Synthesis and electrochemical performance of hollow-structured NiO plus Ni nanofibers wrapped by graphene as anodes for Li-ion batteries

摘要

Hollow-structured NiO + Ni nanofibers wrapped by graphene were designed and successfully fabricated via a simple method. First, solid NiO + Ni nanofibers were prepared by electrospinning followed by calcination. Here, a portion of the metallic Ni was retained to improve the electrochemical performance of NiO by adjusting the calcination temperature. Next, the nanofibers were thoroughly mixed with different amounts of graphene and calcinated once more to form hollow-structured NiO + Ni nanofibers with an extremely high specific surface via the reaction between graphene and NiO on the nanofiber surface and subsequent migration of NiO into the nanofibers. Results showed that the obtained hollow-structured NiO + Ni electrode demonstrates optimal electrochemical performance when the graphene content is controlled to 3 wt%. The first cycle discharge/charge specific capacity of the electrode peaked (1596/1181 mAh center dot g(-1) ) at 100 mA center dot g(-1), with a coulombic efficiency of approximately 74% (60% for 0 wt% graphene, 65% for 1 wt% graphene, and 51% for 4 wt% graphene). It also presented excellent cycling stability after 100 cycles at 100 mA center dot g(-1) on account of its high retained discharge specific capacity (251 mAh center dot g(-1) for 0 wt% graphene, 385 mAh center dot g(-1) for 1 wt% graphene, 741 mAh center dot g(-1) for 3 wt% graphene, and 367 mAh center dot g(-1) for 4 wt% graphene). Moreover, the synthesized electrode possessed outstanding rate capability owing to its large average discharge specific capacity of approximately 546 mAh center dot g(-1) (45 mAh center dot g(-1) for 0 wt% graphene, 256 mAh center dot g(-1) for 1 wt% graphene, and 174 mAh center dot g(-1) for 4 wt% graphene) from 100 mA center dot g(-1) to 2000 mA center dot g(-1). The observed improvement in electrochemical performance could be attributed to the increase in active sites and decrease in charge transport distance in the hollow-structured NiO + Ni nanofibers. Excessive introduction of graphene caused a sharp loss in electrochemical performance due to the agglomeration of graphene sheets on the nanofiber surfaces.