Electrolyte for improving cycling stability of nickel-manganese-based positive electrode material of sodium-ion battery as well as preparation and application of electrolyte

Abstract

The invention belongs to the technical field of sodium ion batteries, and discloses an electrolyte for improving the cycling stability of a nickel-manganese-based positive electrode material as well as preparation and application of the electrolyte. According to the electrolyte, at least one of chain carbonates and cyclic carbonates serves as a solvent, sodium hexafluorophosphate serves as sodium salt, sodium difluoromethane sulfinate and fluoroethylene carbonate are introduced to serve as composite additives, the mass fraction of DFMS ranges from 0.05% to 0.6%, and the mass fraction of FEC ranges from 1% to 3%. When the electrolyte is used in a nickel-manganese-based layered oxide positive electrode / sodium battery, a stable and compact fluorine-containing and sulfur-containing interface film can be constructed on the surface of the positive electrode, the film impedance and the charge transfer impedance are remarkably reduced, the dissolution of transition metal is inhibited, the cycle life of the battery is prolonged, the capacity retention ratio is increased, and the rate capability is improved; compared with basic electrolyte without additives, the electrolyte has obvious comprehensive performance advantages.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and specifically relates to an electrolyte for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries, as well as its preparation and application. Background Technology

[0002] Sodium-ion batteries (SIBs) are considered an important supplement and potential alternative to lithium-ion batteries (LIBs) in large-scale energy storage and some power applications due to their advantages such as abundant sodium resources, low cost, and relatively high system safety. With the increasing grid connection of renewable energy and the growing demand for grid peak shaving and frequency regulation, sodium-ion battery systems that combine high safety and low cost have gradually become a research hotspot in this field.

[0003] Current sodium-ion battery cathode materials mainly include layered transition metal oxides, Prussian blue / white compounds, and multi-anion compounds. Among them, layered oxide cathode materials (especially nickel-manganese layered oxides) have advantages such as high operating voltage, large specific capacity, excellent rate performance, and high compatibility of preparation processes with existing lithium-ion battery production lines, and are generally considered the most promising cathode system for large-scale application of sodium-ion batteries. However, these materials have poor chemical stability in air, are prone to absorbing water and carbon dioxide, leading to the formation of alkaline impurities on the surface; at the same time, they are prone to structural degradation and interfacial side reactions during slurry preparation and cycling, resulting in unstable specific capacity and accelerated cycle decay. In contrast, Prussian blue / white materials, although low in cost, simple in synthesis route, and highly designable, have problems such as difficulty in water removal, limited cycle life, low volumetric energy density, and large voltage polarization; multi-anion compounds have high thermal stability and operating voltage, but limited energy density and relatively high raw material costs. Overall, nickel-manganese layered oxide cathode materials remain the mainstream direction for the current industrialization of sodium-ion batteries.

[0004] Despite some progress, existing sodium-ion batteries still lag significantly behind mature lithium-ion battery systems in key performance indicators such as energy density and cycle life. Compared to lithium-ion batteries, sodium-ion batteries are more prone to side reactions such as electrolyte decomposition and interfacial film dissolution during charge and discharge, leading to instability in the solid-electrolyte interface (SEI) and positive electrode electrolyte interface (CEI), resulting in severe sodium degradation. + Irreversible losses and interfacial impedance continue to increase. For high-nickel content or nickel-manganese-based substrate oxide cathodes, under high voltage and long cycling conditions, problems such as crystal structure phase transition, incomplete surface redox, and transition metal dissolution also occur, further exacerbating capacity decay and coulombic efficiency decline.

[0005] To address the aforementioned issues, existing technologies primarily employ two approaches: first, modifying the cathode material bulk through element doping and coating; and second, optimizing the electrolyte system, particularly by introducing functional electrolyte additives, to regulate the electrode / electrolyte interface reaction behavior. Compared to bulk material modification, rationally designing the electrolyte formulation and the types and amounts of additives can effectively improve the battery's cycle stability and rate performance without significantly increasing material costs and process complexity. Therefore, it is considered one of the most economical and efficient technical routes for improving sodium-ion battery performance.

[0006] Among numerous electrolyte additives, sulfur-containing and fluorine-containing additives have attracted widespread attention. Sulfur-containing additives, due to the strong electronegativity of the central sulfur atom, are more likely to undergo electrochemical reduction on the electrode surface compared to traditional unsaturated carbonate additives, forming a sulfur-rich interfacial film. The corresponding SEI / CEI typically exhibits good elasticity and ionic conductivity, buffering volume changes and inhibiting further solvent decomposition. Fluorine-containing additives, on the other hand, exhibit a significant electron-withdrawing effect, often initiating reduction and decomposition at higher potentials to generate interfacial films with a high proportion of inorganic components such as NaF. This facilitates the construction of a dense, stable, electronically insulating yet ionically conductive protective layer, thereby reducing battery internal resistance and improving cycle life. Fluoroethylene carbonate (FEC) is one of the more widely used fluorine-containing additives, while sodium difluoromethanesulfinate (DFMS) combines fluorine and sulfur characteristics, theoretically potentially allowing the simultaneous introduction of inorganic fluorides and organic sulfur-containing components into the interfacial film, achieving synergistic regulation of the interface. However, there is still a lack of research in the published literature and patents on optimizing the interfacial stability of nickel-manganese base oxide cathode systems by synergistically using fluorine-containing and sulfur-containing additives such as FEC and DFMS, especially in terms of adapting to specific carbonate solvent systems and sodium salt types.

