Lignin composite diaphragm as well as preparation method and application thereof

Abstract

The invention provides a lignin composite diaphragm as well as a preparation method and application thereof, and belongs to the technical field of lithium ion batteries. The lignin composite diaphragm is obtained by dissolving macromolecules and lignin in an organic solvent to obtain a spinning solution, preparing a lignin-based composite fiber membrane through electrostatic spinning, and sequentially carrying out hot-pressing treatment and heat treatment on the lignin-based composite fiber membrane. The ionic conductivity of the lignin composite diaphragm prepared by a hot pressing and heat treatment system is not less than 1.0 mS.cm <-1 >; the electrolyte contact angle is not higher than 20 degrees; the electrolyte absorption rate is not lower than 270%; the thermal decomposition temperature is not lower than 300 DEG C; the tensile strength is not lower than 15 MPa, so that the composite material is suitable for large-scale industrial lithium battery production, and shows extremely wide application prospects in the fields of lithium batteries, sodium batteries and the like.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a lignin composite separator, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, due to their high energy density, long cycle life, and low self-discharge rate, have become core energy storage devices in portable electronic devices, electric vehicles, and large-scale energy storage systems. Within the battery's internal structure, the separator, as a key functional component, plays a crucial role in isolating the positive and negative electrodes to prevent short circuits while simultaneously providing ion transport channels. Its performance directly affects the battery's overall electrochemical performance and safety. An ideal battery separator needs to possess high ion conductivity, good electrolyte wettability and retention capacity, excellent thermal stability, and sufficient mechanical strength.

[0003] Currently, commercially available lithium-ion batteries generally use polyolefin separators, primarily polypropylene (PP) and polyethylene (PE) or their multilayer composites. While these separators possess good chemical stability and certain mechanical properties, their inherent characteristics lead to several significant limitations: First, polyolefins have low surface energy and weak polarity, resulting in poor wetting of commonly used organic electrolytes, thus affecting the uniformity of electrolyte distribution and ion transport efficiency. Second, they suffer from insufficient thermal stability, with melting points typically between 130–165°C. Under higher temperatures or localized overheating conditions, they are prone to softening, melting, or even shrinkage, potentially causing electrode contact issues leading to internal short circuits and increasing the risk of thermal runaway. Third, their raw materials are derived from petrochemical products, meaning their costs are affected by fluctuations in crude oil prices, and their production and disposal processes impose an environmental burden, contradicting sustainable development goals.

[0004] To address these challenges, researchers have recently focused on developing bio-based membrane materials based on renewable resources. Cellulose and its derivatives have attracted attention due to their wide availability, biodegradability, and good hydrophilicity and thermal stability. Lignin, the second most abundant natural polymer, is mainly derived from byproducts of the paper industry and agricultural and forestry waste, offering advantages such as low cost, renewability, and biodegradability. Its molecular structure is rich in oxygen-containing functional groups such as hydroxyl, carboxyl, and methoxy groups, endowing the material with certain polarity and reactivity, which helps enhance the interaction with the electrolyte and improve the wettability and ion transport capacity of the membrane.

[0005] A few studies have explored the application of lignin in lithium-ion battery separators. For example, the reference (A novel high-performance electrospun of polyimide / lignin nanofibers with unique electrochemical properties and its application as lithium-ion battery separators[J], International Journal of Biological Macromolecules, 2023, 246, 25668) introduces a novel high-performance electrospun polyimide / lignin nanofiber with unique electrochemical properties and its application in lithium-ion battery separators. Polyimide (PI) composites, through thermal cross-linking treatment to improve their thermal stability and mechanical strength, result in separators that are less prone to shrinkage at high temperatures and effectively inhibit lithium dendrite growth. Furthermore, lignin, after chemical modification such as sulfonation and phosphorylation, can further enhance its compatibility with the electrolyte and ion transport capacity. The related composite separators exhibit good electrochemical stability in high-voltage battery systems.

[0006] However, the direct application of lignin in battery separators still faces key bottlenecks: its poor chemical stability in organic electrolyte systems makes it prone to dissolution, swelling, or structural delamination during long-term cycling or storage, leading to separator structural damage, functional degradation, and potentially triggering side reactions that severely impact battery cycle life and safety. While existing research has improved lignin stability to some extent through methods such as compounding, cross-linking, and modification, its behavior and durability under long-term electrochemical conditions remain the core issues restricting its practical application.

