Sodium-ion battery electrolyte, preparation method and sodium-ion battery

Abstract

The invention provides a sodium-ion battery electrolyte, a preparation method and a sodium-ion battery, and belongs to the technical field of sodium-ion batteries. The sodium-ion battery electrolyte comprises an electrolyte sodium salt, an organic solvent and a functional additive, the electrolyte sodium salt comprises sodium difluorophosphate and sodium bis (fluorosulfonyl) imide; the organic solvent comprises cyclic carbonate, a sulfonate solvent and fluoroether. According to the invention, sodium difluorophosphate and sodium bis (fluorosulfonyl) imide are adopted as electrolyte sodium salts, so that a stable sodium ion conduction channel can be provided in the charging and discharging process of the battery, the battery is ensured to have good ionic conductivity, the charging and discharging efficiency of the battery is improved, and the performance of the electrolyte is remarkably improved by adding the functional additive.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion batteries, in particular to a sodium ion battery electrolyte, a preparation method and a sodium ion battery. BACKGROUND

[0002] With the gradual depletion of traditional fossil energy and the increasingly serious environmental problems, the development of new renewable energy has become an inevitable trend. Lithium ion batteries have been widely used in electric vehicles, notebook computers and energy storage due to their high energy density, long cycle life and environmental friendliness. However, the resource reserves of lithium are limited and unevenly distributed on the earth, which will be exhausted in the future. Sodium ion batteries, due to their abundant sodium resources and low cost, have become one of the ideal substitutes for lithium ion batteries and have broad application prospects in large-scale energy storage.

[0003] However, due to the high activity of sodium metal in the current sodium ion battery, the interface compatibility with the negative electrode (especially hard carbon or sodium metal) in the conventional carbonate electrolyte is poor, which easily forms an unstable solid electrolyte interface film (SEI film), leading to sodium dendrite growth, short cycle life and low first coulombic efficiency. The conventional electrolyte salt (such as NaPF6) has poor thermal stability and is easily decomposed and catalyzes the decomposition of the solvent at high temperature, resulting in gas production, bulging and rapid capacity decay of the battery, which poses a safety hazard. High-voltage positive electrode materials (such as layered oxides and polyanion compounds) are the key to improving energy density, but the traditional electrolyte is easily oxidized and decomposed at high voltage (>4.0V vs Na + / Na), leading to the deterioration of the positive electrode interface and the increase of impedance.

[0004] In the prior art, the double salt system of NaPF6 / NaDFOB is commonly used to solve the above problems existing in the current sodium ion battery, but its interface stability is still not ideal under extreme high temperature (>80℃) and super high voltage (>4.3V) conditions, and the thermal safety hazard is still a major challenge. SUMMARY

[0005] The present application is made in view of the technical problem that the interface stability of the existing sodium ion battery is not ideal under extreme high temperature and super high voltage conditions, and the purpose is to provide a sodium ion battery electrolyte, a preparation method and a sodium ion battery.

[0006] Specifically, the first aspect of the present application provides a sodium ion battery electrolyte, which comprises an electrolyte sodium salt, an organic solvent and a functional additive; the electrolyte sodium salt comprises sodium difluorophosphate and sodium bisfluorosulfonylimide; The organic solvent comprises a cyclic carbonate, a sulfonate solvent and a fluorinated ether.

[0007] Further, the total molar concentration of the electrolyte sodium salt is 1.0 M-1.5 M; wherein the molar ratio of sodium difluorophosphate to sodium bisfluorosulfonylimide is 6-8:4-2, and / or The organic solvent comprises the following volume percentage of components: 15-25 vol% of cyclic carbonate, 15-25 vol% of sulfonate solvent, 45-55 vol% of fluorinated ether, and 5-15 vol% of chain carboxylate.

[0008] Further, the sulfonate solvent is dimethylsulfoxonium methylide; and the fluorinated ether is one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or bis(2,2,2-trifluoroethyl) ether.

[0009] Further, the functional additive comprises a polymeric positive electrode film-forming additive and an acid scavenger; and the polymeric positive electrode film-forming additive is a vinyl-containing borate compound.

[0010] Further, the polymeric positive electrode film-forming additive is vinyl pinacol borate, and the addition amount of the polymeric positive electrode film-forming additive is 1.0-3.0 vol% of the total volume of the organic solvent; and / or The acid scavenger comprises 1,3-propane sulfolane and tris(trimethylsilyl) phosphite, the addition amount of the 1,3-propane sulfolane is 0.5-2.0 vol%, and the addition amount of the tris(trimethylsilyl) phosphite is 0.5-1.5 vol%.

