Sodium ion electrolyte, preparation method thereof and battery

Abstract

The invention relates to the technical field of sodium ion batteries, in particular to a sodium ion electrolyte, a preparation method thereof and a battery. The sodium ion electrolyte comprises an ester solvent, salts and an additive, wherein the ester solvent comprises chain carboxylic ester, chain carbonic ester and cyclic carbonic ester, and the salt comprises sodium salt. The sodium ion electrolyte provided by the invention has high low-temperature conductivity, not only can improve the low-temperature discharge capacity, but also can improve the low-temperature charging capacity, and can widen the charging window of a conventional commercial sodium battery from 0 DEG C to-30 DEG C.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a sodium ion electrolyte and a preparation method thereof and a battery. Background Art

[0002] In recent years, with the continuous advancement of the industrialization process of sodium-ion batteries, the requirements for the performance of sodium-ion batteries have become increasingly higher, especially in the fields of aerospace, seabed, and scientific exploration in high-latitude areas. As one of the important components of the battery, the electrolyte plays the role of transmitting ions between the positive and negative electrodes, and has an important impact on the battery's cycle life, rate, high and low temperature charging and discharging, safety and other performance. However, in low temperature environments, sodium-ion batteries are difficult to perform their ideal performance due to problems such as decreased ion conductivity, poor compatibility between the electrolyte and the positive and negative electrodes, increased desolvation energy, and poor interface properties between the electrode and the electrolyte. Therefore, it is crucial to deeply study the relationship between the sodium ion solvation structure of the electrolyte, the interface properties between the electrode and the electrolyte, and the low-temperature performance of the electrolyte, and to design low-temperature electrolytes for sodium-ion batteries with lower freezing points, high ionic conductivity, low desolvation energy, and high interface stability based on these principles.

[0003] At present, conventional sodium ion electrolytes generally support low-temperature discharge, and the capacity retention rate of sodium ion batteries can reach more than 70% at a low temperature of -40°C, and the discharge performance at low temperatures can basically cover most application scenarios. However, charging sodium ion batteries at low temperatures is still a major problem in the industry. Conventional electrolytes increase viscosity at low temperatures and have low ion conductivity, which seriously limits the ability of sodium ion batteries to charge at low temperatures. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a high-conductivity low-temperature sodium ion electrolyte and a preparation method thereof. The sodium ion electrolyte provided by the present invention has high low-temperature conductivity, which can not only improve the discharge capacity at low temperatures, but also improve the low-temperature charging capacity, and can widen the charging window of conventional commercial sodium batteries from 0°C to -30°C.

[0005] In the first aspect, the sodium ion electrolyte provided by the present invention comprises an ester solvent, a salt and an additive; wherein the ester solvent comprises a chain carboxylic acid ester, a chain carbonate ester and a cyclic carbonate ester, and the salt comprises a sodium salt. In the present invention, by adopting an optimized ester solvent capable of wide temperature range charge and discharge electrolyte, the sodium ion battery made therefrom can greatly improve the charge and discharge capacity at low temperature, achieve 0.1C charging at -30°C, 0.5C charging at -20°C, and the discharge capacity at -40°C is increased to more than 85%, which can basically cover the applications in aerospace, seabed, scientific exploration in high latitudes and other fields.

[0006] According to the present invention, by adding chain carboxylates and chain carbonates (with low viscosity and melting point) to the sodium ion electrolyte, and adding propylene carbonate (with low melting point, high dielectric constant and strong solvation ability), especially optimizing their types as follows, the low-temperature charge and discharge performance of the sodium ion battery can be further improved.

[0007] Preferably, the chain carboxylic acid ester is selected from one or more of methyl formate (MF), ethyl formate (EF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), ethyl propionate (EP), methyl butyrate (MB), and ethyl butyrate (EB).

[0008] Preferably, the linear carbonate is selected from one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0009] Preferably, the cyclic carbonate is selected from one or both of ethylene carbonate (EC) and propylene carbonate (PC).

[0010] More preferably, the ester solvent is a chain carboxylate, a chain carbonate and a cyclic carbonate in a mass ratio of 10-25:30-40:25-30.

[0011] Preferably, the ester solvent is ethyl propionate (EP), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and propylene carbonate (PC) in a mass ratio of 20-25:20:15:25-30.