[0007] Therefore, developing novel composite separators that can balance high ionic conductivity, excellent thermal stability, good electrolyte compatibility, good chemical and structural stability, environmental friendliness, and controllable cost has become an important research direction in the field of lithium-ion battery materials. Against this backdrop, further optimizing lignin composite and stabilization strategies to construct lignin-based separator systems with long-lasting durability not only has significant scientific research value but also holds important application prospects for promoting the safer and more sustainable development of battery technology. Summary of the Invention

[0008] To address the aforementioned problems in existing technologies, this invention provides a lignin composite separator, its preparation method, and its application. The lignin composite fiber membrane is prepared by electrospinning of polymers and lignin. Further, through hot pressing and low-temperature heat treatment, a lignin-based separator system with long-lasting durability is constructed for use in the preparation of lithium-ion battery separators, enhancing the affinity between the separator and the electrolyte and the Li-ion battery's performance. + The migration number further improves the electrochemical performance of lithium batteries.

[0009] This invention provides a method for preparing a lignin composite membrane, comprising the following steps: (1) The polymer and lignin are dissolved in an organic solvent to obtain a spinning solution, and lignin-based composite fiber membranes are prepared by electrospinning; (2) The lignin-based composite fiber membrane is subjected to hot pressing and heat treatment in sequence to prepare a lignin composite membrane.

[0010] This invention is based on the formation of porous lignin composite membranes by electrospinning and subsequent heat treatment of polymers and lignin. The lignin component in the composite membrane is rich in polar functional groups, such as phenolic hydroxyl groups, carboxyl groups, and methoxy groups. These groups can interact with polar solvent molecules (such as ethylene carbonate EC) and lithium salt anions (such as PF6) in the electrolyte through hydrogen bonds, dipole-dipole interactions, etc. - The lignin composite membrane exhibits strong affinity, effectively reducing the solid-liquid interface energy and promoting the spreading and penetration of the electrolyte on the membrane surface. Furthermore, the porous fiber structure formed through electrospinning, hot pressing, and subsequent heat treatment has a high specific surface area and interconnected pores, which facilitates the rapid penetration of the electrolyte into the interior through capillary action. Hot pressing enhances the smoothness and density of the lignin composite membrane. After the cross-linking reaction occurs during heat treatment, a stable network is formed between the fibers, resulting in a uniform pore structure and unobstructed channels, further enhancing the storage and transport capacity of the electrolyte, reducing dissolution, and improving mechanical strength.

[0011] Lignin possesses advantages such as low cost, renewability, and biodegradability. Its molecular structure is rich in oxygen-containing functional groups such as hydroxyl, carboxyl, and methoxy groups, endowing the material with certain polarity and reactivity. This helps enhance the interaction with the electrolyte, improve the wettability and ion transport capacity of the membrane, and enhance the Li... + The migration number further improves the electrochemical performance of lithium batteries.

[0012] Preferably, the polymer in step (1) is selected from one or more of polyacrylonitrile, polyimide, and polyamide.

[0013] Preferably, in step (1), the mass ratio of polymer to lignin is 1-10:1.

[0014] More preferably, in step (1), the mass ratio of polymer to lignin is 1:1.

[0015] Preferably, the organic solvent in step (1) is selected from one or more of N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0016] More preferably, the organic solvent in step (1) is N,N-dimethylacetamide.

[0017] Preferably, the electrospinning voltage in step (1) is 20-30 kV; The spinning solution injection rate is 1-2.5 mL / h; The spinning temperature is 20-50℃; The spinning humidity is 30-60%.

[0018] Controlling the electrospinning technical parameters within the above range can achieve better spinning results.

[0019] More preferably, in step (1), the electrospinning voltage is 20-30 kV; the spinning solution injection rate is 2.5 mL / h; the spinning temperature is 25℃; and the spinning humidity is 50%.

[0020] Preferably, the lignin-based composite fiber membrane prepared by electrospinning in step (1) has a porous structure with a fiber diameter of 0.5-1.0 μm, a porosity of 40-50%, and an average pore size of 0.6-1.0 μm.

[0021] Preferably, the temperature of the hot pressing treatment in step (2) is 80-150℃, the pressure is 2-20 MPa, and the time is 5-90 min.

[0022] More preferably, the hot pressing temperature in step (2) is 100 °C, the pressure is 10 MPa, and the time is 1 h.

[0023] Preferably, the heat treatment temperature in step (2) is 180-250℃ and the time is 2-5 h.

[0024] At the temperatures described above during heat treatment, a thermal cross-linking reaction occurs. Specifically, the active groups such as phenolic hydroxyl and carboxyl groups in lignin molecules interact with the cyano groups on the polyacrylonitrile molecular chains, forming a stable physical entanglement and partially chemically cross-linked network. This cross-linked structure effectively restricts the migration and dissolution of lignin molecules in the electrolyte, enhances the bonding force between fibers, and thus significantly improves the durability of the membrane in the electrolyte environment.

