Non-aqueous electrolyte and lithium ion secondary battery thereof

Abstract

In order to overcome the existing technology of non-aqueous electrolytic lithium salt using lithium bis(fluorosulfonyl)imide, the battery stability, cycle performance and safety performance are deteriorated, the application provides a non-aqueous electrolyte and a lithium ion secondary battery, the non-aqueous electrolyte comprises electrolyte lithium salt, additive and organic solvent, wherein the electrolyte lithium salt comprises lithium hexafluorophosphate LiPF6 and lithium bis(fluorosulfonyl)imide, the additive comprises unsaturated phosphate ester compound, by adjusting the molar concentration Ca, Cb of lithium hexafluorophosphate LiPF6 and lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte and the mass percentage Mp of the unsaturated phosphate ester compound in the non-aqueous electrolyte, and satisfying the relationship formula, the non-aqueous electrolyte prepared can effectively inhibit the corrosion of lithium bis(fluorosulfonyl)imide to the battery electrode, and further improve the high-temperature cycle performance, initial capacity and safety performance of the lithium ion secondary battery.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, specifically relating to a non-aqueous electrolyte for improving battery safety performance and its lithium-ion secondary battery. Background Technology

[0002] Lithium-ion batteries are widely used in 3C digital products such as mobile phones and laptops, as well as in new energy vehicles, due to their advantages such as high operating voltage, wide operating temperature range, high energy density, no memory effect, and long cycle life. Ternary cathode oxide lithium-ion batteries, with their large reversible capacity and high energy density, are currently considered high-energy-density lithium rechargeable batteries with great potential.

[0003] As the nickel content of the cathode material increases in lithium-ion secondary batteries, their cycle stability deteriorates, and they are prone to safety issues such as thermal runaway. In existing lithium-ion secondary batteries, the lithium salt added to the non-aqueous electrolyte is lithium hexafluorophosphate (LiPF6). While LiPF6 can improve battery cycle performance, it is prone to decomposition during charging and discharging, reducing battery safety.

[0004] Compared to lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI) exhibits better thermal stability as a lithium salt, which can improve battery safety. However, when the molar ratio of lithium bis(fluorosulfonyl)imide (LiFSI) is large, it can corrode the electrode current collector during battery charging and discharging, causing side reactions and deteriorating the stability of lithium-ion secondary batteries, resulting in reduced cycle performance and safety. Conversely, when the molar ratio of lithium bis(fluorosulfonyl)imide (LiFSI) is small, the improvement in cycle performance and safety is not significant. Therefore, there is an urgent need to further develop lithium-ion secondary batteries that combine excellent cycle performance, corrosion resistance, and safety. Summary of the Invention

[0005] In order to address the problems in existing technologies where the lithium salt used in non-aqueous electrolysis is bis(fluorosulfonyl)imide lithium with a large molar ratio, the electrode current collector is corroded, which degrades the battery stability and reduces the battery cycle performance and safety performance, this invention provides a non-aqueous electrolyte and its lithium-ion secondary battery.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] On one hand, the present invention provides a non-aqueous electrolyte comprising a lithium salt, an additive, and an organic solvent, wherein the lithium salt comprises lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, and the additive comprises an unsaturated phosphate ester compound, and the non-aqueous electrolyte satisfies the following relationship:

[0008] ≥5

[0009] Wherein, Ca is the molar concentration of lithium hexafluorophosphate in the non-aqueous electrolyte, in mol / L;

[0010] Cb is the molar concentration of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte, in mol / L.

[0011] Mp is the mass percentage of the unsaturated phosphate ester compound in the total mass of the non-aqueous electrolyte, expressed in units of %.

[0012] Furthermore, the total molar amount of lithium salt in the non-aqueous electrolyte is 0.8~1.4M, and the Ca:Cb ratio is 9:1~4:6.

[0013] Furthermore, the Ca:Cb ratio is 9:1 to 5:5.

[0014] Furthermore, the unsaturated phosphate compound is the compound shown in structural formula 1:

[0015]

[0016] R1 is an alkynyl group with 2 to 5 carbon atoms, and R2 and R3 are each independently selected from a hydrocarbon group with 1 to 5 carbon atoms or a haloalkyl group with 1 to 5 carbon atoms; n1≥0, n2≥0, n3≥0, but n1, n2, and n3 are not all 0 at the same time.

