Lithium battery electrolyte

Abstract

A lithium battery electrolyte relates to the technical field of battery electrolytes, the electrolyte is composed of an organic solvent, a compound lithium salt, a magnesium-based bifunctional additive and fluoroethylene carbonate, the organic solvent is a compound system of cyclic carbonate and chain carbonate, the compound lithium salt can promote adsorption of the magnesium-based additive on the surface of a positive electrode, and the lithium salt can promote adsorption of the magnesium-based additive on the surface of the negative electrode. The magnesium-based bifunctional additive can chelate iron ions through amino alcohol ligand and release ions to be embedded into positive crystal lattices, fluoroethylene carbonate can form a compact interfacial film on a negative electrode, and the magnesium-based bifunctional additive and fluoroethylene carbonate form a positive-negative electrode double-interface protection system. The electrolyte can effectively inhibit the abnormal side reaction of the electrolyte, improve the cycling stability and high-temperature storage performance of the battery and prolong the service life of the battery.

Description

Technical Field

[0001] This invention belongs to the field of battery electrolyte technology, and specifically relates to a lithium battery electrolyte. Background Technology

[0002] Amid the rapid expansion of the global new energy industry, lithium iron phosphate batteries, with their outstanding advantages such as low cost, excellent thermal stability, and long cycle life, have become core energy storage devices in fields such as power batteries and large-scale energy storage systems, playing a crucial role in the electrification of transportation and the transformation of the energy structure. The intrinsically stable crystal structure of lithium iron phosphate, its cathode material, can adapt to large-scale, long-cycle energy storage demands, thus leading to a continuous increase in market application scale and consequently, increasingly stringent requirements for its long-term operational stability.

[0003] However, in actual use, especially under long-term charge-discharge cycles or high-temperature full-charge storage conditions, the cathode material of lithium iron phosphate batteries is prone to micro-distortions in its crystal structure, which in turn leads to the dissolution of iron, forming ferrous (Fe2+) and ferric (Fe3+) ions that enter the electrolyte system. Due to their electronegativity and coordination number characteristics, ferric ions catalyze specific side reactions in the electrolyte, not only intensifying free radical binding reactions but also accelerating the two-electron reduction process, generating a large amount of additional gas. Meanwhile, ferrous ions progressively exacerbate the conventional reduction reaction in the electrolyte. Under the combined effect of these two factors, gas gradually accumulates inside the battery, causing uneven electrolyte wetting and ultimately leading to the deactivation of local active materials. This results in a non-linear capacity decay of the battery, severely limiting its lifespan and safety.

[0004] To address the aforementioned issues, commonly used functional additives in existing electrolyte systems primarily focus on improving performance in a single dimension. While some additives can enhance the density of the negative electrode interfacial film and others can improve the electrolyte's resistance to hydrolysis, they cannot achieve targeted capture and catalytic pathway blocking of ferric ions. Furthermore, these additives struggle to simultaneously achieve synergistic protection against both positive electrode lattice structure stabilization and negative electrode active lithium loss, failing to fundamentally solve the performance degradation problem in lithium iron phosphate batteries caused by iron ion dissolution. These technological bottlenecks urgently require breakthroughs. Summary of the Invention

[0005] In order to overcome the above-mentioned defects of the prior art, the embodiments of the present invention provide a lithium battery electrolyte that solves the problems of the prior art, such as the aggravation of electrolyte side reactions, gas accumulation and nonlinear capacity decay caused by the dissolution of iron ions from the positive electrode, and the inability of conventional additives to achieve targeted chelation of iron ions and synergistic protection of the positive and negative electrodes.

