Non-aqueous electrolyte and lithium ion battery containing same

Abstract

The invention discloses a non-aqueous electrolyte and a lithium ion battery containing the non-aqueous electrolyte. The non-aqueous electrolyte comprises a lithium salt, a non-aqueous organic solvent and additives, the additives comprise a compound A as shown in a structural formula I, a compound B as shown in a structural formula II and a compound B as shown in a structural formula II, M is alkali metal, R is selected from H or C1-C6 alkyl, R1 and R2 are independently selected from H and C1-C6 alkyl, or * represents a connecting end, or R1 and R2 are combined with each other to form a C3-C8 cyclic structure. The additive of the non-aqueous electrolyte comprises the compound A and the compound B, and the high-temperature storage and high-temperature cycle performance of the lithium ion battery can be effectively improved through the synergistic cooperation of the two compounds. Structural formula I and structural formula II

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a non-aqueous electrolyte and a lithium ion battery comprising the same. BACKGROUND

[0002] With the rapid development of new energy vehicles, energy storage and other fields, the market demand for high-performance and low-cost batteries is increasing, and lithium manganese iron phosphate batteries, as a potential new battery technology, have attracted the attention of many enterprises and research institutions, and the market demand is showing a rapid growth trend.

[0003] Lithium manganese iron phosphate batteries not only have broad application prospects in the field of new energy vehicles, but also can be applied to electric two-wheel vehicles, three-wheel vehicles, household energy storage products, emergency products, small and medium-sized backup power storage and other price-sensitive products. In the future, it is expected to be popularized and applied in more fields. However, under high temperature conditions, manganese ions in lithium manganese iron phosphate are more likely to be dissolved from the positive material. After the dissolution of manganese ions, they will migrate to the negative electrode through the separator and be reduced to precipitate at the negative electrode, thereby destroying the solid electrolyte interface film (SEI film) of the negative electrode. The formation and repair of the SEI film will consume lithium ions, leading to a decrease in lithium ions and a decrease in battery capacity, affecting the cycle life and high-temperature cycle stability of the battery. The manganese ions in lithium manganese iron phosphate will undergo Jahn-Teller effect during charging and discharging, especially the high-spin Mn 3+ With only one electron on the doubly degenerate eg orbital, the asymmetric electron distribution will cause the distortion of the MnO6 octahedron, thereby changing the crystal structure of the material. This structural distortion will promote the disproportionation reaction of Mn 3+ , producing Mn 2+ and Mn 4+ , further accelerating the dissolution of manganese and reducing the stability and cycle performance of the material, and the Jahn-Teller effect will be accelerated under high temperature conditions. At the same time, the decomposition of the electrolyte will be accelerated in a high temperature environment, producing some acidic substances. These acidic substances will react with the manganese ions in lithium manganese iron phosphate, accelerating the dissolution and disproportionation of manganese ions, thereby affecting the performance and life of the battery. When the temperature rises, the electrical conductivity of lithium manganese iron phosphate will decrease, leading to an increase in the polarization of the battery, a decrease in the charging and discharging efficiency, and an increase in the heat accumulation inside the battery. This not only affects the high-temperature performance of the battery, but also may cause safety problems. Due to the combined effects of the above factors, the capacity decay rate of lithium manganese iron phosphate batteries will significantly accelerate during high-temperature cycling and high-temperature storage. Compared with normal temperature conditions, the cycle life will be greatly shortened, and the available capacity after storage will also be significantly reduced, which cannot meet the demand of some application scenarios with high requirements for high-temperature performance.

[0004] Therefore, there is an urgent need for a non-aqueous electrolyte and a lithium ion battery comprising the same to solve the problems existing in the prior art. SUMMARY

[0005] In view of the above problems, the purpose of the present application is to provide a non-aqueous electrolyte and a lithium ion battery comprising the same, which can effectively improve the high-temperature cycle and high-temperature storage performance of the lithium ion battery.

