An electrolyte for a sodium-ion battery and a sodium-ion battery

Abstract

To overcome the technical problems of the dissolution of transition metal elements in the positive electrode of existing high-voltage sodium-ion batteries and the reduction of cycling performance, the present application provides an electrolyte for a sodium-ion battery and a sodium-ion battery. The electrolyte includes a first electrolyte salt and a non-aqueous organic solvent. The first electrolyte salt includes sodium aminosulfonate and sodium bis(fluorosulfonyl)imide. Sodium aminosulfonate in the electrolyte can promote the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, generating a passivation layer rich in nitrogen and sulfur elements at the positive electrode interface. Nitrogen and sulfur elements contain lone pairs of electrons, which can attract the electron-deficient transition metal elements in the positive electrode active material, thereby inhibiting the dissolution of transition metal elements in the positive electrode, preventing the decomposition of the passivation layer, improving the high-voltage stability of the battery, enhancing the low-temperature discharge, high-temperature cycling, and high-temperature storage performance of the battery, and inhibiting gas generation in the battery.

Description

Technical Field

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

[0002] In recent years, sodium-ion batteries with a working principle similar to that of lithium-ion batteries have received extensive attention due to their abundant resources and excellent comprehensive performance. Compared with lithium-ion batteries, the main advantages of sodium-ion batteries are as follows: First, the cost is low. Compared with lithium, sodium has abundant reserves and wide distribution, and the cost of lithium salts is about 25 times that of sodium salts. Second, the electrode potential is close to that of lithium. The redox potential of sodium relative to the standard hydrogen electrode is -2.71V, which is only about 0.3V higher than that of lithium (-3.04V). Third, the physical and chemical properties are similar to those of lithium. Therefore, sodium-ion batteries have more advantages in the application of large-scale power station energy storage systems that pay more attention to cost-effectiveness and safety.

[0003] Sodium-ion batteries include a sodium-containing positive electrode material, and the positive electrode material has a lower voltage window and specific capacity, resulting in a low energy density of the sodium-ion battery. In order to improve the energy density of sodium-ion batteries, existing researchers have improved the energy density of sodium-ion batteries by improving the positive electrode active material or by preparing a composite positive electrode material. The above methods mainly improve the battery energy density by studying the positive electrode active material. However, during the charge and discharge process of the battery, the structure attenuation of the positive electrode active material inside the battery accelerates under high voltage conditions, and the dissolution of the positive electrode transition metal element leads to the attenuation of the battery performance and the reduction of the battery cycle stability. Summary of the Invention

[0004] Aiming at the technical problems of the dissolution of the positive electrode transition metal element and the reduction of the cycle performance in existing high-voltage sodium-ion batteries, the present application provides an electrolyte for a sodium-ion battery and a sodium-ion battery.

[0005] The technical solutions adopted by the present invention to solve the above technical problems are as follows:

[0006] On the one hand, the present application provides an electrolyte for a sodium-ion battery, and the electrolyte includes a first electrolyte salt and a non-aqueous organic solvent. The first electrolyte salt includes sodium aminosulfonate and sodium bis(fluorosulfonyl)imide.

[0007] Preferably, the content of sodium aminosulfonate in the electrolyte is 1 to 20 ppm.

[0008] Preferably, the content of sodium aminosulfonate in the electrolyte is 5 to 15 ppm.

[0009] Preferably, based on the content of the electrolyte being 100%, the mass content of sodium bis(fluorosulfonyl)imide is 1% to 10%.

[0010] Preferably, based on the content of the electrolyte being 100%, the mass content of sodium bis(fluorosulfonyl)imide is 2% to 6%.

[0011] Preferably, the electrolyte further includes a second electrolyte salt, and the second electrolyte salt includes at least one of sodium hexafluorophosphate, sodium trifluoromethanesulfonimide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate, and sodium bis(trifluoromethylsulfonyl)imide;

[0012] Based on the mass of the electrolyte being 100%, the mass content of the second electrolyte salt is 5% to 9%.

