A lithium-ion battery electrolyte, a lithium-ion battery, and a method for preparing the same.

Abstract

This invention belongs to the field of lithium-ion battery technology and discloses a lithium-ion battery electrolyte, a lithium-ion battery, and a method for preparing the same. The electrolyte comprises a primary electrolyte and a secondary electrolyte. The primary electrolyte accounts for 50-90% of the total electrolyte mass, and the concentration of fluoroethylene carbonate (FEC) in the primary electrolyte is 1.5-5 wt%. The concentration of FEC in the secondary electrolyte is 5-10 wt%. In the primary electrolyte, 90% of the electrolyte is added first to form a dense SEI film. In the secondary electrolyte, 10% of the electrolyte containing a high concentration of additives is added to suppress gaseous side reactions, resulting in a significant reduction in gas production compared to traditional methods.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium-ion battery electrolyte, a lithium-ion battery, and a method for preparing the same. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] The large-scale application of new energy vehicles and the explosive growth of the energy storage market have jointly driven the demand for high energy density and long cycle stability of lithium-ion batteries. High energy density refers to the characteristic of lithium-ion batteries that can store more electrical energy per unit mass. Long cycle stability refers to the ability of lithium-ion batteries to maintain high capacity, low internal resistance and good electrochemical performance after multiple charge-discharge cycles. It is one of the core indicators for measuring battery life and reliability.

[0004] To address this issue, the industry typically employs lithium replenishment and silicon doping to improve battery energy density. However, while adding lithium replenishing agents such as lithium-rich compounds to the positive electrode can pre-replenish the irreversibly consumed lithium source during the first charge-discharge cycle to compensate for lithium loss during cycling, excessive lithium replenishment can lead to problems such as gas generation and poor slurry processability. Simultaneously, the increased lithium source necessitates silicon doping in graphite to enhance the negative electrode capacity. However, the 300% volume expansion of silicon materials causes continuous rupture and regeneration of the SEI film, constantly consuming active lithium and electrolyte, ultimately resulting in rapid capacity decay and shortened cycle life.

[0005] Therefore, in high-energy-density systems, the stability of the system must be enhanced accordingly; otherwise, the entire system will become unbalanced, leading to a rapid deterioration in cycle life.

[0006] In addition, in commercial lithium-ion batteries, lithium plating not only degrades battery performance and significantly shortens cycle life, but also limits the battery's fast-charging capacity. The risk of lithium plating is further increased, especially during high-rate charging. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a lithium-ion battery electrolyte, a lithium-ion battery, and a method for preparing the same.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a lithium-ion battery electrolyte, comprising a primary electrolyte and a secondary electrolyte, wherein the mass of the primary electrolyte is 50-90 wt% of the total mass of the electrolyte, and the mass percentage a of fluoroethylene carbonate (FEC) in the primary electrolyte is 1.5-5 wt%; and the mass percentage b of fluoroethylene carbonate (FEC) in the secondary electrolyte is 5-10 wt%.

[0009] In a second aspect, the present invention provides a lithium-ion battery, which includes the lithium-ion battery electrolyte, a positive electrode, a negative electrode, and a separator. The positive electrode, negative electrode, and electrolyte satisfy the following formula: k = (X 2 + Y 2 ) / (C×N); Where k is an empirical constant, k is 8-20; X is the value of the mass percentage of lithium supplement in the positive electrode after removing wt%; Y is the value after removing wt% from the mass percentage of silicon-based material (based on elemental silicon) in the negative electrode; C is the value after removing wt% from the mass percentage of additive FEC in the total electrolyte (one filler plus two fillers); N is the NP ratio of the battery (the ratio of the reversible capacity of the negative electrode to the reversible capacity of the positive electrode), and N is 1.10 - 1.15.

[0010] Thirdly, the present invention provides a method for preparing the lithium-ion battery, comprising the following steps: assembling a positive electrode, a negative electrode and a separator into a bare cell, then performing a single injection of electrolyte, formation, standing, performing a second injection of electrolyte, sealing, and capacity testing to obtain the battery.

[0011] Fourthly, the present invention provides an evaluation method for lithium-ion batteries, comprising the following steps: obtaining relevant parameters affecting the long-cycle performance of lithium-ion batteries; Based on the lithium-ion battery model k = (X 2 + Y 2 ) / (C×N), calculate the long-cycle evaluation factor k; The cycle performance of lithium-ion batteries is evaluated based on the numerical values ​​of long-cycle evaluation factors and the set thresholds. Where X is the mass percentage of lithium supplement in the positive electrode after removing wt%; Y is the value after removing wt% from the mass percentage of silicon-based material (based on elemental silicon) in the negative electrode; C is the value after removing wt% from the mass percentage of additive FEC in the total electrolyte (one filler plus two fillers); N is the NP ratio of the battery (the ratio of the reversible capacity of the negative electrode to the reversible capacity of the positive electrode), and N is 1.10 - 1.15.

