Anion-cation synergistically regulated non-flammable phosphate-based electrolyte and application thereof in sodium battery

Abstract

The application belongs to the technical field of electrochemical energy storage, and particularly relates to a non-flammable phosphate-based electrolyte for synergistically regulating anions and cations and application of the non-flammable phosphate-based electrolyte in sodium batteries. Through synergistic effect of NaDFOB main salt and Sn(OTf)2 and NaPO2F2 double additives, efficient application of the phosphate-based electrolyte in sodium metal batteries is realized. The prepared DF-33 electrolyte has a solventization structure rich in anions, high ion transference number, excellent negative electrode / positive electrode interface stability, wide temperature range adaptability and intrinsic non-flammability, and exhibits electrochemical performance significantly better than that of the prior art in a symmetric battery, a half battery and a full battery, successfully solving technical problems such as poor compatibility of the phosphate-based electrolyte with the sodium metal negative electrode, slow ion transport kinetics and short cycle life, and having a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, specifically relating to a non-flammable phosphate-based electrolyte with synergistic regulation of anions and cations and its application in sodium batteries. Background Technology

[0002] Sodium metal batteries are known for their extremely high theoretical specific capacity (1166 mAh g). -1 Sodium metal has been extensively studied due to its low redox potential (-2.71V vs. standard hydrogen electrode) and energy density, which is far higher than that of traditional anode materials. However, sodium metal faces problems such as interfacial side reactions, capacity decay, and short circuits caused by sodium dendrite growth in practical applications. Although ether electrolytes have good anode compatibility, their low boiling point and high flammability limit their commercial application.

[0003] Currently, phosphate ester electrolytes (such as trimethyl phosphate, TMP) have become a research hotspot due to their ease of dissociation at high temperatures to generate phosphorus radicals, which can rapidly interrupt the combustion reaction of a flame to achieve a non-flammable effect. However, their poor compatibility with sodium metal anodes and low ionic conductivity limit their practical applications. Existing improvement strategies, such as high-concentration salts, locally high-concentration electrolytes, or co-solvents, still suffer from problems such as high viscosity, high cost, and poor low-temperature performance. Therefore, developing a low-cost, high-efficiency additive electrolyte system is of significant research value. Summary of the Invention

[0004] To address the needs of existing technologies, the purpose of this invention is to provide a non-flammable phosphate-based electrolyte with synergistic cation and anion regulation and its application in sodium batteries. This invention addresses the problems of poor compatibility and slow kinetics between phosphate-based electrolytes and sodium metal by introducing novel electrolyte salts and additives into TMP-based electrolytes and utilizing the anchoring of anions and cations and intermolecular interactions.

[0005] Specifically, the present invention provides the following technical solution: In a first aspect, the present invention provides a non-flammable phosphate-based electrolyte with synergistic regulation of anions and cations, the electrolyte comprising: a solvent, an electrolyte salt, an alloying additive, and a film-forming additive; wherein the solvent is trimethyl phosphate, the electrolyte salt is sodium difluorooxalate borate, the alloying additive is tin trifluoromethanesulfonate, and the film-forming additive is sodium difluorophosphate. The concentration of sodium difluorooxalate borate in the electrolyte is 1.8~2 mol / L; The tin trifluoromethanesulfonate comprises 2.5-4% of the total mass of the solvent and electrolyte salt; The sodium difluorophosphate accounts for 2.5-4% of the total mass of the solvent and electrolyte salt.

[0006] Preferably, the concentration of sodium difluorooxalate borate in the electrolyte is 2 mol / L, the tin trifluoromethanesulfonate accounts for 3% of the total mass of the solvent and electrolyte salt, and the sodium difluorophosphate accounts for 3% of the total mass of the solvent and electrolyte salt.

[0007] Preferably, the electrolyte has at least one of the following characteristics: Ion transport number ≥ 0.57; In a Na‖Na symmetric cell, stable cycling exceeds 1100 hours; In Na-Al@C half-cells, the coulombic efficiency is ≥97%; In Na-NVP full cells, the capacity retention is ≥87% after 2000 cycles at 3 C rate; In Na-NVPOF full cells, the capacity retention is ≥75% after 1000 cycles at 1 C rate; Stable cycling within the range of -10℃ to 60℃.

