Electrolyte and battery

Abstract

Disclosed are an electrolyte and a battery. The electrolyte comprises sodium hexafluorophosphate (NaPF6), sodium bisfluorosulfonyl imide (NaFSI), and an NaFSI decomposition product, wherein the mass percentage content of NaFSI and NaPF6 in the electrolyte is 3-30%; the mass ratio of NaFSI / (NaFSI+NaPF6) is 3-60%; and the content of the NaFSI decomposition product in the electrolyte is less than 1000 ppm. The electrolyte enables the battery to exhibit excellent cycling performance and storage performance.

Description

An electrolyte and a battery

[0001] This application claims priority to Chinese Patent Application No. 202510018027.1, filed on January 6, 2025, entitled "An Electrolyte and a Battery", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to an electrolyte, and more particularly to an electrolyte and a battery, belonging to the field of secondary battery technology. Background Technology

[0003] Lithium-ion batteries are widely used in consumer electronics, electric vehicles, and energy storage due to their high energy density and cycle performance. However, rising lithium salt prices have limited the development of lithium-ion batteries due to limited lithium resources. Sodium, an element in the same group as lithium, has very similar physical and chemical properties. Moreover, sodium is more abundant on Earth than lithium and has a lower cost, making sodium-ion batteries a better choice for large-scale energy storage. To date, research on sodium-ion battery technology has attracted widespread attention in both academic and industrial fields.

[0004] Sodium bis(fluorosulfonyl)imide is an electrolyte additive with excellent oxidation and thermal stability. It can reduce electrolyte decomposition reactions during battery charging and discharging, thereby effectively improving the cycle stability of sodium-ion batteries. However, electrolytes containing sodium bis(fluorosulfonyl)imide have a narrow electrochemical window at high voltages and are prone to gas generation during high-temperature storage, thus affecting the high-temperature safety performance of the battery.

[0005] Given the above shortcomings, it is essential to develop an electrolyte that improves battery cycle performance and storage performance. Summary of the Invention

[0006] This invention provides an electrolyte that enables batteries to exhibit excellent cycle performance and storage performance.

[0007] The present invention provides a battery comprising the above-mentioned electrolyte, which has advantages such as long cycle life and good storage performance.

[0008] This invention provides an electrolyte comprising sodium hexafluorophosphate (NaPF6), sodium difluorosulfonamide (NaFSI), and NaFSI decomposition products;

[0009] The mass percentage of NaFSI+NaPF6 in the electrolyte is 3% to 30%; the mass ratio of NaFSI / (NaFSI+NaPF6) is 3% to 60%; and the content of NaFSI decomposition products in the electrolyte is less than 1000 ppm.

[0010] The electrolyte as described above further includes a low-boiling-point solvent, wherein the content of the low-boiling-point solvent in the electrolyte is less than 2000 ppm.

[0011] The electrolyte as described above, wherein the NaPF6 content in the electrolyte is 2% to 20% by mass; and / or, the NaFSI content in the electrolyte is 0.1% to 20% by mass.

[0012] The electrolyte as described above further includes a sulfur-based additive, said sulfur-based additive being selected from at least one compound shown in formulas (1-1) to (1-9).

[0013] In the electrolyte as described above, the sulfide additive has a mass percentage content of 0.5% to 5% in the electrolyte.

[0014] The electrolyte as described above further includes other sodium salts, including at least one of sodium perchlorate, sodium difluorophosphate, sodium bis(oxalate-borate), sodium difluorooxalate-borate, sodium trifluoromethanesulfonylimide, sodium difluorodi(oxalate-borate), and sodium tetrafluorooxalate-borate; the other sodium salts constitute 0.1% to 5% by mass in the electrolyte.

[0015] The electrolyte as described above further includes a first additive, which comprises at least one of vinylene carbonate, fluoroethylene carbonate, ethylene ethylene carbonate, tris(trimethylsilane)borate, tris(trimethylsilane)phosphate, tripropylene phosphate, acid anhydride additives, and nitrile additives; the first additive has a mass percentage of 0.1% to 5% in the electrolyte.

[0016] The electrolyte as described above further includes an organic solvent, said organic solvent including at least one of carbonates, carboxylic esters and ethers.