[0012] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows: (1) As an ion transport medium, the composition design of the electrolyte directly affects the interfacial stability. This invention optimizes the electrolyte addition mode by introducing gradient additives. In the first injection, 90% of low-FEC electrolyte is added to form a dense SEI film, and in the second injection, 10% of high-concentration FEC electrolyte is added to suppress gas side reactions, resulting in a significant reduction in gas production compared to the traditional scheme.

[0013] (2) This invention improves the cycle performance of high energy density lithium-ion batteries by synergistic effect of specific positive and negative electrode material formulations, positive and negative electrode design parameters and the amount of key electrolyte components added. It adjusts the relationship between various parameters, improves the charging capacity of lithium-ion batteries, reduces the risk of lithium plating, and ensures the cycle performance of batteries while taking into account high energy density and high energy density.

[0014] By designing a reasonable negative / positive capacity ratio (NP ratio) and matching it with a suitable electrolyte, the energy density can be maximized while ensuring the ultimate cycle life. Attached Figure Description

[0015] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0016] Figure 1 This is a structural diagram of the negative electrode sheet of the lithium-ion battery in Embodiment 1 of the present invention; Figure 2 This is a structural diagram of the positive electrode sheet of the lithium-ion battery in Embodiment 1 of the present invention; Figure 3 These are disassembly photographs of Embodiment 1(a), Comparative Example 1(b), and Comparative Example 3(c) of the present invention. Detailed Implementation

[0017] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0018] To address the technical problems mentioned in the background section, this invention provides a lithium-ion battery electrolyte, comprising a primary electrolyte and a secondary electrolyte. The primary electrolyte comprises 50-90% of the total electrolyte mass, and the mass percentage (a) of fluoroethylene carbonate (FEC) in the primary electrolyte is 1.5-5 wt%. The secondary electrolyte comprises 10% of the total electrolyte mass, and the mass percentage (b) of fluoroethylene carbonate (FEC) in the secondary electrolyte is 5-10 wt%.

[0019] The SEI formed on a Si-C electrode by an electrolyte without FEC mainly consists of fluorinated polymers and inorganic substances (Li₂CO₃ and LiF) generated by EC reduction. The weak intermolecular forces between these fluorinated polymers make them prone to breakage after lithium intercalation in the Si-C electrode, leading to continuous electrolyte decomposition and resulting in low coulombic efficiency and poor cycle stability of the Si-C electrode. In contrast, the SEI formed on a Si-C electrode by an FEC-containing electrolyte mainly consists of fluorinated polymers and inorganic substances (Li₂CO₃ and LiF) generated by FEC reduction. Hydrogen bonds exist between these fluorinated polymers, which make the SEI elastic, buffering the volume changes during lithium intercalation and deintercalation, inhibiting electrolyte decomposition, and improving the coulombic efficiency and cycle stability of the Si-C electrode.

[0020] The electrolyte is added in a gradient of additive concentration. In the first injection, 90% of the electrolyte is added first, and the electrolyte with low concentration of additive FEC (1.5-5wt%) forms a dense SEI film. In the second injection, 10% of the electrolyte with high concentration of additive FEC (5-10wt%) is added to suppress gas side reactions, and the gas production is significantly reduced compared with the traditional method.

[0021] Furthermore, based on the gradient consumption characteristics of additives in the electrolyte (60% consumed in the formation stage), a segmented electrolyte supply was implemented, resulting in a 42% reduction in interfacial impedance. This electrolyte composition can effectively reduce the rate of SEI film impedance growth, thereby improving the long-term cycle performance of the battery.

[0022] In some embodiments, the concentration of fluoroethylene carbonate (FEC) in one electrolyte is 1.5-5 wt%, and the concentration of fluoroethylene carbonate (FEC) in the second electrolyte is 5-10 wt%.

[0023] Preferably, the concentration of fluoroethylene carbonate (FEC) in the first electrolyte is 1.8-2.5 wt%, and the concentration of fluoroethylene carbonate (FEC) in the second electrolyte is 5-8 wt%.

[0024] In some embodiments, the mass of one injection of electrolyte is 80-90 wt% of the total mass of the electrolyte, preferably 85-90 wt%.

[0025] In some embodiments, the primary electrolyte and the secondary electrolyte further include at least one of vinylene carbonate (VC), vinyl sulfate, or tris(trimethylsilyl)phosphate.

