An electrolyte additive combination and a high-temperature resistant lithium-ion battery electrolyte

Abstract

This invention provides an electrolyte additive combination and a high-temperature resistant lithium-ion battery electrolyte. The electrolyte additive combination includes compound A represented by structural formula I and compound B represented by structural formula II. The lithium-ion battery electrolyte includes a negative electrode, a positive electrode, a separator disposed between the negative and positive electrode, and the aforementioned electrolyte. The additive combination of this invention can effectively improve the stability and density of the SEI film formed on the surfaces of the positive and negative electrodes, thereby improving the high-temperature storage performance and high-temperature cycle performance of ternary lithium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of electrolyte technology, and particularly relates to an electrolyte combination additive and a high-temperature resistant lithium-ion battery electrolyte. Background Technology

[0002] Lithium-ion batteries suffer from both slowed kinetics and intensified interfacial side reactions at high temperatures. Therefore, the high-temperature performance of typical lithium-ion batteries is insufficient to meet the growing demands of the power battery and energy storage markets. Electrolytes simultaneously control the Li... + The transport and interfacial reactions of electrolytes largely determine the high-temperature performance of lithium-ion batteries. High-temperature improving additives for lithium-ion battery electrolytes are characterized by small dosage and rapid effect. Adding additives to improve the high-temperature performance of organic electrolytes in lithium-ion batteries, thereby improving the high-temperature performance of lithium-ion batteries, is a current research hotspot and an important future development direction in this field. Therefore, finding and developing high-temperature improving additives is a crucial step in achieving breakthroughs in the high-temperature performance of lithium-ion batteries. Summary of the Invention

[0003] To address the aforementioned technical issues, this invention provides an electrolyte combination additive and a high-temperature resistant lithium-ion battery electrolyte. The combination additive can effectively improve the stability and density of the SEI film formed on the positive and negative electrode surfaces, thereby improving the high-temperature storage performance and high-temperature cycle performance of ternary lithium-ion batteries.

[0004] The present invention proposes an electrolyte additive comprising compound A represented by structural formula I and compound B represented by structural formula II; Formula I; Formula II; In Formula I, R1 is an unsaturated alkenyl group, and R2, R3, and R4 are each independently an alkyl group; In Formula II, R is an ester group or an aromatic group.

[0005] In this invention, a composite electrolyte additive is obtained by compounding silane compounds represented by Formula I and isothiocyanate compounds represented by Formula II. The isothiocyanate compounds have high dielectric constants and low viscosity. The isothiocyanate groups can form CN-Co bonds with the surface of NCM metal oxides, inhibiting direct contact between the electrolyte and the positive electrode, thereby improving the stability of the positive electrode interface. Simultaneously, the isothiocyanate groups have strong electrophilic activity, reacting with water in the electrolyte and active hydrogen in the positive and negative electrode materials, reducing the decomposition of LiPF6 caused by active hydrogen. The silane compounds possess unsaturated double bonds that form organic polymers and a cross-linked network containing Si-O-Si bonds. This composite film can significantly improve the mechanical strength and flexibility of the SEI film. The silane additive provides toughness and effectively prevents breakage. Simultaneously, it forms a cathode electrolyte interphase (CEI) film rich in inorganic substances (such as LiF and SiOx) and organic matter on the cathode surface. This film has strong antioxidant capacity and effectively protects the cathode. Furthermore, isothiocyanate additives typically have higher reduction potentials or lower adsorption energies, preferentially forming films and constructing stable interfaces. This invention, through the combined use of these two additives, achieves a synergistic effect, making it easier to generate thinner and denser CEI and SEI protective films on the NCM cathode and graphite anode surfaces. It also removes residual lithium compounds from the positive and negative electrode surfaces, improving the ionic conductivity of the interfacial film. Compared with traditional electrolyte additives, this significantly improves the battery's high-temperature resistance.

[0006] Compound A and compound B are shown in the following structural formulas: Formula A-1 (CAS: 13688-55-6); Formula A-2 (CAS: 13735-81-4); Formula B-1 (CAS: 24066-82-8); Formula B-2 (CAS: 103-72-0).

[0007] Preferably, R1 is an unsaturated alkenyl group containing an electron-donating group, wherein the electron-donating group is a methoxy group; R is an aromatic group containing an electron-withdrawing group, wherein the electron-withdrawing group is a cyano, fluorine, or nitro group; Preferably, compound A and compound B are compounds with the following structural formulas: Formula A-3 (CAS: 31469-15-5); Formula A-4 (CAS: 54125-02-9); Formula B-3 (CAS: 2719-32-6); Formula B-4 (CAS: 1544-68-9). Formula B-5 (CAS:2131-61-5).