[0025] More preferably, the heat treatment temperature in step (2) is 200 °C and the time is 2 h.

[0026] The present invention also provides a lignin composite membrane prepared by the preparation method described above.

[0027] The lignin composite membrane prepared by this invention has lignin and polymer uniformly dispersed, exhibiting stable physicochemical properties, high porosity, and Li... + Number of migrations.

[0028] Preferably, the lignin composite membrane has at least one of the following properties: ionic conductivity not less than 1.0 mS·cm -1 The electrolyte contact angle is not higher than 20°; the electrolyte absorption rate is not lower than 270%; the thermal decomposition temperature is not lower than 300℃; and the tensile strength is not lower than 15 MPa.

[0029] The present invention also provides the application of the lignin composite separator in the preparation of lithium battery separators.

[0030] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention mixes lignin and polymer and uses electrospinning to make a composite nanofiber membrane. The composite membrane material with nanofiber network structure is obtained by hot pressing and heat treatment. The manufacturing method is simple, green and safe, and can realize large-area continuous production.

[0031] (2) The lignin composite membrane prepared by the present invention improves the ionic conductivity and electrolyte wettability, thereby improving the stability of lignin in lithium batteries.

[0032] (3) In the lignin composite membrane prepared by this invention, lignin is uniformly dispersed within the nanofibers, increasing the active sites, improving the electrolyte absorption rate, and enhancing the thermal decomposition temperature and thermal stability. This addresses the problems of easy leaching and numerous side reactions of traditional lignin in electrolytes. It is suitable for large-scale industrial lithium battery production and shows extremely broad application prospects in lithium batteries, sodium batteries, and other fields. This lignin nanofiber membrane can provide a certain reference for the efficient utilization of bio-based lignin. Attached Figure Description

[0033] Figure 1 A schematic diagram of the process for preparing lignin composite membranes.

[0034] Figure 2 The image shown is a scanning electron microscope (SEM) image of the lignin composite membrane prepared in Example 1.

[0035] Figure 3 The image shows the pore size distribution of the lignin composite membrane prepared in Example 1.

[0036] Figure 4 SEM image of the lignin composite membrane without heat treatment.

[0037] Figure 5Infrared data image of the lignin composite membrane prepared in Example 1.

[0038] Figure 6 The stability of the lignin composite membranes prepared in Example 1 and Comparative Example 1 in the electrolyte.

[0039] Figure 7 The discharge curves of the lignin composite membrane prepared in Example 1 at different rates.

[0040] Figure 8 The cycling curve of the lignin composite membrane prepared in Example 1 at a 2C rate.

[0041] Figure 9 The diagram shows the strength elongation of the lignin composite membrane prepared in Example 1.

[0042] Figure 10 The contact angle between the lignin composite membrane prepared in Example 1 and the electrolyte.

[0043] Figure 11 Thermogravimetric (TG) diagram of the lignin composite membrane prepared in Example 1.

[0044] Figure 12 Differential thermogravimetric diagram (DTG diagram) of the lignin composite membrane prepared in Example 1.

[0045] Figure 13 The lignin composite membrane prepared in Example 1 was tested at 2 mA·cm⁻¹. -2 2 mAh·cm -2 Cyclic performance of Cu-Li batteries.

[0046] Figure 14 The charge-discharge curves of the lignin composite separator prepared in Example 1 when assembled into a Li-Li battery.

[0047] Figure 15 The charge-discharge curves of the lignin composite separator prepared in Example 1 at different charge-discharge rates when assembled into a Li-Li battery. Detailed Implementation

[0048] The present invention will be described in detail below with reference to specific embodiments, and further explained in order to better understand the technical connotation of the present invention. However, the scope of protection of the present invention is not limited to the following implementation scope.

[0049] All reagents used in the examples were analytical grade, and the experimental water was deionized water. Polyacrylonitrile (PAN, P1361) was provided by Siber Chemical Reagent Co., Ltd. (China). N,N-Dimethylformamide (DMF) was provided by Sinopharm Chemical Reagent Co., Ltd. A 1.0 mol / L LiPF6 solution in ethylene carbonate / methyl ethyl carbonate / methyl diethyl carbonate (volume ratio 1:1:1) was purchased from Dodo Chem and used as the electrolyte. The control diaphragm was Celgard 2320, the electrode active material was lithium iron phosphate, and the binder was polyvinylidene fluoride, all from KELU.