[0026] In some embodiments, the solvent in the first and second electrolyte solutions is selected from ethylene carbonate, dimethyl carbonate, or diethyl carbonate.

[0027] In a second aspect, the present invention provides a lithium-ion battery, which includes the lithium-ion battery electrolyte, a positive electrode, a negative electrode, and a separator. The positive electrode, negative electrode, and electrolyte satisfy the following formula: k = (X 2 + Y 2 ) / (C×N); Where k is an empirical constant, k is 8-20; X is the value of the mass percentage of lithium supplement in the positive electrode after removing wt%; Y is the value after removing wt% from the mass percentage of silicon-based material (based on elemental silicon) in the negative electrode; C is the value after removing wt% from the mass percentage of additive FEC in the total electrolyte (one filler plus two fillers); N is the NP ratio of the battery (the ratio of the reversible capacity of the negative electrode to the reversible capacity of the positive electrode), and N is 1.10 - 1.15, preferably 1.11 - 1.13.

[0028] For example, the mass percentage of lithium supplement in the positive electrode is X wt%; the mass percentage of silicon-based material (based on elemental silicon) in the negative electrode is Y wt%. If the mass percentage of one injection of electrolyte in the total electrolyte mass is e, then the mass percentage of FEC in the total electrolyte added is Cwt% = e×a + (1-e)×b.

[0029] Therefore, the above formula is a dimensionless formula.

[0030] This invention adjusts the parameters of the positive and negative electrode plates and electrolyte in the battery cell to satisfy this relationship, so that the lithium delithiation rate of the positive electrode and the lithium insertion rate of the negative electrode are matched, thereby effectively avoiding the safety risks caused by lithium plating in the battery cell and effectively improving the cycle performance of the battery while taking into account high energy density.

[0031] In some embodiments, k is 10-16.

[0032] In some embodiments, X is 1-5, preferably 1.8-3.

[0033] In some embodiments, Y is 3-10, preferably 5-8.

[0034] In some embodiments, C is 2-4, preferably 2-3.

[0035] If the lithium replenishment is too low, the SEI film formed during the first charge and discharge cycle will consume a large amount of active lithium from the positive electrode, and the lithium replenishment agent cannot effectively compensate for this "irreversible capacity loss," resulting in insufficient lithium replenishment. Furthermore, due to the low initial efficiency of the battery, the initial capacity will not reach the theoretical value. More seriously, the available active lithium source will be severely insufficient in the early stages of cycling, leading to a very poor cycle life and a sharp drop in capacity. This problem is exacerbated for highly silicon-doped anodes.

[0036] Excessive lithium addition can lead to severe gas production: Most lithium additives (such as Li5FeO4) will react with moisture in the air or electrolyte to produce a large amount of gas (such as H2, O2), which can cause battery bulging, increased internal pressure, and safety hazards; processing difficulties: excessive lithium additives can make the properties of the positive electrode slurry unstable (such as viscosity changes), making coating difficult and affecting the quality of the electrode sheet; damage to the positive electrode structure: excessive high-activity materials may damage the bulk structure of the positive electrode material, which will lead to a decrease in positive electrode capacity and cycle decay; increased costs: lithium additives are usually expensive, and excessive use is uneconomical.

[0037] If the silicon doping content is too low, the energy density improvement will not be significant, thus negating the purpose of using silicon-based materials; the energy density of the battery (especially the volumetric energy density) will be almost the same as that of traditional graphite anode batteries, which cannot meet the needs of high-end applications.

[0038] If the silicon doping level is too high, the significant volume expansion of silicon will cause the SEI film to continuously rupture and regenerate. With each cycle, the expansion and contraction of silicon particles will break through the surface SEI film, exposing a new silicon surface that reacts with the electrolyte, consuming a large amount of active lithium and electrolyte – this is the root cause of battery life degradation. Simultaneously, repeated volume changes will cause the silicon particles to crack and pulverize, separating from the conductive agent and binder, leading to electrode structure collapse and a sharp increase in internal resistance. The immense stress can also cause the active material to detach from the current collector, resulting in complete battery failure.

[0039] If the NP ratio is too high, i.e., N > 1.15, the positive electrode capacity is significantly higher than the negative electrode capacity. This results in excessive retained undeintercalated lithium, leading to a sacrifice in battery energy density. Simultaneously, a high NP ratio causes a shift in the positive electrode operating voltage, resulting in intensified polarization, particularly in the charge transfer impedance R. ct The increase was significant.