[0008] In this invention, when R1 is an unsaturated alkenyl group containing an electron-donating group and R is an aromatic group containing an electron-withdrawing group, compound A and compound B can respectively act as donor and acceptor to form a charge complex. On the one hand, this can inhibit the reduction of alkenyl silane compound A at the anode interface, ensuring effective film formation at the electrode interface. On the other hand, it can ensure the high ionic conductivity of the electrolyte, ultimately achieving the goal of further improving the high-temperature performance of ternary lithium-ion batteries.

[0009] Preferably, the mass ratio of compound A to compound B is 1:5-5:1.

[0010] The present invention also proposes a high-temperature resistant lithium-ion battery electrolyte, comprising: a conductive lithium salt, the above-mentioned combined additives, other additives, and an organic solvent.

[0011] The proportion of the combined additives is 0.1-5 wt% based on the total mass of the electrolyte.

[0012] Preferably, the other additives are at least one selected from sulfone, tris(trimethylsilane) phosphite (TMSP), vinyl sulfate (DTD), methylene disulfonate (MMDS), vinylene carbonate (VC), fluoroethylene carbonate (FEC), and methylene disulfonate (MMDS).

[0013] Preferably, the proportion of the other additives is 0.5-3 wt% based on the total mass of the electrolyte; more preferably, it can be 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, or 3 wt%. The concentration of the other additives is limited within the above range, which can enable the battery to have both superior electrochemical performance and excellent room temperature cycle performance.

[0014] Preferably, the conductive lithium salt is at least one selected from lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium methanesulfonate, lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium dioxolaneborate (LiBOB), lithium difluorooxolaneborate (LiDFOB), lithium difluorophosphate, lithium difluorobis(oxolane)phosphate, or lithium bis(fluorosulfonyl)imide (LiFSI). Preferably, the concentration of the conductive lithium salt in the electrolyte is 0.5-1.5M, for example, 0.5M, 0.75M, 1M, 1.25M, or 1.5M. Limiting the concentration of the conductive lithium salt within the above range allows the battery to simultaneously achieve better electrochemical performance and room temperature cycle retention.

[0015] Preferably, the organic solvent is at least one of cyclic carbonates, chain carbonates, or carboxylic acid esters; the cyclic carbonate is at least one of ethylene carbonate, propylene carbonate, butenyl carbonate, or γ-butyrolactone; the chain carbonate is at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, or ethyl propyl carbonate; and the carboxylic acid ester is at least one of methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, or ethyl butyrate.

[0016] Preferably, the organic solvent in the electrolyte is at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl propionate (EP).

[0017] Preferably, the organic solvent is selected from at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), or ethyl propionate (EP). The above-mentioned organic solvents have better compatibility with the combined additives of the present invention, thereby enabling the battery to simultaneously achieve superior electrochemical performance and capacity retention.

[0018] Preferably, the method for preparing the electrolyte includes: adding a conductive lithium salt to an organic solvent, and after it is completely dissolved, adding the electrolyte combination additive and other additives to obtain the electrolyte; by preparing the electrolyte by the above method, the electrolyte can have better performance uniformity.

[0019] The present invention also proposes an NCM ternary lithium-ion battery, which includes a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and the electrolyte described above.

[0020] Preferably, the positive electrode sheet comprises a positive electrode material, and the positive electrode material includes LiNi. （1-x-y） Co x Mn y O2, where 0≤x≤1, 0≤y≤1; the negative electrode sheet includes a negative electrode material, which includes graphite. The above-mentioned positive and negative electrode materials are better suited to the additives of this invention, resulting in lithium-ion batteries with superior capacity retention and capacity recovery rates.

[0021] Preferably, the charging voltage of the lithium-ion battery is ≤4.8V, and more preferably 2.8-4.25V.

[0022] Compared with the prior art, the present invention has the following technical effects: The electrolyte additive of the present invention includes compound A shown in structural formula I and compound B shown in structural formula II. Through the combined use of the two, they have a synergistic effect, making it easier to form thinner and denser CEI and SEI protective films on the surfaces of NCM cathode and graphite anode. At the same time, they have functions such as water removal, acid reduction or complexation, which can remove residual lithium compounds on the positive and negative electrode surfaces and improve the ionic conductivity of the interfacial film. Compared with traditional electrolyte additives, they can greatly improve the high-temperature performance of batteries. Attached Figure Description

[0023] Figure 1 This is a graph showing the battery capacity retention rate of Embodiment 1 and Comparative Example 1 after 200 cycles at room temperature. Figure 2 This is a graph showing the battery capacity retention rate after 200 high-temperature cycles in Example 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0024] The present invention will now be described in detail through specific embodiments. However, these embodiments are clearly provided for illustrative purposes and are not intended to limit the scope of the present invention.