[0013] Preferably, the non-aqueous organic solvent includes at least one of carbonates having 3 to 5 carbon atoms, carboxylates having 2 to 6 carbon atoms, and ethers having 4 to 10 carbon atoms.

[0014] Preferably, the electrolyte further includes an additive, and the additive includes at least one of ethylene sulfate, 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, fluoroethylene carbonate, and difluoroethylene carbonate;

[0015] Based on the mass of the electrolyte being 100%, the mass content of the additive is 0.01% to 8%.

[0016] On the other hand, the present application provides a sodium ion battery, including a positive electrode, a negative electrode, and the electrolyte for a sodium ion battery described above.

[0017] Preferably, the positive electrode includes a positive electrode active material, and the positive electrode active material is selected from at least one of layered transition metal oxides, Prussian compounds, phosphate compounds, and sulfate compounds;

[0018] The negative electrode includes a negative electrode active material, and the negative electrode active material includes at least one of hard carbon, soft carbon, carbon nanotubes, graphite, graphene, expanded graphite, or mesophase carbon microspheres.

[0019] Preferably, the chemical formula of the layered transition metal oxide is Na x L y O z , 0 < x ≤ 1, 0 < y ≤ 1, 1 < z ≤ 2, and L can be selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V;

[0020] The chemical formula of the Prussian compound is Na x＇ M[M′(CN)6] y＇·z'H2O, where M is a transition metal, M' is a transition metal, 0 < x' ≤ 2, 0 < y' ≤ 1, 0 < z' ≤ 20:

[0021] The chemical formula of the phosphate compound is Na3(RO 1-x″ PO4)2F 1+2x″ or Na2R'PO4F, where 0 ≤ x″ ≤ 1, R is selected from at least one of Al, V, Ge, Fe, Ga, and R' is selected from at least one of Fe, Mn;

[0022] The chemical formula of the sulfate compound is Na2A(SO4)2·2H2O, where A is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, V.

[0023] Advantageous effects:

[0024] The electrolyte for a sodium-ion battery provided by this application adds sodium aminosulfonate and sodium bis(fluorosulfonyl)imide to the electrolyte. During the charge and discharge process of the sodium-ion battery, sodium aminosulfonate in the electrolyte can promote the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, generating a passivation layer rich in nitrogen and sulfur elements at the positive electrode interface. The nitrogen and sulfur elements contain lone pairs of electrons, which can attract the electron-deficient transition metal elements in the positive electrode active material, thereby inhibiting the dissolution of the positive electrode transition metal elements, preventing the decomposition of the passivation layer, enhancing the high-voltage stability of the battery, improving the low-temperature discharge, high-temperature cycle, and high-temperature storage performance of the battery, and inhibiting the gas generation of the battery. Specific embodiments

[0025] In order to make the technical problems, technical solutions, and advantageous effects solved by the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] On the one hand, this application provides an electrolyte for a sodium-ion battery. The electrolyte includes a first electrolyte salt and a non-aqueous organic solvent. The first electrolyte salt includes sodium aminosulfonate and sodium bis(fluorosulfonyl)imide.

[0027] In existing sodium-ion batteries, the structural attenuation of the positive electrode active material inside the battery accelerates under high-voltage conditions, and the dissolution of transition metal elements in the positive electrode leads to the attenuation of battery performance and the reduction of battery cycle stability. In view of the above problems, through extensive research, the inventors found that by adding sodium aminosulfonate and sodium bis(fluorosulfonyl)imide to the electrolyte used in sodium-ion batteries, during the charge and discharge process of the sodium-ion battery, sodium aminosulfonate in the electrolyte can promote the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, and a passivation layer rich in nitrogen and sulfur elements is formed at the positive electrode interface. Nitrogen and sulfur elements have lone pairs of electrons, which can attract the electron-deficient transition metal elements in the positive electrode active material, thereby inhibiting the dissolution of transition metal elements in the positive electrode, preventing the decomposition of the passivation layer, improving the high-voltage stability of the battery, enhancing the low-temperature discharge, high-temperature cycle and high-temperature storage performance of the battery, and inhibiting gas generation in the battery.

