Flexible lithium manganate positive electrode sheet and preparation method thereof

Abstract

The application discloses a flexible lithium manganate positive pole piece and a preparation method, and belongs to the technical field of lithium ion batteries. The carbon nanofiber membrane with polystyrene sulfonate grafted on the surface and the lithium manganate particles with polystyrene sulfonate adsorbed on the surface are assembled into a filter cake layer in close contact through vacuum filtration, then aniline solution slowly permeates through the filter cake layer and is adsorbed under the condition of no oxidant, so that the aniline monomers are uniformly adsorbed on the polystyrene sulfonate template to form a monomer reaction layer with uniform thickness, and in-situ limited polymerization is carried out, after polymerization and drying, the obtained composite membrane is subjected to gradient heating and hot pressing treatment, rearrangement of polyaniline molecular chains and further compaction at the interface between the carbon fiber and the lithium manganate are realized, and the flexible self-supporting positive pole piece with a positive active film layer free of metal current collectors, a three-dimensional conductive network and a relatively stable conductive state is obtained. The application is suitable for the fields of flexible lithium ion batteries, wearable electronic devices and solid-state lithium batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a flexible lithium manganese oxide positive electrode and its preparation method. Background Technology

[0002] Currently, research on self-supporting positive electrode sheets for flexible lithium-ion batteries mainly focuses on composite systems of carbon nanofiber conductive frameworks and lithium manganese oxide active materials, constructing three-dimensional conductive networks by introducing conductive polymers such as polyaniline. However, existing methods still face challenges such as uneven interfacial coating due to homogeneous nucleation of polyaniline solution during in-situ polymerization, and difficulty in uniformly mixing carbon fibers and lithium manganese oxide particles in the liquid phase due to density differences, which limit the flexibility and electrochemical stability of the electrode sheets.

[0003] To address the above issues, CN121662766A discloses a cathode material, cathode sheet, and battery. This method involves mixing ternary cathode particles, octahedral lithium manganese oxide particles, and spinel-type lithium manganese oxide particles with varying particle sizes. The differences in the shape and size of the different particles are used to increase the compaction density of the electrode sheet. The alkalinity of the ternary material is used to preferentially neutralize the HF in the electrolyte to alleviate manganese leaching. However, this method still relies on traditional coating processes and metal current collectors. The electrode sheet lacks flexible self-supporting capabilities, and the different particles are only physically mixed without chemical bonding or conductive polymer bridging. During charging and discharging, the interface is prone to slippage and detachment. CN121812515A discloses a composite cathode material and its preparation method, cathode sheet, and battery. The method involves co-precipitating a lithium nickel cobalt manganese oxide layer and a lithium iron phosphate layer onto the surface of a lithium manganese oxide core. The physical barrier and chemical neutralization of the double-layer coating inhibit manganese leaching, while the capacity contribution of the coating material is used to improve the overall specific capacity. However, it still requires the addition of conductive agents and binders and coating them onto an aluminum foil current collector, thus failing to achieve a self-supporting electrode without a current collector. Furthermore, the bonding between the coating layer and the core is achieved through high-temperature calcination, which leads to poor interface flexibility and easy cracking during repeated bending, making it unsuitable for dynamic deformation scenarios of flexible devices.

[0004] In summary, existing modification schemes either rely on rigid current collectors and coating processes or lack effective interface design. Therefore, there is an urgent need to develop an electrode that can simultaneously meet the requirements of flexible electrodes for self-support, uniform component distribution, strong interfacial bonding, and bending stability. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes a flexible lithium manganese oxide positive electrode and its preparation method. This invention involves assembling a carbon nanofiber membrane grafted with polystyrene sulfonate and lithium manganese oxide particles adsorbed with polystyrene sulfonate on their surfaces into a tightly contacted filter cake layer via vacuum filtration. Then, aniline solution is slowly permeated through the filter cake layer, and the mixture is allowed to stand and adsorb under oxidant-free conditions. This allows aniline monomers to be uniformly adsorbed onto the polystyrene sulfonate template, forming a monomer reaction layer of uniform thickness. In-situ confined polymerization is then performed on this layer. After polymerization and drying, the resulting composite membrane undergoes gradient-heating hot-pressing treatment to achieve polyaniline molecular chain rearrangement and interfacial compaction. This enhances the physical entanglement, electrostatic interaction, and interfacial contact between the carbon nanofibers and lithium manganese oxide particles, forming a continuous conductive network and a stable self-supporting structure. The resulting flexible, self-supporting positive electrode has a three-dimensional conductive network and a relatively stable conductive state, without metal current collector foil in the positive electrode active film layer. This solves the technical problems of uneven interfacial coating caused by homogeneous nucleation of conductive polymers in solution during in-situ polymerization, and the difficulty in uniform contact between carbon nanofibers and lithium manganese oxide particles in the liquid phase due to density differences.

[0006] This invention proposes a flexible lithium manganese oxide positive electrode sheet, which uses a carbon nanofiber membrane covalently grafted with polystyrene sulfonate as a substrate and loads lithium manganese oxide particles with in-situ polymerized polyaniline on the surface.