[0006] To address the above problems, the present invention provides the following technical solution: A lithium battery electrolyte is composed of an organic solvent, a compounded lithium salt, a magnesium-based bifunctional additive, and fluoroethylene carbonate. The components, by mass percentage, are: organic solvent 75-88%, compounded lithium salt 10-20%, magnesium-based bifunctional additive 2-8%, and fluoroethylene carbonate 1-3%. The organic solvent is a compound system of cyclic carbonate and chain carbonate, and the mass ratio of cyclic carbonate to chain carbonate is 15-30:70-85. The compound lithium salt is a combination of lithium hexafluorophosphate and lithium difluorosulfonyl imide, and the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonyl imide is 85-95:5-15. The total molar concentration of the compound lithium salt in the electrolyte is 1.0-1.2M. The magnesium-based bifunctional additive is selected from one or more of magnesium acetate monoethanolamine, magnesium acetate diethanolamine, and magnesium acetate triethanolamine, and its amino alcohol ligand can chelate through the coordination site. And can release Embedded in the positive electrode lattice of lithium-ion batteries; The fluoroethylene carbonate can form a dense interfacial film at the negative electrode, which, together with the positive electrode protection effect of the magnesium-based bifunctional additive, constitutes a dual-interface protection system for both positive and negative electrodes.

[0007] Furthermore, the cyclic carbonate is selected from one or both of ethylene carbonate and propylene carbonate, and the mass percentage of the cyclic carbonate in the electrolyte is 12-25%. Here, the cyclic carbonate is the basic component for the formation of the negative electrode interface film. This percentage range can ensure the dielectric constant of the electrolyte while avoiding an increase in electrolyte viscosity due to an excessively high percentage, which would affect the ion migration efficiency. At the same time, the selection of ethylene carbonate or propylene carbonate is limited because both can provide a suitable chemical environment for the formation of the interface film.

[0008] Furthermore, when the cyclic carbonate is a compound system of ethylene carbonate and propylene carbonate, and the mass ratio of ethylene carbonate to propylene carbonate is 2:1, the total mass percentage of the two in the electrolyte is 18%. This specific compounding ratio can further optimize the dielectric constant and viscosity balance of the electrolyte. The total percentage of 18% can reduce the overall viscosity of the electrolyte while ensuring the formation of the negative electrode interface film, and the synergistic effect of the two cyclic carbonates can improve the low-temperature tolerance of the electrolyte.

[0009] Furthermore, the chain carbonate is a compound of dimethyl carbonate and ethyl methyl carbonate in a mass ratio of 3:2, and the chain carbonate accounts for 60-75% of the mass of the electrolyte. The chain carbonate here has the advantage of low viscosity. This specific compounding ratio can balance the dielectric constant and viscosity of the electrolyte, ensuring the ionic conductivity of the electrolyte under normal operating conditions, providing a stable channel for ion migration during battery charging and discharging, and avoiding the impact of ion transport obstruction on battery rate performance.

[0010] Furthermore, the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonyl imide in the compound lithium salt is 10:1, and the mass percentage of lithium difluorosulfonyl imide in the compound lithium salt is 5-15%. Lithium difluorosulfonyl imide can promote the adsorption of magnesium-based bifunctional additives on the surface of lithium iron phosphate cathode. This lithium salt ratio not only ensures the basic ionic conductivity of the electrolyte by relying on lithium hexafluorophosphate, but also enhances the electrolyte's resistance to hydrolysis through lithium difluorosulfonyl imide. Moreover, its synergistic adsorption effect with magnesium-based additives can strengthen the enrichment effect of magnesium-based additives at the cathode, improving the chelation efficiency of iron ions at the cathode.

[0011] Furthermore, when the magnesium-based bifunctional additive is a single magnesium acetate monoethanolamine, its mass percentage in the electrolyte is 0.8%-3%; when used in combination with other magnesium acetate-ethanolamine complexes, the mass percentage of magnesium acetate monoethanolamine is 0.3%-1.5%, and the monoethanolamine ligands it contains can achieve [the desired effect]. Coordination chelation. The dosage range here balances the chelation effect and electrolyte compatibility, ensuring the monoethanolamine ligand's ability to target and capture iron ions while avoiding the accumulation of impurities in the electrolyte system due to excessive addition. At the same time, the monodentate coordination form is suitable for cost-sensitive applications.

[0012] Furthermore, when the magnesium-based bifunctional additive is magnesium acetate diethanolamine alone, its mass percentage in the electrolyte is 0.8%-3%; when used in combination with other magnesium acetate-ethanolamine complexes, the mass percentage of magnesium acetate diethanolamine is 0.5%-2%, and the diethanolamine ligand it contains can achieve [the desired effect]. The diethanolamine ligand used here is in a bidentate coordination form, which chelates iron ions more efficiently than monoethanolamine ligands. This dosage range maximizes its chelating advantage while ensuring compatibility with other components of the electrolyte and avoiding adverse reactions between components.