[0028] In some embodiments, the content of sodium aminosulfonate in the electrolyte is 1 to 20 ppm.

[0029] When the content of sodium aminosulfonate in the electrolyte is 1 to 20 ppm, a small amount of sodium aminosulfonate can promote the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, and a passivation layer rich in nitrogen and sulfur elements is formed at the positive electrode interface. Nitrogen and sulfur elements have lone pairs of electrons, which can attract the electron-deficient transition metal elements in the positive electrode active material, thereby inhibiting the dissolution of transition metal elements in the positive electrode, preventing the decomposition of the passivation layer, and improving the high-voltage stability of the battery. If the content of sodium aminosulfonate in the electrolyte is less than 1 ppm, the content of sodium aminosulfonate is too low to promote the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, and it has no effect on improving the high-voltage stability of the battery. If the content of sodium aminosulfonate in the electrolyte is higher than 20 ppm, excessive sodium aminosulfonate will increase the alkalinity of the electrolyte, the stability of the solvent in the electrolyte decreases under alkaline conditions, the decomposition of the solvent in the electrolyte is aggravated, the gas generation in the battery increases, and the battery performance deteriorates. Specifically, the content of sodium aminosulfonate can be 1 ppm, 5 ppm, 8 ppm, 10 ppm, 13 ppm, 15 ppm, 17 ppm, 19 ppm, 20 ppm, as long as the content of sodium aminosulfonate is between 1 and 20 ppm.

[0030] In some preferred embodiments, the content of sodium aminosulfonate in the electrolyte is 5 to 15 ppm. When the content of sodium aminosulfonate is within the preferred range, while promoting the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, it also has a relatively small effect on the decomposition of the solvent in the electrolyte, and has a greater improvement on the high-temperature performance, high-temperature storage performance and low-temperature performance of the battery.

[0031] In some embodiments, based on the content of the electrolyte being 100%, the mass content of sodium bis(fluorosulfonyl)imide is 1% to 10%.

[0032] When the mass content of sodium bis(fluorosulfonyl)imide is higher than 10%, the content of sodium bis(fluorosulfonyl)imide is too high, the viscosity of the electrolyte increases, the thickness of the passivation layer formed at the positive electrode interface is too thick, and there are too many fluorides, reducing the interfacial diffusion rate of ions and resulting in the attenuation of the low-temperature performance of the battery. If the content of sodium bis(fluorosulfonyl)imide is lower than 1%, the content of sodium bis(fluorosulfonyl)imide is too low, the passivation layer formed at the positive electrode interface is incomplete and too thin, the strength and toughness of the passivation layer are reduced, the transition metal in the positive active material dissolves out, destroying the passivation layer, and the cycle performance and storage performance of the battery are reduced, with increased gas production. When the mass content of sodium bis(fluorosulfonyl)imide is in the range of 1% to 10%, reacting with sodium sulfamate, the battery has better high-voltage cycle stability, high-temperature storage performance, and inhibits battery gas production. Specifically, the mass content of sodium bis(fluorosulfonyl)imide can be 1%, 1.5, 2.0%, 2.8%, 3.5%, 4.0%, 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, 10.0%, etc., as long as the mass content of sodium bis(fluorosulfonyl)imide is in the range of 1% to 10%.

[0033] In some preferred embodiments, based on the content of the electrolyte being 100%, the mass content of sodium bis(fluorosulfonyl)imide is 2% to 6%. Within the preferred range, the high-temperature cycle performance and low-temperature performance of the battery are better, and the gas production during high-temperature storage is reduced.