[0017] The electrolyte as described above comprises, by mass percentage: 6%–20% sodium hexafluorophosphate, 1%–10% sodium difluorosulfonamide, 0.5%–3.6% sulfur-based additives, 20 ppm–1000 ppm low-boiling-point solvents, 20–500 ppm NaFSI decomposition products, 60%–87% organic solvents, 1%–5% other sodium salts, and 0.5%–3% first additive.

[0018] The present invention provides a battery comprising the electrolyte described in any of the preceding claims.

[0019] The battery described above includes a positive electrode active material, which comprises NaFeO2, NaCoO2, NaCrO2, NaVO2, and NaTi. 0.5 Ni 0.5 O2, Na x Ni 0.6 Co 0.4 O2, Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2, Na 0.85 Li 0.17 Ni 0.21 Mn 0.64 O2, NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2, Na3V2(PO4)3, Na 0.9 Mn 0.6 Fe 0.4 At least one of PO4, sodium iron pyrophosphate, and sodium ferrocyanide.

[0020] The electrolyte provided by this invention, by limiting the type and content of sodium salts and the content of sodium hexafluorophosphate decomposition products, not only improves the antioxidant stability of the electrolyte, but also effectively inhibits the decomposition rate of sodium hexafluorophosphate, thereby suppressing the problems of gas generation during battery cycle and storage, and enabling the battery to exhibit excellent cycle performance and storage performance.

[0021] The battery provided by this invention is prepared based on the electrolyte described above, and exhibits the characteristics of long cycle life and high storage performance. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0023] To address the issue of NaFSI's tendency to generate gas, the inventors analyzed the causes and discovered that decomposition products during production, transportation, and storage further decompose during battery application. This damages the interfacial film on the electrodes, promotes the continuous decomposition and consumption of the electrolyte, and facilitates the formation of sodium dendrites. This not only generates a large amount of gas but also significantly increases the risk of Al foil corrosion. Therefore, the inventors envisioned that inhibiting NaFSI decomposition would fundamentally solve the gas generation problem, ultimately improving battery cycle performance and storage performance.

[0024] During the exploration process, the inventors discovered that NaPF6 has a certain impact on the stability of NaFSI. However, since NaPF6 is a sodium salt that is very likely to cause gas production in the battery during cycling, if NaPF6 can play a positive role in the stability of NaFSI while minimizing NaPF6 gas production, it will be beneficial to improve the battery's cycle performance and storage performance.

[0025] Based on this, the present invention provides an electrolyte comprising sodium hexafluorophosphate (NaPF6), sodium difluorosulfonamide (NaFSI), and NaFSI decomposition products;

[0026] The mass percentage of NaFSI+NaPF6 in the electrolyte is 3% to 30%; the mass ratio of NaFSI / (NaFSI+NaPF6) is 3% to 60%; and the content of NaFSI decomposition products in the electrolyte is less than 1000 ppm.

[0027] By applying the above-mentioned electrolyte to the battery, not only can the high cycling characteristics of NaFSI be utilized, but the gas production of the battery can also be effectively reduced, giving the battery excellent storage performance.

[0028] On the one hand, this invention screens NaFSI with decomposition products not exceeding 1000 ppm as electrolyte raw materials by detecting the content of NaFSI decomposition products. This effectively reduces the negative impact of NaFSI decomposition products on the stability of the electrode interface film and avoids gas generation caused by continuous electrolyte decomposition and consumption and sodium dendrite formation due to interface film rupture. Specifically, the NaFSI decomposition products in this invention refer to the decomposition products of NaFSI during production, transportation, and storage, including NaFSO3, NH2SO3H, NH2SO3Na, Na2SO4, etc. In one specific embodiment, the content of NaFSI decomposition products can be determined by ion chromatography (IC). It should be noted that NaFSI continuously decomposes in the electrolyte to form decomposition products. Therefore, the decomposition products in the electrolyte will continuously increase during electrolyte storage and transportation or during battery storage and application. This invention refers to the content of NaFSI decomposition products within 30 days after electrolyte production.