[0017] Furthermore, the compound shown in structural formula 1 is selected from one or more of the following compounds:

[0018]

[0019]

[0020] Compound 5.

[0021] Furthermore, the unsaturated phosphate ester compound accounts for 0.001% to 10% of the total mass of the non-aqueous electrolyte by mass Mp.

[0022] Furthermore, the non-aqueous electrolyte also includes auxiliary additives, which include at least one of cyclic sulfate compounds, sulfonyl lactone compounds, cyclic carbonate compounds, and nitrile compounds;

[0023] Based on the total mass of the non-aqueous electrolyte as 100%, the mass percentage of the auxiliary additives is 0.01% to 30%.

[0024] Furthermore, the cyclic sulfate compounds are selected from at least one of vinyl sulfate, propylene sulfate, or methyl vinyl sulfate; the sulfonate lactone compounds include at least one of 1,3-propane sulfonate lactone, 1,4-butane sulfonate lactone, or 1,3-propene sulfonate lactone; the cyclic carbonate compounds include at least one of vinylene carbonate, ethylene ethylene carbonate, or fluoroethylene carbonate; and the nitrile compounds are selected from at least one of succinic anionyl nitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptanonitrile, octadionitrile, azelonitrile, sebaconitrile.

[0025] Furthermore, it includes a positive electrode, a negative electrode, a separator, and the aforementioned non-aqueous electrolyte.

[0026] Furthermore, the positive electrode includes a positive electrode material, which is LiNi. x Co y Mn z L (1-x-y-z) O2, wherein L is at least one of Al, Sr, Mg, Ti, Ca, Zr, Zn, Si, Cu, V or Fe, and 0≤x≤1, 0≤y≤1, 0≤z≤1, 0≤x+y+z≤1.

[0027] The beneficial effects of this invention are:

[0028] The non-aqueous electrolyte provided by this invention comprises an electrolyte lithium salt, additives, and an organic solvent. The electrolyte lithium salt includes lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide, and the additives include unsaturated phosphate compounds. The unsaturated phosphate compounds possess good flame-retardant properties and can form a stable passivation film on the surfaces of the positive and negative electrode layers, suppressing excessive side reactions at the positive electrode. By adjusting the molar concentrations Ca and Cb of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte, and the mass percentage Mp of the unsaturated phosphate compounds in the non-aqueous electrolyte, the desired effect can be achieved, satisfying the following relationship: The non-aqueous electrolyte prepared with ≥5 can effectively inhibit the corrosion of battery electrodes by lithium bis(fluorosulfonyl)imide, and further improve the high-temperature cycle performance, initial capacity utilization and safety performance of lithium-ion secondary batteries. Detailed Implementation

[0029] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0030] This invention provides a non-aqueous electrolyte comprising an electrolyte lithium salt, an additive, and an organic solvent. The electrolyte lithium salt comprises lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI). The additive comprises an unsaturated phosphate ester compound. The non-aqueous electrolyte satisfies the following relationship:

[0031] ≥5

[0032] Wherein, Ca is the molar concentration of lithium hexafluorophosphate in the non-aqueous electrolyte, in mol / L;

[0033] Cb is the molar concentration of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte, in mol / L.

[0034] Mp is the mass percentage of the unsaturated phosphate ester compound in the total mass of the non-aqueous electrolyte.

[0035] The non-aqueous electrolyte provided by this invention comprises an electrolyte lithium salt, additives, and an organic solvent. The electrolyte lithium salt includes lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide, and the additives include unsaturated phosphate compounds. The unsaturated phosphate compounds possess good flame-retardant properties and can form a stable passivation film on the surfaces of the positive and negative electrode layers, suppressing excessive side reactions at the positive electrode. By adjusting the molar concentrations Ca and Cb of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte, and the mass percentage Mp of the unsaturated phosphate compounds in the non-aqueous electrolyte, the desired effect can be achieved, satisfying the following relationship: The non-aqueous electrolyte prepared with ≥5 can effectively inhibit the corrosion of battery electrodes by lithium bis(fluorosulfonyl)imide, and further improve the high-temperature cycle performance, initial capacity utilization and safety performance of lithium-ion secondary batteries.