[0034] In some embodiments, the electrolyte further includes a second electrolyte salt, and the second electrolyte salt includes at least one of sodium hexafluorophosphate, sodium trifluoromethanesulfonimide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate, and sodium bis(trifluoromethylsulfonyl)imide;

[0035] Based on the mass of the electrolyte being 100%, the mass content of the second electrolyte salt is 5% to 9%.

[0036] Adding the second electrolyte salt to the electrolyte has a high solubility and is easily dissociated in the non-aqueous organic solvent to ensure that the electrolyte has a high ionic conductivity and promotes the redox reaction of the battery.

[0037] In some embodiments, the non-aqueous organic solvent includes at least one of a carbonate having 3 to 5 carbon atoms, a carboxylate having 2 to 6 carbon atoms, and an ether having 4 to 10 carbon atoms.

[0038] Preferably, the carbonate solvents having 3 to 5 carbon atoms include cyclic carbonates or chain carbonates having 3 to 5 carbon atoms. The cyclic carbonates include, but are not limited to, at least one of ethylene carbonate (EC), vinylene carbonate (VC), ethylene ethylenecarbonate (VEC), propylene carbonate (PC), γ-butyrolactone (GBL), and butylene carbonate (BC); the chain carbonates may specifically be, but are not limited to, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC).

[0039] Preferably, the carboxylic acid esters having 2 to 6 carbon atoms include, but are not limited to, at least one of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, and propyl propionate (PP). As a preferred embodiment, the non-aqueous electrolyte of the secondary battery further includes vinylene carbonate (VC), ethylene ethylenecarbonate (VEC), and fluoroethylene carbonate (FEC).

[0040] Preferably, the ether solvents having 4 to 10 carbon atoms include cyclic ethers or chain ethers having 4 to 10 carbon atoms. The cyclic ethers include, but are not limited to, at least one of 1,3-dioxolane (DOL), 1,4-dioxane (DX), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), and 2-trifluoromethyltetrahydrofuran (2-CF3-THF); the chain ethers include, but are not limited to, at least one of dimethoxymethane (DMM), 1,2-dimethoxyethane (DME), and diethylene glycol dimethyl ether (TEGDME).

[0041] In some embodiments, the electrolyte further includes an additive, and the additive includes at least one of ethylene sulfate, 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, fluoroethylene carbonate, and difluoroethylene carbonate;

[0042] Based on the mass of the electrolyte being 100%, the mass content of the additive is 0.01% to 8%.

[0043] On the other hand, the present application provides a sodium ion battery, including a positive electrode, a negative electrode, and the electrolyte for a sodium ion battery described above.

[0044] The sodium ion battery provided by the present application has high cycle performance, low-temperature performance, and high-temperature storage performance under high-voltage charging conditions, and the battery generates less gas. It should be noted that the high voltage of the battery means that the charging cut-off voltage of the positive electrode of the battery is 4.0 V or higher.

[0045] In some embodiments, the positive electrode includes a positive electrode active material, and the positive electrode active material is selected from at least one of layered transition metal oxides, Prussian compounds, phosphate compounds, and sulfate compounds;

[0046] The negative electrode includes a negative electrode active material, and the negative electrode active material includes at least one of hard carbon, soft carbon, carbon nanotubes, graphite, graphene, expanded graphite, or mesophase carbon microspheres.

[0047] In some embodiments, the chemical formula of the layered transition metal oxide is Na x L y O z , where 0 < x ≤ 1, 0 < y ≤ 1, 1 < z ≤ 2, and L is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, V.

[0048] In a more preferred embodiment, the chemical formula of the layered transition metal oxide is NaNi m Fe n Mn p O2 (m + n + p = 1, 0 ≤ m ≤ 1, 0 ≤ n ≤ 1, 0 ≤ p ≤ 1), NaNi m Co n Mn p O2 (m + n + p = 1, 0 ≤ m ≤ 1, 0 ≤ n ≤ 1, 0 ≤ p ≤ 1), or at least one of them.