[0029] More importantly, when NaPF6 and NaFSI are combined and the aforementioned content relationship is satisfied, trace amounts of water in the electrolyte will first react with NaPF6 to generate products such as NaPO2F2 in a specific ratio with NaFSI. Under this ratio, products such as NaPO2F2 can increase the chemical energy barrier for NaFSI decomposition, thereby significantly slowing down the decomposition rate of NaFSI and avoiding the gas generation problem caused by the continuous decomposition of NaFSI at high temperatures. In addition, based on the ratio of NaFSI to NaPF6, the difluorosulfonamide anion will promote more sodium ions to combine with organic solvents to form contact ion pairs and aggregates, thus suppressing the gas generation problem of sodium-ion batteries caused by the oxidation reaction of hexafluorophosphate anions and solvents on the positive electrode surface.

[0030] It is worth mentioning that, since the content of NaFSI decomposition products in the electrolyte of the present invention is less than 1000 ppm, and when the above-mentioned content matching relationship between NaPF6 and NaFSI is satisfied, the decomposition rate of NaFSI is also significantly reduced. Therefore, the electrolyte of the present invention will not cause excessive gas production and thus degrade battery performance under long-term storage or application conditions.

[0031] In this invention, ppm refers to parts per million, a unit used to express the content of a substance by mass. For example, if the content of NaFSI decomposition products in the electrolyte is less than 1000 ppm, it means that the mass of NaFSI decomposition products is less than 1000 grams per million grams of electrolyte.

[0032] Furthermore, the electrolyte of the present invention also includes a low-boiling-point solvent, the content of which in the electrolyte is less than 2000 ppm. Here, low-boiling-point solvent refers to solvents with a boiling point below 100°C (mainly dimethyl carbonate (DMC), dichloromethane, diethyl ether, etc.), primarily due to impurities remaining in NaFSI caused by the immature industrial production of NaFSI. The presence of low-boiling-point solvents makes the battery prone to gas generation problems during high-temperature storage, thereby degrading the battery's high-temperature storage performance. Therefore, when the content of low-boiling-point solvents in the electrolyte is less than 2000 ppm, the impact on the degradation of the battery's electrochemical performance is relatively small. Specifically, the mass percentage of low-boiling-point solvents in the present invention can be controlled by using NaFSI of different purities and controlling the amount of NaFSI added. In one specific embodiment, the content of low-boiling-point solvents can be determined by gas chromatography (GC).

[0033] In one specific embodiment, the mass percentage W1 of NaPF6 in the electrolyte is 2% to 20%. The inventors have found that, based on the aforementioned content relationship between NaPF6 and NaFSI, further controlling the mass percentage W1 of NaPF6 in the electrolyte to be 2% to 20% is beneficial to further reduce the decomposition rate of NaFSI, thereby further suppressing the gas generation phenomenon of the battery.

[0034] Furthermore, based on the aforementioned content relationship between NaPF6 and NaFSI, further controlling the mass percentage W2 of NaFSI in the electrolyte to 0.1%–20% can further control the gas generation phenomenon of NaPF6 during cycling, which has a certain effect on improving the storage performance and cycle performance of the battery.

[0035] The electrolyte of the present invention may further include at least one sulfide additive as shown below. Specifically, the sulfide additives shown in formulas (1-1) to (1-9) have the characteristics of high boiling point and high viscosity. These sulfide additives can be matched with the aforementioned low-boiling-point solvents to form a co-solvent, increasing the boiling point of the co-solvent to a suitable value, thereby suppressing gas generation during high-temperature storage of the battery and giving the sodium-ion battery excellent high-temperature storage performance. Simultaneously, the sulfide additives contain sulfonic acid / sulfuric acid groups, which can decompose to obtain slightly acidic decomposition products, appropriately neutralizing the weak alkalinity brought by the FSI groups in the electrolyte, reducing the deterioration effect of the weak alkalinity brought by the FSI groups on the battery during high-temperature storage, thus obtaining a sodium-ion battery with excellent storage performance.

[0036] In particular, formulas (1-5), (1-6), (1-7), and (1-8) contain multiple cyclic structures, which further enhances the activity of the compounds and lowers their energy barriers. This makes the ring-opening reaction of sulfur-based additives easier than that of monocyclic compounds (formulas (1-1), (1-2), (1-3), (1-4), and (1-9)), thus making them more likely to participate in the SEI film formation process and improving the performance of the battery to a certain extent.

