Sodium secondary battery and electrolyte

Abstract

The application relates to the technical field of sodium ion batteries, and discloses an electrolyte for a sodium secondary battery, which comprises a sodium salt, a solvent, and an electrolyte additive with the following structural formula: and an auxiliary additive, wherein the auxiliary additive is lithium bis(oxalato)borate and / or tris(trimethylsilyl)borate. The electrolyte can improve the cycle performance, high-temperature storage performance and low-temperature discharge performance of the sodium ion battery under high voltage. The application further discloses a sodium secondary battery, which can effectively reduce the surface activity of an electrode under high voltage, inhibit the dissolution of vanadium and the occurrence of side reactions between the electrode and the electrolyte, and improve the conduction rate of sodium ions, so that the cycle performance, high-temperature storage performance and low-temperature discharge performance of the battery under high voltage (4.5 V) are improved.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a sodium secondary battery and electrolyte. Background Technology

[0002] Among novel battery systems, sodium-ion batteries are considered one of the most competitive next-generation rechargeable batteries due to their low cost and abundant sodium resources in nature. Sodium is electrochemically and chemically in the same group as Li, thus sodium-ion batteries (SIBs) can follow the successful path of lithium-ion batteries. Furthermore, replacing copper with aluminum as the negative current collector in sodium-ion batteries can significantly reduce the overall weight of the battery, thereby effectively increasing its energy density. Based on these advantages, various sophisticated cathodes and anodes with ingenious designs in terms of form and composition have been successfully developed, many of which have proven to have great commercial application potential. The higher energy density and lower price of sodium-ion batteries remain a relentless pursuit for researchers.

[0003] Currently, the energy density of commercial sodium-ion batteries can be increased by raising the charging voltage. However, with the increase in the upper voltage limit, commercial cathode materials such as sodium vanadium phosphate face problems such as poor high-temperature storage and severe gas generation during cycling. This may be due to two factors: firstly, the newly developed coating or doping technologies may not be perfect; secondly, there is the issue of electrolyte compatibility. Conventional electrolytes will oxidize and decompose on the surface of the battery cathode at a high voltage of 4.3V, especially under high-temperature conditions, which accelerates the oxidation and decomposition of the electrolyte and promotes the deterioration of the cathode material.

[0004] Sodium salt film-forming additives are an important component of electrolyte additives for sodium-ion secondary batteries. High-quality sodium salt film-forming additives, such as NaDFOB, NaFSI, and NaTFSI, form a protective film on the electrode surface. This film is insoluble in organic solvents, allowing sodium ions to freely insert and extract from the electrode while preventing solvent molecules from passing through. This effectively prevents further damage to the electrode from the organic electrolyte and the electrode, thereby improving the battery's room-temperature cycle performance and high- and low-temperature performance. More importantly, compared to the relatively complex and expensive improvement methods of material coating, the use of film-forming additives is simpler and cheaper.

[0005] For example, Chinese Patent 201811353637.3 discloses an electrolyte for lithium metal batteries, which is composed of the following components: lithium salt, additives and non-aqueous solvent; the additives are NaBOB, NaTFSI, NaFSI, NaPF6, NaBF4, and (C3H3NaO2). nThe electrolyte for lithium metal batteries contains one or more of the following: Na2SO4, Mg(FSI)2, Mg(TFSI)2, KFSI, and KTFSI; the concentration of the additive in the electrolyte is 0.2–0.5 mol / L; and the non-aqueous solvent is one or more of the following: carbonate organic solvents, phosphate organic solvents, and ether organic solvents.

[0006] The electrolyte in this invention can form an SEI interface layer on the surface of the lithium metal anode during constant current charging and discharging, thereby improving the battery's safety performance, utilization rate, and cycle stability.

[0007] However, the performance of film-forming additives can vary depending on the type and ratio. Currently, sodium-ion batteries are one of the main research directions for future batteries. In order to further improve the overall performance of sodium-ion batteries, especially cycle performance and high-temperature storage performance, it is necessary to develop an electrolyte additive with better performance for sodium-ion batteries.