[0013] Furthermore, when the magnesium-based bifunctional additive is a single magnesium acetate triethanolamine, its mass percentage in the electrolyte is 0.8%-3%; when used in combination with other magnesium acetate-ethanolamine complexes, the mass percentage of magnesium acetate triethanolamine is 0.2%-1.5%, and the triethanolamine ligand it contains can achieve [the desired effect]. The triethanolamine ligand here is in a multidentate coordination form and has excellent thermal stability. This dosage range allows it to maintain stable chelating ability under high-temperature conditions, making it suitable for high-temperature storage applications, while avoiding the impact of excessive addition on the electrolyte's ionic conductivity.

[0014] Furthermore, the mass ratio of the fluoroethylene carbonate to the magnesium-based bifunctional additive is 1.3-1.6:1. This ratio is the optimal ratio determined based on the protection requirements of the positive and negative electrodes of the electrolyte. It allows the film-forming effect of the fluoroethylene carbonate at the negative electrode interface and the positive electrode protection effect of the magnesium-based additive to form a precise synergy, achieving synchronous protection of the positive and negative electrode interfaces and avoiding battery performance degradation caused by insufficient protection of a single interface.

[0015] Furthermore, when the magnesium-based bifunctional additive is a ternary complex system of magnesium acetate monoethanolamine, magnesium acetate diethanolamine, and magnesium acetate triethanolamine, the mass percentages of magnesium acetate monoethanolamine, magnesium acetate diethanolamine, and magnesium acetate triethanolamine are 0.3%-1.5%, 0.5%-2%, and 0.2%-1.5%, respectively. These three complexes can synergistically achieve the desired effect through different coordination mechanisms. The chelation and stability of the cathode lattice are achieved. The ligands of the three complexes have different coordination forms. After compounding, they can achieve all-round chelation of iron ions. At the same time, the magnesium ions released by the different complexes can be embedded into the cathode lattice from multiple dimensions, which enhances the stability of the cathode structure. This dosage gradient can ensure the synergistic effect of the three components is maximized without destroying the stability of the electrolyte system.

[0016] The lithium battery electrolyte of this invention is designed with the core principle of constructing a synergistic protection system for both positive and negative electrodes. First, an organic solvent system combining cyclic and chain carbonates is used to build a basic electrolyte framework that balances dielectric environment and ion transport efficiency. Then, a lithium salt system combining lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide is introduced. This ensures the basic ionic conductivity of the electrolyte while leveraging lithium bis(fluorosulfonyl)imide to promote the adsorption and enrichment of core functional additives on the positive electrode surface. Next, a magnesium-based bifunctional additive containing amino alcohol ligands is configured, enabling it to target and chelate iron ions through coordination sites and release magnesium ions to embed into the positive electrode lattice, thus stabilizing the positive electrode crystal structure. Finally, fluoroethylene carbonate is added to form a dense interfacial film on the negative electrode surface, synergizing with the positive electrode protection effect of the magnesium-based bifunctional additive. This constructs a comprehensive protection system covering both positive and negative electrodes, fundamentally solving the performance degradation problem of lithium iron phosphate batteries.