[0037] Furthermore, when the mass percentage of sulfide additives in the electrolyte is 0.5%–5%, not only is the co-solvent effect between the sulfide additives and the solvent more significant, but the sulfide additives also form a passivation film with superior stability on the surface of the cathode material. This further prevents the oxidative decomposition of the electrolyte on the cathode surface and the dissolution of transition metal ions in the cathode material, improving the stability of the cathode material structure and interface, making the cathode material less prone to slippage. Moreover, when the sulfide additives are within the above-mentioned limited mass percentage range, they work synergistically with NaFSI to inhibit the decomposition of the passivation film during cycling, further maintaining the stability of the passivation film, enabling the sodium-ion battery to exhibit excellent cycle performance and storage performance.

[0038] In addition to NaPF6 and NaFSI, the electrolyte of this invention may also include other sodium salts. Specifically, the other sodium salts include at least one selected from sodium perchlorate, sodium difluorophosphate, sodium bis(oxalato)borate, sodium difluorooxalatoborate, sodium trifluoromethanesulfonylimide, sodium difluorodi(oxalato)phosphate, and sodium tetrafluorooxalatophosphate; the mass percentage of the other sodium salts in the electrolyte is 0.1% to 5%.

[0039] When the mass percentage of other sodium salts in the electrolyte meets the above-mentioned limits, as an electrolyte additive, it can form a passivation film with significant stability on the positive electrode surface, further preventing the oxidative decomposition of the electrolyte on the positive electrode surface, and suppressing the dissolution of transition metal ions in the positive electrode to a greater extent, thereby further improving the cycle performance and storage performance of the battery. Specifically, the mass percentage of other sodium salts in the electrolyte can be selected from a range of 0.1%, 1%, 3%, 5%, or any combination thereof.

[0040] Furthermore, to further improve the stability of the SEI film on the negative electrode surface of the battery and suppress gas generation, the electrolyte of the present invention also includes at least one of the following additives: vinylene carbonate, fluoroethylene carbonate, ethylene ethylene carbonate, tris(trimethylsilane)borate, tris(trimethylsilane)phosphate, tripropylene phosphate, acid anhydride additives, and nitrile additives, and the first additive has a mass percentage of 0.1% to 5% in the electrolyte. Specifically, the acid anhydride additives include at least one of succinic anhydride and glutaric anhydride, and the nitrile additives include at least one of succinic anhydride, (ethoxy)pentafluorocyclotriphosphazene, acetonitrile, and succinic anhydride.

[0041] Specifically, when the first additive is within the above-mentioned mass percentage range, it can form a stable passivation film on the surface of the positive electrode material, and at the same time preferentially reduce on the surface of the negative electrode to form a SEI film with better performance than that formed by the reduction of existing electrolytes. This suppresses the gas generation problem caused by further reduction and decomposition of the electrolyte, thereby improving the cycle performance and storage performance of the battery.

[0042] The electrolyte of the present invention further includes an organic solvent. In one specific embodiment, the organic solvent includes at least one selected from carbonates, carboxylic esters, and ethers. Within this defined scope, the organic solvent can dissolve sodium salts and additives to form an electrolyte with high conductivity, and can maximize the role of sodium salts and additives in the electrolyte, thereby enabling sodium-ion batteries to achieve better cycle performance and storage performance.

[0043] For example, the carbonate solvent is selected from one or more of ethylene carbonate, propylene carbonate, butene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, or methyl propyl carbonate. Further, the carbonate solvent is a fluorocarbonate solvent, selected from one or more of fluoroethylene carbonate, difluoroethylene carbonate, methyl trifluoromethyl carbonate, methyl trifluoroethyl carbonate, di(2,2,2-trifluoroethyl) carbonate, and (2,2,2-trifluoroethyl) methyl carbonate.

[0044] For example, the carboxylic acid ester solvent is selected from one or more of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, propyl propionate, or ethyl butyrate. Further, the carboxylic acid ester solvent is a fluorocarboxylic acid ester solvent, selected from one or more of ethyl fluoroacetate, methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, 2,2,2-trifluoroethyl difluoroacetate, methyl pentafluoropropionate, or 2,2-difluoroethyl acetate.

