Sodium-ion battery diaphragm, preparation method thereof and sodium-ion battery

Abstract

The invention discloses a sodium-ion battery diaphragm, a preparation method thereof and a sodium-ion battery, and belongs to the technical field of sodium-ion batteries, the sodium-ion battery diaphragm comprises a glass fiber diaphragm and a modified coating coated on one side of the glass fiber diaphragm; the modified coating faces the negative electrode side of the sodium ion battery; the modified coating is prepared from the following raw materials in parts by mass: 10 parts of chitosan, 0.4 to 0.6 part of cellulose nanofiber, 0.02 to 0.04 part of cross-linking agent and 90 to 150 parts of solvent. A large number of polar groups in the chitosan can adsorb sodium ions through electrostatic interaction, so that the difference of local current density is reduced, and the growth of sodium dendrites is effectively inhibited; the cellulose nanofiber is used as a rigid filler and is compounded with the chitosan, so that a compact and uniform network structure can be formed, the mechanical strength of the diaphragm is improved, penetration of sodium dendrites is directly hindered, the pore size of the diaphragm is adjusted, uniform transmission of ions is facilitated, and the cycling stability of the sodium ion battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium-ion batteries, and particularly relates to a separator for sodium-ion batteries, a preparation method thereof, and a sodium-ion battery. Background Art

[0002] With the increasing urgency of the trend to fully develop and utilize clean energy, it is particularly important to vigorously develop new energy storage systems. Electrochemical energy storage has unique advantages in different scenarios and faster response. Lithium-ion batteries are representative of electrochemical energy storage, with characteristics such as long life, high energy density, and strong environmental adaptability. However, the low abundance and high cost of metallic lithium have become the biggest obstacles to sustainable development.

[0003] Sodium-ion batteries with a similar principle have many advantages such as more prominent safety, excellent high and low temperature performance, high fast charging rate, and environmental friendliness, and have become a current research hotspot. Moreover, metallic sodium is widely available and non-toxic, making it more sustainable. As a key part inside a sodium-ion battery, the separator can isolate the positive and negative electrodes of the battery, store the electrolyte for free transmission of sodium ions, play an important role in the electrochemical performance of the battery, and largely determine the safety of the battery.

[0004] The glass fiber separator is made of a large number of inorganic fibers by a non-spinning method, has a rich porous network, high porosity, and thermal stability, and has excellent wettability for the electrolyte of sodium-ion batteries. However, during the long cycle of sodium-ion batteries, the uneven distribution of power density will cause uneven deposition of sodium ions on the surface of the negative electrode, gradually forming needle-shaped or dendritic sodium metal deposits, namely sodium dendrites. The mechanical properties of the glass fiber separator are not good, and the anti-dendrite puncture performance is poor. Dendrites piercing the separator easily cause battery short circuit. Moreover, there are large pores of dozens of micrometers on the glass fiber separator, and the pore size is uneven, which will more easily lead to uneven current density, and then induce the growth of sodium dendrites, which is not conducive to the long-term cycle stability of sodium-ion batteries. Summary of the Invention

[0005] The purpose of the present invention is to provide a separator for sodium-ion batteries, a preparation method thereof, and a sodium-ion battery to solve the problems of poor mechanical properties and poor anti-sodium dendrite puncture performance of the glass fiber separator used in sodium-ion batteries.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] In a first aspect, the present invention provides a separator for sodium-ion batteries, which includes a glass fiber separator and a modified coating applied on one side of the glass fiber separator; the modified coating faces the negative electrode side of the sodium-ion battery;

[0008] The modified coating includes raw materials in the following mass fractions:

[0009] 10 parts of chitosan;

[0010] 0.4 - 0.6 parts of cellulose nanofibers;

[0011] 0.02 - 0.04 parts of cross - linker;

[0012] 90 - 150 parts of solvent.

[0013] Preferably, the coating amount of the modified coating is 0.3 - 0.6 mg / cm 2 .

