Sodium-ion battery electrolyte

Abstract

The invention relates to a formula of a high-performance sodium-ion battery electrolyte, and aims to improve the cycle life, safety and rate capability of a sodium-ion battery. The electrolyte comprises sodium hexafluorophosphate as a sodium salt, carbonic ester as an organic solvent and a plurality of additives (fluoroethylene carbonate, bis (2, 2, 2-trifluoroethyl) carbonate, ethylene sulfate, 1, 3-allyl sultone and sodium difluoro (oxalato) borate). Through the synergistic effect of the additives, a stable positive electrode / electrolyte interface film (CEI) is formed, cation dissolution is reduced, and the sodium ion transmission performance and the high-voltage stability of the battery are improved. In a half cell assembled with a NaNi0. 4Fe0. 2Mn0. 4O2 positive electrode, when the electrolyte works in a voltage range of 2-4.2 V, the specific discharge capacity of more than 160mA. H / g is provided, and the capacity retention ratio exceeds 80% after 200 cycles at the rate of 1C.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium ion batteries, and in particular relates to a high-performance sodium ion battery electrolyte formula. Background Art

[0002] As alkali metal elements of the same main group, sodium and lithium have similar chemical properties. Moreover, compared with lithium-ion batteries, sodium-ion batteries have the advantages of abundant resources, low cost, good low-temperature resistance, and high safety, which fully complement the shortcomings of lithium-ion batteries and can be widely used in large-scale energy storage, two-wheeled vehicles and other fields.

[0003] The electrolyte, as the blood of sodium-ion batteries, plays the role of transporting ions and plays an important role in the battery's cycle life, safety, and rate performance. However, commercial carbonate electrolytes are easily decomposed by transition metal elements at the positive electrode, have a low antioxidant potential, and during the cycle, the unstable positive electrode-electrolyte interface (CEI) cannot completely block the electrolyte and the positive electrode, resulting in continuous interfacial side reactions, continuous consumption of electrolyte, and increased interfacial impedance. This seriously affects the coulombic efficiency and cycle stability of sodium-ion batteries.

[0004] By using additives to oxidatively decompose on the surface of the cathode material under high pressure to construct a dense and stable cathode electrolyte interface (CEI), it is possible to protect the internal cathode material, reduce cation dissolution, improve sodium ion transmission performance, reduce the impedance on the cathode side, and improve cathode cycle stability, rate performance, and battery performance under high pressure. However, there are currently few reports on the composition and addition amount of additives used in high-performance sodium ion battery electrolytes. Therefore, the development of electrolyte formulas that are easy to form stable CEI and high voltage stability is an important direction in the research field of sodium ion batteries. Summary of the invention

[0005] In order to solve the above technical problems, the present invention plans to optimize the formula of the traditional carbonate electrolyte. Specifically, carbonate is used as a solvent, relatively cheap sodium hexafluorophosphate is used as a solute, and a multi-component electrolyte additive is used to solve the problem of poor high-voltage stability of sodium ion batteries in the prior art while ensuring good electrochemical performance and low cost.

[0006] The present invention provides the following technical solutions to solve the above problems:

[0007] A sodium ion battery electrolyte, comprising sodium salt, an organic solvent and an additive;

[0008] The sodium salt is sodium hexafluorophosphate;

[0009] Preferably, the concentration of the sodium salt in the organic solvent is 0.5-2 mol / L.

[0010] The organic solvent is a carbonate, among which propylene carbonate is a typical cyclic carbonate with a wide liquid range and a high dielectric constant, and ethyl methyl carbonate is a typical chain carbonate with a low viscosity. When mixed with high-viscosity propylene carbonate, it is beneficial to reduce the viscosity of the electrolyte, increase the ionic conductivity, and obtain better performance.

[0011] Preferably, the carbonate solvent is one or a combination of propylene carbonate and ethyl methyl carbonate;

[0012] Preferably, the volume ratio of propylene carbonate to ethyl methyl carbonate is 1:0 to 1:2.

