Preparation Method for Lignin-Coated Cellulose Nanofiber Sodium-Ion Battery Separator

Abstract

The present invention discloses a preparation method for a lignin-coated cellulose nanofiber sodium-ion battery separator, pertaining to the technical fields of electrochemistry and battery separators. The preparation method comprises: pretreating bamboo particles via a hydrothermal method; subsequently performing nanonization of the bamboo particles via mechanical disintegration to obtain a lignin-coated cellulose nanofiber suspension; and forming a film from the lignin-coated cellulose nanofibers via vacuum filtration. The preparation process of the cellulose coated with lignin nanoparticles according to the present invention is direct, simple, efficient, and environmentally friendly, eliminating the need for chemical reagents such as acids or alkalis to remove hemicellulose and lignin.Furthermore, the coating of lignin nanoparticles effectively enhances the diffusion rate of sodium ions during charge and discharge processes, as well as the discharge specific capacity and cycling stability.

Description

Technical Field The present invention relates to the technical fields of electrochemistry and battery separators, and specifically discloses an efficient preparation method for a lignin-coated cellulose nanofiber sodium-ion battery separator. Background Art Energy is the core driving force behind the prosperity and progress of human society. However, with social development, issues such as fossil energy shortages and environmental crises caused by energy extraction have become increasingly severe. The key trend in addressing these energy challenges lies in the efficient storage of renewable energy using battery technology. Among these, lithium-ion batteries, as a critical technology, are widely used in various fields such as smartphones and electric vehicles. This extensive application has also led to a supply-demand imbalance for lithium resources, driving up lithium prices. Global lithium resources are unevenly distributed, with China accounting for only about 6% of the world's lithium content. In contrast, China's sodium content is approximately 422 times that of lithium. Therefore, exploring alternative battery technologies, such as sodium-ion batteries, has become a crucial pathway for alleviating lithium shortages and promoting sustainable energy development. As a new type of rechargeable battery, sodium-ion batteries offer advantages such as high energy density, low cost, and broad resource availability. In addition to the cathode, anode, and electrolyte in sodium-ion battery materials, the separator is also an indispensable key component. Its primary function is to isolate the positive and negative electrodes of the battery while allowing sodium ions to freely transport between them. For battery safety, the separator must be an excellent insulator, and for battery performance, it must have adequate pore channels to facilitate ion transport. Thus, the quality of the separator material directly affects the safety, capacity, and other performance metrics of the battery. Since Na⁺ (0.098 nm, 22.99 g·mol⁻¹) has a larger radius and mass than Li⁺ (0.076 nm, 6.94 g·mol⁻¹), the pore size of the separator used should also be larger than that of lithium-ion battery separators. Furthermore, the electrolyte commonly used in sodium-ion batteries is an organic electrolyte prepared with sodium salts such as NaClO₄ or NaPF₆, using propylene carbonate (PC) as the solvent and fluoroethylene carbonate (FEC) as an additive. Propylene carbonate contains only ester carbonyl groups and has a cyclic structure. This unique structure results in poor wetting of the electrolyte on commonly available polymer separators in the market. Therefore, there is an urgent need for a separator material suitable for the pore size requirements and electrolyte wettability of sodium-ion batteries to overcome current limitations. Summary of invention To prepare sustainable, green, and high-performance energy storage materials, the objective of the present invention is to propose an efficient preparation method for a lignin- coated cellulose nanofiber sodium-ion battery separator. This is achieved by controlling the time and temperature of hydrothermal pretreatment to obtain lignocellulose with different lignin concentrations, followed by the use of vacuum filtration to produce the separator. To achieve the above objective, the present invention provides the following technical solution: A preparation method for a lignin-coated cellulose nanofiber sodium-ion battery separator, comprising