Non-aqueous electrolyte and sodium-ion battery comprising same

Abstract

A non-aqueous electrolyte and a sodium-ion battery comprising same. The non-aqueous electrolyte comprises a sodium salt, a non-aqueous organic solvent and an additive, wherein the additive comprises sodium difluorophosphate and a nitrogen-containing cyclic compound, the structure of the nitrogen-containing cyclic compound being as represented by formula 1, formula 2 or formula 3. In the formulae, R1-R9 are each independently selected from a hydrogen atom, an acyl group, an ester group or an alkyl group. Sodium difluorophosphate forms a component rich in NaF, NaPxOy, etc. during the first formation stage, and an SEI film formed after a decomposition product is deposited on the surfaces of positive and negative electrodes is relatively stable; the nitrogen-containing cyclic compound can act as an antioxidant and a dehydrating agent; in addition, the nitrogen-containing cyclic compound can reduce the impedance of the SEI film on the surface of the negative electrode, can form a thin SEI having relatively good toughness, and can reduce the dissolving capacity of the SEI component, such that the high-temperature cycling performance, high-temperature storage performance and rate capability of a sodium-ion battery can be significantly improved by utilizing the synergistic effect of sodium difluorophosphate and the nitrogen-containing cyclic compound.

Description

Non-aqueous electrolyte and sodium ion battery containing the same Technical Field

[0001] The present application belongs to the technical field of secondary batteries, and in particular relates to a non-aqueous electrolyte and a sodium ion battery containing the non-aqueous electrolyte. Background Art

[0002] With the widespread use of lithium-ion batteries, the demand for lithium has increased dramatically. However, lithium is a rare metal, and its production and recycling technologies are still immature, which will limit the further large-scale application of lithium-ion batteries. Therefore, the development of new high-performance secondary battery systems is particularly important. Sodium and lithium belong to the same main group and have similar physical and chemical properties. Due to its abundant resources, low price, and environmental friendliness, sodium has broad application prospects in large-scale energy storage, electric vehicles, electric ships, and special engineering vehicles.

[0003] Additives such as fluoroethylene carbonate and 1,3-propane sultone are often introduced into existing sodium-ion battery electrolytes to improve the electrical properties of sodium-ion batteries. Specifically, these additives form a passivation film on the surface of the negative electrode to protect the negative electrode material and the electrolyte. However, these additives compete with each other during film formation. While playing their respective roles, they also cause some changes in the film-forming effect, resulting in some unfavorable situations such as decreased film strength, increased film thickness, and increased impedance, which in turn leads to insufficient high-temperature performance and deterioration of rate performance of sodium-ion batteries.

[0004] Therefore, there is an urgent need for a non-aqueous electrolyte and a sodium ion battery containing the non-aqueous electrolyte to solve the deficiencies of the existing technical problems.

[0005] Application Contents

[0006] The purpose of this application is to provide a non-aqueous electrolyte that can improve the high temperature performance and rate performance of sodium ion batteries.

[0007] Another object of the present application is to provide a sodium ion battery having good high temperature performance and rate performance.

[0008] To achieve the above objectives, the present application provides a non-aqueous electrolyte comprising a sodium salt, a non-aqueous organic solvent, and an additive, wherein the additive comprises sodium difluorophosphate and a nitrogen-containing cyclic compound, wherein the structure of the nitrogen-containing cyclic compound is shown in Formula 1, Formula 2, or Formula 3:

[0009] Wherein, R1 to R9 are each independently selected from a hydrogen atom, an acyl group, an ester group or a hydrocarbon group.

[0010] Compared with the prior art, the additives of the present invention include sodium difluorophosphate and nitrogen-containing cyclic compounds. Sodium difluorophosphate forms a mixture rich in NaF, NaP in the first formation stage. x O y The SEI film formed after the decomposition product is deposited on the surface of the positive and negative electrodes is relatively stable; the nitrogen-containing cyclic compound is a highly lipophilic and partially hydrophilic compound that can react with hydroxyl radicals and peroxy radicals. Therefore, the nitrogen-containing cyclic compound can act as an antioxidant and dehydrating agent after being dissolved in the electrolyte, thereby improving the stability of the electrolyte itself; at the same time, the nitrogen-containing cyclic compound can reduce the impedance of the SEI film on the surface of the negative electrode, forming a thinner and more resilient SEI film, and reducing the solubility of the SEI component. Under the effect of the above two mechanisms, the electrolyte is inhibited from decomposing during the circulation and storage process, thereby improving the high temperature performance and rate performance of the sodium ion battery. Therefore, the present application utilizes the synergistic effect of sodium difluorophosphate and nitrogen-containing cyclic compounds to enable the non-aqueous electrolyte to significantly improve the high temperature cycle performance, high temperature storage performance and rate performance of the sodium ion battery.

