A high-performance sodium-ion soft-pack battery based on pre-sodiated carbon fiber bipolar

Abstract

This invention belongs to the field of sodium-ion battery technology and discloses a high-performance sodium-ion pouch battery based on a pre-sodium-modified carbon fiber bipolar electrode. Through the synergistic design of a pre-sodium-modified carbon fiber bipolar electrode, a sodium vanadium phosphate cathode, and a PVDF-HFP-based gel electrolyte, this invention achieves significant benefits for sodium-ion batteries in six dimensions: electrochemical performance, structural simplification, safety, low cost, flexibility, and environmental friendliness. It is particularly suitable for cost- and safety-sensitive applications such as large-scale energy storage, low-speed electric vehicles, and flexible electronics.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology and relates to a high-performance sodium-ion pouch battery based on a pre-sodium-modified carbon fiber bipolar electrode. Background Technology

[0002] In recent years, sodium-ion batteries have attracted widespread attention due to their unique advantages in cost and low-temperature performance, and have the potential to replace lithium-ion batteries as the next generation of high-energy-density power batteries, large-scale energy storage, and energy storage devices for consumer electronics. However, the biggest problem with sodium batteries is that during the first charge, the negative electrode consumes a large amount of precious sodium ions to form an SEI film, resulting in a significant reduction in battery capacity and efficiency, which limits the application of pouch sodium-ion batteries and requires further improvement. Therefore, it is crucial to construct flexible electrode materials with suitable voltage windows, high specific reversible capacity, and stable structures.

[0003] Currently, flexible carbon fiber electrodes have become a research hotspot in the field of flexible energy storage. There have been reports on growing Co3O4 nanowire arrays on carbon fiber cloth as anodes for sodium-ion batteries, but there are no relevant literature reports on "three-in-one" electrodes that integrate carbon fiber as a flexible current collector, active material carrier and structural support. Summary of the Invention

[0004] Based on the current state of technology, the purpose of this invention is to provide a flexible, high-energy-density sodium-ion pouch battery. Through pre-sodiumification technology, it compensates for the "first irreversible capacity loss," overcomes the industry problems of low initial efficiency and short cycle life of sodium batteries, and has a development cost close to that of lead-acid batteries, but its performance (lifespan, safety, rate capability) far exceeds that of lead-acid batteries. It is also a cheaper and safer energy storage product than lithium batteries.

[0005] To achieve the objective of this invention, the technical solution is as follows: 1. By pre-storing sodium ions in the carbon fiber anode (pre-sodiumization), the sodium consumed during the formation of the SEI film during the first charge is compensated, thereby significantly increasing the battery's initial coulombic efficiency from less than 60% to over 85%.

[0006] 2. Improved cycle stability: After pre-sodiumification, more sodium ions can be reused, and the battery can still maintain a high capacity after hundreds or thousands of charge-discharge cycles, that is, a longer cycle life.

[0007] 3. Leveraging the advantages of sodium vanadium phosphate (NVP) cathodes: NVP cathodes inherently possess advantages such as high voltage platform, structural stability, and good safety. However, they have poor electronic conductivity and contain little or no sodium. This invention utilizes a highly conductive carbon fiber skeleton to compensate for the conductivity of NVP, while simultaneously using a pre-sodiumized anode to solve the "sodium deficiency" problem of NVP, achieving a perfect synergy.

[0008] The high-performance sodium-ion pouch battery with pre-sodium-modified carbon fiber bipolar structure includes a positive electrode, a negative electrode, and an electrolyte, as shown in the following specific structure: 1. Positive electrode: Sacrificial sodium-rich additives (such as NaCl) are added to the positive electrode slurry containing sodium vanadium phosphate (Na3V2(PO4)3), and then the positive electrode slurry is pressed onto pre-sodium-treated carbon fibers as a self-supporting electrode; 2. Negative electrode: Pre-sodium-treated carbon fiber is used; 3. Electrolyte: A composite formulation based on PVDF-HFP-based gel polymer electrolyte is used, with the following mass ratio of each component: PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer) : NaFSI (sodium bis(fluorosulfonyl)imide) : [Emim][FSI] (1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide salt) = 1:0.5:2; The pre-sodium-treated carbon fiber is prepared by the following method: sodium biphenyl (Na-Bp) or sodium benzophenone (Na-DK) is dissolved in dimethyl ethylene glycol (DME) and stirred under argon protection until completely dissolved to obtain a pre-sodium-treated solution; the carbon fiber is immersed in the pre-sodium-treated solution and reacted at room temperature.

