Halogen Conversion-Intercalation Cathode for Zinc-Ion Battery

Zinc-ion batteries (ZIBs) are a promising class of batteries for grid-scale electrochemical energy storage owing to their low cost, easy fabrication and high operational safety. ZIBs adoption have been hampered by a lack of efficient cathode materials, which is due to the high polarisation of divalent zinc ions, which leads to strong binding with the host lattice in combination with slow solid-state migration. In this contribution, I describe a strategy that eliminates the involvement of Zn 2+ within cathode chemistry. The method employs confined halogen within a carbon structure as the cathode electrode, with the halogen ions acting as both charge carriers and redox centres. The benefits of using confined halogen within the carbon structure as a cathode are (i) elimination of the irreversible binding of Zn 2+ to the host structure within the cathode chemistry, (ii) the provision of substantial extra charge through their conversion-intercalation/adsorption process and (iii) they do not have to diffuse to the surface from bulk electrolyte as they are already inside the carbon structure aqueous ZIBs that utilize a conversion chemistry cathode. I'll illustrate how the cathode composite's Zn halide and carbon structure are completely different, resulting in electrochemical energy storage devices that are fundamentally different (battery vs supercapacitor). The use of graphite in the composite electrode resulted in battery-like behaviour, with the voltage plateau being related to the halogen species' standard potential. Using a mix of electrochemical and in-situ spectroscopic approaches, the halogen reaction process was revealed. This fundamental understanding will be used as a starting point for the development of efficient aqueous ZIBs that utilize a conversion chemistry cathode.