Sodium-ion battery electrolyte for prolonging cycle life of layered oxide

Abstract

The invention discloses a sodium-ion battery electrolyte for prolonging the cycle life of layered oxides, and belongs to the technical field of sodium-ion batteries. The electrolyte is composed of an electrolyte salt, an organic carbonate solvent and an additive containing fluorine and sulfur. The sodium salt is sodium hexafluorophosphate; the organic carbonate solvent is a mixed solvent of cyclic ester and linear ester; the additive comprises a fluorine-containing additive and a sulfur-containing additive, the fluorine-containing additive is one or more of FEC, DFEC, FEC, ETFEC and TFEC, the mass fraction of the fluorine-containing additive is 0.5%-10%, and the sulfur-containing additive is one or more of DTD, PS, PST, ES and DMS, and the mass fraction of the sulfur-containing additive is 0.5%-10%. According to the method designed in the invention, the physical and chemical properties of the electrolyte are changed, and an interfacial film which is thin, uniform, low in impedance, high in ionic conductivity and high in mechanical strength is generated on the surface of the electrode material, so that the continuous decomposition of a solvent can be effectively inhibited, and the cycle performance of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a sodium ion battery electrolyte for improving the cycle life of layered oxides. Background Art

[0002] Sodium-ion batteries, driven by their sustainability and cost advantages, have been recognized as one of the most promising electrochemical energy storage methods. In a battery system, the components that determine battery performance are the positive electrode material, the negative electrode material, and the electrolyte. The electrolyte, the lifeblood of a battery, plays a crucial role in sodium-ion batteries by transporting ions and thus determining the battery's cycling performance, capacity utilization, rate capability, self-discharge, and safety.

[0003] Electrolytes primarily consist of electrolyte salt components, organic solvents, and additives. Electrolyte salts are categorized into inorganic and organic sodium salts. Inorganic sodium salts are currently commonly used, but they suffer from issues such as strong oxidizing properties and easy decomposition. Organic sodium salts, on the other hand, are expensive and can corrode the current collector. Organic solvents are categorized into ether solvents and carbonate solvents. Ether solvents are highly compatible with Na metal anodes, forming a solid electrolyte interphase (SEI) component on the anode side that is highly resistant to Na metal. However, they have low oxidation resistance and are prone to decomposition under high-voltage conditions. Carbonate solvents often have high ionic conductivity and oxidation resistance. Cyclic carbonate solvents, due to their high dielectric constant, can dissolve more electrolyte salts, but this also results in a relatively high viscosity electrolyte system. Therefore, in practical applications, multiple solvents are often mixed to combine their respective advantages, but this inevitably also brings with it their respective disadvantages. Therefore, adjusting the ratio of various solvents and selecting appropriate additives are particularly important.

[0004] During the first charge and discharge process of the battery, the electrolyte will form a positive electrode electrolyte interface film (CEI) on the positive electrode side, and the components of the CEI largely determine the capacity, cycle stability and rate performance of the battery. By adjusting the ratio of the mixed solvent and adding a trace amount of solvent to the electrolyte system, the physicochemical properties of the electrolyte can be changed, the electrochemical window of the electrolyte can be improved, the decomposition order of the electrolyte on the positive electrode side can be changed, the components of the CEI can be changed, the impedance can be reduced, and the electrochemical performance of the battery can be improved. Currently, the commonly used carbonate-based solvents are divided into cyclic esters and linear esters. Cyclic esters have a large dielectric constant and high viscosity, while linear esters have a low dielectric constant and low viscosity. Therefore, a cyclic-line mixing strategy is often used to improve the electrochemical performance of the battery. Currently, the commonly used fluorine additives will form a NaF-rich CEI on the positive electrode side, which can effectively improve the cycle stability of the battery, but the inorganic-rich CEI layer has low toughness and is not conducive to long-term cycling. The present invention introduces a sulfur-containing additive on the basis of a fluorine-containing additive to construct a fluorine-containing inorganic and sulfur-containing organic CEI layer on the positive electrode side, thereby compensating for the shortcoming of low toughness of the inorganic-rich CEI layer and greatly improving the cycle stability. Summary of the Invention

[0005] The present invention provides a sodium ion battery electrolyte for improving the cycle life of layered oxides, aiming to solve the cycle stability of carbonate electrolytes in layered oxide materials.

[0006] In order to solve the above problems, the present invention adopts the following technical solutions:

[0007] The electrolyte components include electrolyte salt, organic carbonate solvent and additives.

[0008] The electrolyte salt includes a mixture of one or more of NaPF6, NaClO4, NaBF4, NaOTf, NaTFSI, and NaFSI.

[0009] The organic carbonate solvent includes: a mixture of one or more of EC, PC, BC, DMC, EMC, DEC, and MPC.