[0007] This invention also proposes a method for preparing a flexible lithium manganese oxide positive electrode, such as... Figure 1 As shown, the specific technical solution is as follows: Step 1: Polyacrylonitrile and polymethyl methacrylate are dissolved in DMF to prepare a spinning solution. The spinning solution is then electrospun into a film, pre-oxidized and carbonized to obtain a carbon nanofiber membrane. After oxygen plasma activation and modification with a silane coupling agent, it is reacted with a carboxylated polystyrene sulfonate solution to obtain a carbon nanofiber membrane (PSS-g-CNF) with polystyrene sulfonate grafted on its surface.

[0008] Step 2: After washing the lithium manganese oxide powder with acid, disperse it in the acidic adsorption solution of polystyrene sulfonate and stir to adsorb. After centrifugation and washing, redisperse it in deionized water to obtain a suspension of lithium manganese oxide particles (PSS@LiMn2O4) with polystyrene sulfonate adsorbed on the surface.

[0009] Step 3: Place the PSS-g-CNF membrane on the filter membrane, vacuum filter the PSS@LiMn2O4 suspension to form a filter cake layer, then permeate with aniline solution and let it stand, then permeate with oxidant solution a second time, and obtain the composite membrane after washing and vacuum drying.

[0010] Step 4: The composite membrane is preheated between stainless steel plates, then heated and hot-pressed, and after natural cooling, a flexible self-supporting positive electrode sheet is obtained.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Since the polymerization reaction is confined to the three-dimensional pores of the filter cake layer, and aniline is pre-distributed uniformly on all solid surfaces through static adsorption, the generated polyaniline grows almost entirely at the interface between carbon nanofibers and lithium manganese oxide particles, rather than forming useless flocculent precipitates in the solution, thus improving the selectivity and uniformity of polyaniline interface coating.

[0012] 2. By pre-assembling the filter cake layer through vacuum filtration, the carbon nanofiber membrane acts as a retention layer to uniformly intercept lithium manganese oxide particles in the gaps between its networks, forming a microstructure in which the particles are wrapped by a fiber mesh. This ensures that the active material and the conductive framework have reached an ideal spatial distribution state before the polymerization begins, and will not separate or agglomerate due to density differences or stirring and shearing during subsequent liquid phase treatment.

[0013] 3. By using a slow, permeable dripping method to add the oxidant solution, the amount of monomer participating in the reaction per unit time is strictly controlled, and the heat of reaction is promptly carried away by the continuously flowing liquid phase and negative pressure filtration. The local temperature surge phenomenon commonly seen in traditional batch mixing polymerization does not occur inside the filter cake layer. This effectively protects the temperature-sensitive polyaniline molecular chain structure and the surface lattice integrity of lithium manganese oxide, and avoids the decrease in polyaniline peroxidation doping degree or the phase transition on the surface of lithium manganese oxide caused by overheating. Attached Figure Description

[0014] Figure 1 Here is a flowchart of the sample preparation process; Figure 2 This is a microscopic morphology image of the sample from Example 1; Figure 3 The images show the microstructure of samples 1-3. Detailed Implementation

[0015] The flexible lithium manganese oxide positive electrode sheet of this application is described in further detail below. This does not limit the scope of protection of this application, which is defined by the claims. Certain specific details disclosed provide a comprehensive understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments can be implemented using other materials, etc., without employing one or more of these specific details.

[0016] This invention proposes a method for preparing a flexible lithium manganese oxide positive electrode, such as... Figure 1 As shown, the specific technical solution is as follows: 1. Preparation of PSS-g-CNF membrane Polystyrene sulfonate was grafted onto the surface of carbon nanofiber membranes by electrospinning a mixed solution of polyacrylonitrile and carboxylated polymethyl methacrylate, followed by carbonization for conductivity, plasma activation, and covalent grafting. To construct a fiber network possessing electronic conductivity, mechanical flexibility, and aniline fixation capabilities to serve as the conductive framework and structural support for the entire electrode, a three-dimensional network structure with interlocking fibers was first obtained through electrospinning. This simultaneously met the requirements for long-range electron transport, particle loading space, and bending flexibility. Since polyacrylonitrile fibers would melt and collapse upon direct high-temperature treatment, pre-oxidation was necessary to achieve high-temperature stability and ensure the integrity of the fiber network morphology after carbonization. Simultaneously, carbonization transformed the fibers from insulators to conductors, while the blended polymethyl methacrylate component decomposed and vaporized during carbonization, leaving nanopores on the fiber surface to increase the specific surface area and provide more active sites.

[0017] Because the carbonized fiber surface is inert and polar liquids cannot adhere to it, it needs to be activated by oxygen plasma to change its surface from hydrophobic and inert to hydrophilic and active. However, the activated surface alone cannot stably retain aniline under subsequent acidic conditions. Therefore, a silane coupling agent is used to provide exposed NH2 on the carbon nanofiber surface. In the presence of EDC / NHS, the -COOH in the carboxylated polystyrene sulfonate is linked to NH2 through amide bonds, thereby fixing the polystyrene sulfonate on the fiber surface. In an acidic environment, the polystyrene sulfonate layer does not dissolve and has dense negative potential sites, which can actively adsorb and retain aniline cations, providing a uniform aniline template for subsequent in-situ polymerization.