[0036] In some embodiments, the total molar amount of lithium salt in the non-aqueous electrolyte is 0.8~1.4M, and the Ca:Cb ratio is 9:1~4:6, that is, the molar ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide is 9:1~4:6.

[0037] The lithium salt content affects the viscosity of non-aqueous electrolytes and also the lithium-ion migration rate. In non-aqueous electrolytes, lithium salts contain lithium ions, which can replenish a small amount of consumed lithium ions during battery charging and discharging, promoting battery cycle performance. Increasing the lithium salt content can improve the lithium-ion migration rate, but excessive lithium salts lead to increased electrolyte viscosity, increased battery polarization, and reduced battery cycle performance. In non-aqueous electrolytes, excessive bis(fluorosulfonyl)imide lithium accelerates the corrosion of the positive electrode current collector. Through extensive experiments, the inventors discovered that limiting the total molar amount of lithium salt in the non-aqueous electrolyte to 0.8~1.4 mol, and the Ca:Cb ratio to 9:1~4:6 (i.e., the molar ratio of lithium hexafluorophosphate to the bis(fluorosulfonyl)imide lithium to 9:1~4:6), can reduce the corrosion of the positive electrode current collector by bis(fluorosulfonyl)imide lithium while simultaneously improving battery cycle performance.

[0038] More preferably, the Ca:Cb ratio is 9:1 to 5:5, that is, the molar ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide is 9:1 to 5:5.

[0039] In some embodiments, the unsaturated phosphate compound is the compound shown in structural formula 1:

[0040]

[0041] R1 is an alkynyl group with 2 to 5 carbon atoms, and R2 and R3 are each independently selected from a hydrocarbon group with 1 to 5 carbon atoms or a haloalkyl group with 1 to 5 carbon atoms; n1≥0, n2≥0, n3≥0, but n1, n2, and n3 are not all 0 at the same time.

[0042] R2 and R3 are selected from hydrocarbon groups or haloalkanes with 1 to 5 carbon atoms. The hydrocarbon group can be a hydrocarbon group containing a carbon-carbon double bond or an alkynyl group. The haloalkane group is a hydrocarbon group in which hydrogen atoms are replaced by halogen elements. The halogen elements can be selected from fluorine, chlorine, bromine, or iodine atoms, and the number of hydrogen atoms substituted is ≥1.

[0043] The mass percentage (Mp) of the unsaturated phosphate ester compound in the total mass of the non-aqueous electrolyte is 0.001 to 10%.

[0044] The inventors discovered that when the mass percentage of the unsaturated phosphate compound shown in Formula 1 in the electrolyte is less than 0.001%, its flame-retardant effect on the battery is not significant; when the mass percentage of the unsaturated phosphate compound shown in Formula 1 in the electrolyte is less than 10%, the battery impedance increases, and the irreversible capacity loss of the battery becomes greater. In non-aqueous electrolytes, a mass percentage of 0.001~10% of the unsaturated phosphate compound shown in Formula 1 can be used to adjust lithium bis(fluorosulfonyl)imide to reduce corrosion reactions and improve battery cycle performance and safety performance.

[0045] In some embodiments, the compound represented by structural formula 1 is selected from one or more of the following compounds:

[0046]

[0047]

[0048] Compound 5.

[0049] The unsaturated phosphate compound accounts for 1% to 8% of the total mass of the non-aqueous electrolyte.

[0050] It should be noted that the compounds shown in the above structural formula 1 are only a few examples of compound types. As long as they satisfy the structural formula shown in structural formula 1, they are all within the scope of unsaturated phosphate compounds described in this application.

[0051] The inventors discovered through research that when the mass percentage of the unsaturated phosphate compound shown in structural formula 1 in the electrolyte is between 1% and 8%, it has a significant flame-retardant effect on the battery, effectively modulates lithium bis(fluorosulfonyl)imide to reduce corrosion reaction, and improves battery cycle performance and safety performance.

[0052] In some embodiments, the non-aqueous electrolyte further includes auxiliary additives, which include at least one of cyclic sulfate compounds, sulfonyl lactone compounds, cyclic carbonate compounds, and nitrile compounds;

[0053] Based on the total mass of the non-aqueous electrolyte as 100%, the mass percentage content of the auxiliary additives is 0.01% to 30%.