[0050] Example 1 A method for preparing a lignin composite membrane, such as Figure 1 As shown, it includes the following steps: 9 g of lignin and 9 g of polyacrylonitrile (PAN) were added in batches to 91 g of N,N-dimethylformamide (DMF), and stirred at 60 °C for 12 h to obtain a spinning solution. PAN fiber-based membranes were prepared using electrospinning under a high-voltage electric field. The electrospinning voltage was 25 kV, the spinning solution injection rate was 2.5 mL / h, and the spinning temperature and humidity were 25 °C and 50%, respectively. The PAN fiber-based membrane was first hot-pressed at 100 °C and 10 MPa for 1 h using a flat vulcanizing machine. Then, the hot-pressed PAN fiber-based membrane was sandwiched between two graphite sheets and placed in an oven for heat treatment at 200 °C for 2 h to obtain a polyacrylonitrile / lignin PL200 composite membrane.

[0051] Figure 2 The image shows a SEM image of the heat-treated PL200 composite membrane. The image reveals that the prepared PL200 composite membrane has a small diameter, resulting in good electrolyte wettability. Furthermore, after heat treatment, both the fiber diameter and pore size are reduced to some extent. Figure 3 The pore size distribution data of the PL200 composite membrane shows that the pore size of the PL200 membrane is mostly concentrated between 600-700 nm.

[0052] Example 2 A method for preparing a lignin composite membrane includes the following steps: 9 g of lignin and 9 g of polyacrylonitrile (PAN) were added in batches to 91 g of N,N-dimethylformamide (DMF), and stirred at 80 °C for 12 h to obtain a spinning solution. PAN fiber-based membranes were prepared using electrospinning under a high-voltage electric field. The electrospinning voltage was 30 kV, the spinning solution injection rate was 2.5 mL / h, and the spinning temperature and humidity were 25 °C and 50%, respectively. The PAN fiber-based membrane was first hot-pressed at 100 °C and 10 MPa for 1 h using a flat vulcanizing machine. Then, the hot-pressed PAN fiber-based membrane was sandwiched between two graphite sheets and placed in an oven for heat treatment at 220 °C for 2 h to obtain a polyacrylonitrile / lignin PL220 composite membrane.

[0053] Example 3 A method for preparing a lignin composite membrane includes the following steps: 9 g of lignin and 9 g of polyamide were added in batches to 91 g of N,N-dimethylformamide (DMF), and stirred at 70 °C for 12 h to obtain a spinning solution. PAN fiber-based membranes were prepared using electrospinning under a high-voltage electric field. The electrospinning voltage was 28 kV, the spinning solution injection rate was 2.5 mL / h, and the spinning temperature and humidity were 25 °C and 50%, respectively. The fiber membrane was first hot-pressed at 100 °C and 10 MPa for 1 h using a flat vulcanizing machine. Then, the hot-pressed PAN fiber-based membrane was sandwiched between two graphite sheets and placed in an oven for heat treatment at 180 °C for 2 h to obtain a polyamide / lignin PL180 composite membrane.

[0054] Example 4 9 g of lignin and 9 g of polyacrylonitrile (PAN) were added in batches to 91 g of N,N-dimethylformamide (DMF), and stirred at 60 °C for 12 h to obtain a spinning solution. PAN fiber-based membranes were prepared using electrospinning under a high-voltage electric field. The electrospinning voltage was 25 kV, the spinning solution injection rate was 2.5 mL / h, and the spinning temperature and humidity were 25 °C and 50%, respectively. The PAN fiber-based membrane was first hot-pressed at 150 °C and 20 MPa for 1 h using a flat vulcanizing machine. Then, the hot-pressed PAN fiber-based membrane was sandwiched between two graphite sheets and placed in an oven for heat treatment at 250 °C for 2 h to obtain a polyacrylonitrile / lignin PL250 composite membrane.

[0055] Comparative Example 1 The preparation method of Comparative Example 1 is the same as that of Example 1, except that no heat treatment is performed. Figure 3 The image shows an SEM image of the PL200 composite membrane without heat treatment. It can be seen from the image that the pore size of the PL membrane without heat treatment is mostly around 1 μm.

