A lithium-ion battery electrolyte system that balances high conductivity at both room temperature and extremely low temperatures, and a lithium-ion battery.

Abstract

This invention relates to the field of lithium-ion battery electrolyte technology, and more particularly to a lithium-ion battery electrolyte system that achieves high conductivity at both room temperature and extremely low temperatures, as well as a lithium-ion battery. This electrolyte system comprises a solvent, a lithium salt, and additives. The solvent consists of an amide-based aprotic polar solvent with a melting point ≤ -50℃ and a dielectric constant ≥ 25 at 25℃, a flame-retardant phosphate solvent with a viscosity ≤ 3.5 mPa·s at 25℃, a fluorocarboxylic acid alkyl ester solvent with a melting point ≤ -70℃ and a viscosity ≤ 1.0 mPa·s at 25℃, and a fluorocyclic carbonate solvent. The lithium salt consists of sulfonylimide lithium salts and inorganic fluoroboronic acid lithium salts. The additives consist of phosphorus- and fluorine-containing inorganic lithium salt additives, cyclic sulfite additives, silicon-containing phosphate additives, and dinitrile compounds. The electrolyte system of this invention can achieve high conductivity at room temperature and low temperature. The ionic conductivity can reach 9.2 mS / cm at 25℃ and ≥ 1.25 mS / cm at -40℃. The applicable voltage window is 3.0~4.7 V.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery electrolyte technology, and in particular to a lithium-ion battery electrolyte system that balances high conductivity at both room temperature and extremely low temperatures, as well as a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries are widely used in new energy vehicles, aerospace, polar equipment, and special energy storage due to their advantages such as high energy density, long cycle life, and no memory effect. However, traditional carbonate electrolytes have problems such as high viscosity at low temperatures, low ionic conductivity, difficulty in lithium-ion desolvation, and a surge in electrode interface impedance. In extremely low temperature environments of -40℃, their capacity retention is extremely low, and they may even be unable to charge and discharge, making it difficult to meet the application requirements of extremely cold environments.

[0003] Existing low-temperature electrolytes mostly adopt routes such as diluting high-concentration systems with fluorinated ethers or compounding single low-temperature solvents, which have drawbacks such as high cost, poor flame retardancy, narrow electrochemical window, and insufficient stability of the negative electrode interface. On the other hand, low-temperature electrolytes based on DMF and phosphate esters have a single solvent combination and have problems such as strong hygroscopicity, many side reactions, and poor high-voltage compatibility, which greatly limits their application in low-temperature high-voltage lithium-ion batteries.

[0004] Patent document CN118198491A discloses a method for preparing and applying a wide-temperature-range battery electrolyte. By reducing the use of carbonates and introducing fluorinated solvents, carboxylic acid ester solvents, and diluents, the viscosity of the electrolyte is reduced over a wide temperature range while maintaining a high dielectric constant, thus improving the electrolyte's conductivity. The use of lithium salts with high conductivity and low low-temperature charge transfer resistance ensures the electrolyte's reaction kinetics. The introduction of effective film-forming additives and weak solvating solvents as wide-temperature additives reduces the charge transfer impedance at the electrode interface, promotes the formation of anion-dominated solid interface film, and improves the battery's wide-temperature performance. Although this prior art achieves a conductivity of 1.034 mS / cm at an extremely low temperature of -60℃, its conductivity at room temperature (25℃) does not exceed 5 mS / cm (as shown in Figure 1). This indicates that the design of maintaining ion conduction at low temperatures sacrifices ion dissociation or migration efficiency at room temperature, lacking practical significance for wide-temperature-range applications. Summary of the Invention

[0005] The present invention aims to solve the above problems by providing a lithium-ion battery electrolyte system that balances high conductivity at both room temperature (25°C) and extremely low temperature (-40°C), as well as a lithium-ion battery.

[0006] The technical solution to the problem solved by this invention is, firstly, to provide a lithium-ion battery electrolyte system that balances high conductivity at both room temperature and extremely low temperatures, comprising a solvent, a lithium salt, and additives.

[0007] The solvent is composed of an amide-based aprotic polar solvent with a melting point ≤ -50℃ and a dielectric constant ≥ 25 at 25℃, a flame-retardant phosphate solvent with a viscosity ≤ 3.5 mPa·s at 25℃, a fluorocarboxylic acid alkyl ester solvent with a melting point ≤ -70℃ and a viscosity ≤ 1.0 mPa·s at 25℃, and a fluorocyclic carbonate solvent.

[0008] The lithium salt is composed of sulfonylimide lithium salt and inorganic fluoroboronic acid lithium salt;

[0009] The additives consist of phosphorus- and fluorine-containing inorganic lithium salts, cyclic sulfite additives, silicon-containing phosphate additives, and dinitrile compounds.

[0010] solvent

[0011] The solvent is used to dissociate the lithium salt and construct a high-speed channel for ion transport. Preferably, the solvent accounts for 50% to 80% of the total mass of the electrolyte. For example, it can be 50%, 55%, 60%, 65%, 70%, 75%, or 80%, preferably 60% to 75%.

