Electrolyte and silicon-based lithium ion battery

Abstract

The invention provides an electrolyte and a silicon-based lithium ion battery. The electrolyte comprises a lithium salt, an organic solvent and an additive, the mass ratio of the additive in the electrolyte is 0.1%-10%, and the additive comprises a sulfur-containing compound and a silazane compound. The sulfur-containing compound has stronger electrochemical reduction tendency and can be preferentially reduced to generate an SEI film, meanwhile, the formed SEI comprises Li2SO3, CH3CH (OSO2Li) and the like, and the components can improve the stability of the SEI film and inhibit the volume expansion of the silicon negative electrode. The silazane compound can form a protective film containing a silicon-nitrogen bond on the surface of the negative electrode, so that the thermal stability and the electrochemical stability of the electrolyte are improved. According to the electrolyte formula disclosed by the invention, the problem of poor cycling stability of the silicon-based lithium ion battery is effectively solved by controlling the types and the concentrations of the additives and utilizing the synergistic effect, and the first effect of the silicon-based lithium ion battery is also improved.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and more particularly, to an electrolyte and a silicon-based lithium-ion battery. Background Art

[0002] The rapid growth of the demand for electric vehicles and large-scale energy storage has put forward higher requirements for the energy density (>350 Wh·kg -1 ) and cost of the next-generation batteries. Considering factors such as comprehensive cost and specific energy, silicon with an ultra-high theoretical energy density is considered to be one of the most promising anode materials. However, the huge volume change (volume change rate is about 300%) during the lithiation / delithiation process of the silicon-based anode will cause the Si particles to be crushed, the SEI to be continuously damaged and rebuilt, resulting in electrolyte depletion and loss of active materials, and ultimately leading to a rapid decay of the battery cycle life. To address the problems such as the volume expansion of the silicon-based anode and the resulting interface damage, the academic and industrial circles mainly focus on two directions: structural design of the silicon-based anode material itself and regulation of the electrode-electrolyte interface by adjusting the electrolyte.

[0003] Structural design of the silicon-based anode material itself: ① Nanostructured silicon anodes to slow down the volume expansion during lithiation / delithiation of the silicon anode; ② Material-coated silicon anodes to improve the mechanical strength, electrochemical performance, etc. of silicon. However, both have problems such as high production cost and poor material homogeneity. In contrast, adding a suitable film-forming additive to preferentially decompose before the reduction and decomposition of other components in the electrolyte to form a high-performance SEI film with better stability, higher mechanical properties, and flexibility to cope with the volume expansion of silicon is a simpler and more efficient direction. Summary of the Invention

[0004] The main object of the present application is to provide an electrolyte and a silicon-based lithium-ion battery to solve the problem in the prior art that the volume expansion of the silicon-based lithium-ion battery during cycling is too large, resulting in an unstable SEI film and poor battery cycling performance.

[0005] To achieve the above object, according to one aspect of the present application, an electrolyte is provided. The electrolyte includes a lithium salt, an organic solvent, and an additive. The mass ratio of the additive in the electrolyte is 0.1% to 10%, and the additive includes a sulfur-containing compound and a silazane compound.

[0006] Furthermore, the mass ratio of the sulfur-containing compound to the silazane compound is (1 to 5):1.

[0007] Further, the sulfur-containing compound is selected from any one or more of sulfide compounds, sulfonate compounds, organic sulfates, organic sulfites, and sulfate ester compounds; further, the sulfur-containing compound is selected from any one or more of allyl sulfide, 1,3-propane sultone, ethylene sulfite, 3,3-diethylenedisulfite, and vinylene sulfate; and / or, the silazane compound is a methylsilazane compound and / or an inorganic polysilazane; further, the silazane compound is selected from any one or more of hexamethyldisilazane, trimethylsilyldiethylamine, trimethylsilyl azide, and inorganic polysilazane.

[0008] Further, the sulfur-containing compound contains at least allyl sulfide; further, the sulfur-containing compound is a combination of allyl sulfide and 3,3-diethylenedisulfite, and the mass ratio of allyl sulfide to 3,3-diethylenedisulfite is 2:(1-5).