[0011] The second aspect of the present application also provides a preparation method of a sodium ion battery electrolyte, which is used for preparing the sodium ion battery electrolyte, and the preparation method comprises the following steps: S1: mixing organic solvents under the protection of an inert atmosphere to obtain a mixed solvent; S2: adding sodium difluorophosphate into the mixed solvent, stirring at a first temperature until completely dissolved to obtain a first solution; S3: adding sodium bisfluorosulfonylimide into the first solution at a second temperature, stirring until completely dissolved to obtain a basic electrolyte; the second temperature is lower than the first temperature; S4: adding a functional additive into the basic electrolyte, and after stirring, performing aging treatment to obtain the sodium ion battery electrolyte.

[0012] Further, in step S2, the first temperature is 20-30℃, the stirring speed is 300-400 rpm, and the dissolution time is 3-4h; and / or In step S3, the second temperature is -5 to 5℃, the stirring speed is 300-400 rpm, and the dissolution time is 4-5h.

[0013] Further, the order of adding the functional additives in step S4 is: first adding the acid scavenger, and then adding the polymeric positive electrode film-forming additive after stirring for 20-40 min.

[0014] Further, the temperature of the ripening treatment in step S4 is 20-30 DEG C, the ripening time is 12-36 h, and the stirring speed is 50-150 rpm.

[0015] The third aspect of the application provides a sodium ion battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the electrolyte is the sodium ion battery electrolyte described above.

[0016] The application has the following beneficial effects: The application replaces the traditional NaPF6 with sodium difluorophosphate (NaPO2F2), which has a much higher thermal decomposition temperature and hydrolysis stability than NaPF6, and can inhibit the generation of HF from the source. However, when used alone, the ionic conductivity is low and the cost is extremely high. The inventors have unexpectedly found that, when NaPO2F2 is combined with sodium bisfluorosulfonylimide (NaFSI) in a specific ratio, the deficiency in ionic conductivity can be compensated for, and the FSI - anion can also synergistically decompose with the PO2F2 - anion at the electrode interface to form a high-strength, high-ionic-conductivity "composite interface film" rich in N, S, P, O and F elements, which can effectively resist the impact of high temperature and high voltage, reduce sodium dendrite growth, and improve the cycle life and initial coulombic efficiency of the battery.

[0017] To improve the solubility of NaPO2F2 and further stabilize the heat-sensitive NaFSI, a sulfonate solvent (such as dimethyl sulfonylmethane) with high dielectric constant is introduced as a "structure solvent", which can effectively dissolve the new salt system due to its strong polarity; at the same time, the proportion of fluoroether is greatly increased, the introduction of the sulfonate solvent enhances the solubility of the electrolyte sodium salt, and the great increase in the proportion of fluoroether forms a continuous phase with flame retardance and oxidation resistance, thereby fundamentally improving the safety of the electrolyte and reducing the safety hazards such as gas production, bulging and rapid capacity decay of the battery.

[0018] The application also uses vinylboronic pinacol ester (VBPE) as a new type of polymeric additive, which can undergo electropolymerization on the surface of the positive electrode during charging to form a dense polymer protective layer (CEI) with self-healing ability, which physically and chemically cooperates with the above-mentioned "composite interface film". The polymer protective layer formed by the vinylboronic pinacol ester cooperates with the composite interface film to effectively inhibit the deterioration of the positive electrode interface and the increase in impedance, so that the battery can also operate stably at high voltage. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be described and explained below in connection with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application. Based on the embodiments provided by the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of the present application.

[0020] Obviously, the following description is only some examples or embodiments of the present application, and for those of ordinary skill in the art, the present application can also be applied to other similar situations without creative labor. In addition, it can also be understood that although the efforts made in this development process can be complex and lengthy, for those of ordinary skill in the art related to the content disclosed in the present application, some design, manufacture or production changes based on the technical content disclosed in the present application are only routine technical means and should not be understood as insufficient disclosure of the present application.

[0021] Unless otherwise specified, "including" and "comprising" mentioned in the present application means open or closed. For example, "including" and "comprising" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0022] Unless otherwise specified, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or A and B are both true (or exist).

[0023] The first aspect of the present application provides a sodium ion battery electrolyte, comprising an electrolyte sodium salt, an organic solvent and a functional additive; the electrolyte sodium salt comprises sodium difluorophosphate (NaPO2F2) and sodium bisfluorosulfonylimide (NaFSI); the organic solvent comprises a cyclic carbonate, a sulfonate solvent and a fluorinated ether. Limitation: when used alone, its solubility in carbonate solvent is limited, the ionic conductivity is low, and the interface film impedance formed is high.

