03, the recess can provide sufficient liquid storage space and expansion buffer space while keeping its impact on energy density within an acceptable range. If T3 / T4 is less than 0.005, the total volume of the recess is too small, resulting in insufficient liquid storage and buffering capacity and a weak improvement effect; if T3 / T4 is greater than 0.03, excessive loss of negative electrode active material will lead to a significant decrease in energy density and may also affect the integrity of the electrode structure.

[0096] In addition, the negative electrode sheet also includes a negative electrode tab, which extends from the negative electrode current collector. By controlling the extension direction of the negative electrode tab to be perpendicular to the thickness direction of the negative electrode sheet, the electrolyte can flow back from the bottom of the cell to the negative electrode sheet along the concave direction during the injection and circulation process, thereby achieving rapid replenishment and uniform distribution of the electrolyte. The concave part is a groove. By controlling the length direction of the groove to be parallel to the extension direction of the negative electrode tab, the electrolyte can be rapidly transported along the groove from the positive electrode and the separator to the first negative electrode active layer of the negative electrode sheet during charge and discharge cycles, thereby avoiding concentration polarization.

[0097] The above settings can significantly improve the uniformity of electrolyte wetting during long-term cycling, ensuring the continuity of lithium-ion transport paths. Even under conditions of high-rate charging and discharging or severe expansion of silicon-based materials, the electrolyte in the groove can serve as a strategic reserve, replenishing areas that are temporarily dried out due to expansion in a timely manner, effectively reducing battery internal resistance and polarization, thereby improving cycle life and fast-charging performance.

[0098] It should be noted that the opening width of the recess and the distance between adjacent recesses can be obtained by micrometer testing.

[0099] The test methods for the dimensions T1 of the recess along the thickness direction of the negative electrode sheet and the thickness T2 of the negative electrode active layer on the surface of the negative electrode current collector include: observing the cross-section of the negative electrode sheet along the thickness direction using a scanning electron microscope, randomly selecting 10 recesses at different locations, measuring T1 and T2 respectively, and taking the arithmetic mean of the 10 measurements as the final test result.

[0100] The testing methods for the total volume T3 of the recess and the volume T4 of the negative electrode active layer on the surface of the negative electrode current collector include: if the battery cell has a wound structure, a 1m long negative electrode sheet is randomly selected for testing; if the battery cell has a stacked structure, a single negative electrode sheet is randomly selected for testing. The total volume T3 of the recess is calculated using the formula T3 = recess length × recess depth (T1) × recess width × number of recesses. The recess depth and width are obtained using a scanning electron microscope, and the recess length is measured using a micrometer. The volume T4 of the negative electrode active layer on the surface of the negative electrode current collector is calculated using the formula T4 = negative electrode sheet length × negative electrode sheet width × negative electrode active layer thickness. Samples at five different locations are tested, and the arithmetic mean of the measured values ​​is used as the final T3 and T4 test results.

[0101] For example, the opening width of the recess can be 0.050mm, 0.061mm, 0.071mm, 0.082mm, 0.093mm, 0.104mm, 0.114mm, 0.125mm, 0.136mm, 0.146mm, 0.157mm, 0.168mm, 0.179mm, 0.189mm, 0.200mm, or a value within the range of any two of the above values; the distance between adjacent recesses can be 0.500mm, 0.821mm, 1.143mm, 1.464mm, 1.786mm, 2.107mm, 2.429mm, 2.750mm, 3.071mm, 3.393mm, 3.714mm, 4.036mm, 4.357mm, 4.679mm, 5. The value of T1 / T2 can be, for example, 0.200, 0.221, 0.243, 0.264, 0.286, 0.307, 0.329, 0.350, 0.371, 0.393, 0.414, 0.436, 0.457, 0.479, 0.500, or a value within the range of any two of the above values. The values ​​within the range; the values ​​of T3 / T4 can be, for example, 0.0050, 0.0068, 0.0086, 0.0104, 0.0121, 0.0139, 0.0157, 0.0175, 0.0193, 0.0211, 0.0229, 0.0246, 0.0264, 0.0282, 0.0300, etc., or values ​​within the range of any two of the above values.

