Sodium-ion battery electrolyte as well as preparation method and application thereof

Abstract

The invention discloses a sodium ion battery electrolyte as well as a preparation method and application thereof, and belongs to the technical field of sodium ions. The sodium ion battery electrolyte provided by the invention comprises a sodium salt, an organic solvent, a main additive and an auxiliary additive, the main additive is one or more of sodium difluorophosphate NaDFP, sodium tetrafluoroborate NaBF4, sodium trifluoroacetate NaTFA or sodium nitrate NaNO3, and the auxiliary additive is one or more of sodium difluorophosphate NaDFP, sodium tetrafluoroborate NaBF4, sodium trifluoroacetate NaTFA or sodium nitrate NaNO3. According to the present invention, one or more of tris (trimethylsilane) borate TMSB, tris (trimethylsilane) phosphate TMSP, (pentafluorophenyl) silane TPFS or tris (trimethylsilane) phosphite TMSPi are adopted, the circulation capacity retention rate in a wide temperature range is significantly improved through the synergistic mechanism in a certain proportion range, the capacity retention rate is high at a high rate of 10 C and above, the rapid charging performance is provided, and the application prospect is broad. The cost is reduced; and commercialization is promoted.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium-ion batteries, and particularly relates to a sodium-ion battery electrolyte, a preparation method thereof, and an application thereof. Background Art

[0002] Compared with the well-known lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) are widely regarded as a powerful supplement to LIBs in the field of electrochemical energy storage due to their cost-effectiveness, rich resources, and safety. They have been regarded as promising candidates for large-scale energy storage devices, thus attracting wide attention. Although certain progress has been made in the research on SIBs under room temperature conditions and partial commercialization has been achieved. However, limited by the kinetic and thermodynamic properties of Na + , problems such as capacity decay and decline in rate performance of SIBs under high-rate charge and discharge or extreme temperature conditions are still huge challenges in their practical applications. When under low temperature and high-rate conditions, the decline in battery performance is mainly related to the migration speed of Na + in the electrolyte and the charge transfer rate, especially the desolvation process of Na + at the interface. While under high temperature conditions, the decline in battery performance is closely related to the stability of the electrode / electrolyte interface (EEI) film. Since the Na-based EEI film formed at the interface has weak Lewis acidity, it is more likely to dissolve with the increase in temperature than the lithium-based EEI film, especially when the operating temperature of SIBs exceeds 45°C, which is more obvious. And the electrolyte, as a key factor affecting the Na + transport kinetics and the formation of a stable electrode / electrolyte interface, its solvation structure plays a decisive role in the desolvation process of Na + and the interface stability. Although the partial electrochemical performance of the battery has been improved to a certain extent through electrolyte design improvement, developing an electrolyte that can operate stably in a wide temperature range and meet high-rate conditions simultaneously is still a challenge.

[0003] The prior art discloses a sodium-ion battery, the electrolyte of which is a non-aqueous electrolyte. The non-aqueous electrolyte includes a first additive and a second additive. The first additive includes at least one of tris(trimethylsilyl) phosphate or tris(trimethylsilyl) borate, and the second additive includes at least one of 1,3-propylene sulfonic acid lactone or ethylene sulfate, which improves the first efficiency and cycle performance of the sodium-ion battery, inhibits gas generation in the battery. However, the cycle capacity retention rate at 25°C and 45°C still needs to be further improved to meet the actual requirements, and the low-temperature retention performance is relatively poor.

[0004] Therefore, developing a sodium-ion battery electrolyte with excellent cycle capacity retention rate in a wide temperature range, high performance capacity retention rate at a high rate of 10C and above, and fast charging performance has important research significance and application value. Summary of the Invention

[0005] In order to solve the technical problem in the prior art that it is difficult for the electrolyte of a sodium-ion battery to achieve high rate and stable operation within a wide temperature range, the primary object of the present invention is to provide an electrolyte for a sodium-ion battery. The electrolyte for a sodium-ion battery provided by the present invention uses a certain type of sodium salt-based additive as the main additive and a certain type of silane additive as the secondary additive. The two have a synergistic effect within a certain proportion range, with excellent cycle capacity retention rate within a wide temperature range, and high performance capacity retention rate at a high rate of 10C and above. The electrolyte for a sodium-ion battery has fast charging performance.

