N-doped graphene quantum dot-decorated MOF-derived yolk-shell ZnO/ NiO hybrids to boost lithium and sodium ion battery performance

Surface engineering at the nanoscale to obtain robust interface between metal oxides and quantum dots is essential for improving the performance and stability of battery materials. Herein, we designed and prepared novel N-doped graphene quantum dot-modified ZnO/NiO anode materials with a well-defined yolk-shell structure for lithium and sodium-ion batteries. NG QDs were assembled on the ZnO/NiO microspheres using three different coupling strategies: solvothermal, direct adsorption and annealing under N2 atmosphere. The presence of NG QDs deposited on the ZnO/NiO hybrids promoted enhanced electrical conductivity, lower chargetransfer resistance and provides more active sites. As a result, NG-ZnO/NiO_s anode obtained by solvothermal route exhibited high reversible delithiation capacity of 912 mAh/g at 18.6 mA g-1 and excellent cycling performance with the average delithiation capacity of 525 mAh/g at 372 mA g-1 over 400 cycles. Moreover, application of the NG-ZnO/NiO_s elecrode in Na-ion batteries revealed decent electrochemical behavior with capacity values reaching 235 mAh/g at 18.6 mA g-1. Importantly, surface properties, morphology and electrochemical behavior of obtained NG-ZnO/NiO hybrids were dependent on the combination route of NG QDs with ZnO/NiO microspheres indicating that quality of heterojunction between composite components has significant impact on the electrode performance.