[0007] Existing sodium-ion battery electrolytes are mainly divided into two categories: carbonate-based and ether-based. Carbonate-based electrolytes are generally composed of a mixture of one or more linear carbonates and one or more cyclic carbonates, such as combinations of dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC). Ether-based electrolytes mostly use ether solvents such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and triethylene glycol dimethyl ether. Commonly used sodium salts include sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium difluorosulfonamide, and sodium di(trifluoromethyl)sulfonamide. In early research on sodium-ion battery cathode materials (such as NVP), carbonate-based electrolytes using EC / DEC, EC / DMC, EC / EMC (volume ratio approximately 1:1), or PC alone as solvent and NaClO4 as sodium salt were frequently used, often with a certain proportion of FEC added as a functional additive. It should be noted that NaClO4 is a hazardous chemical, subject to strict regulation and safety restrictions during the large-scale production and use of energy storage batteries, which is detrimental to the industrialization and promotion of sodium-ion batteries. In contrast, NaPF6 is more suitable for industrial applications in terms of safety, overall electrochemical performance, and process adaptability, and has therefore gradually become one of the preferred sodium salts for carbonate-based sodium-ion battery electrolytes.

[0008] Propylene carbonate (PC) has a high dielectric constant and a low freezing point, which is beneficial for improving the low-temperature performance and conductivity of the electrolyte. Ethyl methyl carbonate (EMC) has low viscosity, which helps to improve the fluidity and high-rate performance of the electrolyte. Therefore, carbonate-based electrolytes constructed with PC / EMC mixed solvents have great application potential in sodium-ion battery systems. However, in traditional electrolyte systems using PC / EMC as solvent, NaPF6 as sodium salt, and without specific optimization for nickel-manganese-based oxide cathodes, the electrode / electrolyte interface is often unstable. On the one hand, the electrolyte on the cathode surface undergoes severe oxidative decomposition, making it difficult to form a dense and stable CEI film. On the other hand, NaPF6 is prone to decomposition under high voltage and long-term cycling conditions, generating corrosive species such as HF, which exacerbates transition metal dissolution and interfacial side reactions, resulting in significantly insufficient cycle stability and coulombic efficiency of the battery, making it difficult to meet the requirements of long-life and high-rate applications.

[0009] In summary, existing technologies lack a design scheme for a high-performance electrolyte system using PC / EMC as a solvent and NaPF6 as a sodium salt for layered oxide cathode materials in nickel-manganese-based sodium-ion batteries. In particular, electrolyte formulations that utilize electrolyte additives to synergistically construct a stable interfacial film, thereby significantly improving battery cycle stability and coulombic efficiency, have not been fully studied and effectively resolved. It is necessary to provide a new electrolyte design approach to overcome the above-mentioned deficiencies. Summary of the Invention

[0010] To overcome the shortcomings and deficiencies of the prior art, the primary objective of this invention is to provide a sodium-ion battery electrolyte for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries. This electrolyte is beneficial for Na… + The insertion / extraction kinetics in cathode materials improve sodium ion conductivity, thereby better improving the cycle stability and rate performance of layered oxides.

[0011] Another objective of this invention is to provide a method for preparing the sodium-ion battery electrolyte described above for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries.

[0012] Another object of the present invention is to provide the application of the sodium-ion battery electrolyte described above for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries in sodium-ion batteries.

[0013] The objective of this invention is achieved through the following solution:

[0014] An electrolyte additive comprising sodium difluoromethanesulfonate (DFMS) and fluoroethylene carbonate (FEC).

[0015] The mass ratio of the fluoroethylene carbonate (FEC) to sodium difluoromethane sulfinate (DFMS) is (1-3):(0.05-0.5); preferably 2:0.1.

[0016] A sodium-ion battery electrolyte for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries, comprising a sodium salt, a solvent, and the aforementioned electrolyte additives.

[0017] The sodium salt is selected from sodium hexafluorophosphate (NaPF6).

[0018] The solvent is selected from at least one of chain carbonates and cyclic carbonates, wherein the cyclic carbonates may be at least one of ethylene carbonate (EC), fluoroethylene carbonate (FEC), and propylene carbonate (PC); and the chain carbonates may be at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC).

[0019] Preferably, the solvent is propylene carbonate (PC) and ethyl methyl carbonate (EMC), with a volume ratio of PC to EMC of 1:1.

[0020] The amount of electrolyte additives used is such that, based on the total mass of sodium salt and solvent, the mass fraction of DFMS is 0.05%–0.6% and the mass fraction of FEC is 1%–3.0%.

[0021] Preferably, the dosage of the electrolyte additive satisfies that, based on the total mass of the sodium salt and the solvent, the mass fraction of DFMS is 0.08%–0.2%, and the mass fraction of FEC is 1.5%–2.5%.

[0022] More preferably, the dosage of the electrolyte additive satisfies that, based on the total mass of the sodium salt and the solvent, the mass fraction of DFMS is 0.1%, and the mass fraction of FEC is 2%.

[0023] The dosage of the sodium salt satisfies that the concentration of the sodium salt in the basic electrolyte formed by the sodium salt and the solvent is 0.8 - 1.2 mol / L, preferably 1 mol / L.

[0024] A preparation method of a sodium-ion battery electrolyte for improving the cycle stability of a nickel-manganese-based cathode material of a sodium-ion battery, which comprises the following steps: If there are multiple solvents, first mix the multiple solvents evenly to obtain a mixed solvent, then add the sodium salt to the mixed solvent and stir to dissolve to form a basic electrolyte, and then add an electrolyte additive to the basic electrolyte and mix and dissolve evenly to obtain the sodium-ion battery electrolyte.