[0006] Another object of the present invention is to provide a preparation method for the above-mentioned electrolyte for a sodium-ion battery.

[0007]

[0007] Another object of the present invention is to provide a sodium-ion battery including the above-mentioned electrolyte for a sodium-ion battery.

[0008] In order to achieve the above-mentioned invention objects, the present invention adopts the following technical solutions:

[0009]

[0009] The present invention protects an electrolyte for a sodium-ion battery, which includes a sodium salt, an organic solvent, a main additive, and a secondary additive;

[0010] Among them, the sodium salt is one or two of sodium hexafluorophosphate NaPF6 or sodium bis(fluorosulfonyl)imide NaFSI;

[0011]

[0010] The main additive is one or more of sodium difluorophosphate NaDFP, sodium tetrafluoroborate NaBF4, sodium trifluoroacetate NaTFA, or sodium nitrate NaNO3;

[0012] The secondary additive is one or more of tris(trimethylsilyl) borate TMSB, tris(trimethylsilyl) phosphate TMSP, (pentafluorophenyl) silane TPFS, or tris(trimethylsilyl) phosphite TMSPi;

[0013]

[0011] The mass ratio of the main additive to the secondary additive is 1:1.5 - 4, the main additive accounts for 0.2% - 2% of the total mass of the electrolyte, and the secondary additive accounts for 0.8% - 3% of the total mass of the electrolyte.

[0014] The present invention provides an electrolyte for a sodium-ion battery, in which the main additive and the secondary additive significantly improve the rate performance of the sodium-ion battery through a synergistic mechanism and can operate stably within a wide temperature range. On the one hand, the main additive is one or more of sodium difluorophosphate (NaDFP), sodium tetrafluoroborate (NaBF4), sodium trifluoroacetate (NaTFA), or sodium nitrate (NaNO3), all of which are sodium salts of strong coordination anions. The strong coordination anions of the main additive preferentially occupy the Na +The primary solvation shell reduces the proportion of solvent in the sodium ion solvation structure, optimizing the solvent-dominated solvation structure into an anion-dominated solvation structure; not only promoting the desolvation of Na + during low-temperature and high-rate charge-discharge processes, but also the anion in the main additive reaches the interface along with Na + and then decomposes to form an EEI film rich in inorganic substances. At the same time, due to its large steric hindrance, the silane-containing co-additive is difficult to enter the solvation shell of sodium ions, firstly avoiding the large desolvation potential energy during the charge-discharge process. On the other hand, the co-additive is selected from one or more of tris(trimethylsilyl) borate TMSB, tris(trimethylsilyl) phosphate TMSP, (pentafluorophenyl) silane TPFS, or tris(trimethylsilyl) phosphite TMSPi. They are all weakly coordinating silane additives, and the hydrophobicity of the silyl group inhibits the generation of HF and the continuous occurrence of a large number of side reactions at high temperatures; at the same time, the silicon-containing compounds generated by the decomposition of the co-additive and the decomposition products of the main additive jointly form a uniform and stable organic-inorganic composite EEI film, avoiding the continuous dissolution and reconstruction of the EEI film and enhancing the interfacial stability of the battery. And the silicon-based component in the EEI film optimizes the Na + transport path, reduces the interfacial impedance, and is conducive to the rapid migration of Na + in the EEI film.

[0015] The synergistic effect of the main additive and the co-additive optimizes the solvation environment through the "anion-dominated + weakly solvated" structure, and balances stability and flexibility through the "inorganic-organic composite interface film", enabling the battery to have high capacity retention and long cycle stability at wide temperature ranges, -20°C to 45°C and high rates, reducing costs and promoting commercialization.

[0016] Preferably, the mass ratio of the main additive to the co-additive is 1:3 to 4.

[0017] More preferably, the mass ratio of the main additive to the co-additive is 1:3.

[0018] Preferably, the main additive is one or both of sodium tetrafluoroborate NaBF4 or sodium trifluoroacetate NaTFA.