[0008] Chinese Patent 201711033014.3 discloses a lithium titanate battery electrolyte with low gas production; it includes an organic solvent, a lithium salt, and additives. The solvent is a mixture of dimethyl carbonate, ethyl chlorocarbonate, tris(trimethylsilyl) phosphate, and chain carboxylic acid esters in a mass ratio of 1:1-1.6:1.1-1.3:5-10; the lithium salt to organic solvent ratio is 10-15%; the additive is at least one selected from sodium benzoate, disodium ethylenediaminetetraacetate, and 10-hydroxy-2-decenoic acid. The lithium titanate battery of this invention has low gas production, high battery safety, improved charging and discharging cycle characteristics, and can suppress gas generation during high-temperature storage.

[0009] The aforementioned patent uses electrolyte additives such as sodium benzoate to enhance the performance of the electrolyte, thereby further improving the overall performance of lithium batteries. This approach can be applied to sodium-ion batteries, but direct application to sodium-ion batteries will result in performance differences. Further improvements are needed to obtain an electrolyte suitable for sodium-ion batteries. Summary of the Invention

[0010] One of the objectives of this invention is to provide an electrolyte suitable for sodium secondary batteries, which can improve the cycle performance, high-temperature storage performance and low-temperature discharge performance of sodium-ion batteries under high voltage.

[0011] Another objective of this invention is to provide a sodium secondary battery that can effectively reduce the surface activity of the electrodes, suppress the dissolution of vanadium and the occurrence of side reactions between the electrodes and the electrolyte under high voltage, and improve the conduction rate of sodium ions, thereby improving the battery's cycle performance, high-temperature storage performance and low-temperature discharge performance under high voltage (4.5V).

[0012] To achieve the above objectives, the present invention provides an electrolyte for sodium secondary batteries, comprising a sodium salt, a solvent, and an electrolyte additive with the following structural formula:

[0013]

[0014] Preferably, the electrolyte additive including the above-described structure also includes auxiliary additives, wherein the auxiliary additives are lithium bis(oxalato)borate and / or tris(trimethylsilane)borate.

[0015] Preferably, the electrolyte additive of the structural formula (I) accounts for 0.1%-1.0% of the total mass of the sodium salt and solvent.

[0016] In the above-mentioned electrolyte, the total mass of the electrolyte additives and auxiliary additives accounts for 0.1%-5% of the total mass of the sodium salt and solvent.

[0017] In the electrolyte described above, the sodium salt is sodium hexafluorophosphate, and the concentration of the sodium salt in the electrolyte is 0.5 mol / L-1.5 mol / L.

[0018] In this invention, it is not excluded that the sodium salt is all or part of sodium nitrate, sodium perchlorate, sodium difluorophosphate, sodium bis(oxalate-borate)borate, sodium difluorooxalate-borate, sodium bis(trifluoromethanesulfonyl)imide, and sodium difluorosulfonylimide.

[0019] In this invention, the sodium salt may also be a combination of one or more of sodium hexafluorophosphate and sodium nitrate, sodium perchlorate, sodium difluorophosphate, sodium bis(oxalate-borate), sodium difluorooxalate-borate, sodium bis(trifluoromethanesulfonyl)imide, and sodium difluorosulfonylimide in any proportion.

[0020] In the electrolyte described above, the solvent includes any one or more of chain carbonate compounds and cyclic carbonate compounds.

[0021] During implementation, chain carbonate compounds and cyclic carbonate compounds can be mixed in any proportion, and none of them will have a decisive impact on the changing trend of the electrolyte of the present invention.

[0022] Preferably, the cyclic carbonates include ethylene carbonate and / or propylene carbonate.

[0023] Preferably, the chain carbonate can be any one or more of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0024] Specifically, the solvent may be one or more of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethyl acetate, butyl acetate, γ-butyrolactone, propyl propionate, difluoroethyl acetate, and ethyl 2,2,2-trifluoroacetate, or any combination thereof.

[0025] The present invention also provides a sodium secondary battery, comprising a positive electrode made of sodium vanadium phosphate, a negative electrode made of hard carbon, and the aforementioned electrolyte.

[0026] Furthermore, the positive electrode material of the present invention can also be selected from Na. x CoO2, Na x MnO2, NaNi 0.33 Fe 0.33 Mn 0.33 One or more of O2, NaFePO4, NaCoPO4, and Na3V2(PO4)3; the negative electrode material is selected from one or more of soft carbon, hard carbon, sodium titanate, and metals that can form alloys with sodium.