[0102] In some embodiments, the electrolyte further includes a carbonate solvent and a lithium salt. Based on the mass of the electrolyte, by controlling the lithium salt content within the range of 8%-18%, sufficient lithium-ion concentration in the electrolyte can be ensured to support the rapid response of the battery under high-rate charge-discharge conditions, while maintaining suitable viscosity to ensure the fluidity and wettability of the electrolyte, thereby optimizing the battery's rate performance and cycle stability. If the lithium salt content is too low (less than 8%), the lithium-ion concentration in the electrolyte is insufficient, making it difficult to meet the rapid lithium-ion demand during high-rate charge-discharge, leading to increased battery polarization, limited capacity utilization, and decreased fast-charging performance. If the lithium salt content is too high (greater than 18%), the electrolyte viscosity increases significantly, the lithium-ion migration rate decreases, and high-concentration lithium salt may exacerbate interfacial side reactions, affecting the battery's fast-charging performance and high-temperature cycle stability.

[0103] It should be noted that the mass content of lithium salts can be obtained by ion chromatography. For example, the mass content of lithium salts can be 8.0%, 8.7%, 9.4%, 10.1%, 10.9%, 11.6%, 12.3%, 13.0%, 13.7%, 14.4%, 15.1%, 15.9%, 16%, 17.3%, 18.0%, or values ​​within any two of the above ranges.

[0104] For example, the carbonate solvent includes at least one of dimethyl carbonate, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, and propylene carbonate. Dimethyl carbonate, as a low-viscosity solvent, can significantly reduce the overall viscosity of the electrolyte, enhance the capillary effect and fluidity of the electrolyte, allowing the electrolyte to penetrate and diffuse more quickly along the concave structure on the surface of the negative electrode. Simultaneously, the introduction of dimethyl carbonate makes the solvation structure of lithium ions more compact and smaller in size, lowering the desolvation energy barrier, thereby accelerating the migration of lithium ions in the separator, electrode pores, and solid electrolyte interface film, and improving lithium ion transport kinetics. The aforementioned low-viscosity solvent and the concave structure on the surface of the negative electrode form a synergistic effect. The concave structure provides a macroscopic flow path, while the low-viscosity electrolyte ensures high-efficiency transport along this path, jointly guaranteeing efficient lithium ion transport during fast charging or cycling, effectively maintaining sufficient wetting of the electrode interface, and significantly improving the rate performance and cycle life of the battery.

[0105] In some embodiments, the carbonate solvent includes dimethyl carbonate (DMC) and ethylene carbonate (EC). Based on the mass of the electrolyte, the mass content (C1%) of dimethyl carbonate is controlled between 5% and 60%. Its low viscosity can significantly reduce the overall viscosity of the electrolyte, enhance the capillary effect and fluidity of the electrolyte, and enable it to penetrate and diffuse more quickly along the concave structure of the negative electrode surface. At the same time, it makes the solvation structure of lithium ions more compact and smaller in size, reduces the desolvation energy barrier, and thus improves the overall lithium ion transport kinetics. If the mass content of dimethyl carbonate (C1%) is too small (less than 5%), the proportion of low-viscosity solvent will be insufficient, the overall viscosity of the electrolyte will be too high, the fluidity will be poor, and it will be difficult to effectively exert the synergistic guiding effect with the concave structure. If the mass content of dimethyl carbonate (C1%) is too large (greater than 60%), it will disrupt the balance with high dielectric constant solvents (such as EC), making the solvation structure of lithium ions too loose, increasing the desolvation energy barrier, and hindering the migration of lithium ions at the interface, thus adversely affecting the fast charging performance.

[0106] Controlling the ethylene carbonate (C2%) content between 5% and 40% allows its high dielectric constant to promote lithium salt dissociation, improve the ionic conductivity of the electrolyte, and participate in the formation of a stable solid electrolyte interface film on the negative electrode surface, enhancing the stability of the electrode interface and thus benefiting the battery's fast-charging performance and cycle life. If the ethylene carbonate (C2%) content is too low (less than 5%), it is difficult to form a dense and stable interface film on the negative electrode surface. The electrolyte is prone to continuous decomposition during cycling, leading to increased consumption of active lithium, decreased interface stability, and deterioration of high-temperature cycling performance. Furthermore, it offers limited improvement to the electrolyte's ionic conductivity, affecting the battery's fast-charging performance. Conversely, if the ethylene carbonate (C2%) content is too high (greater than 40%), the electrolyte viscosity is too high, the lithium-ion migration rate decreases, and the high ethylene carbonate content easily exacerbates side reactions at high temperatures, resulting in an excessively thick or unstable film, which negatively impacts high-temperature cycle life.

