Elucidating proton-intercalation chemistries

Ion intercalation is one of the fundamental reaction mechanisms for rechargeable batteries and the choice of ion charge carriers affects their performance. Among all possibilities, protons stand out as charge carriers due to the smallest ionic radius (at the picometer level) and the capability for fast transport in aqueous media via the unique Grotthuss conduction. As a result, batteries that are based on proton intercalation are intrinsically advantageous in electrochemical reversibility and kinetics, making them highly promising for high-rate and long-life applications, and they are rapidly emerging [1]. An ion intercalation reaction generally involves three main steps: (i) electrolyte ion migration towards the interface, (ii) charge transfer across the electrolyte/electrode interface and then (iii) solid-state diffusion of intercalants in the host electrode. However, the mechanisms that underlieproton intercalation are not yet understood because protons often exhibit exceptional and unconventional behaviors. Regarding proton intercalation, although the first and last steps have been relatively well described, the charge-transfer process remains ambiguous and is the focus of ongoing investigations.