[0040] If the NP ratio is too low, i.e., N < 1.10, the amount of lithium released from the positive electrode exceeds the lithium intercalation capacity limit of the negative electrode. The local current density is too high at the end of charging, which will cause lithium plating on the negative electrode. At the same time, the low NP ratio forces the positive electrode active material to undergo deep delithiation, triggering the release of lattice oxygen and causing the battery performance to deteriorate.

[0041] If the content of fluoroethylene carbonate (FEC) in the electrolyte is too high, the excess FEC will form an excessively thick SEI film with high impedance, leading to a decrease in battery rate performance (poor fast charging capability). At the same time, under certain conditions (such as high temperature and high voltage), the decomposition of FEC may produce fluorine-containing gas, causing battery bulging and performance degradation.

[0042] If the content of fluoroethylene carbonate (FEC) in the electrolyte is too low, an effective protective film cannot be formed on the silicon surface. The SEI film is unstable, porous, and cannot suppress the continuous decomposition of the electrolyte and the volume expansion effect of silicon. The battery's initial efficiency and cycle life will be very poor, offering no advantage compared to batteries without silicon doping but also without protection.

[0043] In some embodiments, the active material of the positive electrode is at least one of lithium iron phosphate, lithium manganese oxide, and lithium nickel cobalt manganese oxide, as well as a lithium replenishing agent.

[0044] In some embodiments, the active material of the negative electrode is at least one of artificial graphite, natural graphite, activated carbon, silicon carbide, hard carbon, soft carbon, mesophase carbon microspheres and lithium titanate, as well as silicon carbide.

[0045] Thirdly, the present invention provides a method for preparing the lithium-ion battery, comprising the following steps: assembling a positive electrode, a negative electrode and a separator into a bare cell, then performing a single injection of electrolyte, formation, standing, performing a second injection of electrolyte, sealing, and capacity testing to obtain the battery.

[0046] In some embodiments, the settling time is 1-3 hours.

[0047] Fourthly, the present invention provides an evaluation method for lithium-ion batteries, comprising the following steps: obtaining relevant parameters affecting the long-cycle performance of lithium-ion batteries; Based on the lithium-ion battery model k = (X 2 + Y 2 ) / (C×N), calculate the long-cycle evaluation factor k; The cycle performance of lithium-ion batteries is evaluated based on the numerical values ​​of long-cycle evaluation factors and the set thresholds. Where X is the mass percentage of lithium supplement in the positive electrode after removing wt%; Y is the value after removing wt% from the mass percentage of silicon-based material (based on elemental silicon) in the negative electrode; C is the value after removing wt% from the mass percentage of additive FEC in the total electrolyte; N represents the NP ratio of the battery.

[0048] In some embodiments, if the calculated long cycle evaluation factor is 8≤k≤20, the expected cycle life is ≥2000 cycles@80%SOC, which significantly improves the cycle performance of lithium-ion batteries while also providing good safety. If k > 20, the electron and ion transport paths in the electrode increase, which increases the internal resistance of the cell and ultimately causes lithium plating in the battery.

[0049] Preferably, if 10≤k≤16, the composition of the battery electrolyte interface film is more complex and stable, which helps to form a denser and more uniform electrolyte interface film, further enhancing the interfacial compatibility between the positive electrode and the electrolyte, and improving the cycle performance and rate performance of the electrochemical device.

[0050] The present invention will be further described below with reference to embodiments: Example 1 A lithium-ion battery includes a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte. The negative electrode is formed by coating a mixture of graphite and 5.0 wt% silicon-carbon as the negative electrode active material onto copper foil, followed by rolling to obtain a negative electrode sheet. The mass ratio of graphite, silicon-carbon, conductive agent (sp), and binder (PAA) in the negative electrode sheet is 92:5:2:1, and the double-sided areal density of the negative electrode is 18.48 mg / cm³. 2 ; A mixture of lithium iron phosphate and 1.8 wt% lithium supplementer was coated onto aluminum foil as the positive electrode active material according to an NP ratio of 1.13. The positive electrode sheet was then obtained by rolling. The mass ratio of lithium iron phosphate, lithium supplementer (LFO), conductive agent (sp), and binder (PVDF) in the positive electrode sheet was 94:1.8:2:2.2, and the areal density of the positive electrode was 45 mg / cm³. 2 .

[0051] Separator: 7μm (polyethylene membrane) + 2μm (ceramic) + 2μm (PVDF); The amount of electrolyte added in the first injection is 90% of the total electrolyte volume, and the FEC mass percentage in the first injection is 2wt%; the amount of electrolyte added in the second injection is 10% of the total electrolyte volume, and the FEC mass percentage in the second injection is 5wt%; the total mass fraction of FEC in the weighted electrolyte is 2.3%, and the total mass of the electrolyte is 11g.