[0018] 2. Preparation of PSS@LiMn2O4 suspension A uniform PSS coating layer was constructed on the surface of lithium manganese oxide through weak acid pretreatment, electrostatic adsorption with acidic polyelectrolytes, and low pH dispersion. The lithium manganese oxide powder was briefly washed with a low-concentration dilute citric acid or acetic acid to remove alkaline residues from the particle surface and improve particle dispersibility, providing a clean substrate for the subsequent uniform coating of polystyrene sulfonate. Because a low-concentration weak acid was used for washing, and the process was completed within minutes, only a trace amount of lithium manganese oxide was consumed upon its addition, rapidly raising the solution pH to weakly acidic. Under these conditions, acid washing only achieved a gentle surface cleaning of the lithium manganese oxide without significant dissolution or consumption.

[0019] The particles, washed with a weak acid, were dispersed in a weakly acidic aqueous solution containing polystyrene sulfonate and stirred for an extended period. This allowed the polystyrene sulfonate molecules to fully diffuse around each particle and cover its entire outer surface, forming a continuous and firmly bonded coating layer. The particles were then repeatedly centrifuged and washed with deionized water to remove free polystyrene sulfonate and detached impurities, retaining only the coating layer tightly bound to the particle surface. The coated particles were then redispersed in a weakly acidic solution to form a stable suspension. The weakly acidic environment maintained the structural stability of the coating layer and inhibited particle aggregation.

[0020] 3. Vacuum filtration assembly and in-situ confined polymerization Vacuum filtration is used to pre-fix the solid phase, followed by slow aniline permeation pre-adsorption and oxidant-confined polymerization to ensure that polyaniline grows only at the solid-phase interface. In the vacuum filtration stage, a carbon fiber membrane is laid on a microporous filter membrane as the filter medium. After pouring in a lithium manganese oxide suspension, the vacuum is activated to rapidly remove the liquid. Lithium manganese oxide particles, due to their larger size than the filter pore size, are trapped and forced to accumulate on the surface and within the pores of the carbon fiber membrane. Because the filtration speed is much faster than the particle settling speed due to density difference, the spatial distribution of the particles is fixed instantaneously during filtration, forming a filter cake layer where the carbon fibers and lithium manganese oxide are in close contact and uniformly mixed. Subsequently, an aniline solution is slowly dripped from the top of the filter cake layer downwards, allowing the aniline liquid to fully wet all solid surfaces within the filter cake layer. After dripping is complete, the vacuum is closed and the mixture is allowed to stand, allowing aniline molecules to migrate from the liquid phase to the polystyrene sulfonate layer on the solid phase surface and adhere stably. This forms a complete and uniformly thick aniline monomer layer on the surface of the carbon fibers and lithium manganese oxide particles.

[0021] The oxidant infiltration and polymerization stages require a slower flow rate. Upon entering the filter cake layer, the oxidant solution immediately contacts the aniline pre-attached to the solid surface and initiates polymerization. The resulting polyaniline is directly deposited on the solid surface. The slow dripping rate allows the oxidant to enter the filter cake layer in discrete, small doses. Each dose is immediately consumed by the surrounding aniline, preventing the accumulation of sufficient oxidant and free aniline concentrations to initiate homogeneous nucleation. Therefore, the polymerization reaction is strictly confined to the solid-liquid interface. Simultaneously, the continuously flowing liquid carries away the heat released during polymerization, preventing temperature rise within the filter cake layer. After polymerization, rinsing and drying yield a three-dimensional composite membrane linked to polyaniline via electrostatic adsorption, hydrogen bonding, or physical entanglement.

[0022] 4. Gradient heating hot pressing treatment A two-stage hot pressing process, involving low-temperature pre-compression venting and high-temperature, high-pressure densification, transforms loose composite membranes into highly dense, self-supporting electrodes. Because trace amounts of moisture and volatile acid remain inside the composite membrane, direct and rapid heating to high temperatures would cause these residues to vaporize and expand instantly, generating bubbles or causing layer delamination. Therefore, to transform the loosely stacked composite membrane into a dense, strong, and self-supporting integrated electrode film, a two-stage heating process is required for hot pressing.

[0023] The first stage involves pre-compression at lower temperatures and pressures, where the polyaniline is not yet softened. Applying pressure initially compresses and reduces the macroscopic gaps between the fibers and particles. Simultaneously, residual volatiles slowly vaporize under heating and are discharged from the membrane edges and pores with the aid of pressure, making the temperature and stress distribution across the entire membrane cross-section more uniform. In the second stage, further heating increases the movement of polyaniline chain segments and improves interfacial compaction. Simultaneously, applying high pressure significantly compresses the membrane thickness, squeezing the micron-sized gaps between the fibers and particles to the nanometer scale or completely closing them. Meanwhile, the softened polyaniline, driven by pressure, fully fills all gaps and firmly bonds the various solid phase surfaces together, forming a continuous, integrated structure with blurred phase boundaries.

