Sodium ion battery and SEI membrane in-situ construction method thereof

Abstract

The invention discloses a sodium ion battery and an SEI membrane in-situ construction method thereof. Electrolyte of the sodium ion battery comprises additives: polytetrafluoroethylene nanoparticles and sodium bis (oxalato) borate; the sodium ion battery is subjected to a three-stage first activation system: the sodium ion battery is charged to 5-15% of the theoretical capacity at the current density of 0.01-0.05 mA / cm < 2 >; charging at the current density of 0.1-0.2 mA / cm < 2 > to 20-40% of the theoretical capacity; and charging to the theoretical capacity at the current density of 0.25-0.5 mA / cm < 2 >. Sodium bis (oxalato) borate and polytetrafluoroethylene nanoparticles are introduced into the electrolyte and are synergistically decomposed under a first activation charging system, and an SEI film is subjected to three action mechanisms of topological structure strengthening of a B-O bond, fluorine element interface passivation effect and dynamic buffering of polytetrafluoroethylene microspheres; the systematic improvement on the interface strength and stability of the sodium metal negative electrode is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a sodium ion battery and an in-situ construction method of an SEI membrane thereof. Background Art

[0002] Sodium-ion batteries, with their high crustal abundance of sodium (approximately 440 times that of lithium), cobalt- and nickel-free cathode materials, and widely adaptable electrolyte systems, offer significant cost advantages in large-scale energy storage and low-speed power applications. They boast rate performance of up to 5C continuous discharge (capacity retention ≥ 95%), a cycle life exceeding 3,000 cycles (at 80% DOD), and a lifecycle environmental impact over 40% lower than that of lithium batteries.

[0003] Although hard carbon anode (theoretical capacity 400-700mAh / g) currently occupies the mainstream market, it is still the most popular cathode in Na + During the deintercalation process, the interlayer expansion rate reaches 15%-20%, which can easily cause grain boundary cracking and electrode pulverization, resulting in accelerated capacity decay in the later stages of the cycle. In contrast, the sodium titanate (Na2Ti3O7) negative electrode is based on the "rock salt-spinel" two-phase transformation mechanism, achieving a low platform potential of 0.3V (vsNa + / Na) and a theoretical capacity of 177mAh / g. Its three-dimensional tunnel structure suppresses volume changes (expansion rate <5%), making it a core development direction for high-stability sodium battery anodes. When paired with a polyanionic cathode (such as NFPP), this material can achieve a full battery energy density of 120-140Wh / kg, meeting the needs of distributed energy storage and two-wheeled vehicle power applications.

[0004] Due to the low charge and discharge potential of sodium titanate, a SEI film (solid electrolyte interface film) will inevitably be produced during the charge and discharge process. The existing electrolyte formula produces an SEI film with insufficient mechanical strength, and the SEI film is more soluble than the SEI film of lithium-ion batteries. The instability during the charge and discharge process will produce continuous side reactions, leading to irreversible capacity loss and even electrolyte depletion, making sodium-ion batteries less stable than lithium-ion batteries, which seriously restricts the practical application of sodium-ion batteries.

[0005] To this end, researchers typically add additives to sodium-ion battery electrolytes to improve their stability. For example, patent CN114497744A discloses a sodium-ion electrolyte and its application, a sodium-ion battery, and a method for preparing the same. The sodium-ion electrolyte includes a sodium tetrafluorooxalophosphate additive, a non-aqueous solvent, and a sodium salt dissolved in the non-aqueous solvent. While the use of sodium tetrafluorooxalophosphate as an additive is proposed, the additive does not address the issues of insufficient SEI film flexibility and stability. Patent CN113809398A discloses an electrolyte additive, an electrolyte, and a sodium secondary battery. The electrolyte additive includes at least one of sodium oxaloborate and sodium fluorooxalophosphate, requiring a high additive amount (greater than 10%) to improve the stability of the passivation film.