[0107] Meanwhile, controlling the ratio C1 / C2 between the mass content of dimethyl carbonate (C1%) and the mass content of ethylene carbonate (C2%) to be between 0.2 and 10 can further balance the fluidity and film-forming properties of the electrolyte, so that the electrolyte has both sufficient low-viscosity components to ensure rapid transport capability and appropriate amounts of high dielectric constant components to maintain good lithium salt dissociation and interfacial film-forming characteristics, thereby achieving synergistic optimization with the negative electrode recess structure. If the C1 / C2 ratio is too small (less than 0.2), the proportion of low-viscosity dimethyl carbonate in the electrolyte is relatively insufficient, resulting in excessively high overall electrolyte viscosity and poor fluidity, which limits the improvement of the battery's fast-charging performance. Alternatively, the content of ethylene carbonate is relatively high, leading to excessively high electrolyte viscosity, decreased lithium-ion migration rate, and the high content of ethylene carbonate can easily exacerbate side reactions at high temperatures, which in turn has an adverse effect on high-temperature cycle life. If the C1 / C2 ratio is too large (greater than 10), the content of high-dielectric-constant ethylene carbonate is relatively insufficient, making it difficult to form a dense and stable interface film on the negative electrode surface. The electrolyte is prone to continuous decomposition during cycling, leading to increased consumption of active lithium, decreased interface stability, and thus deterioration of high-temperature cycle performance. Alternatively, the proportion of low-viscosity dimethyl carbonate is relatively large, which improves the fluidity of the electrolyte and reduces DC internal resistance.

[0108] It should be noted that the mass content of dimethyl carbonate and ethylene carbonate can be obtained by gas chromatography-mass spectrometry (GC-MS). For example, the mass content of dimethyl carbonate can be 5.0%, 8.9%, 12.9%, 16.8%, 20.7%, 24.6%, 28.6%, 32.5%, 36.4%, 40.4%, 44.3%, 48.2%, 52.1%, 56.1%, 60.0%, or values ​​within any two of the above ranges; the mass content of ethylene carbonate can be 5.0%, 7.5%, 10.0%, 12.5%, 15.0%, 17.5%, 20.0%, 22.5%, etc. %, 25.0%, 27.5%, 30.0%, 32.5%, 35.0%, 37.5%, 40.0%, etc., or values ​​within the range of any two of the above values; the value of C1 / C2 can be, for example, 0.200, 0.264, 0.349, 0.462, 0.611, 0.808, 1.068, 1.412, 1.867, 2.468, 3.263, 4.313, 5.702, 7.537, 10.000, etc., or values ​​within the range of any two of the above values.

[0109] In some embodiments, the separator includes a base film, a ceramic layer, and an adhesive layer. The ceramic layer is disposed on at least one surface along the thickness direction of the base film, and the adhesive layer is disposed on the surface of the ceramic layer away from the base film, and / or, the adhesive layer is disposed on the surface of the base film. Specifically, the ceramic layer may be disposed on one surface of the base film, the adhesive layer on the surface of the ceramic layer away from the base film, and the adhesive layer may also be disposed on the other surface of the base film, thereby forming a single-sided ceramic double-sided adhesive structure; or, the ceramic layer may be disposed on one surface of the base film, and the adhesive layer may be disposed only on the surface of the ceramic layer away from the base film, forming a single-sided ceramic single-sided adhesive structure; or, the ceramic layer may be disposed on both surfaces of the base film, and the adhesive layers may be disposed on the surfaces of the ceramic layers on both sides away from the base film, respectively, forming a double-sided ceramic double-sided adhesive structure.