[0019] Preferably, the co-additive is one or more of tris(trimethylsilyl) borate TMSB, tris(trimethylsilyl) phosphate TMSP, or tris(trimethylsilyl) phosphite TMSPi.

[0020] Preferably, the sodium salt is one or both of sodium hexafluorophosphate NaPF6 or sodium bis(fluorosulfonyl)imide NaFSI.

[0021] More preferably, the sodium salt is a compound of sodium hexafluorophosphate NaPF6 and sodium bis(fluorosulfonyl)imide NaFSI in a mass ratio of 1:1.

[0022] Preferably, the concentration of the sodium salt in the organic solvent is 0.8 - 3 mol / L.

[0023] Preferably, the organic solvent is one or both of esters and ethers.

[0024] More preferably, the organic solvent is selected from one or more of ethylene carbonate EC, propylene carbonate PC, dimethyl carbonate DMC, diethyl carbonate DEC, ethyl methyl carbonate EMC, tetrahydrofuran THF, ethylene glycol dimethyl ether DME, diethylene glycol dimethyl ether G2, triethylene glycol dimethyl ether G3, tetraethylene glycol dimethyl ether G4, and their fluorinated derivatives.

[0025] More preferably, the organic solvent is a compound of propylene carbonate PC and ethyl methyl carbonate EMC in a volume ratio of 7:3.

[0026] The present invention also provides a method for preparing an electrolyte for a sodium-ion battery, comprising the following steps:

[0027] In a glove box under argon protection, the sodium salt, the organic solvent, the main additive, and the secondary additive are mixed evenly to obtain the electrolyte for the sodium-ion battery.

[0028] Preferably, the conditions of the glove box under argon protection are: O2 < 0.1 ppm, H2O < 0.1 ppm.

[0029] The present invention also protects a sodium-ion battery, comprising a positive electrode, a negative electrode, and the above-mentioned electrolyte for a sodium-ion battery.

[0030] Preferably, the positive electrode comprises a positive electrode active material, a conductive agent, and a binder.

[0031] Preferably, the negative electrode comprises a negative electrode active material, a conductive agent, and a binder.

[0032] The positive electrode active material is selected with reference to the prior art, and preferably is sodium iron pyrophosphate phosphate.

[0033] The negative electrode active material is selected with reference to the prior art, and preferably is hard carbon.

[0034] Preferably, the conductive agent is Super-P.

[0035] Preferably, the binder is polyvinylidene fluoride (PVDF).

[0036] Preferably, the preparation method of the sodium-ion battery includes the following steps: stacking the positive electrode, separator, and negative electrode in sequence, laminating to obtain a bare battery cell, encapsulating with an aluminum-plastic film, baking, injecting electrolyte, standing, forming, shaping with a jig, secondary encapsulation, and capacity testing to obtain the sodium-ion battery.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The present invention provides a sodium-ion battery electrolyte, which includes a sodium salt, an organic solvent, a main additive, and a secondary additive. The main additive is a sodium salt additive of a certain type, and the secondary additive is a silane additive of a certain type. Through a synergistic mechanism, the rate performance of the sodium-ion battery is significantly improved, and it can operate stably at a wide temperature range. The capacity retention rate after 200 cycles at -20°C is above 81.8%, and the capacity retention rate after 500 cycles at 45°C is above 94%. The capacity retention rate is excellent within a wide temperature range, and the capacity retention rate at a high rate of 10C and above is high, enabling the preparation of a sodium-ion battery with fast charging performance. Specific embodiments

[0039] The present invention will be further described below with reference to embodiments. These embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. The experimental methods without specific conditions in the following examples are generally carried out according to the conventional conditions in the art or according to the conditions recommended by the manufacturer; the raw materials, reagents, etc. used, unless otherwise specified, are raw materials and reagents that can be obtained from commercial channels such as conventional markets. Any non-substantive changes and substitutions made by those skilled in the art based on the present invention fall within the scope of protection required by the present invention.

