A high-voltage sodium-ion battery electrolyte, its preparation method, and the sodium-ion battery itself.

Abstract

This invention belongs to the field of sodium-ion batteries, and relates to an electrolyte for high-voltage sodium-ion batteries, its preparation method, and the sodium-ion battery itself. The electrolyte for high-voltage sodium-ion batteries includes an organic solvent, an electrolyte sodium salt, and a film-forming additive; the organic solvent includes malonic acid esters and carbonate compounds. The sodium-ion battery electrolyte prepared using malonic acid esters exhibits high ionic conductivity and an electrochemical stability window. These malonic acid esters possess a β-dicarbonyl chelate molecular structure, which can regulate the solvation structure, inhibit the oxidative decomposition of the solvent under high pressure, and induce the formation of a thin, dense, uniform electrode-electrolyte interface phase rich in NaF inorganic components. The assembled sodium-ion battery exhibits good rate performance and cycle stability, with a cycle life exceeding 2000 cycles at room temperature at 10C. It achieves cycle stability exceeding 4000 cycles with almost no capacity decay, demonstrating good versatility.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries and relates to the preparation of electrolytes. Background Technology

[0002] Sodium-ion batteries, with their abundant sodium reserves and low manufacturing costs, have shown significant application potential and development prospects in the field of large-scale energy storage. The electrolyte plays a crucial role in the stability of the electrode-electrolyte interface, ion transport kinetics, and battery safety. As sodium-ion batteries develop towards higher energy densities, the cathode material is required to operate stably at higher voltages (>4.3 V). Under such high-voltage conditions, carbonate solvents themselves still face the challenge of insufficient oxidation resistance: they are prone to oxidative decomposition at high potentials, leading to electrolyte consumption and interfacial failure. High-voltage oxidation resistance has become a bottleneck limiting the further application of the system.

[0003] In the non-aqueous electrolyte for lithium-ion batteries disclosed in patent CN 109346772 A, fluorinated carbonate solvents can reduce viscosity, improve wettability, and broaden the electrochemical window. However, their compatibility with sodium salts is poor, easily leading to sodium dendrite growth and increased interfacial impedance, making it difficult to meet the long-cycle requirements of high-voltage sodium-ion batteries. Compared to traditional carbonate solvents, carboxylic acid ester solvents have moderate dielectric constants and viscosities, balancing sodium salt solubility and ion transport rates. Patent CN114520371 A discloses a non-aqueous electrolyte using a multi-component solvent system of carbonates and carboxylic acid esters, combined with nitrile compounds and film-forming additives, aiming to balance high and low temperature performance with high voltage stability. However, its solvent system is complex and relies on the synergistic effect of the carbonate component and lithium salt; its long-term cycle stability under high voltage in sodium batteries still needs to be verified. The single-component electrolyte for lithium batteries disclosed in patent CN117976989 A improves the antioxidant stability of the electrolyte and enhances the compatibility of the high-voltage positive electrode by fluorinating the molecular structure of carboxylic acid ester solvents. However, the single solvent has limited compatibility with the sodium negative electrode interface, making it difficult to form a dense and stable SEI film, and the high-voltage cycle stability still needs to be improved.

[0004] It is evident that existing technologies, in order to address the problems of insufficient antioxidant stability and poor interfacial compatibility of carbonate electrolytes in high-voltage sodium-ion batteries, often employ strategies such as fluorinated solvents, multi-component solvent blends, or synergistic effects of multiple film-forming additives. However, these approaches generally suffer from drawbacks such as complex compositions, mutual constraints between functional additives, and limited interfacial compatibility with the sodium anode, making it difficult to simultaneously achieve a dense electrode-electrolyte interface film and long-term cycle stability during high-voltage cycling. Therefore, it is imperative to seek a sodium-ion electrolyte that combines high antioxidant stability with good interfacial film-forming ability. This patent introduces acrylate compounds as co-solvents into the carbonate-based electrolyte system, utilizing their β-dicarbonyl double-coordination chelate structure to regulate the Na⁺ solvation sheath layer, fundamentally suppressing solvent decomposition under high voltage and inducing the formation of a stable electrode-electrolyte interface film, enabling sodium-ion batteries to achieve both long cycle life and high safety under high voltage. Summary of the Invention

[0005] To address the problem of insufficient antioxidant properties of carbonate-based electrolytes under high voltage, this invention proposes a high-voltage sodium-ion battery electrolyte, its preparation method, and a sodium-ion battery. A class of mixed ester solvent electrolytes was constructed, and a novel high-voltage sodium-ion battery electrolyte with both high voltage stability and interfacial compatibility was developed.

