Electrolyte additive, high-temperature stable electrolyte and lithium ion battery

Abstract

The application discloses an electrolyte additive, a high-temperature stable electrolyte and a lithium ion battery, and belongs to the technical field of organic synthesis. The problem of gas generation of hexamethyldisilazane (HMDS) in an electrolyte is solved by carrying out a nucleophilic substitution reaction on a silazane and a borate ester. The obtained new compound can form a film on a positive electrode surface as an electrolyte additive, reduces catalytic decomposition of the electrolyte by the positive electrode, and improves high-temperature stability of the electrolyte.

Description

Technical Field

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

[0002] With the rapid development of the low-altitude economy, the demand for eVTOL (electric vertical takeoff and landing) aircraft has surged, placing higher performance requirements on high-energy-density, high-power batteries. High-energy-density batteries extensively utilize silicon-based anode materials due to their high theoretical specific capacity (approximately 4200 mAh / g, far exceeding the 372 mAh / g of graphite anodes), making them a research hotspot. However, silicon anodes suffer from severe volume expansion during charge and discharge, leading to repeated growth of the solid electrolyte interphase (SEI) film, continuously consuming active lithium in the electrolyte, and severely impacting battery cycle performance. To suppress the severe volume expansion of silicon, silazane-based electrolyte additives have been applied. Patent CN120127223A discloses that hexamethyldisilazane can improve the stability of the SEI film, suppress the volume expansion of the silicon anode, and simultaneously remove moisture from the electrolyte, improving the thermal and electrochemical stability of the electrolyte. Patent CN119495819A also discloses that silazane additives can improve charge and discharge capacity and significantly enhance cycle performance.

[0003] However, the aforementioned silazane additives still have significant problems. For example, hexamethyldisilazane readily reacts to produce ammonia (NH3), causing the battery to bulge due to gas production. When left in the electrolyte for a long time, it is prone to producing white precipitates, affecting the quality of the electrolyte and reducing the battery's safety and cycle performance. Furthermore, it cannot form an effective protective film on the positive electrode surface, leading to instability at the positive electrode interface. Summary of the Invention

[0004] This invention provides an electrolyte additive, a high-temperature stable electrolyte, and a lithium-ion battery. By performing a nucleophilic substitution reaction between silazane and borate ester, the problem of gas generation by hexamethyldisilazane (HMDS) in the electrolyte is improved. The resulting new compound can form a film on the positive electrode surface as an electrolyte additive, reducing the catalytic decomposition of the electrolyte by the positive electrode and improving the high-temperature stability of the electrolyte.

[0005] The electrolyte additive proposed in this invention has the following structural formula I:

[0006] R1 and R2 are each independently selected from alkyl, phenyl or cycloalkyl groups with 1-6 carbon atoms.

[0007] In this invention, the additive improves the problem of easy gas generation in HMDS by modifying the borate ester functional group, and at the same time forms a stable and low-resistance CEI film (cathode electrolyte interphase) on the positive electrode. The electrolyte / electrode interface layer is relatively stable and can effectively suppress the catalytic decomposition of the electrolyte by the strong oxidizing properties of the high nickel positive electrode. The additive has relatively low film-forming impedance, which can also meet the low impedance requirement of the battery under fast charging conditions.

[0008] Preferably, the structural formula of the electrolyte additive is as follows: .

[0009] In this invention, experimental results demonstrate that, compared to compounds B and C, compound A has the least steric hindrance and the best film-forming performance on the positive electrode surface. On the one hand, it can more effectively prevent the electrolyte from reacting with the positive electrode to generate gas, and on the other hand, it can more effectively improve the stability of the electrolyte.

[0010] Preferably, the electrolyte additive is obtained by nucleophilic substitution reaction of hexamethyldisilazane and borate ester, and the specific synthetic route is shown below:

[0011] Preferably, the borate ester is one of trimethyl borate, triethyl borate, tripropyl borate, triisopropyl borate, tributyl borate, tritert-butyl borate, or triphenyl borate.