[0040] The preparation methods of the positive and negative electrodes of the sodium-ion batteries in the embodiments and comparative examples of the present invention are as follows:

[0041] Preparation of the positive electrode: The positive electrode material sodium iron pyrophosphate phosphate (Na4Fe3(PO4)2P2O7), binder PVDF (polyvinylidene fluoride), conductive agent Super-P, and dispersant are dispersed in an NMP (N-methylpyrrolidone) organic solvent in a mass ratio of 90:4.6:5:0.4, and stirred to be stable and uniform under the action of a vacuum mixer, and then uniformly coated on a water-based carbon-coated aluminum foil with a thickness of 13 μm. Subsequently, it is transferred to a blast drying oven at 110°C and dried for 12 h, and then made into a positive electrode sheet through cold pressing and die cutting;

[0042] Negative electrode preparation: The negative electrode materials, hard carbon (HC), binder PVDF (polyvinylidene fluoride), and conductive agent Super-P, were dispersed in an organic solvent NMP (N-methylpyrrolidone) at a mass ratio of 90:5:5. Under the action of a vacuum mixer, it was stirred until stable and uniform, and then uniformly coated on an aluminum foil with a thickness of 12 μm. Subsequently, it was transferred to a forced-air oven at 110 °C and dried for 24 h, and then made into a negative electrode sheet through cold pressing and die cutting.

[0043] I. Experimental methods

[0044] Example 1 An electrolyte for a sodium-ion battery, its preparation method and application

[0045] The electrolyte was prepared in a glove box filled with argon (O2 < 0.1 ppm, H2O < 0.1 ppm). The sodium salt was a 1:1 molar ratio mixture of NaFSI and NaPF6, and the organic solvents were a 7:3 volume ratio mixture of PC and EMC. The sodium salt and the solvents were mixed evenly by magnetic stirring. The concentration of the sodium salt in the organic solvent was 1.2 mol / L. The main additive was NaDFP, and the co-additives were a 1:1 mass ratio mixture of TMSB and TMSP. The main additive accounted for 1 wt% of the total mass of the electrolyte, and the co-additives accounted for 3 wt% of the total mass of the electrolyte. After complete dissolution, the electrolyte was obtained.

[0046] Sodium-ion battery preparation: The positive electrode, separator, and negative electrode were stacked in sequence to obtain a bare battery cell, which was then encapsulated with an aluminum-plastic film, baked, filled with electrolyte, allowed to stand, formed, shaped with a fixture, second-sealed, and capacity tested to complete the preparation of the sodium-ion battery.

[0047] Example 2 An electrolyte for a sodium-ion battery, its preparation method and application

[0048] The experimental method was the same as that of Example 1, except that the main additive accounted for 1.3 wt% of the total mass of the electrolyte, the co-additives accounted for 2.6 wt% of the total mass of the electrolyte, and the mass ratio of the main additive to the co-additives was 1:2.

[0049] Example 3 An electrolyte for a sodium-ion battery, its preparation method and application

[0050] The experimental method was the same as that of Example 1, except that the mass of the main additive was 0.8 wt% and the mass of the co-additives was 3.2 wt%, and the mass ratio of the main additive to the co-additives was 1:4.

[0051] Example 4 An electrolyte for a sodium-ion battery, its preparation method and application

[0052] The experimental method was the same as that of Example 1, except that the main additive NaDFP was replaced with an equal amount of NaNO3.

[0053] Example 5 An electrolyte for a sodium-ion battery, a preparation method thereof, and an application

[0054] The experimental method was the same as that in Example 1, except that the main additives were a compound of NaBF4 and NaTFA with a mass ratio of 1:1.

[0055] Example 6 An electrolyte for a sodium-ion battery, a preparation method thereof, and an application

[0056] The experimental method was the same as that in Example 1, except that the co-additives were a compound of TMSB, TMSP, and TMSPi with a mass ratio of 1:1:1.

[0057] Example 7 An electrolyte for a sodium-ion battery, a preparation method thereof, and an application

[0058] The experimental method was the same as that in Example 1, except that the co-additive was TMSP.

[0059] Example 8 An electrolyte for a sodium-ion battery, a preparation method thereof, and an application

[0060] The experimental method was the same as that in Example 1, except that the main additives accounted for 2 wt% of the total mass of the electrolyte, the co-additives accounted for 3 wt% of the total mass of the electrolyte, and the mass ratio of the main additives to the co-additives was 1:1.5.