[0110] With the above settings, the distribution of the ceramic layer and the adhesive layer can be flexibly adjusted according to actual needs. When the ceramic layer is disposed on at least one side of the base film thickness direction, the ceramic layer faces the positive electrode sheet to improve the heat resistance of the separator and prevent positive electrode burrs from piercing the separator, thereby enhancing the thermal stability and safety of the separator. The adhesive layer is used to enhance the adhesion between the separator and the electrode sheet, ensuring the initial stability of the electrode interface. At the same time, by controlling the coverage of the adhesive layer on one side between 5% and 95%, sufficient adhesion between the separator and the electrode sheet can be ensured while reserving sufficient channels for electrolyte transport, thus forming an efficient electrolyte transport network. If the coverage of the adhesive layer on one side is too low (below 5%), insufficient adhesion will easily lead to interface peeling; if the coverage of the adhesive layer on one side is too high (above 95%), it will block the transport channel, affecting the fast charging performance and cycle life of the battery.

[0111] It should be noted that the testing method for the coverage rate of the adhesive layer on one side includes: measuring the local air permeability of the diaphragm using an air permeability tester, and calculating the adhesive layer coverage rate based on the difference in air permeability between the areas with and without the adhesive layer; or, observing the diaphragm surface using a scanning electron microscope, randomly selecting 5 different fields of view, calculating the percentage of the area of ​​the adhesive layer area to the total field of view area, and using the arithmetic mean of the 5 measurements as the final adhesive layer coverage rate. For example, the coverage rate of the adhesive layer on one side can be 5.0%, 11.4%, 17.9%, 24.3%, 30.7%, 37.1%, 43.6%, 50.0%, 56.4%, 62.9%, 69.3%, 75.7%, 82.1%, 88.6%, 95.0%, or a value within any two of the above ranges.

[0112] In some embodiments, when the ceramic layer is disposed on at least one surface in the thickness direction of the base film, controlling the thickness of the ceramic layer to be 0.5 μm-2 μm can effectively improve the heat resistance of the separator and prevent positive electrode burrs from puncturing the separator. If the ceramic layer thickness is too thin (less than 0.5 μm), the heat resistance and puncture resistance are insufficient, making it difficult to effectively protect the separator; if the ceramic layer thickness is too thick (greater than 2 μm), it will increase the ion transport impedance, affecting the rate performance and cycle life of the battery.

[0113] It should be noted that the method for testing the thickness of the ceramic layer includes: observing the cross-section of the diaphragm along the thickness direction using a scanning electron microscope, randomly selecting five different locations, and measuring the thickness of the ceramic layer at each location. The arithmetic mean of the five measurements is taken as the final ceramic layer thickness. For example, the thickness of the ceramic layer can be 0.50 μm, 0.61 μm, 0.71 μm, 0.82 μm, 0.93 μm, 1.04 μm, 1.14 μm, 1.25 μm, 1.36 μm, 1.46 μm, 1.57 μm, 1.68 μm, 1.79 μm, 1.89 μm, 2.00 μm, or a value within any two of the above ranges.

[0114] In some implementations, by controlling the porosity of the separator between 30% and 60%, ample storage and migration channels can be provided for the electrolyte, synergistically forming an efficient ion transport network. If the porosity is too low (below 30%), ion transport is hindered, and rate performance decreases; if the porosity is too high (above 60%), the mechanical strength of the separator decreases, affecting battery safety.

[0115] It should be noted that the porosity of the membrane can be obtained by mercury porosimetry. For example, the porosity of the membrane can be 30.0%, 32.1%, 34.3%, 36.4%, 38.6%, 40.7%, 42.9%, 45.0%, 47.1%, 49.3%, 51.4%, 53.6%, 55.7%, 57.9%, 60.0%, or a value within any two of the above ranges.

[0116] The present application will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Reagents or instruments used, unless otherwise specified, are all commercially available conventional reagent products.