[0054] In non-aqueous electrolytes, auxiliary additives can improve electrolyte conductivity, increase lithium-ion migration rate, and enhance battery electrochemical reaction rate. Simultaneously, these additives, in synergy with the unsaturated phosphate ester compound, lithium hexafluorophosphate, and lithium bis(fluorosulfonyl)imide shown in Formula 1, react to produce a more stable passivation film on the positive and negative electrode surfaces, improving electrolyte stability, inhibiting corrosion of the positive electrode current collector, and enhancing battery cycle performance, initial capacity utilization, and safety performance.

[0055] In non-aqueous electrolysis, adding a small amount of auxiliary additives can play a role. The amount of auxiliary additives added affects the conductivity of the electrolyte. Too much additive increases the cost of the electrolyte, while too little additive cannot play its role and cannot better improve the cycle performance and safety performance of the battery.

[0056] It should be noted that, unless otherwise specified, the amount of any one of the optional substances in the auxiliary additives added to the non-aqueous electrolyte is generally less than 10%, preferably 0.1-5%, and more preferably 0.1% to 2%. Specifically, the amount of any one of the optional substances in the auxiliary additives can be 0.05%, 0.08%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 7.8%, 8%, 8.5%, 9%, 9.5%, or 10%.

[0057] In some embodiments, when the auxiliary additive is selected from fluoroethylene carbonate, the amount of fluoroethylene carbonate added is 0.05% to 30% based on the total mass of the non-aqueous electrolyte as 100%.

[0058] In some embodiments, the cyclic sulfate compounds are selected from at least one of vinyl sulfate, propylene sulfate, or methyl vinyl sulfate; the sulfonate lactones include at least one of 1,3-propane sulfonate lactone, 1,4-butane sulfonate lactone, or 1,3-propene sulfonate lactone; the cyclic carbonate compounds include at least one of vinylene carbonate, ethylene ethylene carbonate, or fluoroethylene carbonate; and the nitrile compounds are selected from at least one of succinic anionyl nitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptacyanide, octanoic anionyl nitrile, azelaic anionyl nitrile, and sebacic anionyl nitrile.

[0059] In some embodiments, the organic solvent includes one or more of cyclic carbonate compounds, linear carbonate compounds, carboxylic acid ester compounds, and ether compounds.

[0060] In some embodiments, the cyclic carbonate may be, but is not limited to, one or more of ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), and butylene carbonate (BC); the chain carbonate may be, but is not limited to, one or more of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC). The content of the cyclic carbonate is not particularly limited and is arbitrary within a range that does not significantly impair the performance of the secondary battery of the present invention. However, when using only one type, its content is typically 3% or more, preferably 5% or more, by volume relative to the total amount of solvent in the non-aqueous electrolyte. By setting this range, a decrease in conductivity due to a decrease in the dielectric constant of the non-aqueous electrolyte can be avoided, making it easier to achieve good high-current discharge characteristics, stability relative to the negative electrode, and cycle characteristics of the non-aqueous electrolyte battery. Furthermore, the upper limit is typically 90% or less by volume, preferably 85% or less, and more preferably 80% or less by volume. By setting this range, the oxidation / reduction resistance of the non-aqueous electrolyte can be improved, thereby contributing to enhanced stability during high-temperature storage. The content of the chain carbonate is not particularly limited, but relative to the total amount of solvent in the non-aqueous electrolyte, it is typically 15% or more by volume, preferably 20% or more, and more preferably 25% or more. Furthermore, it is typically 90% or less by volume, preferably 85% or less, and more preferably 80% or less. By keeping the chain carbonate content within the above range, it is easier to achieve an appropriate viscosity for the non-aqueous electrolyte, suppressing the decrease in ionic conductivity, and thus contributing to achieving a good range of output characteristics for the non-aqueous electrolyte battery. When using two or more chain carbonates in combination, it is sufficient to ensure that the total amount of chain carbonate meets the above range.