the following steps: (1) Pretreatment of bamboo particles by hydrothermal method; The bamboo particles have a size of 20 - 80 mesh; the pretreatment process comprises: soaking in water for a period, heating to 120 - 240°C, and conducting a hydrothermal reaction for 30 - 180 minutes to obtain a bamboo particle suspension; (2) Subsequently, mechanical disintegration is employed to nanonize the bamboo particles to obtain a lignin-coated cellulose nanofiber suspension; A bamboo particle suspension with a solid content of 5% - 10% is prepared, ground several times using a disk grinder to obtain the lignin-coated cellulose nanofiber suspension. The suspension is then mixed with water and an isopropanol solution in a volume ratio of 15 - 50 : 4 : 32 to form a homogeneous mixture. The mixture is ultrasonicated and magnetically stirred at room temperature to ensure uniform dispersion; (3) The lignin-coated cellulose nanofiber suspension is then formed into a film via vacuum filtration; A vacuum filtration device is used to filter the suspension, yielding an initial lignin-coated cellulose nanofiber membrane. The obtained initial membrane is immersed in tert-butanol for a period, followed by vacuum drying to obtain the lignin-coated cellulose nanofiber membrane, i.e., the sodium-ion battery separator. Further, in step (1), the bamboo particles have a size of 30 - 60 mesh; the pretreatment process comprises: soaking in water for a period, heating to 160 - 220°C, and conducting a hydrothermal reaction for 30 - 90 minutes. Further, in step (1), the bamboo particles have a size of 40 mesh; the pretreatment process comprises: soaking in water for a period, heating to 180°C, and conducting a hydrothermal reaction for 60 minutes. Further, in step (1), the bamboo particles are prepared by cleaning bamboo strips with distilled water to remove surface dust and dirt, crushing the bamboo strips in a high-speed blender, and sieving through a 20 - 80 mesh screen. Further, in step (2), the disk grinding method employs a grinder with a gap of 0.1 - 2 μm for ultrafine grinding, and the grinding is performed 30 - 40 times. Further, in step (2), a bamboo particle suspension with a solid content of 5% is prepared, and the lignin-coated cellulose nanofiber suspension is mixed with water and an isopropanol solution in a volume ratio of 15 : 2 : 8 to form a homogeneous mixture. Further, in step (3), the vacuum filtration device is used for filtration, and the obtained initial membrane is immersed in tert-butanol for 12 - 24 hours, ultrasonicated for 1 - 2 hours, and magnetically stirred for 6 - 12 hours. After vacuum drying, the lignin-coated cellulose nanofiber sodium-ion battery separator is obtained. The present invention also provides a sodium-ion battery separator material, which is the lignin-coated cellulose nanofiber sodium-ion battery separator prepared by any of the above- described methods. Further, in the sodium-ion battery separator material, the lignin-coated cellulose nanofibers have an average diameter of 10 - 20 nm, and the lignin particles have an average particle size of 25 - 40 nm. Finally, the present invention also provides a sodium-ion battery, wherein the separator material of the sodium-ion battery is the aforementioned sodium-ion battery separator material. Compared with the prior art, the advantages of the present invention are as follows: The raw materials used in the present invention are widely available, easily obtainable, and renewable. Typically, nanocellulose is prepared by chemically removing lignin from biomass fibres, a process that is time-consuming, labour-intensive, and environmentally polluting. In contrast, the raw materials used in this invention do not require lignin removal. The hydrothermal method employed offers distinct advantages, including simple processing, low cost, and high yield. The lignin-coated cellulose nanofiber sodium-ion battery separator prepared by this invention exhibits benefits such as a large specific surface area and high electrolyte wettability, effectively promoting sodium-ion transport, improving the battery's initial discharge specific capacity, and making a significant contribution to achieving efficient and sustainable energy storage solutions. Brief Description of the Drawings Fig. 1 is a TEM (Transmission Electron Microscopy) image of the lignin-coated cellulose nanofibers in the present invention. Fig. 2 is an SEM (Scanning Electron Microscopy) image of the lignin-coated cellulose nanofiber sodium-ion battery separator in the present invention. Embodiments The present invention is further described