A NaFSI-based gel electrolyte, sodium-ion power battery and its preparation method

Abstract

This invention relates to the field of sodium-ion batteries, and more particularly to a NaFSI-based gel electrolyte, a sodium-ion power battery, and a method for preparing the same. This application uses a NaFSI-based gel electrolyte to replace the NaPF6-based electrolyte. The high dissociation degree of NaFSI provides higher intrinsic ionic conductivity. The "liquid" ion transport channels in the gel electrolyte and the low interfacial impedance resulting from in-situ polymerization together ensure that the entire battery system has excellent fast-charging capability. NaFSI, the in-situ constructed polymer framework, and the NaDFOB additive synergistically enhance interfacial stability and fast-charging performance at high temperatures.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion batteries, and more particularly to a NaFSI-based gel electrolyte, a sodium-ion power battery, and a method for preparing the same. Background Technology

[0002] Sodium-ion batteries are considered one of the most promising alternative technologies for large-scale energy storage and power batteries due to the abundance and low cost of sodium resources and their similar electrochemical working mechanism to lithium-ion batteries. Currently, mainstream sodium-ion battery systems for power battery applications typically employ a combination of layered ternary cathode materials, a hard carbon anode, and a NaPF6-based carbonate electrolyte. However, this technology faces several key bottlenecks in its practical application to the power battery field, specifically in the following aspects.

[0003] Firstly, the environmental moisture sensitivity of hard carbon anodes and the hydrolysis problem of NaPF6 create a combined effect. Due to their abundant nanoporous structure and surface defects, hard carbon anodes readily adsorb and introduce moisture from the environment during battery fabrication. Simultaneously, NaPF6, as the mainstream sodium salt in current sodium-ion batteries, is highly sensitive to trace amounts of moisture. Studies have shown that even in electrolytes prepared using battery-grade carbonate solvents (with a water content below 20 ppm), the hydrolysis reaction of NaPF6 can still be clearly observed. NaPF6 undergoes hydrolysis upon contact with water, generating corrosive substances such as hydrofluoric acid (HF). The generation of HF not only corrodes the positive electrode active material and causes the dissolution of transition metal ions, but also damages the electrode / electrolyte interface structure, accelerating battery performance degradation. Furthermore, the gas generated during hydrolysis can cause battery swelling, severely affecting the battery's cycle life and safety performance.

[0004] Secondly, the insufficient thermal stability of NaPF6 triggers a chain of side reactions. NaPF6 itself has poor thermal stability and easily decomposes to produce phosphorus pentafluoride (PF5) under heating conditions. As a strong Lewis acid, PF5 further catalyzes the decomposition of organic solvents in the electrolyte, initiating autocatalytic degradation of the electrolyte. This heat-induced degradation process is particularly prominent under the high-temperature operating conditions of power batteries, severely restricting the battery's long-term stable operation in high-temperature environments.

[0005] Third, the limited ionic conductivity of NaPF6-based electrolytes restricts the fast-charging performance of batteries. Power battery applications place stringent demands on fast-charging capabilities, and the limited ionic conductivity of NaPF6-based electrolytes is one of the key factors restricting the fast-charging performance of sodium-ion batteries. Insufficient ion transport efficiency leads to increased polarization of the battery under high-current charge and discharge conditions, making it difficult to meet the actual needs of power batteries for high power density and rapid energy replenishment.

[0006] To address the aforementioned issues, researchers have attempted improvements from multiple angles. For example, CN115692844A proposes an electrolyte additive strategy that combines a benzene-containing main additive with auxiliary additives such as lithium bis(trimethylsilane)borate and / or tris(trimethylsilane)borate. This strategy aims to effectively reduce electrode surface activity, inhibit vanadium dissolution and side reactions between the electrode and electrolyte, and improve sodium ion conductivity, thereby enhancing the battery's cycle performance at high voltage (4.5V), high-temperature storage performance, and low-temperature discharge performance.

[0007] CN115947336A focuses on the modification of hard carbon anode materials. By coating the surface of hard carbon with caffeic acid, the caffeic acid modifier undergoes in-situ self-polymerization on the surface of the hard carbon material through electrochemical action to form a coating layer, thereby comprehensively improving the overall performance of the hard carbon anode. However, these solutions are still local optimizations within the framework of NaPF6-based electrolyte systems and fail to fundamentally solve the intrinsic defects of NaPF6 itself, such as its hydrolytic sensitivity, insufficient thermal stability, and limited ionic conductivity.