[0049] In some embodiments, the molecular formula of the Prussian compound is Na x＇ M[M′(CN)6] y '·z'H2O, where M is a transition metal, M′ is a transition metal, 0 < x' ≤ 2, 0 < y' ≤ 1, 0 < z' ≤ 20.

[0050] In a more preferred embodiment, the Prussian compound is Na x 'Mn[Fe(CN)6] y '·z'H2O or Na x 'Fe[Fe(CN)6] y '·z'H2O, 0 < x' ≤ 2, 0 < y' ≤ 1, 0 < z' ≤ 20.

[0051] In some embodiments, the chemical formula of the phosphate compound is Na3(RO 1-x″ PO4)2F 1+2x″ or Na2R′PO4F, where 0 ≤ x″ ≤ 1, R is selected from at least one of Al, V, Ge, Fe, Ga, and R′ is selected from at least one of Fe, Mn.

[0052] In a more preferred embodiment, the chemical formula of the phosphate compound is Na3(VPO4)2F3, Na3(VOPO4)2F, Na2FePO4F, Na2MnPO4F.

[0053] In some embodiments, the chemical formula of the sulfate compound is Na2A(SO4)2·2H2O, and A is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0054] The present invention will be further described below through examples.

[0055] Example 1

[0056] This example is used to illustrate the electrolyte for a sodium-ion battery and the sodium-ion battery disclosed in the present application.

[0057] Electrolyte: A solution is prepared by mixing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate as solvents in a mass ratio of 30:40:30, and then a first electrolyte salt is added to the solution to obtain a mixture. The first electrolyte salt is sodium aminosulfonate and sodium bis(fluorosulfonyl)imide (NaFSI), and the specific contents of sodium aminosulfonate and sodium bis(fluorosulfonyl)imide (NaFSI) are shown in Table 1.

[0058] To prepare a secondary sodium-ion battery, the following steps are included:

[0059] (1) Preparation of the positive electrode: The positive electrode active material NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, conductive carbon black Super-P, and binder polyvinylidene fluoride (PVDF) are mixed, and then they are dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry; the obtained slurry is uniformly coated on both sides of an aluminum foil, dried, calendered, and vacuum dried, and then an aluminum lead wire is welded with an ultrasonic welder to obtain a positive electrode plate.

[0060] (2) Preparation of the negative electrode: According to a mass ratio of 94:1:2.5:2.5, the negative electrode active material hard carbon with a specific surface area of 5m 2 / g, conductive carbon black Super-P, binder styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are mixed, and then they are dispersed in an appropriate amount of deionized water to obtain a negative electrode slurry; the slurry is coated on both sides of a copper foil, dried, calendered, and vacuum dried, and then a nickel lead wire is welded with an ultrasonic welder to obtain a negative electrode plate.

[0061] (3) Preparation of sodium-ion secondary battery: Place a separator between the prepared positive electrode plate and negative electrode plate, then wind the sandwich structure composed of the positive electrode plate, negative electrode plate and separator, flatten the wound body and put it into an aluminum foil packaging bag, and bake it in vacuum at 75 °C for 48 h to obtain the battery cell to be injected with electrolyte. In a glove box with the dew point controlled below -40 °C, inject the prepared electrolyte into the battery cell, perform vacuum packaging, and let it stand for 24 h.

[0062] Examples 2-21 and Comparative Examples 1-6

[0063] The difference between the examples and comparative examples and Example 1 is that a second electrolyte salt and an additive are further added to the mixture solution prepared in Example 1, and the types and contents of the first electrolyte salt, second electrolyte salt and additive are different. See Table 1 for details. The rest of the preparation method is the same as that of Example 1.

[0064] Performance test

[0065] Perform the following performance tests on the sodium-ion batteries prepared in the above examples and comparative examples.