[0006] To achieve the above-mentioned purpose, in one aspect, the present application provides a non-aqueous electrolyte comprising a lithium salt, a non-aqueous organic solvent and an additive, wherein the additive comprises a compound A represented by structural formula I and a compound B represented by structural formula II,

[0007] Structural formula I Structural formula II wherein M is an alkali metal, R is selected from H or C1-C6 alkyl, R1 and R2 are each independently selected from H, C1-C6 alkyl, or , and * represents a connecting end, or R1 and R2 combine with each other to form a C3-C8 cyclic structure. Compared with the prior art, the non-aqueous electrolyte of the application comprises a lithium salt, a non-aqueous organic solvent and an additive, the additive comprises compound A and compound B, wherein compound A is an unsaturated cyclic sulfimide salt, can polymerize to form an SEI layer at the negative electrode interface, and improves the cycle performance and high-temperature storage performance of the battery. Meanwhile, compound A can also form a stable, dense and thick CEI film at the positive electrode interface, the main components of the CEI film are LiF and Li2SO4, which can significantly improve the high-temperature storage performance, but the LiF contained in the CEI film has high ionic conductivity, but the film formed on the electrode surface is relatively dense, and the transmission of lithium ions in the film may be hindered to a certain extent, and the electronic conductivity of LiF is low, which makes the transmission of electrons in the film difficult, thereby increasing the impedance of the CEI film and making the positive electrode potential too high and deteriorating the high-temperature cycle performance. Compound B is a cyclic sulfate compound, can generate a stable SEI film at the negative electrode interface, the SEI film is rich in lithium alkyl sulfonate, can effectively prevent the side reaction of the negative electrode surface with the electrolyte, and improves the cycle performance of the battery. Meanwhile, compound B can also form a dense and thin CEI film at the positive electrode, the CEI film is not easy to decompose at high voltage and has low impedance, the main reason is that the main components of the CEI film, lithium sulfate (Li2SO4) and lithium sulfide (Li2S), have relatively small hindrance in ion and electron transmission, and have less influence on the CEI impedance than LiF, and then the substance can reduce the impedance on the positive electrode side in high-voltage high-temperature cycle, thereby reducing the positive electrode polarization, avoiding the decomposition of the positive electrode active material caused by the too high positive electrode potential, and improving the high-temperature cycle performance. However, the cyclic sulfate compound has high reactivity and can quickly react with the components in the active material or electrolyte on the positive electrode surface to form an initial CEI film. The rapid reaction makes the reaction products cover the electrode surface in a short time, limits the subsequent reaction, and thus the thickness of the finally formed CEI film is relatively thin, so the effect of improving the high-temperature storage is not obvious. Therefore, when compound A and compound B are used together, the microstructure of the interface film can be optimized, the thermal stability of the whole CEI film can be improved, and the decomposition and damage of the interface film at high temperature can be reduced, thereby the high-temperature storage and high-temperature cycle performance of the lithium ion battery can be effectively improved. Further, R is selected from C1-C3 alkyl, R1 and R2 are each independently selected from H, C1-C3 alkyl, or , * represents a connecting end, or R1 and R2 combine with each other to form a C3-C6 cycloalkane.

[0008] Further, R1 and R2 are each independently selected from H, ethyl, or and R7 and R8 are not simultaneously or represents a connecting terminal, or R7and R8combine with each other to form a cyclohexane.

[0009] Further, the compound A of the present application is selected from at least one of compound 1 to compound 5,

[0010] .

[0011] Further, the compound B of the present application is selected from at least one of compound 6 to compound 9,

[0012] Compound 6 (CAS: 1431298-10-0) Compound 7 (CAS: 6970-90-7)

[0013] Compound 8 (CAS: 124535-97-3) Compound 9 (CAS: 2507955-35-1).

[0014] Further, the mass percentage of the compound A of the present application in the nonaqueous electrolyte is 0.05% to 5%. Preferably, the mass percentage of the compound A in the nonaqueous electrolyte is 0.1% to 4%, and more preferably, the mass percentage of the compound A in the nonaqueous electrolyte is 0.5% to 4%. As an example, the mass percentage of the compound A in the nonaqueous electrolyte can be, but is not limited to, 0.05%, 0.1%, 0.2%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.8%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%.