[0012] The solvent, through the selection of its components, effectively dissociates lithium salts and ensures lithium-ion migration at both room temperature and extremely low temperatures. Specifically:

[0013] Amide-based aprotic polar solvents The product has the following characteristics: melting point ≤ -50℃, ensuring it does not solidify at extremely low temperatures of -40℃; dielectric constant ≥ 25, ensuring sufficient dissociation of the lithium salt and providing charge carriers; and moderate coordination strength between the amide group and lithium ions, avoiding interference with lithium ion migration or difficulty in dissociating the lithium salt. In this invention, amide-based aprotic polar solvents are used to ensure high carrier concentrations at both room temperature and extremely low temperatures.

[0014] For example, the amide-based aprotic polar solvent can be at least one of N,N-dimethylformamide (DMF) and N,N-diethylformamide (DEF). N,N-diethylformamide has a relatively low dielectric constant and its performance in the electrolyte system of this invention is inferior to that of N,N-dimethylformamide.

[0015] As a preferred embodiment of the present invention, the amide-based aprotic polar solvent is N,N-dimethylformamide.

[0016] Flame-retardant phosphate solvent Besides providing flame retardancy and high-pressure stability through P=O bonds, it also inhibits the reduction and decomposition of DMF by participating in film formation on the graphite anode surface, preferably with amide-based aprotic polar solvents (especially DMF) due to its electrochemical stability. Simultaneously, it requires the lowest possible viscosity to avoid affecting lithium-ion migration. Without flame-retardant phosphate solvents, DMF is prone to irreversible reduction and decomposition on the graphite anode, leading to a sharp drop in initial coulombic efficiency and cycle life.

[0017] For example, the flame-retardant phosphate solvent can be at least one of trimethyl phosphate (TMP) and triethyl phosphate (TEP). Among them, trimethyl phosphate has a lower viscosity and is more advantageous at low temperatures.

[0018] As a preferred embodiment of the present invention, the flame-retardant phosphate solvent is trimethyl phosphate.

[0019] Furthermore, the optimal ratio of amide-based aprotic polar solvent to flame-retardant phosphate solvent is limited. If the amount of flame-retardant phosphate solvent is too low, it will not be sufficient to form an effective protective layer on the negative electrode, leading to the decomposition of the amide-based aprotic polar solvent. If the amount is too high, the viscosity of the system will increase, resulting in impaired low-temperature conductivity.

[0020] As a preferred embodiment of the present invention, the volume ratio of the amide-based aprotic polar solvent to the flame-retardant phosphate solvent is 1:(1.1 to 1.3); for example, it can be 1:1.1, 1:1.15, 1:1.2, 1:1.25, or 1:1.3.

[0021] Fluorocarboxylic acid alkyl ester solvents With a viscosity ≤ 1.0 mPa·s, it is used to reduce the overall viscosity. The fluorocarboxylic acid alkyl ester group ensures weak solvation characteristics. Its dual characteristics of low viscosity and weak solvation enable it to create a highly fluid environment outside the lithium-ion solvation sheath, reducing the viscous resistance to lithium-ion migration at low temperatures. It is a key component for increasing the ionic conductivity to ≥1.25 mS / cm at -40℃. If the fluorocarboxylic acid alkyl ester solvent is lacking, the low-temperature migration resistance will be too high, and the ionic conductivity at -40℃ will not meet the target.

[0022] For example, the alkyl fluorinated carboxylic acid solvent can be at least one of ethyl trifluoroacetate (TFAc), methyl trifluoroacetate (TFAm), ethyl pentafluoropropionate, and methyl heptafluorobutyrate. Methyl heptafluorobutyrate has a weak solvation degree, which can hinder the hopping transport of lithium ions between solvated sheaths; meanwhile, ethyl pentafluoropropionate and methyl heptafluorobutyrate have high viscosity and large molecular volume, which are not conducive to lithium ion migration, and their compatibility with DMF (at low temperatures) is relatively poor; while methyl trifluoroacetate has a low boiling point and is prone to volatilization during the exothermic dissolution of lithium salts, and may also compete with DMF; therefore, ethyl trifluoroacetate is the optimal choice in the electrolyte system of this invention.

[0023] As a preferred embodiment of the present invention, the fluorocarboxylic acid alkyl ester solvent is ethyl trifluoroacetate.

[0024] Furthermore, the amount of amide-based aprotic polar solvent and fluorocarboxylic acid alkyl ester solvent is preferably limited. If there is too little amide-based aprotic polar solvent, the lithium salt will not dissociate sufficiently, the carrier concentration will decrease, and the conductivity at 25 °C will be impaired. If there is too little fluorocarboxylic acid alkyl ester solvent, the bulk viscosity will increase, desolvation will be difficult, and the conductivity at -40 °C will be impaired. It is impossible to achieve both high conductivity at room temperature and extremely low temperature.