[0006] The technical solution of this invention is implemented as follows:

[0007] In a first aspect, the present invention provides an electrolyte for high-voltage sodium-ion batteries, comprising an organic solvent, an electrolyte sodium salt, and additives. The organic solvent includes malonate compounds and carbonate compounds. The present invention introduces malonate compounds with a dicarbonyl structure into a carbonate electrolyte system, constructing a type of mixed ester solvent electrolyte.

[0008] Further, the malonic acid ester compounds include one or more of diethyl isopropyl malonate (iPrM), diisopropyl malonate (DIPM), di-tert-butyl malonate (DTBM), and diethyl phenyl malonate (PM); the carbonate compounds include one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC).

[0009] Furthermore, the electrolyte sodium salt is one or more of sodium hexafluorophosphate (NaPF6), sodium difluorooxalate borate (NaDFOB), sodium perchlorate (NaClO4), and sodium trifluoromethanesulfonate (NaOTf).

[0010] Furthermore, the film-forming additive is fluoroethylene carbonate (FEC) or N,N-dimethyltrifluoroacetamide (FDMA).

[0011] Further, the molar volume ratio of the electrolyte sodium salt to the organic solvent is 1-2 mol: 1L; the malonic acid ester compound accounts for 20%-80% of the total volume of the organic solvent; and the amount of the film-forming additive is 5-10 wt% of the total mass of the electrolyte.

[0012] Secondly, the present invention also provides a method for preparing the above-mentioned malonate-based high-voltage sodium-ion battery electrolyte, comprising the following steps:

[0013] (1) After drying and removing water from the organic solvent malonate compound and carbonate compound, the organic solvent is prepared by mixing them;

[0014] (2) Under a dry inert gas protective atmosphere, electrolyte sodium salt and film-forming additive are added to the organic solvent in sequence and mixed evenly to prepare the electrolyte for the high-voltage sodium-ion battery.

[0015] Furthermore, in step (1), all organic solvents are used after being dehydrated by passing them through a 4-Å molecular sieve; the method for preparing the organic solvent is to mix and stir one or more of the malonate esters and carbonates until the solution is clear and transparent.

[0016] Further, in step (2), the conditions for the dry inert atmosphere are H2O < 1 ppm and O2 < 1 ppm; the inert gases include argon, nitrogen and helium; in step (1), first add the electrolyte sodium salt and stir until it is mixed evenly, remove impurities, and obtain a transparent solution; then add the film-forming additive and stir until it is transparent.

[0017] Thirdly, the present invention also provides a sodium-ion battery, comprising a positive electrode, a separator, a negative electrode, and the malonate-based high-voltage sodium-ion battery electrolyte.

[0018] Furthermore, the active material of the positive electrode includes any one of sodium vanadium fluorophosphate (NVPF), sodium vanadium phosphate (NVP), sodium iron pyrophosphate (NFP), sodium iron pyrophosphate (NFPP), sodium vanadium fluorophosphate (NVOPF), sodium vanadium phosphate (NVOP), sodium ferrous sulfate (NFS), layered oxides, and organic carbonyl cathode materials.

[0019] Furthermore, the negative electrode is a metallic sodium negative electrode or hard carbon.

[0020] Furthermore, the sodium-ion battery includes a coin cell sodium-ion battery, a pouch sodium-ion battery, a square sodium-ion battery, or a cylindrical sodium-ion battery.

[0021] The present invention has the following beneficial effects:

[0022] 1. In the high-voltage sodium-ion battery electrolyte provided by the present invention, malonate compounds with a double carbonyl structure, especially diethyl isopropylmalonate, are introduced into the carbonate system, which makes the electrolyte have high antioxidant stability and good interfacial film-forming ability, thereby improving the overall performance of the electrolyte.