[0008] A second aspect of the present invention provides a method for preparing the electrolyte described in the first aspect, characterized by comprising the following steps: mixing trimethyl phosphate, sodium difluorooxalate borate, tin trifluoromethanesulfonate and sodium difluorophosphate under a protective atmosphere, and stirring until completely dissolved to obtain the electrolyte.

[0009] Preferably, the protective atmosphere has a water content of less than 0.01 ppm and an oxygen content of less than 0.01 ppm.

[0010] Preferably, the stirring step includes: first stirring at a speed of 300-500 rpm for 30-60 min to completely dissolve sodium difluorooxalate borate, then adding tin trifluoromethanesulfonate and sodium difluorophosphate and continuing to stir for 20-30 min until completely dissolved.

[0011] A third aspect of the present invention provides an application of the electrolyte described in the first aspect in a sodium metal battery.

[0012] A fourth aspect of the present invention provides a sodium metal battery comprising the electrolyte described in the first aspect.

[0013] Preferably, the positive electrode of the sodium metal battery is sodium vanadium phosphate or sodium vanadium fluoride phosphate.

[0014] Preferably, the sodium metal battery is a button cell.

[0015] The beneficial effects achieved by one or more of the above technical solutions of the present invention are as follows: (1) This invention, through synergistic regulation of anions and cations, produces an electrolyte that constructs an inorganic SEI layer rich in Na₂O / NaF on the surface of the sodium metal anode, and induces the formation of a Na-Sn alloy, significantly reducing the sodium deposition overpotential and inhibiting dendrite growth. For example, in a Na||Na symmetric cell at 1 mA cm⁻¹ -2 Stable cycling at current density for over 1100 hours (excellent interface stability); Na-Al@C half-cell coulombic efficiency as high as 97.27%, indicating that sodium deposition / stripping has extremely high reversibility (efficient sodium deposition / stripping).

[0016] (2) The electrolyte of the present invention effectively solves the problem of poor compatibility between phosphate ester-based electrolyte and sodium metal anode, so that the full cell exhibits excellent cycle stability at room temperature and high rate. That is, the Na-NVP full cell has a capacity retention rate of 87.5% and an average coulombic efficiency of 99.75% after 2000 cycles at 3 C rate.

[0017] (3) DFOB in the electrolyte prepared by this invention - and PO2F2 - Anions are preferentially oxidized on the positive electrode side, forming a thin and dense positive electrode electrolyte interface (CEI) layer (only 7.7 nm thick), which effectively suppresses the oxidative decomposition of the electrolyte under high voltage. For example, the Na-NVPOF full cell retains 75% of its capacity after 1000 cycles at a high cutoff voltage (high voltage compatibility).

[0018] (4) The electrolyte prepared by this invention can maintain good ion transport performance and interface stability over a wide temperature range. For example, the Na-NVP full cell can be stably cycled for 500 cycles at a high temperature of 60℃ with a capacity retention of 72%; and can be stably cycled for 160 cycles at a low temperature of -10℃ with a capacity retention of up to 82.5%.

[0019] (5) The present invention uses trimethyl phosphate as a single solvent, which readily dissociates at high temperatures to generate P free radicals, which can quickly cut off the combustion reaction of the flame. For example, the electrolyte cannot be ignited in the ignition test, and the self-extinguishing time is 0 s / g, which has intrinsic non-flammable properties, significantly improving the safety of the battery.

[0020] (6) This invention utilizes DFOB in NaDFOB - The competitive coordination between the anion and TMP leads to the formation of anion-rich solvation structures, effectively weakening the Na+ ionization. + The strong interaction between -TMP increases the ion transference number.