[0015] Further, the mass percentage of the compound B of the present application in the nonaqueous electrolyte is 0.5% to 4%. Preferably, the mass percentage of the compound B in the nonaqueous electrolyte is 0.5% to 3%. As an example, the mass percentage of the compound A in the nonaqueous electrolyte can be, but is not limited to, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.8%, 2%, 2.5%, 3%, 3.5%, 4%.

[0016] Further, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium trifluoromethylsulfonate (LiCF3SO3), lithium bis-trifluoromethylsulfonimide (LiTFSI), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium fluorosulfonate (LiSO3F), lithium difluorobis(oxalato)phosphate (C2BF2LiO4), lithium lower aliphatic carboxylate, lithium difluorodioxalate phosphate (LiDFBP), and lithium bisfluorosulfonimide (LiFSI).

[0017] Further, the mass percentage of the lithium salt in the non-aqueous electrolyte solution of the present application is 5% to 25%, preferably, the mass percentage of the lithium salt in the non-aqueous electrolyte solution is 8% to 20%, more preferably, the mass percentage of the lithium salt in the non-aqueous electrolyte solution is 10% to 15%, as an example, the mass percentage of the lithium salt in the non-aqueous electrolyte solution can be but not limited to 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 20%, 25%.

[0018] Further, the non-aqueous organic solvent of the present application comprises at least one of carbonates and carboxylic acid esters.

[0019] Further, the non-aqueous organic solvent of the present application comprises at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), butyl acetate (n-Ba), γ-butyrolactone (γ-Bt), propyl propionate (n-PP), ethyl propionate (EP), and ethyl butyrate (Eb).

[0020] Another aspect of the present application provides a lithium ion battery, comprising a positive electrode material and a negative electrode material, and further comprising the aforementioned non-aqueous electrolyte solution.

[0021] Further, the positive electrode material of the present application is selected from lithium manganese iron phosphate materials. Specifically, the chemical formula of the lithium manganese iron phosphate material is LiMnxFe1-xPO4, 0 x Fe 1-x PO4, 0 0.6 Fe 0.4 PO4.

[0022] Further, the negative electrode material of the present application is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite material, and silicon monoxide. As an example, the negative electrode material is artificial graphite, but not limited thereto. DETAILED DESCRIPTION

[0023] In order to better illustrate the purpose, technical solutions and beneficial effects of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be noted that the following implementation method is a further explanation of the present invention and should not be used as a limitation of the present invention.

[0024] Example 1 (1) Preparation of non-aqueous electrolyte Under an argon atmosphere and in a vacuum glove box with a moisture content of <1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio of EC:EMC=3:7, and then additives Compound 1 and Compound 6 were added. After dissolving and stirring thoroughly, lithium hexafluorophosphate (LiPF6) was added and mixed evenly to obtain a non-aqueous electrolyte.

[0025] (2) Preparation of positive electrode Lithium iron manganese phosphate LiMn 0.6 Fe 0.4 PO4, adhesive PVDF and conductive agent SuperP are evenly mixed in a mass ratio of 95:1:4 to form a battery positive electrode slurry with a certain viscosity. The mixed slurry is coated on both sides of the aluminum foil, dried and rolled to obtain the positive electrode sheet.

[0026] (3) Preparation of negative electrode The negative electrode material artificial graphite, the binder PVDF and the conductive agent SuperP are evenly mixed in a mass ratio of 90:2:8 to prepare a lithium secondary battery negative electrode slurry with a certain viscosity. The mixed slurry is coated on both sides of the copper foil, dried and rolled to obtain the negative electrode sheet.