[0024] After hot pressing, natural cooling is required under pressure to freeze the polyaniline molecular chains in the compressed compact structure, preventing the film from generating internal stress or springing back and thickening due to thermal shrinkage, and finally obtaining the required self-supporting flexible lithium manganese oxide positive electrode sheet.

[0025] The following are some specific embodiments of the present invention, and Table 1 shows the raw material information required for each embodiment.

[0026] Table 1 Raw Material Information Table

[0027] Example 1 S1: 7g of polyacrylonitrile and 3g of polymethyl methacrylate were dissolved in 90g of N,N-dimethylformamide and stirred at 60℃ for 12h to obtain a spinning solution. 20mL of the spinning solution was taken and electrospun under the conditions of 18kV voltage, 15cm receiving distance, and 0.8mL / h feed rate. The fiber membrane was collected with aluminum foil and then vacuum dried at 60℃ for 12h. Subsequently, it was pre-oxidized in a tube furnace under air atmosphere by heating to 260℃ at 2℃ / min and holding for 2h. Then, it was switched to nitrogen atmosphere and heated to 900℃ at 5℃ / min and held for 3h. After natural cooling to room temperature, a carbon nanofiber membrane was obtained. The carbon nanofiber membrane was activated by oxygen plasma treatment in an oxygen plasma treatment device under the conditions of 150W power, 50sccm oxygen flow rate, and 20Pa chamber pressure for 4min. The activated membrane was then immersed in a solution containing 2 vol%. The KH-550 was stirred in an ethanol / water solution at 40°C for 4 hours. After removal, it was washed three times with ethanol and twice with deionized water. It was then vacuum dried at 60°C for 8 hours. 0.5 g of sodium poly(4-styrene sulfonate-copoly-maleic acid) was dissolved in 100 mL of MES buffer, and then 96 mg of EDC and 35 mg of NHS were added. The mixture was stirred at room temperature for 30 minutes to prepare an activation solution. The dried carbon nanofiber membrane was immersed in the activation solution and stirred at room temperature for 12 hours. After removal, it was washed three times with dilute hydrochloric acid solution at pH=2 and then washed with deionized water until the conductivity of the washing solution did not exceed 10 μS / cm. The membrane was then vacuum dried at 60°C for 8 hours to obtain a carbon nanofiber membrane with surface grafted polystyrene sulfonate, with a membrane thickness of approximately 60~80 μm.

[0028] S2: Take 5g of lithium manganese oxide powder and add it to 100mL of 0.03mol / L citric acid solution. After ultrasonic dispersion for 10min, centrifuge at 4000rpm for 5min, discard the supernatant and wash once more. Dissolve 1g of sodium polystyrene sulfonate in 100mL of deionized water and adjust the pH to 4.5 with dilute citric acid as the adsorption solution. Disperse the washed lithium manganese oxide particles in the adsorption solution and stir at 200rpm for 8h at room temperature. After centrifugation, wash the precipitate three times with deionized water. Then redisperse the precipitate in 50mL of deionized water with the pH adjusted to 4.5 with dilute citric acid to obtain a lithium manganese oxide suspension with polystyrene sulfonate adsorbed on the surface.

[0029] S3: Take 1.5 mL of aniline, 17.1 mL of glacial acetic acid, and 50 mL of anhydrous ethanol, add them to 800 mL of deionized water and stir to mix. Then, make up to 1000 mL as the permeate. Cut the fiber membrane prepared in S1 into 25 cm sections. 2 The solution was spread evenly on a PVDF filter membrane, and then the membrane was pressed tightly against the Buchner funnel. Under vacuum filtration at -0.09 MPa, the suspension prepared by S2 was slowly poured into the funnel and filtered for 10 minutes, controlling the lithium manganese oxide loading at 8 mg / cm³.2 The funnel containing the filter cake was then transferred to a fume hood and connected to a peristaltic pump. 10 mL of permeate was added dropwise from the top of the filter cake at a flow rate of 0.2 mL / min, while maintaining vacuum filtration during the addition. After the addition was complete, filtration continued for 5 minutes, then the vacuum was turned off and the mixture was allowed to stand in an ice bath at 2°C for 30 minutes. Then, while maintaining the ice bath, 0.85 g of ammonium persulfate and 5.7 mL of glacial acetic acid were dissolved in 150 mL of deionized water by stirring. The solution was then brought to a final volume of 200 mL to prepare the polymerization solution. 10 mL of polymerization solution was added dropwise from the top of the filter cake at a flow rate of 0.1 mL / min, while maintaining vacuum filtration during the addition. After the addition was complete, filtration was continued for 30 min. Then, 50 mL of deionized water was added dropwise from the top of the filter cake at a flow rate of 1 mL / min to wash it. After washing, filtration was continued for 30 min. The funnel was then transferred to a vacuum drying oven and dried at 60 °C and a vacuum of -0.09 MPa for 4 h. The filter cake was then peeled off to obtain a carbon nanofiber / lithium manganese oxide self-supporting composite membrane connected to polyaniline.

