A cyanosilicate compound, electrolyte and lithium ion battery

Abstract

The present invention discloses a cyanosilicate compound, an electrolyte, and a lithium-ion battery. The present invention uses a cyano compound with "Si" as the central atom as a lithium-ion battery electrolyte additive. Its molecular structure contains four "-CN" atoms, which are more effective in inhibiting the dissolution of transition metal ions in the high-voltage state of lithium batteries than currently commercialized nitrile additives. Compared with HTCN, the "Si-O" atoms in the skeleton structure improve lithium-ion conductivity and effectively reduce battery impedance. Furthermore, the "-CN" and "Si-O" functional groups have good affinity for H protons, giving the cyanosilicate additive a water- and acid-removing effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a cyanosilicate compound, an electrolyte and a lithium ion battery. Background Art

[0002] The statements herein merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] In recent years, lithium-ion batteries have been widely used in digital products, power and energy storage and other fields. High energy density has always been one of the key directions for the development of power lithium-ion battery technology, because it is an important way to solve the user's mileage anxiety problem. In high-energy-density battery systems, the current commercial cathode materials are mainly LiNixCoyMn(1-xy)O2 and LiCoO2. Both have layered structures. The unstable material structure under high-voltage conditions can easily cause high-valent metal ions to dissolve, resulting in a sharp deterioration in battery performance. In addition, a large increase in charging voltage will also cause an increase in electrolyte side reactions, such as EC decomposition to produce gas, FEC (fluoroethylene carbonate) decomposition leading to increased acidity, and increased water and HF content leading to increased decomposition of LiPF6. Therefore, the development of electrolytes that are both resistant to high voltage and high temperature has become an important research topic for upstream and downstream electrolyte companies.

[0004] The cyano group of nitrile additives has a strong electronegativity and a strong coordination effect with transition metal ions, which can inhibit the dissolution of metal ions. At the same time, it preferentially captures H protons when oxidation side reactions occur in the electrolyte, inhibiting the decomposition of LiPF6 and FEC. Nitrile additives such as succinonitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrinitrile (HTCN), and 1,2-bis(2-cyanoethoxy)ethane (DENE) have been used in large quantities and have achieved good results in improving the high voltage, high temperature and cycle performance of batteries. Extensive use and comparative verification have shown that 1,3,6-hexanetrinitrile has the best performance among the four nitrile additives used in large quantities. The reason is that there are three cyano groups in a single molecule, which makes its coordination effect with transition metal ions stronger. However, the main chain structure of HTCN is all C atoms, which greatly limits the Li + transmission.

[0005] FEC is a common electrolyte component, mainly used as a negative electrode film-forming additive and high-voltage solvent. When FEC is used as a film-forming additive for silicon-based negative electrodes, the addition amount must be above 5% to meet the repeated growth requirements of the SEI film during the lithium insertion and removal process of the silicon-based negative electrode. FEC has a higher oxidation potential than EC and can be used as a high-voltage solvent, and the addition amount is generally greater than 10%. However, FEC has problems such as water sensitivity and high-temperature instability, which have a significant impact on the high temperature and cycle performance of the battery. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the present invention aims to provide a cyanosilicate compound, an electrolyte and a lithium ion battery.

[0007] In order to achieve the above object, the present invention is implemented through the following technical solutions:

[0008] In a first aspect, the present invention provides a cyanosilicate compound, the structural formula of which is shown below:

[0009]

[0010] Wherein, the compound of formula I is named tetra(2-cyanoethyl)silicate; the compound of formula II is named tetracyanomethylsilicate.

[0011] In some embodiments, the method for preparing the cyanosilicate compound comprises the following steps:

[0012] 1) mixing 3-hydroxypropionitrile or hydroxyacetonitrile and a hydrogen chloride chelating agent to obtain a system solution;

[0013] 2) adding a silicon tetrachloride organic solution to the system solution obtained in the above step to carry out a low-temperature reaction to obtain a cyanosilicate;

[0014] 3) The reaction product is filtered, extracted, and molecularly distilled to obtain a target product with a purity of ≥99.5%.

[0015] In some embodiments, the mixing temperature is -10 to -20°C.

[0016] In some embodiments, the low-temperature reaction temperature is 25-45° C., and the reaction time is 3-7 h.

[0017] In a second aspect, the present invention provides a lithium-ion battery electrolyte comprising an organic solvent, a lithium salt and an additive, wherein the additive comprises FEC and at least one of the cyanosilicate compounds, and the amount of the cyanosilicate compound added is 0.1%-3%, where % is the mass percentage.