[0024] NaPO2F2 (thermal stability and phosphorus source base stone): its PO2F2 - The anion has very high thermodynamic stability (decomposition temperature > 300℃) and hydrolysis resistance, which eliminates the way of producing HF like NaPF6 from the source, providing an intrinsic thermal safety basis for the electrolyte. Limitation: relatively poor thermal stability, potential corrosion risk to aluminum current collector.

[0025] NaFSI (high conductivity and nitrogen-sulfur synergistic agent): FSI - Anions have lower lattice energy and higher dissociation constant, which can provide extremely high ionic conductivity to make up for the shortcomings of NaPO2F2. More importantly, the reduction and oxidation decomposition products of FSI (such as NaF, Na2SO4, Na3N, etc.) are excellent SEI / CEI film components. -

[0026] The high conductivity of NaFSI perfectly makes up for the shortcomings of NaPO2F2, while the high thermal stability of NaPO2F2 inhibits the possible decomposition of NaFSI at high temperatures, and the two complement each other in basic performance.

[0027] Co-decomposition and composite interface film formation: At the electrode interface (especially the positive electrode interface), PO2F2 - and FSI - will undergo synergistic co-decomposition to generate a brand new "composite interface film" rich in NaF, Na3PO4, Na2SO4, Na3N and other inorganic substances. This interface film: high mechanical strength: a variety of chemical bonds (P-O, S-O, N≡) are interwoven, which can effectively inhibit the lattice distortion and oxygen precipitation of the positive electrode material at high pressure. High ionic conductivity: abundant NaF, Na3N, etc. provide a fast Na + transport channel. The staggered filling of different decomposition products makes the interface film extremely dense, which can completely isolate the direct contact between the electrolyte and the high-activity electrode material.

[0028] Therefore, the combination of sodium difluorophosphate (NaPO2F2) and sodium bisfluorosulfonylimide (NaFSI) in the electrolyte sodium salt not only solves the limitations of each when used alone, but also forms a composite interface film with excellent performance through synergistic effect, greatly improving the interface stability of sodium-ion batteries under extreme high temperature and ultra-high voltage conditions.

[0029] In the present embodiment, the total molar concentration of the electrolyte sodium salt is 1.0 M-1.5 M, preferably, the total concentration of sodium salt is 1.2 M; wherein the molar ratio of sodium difluorophosphate to sodium bisfluorosulfonylimide is 6-8:4-2, preferably, the molar ratio is 7:3, which ensures that there is enough PO2F2 - to provide thermal stability and P / O elements, and at the same time there is enough FSI - to provide high conductivity and N / S elements, so as to generate an interface film with the best chemical composition, which is the key to balancing conductivity, cost and interface film performance. Deviation of the ratio will lead to imbalance of interface film components and performance decline.

[0030] ​In the present embodiment, the organic solvent comprises the following components in volume percentage: cyclic carbonate (EC) 15-25 vol%, sulfonate solvent (PC) 15-25 vol%, fluorinated ether (TTE) 45-55 vol%, chain carboxylate (EP) 5-15 vol%. The cyclic carbonate in the organic solvent provides high dielectric constant, helps the dissolution and ion dissociation of the electrolyte sodium salt. The cyclic carbonate has high polarity, can form strong interaction with the ions in the electrolyte sodium salt, promote the solvation process of the ions, make the electrolyte sodium salt better dissolve in the organic solvent, thereby improving the ion conduction ability of the electrolyte. At the same time, it can also stabilize the electrode interface to a certain extent, help to form a stable solid electrolyte interface film (SEI film), reduce the growth of sodium dendrites, and improve the cycle stability of the battery. The addition of sulfonate solvent such as dimethyl sulfoxide methane (MSM) can further enhance the dissolution ability of the electrolyte sodium salt. It cooperates with the cyclic carbonate to optimize the dissolution performance of the organic solvent, ensuring that the electrolyte sodium salt can be uniformly dispersed in the electrolyte. Moreover, the sulfonate solvent can also participate in the electrode interface reaction, positively affect the structure and performance of the formed interface film, and improve the stability and ion conductivity of the interface film. The proportion of fluorinated ether (TTE) in the organic solvent is large, which is 45-55 vol%. Fluorinated ether has good flame retardance and oxidation resistance, and builds a continuous phase of flame retardance and oxidation resistance, which fundamentally improves the safety of the electrolyte. It can reduce the flammability of the electrolyte and reduce the risk of combustion or explosion of the battery under abnormal conditions such as high temperature or overcharge. In addition, fluorinated ether can also improve the viscosity and wettability of the electrolyte, making the electrolyte better wet the electrode material and improving the charge and discharge performance of the battery. The volume percentage of chain carboxylate (EP) is 5-15 vol%, which is to adjust the viscosity and flowability of the organic solvent. An appropriate amount of chain carboxylate can reduce the viscosity of the electrolyte and improve the migration rate of ions, thereby improving the rate performance of the battery. At the same time, it can also interact with other solvent components to optimize the overall performance of the electrolyte, so that the battery can maintain good performance under different working conditions.

