Preparation And Application Of Core-shell Structured Composite Nanofiber As Li~+/Na~+ Battery Anode Material

As energy storage device,lithium/sodium ion battery has excellent performance such as high power and energy density.Compared with lithium-ion battery,sodium-ion battery is considered as a potential substitute for energy storage material because of its low cost,better temperature adaptability and environmental friendliness.However,commercial graphite anode with low theoretical capacity(372 m Ah g^-1)can no longer meet the development of energy storage anode materials.Therefore,high performance anode materials need to be designed.As anode materials for lithium/sodium ion batteries,transition metal compounds are currently the focus of research.However,their large volume expansion and poor electrical conductivity as anode in the process of charge and discharge will lead to poor rate performance and cycle stability.In this work,to address these problems,carbon composite nanofibers with controllable core-shell structure as anode were prepared by electrospinning.The transition metal compounds with high specific capacity（Fe₂P and MnSe）and conductive materials with structure stability（TiO₂ and TiC）were used as the core and shell of composite nanofiber anode materials,respectively.The anode material of lithium/sodium-ion battery with excellent electrochemical performance was obtained.The lithium/sodium ion storage mechanisms were studied in detail.This study consisted of two parts:（1）Iron phytate nanoparticles was embedded in TiO₂（IP/TiO₂）by coaxial electrospinning with the aid of polyacrylonitrile（PAN）and polyvinylpyrrolidone（PVP）.N,P-doped Fe₂P@TiO₂ core-shell carbon nanofibers（Fe₂P@TiO₂/CNF）were prepared after heat treatment.When Fe₂P@TiO₂/CNF were introduced as a self-supporting anode in lithium/sodium-ion batteries,Fe₂P core supplied high energy density,and N,P-doped carbon fiber could induce the structural defects,facilitate electron transport and improve electrical conductivity.Moreover,TiO₂ shell inhibited volumetric expansion and pulverization of Fe₂P core and improved the cycling stability of the anode.The Fe₂P@TiO₂/CNF electrode exhibited a large reversible capacity(LIB:1175.7 and SIB:519.8 m Ah g^-1),excellent cycling ability and rate performance.As anode in LIBs for Fe₂P@TiO₂/CNF,capacitance and diffusion control played important roles in Li⁺intercalation and deintercalation,simultaneously.When Fe₂P@TiO₂/CNF anode was used inSIBs,capacitive control behavior played a major role and displayed outstanding electrochemical performance.（2）MnSe and TiC were selected as the anode materials for lithium/sodium-ion batteries.Manganese acetate（MA）@TiC/PVP composite fibers were prepared by coaxial electrospinning.The effects of different carbonization temperatures for PVP on the morphology of MnSe@TiC core-shell carbon nanofibers（MnSe@TiC/CNF）were studied.MnSe had high theoretical capacity as anode material.Compared to Fe₂P,MnSe had larger layer spacing,which was easier to achieve sodium ion insertion and extraction.TiC shell had good mechanical properties and electrochemical stability.Compared with TiO₂,it had higher conductivity and theoretical capacity.The result showed that MnSe@TiC/CNF as anode material exhibited excellent cycle stability(LIBs:791.4 m Ah g^-1,SIBs:515.3 m Ah g^-1)and rate performance.