The Preparation And Electrochemical Properties Of One-Dimensional Metal Oxide/Graphene Nanoroll Core-Shell Composites

Due to the high energy density,low self-discharge,no memory effect and long service life of lithium-ion batteries,it has become an energy source for hybrid vehicles and portable electronic devices.With the increasing demands for electrical vehicles and portable electronic devices high-performance,the search for promising anode alternatives with high specific capacity,good rate capability and long-term cyclability is urgent for the next-generation lithium ion batteries（LIBs）.Graphite has a low operating voltage（⁰ V vs.Li⁺/Li）,which is beneficial for enhancing the energy density of the full battery.Unfortunately,its low theoretical capacity（372 mAh/g）limits further applications in high-performance energy storage devices.Transition metal oxide has high specifc capacities,high power densities and environmental benignity,making it to be a promising anode material for LIBs.However,huge volume expansion during the conversion reaction and the intrinsically low conductivity degrade the electrochemical activity of manganese oxides.Therebefore,the graphene nanoscroll was compounded with transition metal oxide.Meanwhile,the structure of transition metal oxide was modified.Then,the electrode materials with high electrochemical properties are synthesized.The research contents were illustrated in detail as follows:（1）The ultra-long MnO₂ nanofibers were prepared by hydrothermal synthesis,and then,the MnO₂ nanofibers were combined with graphene oxide by a"rapid spray freezing"method.Finally,the porous MnO_x@graphene core-shell nanofibers electrode was obtained after the reduction of hydrazine.Such an elastic graphene shell could not only effectively accommodate the volume variation of MnOx in the lithiation process,but also provide direct and rapid electron transport pathways,ensuring the structural integrity of electrode.The interaction of manganese mixed-valence can further enhance the electrical conductivity of the electrode.Moreover,the diffusion of Li ions is significantly enhanced by vertically aligned single-crystalline and mesoporous structure of MnOx.As a result,the PMnOx@G exhibits ultrahigh rate performance(306 mA h g^-1 at 0.1 A g^-1,141 mA h g^-1 at 2.0 A g^-1),as well as excellent cycling stability(144.5 mA h g^-1 at 2 A g^-1 over 200 cycles)as a negative electrode material for lithium ion batteries.（2）The SnO₂ nanotubes were synthesized by redox reaction with the template of MnO₂nanofibers and the tin source of SnSO₄.And then,the SnO₂ nanotubes were combined with graphene oxide by a"rapid spray freezing"method.Finally,the SnO₂@graphene core-shell nanotubes electrode was obtained after the calcination reduction.Such an elastic graphene shell could not only effectively accommodate the volume variation of SnO₂ in the constantly cycles,but also provide direct and rapid electron transport pathways,ensuring the structural integrity of electrode.Meanwhile,the hollow structure of the SnO₂ nanotube accelerates the transmission rate of sodium ions.As a result,the SnO₂@G exhibits high rate performance(1072 mA h g^-1 at 0.1 A g^-1,419 mA h g^-1 at 10 A g^-1),as well as excellent cycling stability(1162 mA h g^-1 at 2 A g^-1 over 500 cycles)as a negative electrode material for sodium ion batteries.