[0031] In the present embodiment, the sulfonate solvent is dimethyl sulfoxide methane (MSM); the fluorinated ether is one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) or bis(2,2,2-trifluoroethyl) ether.

[0032] Dimethyl sulfoxide methane has very high dielectric constant and DN value, is a key solvent for dissolving high-difficulty sodium salt, and ensures high ion migration number of the electrolyte.

[0033] 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) in fluoroethers: its proportion is raised to 50%, making it change from an additive to a bulk solvent. TTE itself is non-flammable and has very strong oxidation resistance (>5.0 V vs Na + / Na). It constitutes a safe continuous phase of the electrolyte, surrounding other solvent molecules, greatly improving the flash point and oxidation limit of the electrolyte as a whole.

[0034] MSM has a partial positive charge on the sulfur atom and a partial negative charge on the oxygen atom in its molecular structure, forming a strong polarity dipole moment. This strong polarity enables dimethyl sulfoxyl methane to interact strongly with the anions and cations in the sodium salt of the electrolyte, separating the ions by electrostatic attraction, thereby promoting the dissociation of the sodium salt; TTE acts as a "shell" to wrap and stabilize the entire solvation sheath structure, while providing excellent oxidation resistance and flame retardant properties. MSM solves the problem of poor solubility of TTE, and TTE solves the problem of insufficient oxidation resistance of MSM itself. High proportions of TTE and MSM work together to reduce the coordination number of solvent molecules with Na + <>, increase the coordination proportion of anions, and form the characteristics of a weakly solvated / quasi-concentrated salt electrolyte, which is beneficial to the formation of a thinner and more stable interfacial film.

[0035] In the embodiment, the functional additives include a polymeric positive electrode film-forming additive and an acid scavenger; the polymeric positive electrode film-forming additive is a vinyl-containing borate compound. The polymeric positive electrode film-forming additive is vinyl boron pinacol ester (VBPE), and the addition amount is 1.0-3.0 vol% of the total volume of the organic solvent, preferably 2.0 vol%; the acid scavenger includes 1,3-propane sulfone lactone (PS) and tris (trimethylsilyl) phosphite (TTSPi), the addition amount of the 1,3-propane sulfone lactone is 0.5-2.0 vol%, preferably 1.0 vol%, and the addition amount of the tris (trimethylsilyl) phosphite is 0.5-1.5 vol%, preferably 0.5 vol%.

[0036] VBPE as a polymeric positive electrode film-forming agent, the vinyl group in its molecule can undergo electropolymerization at high voltage (>4.0 V) to form a cross-linked polymer protective layer (CEI) rich in B-O bonds on the surface of the positive electrode material. The polymer layer has excellent toughness and can adapt to the volume change of the positive electrode material during charging and discharging, and even has self-healing ability, which cannot be achieved by small molecule additives.

[0037] 1,3-propane sultone (PS) and tri (trimethylsilyl) phosphate (TTSPi) constitute a dual acid scavenging mechanism. PS, as an acid scavenger and co-promoter of film-forming agent, reacts preferentially with H2O, and TTSPi, as a high-performance water / acid scavenger, cooperates with PS, and TTSPi efficiently captures HF. The products generated by the reaction of PS with water can further interact with the products generated after TTSPi captures HF to form stable compounds, thereby effectively removing HF and water from the electrolyte system. The sulfonic acid group generated by the hydrolysis of PS can undergo esterification with silanol and other substances generated after TTSPi captures HF to form stable sulfonate compounds, reducing the damage of free HF and water to the performance of the battery. This dual acid scavenging mechanism not only can timely remove trace amounts of HF and water generated in the electrolyte, but also can form a film structure with certain protective effect at the electrode interface. In the process of reacting with water, PS will form a film containing sulfonic acid groups on the surface of the electrode, which can prevent unnecessary side reactions between other components in the electrolyte and the electrode material. The products formed after TTSPi captures HF also participate in the formation of the film, further enhancing the stability and protective performance of the film. The synergistic scavenging effect of PS and TTSPi provides a pure chemical environment for the electro-polymerization of VBPE and the growth of the composite interface film, ensuring the high quality of the protective layer.