[0023] 2. The high-voltage sodium-ion battery electrolyte provided by this invention utilizes a synergistic mechanism of "bidentant coordination + steric hindrance" of malonic acid ester compounds to construct a Na⁺ solvation structure with low coordination number, loose structure, and high anion participation. This reduces the Na⁺ desolvation energy barrier and interfacial transport impedance, while simultaneously promoting the reduction and decomposition of PF6⁻. This forms a dense CEI film rich in NaF and carboxylates at the cathode interface, effectively suppressing the oxidative decomposition of the electrolyte under high voltage. Consequently, this achieves excellent long-cycle stability of the sodium-ion battery under high voltage, making it suitable for long-life sodium-ion batteries using various high-voltage cathode material systems.

[0024] 3. The malonate-based high-voltage sodium-ion battery electrolyte provided by this invention has wide applicability and can be well matched with a variety of cathode materials, including high-voltage cathode materials such as NVPF, NVOPF, and NFS, as well as other polyanionic cathode materials such as NVP, NFP, NFPP, and NVOP, P2 layered oxide cathode materials, and organic carbonyl cathode materials.

[0025] 4. The malonate-based high-voltage sodium-ion battery electrolyte provided by this invention exhibits excellent long-term cycling stability. The assembled Na||iPrM-PF||NVPF battery retains 89.18% capacity after 2000 cycles at 10 C within a voltage range of 2-4.5 V; the assembled Na||NFPP battery shows almost no capacity decay after 4000 cycles. This electrolyte possesses significant research value and application prospects.

[0026] 5. The malonate-based high-voltage sodium-ion battery electrolyte provided by this invention uses sodium hexafluorophosphate, sodium difluorooxalate borate, sodium perchlorate, and sodium trifluoromethanesulfonate as the electrolyte sodium salt, or one or a mixture thereof. The selected sodium salts all possess high oxidation decomposition potentials and are compatible with high-voltage cathode materials such as sodium vanadium fluorophosphate, sodium vanadium oxyphosphate, sodium vanadium oxyphosphate, and sodium ferrous sulfate, thereby improving the battery's energy density.

[0027] 6. The electrolyte preparation process for high-voltage sodium-ion batteries provided by this invention is simple and easy to implement, has no special or demanding requirements for production equipment, is highly compatible with existing industrial preparation processes, and has excellent value for industrial mass production applications. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 Linear sweep voltammetry curves of the electrolytes in Example 1 and Comparative Example 1 are shown.

[0030] Figure 2 Rate performance of Na||NVPF batteries assembled with electrolytes from Example 1 and Comparative Example 1 at room temperature.

[0031] Figure 3 The graph shows the cycling performance of Na||NVPF batteries assembled with electrolytes from Example 1 and Comparative Example 1 at a rate of 10 C at room temperature.

[0032] Figure 4 The charge-discharge curves of the Na||NVPF battery assembled with the electrolyte in Example 1 at 10 C rate at room temperature are shown.

[0033] Figure 5 The graph shows the cycling performance of the HC||NVPF battery assembled with the electrolyte in Example 1 at a rate of 0.5 C at room temperature.

[0034] Figure 6 The graph shows the cycling performance of the Na||NFPP battery assembled with the electrolyte in Example 1 at 5 C rate and room temperature.

[0035] Figure 7 The graph shows the cycling performance of the Na||NFS battery assembled with the electrolyte in Example 1 at 5 C rate and room temperature.

[0036] Figure 8 The graph shows the cycling performance of the Na||NFS battery assembled with the electrolyte in Example 1 at 10 C rate at room temperature.

[0037] Figure 9 The images show the electrochemical impedance spectroscopy of the electrolytes in Examples 1, 2, 3, and 4, with insets showing a comparison of ionic conductivity.

[0038] Figure 10 The linear sweep voltammetry curves for the electrolyte in Example 2 are shown.

[0039] Figure 11 Rate performance of Na||NVPF battery assembled with electrolyte in Example 3 at room temperature. Detailed Implementation

[0040] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0041] Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; the materials and reagents used are commercially available unless otherwise specified.

[0042] The malonate-based high-voltage sodium-ion battery electrolyte provided by this invention comprises an organic solvent, an electrolyte sodium salt, and additives. The organic solvent includes malonate compounds and carbonate compounds.

[0043] The malonic acid ester organic solvents provided in the embodiments of the present invention include diethyl isopropyl malonate (iPrM), diisopropyl malonate (DIPM), di-tert-butyl malonate (DTBM), and diethyl phenyl malonate (PM).