[0014] Preferably, the fiber length of the cellulose nanofibers is 100 - 500 nm, and the fiber diameter is 5 - 10 nm.

[0015] Preferably, the cross - linker includes one or a combination of more of genipin, tannic acid, and carbodiimide.

[0016] Preferably, the pore diameter of the glass fiber separator is 1 - 5 μm, and the thickness is 100 - 400 μm.

[0017] Preferably, the solvent includes any one of aqueous acetic acid solution, aqueous acetic acid solution, aqueous hydrochloric acid solution, and aqueous lactic acid solution with a mass fraction of 0.5 - 2%.

[0018] By adopting the above - mentioned technical solution, a layer of modified coating is coated on one side of the glass fiber separator close to the negative electrode of the battery. The modified coating is mainly composed of a composite of chitosan and cellulose nanofibers. Since the formation of sodium dendrites in sodium - ion batteries is mainly due to the uneven distribution of the electric field on the electrode surface and the uneven distribution of ion nucleation sites, resulting in the deposition of sodium ions to form sodium dendrites. A modified coating is coated on the surface of the separator close to the negative electrode, that is, the end where sodium dendrites grow. The formed chitosan composite cellulose nanofiber layer can form a dense microstructure. On the one hand, it can directly prevent the penetration of sodium dendrites. On the other hand, it can regulate the pore size structure of the glass fiber separator, making the pore size distribution on the surface of the glass fiber separator more uniform. The uniform pore size can help the ions in the electrolyte to be uniformly transported, thereby avoiding uneven current density and inducing the growth of sodium dendrites, and indirectly inhibiting the growth rate of sodium dendrites in sodium - ion batteries, thus improving the long - term cycle stability of sodium - ion batteries.

[0019] And chitosan contains a large number of polar groups such as amino and hydroxyl groups. These polar groups can adsorb sodium ions through electrostatic interaction, thereby inhibiting the growth of the tips of sodium dendrites caused by local concentration gradients, thus significantly delaying dendrite nucleation. The introduction of cellulose nanofibers can form a three - dimensional network on the surface of the glass fiber separator, which can extend the diffusion path of sodium ions and uniform the ion distribution, thereby inhibiting the growth of sodium dendrites.

[0020] Moreover, the cellulose nanofibers in the modified coating can act as rigid fillers. Through the action of a cross-linking agent, an interpenetrating network structure is formed between the cellulose nanofibers and the chitosan matrix, greatly enhancing the mechanical strength of the glass fiber separator. Moreover, the cellulose nanofibers are evenly dispersed on the surface of the glass fiber separator, and the aspect ratio characteristic further enhances the stress transfer ability of the glass fiber separator, improving the tensile properties of the glass fiber separator. There can also be hydrogen bond or electrostatic interactions between the modified coating and the glass fiber separator, increasing the interfacial bonding force between the two, making it difficult for the modified coating to fall off from the surface of the glass fiber separator, inhibiting crack propagation, and improving the mechanical strength and sodium dendrite puncture resistance of the glass fiber separator.

[0021] At the same time, the hydrophilicity of chitosan and the porous structure of cellulose nanofibers do not affect the wettability of the glass fiber separator. Instead, they will improve the wettability and liquid retention of the separator to a certain extent, thereby reducing the interfacial impedance and improving the cycle performance of the sodium-ion battery.

[0022] Preferably, the chitosan is modified with sodium tripolyphosphate; the mass ratio of the chitosan to the sodium tripolyphosphate is 1:(0.2 - 0.3).

[0023] By adopting the above technical solution, the sodium tripolyphosphate contains negatively charged phosphate groups. After the chitosan is modified with sodium tripolyphosphate, the phosphate groups will form ionic cross-linking agents with the amino and hydroxyl groups in the chitosan. This can not only enhance the mechanical strength of the chitosan and indirectly improve the mechanical strength of the glass fiber separator, but also introduce a large number of negatively charged phosphate ions into the modified coating. The phosphate ions can enhance the sodium ion adsorption ability of the glass fiber separator, stabilize the adsorption of sodium ions on the separator surface, thereby reducing the interfacial impedance and improving the sodium affinity of the modified coating.