[0013] The additive is a combination of fluoroethylene carbonate, bis(2,2,2-trifluoroethyl) carbonate, vinyl sulfate, 1,3-propylene sultone, and sodium difluorooxalatoborate;

[0014] According to the above sodium ion battery electrolyte, optionally, the content of the fluoroethylene carbonate accounts for 1 vol% to 10 vol% of the total volume of the organic solvent. Preferably, the content of the fluoroethylene carbonate accounts for 3 vol% to 5 vol% of the total volume of the organic solvent.

[0015] According to the above-mentioned sodium ion battery electrolyte, optionally, the content of bis(2,2,2-trifluoroethyl) carbonate accounts for 1 vol% to 5 vol% of the total volume of the organic solvent. Preferably, the content of bis(2,2,2-trifluoroethyl) carbonate accounts for 2 vol% to 3 vol% of the total volume of the organic solvent.

[0016] According to the above sodium ion battery electrolyte, optionally, the content of vinyl sulfate accounts for 0.1wt% to 5wt% of the total mass of the organic solvent. Preferably, the content of vinyl sulfate accounts for 0.5wt% to 3wt% of the total mass of the organic solvent.

[0017] According to the above sodium ion battery electrolyte, optionally, the content of 1,3-propene sultone accounts for 0.1wt% to 5wt% of the total mass of the organic solvent. Preferably, the content of 1,3-propene sultone accounts for 0.5wt% to 3wt% of the total mass of the organic solvent.

[0018] According to the above sodium ion battery electrolyte, optionally, the content of sodium difluorooxalatoborate accounts for 0.01wt% to 5wt% of the total mass of the organic solvent, and preferably, the content of 1,3-propene sultone accounts for 0.2wt% to 2wt% of the total mass of the organic solvent.

[0019] Under the synergistic effect of fluorine-containing additives (fluoroethylene carbonate, bis(2,2,2-trifluoroethyl) carbonate), sulfur-containing additives (vinyl sulfate, 1,3-propylene sultone), and inorganic salt additives (sodium difluorooxalatoborate), a multilayer CEI film rich in fluorine, sulfur, and boron is formed on the positive electrode side, giving it better mechanical properties and chemical stability, inhibiting side reactions and enhancing interface stability, and significantly reducing the dissolution of positive electrode cations. This innovative multi-additive synergistic mechanism provides an important theoretical basis and practical application prospects for the development of the next generation of high-performance sodium-ion batteries.

[0020] According to the sodium ion battery electrolyte of the present invention and NaNi 0.4 Fe 0.2 Mn 0.4 The O2 positive electrode assembled half-cell can provide a discharge specific capacity of >160mA·h / g when operating in the voltage range of 2 to 4.2V, and can still provide a reversible discharge capacity of more than 140mA·h / g when cycling at a 1C rate. It can still provide a capacity retention rate of more than 80% after cycling 200 times under 1C conditions.

[0021] Beneficial effects of the present invention:

[0022] 1. The present invention optimizes the traditional carbonate electrolyte formula, uses carbonate as a solvent, relatively cheap sodium hexafluorophosphate as a solute, and combines a multi-component electrolyte additive. Under the premise of ensuring good electrochemical performance and low cost, the electrode interface is optimized, which significantly reduces the dissolution of positive electrode cations and improves the cycle stability of the sodium ion battery.

[0023] 2. The present invention forms a multilayer CEI film rich in fluorine, sulfur and boron on the positive electrode side with the help of the synergistic effect of additives. Among them, fluorine-containing additives such as fluoroethylene carbonate and bis(2,2,2-trifluoroethyl) carbonate significantly inhibit the decomposition of PC solvents in the electrolyte and form a protective layer rich in NaF and other fluorides on the positive electrode. Sodium difluorooxalatoborate has a higher HOMO energy level and successfully constructs a uniform and dense interface film rich in fluoride and boride on the surface of the positive electrode; the anions generated by the decomposition of vinyl sulfate and 1,3-propylene sulfone sulfur-containing additives during the charge and discharge process combine with transition metal ions to form insoluble transition metal sulfates or sulfites, further constructing a dense CEI film.

[0024] 3. The sodium ion battery electrolyte system designed by the present invention, through the synergistic effect of multiple additives, helps to form a multilayer CEI film rich in organic and inorganic components with stable chemical and mechanical properties on the positive electrode side, optimizes the electrode interface, and significantly reduces the dissolution of positive electrode cations.