[0061] Example 9 An electrolyte for a sodium-ion battery, a preparation method thereof, and an application

[0062] The experimental method was the same as that in Example 1, except that the mass of the added main additives was 0.2 wt%, the mass of the added co-additives was 0.8 wt%, and the mass ratio of the main additives to the co-additives was 1:4.

[0063] Comparative Example 1 An electrolyte for a sodium-ion battery, a preparation method thereof, and an application

[0064] The experimental method was the same as that in Example 1, except that no main additives were added, 3 wt% of co-additives were added, and the co-additives were a compound of TMSB and TMSP with a mass ratio of 1:1.

[0065] Comparative Example 2 An electrolyte for a sodium-ion battery, a preparation method thereof, and an application

[0066] The experimental method was the same as that in Example 1, except that no co-additives were added, 1 wt% of main additives were added, and the main additive was NaDFP.

[0067] Comparative Example 3 An electrolyte for a sodium-ion battery, a preparation method thereof, and an application

[0068] The experimental method was the same as that in Example 1, except that the main additive was sodium bis(trifluoromethanesulfonyl)imide (NaTFSI).

[0069] Comparative Example 4: A Sodium-Ion Battery Electrolyte, Its Preparation Method and Application

[0070] The experimental method was the same as that in Example 1, except that the secondary additive was perfluorobenzene (HFB).

[0071] Comparative Example 5: A Sodium-Ion Battery Electrolyte, Its Preparation Method and Application

[0072] The experimental method was the same as that in Example 1, except that the mass of the main additive added was 3 wt%, and the mass of the secondary additive added was 3 wt%.

[0073] Comparative Example 6: A Sodium-Ion Battery Electrolyte, Its Preparation Method and Application

[0074] The experimental method was the same as that in Example 1, except that the mass of the main additive added was 1 wt%, and the mass of the secondary additive added was 4 wt%.

[0075] Comparative Example 7: A Sodium-Ion Battery Electrolyte, Its Preparation Method and Application

[0076] The experimental method was the same as that in Example 1, except that equimolar sodium difluorophosphate was used to replace NaFSI and NaPF6.

[0077] II. Test Indicators

[0078] (1) Room Temperature Cycling Test: The battery was placed in an oven at a constant temperature of 25 °C for 4 hours, then charged at a constant current of 0.2C to 3.25V, then charged at a constant voltage until the current dropped to 0.05C, and then discharged at a constant current of 0.5C to 1.5V. Such cycling was carried out, and the initial capacity of the battery and the discharge capacity of the last cycle (the 500th cycle) were recorded;

[0079] Capacity Retention Rate = Discharge Capacity of the Last Cycle (the 800th Cycle) / Initial Capacity × 100%;

[0080] (2) Low Temperature Cycling Test: The battery was placed in an oven at a constant temperature of -20 °C for 4 hours, then charged at a constant current of 0.5C to 3.25V, then charged at a constant voltage until the current dropped to 0.05C, and then discharged at a constant current of 0.5C to 1.5V. Such cycling was carried out, and the initial capacity of the battery and the discharge capacity of the last cycle (the 200th cycle) were recorded;

[0081] Capacity Retention Rate = Discharge Capacity of the Last Cycle (the 200th Cycle) at -20 °C / Initial Capacity × 100%;

[0082] (3) High-temperature cycling test: Place the battery in an oven at a constant temperature of 45°C for 4 hours, then charge it at a constant current of 0.5C until 3.25V, then charge it at a constant voltage until the current drops to 0.05C, and then discharge it at a constant current of 0.5C until 1.5V. Repeat this cycle, and record the initial capacity of the battery and the discharge capacity of the last cycle (the 300th cycle).

[0083] Capacity retention rate = discharge capacity of the last cycle (the 500th cycle) at 45°C / initial capacity × 100%;

[0084] (4) Rate performance: Charge the battery at a constant current of 0.5C until 3.25V in an environment of 25°C, then charge it at a constant voltage until the current drops to 0.05C, and then discharge it at a constant current of 0.2C / 0.5C / 1C / 5C / 10C / 30C until 1.5V respectively. Record the discharge capacity of the battery cell;

[0085] Capacity retention rate at different rates = discharge capacity at different rates / initial capacity at 0.2C × 100%.