[0052] The specific composition of the electrolyte is 15% LiPF6 as lithium salt, 2% VC, 2% FEC, 1% DTD as additives, and EC:DMC:EMC in a volume ratio of 3:4:3 as solvent.

[0053] The specific composition of the second electrolyte is 15% LiPF6 as lithium salt, 5% FEC as additive, and EC:DMC:EMC in a volume ratio of 3:4:3 as solvent.

[0054] The prepared positive electrode, negative electrode and separator are assembled into a bare cell by stacking assembly. Then, a single injection of electrolyte is used for formation, standing, a second injection of electrolyte is used for sealing and capacity testing to obtain a lithium-ion battery (the assembly, formation, sealing and capacity testing of lithium-ion batteries are all carried out using conventional operations in the field, and the operation of this step is the same in the examples and comparative examples).

[0055] Example 2 A lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; wherein the negative electrode is formed by coating a mixture of graphite and 6.0 wt% silicon-carbon as the negative electrode active material onto copper foil, and then rolling to obtain a negative electrode sheet. In the negative electrode sheet, the mass ratio of graphite, silicon-carbon, conductive agent (sp), and binder (PAA) is 92:6:1:1, and the double-sided areal density of the negative electrode is 17.93 mg / cm³. 2 ; A mixture of lithium iron phosphate and 2.2 wt% lithium supplementer was coated onto aluminum foil as the positive electrode active material according to an NP ratio of 1.12. The positive electrode sheet was then obtained by rolling. The mass ratio of lithium iron phosphate, lithium supplementer (LFO), conductive agent (sp), and binder (PVDF) in the positive electrode sheet was 94:2.2:1.6:2.2. The areal density of the positive electrode was 45 mg / cm³. 2 .

[0056] Separator: 7μm (polyethylene membrane) + 2μm (ceramic) + 2μm (PVDF); The amount of electrolyte added in the first injection is 90% of the total electrolyte volume, and the FEC mass percentage in the first injection is 2.3 wt%. The amount of electrolyte added in the second injection is 10% of the total electrolyte volume, and the FEC mass percentage in the second injection is 7 wt%. After weighting, the total FEC mass fraction in the electrolyte is 2.8%, and the total electrolyte mass is 11 g.

[0057] The specific composition of the electrolyte is 15% LiPF6 as lithium salt, 2% VC, 2.3% FEC, and 1% DTD as additives, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0058] The specific composition of the second electrolyte is 15% LiPF6 as lithium salt, 7% FEC as additive, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0059] The prepared positive electrode, negative electrode and separator are assembled into a bare cell by stacking. Then, a single injection of electrolyte is used for formation, settling, a second injection of electrolyte is used for sealing and capacity testing to obtain a lithium-ion battery.

[0060] Example 3 A lithium-ion battery includes a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte. The negative electrode is formed by coating a mixture of graphite and 6.0 wt% silicon-carbon as the negative electrode active material onto copper foil, followed by rolling to obtain a negative electrode sheet. The mass ratio of graphite, silicon-carbon, conductive agent (SP), and binder (PAA) in the negative electrode sheet is 91:6:2:1, and the double-sided areal density of the negative electrode is 18.11 mg / cm³. 2 .

[0061] A mixture of lithium iron phosphate and 2.8 wt% lithium supplementer was coated onto aluminum foil as the positive electrode active material according to an NP ratio of 1.11. The positive electrode sheet was then obtained by rolling. The mass ratio of lithium iron phosphate, lithium supplementer (LFO), conductive agent (sp), and binder (PVDF) in the positive electrode sheet was 94:2.8:1.6:1.6, and the areal density of the positive electrode was 45 mg / cm³. 2 .

[0062] Separator: 7μm (polyethylene membrane) + 2μm (ceramic) + 2μm (PVDF); The amount of electrolyte added in the first injection is 90% of the total electrolyte volume, and the FEC mass percentage in the first injection is 2wt%. The amount of electrolyte added in the second injection is 10% of the total electrolyte volume, and the FEC mass percentage in the second injection is 5wt%. After weighting, the total FEC mass fraction in the electrolyte is 2.3%, and the total electrolyte mass is 11g.

[0063] The specific composition of the electrolyte is 15% LiPF6 as lithium salt, 2% VC, 2% FEC, and 1% DTD as additives, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0064] The specific composition of the second electrolyte is 15% LiPF6 as lithium salt, 5% FEC as additive, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0065] The prepared positive electrode, negative electrode and separator are assembled into a bare cell by stacking. Then, a single injection of electrolyte is used for formation, settling, a second injection of electrolyte is used for sealing and capacity testing to obtain a lithium-ion battery.