[0008] However, while the above processes can alleviate the problems of water absorption by hard carbon and poor stability and rate capability of NaPF6, they cannot fundamentally overcome the limitations of poor heat resistance and limited ionic conductivity of the material system. In addition, the process of modifying the anode material is complex and costly, and cannot completely eliminate the water absorption of the material.

[0009] Sodium bis(fluorosulfonyl)imide (NaFSI, chemical formula F₂NNaO₄S₂) has attracted widespread attention in recent years as an emerging sodium salt material. Studies have shown that NaFSI has the potential to replace NaPF₆ and exhibits excellent comprehensive electrochemical performance. Specifically, NaFSI has the following outstanding advantages: In terms of ionic conductivity, NaFSI-based electrolytes exhibit superior ionic conductivity compared to NaPF₆ in various solvent systems, such as approximately 6.5 mS / cm in triethyl phosphate (TEP) solvent; in terms of thermal stability, NaFSI helps form a more stable solid-state electrolyte interphase (SEI) film, improving the high-temperature stability of the battery; in terms of interfacial stability, NaFSI can promote the formation of an inorganic-rich SEI layer, effectively suppressing side reactions and inhibiting the growth of sodium dendrites. Furthermore, NaFSI-based electrolytes also exhibit significantly better electrochemical performance than the NaPF₆ system under low-temperature conditions.

[0010] To address the shortcomings of the existing technologies, this invention aims to provide a sodium-ion power battery solution based on a fully NaFSI-based gel electrolyte. Specifically, it addresses the following key technical issues: achieving stable application of the fully NaFSI-based electrolyte in power sodium batteries, fully leveraging the advantages of NaFSI's high ionic conductivity, high thermal stability, and low hygroscopicity to replace the traditional NaPF6-based electrolyte system. This fundamentally overcomes the inherent defects of the NaPF6-based system in terms of hydrolysis sensitivity, thermal stability, and ionic conductivity, enabling sodium-ion batteries to truly meet the practical requirements of power batteries for fast charging performance and long-term stable operation under high-temperature conditions. Summary of the Invention

[0011] To address the shortcomings of the existing technologies, this invention aims to provide a NaFSI-based gel electrolyte, a sodium-ion power battery, and its preparation method, thereby enabling the stable application of all-NaFSI-based electrolytes in power sodium batteries and fully leveraging the advantages of NaFSI's high ionic conductivity, high thermal stability, and low hygroscopicity to replace the traditional NaPF6-based electrolyte system.

[0012] In a first aspect, this application provides a NaFSI-based gel electrolyte, wherein the gel electrolyte is a three-dimensional network structure formed by in-situ polymerization of the following components in parts by mass under thermal initiation:

[0013] Precursor components include

[0014] NaFSI: 10-40 portions;

[0015] Organic solvent: 30-70 parts;

[0016] Polymer monomer: 5-30 parts;

[0017] Crosslinking agent: 0.5-3 parts;

[0018] Thermal polymerization initiator: 0.01-1 part.

[0019] Furthermore, the carbonate organic solvent includes any one or a combination of several of ethylene carbonate, propylene carbonate, and dimethyl carbonate.

[0020] Furthermore, the polymer monomer is a free radical polymeric monomer containing unsaturated double bonds.

[0021] Furthermore, the polymer monomer is an acrylate monomer.

[0022] The preferred acrylate monomer is polyethylene glycol diacrylate, which has a mass fraction of 10% in the polymer precursor.

[0023] Furthermore, the crosslinking agent is triethylene glycol diacrylate.

[0024] Furthermore, the precursor component also includes 0.1-0.5 parts by weight of sodium difluorooxalate borate.

[0025] Sodium difluorooxalate borate is used to assist in the formation of a stable passivation layer on the surface of Al current collectors.

[0026] Furthermore, the gel electrolyte system can inhibit the oxidation and corrosion of Al current collectors at 4.2V, and no obvious pitting corrosion is observed on the Al surface after 100 cycles.

[0027] Secondly, this application provides a sodium-ion power battery comprising the gel electrolyte described in this application.

[0028] Furthermore, the positive electrode material is selected from layered transition metal oxides; the negative electrode material is hard carbon; and both the positive electrode current collector and the negative electrode current collector are aluminum foil.

[0029] Thirdly, this application provides a method for preparing the aforementioned sodium-ion power battery, comprising the following steps:

[0030] S1. Dissolve NaFSI conductive salt in carbonate organic solvent and stir until completely dissolved;

[0031] S2. Add polymer monomers, crosslinking agents, thermal polymerization initiators and functional additives, and stir until homogeneous to obtain a gel electrolyte precursor solution;

[0032] S3. After assembling the positive electrode, negative electrode and separator into a battery cell, inject the precursor solution;

[0033] S4. Place the injected battery cell in an environment of 60-90℃ for 1-6 hours to initiate monomer polymerization reaction and form gel electrolyte;

[0034] S5. Formation, aging, and capacity testing are performed to obtain the finished battery.