[0066] High-temperature cycle performance test:

[0067] Let the formed battery stand at 45 °C for 2 h, charge it at a constant current rate of 0.5C to 4.0V, then charge it at a constant voltage until the current is 0.03C, and then discharge it at a constant current of 1C to 1.5V, and record the discharge capacity D1. Charge and discharge the battery 200 times according to this charge and discharge method, and record the discharge capacity D2 of the 200th discharge.

[0068] The calculation method is as follows: High-temperature cycle capacity retention rate (%) = D2 / D1×100%.

[0069] High-temperature storage test:

[0070] Let the formed battery stand at 25 °C for 2 h, charge it at a constant current rate of 0.5C to 4.0V, then charge it at a constant voltage until the current is 0.03C, and measure the battery volume V1; place the battery in storage at 60 °C for 14 d, and measure the battery volume V2.

[0071] The calculation method is as follows: High-temperature storage volume change rate (%) = (V2 - V1) / V1×100%.

[0072] Low-temperature discharge test:

[0073] The formed battery is left standing at 25°C for 2 h, then charged at a constant current of 0.5 C until 4.0 V, then charged at a constant voltage until the current reaches 0.03 C, and then discharged at a constant current of 0.3 C until 1.5 V, and the discharge capacity D3 is recorded. It is charged at a constant current of 0.5 C until 4.0 V, and then charged at a constant voltage until the current reaches 0.03 C. Then the battery is left standing at -20°C for 5 h and discharged at a constant current of 0.5 C until 1.5 V, and the discharge capacity D4 is recorded.

[0074] The calculation method is as follows:

[0075] Low-temperature discharge capacity retention rate (%) = D4 / D3 × 100%.

[0076] The test results are shown in Table 1.

[0077] Table 1

[0078]

[0079]

[0080] Comparing Comparative Examples 1-2 with Example 1, in Comparative Example 1, the electrolyte does not contain sodium aminosulfonate, and the electrolyte only contains sodium bis(fluorosulfonyl)imide. The high-temperature cycle capacity retention rate of the battery is only 71%, and the volume change rate of the battery during high-temperature storage is high. In Comparative Example 2, there is no sodium bis(fluorosulfonyl)imide, the high-temperature cycle capacity retention rate of the battery is further reduced, the gas generation during high-temperature storage increases, and the low-temperature capacity retention rate decreases; comparing Example 2 with Comparative Examples 3-4, the positive electrode voltage of the battery is further increased. The high-temperature cycle performance of the batteries in Comparative Examples 3 and 4 is worse, and the batteries generate more gas; it is speculated that adding sodium aminosulfonate to the electrode liquid can promote the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, and a passivation layer rich in nitrogen and sulfur elements is formed at the positive electrode interface. Nitrogen and sulfur elements contain lone pair electrons, which can attract the electron-deficient transition metal elements in the positive electrode active material, thereby inhibiting the dissolution of the positive electrode transition metal elements and preventing the decomposition of the passivation layer, improving the high-voltage stability of the battery. It shows that sodium aminosulfonate and sodium bis(fluorosulfonyl)imide have a synergistic effect in improving the high-temperature cycle performance, high-temperature storage performance and low-temperature performance of the battery, and neither of them can be missing. Comparing Example 1 with Example 2, increasing the positive electrode voltage, the high-temperature cycle performance and high-temperature storage performance of the battery both decline. Due to the electrolyte containing sodium aminosulfonate and NaFSI, the attenuation amplitude of the battery at a higher voltage is smaller, and the comprehensive performance of the battery in high-temperature storage, high-temperature cycle and low temperature is better.

[0081] In Examples 1, 3 - 13, the sodium aminosulfonate in the electrolyte is adjusted to be between 1 and 20 ppm, and the mass content of NaFSI is between 1% and 10%. The battery has relatively high high - temperature cycling performance and low - temperature capacity retention rate, and the volume change rate during high - temperature storage of the battery is low. When the mass content of NaFSI in the electrolyte is between 2% and 6%, or the sodium aminosulfonate is between 5 and 15 ppm, the high - temperature cycling performance and low - temperature capacity retention rate of the battery are even higher, and the volume change rate during high - temperature storage of the battery is even lower. When the mass content of NaFSI in the electrolyte is in the range of 2% - 6% and the sodium aminosulfonate is in the range of 5 - 15 ppm, the high - temperature cycling and low - temperature cycling capacity retention rates of the battery are higher, and the high - temperature storage performance is better.