[0066] Comparative Example 1 The difference from Example 2 is that in the negative electrode sheet, the mass ratio of graphite, silicon carbide, conductive agent (sp), and binder (PAA) is 90:8:1:1; and the mass percentage of silicon carbide is 8 wt%. In the positive electrode sheet, the mass ratio of lithium iron phosphate, lithium supplement (LFO), conductive agent (sp), and binder (PVDF) is 92.9:3.3:1.6:2.2; the mass percentage of lithium supplement is 3.3 wt%. The amount of electrolyte added in the first injection is 90% of the total electrolyte volume, and the FEC mass percentage in the first injection is 2wt%. The amount of electrolyte added in the second injection is 10% of the total electrolyte volume, and the FEC mass percentage in the second injection is 5wt%. After weighting, the total FEC mass fraction in the electrolyte is 2.3%, and the total electrolyte mass is 11g.

[0067] The specific composition of the electrolyte is 15% LiPF6 as lithium salt, 2% VC, 2% FEC, and 1% DTD as additives, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0068] The specific composition of the second electrolyte is 15% LiPF6 as lithium salt, 5% FEC as additive, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0069] Comparative Example 2 The difference from Example 2 is that in the negative electrode sheet, the mass ratio of graphite, silicon carbide, conductive agent (sp), and binder (PAA) is 95:3:1:1; and the mass percentage of silicon carbide is 3 wt%. In the positive electrode sheet, the mass ratio of lithium iron phosphate, lithium supplementer (LFO), conductive agent (sp), and binder (PVDF) is 94.2:2:1.6:2.2; the mass percentage of lithium supplementer is 2 wt%. The amount of electrolyte added in the first injection is 90% of the total electrolyte volume, and the FEC mass percentage in the first injection is 2wt%. The amount of electrolyte added in the second injection is 10% of the total electrolyte volume, and the FEC mass percentage in the second injection is 5wt%. After weighting, the total FEC mass fraction in the electrolyte is 2.3%, and the total electrolyte mass is 11g.

[0070] The specific composition of the electrolyte is 15% LiPF6 as lithium salt, 2% VC, 2% FEC, and 1% DTD as additives, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0071] The specific composition of the second electrolyte is 15% LiPF6 as lithium salt, 5% FEC as additive, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0072] Comparative Example 3 The difference from Example 3 is that in the negative electrode sheet, the mass ratio of graphite, silicon carbide, conductive agent (sp), and binder (PAA) is 93:5:1:1; and the mass percentage of silicon carbide is 5 wt%. In the positive electrode sheet, the mass ratio of lithium iron phosphate, lithium supplementer (LFO), conductive agent (sp), and binder (PVDF) is 94.2:2:1.6:2.2; the mass percentage of lithium supplementer is 2 wt%. The amount of electrolyte added in the first injection is 90% of the total electrolyte volume, and the FEC mass percentage in the first injection is 4wt%. The amount of electrolyte added in the second injection is 10% of the total electrolyte volume, and the FEC mass percentage in the second injection is 10wt%. After weighting, the total FEC mass fraction in the electrolyte is 4.6%, and the total electrolyte mass is 11g.

[0073] The specific composition of the electrolyte is 15% LiPF6 as lithium salt, 2% VC, 4% FEC, and 1% DTD as additives, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0074] The specific composition of the second electrolyte is 15% LiPF6 as lithium salt, 10% FEC as additive, and EC:DMC:EMC mixed in a volume ratio of 3:4:3 as solvent.

[0075] Comparative Example 4 The difference from Example 1 is that the electrolyte is added all at once, with an FEC content of 2wt%, and the specific composition is the same as that of the electrolyte in Example 1.

[0076] Comparative Example 5 The difference from Example 1 is that the electrolyte is added all at once, with an FEC content of 5 wt%, and the specific composition is the same as the two-pour electrolyte in Example 1.

[0077] Battery performance test: 1. Cyclic test: The lithium battery was subjected to a cyclic test in accordance with the requirements of GB / T 31484-2015 "Requirements and test methods for cycle life of power batteries for electric vehicles".

[0078] 2. Interface effect confirmation: Discharge the obtained lithium-ion battery at a rate of 0.5 C to the discharge cutoff voltage, let it stand for 30 minutes; then charge it at a rate of 2 C with constant current and constant voltage to the charging cutoff voltage; then, disassemble the negative electrode and observe the residual floating lithium on the surface of the negative electrode.