[0012] The present invention also proposes a high-temperature stable electrolyte, comprising an electrolyte lithium salt, an organic solvent, and the above-mentioned electrolyte additives.

[0013] Preferably, the electrolyte lithium salt is at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluoromethanesulfonyl)imide (LiFSI), lithium dioxolaneborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorooxolaneborate (LiDFOB), lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, or lithium perchlorate.

[0014] Preferably, the organic solvent is at least one of organic esters, alkyl ethers, cyclic ethers, sulfones, dinitriles, or ionic liquids; The organic esters are at least one of ethylene carbonate (EC), propylene carbonate (PC), butenyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl formate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate, or ethyl butyrate; the alkyl ethers are at least one of dimethyl ether, diethyl ether, or methyl ethyl ether; the sulfones are at least one of dimethyl sulfoxide, dimethyl sulfone, or sulfolane; the dinitriles are at least one of adiponitrile, succinate, or glutaronitrile; and the ionic liquids are at least one of imidazole or pyrrole ionic liquids.

[0015] Preferably, based on the total mass of the electrolyte, the mass percentage of the electrolyte lithium salt is 8-26%, the mass percentage of the organic solvent is 72-90%, and the mass percentage of the electrolyte additive is 0.1-5%.

[0016] Preferably, the mass of the electrolyte lithium salt accounts for 8-26% of the total mass of the electrolyte, and more preferably 10-16%.

[0017] Preferably, the organic solvent accounts for 72-90% of the total mass of the electrolyte, and more preferably 80-90%.

[0018] Preferably, the electrolyte further includes other additives; The other additives are at least one of fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone, vinylene carbonate, ethylene ethylene carbonate, ethylene sulfate, or ethylene sulfite.

[0019] Preferably, the electrolyte additive accounts for 0.1-5% of the weight of the electrolyte, and the total mass of the additives accounts for 0.1-20% of the weight of the electrolyte.

[0020] The present invention also proposes a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and the above-mentioned high-temperature stable electrolyte.

[0021] Preferably, the positive electrode active material is any one of lithium cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, or lithium manganese iron phosphate; preferably, it is lithium nickel cobalt manganese oxide.

[0022] Preferably, the negative electrode active material is any one of graphite, silicon oxide, silicon carbon, lithium metal or lithium titanate; preferably, it is a negative electrode composed of silicon carbon and graphite, with a specific capacity of about 600 mAh / g.

[0023] Preferably, the diaphragm is a polypropylene (PP), polyethylene (PE) diaphragm or a glass fiber diaphragm; preferably, it is a polypropylene (PP) diaphragm.

[0024] Compared with the prior art, the present invention has the following technical effects: (1) This invention essentially proposes a novel additive to suppress electrolyte gas production, which can improve the stability of the SEI film, suppress the volume expansion of the silicon anode, and reduce the generation of other substances. The generated CEI film is rich in boron and silicon elements, forming a compatible SEI film on the silicon anode. Compared with commonly used high-temperature unsaturated additives on the market, the film impedance is lower. Compared with silazane additives, it has better high-temperature stability, which can meet the requirements of electrolyte stability during fast charging.

[0025] (2) The present invention also proposes a lithium-ion battery electrolyte that has good compatibility with the positive and negative electrodes, can construct a stable high-temperature resistant CEI film on the positive and negative electrodes, inhibit electrolyte decomposition, slow down the expansion of lithium-ion batteries, and improve the cycle stability of the battery. Attached Figure Description

[0026] Figure 1 The 1H NMR spectrum of compound A described in this invention; Figure 2 The above are rate curves of the electrolyte used in lithium-ion batteries according to the embodiments and comparative examples of the present invention. Detailed Implementation

[0027] 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.

