Effects Of Graphene-coating On Electrochemical Performance Of NiO/Co₃O₄ Nano-composites Via Electrospinning Technique

As anode materials for lithium ion batteries,transition metal oxides show usually higher energy density,but poor electrical conductivity and volume expansion during lithiation that limit their practical applications.In this work,to overcome those intrinsic drawbacks,tubular nanosized NiO/Co₃O₄ composites are designed and assembled via electrospinning technique.The tubular structure not only provides more spaces for volume expansion,but also facilitates lithium ion transportation.Furthermore,NiO/Co₃O₄@r GO with three dimension network was constructed,which the external encapsulated graphene sheets can provide high conductive path for electrons.The as-obtained NiO/Co₃O₄ nanotubes encapsulated with graphene show excellent electrochemical performance,including large lithium ion storage capability,high rate capability and cycling stability.The detailed research contents are as follows:(1)Via electrospinning of DMF solution containing PAN/PVP and metal salts followed by controllable calcination,one-dimensional tubular and fibrous nano-sized NiO/Co₃O₄composites were synthesized.Meanwhile granular nano-sized NiO/Co₃O₄composites were obtained via a sol-gel method without electrospinning.Compared with nanofibers and nanoparticles,the internal cavity of tubular structure alleviates the volume change in lithiation/delithiation and accelerate Li⁺diffusion affording finally superior cycling performance and higher rate capability.(2)To coat with graphene,the tubular NiO/Co₃O₄ was functionalized with amino groups using APTES.Considering the oxygen containing groups,such as carboxylic groups,on the surface of chemically reduced graphene oxide,the NiO/Co₃O₄were encapsulated with graphene simply through a self-assembly process.It is demonstrated that the graphene sheets were uniformly coated on tubular NiO/Co₃O₄The as-prepared graphene-encapsulated NiO/Co₃O₄ shows large lithium ion capacity(?1206 m A h g^-1 after 100 cycles at 0.1 A g^-1),long cycling capability(?78%capacity retention ratio after 700 cycles at 0.5 A g^-1)and superior rate capability(?467 m A h g^-1 at 10.0 A g^-1).For comparison,the effects of the graphene sheets on NiO/Co₃O₄ nanofibers and individual particles were also studied.The electrochemical kinetic analysis was performed and found that the tubular structure and graphene-coating both affect the lithium ion storage mechanism and also the structural stability of the NiO/Co₃O₄.