[0009] Further, the silazane compound contains at least hexamethyldisilazane; further, the silazane compound is a combination of hexamethyldisilazane and trimethylsilyldiethylamine, and the mass ratio of hexamethyldisilazane to trimethylsilyldiethylamine is (3-7):(3-7).

[0010] Further, the ionic conductivity of the electrolyte at 25 °C is 10.4-12.8 mS / cm.

[0011] Further, the mass ratio of the organic solvent in the electrolyte is 60-85%; further, the organic solvent is a carbonate compound.

[0012] Further, the carbonate compound is selected from any one or more of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate; further, the carbonate compound is a combination of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate, and the volume ratio of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate is (3-8):(1-5):(1-2).

[0013] Further, the concentration of the lithium salt in the electrolyte is 0.1-2 mol / L; further, the lithium salt is selected from any one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(difluoromethanesulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0014] According to another aspect of the present application, a silicon-based lithium-ion battery is provided, including a positive electrode sheet, an electrolyte, and a silicon-based negative electrode sheet, and the electrolyte is the aforementioned electrolyte.

[0015] Applying the technical solution of the present application, sulfur-containing compounds have a stronger tendency for electrochemical reduction, can preferentially reduce to form a SEI film, protect the solvent from decomposition, and at the same time, the components of the formed SEI are Li 2 SO 3 、CH 3 CH(OSO 2 Li), etc. These components can improve the stability of the SEI film, inhibit the volume expansion of the silicon anode, and thus contribute to improving the cycle stability of the silicon-based lithium-ion battery. Silazane compounds can form a protective film containing silicon-nitrogen bonds on the surface of the anode, improving the thermal stability and electrochemical stability of the electrolyte. Among them, the Si-N-Si active groups in silazane compounds can combine with the hydroxyl groups on the surface of the silicon-based anode through nucleophilic substitution reactions to form Si-O-Si covalent bonds. This reaction can generate a dense hydrophobic layer, which helps to reduce the surface energy of the silicon-based anode, reduce the agglomeration and pulverization of silicon particles during charge and discharge, and maintain the integrity of the electrode structure. At the same time, it reduces the adsorption of trace moisture in the electrolyte, inhibits the corrosion of the electrolyte to the electrode, and improves the interfacial chemical stability. In addition, during the first lithium intercalation process, silazane compounds undergo reductive decomposition at a low potential (<1.0V vs. Li / Li+), generating silicon-containing oligomers and nitrogen-containing components. The inorganic-organic composite SEI film formed by these products can adapt to the volume expansion of silicon particles and relieve the rupture of the SEI caused by mechanical stress. When sulfur-containing compounds and silazane compounds are used in combination, the decomposition products of silazane compounds can preferentially cover the highly active areas on the electrode surface, while the Li 2 SO 3 、CH 3 CH(OSO 2 Li) and other components in the sulfur-containing compounds contribute to enhancing the mechanical strength of the SEI. The two form a gradient structure, which helps to improve the overall stability of the SEI film, and thus helps to improve the cycle stability of the silicon-based lithium-ion battery. And when sulfur-containing compounds and silazane compounds are used in combination, it helps to improve the ionic conductivity of the electrolyte. Therefore, the electrolyte formulation of the present application effectively solves the problem of poor cycle stability of silicon-based lithium-ion batteries by controlling the types and concentrations of additives and utilizing the synergistic effect, and also improves the first efficiency of silicon-based lithium-ion batteries. Detailed implementation manners

[0016] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The following will describe the present application in detail with reference to the embodiments.

[0017] As analyzed in the background art of the present application, in the prior art, the silicon-based lithium-ion battery has a problem of poor cycle performance due to excessive volume expansion during cycling, which causes the SEI film to be unstable. The present application provides an electrolyte and a silicon-based lithium-ion battery.

[0018] In a typical embodiment of the present application, an electrolyte is provided. The electrolyte includes a lithium salt, an organic solvent, and an additive. The mass ratio of the additive in the electrolyte is 0.1% to 10%. The additive includes a sulfur-containing compound and a silazane compound.