[0024] When the modified coating is coated on the surface of the glass fiber separator, correspondingly, it will also enhance the sodium affinity of the separator surface near the negative electrode end. The improvement of sodium affinity can reduce the nucleation barrier of sodium ions, inhibit the growth of sodium dendrites, and the uniform adsorption of sodium ions on the sodium-affine surface can eliminate the accumulation of local charges. When the charges on the separator surface are evenly distributed, the growth of sodium dendrites can be greatly inhibited, avoiding the short-circuit problem caused by the puncture of sodium dendrites.

[0025] After being modified with sodium tripolyphosphate, it can also help form a denser network structure, reduce the difference in local current density, guide the sodium ions to transport along a uniform path, avoid the formation of an ion concentration gradient on the surface of the glass fiber separator, and effectively inhibit the nucleation of sodium dendrites. While improving the mechanical strength of the glass fiber separator, the addition of sodium tripolyphosphate can enhance the liquid retention of the material and maintain the infiltration effect of the electrolyte.

[0026] Preferably, the sodium tripolyphosphate modified chitosan is prepared according to the following method:

[0027] Pretreatment of chitosan: Dissolve chitosan in a weak acid solution, add an oxidant, and stir and mix at 50-55 °C for 4-5 h to obtain a pretreated chitosan solution;

[0028] Sodium tripolyphosphate modified chitosan: Dropwise add a sodium tripolyphosphate solution to the pretreated chitosan solution, stir and react at 40-50 °C for 30-60 min, and finally obtain the product through centrifugation, washing and freeze-drying.

[0029] Preferably, the weak acid solution includes any one of acetic acid solution and acetic acid solution with a mass fraction of 1-5%.

[0030] Preferably, the oxidant includes one or a combination of hydrogen peroxide, hydrogen peroxide and sodium hypochlorite.

[0031] By adopting the above technical solution, chitosan is first pretreated with an oxidant. After the oxidation treatment, the number of polar groups in chitosan can be increased, more active sites can be introduced, and the crystal structure of chitosan can be destroyed through the oxidation treatment, improving the flexibility of the chitosan molecular chain, which is beneficial to the further reaction and crosslinking with sodium tripolyphosphate.

[0032] Then, a sodium tripolyphosphate solution is dropwise added. The protonated amino group in chitosan can form an ionic bond with the phosphate ion in sodium tripolyphosphate, thus forming a stable modified compound. The hydrogen bond formed between the two can also help further stabilize the crosslinked structure, transforming the linear structure of chitosan into a three-dimensional network, enhancing the mechanical strength of chitosan, and indirectly improving the improvement effect of the modified coating on the mechanical properties of the glass fiber separator. The modified layer formed by the obtained modified coating on the surface of the glass fiber separator can guide the uniform deposition of sodium ions, reduce the local current density difference, and inhibit the growth of sodium dendrites.

[0033] After the modification treatment with sodium tripolyphosphate, the binding sites between chitosan and cellulose nanofibers also increase. Through electrostatic attraction and hydrogen bond interaction between the two, the interfacial binding force is improved, further enhancing the interfacial stress transfer effect, thereby improving the mechanical strength of the separator and forming a more uniform ion transport channel.

[0034] In the second aspect, the present invention provides a preparation method of a sodium ion battery separator, including the following process steps:

[0035] S1. Add chitosan to a solvent, stir and mix for 4-6 h, then add cellulose nanofibers, stir and disperse for 20-40 min, add a crosslinking agent, and continue to stir for 30-60 min to obtain a modified coating;

[0036] S2. Coating the modified coating on one side of the glass fiber separator, and drying it at 40 - 50 °C for 2 - 3 h to obtain the sodium-ion battery separator.