[0038] The addition of the functional additives further improves the performance of the sodium-ion battery electrolyte. The vinyl-containing borate compound as a polymerization-type positive electrode film-forming additive can form a stable and self-healing polymer protective layer on the surface of the positive electrode, effectively inhibiting the deterioration of the positive electrode interface and the increase in impedance. The acid scavenger can timely remove acidic impurities in the electrolyte to prevent damage to the interface film, providing a guarantee for the stable operation of the battery. When these functional additives work together, they cooperate and work synergistically, making the sodium-ion battery electrolyte exhibit excellent performance in multiple aspects. In a high-temperature environment, the composite interface film and the polymer protective layer can effectively resist the impact of high temperature and reduce the performance degradation of the battery; under high voltage conditions, they can inhibit the lattice distortion and oxygen precipitation of the positive electrode material, improving the safety and cycle life of the battery. In addition, the reasonable amount of functional additives is also crucial. The addition amount of pinacyl ethylene glycol borate is controlled at 1.0-3.0 vol% of the total volume of the organic solvent, which can ensure the formation of a polymer protective layer with sufficient thickness and quality, and will not affect the other performance of the electrolyte due to excessive addition. The addition amount of 1,3-propane sultone and tri (trimethylsilyl) phosphate is in the range of 0.5-2.0 vol% and 0.5-1.5 vol%, respectively, which can effectively remove acidic impurities while avoiding unnecessary side reactions caused by excessive addition.

[0039] The second aspect of the present application also provides a preparation method of a sodium-ion battery electrolyte, for preparing the sodium-ion battery electrolyte, the preparation method comprising the following steps: Preparation stage: all operations are carried out in a glove box filled with high-purity argon (H2O < 0.5 ppm, O2 < 0.5 ppm).

[0040] Sodium salt: NaPO2F2 and NaFSI are dried at 80±2℃ under high vacuum (-0.098 MPa) for 48h, and immediately transferred to a glove box and cooled to 25℃ for standby.

[0041] Solvent: EC / PC / EP: After being soaked with 4A molecular sieves for 72h, calcium hydride (CaH2) is added for stirring and refluxing for 6h, and the intermediate fraction is collected by distillation, with the moisture controlled to be <8ppm.

[0042] MSM: requires double drying treatment with 3A molecular sieves and calcium oxide (CaO, added amount 15g / L) for 96h, and magnetic stirring (50 rpm).

[0043] TTE: the reagent stored with 4A molecular sieves is directly used.

[0044] Additive: PS, TTSPi and VBPE are stored with 4A molecular sieves for at least 48h before use.

[0045] S1: organic solvents are mixed to obtain a mixed solvent under the protection of an inert atmosphere; S2: sodium difluorophosphate is added to the mixed solvent, and stirred at a first temperature until completely dissolved to obtain a first solution; S3: sodium bisfluorosulfonylimide is added to the first solution at a second temperature, and stirred until completely dissolved to obtain a basic electrolyte; the second temperature is lower than the first temperature; S4: functional additives are added to the basic electrolyte, and after stirring, aging treatment is performed to obtain the sodium-ion battery electrolyte.

[0046] In this embodiment, step S1 takes a 2L jacketed glass reactor, which is connected with a circulating water bath (set to 25.0±0.5℃). According to the volume ratio, use a precision pipette to add: EP (100 ml) -> MSM (200 ml) -> EC (100ml) + PC (100 ml) in sequence. Start magnetic stirring, and the stirring speed is controlled to be 200 rpm, and the stirring is continued for 60 min to make the solvents fully miscible.

[0047] In this embodiment, the first temperature in step S2 is 20-30°C, the stirring speed is 300-400 rpm, and the dissolution time is 3-4 h; the second temperature in step S3 is -5 to 5°C, the stirring speed is 300-400 rpm, and the dissolution time is 4-5 h.

[0048] Specifically, dissolve NaPO2F2: maintain 25°C and 200 rpm stirring. Weigh NaPO2F2 (70.2 g, 0.84 mol) with a precision electronic balance, and slowly add it through a feeding funnel at a rate of -1.0 g / min. After the feeding is completed, increase the stirring speed to 300 rpm, and continue stirring at 25°C for 180 min until complete dissolution.

[0049] Dissolve NaFSI (low-temperature operation): Set the jacket water temperature of the reaction kettle to -5°C, and reduce the solution temperature to 0±2°C. Weigh NaFSI (64.8 g, 0.36 mol), and add it at a rate of -0.5 g / min under the condition of maintaining 300 rpm stirring, which takes about 130 min. After the feeding is completed, continue stirring at 0°C and 300 rpm for 240 min, and a clear and transparent light yellow base electrolyte is obtained.