[0009] No separate diaphragm is required; the gel electrolyte membrane also functions as a diaphragm, simplifying the structure and reducing internal resistance. It is packaged in an aluminum-plastic film, making it lightweight and customizable in shape, suitable for roll-to-roll continuous production.

[0010] This invention is applicable to scenarios with extremely high requirements for cost, safety, and lifespan, but relatively relaxed requirements for energy density. Examples include low-speed electric vehicles (two-wheeled / three-wheeled / forklifts), construction machinery, and starting power supplies.

[0011] The core features of this invention are: 1. All-carbon self-supporting electrode: Eliminates the need for copper foil, aluminum foil, and binder, simplifying the coating process. 2. Chemical pre-sodiuming: Pre-addition of sodium on the negative electrode side improves initial efficiency to 85-92%. 3. Integrated gel electrolyte: Eliminates the need for a separator, improving safety and flexibility. 4. Flexible packaging: Lightweight and customizable shape.

[0012] The innovation and advantages of this invention are as follows: 1. All-carbon, binder-free, and metal-free current collector design: Both positive and negative electrodes use carbon fiber as the conductive skeleton and support, eliminating the need for copper foil, aluminum foil, and PVDF binder, reducing cost and weight, and improving flexibility.

[0013] 2. Pre-sodium-modified carbon fiber bipolar technology: negative electrode pre-sodiumification: compensates for irreversible capacity loss in the first cycle; positive electrode carbon fiber pre-sodiumification: enhances the electronic conductivity of sodium vanadium phosphate, and synergistically improves the first coulombic efficiency to 85-92%.

[0014] 3. Ionic liquid-based gel electrolyte: PVDF-HFP + NaFSI + [EMIM][FSI] ternary system, which combines the high ionic conductivity of liquid and the safety of solid, without the need for a separate separator, simplifying the battery structure.

[0015] 4. Self-supporting electrode integrated molding: The active material is directly loaded onto the carbon fiber skeleton, avoiding traditional processes such as coating and rolling, and is suitable for roll-to-roll continuous production.

[0016] Compared with lithium iron phosphate batteries, the advantages of this invention are: 1. Extremely low cost (abundant sodium resources, controllable carbon fiber cost) and moderate cost (limited lithium resources, large price fluctuations) to replace lead-acid, reducing costs and providing extremely high safety (gel electrolyte is non-flammable, NVP has good thermal stability), suitable for applications where safety is extremely sensitive, with excellent rate performance (carbon fiber skeleton + gel electrolyte, supports fast charging and discharging), meeting the needs of fast charging and high current. 2. Excellent low-temperature performance (sodium salts and specific electrolytes perform better than lithium at low temperatures) (lithium batteries have poor low-temperature performance, with severe capacity decay below 0°C), while sodium batteries are suitable for cold regions. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the preparation process of the sodium-ion pouch cell of this invention. Detailed Implementation

[0018] To better illustrate the present invention, the following embodiments are provided: Example

[0019] The method for preparing the high-performance sodium-ion pouch cell with the pre-sodium-modified carbon fiber bipolar electrode is as follows: I. Pre-sodiuming operation procedure for positive and negative electrodes 1. Positive electrode additive method Operating method: During the preparation of the positive electrode slurry, a sacrificial sodium-rich additive (such as NaCl) is directly added. After the cell is assembled, the additive will irreversibly decompose during the first charge, and the released sodium ions will just compensate for the consumption of the negative electrode in the formation of the SEI film.

[0020] Technical evaluation: Its biggest advantage is that it is perfectly compatible with existing production lines. Only the positive electrode formula needs to be adjusted, and almost no changes to the process and equipment are required, making it suitable for industrial applications.

[0021] 2. Preparation of negative electrode by chemical pre-sodiumization method Operation method: (1) Preparation of pre-sodiumization solution: Sodium biphenyl (Na-Bp) or sodium benzophenone (Na-DK) is dissolved in dimethyl ethylene glycol (DME) and stirred until completely dissolved under argon protection. (2) Pre-sodiumization operation: Carbon fiber is immersed in pre-sodiumization solution and reacted at room temperature to prepare negative electrode.

[0022] Technical evaluation: Highly applicable.

[0023] The assembly process of the two batteries The manufacturing process of the sodium-ion soft-pack battery of this invention, which is "all carbon, without current collectors, without binders, and with gel electrolyte," is as follows: 1. Process Overview Carbon fiber pretreatment → Positive electrode preparation → Negative electrode preparation → Gel electrolyte membrane preparation → Stacking and assembly → Encapsulation → Liquid injection / wetting → Formation → Degassing and final sealing → Aging test. Among these, pre-sodiumization and gel electrolyte membrane formation are the two most critical process steps.