[0082] Comparing Examples 1, 3 - 13 with Examples 14 - 15, in Example 14, the content of sodium aminosulfonate in the electrolyte is higher than 20 ppm, and the volume change rate during high - temperature storage of the battery is as high as 58.4%. The gas generation of the battery increases. It is speculated that too much sodium aminosulfonate in the electrolyte increases the alkalinity of the electrolyte, and the stability of the solvent in the electrolyte decreases under alkaline conditions, resulting in increased electrolyte decomposition and increased gas generation of the battery. In Example 15, the content of sodium aminosulfonate in the electrolyte is lower than 1 ppm, the gas generation of the battery increases, and the high - temperature cycling capacity retention rate decreases. It is guessed that the content of sodium aminosulfonate in the electrolyte is too low to play a role in promoting the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, unable to inhibit the dissolution of transition metal elements in the positive electrode, the decomposition of the electrode interface film, and the reduction of electrical performance.

[0083] Comparing Examples 3 - 6 with Example 16, when the mass content of sodium bis(fluorosulfonyl)imide in the electrolyte is higher than 10%, the low - temperature discharge capacity retention rate of the battery decreases and the gas generation increases. It is guessed that with the increase of the content of NaFSI, the viscosity of the electrolyte increases, there are too many fluorides in the passivation layer, reducing the interfacial diffusion of ions, resulting in the attenuation of the low - temperature capacity retention rate of the battery and increased gas generation. Comparing Example 3 with Example 17, when the content of NaFSI in the electrolyte is lower than 1%, it is guessed that the content of sodium bis(fluorosulfonyl)imide is too low, the passivation layer formed at the positive electrode interface is incomplete and too thin, the strength and toughness of the passivation layer are reduced, the transition metals in the positive active material dissolve out, destroying the passivation layer, and the cycling performance and storage performance of the battery are reduced, and the gas generation increases.

[0084] Comparing Example 1 with Examples 18 - 20, when FEC, DTD or PS is added to the electrolyte, the high - temperature cycling capacity retention rate of the battery is further improved and the gas generation is reduced. It is speculated that the additives FEC, DTD or PS added to the electrolyte have the effect of synergistically with sodium aminosulfonate and promoting the interfacial passivation of sodium bis(fluorosulfonyl)imide at the positive electrode, forming a uniform, complete and structurally stable passivation film, better inhibiting the dissolution of transition metals in the positive electrode, further reducing the gas generation of the battery and improving the high - temperature cycling performance.

[0085] Comparing Example 1 with Example 21, sodium hexafluorophosphate, a second electrolyte salt, is further added to the electrolyte. The high-temperature cycle capacity retention rate and low-temperature discharge capacity retention rate of the battery are improved, indicating that adding the second electrolyte salt to the electrolyte synergizes with sodium aminosulfonate and sodium bis(fluorosulfonyl)imide, increasing the conductivity of the electrolyte, forming a complete and structurally stable passivation film at the positive electrode interface, and improving the high-temperature cycle and low-temperature performance of the battery.