[0050] In this embodiment, the temperature of the reaction kettle is restored to 25°C in step S4, and the stirring speed is adjusted to 150 rpm.

[0051] Add the following additives in sequence with a microsyringe: PS (10.0 ml, 1.0 vol%), dropwise addition time >5 min, and stir for 30 min after the addition is completed.

[0052] TTSPi (5.0 ml, 0.5 vol%), dropwise addition time >5 min, and stir for 30 min after the addition is completed.

[0053] VBPE (20.0 ml, 2.0 vol%), dropwise addition time >10 min, and stir for 30 min after the addition is completed.

[0054] After the addition of all the additives is completed, maintain the temperature at 20-30°C and the low-speed stirring at 50-150 rpm for 12-36 h, preferably 24 h of long-time aging, which is a key process for the reorganization of the solvation structure and the pre-arrangement of the additive molecules.

[0055] The matured electrolyte is filtered through a set of pre-dried polytetrafluoroethylene (PTFE) microporous filter membranes with a pore size of 0.22 μm. The filtered electrolyte is divided into brown pressure-resistant glass bottles, vacuumed to -0.09 MPa, and sealed with a butyl rubber plug coated with a fluorine film and an aluminum cap. The finished product is stored in an explosion-proof refrigerator at a temperature of 10-15 °C and stored in the dark.

[0056] The third aspect of the present application provides a sodium-ion battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the electrolyte is the sodium-ion battery electrolyte described above.

[0057] The unique formula and performance advantages of the electrolyte enable the sodium-ion battery to exhibit more excellent performance during charging and discharging. In the charging and discharging cycle, the electrolyte can effectively reduce the side reactions inside the battery, reduce energy loss, and improve the charging and discharging efficiency of the battery. Its good ion conduction capacity ensures the rapid migration of sodium ions between the positive and negative electrodes, enabling the battery to quickly respond to charging and discharging demands and improving the power density of the battery. In different working temperature environments, this electrolyte also has good adaptability. In a low-temperature environment, it can still maintain a certain ion conductivity, reduce the increase of internal resistance of the battery, avoid a significant decline in battery performance, and ensure that the battery can still work normally in cold conditions. In a high-temperature environment, the flame retardance and oxidation resistance of the electrolyte play an important role in preventing safety accidents caused by overheating of the battery, and the stable interface film structure can also reduce the damage of high temperature to the performance of the battery and prolong the service life of the battery.

[0058] Embodiment The following examples more specifically describe the present disclosure, which are merely illustrative and not restrictive, as various modifications and changes can be obvious to those skilled in the art within the scope of the present disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by weight. Unless otherwise stated, all reagents used in the examples are available by conventional commercial routes or synthesized according to conventional methods and can be used directly without further treatment. Unless otherwise stated, the instruments used in the examples are available by conventional commercial routes.

[0059] Example 1 A sodium-ion battery electrolyte, characterized in that it comprises an electrolyte sodium salt, an organic solvent and a functional additive; the electrolyte sodium salt comprises sodium difluorophosphate (NaPO2F2) and sodium bisfluorosulfonylimide (NaFSI); the total molar concentration of the electrolyte sodium salt is 1.2 M; wherein the molar ratio of NaPO2F2 to NaFSI is 7:3; The organic solvent includes cyclic carbonate 20 vol%, dimethylsulfone (MSM) 20 vol%, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) 50 vol%, chain carboxylic acid ester 10 vol%; The functional additives include vinylboronic acid pinacol ester (VBPE) 2.0 vol% and 1,3-propane sultone (PS) 1.0 vol%, tris(trimethylsilyl) phosphite (TTSPi) 0.5 vol%; The preparation method of the sodium ion battery electrolyte includes the following steps: S1: NaPO2F2 and NaFSI are vacuum dried at 80°C and -0.098 MPa for 48h; the solvents EC, PC, EP, MSM, and TTE are deeply dehydrated to less than 10 ppm of water by molecular sieves and chemical reagents; at 25°C, 250 rpm stirring, EP (100ml), MSM (200ml), EC (100ml), PC (100ml) are sequentially added to the reactor, and finally TTE (500ml) is added, stirring for 60min, to obtain a mixed solvent; S2: NaPO2F2 is added to the mixed solvent, stirred at 350 rpm at 25°C until completely dissolved, and the dissolution time is 3h, to obtain a first solution; S3: NaFSI is added to the first solution at 0°C, stirred at 350 rpm until completely dissolved, and the dissolution time is 4h, to obtain a base electrolyte; S4: functional additives are added to the base electrolyte, PS is added first, then TTSPi is added after stirring, and finally VBPE is added, after stirring, aging treatment is carried out, the aging treatment temperature is 25°C, the aging time is 24h, and the stirring speed is 100rpm, the aged electrolyte is filtered under inert atmosphere, the filter pore size is 0.2μm, to obtain the sodium ion battery electrolyte.