[0030] S4: The composite membrane obtained in S3 is laid flat on a stainless steel plate. A layer of polytetrafluoroethylene cloth is covered on the membrane, and then another layer of stainless steel plate is placed on top. The membrane is then placed in a flat hot press. It is first preheated at 80°C and 2MPa pressure for 10 minutes, then heated to 120°C and hot-pressed at 10MPa pressure for 5 minutes. After the heating is turned off, the membrane is kept at 5MPa pressure and allowed to cool naturally to 25°C. The steel plate is then removed and the polytetrafluoroethylene cloth is peeled off to obtain the flexible self-supporting lithium manganese oxide positive electrode sheet.

[0031] Example 2 The difference from the preparation method in Example 1 is as follows: S1: The spinning solution stirring temperature is 40℃, the stirring time is 6h, the pre-oxidation temperature is 240℃, the holding time is 1.5h, the oxygen plasma power is 100W, and the KH-550 concentration is 1.5 vol%. S2: Citric acid concentration is 0.01 mol / L, pH is adjusted to 4.0, stirring speed is 150 rpm, and stirring time is 6 h; S3: The flow rate of the leachate is 0.15 mL / min, the ice bath standing time is 20 min, the flow rate of the polymerization solution is 0.08 mL / min, and the vacuum drying time is 3 h; S4: The preheating pressure is 1.5MPa and the temperature is 60℃. Then the temperature is raised to 100℃ and the pressure is 8MPa. The hot pressing time is 3min. The pressure during the holding period is 3MPa. All other steps are the same.

[0032] Example 3 The difference from the preparation method in Example 1 is as follows: S1: The spinning solution stirring temperature is 80℃, the stirring time is 18h, the pre-oxidation temperature is 280℃, the holding time is 2.5h, the oxygen plasma power is 200W, and the KH-550 concentration is 2.5 vol%. S2: Citric acid concentration is 0.05 mol / L, pH is adjusted to 5.0, stirring speed is 300 rpm, and stirring time is 12 h; S3: Permeate flow rate is 0.3 mL / min, ice bath standing time is 40 min, polymerization solution flow rate is 0.12 mL / min, and vacuum drying time is 6 h; S4: The preheating pressure is 3MPa and the temperature is 90℃. Then the temperature is raised to 140℃ and the pressure is 12MPa. The hot pressing time is 8min. The pressure during the holding period is 8MPa. All other steps are the same.

[0033] Example 4 The difference from the preparation method in Example 1 is as follows: S1: The stirring temperature of the spinning solution is 50℃, the stirring time is 15h, the pre-oxidation temperature is 250℃, the holding time is 2.2h, the oxygen plasma power is 180W, and the KH-550 concentration is 1.8 vol%. S2: Citric acid concentration is 0.02 mol / L, pH is adjusted to 4.8, stirring speed is 250 rpm, and stirring time is 10 h; S3: Permeate flow rate is 0.25 mL / min, ice bath standing time is 25 min, polymerization solution flow rate is 0.09 mL / min, and vacuum drying time is 5 h; S4: The preheating pressure is 2.5MPa and the temperature is 70℃. Then the temperature is raised to 110℃ and the pressure is 9MPa. The hot pressing time is 6min. The pressure during the holding period is 7MPa. All other steps are the same.

[0034] Example 5 The difference from the preparation method in Example 1 is as follows: S1: KH-550 is replaced with KH-560; S2: Citric acid is replaced with glacial acetic acid; S3: Replace the PVDF filter membrane with a PTFE filter membrane; the remaining steps are the same.

[0035] Example 6 The difference from the preparation method in Example 1 is as follows: S1: KH-550 is replaced with KH-792; S2: Citric acid is replaced with oxalic acid; S3: Replace the PVDF filter membrane with a PI filter membrane; the remaining steps are the same.

[0036] Comparative Example 1 The difference from the preparation method in Example 1 is as follows: S3: Instead of adding the permeate and letting it stand, mix aniline, ammonium persulfate and glacial acetic acid in the same proportion, and then add the mixed solution dropwise from the top of the filter cake. All other steps are the same.

[0037] This comparative example prepares an electrode without polyaniline pre-adsorption.

[0038] Comparative Example 2 The difference from the preparation method in Example 1 is as follows: S3: Replace vacuum filtration with gravity filtration, the rest of the steps are the same.

[0039] This comparative example prepares electrode sheets that do not rely on vacuum filtration of pre-assembled filter cake layers.

[0040] Comparative Example 3 The difference from the preparation method in Example 1 is as follows: S4: Directly place the composite film obtained in S3 into a hot press and hot press it at 120℃ and 10MPa for 15 minutes. All other steps are the same.

[0041] This comparative example shows the preparation of electrode sheets without gradient heating and hot pressing.

[0042] Comparative Example 4 The difference from the preparation method in Example 1 is as follows: S2: Replace citric acid with deionized water, do not adjust pH, and keep the rest of the steps the same.

[0043] This comparative example shows the preparation of electrode sheets obtained without adsorption of polystyrene sulfonate under acidic conditions.

[0044] Comparative Example 5 The difference from the preparation method in Example 1 is as follows: S1: The spinning solution is prepared using only polyacrylonitrile without the addition of polymethyl methacrylate, and the remaining steps are the same.

[0045] This comparative example shows the preparation of electrode sheets from carbon nanofiber membranes without pore-forming agents.