[0025] As a preferred embodiment of the present invention, the volume ratio of the amide-based aprotic polar solvent to the fluorocarboxylic acid alkyl ester solvent is (0.5–2):1; for example, it can be 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, or 2:1; preferably 1:1.

[0026] Fluorinated cyclic carbonate solvents Its high reduction potential allows it to preferentially reduce DMF and TMP on the negative electrode surface, producing a dense SEI with low interfacial resistance at low temperatures, thus preventing direct contact between the two and the graphite negative electrode. Simultaneously, in synergy with TMP, it forms a dense LiF inner layer and a flexible organic outer layer on the graphite surface, blocking DMF while providing flexible buffering and accommodating volume expansion. Without a fluorinated cyclic carbonate solvent, the negative electrode SEI is insufficient to prevent DMF permeation and decomposition, and the system's initial coulombic efficiency and cycle stability fail to meet practical requirements.

[0027] For example, the fluorocyclic carbonate solvent can be at least one of fluoroethylene carbonate (FEC) and difluoroethylene carbonate. However, considering film-forming effect, low-temperature performance and cost, fluoroethylene carbonate is the best choice.

[0028] As a preferred embodiment of the present invention, the fluorocyclic carbonate solvent is selected from fluoroethylene carbonate.

[0029] Furthermore, the amount of fluorinated cyclic carbonate solvent in the solvent is preferably limited. Too much fluorinated cyclic carbonate solvent will affect the viscosity of the system, while too little will result in insufficient SEI to protect the DMF.

[0030] As a preferred embodiment of the present invention, the volume concentration of the fluorinated cyclic carbonate solvent in the solvent is 3% to 8%; for example, it can be 3%, 4%, 5%, 6%, 7%, or 8%.

[0031] In some preferred embodiments, the volume ratio of the amide-based aprotic polar solvent, the flame-retardant phosphate solvent, the fluorocarboxylic acid alkyl ester solvent, and the fluorocyclic carbonate solvent is (20-40):(20-45):(15-40):(2-10); preferably (25-35):(30-40):(25-35):(3-8); and preferably (28-32):(33-37):(28-32):(4-6).

[0032] In some preferred embodiments, the solvent is composed of N,N-dimethylformamide, trimethyl phosphate, ethyl trifluoroacetate, and fluoroethylene carbonate, forming a synergistic system with low melting point, low viscosity, and high dissociation.

[0033] lithium salts

[0034] Lithium salts are lithium ion providers, and high concentrations of lithium salts help improve ion transport and interfacial stability at low temperatures. Preferably, the total concentration of the lithium salt in the electrolyte is 1.5–3.5 mol / L. Examples include 1.5 mol / L, 1.75 mol / L, 2 mol / L, 2.25 mol / L, 2.5 mol / L, 2.75 mol / L, 3 mol / L, 3.25 mol / L, and 3.5 mol / L.

[0035] The choice of lithium salt in the solvents for the aforementioned quaternary blends is limited, including:

[0036] Sulfonylimide lithium salts As the main salt, in the aforementioned weakly solvated, high dielectric constant solvent systems, sulfonylimide lithium salts can still pass through the S=O group and N... - By highly delocalizing the negative charge, an extremely high effective carrier concentration is provided at room temperature.

[0037] In principle, lithium sulfonylimide salts can be selected from at least one of lithium bisfluorosulfonylimide (LiFSI) and lithium bistrifluoromethanesulfonylimide. However, due to the slightly low conductivity of lithium bistrifluoromethanesulfonylimide ions and its corrosive effect on Al current collectors, although its conductivity is still quite good, it is not suitable for high-voltage battery environments.

[0038] As a preferred embodiment of the present invention, the sulfonylimide lithium salt is selected from lithium bisfluorosulfonylimide.

[0039] Inorganic fluoroborate lithium salts As a co-salt, via BF4 - It attracts and polarizes surrounding solvent molecules, modulates the composition and binding strength of the solvated sheath, and improves the low-temperature desolvation kinetics of sulfonylimide lithium salts.

[0040] As a preferred embodiment of the present invention, the inorganic fluoroborate lithium salt is lithium tetrafluoroborate (LiBF4).

[0041] Furthermore, the amounts of sulfonylimide lithium salts and inorganic fluoroboronic acid lithium salts are preferably limited, as excessive use of inorganic fluoroboronic acid lithium salts can easily lead to the formation of BF4. - The dominant tight ion pairs cause the desolvation energy barrier to rise instead of fall; too few ions also cannot effectively regulate the solvation sheath.

[0042] As a preferred embodiment of the present invention, the molar ratio of the sulfonylimide lithium salt to the inorganic fluoroboronic acid lithium salt is (2-6):1. Examples include 2:1, 3:1, 4:1, 5:1, and 6:1.

[0043] In some preferred embodiments, the lithium salt is composed of lithium bis(fluorosulfonyl)imide and lithium tetrafluoroborate.

[0044] additive

[0045] The additive is used to optimize the electrode interface and ensure stable battery cycling. Preferably, the additive is 0.5% to 3% of the total electrolyte mass. Examples include 0.5%, 0.75%, 1%, 1.25%, 1.5%, 1.75%, 2%, 2.25%, 2.5%, 2.75%, and 3%.