[0017] Compared with the prior art, the advantages of the present invention are as follows: (1) The present invention can achieve targeted chelation of iron ions dissolved from the positive electrode and stabilize its crystal structure by embedding magnesium ions into the positive electrode lattice, thereby suppressing the side reactions of the electrolyte catalyzed by iron ions from the source, reducing the generation of abnormal gases inside the battery, and avoiding the problem of uneven electrolyte wetting caused by gas accumulation. (2) The present invention constructs a dual-interface synergistic protection system for positive and negative electrodes. Fluoroethylene carbonate can form a dense interfacial film on the negative electrode to reduce the loss of active lithium. Magnesium-based bifunctional additives can enhance the structural stability of the positive electrode, realize synchronous protection of the positive and negative electrode interfaces, and improve the overall reliability of the battery interface. (3) The compound lithium salt system of the present invention takes into account both the basic ionic conductivity and hydrolysis resistance of the electrolyte. At the same time, lithium difluorosulfonyl imide can promote the adsorption and enrichment of magnesium-based additives on the positive electrode surface, enhance the efficiency of the core functional components, and ensure the balance of the electrochemical performance of the electrolyte. (4) The optimized organic solvent compound system of the present invention achieves a precise balance between dielectric constant and viscosity, which not only provides a suitable environment for the formation of negative electrode interface film, but also ensures the smoothness of ion migration channels, and at the same time broadens the applicable temperature range of the battery and enhances environmental adaptability. (5) The magnesium-based bifunctional additive of the present invention can flexibly select a single component or a multi-component compound system. The coordination forms of different ligands can adapt to the protection requirements of different application scenarios, improve the flexibility of electrolyte scenario adaptation, and meet the diverse battery usage conditions. (6) The present invention can effectively improve the nonlinear capacity decay problem during the long-term charge and discharge cycle of the battery, improve the structural and performance stability of the battery under high temperature storage conditions, extend the overall service life of the battery, and enhance its application value in the fields of energy storage and power. Attached Figure Description

[0018] Figure 1 This is a flowchart of a lithium battery electrolyte according to the present invention. Detailed Implementation

[0019] To facilitate understanding of the present invention, the invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0020] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0021] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0022] Example 1 A lithium battery electrolyte of the present invention comprises an organic solvent, a compounded lithium salt, a magnesium-based bifunctional additive, and fluoroethylene carbonate. The components, by mass percentage, are: 80% organic solvent, 15% compounded lithium salt, 3% magnesium-based bifunctional additive, and 2% fluoroethylene carbonate. The organic solvent is a compound system of cyclic carbonates and chain carbonates. The cyclic carbonate is ethylene carbonate, and the chain carbonate is a compound of dimethyl carbonate and ethyl methyl carbonate in a mass ratio of 3:2. The mass ratio of carbonates is 15:85, with ethylene carbonate accounting for 15% of the electrolyte and chain carbonates accounting for 65%. The compound lithium salt is a combination of lithium hexafluorophosphate and lithium bisfluorosulfonyl imide, with a mass ratio of 10:1. The total molar concentration of the compound lithium salt in the electrolyte is 1.1M, and lithium bisfluorosulfonyl imide accounts for 9.1% of the mass of the compound lithium salt. The magnesium-based bifunctional additive is magnesium acetate diethanolamine. The mass ratio of fluoroethylene carbonate to the magnesium-based bifunctional additive is 1.5:1.

[0023] refer to Figure 1 The method for preparing the lithium battery electrolyte in this embodiment includes the following steps: S1. In an argon atmosphere glove box, weigh out ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate according to the above proportions, place them in a dry reaction container, and stir for 30 minutes until they are mixed evenly to obtain an organic solvent system. S2. Add lithium hexafluorophosphate and lithium difluorosulfonyl imide in the corresponding proportion to the organic solvent system obtained in step S1, and stir at 300 r / min for 60 min until the lithium salt is completely dissolved. S3. Add magnesium acetate diethanolamine and fluoroethylene carbonate sequentially to the system in step S2, and stir continuously at a speed of 300 r / min for 120 min until the system is homogeneous and transparent to obtain the target electrolyte.

[0024] Example 2 The difference between this embodiment and Example 1 is that the cyclic carbonate in the organic solvent is a mixture of ethylene carbonate and propylene carbonate in a mass ratio of 2:1, and the total mass percentage of the two in the electrolyte is 18%. The chain carbonate accounts for 62% of the electrolyte, and the mass ratio of cyclic carbonate to chain carbonate is 18:82. The remaining components and preparation methods are completely consistent with Example 1.

[0025] Example 3 The difference between this embodiment and Example 1 is that the magnesium-based bifunctional additive is a ternary compound system of magnesium acetate monoethanolamine, magnesium acetate diethanolamine and magnesium acetate triethanolamine, wherein magnesium acetate monoethanolamine accounts for 0.5% of the electrolyte, magnesium acetate diethanolamine accounts for 1.5% of the electrolyte, and magnesium acetate triethanolamine accounts for 1.0% of the electrolyte, with a total mass ratio of 3%. The remaining components and preparation methods are completely consistent with Example 1.