[0006] Existing electrolyte additives are mostly sodium salt additives (such as boron-containing sodium salts or fluorine-containing sodium salts). The SEI film formed has poor chemical stability and cannot meet the requirements of flexibility and rigidity. The battery's initial coulombic efficiency (ICE) is generally lower than 80%, and sodium loss is serious. Summary of the Invention

[0007] In response to the deficiencies in the prior art, the present invention provides a sodium ion battery and an in-situ SEI membrane construction method thereof. Sodium bis(oxalatoborate) (NaBOB) and polytetrafluoroethylene (PTFE) nanoparticles are introduced into the electrolyte, which synergistically decompose under the initial activation charging system to in-situ construct an SEI membrane with both a rigid-flexible composite structure, thus solving the problems of insufficient strength and poor stability of the SEI membrane of the sodium titanate negative electrode.

[0008] In order to solve the above technical problems, the first aspect of the present invention provides a method for in-situ construction of a SEI membrane in a sodium ion battery, wherein the electrolyte of the sodium ion battery includes additives: polytetrafluoroethylene nanoparticles and sodium bis(oxalatoborate); the sodium ion battery implements a three-stage initial activation system:

[0009] S1, 0.01-0.05mA / cm 2 The current density is continuously charged to 5-15% of the theoretical capacity;

[0010] S2, 0.1-0.2mA / cm 2 The current density is continuously charged to 20-40% of the theoretical capacity;

[0011] S3, 0.25-0.5mA / cm 2 The current density is kept constant and the battery is charged to the theoretical capacity.

[0012] The present invention adds NaBOB and PTFE to the electrolyte and synergistically decomposes them under a three-stage primary activation system, specifically:

[0013] In the first, low-voltage range, NaBOB first decomposes to form a BO structure (bond energy reaches 523 kJ / mol), forming a diamond-like topology at the atomic scale. This rigid framework, dominated by covalent bonds, increases the elastic modulus of the SEI film, effectively inhibiting the growth of sodium dendrites and film rupture. Low current density charging is used in this stage, giving NaBOB sufficient time to decompose and build a rigid BO support layer at the bottom, enhancing the mechanical strength and ion conductivity of the SEI film.

[0014] Following the initial rise in voltage and capacity, in the second stage, the fluorine-containing sodium salt of the electrolyte salt controllably decomposes to form NaF nanocrystals (approximately 5-8 nm in size), which react with PTFE to form a CF composite chemical passivation layer. PTFE nanospheres are adsorbed on the outside, and a gradient distribution of fluorine regulates interfacial reactions, achieving the dual effects of increasing the sodium ion transfer rate and reducing the rate of interfacial side reactions. Simultaneously, the BO and CF bonds chemically crosslink to form a three-dimensional network structure, achieving a combination of rigidity (compression resistance) and flexibility (tensile resistance). Simultaneously, the PTFE nanospheres act as stress buffers, absorbing the mechanical stress generated by volume changes through a reversible deformation of 20-35%, thereby reducing the stress concentration factor within the SEI membrane. This triple composite structure, through a multi-stage synergistic effect of mechanical, chemical, and dynamic forces, reduces the SEI membrane thickness from the conventional 120 nm to approximately 45 nm, thereby lowering the activation energy for ion transport.

[0015] In addition, in the third stage, high current density charging is used to suppress solvent decomposition and make more electrical energy for electrochemical deintercalation reaction.

[0016] Furthermore, the concentration of the polytetrafluoroethylene in the electrolyte is 0.1-2 wt %, and the concentration of the sodium bis(oxalatoborate) in the electrolyte is 1-4 wt %.

[0017] Furthermore, the particle size of the polytetrafluoroethylene is 50-200 nm.

[0018] Furthermore, the electrolyte solution further comprises an electrolyte salt: a fluorine-containing sodium salt, wherein the electrolyte salt is one or more of sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoromethanesulfonate, sodium bistrifluoromethanesulfonyl imide and sodium bisfluorosulfonyl imide.

[0019] Furthermore, the concentration of the electrolyte salt in the electrolyte is 0.1-10 mol / L.

[0020] Furthermore, the electrolyte also includes a solvent, and the solvent is one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, trimethyl phosphate, triethyl phosphate and diethylene glycol dimethyl ether.

[0021] Furthermore, the sodium ion battery also includes a positive electrode and a negative electrode.

[0022] Furthermore, the active material polyanion compound of the positive electrode is selected from Na4Fe3(PO4)2(P2O7), Na4Mn3(PO4)2(P2O7), Na3MnTi(PO4)3, Na2Fe2(SO4)3, Na4Fe 3-x Mn x One or more of (PO4)2(P2O7), wherein 0<x<3.