[0046] The additives that can be used in the above-mentioned quaternary blend solvents and binary blend lithium salts are limited, among which:

[0047] Phosphorus and fluorine-containing inorganic lithium salt additives It is used to further preferentially reduce and decompose TMP to produce a dense, low-impedance SEI rich in LiF, and together with FEC and LiBF4, it can be used to construct a low-temperature SEI protection.

[0048] For example, the phosphorus- and fluorine-containing inorganic lithium salt additives can be at least one of lithium difluorophosphate (LiPO2F2), lithium difluorodioxanol phosphate, and lithium tetrafluorooxanol phosphate. Among them, lithium difluorodioxanol phosphate has a slightly higher film-forming impedance, which affects the low-temperature conductivity; lithium tetrafluorooxanol phosphate has a relatively high fluorine content, which can easily lead to an excessively thick SEI film and increase charge transfer impedance.

[0049] As a preferred embodiment of the present invention, the phosphorus- and fluorine-containing inorganic lithium salt additive is lithium difluorophosphate.

[0050] Cyclic sulfite additives Used to enhance film formation and suppress battery low-temperature gas expansion (TMP and DMF may undergo trace amounts of uncontrollable decomposition during repeated charging and discharging, generating trace amounts of gas that cause battery expansion).

[0051] In principle, cyclic sulfites can be selected from at least one of vinyl sulfite (ES) and propylene sulfite. However, because propylene sulfite has a methyl side chain, it can easily lead to reduced film-forming efficiency and a loose film layer. While its short-term cycling performance at low temperatures is acceptable, it provides insufficient long-term protection for DMF and exhibits significant long-term cycling degradation.

[0052] As a preferred embodiment of the present invention, the cyclic sulfite additive is selected from vinyl sulfite.

[0053] Phosphorus- and fluorine-containing inorganic lithium salt additives and cyclic sulfite additives are mainly used for anode interface stabilization. Since the main risk of DMF (disulfide-induced metallization) lies at the anode, their dosage is relatively high. Preferably, the total mass of the phosphorus- and fluorine-containing inorganic lithium salt additives and cyclic sulfite additives is 60% to 80% of the additive mass. For example, it can be 60%, 65%, 70%, 75%, or 80%.

[0054] The preferred ratio of phosphorus- and fluorine-containing inorganic lithium salt additives to cyclic sulfite additives is limited. Insufficient phosphorus- and fluorine-containing inorganic lithium salt additives can easily lead to insufficient inorganic LiF, increasing the low-temperature impedance of the SEI. Preferably, the mass ratio of the phosphorus- and fluorine-containing inorganic lithium salt additives to the cyclic sulfite additives is (1.2–1.8):1, for example, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, or 1.8:1.

[0055] Additives containing silicon phosphate It removes HF by reacting with HF through Si-O bonds and forms a film on the positive electrode through oxidation, thereby modifying the positive electrode interface, inhibiting the dissolution of transition metals, and stabilizing the positive electrode interface.

[0056] As a preferred embodiment of the present invention, the silicon-containing phosphate additive is selected from tris(trimethylsilyl) phosphate (TMSP).

[0057] dinitrile compounds It is used to form stable complexes with transition metal ions in the electrolyte, preventing metal dissolution and subsequent high-voltage electrolyte decomposition.

[0058] For example, the dinitrile compound can be at least one of adiponitrile (ADN) and glutaronitrile (GN). Among them, the oxidative stability of the GN molecular skeleton is weaker than that of ADN. GN may undergo uncontrolled oxidative polymerization at a high voltage of 4.7 V, while ADN has better antioxidant capacity.

[0059] As a preferred embodiment of the present invention, the dinitrile compound is selected from adiponitrile.

[0060] The silica-containing phosphate additives and dinitrile compounds constitute a complete interface protection, mainly used for positive electrode stabilization. Preferably, the total mass of the silica-containing phosphate additives and dinitrile compounds is 20% to 40% of the additive mass. For example, it can be 20%, 25%, 30%, 35%, or 40%.

[0061] The amount of silicon phosphate additives and dinitrile compounds is preferably limited, and the proportion of dinitrile compounds should not be too high, as excessive nitrile compounds may affect the SEI of the negative electrode. Preferably, the mass ratio of the silicon phosphate additives to the dinitrile compounds is (1.2–1.8):1, and for example, it can be 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, or 1.8:1.

[0062] In some preferred embodiments, the mass ratio of the phosphorus- and fluorine-containing inorganic lithium salt additives, cyclic sulfite additives, silicon-containing phosphate additives, and dinitrile compounds is (3-10):(2-8):(1-6):(0.5-5); preferably (5-8):(3-5):(2-4):(1-3).

[0063] In some preferred embodiments, the additive consists of lithium difluorophosphate, vinyl sulfite, tris(trimethylsilyl)phosphate, and adiponitrile.