[0117] Example 1 This embodiment provides a method for preparing a lithium-ion secondary battery, including the following steps: Step 1: Preparation of the positive electrode sheet LiNi 0.68 Co 0.08 Mn 0.23 Al 0.01O2 single-crystal powder, lithium iron phosphate, polyvinylidene fluoride, and carbon black were added to a vacuum mixer in a mass ratio of 91:5:2:2. N-methylpyrrolidone was then added, and the mixture was thoroughly mixed under vacuum to form a uniform, highly fluid cathode slurry with a solid content of 55 wt%. This cathode slurry was then uniformly coated onto a 12 μm thick aluminum foil, with a single-sided coating density of 7.0 mg / cm³. 2 This forms the first positive electrode active layer. Using the same method, LiNi... 0.68 Co 0.08 Mn 0.23 Al 0.01 O2 single crystal powder, polyvinylidene fluoride, and carbon black were mixed in a mass ratio of 96:2:2 to prepare a second positive electrode active layer slurry, which was then coated onto the surface of the first positive electrode active layer, with the total coating surface density controlled at 14 mg / cm³. 2 After drying and rolling, the total thickness of the positive electrode active material layer is 40.1 μm.

[0118] Step 2: Preparation of the negative electrode Graphite, silicon carbide, styrene-butadiene rubber, sodium carboxymethyl cellulose, acetylene black, and carbon nanotubes from the first negative electrode active material were added to a vacuum mixer in a mass ratio of 87:10:1:1:0.8:0.2. Deionized water was added, and the mixture was thoroughly mixed under vacuum to form a homogeneous, highly fluid negative electrode slurry with a solid content of 45 wt%. This negative electrode slurry was then uniformly coated onto a 6 μm thick carbon-coated copper foil, with a single-sided coating density of 7.5 mg / cm³. 2 This forms the first negative electrode active layer. Using the same method, graphite, styrene-butadiene rubber, sodium carboxymethyl cellulose, and acetylene black from the first negative electrode active material are mixed in a mass ratio of 97:1:1:1 to prepare a second negative electrode active layer slurry. This slurry is then uniformly coated onto the surface of the first negative electrode active layer, with a single-sided coating density of 7.5 mg / cm³. 2 After drying and rolling, the total thickness of the negative electrode active material layer is 52μm, thus obtaining the negative electrode sheet.

[0119] Step 3: Preparation of electrolyte In an argon-filled glove box with a water content below 0.1 ppm and an oxygen content below 0.1 ppm, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were mixed uniformly at a mass ratio of 30:35:35 to obtain a mixed solvent. Thoroughly dried lithium hexafluorophosphate was added to the mixed solvent, controlling the mass fraction of lithium hexafluorophosphate to be 12.5%, and stirred until completely dissolved. Subsequently, fluoroethylene carbonate, lithium difluorophosphate, lithium difluorosulfonyl imide, mannitol carbonate sulfate, and 1,3-propane sulpholol were added to the electrolyte at amounts of 0.5%, 0.5%, 1%, 1.5%, and 1.5%, respectively. After stirring until homogeneous and passing physical property testing, the electrolyte was obtained.

[0120] Step 4: Preparation of lithium-ion secondary batteries The positive electrode sheet obtained in the first step, the negative electrode sheet obtained in the second step, and the polypropylene separator are assembled into a bare cell through a stacking process. After the bare cell is welded with tabs, it is placed in the battery casing for encapsulation. Then, the electrolyte prepared in the third step is injected into the dried cell. After processes such as standing, aging, formation, secondary encapsulation, aging, and sorting, a lithium-ion secondary battery is obtained.

[0121] The preparation methods of Examples 2-46 and Comparative Examples 1-8 are basically the same as those of Example 1. The differences are shown in Tables 1-3.

[0122] Table 1

[0123] Table 2

[0124] Table 3

[0125] Test case 1. Fast charging cycle performance test At 25℃, the battery was charged at 0.33C to the upper limit voltage of 4.4V, then discharged at 0.33C to the lower limit voltage of 2.5V, repeated twice. The discharge capacity of the second discharge was taken as the initial discharge capacity C0. Subsequently, the battery was subjected to stepped charging: first, constant current charging at 1.0C to 10% SOC, then constant current charging at 5.0C to 60% SOC, then constant current charging at 4.5C to 85% SOC, then constant current charging at 4.0C to 90% SOC, and finally constant current and constant voltage charging at 1.0C to the upper limit voltage of 4.4V, with a cutoff current of 0.05C. After charging, the battery was allowed to stand for 30 minutes, then discharged at 1.0C constant current to the lower limit voltage. This cycle was repeated 1200 times, and the discharge capacity C(1200T) of the 1200th cycle was recorded. The discharge capacity retention rate was calculated as: Discharge capacity retention rate % = C(1200T) / C0 × 100%.