[0045] For example, the ether solvent is selected from one or more of tetrahydrofuran, 1,3-dioxapentane, diethyl ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, or tetraethylene glycol dimethyl ether. Further, the ether solvent is a fluorinated ether solvent, selected from one or more of bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0046] In one specific embodiment, the electrolyte comprises, by mass percentage: 6%–20% sodium hexafluorophosphate, 1%–10% sodium difluorosulfonyl imide, 0.5%–3.6% sulfur-based additives, 20 ppm–1000 ppm low-boiling-point solvents, 20–500 ppm NaFSI decomposition products, 60–87% organic solvents, 1%–5% other sodium salts, and 0.5%–3% first additive. With this composition, the electrolyte exhibits high antioxidant properties, resulting in high cycle performance and high storage capacity for the battery.

[0047] The present invention does not limit the preparation method of the electrolyte. In one specific embodiment, it is sufficient to mix NaPF6, NaFSI and an organic solvent in a specified ratio.

[0048] Furthermore, when the electrolyte also includes additives, NaPF6, NaFSI, organic solvents, and additives can be mixed in the prescribed proportions.

[0049] The present invention also provides a battery comprising the electrolyte described above. Based on the electrolyte provided by the present invention, the battery provided by the present invention has excellent cycle performance and storage performance.

[0050] In one specific embodiment, the battery includes a positive electrode active material, which includes NaFeO2, NaCoO2, NaCrO2, NaVO2, and NaTi. 0.5 Ni 0.5 O2, Na x Ni 0.6 Co 0.4 O2, Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2, Na 0.85 Li 0.17 Ni 0.21 Mn 0.64 O2, NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2, Na3V2(PO4)3, Na 0.9 Mn 0.6 Fe 0.4At least one of PO4, sodium iron pyrophosphate, and sodium ferrocyanide. Specifically, the battery includes a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on the surface of the positive current collector. The positive active material layer includes a positive active material, a conductive agent, and a binder. The positive current collector is generally aluminum foil, and the positive active material includes NaFeO2, NaCoO2, NaCrO2, NaVO2, and NaTi. 0.5 Ni 0.5 O2, Na x Ni 0.6 Co 0.4 O2, Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2, Na 0.85 Li 0.17 Ni 0.21 Mn 0.64 O2, NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2, Na3V2(PO4)3, Na 0.9 Mn 0.6 Fe 0.4 At least one of PO4, sodium iron pyrophosphate, and sodium ferrocyanide. It is understood that applying the above electrolyte to sodium-ion batteries containing the above positive electrode active material allows the components in the electrolyte to work synergistically with the positive electrode active material, improving the stability of the positive electrode active material and thus obtaining a battery with high cycle performance, excellent storage performance, and superior safety performance.

[0051] In one specific embodiment, in addition to the electrolyte and positive electrode provided by the present invention, the battery also includes a negative electrode and a separator, specifically:

[0052] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector. The negative active material layer includes a negative active material, a conductive agent, and a binder. The negative current collector is generally aluminum foil, and the negative active material is selected from one or more of carbonaceous materials, silicon carbide materials, alloy materials, and sodium-containing metal composite oxides.

[0053] The conductive agents and binders used in both the positive and negative electrode active material layers can be conventional materials in this field.

[0054] The separator is a separator known in the art that can be used in batteries and is stable to the electrolyte used. It may include one or more of polyolefins, aromatic polyamides, polytetrafluoroethylene, and polyethersulfone, and may be configured as needed.

[0055] The present invention will be further described in detail below through specific embodiments.

[0056] Example

[0057] Example 1

[0058] The electrolyte provided in this embodiment comprises, by mass percentage: 14.55% sodium hexafluorophosphate, 0.45% sodium difluorosulfonamide, 56 ppm low-boiling-point solvent, and the balance being organic solvents ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC), with EC:PC:EMC in a ratio of 1:1:4.

[0059] The preparation method of the electrolyte in this embodiment includes: adding sodium hexafluorophosphate and sodium difluorosulfonamide to an organic solvent under a dry environment with a moisture content of less than 5 ppm and stirring at a constant speed until homogeneous; then adding an additive to the above mixed solution and mixing evenly to obtain the electrolyte. Gas chromatography analysis of the electrolyte revealed that the content of the low-boiling-point solvent was 56 ppm.