[0027] Beneficial effects

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) When a compound with a specific structure is used as an electrolyte additive, acetate and sodium ions will coordinate, resulting in a stable interfacial film between the additive molecules covering the positive and negative electrode surfaces. Furthermore, through the stable structure combined with the benzene ring, it has higher thermodynamic stability. Moreover, the compound with this specific structure is conducive to the conduction of sodium ions, which can effectively improve the cycle performance, high temperature storage performance and low temperature discharge performance of sodium ion batteries under high voltage.

[0030] (2) By using compounds with specific structural formulas as electrolyte additives and mixing them with lithium bis(trimethylsilane)borate and / or tri(trimethylsilane)borate to form a new electrolyte formulation, a synergistic effect can be generated, thereby improving the cycle performance, high-temperature storage performance and low-temperature discharge performance of sodium-ion batteries under high voltage, making sodium-ion batteries have better performance.

[0031] (3) By using specific electrolyte additives in a certain mass ratio, the electrolyte can have better performance, effectively improving the cycle performance, high-temperature storage performance and low-temperature discharge performance of sodium-ion batteries under high voltage.

[0032] (4) When the electrolyte of the present invention is used in a sodium secondary battery with sodium vanadium phosphate as the positive electrode and hard carbon as the negative electrode, it can effectively reduce the surface activity of the electrode, inhibit the dissolution of vanadium and the occurrence of side reactions between the electrode and the electrolyte, and improve the conduction rate of sodium ions, thereby improving the cycle performance, high temperature storage performance and low temperature discharge performance of the battery under high voltage (4.5V). Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1

[0035] A sodium secondary battery specifically includes the following preparation steps:

[0036] (1) Preparation of electrolyte: Ethyl carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) are mixed in a mass ratio of EC:DEC:EMC = 1:1:1. After mixing, 1 mol of sodium hexafluorophosphate (NaPF6) is added to make the sodium salt concentration in the electrolyte reach 1M. After the sodium salt is completely dissolved, an electrolyte additive with a mass of 0.1% of the total mass of sodium salt and solvent is added. The electrolyte additive is an electrolyte additive with structural formula (I).

[0037] (2) Preparation of positive electrode sheet: Sodium vanadium phosphate, conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly at a mass ratio of 95:2.3:2:0.7 to prepare a sodium-ion battery positive electrode slurry with a certain viscosity. The slurry is coated on aluminum foil for current collector with a coating amount of 30g / m2. After drying at 85℃, it is cold pressed. Then, the edges are cut, the sheets are cut and slit. After slit, the sheets are dried at 85℃ for 4 hours under vacuum. The tabs are then welded to produce a sodium-ion battery positive electrode sheet that meets the requirements.

[0038] (3) Preparation of negative electrode sheet: Hard carbon, conductive agent SuperP, thickener CMC, and binder SBR (styrene-butadiene rubber emulsion) are mixed in a mass ratio of 95:1.5:1.0:2.5 to form a slurry. The mixture is evenly coated on both sides of copper foil, dried, and rolled to obtain a negative electrode sheet, thus producing a sodium-ion battery negative electrode sheet that meets the requirements.

[0039] (4) Preparation of sodium-ion battery: The positive electrode, negative electrode and separator prepared according to the above process are stacked to form a sodium-ion battery with a thickness of 4.7 mm, a width of 55 mm and a length of 60 mm. The battery is vacuum baked at 75 °C for 10 hours and injected with the above electrolyte. After standing for 24 hours, it is charged to 4.5 V with a constant current of 0.1 C (180 mA) and then charged with a constant voltage of 4.5 V until the current drops to 0.05 C (90 mA). Then it is discharged to 3.0 V with 0.2 C (180 mA). The charge and discharge are repeated twice. Finally, the battery is charged to 3.8 V with 0.2 C (180 mA) to complete the battery fabrication.

[0040] Example 2

[0041] It is basically the same as Example 1, except that the mass of the electrolyte additive is 0.2% of the total mass of sodium salt and solvent.