[0126] 2. High-temperature cycling performance test At 45℃, the battery is charged at 1C to the upper limit voltage of 4.4V, and then discharged at 1C to the lower limit voltage of 2.5V. This cycle is repeated twice, and the discharge capacity of the second discharge is selected as the initial discharge capacity C0. At 45℃, the battery is then subjected to step charging: charged at 1.0C constant current and constant voltage to 97% SOC, allowed to stand for 30 minutes, and then discharged at 1C to the lower limit voltage. This cycle is repeated 1000T, and the discharge capacity C(1000T) is read. The discharge capacity retention rate is calculated as C(1000T) / C0 × 100%.

[0127] 3. Impedance test At 25℃, the battery was charged at 0.33C to the upper limit voltage of 4.4V, and then discharged at 0.33C to the lower limit voltage of 2.5V. This cycle was repeated twice, and the discharge capacity of the second discharge was selected as the initial discharge capacity C0. The battery was charged at a constant current of 1.0C0 to 0.3% SOC, allowed to stand for 60 minutes, and the final voltage V1 was recorded. Then, it was discharged at a constant current of 1C0 for 10 seconds, with the voltage collected every 0.1 seconds, and the final voltage V2 of the constant current discharge was recorded. The DC internal resistance (DCR) was calculated as (V1-V2) / I×1000 mΩ. Here, I refers to the discharge current, specifically the current value corresponding to 1C0 (in A).

[0128] The above tests were conducted under conditions ensuring a battery mass energy density of 250Wh / kg-270Wh / kg. The test results are shown in Table 4.

[0129] Table 4

[0130] As can be seen from Tables 1-4, this application achieves a balance between the potentials of the positive and negative electrodes through the layered design of the positive / negative electrode active layers, the synergistic regulation of silicon / iron element content, and the optimized matching of the electrolyte. This stabilizes the voltage platform, increases the battery's operating voltage range, improves energy output efficiency, and enhances the stability of the positive electrode interface. As a result, the battery achieves a synergistic improvement in voltage platform stability, fast charging capability, and cycle life.

[0131] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A lithium-ion secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises a positive current collector and a positive active layer disposed on at least one surface of the positive current collector in the thickness direction, the positive active layer comprising a positive active material, the positive active material comprising lithium iron phosphate material and nickel-cobalt-manganese ternary material; the negative electrode comprises a negative current collector and a negative active layer disposed on at least one surface of the negative current collector in the thickness direction, the negative active layer comprising a negative active material, the negative active material comprising silicon-based material and graphite material; characterized in that: The positive electrode active layer includes a first positive electrode active layer and a second positive electrode active layer stacked together. The first positive electrode active layer is disposed between the positive electrode current collector and the second positive electrode active layer. The lithium iron phosphate material is located in the first positive electrode active layer, and the nickel-cobalt-manganese ternary material is located at least in the second positive electrode active layer. Based on the mass of the positive electrode active layer, the mass content of iron element is m%, satisfying 0.2≤m≤4. The negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer stacked together. The first negative electrode active layer is disposed between the negative electrode current collector and the second negative electrode active layer. The silicon-based material is located in the first negative electrode active layer, and the graphite material is located at least in the second negative electrode active layer. Based on the mass of the negative electrode active layer, the mass content of silicon element is w%, satisfying: 0.5≤w≤15 and 0.5≤w / m≤20. The electrolyte includes a sulfur-containing additive, and the mass content of the sulfur-containing additive is a%, based on the mass of the electrolyte, satisfying 0.01≤a≤5.

