Preparation And Properties Of Metal Fluoride-doped Porous Carbon Nanofibers/sulfur Electrodes

Lithium-sulfur(Li-S)batteries are expected to replace lithium-ion batteries as the next-generation ideal energy battery with the theoretical specific capacity of 1675m Ah/g and mass density of 2600 Wh/kg.However,Li-S batteries still face many challenges,mainly as follows:the insulation of sulfur element;the volume expansion(contraction)of up to 79%generated during charging and discharging;"shuttle effect"of the intermediate product lithium polysulfide(Li₂S_x,3<x?8).Therefore,designing functional cathode materials with high electrochemical activity and excellent physical structure has become an effective strategy to solve the above problems,in which carbon materials are the main research focus.In this paper,YF₃-doped semi-closed carbon nanofibers(SC-YF₃doped-CNFs)and YF₃/CoF₃co-doped carbon nanofibers were prepared as high-performance sulfur host based on self-made electro-blown(EBS)technology.The main research contents are as follows:(1)Polyvinylpyrrolidone(PVP)was selected as carbon source and PTFE asporogen.SC-YF₃doped-CNFs were prepared by self-made high-efficiency EBS technology,pre-oxidation and carbonization processes using PVP/PTFE/Y(NO₃)₃ spinning solution.After that,SEM,TEM,XRD,XPS and other electrochemical tests were used to investigate the influence of Y(NO₃)₃doping on the morphology and long cycle stability of semi-closed carbon nanofibers.The results show that the SC-YF₃doped-CNFs has 3D porous network structure inside,and the outside is covered by microporous carbon layer,forming the unique semi-closed structure.This further effectively hinders the diffusion of polysulfide while ensuring the conductivity of the positive electrode and adapting to volume expansion.When the doping amount of Y(NO₃)₃ is 2g,the comprehensive performance of Li-S battery is the best.Therefore,the assembled Li-S batteries exhibit high reversible discharge capacity of 954.2m Ah/g with a mere decay of 0.043%per cycle after 600 cycles at 1 C rate.Moreover,the discharge capacity decay can be as low as 0.029%per cycle during 800 cycles at high current density of 2 C rate.Even at a high rate of 5 C,the cells still possess a favorable capacity of 636.5 m Ah/g while steadily operating 700 cycles with a mere capacity decay rate of 0.056%,implying the great potential of this stable semi-closed cathode structure for industrialization.(2)Based on the PVP/PTFE/Y(NO₃)₃spinning system,Co(NO₃)₂was added and YF₃/CoF₃co-doped semi-closed carbon nanofibers were prepared by the same process.The fiber retains a special semi-closed physical structure that acts as a micro-reactor,effectively confining polysulfides to this area.At the same time,the cathode can exert the catalytic effect of Co and accelerate the redox kinetics.It is found that when the doping amount of Co(NO₃)₂is 1g,the comprehensive performance of Li-S battery is optimal.Therefore,the assembled Li-S batteries exhibit high reversible discharge capacity of 1055.2 m Ah/g with a capacity retention of 91.2%after 300 cycles at 0.5 C rate.At 1 C rate,the batteries exhibit high reversible discharge capacity of 1038m Ah/g with a mere decay of 0.05% per cycle during 1000 cycles.