Structure Regulation,Electrochemical Performance And Application Of Electrospun Composite Nanofiber Membrane Electrodes

The development of portable,wearable electronic technology has put forward higher requirements for the rapid charge and discharge capability and flexibility of mobile power supplies.The development of highly conductive self-supporting structure electrode materials is one of the frontier topics in the field of energy materials and devices.Electrospun nanofiber electrodes have continuous electron transmission channels and abundant ion transportation paths,which helps to improve the electronic conductance and ion conductance of the electrode,with enhancement of the rapid charge and discharge performance of the electrode.At the same time,the flexibility of the nanofiber can meet the requirements of flexible electrodes.In this work,the composite nanofiber membrane cathode materials(Li Fe PO₄/C,Li₃V₂(PO₄)₃/C)and composite nanofiber membrane anode materials(Fe Ti O₃/Ti O₂/C,Li₄Ti₅O₁₂/C)were prepared based on the electrospinning and heat treatment technology.The composition,microstructure and electrochemical properties of the materials have been characterized and analyzed by various advanced methods.The all-fiber-based gel state full batteries have been designed and assembled,and the application of composite nanofiber membranes in the lithium-sulfur batteries has been also explored.The main research contents and results obtained are as follows:(1)Using polyacrylonitrile(PAN)as the polymer,iron nitrate and lithium acetate as the main raw materials,the Li Fe PO₄/C composite nanofiber prepared by electrospinning and heat treatment technology has a sheath-core cable structure,the surface layer of which is mainly amorphous carbon and the core is majorly composed of nano-Li Fe PO₄.Although the sheath-core cable structure nanofibers have good flexibility and self-supporting properties,their electrochemical performance is unsatisfactory because the carbon layer covers Li Fe PO₄ and hinders the contact between Li Fe PO₄ and the electrolyte.The research results show that the specific discharge capacity of Li Fe PO₄/C composite nanofiber membrane electrode is only about 85 m Ah/g.(2)Based on the different migration and thermal decomposition characteristics of polyvinylpyrrolidone(PVP)and PAN polymer during the heat treatment of the composite nanofiber membrane precursor,innovatively using PAN and PVP as the polymer,iron nitrate and lithium acetate as the main raw materials,the modified Li Fe PO₄/C composite nanofiber membrane cathode material with a good electrochemical performance was successfully prepared by optimizing the electrospinning and heat treatment process.The battery composed of the material and the metal lithium negative electrode can be charged and discharged normally at0.5 C to 10 C.After 700 cycles at 1 C,the specific discharge capacity remains 125m Ah/g,and it can be cycled stably for 1000 cycles at 10 C.The study finds that in the modified Li Fe PO₄/C composite nanofiber structure,Li Fe PO₄ nanoparticles are mainly distributed on the surface of the fiber,while amorphous carbon is majorly concentrated on the fiber core,which is beneficial to the transportation of lithium ions and electrons during the electrochemical reaction,while maintaining a stable electrode structure.(3)The high-voltage(4.8 V)Ni-doped Li₃V₂(PO₄)₃/C composite nanofiber membrane cathode material was prepared by the optimized electrospinning method.Studies have found that Ni doping has a significant effect on the morphology,microstructure and electrochemical properties of Li₃V₂(PO₄)₃/C composite nanofibers.Within the range of Ni doping content(0-5‰),1‰Ni doping has little effect on the crystal structure of Li₃V₂(PO₄)₃,and due to the catalytic effect of nano-Ni particles,Li₃V₂(PO₄)₃ grows on the surface of carbon-based fibers to form needle-like Li₃V₂(PO₄)₃ nanorods,which is conducive to the formation of a 3D network hybrid structure composed of Li₃V₂(PO₄)₃/C composite nanofibers and needle-like nanorods.The battery composed of this self-supporting composite nanofiber membrane electrode and metal lithium anode has a good electrochemical performance at 1 C and 5 C,which is mainly attributed to the continuous unique 3D conductive carbon network.At the same time,the high porosity is helpful to electrolyte penetration and lithium ion migration,and the stable structure of the electrode improving its redox reaction process.(4)Fe Ti O₃/Ti O₂/C,Li₄Ti₅O₁₂/C(LTO/C)composite nanofiber membrane anode materials with a stable structure have been prepared by the electrospinning process.The microstructure,rate and cycling performance of the electrode were analyzed.The study finds out that combining Ti O₂ and Fe Ti O₃,the synergy of Ti O₂,Fe Ti O₃with 3D conductive carbon network,high porosity and self-supporting structure during the charging and discharging process overcome the inherent shortcomings of single-phase Ti O₂ and Fe Ti O₃,and improve the specific capacity and structure of the electrode stability.Li₄Ti₅O₁₂/C(LTO/C)composite nanofiber membrane electrode improves the conductivity of Li₄Ti₅O₁₂ and its rate performance with the help of a3D conductive carbon network,which can stabilize the cycle performance more than700 cycles,with the discharge specific capacity decreasing from the initial?135m Ah/g to?125 m Ah/g.Its Coulomb efficiency is close to 100%,showing a good cycle stability and charge/discharge reversibility.(5)On the basis of the composite nanofiber membrane cathode materials and anode materials,the whole fiber-based lithium-ion battery has been designed and assembled by use of LFP/C composite nanofiber membrane as the cathode and LTO/C composite nanofiber membrane as the anode,and its electrochemical performance is characterized.The whole cell of LFP/C//LTO/C works 800 cycles at1 C,as the specific capacity remains above 100 m Ah/g,and the Coulomb efficiency is close to 100%.Up to now,this cycle-life for the all-fiber based cells is the highest reported publically.Although the all-fiber-based simulated soft-pack battery is bent200 times under a bending radius of 4 mm with good flexibility,the resistance and specific capacity of the battery are basically stable.The excellent performance can be attributed to the good electronic and ionic conductivity of the three-dimensional network structure of the self-supporting electrode,which provides a technical reference for the development of flexible lithium ion battery.(6)The composite nanofiber membrane materials have been successfully applied to lithium-sulfur batteries.The rate performance and charge-discharge cycling characteristics of lithium-sulfur batteries are characterized.The results show that after the addition of the interlayer of Li₃V₂(PO₄)₃(LVP)/C composite nanofiber membrane,the cycle and rate performance of the Li-S battery are obviously improved,and the initial discharge specific capacity of the battery reaches 725m Ah/g under 2 C,and 490 m Ah/g is retained after 500 cycles,with the Coulomb efficiency close to 99%.The composite nanofiber membrane inhibits the shuttle of polysulfides mainly due to the unique porous network structure of carbon-based nanofibers,and the uniform and fine mesh can hinder the diffusion of polysulfides to the negative electrode.The nano oxide particles in the fiber can form a strong chemical bond adsorption with polysulfide.The highly conductive 3D network structure carbon fiber acts as a secondary current collector,which can quickly convert the adsorbed polysulfide into active sulfur under the catalytic action of Li₃V₂(PO₄)₃ and Ti O₂,thereby improving the utilization rate of sulfur,so that the cycle stability of the battery is significantly improved.