[0019] The sulfur-containing compound has a stronger electrochemical reduction tendency and can preferentially reduce to form a SEI film, protecting the solvent from decomposition. At the same time, the components of the formed SEI are Li 2 SO 3 、CH 3 CH(OSO 2 Li), etc. These components can improve the stability of the SEI film and inhibit the volume expansion of the silicon negative electrode, thus helping to improve the cycle stability of the silicon-based lithium-ion battery. The silazane compound can form a protective film containing silicon-nitrogen bonds on the surface of the negative electrode, enhancing the thermal stability and electrochemical stability of the electrolyte. Among them, the Si-N-Si active group in the silazane compound can combine with the hydroxyl group on the surface of the silicon-based negative electrode through a nucleophilic substitution reaction to form a Si-O-Si covalent bond. This reaction can generate a dense hydrophobic layer, which helps to reduce the surface energy of the silicon-based negative electrode, reduce the agglomeration and pulverization of silicon particles during charge and discharge, and maintain the integrity of the electrode structure. At the same time, it reduces the adsorption of trace moisture in the electrolyte, inhibits the corrosion of the electrolyte to the electrode, and enhances the interfacial chemical stability. In addition, during the first lithiation process, the silazane compound undergoes reductive decomposition at a low potential (<1.0V vs. Li / Li+), generating silicon-containing oligomers and nitrogen-containing components. The inorganic-organic composite SEI film composed of these products can adapt to the volume expansion of silicon particles and relieve the rupture of the SEI caused by mechanical stress. When the sulfur-containing compound and the silazane compound are used in combination, the decomposition products of the silazane compound can preferentially cover the highly active areas on the electrode surface, while the Li 2 SO 3 、CH 3 CH(OSO 2 Li) and other components in the sulfur-containing compound help to enhance the mechanical strength of the SEI. The two form a gradient structure, which helps to improve the overall stability of the SEI film and further helps to improve the cycle stability of the silicon-based lithium-ion battery. And when the sulfur-containing compound and the silazane compound are used in combination, it helps to improve the ionic conductivity of the electrolyte. Therefore, the electrolyte formulation of the present application effectively solves the problem of poor cycle stability of the silicon-based lithium-ion battery by controlling the types and concentrations of additives and utilizing the synergistic effect, and also improves the first efficiency of the silicon-based lithium-ion battery.

[0020] In an embodiment of the present application, the mass ratio of the additive in the above electrolyte is 3% to 7%. Specifically, it can be 3%, 4%, 5%, 6%, 7%, and the range values between any two values, which helps to further optimize the performance of the SEI film and improve the ionic conductivity of the electrolyte.

[0021] In an embodiment of the present application, the mass ratio of the sulfur-containing compound to the silazane compound is (1 to 5):1. In an embodiment of the present application, the mass ratio of the sulfur-containing compound to the silazane compound is (2 to 4):1. Specifically, it can be 2:1, 3:1, 4:1, and the range values between any two ratios.

[0022] The decomposition products of the silazane compound can preferentially cover the highly active areas on the electrode surface, while the components such as Li 2 SO 3 、CH 3 CH(OSO 2 Li) generated by the sulfur-containing compound help to enhance the mechanical strength of the SEI. Controlling the mass ratio of the sulfur-containing compound to the silazane compound within the above range helps to give full play to the mutual synergistic effect between the two, thus helping to form an SEI film with better mechanical strength and flexibility, further helping to alleviate the influence of the volume change of silicon particles on the battery performance, and also helping to further improve the ionic conductivity of the electrolyte, thereby helping to further improve the initial efficiency of the battery.

[0023] In an embodiment of the present application, the above sulfur-containing compound is selected from any one or more of thioether compounds, sulfonate compounds, organic sulfates, organic sulfites, and sulfate ester compounds; in an embodiment of the present application, the above sulfur-containing compound is selected from any one or more of allyl sulfide, 1,3-propane sultone, ethylene sulfite, 3,3-diethylenedisulfite, and vinylene sulfate; and / or, the silazane compound is a methylsilazane compound and / or an inorganic polysilazane; in an embodiment of the present application, the above silazane compound is selected from any one or more of hexamethyldisilazane, trimethylsilyldiethylamine, trimethylsilyl azide, and inorganic polysilazane.

[0024] Controlling the type of sulfur-containing compound within the above range helps to form more Li 2 SO 3 and CH 3 CH(OSO 2(Li), which helps to further improve the stability of the SEI film. Controlling the types of silazane compounds within the above range helps to provide more Si-N-Si active groups, thus contributing to the formation of a denser hydrophobic layer. And controlling the types of sulfur-containing compounds and silazane compounds within the above range helps to enhance the synergistic effect between the two, thereby contributing to further improving the ionic conductivity of the electrolyte and forming a more stable SEI film.