[0060] Example 2 This example is basically the same as Example 1, except that the molar ratio of NaPO2F2 to NaFSI is 6:4.

[0061] Example 3 This example is basically the same as Example 1, except that the molar ratio of NaPO2F2 to NaFSI is 8:2.

[0062] Example 4 This example is basically the same as Example 1, except that the addition amount of vinylboronic acid pinacol ester is 1.0 vol%.

[0063] Example 5 This example is substantially the same as Example 1, except that the amount of added vinylboronic acid pinacol ester is 3.0 vol%.

[0064] Comparative Example 1 A conventional electrolyte: 1.0 M NaPF6in EC:PC:DEC = 1:1:1 (vol%) with 2% FEC additive was used.

[0065] Comparative Example 2 This comparative example is substantially the same as Example 1, except that the electrolyte salt is only 1.2 M NaPO2F2, without NaFSI.

[0066] Comparative Example 3 This comparative example is substantially the same as Example 1, except that the electrolyte salt is only 1.2 M NaFSI, without NaPO2F2.

[0067] Comparative Example 4 This comparative example is substantially the same as Example 1, except that the solvent system is EC:PC:EP = 3:1:6 (vol%), without MSM and TTE.

[0068] Comparative Example 5 This comparative example is substantially the same as Example 1, except that no functional additives (PS, TTSPi, VBPE) are added.

[0069] Experimental Cases Battery assembly and testing: High-voltage sodium electrode material NaNi 0.3 Fe 0.3 Mn 0.3 O2(NFM), conductive agent Super P, binder PVDF were mixed in a mass ratio of 8:1:1 to make a slurry, coated on an aluminum foil to prepare a positive electrode sheet. A hard carbon material was used as the negative electrode, a glass fiber membrane was used as the separator, and the electrolyte was the electrolyte in Examples 1-5 and Comparative Examples 1-5. CR2032 type button cells were assembled in a glove box, and the test results are shown in Table 1.

[0070]

[0071] As can be seen from Table 1, Example 1 shows relatively good comprehensive performance in various aspects. Compared with other examples and comparative examples, its interface impedance grows slowly, indicating that the internal resistance change of the battery during use is relatively stable, which is conducive to the long-term performance of the battery. Its CEI film is thin and uniform, and such high-quality interface film structure can effectively protect the electrode and reduce the occurrence of side reactions. The solid composite interface film constructed by NaPO2F2 and NaFSI combines various advantages such as thermal stability and high conductivity, providing a strong guarantee for the performance of the battery. Due to the high proportion of NaFSI in Example 2, the interface film has too much organic component, and the stability decreases. This may make the interface film more easily damaged in long-term use or complex environment, thereby affecting the overall performance of the battery. In Example 3, the proportion of NaPO2F2 is high, although it may have certain advantages in terms of thermal stability, but the ionic conductivity is slightly lower, which limits the performance of the battery in high-rate charging and discharging, so that the battery cannot fully exert its capacity in the process of rapid charging and discharging. Because of the less VBPE in Example 4, the positive electrode protection layer is incomplete, which will cause the positive electrode to be easily damaged during the cycle process, thereby affecting the cycle life of the battery. Example 5 has more VBPE, which increases the internal resistance and has a certain negative impact on the high-temperature performance. In a high-temperature environment, the increase of internal resistance will cause the battery to heat up, further reducing the performance and safety of the battery.