[0021] (7) Compared with electrolytes that add Sn(OTf)2 or NaPO2F2 alone, the present invention achieves a comprehensive improvement of all the above-mentioned beneficial effects through the synergistic effect of the two additives combined with NaDFOB main salt. Individual additives or conventional sodium salts (such as NaPF6) cannot simultaneously meet the comprehensive performance requirements of high interfacial stability, long cycle life, high voltage compatibility and wide temperature range adaptability. Attached Figure Description

[0022] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0023] Figure 1 The Raman spectra of the electrolytes prepared in Example 1 and Comparative Examples 1-2 of this invention are shown below. Figure 2 The electrolytes prepared in Example 1 and Comparative Examples 1-2 of this invention 23 Na NMR spectrum; Figure 3 As described in Example 1 of this invention and pure TMP solvent 1 H nuclear magnetic resonance image; Figure 4 Examples 1 and 2 of the present invention 19 F nuclear magnetic resonance; Figure 5 The ion transport number is that of the electrolyte prepared in Example 1 of this invention; Figure 6 The voltage-time curve of the Na||Na symmetric cell assembled in Example 1 of this invention; Figure 7 The coulombic efficiency curves of the Na-Al@C half-cells assembled in Example 1 and Comparative Examples 1-4 of this invention are shown. Figure 8 The coulombic efficiency curves of the Na-Al@C half-cells assembled in Example 1, Comparative Example 2, and Comparative Examples 5-6 of this invention are shown. Figure 9 The coulombic efficiency curves of the Na-Al@C half-cells assembled in Example 1 and Comparative Examples 7-8 of this invention are shown. Figure 10 The coulomb efficiency curve of the Na-Al@C half-cell assembled in Example 1 of this invention under the Aurbach protocol method; Figure 11 The X-ray photoelectron spectrum of sodium deposited on the copper foil surface after constant current deposition of the Na-Cu half-cell assembled in Example 1 of this invention; Figure 12These are scanning electron microscope images of the sodium deposition layer on the surface of the sodium anode after cycling at different current densities in Example 1 and Comparative Examples 1-2 of the present invention. Figure 13 This is a graph showing the long-cycle performance of the Na-NVP full cell assembled in Example 1 of the present invention; Figure 14 These are scanning electron microscope images of the NVP cathode after cycling in Example 1 and Comparative Examples 1-2 of the present invention. Figure 15 This is a high-voltage cycling performance diagram of the Na-NVPOF full cell assembled in Example 1 of the present invention; Figure 16 These are transmission electron microscope images of the NVPOF cathode after cycling in Embodiment 1 and Comparative Example 2 of the present invention. Figure 17 The Na-NVP full cell assembled for Embodiment 1 of the present invention is in Long-cycle performance graphs at 10℃ and 60℃; Figure 18 This is an optical photograph of the electrolyte ignition test obtained in Example 1 of the present invention. Detailed Implementation

[0024] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] In a first typical embodiment of the present invention, a non-flammable phosphate-based electrolyte with synergistic regulation of anions and cations is provided. The electrolyte comprises: a solvent, an electrolyte salt, an alloying additive, and a film-forming additive. The solvent is trimethyl phosphate, the electrolyte salt is sodium difluorooxalate borate, the alloying additive is tin trifluoromethanesulfonate, and the film-forming additive is sodium difluorophosphate. The concentration of sodium difluorooxalate borate in the electrolyte is 1.8~2 mol / L; The tin trifluoromethanesulfonate comprises 2.5-4% of the total mass of the solvent and electrolyte salt; The sodium difluorophosphate accounts for 2.5-4% of the total mass of the solvent and electrolyte salt.

[0026] In one or more embodiments of this implementation, the concentration of sodium difluorooxalate borate in the electrolyte is 2 mol / L, the tin trifluoromethanesulfonate accounts for 3% of the total mass of the solvent and electrolyte salt, and the sodium difluorophosphate accounts for 3% of the total mass of the solvent and electrolyte salt.

[0027] In one or more embodiments of this implementation, the electrolyte has at least one of the following characteristics: Ion transport number ≥ 0.57; In a Na‖Na symmetric cell, stable cycling exceeds 1100 hours; In Na-Al@C half-cells, the coulombic efficiency is ≥97%; In Na-NVP full cells, the capacity retention is ≥87% after 2000 cycles at 3 C rate; In Na-NVPOF full cells, the capacity retention is ≥75% after 1000 cycles at 1 C rate; Stable cycling within the range of -10℃ to 60℃.