[0025] The solvent is preferably used after being dehydrated and purified by a molecular sieve, and the time for molecular sieve purification is 12–48 hours.

[0026] Application of the above-mentioned sodium-ion battery electrolyte for improving the cycle stability of a nickel-manganese-based cathode material of a sodium-ion battery in a sodium-ion battery. When this electrolyte is used in a nickel-manganese-based layered oxide cathode / sodium battery, a stable and dense fluorine- and sulfur-containing interface film can be constructed on the cathode surface, significantly reducing the film impedance and charge transfer impedance, inhibiting the dissolution of transition metals, and improving the cycle life, capacity retention rate and rate performance of the battery, showing obvious comprehensive performance advantages compared with the basic electrolyte without additives.

[0027] A secondary battery, which comprises a cathode, an anode, an electrolyte and a separator, and the electrolyte is the above-mentioned sodium-ion battery electrolyte for improving the cycle stability of a nickel-manganese-based cathode material of a sodium-ion battery.

[0028] The cathode material is a nickel-manganese-based sodium-ion cathode material, and the nickel-manganese-based sodium-ion cathode material is Na x Li y Ni z Mn 1-z O2, where 0.5 ≤ x ≤ 1.0, 0 ≤ y ≤ 0.2, 0 < z ≤ 0.5; Preferably, the nickel-manganese-based sodium-ion cathode material is Na 0.8 Li x Ni y Mn 0.65 O2, where x = 0.05 - 0.2; y = 0.15 - 0.5;

[0029] The negative electrode is a metallic sodium negative electrode and / or a carbon-based negative electrode.

[0030] The secondary battery is a sodium-ion coin cell or a sodium-ion full cell.

[0031] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0032] The additive DFMS was used for the first time in sodium-ion batteries. Performance tests comparing nickel-manganese-based sodium cathodes / sodium half-cells showed that the sodium-nickel-manganese-based cathode was superior to the sodium-based half-cell. 0.8 Li x Ni y Mn 0.65 O2 was used to confirm that DFMS additives improved the cycle stability and rate performance of sodium nickel manganese-based cathodes / sodium half-cells, as well as stabilized the coulombic efficiency. Characterization test results confirmed that DFMS additives effectively reduced electrode interface impedance and improved battery performance by modifying and optimizing the interface of the nickel manganese-based cathode in sodium-ion batteries. Attached Figure Description

[0033] Figure 1 The figures show a comparison of the electrolytes prepared in Examples 1, 2, 3, 4, 5, 6, 7 and Comparative Example 1, when applied to sodium-nickel-manganese-based cathode / Na half-cells and cycled 200 times at 1C.

[0034] Figure 2 Comparative graphs show the electrolytes prepared for Examples 7, 8, 9 and Comparative Example 1 being tested in sodium-nickel-manganese based cathode / Na half-cells after 200 cycles at 1C.

[0035] Figure 3 The cycling rate diagrams for the electrolytes prepared in Examples 2, 5, 7 and Comparative Example 1 at 0.3C, 1C, 2C, 3C and 5C in sodium nickel manganese-based cathode / Na half-cells.

[0036] Figure 4 Cycling rate diagrams of the electrolytes prepared in Examples 7, 8, 9 and Comparative Example 1 at 0.3C, 1C, 2C, 3C and 5C in sodium nickel manganese-based cathode / Na half-cells.

[0037] Figure 5 Scanning electron microscope images of the electrodes of the sodium nickel manganese-based cathode / Na half-cells prepared in Examples 2, 5, 7 and Comparative Example 1 after 200 cycles of the electrolytes prepared in Examples 2, 5, 7 and Comparative Example 1.

[0038] Figure 6 High-resolution transmission electron microscope images of the electrodes prepared for Examples 2, 5, 7 and Comparative Example 1 after cycling the sodium nickel manganese-based cathode / Na half-cell for 200 cycles.

[0039] Figure 7 The image shows a 3D comparison of the amount of transition metal deposited on the sodium metal surface after 3 and 200 cycles of the sodium nickel manganese-based cathode / Na half-cells prepared in Examples 2, 5, 7 and Comparative Example 1, after the electrolytes were cycled 200 times in a sodium nickel manganese-based cathode / Na half-cell.

[0040] Figure 8 The image shows a 3D comparison of the amount of transition metal deposited on the sodium metal surface after 3 and 200 cycles of the sodium nickel manganese-based cathode / Na half-cells prepared in Examples 7, 8, 9 and Comparative Example 1, after the electrolytes were cycled 200 times in a sodium nickel manganese-based cathode / Na half-cell. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used, unless otherwise specified, are all commercially available products.

[0042] Unless otherwise specified, all reagents used in the examples are commercially available. The CAS number of the cyclic carbonate solvent propylene carbonate (PC) in the electrolyte of this invention is 108-32-7, the CAS number of the linear carbonate solvent ethyl methyl carbonate (EMC) is 623-53-0, the CAS number of the additive fluoroethylene carbonate (FEC) is 114435-02-8, and the CAS number of the additive sodium difluoromethane sulfinate (DFMS) is 275818-95-6.