[0087] III. Test results

[0088] Table 1 Test results of examples and comparative examples

[0089]

[0090]

[0091] According to Table 1, it can be seen that the sodium-ion batteries prepared with the sodium-ion battery electrolytes of Examples 1-9 have high capacity retention rate and long cycle stability at a wide temperature range of -20°C to 45°C and high rates, and Example 1 has the best effect.

[0092] For Comparative Example 1, without adding the main additive, the cycle capacity retention rate at -20°C to 45°C decreases significantly. The low-rate performance is similar to that of Example 1, but the capacity retention rate decreases at high rates of 10C and above; for Comparative Example 2, without adding the secondary additive, the cycle performance decreases more at -20°C to 45°C, and the high-rate performance is poor; for Comparative Example 3, the main additive is not within the selected range, and the performance decreases comprehensively; for Comparative Example 4, the secondary additive does not contain silane; for Comparative Example 5, there is too much main additive; for Comparative Example 6, there is too much secondary additive; for Comparative Example 7, it is proved that other types of sodium salts cannot be selected.

[0093] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A sodium-ion battery electrolyte, characterized in that, It includes sodium salt, organic solvent, main additive and auxiliary additive; Among them, the sodium salt is one or both of sodium hexafluorophosphate NaPF6 or sodium bis(fluorosulfonyl)imide NaFSI; The main additive is one or more of sodium difluorophosphate NaDFP, sodium tetrafluoroborate NaBF4, sodium trifluoroacetate NaTFA or sodium nitrate NaNO3; The auxiliary additive is one or more of tris(trimethylsilyl) borate TMSB, tris(trimethylsilyl) phosphate TMSP, (pentafluorophenyl)silane TPFS or tris(trimethylsilyl) phosphite TMSPi; The mass ratio of the main additive to the auxiliary additive is 1:1.5 - 4, the main additive accounts for 0.2% - 2% of the total mass of the electrolyte, and the auxiliary additive accounts for 0.8% - 3% of the total mass of the electrolyte.

2. The sodium-ion battery electrolyte according to claim 1, wherein The auxiliary additive is one or more of tris(trimethylsilyl) borate TMSB, tris(trimethylsilyl) phosphate TMSP or tris(trimethylsilyl) phosphite TMSPi.

3. The sodium-ion battery electrolyte according to claim 1, wherein The mass ratio of the main additive to the auxiliary additive is 1:3 - 4.

4. The sodium-ion battery electrolyte according to claim 1, wherein The concentration of the sodium salt in the organic solvent is 0.8 - 3 mol / L.

5. The sodium-ion battery electrolyte according to claim 1, wherein The main additive is one or both of sodium tetrafluoroborate NaBF4 or sodium trifluoroacetate NaTFA.

6. The sodium-ion battery electrolyte according to claim 1, wherein The organic solvent is one or both of esters or ethers.

7. The sodium-ion battery electrolyte according to claim 1, wherein The organic solvent is selected from one or more of ethylene carbonate EC, propylene carbonate PC, dimethyl carbonate DMC, diethyl carbonate DEC, ethyl methyl carbonate EMC, tetrahydrofuran THF, ethylene glycol dimethyl ether DME, diethylene glycol dimethyl ether G2, triethylene glycol dimethyl ether G3 or tetraethylene glycol dimethyl ether G4 and their fluorinated derivatives.

8. The sodium ion battery electrolyte according to claim 7, wherein, The organic solvent is a mixture of propylene carbonate PC and ethyl methyl carbonate EMC with a volume ratio of 7:

3.

9. The preparation method of the sodium ion battery electrolyte according to any one of claims 1 to 8, characterized in that, It includes the following steps: In a glove box protected by argon, mix the sodium salt, organic solvent, main additive and auxiliary additive evenly to obtain the sodium-ion battery electrolyte.

10. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode and the sodium-ion battery electrolyte according to any one of claims 1 - 8.

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