[0061] In some embodiments, fluorine-containing chain carbonates (hereinafter referred to as "fluorinated chain carbonates") are also preferably used. There is no particular limitation on the number of fluorine atoms in a fluorinated chain carbonate as long as it is 1 or more, but it is generally 6 or less, preferably 4 or less. When a fluorinated chain carbonate has multiple fluorine atoms, these fluorine atoms can be bonded to the same carbon atom or to different carbon atoms. Examples of fluorinated chain carbonates include dimethyl fluorinated carbonate derivatives, methyl ethyl fluorinated carbonate derivatives, and diethyl fluorinated carbonate derivatives.

[0062] Carboxylic acid ester solvents include cyclic carboxylic acid esters and / or chain carbonates. Examples of cyclic carboxylic acid esters include one or more of γ-butyrolactone, γ-valerolactone, and δ-valerolactone. Examples of chain carbonates include one or more of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, propyl propionate (PP), and butyl propionate.

[0063] In some embodiments, the ether solvent includes cyclic ethers or chain ethers, preferably chain ethers with 3 to 10 carbon atoms and cyclic ethers with 3 to 6 carbon atoms. The cyclic ethers may specifically be, but are not limited to, one or more of 1,3-dioxolane (DOL), 1,4-dioxane (DX), crown ethers, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), and 2-trifluoromethyltetrahydrofuran (2-CF3-THF). The chain ethers may specifically be, but are not limited to, dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether. Because chain ethers have high solvation ability with lithium ions and can improve ion dissociation, dimethoxymethane, diethoxymethane, and ethoxymethoxymethane, which have low viscosity and can impart high ionic conductivity, are particularly preferred. Ether compounds can be used alone or in any combination and ratio of two or more. There are no particular restrictions on the amount of ether compounds added; it is arbitrary as long as it does not significantly impair the performance of the high-pressure lithium-ion battery of this invention. Typically, the volume ratio is 1% or more, preferably 2% or more, and more preferably 3% or more when the non-aqueous solvent volume ratio is 100%. Furthermore, the volume ratio is typically 30% or less, preferably 25% or less, and more preferably 20% or less. When using two or more ether compounds in combination, the total amount of ether compounds should meet the above-mentioned range. When the amount of ether compounds added is within the above-mentioned preferred range, it is easy to ensure the improved ionic conductivity resulting from the increased lithium-ion dissociation degree and reduced viscosity of the chain ethers. Additionally, when the negative electrode active material is a carbon material, the phenomenon of co-intercalation between the chain ethers and lithium ions can be suppressed, thus enabling the input / output characteristics and charge / discharge rate characteristics to reach an appropriate range.

[0064] Preferably, the solvent is a composition of ethylene carbonate, diethyl carbonate and methyl ethyl carbonate.

[0065] In non-aqueous electrolytes, the organic solvent can be one of cyclic carbonate compounds, linear carbonate compounds, carboxylic acid ester compounds, or ether compounds, or a combination of two or more of them.

[0066] On the other hand, the present invention provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, a separator, and the non-aqueous electrolyte described above.

[0067] Non-aqueous electrolytes are used in lithium-ion secondary batteries. The unsaturated phosphate compound shown in Formula 1 can effectively improve the flame retardant performance of the battery. The unsaturated phosphate compound shown in Formula 1 can be used to adjust lithium bis(fluorosulfonyl)imide to reduce corrosion of the positive electrode current collector. The contents of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and the unsaturated phosphate compound shown in Formula 1 in the electrolyte satisfy the following relationship: A value of ≥5 can effectively improve the battery's high-temperature cycle performance, initial capacity utilization, and safety performance.

[0068] In some embodiments, the positive electrode includes a positive electrode material, wherein the positive electrode material is LiNi. x Co y Mn z L (1-x-y-z) O2, wherein L is at least one of Al, Sr, Mg, Ti, Ca, Zr, Zn, Si, Cu, V or Fe, and 0≤x≤1, 0≤y≤1, 0≤z≤1, 0≤x+y+z≤1.

[0069] The negative electrode includes a negative electrode material, and the negative electrode active material is graphite, wherein the graphite is at least one of artificial graphite or natural graphite.

[0070] The diaphragm includes at least one of a polymer diaphragm and a nonwoven fabric, wherein the polymer diaphragm includes at least one of a single-layer PP, a single-layer PE, a double-layer PP / PE, a double-layer PP / PP, or a triple-layer PP / PE / PP.