[0037] Preferably, the technical effects of the present invention can also be achieved without the modification treatment of chitosan with sodium tripolyphosphate.

[0038] By adopting the above technical solution, chitosan, cellulose nanofibers and a crosslinking agent are mixed in an acidic solution to form a homogeneous dispersion system. The obtained modified coating is coated on one side of the glass fiber separator. After the solvent evaporates, the chitosan segments and the cellulose nanofibers gradually condense through hydrogen bonding and the action of the crosslinking agent to form a dense and stable porous structure. The cellulose nanofibers are embedded in the chitosan matrix as rigid nano-fillers and form an interpenetrating network through physical entanglement, further improving the mechanical strength.

[0039] When coated on the surface of the glass fiber separator, the modified coating can also combine with the polar groups on the glass fiber separator through the hydrogen bonding and van der Waals forces of the contained polar groups to form a firm interfacial layer, improving the bonding force between the modified coating and the glass fiber separator and enhancing the interfacial stress transfer effect.

[0040] By coating the modified coating on the surface of the glass fiber separator at one end close to the negative electrode side, the mechanical properties of the separator can be significantly improved, which is directly manifested as an increase in the tensile strength of the separator and the effective resistance to the mechanical penetration of sodium dendrites. The aspect ratio of the cellulose nanofibers can also disperse local stress through stress transfer and avoid local fracture.

[0041] At the same time, the pore structure on the surface of the separator can be regulated to help form a uniformly distributed physical barrier on the surface of the separator, which is beneficial to the uniform transmission of sodium ions and reduces local polarization. At the same time, the excellent wettability and liquid retention of the glass fiber separator can be maintained, reducing the interfacial impedance.

[0042] In the third aspect, the present invention provides a sodium-ion battery, the separator of which uses the obtained sodium-ion battery separator described above.

[0043] The beneficial effects of the present invention:

[0044] 1. The sodium-ion battery separator of the present invention uses the glass fiber separator as the base material, and coats the modified coating at one end of the glass fiber separator close to the negative electrode. A large number of polar groups are contained in the chitosan in the modified coating, which can adsorb sodium ions through electrostatic action, reduce the difference in local current density, and effectively inhibit the growth of sodium dendrites; the cellulose nanofibers, as rigid fillers, are compounded with chitosan, can form a dense and uniform network structure, improve the mechanical strength of the separator, can also directly hinder the puncture of sodium dendrites, can regulate the pore size of the separator, help the ions to be uniformly transmitted, reduce the growth of sodium dendrites, and improve the cycle stability of the sodium-ion battery.

[0045] 2. In the modified coating applied to the surface of the sodium-ion battery separator of the present invention, chitosan can be modified by sodium tripolyphosphate. The introduction of sodium tripolyphosphate can significantly enhance the sodium affinity of the modified coating, thereby reducing the interfacial impedance, eliminating the accumulation of local charges, inhibiting the growth of sodium dendrites, and avoiding the short-circuit problem caused by the puncture of sodium dendrites. At the same time, it can further improve the mechanical strength of the modified coating, the uniformity of ion transport, and the puncture resistance. Detailed implementation mode

[0046] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0047] Preparation example

[0048] Preparation example 1, a sodium tripolyphosphate-modified chitosan, is prepared according to the following method:

[0049] Pretreatment of chitosan: Dissolve 10 g of chitosan in 500 mL of acetic acid solution with a mass fraction of 3%, add 5 mL of hydrogen peroxide, and stir and mix at 55 °C for 4 h to obtain a pretreated chitosan solution;

[0050] Sodium tripolyphosphate-modified chitosan: Dropwise add a 10% sodium tripolyphosphate solution (where the addition amount of sodium tripolyphosphate is 2 g) to the pretreated chitosan solution, stir and react at 50 °C for 40 min, and finally obtain it through centrifugation, washing, and freeze-drying.