[0079] Specifically, if the area of ​​the residual lithium floating region on the negative electrode surface is less than or equal to 0%, it is considered a good interface; if the area of ​​the residual lithium floating region on the negative electrode surface is less than 5%, it is considered slight lithium plating; if the area of ​​the residual lithium floating region on the negative electrode surface is 5-50%, it is considered moderate lithium plating; and if the area of ​​the residual lithium floating region on the negative electrode surface is greater than 50%, it is considered severe lithium plating. The results are as follows... Figure 3 As shown, from Figure 3 It can be seen that the interface of Example 1 is good, the interface of Comparative Example 1 is moderately lithium-plated, and the interface of Comparative Example 3 is severely lithium-plated.

[0080] 4. High temperature test: According to GB / T 31484-2015 "Requirements and test methods for cycle life of power batteries for electric vehicles", the discharge capacity retention rate is tested after storage at 45°C±2°C for 28 days.

[0081] Specifically, the relevant parameters and performance test results under the same conditions for the embodiments and comparative examples are shown in Table 1 below.

[0082] Table 1. Comparison of phase properties of examples and comparative examples.

[0083] As can be seen from the test data in Table 1, Examples 1-3, within the scope of this invention, can effectively improve the room temperature and high temperature cycle performance of lithium-ion batteries and mitigate the risk of lithium plating in the cell. Compared to Example 1, Example 2 increases the total mass percentage of FEC in the electrolyte. In the first week, 90% low-concentration FEC electrolyte is injected to form a dense SEI film, followed by the addition of 10% high-concentration FEC electrolyte to suppress gaseous side reactions, thereby effectively improving the cell's storage and cycle performance. In Example 3, the NP ratio of the battery is reduced, decreasing lithium redundancy and allowing lithium ions to be promptly embedded in the active material, significantly reducing lithium plating and thus improving the cell's long-cycle performance.

[0084] like Figure 3 As shown, in Comparative Example 1, due to excessive lithium replenishment, most lithium replenishing agents (such as Li5FeO4) will undergo side reactions with moisture in the air or the electrolyte, producing a large amount of gas (such as H2 and O2). This can lead to battery bulging, increased internal pressure, and safety hazards. Simultaneously, excessive highly active materials may damage the bulk structure of the cathode material, resulting in a decrease in cathode capacity and cycle degradation. In Comparative Example 2, although there is no disadvantage in cycle performance, the silicon doping level is too low, resulting in low capacity utilization and insignificant energy density improvement. This defeats the purpose of using silicon-based materials, and the battery's energy density (especially volumetric energy density) will be comparable to that of traditional graphite anode batteries, failing to meet the demands of high-end applications. In Comparative Example 3, the electrolyte contains excessively high levels of FEC additives. Excessive FEC will form an excessively thick and highly impedance SEI film, leading to a decrease in battery rate performance (poor fast charging capability) and significant deterioration in high-temperature cycling. Furthermore, under certain conditions (such as high temperature and high voltage), FEC decomposes to produce fluorine-containing gas, causing battery bulging and performance degradation. Comparative Example 4 did not employ a secondary electrolyte injection method. 60% of the FEC was consumed during the formation stage, resulting in insufficient FEC content in the later stages. This led to incomplete SEI film formation and a large exposed area on the graphite surface, causing continuous consumption of active lithium and affecting the battery's long-term cycle performance. Comparative Example 5 also did not employ a secondary electrolyte injection method and used the two-electrolyte formulation from Example 1. This formulation had a high FEC content but lacked additives such as VC and DTD required for film formation. The initial film formation was unstable and prone to breakage, resulting in poor electrical performance.

[0085] Based on the above analysis, it can be seen that the present invention improves the cycle performance of high-energy-density lithium-ion batteries through the synergistic effect of specific positive and negative electrode material formulations, positive and negative electrode design parameters, and the amount of key electrolyte components added. By adjusting the relationship between various parameters, the charging capability of lithium-ion batteries is improved, the risk of lithium plating is reduced, and the cycle performance of batteries is guaranteed while taking into account high energy density and high compaction.

[0086] This invention employs a dynamic electrolyte to adapt to a high-energy-density, long-cycle system: 90% electrolyte is injected in the first cycle to form a dense SEI film, followed by a second injection of 10% high-concentration FEC electrolyte to suppress gaseous side reactions, significantly reducing gas production compared to traditional methods. The SEI formed on the Si-C electrode using an FEC-containing electrolyte is mainly composed of fluorinated polymers and inorganic substances (Li₂CO₃ and LiF) generated from FEC reduction. Hydrogen bonds exist between the fluorinated polymers, giving the SEI elasticity, which buffers the volume changes during lithium insertion / extraction, suppresses electrolyte decomposition, and improves the coulombic efficiency and cycle stability of the Si-C electrode.