[0023] Furthermore, the active material of the negative electrode is Na2Ti3O7.

[0024] The second aspect of the present invention provides a sodium ion battery prepared by the method described in the first aspect.

[0025] Beneficial effects of the present invention:

[0026] The present invention introduces sodium bis(oxalatoborate) and polytetrafluoroethylene nanoparticles into the electrolyte, synergistically decomposes them under the initial activation charging system, and synergistically optimizes the three dimensions of mechanical stability, chemical stability and dynamic adaptability. The SEI film achieves a systematic improvement in the interface strength and stability of the sodium metal negative electrode through the triple action mechanism of the topological structure strengthening of the BO bond, the interface passivation effect of the fluorine element, and the dynamic buffering of the polytetrafluoroethylene microspheres. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0028] Figure 1 Schematic diagram of the SEI film structure on the negative electrode surface of the present invention;

[0029] Explanation of the numbers in the figure: 1. negative electrode, 2. BO rigid support layer, 3. chemical passivation layer, 4. PTFE flexible layer. DETAILED DESCRIPTION

[0030] The following will provide a clear and complete description of the technical solutions of the present invention in conjunction with specific embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0031] This embodiment relates to a method for in-situ construction of a SEI membrane in a sodium ion battery. The electrolyte of the sodium ion battery includes additives: polytetrafluoroethylene nanoparticles and sodium bis(oxalatoborate). The sodium ion battery implements a three-stage initial activation system:

[0032] S1, 0.01-0.05mA / cm 2 The current density is continuously charged to 5-15% of the theoretical capacity;

[0033] S2, 0.1-0.2mA / cm 2 The current density is continuously charged to 20-40% of the theoretical capacity;

[0034] S3, 0.25-0.5mA / cm 2 The current density is kept constant and the battery is charged to the theoretical capacity.

[0035] In this embodiment, NaBOB and PTFE are added to the electrolyte and synergistically decomposed under a three-stage initial activation system. Specifically, in the low voltage range of the first stage, NaBOB first decomposes to form a BO structure (bond energy reaches 523 kJ / mol), forming a diamond-like topological configuration at the atomic scale. This rigid skeleton dominated by covalent bonds increases the elastic modulus of the SEI film, effectively inhibiting the growth of sodium dendrites and film rupture. In this stage, low current density charging is used to give NaBOB sufficient time to decompose and form a bottom BO rigid support layer 2 on the surface of the negative electrode 1. Figure 1 As shown, the SEI membrane's mechanical strength and ion conductivity are enhanced. Following the initial rise in voltage and capacity, the electrolyte salt, containing fluorine, decomposes in a controlled manner in the second stage, generating NaF nanocrystals (approximately 5-8 nm in size). These react with PTFE to form a CF composite chemical passivation layer 3. PTFE nanospheres are adsorbed on the outer surface to form a PTFE flexible layer 4. The gradient distribution of fluorine allows for interfacial reaction regulation, achieving the dual effects of increasing sodium ion transport and reducing interfacial side reaction rates. Simultaneously, the BO and CF bonds chemically crosslink to form a three-dimensional network structure, achieving a combination of rigidity (compression resistance) and flexibility (tensile resistance). Furthermore, the PTFE nanospheres act as stress buffers, absorbing the mechanical stress generated by volume changes through a reversible deformation of 20-35%, thereby reducing the stress concentration factor within the SEI membrane. This triple composite structure, through a multi-stage synergistic effect of mechanical, chemical, and dynamic forces, reduces the SEI membrane thickness from the conventional 120 nm to approximately 45 nm, thereby lowering the activation energy for ion transport. In addition, in the third stage, high current density charging is used to suppress solvent decomposition and make more electrical energy for electrochemical deintercalation reaction.

[0036] As a preferred embodiment, the concentration of the polytetrafluoroethylene in the electrolyte is 0.1-2 wt %, the concentration of the sodium bis(oxalatoborate) in the electrolyte is 1-4 wt %; and the particle size of the polytetrafluoroethylene is 50-200 nm.