[0024] 2. Detailed Explanation of Step-by-Step Process Step 1: Carbon fiber pretreatment (shared by positive and negative electrodes) removes surface impurities, increases oxygen-containing functional groups, and improves hydrophilicity and binding force with active materials. (1) Select T700 grade carbon fiber cloth / felt (thickness 0.3-0.5mm, surface density 80-120g / m²), ultrasonic cleaning with acetone for 30 minutes → cleaning with deionized water → cleaning with ethanol, and drying at 80℃ under vacuum for 12 hours.

[0025] (2) Surface activation: Treat with oxygen plasma for 5-10 minutes or soak in concentrated nitric acid at 60°C for 2 hours.

[0026] Step 2: Cathode preparation (sodium vanadium phosphate lamination) (1) Slurry preparation: Weigh the ingredients according to the ratio of NVP: conductive carbon black: binder = 85:10:5 (weight ratio), add solvent NMP (N-methylpyrrolidone) or water (depending on the type of binder), and then add sodium-rich additive NaCl. The solid content is 30-40%, and vacuum degassing is performed for 30 minutes. (2) Coating / Impregnation: The above slurry is coated or impregnated onto the pretreated carbon fibers using a blade coating method (interval coating machine) or a dip-coating method to form a self-supporting positive electrode with a target areal capacity of 3.5-4.5 mAh / cm². (3) Drying: 80℃ oven drying for 2 hours → 120℃ vacuum drying for 12 hours, roller pressing (optional): linear pressure 50-100 kg / cm, compaction density controlled at 2.0-2.4 g / cm³.

[0027] Step 3: Anode preparation (pre-sodium-treated carbon fiber) (1) Preparation of pre-sodium solution: Dissolve 0.5-1.0 mol / L sodium biphenyl (Na-Bp) or sodium benzophenone (Na-DK) in dimethyl ethylene glycol (DME) and stir under argon protection until completely dissolved.

[0028] (2) Pre-sodiumization operation: Immerse the carbon fiber in the pre-sodiumization solution and react at room temperature for 30-120 minutes (the longer the time, the greater the amount of pre-sodiumization).

[0029] (3) Cleaning: After taking it out, clean it 3 times with anhydrous DME or THF to remove surface residue, and dry it at 60℃ under vacuum for 2 hours.

[0030] (4) Passivation (optional) Place the surface naturally in a dry room for 12-24 hours to form a stable SEI film.

[0031] (5) Pre-sodiumization degree control: The amount of pre-sodiumization is controlled by adjusting the reaction time, solution concentration, and temperature. Target: To achieve an initial coulombic efficiency of ≥90% for the negative electrode.

[0032] Step 4: Preparation of gel electrolyte membrane (PVDF-HFP based) (1) Solution preparation: Dissolve PVDF-HFP (molecular weight 400,000-600,000) in acetone, add NaFSI and [Emim][FSI], with a mass ratio of PVDF-HFP : NaFSI : [Emim][FSI] = 1 : 0.5 : 2. The solid content is 10-15%, and the solution is stirred until completely dissolved.

[0033] (2) Degassing: Let stand or vacuum degas for 30 minutes.

[0034] (3) Film formation: The film is coated on the release film by the blade coating method (interval coating machine), and the wet film thickness is 200-400μm.

[0035] (4) Drying: evaporate at room temperature for 30 minutes → vacuum dry at 60℃ for 12 hours, with a final film thickness of 30-60μm.

[0036] The gel electrolyte membrane also functions as a separator, thus eliminating the need for an additional PP / PE separator. If mechanical strength is insufficient, a thin layer of PP nonwoven fabric can be laminated on top as a support.

[0037] Step 5: Stacking and Assembling Stacking order: from top to bottom: gel electrolyte membrane → negative electrode → gel electrolyte membrane → positive electrode → gel electrolyte membrane → negative electrode... (the negative electrode has one more layer than the positive electrode).

[0038] Number of stacked layers: determined according to the target capacity (e.g., 40 negative electrode layers, 39 positive electrode layers).

[0039] Pre-welding of electrode tabs: The positive electrode tab (aluminum strip) is ultrasonically spot-welded to the positive electrode sheet, and the negative electrode tab (nickel strip or nickel-plated copper) is spot-welded to the negative electrode sheet. Alignment control: The alignment deviation of the positive and negative electrode edges is ≤0.5mm. Fixing: Temporarily fix the stacked pieces with high-temperature resistant tape.