[0064] In some preferred embodiments: the solvent is composed of N,N-dimethylformamide, trimethyl phosphate, ethyl trifluoroacetate, and fluoroethylene carbonate; the volume ratio of DMF, TMP, TFAc, and FEC is (20–40):(20–45):(15–40):(2–10); and the solvent accounts for 50%–80% of the total mass of the electrolyte. The lithium salt is composed of lithium difluorosulfonylimide and lithium tetrafluoroborate; the molar ratio of LiFSI to LiBF4 is (2–6):1; and the total concentration of the lithium salt in the electrolyte is 1.5–3.5 mol / L. The additive is composed of lithium difluorophosphate, vinyl sulfite, tris(trimethylsilyl)phosphate, and adiponitrile. The mass ratio of LiPO2F2, ES, TMSP, and ADN is (3–10):(2–8):(1–6):(0.5–5); and the additive accounts for 0.5%–3.0% of the total mass of the electrolyte.

[0065] Secondly, another objective of this invention is to provide a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the electrolyte system described above.

[0066] As a preferred embodiment of the present invention, the negative electrode is a graphite negative electrode.

[0067] As a preferred embodiment of the present invention, the cathode is at least one of a ternary cathode and a lithium iron phosphate cathode.

[0068] The beneficial effects of this invention are:

[0069] 1. This invention provides an electrolyte system that can achieve high conductivity at both room temperature and low temperature. In some embodiments, the ionic conductivity can reach 9.2 mS / cm at 25°C and ≥ 1.25 mS / cm at -40°C, with an applicable voltage window of 3.0–4.7 V.

[0070] 2. This invention provides a lithium-ion battery using this electrolyte system, suitable for graphite anodes. In some embodiments, the capacity retention rate is ≥ 78% after 100 cycles at -40℃, ≥ 62% after 200 cycles, and ≥ 88% after 100 cycles at -20℃; it can operate stably in a wide temperature range of -40 to 60℃. Detailed Implementation

[0071] The following are specific embodiments of the present invention, and the technical solutions of the present invention will be further described, but the present invention is not limited to these embodiments.

[0072] The detection methods involved in the examples and comparative examples are as follows:

[0073] Conductivity: Electron impedance spectroscopy (EIS) was used. Impedance was measured at 25°C using a 0.1 mol / L standard KCl solution, and the cell constant K was calculated. Electrolyte was injected into a clean, dry conductivity electrode cell in a glove box (H₂O < 0.1 ppm, O₂ < 0.1 ppm), ensuring complete immersion of the electrode, and then sealed. The sealed cell was placed in a high-low temperature chamber and held at 25°C, 0°C, -20°C, and -40°C for 2 hours each. Electron impedance spectroscopy was performed at each temperature, with a frequency range of 1 MHz to 100 Hz and a perturbation amplitude of ±10 mV. The real intercept of the high-frequency region of the Nyquist plot was taken as the bulk resistance R. b Calculate the conductivity: σ = K / R b .

[0074] Voltage window: Assembled as a Li / NCM coin cell, scanned from open circuit potential towards anode to 5.0 V (vs. Li / Li + The scan rate was 0.1 mV / s. Current-voltage curves were recorded, and the potential at which the current density began to rise sharply was taken as the oxidation decomposition potential. The cells were assembled into Li / graphite coin cells, and the scan was performed from the OCV towards the cathode to 0 V (vs. Li / Li). + ), and record the restoration current.

[0075] Low-temperature cycling: Assembled as CR2032 coin cells with 100 μL electrolyte. Charged at 25°C at 0.1C constant current to 4.2 V (NCM) or 3.65 V (LFP), then charged at constant voltage until current < 0.05C. Discharged at 0.1C to 3.0 V (NCM) or 2.5 V (LFP). Repeated 2–3 times, with the last cycle used as the initial capacity C0.

[0076] Set the temperature of the high and low temperature test chamber. Inside the high and low temperature test chamber, first let it stand for more than 4 hours, then charge it at 0.2C to 4.2 V (NCM) or 3.65 V (LFP) with a cutoff current of 0.05C; then discharge it at 0.2C to 3.0 V (NCM) or 2.5 V (LFP); cycle 100 or 200 times; calculate the capacity retention rate.

[0077] Example 1

[0078] A lithium-ion battery electrolyte system that balances high conductivity at both room temperature and extremely low temperatures includes a solvent, lithium salt, and additives.

[0079] The solvent is a mixture of N,N-dimethylformamide, trimethyl phosphate, ethyl trifluoroacetate, and fluoroethylene carbonate in a volume ratio of 30:35:30:5, and the solvent accounts for 63% of the total mass of the electrolyte.

[0080] The lithium salt is composed of lithium bis(fluorosulfonyl)imide and lithium tetrafluoroborate in a molar ratio of 4:1, and the total concentration of the lithium salt in the electrolyte is 2.5 mol / L.

[0081] The additive is composed of lithium difluorophosphate, vinyl sulfite, tris(trimethylsilyl)phosphate, and adiponitrile in a mass ratio of 6:4:3:2, and the additive accounts for 1.5% of the total mass of the electrolyte.