[0025] In one embodiment of the present application, at least allyl sulfide is contained in the above sulfur-containing compound; in one embodiment of the present application, the above sulfur-containing compound is a combination of allyl sulfide and 3,3-diethylenedisulfite, and the mass ratio of allyl sulfide to 3,3-diethylenedisulfite is 2:(1-5), specifically it can be 2:5, 2:4, 2:3, 2:2, 2:1 and the range values between any two ratios.

[0026] Allyl sulfide has a relatively high LUMO value and a relatively low HOMO value, enabling allyl sulfide to be further reduced to participate in the formation of the SEI film after the solvent decomposes to form the SEI film, and regulating the composition of the SEI film to form a more stable SEI film. The addition of 3,3-diethylenedisulfite and controlling the mass ratio of allyl sulfide to 3,3-diethylenedisulfite within the above range helps to form a more stable film and a smaller impedance, better regulate the positive and negative electrode interfaces, and thus promotes the stable deintercalation and intercalation of Li + In addition, the formed interfacial film can effectively inhibit the dissolution of transition metal elements and protect the structural stability of the material, and inhibit the volume expansion of the silicon negative electrode.

[0027] In one embodiment of the present application, at least hexamethyldisilazane is contained in the above silazane compound; in one embodiment of the present application, the above silazane compound is a combination of hexamethyldisilazane and trimethylsilyldiethylamine, and the mass ratio of hexamethyldisilazane to trimethylsilyldiethylamine is (3-7):(3-7), specifically it can be 3:7, 3:5, 3:3, 4:7, 4:5, 4:3, 5:7, 5:6, 5:4, 5:3, 6:7, 6:5, 6:4, 6:3, 7:6, 7:5, 7:4, 7:3 and the range values between any two ratios.

[0028] The Si-N-Si active groups in hexamethyldisilazane can combine with a large number of hydroxyl groups existing on the surface of the silicon-based anode through nucleophilic substitution reactions with the hydroxyl groups to form Si-O-Si covalent bonds. This reaction can generate a dense trimethylsilyloxy hydrophobic layer, which can reduce the surface energy of silicon, reduce the agglomeration and pulverization of silicon particles during charge and discharge, and maintain the integrity of the electrode structure. In addition, during the first lithiation process, hexamethyldisilazane undergoes reductive decomposition at a low potential (<1.0 V vs. Li / Li+), generating polysiloxane and Li 3 N. The inorganic-organic composite SEI film composed of these products can adapt to the volume expansion of silicon particles (~300%) and relieve the rupture of the SEI caused by mechanical stress. The addition of trimethylsilyldiethylamine and controlling the mass ratio of hexamethyldisilazane to trimethylsilyldiethylamine within the above range helps to enhance the thermal stability of the electrolyte and delay the decomposition reaction of the electrolyte at high temperatures; in addition, the amino group (-N(C 2 H 5 ) 2 ) in TMSDEA is alkaline and can effectively neutralize the residual acidic substances (such as HF) in the electrolyte, which helps to reduce the corrosion of the interfacial film and maintain the stability of the positive and negative electrode structures.

[0029] In an embodiment of the present application, the ionic conductivity of the above electrolyte at 25 °C is 10.4 - 12.8 mS / cm. In an embodiment of the present application, the ionic conductivity of the above electrolyte at 25 °C is 11.2 - 12.8 mS / cm.

[0030] The electrolyte with the above ionic conductivity helps to improve the charge-discharge specific capacity and the first efficiency of the battery.

[0031] In an embodiment of the present application, the mass ratio of the organic solvent in the above electrolyte is 60 - 85%; in an embodiment of the present application, the above organic solvent is a carbonate compound.

[0032] Controlling the mass ratio of the organic solvent in the electrolyte within the above range helps to improve the stability of the electrolyte, the solubility and electrochemical activity of the lithium salt and additives in the electrolyte. The carbonate compound has a stable molecular structure and a wide electrochemical window, and its use as an organic solvent helps to improve the stability of the electrolyte during charge and discharge.