[0040] Step 6: Flexible Packaging (1) Aluminum-plastic film perforation: Select the perforation depth according to the cell thickness (including the stacked body + gel film) (usually 0.5-1mm thicker than the cell).

[0041] (2) Insertion: Place the stacked sheet into the punched aluminum-plastic film pit, top seal + side seal, heat seal temperature 180-200°C, time 3-5 seconds, pressure 0.3-0.5MPa, leave one side unsealed as the liquid injection port.

[0042] (3) Pre-applying adhesive to the tabs: Apply hot melt adhesive (CPP) to the tabs to prevent short circuits and leakage.

[0043] Step 7: Injection and Immersion Because this invention uses a gel electrolyte, conventional liquid injection methods are not applicable. The proposed method (pre-formed film method) involves pre-preparing the gel electrolyte membrane, which is then directly stacked. No liquid injection step is required. The process is simple, but the interfacial contact may be poor, necessitating hot-pressing optimization. Hot-pressing parameters: 60-80°C, pressure 0.5-1.0 MPa, time 5-10 minutes.

[0044] Step 8: Transformation Standing: Let stand at 40℃ for 12-24 hours to allow the gel to fully contact the electrode, form the SEI film, and fully intercalate sodium.

[0045] Step 9: Degassing and final sealing (1) Vacuum degassing: Under vacuum conditions (-90 kPa), puncture the gas bag and remove the gas generated by the chemical reaction.

[0046] (2) Final sealing: Heat seal the injection port / air bag edge at 180-200°C for 3-5 seconds.

[0047] (3) Trimming: Remove excess air bags and seal edges to form the battery.

[0048] The technical parameters of the sodium-ion battery prepared by this invention, as determined by testing, are as follows: Parameter target value Single unit capacity 10 ~ 100 Ah Nominal voltage 3.2 V Energy density 180 ~ 220 Wh / kg (cell level) First-pass coulomb efficiency: 85% ~ 92% Cycle life 500 cycles @ 80% capacity retention Operating temperature: -20°C ~ 80°C Rate performance: 3C / 0.2C capacity ratio ≥ 80% Safety is ensured by needle puncture and overcharge tests (the gel is non-flammable).

[0049] It was compared with commercially available batteries: As can be seen, the battery of this invention significantly improves electrochemical performance, with a substantial increase in initial coulombic efficiency, from 60-75% in traditional sodium batteries to 85-92%; cycle life is significantly extended, with capacity retention ≥80% after 500 cycles, while the electron conductivity of ordinary NVP cathodes is poor. Furthermore, the battery of this invention exhibits wide temperature adaptability, operating normally from -20°C to 80°C, with excellent thermal stability; thermal decomposition >300°C far exceeds that of liquid electrolytes (decomposition at ~150°C), and traditional carbonate electrolytes fail at low temperatures.

[0050] Principle: Pre-sodiumification of the negative electrode compensates for the sodium ions consumed in the formation of the SEI film; the carbon fiber 3D network enhances the electronic conductivity of the positive electrode; the ionic liquid-based gel electrolyte combines high ionic conductivity with a wide electrochemical window. The [EMIM][FSI] ionic liquid itself is non-flammable; the solid-state properties of the gel eliminate the risk of leakage; the PVDF-HFP matrix provides mechanical strength.

[0051] The sodium-ion battery prepared by this invention has an all-carbon structure, no brittle metal foil, is bendable, and has unique flexibility / customizability, making it suitable for wearable devices and flexible electronic products; it is also lightweight and thin, eliminating the need for metal current collectors and separators, with a thickness as low as 1-2 mm, making it suitable for ultra-thin battery applications.

Claims

1. A sodium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that, The positive electrode is prepared by adding sodium-rich additive NaCl to a positive electrode slurry containing sodium vanadium phosphate, and then pressing the positive electrode slurry onto pre-sodium-treated carbon fibers as a self-supporting electrode. The negative electrode uses pre-sodium-treated carbon fiber; The electrolyte adopts a composite formulation based on PVDF-HFP-based gel polymer electrolyte, and the mass ratio of each component is as follows: polyvinylidene fluoride-hexafluoropropylene copolymer: sodium bis(fluorosulfonyl)imide: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide salt = 1:0.5:2; The pre-sodium-treated carbon fiber is prepared by the following method: dissolving sodium biphenyl or sodium benzophenone in ethylene glycol dimethyl ether and stirring under argon protection to obtain a pre-sodium-treated solution; immersing the carbon fiber in the pre-sodium-treated solution and reacting at room temperature.

2. The sodium-ion battery as described in claim 1, characterized in that, Without a separate diaphragm, the gel electrolyte membrane also functions as a diaphragm.

Images