below with reference to specific examples. These examples provide an efficient preparation method for a lignin-coated cellulose nanofiber sodium-ion battery separator, and are merely illustrative and do not limit the scope of the invention in any way. All reagents used in the invention are of analytical grade, and all materials can be purchased commercially. The vacuum filtration apparatus used (model ET-2535H) is equipped with a G3 glass filter funnel (60 ml). Embodiment 1 1. Clean the surface dust and dirt from strips of Dendrocalamus sinicus bamboo with distilled water. Place the bamboo strips into a high-speed blender for crushing, and sieve through a 40-mesh screen to obtain Dendrocalamus sinicus bamboo particles. 2. Fully soak the Dendrocalamus sinicus bamboo particles in deionized water at room temperature for 24 hours. Subsequently, subject them to a hydrothermal reaction at 180°C for 60 minutes to obtain a Dendrocalamus sinicus bamboo particle suspension. 3. Obtain a Dendrocalamus sinicus bamboo particle suspension with a solid content of 5% by adjusting the solid-liquid ratio. Then, subject the suspension to mechanical disintegration by grinding it 30 times using a disk grinder with a 1 μm gap to obtain a lignin-coated cellulose nanofiber suspension. Characterization and size measurement analysis of the lignocellulose using TEM (Transmission Electron Microscopy) revealed that the lignin-coated cellulose nanofibers have an average diameter of 10 - 20 nm, and the lignin nanoparticles have an average particle size of 25 - 40 nm, as shown in Figure 1. 4. Sequentially add 15 ml of the lignin-coated cellulose nanofiber suspension, 4 ml of water, and 32 ml of an isopropanol solution to form a mixture in a volume ratio of 15 : 4 : 32. Ultrasonicate the mixture for 30 minutes at room temperature and magnetically stir it for 6 hours to achieve uniform dispersion of the lignin-coated cellulose nanofiber suspension. 5. Use a vacuum filtration apparatus for filtration to obtain an initial lignin-coated cellulose nanofiber membrane. Immerse the obtained initial membrane in tert-butanol for 24 hours, and finally vacuum-dry it for 6 hours to obtain the lignin-coated cellulose nanofiber membrane designated as SP-1, as shown in Figure 2. 6. Cut SP-1 into discs with a diameter of 19 mm and assemble them into coin cells in a vacuum glove box for electrochemical testing. 7. Sodium coin cell assembly method: First, prepare materials. Use the prepared lignin-coated cellulose nanofiber membrane (SP- 1) as the separator material. Cut a sodium metal sheet and a bamboo carbon anode sheet into discs with a diameter of 14 mm to serve as the cathode and anode materials, respectively. Prepare an electrolyte primarily composed of sodium perchlorate (NaClO₄). Gather basic components such as gaskets, positive and negative electrode shells, and the separator. Place all the above materials into a vacuum glove box for use. Next, assemble the battery. Place the negative electrode shell at the bottom layer. Sequentially add the anode material (bamboo carbon anode sheet), electrolyte (primarily NaClO₄), separator (lignin- coated cellulose nanofiber membrane SP-1), electrolyte (primarily NaClO₄), cathode material (sodium sheet), gasket, and positive electrode shell. Finally, use a crimping machine to seal the assembled battery to obtain a complete sodium coin cell. 8. Preparation method for the bamboo carbon anode sheet: Weigh 20 g of bamboo powder and carbonize it in a tube furnace to obtain bamboo carbon powder. Subsequently, mix the bamboo carbon powder, conductive acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 8 : 1 : 1 (i.e., 0.8 g bamboo carbon powder, 0.1 g carbon black, and 0.1 g PVDF). Add a certain amount of N-methyl-2-pyrrolidone (NMP) and a small amount of anhydrous ethanol, and grind into a uniform paste. Uniformly coat this paste onto a copper foil, and dry at 120°C for 6 hours to obtain the bamboo carbon anode sheet. 