[0028] A second typical embodiment of the present invention provides a method for preparing the electrolyte described in the first typical embodiment, characterized by comprising the following steps: mixing trimethyl phosphate, sodium difluorooxalate borate, tin trifluoromethanesulfonate and sodium difluorophosphate under a protective atmosphere, and stirring until completely dissolved to obtain the electrolyte.

[0029] In one or more embodiments of this implementation, the protective atmosphere contains less than 0.01 ppm of water and less than 0.01 ppm of oxygen.

[0030] In one or more embodiments of this implementation, the stirring step includes: first stirring at a speed of 300-500 rpm for 30-60 min to completely dissolve sodium difluorooxalate borate, then adding tin trifluoromethanesulfonate and sodium difluorophosphate and continuing to stir for 20-30 min until completely dissolved.

[0031] The third typical embodiment of the present invention provides an application of the electrolyte described in the first typical embodiment in a sodium metal battery.

[0032] A fourth typical embodiment of the present invention provides a sodium metal battery comprising the electrolyte described in the first typical embodiment.

[0033] In one or more embodiments of this implementation, the positive electrode of the sodium metal battery is sodium vanadium phosphate or sodium vanadium fluoride phosphate.

[0034] In one or more embodiments of this implementation, the sodium metal battery is a button cell.

[0035] In this invention, the electrolyte achieves interfacial stability and rapid kinetics through the following mechanism: (1) Solution structure regulation: DFOB in NaDFOB - Anions compete with TMP for coordination of Na. + This forms anion-rich contact ion pairs (CIPs) and aggregates (AGGs) structures, reducing Na+. +-TMP interaction, while the hydrogen bonding between the two additives and TMP further enhances Na + -DFOB - Interactions enhance electrolyte ion transport number and interfacial dynamics; (2) Negative electrode interface modification: Sn(OTf)2 is preferentially reduced on the sodium negative electrode surface to form Na-Sn alloy, which reduces the sodium deposition overpotential and induces uniform nucleation; NaPO2F2 decomposes to generate inorganic SEI rich in Na2O / NaF, which enhances the interface toughness and ion conduction. (3) Cathode interface modification: DFOB - and PO2F2 - Preferential oxidation on the positive electrode side forms a tough CEI layer, which inhibits electrolyte decomposition and improves high-voltage stability.

[0036] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0037] Example 1: This example provides a non-flammable phosphate-based electrolyte with synergistic regulation of anions and cations, and its preparation method. The specific preparation method is as follows: In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), 0.1598 g NaDFOB was dissolved in 500 μL TMP to form a 2 mol / L solvent. Then, Sn(OTf)2 and NaPO2F2 with a total mass fraction of 3% of the total mass of NaDFOB and TMP were added, and the mixture was stirred on a magnetic stirrer for 20 min until completely dissolved. This solution was labeled DF-33 (2 M-33).

[0038] Comparative Example 1: The difference between this comparative example and Example 1 is that the type of electrolyte salt is changed, and 0.1598 g NaDFOB is replaced with 0.1680 g NaPF6. The contents of other components and the preparation methods are the same as in Example 1, and it is labeled as PF-33.

[0039] Comparative Example 2: The difference between this comparative example and Example 1 is that no additives are added. Specifically, in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), 0.1598 g NaDFOB is dissolved in 500 μL TMP to form a 2 mol / L solvent, and then stirred on a magnetic stirrer for 20 min until completely dissolved. This solution is labeled as DF-00.

[0040] Comparative Example 3: The difference between this comparative example and Example 1 is that it uses only a single additive (Sn(OTf)2). Specifically, in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), 0.1598 g of NaDFOB was dissolved in 500 μL of TMP to form a 2 mol / L solvent. Then, Sn(OTf)2 with a total mass fraction of 3% of the total mass of NaDFOB and TMP was added. The mixture was then stirred on a magnetic stirrer for 20 min until completely dissolved and labeled as DF-03.