[0012] In the present invention, the specific ester solvents in the above preferred ratios have the best effect.

[0013] Preferably, the salts include sodium hexafluorophosphate (NaPF 6 ), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(oxalatoborate) (NaBOB), sodium difluorooxalatoborate (NaODFB), sodium difluorophosphate (NaPO 2 F 2 ), sodium tetrafluoroborate (NaBF 4 )

[0014] Preferably, the salts include sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium difluorooxalate borate and sodium difluorophosphate; the mass ratio of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium difluorooxalate borate and sodium difluorophosphate is 4-6:7-9:0.5-1:0.5-1, preferably 6:8:0.5:0.5. In the present invention, sodium hexafluorophosphate (NaPF60) with high electrochemical stability and conductivity is added to the sodium ion electrolyte system of the present invention. 6), and at the same time, sodium bis(fluorosulfonyl)imide (NaFSI) with higher oxidation potential and better solubility was added, and a small amount of sodium difluorooxalatoborate (NaODFB), sodium difluorophosphate (NaPO 2 F 2 ) can significantly improve the film-forming effect and is beneficial to the improvement of low-temperature performance, and the effect is better under the preferred ratio.

[0015] Preferably, the additive is selected from one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), propylene sulfite (PS), vinyl sulfite (ES), dimethyl sulfate (DMS), vinyl sulfate (DTD), tris(trimethylsilyl) phosphate (TMSP), and tris(trimethylsilyl) borate (TMSB).

[0016] Preferably, the additives are fluoroethylene carbonate, vinylene carbonate, propylene sulfite and tris(trimethylsilyl) phosphate; the mass ratio of fluoroethylene carbonate, vinylene carbonate, propylene sulfite and tris(trimethylsilyl) phosphate is 2-3:0.2-0.8:1-2:0.2-0.8, preferably 2.5:0.5:1.5:0.5.

[0017] In the present invention, by optimizing the additives, fluoroethylene carbonate (FEC), vinylene carbonate (VC), and propylene sulfite (PS) with higher reduction potentials that can preferentially react to generate lower SEI film impedance, and at the same time, an additive selected from tris(trimethylsilyl) phosphate (TMSP) and tris(trimethylsilyl) borate (TMSB) that can be complexed on the positive electrode surface is added, which can better protect the positive electrode material and interact with other ester solvents and sodium salt components, thereby greatly improving the low-temperature conductivity, low-temperature discharge capacity, and low-temperature charging capacity.

[0018] Preferably, the mass ratio of the ester solvent, the salt and the additive is 75-85:10-20:1-10, preferably 80:15:5.

[0019] In a second aspect, the present invention provides a method for preparing a sodium ion electrolyte, comprising mixing the ester solvent, salts and additives in sequence.

[0020] In a third aspect, the sodium ion battery provided by the present invention comprises the sodium ion electrolyte or the sodium ion electrolyte obtained by the preparation method.

[0021] Preferably, the positive electrode of the sodium ion battery is sodium nickel iron manganese oxide, and the negative electrode is hard carbon.

[0022] The beneficial effect of the present invention is at least that: the sodium ion battery made of the wide temperature range charge and discharge electrolyte provided by the present invention can greatly improve the charge and discharge capacity at low temperature, achieve 0.1C charging at -30°C, 0.5C charging at -20°C, and the discharge capacity at -40°C is increased to more than 85%, which can cover applications in aerospace, seabed, high-latitude scientific exploration and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0024] Figure 1 Schematic diagram of capacity retention rate under -20°C low temperature cycle in an embodiment of the present invention.

[0025] Figure 2 Schematic diagram of capacity retention rate under -30°C low temperature cycle in an embodiment of the present invention. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products or can be prepared by known methods. If no specific techniques or conditions are specified in the examples, they are all conventional methods or the techniques or conditions described in the literature in this field, or according to the product instructions. If the manufacturers of the reagents and instruments are not specified, they are all conventional products that can be purchased through regular channels.

[0028] The present invention will be further described below with reference to the embodiments.