[0060] The cathode material NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2, conductive material acetylene black, and binder (polyvinylidene fluoride) PVDF are dispersed in solvent N-methylpyrrolidone (NMP) at a mass ratio of 90:5:5 to obtain a positive electrode active material layer slurry. The positive electrode active material layer slurry is uniformly coated on the surface of the positive electrode current collector aluminum foil. After drying, rolling, baking, slitting, and spot welding of electrode tabs, a positive electrode sheet is obtained with a total thickness of 134 μm.

[0061] The negative electrode active material hard carbon, conductive agent conductive carbon black super-p, and binder (polyvinylidene fluoride) PVDF are dispersed in deionized water at a mass ratio of 89:1:10 and stirred evenly to obtain a negative electrode active material layer slurry. The negative electrode active material layer slurry is uniformly coated on the surface of the negative electrode current collector copper foil. After drying, rolling, baking, slitting, and spot welding of electrode tabs, a negative electrode sheet is obtained with a total thickness of 150μm.

[0062] The prepared positive electrode, negative electrode and separator are stacked in sequence. The separator is a three-layer separator with a thickness of 20μm. The separator is placed between the positive electrode and negative electrode, rolled up and flattened and put into an aluminum foil packaging bag. After vacuum baking at 85℃ for 48h, the cell is obtained. The electrolyte is injected into the cell in a glove box. After encapsulation, formation, aging and capacity testing, the sodium-ion battery is prepared.

[0063] The electrolyte composition and battery preparation provided in Examples 2-44 and Comparative Examples 1-7 are basically the same as those in Example 1. Specific parameters are shown in Table 1.

[0064] Table 1

[0065] Test case

[0066] 1. The content of NaFSI decomposition products A1 in the freshly prepared electrolyte was detected, and the content of NaFSI decomposition products A2, A3, and A4 in the electrolyte was detected after standing at 45℃ for 7d, 14d, and 30d respectively. The results are shown in Table 2.

[0067] Table 2

[0068] 2. The following battery performance tests were conducted on the sodium batteries containing the electrolytes prepared in Examples 1-44 and Comparative Examples 1-7, and the test methods are as follows:

[0069] High-temperature cycle performance test: The battery was charged at a constant current of 1C to 4.0V at 45℃, then charged at a constant voltage of 4.0V to the cutoff current of 0.05C, and then discharged at 1C to 2.0V. The discharge capacity was recorded as E1. The thickness was measured using a thickness gauge and recorded as B1. After 500 cycles of charge and discharge, the discharge capacity of the 500th cycle was recorded as E2, and the thickness was measured while still hot and recorded as B2. The capacity retention rate at 45℃ η1 = E2 / E1*100%, and the thickness growth rate D1 = (B2-B1) / B1*100%, as shown in Table 3.

[0070] High-temperature storage performance test: The battery was charged at room temperature (25℃) with a constant current of 1C to 4.0V, and then charged at a constant voltage of 4.0V to a cutoff current of 0.05C. The battery was then discharged at a constant current of 1C, and the discharge capacity was recorded as E3. The thickness was measured using a thickness gauge and recorded as B3. At room temperature (25℃), the battery was charged at a constant current of 1C to 4.0V, and then charged at a constant voltage of 4.0V to a cutoff current of 0.05C. The battery was then transferred to a high temperature of 60℃ and left for 14 days. The thickness was measured while still hot and recorded as B4. The battery was then discharged at a constant current of 1C, and the discharge capacity was recorded as E4. The battery was then charged at a constant current of 1C to 4.0V at room temperature (25℃), and then charged at a constant voltage of 4.0V to a cutoff current of 0.05C. It was then discharged at 1C to 2.0V, and this charge-discharge cycle was repeated for 3 weeks. The discharge capacity of the third cycle was recorded and denoted as E5. The capacity retention rate at 60℃ was η2 = E4 / E3 * 100%, the capacity recovery rate was η3 = E5 / E3 * 100%, and the thickness growth rate was D2 = (B4 - B3) / B3 * 100%, as shown in Table 3.