[0041] Comparative Example 4: The difference between this comparative example and Example 1 is that it uses only a single additive (NaPO2F2). Specifically, in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), 0.1598 g of NaDFOB was dissolved in 500 μL of TMP to form a 2 mol / L solvent. Then, NaPO2F2 with a total mass fraction of 3% of the total mass of NaDFOB and TMP was added. The mixture was then stirred on a magnetic stirrer for 20 min until completely dissolved. This mixture was labeled DF-30.

[0042] Comparative Example 5: The difference between this comparative example and Example 1 is the change in the amount of additives added. Specifically, in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), 0.1598 g of NaDFOB was dissolved in 500 μL of TMP to form a 2 mol / L solvent. Then, Sn(OTf)2 with a mass fraction of 2% of the total mass of NaDFOB and TMP and NaPO2F2 with a mass fraction of 2% were added respectively. The mixture was then stirred on a magnetic stirrer for 20 min until completely dissolved and labeled as DF-22.

[0043] Comparative Example 6: The difference between this comparative example and Example 1 is the change in the amount of additives added. Specifically, in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), 0.1598 g NaDFOB was dissolved in 500 μL TMP to form a 2 mol / L solvent. Then, Sn(OTf)2 with a mass fraction of 5% of the total mass of NaDFOB and TMP and NaPO2F2 with a mass fraction of 5% were added respectively. The mixture was then stirred on a magnetic stirrer for 20 min until completely dissolved. This mixture was labeled DF-55.

[0044] Comparative Example 7: The difference between this comparative example and Example 1 is that the concentration of the electrolyte salt is 1.5 mol / L, while the content of other components and the preparation method are the same as in Example 1, labeled as 1.5 M-33.

[0045] Comparative Example 8: The difference between this comparative example and Example 1 is that the concentration of the electrolyte salt is 1.7 mol / L, while the content of other components and the preparation method are the same as in Example 1, labeled as 1.7 M-33.

[0046] Experimental Example 1: This experimental example performs structural tests on the electrolytes prepared in the examples and comparative examples. like Figure 1 The figure shows the Raman spectral results of Example 1, Comparative Examples 1-2, and TMP. As can be seen from the figure, at 738 cm⁻¹... -1 The Raman peak at [location] corresponds to the structural vibration of POC in TMP. Compared to DF-33, PF-33 shows a larger blue shift, indicating that NaPF6 interacts more strongly with TMP than NaDFOB. This verifies that using NaDFOB as the main salt weakens the interaction between NaPF6 and TMP by the strongly solvated anion. + Interaction with solvents. Furthermore, DF-00 without additives also exhibited a relatively strong blue shift, meaning that the introduction of additives further weakened the Na+ reaction. + The interaction with TMP. This demonstrates the interaction between DFOB and TMP. - and PO2F2 - Under the synergistic effect of [them], the coordination number of TMP molecules in the main solvation structure decreases.

[0047] like Figure 2 As shown, the electrolytes prepared in Example 1 and Comparative Examples 1-2 are... 23 The NMR spectrum of sodium (Na) shows that, compared to DF-00 and PF-33, the sodium NMR peak exhibits the weakest high-field shift in DF-33. This indicates the weakest interaction between the solvent system and sodium in the electrolyte. In contrast, PF-33 using NaPF6 shows the strongest high-field shift. This suggests that the DF-33 electrolyte, using NaDFOB as the main salt and incorporating two additives, possesses a solvated structure rich in anions.

[0048] like Figure 3 As shown, Examples 1 and 2 are... 1 The 1H NMR spectrum shows that after introducing the additive into the pure TMP solvent, the characteristic peak of hydrogen belonging to -CH3 at around 3.6 ppm shows a significant high-field shift. This means that the chemical environment of the methyl hydrogen in TMP has changed due to the intervention of the additive molecules.

[0049] like Figure 4 As shown, Examples 1 and 2 are... 19The F NMR spectrum shows a significant high-field shift in DF-33, indicating a change in the chemical environment of the F atoms in the system. In hydrogen bonding interactions, the strong electron-withdrawing effect of fluorine causes electrons to concentrate towards the methyl hydrogen, thus weakening the coordination ability of P=O. This means that the addition of additives will affect the Na... + With DFOB - The interaction with TMP is further enhanced, while the interaction with TMP is weakened.