[0042] Example 3

[0043] It is basically the same as Example 1, except that the mass of the electrolyte additive is 0.5% of the total mass of sodium salt and solvent.

[0044] Example 4

[0045] This is essentially the same as Example 1, except that the mass of the electrolyte additive is 1% of the total mass of the sodium salt and solvent.

[0046] Example 5

[0047] The process is basically the same as in Example 1, except that the electrolyte additive is a mixture of the compound shown in structural formula (I) and lithium bis(oxalato)borate, with the amount of the compound shown in structural formula (I) additive being 0.2% and the amount of lithium bis(oxalato)borate being 0.3%.

[0048] Example 6

[0049] The process is basically the same as in Example 1, except that the electrolyte additive is a mixture of the compound shown in structural formula (I) and tris(trimethylsilane)borate, with the amount of the compound shown in structural formula (I) additive being 0.2% and the amount of tris(trimethylsilane)borate being 0.3%.

[0050] Example 7

[0051] The process is basically the same as in Example 1, except that the electrolyte additive is a mixture of the compound shown in structural formula (I), lithium bis(oxalato)borate and tris(trimethylsilane)borate, with a weight ratio of 4:3:3 and a total weight of 0.5%.

[0052] Example 8

[0053] It is largely the same as Example 3, except that its solvent is EC:PC:DEC:EMC = 1:1:2:2.

[0054] Example 9

[0055] It is largely the same as Example 3, except that its solvent is EC:DMC:EMC = 1:1:1.

[0056] The comparative examples used the following compounds:

[0057]

[0058]

[0059] Comparative Example 1

[0060] It is basically the same as Example 2, except that the electrolyte additive is Comparative Compound 1.

[0061] Comparative Example 2

[0062] It is basically the same as Example 2, except that the electrolyte additive is Comparative Compound 2.

[0063] Comparative Example 3

[0064] It is basically the same as Example 2, except that the electrolyte additive is Comparative Compound 3.

[0065] Comparative Example 4

[0066] It is basically the same as Example 5, except that the electrolyte additive is lithium bis(oxalato)borate, which is equivalent to 0.5% of the total mass of sodium salt and solvent.

[0067] Comparative Example 5

[0068] It is basically the same as Example 6, except that the electrolyte additive is tris(trimethylsilane)borate, which is equivalent to 0.5% of the total mass of sodium salt and solvent.

[0069] Comparative Example 6

[0070] The process is basically the same as in Example 7, except that the electrolyte additive is a mixture of tris(trimethylsilane)borate and lithium bis(oxalato)borate, each of which is equivalent to 0.25% of the total mass of the sodium salt and solvent.

[0071] Comparative Example 7

[0072] It is basically the same as Example 2, except that the electrolyte additive is NaDFOB.

[0073] Comparative Example 8

[0074] It is basically the same as Example 2, except that the electrolyte additive is NaTFSI.

[0075] Comparative Example 9

[0076] It is basically the same as Example 2, except that the electrolyte additive is NaFSI.

[0077] Performance testing

[0078] Sodium-ion battery performance testing

[0079] Test Method 1:

[0080] 25℃ 0.5C / 0.5C room temperature cycle test: At 25℃, the battery is charged at a constant current of 0.5C to 4.5V, then charged at a constant voltage of 4.5V to the cutoff current of 0.05C. The battery is then discharged at a constant current of 0.5C, and the discharge capacity is recorded as C0. This charge-discharge cycle is repeated 400 times to obtain the discharge capacity C on the 400th cycle. 400 Capacity retention rate = C 400 / C0*100%.

[0081] Test Method 2:

[0082] Capacity retention test after 14 days of storage at 60℃: At 25℃, the battery was charged to 4.5V at a constant current of 0.5C and then charged to a cutoff current of 0.05C at a constant voltage of 4.5V. The battery was then discharged at a constant current of 0.5C, and the discharge capacity was recorded as C0. At 25℃, the battery was charged to 4.5V at a constant current of 0.5C and then charged to a cutoff current of 0.05C at a constant voltage of 4.5V. The battery was then transferred to 60℃ and stored for 14 days. The battery was then discharged at a constant current of 0.5C, and the discharge capacity was recorded as C1. The capacity retention rate after 14 days of storage at 60℃ = C1 / C0 * 100%.