[0026] Example 4 The difference between this embodiment and Example 1 is that the mass ratio of fluoroethylene carbonate to magnesium-based bifunctional additive is 1.4:1, while the remaining components and preparation methods are completely consistent with Example 1.

[0027] Comparative Example 1 The difference between this comparative example and Example 1 is that the magnesium-based bifunctional additive is removed and the mass ratio of the organic solvent is adjusted to 83%. This change makes the system no longer meet the core limitation of the magnesium-based bifunctional additive in claim 1. The remaining components and preparation methods are completely consistent with Example 1.

[0028] Comparative Example 2 The difference between this comparative example and Example 1 is that the magnesium-based bifunctional additive, which accounts for 3% by mass, is replaced with an equal mass percentage of the conventional cathode additive, vinylene carbonate. Vinylene carbonate is a conventional cathode film-forming additive and does not have the bifunctional properties of the magnesium-based bifunctional additive. The remaining components and preparation methods are completely consistent with those of Example 1.

[0029] Comparative Example 3 The difference between this comparative example and Example 1 is that the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonyl imide in the compound lithium salt is adjusted to 99:1, and the mass ratio of lithium difluorosulfonyl imide in the compound lithium salt is 1%, which is lower than the lower limit of the 5-15% range defined in claim 5. The remaining components and preparation methods are completely consistent with those in Example 1.

[0030] Comparative Example 4 The difference between this comparative example and Example 1 is that the mass ratio of fluoroethylene carbonate to magnesium-based bifunctional additive is adjusted to 1:2, which exceeds the range of 1.3-1.6:1 defined in claim 9. The remaining components and preparation methods are completely consistent with those in Example 1.

[0031] Comparative Example 5 The difference between this comparative example and Example 1 is that fluoroethylene carbonate is removed and the mass percentage of the organic solvent is adjusted to 82%. This change makes the system no longer meet the core limitation of fluoroethylene carbonate in claim 1. The remaining components and preparation methods are completely consistent with Example 1.

[0032] The testing method is as follows: 3000-cycle capacity retention: Electrolyte injection for each case A graphite battery cell was charged to 3.65V at 1C current (constant voltage until current ≤0.05C) and discharged to 3.0V at 1C current in an environment of 25℃, and the cycle was repeated 3000 times. The percentage of the discharge capacity after the 3000th discharge cycle to the initial discharge capacity was calculated. 180-day storage capacity recovery rate: After charging the above cells to 3.65V at 1C (constant voltage until current ≤0.05C), they are sealed and stored in a 60℃ constant temperature chamber for 180 days. After storage, they are discharged to 3.0V at 25℃ at 1C and the capacity after storage is recorded. Then, they are charged and discharged once at 1C and the recovered capacity is recorded. The percentage of recovered capacity to initial capacity is calculated. Abnormal gas production (mL): Under 60℃ environment, the battery completes 200 charge-discharge cycles at 1C rate. The total volume of gas generated during the cycle is measured by a sealed gas collection device combined with the water displacement method. Positive electrode interface impedance (Ω) after 3000 cycles: After the battery completed 3000 1C charge-discharge cycles, it was disassembled, and the positive electrode sheet was used to prepare the working electrode. The positive electrode interface impedance value was obtained by AC impedance spectroscopy (EIS) using an electrochemical workstation. Initial capacity loss (%): Under 25℃ conditions, the battery is charged at a constant current and constant voltage of 1C to the rated voltage, and then discharged at a constant current of 1C to the cutoff voltage. The difference between the initial charge capacity and the initial discharge capacity is divided by the initial charge capacity and then multiplied by 100 to obtain the initial capacity loss.

[0033] Table 1: Experimental Results of Examples 1-4 and Comparative Examples 1-5

[0034] In summary, referring to Table 1, the embodiments, due to the adoption of a dual-interface protection system for positive and negative electrodes containing magnesium-based bifunctional additives, a reasonable lithium salt ratio, and an organic solvent compounding scheme, all exhibit significantly better performance indicators than the comparative examples that deviate from the core process characteristics of this invention.

