Electrolyte for pure silicon negative electrode, preparation method of electrolyte and battery

Abstract

The invention belongs to the field of batteries, and particularly relates to an electrolyte for a pure silicon negative electrode, a preparation method of the electrolyte and a battery. The additive comprises a solvent additive and a lithium salt additive; the solvent additive at least comprises a fluorine-containing additive; the fluorine-containing additive is one or a mixture of fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC). According to the technical scheme, the problem of gas production caused by excessive side reactions of a pure silicon negative electrode is solved, and the performance of the battery in a high-temperature storage test is improved. And meanwhile, the ion migration rate can be increased, the battery impedance is reduced, and the problems of fast cycle life attenuation and poor rate capability of the battery caused by high internal resistance of a pure silicon negative electrode are solved.

Description

Technical Field

[0001] The present invention belongs to the field of batteries, and particularly relates to an electrolyte for a pure silicon anode, a preparation method thereof, and a battery. Background Art

[0002] The growing electric vehicle industry and consumers' pursuit of the performance of electric vehicles have put forward higher requirements for the performance of lithium-ion batteries, such as energy density, cycle life, and fast charging. Developing electrode active materials with higher capacity and more stable electrochemical performance is the key to comprehensively improving battery performance. Compared with traditional graphite anodes (theoretical specific capacity of 372 mAh g-1), silicon-based anodes have a higher specific capacity (4200 mAh g-1), which can effectively promote the improvement of battery energy density and have broad application prospects.

[0003] However, during the electrochemical cycling process of silicon-based anodes, different from the graphite intercalation reaction, silicon undergoes an alloying reaction with lithium, resulting in a huge volume effect. The volume of silicon after complete lithium intercalation is about three times that of the original silicon. Even with structural design optimization and surface doping and coating modification of silicon materials, it is still impossible to completely avoid the excessive volume change of silicon particles due to their intrinsic properties. This change causes silicon particles to continuously crack during cycling, exposing fresh interfaces and continuously reacting with the electrolyte. During this process, the electrolyte is rapidly consumed, and the side reaction products cause an increase in the internal resistance of the electrode, ultimately leading to battery failure and seriously affecting the battery life.

[0004] In addition, silicon is a semiconductor material with a conductivity of only 10 mS / cm, and the ion diffusion coefficient is between 10-10 and 10-13 cm2 / s (for graphite, it is 10 -6 to 10 -11 cm2 / s). Therefore, the ability of silicon anodes to conduct electrons and Li+ is inferior to that of graphite anodes, affecting the charge transfer kinetics during battery charging and discharging and limiting the rate performance of the battery. The above disadvantages limit the practical application of single silicon anodes.

[0005] Currently, the optimization applications for silicon-based batteries are divided into the following three directions: mixing of anode materials, modification of anode materials, design of anode electrode structures, and optimization of electrolytes.

[0006] In the direction of mixing anode materials, a certain amount of silicon-based anode (usually with a proportion less than 30%) is added to the existing pure graphite anode system. In this way, the energy density of the battery can be increased by about 25% at most without affecting the conductivity. However, since the lithium intercalation potential of silicon materials is higher than that of graphite materials, the composite anode used in a mixed manner still cannot suppress the excessive volume effect of silicon materials, resulting in the battery life being much lower than that of the traditional pure graphite system. Therefore, this method cannot effectively solve the problems faced by silicon-based anodes.

[0007] In terms of the modification of the anode material, people mainly focus on the surface doping and coating and structural optimization of silicon materials to inhibit the cracking of silicon particles and side reactions of the electrolyte. Although this method can alleviate the problems such as material cracking caused by the volume effect of silicon materials to a certain extent, the phenomenon that fresh interfaces are continuously generated on silicon particles still cannot be completely avoided. Therefore, it is necessary to match with a suitable electrolyte to improve the electrochemical performance of silicon-based anode batteries.

[0008] In the direction of electrolyte optimization, film-forming additives have always received great attention. Patent CN113764730 B provides a lithium-ion battery electrolyte containing an additive with a carbon-nitrogen double bond and a heteroatom connected adjacent to the double bond, which forms a SEI film containing polymer fragments on the anode surface and improves the cycling performance of the lithium-ion battery. However, during the high-temperature storage of the electrolyte provided by this patent, the battery volume expands greatly, indicating that there are still many side reactions between the electrolyte and the electrode, and further improvement is needed. Patent CN116315105A provides a silane additive. The addition of this additive can inhibit the increase in impedance during cycling, improve the cycling stability performance, gas generation during cycling and high-temperature storage of the battery, etc., but it still cannot reach the level of practical application. Moreover, the ionic conductivity of the electrolyte using this additive is relatively low (about 7.5 mS / cm), which also has an adverse effect on the power performance. Summary of the Invention

[0009] The purpose of the present invention is to overcome the shortcomings in the prior art and provide an electrolyte for a pure silicon anode, its preparation method and a battery.