[0056] Comparative Example 2 The preparation method of Comparative Example 2 is the same as that of Example 1, except that hot pressing was not performed. Details are as follows: 9 g of lignin and 9 g of polyacrylonitrile (PAN) were added in batches to 91 g of N,N-dimethylformamide (DMF), and stirred at 60 °C for 12 h to obtain a spinning solution. PAN fiber-based membranes were prepared by electrospinning under a high-voltage electric field. The electrospinning voltage was 25 kV, the spinning solution infusion rate was 2.5 mL / h, and the spinning temperature and humidity were 25 °C and 50%, respectively. The obtained PAN fiber-based membrane was sandwiched between two graphite sheets and placed in an oven for heat treatment at 200 °C for 2 h to obtain an untreated polyacrylonitrile / lignin PL200 composite membrane.

[0057] Detection Example 1 PL200 composite membranes of polyacrylonitrile / lignin have a thickness of 1750~1000 cm⁻¹. -1 Infrared spectral analysis in the wavenumber range showed: 1750~1650 cm⁻¹ -1 The absorption peaks in the range are attributed to the stretching vibration of the carbonyl group (-C=O), a feature originating from ester groups (such as ferulic acid ester and p-coumaric acid ester) in the lignin structure and oxygen-containing functional groups remaining during the preparation of polyacrylonitrile; 1650~1550 cm⁻¹ -1 The absorption peaks in this region correspond to the stretching vibrations of the carbon-carbon double bond (C=C), mainly originating from the skeletal vibrations of the lignin aromatic ring, but also contributing to the unsaturated bonds in the polyacrylonitrile side chains; 1550~1450 cm⁻¹ -1 The absorption peak at 1300–1200 cm⁻¹ is due to the bending vibration of the methylene group (-CH₂-), reflecting the aliphatic carbon chain structure of the polyacrylonitrile backbone and the presence of lignin alkyl side chains; -1 The strong absorption peak is a characteristic stretching vibration of the aryl ether bond (Ar-OC), which is a typical linking bond between lignin phenylpropane units (such as the guaiacol-syringyl ether bond), confirming the retention of lignin in the composite membrane; 1150~1050 cm⁻¹ -1 The absorption peak corresponds to the stretching vibration of the methoxy group (-OCH3), further verifying the methoxy substitution characteristics of the guaiacol and syringyl units in the lignin structure. These characteristic peaks collectively indicate that the PL200 composite membrane of polyacrylonitrile / lignin successfully retains the characteristic structures of lignin, such as the aromatic ring, aryl ether bond, and methoxy group, while also incorporating the aliphatic carbon chain and carbonyl functional group of polyacrylonitrile, confirming the effective composite of lignin and polyacrylonitrile at the molecular level.

[0058] Detection Example 2 To further evaluate the chemical stability of the composite separator in the battery operating environment, a systematic comparative experiment on electrolyte tolerance was conducted between the untreated PL separator (Comparative Example 1) and the separator treated with high temperature. The specific steps were as follows: Both separators were cut into rectangular samples of 1 cm × 3 cm and placed in glass sample vials containing equal amounts of LiPF6-based organic electrolyte (EC / DMC = 1:1, v / v), ensuring complete immersion of the separators. The experiment was conducted at room temperature (25℃) under an inert atmosphere to simulate the internal environment of a real battery.

[0059] By observing and recording the changes in diaphragm morphology and electrolyte color at regular intervals, the results are as follows: Figure 6 As shown, the untreated polyacrylonitrile-based PL membrane (Comparative Example 1) showed significant changes in the initial soaking stage: after 1 hour of soaking, the membrane edges began to swell, and the electrolyte gradually turned pale yellow; after 12 hours of soaking, the membrane structure became noticeably loose, and the electrolyte color deepened to brownish-yellow, indicating that a large amount of lignin had dissolved. In contrast, the heat-treated PL200 membrane maintained its morphology intact within the same time frame, without significant swelling or disintegration, and the electrolyte remained clear and transparent, with only slight color changes after prolonged soaking (>7 days).

[0060] These results clearly demonstrate that heat treatment significantly improves the chemical and structural stability of the composite membrane in the electrolyte. The mechanism primarily stems from the thermal cross-linking reaction that occurs during heat treatment: at 180-250℃, the active groups such as phenolic hydroxyl and carboxyl groups in lignin molecules interact with the cyano groups on the polyacrylonitrile molecular chains, forming a stable physical entanglement and partially chemically cross-linked network. This cross-linked structure effectively restricts the migration and dissolution of lignin molecules in the electrolyte, enhances the bonding force between fibers, and thus significantly improves the membrane's durability in the electrolyte environment.

[0061] Detection Example 3 Next, the electrochemical performance of lithium-ion batteries prepared using the lignin composite membrane, PAN-based membrane (Specture, P1361), and commercially available Celgard membrane (CELGARD, 2320) prepared in the examples and comparative examples was tested.