[0035] Comparative Example 1 was unable to chelate due to the removal of the magnesium-based bifunctional additive. This leads to an exacerbation of side reactions in the catalytic electrolyte, a surge in abnormal gas production, and a lack of positive electrode lattice. The embedded stability and structural distortion resulted in a significant decrease in capacity retention after 3000 cycles and a reduced capacity recovery rate after 180 days. The positive electrode interface impedance increased significantly after 3000 cycles, and there was no synergistic effect between the negative electrode film and the positive electrode protection, leading to increased initial capacity loss. Comparative Example 2 used a conventional positive electrode additive, vinylene carbonate, instead of the magnesium-based bifunctional additive. This additive does not contain amino alcohol ligands. Coordination chelation ability can only slightly improve the film formation on the negative electrode and cannot inhibit it. The abnormal gas production caused by this was relatively high, resulting in a lower overall abnormal gas production. The capacity retention rate after 3000 cycles and the capacity recovery rate after 180 days of storage were lower than in the example. The decrease in cathode interface impedance after 3000 cycles was limited, and the initial capacity loss remained high. In Comparative Example 3, the proportion of lithium difluorosulfonylimide in the compounded lithium salt was below the specified range, making it difficult to promote the adsorption and enrichment of magnesium-based bifunctional additives on the cathode surface. Insufficient chelation efficiency resulted in a higher total abnormal gas production than in the example, lower capacity retention after 3000 cycles and lower capacity recovery after 180 days, higher positive electrode interface impedance after 3000 cycles, and slightly larger initial capacity loss. In Comparative Example 4, the mass ratio of fluoroethylene carbonate to magnesium-based bifunctional additive exceeded the specified range, leading to an imbalance in the synergistic protection of the positive and negative electrode interfaces. Insufficient density of the negative electrode film increased active lithium loss, resulting in a decrease in capacity retention after 3000 cycles and lower capacity recovery after 180 days, an increase in the total abnormal gas production, higher positive electrode interface impedance after 3000 cycles, and a larger initial capacity loss. In Comparative Example 5, the removal of fluoroethylene carbonate resulted in the absence of negative electrode interface protection, leading to significant active lithium loss and the inability to form synergistic protection with the magnesium-based bifunctional additive. The catalytic side reactions were not effectively suppressed, the total abnormal gas production was relatively large, the capacity retention rate after 3000 cycles and the capacity recovery rate after 180 days of storage were low, the positive electrode interface impedance was high after 3000 cycles, and the initial capacity loss was relatively large.

[0036] In Example 1, the core components form a basic synergistic effect, the magnesium-based bifunctional additive achieves iron ion chelation, and the appropriate lithium salt ratio ensures the basic stability of the electrolyte, thus obtaining balanced basic high performance data.

[0037] Example 2 uses a compound cyclic carbonate system with a mass ratio of ethylene carbonate to propylene carbonate of 2:1, which improves the overall thermal stability of the electrolyte without affecting the iron ion chelation ability. Therefore, the cycle capacity retention rate and thermal decomposition temperature are better than those of Example 1.

[0038] Example 3 uses a ternary compound magnesium-based bifunctional additive. The multi-coordination form achieves a stronger iron ion chelation effect, effectively inhibiting the corrosion of the electrode by iron ions. Therefore, the cycle capacity retention rate and iron ion chelation amount are the best among all examples.

[0039] Example 4: The fluoroethylene carbonate and magnesium-based bifunctional additive were mixed in a ratio of 1.4:1, which optimized the density of the negative electrode interface film and reduced interfacial side reactions. Therefore, the initial coulombic efficiency was the highest and the density level of the negative electrode interface film was the best.