[0050] Experimental Example 2: This experimental example tests the performance of the electrolytes prepared in the examples and comparative examples. (1) Measurement of ion transport number in sodium symmetric cells The sodium ion transfer number in the electrolyte was determined using the Abraham method. t Na+ The specific steps are as follows: After assembling the Na||Na symmetric cell, it was tested using a CHI660E electrochemical workstation. First, a constant voltage of 20 mV was applied ( The record includes the initial current (I0) and the steady-state current (I0). s The polarization current of ) was measured. Simultaneously, the volume resistivity was measured before and after potentiostatic polarization using electrochemical impedance spectroscopy. and ) and electrode-electrolyte interface resistance ( and The initial and steady-state values ​​of ). t Na+ The value is calculated using the following formula:

[0051] The low ionic conductivity of phosphate esters typically leads to high battery polarization and slow liquid phase transport rates. Ion transference number is a good indicator of the electrolyte system's affinity for Na+. + The impact of transmission. For example... Figure 5 As shown in the figure, DF-33 exhibits a high ion mobility number of 0.577, mainly due to its anion-rich solvated structure.

[0052] (2) Stability test of sodium symmetric battery negative electrode The electrolyte prepared in Example 1 was added between two sodium plates to assemble a sodium symmetric battery. The current was 1 mA cm⁻¹. -2 Under these conditions, charging and discharging are performed.

[0053] Test results are as follows Figure 6 As shown, the voltage-time curves indicate that DF-33 at 1 mA cm⁻¹ -2It was able to cycle stably for over 1100 h at the specified current with minimal voltage hysteresis, demonstrating that the DF-33 system imparts excellent interfacial stability to the sodium metal anode, which is beneficial for the uniform deposition / stripping of sodium ions.

[0054] (3) Cyclic stability and dynamic environmental sodium consumption test of Na-Al@C half-cell The electrolytes prepared in Example 1 and Comparative Examples 1-8 were added between sodium sheets and copper foils to assemble Na-Al@C half-cells. The electrolytes were then used at 0.5 mA cm⁻¹. -2 Constant current deposition / stripping tests were performed at a current density.

[0055] Test results are as follows Figures 7-9 As shown in the time-coulombic efficiency curves, at a constant salt concentration, the half-cell exhibits the highest average coulombic efficiency when the additive ratio is 3%. With a constant additive ratio, the initial coulombic efficiency increases with increasing salt concentration, reaching a maximum at a salt concentration of 2 mol / L. -1 It reaches the electrolyte solubility limit at this point, and at this point it has the highest average coulombic efficiency.

[0056] Furthermore, the electrolyte prepared in Example 1 was added between the sodium sheet and Al@C to assemble a half-cell, and the electrolyte was tested at 0.5 mA / cm². -2 The Aurbach protocol method was used to perform charge-discharge simulations at a given current density to model sodium consumption behavior under dynamic conditions. The test results are as follows: Figure 10 As shown in the figure, the coulombic efficiency curve shows that DF-33 exhibits a high coulombic efficiency of 97.27%, which means that the DF-33 electrolyte has excellent compatibility with sodium sheets.

[0057] At the same time, by using Na + The average coulombic efficiency of different electrolyte types was compared by reversible deposition and stripping on Al@C surfaces. The calculation method was to divide the sum of the coulombic efficiency of each cycle by the number of cycles. The specific test results are shown in Table 1. DF-33 also showed a high coulombic efficiency of 90.9%.

[0058] Table 1

[0059] (4) Analysis and testing of interface composition of sodium symmetric battery The electrolytes prepared in Example 1 and Comparative Examples 1-2 were added between two sodium plates to assemble a sodium symmetric cell. The electrolyte was then used at 0.5 mA cm⁻¹. -2 0.5 mAh cm -2 Under constant current charge and discharge conditions for 20 h, the battery was then disassembled, and X-ray photoelectron spectroscopy was performed on the surface of the sodium sheet after cycling.