[0046] Experimental Example 1 The electrode sheets prepared in Examples 1-6 and Comparative Examples 1-5 were cut into rectangular samples with a width of 10 mm and a length of 60 mm. They were placed in an environment of 25°C and 55% relative humidity for 8 hours. Cylindrical metal mandrels with radii of 10 mm, 5 mm, 3 mm, 2 mm, 1 mm, and 0.5 mm were used to bend the sample 180° along the length direction of the mandrel. After holding the bend for 5 seconds, the sample was removed. The bending area was observed under an optical microscope to check for through cracks, surface cracks, interlayer delamination, or powder shedding. At the same time, the surface resistance of the bending area was measured with a four-probe resistance meter. If there were no defects in appearance and the resistance change rate was ≤15%, the sample was considered to have passed through the radius. The smallest radius that could pass through was taken as the minimum bending radius of the sample.

[0047] The electrodes prepared in Examples 1-6 and Comparative Examples 1-5 were cut into rectangular specimens with a width of 10 mm and a length of 100 mm. They were placed in an environment of 25°C and 55% relative humidity for 8 hours. A dynamic bending fatigue testing machine was used, with the bending radius set as the minimum bending radius of the specimen, the bending angle set to ±180°, and the bending frequency set to 30 times / min. The two ends of the specimen were fixed on the fixture, and the testing machine was started. The bending area was observed with a microscope every 500 tests. When a through crack, delamination, or large-area powdering and flaking was observed, the number of cycles was recorded as the bending fatigue life of the sample.

[0048] Examples 1-6 and Comparative Examples 1-5 were cut into rectangular specimens with a width of 25 mm and a length of 150 mm along the length direction. Using a universal testing machine equipped with a T-shaped peeling clamp, a 20 mm long layer was manually peeled off from the middle layer of one end of the specimen along the length direction. The upper and lower layers after peeling were clamped in the upper and lower clamps of the testing machine, respectively, ensuring that the specimen was aligned with the center line of the clamp. The peeling angle was 180°, the peeling speed was set to 50 mm / min, the testing machine was started, and the peeling was continuously performed for at least 100 mm. The force curve during the peeling process was recorded, and the average peeling force of the middle 60 mm was recorded. The interlayer bond strength was calculated according to the formula: average peeling force / specimen width.

[0049] Table 2 Mechanical and mechanical properties of the examples and comparative examples

[0050] As shown in Table 2, all the sample examples exhibit good mechanical properties, indicating that the integrated technology of hierarchical confinement construction and interface bonding constructed in this invention is beneficial for the samples to exhibit excellent mechanical flexibility and interface bonding performance. Comparative Example 1 eliminated the aniline pre-adsorption process, resulting in homogeneous nucleation after aniline and oxidant were mixed simultaneously. Polyaniline was generated in the bulk solution rather than uniformly coated on the solid surface, leading to discontinuous interface coating. Therefore, the flexibility and interlayer bonding strength were low, and the sample was prone to cracking upon bending. Comparative Example 2 used gravity filtration instead of vacuum filtration for pre-fixation, causing severe delamination between the carbon fiber and lithium manganese oxide due to density differences. The electrode exhibited a loose, separated structure with extremely weak interlayer bonding, resulting in a very large minimum bending radius and extremely low interlayer bonding strength. Comparative Example 3 eliminated gradient heating and hot pressing, using a one-time high-temperature and high-pressure hot pressing method. This caused the residual volatiles inside the composite film to vaporize instantaneously, generating bubbles and delamination. The polyaniline segments were not fully rearranged, and the interface tightness was insufficient, resulting in poor flexibility and reduced fatigue life. In Comparative Example 4, the pH was not adjusted to acidity with citric acid during the polystyrene sulfonate adsorption process, resulting in insufficient protonation of the lithium manganese oxide surface, weakened electrostatic adsorption driving force, low polystyrene sulfonate adsorption, and uneven coating layer. Consequently, the interfacial bonding strength was low, and delamination was easy during bending. In Comparative Example 5, no pore-forming agent was added during the preparation of carbon nanofibers, resulting in a small fiber specific surface area, low porosity, reduced polystyrene sulfonate grafting, and sparse conductive network. Therefore, both the flexibility and interfacial bonding strength were lower than those of the examples.

[0051] Experiment Example 2 The electrodes prepared in Examples 1-6 and Comparative Examples 1-5 were used as positive electrodes, the negative electrode was a lithium metal / carbon fiber composite negative electrode, and the separator was Celgard. 2400, using 1 mol / L LiPF6 dissolved in EC / DMC at a volume ratio of 1:1, and encapsulated in an aluminum-plastic film to form a soft-pack battery. In a 25°C constant-temperature chamber, the assembled battery is first charged at a constant current of 0.1C to 4.3V, then charged at a constant voltage of 4.3V until the current drops to 0.05C. After resting for 10 minutes, it is discharged at a constant current of 0.1C to 3.0V. This charge-discharge cycle is repeated three times, and the capacity of the third discharge is the initial capacity C0. Subsequently, the battery is charged at a constant current of 1C to 4.3V, then charged at a constant voltage of 4.3V until the current drops to 0.05C. After resting for 10 minutes, it is discharged at a constant current of 1C to 3.0V, and then rested for 10 minutes. This charge-discharge cycle is repeated. After 1000 cycles, the capacity is calibrated by three standard charge-discharge cycles at 0.1C, and the highest value among the three is taken as the remaining capacity C. n The capacity retention rate is calculated using the formula C. n / C0×100%.