[0044] The sodium-ion battery provided in this embodiment of the invention includes a positive electrode, a separator, a negative electrode, and the above-mentioned malonate-based high-voltage sodium-ion battery electrolyte.

[0045] In some embodiments, the active material used in the positive electrode includes one of sodium vanadium fluorophosphate, sodium vanadium phosphate, sodium iron pyrophosphate, sodium iron pyrophosphate, sodium vanadium fluorophosphate, sodium vanadium phosphate, sodium ferrous sulfate, and layered oxides.

[0046] In some embodiments, the negative electrode is a metallic sodium negative electrode or hard carbon.

[0047] In some embodiments, the sodium-ion battery includes a coin cell sodium-ion battery, a pouch sodium-ion battery, a square sodium-ion battery, or a cylindrical sodium-ion battery.

[0048] The battery manufacturing steps used in this invention are as follows:

[0049] I. Preparation of Battery Electrodes

[0050] 1. Preparation of positive electrode sheets such as NVPF / NVP / NVOP / NVOPF / NFS / NFP / NFPP, etc.

[0051] The positive electrode active material, conductive carbon (Super P Li), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1, with 1-methyl-2-pyrrolidone (NMP) as the solvent. After thorough mixing, the mixture was coated onto a 17 μm thick aluminum foil using a doctor blade and dried in a vacuum oven at 120 °C for 8 h. After drying, the foil was cut into circular electrodes with a diameter of 13 mm using a die-cutting machine. The active material loading of the electrodes was 1.3–2.0 mg cm⁻¹. -2 .

[0052] 2. Preparation of hard carbon negative electrode sheet

[0053] Hard carbon, Super P Li, and PVDF were mixed in a mass ratio of 8:1:1, using NMP as a solvent. After thorough mixing, the mixture was coated onto a 17 μm thick copper foil using a doctor blade and dried overnight at 60°C in a vacuum oven. After drying, the coated electrodes were cut into circular sheets with a diameter of 13 mm using a die-cutting machine. The active material loading of the electrodes was 0.8–2 mg cm⁻¹. -2 .

[0054] II. Battery Assembly

[0055] The positive electrode material is one or more of the materials mentioned above, the negative electrode material is sodium metal or hard carbon, the current collector is aluminum foil or copper foil, the separator is glass fiber (Whatman, pore size 2.7 μm), and the electrolyte volume is 100-120 μL. A sodium-ion battery is assembled. All operations during battery assembly are performed in a glove box (O2 and H2O < 1 ppm).

[0056] The implementation method of the present invention will be explained below with reference to specific embodiments:

[0057] Example 1

[0058] The electrolyte formulation for the high-voltage sodium-ion battery in this embodiment is as follows: the molar volume ratio of NaPF6 to organic solvent is 1 mol: 1 L, the organic solvent is a 1:1 volume ratio of iPrM (diethyl isopropyl malonate) / PC (propylene carbonate) mixed solvent, and 5 wt.% FEC (fluoroethylene carbonate) is added to the total electrolyte mass.

[0059] The specific manufacturing steps of the embodiment are as follows:

[0060] Step S1: Prepare the solvent by adding 50 parts of diethyl isopropyl malonate and then 50 parts of propylene carbonate, and stirring for 10 minutes until the mixture is homogeneous.

[0061] Step S2: Add sodium salt. Weigh sodium hexafluorophosphate according to the molar volume ratio of sodium salt to solvent of 1 mol: 1 L. After adding sodium salt to solvent, stir at a constant speed for 30 min until the mixture is uniform and a transparent solution is obtained.

[0062] Step S3: Add film-forming additives. Select FEC and add it to the solvent at 5% of the total mass of the prepared electrolyte. Stir at a constant speed for 10 minutes until the solution is transparent.