[0043] The nickel-manganese based cathode material Na used in the examples 0.8 Li 0.1 Ni 0.3 Mn 0.65 The method for preparing O2 is as follows:

[0044] (1) Raw material weighing and proportioning: The cathode material is prepared by high-temperature solid-state method. Sodium source, nickel source, lithium source and manganese source raw materials are accurately weighed according to the target stoichiometric ratio. The sodium source is Na2CO3, the lithium source can be LiOH·H2O, the nickel source is NiO, and the manganese source is MnO2. To compensate for sodium volatilization during high-temperature sintering, sodium source is added in excess of 2 mol% according to the theoretical amount.

[0045] (2) Preparation of precursor by ball milling: The raw materials weighed in step (1) are first pre-mixed in an agate mortar for 30 minutes, and then transferred to a high-energy ball mill jar. Agate beads with a diameter of 5 mm are used for milling. The ball mill jar is an agate jar with argon as the protective atmosphere. The ball milling speed is controlled at 500 rpm, the ball milling time is 5 hours, and the ball-to-material ratio is maintained at 20:1. After the ball milling is completed, the precursor is loaded into a ceramic crucible and awaits sintering.

[0046] (3) Sintering in a pure oxygen atmosphere: The crucible containing the precursor is placed in a tube furnace and sintered in a pure oxygen atmosphere at a pressure of 1 atm and an oxygen flow rate of 100 mL / min. The sintering procedure is as follows: the temperature is increased to 900℃ at a heating rate of 1.5℃ / min, held at that temperature for 12 hours for a high-temperature solid-phase reaction, and cooled to room temperature at a cooling rate of 1℃ / min. After removing the sintered product, it is ground and sieved to obtain the final cathode material.

[0047] In this embodiment, the sodium-nickel-manganese-based positive electrode was prepared using a degassing slurry. The binder used was polyvinylidene fluoride (PVDF), the conductive agent was acetylene black, and the solvent was N-methylpyrrolidone (NMP). The ratio of positive electrode material to PVDF to acetylene black was 8:1:1 (mass ratio), and the ratio of PVDF to NMP was 0.1:3 (volume ratio). The degassing slurry preparation time was 40 minutes. The resulting sodium-nickel-manganese-based positive electrode material (Na... 0.8 Li 0.15 Ni 0.23 Mn 0.65 O2 slurry was applied to aluminum foil with a coating thickness of 70 µm. The foil was first dried in an 80℃ forced-air drying oven for 30 minutes, then dried in a 120℃ vacuum drying oven for 12 hours. After being rolled three times by a roller press, the foil was cut into 14 mm electrode sheets for later use.

[0048] The sodium-nickel-manganese-based positive electrode / sodium half-cell assembled in the embodiment uses a 2032 model battery case, the diameter of the sodium-nickel-manganese-based positive electrode is 12 mm, the diameter of the sodium sheet is kept consistent with the steel gasket (15.6 mm), the amount of electrolyte used is 140 µL, and the separator used is GF / D model glass fiber with a diameter of 18 mm.

[0049] In the embodiments, the assembled sodium-nickel-manganese-based cathode / sodium half-cell was subjected to constant current charge-discharge testing using a blue electric tester to evaluate cycle stability. The constant current charge-discharge cycle stability voltage test range for the sodium-nickel-manganese-based cathode / sodium half-cell was 2.0V-4.0V. The first three cycles were activated using 0.3C, and long cycles were performed using 1C.

[0050] In the embodiments, the sodium-nickel-manganese-based cathode / sodium battery assembled was subjected to charge-discharge tests at different rates using a blue electric tester to evaluate its rate performance. The test voltage range was 2.0V-4.0V, and the batteries were cycled at 0.3C, 1C, 2C, 3C, 4C, and 5C (1C=100 mAh / g).

[0051] In this embodiment, the impedance test object is a sodium-nickel-manganese-based cathode / sodium half-cell that has completed 3 and 200 cycles and is then charged with 1C. The frequency range used in the test is 100,000 Hz to 0.01 Hz.

[0052] In this embodiment, sodium-nickel-manganese-based cathode / sodium batteries that had been cycled to a certain number of times were disassembled. The electrodes were removed, cleaned, dried, and sealed for characterization. All disassembly of sodium-nickel-manganese-based cathode / sodium coin cells that had been cycled to a certain number of times was carried out in a glove box. The disassembled sodium-nickel-manganese-based cathode sheets were cleaned and soaked in ethyl methyl carbonate (EMC) solvent to remove residual salts, membrane fibers, and other impurities from the surface. After the EMC solvent had completely evaporated, the dried sodium-nickel-manganese-based cathode sheets were placed in 5 ml centrifuge tubes for storage.

[0053] In this embodiment, the sodium-nickel-manganese-based positive electrode sheet tested by atomic force microscopy was disassembled from uncycled sodium-nickel-manganese-based positive electrode sheets and sodium-nickel-manganese-based positive electrode / sodium batteries that had been cycled to a certain number of cycles. The electrodes were removed, cleaned, dried, and sealed for characterization. Before use, the sodium-nickel-manganese-based positive electrode sheets were stored in centrifuge tubes in a glove box and the mouths of the centrifuge tubes were sealed with adhesive. One-quarter of the electrode sheet was used for testing, and the morphology and Young's modulus of the sodium-nickel-manganese-based positive electrode sheet were tested by atomic force microscopy.