[0010] The additive is a fluorine-containing or sulfur-containing organic solvent: one or more of the fluorine-containing additives FEC, FEMC, ETFEC, and TFPC; and one or more of the sulfur-containing additives DTD, PS, PST, ES, and DMS.

[0011] Preferably, the mass fraction of the electrolyte salt in the electrolyte is 10% to 20%.

[0012] Preferably, the mass fraction of the organic carbonate solvent is 60% to 90%.

[0013] Preferably, the mass fraction of the fluorine- and sulfur-containing additives is 1% to 10%.

[0014] The present invention also provides a method for preparing a carbonate-based electrolyte, comprising the following steps:

[0015] (1) Under an argon atmosphere, one or more organic carbonate solvents are fully mixed in a volume ratio to obtain a pre-purified solution.

[0016] (2) Adding fluorine-containing and sulfur-containing additives to step (1) according to the mass percentage of the solvent and the additive to obtain a pre-purified solution.

[0017] (3) According to the mass ratio between the sodium salt and the solvent, a certain amount of sodium salt is added to the pre-purified solution mixed in step (2) to dissolve and obtain the carbonate electrolyte.

[0018] Compared with the prior art, the present invention has the following significant beneficial effects:

[0019] The fluorine- and sulfur-containing carbonate-based electrolyte designed in this invention modifies the physicochemical properties of the electrolyte system, forming a fluorine-rich inorganic and sulfur-rich organic CEI on the surface of the positive electrode material. This synergistic fluorine-sulfur CEI compensates for the insufficient toughness of the fluorine-rich inorganic while also providing the strong mechanical strength of the inorganic-rich CEI, effectively inhibiting the sustained oxidative decomposition of the carbonate-based electrolyte on the electrode surface. The electrolyte system designed in this invention exhibits a better electrochemical window than commercial carbonate-based electrolyte systems. Its application to layered oxide materials can significantly improve their cycling stability, providing a feasible solution for the further commercial development of sodium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a test chart of the cycle performance of sodium ion batteries assembled with the electrolytes described in Examples 1 and 2 of the present invention and Comparative Examples 1, 2, and 3.

[0021] Figure 2 1 is a charge and discharge curve diagram of the electrolyte described in Example 1 and Example 2 of the present invention at room temperature.

[0022] Figure 3 1 is the linear sweep voltammogram of the electrolytes described in Example 1 and Example 2 of the present invention at room temperature.

[0023] Figure 4 These are impedance diagrams of sodium ion batteries assembled with the electrolytes described in Examples 1 and 2 of the present invention after 90 cycles, and impedance diagrams of sodium ion batteries assembled with the electrolyte described in Example 2 after 20, 60, and 80 cycles, respectively.

[0024] Figure 5This is a diagram of the conductivity performance of the sodium ion battery assembled with the electrolyte described in Example 2 of the present invention at different temperatures. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments and comparative examples of the present invention are described in further detail below, but the present invention is not limited to the embodiments described below.

[0026] Example 1

[0027] PC and EMC were mixed in a volume ratio of 3:7 to obtain a PC / EMC mixed solution. After mixing evenly, 1 mol of NaPF6 was added, and finally 2% by mass of FEC was added.

[0028] Example 2

[0029] PC and EMC were mixed in a volume ratio of 3:7 to obtain a PC / EMC mixed solution. After mixing evenly, 1 mol of NaPF6 was added, and finally 2% by mass of FEC and 5% by mass of PS were added.

[0030] Comparative Example 1

[0031] EC and DMC were mixed in a volume ratio of 1:1 to obtain a PC / EMC mixed solution. After mixing evenly, 1 mol of NaPF6 was added.

[0032] Comparative Example 2

[0033] EC, EMC and DMC were mixed in a volume ratio of 1:1:1 to obtain a mixed solution of EC / EMC / DMC. After mixing evenly, 1 mol of NaPF6 was added, and finally 2% by mass of FEC was added.

[0034] Comparative Example 3

[0035] EC and DEC were mixed in a volume ratio of 1:1 to obtain an EC / DEC mixed solution. After mixing evenly, 1 mol of NaClO4 was added, and finally 5% by mass of FEC was added.

[0036] Sodium-ion batteries were assembled using the carbonate-based sodium-ion battery electrolyte containing the aforementioned fluorine- and sulfur-containing binary additives. The positive electrode used a layered transition metal oxide, sodium nickel iron manganese oxide, and the negative electrode used metallic sodium. The sodium-ion batteries were assembled in an argon atmosphere. The assembled batteries were subjected to a room-temperature cycle test at 25°C under the following conditions: three cycles of activation at 0.2C, followed by constant current charge and discharge at 2.0C (1.0C = 100 mAh / g). The voltage condition was 2-4V.