[0063] The electrolyte described in Example 1 has a high decomposition voltage of 4.5 V ( Figure 1 This technology can be matched with high-voltage cathode materials. The assembled Na||NVPF battery exhibits excellent rate performance and long-term cycle stability, with an average discharge specific capacity of 124.0 mAhg at room temperature at rates of 0.1 C, 0.2 C, 0.5 C, 1.0 C, 2.0 C, 3.0 C, 4.0 C, 5.0 C, and 10.0 C. -1 122.0 mAh g -1 120.7 mAh g -1 120.0 mAh g -1 119.3 mAh g -1 118.9 mAh g -1 118.8mAh g -1 118.3 mAh g - 1 and 115.7 mAh g -1 After returning from 10C to 0.1C, the discharge specific capacity remains at 124.0 mAh g. -1 This indicates that the electrolyte has good rate reversibility. Figure 2 ). Figure 3 The graph shows the cycling performance at a 10 C rate. The discharge specific capacity after 2000 cycles is 101.82 mAh g. -1 The capacity retention rate was 89.18%, demonstrating good cycle stability. The corresponding charge-discharge curves ( Figure 4 The results showed that the charge-discharge plateau of the battery at 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800 and 2000 cycles did not show significant shortening or shift during long cycles, exhibiting smaller polarization and better reversibility.

[0064] Furthermore, the high-voltage sodium-ion battery electrolyte formulated in this case study also demonstrated certain potential in practical applications. The assembled HC||NVPF full cell maintained a capacity of 107.07 mAh g⁻¹ after 236 cycles at 0.5C. -1 The discharge specific capacity, coulombic efficiency reached 99.47%, and capacity retention was 84.08%. Figure 5 It exhibits good cycle stability and electrochemical reversibility.

[0065] The malonate-based high-voltage sodium-ion battery electrolyte formulated in this case can be matched with various cathode materials. Besides NVPF, batteries assembled with NFPP and NFS and sodium metal also exhibit good cycle stability. The assembled Na||NFPP battery can stably cycle for over 4000 cycles at 5 C rate at room temperature, with no significant decrease in discharge specific capacity. Figure 6 The assembled Na||NFS cell retained 94.69% of its capacity after 500 cycles at 5 C rate at room temperature. Figure 7 Furthermore, after 300 cycles at a high current of 10C, the capacity retention rate is 90.99%. Figure 8 The electrolyte system in this embodiment still exhibits good cycle stability, indicating that it is well-suited to different cathode materials.

[0066] Furthermore, the electrolyte system prepared in this case exhibited the lowest Z' value and 4.1 mS cm⁻¹. -1 High ionic conductivity ( Figure 9 This indicates that it has good compatibility with high-voltage cathode interfaces and excellent ion transport kinetics.

[0067] Comparative Example 1

[0068] The electrolyte formulation for this comparative sodium-ion battery is as follows:

[0069] The molar volume ratio of NaPF6 to the organic solvent is 1 mol: 1 L, the organic solvent is PC (propylene carbonate), and 5 wt.% FEC (fluoroethylene carbonate) of the total electrolyte mass is added.

[0070] The specific steps for creating the comparison scale are as follows:

[0071] Step S1: Prepare the solvent by adding 100 parts of propylene carbonate;

[0072] Step S2: Add sodium salt. Weigh sodium hexafluorophosphate according to the molar volume ratio of sodium salt to solvent of 1 mol: 1 L. After adding sodium salt to solvent, stir at a constant speed for 30 min until the mixture is uniform and a transparent solution is obtained.

[0073] Step S3: Add film-forming additives. Select FEC and add it to the solvent at 5% of the total mass of the prepared electrolyte. Stir at a constant speed for 10 minutes until the solution is transparent.

[0074] The decomposition voltage of the electrolyte in Comparative Example 1 was only 3.5 V. Figure 1 The electrolyte is not sufficiently compatible with the high-voltage cathode material and cannot meet the requirements of high-voltage systems. In multi-rate tests at room temperature (0.1C~10.0C), the average discharge specific capacity of the assembled Na||NVPF battery was inferior to that of Example 1 (…). Figure 2). Figure 3 The cycling performance diagram at 10 C rate shows that the capacity retention after 2000 cycles is only 69.68%, which is significantly lower than that of Example 1. Moreover, the coulombic efficiency of the battery in Comparative Example 1 is extremely unstable.

[0075] The battery performance of Implementation 1 and Comparative Example 1 is summarized in Table 1:

[0076] Table 1. Performance summary of sodium-ion batteries assembled in Example 1 and Comparative Example 1

[0077]

[0078] As can be seen from Table 1, Example 1 can be matched with a variety of cathode materials. Sodium-ion half-cells and full cells assembled with the high-voltage cathode material NVPF have good cycle stability, and sodium-ion batteries assembled with other high-voltage cathode materials NFS and NFPP also have good cycle performance.