[0028] Synthesis of compound A: Dry the glassware, including the three-necked flask, water separator, and condenser, thoroughly in an oven. Assemble while hot, and then purge the system with an inert gas (nitrogen) for 10 minutes to displace the air. Connect an anhydrous calcium chloride drying tube to the upper end of the condenser. Under continuous inert gas flow protection, add 8.95 g (10.4 mL, 50.0 mmol) of hexamethyldisilazane, 5.70 g (5.4 mL, 55.0 mmol) of trimethyl borate, 10 mg of p-toluenesulfonic acid catalyst, and 30 mL of anhydrous toluene solvent (as an azeotropic carrier) to the three-necked flask. Assemble the reaction apparatus (connect one end of the three-necked flask to the water separator, and the water separator to the reflux condenser). Turn on the stirrer and begin heating, maintaining a steady reflux of the reaction mixture (toluene boiling point is approximately 110°C). Periodically release the lower layer of methanol from the valve at the bottom of the water separator and record the volume. Continue heating until no new methanol accumulates in the water separator (usually 12-24 hours). After h), indicating that the reaction was essentially complete, the heat source was removed, and the reaction solution was allowed to cool to room temperature. The reaction solution was transferred to a round-bottom flask, and toluene was removed by vacuum distillation on a rotary evaporator (water bath temperature < 40℃). The remaining liquid was then subjected to high-vacuum fractionation, with the target product distilled off within the range of 50-70℃ / 1-2 mmHg, yielding compound A. Its 1H NMR spectrum is shown in the figure below. Figure 1 As shown, refer to Figure 1 It can be seen that its 1 H NMR (400MHz, CDCl3) δ 3.37 (s, 6H), 0.07 (s, 18H).

[0029] Following the same synthetic method described above, the difference lies in replacing trimethyl borate with an equimolar amount of triethyl borate to obtain compound B. 1 H NMR (400MHz, CDCl3) δ 3.91 (q, J=4.4Hz, 4H), 1.13 (t, J=4.6Hz, 6H), 0.08 (s, 18H).

[0030] Following the same synthetic method described above, the difference lies in replacing trimethyl borate with an equimolar amount of triethyl borate to obtain compound C. 1 H NMR (400MHz, CDCl3) δ 7.31 (d, J=7.8Hz, 4H), 7.03-6.94 (m, 6H), 0.06 (s, 18H).

[0031] Example 1 This embodiment presents a high-temperature stable electrolyte, which is prepared by the following method: In an argon glove box with water and oxygen content ≤0.1 ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a mass ratio of 3:7 to obtain an organic solvent. Lithium hexafluorophosphate (LiPF6) is then slowly added to the organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and compound A are added and stirred until homogeneous to obtain the electrolyte. The amounts of lithium hexafluorophosphate, organic solvent, FEC, and compound A added are 13%, 80%, 6%, and 1% of the total mass of the electrolyte, respectively.

[0032] This embodiment also presents a lithium-ion battery, which is prepared by the following method: The positive electrode material lithium nickel cobalt manganese oxide (NCM90), conductive agent carbon black (SuperP), and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:3:3. After mixing, N-methylpyrrolidone was added, and the theoretical solid content was controlled to be 55%. The mixture was homogenized using a vacuum defoamer to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated onto an aluminum foil with a thickness of 12 μm. After drying, rolling, and cutting, the positive electrode sheet was obtained. A silicon-carbon and graphite composite anode material (specific capacity approximately 600 mAh / g), conductive agent SuperP, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) were mixed in a mass ratio of 94:1.5:1.5:3. After mixing, deionized water was added, and the theoretical solid content was controlled to be 52%. The mixture was homogenized using a vacuum defoamer to obtain an anode slurry. The anode slurry was then uniformly coated onto an 8 μm thick copper foil. After drying, rolling, and cutting, the anode sheet was obtained. The N / P ratio of the positive and negative electrodes was 1.1. A soft-pack stacked battery is manufactured in an environment with a dew point temperature below -60 ℃. The positive electrode, separator (polypropylene PP) and negative electrode are stacked in sequence to ensure that the separator completely separates the positive and negative electrodes. Before electrolyte injection, the moisture content of the battery is baked to below 200 ppm. The above-mentioned electrolyte is then injected, and the battery is sealed, formed and tested for capacity to obtain the lithium-ion battery.