[0054] In the embodiments, the sodium-nickel-manganese-based positive electrode sheets tested by scanning electron microscopy were disassembled from sodium-nickel-manganese-based positive electrode sheets that had not been cycled and sodium-nickel-manganese-based positive electrode / sodium batteries that had been cycled to a certain number of cycles. The electrodes were removed, cleaned, dried, and sealed for characterization. Before use, the sodium-nickel-manganese-based positive electrode sheets were stored in centrifuge tubes in a glove box and the mouths of the centrifuge tubes were sealed with adhesive. One-quarter of the electrode sheet was used for testing, and the microstructure of the sodium-nickel-manganese-based positive electrode sheets was tested by scanning electron microscopy.

[0055] In the examples, the mass fraction of the additives refers to the percentage of the mass of the additives relative to the mass of the base electrolyte (i.e., the total mass of sodium salt and solvent).

[0056] Example 1

[0057] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0058] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0059] (3) Ensure that the mass fraction of the additive is 0.05%. Calculate and weigh a certain mass of sodium difluoromethane sulfinate (DFMS) (CAS No.: 275818-95-6) additive and add it to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 0.05wt% DFMS.

[0060] (4) Sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 0.05 wt% DFMS obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0061] Example 2:

[0062] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0063] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0064] (3) Ensure that the mass fraction of the additive is 0.1%. Calculate and weigh a certain mass of sodium difluoromethane sulfinate (DFMS) (CAS No.: 275818-95-6) additive and add it to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 0.1 wt% DFMS.

[0065] (4) Sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 0.1 wt% DFMS obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0066] (5) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 0.1% DFMS obtained in (3). After standing for 8 hours, it was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, rate tests were performed at different rates to evaluate the rate performance of the battery under this electrolyte.

[0067] (6) Perform AC impedance testing on the batteries that have been cycled to a certain number of cycles in (4) to evaluate the effect of electrolyte containing 0.1% DFMS on battery impedance.

[0068] (7) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual salt and impurities on the surface of the sodium nickel manganese-based positive electrode with the appropriate solvent and dry it. Then characterize the morphology of the cycled sodium nickel manganese-based positive electrode by scanning electron microscopy.

[0069] (8) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the integrity of the sodium nickel manganese-based positive electrode structure after cycling by X-ray diffraction (XRD).

[0070] (9) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the SEI film composition on the surface of the sodium nickel manganese-based positive electrode after cycling by X-ray photoelectron spectroscopy (XPS).

[0071] (10) Disassemble the battery that has been cycled to a certain number of cycles in (4) and remove the sodium sheet. Wash away the residual electrolyte and impurities on the surface of the sodium sheet with the appropriate solvent and dry it. Then, characterize the amount of transition metal ions deposited on the surface of the sodium sheet after cycling by inductively coupled plasma (ICP).

[0072] Example 3:

[0073] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0074] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0075] (3) Ensure that the mass fraction of the additive is 0.2%. Calculate and weigh a certain mass of sodium difluoromethane sulfinate (DFMS) (CAS No.: 275818-95-6) additive and add it to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 0.2wt% DFMS.

[0076] (4) Sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 0.2 wt% DFMS obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0077] Example 4:

[0078] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0079] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0080] (3) Ensure that the mass fraction of the additive is 1%. Calculate and weigh a certain mass of fluoroethylene carbonate (FEC) (CAS No.: 114435-02-8) additive and add it to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 1 wt% FEC.

[0081] (4) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 1 wt% FEC obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0082] Example 5:

[0083] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0084] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0085] (3) Ensure that the mass fraction of the additive is 2%. Calculate and weigh a certain mass of fluoroethylene carbonate (FEC) (CAS No.: 114435-02-8) additive and add it to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 2wt% FEC.

[0086] (4) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 2 wt% FEC obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0087] (5) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 2% FEC obtained in (3). After standing for 8 hours, it was subjected to 3 cycles of rate charge and discharge at 0.3C to form a film. Then, rate tests were performed at different rates to evaluate the rate performance of the battery under this electrolyte.

[0088] (6) Perform AC impedance testing on the batteries that have been cycled to a certain number of cycles in (4) to evaluate the effect of electrolyte containing 2% FEC on battery impedance.

[0089] (7) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual salt and impurities on the surface of the sodium nickel manganese-based positive electrode with the appropriate solvent and dry it. Then characterize the morphology of the cycled sodium nickel manganese-based positive electrode by scanning electron microscopy.

[0090] (8) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the integrity of the sodium nickel manganese-based positive electrode structure after cycling by X-ray diffraction (XRD).

[0091] (9) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the SEI film composition on the surface of the sodium nickel manganese-based positive electrode after cycling by X-ray photoelectron spectroscopy (XPS).

[0092] (10) Disassemble the battery that has been cycled to a certain number of cycles in (4) and remove the sodium sheet. Wash away the residual electrolyte and impurities on the surface of the sodium sheet with the appropriate solvent and dry it. Then, characterize the amount of transition metal ions deposited on the surface of the sodium sheet after cycling by inductively coupled plasma (ICP).

[0093] Example 6:

[0094] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0095] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0096] (3) Ensure that the mass fraction of the additive is 3%. Calculate and weigh a certain mass of fluoroethylene carbonate (FEC) (CAS No.: 114435-02-8) additive and add it to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 3wt% FEC.

[0097] (4) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 3 wt% FEC obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0098] Example 7:

[0099] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0100] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0101] (3) Ensure that the mass fraction of the additive is 0.1% DFMS + 2% FEC. Calculate and weigh a certain mass of sodium difluoromethane sulfinate (DFMS) (CAS No.: 275818-95-6) and fluoroethylene carbonate (FEC) (CAS No.: 114435-02-8) and add them to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 0.1% DFMS + 2% FEC.