[0052] The assembled pouch battery was placed in a 25°C constant temperature chamber. It was first charged at a constant current of 0.1C to 4.3V, then charged at a constant voltage of 4.3V until the current dropped to 0.05C. After resting for 10 minutes, it was discharged at a constant current of 0.1C to 3.0V. The discharge capacity was recorded as the baseline capacity C. 0.1 The battery was then charged to 4.3V at a rate of 0.1C, left to stand for 10 minutes, and then discharged sequentially at constant currents of 0.2C, 0.5C, 1C, 2C, and 5C to 3.0V. The rate performance was calculated using the formula: C = x / C0×100%, where x is the discharge rate.

[0053] Table 3 Electrochemical stability of the examples and comparative examples

[0054] As shown in Table 3, the samples in the examples all exhibited high cycle capacity retention and high-rate discharge performance, indicating that the multi-level confinement and interface enhancement integrated technology constructed in this invention is beneficial for the samples to exhibit excellent electrochemical cycle stability and rate characteristics. Comparative Example 1 eliminated the aniline pre-adsorption process, resulting in homogeneous nucleation after aniline and oxidant were mixed simultaneously. Polyaniline was generated in the solution rather than uniformly coated on the solid surface, leading to discontinuous interface coating and increased interfacial resistance. Therefore, its cycle retention and rate performance were significantly lower than those of the examples. In Comparative Example 2, the carbon fiber and lithium manganese oxide underwent severe delamination due to density differences, causing the internal conductive network of the electrode to collapse and the active material to detach from the conductive framework, resulting in the worst electrochemical performance among all samples. In Comparative Example 3, the volatiles inside the composite membrane instantly vaporized, generating bubbles and delamination. The polyaniline segments were not fully rearranged, and the interface tightness was insufficient, leading to accelerated structural degradation during charge and discharge. Both the cycle stability and rate performance were lower than those of the examples. In Comparative Example 4, the lithium manganese oxide surface lacked sufficient protonation, resulting in weakened electrostatic adsorption driving force, low polystyrene sulfonate adsorption, uneven coating layer, weak interfacial bonding, and a significant decrease in electrochemical performance. In Comparative Example 5, the absence of polymethyl methacrylate (PMMA) pore-forming agent in the carbon nanofiber preparation led to a small fiber specific surface area, low porosity, reduced polystyrene sulfonate grafting, sparse conductive network, and the inability of the active material to fully exert its electrochemical activity.

[0055] Experimental Example 3 To observe the differences in the microstructure of the samples, SEM, EDS, and TEM analyses were performed on samples from Example 1 and Comparative Examples 1-3. SEM samples were directly fixed to the sample stage with conductive adhesive. Interface samples were obtained through liquid nitrogen cryogenic fracturing and argon ion cross-section polishing and then vertically fixed. EDS samples were collected from the same cross-sectional area at an accelerating voltage of 15 kV, with elemental channels including C, Mn, O, N, and S. TEM samples were prepared using slight ultrasonic dispersion and FIB method, dropped onto a lacey carbon copper grid, and observed using a 200 kV field emission transmission electron microscope. The observation results are as follows: Figures 2-2 As shown.

[0056] Figure 2 This is a microscopic morphology image of the sample from Example 1. Figure 2 (a) is a surface SEM image, showing that lithium manganese oxide particles are uniformly embedded in the three-dimensional conductive network of carbon nanofibers. (b) is a cross-sectional SEM image of the sample, showing that the sample has no obvious stratification and presents a dense structure. (c) is a morphological reference image of the EDS sample of Example 1. Figures (d) to (h) show the cross-sectional mapping of C, Mn, O, N, and S elements, respectively. It can be seen that carbon fibers and polyaniline form a continuous conductive skeleton, lithium manganese oxide is uniformly distributed along the thickness, and N and S elements are co-located around the particles and fibers, indicating that polyaniline / polystyrene sulfonate is uniformly coated on the two interfaces. (i) and (j) are local high-magnification SEM and TEM images of the sample, respectively. It can be seen that lithium manganese oxide and carbon fibers form a tight contact and bridging structure through polyaniline / polystyrene sulfonate. Figure (k) shows clear lithium manganese oxide lattice stripes and an outer amorphous polyaniline / polystyrene sulfonate coating layer.

[0057] Figure 3 The images show the microstructure of samples 1-3. Figure 3 (a) and (d) are cross-sectional SEM and high-magnification SEM images of Comparative Example 1. It can be seen that the polyaniline aggregates in the comparative example sample and the coating is discontinuous. Figure 3 (b) and (e) are superimposed SEM and EDS elemental distribution images of the cross-section of the sample in Comparative Example 2. It can be seen that there is a clear misalignment of the Mn / C thickness distribution in the sample. Figure 3 (c) and (f) are cross-sections and high-magnification SEM images of Comparative Example 3 sample, showing that pores, cracks and interface peeling exist in Comparative Example 3 sample.