2. The lithium-ion secondary battery according to claim 1, characterized in that, The thickness of the first positive electrode active layer is h1 μm, and the thickness of the second positive electrode active layer is h2 μm, satisfying: 15≤h1≤35, 15≤h2≤35, 0.8≤h1 / h2≤2.2; And / or, the thickness of the first negative electrode active layer is h3 μm, and the thickness of the second negative electrode active layer is h4 μm, satisfying: 20≤h3≤40, 20≤h4≤40, 0.8≤h3 / h4≤1.8; And / or, the sulfur-containing additive includes at least one of 1,3-propanesulfonyl lactone, mannitol sulfate, vinyl sulfate, vinyl sulfite, propylene-1,3-sulfonyl lactone, and methanedisulfonate; Preferably, the sulfur-containing additive includes 1,3-propane sulfonyl lactone and mannitol carbonate sulfate, and based on the mass of the electrolyte, the content of 1,3-propane sulfonyl lactone is a2%, and the content of mannitol carbonate sulfate is a1%, satisfying 0.02≤a1+a2≤5, 0.01≤a1≤3, and 0.01≤a2≤3.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized in that, The first positive electrode active layer includes a first positive electrode active material, which is a lithium iron phosphate material and a nickel-cobalt-manganese ternary material; And / or, the second positive electrode active layer includes a second positive electrode active material, wherein the second positive electrode active material is a nickel-cobalt-manganese ternary material; And / or, the nickel-cobalt-manganese ternary material in the first positive electrode active layer has a single crystal structure with an average particle size of 2μm-5μm; And / or, the lithium iron phosphate material comprises primary particles with an average particle size of 400nm-700nm.

4. The lithium-ion secondary battery according to claim 1, characterized in that, The first negative electrode active layer includes a first negative electrode active material, which is a silicon-based material and a graphite material; And / or, the second negative electrode active layer includes a second negative electrode active material, which is a graphite material; And / or, the average particle size of the graphite material in the first negative electrode active layer is 10μm-15μm; And / or, the average particle size of the graphite material in the second negative electrode active layer is 5μm-10μm; And / or, the average particle size of the silicon-based material is 5μm-10μm.

5. The lithium-ion secondary battery according to claim 1 or 4, characterized in that, The graphite material in the first negative electrode active layer has a carbon coating layer with a thickness of 1 nm-20 nm. And / or, the silicon-based material includes a silicon-carbon material, which includes porous carbon and silicon particles located within the porous carbon channels.

6. The lithium-ion secondary battery according to claim 1, 2, or 4, characterized in that, The second negative electrode active layer has a recess on the surface away from the negative electrode current collector, satisfying at least one of the following conditions: (1) The opening width of the recess is 0.05mm-0.2mm; (2) The distance between adjacent recesses is 0.5mm-5mm; (3) The dimension of the recess along the thickness direction of the negative electrode sheet is T1, and the thickness of the negative electrode active layer on the surface of the negative electrode current collector is T2, 0.2≤T1 / T2≤0.5; (4) The total volume of the recess is T3, and the volume of the negative electrode active layer on the surface of the negative electrode current collector is T4, 0.005≤T3 / T4≤0.03; (5) The negative electrode sheet also includes a negative electrode tab, which extends from the negative electrode current collector and the extension direction of the negative electrode tab is perpendicular to the thickness direction of the negative electrode sheet; the recess is a groove, and the length direction of the groove is parallel to the extension direction of the negative electrode tab.

7. The lithium-ion secondary battery according to claim 1, characterized in that, The electrolyte also includes carbonate solvents and lithium salts, wherein the lithium salt content is 8%-18% based on the mass of the electrolyte. The carbonate solvents include at least one of dimethyl carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, and propylene carbonate.

8. The lithium-ion secondary battery according to claim 1 or 7, characterized in that, The carbonate solvent includes dimethyl carbonate and ethylene carbonate. Based on the mass of the electrolyte, the mass content of dimethyl carbonate is C1% and the mass content of ethylene carbonate is C2%, satisfying: 5≤C1≤60, 5≤C2≤40, 0.2≤C1 / C2≤10.

9. The lithium-ion secondary battery according to claim 1, characterized in that, The diaphragm includes a base membrane, a ceramic layer, and an adhesive layer. The ceramic layer is disposed on at least one side surface along the thickness direction of the base membrane, and the adhesive layer is disposed on the side surface of the ceramic layer away from the base membrane, and / or, the adhesive layer is disposed on the surface of the base membrane. The coverage of the adhesive layer on one side is 5%-95%.

10. The lithium-ion secondary battery according to claim 9, characterized in that, When the ceramic layer is disposed on at least one side surface in the thickness direction of the base film, the ceramic layer faces the positive electrode, and the thickness of the ceramic layer is 0.5 μm-2 μm; and / or, the porosity of the separator is 30%-60%.

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