[0037] Table 1 Cycling performance test results of sodium ion batteries assembled with the electrolytes of Examples 1-2 and Comparative Examples 1-3.

[0038]

[0039] The calculation formula for the 250-cycle capacity retention rate is:

[0040] Discharge capacity after 250 cycles / discharge capacity of the first cycle after activation × 100%.

[0041] Figure 1 Graph showing the cycle performance of sodium ion batteries assembled using the electrolytes described in Examples 1 and 2 and Comparative Examples 1, 2, and 3; Figure 3 The linear sweep voltammogram of the sodium ion battery assembled with the electrolytes described in Example 1 and Example 2. The experimental results show that the electrolyte system containing FEC and PS additives in the adjusted solvent ratio exhibits better electrochemical performance, improves the cycle stability of the battery and has a wider electrochemical window. Figure 1 The test results show that under the conditions of 2-4V, 0.2C activation and 2.0C cycling, the capacity retention rates of sodium ion batteries containing 2wt% FEC and 2wt% FEC+5wt% PS additives are 76.28% and 99.13% respectively after 250 cycles.

[0042] Figure 4 The impedance diagrams of the sodium ion battery assembled with the electrolytes described in Examples 1 and 2 after 80 cycles and the impedance diagrams of the sodium ion battery assembled with the electrolyte described in Example 2 after 20, 60 and 80 cycles. Figure 4 The experimental results show that the addition of PS additives reduces the impedance of the formed CEI film, and as the cycle progresses, R sEI Example 2 of the present invention shows that the strategy of using fluorine and sulfur additives achieves excellent cycle stability.

[0043] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A sodium ion battery electrolyte for improving the cycle life of layered oxides, characterized in that: The electrolyte comprises: an electrolyte salt component; a solvent component; and fluorine-containing and sulfur-containing additive components; wherein the fluorine-containing additive is one or more of FEC (fluoroethylene carbonate), DFEC (difluoroethylene carbonate), FEMC (methyl trifluoroethyl carbonate), ETFEC (ethyl trifluoroethyl carbonate), and TFPC (3,3,3-trifluoropropylene carbonate), with a mass fraction of 0.5%-10%; and the sulfur-containing additive is one or more of DTD (vinyl sulfate), PS (1,3-propane sultone), PST (propenyl-1,3-sultone), ES (ethylene sulfite), and DMS (dimethyl sulfite), with a mass fraction of 0.5%-10%.

2. The sodium ion battery electrolyte for improving the cycle life of layered oxide according to claim 1, characterized in that: The electrolyte salt includes a mixture of one or more of NaPF6 (sodium hexafluorophosphate), NaClO4 (sodium perchlorate), NaBF4 (sodium tetrafluoroborate), NaOTf (sodium trifluoromethanesulfonate), NaTFSI (sodium bis(trifluoromethylsulfonyl)imide), and NaFSI (sodium bis(fluorosulfonyl)imide).

3. The sodium ion battery electrolyte containing a layered oxide for improving the cycle life according to claim 1, characterized in that: The solvent component includes a mixture of one or more of EC (ethylene carbonate), PC (propylene carbonate), BC (butylene carbonate), DMC (dimethyl carbonate), EMC (ethyl methyl carbonate), DEC (diethyl carbonate), and MPC (methylpropyl carbonate).

4. The sodium ion battery electrolyte for improving the cycle life of layered oxide according to claim 1, characterized in that: The mass fraction of one or more of the fluorine-containing additives FEC, FEMC, ETFEC, and TFPC is 1%-5%, and the mass fraction of one or more of the sulfur-containing additives DTD, PS, PST, ES, and DMS is 1%-5%.

5. The sodium ion battery electrolyte for improving the cycle life of layered oxide according to claim 1, characterized in that: The mass fraction of the electrolyte salt component is 10%-20%; the mass fraction of the solvent is 60%-90%; and the mass fraction of the additive is 1%-10%.

6. A sodium ion battery electrolyte for improving the cycle life of layered oxides according to claims 1-5, characterized in that: The method includes: After the organic carbonate solvent is uniformly mixed according to the designed volume ratio, the additive is added to the organic carbonate solvent according to the mass percentage, and then the electrolyte salt component is added to the organic carbonate solvent containing fluorine and sulfur additives according to the mass percentage to obtain the sodium ion battery electrolyte containing fluorine and sulfur additives.

7. A sodium ion secondary battery for improving the cycle life of layered oxides according to claims 1-5, characterized in that: The battery includes: The positive electrode is a layered oxide positive electrode material; The negative electrode is a sodium metal sheet with a diameter of 13 mm; The diaphragm is located between the positive electrode and the negative electrode, which is used to separate the positive and negative electrodes and transmit sodium ions; The battery casing is used to assemble the positive electrode, negative electrode, separator, and electrolyte into a complete battery.

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