[0071] The present invention will be further illustrated by the following examples.

[0072] The unsaturated phosphate compound used as an additive in the non-aqueous electrolyte in Examples 1-5 and Comparative Examples 1-2 is Compound 1, and the saturated phosphate compound used in Example 6 is Compound 2. The specific structures are as follows:

[0073]

[0074] Examples 1-6 and Comparative Examples 1-2

[0075] This embodiment illustrates the battery and its preparation method disclosed in this invention, using LiNi as the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2, taking graphite as an example of the negative electrode active material, includes the following operation steps:

[0076] (1) Preparation of positive electrode plate

[0077] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1O2, conductive carbon black Super-P, and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 93:4:3, and then dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The slurry is uniformly coated on both sides of an aluminum foil, and after drying, calendering, and vacuum drying, a positive electrode is obtained. Aluminum leads are then welded on using an ultrasonic welder to obtain a positive electrode plate with a thickness of 120-150 μm.

[0078] (2) Preparation of negative electrode plate

[0079] Artificial graphite (anode active material), Super-P conductive carbon black, styrene-butadiene rubber (SBR) binder, and carboxymethyl cellulose (CMC) were mixed in a mass ratio of 94:1:2.5:2.5 and then dispersed in deionized water to obtain anode slurry. The slurry was coated on both sides of copper foil, and after drying, calendering, and vacuum drying, anode was obtained. Nickel leads were then welded on using an ultrasonic welder to obtain anode plate with a thickness of 120-150 μm.

[0080] (3) Preparation of electrolyte

[0081] Ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of EC:DEC:EMC = 1:1:1. Then, lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide were added according to the proportions in Table 1 until the total lithium salt molar concentration was 1 mol / L. Then, compound 1 was added based on the total mass of the electrolyte. The specific amount added is shown in Table 1.

[0082] (4) Preparation of lithium-ion secondary batteries

[0083] Battery fabrication: A three-layer separator with a thickness of 20 μm is placed between the positive and negative plates. Then, the sandwich structure composed of the positive plate, negative plate and separator is wound up. The wound body is flattened and placed in an aluminum foil packaging bag. It is then vacuum baked at 75°C for 48 hours to obtain the battery to be injected with electrolyte.

[0084] Battery electrolyte filling and formation: In a glove box with the dew point controlled below -40°C, the electrolyte prepared above is injected into the cell, vacuum sealed, and left to stand for 24 hours.

[0085] The battery is formed for its first charge using the following steps: constant current charging at 0.05C for 180 minutes, constant current charging at 0.2C to 3.95V, secondary vacuum sealing, and then further constant current charging at 0.2C to the cutoff voltage of 4.3V. After being left at room temperature for 24 hours, it is discharged at 0.2C to 3.0V.

[0086] Performance testing

[0087] The lithium-ion secondary batteries prepared above were subjected to the following performance tests:

[0088] 1. High-temperature cycle performance test: At 45℃, the formed battery is charged with 1C constant current and constant voltage to the cutoff voltage, then charged with constant voltage until the current drops to 0.02C, and then discharged with 1C constant current to 3.0V. This cycle is repeated 500 times, and the discharge capacity of the first cycle and the discharge capacity of the last cycle are recorded.

[0089] Calculate the capacity retention during high-temperature cycling using the following formula:

[0090] Capacity retention rate = Discharge capacity in the last cycle / Discharge capacity in the first cycle × 100%.

[0091] 2. Needle prick test

[0092] At 25℃, the capacity-balanced batteries were charged to a full voltage of 4.3V using a 1C constant current and constant voltage method, with a cutoff current of 0.05C. Then, the lithium-ion secondary batteries were placed in an explosion-proof box, and steel needles with a diameter of 3mm were used for piercing at a speed of 25mm / s for 1 hour.

[0093] 3. Corrosion test

[0094] At 25°C, the lithium-ion secondary battery was discharged to 3V at 0.5C after 500 cycles. It was then disassembled in a glove box, and the positive electrode current collector was subjected to scanning electron microscopy (SEM) to observe the corrosion of the current collector.

[0095] The battery performance test results obtained from Examples 1-6 and Comparative Examples 1-2 are filled in Table 2.