[0018] The addition amount here is the percentage of the total mass of the electrolyte.

[0019] In some embodiments, the amount of FEC added is 0.01%-15%, where % is mass percentage and the amount added here is the percentage of the total mass of the electrolyte.

[0020] Preferably, the added amount of FEC is 5%-15%, where % is mass percentage.

[0021] Preferably, the cyanosilicate compound is tetrakis(2-cyanoethyl)silicate, and the addition amount thereof is 1-2%, where % is mass percentage; the addition amount of FEC is 7%-12%, where % is mass percentage.

[0022] More preferably, the cyanosilicate compound is tetrakis(2-cyanoethyl)silicate, and the addition amount thereof is 1.5%, where % is mass percentage; the addition amount of FEC is 10%, where % is mass percentage.

[0023] In some embodiments, the electrolyte further includes a third additive, which is selected from one of 1,3-propane sultone (PS), 1,4-butane sultone (BS), 1,3-propylene sulfonic acid (PST), methylene methanedisulfonate (MMDS), tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) phosphite (TMSPi), tris(trimethylsilyl) borate (TMSB), succinonitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrinitrile (HTCN), 1,2-bis(2-cyanoethoxy)ethane (DENE), 1,2,3-tris(cyanoethoxy)propane, tris(2-cyanoethyl)borate, tris(2-cyanoethyl) phosphate, tris(2-cyanoethyl) phosphite, vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl sulfate (DTD), vinyl sulfite (ES) or vinyl fluorosulfate, or a combination thereof.

[0024] Preferably, the amount of the third additive added is 0.1%-8%, where % is the mass percentage and the amount added here is the percentage of the total mass of the electrolyte.

[0025] In some embodiments, the organic solvent is selected from at least two of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), γ-butyrolactone (GBL), propyl propionate (PP), and ethyl propionate (EP).

[0026] Preferably, the organic solvent accounts for 50%-80% of the total mass of the electrolyte.

[0027] In some embodiments, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluorooxalatoborate (LiODFB), lithium difluorobis(oxalatophosphate) (LiDFBP), lithium difluorophosphate (LiPO2F2), and lithium tetrafluoroborate (LiBF4).

[0028] Preferably, the lithium salt accounts for 10%-20% of the total mass of the electrolyte.

[0029] In a third aspect, the present invention provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the electrolyte is the lithium-ion battery electrolyte.

[0030] The beneficial effects achieved by one or more embodiments of the present invention are as follows:

[0031] This invention uses a cyano compound with "Si" as its central atom as a lithium-ion battery electrolyte additive. Its molecular structure contains four "-CN" groups. Compared to currently commercialized nitrile additives, it is more effective in inhibiting the dissolution of transition metal ions at high voltages in lithium batteries. Compared to HTCN, the "Si-O" group in the backbone structure improves lithium-ion conductivity and effectively reduces battery impedance. Furthermore, the "-CN" and "Si-O" functional groups have a good affinity for hydrogen protons, making this cyanosilicate additive effective in removing water and acid.

[0032] The present invention also provides an electrolyte solution that uses a cyanosilicate as a first additive and FEC as a second additive or solvent. Through the synergistic effect of these two additives, the FEC decomposition side reaction is suppressed, thereby improving the overall performance of the electrolyte solution. Validation of multiple battery systems demonstrated that the electrolyte solution exhibited significant effects in improving both high-voltage battery systems and high-nickel ternary battery systems with silicon-based anodes. DETAILED DESCRIPTION

[0033] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0034] The present invention will be further described below with reference to the embodiments.

[0035] Example

[0036] The preparation method of tetrakis(2-cyanoethyl)silicate (Formula I) is:

[0037]

[0038] (1) Add 0.4 mol of anhydrous 3-hydroxypropionitrile and 0.3 mol of triethylamine mixed solution into a three-necked flask, and control the reaction temperature at -15°C;

[0039] (2) 0.1 mol of silicon tetrachloride and 100 ml of dimethyl carbonate were mixed and placed in a constant pressure dropping funnel;

[0040] (3) adding the mixed solution of step (2) dropwise to the mixed solution of step (1) at a rate of 10 ml / h;

[0041] (4) After the addition is complete, the temperature is raised to 40° C. and the reaction is continued for 5 h; triethylamine hydrochloride is filtered out and the filtrate is distilled under reduced pressure to obtain a crude product;

[0042] (5) The crude reaction product was added to ice water for dissolution, and then xylene was added and stirred thoroughly. The organic phase was separated using a separatory funnel. The organic phase was washed with ice water for 3 to 4 times, and the xylene was removed by rotary evaporation to obtain tetra(2-cyanoethyl)silicate with a yield of 66.7%. After molecular distillation, the target product with a purity of 99.5% was obtained.