[0010] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0011] An electrolyte for a pure silicon anode includes a lithium salt, a solvent and an additive; the additive includes a solvent additive and a lithium salt additive; at least one fluorine-containing additive is included in the solvent additive; the fluorine-containing additive is one or a mixture of fluoroethylene carbonate FEC and difluoroethylene carbonate DFEC.

[0012] The mass content of the fluorine-containing additive is 10%-20%; preferably 15%; preferably, the fluorine-containing additive is a mixture of fluoroethylene carbonate FEC and difluoroethylene carbonate DFEC, and more preferably, the mass ratio of the two is 10:5.

[0013] The solvent is one or a mixture of at least two of ethylene carbonate EC, propylene carbonate PC, diethyl carbonate DEC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC; preferably, the solvent is a mixture of ethylene carbonate EC, propylene carbonate PC and ethyl methyl carbonate EMC; preferably, EC:PC:EMC = (5-6):(8-10):(49-56).

[0014] The solvent additive also includes sultone additives; the sultone additives include one or a mixture of propylene sulfate (PS) or hydroquinone sulfate (PST);

[0015] The mass content of the sultone additives is 1-3%; preferably 2%; more preferably, the sultone additives are a mixture of propylene sulfate (PS) and hydroquinone sulfate (PST), and the ratio of the two is 1:1.

[0016] The solvent additive also includes phosphate esters; preferably, the phosphate ester is triphenyl phosphate; preferably, the mass content of the phosphate ester additive is 1%.

[0017] The solvent additive also includes one or a mixture of hexamethylene diisocyanate (HDI), tetraethylsilane (TVSI) or succinic anhydride (SA).

[0018] The lithium salt additive includes one or a mixture of lithium bis(oxalato)borate (LiODFB), lithium bis(trifluoromethanesulfonyl)imide (LiOTFP), lithium bis(fluorosulfonyl)imide (LiFSI); preferably a mixture of lithium bis(oxalato)borate (LiODFB) and lithium bis(trifluoromethanesulfonyl)imide (LiOTFP), more preferably, the mass ratio of the two is 0.6:0.4;

[0019] Preferably, the mass content of the lithium salt additive is 3%;

[0020] Preferably, the lithium salt is lithium hexafluorophosphate (LiPF6), preferably, the mass content of the lithium salt is 13%.

[0021] The present invention also includes a method for preparing the electrolyte for the pure silicon negative electrode, including the following steps: adding the lithium salt and the additive to the solvent and stirring evenly.

[0022] The present invention also includes a battery, including a positive electrode sheet, a negative electrode sheet and the electrolyte; the active material in the negative electrode sheet is pure silicon.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] The technical solution of this application adopts a high-entropy electrolyte solution that combines a fluorinated additive solvent with a variety of lithium salts and other organic functional additives (for acid and water removal, film formation). It can effectively improve the stability of the battery at high voltages, protect the electrolyte-electrode interface, inhibit the dissolution of transition metals, reduce the side reactions of the electrolyte caused by moisture, free acids, and transition metals, solve the gas generation problem of pure silicon anodes due to excessive side reactions, and improve the performance of the battery in high-temperature storage tests. At the same time, it can also increase the ion migration rate, reduce the battery impedance, and solve the problems of rapid decline in the cycle life and poor rate performance of the battery caused by the high internal resistance of pure silicon anodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a side view of the thickness expansion during high-temperature storage for the embodiments and comparative examples of the present invention;

[0026] Figure 2 The 2C rate performance graphs for the embodiments and comparative examples of the present invention;

[0027] Figure 3 It is a graph of the -20°C performance test results for the embodiments and comparative examples of the invention;

[0028] Figure 4 It is a graph of the 0.5C room temperature cycle test results for the embodiments and comparative examples of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0029] In order to enable those skilled in the art of this technology to better understand the technical solution of the present invention, the present invention will be further described in detail below in conjunction with the drawings and the best embodiments.