[0027] (4) Preparation of lithium-ion batteries The positive electrode sheet, separator, and negative electrode sheet are stacked in order and then stacked as needed. After the tabs are welded, they are placed in the aluminum-plastic film of the battery packaging. The prepared non-aqueous electrolyte is injected into the dried bare cell. The cells are then vacuum packaged, allowed to stand, formed (charged at a constant current of 0.05C to 3.0V, then at a constant current of 0.1C to 3.3V), shaped, and tested for capacity. Finally, a 1Ah soft-pack lithium secondary battery is obtained.

[0028] The non-aqueous electrolyte formulations of Examples 1 to 15 and Comparative Examples 1 to 3 are shown in Table 1. The steps for preparing the electrolytes and manufacturing the batteries are the same as those of Example 1.

[0029] Table 1 Formula of non-aqueous electrolyte

[0030] The lithium-ion batteries prepared in Examples 1 to 15 and Comparative Examples 1 to 3 were subjected to high-temperature storage tests and high-temperature cycle tests, respectively. The specific test conditions are as follows. The performance test results are shown in Table 2.

[0031] High temperature storage performance test At room temperature (25°C), the lithium ion battery was charged and discharged at 0.2C / 0.2C for one time (the discharge capacity of the battery was recorded as C0), and the upper limit voltage was 4.2V; the battery was placed in a 60°C oven for 30d, the battery was taken out, and the battery was placed in a 25°C environment, and discharged at 0.2C, and the discharge capacity was recorded as C1; then the lithium ion battery was charged and discharged at 0.2C / 0.2C for one time (the discharge capacity of the battery was recorded as C2), and the capacity retention rate and the capacity recovery rate of the lithium ion battery were calculated using the following formula: Capacity retention rate = C1 / C0 x 100% Capacity recovery rate = C2 / C0 x 100% High temperature cycling performance test The lithium ion battery was placed in a 45°C thermostat for 30 minutes, and the lithium ion battery was allowed to reach a constant temperature, and was charged at 4C constant current to a voltage of 4.2V, and then was charged at 4.2V constant voltage to a current of 0.05C, and then was discharged at 4C constant current to a voltage of 2.5V, and the first cycle discharge capacity of the battery was recorded as C0, which was one charge-discharge cycle. Then the battery was charged and discharged at 1C / 1C for 1000 cycles at 45°C, and the discharge capacity was recorded as C1, and the capacity retention rate of the lithium ion battery was calculated using the following formula.

[0032] Capacity retention rate = C1 / C0 x 100% Table 2: Results of performance test of lithium ion battery

[0033] From the results in Table 2, it can be seen that the lithium ion batteries of Examples 1-15 have more excellent high-temperature storage performance and high-temperature cycle performance compared with Comparative Examples 1-3.