[0096] Table 1. Parameters of Examples 1-6 and Comparative Examples 1-2

[0097]

[0098] Table 2 Battery performance test results of Examples 1-6 and Comparative Examples 1-2

[0099]

[0100] Examples 1-6 and Comparative Examples 1-2 show that, under the same positive electrode active material conditions, the molar concentration of LiPF6 and the concentration of lithium bis(fluorosulfonyl)imide (LiFSI) in the non-aqueous electrolyte of a lithium-ion secondary battery satisfy the following conditions: At ≥5, the prepared battery emitted smoke but did not ignite during the nail penetration test, and no corrosion was observed at the battery tabs after high-temperature cycling. Examples 1-5 show that LiPF6, LiFSI, and the compound shown in structural formula 1 in the non-aqueous electrolyte exhibited the desired effect according to the relationship... As the value increases, the battery surface temperature gradually decreases during the nail penetration test. This may be because the unsaturated phosphate ester compounds in the non-aqueous electrolyte can improve the battery's flame retardant performance. The battery's high-temperature cycle capacity retention rate gradually increases, and the battery's initial capacity also gradually increases. This may be because the compound shown in Structural Formula 1 can be used to adjust lithium bis(fluorosulfonyl)imide to reduce corrosion reaction, synergistically improving the battery's high-temperature cycle performance, initial capacity utilization, and safety performance. Examples 5-6 involve adding different unsaturated phosphate ester compounds shown in Structural Formula 1 to the non-aqueous electrolyte. The battery's electrical and safety performance were similar, indicating that as long as the compound shown in Structural Formula 1 is satisfied, the effect is the same. Comparing Examples 1-6 with Comparative Example 1, it is found that the non-aqueous electrolyte does not contain the compound shown in Structural Formula 1. The prepared battery smoked and caught fire during the nail penetration test, exhibited low battery safety performance, poor high-temperature cycle performance, and slight corrosion in the corrosion test. Comparing Examples 1-6 with Comparative Example 2, it is found that LiPF6, LiFSI, and the compound shown in Structural Formula 1 in the non-aqueous electrolyte do not satisfy the relationship. Under conditions ≥5, the battery still exhibits poor flame retardancy, high-temperature cycle performance, low initial capacity, and safety performance.

[0101] Examples 7-10 and Comparative Examples 3-5

[0102] Compared with Example 1, Examples 7-10 and Comparative Examples 3-5 use different positive electrode materials. The specific selection of positive electrode materials is shown in Table 3. The preparation method of the positive electrode sheet is the same as in Example 1, and the preparation method of the negative electrode sheet is also the same. Because the positive electrode materials in Examples 7-10 and Comparative Examples 3-5 are different, the charging cut-off voltage of the batteries prepared is different from that in Example 1. The cut-off voltage for the different positive electrode materials is 4.4V for LiNi. 0.6 Co 0.2 Mn 0.2 O2, 4.35V LiNi 0.7 Co 0.1 Mn 0.2 O2, 4.3VLiNi 0.8 Co 0.1 Mn 0.2 O1, the battery charging cut-off voltage, the battery full-charge cut-off voltage during high-temperature cycle performance testing, and the battery full-charge cut-off voltage during nail penetration testing are all based on the cut-off voltage of different cathode materials, 4.4V LiNi. 0.6 Co 0.2 Mn 0.2 O2, 4.35V LiNi 0.7 Co 0.1 Mn 0.2 O2, 4.3V LiNi 0.8 Co 0.1 Mn 0.2O1 was selected accordingly, and the rest were the same as in Example 1. The amount of different compounds added to the electrolytes of Examples 7-10 and Comparative Examples 3-5 is shown in Table 3, and the battery performance test results are shown in Table 4.