[0086] Comparing Example 21 with Comparative Examples 5-6, in Comparative Example 5, there is no sodium aminosulfonate, resulting in a decrease in the high-temperature cycle and low-temperature performance of the battery and an increase in gas production. In Comparative Example 6, there is no sodium bis(fluorosulfonyl)imide, leading to an even greater decrease in the high-temperature cycle and low-temperature performance of the battery and a further increase in gas production. It is speculated that although the second electrolyte salt is added to the electrolyte, without sodium fluorosulfonylimide or sodium aminosulfonate in the electrolyte, a better passivation film for suppressing the dissolution of transition metals at the positive electrode interface cannot be formed. This shows that sodium aminosulfonate and sodium bis(fluorosulfonyl)imide have synergy, and only when they work together can a passivation layer rich in nitrogen and sulfur elements be formed at the positive electrode interface. Nitrogen and sulfur elements have lone pairs of electrons, which can attract the electron-deficient transition metal elements in the positive electrode active material, forming a passivation film to inhibit the dissolution of transition metal elements at the positive electrode, enhancing the high-voltage stability of the battery, and improving the high-temperature cycle performance, high-temperature storage performance, and low-temperature performance of the battery. Neither of them can be missing.

[0087] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An electrolyte for a sodium-ion battery, characterized in that, The electrolyte includes a first electrolyte salt and a non-aqueous organic solvent. The first electrolyte salt includes sodium aminosulfonate and sodium bis(fluorosulfonyl)imide. The content of sodium aminosulfonate in the electrolyte is 1 to 20 ppm. Based on the content of the electrolyte being 100%, the mass content of sodium bis(fluorosulfonyl)imide is 1% to 10%. The charging cut-off voltage of the battery positive electrode is 4.0 V or above.

2. The electrolyte for a sodium-ion battery according to claim 1, wherein The content of sodium aminosulfonate in the electrolyte is 5 to 15 ppm.

3. The electrolyte for a sodium-ion battery according to claim 1, wherein Based on the content of the electrolyte being 100%, the mass content of sodium bis(fluorosulfonyl)imide is 2% to 6%.

4. The electrolyte for a sodium-ion battery according to claim 1, wherein The electrolyte further includes a second electrolyte salt, which includes at least one of sodium hexafluorophosphate, sodium trifluoromethanesulfonimide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate, and bis(trifluoromethylsulfonyl)imide sodium. Based on the mass of the electrolyte being 100%, the mass content of the second electrolyte salt is 5% to 9.

5. The electrolyte for a sodium-ion battery according to claim 1, characterized in that, The non-aqueous organic solvent includes at least one of carbonates with 3 to 5 carbon atoms, carboxylates with 2 to 6 carbon atoms, and ethers with 4 to 10 carbon atoms.

6. The electrolyte for a sodium ion battery according to claim 1, characterized in that, The electrolyte further includes an additive, which includes at least one of vinylene sulfate, 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, fluorinated ethylene carbonate, and difluorinated ethylene carbonate. Based on the mass of the electrolyte being 100%, the mass content of the additive is 0.01% to 8%.

7. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, and the electrolyte for a sodium-ion battery according to any one of claims 1-6.

8. The sodium ion battery according to claim 7, characterized in that, The positive electrode includes a positive electrode active material, which includes at least one of layered transition metal oxides, Prussian compounds, phosphate compounds, and sulfate compounds. The negative electrode includes a negative electrode active material, which includes at least one of hard carbon, soft carbon, carbon nanotubes, graphite, graphene, expanded graphite, or mesophase carbon microspheres. The chemical formula of the layered transition metal oxide is Na x L y O z , where 0 < x ≤ 1, 0 < y ≤ 1, 1 < z ≤ 2, and L can be selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V; The chemical formula of the Prussian compound is Na x＇ M[M′(CN)6] y＇ ·z'H2O, where M is a transition metal, M′ is a transition metal, 0 < x' ≤ 2, 0 < y' ≤ 1, 0 < z' ≤ 20: The chemical formula of the phosphate compound is Na3(RO 1-x＇＇ PO4)2F 1+2x＇＇ or Na2R´PO4F, where 0 ≤ x'' ≤ 1, R is selected from at least one of Al, V, Ge, Fe, and Ga, and R´ is selected from at least one of Fe and Mn; The chemical formula of the sulfate compound is Na2A(SO4)2·2H2O, and A is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

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