[0079] Example 2

[0080] The electrolyte formulation for the high-voltage sodium-ion battery in this embodiment is as follows:

[0081] The molar volume ratio of NaPF6 to the organic solvent is 1 mol: 1 L. The organic solvent is a DIPM (diisopropyl malonate) / PC (propylene carbonate) mixed solvent with a volume ratio of 1:1, and 5 wt.% FEC (fluoroethylene carbonate) is added to the total electrolyte mass.

[0082] The specific manufacturing steps of the embodiment are as follows:

[0083] Step S1: Prepare the solvent by adding 50 parts of DIPM (diisopropyl malonate) and then adding 50 parts of propylene carbonate. Stir for 10 minutes until the mixture is homogeneous.

[0084] Step S2: Add sodium salt. Weigh sodium hexafluorophosphate according to the molar volume ratio of sodium salt to solvent of 1 mol: 1 L. After adding sodium salt to solvent, stir at a constant speed for 30 min until the mixture is uniform and a transparent solution is obtained.

[0085] Step S3: Add film-forming additives. Select FEC and add it to the solvent at 5% of the total mass of the prepared electrolyte. Stir at a constant speed for 10 minutes until the solution is transparent.

[0086] The electrolyte described in Example 2 has an ionic conductivity of 3.3 mS / cm at room temperature. -1 ( Figure 9 ), and has a high decomposition voltage of 4.4 V ( Figure 10 It can be matched with high-voltage cathode materials.

[0087] Example 3

[0088] The electrolyte formulation for the high-voltage sodium-ion battery in this embodiment is as follows:

[0089] The molar volume ratio of NaPF6 to the organic solvent is 1 mol: 1 L. The organic solvent is a 1:1 volume ratio of PM (diethyl phenylmalonate) / PC (propylene carbonate) mixed solvent, and 5 wt.% FEC (fluoroethylene carbonate) of the total electrolyte mass is added.

[0090] The specific manufacturing steps of the embodiment are as follows:

[0091] Step S1: Prepare the solvent by adding 50 parts of PM (diethyl phenylmalonate) and then adding 50 parts of propylene carbonate. Stir for 10 minutes until the mixture is homogeneous.

[0092] Step S2: Add sodium salt. Weigh sodium hexafluorophosphate according to the molar volume ratio of sodium salt to solvent of 1 mol: 1 L. After adding sodium salt to solvent, stir at a constant speed for 30 min until the mixture is uniform and a transparent solution is obtained.

[0093] Step S3: Add film-forming additives. Select FEC and add it to the solvent at 5% of the total mass of the prepared electrolyte. Stir at a constant speed for 10 minutes until the solution is transparent.

[0094] The electrolyte described in Example 3 has an ionic conductivity of 2.5 mS / cm at room temperature. -1 ( Figure 9 The assembled Na||NVPF cells had an average specific capacity of 121.5 mAh g at room temperature and at rates of 0.5 C, 1.0 C, 2.0 C, 3.0 C, 4.0 C, and 5.0 C. -1 121.6 mAh g -1 116.9 mAh g -1 115.4 mAh g -1 114.6 mAh g -1 and 113.8 mAhg -1 ( Figure 11 This indicates good rate capability.

[0095] Example 4

[0096] The electrolyte formulation for the high-voltage sodium-ion battery in this embodiment is as follows:

[0097] The molar volume ratio of NaPF6 to the organic solvent is 1 mol: 1 L. The organic solvent is a 1:1 volume ratio of DTBM (di-tert-butyl malonate) / PC (propylene carbonate) mixed solvent, and 5 wt.% FEC (fluoroethylene carbonate) is added to the total electrolyte mass.

[0098] The specific manufacturing steps of the embodiment are as follows:

[0099] Step S1: Prepare the solvent by adding 50 parts of DTBM (di-tert-butyl malonate) and then adding 50 parts of propylene carbonate. Stir for 10 minutes until the mixture is homogeneous.

[0100] Step S2: Add sodium salt. Weigh sodium hexafluorophosphate according to the molar volume ratio of sodium salt to solvent of 1 mol: 1 L. After adding sodium salt to solvent, stir at a constant speed for 30 min until the mixture is uniform and a transparent solution is obtained.

[0101] Step S3: Add film-forming additives. Select FEC and add it to the solvent at 5% of the total mass of the prepared electrolyte. Stir at a constant speed for 10 minutes until the solution is transparent.