[0062] Figure 7 and Figure 8The discharge curves of lithium-ion batteries assembled with composite fiber separator PL (Comparative Example 1) and PL200, PAN base film and Celgard separator at different rates (0.2C, 0.5C, 1C, 2C, 5C and 10C) and the cycle performance curve at 2C rate are presented. As the rate increases, the discharge plateau of the lithium-ion batteries assembled with PL (Comparative Example 1) and PL200 separator, PAN base film and Celgard separator all decrease, and the discharge specific capacity all decrease. The reason for this phenomenon is related to the polarization of the battery. The lithium-ion battery assembled with the PL200 separator exhibited discharge specific capacities of 168 mAh / g, 165 mAh / g, 152 mAh / g, 147 mAh / g, and 126 mAh / g at rates of 0.2C, 0.5C, 1C, 2C, 5C, and 10C, respectively. The battery assembled with the PL200 separator demonstrated a higher discharge specific capacity than those assembled with the Celgard separator and the PAN-based membrane. This indicates that the electrochemical performance of the lignin-incorporated PL200 composite fiber separator is significantly superior to that of the original PAN-based membrane and the commercially available Celgard separator. Figure 8 As can be seen, their performance is relatively stable at a 2C rate.

[0063] Detection Example 4 After completing the physicochemical characterization of the separator, a systematic mechanical property test was conducted on the PL200 composite fiber separator. The testing process strictly followed the standard material mechanics evaluation procedure: First, the PL200 composite fiber separator sample prepared in Example 1 was cut into standard strips with dimensions of 1 mm × 50 mm, ensuring that its long axis direction was consistent with the fiber orientation. Subsequently, a uniaxial tensile test was performed using a high-precision fiber strength tester at a tensile rate of 5 mm / min. The test was conducted under normal temperature and humidity (25°C, 50% RH) conditions to evaluate the mechanical stress that the separator would withstand during simulated battery assembly and use.

[0064] Test results are as follows Figure 9 As shown, its stress-strain curves reveal that the PL200 composite fiber separator has a tensile strength of approximately 15 MPa and an elongation at break of approximately 6%. This data indicates that the separator possesses high mechanical strength, meeting the basic requirements for lithium-ion battery separators to withstand internal tension and external extrusion in practical applications. However, its relatively low elongation reflects limited material toughness and generally poor flexibility.

[0065] This mechanical behavior is mainly attributed to the material composition and structural characteristics of the diaphragm. On the one hand, lignin and polyacrylonitrile undergo a thermal cross-linking reaction during heat treatment at 200°C, forming stable physical and chemical bonds between the fibers. This enhances the integrity of the fiber network and the load transfer efficiency, thereby significantly improving the tensile strength of the material. On the other hand, the benzene ring groups abundant in the lignin molecular structure, while imparting high rigidity and strength to the material, also restrict the mobility and deformability of the molecular chains. This results in the material being less prone to plastic deformation under stress, leading to lower elongation at break and insufficient toughness.

[0066] Case 5 To further evaluate the interfacial compatibility between the PL200 diaphragm and the electrolyte, a systematic wettability analysis of the diaphragm surface was performed using a contact angle meter. The test was conducted at room temperature (25℃) and in a dry atmosphere. A fixed volume (2 μL) of LiPF6-based electrolyte (EC / DMC, 1:1 v / v) was gently deposited onto the diaphragm surface, and the droplet morphology changes were recorded using high-speed imaging. The results are as follows: Figure 10 As shown, the PL200 membrane exhibits excellent electrolyte wetting behavior, with a static contact angle of approximately 18°, which is significantly lower than that of the commercial Celgard membrane (approximately 48.7°) and the pure PAN-based membrane, indicating that the composite membrane has a strong affinity for the electrolyte.

[0067] This excellent wetting performance is mainly attributed to the following two factors: (1) The lignin component in the PL200 membrane is rich in a large number of polar functional groups, such as phenolic hydroxyl, carboxyl, and methoxy groups. These groups can interact with polar solvent molecules (such as ethylene carbonate EC) and lithium salt anions (such as PF6) in the electrolyte through hydrogen bonds, dipole-dipole interactions, etc. - (1) It generates a strong affinity, thereby effectively reducing the solid-liquid interface energy and promoting the spread and penetration of electrolyte on the membrane surface. (2) The porous fiber structure formed by electrospinning and subsequent heat treatment of the membrane has a high specific surface area and interconnected pores, which is conducive to the rapid penetration of electrolyte into the interior through capillary action. In particular, after thermal cross-linking, a stable network is formed between the fibers, the pore structure is uniform and the channels are unobstructed, which further enhances the storage and transmission capacity of electrolyte.