[0071] DCIR Test: After capacity grading, the battery is charged to 4.0V at 1C at room temperature, left to rest for 5 minutes, then discharged at 1C for 30 minutes, left to rest for 1 hour, and then discharged at 2C for 10 seconds. The DCIR at 50% SOC is calculated and recorded as C1. For batteries that have completed 14 days of high-temperature storage performance testing at 60℃, the battery is charged to 4.0V at 1C at room temperature, left to rest for 5 minutes, then discharged at 1C for 30 minutes, left to rest for 1 hour, and then discharged at 2C for 10 seconds. The DCIR at 50% SOC is calculated and recorded as C2. The DC resistance growth rate η4 = (C2 - C1) / C1 * 100%, see Table 3.

[0072] Table 3

[0073] As shown in Tables 2 and 3, mixing NaPF6 and NaFSI in a specific ratio can effectively suppress gas production in the system and improve the cycle performance and storage performance of the battery.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrolyte, characterized in that, Including sodium hexafluorophosphate (NaPF6), sodium difluorosulfonamide (NaFSI), and NaFSI decomposition products; The mass percentage of NaFSI+NaPF6 in the electrolyte is 3% to 30%; the mass ratio of NaFSI / (NaFSI+NaPF6) is 3% to 60%; and the content of NaFSI decomposition products in the electrolyte is less than 1000 ppm.

2. The electrolyte according to claim 1, characterized in that, The electrolyte also includes a low-boiling-point solvent, the content of which in the electrolyte is less than 2000 ppm.

3. The electrolyte according to claim 1 or 2, characterized in that, The NaPF6 content in the electrolyte is 2% to 20% by mass; and / or, the NaFSI content in the electrolyte is 0.1% to 20% by mass.

4. The electrolyte according to any one of claims 1-3, characterized in that, It also includes sulfur-based additives, said sulfur-based additives being selected from at least one of the compounds shown in formulas (1-1) to (1-9).

5. The electrolyte according to claim 4, characterized in that, The sulfur-based additive has a mass percentage of 0.5% to 5% in the electrolyte.

6. The electrolyte according to any one of claims 1-5, characterized in that, The electrolyte also includes other sodium salts, including at least one of sodium perchlorate, sodium difluorophosphate, sodium bis(oxalate)borate, sodium difluorooxalateborate, sodium trifluoromethanesulfonylimide, sodium difluorodi(oxalate)phosphate, and sodium tetrafluorooxalate phosphate; the other sodium salts in the electrolyte have a mass percentage of 0.1% to 5%.

7. The electrolyte according to any one of claims 1-6, characterized in that, The electrolyte further includes a first additive, which includes at least one of vinylene carbonate, fluoroethylene carbonate, ethylene ethylene carbonate, tris(trimethylsilane)borate, tris(trimethylsilane)phosphate, tripropylene phosphate, acid anhydride additives, and nitrile additives; the first additive has a mass percentage of 0.1% to 5% in the electrolyte.

8. The electrolyte according to any one of claims 1-7, characterized in that, It also includes organic solvents, which include at least one of carbonates, carboxylic esters and ethers.

9. The electrolyte according to any one of claims 1-8, characterized in that, The electrolyte comprises, by mass percentage: 6%–20% sodium hexafluorophosphate, 1%–10% sodium difluorosulfonamide, 0.5%–3.6% sulfur-based additives, 20 ppm–1000 ppm low-boiling-point solvents, 20–500 ppm NaFSI decomposition products, 60%–87% organic solvents, 1%–5% other sodium salts, and 0.5%–3% first additive.

10. A battery, characterized in that, The battery comprises the electrolyte according to any one of claims 1-9.

11. The battery according to claim 10, characterized in that, The battery includes a positive electrode active material, which includes NaFeO2, NaCoO2, NaCrO2, NaVO2, and NaTi. 0.5 Ni 0.5 O2, Na x Ni 0.6 Co 0.4 O2, Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2, Na 0.85 Li 0.17 Ni 0.21 Mn 0.64 O2, NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2, Na3V2(PO4)3, Na 0.9 Mn 0.6 Fe 0.4 At least one of PO4, sodium iron pyrophosphate, and sodium ferrocyanide.

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