[0025] 4. By carefully selecting and optimizing different types of additives, the present invention solves the problem of poor stability of existing sodium ion batteries under high-voltage environments and provides an effective solution for the practical application of sodium ion batteries.

[0026] 5. The present invention adopts relatively cheap materials and a simplified preparation process, which not only reduces the production cost but also has excellent battery performance, so that sodium ion batteries have broader application prospects and market competitiveness in the fields of energy storage and power batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the charge and discharge curve for the first three cycles of the sodium ion battery in Example 1 of the present application.

[0028] Figure 2 This is a cycle performance diagram of the sodium ion battery at 1C rate in Examples 1, 2, 3 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0029] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0030] The present application provides a sodium ion battery electrolyte, which includes a sodium salt, a solvent and an additive, wherein the sodium salt is used as a part of the charge carrier to transfer sodium ions between electrodes; the solvent is used as a medium for soluble sodium salts to determine the solvation structure of sodium ions; the additive accounts for a relatively low proportion in the electrolyte, but directly participates in the formation of the electrode / electrolyte interface layer. The present application example explores the composition of the electrolyte through the additive composition, additive content, etc.

[0031] In order to test the application effect of the sodium ion battery electrolyte proposed in this application in sodium ion batteries, 0.4 Fe 0.2 Mn 0.4 O2 positive electrode assembled half-cell. The electrolyte and sodium half-cell of the following examples were assembled in an argon atmosphere glove box with a moisture and oxygen content of less than 0.1ppm, and the model was CR2032 button cell. After the assembly was completed, it was left for 6 to 12 hours and placed on the LAND battery test system provided by Wuhan Landian Electronics Co., Ltd. for testing. The cut-off voltage was 2 to 4.2V, and constant current charge and discharge tests were performed at current densities of 0.1C and 1C, respectively.

[0032] Example 1

[0033] The sodium ion battery electrolyte provided in this embodiment includes sodium salt, organic solvent and additives.

[0034] Among them, the sodium salt in this embodiment is sodium hexafluorophosphate, the organic solvent is propylene carbonate and ethyl methyl carbonate, the volume ratio of the organic solvent is 1:1, the additive is fluoroethylene carbonate, the amount of the additive is 5vol%; vinyl sulfate, the amount of the additive is 1wt%; sodium difluorooxalatoborate, the amount of the additive is 0.5wt%, and the concentration of the sodium salt in the electrolyte is 1mol / L.

[0035] The above electrolyte and NaNi 0.4 Fe 0.2 Mn 0.4 The O2 positive electrode was assembled into a CR2032 button cell in an argon-filled glove box and the electrochemical performance was tested. Figure 1 , Figure 2 As shown, the discharge specific capacities of the assembled battery at 0.1C and 1C rates at room temperature are 159.13 and 146.71 mA·h / g, respectively, and the capacity retention rate is 82.78% after 200 cycles at 1C.

[0036] Example 2

[0037] The sodium ion battery electrolyte provided in this embodiment includes sodium salt, organic solvent and additives.

[0038] Among them, the sodium salt in this embodiment is sodium hexafluorophosphate, the organic solvent is propylene carbonate and ethyl methyl carbonate, the volume ratio of the organic solvent is 1:1, the additive is fluoroethylene carbonate, the amount of the additive is 5vol%; vinyl sulfate, the amount of the additive is 1wt%; 1,3-propylene sultone, the amount of the additive is 1wt%, and the concentration of the sodium salt in the electrolyte is 1mol / L.

[0039] The above electrolyte and NaNi 0.4 Fe 0.2 Mn 0.4 The O2 positive electrode was assembled into a CR2032 button cell in an argon-filled glove box and the electrochemical performance was tested. Figure 2 As shown, the discharge specific capacity of the assembled battery at 1C rate at room temperature is 147.16 mA·h / g, and the capacity retention rate is 71.67% after 200 cycles at 1C.

[0040] Example 3

[0041] The sodium ion battery electrolyte provided in this embodiment includes sodium salt, organic solvent and additives.