[0025] Example 1 A high-temperature resistant lithium-ion battery electrolyte is prepared by the following method: In an argon-filled glove box, ethylene carbonate (EC), ethyl propionate (EP), and ethyl methyl carbonate (EMC) were mixed evenly at a mass ratio of EC:EP:EMC = 1:1:2. Then, 0.7 mol of lithium hexafluorophosphate (LiPF6) and 0.3 mol of lithium difluorosulfonyl imide (LiFSI) were added and stirred until completely dissolved to obtain a 1 mol / L electrolyte. Next, vinylene carbonate (VC), ethylene sulfate (DTD), and compounds shown in Formula A-1 and Formula B-1 were added. Based on the total mass of the electrolyte being 100%, the mass percentages of VC, DTD, and the compound shown in Formula A-1 were 0.5%, 2%, and 1%, respectively. After stirring and mixing evenly, the high-temperature resistant lithium-ion battery electrolyte was obtained. An NCM ternary lithium-ion battery is prepared by the following method: (1) LiNi nickel cobalt manganese ternary material 0.7 Co 0.1 Mn 0.2 O2, conductive agent Super P, binder PVDF, and carbon nanotubes (CNTs) were mixed evenly at a mass ratio of 97.5:0.5:1:1 to obtain a lithium-ion battery positive electrode slurry. This slurry was then coated onto aluminum foil for current collectors, with a coating weight of 326 g / m². 2After drying at 85℃, the material is cold-pressed; then slit and sliced, and dried under vacuum at 85℃ for 4 hours to obtain the positive electrode sheet for lithium-ion batteries; artificial graphite is mixed with conductive agent Super P, thickener CMC, and binder SBR (styrene-butadiene rubber latex) at a mass ratio of 95:1.5:1.0:2.5 to obtain the negative electrode slurry for lithium-ion batteries, which is then coated on both sides of copper foil with a coating amount of 216 g / m². 2 After drying at 85°C, the material is cold-pressed; then slit, sliced, and dried under vacuum at 85°C for 4 hours to obtain lithium-ion battery anode sheets. (2) The positive electrode, negative electrode and separator prepared above are stacked to form a cell with a thickness of 0.5 mm, a width of 8 mm and a length of 10 mm. The cell is vacuum baked at 85°C for 48 h and injected with the electrolyte to obtain a lithium-ion battery with a capacity of 3 Ah.

[0026] Examples 2-18 and Comparative Examples 1-8 were prepared according to the preparation method described in Example 1, except that the components in the electrolyte were changed, as shown in Table 1 below. The performance of the lithium-ion batteries in the examples and comparative examples was tested. The test items and methods are as follows: (1) Cyclic performance test: The lithium-ion batteries corresponding to Example 1 and Comparative Example 1 were charged and discharged at 0.1C for three weeks under normal temperature (25℃) and high temperature (45℃) conditions, respectively, and then charged and discharged at 0.5C and discharged at 1C for 200 cycles. The results are as follows. Figure 1 , 2 As shown; (2) High-temperature capacity retention rate: After capacity testing, the lithium-ion batteries described in the examples and comparative examples were charged and discharged at 0.33C for three weeks at room temperature. The discharge capacity of the last cycle was recorded as C1. The batteries were then fully charged at 0.33C current I, left to stand at 60°C for 7 days, and then discharged at 0.33C current I until empty. The discharge capacity was recorded as C2. The high-temperature capacity retention rate of the experimental batteries was calculated. The results are shown in Table 1.

[0027] Table 1. Electrolyte composition and battery performance test results of the examples and comparative examples.