[0011] Preferably, R1 to R9 in the present application are each independently selected from a hydrogen atom, an acyl group having 2 to 6 carbon atoms, an ester group having 2 to 6 carbon atoms, or a hydrocarbon group having 1 to 6 carbon atoms.

[0012] Preferably, R1 to R9 in the present application are each independently selected from a hydrogen atom, an alkanoyl group having 2 to 4 carbon atoms, an alkyl ester group having 2 to 4 carbon atoms, an alkyl group having 1 to 6 carbon atoms, or a phenyl group.

[0013] Preferably, the nitrogen-containing cyclic compound of the present application includes at least one of Compounds 1 to 5:

[0014] Preferably, the mass percentage of the sodium difluorophosphate of the present application in the non-aqueous electrolyte is 0.01-0.5%, and the mass percentage of the nitrogen-containing cyclic compound in the non-aqueous electrolyte is 0.01-2%. Specifically, the mass percentage of the sodium difluorophosphate of the present application in the non-aqueous electrolyte may be, but is not limited to, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.08%, 0.1%, 0.15%, 0.2%, 0.26%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, and the mass percentage of the nitrogen-containing cyclic compound in the non-aqueous electrolyte may be, but is not limited to, 0.0 1%, 0.03%, 0.05%, 0.08%, 0.1%, 0.15%, 0.2%, 0.26%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2%.

[0015] Preferably, the sodium salt of the present application is selected from at least one of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium trifluoromethanesulfonate (NaCF3SO3), sodium bistrifluoromethanesulfonyl imide (NaN(CF3SO2)2), sodium bisoxalatoborate (C4BLiO8), sodium difluorophosphate (NaPO2F2), sodium difluorooxalatoborate (C2BF2NaO4), sodium difluorobisoxalatophosphate (NaDFBP) and sodium bisfluorosulfonyl imide (NaFSI). Preferably, the sodium salt of the present application is sodium hexafluorophosphate.

[0016] Preferably, the mass percentage of the sodium salt of the present application in the non-aqueous electrolyte is 6.5-15.5%. Specifically, the mass percentage of the sodium salt of the present application in the non-aqueous electrolyte may be, but is not limited to, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.2%, 11.5%, 12%, 12.3%, 12.5%, 13%, 13.5%, 13.7%, 14%, 14.5%, 15%, 15.5%. Preferably, the mass percentage of the sodium salt of the present application in the non-aqueous electrolyte is 8-15%.

[0017] Preferably, the non-aqueous organic solvent of the present application is at least one of ethylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), butyl acetate (n-Ba), γ-butyrolactone (γ-Bt), propyl propionate (n-Pp), ethyl propionate (EP), and ethyl butyrate (Eb). Preferably, the non-aqueous organic solvent of the present application is a mixture of ethyl propionate (EP), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) to achieve relatively stable cycling performance.

[0018] To achieve the above objectives, the present application provides a sodium ion battery, including a positive electrode material, a negative electrode material, and the above-mentioned non-aqueous electrolyte, wherein the positive electrode material is a layered oxide, and the chemical formula of the layered oxide is Na x M (1-y-z) Fe y Mn z O2, wherein M is independently selected from at least one of Co, Ni, Cu, Mg, Zn, Al, Sn, Ga, Cr, Sr, V and Ti, 0 <x≤1,0≤y≤1,0≤z≤1,y+z≤1。

[0019] Compared with the prior art, the sodium ion battery of the present application includes sodium difluorophosphate and nitrogen-containing cyclic compounds. The synergistic effect of sodium difluorophosphate and nitrogen-containing cyclic compounds can significantly improve the high-temperature cycle performance, high-temperature storage performance and rate performance of the sodium ion battery.

[0020] Preferably, in the present application, x=1, y=1 / 3, z=1 / 3, and M is Ni.

[0021] Preferably, the negative electrode material of the present application is at least one of soft carbon and hard carbon. Soft carbon is amorphous carbon that can graphitize at temperatures above 2500°C, while hard carbon is difficult to graphitize even after high-temperature treatment, resulting in a stronger sodium storage capacity and a lower operating potential. Therefore, the negative electrode material of the present application is preferably hard carbon. DETAILED DESCRIPTION

[0022] To further understand the present application, the following will clearly and completely describe the technical solutions of the present application in conjunction with the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments of the present application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0023] All raw materials in this application are not particularly limited in their sources and can be purchased from the market or prepared according to conventional methods well known to those skilled in the art.

[0024] There is no particular limitation on the purity of all raw materials in this application. This application preferably uses analytically pure materials or conventional purities in the field of sodium ion battery electrolytes.