[0033] Example 2 This embodiment proposes a high-temperature stable electrolyte, which is prepared by the method described in Example 1. The difference from Example 1 is that the amounts of lithium hexafluorophosphate, organic solvent, FEC, and compound A added are 13%, 80.5%, 6%, and 0.5% of the total mass of the electrolyte, respectively.

[0034] This embodiment also proposes a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0035] Example 3 This embodiment proposes a high-temperature stable electrolyte, which is prepared by the method described in Example 1. The difference from Example 1 is that the amounts of lithium hexafluorophosphate, organic solvent, FEC, and compound A added are 13%, 79.5%, 6%, and 1.5% of the total mass of the electrolyte, respectively.

[0036] This embodiment also proposes a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0037] Example 4 This embodiment proposes a high-temperature stable electrolyte, which is prepared by the method described in Example 1. The difference from Example 1 is that compound B is used instead of compound A.

[0038] This embodiment also proposes a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0039] Example 5 This embodiment proposes a high-temperature stable electrolyte, which is prepared by the method described in Example 1. The difference from Example 1 is that compound C is used instead of compound A.

[0040] This embodiment also proposes a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0041] Comparative Example 1 This comparative example presents an electrolyte prepared by the method described in Example 1. The difference from Example 1 is that compound A is omitted, and the amounts of lithium hexafluorophosphate, organic solvent, and FEC added are 13%, 81%, and 6% of the total mass of the electrolyte, respectively.

[0042] This comparative example also presents a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0043] Comparative Example 2 This comparative example presents an electrolyte prepared by the method described in Example 1, except that hexamethyldisilazane (HMDS) is used instead of compound A.

[0044] This comparative example also presents a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0045] Comparative Example 3 This comparative example presents an electrolyte prepared by the method described in Example 1, except that trimethyl borate (TMB) is used instead of compound A.

[0046] This comparative example also presents a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0047] Comparative Example 4 This comparative example presents an electrolyte prepared by the method described in Example 1, except that HMDS and TMB in a mass ratio of 1.5:1 are used instead of compound A.

[0048] This comparative example also presents a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0049] Comparative Example 5 This comparative example presents an electrolyte prepared by the method described in Example 1, except that tris(trimethylsilane)phosphate (TMSP) is used instead of compound A.

[0050] This comparative example also presents a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0051] Comparative Example 6 This comparative example presents an electrolyte prepared by the method described in Example 1, except that tris(trimethylsilane)borate ester (TMAB) is used instead of compound A.

[0052] This comparative example also presents a lithium-ion battery, which is prepared by the method described in Example 1. The difference from Example 1 is that the electrolyte described in Example 1 is used instead of the electrolyte described in Example 1.

[0053] (1) Battery rate charging test The lithium-ion battery prepared above was charged at a constant current of 1C to a voltage of 4.2V in a constant temperature chamber at an ambient temperature of 25 ℃, then charged at a constant voltage of 4.2V to a current of 0.05C, and then discharged at a constant current of 1C to a voltage of 2.8V. This cycle was repeated for 3 weeks, and the discharge capacity of the third week was recorded as C0. Then, the battery was charged at different rates of 0.33C, 0.5C, 1C, 2C, 4C, and 6C to a voltage of 4.2V, and then discharged at a constant current of 1C to a voltage of 2.8V. Each rate was cycled three times. The rate-based capacity retention rate (%) = (charge capacity at different rates / first-cycle discharge capacity) The results are shown in Table 1 below. Figure 2 As shown; (2) Battery high temperature cycle test The lithium-ion battery prepared above was tested in a constant temperature chamber at an ambient temperature of 45 ℃. It was charged at a constant current of 1C to a voltage of 4.2V, then charged at a constant voltage of 4.2V to a current of 0.05C, and then discharged at a constant current of 1C to a voltage of 2.8V. The cycle was repeated for 300 cycles, and the capacity retention rate was recorded. The capacity retention rate (%) of the nth cycle was calculated as (discharge capacity of the nth cycle / discharge capacity of the first cycle). The results are shown in Table 1 below. (3) Battery volume expansion rate test The lithium-ion battery prepared above was placed in a constant temperature chamber at an ambient temperature of 45℃, and the battery volume V0 was measured. After capacity testing, the battery volume V1 was measured. Then, it was charged at a constant current of 1C to a voltage of 4.2V, then charged at a constant voltage of 4.2V to a current of 0.05C, and then discharged at a constant current of 1C to a voltage of 2.8V. After 200 cycles, the battery volume V2 was measured. The volume expansion rate was calculated based on the values ​​of V0, V1, and V2: (V2-V1) / V0×100%. The results are shown in Table 1 below. Table 1. Electrolyte composition and corresponding battery performance of each embodiment and comparative example.