[0033] In an embodiment of the present application, the above carbonate compound is selected from any one or more of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate; in an embodiment of the present application, the above carbonate compound is a combination of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate, and the volume ratio of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate is (3 - 8):(1 - 5):(1 - 2).

[0034] Ethylene carbonate has a relatively high dielectric constant, can effectively dissolve lithium salts and preferentially form a SEI film on the electrode surface. This film can significantly improve the electrochemical stability of the battery, reduce the decomposition of the electrolyte and the loss of active materials, and extend the battery life. Diethyl carbonate has a relatively low viscosity, can improve the fluidity of the electrolyte, helps the rapid transmission of lithium ions, and enhances the power density and charge-discharge efficiency of the battery. The addition of methyl ethyl carbonate can make the SEI film more flexible, better adapt to the volume change of the silicon-based anode during charge and discharge, alleviate the risk of SEI film rupture, and increase the cycle stability of the battery. Controlling the volume ratio of ethylene carbonate, diethyl carbonate and methyl ethyl carbonate within the above range helps to integrate the advantages of the three, form a dense and flexible SEI film, optimize the lithium ion transmission path, and improve the ionic conductivity and electrochemical stability of the electrolyte. Allyl sulfide has the highest LUMO value compared with the above organic solvents, and at the same time its HOMO value is lower than that of methyl ethyl carbonate, indicating that allyl sulfide can further participate in the formation of the SEI film after methyl ethyl carbonate decomposes to form the SEI film, regulate the composition of the SEI film to form a more stable SEI film and help reduce the reduction decomposition of ethylene carbonate and diethyl carbonate.

[0035] In an embodiment of the present application, the concentration of the lithium salt in the above electrolyte is 0.1-2 mol / L; in an embodiment of the present application, the above lithium salt is selected from any one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(difluoromethanesulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide.

[0036] Controlling the concentration of the lithium salt in the electrolyte within the above range helps to keep the electrolyte with high ionic conductivity while avoiding too high viscosity, thereby helping to improve the transmission efficiency of lithium ions in the electrolyte, and further helping to improve the initial efficiency of the battery. Controlling the type of lithium salt within the above range helps to improve the interaction between the lithium salt and each component, thereby helping to further improve the ionic conductivity of the electrolyte.

[0037] In an embodiment of the present application, the above electrolyte is composed of a lithium salt, an organic solvent, and an additive. The lithium salt is lithium hexafluorophosphate, the concentration of the lithium salt in the electrolyte is 1 mol / L, the organic solvent is a combination of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate, the volume ratio of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate is 3:5:2, the additive is a combination of a sulfur-containing compound and a silazane compound, the mass ratio of the sulfur-containing compound to the silazane compound is 3:1, the sulfur-containing compound is a combination of allyl sulfide and 3,3-diethylenedisulfite, and the mass ratio of allyl sulfide to 3,3-diethylenedisulfite is 2:1. The silazane compound is a combination of hexamethyldisilazane and trimethylsilyldiethylamine, and the mass ratio of hexamethyldisilazane to trimethylsilyldiethylamine is 3:7.

[0038] In an embodiment of the present application, the above electrolyte is composed of a lithium salt, an organic solvent, and an additive. The lithium salt is lithium hexafluorophosphate, the concentration of the lithium salt in the electrolyte is 1 mol / L, the organic solvent is a combination of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate, the volume ratio of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate is 8:1:1, the additive is a combination of a sulfur-containing compound and a silazane compound, the mass ratio of the sulfur-containing compound to the silazane compound is 2:1, the sulfur-containing compound is a combination of allyl sulfide and 3,3-diethylenedisulfite, and the mass ratio of allyl sulfide to 3,3-diethylenedisulfite is 2:5. The silazane compound is a combination of hexamethyldisilazane and trimethylsilyldiethylamine, and the mass ratio of hexamethyldisilazane to trimethylsilyldiethylamine is 7:3.

[0039] In a typical embodiment of the present application, a method for preparing an electrolyte is provided. The method includes: in a glove box with an oxygen content < 10 ppm and a moisture content < 1 ppm, an organic solvent is prepared, and a molecular sieve cooled to 110 °C is placed therein for 48 h to remove moisture. The lithium salt is dissolved in the dehydrated organic solvent, and then the additive is added to the organic solvent, and stirred with a magnetic stirrer for 12 h until the additive is completely dissolved to obtain the electrolyte.