[0042] Among them, the sodium salt in this embodiment is sodium hexafluorophosphate, the organic solvent is propylene carbonate and ethyl methyl carbonate, the volume ratio of the organic solvent is 1:1, the additive is fluoroethylene carbonate, the amount of the additive is 5 vol%; vinyl sulfate, the amount of the additive is 1 wt%; bis (2,2,2-trifluoroethyl) carbonate, the amount of the additive is 2 vol%, and the concentration of the sodium salt in the electrolyte is 1 mol / L.

[0043] The above electrolyte and NaNi 0.4 Fe 0.2 Mn 0.4 The O2 positive electrode was assembled into a CR2032 button cell in an argon-filled glove box and the electrochemical performance was tested. Figure 2 As shown, the discharge specific capacity of the assembled battery at 1C rate at room temperature is 144.69 mA·h / g, and the capacity retention rate is 76.82% after 200 cycles at 1C.

[0044] Comparative Example 1

[0045] The sodium ion battery electrolyte provided in this embodiment includes sodium salt, organic solvent and additives.

[0046] The sodium salt in this embodiment is sodium hexafluorophosphate, the organic solvent is propylene carbonate and ethyl methyl carbonate, the volume ratio of the organic solvent is 1:1, the additive is fluoroethylene carbonate, the amount of the additive is 5 vol%, and the concentration of the sodium salt in the electrolyte is 1 mol / L.

[0047] The above electrolyte and NaNi 0.4 Fe 0.2 Mn 0.4 The O2 positive electrode was assembled into a CR2032 button cell in an argon-filled glove box and the electrochemical performance was tested. Figure 2 As shown, the discharge specific capacity of the assembled battery at 1C rate at room temperature is 147.02A·h / g, and the capacity retention rate is only 65.62% after 120 cycles at 1C.

Claims

1. A sodium ion battery electrolyte, characterized in that: Including sodium salts, organic solvents and additives; The concentration of the sodium salt in the organic solvent is 0.5 to 2 mol / L; The organic solvent is propylene carbonate and / or ethyl methyl carbonate.

2. A sodium ion battery electrolyte according to claim 1, characterized in that: The sodium salt is sodium hexafluorophosphate.

3. A sodium ion battery electrolyte according to claim 1, characterized in that: The volume ratio of the propylene carbonate to the ethyl methyl carbonate is 1:0 to 1:

2.

4. A sodium ion battery electrolyte according to claim 1, characterized in that: The additive is a combination of fluoroethylene carbonate, bis(2,2,2-trifluoroethyl) carbonate, vinyl sulfate, 1,3-propylene sultone, and sodium difluorooxalatoborate.

5. The sodium ion battery electrolyte according to claim 1, characterized in that The content of the fluoroethylene carbonate is 1 vol% to 10 vol%, preferably 3 vol% to 5 vol%, based on the total volume of the organic solvent.

6. The sodium ion battery electrolyte according to claim 1, characterized in that: The content of bis(2,2,2-trifluoroethyl) carbonate is 1 vol% to 5 vol%, preferably 2 vol% to 3 vol%, based on the total volume of the organic solvent.

7. The sodium ion battery electrolyte according to claim 1, characterized in that: The content of the vinyl sulfate accounts for 0.1 wt% to 5 wt% of the total mass of the organic solvent, and preferably 0.5 wt% to 3 wt%.

8. The sodium ion battery electrolyte according to claim 1, characterized in that: The content of 1,3-propene sultone accounts for 0.1wt% to 5wt% of the total mass of the organic solvent, and preferably 0.5wt% to 3wt%.

9. The sodium ion battery electrolyte according to claim 1, characterized in that: The content of the sodium difluorooxalatoborate is 0.01 wt% to 5 wt%, preferably 0.2 wt% to 2 wt%, based on the total mass of the organic solvent.

10. The use of a sodium ion battery electrolyte according to any one of claims 1 to 9, characterized in that: The sodium ion battery electrolyte and NaNi 0.4 Fe 0.2 Mn 0.4 The O2 positive electrode assembled half-cell can provide a discharge specific capacity of nearly 160mA·h / g when operating in the voltage range of 2 to 4.2V, and can still provide a reversible discharge capacity of nearly 150mA·h / g when cycling at a 1C rate, and provide a capacity retention rate of nearly 80% after cycling 200 times under 1C conditions.

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