[0082] A lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0083] The negative electrode is a commercial graphite negative electrode sheet, and the positive electrode is a commercial NCM positive electrode sheet, with an areal capacity of approximately 2 mAh / cm². 2 The N / P ratio is 1.2; the diaphragm is a PP / PE diaphragm with a thickness of 25 μm; the electrolyte is the above-mentioned electrolyte system.

[0084] Example 2

[0085] This embodiment is basically the same as Embodiment 1, except that the positive electrode of the battery is different.

[0086] Specifically: the negative electrode is a commercial graphite negative electrode sheet, and the positive electrode is a commercial LiFePO4 positive electrode sheet, with an area capacity of 2 mAh / cm². 2The N / P ratio is 1.2; the diaphragm is a PP / PE diaphragm with a thickness of 25 μm; the electrolyte is the electrolyte system obtained in Example 1.

[0087] Example 3

[0088] This embodiment is basically the same as Example 1, except that N,N-dimethylformamide in the electrolyte solvent is replaced with an equal amount of N,N-diethylformamide.

[0089] Example 4

[0090] This embodiment is basically the same as Embodiment 1, except that the trimethyl phosphate in the electrolyte solvent is replaced with an equal amount of triethyl phosphate.

[0091] Example 5

[0092] This embodiment is basically the same as Example 1, except that the ethyl trifluoroacetate in the electrolyte solvent is replaced with an equal amount of methyl trifluoroacetate.

[0093] Example 6

[0094] This embodiment is basically the same as Embodiment 1, except that lithium difluorophosphate in the electrolyte additive is replaced with an equal amount of lithium difluorodioxarate phosphate.

[0095] Example 7

[0096] This embodiment is basically the same as Embodiment 1, except that adiponitrile in the electrolyte additive is replaced with an equal amount of glutaronitrile.

[0097] Example 8

[0098] This embodiment is basically the same as Example 1, except that the proportions of the components in the electrolyte solvent are different. Specifically, the solvent is a mixture of N,N-dimethylformamide, trimethyl phosphate, ethyl trifluoroacetate, and fluoroethylene carbonate in a volume ratio of 29:37:29:5.

[0099] Example 9

[0100] This embodiment is basically the same as Example 1, except that the proportions of the components in the electrolyte solvent are different. Specifically, the solvent is a mixture of N,N-dimethylformamide, trimethyl phosphate, ethyl trifluoroacetate, and fluoroethylene carbonate in a volume ratio of 25:29:41:5.

[0101] Example 10

[0102] This embodiment is basically the same as Example 1, except that the proportions of the components in the electrolyte solvent are different. Specifically, the solvent is a mixture of N,N-dimethylformamide, trimethyl phosphate, ethyl trifluoroacetate, and fluoroethylene carbonate in a volume ratio of 35:41:19:5.

[0103] Comparative Example 1

[0104] This comparative example is basically the same as Example 1, except that the electrolyte used is a commercial carbonate electrolyte.

[0105] Comparative Example 2-1

[0106] This comparative example is basically the same as Example 1, except that the electrolyte solvent does not contain N,N-dimethylformamide. Specifically, the solvent is a mixture of trimethyl phosphate, ethyl trifluoroacetate, and fluoroethylene carbonate in a volume ratio of 51:44:5.

[0107] Comparative Example 2-2

[0108] This comparative example is basically the same as Example 1, except that N,N-dimethylformamide in the electrolyte solvent is replaced with an equal amount of propylene carbonate.

[0109] Comparative Examples 2-3

[0110] This comparative example is basically the same as Example 1, except that N,N-dimethylformamide in the electrolyte solvent is replaced with an equal amount of acetonitrile.

[0111] Comparative Example 3

[0112] This comparative example is basically the same as Example 1, except that the electrolyte solvent does not contain trimethyl phosphate. Specifically, the solvent is a mixture of N,N-dimethylformamide, ethyl trifluoroacetate, and fluoroethylene carbonate in a volume ratio of 47.5:47.5:5.

[0113] Comparative Example 4

[0114] This comparative example is basically the same as Example 1, except that the electrolyte solvent does not contain ethyl trifluoroacetate. Specifically, the solvent is a mixture of N,N-dimethylformamide, trimethyl phosphate, and fluoroethylene carbonate in a volume ratio of 44:51:5.

[0115] Comparative Example 5

[0116] This comparative example is essentially the same as Example 1, except that the electrolyte solvent does not contain fluoroethylene carbonate. Specifically, the solvent is a mixture of N,N-dimethylformamide, trimethyl phosphate, and ethyl trifluoroacetate in a volume ratio of 31.6:36.8:31.6.

[0117] Comparative Example 6

[0118] This comparative example is basically the same as Example 1, except that the electrolyte lithium salt does not contain lithium tetrafluoroborate. Specifically, the lithium salt is lithium bis(fluorosulfonyl)imide, and the total concentration of lithium salt in the electrolyte is 2.5 mol / L.