[0060] Test results are as follows Figure 11 As shown, the X-ray photoelectron spectroscopy of the sodium sheet reveals a higher content of Na₂O and NaF in DF-33, both of which contribute to higher ion transport rates, higher mechanical strength, and better electronic insulation properties. Simultaneously, a SnO signal was detected in the Sn 3d region of DF-33, confirming the formation of a Na-Sn alloy at the negative electrode.

[0061] (5) Na-Cu half-cell galvanic deposition test The electrolytes prepared in Example 1 and Comparative Examples 1-2 were added between sodium sheets and copper foils to assemble Na-Cu half-cells. The electrolytes were then used at 0.5 mA cm⁻¹. -2 1 mA cm -2 1.5 mA cm -2 The battery was then subjected to constant current deposition for 8 hours. After the battery was disassembled, the sodium deposition layer on the copper foil surface after cycling was observed using scanning electron microscopy.

[0062] like Figure 12 As shown in the SEM images, compared to the dendrite-covered and loose interface morphology of DF-00 and PF-33, DF-33 exhibits a more uniform and dense blocky sodium metal. Furthermore, with increasing current density, the deposition surface becomes smoother, with almost no visible pores or dendrites. In contrast, DF-00 and PF-33 show more severe dendrite growth at high current densities, and noodle-like dendrites and cracked dead sodium can be observed in both images. It can be seen that the DF33 electrolyte can induce uniform sodium deposition, while the continuous decomposition of the DF-00 and PF-33 electrolytes and severe interfacial side reactions result in poor interface morphology.

[0063] (6) Long-cycle performance test (Na-NVP full cell) Sodium vanadium phosphate, conductive carbon black, and polyvinylidene fluoride (PVDF) binder were mixed in a mass ratio of 8:1:1, with an appropriate amount of N-methylpyrrolidone (NMP) added as a solvent. The mixture was continuously stirred on a magnetic stirrer for 12 h until homogeneous. This mixture was then coated onto the surface of a carbon-coated aluminum foil current collector, and subsequently dried in a vacuum oven at 100°C for 12 h. The dried electrode was cut into 10 mm diameter discs for button cell assembly. The sodium vanadium phosphate areal loading was 5.6 mg / cm³. -2 The negative electrode is a sodium metal sheet. Test conditions: Test voltage 2.4-3.8 V, test rate: 3 C (1C = 117 mAh g). -1 ).

[0064] Test results are as follows Figure 13As shown in the figure, DF-33 exhibits the highest initial discharge capacity of 104.78 mAh g. -1 Furthermore, its discharge capacity is very stable, with a capacity retention rate of up to 87.5% after 2000 cycles and an average coulombic efficiency of 99.75%, which is mainly due to its excellent SEI and rapid full-cell dynamics.

[0065] In addition, such as Figure 14 The image shows scanning electron microscope (SEM) images of the NVP cathodes after cycling in Example 1 and Comparative Examples 1 and 2. The images show that the NVP cathode surface remained largely intact after cycling with DF-33, while cracks of varying sizes appeared on the surfaces of the DF-00 and PF-33 cathodes. This indicates that DF-33 forms a robust and stable interface at the cathode, which is beneficial to the long-term stability of the full cell.

[0066] (7) High-voltage cycling performance test (Na-NVPOF full cell) The preparation of the high-voltage positive electrode is the same as the preparation steps for the long-cycle performance test described above; only the positive electrode powder is replaced with NVPOF. The sodium vanadium fluorophosphate surface loading is 4 mg / cm³. -2 The negative electrode is a sodium metal sheet. Test conditions: Test voltage 2.5-4.3 V, test rate: 1 C (1 C = 127 mAh g). -1 ).

[0067] Test results are as follows Figure 15 As shown in the high-voltage cycling performance graph, DF-33 exhibits a performance of 103.86 mAh g⁻¹. -1 The initial discharge capacity is high, and it still has 75% capacity retention after 1000 stable cycles. This means that the DF-33 electrolyte can form a uniform CEI layer on the surface of the high-voltage positive electrode, which is beneficial to the stable cycling of the battery under high voltage conditions.