[0054] As shown in Table 1 above, the volume expansion rate of the pouch battery prepared in the examples is significantly lower than that of the pouch battery in the comparative example, indicating that compounds A, B, and C form a stable SEI film on the positive electrode surface, suppressing gas generation during battery cycling. Furthermore, the capacity retention rate of the examples is better than that of the comparative example, indicating that compounds A, B, and C effectively improve the high-temperature cycling performance of the pouch battery, and the formed SEI film is relatively stable at high temperatures; among these, compound A is the best. Data from 6C rate charging shows that the examples improve the capacity retention rate during rate charging compared to the comparative example, especially compound A (1%), which exhibits the best performance.

[0055] 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, Its structural formula is shown in Equation I below: R1 and R2 are each independently selected from alkyl, phenyl or cycloalkyl groups with 1-6 carbon atoms.

2. The electrolyte additive according to claim 1, characterized in that, The electrolyte additive is compound A, B, or C, which is represented by any of the following structural formulas: 。 3. The electrolyte additive according to claim 1 or 2, characterized in that, The electrolyte additive is obtained by nucleophilic substitution reaction of hexamethyldisilazane and borate ester, and the specific synthetic route is shown below: 。 4. The electrolyte additive according to claim 3, characterized in that, The borate ester is one of trimethyl borate, triethyl borate, tripropyl borate, triisopropyl borate, tributyl borate, tritert-butyl borate, or triphenyl borate.

5. A high-temperature stable electrolyte, characterized in that, It includes lithium electrolyte salts, organic solvents, and electrolyte additives as described in any one of claims 1-4.

6. The high-temperature stable electrolyte according to claim 5, characterized in that, The electrolyte lithium salt is at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium dioxaborate, lithium difluorophosphate, lithium difluorooxaborate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, or lithium perchlorate.

7. The high-temperature stable electrolyte according to claim 5 or 6, characterized in that, The organic solvent is at least one of organic esters, alkyl ethers, cyclic ethers, sulfones, dinitriles, or ionic liquids; The organic esters are at least one of ethylene carbonate, propylene carbonate, butene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl formate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate, or ethyl butyrate; the alkyl ethers are at least one of dimethyl ether, diethyl ether, or methyl ethyl ether; the sulfones are at least one of dimethyl sulfoxide, dimethyl sulfone, or sulfolane; the dinitriles are at least one of adiponitrile, succinate, or glutaronitrile; and the ionic liquids are at least one of imidazole or pyrrole ionic liquids.

8. The high-temperature stable electrolyte according to claim 5 or 6, characterized in that, Based on the total mass of the electrolyte, the mass percentage of the electrolyte lithium salt is 8-26%, the mass percentage of the organic solvent is 72-90%, and the mass percentage of the electrolyte additive is 0.1-5%.

9. The high-temperature stable electrolyte according to claim 5 or 6, characterized in that, The electrolyte also includes other additives; The other additives are at least one of fluoroethylene carbonate, 1,3-propanesulfonate lactone, ethylene carbonate, ethylene ethylene carbonate, ethylene sulfate, or ethylene sulfite.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and the high-temperature stable electrolyte as described in any one of claims 5-9.

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