[0068] Good electrolyte wettability has a significant impact on battery performance: on the one hand, it ensures uniform electrolyte distribution within the separator, reducing interfacial impedance; on the other hand, it facilitates efficient lithium-ion transport during charging and discharging, thereby improving the battery's rate performance and cycle stability. In summary, the contact angle test results, from an interface science perspective, confirm that the PL200 separator possesses excellent electrolyte compatibility, providing key interfacial characteristics to support its application in high-efficiency lithium-ion batteries.

[0069] Case 6 Thermogravimetric analysis (TGA) was then performed on the PL200 membrane and its comparative samples to systematically evaluate its thermal stability and component decomposition behavior. The tests were conducted under a nitrogen atmosphere with a heating rate of 10 °C / min, ranging from room temperature to 800 °C. The obtained thermogravimetric TG (TG) values ​​were... Figure 11 ) and differential thermogravimetric analysis (DTG) Figure 12 The curve is shown in the figure.

[0070] TG curve analysis shows that the PL200 membrane exhibits significantly better thermal stability than commercial Celgard membranes and pure PAN membranes. Its main weight loss phase begins at approximately 350°C, a temperature much higher than the melt decomposition temperatures of polyethylene (approximately 135°C) and polypropylene (approximately 165°C) in the Celgard membrane, and also higher than the initial decomposition temperature of the pure PAN membrane (approximately 300°C). At 800°C, the char residue of the PL200 membrane is approximately 55%, higher than that of the comparative example 1 PL membrane (approximately 50%) and the PAN membrane (approximately 45%), indicating that it has higher thermal stability and charring ability.

[0071] DTG curves further revealed the decomposition kinetics of the material. The PL200 membrane exhibited a relatively flat and broad decomposition peak, with the maximum weight loss rate corresponding to a temperature range of 380-450℃. This contrasts sharply with the sharp double-melting decomposition peak of the Celgard membrane and the stepwise decomposition peaks of the PAN membrane near 300℃ and 440℃. The broadening of the PL200 membrane's decomposition peak indicates that its decomposition process is slower and more controlled, mainly attributed to the cross-linked network structure formed between lignin and PAN. This cross-linked structure effectively hinders the rapid dissociation and volatilization of polymer chains upon heating, improving the overall thermodynamic stability of the material.

[0072] Case 7 To systematically evaluate the impact of separator materials on the long-term cycle stability of batteries, CR2032 coin cells were assembled using LiFePO4 (LFP) cathodes and lithium metal anodes. PL (Comparative Example 1), PL200 separators, PAN, and commercially available Celgard2032 separators were used as battery separator components. All batteries were assembled under identical conditions and subjected to constant current charge-discharge cycle tests at 2C rate at an ambient temperature of 25°C, with a cutoff voltage range of 2.5–4.2 V, to investigate the effect of the separator on battery capacity retention and cycle stability.

[0073] The results of the loop test are as follows Figure 13As shown in the figure, under the same test conditions, the cycle performance of batteries assembled with the four types of separators showed significant differences. Among them, the battery with the untreated PL separator (Comparative Example 1) exhibited the most significant capacity decay, with a rapid capacity drop in the early stages of cycling and large fluctuations in coulombic efficiency. In contrast, the heat-treated PL200 separator demonstrated excellent cycle stability, with the highest capacity retention and slow decay during long-term cycling, and the coulombic efficiency remained close to 100%, showing highly reversible electrochemical behavior. The cycle stability of batteries assembled with PAN and Celgard separators was between the two, but both were inferior to those with the PL200 separator.

[0074] The aforementioned differences mainly stem from the structural stability and interfacial characteristics of the diaphragm in the electrolyte environment. For the untreated PL diaphragm (Comparative Example 1), the lignin component gradually dissolves during cycling, leading to the following problems: The membrane structure deteriorates, the pore structure collapses or becomes blocked, and the ion transport channels are damaged. The dissolved lignin may undergo irreversible adsorption or side reactions on the electrode surface, increasing interfacial impedance and consuming active lithium; Continuous leaching of substances leads to a decrease in the mechanical strength of the diaphragm, making it unable to maintain effective electrode isolation function during long-term cycling.

[0075] The heat-treated PL200 separator, due to the formation of a stable thermal cross-linking network between lignin and PAN, effectively anchors lignin within the fiber skeleton, thus significantly inhibiting its dissolution and migration in the electrolyte. Furthermore, the abundant polar functional groups such as hydroxyl and carboxyl groups in the lignin molecules not only enhance the separator's affinity for the electrolyte and promote uniform lithium-ion transport, but also contribute to the formation of a more stable solid electrolyte interphase (SEI) film at the electrode-electrolyte interface, further improving the battery's cycle reversibility.