[0030] Embodiment

[0031] An electrolyte for a pure silicon anode includes a lithium salt, a solvent, and an additive; the additive includes a solvent additive and a lithium salt additive; at least one fluorine-containing additive is included in the solvent additive; the fluorine-containing additive is one or a mixture of fluoroethylene carbonate FEC and difluoroethylene carbonate DFEC.

[0032] The solvent is one or a mixture of at least two of ethylene carbonate EC, propylene carbonate PC, diethyl carbonate DEC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC; preferably, the solvent is a mixture of ethylene carbonate EC, propylene carbonate PC, and ethyl methyl carbonate EMC

[0033] The solvent additive further includes a sultone additive; the sultone additive includes one or a mixture of propylene sulfate PS or hydroquinone sulfate PST.

[0034] The solvent additive further includes phosphate ester additives; preferably, the phosphate ester additive is triphenyl phosphate;

[0035] The solvent additive further includes one or a mixture of hexamethylene diisocyanate HDI, tetraethylenesilane TVSI, or succinic anhydride SA.

[0036] The lithium salt additive includes one or a mixture of lithium bis(oxalato)borate LiODFB, lithium bis(trifluoromethanesulfonyl)imide LiOTFP, and lithium bis(fluorosulfonyl)imide LiFSI; preferably, it is a mixture of lithium bis(oxalato)borate LiODFB and lithium bis(trifluoromethanesulfonyl)imide LiOTFP, and the lithium salt is lithium hexafluorophosphate LiPF6.

[0037] The preparation method of the electrolyte for the pure silicon anode includes the following steps: adding the lithium salt and the additive to the solvent and stirring evenly.

[0038] The formulation components and contents of different examples are given in Table 1.

[0039] Table 1

[0040] Solution EC PC EMC FEC DFEC PS PST TPP HDI TVSI SA LiODFB LiOTFP LiFSI <![CDATA[LiPF6]]> Comparative Example 6 10 56 10 2 1 2 13 Example 1 6 9 52 15 2 1 2 13 Example 2 5 8 49 20 2 1 2 13 Example 3 6 9 52 15 2 1 2 13 Example 4 6 9 52 15 1 1 1 2 13 Example 5 6 9 52 10 5 1 1 1 2 13 Example 6 6 9 51 10 5 1 1 1 1 2 13 Example 7 6 9 50 10 5 1 1 1 1 1 2 13 Example 8 6 9 49.5 10 5 1 1 1 0.5 1 1 2 13 Example 9 6 9 49.5 10 5 1 1 1 0.5 1 1 2 13 Example 10 6 9 49.5 10 5 1 1 1 0.5 1 0.6 0.4 2 13

[0041] Examples 1-3: Gradient tests were conducted on the addition amount of FEC, and the addition amounts were 10%, 15%, and 20% of the total mass of the electrolyte, respectively. As a film-forming additive for the anode that can continuously form a film, FEC is an essential component of the electrolyte for silicon anode batteries. There is little research on the electrolyte of pure silicon anodes, and there is no research on the addition amount of FEC. Therefore, gradient experiments were conducted on FEC to screen for a suitable addition amount.

[0042] Examples 1, 3, and 4: The sulfonic acid lactone additive PS can form an SEI on the anode through ring-opening polymerization, effectively protecting the anode. The analogue of PS, PST, has one more C=C double bond structure than PS and can form a more stable and tough SEI. Comparative tests were conducted on these two similar additives, and the addition amount was optimized.

[0043] Example 5: Fluorinated solvents such as FEC can continuously form a stable SEI on the silicon anode, maintain the inertness of the anode interface during cycling, and extend the cycle life of the battery. The difluorinated DFEC is further fluorinated on the basis of FEC, and its oxidation resistance is further improved. At the same time, the fluorinated solvent has good contact with the electrode interface and a lower freezing point, which can improve the low-temperature and rate performance of the electrolyte.

[0044] Example 6: The phosphate ester additive TPP contains multiple highly unsaturated bonds in its chemical structure and has relatively high HOMO and low LUMO energy levels. Therefore, a stable and tough interfacial film can be formed on both the positive and negative electrodes, enhancing the interfacial stability, thereby improving the high-temperature performance of the battery and its stability in a relatively high voltage range.