[0037] As a preferred embodiment, the electrolyte further includes an electrolyte salt: a fluorine-containing sodium salt, the electrolyte salt being one or more of sodium tetrafluoroborate, sodium hexafluorophosphate (NaPF6), sodium trifluoromethanesulfonate, sodium bistrifluoromethanesulfonyl imide and sodium bisfluorosulfonyl imide; the concentration of the electrolyte salt in the electrolyte is 0.1-10 mol / L; the electrolyte further includes a solvent, the solvent being one or more of ethyl methyl carbonate, diethyl carbonate (DEC), propylene carbonate, ethylene carbonate (EC), dimethyl carbonate, trimethyl phosphate, triethyl phosphate and diethylene glycol dimethanol ether.

[0038] As a preferred embodiment, the sodium ion battery further comprises a positive electrode and a negative electrode; the active material polyanion compound of the positive electrode is selected from Na4Fe3(PO4)2(P2O7), Na4Mn3(PO4)2(P2O7), Na3MnTi(PO4)3, Na2Fe2(SO4)3, Na4Fe 3-x Mn x One or more of (PO4)2(P2O7), wherein 0<x<3; the active material of the negative electrode is Na2Ti3O7.

[0039] Another embodiment provides a sodium ion battery prepared by the method described in the above embodiment.

[0040] Example 1

[0041] This embodiment relates to a method for in-situ construction of a SEI membrane in a sodium ion battery, comprising the following steps:

[0042] (1) NaBOB and PTFE nanoparticles were added to a mixed solvent of EC / DEC (volume ratio of 1:1) and ultrasonically dispersed for 2 h;

[0043] (2) adding NaPF6, stirring until completely dissolved, and standing for 24 hours to obtain a uniform electrolyte solution, wherein the concentration of NaPF6 in the electrolyte is 1M, the concentration of NaBOB is 2wt%, and the concentration of PTFE is 1%;

[0044] (3) Using Na4Fe3(PO4)2(P2O7) as the positive electrode active material, Na2Ti3O7 as the negative electrode active material, acetylene black as the conductive agent, and 5 wt% PVDF (polyvinylidene fluoride) in NMP (N-methylpyrrolidone) as the binder, the positive and negative electrode slurries were mixed according to the mass ratio of active material: conductive agent: binder = 90:5:5, and the positive and negative electrode sheets were prepared by manual coating. The positive and negative electrode sheets were assembled into a soft pack battery, and the electrolyte of step (2) was injected and then packaged;

[0045] (4) Battery activation: 0.03 mA / cm 2 The current density was charged to 10% of the theoretical capacity, and then the 2 The current density was charged to 30% of the theoretical capacity, and finally the current density was 0.25 mA / cm 2 The current density is charged to the theoretical capacity to obtain a sodium ion battery.

[0046] Example 2

[0047] The difference between this embodiment and embodiment 1 is that the concentration of NaBOB is adjusted to 1%, the concentration of PTFE is adjusted to 0.5%, and the other steps and parameters remain unchanged.

[0048] Example 3

[0049] The difference between this embodiment and embodiment 1 is that the concentration of NaBOB is adjusted to 4%, the concentration of PTFE is adjusted to 2%, and the other steps and parameters remain unchanged.

[0050] Comparative Example 1

[0051] The difference between this comparative example and Example 1 is that the concentration of NaBOB is adjusted to 5%, and the other steps and parameters remain unchanged.

[0052] Comparative Example 2

[0053] The difference between this comparative example and Example 1 is that the concentration of PTFE is adjusted to 3%, and the other steps and parameters remain unchanged.

[0054] Comparative Example 3

[0055] The difference between this comparative example and Example 1 is that the concentration of NaBOB is adjusted to 0%, and the other steps and parameters remain unchanged.

[0056] Comparative Example 4

[0057] The difference between this comparative example and Example 1 is that the concentration of PTFE is adjusted to 0%, and the other steps and parameters remain unchanged.

[0058] Comparative Example 5

[0059] The difference between this comparative example and Example 1 is that the first stage of battery activation in step (4) is omitted, specifically: 2 The current density was charged to 30% of the theoretical capacity, and then the 2 The current density is charged to the theoretical capacity, and the other steps and parameters remain unchanged.