[0040] In another typical embodiment of the present application, a silicon-based lithium-ion battery is provided, including a positive electrode sheet, an electrolyte, and a silicon-based negative electrode sheet, and the electrolyte is the aforementioned electrolyte.

[0041] Since the aforementioned electrolyte is used in the silicon-based lithium-ion battery, the silicon-based lithium-ion battery has high cycle stability and initial efficiency.

[0042] The beneficial effects of the present application will be further described below in conjunction with embodiments.

[0043] Example 1

[0044] In a glove box with an oxygen content < 10 ppm and a moisture content < 1 ppm, an organic solvent was prepared according to ethylene carbonate EC / diethyl carbonate DEC / ethyl methyl carbonate EMC = 3:5:2 (v:v:v). The organic solvent was cooled to 110 °C and molecular sieves were placed in it for 48 h to remove moisture. Lithium hexafluorophosphate LiPF 6 was weighed in a high-purity argon glove box and dissolved in the moisture-removed organic solvent at a molar concentration of 1 mol / L. Then, allyl sulfide AS and hexamethyldisilazane HDMS were weighed respectively, and the mass ratio of allyl sulfide AS to hexamethyldisilazane HDMS was 3:1. They were added thereto and stirred with a magnetic stirrer for 12 h until the additives were completely dissolved to obtain an electrolyte. The mass fraction of the organic solvent in the electrolyte was 85%, and the mass fraction of the additive was 5%.

[0045] Example 2

[0046] The difference from Example 1 was that the mass ratio of allyl sulfide to hexamethyldisilazane was 2:1, and finally an electrolyte was obtained.

[0047] Example 3

[0048] The difference from Example 1 was that the mass ratio of allyl sulfide to hexamethyldisilazane was 1:1, and finally an electrolyte was obtained.

[0049] Example 4

[0050] The difference from Example 1 was that the mass ratio of allyl sulfide to hexamethyldisilazane was 5:1, and finally an electrolyte was obtained.

[0051] Example 5

[0052] The difference from Example 1 was that the mass ratio of allyl sulfide to hexamethyldisilazane was 6:1, and finally an electrolyte was obtained.

[0053] Example 6

[0054] The difference from Example 1 was that the combination of allyl sulfide and 3,3-diethylenedisulfite was used to replace allyl sulfide, and the mass ratio of allyl sulfide to 3,3-diethylenedisulfite in the combination of allyl sulfide and 3,3-diethylenedisulfite was 2:5, and finally an electrolyte was obtained.

[0055] Example 7

[0056] The difference from Example 1 is that the combination of allyl sulfide and sodium 3,3-diethylenedisulfonate is used to replace allyl sulfide, and the mass ratio of allyl sulfide to sodium 3,3-diethylenedisulfonate in the combination of allyl sulfide and sodium 3,3-diethylenedisulfonate is 2:1, and finally an electrolyte is obtained.

[0057] Example 8

[0058] The difference from Example 1 is that the combination of hexamethyldisilazane and trimethylsilyldiethylamine is used to replace hexamethyldisilazane, and the mass ratio of hexamethyldisilazane to trimethylsilyldiethylamine in the combination of hexamethyldisilazane and trimethylsilyldiethylamine is 3:7, and finally an electrolyte is obtained.

[0059] Example 9

[0060] The difference from Example 1 is that the combination of hexamethyldisilazane and trimethylsilyldiethylamine is used to replace hexamethyldisilazane, and the mass ratio of hexamethyldisilazane to trimethylsilyldiethylamine in the combination of hexamethyldisilazane and trimethylsilyldiethylamine is 7:3, and finally an electrolyte is obtained.

[0061] Example 10

[0062] The difference from Example 1 is that 1,3-propanesultone is used to replace allyl sulfide, and finally an electrolyte is obtained.

[0063] Example 11

[0064] The difference from Example 1 is that trimethylsilyldiethylamine is used to replace hexamethyldisilazane, and finally an electrolyte is obtained.

[0065] Example 12

[0066] The difference from Example 1 is that the volume ratio of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate is 8:1:1, and finally an electrolyte is obtained.