[0028] Note: Formula A'-1; Formula A'-2; Formula A'-3; Based on Table 1, a comparison of the test results of Examples 1-18 and Comparative Examples 2-3 shows that, compared to simply adding Compound A or Compound B as an additive, the combined additive of Compound A and Compound B results in a higher high-temperature cycle capacity retention rate for lithium-ion batteries, demonstrating the effect of the combined additive in enhancing the stability of the electrolyte interface film. The test results of Examples 1-18 and Comparative Examples 6-7 show that, although the high-temperature cycle capacity retention rate of lithium-ion batteries obtained when Compound A' and Compound B, which do not meet the requirements of Structural Formula I, are combined as additives, is relatively higher than that of Compound A or Compound B alone as additives, it is far less than the high-temperature resistance effect of the combined additive of Compound A and Compound B, which meets the requirements of Structural Formula I. This demonstrates the effect of the combined additive of Compound A and Compound B in enhancing the stability of the electrolyte interface film. The specific synergistic effect between compounds B indicates that not all silane and isothiocyanate compounds can effectively improve the high-temperature resistance of the electrolyte. Test results from Examples 1-12 and 13-18 show that when compounds A and B are combined with electron-donating and electron-withdrawing groups respectively, the high-temperature cycle capacity retention of the resulting lithium-ion battery is further improved compared to the previous results. However, test results from Comparative Example 8 show that when both compounds A and B are combined with electron-withdrawing groups, the high-temperature cycle capacity retention of the resulting lithium-ion battery cannot be further improved. This demonstrates that specific selection of substituents in compounds A and B is necessary to further optimize the high-temperature resistance of the electrolyte.

[0029] Figure 1 , Figure 2 The figures are battery capacity retention curves for Example 1 and Comparative Example 1 after 200 cycles at room temperature (25°C) and high temperature (45°C), respectively. (Refer to...) Figure 1 , 2 It can be seen that Example 1 has a capacity retention rate of ≥98% after 200 cycles at 25℃ and ≥96% at 45℃, which are significantly improved compared with Comparative Example 1, demonstrating the effect of additives in enhancing the stability of the interfacial film.

[0030] In summary, it can be seen that by combining the silane compounds shown in Formula I and the isothiocyanate compounds shown in Formula II in this invention, the high-temperature performance of the battery can be significantly improved compared with traditional additives. This is because the introduction of the above-mentioned combined additives has a synergistic effect, making the positive and negative electrode films more dense and stable, reducing the interfacial impedance of the positive and negative electrodes, and at the same time removing by-products on the surface of the positive and negative electrodes, improving the ionic conductivity of the interfacial film, thereby greatly improving the high-temperature performance of the battery.

[0031] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An electrolyte additive, characterized in that, This includes compound A, which is represented by structural formula I, and compound B, which is represented by structural formula II; Formula I; Formula II; In Formula I, R1 is an unsaturated alkenyl group, and R2, R3, and R4 are each independently a C1-C4 alkyl group; In Formula II, R is an alkyl-substituted ester group or a phenyl group; The mass ratio of compound A to compound B is 1:5-5:

1.

2. The electrolyte additive according to claim 1, characterized in that, Compound A and compound B are shown in the following structural formulas: Formula A-1; Formula A-2; Formula B-1; Formula B-2.

3. The electrolyte additive according to claim 1, characterized in that, R1 is an unsaturated alkenyl group containing an electron-donating group, wherein the electron-donating group is a methoxy group; R is an aromatic group containing an electron-withdrawing group, wherein the electron-withdrawing group is a cyano, fluorine, or nitro group; Compound A and compound B are shown in the following structural formulas: Formula A-3; Formula A-4; Formula B-3; Formula B-4; Formula B-5.

4. A high-temperature resistant lithium-ion battery electrolyte, characterized in that, include: Conductive lithium salt, the combined additives according to any one of claims 1-3, other additives, and organic solvents.

5. The high-temperature resistant lithium-ion battery electrolyte according to claim 4, characterized in that, The proportion of the combined additives is 0.1-5 wt% based on the total mass of the electrolyte.

6. The high-temperature resistant lithium-ion battery electrolyte according to claim 4 or 5, characterized in that, The other additives are at least one of sulfone, tris(trimethylsilane) phosphite, vinyl sulfate, methylene disulfonate, vinylene carbonate, or fluorovinyl carbonate. The proportion of the other additives is 0.5-3 wt% based on the total mass of the electrolyte.

7. The high-temperature resistant lithium-ion battery electrolyte according to claim 4 or 5, characterized in that, The conductive lithium salt is at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium dioxalate borate, lithium difluorooxalate borate, lithium difluorophosphate, lithium difluorobis(oxalate) phosphate, or lithium bis(fluorosulfonyl)imide.

8. The high-temperature resistant lithium-ion battery electrolyte according to claim 4 or 5, characterized in that, The organic solvent is at least one of cyclic carbonates, chain carbonates, or carboxylic acid esters. The cyclic carbonate is at least one of ethylene carbonate, propylene carbonate, butenyl carbonate, or γ-butyrolactone. The chain carbonate is at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, or ethyl propyl carbonate. The carboxylic acid ester is at least one of methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, or ethyl butyrate.

9. An NCM ternary lithium-ion battery, characterized in that, It includes a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and the electrolyte according to any one of claims 4-8.

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