Tunable oxygen-vacancy mediated NiCo2O4 microspheres coupled by holey graphene framework with superior lithium-ion storage properties

Abstract

Homogeneous NiCo2O4 (NCO) microspheres encapsulated by holey graphene framework (HGF) are synthesized by using solvothermal and further annealing approaches, respectively. By tuning the solvothermal conditions involved in the synthesis process, the specific oxygen vacancies induced into NCO microspheres with different ratios can be researched comprehensively. Thus, lower (NCO/HGF-1) and higher (NCO/HGF-2) oxygen vacancy forms of hybrid NCO/HGF microspheres are prepared, respectively. Specifically, those hybrid NCO/HGF microspheres exhibit a uniformly spherical structure along with the average particle size around 600 nm. Meanwhile, the inner NCO kernels are covered by the superficial HGF layers. More importantly, those NCO/HGF electrodes show excellent reversibly specific discharging capacity for lithium-ion batteries (LIBs) (554.71 mAh g−1 and 591.84 mAh g−1 for NCO/HGF-1 electrodes and NCO/HGF-2 electrodes at 2 A g−1, respectively) and relatively higher Li+ diffusion coefficient (9.76*10−12 cm2 s−1 for NCO/HGF-1 electrodes and 5.53*10−12 cm2 s−1 for NCO/HGF-2 electrodes, respectively). The superior electrochemical properties of the hybrid NCO/HGF electrodes can be explained to the specific holes in HGF layers facilitating Li+ transport and the oxygen vacancies in NCO enhancing electronic conductivity. The result is further verified by the density functional theory (DFT) analysis. This work presents a new strategy and promising potential of electrodes preparation for metal-ion batteries.