9. Electrochemical performance testing method: The coin cells are primarily tested for cycle life and constant current charge-discharge performance using a Land test system. Embodiment 2 The basic method is consistent with Example 1, with the difference being that the hydrothermal treatment temperature is 160°C for a reaction time of 60 minutes. Embodiment 3 The basic method is consistent with Example 1, with the difference being that the hydrothermal treatment temperature is 200°C for a reaction time of 60 minutes. Embodiment 4 The basic method is consistent with Example 1, with the difference being that the hydrothermal treatment temperature is 220°C for a reaction time of 60 minutes. Embodiment 5 The basic method is consistent with Example 1, with the difference being that the hydrothermal treatment is conducted at 180°C for a reaction time of 30 minutes. Embodiment 6 The basic method is consistent with Example 1, with the difference being that the hydrothermal treatment is conducted at 180°C for a reaction time of 90 minutes. Embodiment 7 The basic method is consistent with Example 1, with the difference being that the disk grinder gap used is 0.1 μm. Embodiment 8 The basic method is consistent with Example 1, with the difference being that the disk grinder gap used is 2 μm. Embodiment 9 The basic method is consistent with Example 1, with the difference being that the grinding is performed 40 times. Embodiment 10 The basic method is consistent with Example 1, with the difference being that the solid content of the Dendrocalamus sinicus bamboo slurry is 10%. Comparative Example 1 This comparative example primarily uses a commercial glass fibre (GF) separator (purchased from Shanghai Titan Scientific Co., Ltd., model GF / A-1820). Comparative Example 2 This comparative example primarily uses a commercial polypropylene (PP) separator (purchased from Shanghai Dingze Industrial Co., Ltd., model Celgard 2500). Comparative Example 3 This comparative example primarily uses a commercial ceramic separator (purchased from Shanghai Dingze Industrial Co., Ltd., model 12+2 μm ceramic membrane, PE single-sided coated with alumina particles). The electrochemical performance indicators of the sodium-ion batteries obtained from the above examples and comparative examples are compared as shown in Table 1 below: Table 1: Performance Indicators of Products from Each Example Embodiment Electrolyte Initial Capacity Rate Performance No. Contact Discharge Retention Angle (°) Specific Rate After -1 -1 100mA g 1000mA g Capacity 500 Cycles (%) (mAh g⁻¹) Embodiment 1 = 0 298.3 98.4% 298.3 221.4 Embodiment 2 = 0 272.3 96.5% 272.3 193.9 Embodiment 3 = 0 258.4 94.6% 258.4 178.2 Embodiment 4 = 0 246.8 92.8% 246.8 169.1 Embodiment 5 = 0 280.7 97.4% 280.7 200.4 Embodiment 6 = 0 265.2 91.9% 265.2 180.7 Embodiment 7 = 0 288.4 97.6% 288.4 206.8 Embodiment 8 = 0 284.3 97.8% 284.3 202.4 Embodiment 9 = 0 291.6 98.1% 291.6 212.9 Embodiment 10 = 0 273.2 96.8% 273.2 192.6 Comparative = 5 132.2 90.4% 132.2 52.8 Example 1 Comparative 61.25 102.7 86.1% 92.7 39.4 Example 2 Comparative 49.25 83.5 84.3% 83.5 25.9 Example 3 As can be seen from the performance indicators of the products prepared in each example in Table 1, the electrolyte affinity of the separator material prepared according to the present invention is significantly higher than that of the two commercial separators, PP and ceramic. Furthermore, when the separator material prepared by the present invention is assembled into a sodium-ion coin cell, the initial discharge specific capacity, cycling performance at various cycle numbers (discharge specific capacity at the same current density), and rate capability (discharge specific capacity at different current densities) are all extremely significantly superior to those of the three commercial separators (GF, PP, and ceramic). In summary, the raw materials of the present invention are abundant in source and possess green renewability. By employing vacuum filtration technology, the process is simple, streamlined, and efficient. It can also provide the biomass-based separator material with high porosity, which is beneficial for electrochemical reactions and sodium ion transport. Retaining lignin on the cellulose basis eliminates the time, labour, and resource consumption associated with lignin removal, and confers a favourable electrochemical specific surface area to the prepared separator material, enhancing sodium ion transport channels. Simultaneously, it overcomes issues such as poor electrolyte wettability of traditional polyolefin separators. When the prepared lignin-coated cellulose nanofiber membrane is used as the separator material, the battery exhibits outstanding performance in terms of specific capacity, cycling stability, and rate capability. The above description is merely a further detailed explanation of the present invention and does not constitute a limitation thereof. For those skilled in the art related to the present invention, several variations or substitutions based on the inventive concept can be made without departing from it. All such equivalent variations or substitutions are encompassed within the scope defined by the claims of this application and are protected by patent law.

