Preparation And Performance Study Of PVP-based Carbon Nanofiber Interlayer Materials For Lithium Metal Batterie

Lithium（Li）metal is regarded as the ideal anode material for next-generation high-energy-density batteries due to its ultra-high theoretical specific capacity(3860m Ah g^-1)and lowest redox electrochemical potential（-3.04 V vs standard hydrogen electrode）.However,the uncontrollable growth of Li dendrites in Li metal anode during charge-discharge process severely limit the practical application of Li metal batteries.Carbon nanofiber materials possess the advantages of large specific surface area,outstanding electrical conductivity and designable structure,and show great advantages in inhibiting the growth of Li dendrites and protecting Li anode;polar metal compounds as the lithiophilic sites can regulate the Li nucleation behavior,thereby guiding the uniform Li deposition and suppressing the growth of Li dendrites.In this text,based on electrostatic solution blowing spinning and microfluidic spinning technology,water-soluble polymer polyvinylpyrrolidone（PVP）was used as the carbon source,and by means of physical structure design and chemical functional doping,building with different structures and metal fluoride/sulfide-doped carbon nanofiber-based interlayer materials to achieve rapid migration and uniform deposition of Li ions,inhibit Li dendrites growth and improve the electrochemical performance of Li metal batteries.The main research contents are as follows:（1）Based on the PVP/Zn（CH₃COO）₂/PTFE spinning system,the Zn F₂-PCNFs coated interlayer was prepared by electrostatic solution blowing spinning technology and high temperature carbonization process.The effects of calcination temperature on the fiber structure,graphitization degree,electrical conductivity,etc.,and the electrochemical performance of the assembled cells were systematically studied.The results showed that Zn F₂-PCNFs-700 possessed the best comprehensive performance.Benefiting from the physical regulation of PCNFs and the chemical adsorption of Zn F₂,the interlayer could not only effectively induce uniform Li deposition,but also inhibited the shuttle effect of polysulfides.The capacity retention rate of the assembled Li|LFP battery was 96.4%after 500 cycles at 1 C,and the capacity retention rate of the assembled Li|S battery was 93.3%after 200 cycles.（2）In order to further improve the redox reaction kinetics of cells with Zn F₂-PCNFs interlayer,based on the above PVP/Zn（CH₃COO）₂/PTFE spinning system and the explored optimal calcination temperature,choosed the pre-oxidized PVP/Zn（CH₃COO）₂/PTFE nanofibers（NFs）as the matrix to prepare ZIF-8/Zn S/NFs precursors combined with hydrothermal reaction,and then constructed Zn F₂/Zn S heterostructure and N doped carbon（NC）decorated PCNFs（Zn F₂/Zn S/NC/PCNFs）coated interlayer via carbonization process.The obtained Zn F₂/Zn S/NC/PCNFs interlayer exhibited large specific surface area,hierarchical porous structure,excellent electrical conductivity and electrolyte affinity.The existence of the Zn F₂/Zn S heterostructure could significantly improve the diffusion kinetics of Li ions and inhibit the Li dendrites growth and polysulfides shuttle.The Zn F₂/Zn S/NC/PCNFs interlayer possessed high ionic conductivity(1.812×10^-3 S cm^-1)and low charge transfer resistance（Li|LFP,2.8Ω）,and the assembled Li|LFP battery possessed the capacity retention rate of 93.6%after 600 cycles at 1 C,the Li|S battery had a capacity retention rate of 91.4%after 200 cycles at 0.5 C.（3）Based on microfluidic spinning technology,high temperature carbonization process and solution casting method,using PVP/Mn SO₄ as spinning solution and cellulose nanocrystals（CNC）as casting system,the ordered Mn S@CNC/CNFs flexible self-supporting interlayer was prepared.The synergistic effect between the well-arranged carbon framework and polar Mn S particles and CNC rich in polar-OH could induce more uniform Li deposition to inhibit Li dendrites growth,and had strong adsorption to polysulfides.The assembled Li|LFP battery with the interlayer exhibited excellent rate performance,the initial discharge specific capacity of Li|LFP battery was148.2 m Ah g^-1 at 1 C,and the specific capacity remained 123.1 m Ah g^-1 after 600 cycles.The assembled Li|S battery also exhibited low charge transfer impedance（11.4Ω）and good cycling stability,while the Li|Li symmetric battery could cycle steadily for 500 h at 1 m A cm^-2.（4）In order to further promote the uniform deposition of Li ions in the direction perpendicular to the electrode,based on the above-mentioned PVP/Mn SO₄ spinning system,and adding PTFE as porogen and F source,the concentration gradient distributed Mn S/Mn F₂ nanoparticles-doped PCNFs（Mn S/Mn F₂-PCNFs-G）flexible self-supporting interlayer was designed and prepared by layer-by-layer electrostatic solution blowing technology and carbonization process.The flexible interlayer exhibited low charge transfer resistance（Li|S,1.3Ω）,excellent rate recovery performance and reaction kinetics,and could effectively suppress the Li dendrites growth and polysulfides shuttle.The Li|LFP battery assembled with the Mn S/Mn F₂-PCNFs-G interlayer had a capacity retention rate of 90.7%after 400 cycles at 2 C,and the Li|S battery remained 771.6 m Ah g^-1 after 200 cycles at 0.5 C.