[0083] Test Method 3:

[0084] -10℃ Low Temperature Discharge Test: At 25℃, the battery is charged at a constant current of 0.5C to 4.5V, then charged at a constant voltage of 4.5V to a cutoff current of 0.05C. The battery is then discharged at a constant current of 0.5C, and the discharge capacity is recorded as C0. At 25℃, the battery is charged at a constant current of 0.5C to 4.5V, then charged at a constant voltage of 4.5V to a cutoff current of 0.05C. The battery is then transferred to -10℃ and left to stand for 240 minutes. The battery is then discharged at a constant current of 0.5C, and the discharge capacity is recorded as C2. The -10℃ discharge rate = C2 / C0 * 100%.

[0085] Examples 1-7 and Comparative Examples 1-13 described above were tested using the three test methods described above to assess the room temperature cycle performance, high temperature cycle performance, and low temperature discharge performance of sodium-ion batteries. The results are shown in Table 1.

[0086] Table 1: Performance Test Results of Sodium-ion Batteries

[0087]

[0088]

[0089] The data in Table 1 shows that:

[0090] 1. As can be seen from the comparison between Example 2 and Comparative Examples 1-3, the amount of formate has a significant effect on the electrochemical performance. When the compound shown in structural formula (I) is used as an electrolyte additive, it has a significant effect on the electrochemical performance of the electrolyte.

[0091] 2. As can be seen from the comparison between Example 2 and Comparative Examples 7-9, the compound shown in structural formula (I) has a better effect on improving the electrochemical performance of electrolytes than other conventional electrolyte additives.

[0092] 3. By comparing Example 5 with Comparative Example 4, it can be seen that the compound shown in structural formula (I) and lithium dioxaborate produced a synergistic effect, thereby improving the cycle performance, high temperature storage performance and low temperature discharge performance of sodium-ion batteries under high voltage, making sodium-ion batteries have better performance.

[0093] 4. By comparing Example 6 with Comparative Example 5, it can be seen that the compound shown in structural formula (I) and tris(trimethylsilane)borate ester have a synergistic effect, thereby improving the cycle performance, high temperature storage performance and low temperature discharge performance of sodium-ion batteries under high voltage, making sodium-ion batteries have better performance.

[0094] 5. As can be seen from the comparison between Example 7 and Comparative Example 6, the synergistic effect produced by the compound shown in structural formula (I) after being mixed with lithium bis(oxalato)borate and tris(trimethylsilane)borate significantly improves the electrochemical performance of the electrolyte compared with the mixture of lithium bis(oxalato)borate and tris(trimethylsilane)borate.

[0095] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An electrolyte for a sodium secondary battery, comprising a sodium salt, a solvent, characterized in that, The electrolyte additive of the structural formula (I) is also included. Formula (I); The auxiliary additive is lithium bisoxalate borate and / or tris(trimethylsilyl) borate.

2. The electrolyte for a sodium secondary battery according to claim 1, characterized by, The mass of the electrolyte additive of the structural formula (I) accounts for 0.1%-1.0% of the total mass of the sodium salt and the solvent.

3. The electrolyte for a sodium secondary battery according to claim 1 or 2, characterized by, The sum of the mass of the electrolyte additive and the auxiliary additive accounts for 0.1%-5% of the total mass of the sodium salt and the solvent.

4. The electrolyte for a sodium secondary battery according to claim 3, characterized by, The sodium salt is sodium hexafluorophosphate, and the concentration of the sodium salt in the electrolyte is 0.5mol / L-1.5mol / L.

5. The electrolyte for a sodium secondary battery according to claim 3, characterized by, The solvent includes any one or more of chain and cyclic carbonate compounds.

6. The electrolyte for a sodium secondary battery according to claim 5, characterized by, The cyclic carbonate compound includes ethylene carbonate and / or propylene carbonate.

7. The electrolyte for a sodium secondary battery according to claim 5, wherein The chain carbonate compound is any one or more of dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

8. A sodium secondary battery comprising a positive electrode of which material is sodium vanadium phosphate and a negative electrode of which material is hard carbon, characterized in that, The electrolyte as claimed in any one of claims 1-7 is also included.

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