[0087] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium-ion battery electrolyte, characterized in that: It includes a first electrolyte and a second electrolyte. The mass of the first electrolyte is 50-90 wt% of the total electrolyte mass. The concentration of fluoroethylene carbonate (FEC) in the first electrolyte is 1.5-5 wt%. The concentration of fluoroethylene carbonate (FEC) in the second electrolyte is 5-10 wt%.

2. The lithium-ion battery electrolyte according to claim 1, characterized in that: The concentration of fluoroethylene carbonate (FEC) in the first electrolyte bath is 1.5-5 wt%, and the concentration of fluoroethylene carbonate (FEC) in the second electrolyte bath is 5-10 wt%. Preferably, the concentration of fluoroethylene carbonate (FEC) in the first electrolyte is 1.8-2.5 wt%, and the concentration of fluoroethylene carbonate (FEC) in the second electrolyte is 5-8 wt%.

3. The lithium-ion battery electrolyte according to claim 1, characterized in that: The mass of one injection of electrolyte is 80-90 wt% of the total mass of electrolyte, preferably 85-90 wt%.

4. The lithium-ion battery electrolyte according to claim 1, characterized in that: The primary and secondary electrolytes also include at least one of vinylene carbonate, vinyl sulfate, or tris(trimethylsilyl)phosphate; Alternatively, the solvents in the first and second electrolyte solutions may be selected from ethylene carbonate, dimethyl carbonate, or diethyl carbonate.

5. A lithium-ion battery, characterized in that: It includes the lithium-ion battery electrolyte, positive electrode, negative electrode, and separator as described in any one of claims 1-4; The positive electrode, negative electrode, and electrolyte satisfy the following formula: k = (X 2 + And 2 ) / (C×N); Where k is an empirical constant, k is 8-20; X is the value of the mass percentage of lithium supplement in the positive electrode after removing wt%; Y is the value after removing wt% from the mass percentage of silicon-based material (based on elemental silicon) in the negative electrode; C is the value after removing wt% from the mass percentage of additive FEC in the total electrolyte; N is the NP ratio of the battery, and N is 1.10 - 1.15, preferably 1.11 - 1.

13.

6. The lithium-ion battery according to claim 5, characterized in that: k is 10-16; Alternatively, X can be 1-5, preferably 1.8-3; Alternatively, Y can be 3-10, preferably 5-8; Alternatively, C can be 2-4, preferably 2-3.

7. The lithium-ion battery according to claim 5, characterized in that: The active material of the positive electrode is at least one of lithium iron phosphate, lithium manganese oxide, and lithium nickel cobalt manganese oxide, as well as a lithium replenishing agent; Alternatively, the active material of the negative electrode may be at least one of artificial graphite, natural graphite, activated carbon, silicon carbide, hard carbon, soft carbon, mesophase carbon microspheres, and lithium titanate, as well as silicon carbide.

8. The method for preparing the lithium-ion battery according to any one of claims 5-7, characterized in that: The process includes the following steps: assembling the positive electrode, negative electrode, and separator into a bare cell; then injecting one electrolyte, forming, allowing it to stand; injecting two electrolytes, sealing, and dividing the capacity to obtain the final product.

9. A method for evaluating lithium-ion batteries, characterized in that: The lithium-ion battery uses any of the lithium-ion battery electrolytes described in claims 1-4, and the evaluation method includes the following steps: obtaining relevant parameters that affect the long-cycle performance of the lithium-ion battery; Based on the lithium-ion battery model k = (X 2 + Y 2 ) / (C×N), calculate the long-cycle evaluation factor k; The cycle performance of lithium-ion batteries is evaluated based on the numerical values ​​of long-cycle evaluation factors and the set thresholds. Where X is the mass percentage of lithium supplement in the positive electrode after removing wt%; Y is the value after removing wt% from the mass percentage of silicon-based material (based on elemental silicon) in the negative electrode; C is the value after removing wt% from the mass percentage of additive FEC in the total electrolyte; N represents the NP ratio of the battery.

10. The evaluation method for lithium-ion batteries according to claim 9, characterized in that: If the calculated long-cycle evaluation factor is 8≤k≤20, the expected cycle life is ≥2000 cycles@80%SOC, which significantly improves the cycle performance of lithium-ion batteries while also providing good safety. If k > 20, the electron and ion transport paths in the electrode increase, which increases the internal resistance of the cell and ultimately causes lithium plating in the battery. Preferably, if 10≤k≤16, the composition of the battery electrolyte interface film is more complex and stable, which helps to form a denser and more uniform electrolyte interface film, further enhancing the interfacial compatibility between the positive electrode and the electrolyte, and improving the cycle performance and rate performance of the electrochemical device.

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