[0067] Example 13

[0068] The difference from Example 1 is that the volume ratio of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate is 1:6:3, and finally an electrolyte is obtained.

[0069] Example 14

[0070] The difference from Example 1 is that it is added according to the proportion that the total mass of allyl sulfide and hexamethyldisilazane accounts for 7% in the electrolyte, and finally an electrolyte is obtained.

[0071] Example 15

[0072] The difference from Example 1 is that the electrolyte is finally obtained by adding the total mass of allyl sulfide and hexamethyldisilazane at a proportion of 10% in the electrolyte.

[0073] Example 16

[0074] The difference from Example 1 is that the electrolyte is finally obtained by adding the total mass of allyl sulfide and hexamethyldisilazane at a proportion of 0.1% in the electrolyte.

[0075] Comparative Example 1

[0076] The difference from Example 1 is that the electrolyte is finally obtained without adding allyl sulfide and hexamethyldisilazane.

[0077] Comparative Example 2

[0078] The difference from Example 1 is that the electrolyte is finally obtained without adding allyl sulfide.

[0079] Comparative Example 3

[0080] The difference from Example 1 is that the electrolyte is finally obtained without adding hexamethyldisilazane.

[0081] Comparative Example 4

[0082] The difference from Example 1 is that the electrolyte is finally obtained by adding the total mass of allyl sulfide and hexamethyldisilazane at a proportion of 15% in the electrolyte.

[0083] Ionic conductivity test

[0084] Take 5 mL of the electrolytes prepared in the examples and comparative examples and inject them into a polytetrafluoroethylene reagent bottle. After inserting the conductivity electrode, place it in a constant temperature test chamber. Test procedure: At 25°C, let the electrolyte stand for 1 h, and then measure it with a conductivity meter. The test results are shown in Table 1.

[0085] Battery performance test

[0086] Use a silicon / graphite / carbon composite material as the negative electrode active material, and make a negative electrode sheet according to the ratio of negative electrode active material: conductive agent: binder = 8:1:1 (mass ratio), and place it in a glove box for standby. Then use a lithium metal sheet as the counter electrode and a 25-μm polyethylene separator as the separator, and add 100 μL of the electrolytes prepared in the examples and comparative examples respectively, and assemble CR2032 type button batteries in a glove box (LS800D type, oxygen and water content ≤ 0.1 ppm) and seal them. Place the assembled batteries at room temperature for 10 h and conduct electrochemical tests.

[0087] The assembled battery was tested for charge-discharge cycle performance using a Wuhan Blue Electric CT2001A battery test system, and the test voltage range was set to 2.4 - 4.2 V (vs. Li + / Li). The test results are shown in Table 1.

[0088] Table 1

[0089]

[0090]

[0091] It can be seen from Table 1 that by comparing the ionic conductivity of the electrolytes and the cycle data of the batteries in the examples and comparative examples, the combined use of sulfur-containing compounds and silazane compounds helps to improve the conductivity of the electrolyte, enhance the ion transport efficiency, and contribute to the formation of a more stable SEI film, thereby helping to improve the energy density of silicon-based lithium-ion batteries, improve the cycle stability, and extend the service life.

[0092] From the above description, it can be seen that the above embodiments of the present application achieve the following technical effects:

[0093] Sulfur-containing compounds have a stronger tendency for electrochemical reduction and can preferentially reduce to form an SEI film to protect the solvent from decomposition. At the same time, the components of the formed SEI are Li 2 SO 3 、CH 3 CH(OSO 2 Li), etc. These components can improve the stability of the SEI film and inhibit the volume expansion of the silicon negative electrode, thereby contributing to improving the cycle stability of silicon-based lithium-ion batteries. Silazane compounds can form a protective film containing silicon-nitrogen bonds on the surface of the negative electrode, enhancing the thermal stability and electrochemical stability of the electrolyte. Among them, the Si-N-Si active groups in the silazane compounds can combine with the hydroxyl groups on the surface of the silicon-based negative electrode through nucleophilic substitution reactions to form Si-O-Si covalent bonds. This reaction can generate a dense hydrophobic layer, which helps to reduce the surface energy of the silicon-based negative electrode, reduce the aggregation and pulverization of silicon particles during charge and discharge, and maintain the integrity of the electrode structure. At the same time, it reduces the adsorption of trace moisture in the electrolyte, inhibits the corrosion of the electrolyte to the electrode, and enhances the interfacial chemical stability. In addition, during the first lithiation process, the silazane compounds undergo reductive decomposition at a low potential (<1.0 V vs. Li / Li+), generating silicon-containing oligomers and nitrogen-containing components. The inorganic-organic composite SEI film formed by these products can adapt to the volume expansion of silicon particles and relieve the rupture of the SEI caused by mechanical stress. When sulfur-containing compounds and silazane compounds are used in combination, the decomposition products of the silazane compounds can preferentially cover the highly active regions on the electrode surface, while the Li 2 SO 3, CH 3 CH(OSO 2 Components such as Li) help to enhance the mechanical strength of the SEI. The two form a gradient structure, which helps to improve the overall stability of the SEI film, and further helps to improve the cycle stability of the silicon-based lithium-ion battery. And when the sulfur-containing compound and the silazane compound are used in combination, it helps to improve the ionic conductivity of the electrolyte. Therefore, the electrolyte formulation of the present application effectively solves the problem of poor cycle stability of the silicon-based lithium-ion battery by controlling the types and concentrations of additives and the utilization of the synergistic effect, and also improves the initial efficiency of the silicon-based lithium-ion battery.

[0094] The above are only the preferred embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. An electrolyte, characterized in that: The electrolyte comprises lithium salt, organic solvent and additives, wherein the mass proportion of the additives in the electrolyte is 0.1% to 10%, and the additives comprise sulfur-containing compounds and silazane compounds.

2. The electrolyte according to claim 1, characterized in that The mass ratio of the sulfur-containing compound to the silazane compound is (1-5):

1.

3. The electrolyte according to claim 1 or 2, characterized in that The sulfur-containing compound is selected from any one or more of thioether compounds, sulfonate compounds, organic sulfates, organic sulfites and sulfate compounds; preferably, the sulfur-containing compound is selected from any one or more of allyl sulfide, 1,3-propane sultone, ethylene sulfite, 3,3-diethylene disulfite and vinyl sulfate; And / or, the silazane compound is a methylsilazane compound and / or an inorganic polysilazane; preferably, the silazane compound is selected from any one or more of hexamethyldisilazane, trimethylsilyldiethylamine, trimethylsilyl azide and the inorganic polysilazane.

4. The electrolyte according to claim 3, characterized in that The sulfur-containing compound contains at least the allyl sulfide; preferably, the sulfur-containing compound is a combination of the allyl sulfide and the 3,3-diethylene disulfite, and the mass ratio of the allyl sulfide to the 3,3-diethylene disulfite is 2:(1-5).

5. The electrolyte according to claim 3 or 4, characterized in that The silazane compound contains at least the hexamethyldisilazane; preferably, the silazane compound is a combination of the hexamethyldisilazane and the trimethylsilyldiethylamine, and the mass ratio of the hexamethyldisilazane to the trimethylsilyldiethylamine is (3-7): (3～7)。 6. The electrolyte according to any one of claims 1 to 5, characterized in that The ionic conductivity of the electrolyte at 25° C. is 10.4 to 12.8 mS / cm.

7. The electrolyte according to any one of claims 1 to 6, characterized in that The mass percentage of the organic solvent in the electrolyte is 60-85%; preferably, the organic solvent is a carbonate compound.

8. The electrolyte according to claim 7, characterized in that The carbonate compound is selected from any one or more of ethylene carbonate, diethyl carbonate and methyl ethyl carbonate; further preferably, the carbonate compound is a combination of ethylene carbonate, diethyl carbonate and methyl ethyl carbonate, and the volume ratio of the ethylene carbonate, the diethyl carbonate and the methyl ethyl carbonate is (3-8):(1-5):(1-2).

9. The electrolyte according to any one of claims 1 to 8, characterized in that The concentration of the lithium salt in the electrolyte is 0.1-2 mol / L; preferably, the lithium salt is selected from any one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide) and lithium bis(trifluoromethylsulfonylimide).

10. A silicon-based lithium-ion battery, comprising a positive electrode sheet, an electrolyte and a silicon-based negative electrode sheet, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 9.

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