[0072] The by-reaction of Comparative Example 1 is severe under high pressure, the CEI film is thick and unstable, and it collapses comprehensively under the double test of high pressure and high temperature, which may be caused by the composition and structure of the traditional electrolyte being difficult to withstand the extreme conditions of high pressure and high temperature. The solvent and electrolyte of the traditional electrolyte are prone to decomposition and oxidation under high pressure, resulting in the continuous thickening and loose structure of the CEI film, which cannot effectively protect the electrode. The ion conductivity of Comparative Example 2 is extremely low, and the polarization is large, which cannot be normally cycled, because the single NaPO2F2 electrolyte salt is difficult to form an efficient ion conduction channel, which hinders the migration of sodium ions between the positive and negative electrodes, and a large polarization phenomenon occurs in the battery, so that effective charge and discharge cycles cannot be achieved. Comparative Example 3 has poor thermal stability and interface corrosion because of the absence of NaPO2F2. This is because NaPO2F2 plays a key role in building a stable interface film and providing thermal stability, and the absence of it will make the battery interface vulnerable to external factors during the charge and discharge process, resulting in rapid decline in battery performance. Comparative Example 4 has poor high-pressure performance due to the absence of TTE / MSM and the easy oxidation of the solvent. TTE and MSM respectively endow the electrolyte with intrinsic safety, oxidation resistance and good salt solubility. The absence of them will make the electrolyte more prone to oxidation under high pressure, reducing the high-pressure performance of the battery. Comparative Example 5 has many interface side reactions and poor cleanliness due to the absence of additives. Additives play an important role in reducing internal side reactions, building stable interface films, etc. Without them, the battery interface will have a large number of side reactions, produce impurities and gas, and affect the performance and safety of the battery. Through the analysis of these examples and comparative examples, the role of each component of the sodium-ion battery electrolyte in the present application and its influence on the performance of the battery can be more clearly understood, providing an important reference for further optimizing the electrolyte formula and improving the performance of sodium-ion batteries. It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are only examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications that can be thought of by those skilled in the art, and other ways constructed by combining part of the components of the embodiments are also included in the scope of the present application.

Claims

1. A sodium-ion battery electrolyte, characterized in that, It includes sodium electrolyte salts, organic solvents, and functional additives; the sodium electrolyte salts include sodium difluorophosphate and sodium difluorosulfonyl imide. The organic solvents include cyclic carbonates, sulfonate solvents, and fluoroethers.

2. The sodium-ion battery electrolyte according to claim 1, characterized in that, The total molar concentration of the electrolyte sodium salt is 1.0 M-1.5 M; wherein the molar ratio of sodium difluorophosphate to sodium difluorosulfonamide is 6-8:4-2, and / or The organic solvent comprises the following components by volume percentage: 15-25 vol% cyclic carbonates, 15-25 vol% sulfonate solvents, 45-55 vol% fluoroethers, and 5-15 vol% chain carboxylic esters.

3. The sodium-ion battery electrolyte according to claim 2, characterized in that, The sulfonate solvent is dimethylsulfonylmethane; the fluorinated ether is one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or bis(2,2,2-trifluoroethyl) ether.

4. The sodium-ion battery electrolyte according to claim 1, characterized in that, The functional additives include polymeric positive electrode film-forming additives and acid scavengers; the polymeric positive electrode film-forming additives are vinyl-containing borate ester compounds.

5. The sodium-ion battery electrolyte according to claim 4, characterized in that, The polymeric positive electrode film-forming additive is vinyl borate pinacol ester, and its addition amount is 1.0-3.0 vol% of the total volume of the organic solvent; and / or The acid scavenger comprises 1,3-propanesulfonyl lactone and tris(trimethylsilane) phosphite, wherein the amount of 1,3-propanesulfonyl lactone added is 0.5-2.0 vol%, and the amount of tris(trimethylsilane) phosphite added is 0.5-1.5 vol%.

6. A method for preparing a sodium-ion battery electrolyte, characterized in that, The method for preparing the sodium-ion battery electrolyte according to any one of claims 1-5 comprises the following steps: S1: Under an inert atmosphere, organic solvents are mixed to obtain a mixed solvent; S2: Add sodium difluorophosphate to the mixed solvent and stir at a first temperature until completely dissolved to obtain a first solution; S3: Add sodium difluorosulfonamide to the first solution at a second temperature and stir until completely dissolved to obtain the basic electrolyte; the second temperature is lower than the first temperature; S4: Add functional additives to the basic electrolyte, stir and then perform aging treatment to obtain the sodium-ion battery electrolyte.

7. The method for preparing the sodium-ion battery electrolyte according to claim 6, characterized in that, In step S2, the first temperature is 20-30℃, the stirring speed is 300-400 rpm, and the dissolution time is 3-4 hours; and / or In step S3, the second temperature is -5 to 5°C, the stirring speed is 300-400 rpm, and the dissolution time is 4-5 hours.

8. The method for preparing the sodium-ion battery electrolyte according to claim 6, characterized in that, The order of adding functional additives in step S4 is as follows: first add the acid scavenger, stir for 20-40 minutes, and then add the polymeric positive electrode film-forming additive.

9. The method for preparing the sodium-ion battery electrolyte according to claim 6, characterized in that, The aging process in step S4 is carried out at a temperature of 20-30℃, for a aging time of 12-36 hours, and with a stirring speed of 50-150 rpm.

10. A sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The electrolyte is the sodium-ion battery electrolyte as described in any one of claims 1-6.

Images