[0058] In summary, Example 1 confirmed through SEM / TEM / EDS that carbon nanofibers form a three-dimensional conductive network, lithium manganese oxide is uniformly embedded, and polyaniline / polystyrene sulfonate forms a continuous coating and bridging structure on the particle and fiber surfaces. Comparative Examples 1-3, on the other hand, showed defects such as polyaniline agglomeration, misalignment of Mn / C distribution, and pores and cracks due to the removal of aniline pre-adsorption, gravity filtration, and gradient hot pressing, respectively. This indicates that aniline pre-adsorption, vacuum filtration pre-assembly, and gradient hot pressing are key steps to achieve a uniform and dense interface.

Claims

1. A flexible lithium manganese oxide positive electrode sheet, characterized in that: The electrode is based on a carbon nanofiber membrane; sodium polystyrene sulfonate is covalently grafted onto the surface of the carbon nanofiber membrane, and lithium manganese oxide particles are uniformly loaded in its three-dimensional porous structure; sodium polystyrene sulfonate is adsorbed on the surface of the lithium manganese oxide particles, and polyaniline is grown in situ through polymerization; the polyaniline and the sodium polystyrene sulfonate form an interpenetrating composite conductive network through electrostatic adsorption, hydrogen bonding and physical entanglement.

2. The flexible lithium manganese oxide positive electrode sheet according to claim 1, characterized in that: Using the flexible lithium manganese oxide positive electrode as the positive electrode and the lithium metal / carbon fiber composite electrode as the negative electrode, and encapsulating the outer shell with an aluminum-plastic film, a flexible soft-pack battery is prepared; the capacity retention rate of the flexible soft-pack battery is >80% after 1000 charge-discharge cycles.

3. A method for preparing a flexible lithium manganese oxide positive electrode sheet according to any one of claims 1 to 2, characterized in that, Includes the following steps: S1: Polyacrylonitrile and polymethyl methacrylate are dissolved in DMF to prepare a spinning solution. The spinning solution is electrospun into a film, pre-oxidized and carbonized to obtain a carbon nanofiber membrane. Then, after oxygen plasma activation and silane coupling agent modification, it is reacted with polystyrene sulfonate solution to obtain a carbon nanofiber membrane with polystyrene sulfonate grafted on the surface. S2: After washing the lithium manganese oxide powder with acid, it is dispersed in the acidic adsorption solution of polystyrene sulfonate and stirred for adsorption. After centrifugation and washing, it is redispersed in deionized water to obtain a suspension of lithium manganese oxide particles with polystyrene sulfonate adsorbed on the surface. S3: A carbon nanofiber membrane grafted with polystyrene sulfonate is placed on a filter membrane, and a lithium manganese oxide particle suspension is vacuum filtered to form a filter cake layer. Then, it is permeated with aniline solution and allowed to stand. Then, it is permeated with oxidant solution a second time. After washing and vacuum drying, a composite membrane is obtained. S4: The composite film is sandwiched between stainless steel plates and preheated and pressed, then heated and pressed, and after maintaining the pressure and natural cooling, a flexible self-supporting positive electrode sheet is obtained.

4. The method for preparing a flexible lithium manganese oxide positive electrode sheet according to claim 3, characterized in that: The silane coupling agent mentioned in S1 is one or more of KH-550, KH-560, and KH-792.

5. The method for preparing a flexible lithium manganese oxide positive electrode sheet according to claim 3, characterized in that: The temperature for preparing the spinning solution in S1 is 40~80℃, and the time is 6~18h; the pre-oxidation temperature is 240~280℃, and the time is 1.5~2.5h; the oxygen plasma activation power is 100~200W; and the concentration of the silane coupling agent is 1.5vol%~2.5vol%.

6. The method for preparing a flexible lithium manganese oxide positive electrode sheet according to claim 3, characterized in that: The pickling described in S2 is performed using one or more of citric acid, glacial acetic acid, and oxalic acid.

7. The method for preparing a flexible lithium manganese oxide positive electrode sheet according to claim 3, characterized in that: The acid concentration during pickling in S2 is 0.01~0.05mol / L; the pH of the acidic adsorption solution is 4.0~5.0; the stirring speed is 150~300rpm; and the adsorption time is 6~12h.

8. The method for preparing a flexible lithium manganese oxide positive electrode sheet according to claim 3, characterized in that: The filter membrane mentioned in S3 is one of PVDF, PTFE, or PI filter membranes.

9. The method for preparing a flexible lithium manganese oxide positive electrode sheet according to claim 3, characterized in that: The flow rate of the permeation aniline solution in S3 is 0.15~0.3 mL / min; the standing time is 20~40 min; the secondary permeation flow rate is 0.08~0.12 mL / min; and the vacuum drying time is 3~6 h.

10. The method for preparing a flexible lithium manganese oxide positive electrode sheet according to claim 3, characterized in that: The preheating pressure of S4 is 1.5~3MPa, and the temperature is 60~90℃; the heating and pressing temperature is 100~140℃, the pressure is 8~12MPa, and the heating and pressing time is 3~8min; the holding pressure is 3~8MPa.

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