[0035] Beneficial effects: 1. This application uses NaFSI-based gel electrolyte to replace NaPF6-based electrolyte. The high degree of dissociation of NaFSI provides higher intrinsic ionic conductivity. The "liquid" ion transport channels in the gel electrolyte and the low interfacial impedance brought about by in-situ polymerization together ensure that the entire battery system has excellent fast charging capability. The excellent thermal stability of NaFSI and the in-situ constructed polymer skeleton work together to improve the interfacial stability, corrosion resistance and fast charging performance at high temperature.

[0036] 2. Furthermore, NaDFOB, NaFSI, and the gel network are used in synergy. NaDFOB preferentially decomposes to construct a stable passivation layer rich in Al-F and BF on the surface of the aluminum current collector, which effectively inhibits aluminum foil corrosion under high voltage and widens the electrochemical window. This promotes NaFSI to provide high ionic conductivity, and synergistically improves the interface stability and fast charging performance at high temperature.

[0037] 3. This application replaces the NaPF6-based electrolyte with a NaFSI-based gel electrolyte, which solves the contradiction between the introduction of water in conventional hard carbon anode and layered oxide ternary cathode systems and the water sensitivity of conventional NaPF6, as well as the inherent defect of poor thermal stability of NaPF6 itself. Furthermore, NaFSI compensates for the kinetic loss of electrolyte gelation, thereby achieving high rate capability, high thermal stability, and high cycle stability of sodium-ion power batteries. Detailed Implementation

[0038] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments.

[0039] NaPF6, sodium hexafluorophosphate; NaFSI, sodium difluorosulfonamide; EC, ethylene carbonate; PC, propylene carbonate;

[0040] DMC, dimethyl carbonate; PEGDA, polyethylene glycol diacrylate; TEGDA, triethylene glycol diacrylate; AIBN, azobisisobutyronitrile; NaDFOB, sodium difluorooxalate borate.

[0041] Example 1: A method for preparing a sodium-ion power battery based on a full NaFSI gel electrolyte. The raw material formulation and dosage are shown in Table 1, and the preparation steps are as follows:

[0042] S1. Dissolve NaFSI in a mixed solvent of EC / PC / DMC and stir until completely dissolved.

[0043] S2, add PEGDA monomer, TEGDA crosslinking agent, AIBN initiator and NaDFOB additive, and stir until homogeneous.

[0044] S3, inject the precursor solution into the assembled battery cell (layered oxide positive electrode / hard carbon negative electrode, both positive and negative electrode current collectors are Al foil).

[0045] S4 was heated at 80°C for 3 hours to initiate in-situ polymerization.

[0046] S5, formation, aging, and capacity separation.

[0047] Table 1, List of raw material formulas and dosages for Example 1

[0048] lithium salts NaFSI 20% solvent EC:PC:DMC (1:1:2) 68.50% monomer PEGDA 10.00% Crosslinking agent TEGDA 1.00% Initiator AIBN 0.50% additive NaDFOB 0.50%

[0049] Example 2: A method for preparing a sodium-ion power battery based on a full NaFSI gel electrolyte. Compared with Example 1, no NaDFOB was added. The gel electrolyte formulation and dosage are shown in Table 2, and the preparation steps are as follows:

[0050] Table 2, List of raw material formulas and dosages for Example 2

[0051] lithium salts NaFSI 20.00% solvent EC:PC:DMC (1:1:2) 69.00% monomer PEGDA 10.00% Crosslinking agent TEGDA 1.00% Initiator AIBN 0.50% additive none 0.00%

[0052] Comparative Example 1: A method for preparing a sodium-ion power battery based on a fully NaFSI gel electrolyte. The raw material formulation and dosage are shown in Table 3. Compared with Example 1, the electrolyte is a liquid electrolyte, and the preparation steps are as follows:

[0053] S1, Dissolve the lithium salt in a mixed solvent of EC / PC / DMC and stir until completely dissolved.

[0054] S2, add NaDFOB additive, stir evenly to form electrolyte.

[0055] S3, inject the electrolyte into the assembled battery cell (layered oxide positive electrode / hard carbon negative electrode, both positive and negative electrode current collectors are Al foil).

[0056] S4, formation, aging, and capacity separation.

[0057] Table 3. List of raw material formulations and dosages for the liquid electrolyte in Comparative Example 1.