[0060] Comparative Example 6

[0061] The difference between this comparative example and Example 1 is that the second stage of battery activation in step (4) is omitted, specifically: 2 The current density was charged to 10% of the theoretical capacity, and then the 2 The current density is charged to the theoretical capacity, and the other steps and parameters remain unchanged.

[0062] Comparative Example 7

[0063] The difference between this comparative example and Example 1 is that the first and second stages of battery activation are omitted in step (4). Specifically, the battery is activated directly at 0.25 mA / cm 2 The current density is charged to the theoretical capacity, and the other steps and parameters remain unchanged.

[0064] Test Case

[0065] The sodium ion batteries formed in the examples and comparative examples were subjected to the following electrical performance tests, and the results are shown in Table 1.

[0066] The initial coulombic efficiency (ICE) is the ratio of the discharge capacity to the charge capacity during the first charge and discharge process of a sodium-ion battery, reflecting the quality of the film formation at the electrode interface and the utilization rate of the active material. The assembled battery was placed in a thermostat and charged at a constant current of 0.1C (C is the current corresponding to the theoretical capacity) to a cutoff voltage of 3.9V. After standing for 30 minutes, it was discharged at the same current to 1.5V, and the ICE data was recorded.

[0067] Cycling stability: First, perform a 0.1C capacity calibration and record the discharge capacity as C0. Then, use 10C charge and discharge (charge to 3.9V, discharge to 1.5V). Perform a 0.1C capacity calibration every 1000 cycles and record the capacity retention rate at the 1000th cycle. The discharge capacity of the 10th 0.1C capacity calibration is C10. The capacity retention rate R1 = C10 / C0*100% is used to evaluate the cycling stability of the battery for 10,000 cycles.

[0068] Low-temperature performance: The battery was preconditioned in a thermostat at -40°C for 8 hours to ensure uniform internal temperature. Charge and discharge were performed at a current of 0.1C to the cutoff voltage, and the capacity was recorded. The ratio of this current to the discharge capacity at room temperature was the -40°C capacity retention rate. Test conditions: 0.1C charge and discharge, voltage range 1.5-4.2V.

[0069] Table 1

[0070] First effect / % Cycling stability / % -40℃ capacity retention rate / % Example 1 92.3 89.5 75.2 Example 2 91.8 88.5 74.6 Example 3 92.1 89.6 75.4 Comparative Example 1 89.4 88.4 68.9 Comparative Example 2 92.3 89.5 75.2 Comparative Example 3 79.4 49.3 49.7 Comparative Example 4 87.5 58.7 68.4 Comparative Example 5 91.4 60.3 60.0 Comparative Example 6 90.7 64.7 41.9 Comparative Example 7 68.0 34.8 30.7

[0071] A comparison of Examples 1-3 shows that with increasing NaBOB and PTFE content, the battery's initial efficiency, cycle stability, and low-temperature performance improve. However, when NaBOB is greater than 2% and PTFE is greater than 1%, the rate of improvement slows. Comparative Examples 1-2 show that when NaBOB is greater than 4%, supersaturated precipitation of the borate precursor is triggered during the electrochemical reduction process, and its interface regulation effect will undergo a phase transition from ordered construction to disordered stacking. This structural heterogeneity increases the activation energy of ion transport and reduces the sodium ion transport efficiency. The ultra-thick SEI film significantly increases the sodium ion transport barrier, thereby reducing the initial efficiency and low-temperature performance. When PTFE is greater than 2%, due to its insulating properties, a steric hindrance effect is generated during the SEI film formation process. The excessive introduction of PTFE causes the reconstruction of the three-dimensional ion conduction network of the SEI film to fail, which affects the transport of Na ions and reduces the electrochemical performance. From the comparison of Comparative Examples 3-4, it can be seen that when NaBOB is not added, the SEI film is generated solely by fluorine-containing sodium salts, which consumes a large amount of Na ions in the electrolyte, so the initial efficiency is the lowest; since the SEI film does not form BO bonds, its strength is greatly reduced, and it ruptures and regrows, resulting in reduced cycle performance and low-temperature performance; the addition of PTFE mainly provides a buffer layer to reduce stress concentration within the SEI film. When it is not added, the SEI film ruptures more frequently, resulting in a reduction in cycle stability to 58.7% and a significant decrease in low-temperature performance.