[0045] Example 7: High specific energy batteries require cathode materials with a high nickel content. Due to the lithium-nickel mixing effect, nickel-rich cathode materials contain higher residual alkali, which will increase the battery impedance, enhance the polarization effect, and affect the cycling performance of the battery. Adding anhydride additives can neutralize the residual alkali in the cathode material and reduce the adverse effects brought by the residual alkali.

[0046] Examples 8 and 9: In Example 8, an N-containing additive is introduced, and in Example 9, an Si-containing additive is introduced as a film-forming additive. Introducing more different elements to form a high-entropy electrolyte can produce high-entropy effects and cocktail effects, and improve the battery performance through synergistic effects.

[0047] Example 10: On the basis of Example 8, a new additive LiOTFP is introduced to partially replace LiODFB. This additive can act together with LiODFB to form a tough and effective interfacial film rich in B, P, and LiF on the surfaces of the positive and negative electrodes, helping to improve the cycling performance of the battery.

[0048] Battery composition: It includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

[0049] The positive electrode sheet is prepared in the following manner: The ternary positive electrode active material, the conductive agent SuperP, and the binder PVDF are uniformly mixed with N-methylpyrrolidone (NMP) in a mass ratio of 96:2.0:2.0 to form a positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and the coating amount is 0.018 g / cm2. After drying at 85°C, cold pressing is carried out; after slicing, trimming, and slitting, the tab is welded to make the positive electrode sheet of the lithium-ion secondary battery.

[0050] The negative electrode sheet is prepared in the following manner: The Si negative electrode, the conductive agent CNT, and the binder PAA are uniformly mixed with pure water in a mass ratio of 91:2.0:7.0 to form a negative electrode slurry; the negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, and the coating amount is 0.009 g / cm2 and dried at 85°C, and then after cold pressing, slicing, and slitting, the tab is welded to make the negative electrode sheet of the lithium-ion secondary battery.

[0051] The separator is a 14-μm ceramic-coated polyethylene separator.

[0052] The test results of different examples and comparative examples are shown in Table 2. Figure 1 It is a side view of the thickness expansion during high-temperature storage of the examples and comparative examples of the present invention;Figure 2 Performance graphs at 2C rate for the embodiments and comparative examples of the present invention; Figure 3 Test result graphs at -20°C for the embodiments and comparative examples of the invention; Figure 4 Test result graphs of 0.5C normal temperature cycling for the embodiments and comparative examples of the invention.

[0053] Table 2

[0054]

[0055] For the optimized solution, we conducted electrical performance tests on soft-pack full batteries. The rate performance was compared in terms of the discharge capacity under the condition of discharging at 2C current. The normal temperature cycling was tested at 25°C. The cycling mode was constant current charging at 0.5C to 4.2V, followed by constant voltage charging with a cut-off current of 0.05C. After charging to 100% SOC, it was left standing for 0.5h and then discharged at 0.5C to 3.0V. The high-temperature storage test of the battery was carried out at 60°C. The thickness of the battery was tested every 7 days, and the residual capacity of the battery was tested on the 28th day.

[0056] Adding the negative electrode film-forming additive FEC can improve the battery cycle performance (Examples 1-2). However, FEC will generate more HF at high temperatures, corrode the positive and negative electrode materials, cause the electrolyte to undergo a double decomposition reaction and produce gas. Therefore, excessive addition of FEC will lead to a significant increase in gas production under high-temperature conditions (Example 2). Replacing the negative electrode film-forming additive PS with PST can form a denser and tougher SEI on the negative electrode side, effectively improving the stability of the battery at high temperatures. However, the dense SEI will also increase the impedance, thus having a negative impact on the low-temperature performance and rate discharge performance of the battery (Example 3). Using two sulfur-containing additives PS and PST in combination can improve the cycle and high-temperature storage performance of the battery, while not affecting the rate and low-temperature performance (Example 4). Replacing part of FEC with DFEC with a higher fluorination rate can improve the electrochemical stability of the battery, improve the battery cycle and high-temperature storage performance. However, due to the high viscosity of DFEC, adding DFEC will reduce the ionic conductivity of the electrolyte, resulting in the rate and low-temperature discharge performance of the battery (Example 5). Adding the film-forming additive TPP to strengthen the protection of the positive and negative electrode interfaces can slightly improve the battery cycle and high-temperature storage performance, but it deteriorates the rate performance (Example 6). In Example 7, adding the weak acid additive succinic anhydride (SA) to neutralize the residual alkali of the ternary negative electrode can effectively improve the battery cycle and high-temperature storage performance. In Examples 8 and 9, additives containing different heteroelements were introduced for testing respectively. The results show that the additive HDI with an N=C=O group can help improve the battery cycle, high-temperature storage and low-temperature discharge performance, while not affecting the rate performance. Based on Example 8, a new additive LiOTFP was introduced. This additive is rich in F elements and can act together with LiODFB to form an interfacial film with multiple heteroelements on the positive and negative electrode surfaces. At the same time, the combination of various anions in the electrolyte leads to the diversity of the solvation structure, forming a "high-entropy system", reducing the dissolution strength between lithium ions and solvents / anions, promoting the diffusion of lithium ions and the formation of a stable interfacial passivation layer, and improving the electrochemical performance of the battery.