[0058] lithium salts NaPF6 20.00% solvent EC:PC:DMC (1:1:2) 79.50% monomer none 0.00% Crosslinking agent none 0.00% Initiator none 0.00% additive NaDFOB 0.50%

[0059] Comparative Example 2 is a method for preparing a sodium-ion power battery based on a full NaFSI gel electrolyte. Compared with Comparative Example 1, the electrolyte formulation is different, as shown in Table 4.

[0060] Table 4. List of raw material formulations and dosages for the liquid electrolyte in Comparative Example 2.

[0061] lithium salts NaPF6 13.34% lithium salts NaFSI 6.66% solvent EC:PC:DMC (1:1:2) 79.50% monomer none 0.00% Crosslinking agent none 0.00% Initiator none 0.00% additive NaDFOB 0.50%

[0062] Comparative Example 3 is a method for preparing a sodium-ion power battery based on a full NaFSI gel electrolyte. Compared with Comparative Example 1, the electrolyte formulation is different, as shown in Table 5.

[0063] Table 5. List of raw material formulations and dosages for the liquid electrolyte in Comparative Example 3.

[0064] lithium salts NaFSI 20.00% solvent EC:PC:DMC (1:1:2) 79.50% monomer none 0.00% Crosslinking agent none 0.00% Initiator none 0.00% additive NaDFOB 0.50%

[0065] Comparative Example 4: A method for preparing a sodium-ion power battery based on a full NaFSI gel electrolyte. Compared with Example 1, the gel electrolyte formulation is different, as shown in Table 6.

[0066] Table 6. List of gel electrolyte raw material formulations and dosages for Comparative Example 4

[0067] lithium salts NaFSI 20.00% solvent EC:PC:DMC (1:1:2) 68.50% monomer PEGDA 10.00% Crosslinking agent TEGDA 1.00% Initiator AIBN 0.50% additive NaBF4 0.50%

[0068] Performance testing

[0069] The following examples and comparative data illustrate the safety and electrochemical performance of the sodium-ion battery and corresponding electrolyte of the present invention. The test results are shown in Table 7.

[0070] (1) Room temperature ionic conductivity test

[0071] The electrolyte or gel electrolyte precursor was injected into the model electrode (the gel electrolyte needs to be cured under the same temperature and time conditions as in the corresponding example). Electrochemical impedance spectroscopy (EIS) was performed on the model electrode at 25°C using an electrochemical workstation, with an AC perturbation voltage of 5 mV and a frequency range of 10... 5 -10 -2 The electrolyte impedance R was measured at Hz, and the thickness L and surface area S of the electrolyte were tested.

[0072] The ionic conductivity (mS / cm) of a solid electrolyte is equal to L / RS.

[0073] (2) Room temperature cycling test

[0074] At 25°C, charge at a constant current of 1C to 4.2V, charge at a constant voltage of 4.2V to 0.05C, and discharge at a constant current of 1C to 2.0V. Repeat this process 200 times and record the discharge capacity Qn (mAh) on the nth cycle.

[0075] Battery capacity retention rate after 200 cycles (%) = (Q200 / Q1)*100%.

[0076] (3) High temperature cycling test

[0077] At 55℃, charge at a constant current of 1C to 4.2V, charge at a constant voltage of 4.2V to 0.05C, and discharge at a constant current of 1C to 2.0V. Repeat this process 200 times and record the discharge capacity Qn (mAh) on the nth cycle.

[0078] Battery capacity retention rate after 200 cycles (%) = (Q200 / Q1)*100%.

[0079] (4) Ratio performance test

[0080] At 25°C, it is charged to 4.2V at a constant current of 0.2C, charged to 0.05C at a constant voltage of 4.2V, and discharged to 2.0V at 0.2C / 5C respectively.

[0081] Discharge capacity retention rate (%) = (Q0.2C / Q5C)*100%.

[0082] (5) High-temperature storage test

[0083] At 25℃, the cell was charged at a constant current of 0.2C to 4.2V, then charged at a constant voltage of 4.2V to 0.05C, and discharged at 0.2C to 2.0V. The discharge capacity was recorded as Q0. Then, the cell was charged at a constant current of 0.2C to 4.2V, then charged at a constant voltage of 4.2V to 0.05C. After being stored at 55℃ for 7 days, it was left to stand at room temperature for 6 hours and then discharged at a constant current of 0.2C to 2.0V. The discharge capacity was recorded as Q7.

[0084] Storage capacity retention rate (%) = (Q0 / Q7) * 100%.

[0085] (6) Corrosion characterization of current collectors

[0086] After 200 cycles at high temperature, the battery cell was disassembled, and the main material on the surface of the positive electrode was wiped off with NMP. The corrosion of the current collector was characterized by SEM.