Claims

1. A lithium battery electrolyte, characterized in that: It is composed of organic solvent, compound lithium salt, magnesium-based bifunctional additive and fluoroethylene carbonate, and the components by mass percentage are as follows: organic solvent 75-88%, compound lithium salt 10-20%, magnesium-based bifunctional additive 2-8%, fluoroethylene carbonate 1-3%; The organic solvent is a compound system of cyclic carbonate and chain carbonate, and the mass ratio of cyclic carbonate to chain carbonate is 15-30:70-85. The compound lithium salt is a combination of lithium hexafluorophosphate and lithium difluorosulfonyl imide, and the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonyl imide is 85-95:5-15. The total molar concentration of the compound lithium salt in the electrolyte is 1.0-1.2M. The magnesium-based bifunctional additive is selected from one or more of magnesium acetate monoethanolamine, magnesium acetate diethanolamine, and magnesium acetate triethanolamine, and its amino alcohol ligand can chelate through the coordination site. And can release Embedded in the positive electrode lattice of lithium-ion batteries; The fluoroethylene carbonate can form a dense interfacial film at the negative electrode, which, together with the positive electrode protection effect of the magnesium-based bifunctional additive, constitutes a dual-interface protection system for both positive and negative electrodes.

2. The lithium battery electrolyte according to claim 1, characterized in that: The cyclic carbonate is selected from one or two of ethylene carbonate and propylene carbonate, and the mass percentage of the cyclic carbonate in the electrolyte is 12-25%.

3. The lithium battery electrolyte according to claim 1, characterized in that: When the cyclic carbonate is a compound system of ethylene carbonate and propylene carbonate, and the mass ratio of ethylene carbonate to propylene carbonate is 2:1, the total mass percentage of the two in the electrolyte is 18%.

4. The lithium battery electrolyte according to claim 1, characterized in that: The chain carbonate is a compound of dimethyl carbonate and ethyl methyl carbonate in a mass ratio of 3:2, and the chain carbonate accounts for 60-75% of the mass of the electrolyte.

5. The lithium battery electrolyte according to claim 1, characterized in that: The mass ratio of lithium hexafluorophosphate to lithium difluorosulfonyl imide in the compound lithium salt is 10:1, and the mass percentage of lithium difluorosulfonyl imide in the compound lithium salt is 5-15%. Lithium difluorosulfonyl imide can promote the adsorption of magnesium-based bifunctional additives on the surface of lithium iron phosphate cathode.

6. The lithium battery electrolyte according to claim 1, characterized in that: When the magnesium-based bifunctional additive is magnesium acetate monoethanolamine alone, its mass percentage in the electrolyte is 0.8%-3%; when used in combination with other magnesium acetate-ethanolamine complexes, the mass percentage of magnesium acetate monoethanolamine is 0.3%-1.5%, and the monoethanolamine ligands it contains can achieve [the desired effect]. Coordination chelation.

7. The lithium battery electrolyte according to claim 1, characterized in that: When the magnesium-based bifunctional additive is magnesium acetate diethanolamine alone, its mass percentage in the electrolyte is 0.8%-3%; when used in combination with other magnesium acetate-ethanolamine complexes, the mass percentage of magnesium acetate diethanolamine is 0.5%-2%, and the diethanolamine ligands it contains can achieve [specific effects related to electrolyte composition]. Coordination chelation.

8. The lithium battery electrolyte according to claim 1, characterized in that: When the magnesium-based bifunctional additive is magnesium acetate triethanolamine alone, its mass percentage in the electrolyte is 0.8%-3%; when used in combination with other magnesium acetate-ethanolamine complexes, the mass percentage of magnesium acetate triethanolamine is 0.2%-1.5%, and the triethanolamine ligand it contains can achieve [the desired effect]. Coordination chelation.

9. The lithium battery electrolyte according to claim 1, characterized in that: The mass ratio of the fluoroethylene carbonate to the magnesium-based bifunctional additive is 1.3-1.6:

1.

10. The lithium battery electrolyte according to claim 1, characterized in that: When the magnesium-based bifunctional additive is a ternary complex system of magnesium acetate monoethanolamine, magnesium acetate diethanolamine, and magnesium acetate triethanolamine, the mass percentages of magnesium acetate monoethanolamine, magnesium acetate diethanolamine, and magnesium acetate triethanolamine are 0.3%-1.5%, 0.5%-2%, and 0.2%-1.5%, respectively. The three complexes can synergistically achieve the desired effect through different coordination mechanisms. The chelation and stability of the positive electrode lattice.

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