[0043] The preparation method of tetracyanomethylsilicate (Formula II) is:

[0044] (1) Add 0.5 mol of anhydrous hydroxyacetonitrile and 0.3 mol of triethylamine mixed solution into a three-necked flask, and control the reaction temperature at -20°C;

[0045] (2) 0.1 mol of silicon tetrachloride and 100 ml of dimethyl carbonate were mixed and placed in a constant pressure dropping funnel;

[0046] (3) adding the mixed solution of step (2) dropwise to the mixed solution of step (1) at a rate of 10 ml / h;

[0047] (4) After the addition is complete, the temperature is raised to 30° C. and the reaction is continued for 6 h; triethylamine hydrochloride is filtered out and the filtrate is distilled under reduced pressure to obtain a crude product;

[0048] (5) The crude product was added to ice water for dissolution, and then xylene was added and stirred thoroughly. The organic phase was separated using a separatory funnel. The organic phase was washed with ice water for 3 to 4 times, and xylene was removed by rotary evaporation to obtain tetramethyl silicate with a yield of 50.8%. After molecular distillation, the target product with a purity of 99.2% was obtained.

[0049] The following provides the specific preparation process of electrolyte, positive electrode sheet, negative electrode sheet and lithium battery:

[0050] (1) Preparation of electrolyte: In a glove box filled with argon, the corresponding solvents were mixed uniformly in a predetermined ratio and stirred continuously. A predetermined amount of electrolyte lithium salt and additives were slowly added to the mixed solvent to obtain Examples 1 to 8 and Comparative Examples 1 to 6. The electrolyte formula is shown in Table 1.

[0051] (2) Preparation of positive electrode sheets: Polyvinylidene fluoride (PVDF), conductive agent and positive electrode material are added to N-methylpyrrolidone (NMP) in a mass ratio of 1.5%:1.5%:97%, and the mixture is thoroughly stirred and evenly mixed. The slurry is coated on an aluminum foil current collector, dried, cold pressed and punched to obtain positive electrode sheets.

[0052] In Examples 1 to 2 and Comparative Examples 1 to 2, the positive electrode material is high-voltage lithium cobalt oxide; in Examples 3 to 8 and Comparative Examples 3 to 6, the positive electrode material is high-nickel ternary material (811).

[0053] (3) Preparation of negative electrode sheets: Sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber latex (SBR), conductive agent, graphite and silicon oxide are added to deionized water in the mass ratio of 1.3%: 2%: 1%: 90%: 6.7% in sequence, and the mixture is thoroughly stirred and evenly mixed. The slurry is coated on a copper foil current collector, dried, cold pressed and punched to obtain a negative electrode sheet.

[0054] (4) Preparation of lithium battery: The separator, positive electrode sheet and negative electrode sheet are stacked in a "Z" shape to obtain a bare cell to be filled with liquid, and then the cell to be filled with liquid is packaged with aluminum plastic film. After baking, the electrolyte is injected at an addition amount of 2.8g / Ah to obtain a lithium-ion battery with a nominal capacity of 3Ah to be tested.

[0055] (5) Testing of lithium-ion batteries:

[0056] DCR test: Charge the battery to 4.0V using constant current and constant voltage. After standing for 6 hours, test the battery's DC internal resistance.

[0057] Cycling performance test: At different temperatures, the capacity retention rate under different numbers of 1C charge and discharge cycles was tested. For Examples 1 to 2 and Comparative Examples 1 to 2, the charge and discharge cut-off voltage was 2.8V to 4.5V; for Examples 3 to 8 and Comparative Examples 3 to 6, the charge and discharge cut-off voltage was 2.8V to 4.2V.

[0058] High-temperature storage performance test: The fully charged battery is stored at 60°C for 7 days to test the capacity retention rate and capacity recovery rate.

[0059] (6) Negative electrode transition metal deposition test: After the battery was cycled at high temperature for 500 times, it was discharged to 3.0 V, the battery was disassembled, the negative electrode powder was collected, and the ICP test was performed. For Examples 1 to 2 and Comparative Examples 1 to 2, the Co element content was recorded; for Examples 3 to 8 and Comparative Examples 3 to 6, the Ni element content was recorded.