[0087] Table 7, List of Test Results

[0088]

[0089] Observing Comparative Examples 1, 2, and 3, it can be seen that as the proportion of NaFSI in the lithium salt increases, the ionic conductivity of the electrolyte increases monotonically, and the rate performance of the battery also increases accordingly, confirming that NaFSI has a higher ionic conductivity than NaPF6 as an electrolyte conductive salt. Observing Example 1 and Comparative Examples 1 and 3, it can be seen that converting the electrolyte to a gel electrolyte leads to a decrease in ionic conductivity. However, by replacing the sodium salt from NaPF6 to NaFSI, the loss of conductivity is basically compensated, and the NaFSI-based gel battery achieves a higher rate performance than the NaPF6-based liquid battery. Observing Example 1, 2, and 4, it can be seen that NaDFOB lithium salt has the effect of assisting in the construction of a more stable SEI film, which can further improve the overall performance of the NaFSI-based gel battery; NaBF4 can also play a role in constructing a stable SEI film, but at the same time, it seriously affects the ionic conductivity of the gel electrolyte.

[0090] Observations of Examples 1, 2, and 3 show that increasing the proportion of NaFSI in the lithium salt leads to a decrease in cycle stability, and this effect is more pronounced in high-temperature cycling than in room-temperature cycling. It also affects the high-temperature storage performance of the battery. SEM observation of the current collector after high-temperature cycling reveals that increasing the proportion of NaFSI leads to corrosion deterioration of the aluminum current collector, which is a significant cause of cell performance degradation. Comparing Examples 1, 2, and 4, it is evident that the gel electrolyte can independently inhibit the corrosion of the current collector by NaFSI lithium salt, thus exhibiting superior cycle stability and high-temperature storage performance compared to liquid batteries. Furthermore, combining it with NaDFOB allows for the construction of a boron- and fluorine-containing SEI film, improving the ion transport kinetics and structural stability of the electrode, thereby achieving higher overall performance than gel batteries without NaDFOB additives. Combining it with NaBF4, by constructing a boron-containing SEI film, can also improve the storage performance and high-temperature cycle performance of gel batteries; however, due to the poor kinetic performance of the electrolyte, room-temperature rate capability and cycle performance show significant degradation.

[0091] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A NaFSI-based gel electrolyte, characterized in that, The gel electrolyte is a three-dimensional network structure formed by the in-situ polymerization of the following precursor components under thermal initiation: Precursor components include NaFSI: 10-40 portions; Organic solvent: 30-70 parts; Polymer monomer: 5-30 parts; Crosslinking agent: 0.5-3 parts; Thermal polymerization initiator: 0.01-1 part.

2. The gel electrolyte according to claim 1, characterized in that, The organic solvent includes any one or a combination of several of ethylene carbonate, propylene carbonate, and dimethyl carbonate.

3. The gel electrolyte according to claim 1, characterized in that, The polymer monomer is a free radical polymerizable monomer containing unsaturated double bonds.

4. The gel electrolyte according to claim 3, characterized in that, The polymer monomer is an acrylate monomer.

5. The gel electrolyte according to claim 1, characterized in that, The crosslinking agent is triethylene glycol diacrylate.

6. The gel electrolyte according to claim 1, characterized in that, The precursor components also include 0.1-0.5 parts by weight of sodium difluorooxalate borate.

7. The gel electrolyte according to any one of claims 1-6, characterized in that, The gel electrolyte system can inhibit the oxidation and corrosion of Al current collectors at 4.2V, and no obvious pitting corrosion is observed on the Al surface after 100 cycles.

8. A sodium-ion power battery, characterized in that, It includes the gel electrolyte according to any one of claims 1-7.

9. The sodium-ion power battery according to claim 8, characterized in that, The positive electrode material is selected from layered transition metal oxides; the negative electrode material is hard carbon; and both the positive and negative current collectors are aluminum foil.

10. A method for preparing the sodium-ion power battery according to claim 8 or 9, characterized in that, Includes the following steps: S1. Dissolve NaFSI conductive salt in carbonate organic solvent and stir until completely dissolved; S2. Add polymer monomers, crosslinking agents, thermal polymerization initiators and functional additives, and stir until homogeneous to obtain a gel electrolyte precursor solution; S3. After assembling the positive electrode, negative electrode and separator into a battery cell, inject the precursor solution; S4. Place the injected battery cell in an environment of 60-90℃ for 1-6 hours to initiate monomer polymerization reaction and form gel electrolyte; S5. Formation, aging, and capacity testing are performed to obtain the finished battery.

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