Aqueous Zn-ion batteries: Cathode materials and analysis

Reversible aqueous Zn-ion electrochemistry has revived the interest in aqueous batteries, thanks to the attractive features conferred by the energy dense metallic zinc anode and safe and inexpensive aqueous electrolytes. Ultimately, the practical development of the technology would depend significantly on the cathode hosts' electrochemistry, which is strongly influenced by the structural and functional attributes of host materials. We introspect that here while reviewing different inorganic host chemistries and some notable findings that can potentially shape future developments. An assessment of the energy density and its relation to the inactive components of the cell is also presented. Finally, we take a critical look at the evolving ambiguity around the analysis of the charge storage mechanism and raise a few questions on mechanistic understanding and lack thereof, which are crucial to address shortcomings in the cathode performance.