[0051] Preparation example 2, a sodium tripolyphosphate-modified chitosan, is only different from preparation example 1 in that the addition amount of sodium tripolyphosphate is 3 g.

[0052] Preparation example 3, a sodium tripolyphosphate-modified chitosan, is only different from preparation example 1 in that the addition amount of sodium tripolyphosphate is 1 g.

[0053] Preparation example 4, a sodium tripolyphosphate-modified chitosan, is only different from preparation example 1 in that the addition amount of sodium tripolyphosphate is 4 g.

[0054] Example

[0055] Example 1, a sodium-ion battery separator, is prepared according to the following steps:

[0056] S1. Add 10 g of chitosan to 120 g of acetic acid solution with a mass fraction of 1%, stir and mix for 4 h, then add 0.5 g of cellulose nanofibers (average length of 300 nm and average diameter of 6 nm), stir and disperse for 30 min, add 0.02 g of genipin, and continue to stir for 60 min to obtain a modified coating;

[0057] S2. Coat the modified coating on one side of a glass fiber separator (average pore size of 2.8 μm and average thickness of 260 μm), where the coating amount is 0.5 mg / cm 2 , and then dry it at 40 °C for 3 h to obtain a sodium-ion battery separator.

[0058] Examples 2 and 3, a sodium-ion battery separator, which is only different from Preparation Example 1 in that the raw material ratio of the modified coating is adjusted, as shown in Table 1 specifically:

[0059] Table 1 Raw material ratio of the modified coating in Examples 1 to 3

[0060] Example 1 Example 2 Example 3 Chitosan / g 10 10 10 Cellulose nanofibers / g 0.5 0.4 0.6 Genipin / g 0.02 0.02 0.03 Acetic acid solution / g 120 90 150

[0061] Among them, the coating amount of the modified coating on the surface of the glass fiber separator in Example 2 is 0.6 mg / cm 2 ; the coating amount of the modified coating on the surface of the glass fiber separator in Example 3 is 0.3 mg / cm 2 .

[0062] Example 4, a sodium-ion battery separator, which is only different from Example 1 in that the chitosan is replaced with an equal amount of sodium tripolyphosphate modified chitosan prepared in Preparation Example 1.

[0063] Example 5, a sodium-ion battery separator, which is only different from Example 1 in that the chitosan is replaced with an equal amount of sodium tripolyphosphate modified chitosan prepared in Preparation Example 2.

[0064] Example 6, a sodium-ion battery separator, which is only different from Example 1 in that the chitosan is replaced with an equal amount of sodium tripolyphosphate modified chitosan prepared in Preparation Example 3.

[0065] Example 7, a sodium-ion battery separator, which is only different from Example 1 in that the chitosan is replaced with an equal amount of sodium tripolyphosphate modified chitosan prepared in Preparation Example 8.

[0066] Example 8, a sodium-ion battery separator, which is only different from Example 1 in that the coating amount of the modified coating on the surface of the glass fiber separator is 0.8 mg / cm 2 .

[0067] Comparative Example

[0068] Comparative Example 1, a sodium-ion battery separator, which is only different from Example 1 in that the addition amount of cellulose nanofibers in the modified coating is 0.2 g.

[0069] Comparative Example 2, a sodium-ion battery separator, which is only different from Example 1 in that the addition amount of cellulose nanofibers in the modified coating is 0.8 g.

[0070] Comparative Example 3, a sodium-ion battery separator, which is only different from Example 1 in that no cellulose nanofibers are added to the modified coating.

[0071] Performance detection test

[0072] 1. Mechanical property test: According to the method for testing tensile strength recorded in GB / T 36363-2018 "Polyolefin Separators for Lithium-Ion Batteries", the tensile strength of the sodium-ion battery separators obtained in the examples and comparative examples of the present invention was tested, and the glass fiber separator without coating treatment (average pore size: 2.8 μm, average thickness: 260 μm) was set as Control Group 1;

[0073] The test results are shown in Table 2.