[0034] It can be further known from the comparison of Example 1 and Comparative Examples 2-3 that, in the case of using only compound A or compound B, the high-temperature storage and cycle performance of the lithium ion battery cannot be simultaneously considered, and the improvement effect is lower than that of Example 1, which may be because compound A is an unsaturated cyclic sulfimide salt, which can polymerize to form an SEI layer at the negative electrode interface, thereby improving the cycle performance and high-temperature storage performance of the battery. At the same time, compound A can also form a stable, dense and thick CEI film at the positive electrode interface, and the main components of the CEI film are LiF and Li2SO4, which can significantly improve the high-temperature storage performance. Although LiF contained in the CEI film has high ionic conductivity, the film formed on the electrode surface is relatively dense, and the transmission of lithium ions in it may be hindered to a certain extent, and the electronic conductivity of LiF is relatively low, which makes the transmission of electrons in the film difficult, thereby increasing the impedance of the CEI film, and the positive electrode potential is too high to deteriorate the high-temperature cycle performance. Compound B is a cyclic sulfate compound, which can generate a stable SEI film at the negative electrode interface, and the SEI film is rich in lithium alkyl sulfonate, which can effectively prevent the negative electrode surface from reacting with the electrolyte, thereby improving the cycle performance of the battery. At the same time, compound B can also form a dense and thin CEI film at the positive electrode, which is not easy to decompose at high voltage and has low impedance. The main reason is that the main components of the CEI film, lithium sulfate (Li2SO4) and lithium sulfide (Li2S), have relatively small hindrance in ion and electron transmission, and the influence on CEI impedance is not as significant as LiF. Therefore, this substance can reduce the impedance on the positive electrode side in high-voltage high-temperature cycling, thereby reducing the positive electrode polarization and avoiding the decomposition of the positive active material caused by the high positive electrode potential, thereby improving the high-temperature cycle performance. However, due to the high reactivity of the cyclic sulfate compound, it can quickly react with the components in the active material or electrolyte on the positive electrode surface to form an initial CEI film. This rapid reaction causes the reaction products to cover the electrode surface in a short time, limiting the subsequent reaction, thereby resulting in a thin CEI film, which has no obvious effect on improving the high-temperature storage performance. Therefore, when compound A and compound B are used together, the microstructure of the interface film can be optimized, the thermal stability of the CEI film as a whole can be improved, and the decomposition and destruction of the interface film at high temperature can be reduced, thereby effectively improving the high-temperature storage and high-temperature cycle performance of the lithium ion battery. Therefore, by introducing compound A and compound B into the electrolyte of the lithium ion battery, a SEI film with uniform structure and uniform distribution can be formed on the negative electrode surface, the SEI film has excellent lithium ion transmission performance, a dense and moderately thick CEI film can be formed on the positive electrode, and the high-temperature cycle performance and high-temperature storage performance of the battery can be improved through the synergistic effect of the two.

[0035] It should be pointed out finally that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit the protection scope of the present application, and although the present application has been described in detail with reference to the preferred embodiments, it is not limited to the listed in the embodiments, and those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A non-aqueous electrolyte comprising a lithium salt, a non-aqueous organic solvent and an additive, characterized in that: The additive includes compound A shown in structural formula I and compound B shown in structural formula II, Structural Formula I Structural Formula II Wherein, M is an alkali metal, R is selected from H or a C1~C6 alkyl group, R1 and R2 are each independently selected from H, a C1~C6 alkyl group, or , * represents the connecting end, or R1 and R2 are combined with each other to form a C3~C8 ring structure.

2. A non-aqueous electrolyte according to claim 1, characterized in that R is selected from C1~C3 alkyl, R1 and R2 are each independently selected from H, C1~C3 alkyl, or , * represents the connecting end, or R1 and R2 are combined with each other to form a C3~C6 cycloalkane.

3. A non-aqueous electrolyte according to claim 1, characterized in that The compound A is selected from at least one of compound 1 to compound 5, 。 4. The non-aqueous electrolyte according to claim 1, wherein The compound B is selected from at least one of compound 6 to compound 9, Compound 6 Compound 7 Compound 8 Compound 9.

5. The non-aqueous electrolyte according to claim 1, wherein The mass percentage of the compound A in the non-aqueous electrolyte is 0.05%-5%, and the mass percentage of the compound B in the non-aqueous electrolyte is 0.5%-4%.

6. The non-aqueous electrolyte according to claim 1, wherein The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bistrifluoromethanesulfonyl imide, lithium bisoxalatoborate, lithium difluorophosphate, lithium fluorosulfonate, lithium difluorooxalatoborate, lithium lower aliphatic carboxylate, lithium difluorobisoxalatophosphate and lithium bisfluorosulfonyl imide.

7. The non-aqueous electrolyte according to claim 1, wherein The non-aqueous organic solvent includes at least one of a carbonate ester and a carboxylate ester.

8. A non-aqueous electrolyte according to claim 7, characterized in that: The non-aqueous organic solvent includes at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate and ethyl butyrate.

9. A lithium-ion battery, characterized in that: The invention comprises a positive electrode material and a negative electrode material, and also comprises the non-aqueous electrolyte according to any one of claims 1 to 8.

10. The lithium-ion battery according to claim 9, wherein The positive electrode material is selected from lithium manganese iron phosphate material.

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