[0102] (4) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 0.1% DFMS + 2% FEC obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0103] (5) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 0.1% DFMS + 2% FEC obtained in (3). After standing for 8 hours, it was subjected to 3 cycles of rate charge and discharge at 0.3C to form a film. Then, rate tests were performed at different rates to evaluate the rate performance of the battery under this electrolyte.

[0104] (6) Perform AC impedance testing on the batteries that have been cycled to a certain number of cycles in (4) to evaluate the effect of electrolyte containing 0.1% DFMS + 2% FEC on battery impedance.

[0105] (7) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual salt and impurities on the surface of the sodium nickel manganese-based positive electrode with the appropriate solvent and dry it. Then characterize the morphology of the cycled sodium nickel manganese-based positive electrode by scanning electron microscopy.

[0106] (8) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the integrity of the sodium nickel manganese-based positive electrode structure after cycling by X-ray diffraction (XRD).

[0107] (9) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the SEI film composition on the surface of the sodium nickel manganese-based positive electrode after cycling by X-ray photoelectron spectroscopy (XPS).

[0108] (10) Disassemble the battery that has been cycled to a certain number of cycles in (4) and remove the sodium sheet. Wash away the residual electrolyte and impurities on the surface of the sodium sheet with the appropriate solvent and dry it. Then, characterize the amount of transition metal ions deposited on the surface of the sodium sheet after cycling by inductively coupled plasma (ICP).

[0109] Example 8:

[0110] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0111] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0112] (3) Ensure that the mass fraction of the additive is 2.1% FEC. Calculate and weigh a certain mass of fluoroethylene carbonate (FEC) (CAS No.: 114435-02-8) additive and add it to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 0.1% DFMS + 2% FEC.

[0113] (4) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 2.1% FEC obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0114] (5) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 2.1% FEC obtained in (3). After standing for 8 hours, it was subjected to 3 cycles of rate charge and discharge at 0.3C to form a film. Then, rate tests were performed at different rates to evaluate the rate performance of the battery under this electrolyte.

[0115] (6) Perform AC impedance testing on the batteries that have been cycled to a certain number of cycles in (4) to evaluate the effect of electrolyte containing 2.1% FEC on battery impedance.

[0116] (7) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual salt and impurities on the surface of the sodium nickel manganese-based positive electrode with the appropriate solvent and dry it. Then characterize the morphology of the cycled sodium nickel manganese-based positive electrode by scanning electron microscopy.

[0117] (8) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the integrity of the sodium nickel manganese-based positive electrode structure after cycling by X-ray diffraction (XRD).

[0118] (9) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the SEI film composition on the surface of the sodium nickel manganese-based positive electrode after cycling by X-ray photoelectron spectroscopy (XPS).

[0119] (10) Disassemble the battery that has been cycled to a certain number of cycles in (4) and remove the sodium sheet. Wash away the residual electrolyte and impurities on the surface of the sodium sheet with the appropriate solvent and dry it. Then, characterize the amount of transition metal ions deposited on the surface of the sodium sheet after cycling by inductively coupled plasma (ICP).

[0120] Example 9:

[0121] (1) The cyclic carbonate solvent propylene carbonate (PC) (CAS No.: 108-32-7) and the linear carbonate solvent ethyl methyl carbonate (EMC) (CAS No.: 623-53-0) were mixed at a volume ratio of PC:EMC=1:1 (vol%) and purified by molecular sieve.

[0122] (2) Weigh a certain amount of sodium hexafluorophosphate by calculation and add it to the mixed solvent in (1). After the sodium hexafluorophosphate is completely dissolved, a basic electrolyte with a molar concentration of 1 mol / L is obtained and left to stand for one day before use.

[0123] (3) Ensure that the mass fraction of the additive is 2.1% DFMS. Calculate and weigh a certain mass of sodium difluoromethane sulfinate (DFMS) (CAS No.: 275818-95-6) additive and add it to the basic electrolyte in (2). Shake evenly to completely dissolve the additive and obtain an electrolyte containing 2.1% DFMS.

[0124] (4) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 2.1% DFMS obtained in (3). After standing for 8 hours, the cell was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, a long-cycle charge-discharge test was performed at 1C to evaluate the cycle stability of the cell under this electrolyte.

[0125] (5) The sodium nickel manganese-based cathode / sodium coin cell was assembled with the electrolyte containing 2.1% DFMS obtained in (3). After standing for 8 hours, it was subjected to 3 cycles of charge-discharge at 0.3C to form a film. Then, rate tests were performed at different rates to evaluate the rate performance of the battery under this electrolyte.

[0126] (6) Perform AC impedance testing on the batteries that have been cycled to a certain number of cycles in (4) to evaluate the effect of electrolyte containing 2.1% DFMS on battery impedance.

[0127] (7) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual salt and impurities on the surface of the sodium nickel manganese-based positive electrode with the appropriate solvent and dry it. Then characterize the morphology of the cycled sodium nickel manganese-based positive electrode by scanning electron microscopy.

[0128] (8) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the integrity of the sodium nickel manganese-based positive electrode structure after cycling by X-ray diffraction (XRD).

[0129] (9) Disassemble the battery that has been cycled to a certain number of cycles in (4) and take out the sodium nickel manganese-based positive electrode. Wash away the residual electrolyte and impurities on the surface of the positive electrode with the appropriate solvent and dry it. Then characterize the SEI film composition on the surface of the sodium nickel manganese-based positive electrode after cycling by X-ray photoelectron spectroscopy (XPS).