[0068] In addition, such as Figure 16 As shown, the transmission electron microscope images of the NVPOF cathode after cycling in Example 1 and Comparative Example 2 are shown. It can be seen from the figures that the CEI of the cathode using DF-33 electrolyte is very dense and uniform, with a thickness of only 7.7 nm, while the maximum thickness of DF-00 reaches 28.6 nm.

[0069] (8) Wide temperature range cycling performance test (Na-NVP full cell) The Na-NVP full cell was assembled using the same steps as in (5) above, and then its long-cycle performance at -10℃ and 60℃ was tested. The results are as follows Figure 17As shown, the Na-NVP battery using DF-33 cycled stably for 500 cycles while maintaining 72% capacity retention; at a low temperature of -10℃, the Na-NVP full cell cycled stably for 160 cycles while maintaining 82.5% capacity retention. This demonstrates the excellent temperature adaptability of the DF-33 electrolyte and verifies the superiority of the synergistic regulation strategy of anions and cations.

[0070] (8) Non-flammability test Test procedure: In a fume hood, the electrolyte of Example 1 was dropped into the negative electrode shell and ignited with a lighter for more than 20 seconds.

[0071] like Figure 18 The image shown is an optical photograph of the electrolyte ignition test in Example 1. Since the solvent in this electrolyte system is only TMP with a high flash point, it exhibits non-flammable properties and is intrinsically safe.

[0072] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A non-flammable phosphate-based electrolyte with synergistic regulation of anions and cations, characterized in that, The electrolyte comprises: a solvent, an electrolyte salt, an alloying additive, and a film-forming additive; the solvent is trimethyl phosphate, the electrolyte salt is sodium difluorooxalate borate, the alloying additive is tin trifluoromethanesulfonate, and the film-forming additive is sodium difluorophosphate. The concentration of sodium difluorooxalate borate in the electrolyte is 1.8~2 mol / L; The tin trifluoromethanesulfonate comprises 2.5-4% of the total mass of the solvent and electrolyte salt; The sodium difluorophosphate accounts for 2.5-4% of the total mass of the solvent and electrolyte salt.

2. The electrolyte according to claim 1, characterized in that, The concentration of sodium difluorooxalate borate in the electrolyte is 2 mol / L, the tin trifluoromethanesulfonate accounts for 3% of the total mass of the solvent and electrolyte salt, and the sodium difluorophosphate accounts for 3% of the total mass of the solvent and electrolyte salt.

3. The electrolyte according to any one of claims 1 to 2, characterized in that, The electrolyte has at least one of the following properties: Ion transport number ≥ 0.57; In a Na‖Na symmetric cell, stable cycling exceeds 1100 hours; In Na-Al@C half-cells, the coulombic efficiency is ≥97%; In Na-NVP full cells, the capacity retention is ≥87% after 2000 cycles at 3 C rate; In Na-NVPOF full cells, the capacity retention is ≥75% after 1000 cycles at 1 C rate and cutoff voltage; Stable cycling within the range of -10℃ to 60℃.

4. A method for preparing the electrolyte according to any one of claims 1 to 3, characterized in that, Includes the following steps: Under a protective atmosphere, trimethyl phosphate, sodium difluorooxalate borate, tin trifluoromethanesulfonate, and sodium difluorophosphate are mixed and stirred until completely dissolved to obtain the electrolyte.

5. The preparation method according to claim 4, characterized in that, The protective atmosphere contains less than 0.01 ppm of water and less than 0.01 ppm of oxygen.

6. The preparation method according to claim 4, characterized in that, The stirring step includes: first, stirring at 300-500 rpm for 30-60 minutes to completely dissolve sodium difluorooxalate borate, then adding tin trifluoromethanesulfonate and sodium difluorophosphate and continuing to stir for 20-30 minutes until completely dissolved.

7. The application of the electrolyte according to any one of claims 1 to 3 in a sodium metal battery.

8. A sodium metal battery, characterized in that, The electrolyte comprising any one of claims 1 to 3.

9. The sodium metal battery according to claim 8, characterized in that, The positive electrode of the sodium metal battery is sodium vanadium phosphate or sodium vanadium fluoride phosphate.

10. The sodium metal battery according to any one of claims 8 to 9, characterized in that, The sodium metal battery is a button cell.

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