[0074] 2. Long-term cycle stability test:

[0075] Sample preparation: Hard carbon, sodium polyacrylate, and carbon black were mixed evenly in deionized water according to a mass ratio of 93:4:3 to obtain a negative electrode slurry. Then the negative electrode slurry was coated on aluminum foil and obtained a negative electrode sheet after baking and rolling; Sodium iron phosphate, polyvinylidene fluoride, and carbon black were mixed evenly in an appropriate amount of NMP according to a mass ratio of 92:4:4 to obtain a positive electrode slurry; Then the positive electrode slurry was coated on aluminum foil and obtained a positive electrode sheet after baking and rolling;

[0076] The obtained negative electrode sheet, positive electrode sheet, and the sodium-ion battery separators obtained in the examples and comparative examples were respectively assembled into sodium-ion batteries, where the side coated with the modified coating was close to the negative electrode sheet of the sodium-ion battery, and the electrolyte of the sodium-ion battery was NaPF6 and propylene carbonate with a mass ratio of 2:1.

[0077] And the sodium-ion battery prepared with the uncoated glass fiber separator was set as Control Group 1; The sodium-ion battery separator obtained in Example 1 with the side coated with the modified coating facing the positive electrode sheet was assembled into a sodium-ion battery and set as Control Group 2.

[0078] Test: The sample batteries of the examples, comparative examples, and Control Groups 1 and 2 were subjected to a cycle performance test. Long-term cycling was carried out at 25 °C and a 3C rate, and the cycle voltage was 0.3 - 1.9 V. The capacity retention rate of the sample batteries after 500 cycles was tested, and the capacity retention rate was calculated according to the following method:

[0079]

[0080] The test results are shown in Table 3.

[0081] Table 2 Tensile Strength Test Results

[0082]

[0083] Table 3 Cycling Stability Test Results

[0084]

[0085] According to Table 2 and Table 3, in combination with Example 1 and Example 4, it can be seen that the capacity retention rate and tensile strength of Example 4 are both increased compared with Example 1. The reason is that in the modified coating of Example 4, after chitosan is modified by sodium tripolyphosphate, the cross-linking density of chitosan is increased, thereby improving the mechanical properties, and the sodium affinity of the modified coating can be improved, reducing the interfacial impedance, eliminating the accumulation of local charges, and inhibiting the growth of sodium dendrites, so that the battery performance can be maintained stable during long-term cycling.

[0086] In combination with Example 4, Example 6 and Example 7, it can be seen that the performance of Example 6 and Example 7 is decreased compared with Example 4. The reason is that in Example 6, the addition amount of sodium tripolyphosphate is reduced, and the modification effect on chitosan is accordingly decreased; in Example 7, the addition amount of sodium tripolyphosphate is increased, resulting in an increase in the cross-linking density when the modified chitosan is compounded with cellulose nanofibers, causing partial blockage of the pores, which is not conducive to ion transport and will instead affect the battery performance.

[0087] In combination with Example 1 and Example 8, it can be seen that the performance of Example 8 is decreased compared with Example 1. The reason is that in Example 8, the coating amount of the modified coating on the surface of the glass fiber separator is increased, which is not conducive to regulating the pore uniformity on the separator surface, and the pores are blocked by the coating, which is not conducive to ion transport.

[0088] Combined with Example 1 and Comparative Examples 1 to 3, it can be seen that the performance of Comparative Examples 1 to 3 has significantly decreased compared to Example 1. The reason is that in Comparative Examples 1 to 3, the addition amount of cellulose nanofibers in the modified coating is changed. In Comparative Example 1, the content of cellulose nanofibers is reduced, so the content of rigid fillers decreases, the improvement effect on the mechanical strength of the separator decreases, the ability to resist stress and strain decreases, and the puncture resistance also decreases. In Comparative Example 3, no cellulose nanofibers are added, and the performance degradation is more obvious. In Comparative Example 2, the content of cellulose nanofibers is increased. Due to the self-polarity of excessive cellulose nanofibers, agglomeration will occur in the coating, which will block the pores when coated on the surface of the separator, affecting ion transport, and stress concentration points will be introduced, which is not conducive to the improvement of mechanical strength.

