A novel CoO hierarchical morphologies on carbon nanofiber for improved reversibility as binder-free anodes in lithium/sodium ion batteries

Abstract In this work, hierarchical CNF/CoO composite films are synthesized through a reflux method in which cobalt compounds can grow on the substrate of CNFs under low temperature of 120 °C and ambient pressure without special react vessel for large-scaled output. The crystal morphologies of CoO nanoflakes are facilely controlled by adjusting the weight ratio of starting cobalt salt. The newly hierarchical structure of nanoparticle-nanoflake-micron particle plays a key role in achieving extraordinary electrochemical performance. CNF with good conductivity and stable structure provides fast electronic conductive pathway in the composite electrode, which is beneficial for the high rate performance and cyclic performance. CoO with the hierarchical structure presents the enhancement of capacity. By combining the advantages of CNF and CoO, CNF/CoO-4 sample with a proper amount of CoO in the composite film achieves an excellent electrochemical performance in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). CNF/CoO-4 sample presents capacity of 530 mAh g−1 after 100 cycles at 0.2 A g−1 in LIBs, and delivers 193 mAh g−1 after 50 cycles at 0.05 A g−1 in SIBs. Furthermore, in CNF/CoO//LiCoO2/Al coin-type full cell, CNF/CoO-4 delivers a high reversible discharge capacity of 430 mAh g−1 after 55 cycles at 0.1 A g−1. The reflux method can be used to synthesize other CNF supported metal oxides with various crystal morphologies for highly reversible energy storage.