[0102] The electrolyte prepared in Example 4 has an ionic conductivity of 2.7 mS / cm at room temperature. -1 ( Figure 9 Its low interfacial impedance and high ionic conductivity indicate that it can achieve excellent cycle stability and rate performance at high cutoff voltages.

[0103] Comparative Example 2

[0104] The electrolyte formulation for this comparative sodium-ion battery is as follows:

[0105] The molar volume ratio of NaPF6 to organic solvent is 1 mol: 1 L, the organic solvent is iPrM (diethyl isopropyl malonate), and 5 wt.% FEC (ethylene fluorocarbonate) of the total electrolyte mass is added.

[0106] The specific steps for creating the comparison scale are as follows:

[0107] Step S1: Prepare the solvent by adding 100 parts of diethyl isopropyl malonate;

[0108] Step S2: Add sodium salt. Weigh sodium hexafluorophosphate according to the molar volume ratio of sodium salt to solvent of 1 mol: 1 L. After adding sodium salt to solvent, stir at a constant speed for 30 min. It was found that the sodium salt did not dissolve completely and a transparent solution could not be obtained.

[0109] Step S3: Add film-forming additives. Select FEC and add it to the solvent at 5% of the total mass of the electrolyte. Stir at a constant speed for 10 minutes or until a uniform and transparent solution is obtained.

[0110] Therefore, simple malonic acid ester solvents are not suitable for use as electrolytes.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An electrolyte for high-voltage sodium-ion batteries, characterized by: It includes organic solvents, sodium electrolyte salts, and film-forming additives; the organic solvents include malonic acid esters and carbonate compounds.

2. The electrolyte for high-voltage sodium-ion batteries according to claim 1, characterized in that: The malonic acid esters are one or more of diethyl isopropyl malonate, diisopropyl malonate, di-tert-butyl malonate, and diethyl phenyl malonate; the carbonates are one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

3. The electrolyte for high-voltage sodium-ion batteries according to claim 2, characterized in that: The electrolyte sodium salt is one or more of sodium hexafluorophosphate, sodium difluorooxalate borate, sodium perchlorate, and sodium trifluoromethanesulfonate.

4. The electrolyte for high-voltage sodium-ion batteries according to claim 3, characterized in that: The film-forming additive is fluoroethylene carbonate or N,N-dimethyltrifluoroacetamide.

5. The electrolyte for high-voltage sodium-ion batteries according to claim 4, characterized in that: The molar volume ratio of the electrolyte sodium salt to the organic solvent is 1-2 mol: 1L; the malonic acid ester compound accounts for 20%-80% of the total volume of the organic solvent; and the amount of film-forming additive is 5 wt%-10 wt% of the total mass of the electrolyte.

6. The method for preparing the electrolyte for a high-voltage sodium-ion battery according to any one of claims 1 to 5, characterized in that, The steps are as follows: (1) Prepare an organic solvent by drying and removing water from malonate compounds and carbonate compounds; (2) Under a dry and inert atmosphere, electrolyte sodium salt and film-forming additive are added to the organic solvent in step (1) in sequence and mixed evenly to obtain electrolyte for high-voltage sodium-ion batteries.

7. The method for preparing the electrolyte for a high-voltage sodium-ion battery according to claim 6, characterized in that: In step (2), the dry inert atmosphere contains H2O < 1 ppm and O2 < 1 ppm, and the inert atmosphere is argon, nitrogen or helium.

8. The method for preparing the electrolyte for a high-voltage sodium-ion battery according to claim 7, characterized in that: The electrolyte used in the high-voltage sodium-ion battery is transparent.

9. A sodium-ion battery, characterized in that: It includes a positive electrode, a separator, a negative electrode, and the electrolyte for a high-voltage sodium-ion battery as described in any one of claims 1 to 5.

10. The sodium-ion battery according to claim 9, characterized in that: The positive electrode active material is any one of sodium vanadium fluorophosphate, sodium vanadium phosphate, sodium iron pyrophosphate, sodium iron pyrophosphate, sodium vanadium fluorophosphate, sodium vanadium oxyfluoride, sodium vanadium oxyfluoride, sodium ferrous sulfate, layered oxides, and organic carbonyl groups; the negative electrode is a metallic sodium negative electrode or hard carbon.

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