[0089] Combined with Example 1 and Control Group 1, it can be seen that the performance of Control Group 1 has significantly decreased compared to Example 1, indicating that coating the modified coating on one side of the glass fiber separator in Example 1 can significantly improve the mechanical strength and puncture resistance of the separator, thereby enhancing the long-term cycling stability of the sodium-ion battery.

[0090] Combined with Example 1 and Control Group 2, it can be seen that the capacity retention rate of Control Group 2 has decreased compared to Example 1. The reason is that in Control Group 2, the side with the modified coating is close to the positive electrode sheet, while the growth of sodium dendrites mostly deposits and nucleates on the negative electrode sheet, and it cannot provide good protection for the separator.

[0091] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0092] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A sodium-ion battery separator, characterized in that, The sodium-ion battery separator includes a glass fiber separator and a modified coating applied on one side of the glass fiber separator; the modified coating faces the negative electrode side of the sodium-ion battery; The modified coating includes raw materials in the following mass parts: 10 parts of chitosan; 0.4 - 0.6 part of cellulose nanofibers; 0.02 - 0.04 part of crosslinking agent; 90 - 150 parts of solvent.

2. The sodium ion battery separator according to claim 1, characterized in that, The coating amount of the modified coating is 0.3 to 0.6 mg / cm 2 .

3. The sodium-ion battery separator according to claim 1, characterized in that, The chitosan is modified by sodium tripolyphosphate; the mass ratio of chitosan to sodium tripolyphosphate is 1:(0.2 - 0.3).

4. The sodium ion battery separator according to claim 3, characterized in that, The sodium tripolyphosphate-modified chitosan is prepared by the following method: Pretreatment of chitosan: Dissolve chitosan in a weak acid solution, add an oxidizing agent, and stir and mix at 50 - 55°C for 4 - 5 h to obtain a pretreated chitosan solution; Sodium tripolyphosphate-modified chitosan: Dropwise add a sodium tripolyphosphate solution into the pretreated chitosan solution, stir and react at 40 - 50°C for 30 - 60 min, and finally obtain the product through centrifugation, washing, and freeze-drying.

5. The sodium ion battery separator according to claim 4, characterized in that, The oxidizing agent includes one or a combination of more than one of hydrogen peroxide, hydrogen peroxide, and sodium hypochlorite.

6. The sodium ion battery separator according to claim 1, wherein The fiber length of the cellulose nanofibers is 100 - 500 nm, and the fiber diameter is 5 - 10 nm.

7. The sodium-ion battery separator according to claim 1, wherein The crosslinking agent includes one or a combination of more than one of genipin, tannic acid, and carbodiimide.

8. The sodium ion battery separator according to claim 1, wherein The pore size of the glass fiber separator is 1 - 5 μm, and the thickness is 100 - 400 μm.

9. The preparation method of the sodium ion battery separator according to any one of claims 1 to 8, characterized in that, It includes the following process steps: S1. Add chitosan to the solvent, stir and mix for 4 - 6 h, then add cellulose nanofibers, stir and disperse for 20 - 40 min, add the crosslinking agent, and continue to stir for 30 - 60 min to obtain a modified coating; S2. Coat the modified coating on one side of the glass fiber separator, and dry it at 40 - 50°C for 2 - 3 h to obtain the sodium-ion battery separator.

10. A sodium-ion battery, characterized in that, The sodium-ion battery uses the sodium-ion battery separator described in any one of claims 1 - 8.

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