[0119] Comparative Example 7-1

[0120] This comparative example is basically the same as Example 1, except that lithium difluorosulfonamide in the electrolyte lithium salt is replaced with an equal amount of lithium hexafluorophosphate.

[0121] Comparative Example 7-2

[0122] This embodiment is basically the same as Embodiment 1, except that lithium bisfluorosulfonylimide in the electrolyte lithium salt is replaced with an equal amount of lithium bistrifluoromethylsulfonylimide.

[0123] Comparative Example 8-1

[0124] This embodiment is basically the same as Embodiment 1, except that the vinyl sulfite in the electrolyte additive is replaced with an equal amount of propylene sulfite.

[0125] Comparative Example 8-2

[0126] This comparative example is basically the same as Example 1, except that the vinyl sulfite in the electrolyte additive is replaced with an equal amount of 1,3-propanesulfonyl lactone.

[0127] Comparative Example 9-1

[0128] This comparative example is basically the same as Example 1, except that the tris(trimethylsilyl)phosphate in the electrolyte additive is replaced with an equal amount of tris(trimethylsilyl)boronic acid ester.

[0129] Comparative Example 9-2

[0130] This comparative example is basically the same as Example 1, except that the tris(trimethylsilyl)phosphate in the electrolyte additive is replaced with an equal amount of tris(trimethylsilyl)phosphite.

[0131] The results of the examples and comparative examples are shown in Table 1 below.

[0132] Table 1.

[0133] experimental group change Conductivity at 25°C (mS / cm) Conductivity at -40°C (mS / cm) Voltage window (V) Circulate at -40°C for 100 cycles -40°C, 200 cycles Example 1 Graphite / NCM 9.20 1.25 3.0～4.7 78% 62% Example 2 Graphite / LFP 9.20 1.25 3.0～3.65 82% 72% Example 3 DMF→DEF 8.55 1.17 3.0～4.7 72% 52% Example 4 TMP→TEP 9.02 1.05 3.0～4.7 70% 48% Example 5 TFAc→TFAm 8.84 1.22 3.0～4.7 74% 55% Example 6 <![CDATA[LiPO2F2→LiDFOP]]> 9.10 1.13 3.0～4.7 72% 52% Example 7 ADN→GN 9.20 1.25 3.0～4.5 70% 48% Example 8 29:37:29:5 9.20 1.23 3.0～4.7 77% 61% Example 9 25:29:41:5 8.81 1.29 3.0～4.7 75% 60% Example 10 35:41:19:5 9.14 1.05 3.0～4.7 69% 42% Comparative Example 1 Commercial carbonate electrolyte 8.50 0.30 3.0～4.5 30% <15% Comparative Example 2-1 No DMF 8.03 0.45 3.0～4.7 35% <15% Comparative Example 2-2 DMF→PC 8.82 0.60 3.0～4.6 40% <20% Comparative Examples 2-3 DMF → Acetonitrile 7.55 0.58 3.0～4.5 35% <15% Comparative Example 3 No TMP 9.01 1.11 3.0～4.7 — — Comparative Example 4 No TFAc 9.03 0.55 3.0～4.7 42% <20% Comparative Example 5 No FEC 9.10 1.17 3.0～4.7 <50% <25% Comparative Example 6 <![CDATA[Without LiBF4]]> 9.20 1.09 3.0～4.7 65% 38% Comparative Example 7-1 <![CDATA[LiFSI→LiPF6]]> 7.00 0.42 3.0～4.5 25% <10% Comparative Example 7-2 LiFSI→LiTFSI 8.82 1.07 3.0～3.8 — — Comparative Example 8-1 ES → Propylene sulfite 8.96 1.20 3.0～4.7 68% 40% Comparative Example 8-2 ES→PS 9.12 1.25 3.0～4.7 69% 50% Comparative Example 9-1 TMSP→TMSB 9.20 1.24 3.0～4.7 70% 42% Comparative Example 9-2 TMSP→TMSPi 9.20 1.25 3.0～4.7 73% 50%

[0134] As shown in Table 1, the electrolyte system of the present invention can be applied to graphite anode lithium-ion battery systems, achieving the effect of balancing high ionic conductivity at room temperature (9.2 mS / cm at 25℃) and high ionic conductivity at extremely low temperature (1.25 mS / cm at -40℃). It also has a wide voltage window of 3.0 to 4.7 V and excellent low-temperature long-cycle stability (62% capacity retention after 200 cycles at -40℃ 0.2C / 0.2C).

[0135] As can be seen from the comparison of Example 1 and the comparative example, each component in the electrolyte system of the present invention is indispensable:

[0136] In Comparative Examples 2-1, 2-2, and 2-3, the removal of DMF resulted in the loss of strongly dissociating components in the solvent, leading to a significant deficiency in carrier concentration. The conductivity at -40°C plummeted from 1.25 mS / cm to 0.45 mS / cm. Comparative Examples 2-2 and 2-3 further demonstrate that even when DMF was replaced with propylene carbonate (PC) or acetonitrile, both possessing high dielectric constants, the conductivity at -40°C was only 0.60 and 0.58 mS / cm, respectively. PC suffers from high viscosity and co-intercalation with the graphite anode, while acetonitrile exhibits insufficient lithium salt dissociation due to its low donor number. This indicates that a high dielectric constant alone is insufficient to replace DMF; the appropriate number of donors provided by the amide groups is crucial for achieving adequate dissociation.