[0057] In summary, the high-entropy electrolyte solution scheme of compounding multiple fluorinated solvents with multiple lithium salts and organic functional additives (acid removal, water removal, film formation) can effectively improve the stability of the battery at high voltages, protect the electrolyte-electrode interface, inhibit the dissolution of transition metals, reduce the side reactions of the electrolyte caused by moisture, free acids and transition metals, solve the gas production problem caused by excessive side reactions of the pure silicon negative electrode, and improve the performance of the battery in high-temperature storage tests. At the same time, it can also increase the ion migration rate, reduce the battery impedance, and solve the problems of fast decay of the battery cycle life and poor rate performance caused by the high internal resistance of the pure silicon negative electrode.

[0058] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. An electrolyte for a pure silicon negative electrode, characterized in that, It includes a lithium salt, a solvent, and an additive; the additive includes a solvent additive and a lithium salt additive; at least one fluorine-containing additive is included in the solvent additive; the fluorine-containing additive is one or a mixture of fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC).

2. The electrolyte for a pure silicon negative electrode according to claim 1, characterized in that, The mass content of the fluorine-containing additive is 10%-20%; preferably 15%; preferably, the fluorine-containing additive is a mixture of fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC), and more preferably, the mass ratio of the two is 10:

5.

3. The electrolyte for a pure silicon negative electrode according to claim 1, wherein The solvent is one or a mixture of at least two of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC); preferably, the solvent is a mixture of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC); preferably, EC:PC:EMC=(5-6):(8-10):(49-56).

4. The electrolyte for a pure silicon negative electrode according to claim 1, characterized in that, The solvent additive further includes a sultone additive; the sultone additive includes one or a mixture of propylene sulfate (PS) and hydroquinone sulfate (PST).

5. The electrolyte for a pure silicon negative electrode according to claim 5, wherein, The mass content of the sultone additive is 1%-3%; preferably 2%; more preferably, the sultone additive is a mixture of propylene sulfate (PS) and hydroquinone sulfate (PST), and the ratio of the two is 1:

1.

6. The electrolyte for a pure silicon negative electrode according to claim 1, wherein, The solvent additive further includes a phosphate ester additive; preferably, the phosphate ester additive is triphenyl phosphate; preferably, the mass content of the phosphate ester additive is 1%.

7. The electrolyte for a pure silicon negative electrode according to claim 1, characterized in that, The solvent additive further includes one or a mixture of hexamethylene diisocyanate (HDI), tetraethylsilane (TVSI), or succinic anhydride (SA).

8. The electrolyte for a pure silicon negative electrode according to claim 1, characterized in that, The lithium salt additive includes one or a mixture of lithium bis(oxalato)borate (LiODFB), lithium bis(trifluoromethanesulfonyl)imide (LiOTFP), and lithium bis(fluorosulfonyl)imide (LiFSI); preferably, it is a mixture of lithium bis(oxalato)borate (LiODFB) and lithium bis(trifluoromethanesulfonyl)imide (LiOTFP), and more preferably, the mass ratio of the two is 0.6:0.4; Preferably, the mass content of the lithium salt additive is 3%; Preferably, the lithium salt is lithium hexafluorophosphate (LiPF6), and preferably, the mass content of the lithium salt is 13%.

9. A method for preparing an electrolyte for a pure silicon negative electrode according to any one of claims 1-8, characterized in that, It includes the following steps: adding the lithium salt and the additive into the solvent and stirring evenly to obtain it.

10. A battery, characterized in that, It includes a positive electrode sheet, a negative electrode sheet, and the electrolyte according to any one of claims 1-8; the active substance in the negative electrode sheet is pure silicon.

Images