[0130] (10) Disassemble the battery that has been cycled to a certain number of cycles in (4) and remove the sodium sheet. Wash away the residual electrolyte and impurities on the surface of the sodium sheet with the appropriate solvent and dry it. Then, characterize the amount of transition metal ions deposited on the surface of the sodium sheet after cycling by inductively coupled plasma (ICP).

[0131] Comparative Example 1: No additives were added to the electrolyte, and the rest of the operation was the same as in Example 7.

[0132] Comparison of effects:

[0133] like Figure 1 This is a cycle performance graph showing the electrolytes prepared in Examples 1, 2, 3, 4, 5, 6, 7, and Comparative Example 1, assembled into sodium-nickel-manganese-based cathode / sodium half-cells, and subjected to charge-discharge cycle tests at a 1C rate using a Blue Electric testing system. The experimental results of Examples 1, 2, and 3 show that when the DFMS additive content is 0.1 wt%, the cycle stability of the sodium-nickel-manganese-based cathode / sodium half-cell is significantly improved. When the concentration of the composite additive is 0.1% DFMS + 2% FEC (Example 7), the discharge specific capacity after 200 cycles is 81.7 mAh g⁻¹. -1 In contrast, the discharge specific capacity in the Comparative Example 1 system was only 16.6 mAh g after 200 cycles. -1 .

[0134] like Figure 2 The graphs show the cycle performance of sodium-nickel-manganese-based cathode / sodium half-cells assembled with the electrolytes prepared in Examples 7, 8, and 9, as well as Comparative Example 1, under 1C charge-discharge cycle testing using the Blue Electric Test System. The experimental results of Examples 7, 8, and 9 show that when the concentration of the composite additive is 0.1% DFMS + 2% FEC (Example 7), the discharge specific capacity after 200 cycles is 81.7 mAh g⁻¹. -1 In Examples 8 and 9, when two additives of equal mass fraction were added individually, the long-cycle capacity performance was inferior to that of the composite additive, and the discharge specific capacity was only 16.6 mAh g after 200 cycles in the Comparative Example 1 system. -1 .

[0135] like Figure 3The sodium-nickel-manganese-based cathode / sodium batteries assembled with the electrolytes prepared in Examples 2, 5, and 7 and Comparative Example 1 were tested for rate performance at 0.3C, 1C, 2C, 3C, 4C, 5C, and 0.3C using the Blue Electric testing system. The figures show that the additive system containing 0.1% DFMS + 2% FEC (Example 7) significantly improves the rate performance of the battery. At 5C, the battery's discharge specific capacity is 80.30 mAh g⁻¹. -1 In contrast, the discharge specific capacity of the system in Comparative Example 1 was only 39.3 mAh g⁻¹. -1 .

[0136] like Figure 4 The sodium-nickel-manganese-based cathode / sodium batteries assembled with the electrolytes prepared in Examples 7, 8, and 9, and Comparative Example 1, were tested for rate performance at 0.3C, 1C, 2C, 3C, 4C, 5C, and 0.3C using the Blue Electric testing system. The figures show that the additive system containing 0.1% DFMS + 2% FEC (Example 7) significantly improves the rate performance of the battery. At 5C, the battery's discharge specific capacity is 80.30 mAh g⁻¹. -1 In Examples 8 and 9, when two additives of equal mass fraction were added alone, the rate performance was lower than that of Example 7 (composite additive), and the discharge specific capacity in Comparative Example 1 was only 39.3 mAh g⁻¹. -1 .

[0137] like Figure 5 The images are scanning electron microscope (SEM) images of uncirculated sodium-nickel-manganese (CNM)-based positive electrode sheets and CNM-based positive electrode sheets after 200 cycles in the electrolytes prepared in Examples 5, 7, and Comparative Example 1. The image from Comparative Example 1 shows that the CNM-based positive electrode sheet circulated in the basic electrolyte (Comparative Example 1) formed a thicker layer of non-uniform decomposition products on its surface, and cracks appeared on the positive electrode surface. This indicates that the electrolyte continuously decomposes in the basic electrolyte system, failing to construct a uniform SEI film. In contrast, the CNM-based positive electrode surface is smoother in the system containing 0.1% DFMS + 2% FEC composite additive (Example 7), with fewer surface decomposition products, and a more uniform and thin CEI film can be constructed at the positive electrode and electrolyte interface.

[0138] Figure 6The images are transmission electron microscope (TEM) images of uncirculated sodium-nickel-manganese (CNM)-based cathode plates and CNM-based cathode plates after 200 cycles in the electrolytes prepared in Examples 5, 7, and Comparative Example 1. It can be seen that the CNM-based cathode plates circulated in the basic electrolyte have disordered lattice fringes and indistinct diffraction peaks, indicating that the cathode structure is severely damaged in the basic electrolyte system. In contrast, in the system containing 0.1% DFMS + 2% FEC composite additive (Example 7), the CNM-based cathode plates have clear and distinct lattice fringes and uniformly arranged lattice diffraction peaks, indicating that the composite additive can protect the CNM-based cathode from severe damage.

