Hollow Metal Oxide/Carbon Nanofibers As Flexible Composite Electrodes For Lithium Ion Storage

Carbon nanofibers are common electrode carrier because its excellent electrical conductivity and stability.But limited capacity limits their further development.Conversely,most metal oxides have high theoretical capacity but poor cycle stability.This is a hotspots of lithium-ion batteries research that scientifically combine metal oxides with carbon nanofibers,then obtain high-capacity stable electrode materials.In order to improve the lithium storage performance of metal oxide/carbon nanofiber,the fiber was designed as hollow structure.Hollow Co₃O₄/carbon nanofiber membrane and hollow Mo O₂/carbon nanofiber membrane were prepared to improve the lithium storage performance of composite electrode.The details are as follows:（1）The best process of hollow carbon nanofibers is determined by single factor experimental test.12 wt%PAN shell solution,30 wt%PMMA core solution,21 cm receiving distance,21 k V electrostatic voltage and 600℃carbonization temperature can prepare hollow carbon nanofibers with light weight and large internal space.The fiber diameter is mostly 200～250 nm,which is conducive to contact with electrolyte and packaging metal oxides.The fiber after carbonization is composed of amorphous carbon with good conductivity.Therefore,the hollow carbon nanofiber based on PAN/PMMA is an ideal electrode material.（2）Through coaxial electrospinning,carbonization and oxidation,Co₃O₄ was encapsulated in the hollow carbon nanofiber wall and obtained hollow Co₃O₄/carbon nanofiber has large cavity and no particles on the surface.Compared with the solid structure,（1）the hollow Co₃O₄/carbon nanofiber composite electrode shows higher initial specific capacity（1619.85 m Ah/g>1089.46 m Ah/g at 0.1 A/g;949.20 m Ah/g>811.70m Ah/g at 1 A/g）and lower impedance.This is due to the hollow structure provides more surface area to close contact with electrolysis,and brings higher discharge capacity.（2）The hollow Co₃O₄/carbon nanofiber composite electrode has a more stable cycle.Its capacity attenuation rate is 19.6%after circulating 40 cycles at 0.1 A/g,which is lower than 48.9%of the solid structure.This is help to the fact that the carbon wall alleviates the excessive expansion of Co₃O₄,and the reserved cavity effectively buffers the stress caused by the volume expansion of Co₃O₄,ensuring the integrity of the composite electrode and the stability of the cycle.Compared with solid Co₃O₄/carbon nanofibers,the cyclic performance of hollow composite electrode has been greatly improved,which provides a direction for the structural design of the composite electrode.（3）In order to avoid the damage of oxidation in the upper part,the types of metal oxides were changed and the synthetic route was simplified.After a simple coaxial electrospinning and carbonization process,Mo O₂ was successfully encapsulated in the hollow carbon nanofiber wall,and the lithium storage performance of the hollow composite electrode was explored.The hollow Mo O₂/carbon nanofiber composite electrode is proved has higher capacity and stronger stability,which are derived from the reserved space and large specific surface area.（1）At 0.1 A/g,the initial discharge capacity of hollow Mo O₂/carbon nanofiber composite electrode is 1328.70 m Ah/g and the reversible capacity is 934.70 m Ah/g.After 80 cycles,the capacity remains at 879.37m Ah/g,which is higher than that of solid structure 596.95 m Ah/g;（2）Under the high current density of 1 A/g,the capacity of solid composite electrode decreases rapidly to less than 100 m Ah/g,while the specific capacity of hollow composite electrode remains stable at 283.50 m Ah/g after 1000 long cycles with no obvious landslide.（3）Under the impact of mechanical stress,it can still maintain stable output,showing considerable practical application potential.