[0029] Example 1 In a glove box filled with argon with an oxygen content of ≤1ppm and a water content of ≤1ppm, ester solvents ethyl propionate (EP), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and propylene carbonate (PC) were stirred and mixed in a mass ratio of 20%wt: 20%wt: 15%wt: 25%wt, and the ester solvent accounted for 80% of the total mass of the electrolyte. Subsequently, 6%wt of sodium hexafluorophosphate (NaPF6 ), 8%wt sodium bis(fluorosulfonyl)imide (NaFSI), 0.5%wt sodium difluorooxalatoborate (NaODFB), 0.5%wt sodium difluorophosphate (NaPO 2 F 2 ) and stir evenly. After the mixed solution returns to room temperature, 2.5%wt of fluoroethylene carbonate (FEC), 0.5%wt of vinylene carbonate (VC), 1.5%wt of propylene sulfite (PS) and 0.5%wt of tris(trimethylsilyl) phosphate (TMSP) are added in sequence and stirred evenly.

[0030] The above electrolyte is injected into a 2Ah soft-pack sodium-ion battery and sealed. The positive electrode of the sodium-ion battery is sodium nickel iron manganese oxide, and the negative electrode is hard carbon. The preparation is completed after aging, formation, and capacity separation.

[0031] Example 2 In a glove box filled with argon with an oxygen content of ≤1ppm and a water content of ≤1ppm, ester solvents ethyl propionate (EP), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and propylene carbonate (PC) were stirred and mixed in a mass ratio of 15%wt: 20%wt: 15%wt: 30%wt, and the ester solvent accounted for 80% of the total mass of the electrolyte. Subsequently, 6%wt of sodium hexafluorophosphate (NaPF 6 ), 8%wt sodium bis(fluorosulfonyl)imide (NaFSI), 0.5%wt sodium difluorooxalatoborate (NaODFB), 0.5%wt sodium difluorophosphate (NaPO 2 F 2 ) and stir evenly. After the mixed solution returns to room temperature, 2.5%wt of fluoroethylene carbonate (FEC), 0.5%wt of vinylene carbonate (VC), 1.5%wt of propylene sulfite (PS) and 0.5%wt of tris(trimethylsilyl) phosphate (TMSP) are added in sequence and stirred evenly.

[0032] The above electrolyte is injected into a 2Ah soft-pack sodium-ion battery and sealed. The positive electrode of the sodium-ion battery is sodium nickel iron manganese oxide, and the negative electrode is hard carbon. The preparation is completed after aging, formation, and capacity separation.

[0033] Example 3 In a glove box filled with argon with an oxygen content of ≤1ppm and a water content of ≤1ppm, ester solvents ethyl propionate (EP), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and propylene carbonate (PC) were stirred and mixed in a mass ratio of 20%wt: 20%wt: 15%wt: 25.5%wt, and the ester solvent accounted for 80.5% of the total mass of the electrolyte. Subsequently, 6%wt of sodium hexafluorophosphate (NaPF 6), 8%wt sodium bis(fluorosulfonyl)imide (NaFSI), 0.5%wt sodium difluorooxalatoborate (NaODFB), 0.5%wt sodium difluorophosphate (NaPO 2 F 2 ) and stir evenly. After the mixed solution returns to room temperature, 2.5%wt fluoroethylene carbonate (FEC), 0.5%wt vinylene carbonate (VC), and 1.5%wt propylene sulfite (PS) are added in sequence and stirred evenly.

[0034] The above electrolyte is injected into a 2Ah soft-pack sodium-ion battery and sealed. The positive electrode of the sodium-ion battery is sodium nickel iron manganese oxide, and the negative electrode is hard carbon. The preparation is completed after aging, formation, and capacity separation.

[0035] Comparative Example 1 In a glove box filled with argon with an oxygen content of ≤1ppm and a water content of ≤1ppm, ester solvents ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and propylene carbonate (PC) were stirred and mixed in a mass ratio of 30%wt: 20%wt: 31.5%wt, and the ester solvent accounted for 81.5% of the total mass of the electrolyte. Then, 14%wt of sodium hexafluorophosphate (NaPF 6 ), add 2.5%wt fluoroethylene carbonate (FEC), 0.5%wt vinylene carbonate (VC), and 1.5%wt propylene sulfite (PS) in sequence and stir well.