[0139] Figure 7 This is a graph showing the deposition of transition metal ions on the surface of sodium sheets after 3 and 200 cycles in the electrolytes prepared in Examples 2, 5, 7, and Comparative Example 1, after washing away residual electrolyte and impurities and drying. The deposition was characterized by inductively coupled plasma (ICP). The graph shows that after 3 cycles, the deposition amounts of Ni and Mn transition metal ions detected in the sodium sheets after cycling in the basic electrolyte (Comparative Example 1) were 0.21 and 0.73 mg / L, respectively. In the system with 0.1% DFMS + 2% FEC composite additive (Example 7), after 3 cycles, the deposition amounts of Ni and Mn transition metal ions detected in the sodium sheets were 0.03 and 0.09 mg / L, respectively. After 200 cycles, the Ni and Mn transition metal ion deposition amounts detected in the sodium sheet after cycling in the basic electrolyte (Comparative Example 1) were 0.97 and 0.70 mg / L, respectively. In the system with 0.1% DFMS + 2% FEC composite additive (Example 7), the Ni and Mn transition metal ion deposition amounts detected in the sodium sheet after 200 cycles were 0.07 and 0.33 mg / L, respectively. ICP results after 3 and 200 cycles showed that the addition of the composite additive could protect sodium, nickel, and manganese transition metal ions and reduce their dissolution.

[0140] Figure 8The figures show the deposition amounts of transition metal ions on the sodium sheets after 3 and 200 cycles in the electrolytes prepared in Examples 7, 8, 9, and Comparative Example 1, after washing away residual electrolyte and impurities and drying. The deposition amounts were characterized by inductively coupled plasma (ICP). The figures show that after 3 cycles, the Ni and Mn transition metal ion deposition amounts detected in the sodium sheets after cycling in Examples 8 and 9 were 0.06 and 0.13 mg / L (Example 8) and 0.15 and 0.35 mg / L (Example 9), respectively. In the system with 0.1% DFMS + 2% FEC composite additive (Example 7), the Ni and Mn transition metal ion deposition amounts detected in the sodium sheets after 3 cycles were 0.03 and 0.09 mg / L, respectively. After 200 cycles, the Ni and Mn transition metal ion deposition amounts detected in the sodium sheets after cycling in Examples 8 and 9 were 0.15 and 0.46 mg / L (Example 8) and 0.20 and 0.52 mg / L (Example 9), respectively. In the system with 0.1% DFMS + 2% FEC composite additive (Example 7), the Ni and Mn transition metal ion deposition amounts detected in the sodium sheets after 200 cycles were 0.07 and 0.33 mg / L, respectively. ICP results after 3 and 200 cycles showed that the addition of the composite additive reduced the dissolution of nickel and manganese transition metal ions more effectively than adding any single additive.

[0141] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An electrolyte additive, characterized in that... Including sodium difluoromethanesulfonate and fluoroethylene carbonate.

2. The electrolyte additive according to claim 1, characterized in that: The mass ratio of the fluoroethylene carbonate and sodium difluoromethanesulfonate is (1-3):(0.05-0.5); preferably 2:0.

1.

3. A sodium-ion battery electrolyte for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries, characterized in that... Includes sodium salts, solvents, and electrolyte additives as described in claim 1 or 2.

4. The sodium-ion battery electrolyte for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries according to claim 3, characterized in that: The sodium salt is selected from sodium hexafluorophosphate; The solvent is selected from at least one of chain carbonates and cyclic carbonates, wherein the cyclic carbonates are at least one of ethylene carbonate, fluoroethylene carbonate, and propylene carbonate; and the chain carbonates are at least one of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Preferably, the solvent is propylene carbonate and ethyl methyl carbonate, and the volume ratio of propylene carbonate to ethyl methyl carbonate is 1:

1.

5. The sodium-ion battery electrolyte for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries according to claim 3, characterized in that: The amount of electrolyte additives used is such that, based on the total mass of sodium salt and solvent, the mass fraction of DFMS is 0.05%–0.6% and the mass fraction of FEC is 1%–3.0%.

6. The sodium-ion battery electrolyte for improving the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries according to claim 3, characterized in that: The amount of sodium salt used is such that the concentration of sodium salt in the basic electrolyte formed by sodium salt and solvent is 0.8-1.2 mol / L, preferably 1 mol / L.

7. A method for preparing a sodium-ion battery electrolyte according to any one of claims 3-6, which improves the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries, characterized in that... Includes the following steps: If there are multiple solvents, first mix the multiple solvents evenly to obtain a mixed solvent, then add the sodium salt to the mixed solvent and stir to dissolve to form a basic electrolyte, then add electrolyte additives to the basic electrolyte, mix and dissolve evenly to obtain the sodium-ion battery electrolyte.

8. The application of the sodium-ion battery electrolyte according to any one of claims 3-6, which improves the cycle stability of nickel-manganese-based cathode materials in sodium-ion batteries, in sodium-ion batteries.

9. A secondary battery, characterized in that... It includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the electrolyte is the sodium-ion battery electrolyte according to any one of claims 3-6 for improving the cycle stability of nickel-manganese-based positive electrode materials in sodium-ion batteries.

10. The secondary battery according to claim 9, characterized in that: The positive electrode material described is a nickel-manganese-based sodium-ion positive electrode material, and the nickel-manganese-based sodium-ion positive electrode material is Na x Li y Ni z Mn 1-z O2, where 0.5 ≤ x ≤ 1.0, 0 ≤ y ≤ 0.2, 0 < z ≤ 0.5; Preferably, the nickel-manganese-based sodium-ion positive electrode material is Na 0.8 Li x Ni y Mn 0.65 O2, where x = 0.05 - 0.2; y = 0.15 - 0.5; The negative electrode is a metallic sodium negative electrode and / or a carbon-based negative electrode; The secondary battery is a sodium-ion coin cell or a sodium-ion full cell.

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