[0060] Table 1 and Table 2 provide the specific compositions of the electrolytes of Examples 1 to 3 and Comparative Examples 1 to 2, and Examples 3 to 8 and Comparative Examples 3 to 6, respectively.

[0061] Table 1 Electrolyte compositions of Examples 1-2 and Comparative Examples 1-2

[0062]

[0063]

[0064] Table 2 Electrolyte compositions of Examples 3 to 8 and Comparative Examples 3 to 6

[0065]

[0066] Tables 3 and 4 provide the performance test results of batteries of different embodiments and comparative examples.

[0067] Table 3

[0068]

[0069]

[0070] Table 4

[0071]

[0072]

[0073] Result analysis:

[0074] Combining the electrolyte formulations of the embodiment and comparative example in Table 1 and the battery test performance results in Table 3, it can be seen that the addition of nitrile additives can significantly improve the cycle performance and storage performance of high-voltage lithium cobalt oxide batteries and inhibit the precipitation of Co ions at the negative electrode.

[0075] Compared with Comparative Example 2, Examples 1 and 2 have lower electrochemical resistance (DCR) because the "Si-O" in the cyanosilicate compound has better lithium ion conductivity than the carbon chain structure of HTCN. The battery's high-temperature cycle performance is greatly improved. In addition to the improvement in DCR, this is mainly attributed to the cyanosilicate compound reducing the acidity of the electrolyte, thereby effectively inhibiting the decomposition of FEC. The reaction formula is as follows:

[0076]

[0077] in conclusion

[0078] Combining the electrolyte formulas of the embodiments and comparative examples in Table 3 and the battery test performance results in Table 4, it can be seen that under the high nickel and silicon-based negative electrode system, the battery as a whole exhibits electrochemical performance results with the same trend as high-voltage lithium cobalt oxide batteries.

[0079] In this system, the introduction of the cyanosilicate additive significantly improved the battery's high-temperature performance. ICP test results of the negative electrode after cycling indicate that nickel ion dissolution is somewhat suppressed. Under the same addition ratio, the addition of tetramethyl silicate in a higher molar amount than tetra(2-cyanoethyl) silicate resulted in a more pronounced improvement in high-temperature performance, but a relative deterioration in DCR.

[0080] Based on the overall test results of the embodiment and comparative example batteries, it can be found that when the addition amount of the first additive tetra(2-cyanoethyl)silicate is 1.5% and the FEC addition amount is 10%, the overall performance of the battery is optimal, which proves that the two have a good synergistic effect and have good use value in high-voltage battery systems and silicon-based negative electrode systems.

[0081] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A lithium-ion battery electrolyte, characterized in that: The method comprises an organic solvent, a lithium salt and an additive, wherein the additive comprises at least one of a cyanosilicate compound and FEC. The addition amount of the cyanosilicate compound is 1.5%, where % is the mass percentage; The amount of FEC added is 7%-10%, where % is the mass percentage; The structural formula of the cyanosilicate compound is shown below: or .

2. The lithium-ion battery electrolyte according to claim 1, wherein: The electrolyte further includes a third additive, which is selected from one of 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sulfonic acid, methylene methanedisulfonate, tris(trimethylsilane) phosphate, tris(trimethylsilane) phosphite, tris(trimethylsilane) borate, succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile, 1,2-bis(2-cyanoethoxy)ethane, 1,2,3-tris(cyanoethoxy)propane, tris(2-cyanoethyl)borate, tris(2-cyanoethyl) phosphate, tris(2-cyanoethyl) phosphite, vinylene carbonate, vinyl ethylene carbonate, vinyl sulfate, vinyl sulfite or vinyl fluorosulfate, or a combination thereof.

3. The lithium-ion battery electrolyte according to claim 2, wherein: The addition amount of the third additive is 0.1%-8%, where % is mass percentage.

4. The lithium-ion battery electrolyte according to claim 1, wherein: The organic solvent is selected from at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, γ-butyrolactone, propyl propionate, and ethyl propionate.

5. The lithium-ion battery electrolyte according to claim 4, wherein: The organic solvent accounts for 50%-80% of the total mass of the electrolyte.

6. The lithium-ion battery electrolyte according to claim 1, wherein: The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), lithium difluorophosphate, and lithium tetrafluoroborate.

7. The lithium-ion battery electrolyte according to claim 6, wherein: The lithium salt accounts for 10%-20% of the total mass of the electrolyte.

8. A lithium-ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the electrolyte is the lithium-ion battery electrolyte according to claim 1.

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