[0072] A comparison between Example 1 and Comparative Examples 5-7 shows that in Comparative Example 5, NaBOB was not given sufficient time to decompose, the BO rigid support layer was not established, the SEI film strength was insufficient, and dissolution and reconstruction would continue in the later stage, resulting in a decrease in cycle stability; Comparative Example 6 omitted the second stage, resulting in a decrease in CF layer coverage and low-temperature performance; Comparative Example 7 directly performed high-current activation, without sufficient time and voltage to form an SEI film. In the absence of an SEI film, solvent decomposition was accelerated, resulting in a sharp decrease in the initial efficiency.

[0073] In summary, the present invention introduces sodium bis(oxalatoborate) and polytetrafluoroethylene nanoparticles into the electrolyte, synergistically decomposes them under the initial activation charging system, and synergistically optimizes the three dimensions of mechanical stability, chemical stability and dynamic adaptability. The SEI film achieves a systematic improvement in the interface strength and stability of the sodium metal negative electrode through the triple action mechanism of topological structure strengthening of BO bonds, interface passivation effect of fluorine elements, and dynamic buffering of polytetrafluoroethylene microspheres. Among them, the battery ICE is increased to more than 92%, the cycle life is extended to 10,000 times, and the capacity retention rate is >88%. The capacity retention rate at a low temperature of -40°C is more than 75%.

[0074] The present invention has been described in detail above with reference to specific embodiments and exemplary examples. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art will appreciate that various equivalent substitutions, modifications, or improvements may be made to the technical solutions and implementations of the present invention without departing from the spirit and scope of the present invention, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for in-situ construction of a sodium ion battery SEI membrane, characterized in that: The electrolyte of the sodium ion battery includes additives: polytetrafluoroethylene nanoparticles and sodium bis(oxalatoborate); the sodium ion battery implements a three-stage initial activation system: S1, 0.01-0.05mA / cm 2 The current density is continuously charged to 5-15% of the theoretical capacity; S2, 0.1-0.2mA / cm 2 The current density is continuously charged to 20-40% of the theoretical capacity; S3, 0.25-0.5mA / cm 2 The current density is kept constant and the battery is charged to the theoretical capacity.

2. The in-situ construction method of the SEI membrane of a sodium ion battery according to claim 1, characterized in that: The concentration of the sodium bis(oxalatoborate) in the electrolyte is 1-4 wt %, and the concentration of the polytetrafluoroethylene in the electrolyte is 0.1-2 wt %.

3. The in-situ construction method of the SEI membrane of a sodium ion battery according to claim 1, wherein: The particle size of the polytetrafluoroethylene is 50-200 nm.

4. The in-situ construction method of the SEI membrane of a sodium ion battery according to claim 1, wherein: The electrolyte further comprises an electrolyte salt: a fluorine-containing sodium salt.

5. The in-situ construction method of the sodium ion battery SEI membrane according to claim 4, characterized in that: The fluorine-containing sodium salt is one or more of sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethanesulfonyl imide) and sodium bis(fluorosulfonyl imide).

6. The in-situ construction method of the SEI membrane of a sodium ion battery according to claim 4, characterized in that: The concentration of the electrolyte salt in the electrolyte is 0.1-10 mol / L.

7. The in-situ construction method of the SEI membrane of a sodium ion battery according to claim 1, wherein: The electrolyte further includes a solvent, which is one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, trimethyl phosphate, triethyl phosphate and diethylene glycol dimethyl ether.

8. The in-situ construction method of the SEI membrane of a sodium ion battery according to claim 1, wherein: The sodium ion battery further comprises a positive electrode and a negative electrode, wherein the active material of the positive electrode is a polyanion compound, and the active material of the negative electrode is Na2Ti3O7.

9. The in-situ construction method of the SEI membrane of a sodium ion battery according to claim 8, characterized in that: The active materials of the positive electrode are Na4Fe3(PO4)2(P2O7), Na4Mn3(PO4)2(P2O7), Na3MnTi(PO4)3, Na2Fe2(SO4)3, Na4Fe 3-x Mn x One or more of (PO4)2(P2O7), wherein 0<x<3.

10. A sodium ion battery prepared by the method according to any one of claims 1 to 9.

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