[0036] The above electrolyte is injected into a 2Ah soft-pack sodium-ion battery and sealed. The positive electrode of the sodium-ion battery is sodium nickel iron manganese oxide, and the negative electrode is hard carbon. The preparation is completed after aging, formation, and capacity separation.

[0037] Performance Testing The test method is as follows.

[0038] 1. -40℃ discharge At 25℃, charge at 0.5C and discharge at 0.5C for 3 weeks, then fully charge at 0.5C. The discharge capacity in the third week of the cycle is recorded as the initial capacity of the battery. Place the battery at -40℃ for 4 hours and discharge at 0.5C. The discharge capacity is taken as the discharge capacity at -40℃, and the discharge capacity ratio is calculated.

[0039] 2.-30℃ cycle The battery to be tested was placed in a -30℃ environment for 4h, and a low-temperature cycle test was performed with 0.1C constant current charging / 0.5C constant current discharging to the cut-off voltage, and the capacity retention rate per week was calculated.

[0040] 3.-20℃ cycle The battery to be tested was placed in a -20℃ environment for 4h, and a low-temperature cycle test was performed with 0.5C constant current charging / 0.5C constant current discharging to the cut-off voltage, and the capacity retention rate per week was calculated.

[0041] The test results are shown in Table 1 and Figure 1-2 As shown (in the figure, Examples 1-4 represent Examples 1-3 and Comparative Example 1, respectively).

[0042] Table 1

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A sodium ion electrolyte, characterized in that It includes ester solvents, salts and additives; wherein the ester solvents include chain carboxylic acid esters, chain carbonates and cyclic carbonates, and the salts include sodium salts.

2. The sodium ion electrolyte according to claim 1, characterized in that The chain carboxylic acid ester is selected from one or more of methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, ethyl propionate, methyl butyrate, and ethyl butyrate.

3. The sodium ion electrolyte according to claim 1 or 2, characterized in that The chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

4. The sodium ion electrolyte according to any one of claims 1 to 3, characterized in that The cyclic carbonate is selected from one or both of ethylene carbonate and propylene carbonate.

5. The sodium ion electrolyte according to any one of claims 1 to 4, characterized in that The ester solvent is a chain carboxylate, a chain carbonate and a cyclic carbonate in a mass ratio of 10-25:30-40:25-30; Preferably, the ester solvent is ethyl propionate, ethyl methyl carbonate, diethyl carbonate and propylene carbonate in a mass ratio of 20-25:20:15:25-30.

6. The sodium ion electrolyte according to any one of claims 1 to 5, characterized in that The salts include one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(oxalatoborate), sodium difluorooxalatoborate, sodium difluorophosphate, and sodium tetrafluoroborate; Preferably, the salts include sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium difluorooxalate borate and sodium difluorophosphate; the mass ratio of the sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium difluorooxalate borate and sodium difluorophosphate is 4-6:7-9:0.5-1:0.5-1.

7. The sodium ion electrolyte according to any one of claims 1 to 6, characterized in that The additive is selected from one or more of fluoroethylene carbonate, vinylene carbonate, propylene sulfite, vinyl sulfite, dimethyl sulfate, vinyl sulfate, tris(trimethylsilyl)phosphate, and tris(trimethylsilyl)borate; Preferably, the additives are fluoroethylene carbonate, vinylene carbonate, propylene sulfite and tris(trimethylsilyl)phosphate; the mass ratio of fluoroethylene carbonate, vinylene carbonate, propylene sulfite and tris(trimethylsilyl)phosphate is 2-3:0.2-0.8:1-2:0.2-0.

8.

8. The sodium ion electrolyte according to any one of claims 1 to 7, characterized in that The mass ratio of the ester solvent, the salt and the additive is 75-85:10-20:1-10.

9. The method for preparing the sodium ion electrolyte according to any one of claims 1 to 8, characterized in that: The method comprises mixing ester solvents, salts and additives in sequence.

10. A sodium ion battery, characterized in that: It comprises the sodium ion electrolyte according to any one of claims 1 to 8 or the sodium ion electrolyte obtained by the preparation method according to claim 9; preferably, the positive electrode of the sodium ion battery is sodium nickel iron manganese oxide, and the negative electrode is hard carbon.

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