[0025] Example 1

[0026] Preparation of non-aqueous electrolyte:

[0027] Under an argon atmosphere, the electrolyte was prepared in a vacuum glove box with a moisture content of <1 ppm. In a dry argon atmosphere glove box, ethyl propionate (EP), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a weight ratio of EP:EMC:DEC = 1:2:3. Then, compound 1 (CAS No.: 73-31-4) and sodium difluorophosphate were added. After dissolution and thorough stirring, sodium salt was added and mixed evenly to obtain a non-aqueous electrolyte.

[0028] Preparation of positive electrode:

[0029] The sodium nickel cobalt aluminate ternary material NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, adhesive PVDF and conductive agent SuperP are evenly mixed in a mass ratio of 95:1:4 to form a sodium ion battery positive electrode slurry with a certain viscosity. The mixed slurry is coated on both sides of the aluminum foil, dried and rolled to obtain the positive electrode sheet.

[0030] Preparation of negative electrode sheet:

[0031] Hard carbon, conductive agent SuperP, thickener CMC, and adhesive SBR (styrene-butadiene rubber emulsion) are prepared into a slurry in a mass ratio of 95:1.5:1.0:2.5, mixed evenly, and the mixed slurry is coated on both sides of the copper foil, dried, and rolled to obtain a negative electrode sheet.

[0032] Preparation of sodium ion batteries:

[0033] The positive electrode sheet, separator and negative electrode sheet are stacked to form a square battery cell, which is then packaged with a polymer and filled with the prepared sodium ion battery non-aqueous electrolyte. After the formation and capacity separation processes, a sodium ion battery with a capacity of 1400mAh is produced.

[0034] The non-aqueous electrolyte formulations of the embodiments and comparative examples are shown in Table 1. The steps for preparing the non-aqueous electrolyte and preparing the sodium ion battery are the same as those of Example 1.

[0035] Table 1 Non-aqueous electrolyte components of various examples and comparative examples

[0036] The sodium ion batteries prepared in Examples 1 to 10 and Comparative Examples 1 to 3 were subjected to high-temperature storage performance tests, high-temperature cycle tests, and rate performance tests according to the following specific test conditions. The performance test results are shown in Table 2.

[0037] Sodium ion battery high temperature storage test:

[0038] At room temperature (25°C), the sodium ion battery was charged and discharged at 0.5C / 0.5C (the battery discharge capacity was recorded as C0), with an upper limit voltage of 4.0V; the battery was placed in a 60°C oven for 30 days, the battery was taken out, and the battery was placed in a 25°C environment and discharged at 0.5C, with the discharge capacity recorded as C1; then the sodium ion battery was charged and discharged at 0.5C / 0.5C (the battery discharge capacity was recorded as C2);

[0039] Capacity retention rate = (C1 / C0)*100%

[0040] Capacity recovery rate = (C2 / C0)*100%.

[0041] Sodium ion battery high temperature cycle test:

[0042] Place the sodium ion battery in a 45°C constant temperature box and let it stand for 30 minutes to allow the sodium ion battery to reach a constant temperature. Charge at a constant current of 1C to a voltage of 4.0V, then charge at a constant voltage of 4.0V to a current of 0.05C, and then discharge at a constant current of 1C to a voltage of 2.0V, and record the first-cycle discharge capacity of the battery. This is one charge and discharge cycle. Repeat this cycle for 400 weeks, record the discharge capacity of the first cycle and the discharge capacity of the last cycle, and calculate the capacity retention rate according to the following formula;

[0043] Capacity retention rate = discharge capacity of the last cycle / discharge capacity of the first cycle × 100%.

[0044] Rate performance test of sodium ion battery:

[0045] The sodium ion battery is charged to 4.0V at 0.5C constant current at 25℃, then charged to 0.05C at constant voltage, and then discharged to 2.0V at 0.5C constant current. This is a charge and discharge cycle. Repeat the charge and discharge three times. The discharge capacity of the last cycle is C0. Then charge to 4.0V at 0.5C constant current, then charge to 0.05C at constant voltage, and then discharge to 2.0V at 5C constant current. The discharge capacity is C1.

[0046] Capacity retention rate = C1 / C0*100%.