[0137] In Comparative Example 3, the lack of TMP to protect the DMF led to severe DMF decomposition, preventing the completion of 100 cycles. In Comparative Example 4, the lack of TFAc to reduce system viscosity resulted in high lithium-ion migration resistance at low temperatures, affecting low-temperature performance. In Comparative Example 5, the lack of FEC to form a dense SEI film led to slow DMF permeation and decomposition, resulting in a significant decrease in cycle performance. In Comparative Example 6, the lack of LiBF4 led to increased interfacial impedance and significant low-temperature capacity decay.

[0138] In Comparative Examples 7-1 and 7-2, the non-sulfonamide lithium salt LiPF6 is unstable in DMF and has poor dissociation at low temperatures; although LiTFSI is also a sulfonamide lithium salt and can provide appropriate conductivity, it corrodes the aluminum current collector at >3.8 V, and the battery fails rapidly under high voltage charging conditions and cannot cycle normally.

[0139] Since the additives account for only 1.5 wt% of the electrolyte, replacing one additive alone has little impact on the bulk ionic conductivity. Adjustments to other additives mainly affect the quality of the SEI film, which in turn affects the battery's cycle performance. In Comparative Examples 8-1 and 8-2, the non-sulfite additive PS exhibits high film brittleness; while the steric hindrance of the methyl groups on the propylene sulfite side easily leads to a loose SEI, resulting in insufficient long-term protection of the DMF. In Comparative Examples 9-1 and 9-2, TMSB hydrolyzes rapidly; and P(III) is prone to uncontrollable reduction at the negative electrode, interfering with the SEI; all of these factors contribute to a decrease in long-term cycle capacity at low temperatures.

[0140] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A lithium-ion battery electrolyte system that balances high conductivity at both room temperature and extremely low temperatures, comprising a solvent, a lithium salt, and additives, characterized in that: The solvent is composed of an amide-based aprotic polar solvent with a melting point ≤ -50℃ and a dielectric constant ≥ 25 at 25℃, a flame-retardant phosphate solvent with a viscosity ≤ 3.5 mPa·s at 25℃, a fluorocarboxylic acid alkyl ester solvent with a melting point ≤ -70℃ and a viscosity ≤ 1.0 mPa·s at 25℃, and a fluorocyclic carbonate solvent. The lithium salt is composed of sulfonylimide lithium salt and inorganic fluoroboronic acid lithium salt; The additives consist of phosphorus- and fluorine-containing inorganic lithium salts, cyclic sulfite additives, silicon-containing phosphate additives, and dinitrile compounds.

2. The lithium-ion battery electrolyte system according to claim 1, which combines high conductivity at both room temperature and extremely low temperatures, is characterized in that: The solvent is composed of N,N-dimethylformamide, trimethyl phosphate, ethyl trifluoroacetate, and fluoroethylene carbonate; The lithium salt is composed of lithium bis(fluorosulfonyl)imide and lithium tetrafluoroborate; The additive consists of lithium difluorophosphate, vinyl sulfite, tris(trimethylsilyl)phosphate, and adiponitrile.

3. A lithium-ion battery electrolyte system according to claim 1 or 2, characterized in that: The volume ratio of the amide-based aprotic polar solvent, flame-retardant phosphate solvent, fluorocarboxylic acid alkyl ester solvent, and fluorocyclic carbonate solvent is (20-40):(20-45):(15-40):(2-10).

4. A lithium-ion battery electrolyte system according to claim 1 or 2, characterized in that: The solvent accounts for 50% to 80% of the total mass of the electrolyte.

5. A lithium-ion battery electrolyte system according to claim 1 or 2, characterized in that: The molar ratio of the sulfonylimide lithium salt to the inorganic fluoroboronic acid lithium salt is (2-6):

1.

6. A lithium-ion battery electrolyte system according to claim 1 or 2, characterized in that: The total concentration of the lithium salt in the electrolyte is 1.5–3.5 mol / L.

7. A lithium-ion battery electrolyte system according to claim 1 or 2, characterized in that: The mass ratio of the phosphorus- and fluorine-containing inorganic lithium salt additives, cyclic sulfite additives, silicon-containing phosphate additives, and dinitrile compounds is (3-10):(2-8):(1-6):(0.5-5).

8. A lithium-ion battery electrolyte system according to claim 1 or 2, characterized in that: The additive is 0.5% to 3% of the total mass of the electrolyte.

9. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that: The electrolyte is the electrolyte system as described in any one of claims 1 to 8.

10. A lithium-ion battery according to claim 9, characterized in that: The negative electrode is a graphite negative electrode.