[0103] Table 3. Parameters of Examples 7-10 and Comparative Examples 3-5

[0104]

[0105]

[0106] Table 4 Battery performance test results for Examples 7-10 and Comparative Examples 3-5

[0107]

[0108]

[0109] Examples 3-4 and 7-10 show that the concentrations of LiPF6 and LiFSI in the non-aqueous electrolyte satisfy the relationship (Cb×Mp) / Ca×1000≥5 with respect to the compound of structural formula 1. Batteries with different cathode materials that satisfy the cathode material relationship formula of this application also have high cycle performance, no smoke or fire, no corrosion of the cathode current collector, and high initial capacity. In the lithium-ion secondary batteries of Comparative Examples 3 and 5, the electrolyte does not contain the compound of structural formula 1. The maximum surface temperature of the lithium-ion secondary batteries increased significantly during the nail penetration test, and smoke and fire occurred, indicating low safety performance. In the lithium-ion secondary batteries of Comparative Examples 2, 4, and 6, the molar concentrations of LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte do not satisfy the preset relationship (Cb×Mp) / Ca×1000≥5 with respect to the compound of structural formula 1, resulting in smoke and corrosion of the batteries.

[0110] As shown in Tables 1-4 above, when the concentrations of LiPF6 and LiFSI in the non-aqueous electrolyte satisfy the relationship (Cb×Mp) / Ca×1000≥5 with the compound of structural formula 1, the prepared lithium-ion secondary battery has high safety performance, high initial capacity, no smoke or fire phenomenon or corrosion phenomenon, and high temperature cycle performance is improved to a certain extent.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-aqueous electrolyte, characterized in that, The electrolyte comprises lithium salts, additives, and organic solvents, wherein the lithium salts include lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, the additives include unsaturated phosphate ester compounds, and the non-aqueous electrolyte satisfies the following relationship: ≥5 Wherein, Ca is the molar concentration of lithium hexafluorophosphate in the non-aqueous electrolyte, in mol / L; Cb is the molar concentration of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte, in mol / L. Mp is the mass percentage of the unsaturated phosphate ester compound in the total mass of the non-aqueous electrolyte, expressed in % (%). The unsaturated phosphate compound is the compound shown in structural formula 1: Structural Formula 1 Where R1 is an alkynyl group with 2 to 5 carbon atoms, and R2 and R3 are each independently selected from a hydrocarbon group with 1 to 5 carbon atoms or a haloalkyl group with 1 to 5 carbon atoms; n1≥0, n2≥0, n3≥0, but n1, n2, and n3 are not all 0 at the same time; The total molar amount of lithium salt in the non-aqueous electrolyte is 0.8~1.4M, and the Ca:Cb ratio is 9:1~4:6; the mass percentage (Mp) of the unsaturated phosphate ester compound in the total mass of the non-aqueous electrolyte is 0.001~10%.

2. The non-aqueous electrolyte according to claim 1, characterized in that, The Ca:Cb ratio is 9:1 to 5:

5.

3. The non-aqueous electrolyte according to claim 1, characterized in that, The compound shown in structural formula 1 is selected from one or more of the following compounds: Compound 5.

4. The non-aqueous electrolyte according to claim 1, characterized in that, The non-aqueous electrolyte also includes auxiliary additives, which include at least one of cyclic sulfate compounds, sulfonyl lactone compounds, cyclic carbonate compounds, and nitrile compounds. Based on the total mass of the non-aqueous electrolyte as 100%, the mass percentage of the auxiliary additives is 0.01% to 30%.

5. The non-aqueous electrolyte according to claim 4, characterized in that, The cyclic sulfate compounds are selected from at least one of vinyl sulfate, propylene sulfate, or methyl vinyl sulfate; the sulfonate lactone compounds include at least one of 1,3-propane sulfonate lactone, 1,4-butane sulfonate lactone, or 1,3-propene sulfonate lactone; the cyclic carbonate compounds include at least one of vinylene carbonate, ethylene ethylene carbonate, or fluoroethylene carbonate; and the nitrile compounds are selected from at least one of succinic anionyl nitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptacyanide, octadionitrile, azelaic anionyl nitrile, and sebacate.

6. A lithium-ion secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and the non-aqueous electrolyte as described in any one of claims 1-5.

7. The lithium-ion secondary battery according to claim 6, characterized in that, The positive electrode includes a positive electrode material, which is LiNi. x Co y Mn z L (1-x-y-z) O2, wherein L is at least one of Al, Sr, Mg, Ti, Ca, Zr, Zn, Si, Cu, V or Fe, and 0≤x≤1, 0≤y≤1, 0≤z≤1, 0≤x+y+z≤1.

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