Unveiling the mechanism of sodium ion storage for needle-shaped Zn _{: X} Co _{3- x} O ₄ nanosticks as anode materials

The interest in the development of micro-nanostructured metal oxides has been increasing recently because of their advantages as electrode materials in energy storage applications. In this study, dandelion-like Zn _x Co _3-x O ₄ microspheres assembled with porous needle-shaped nanosticks were synthesized by co-precipitation followed by a post-annealing treatment. The open space between neighboring nanosticks enables easy infiltration of the electrolyte; therefore, each nanostick is surrounded by the electrolyte solution, which ensures proper utilization of the active material during the electrochemical reaction. The dandelion-like Zn _x Co _3-x O ₄ hierarchical microspheres exhibit a greatly improved electrochemical performance with a high capacity and good cyclability as anodes for sodium-ion batteries (SIBs). A high initial reversible capacity of 612 mA h g ^-1 (at 35 mA g ^-1 , ∼0.04C) is obtained and a capacity of 349 mA h g ^-1 is retained after 200 cycles. Meanwhile, the electrode shows a high rate performance with a capacity of 246 mA h g ^-1 at 2.0C-rate. The conversion of Zn _x Co _3-x O ₄ with Na is followed by ex situ X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) in different sodiation/de-sodiation states during electrochemical cycling. These analyses reveal that Na insertion/extraction is followed by complete reduction/oxidation of both metallic cobalt and zinc. The development of metallic Co and Zn after complete discharge and the formation of Co ₃ O ₄ and ZnO when the electrode was fully recharged were identified by ex situ TEM analysis. In addition, the Zn _x Co _3-x O ₄ anode demonstrates feasible operation in a full cell by pairing with a NaNi _2/3 Bi _1/3 O ₂ cathode, affording a sodium-ion battery with an average working voltage of 2.6 V.