[0047] Table 2 Sodium ion battery performance test results

[0048] From the performance test results of sodium ion batteries in Table 2, it can be seen that the additives of the present application can significantly improve the high temperature storage performance, high temperature cycle performance and rate performance of sodium ion batteries. This is because the additives of the present application include sodium difluorophosphate and nitrogen-containing cyclic compounds. Sodium difluorophosphate forms a complex rich in NaF, NaP in the first formation stage. x O y The SEI film formed after the decomposition product is deposited on the surface of the positive and negative electrodes is relatively stable; nitrogen-containing cyclic compounds are highly lipophilic and partially hydrophilic compounds, which can react with hydroxyl radicals and peroxyl radicals. Therefore, nitrogen-containing cyclic compounds can act as antioxidants and water scavengers after being dissolved in the electrolyte, thereby improving the stability of the electrolyte itself; at the same time, nitrogen-containing cyclic compounds can reduce the impedance of the SEI film on the surface of the negative electrode, forming a thinner and more resilient SEI, and reducing the solubility of the SEI components. Under the combined effect of the above two mechanisms, the decomposition of the electrolyte during circulation and storage is inhibited, thereby improving the high temperature performance and rate performance of the sodium ion battery.

[0049] From the results of Example 1 and Comparative Examples 2 to 3, it can be seen that the use of a non-aqueous electrolyte containing sodium difluorophosphate and a nitrogen-containing cyclic compound can significantly improve the high-temperature storage performance, high-temperature cycle performance and rate performance of the sodium ion battery compared to the use of a non-aqueous electrolyte containing only sodium difluorophosphate or a nitrogen-containing cyclic compound. Therefore, the present application utilizes the synergistic effect of sodium difluorophosphate and nitrogen-containing cyclic compounds to enable the non-aqueous electrolyte to significantly improve the high-temperature cycle performance, high-temperature storage performance and rate performance of the sodium ion battery.

[0050] The above describes in detail the non-aqueous electrolyte and the sodium ion battery containing the non-aqueous electrolyte provided by the present application. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only intended to help understand the method and its core ideas of the present application, including the best mode, and also to enable any person skilled in the art to practice the present application, including making and using any device or system, and implementing any combined method. It should be noted that for those skilled in the art, without departing from the principles of the present application, several improvements and modifications can be made to the present application, and these improvements and modifications also fall within the scope of protection of the claims of the present application. The scope of patent protection of this application is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to the literal description of the claims, or if they include equivalent structural elements that are not substantially different from the literal description of the claims, then these other embodiments should also be included in the scope of the claims.

Claims

1. A non-aqueous electrolyte comprising a sodium salt, a non-aqueous organic solvent and an additive, characterized in that: The additive includes sodium difluorophosphate and a nitrogen-containing cyclic compound, and the structure of the nitrogen-containing cyclic compound is shown in Formula 1, Formula 2 or Formula 3: Wherein, R1 to R9 are each independently selected from a hydrogen atom, an acyl group, an ester group or a hydrocarbon group.

2. The non-aqueous electrolyte according to claim 1, characterized in that R1 to R9 are each independently selected from a hydrogen atom, an acyl group having 2 to 6 carbon atoms, an ester group having 2 to 6 carbon atoms, or a hydrocarbon group having 1 to 6 carbon atoms.

3. The non-aqueous electrolyte according to claim 1, characterized in that R1 to R9 are each independently selected from a hydrogen atom, an alkanoyl group having 2 to 4 carbon atoms, an alkyl ester group having 2 to 4 carbon atoms, an alkyl group having 1 to 6 carbon atoms, or a phenyl group.

4. The non-aqueous electrolyte according to claim 1, characterized in that The nitrogen-containing cyclic compound includes at least one of Compounds 1 to 5:

5. The non-aqueous electrolyte according to claim 1, characterized in that The mass percentage of the sodium difluorophosphate in the non-aqueous electrolyte is 0.01-0.5%, and the mass percentage of the nitrogen-containing cyclic compound in the non-aqueous electrolyte is 0.01-2%.

6. The non-aqueous electrolyte according to claim 1, characterized in that The sodium salt is selected from at least one of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium bistrifluoromethanesulfonyl imide, sodium bisoxalatoborate, sodium difluorophosphate, sodium difluorooxalatoborate, sodium difluorobisoxalatophosphate and sodium bisfluorosulfonyl imide.

7. The non-aqueous electrolyte according to claim 1, characterized in that The mass percentage of the sodium salt in the non-aqueous electrolyte is 6.5-15.5%.

8. The non-aqueous electrolyte according to claim 1, characterized in that The non-aqueous organic solvent is at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate and ethyl butyrate.

9. A sodium ion battery, comprising a positive electrode material and a negative electrode material, characterized in that: The non-aqueous electrolyte according to any one of claims 1 to 8 is also included, wherein the positive electrode material is a layered oxide, and the chemical formula of the layered oxide is Na x M (1-y-z) Fe y Mn z O2, wherein M is independently selected from at least one of Co, Ni, Cu, Mg, Zn, Al, Sn, Ga, Cr, Sr, V and Ti, 0 <x≤1,0≤y≤1,0≤z≤1,y+z≤1。 10. The sodium ion battery according to claim 9, characterized in that: The negative electrode material is at least one of soft carbon and hard carbon.

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