[0076] Therefore, the PL200 separator effectively improves the cycle performance of the battery through the dual effects of structural immobilization and interface functionalization, which provides a key material basis for its application in high-performance, long-life lithium metal batteries.

[0077] Detection Example 8 To comprehensively evaluate the role of the separator in suppressing lithium dendrites and improving the cycle stability of the lithium metal anode, copper-lithium (Cu|Li) half-cells and lithium-lithium (Li|Li) symmetric cells were assembled and their electrochemical performance was tested. Relevant cycle performance data are as follows: Figures 13 to 15 As shown.

[0078] In Cu‖Li batteries ( Figure 13 In the test, at 2 mA·cm -2 The current density is 2 mAh·cm-2 The capacity of the lithium metal anode was subjected to lithium deposition / stripping cycles to investigate the reversibility of the lithium metal anode under these conditions. Test results showed that the battery using the PL200 separator exhibited the highest and most stable coulombic efficiency (average >98%), maintaining this efficiency for over 120 cycles without significant degradation. In contrast, the battery using the untreated PL separator (Comparative Example 1) showed large fluctuations and rapid degradation in coulombic efficiency, while the batteries using PAN or Celgard separators exhibited a continuous decrease and drastic fluctuations in efficiency around 80 cycles. This demonstrates that the PL200 separator effectively promotes uniform lithium-ion deposition and stripping, significantly reducing irreversible loss of active lithium caused by dendrite growth and "dead lithium" formation.

[0079] In Li‖Li symmetric cells ( Figure 14 , 15 This advantage was further validated in long-cycle testing under the same current density and capacity conditions (1 mA·cm). -2 1 mAh·cm -2 Symmetrical cells using PL200 separators exhibit extremely low and stable overpotentials (stabilizing at approximately 20 mV in the later stages of cycling), with flat and regular voltage-time curves. In contrast, cells using PAN or Celgard separators show significantly higher overpotentials (initially approximately 120 mV), and exhibit irregular fluctuations and a continuous increase with cycling. This is typical of lithium dendrite growth, intensified interfacial side reactions, and unstable thickening of the SEI film.

[0080] In summary, the cycle test results of Cu‖Li and Li‖Li batteries strongly demonstrate that the PL200 composite separator can significantly improve the cycle reversibility and interfacial stability of the lithium metal anode. This is especially true at 2 mA·cm⁻¹. -2 and at higher current densities ( Figure 15 The results show its performance at 1-10 mA·cm -2 Despite its excellent rate performance within the range, it still maintains low overpotential and high cycle efficiency, which provides key electrochemical performance evidence for its application in lithium metal battery systems with high energy density and long cycle life.

[0081] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a lignin composite membrane, characterized in that, Includes the following steps: (1) The polymer and lignin are dissolved in an organic solvent to obtain a spinning solution, and lignin-based composite fiber membranes are prepared by electrospinning; (2) The lignin-based composite fiber membrane is subjected to hot pressing and heat treatment in sequence to prepare a lignin composite membrane.

2. The preparation method according to claim 1, characterized in that, The polymer in step (1) is selected from one or more of polyacrylonitrile, polyimide, and polyamide.

3. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of polymer to lignin is 1-10:

1.

4. The preparation method according to claim 1, characterized in that, The organic solvent in step (1) is selected from one or more of N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

5. The preparation method according to claim 1, characterized in that, In step (1), the electrospinning voltage is 20-30 kV; The spinning solution injection rate is 1-2.5 mL / h; The spinning temperature is 20-50℃; The spinning humidity is 30-60%.

6. The preparation method according to claim 1, characterized in that, The lignin-based composite fiber membrane prepared by electrospinning in step (1) has a porous structure with a fiber diameter of 0.5-1.0 μm, a porosity of 40-50%, and an average pore size of 0.6-1.0 μm.

7. The preparation method according to claim 1, characterized in that, In step (2), the hot pressing temperature is 80-150℃, the pressure is 2-20 MPa, and the time is 5-90 min.

8. The preparation method according to claim 1, characterized in that, In step (2), the heat treatment temperature is 180-250℃ and the time is 2-5 h.

9. A lignin composite membrane prepared by the preparation method according to any one of claims 1-8.

10. The application of the lignin composite separator according to claim 9 in the preparation of lithium battery separators.

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