Claims

1. A method for preparing a separator for sodium-ion batteries of with lignin-coated cellulose nanofibers, which includes the following steps: (1) pretreatment of bamboo particles via a hydrothermal method, whereby  the bamboo particles have a size of 20 - 80 mesh;  the pretreatment process includes:  a period of weeks in water,  heat to 120 - 240 °C, and  carrying out a hydrothermal reaction for 30 - 180 minutes, to obtain a bamboo particle suspension; (2) subsequently performing a nanonization of the bamboo particles via mechanical disintegration to obtain lignin-coated cellulose nanofibers, comprising:  the preparation of a bamboo particulate suspension with a solid content of 5% - 10%,  grinding repeatedly using a disc mill to form a suspension of to obtain lignin-coated cellulose nanofibers, and subsequently  the successive addition of the suspension of the lignin-coated cellulose nanofibers, water and an isopropanol solution in a volume ratio from 15 - 50 : 4 : 32 and mix thoroughly to form a mixture;  subjecting the mixture to ultrasonic vibrations and magnetic stirring room temperature to obtain a uniform dispersion of the suspension; (3) removed by vacuum filtration from the suspension of the lignin-coated cellulose nanofibers forming a film, comprising:  the application of a vacuum filtration device for filtration to an initial with to obtain a membrane of lignin-coated cellulose nanofibers;  immersing the obtained first in tert-butanol for a period of time membrane; and finally  vacuum drying of the membrane of the lignin-coated cellulose nanofibers available, i.e. the separator for sodium-ion batteries.

2. The method for preparing according to conclusion 1, where in step (1):  the bamboo particles have a size of 30 - 60 mesh; and  the pretreatment process includes: soaking in water for a certain period of time, heat to 160 - 220 °C and carry out a for 30 - 90 minutes hydrothermal reaction.

3. The method for preparing according to conclusion 2, where in step (1):  the bamboo particles have a size of 40 mesh; and  the pretreatment process includes: soaking in water for a certain period of time, heat to 180 °C and carry out a hydrothermal for 60 minutes reaction 4. The method for preparation according to conclusion 1, whereby in step (1) the bamboo particles are prepared by:  removing dust and dirt from the surface of with distilled water bamboo strips,  grinding the bamboo strips in a high-speed blender, and  sieving through a sieve with a mesh size of 20 - 80 mesh.

5. The method for preparing according to conclusion 1, whereby in step (2):  for grinding with the disc mill, use is made of a grinding mill with a opening of 0.1 - 2 μm for ultrafine grinding, and  the grinding is performed 30 - 40 times.

6. The method for preparing according to conclusion 1, whereby in step (2):  a suspension of bamboo particles with a solid content of 5% is prepared, and  the suspension of the lignin-coated cellulose nanofibers, water and isopropanol solution be added successively in a volume ratio of 15 : 2 : 8 and be mixed completely to form the mixture.

7. The method for preparing according to conclusion 1, whereby in step (3):  the vacuum filtration device is used for filtration,  the obtained initial membrane is immersed in tert-butanol for 12 - 24 hours submerged, for 1 - 2 hours to ultrasonic vibrations and for 6 - 12 hours is subjected to magnetic stirring, and is vacuum-dried before the obtaining the separator for sodium-ion batteries made of lignin-coated cellulose nanofibers.

8. A separation material for sodium-ion batteries, being the separator for sodium-ion batteries of lignin-coated cellulose nanofibers, prepared according to the method for preparing according to any of claims 1 - 7.

9. The separation material for sodium-ion batteries, conclusion 8, where in the separator of the sodium-ion batteries:  the lignin-coated cellulose nanofibers have an average diameter of 10 - 20 nm, and  the lignin particles have an average particle size of 25 - 40 nm.

10. A sodium-ion battery, where the separator material of the sodium-ion battery the is separation material for sodium-ion batteries according to claim 8 or 9. Fig. 1 Fig. 2

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