Multivalent-Ion versus Proton Insertion into Battery Electrodes

For a sustainable future in energy storage technology, alternative battery chemistries, such as those based on the insertion chemistry of multivalent (MV) ions (i.e., Mg, Zn, Ca, and Al ions), have received significant interest over the years. For their future development, a clear understanding of the underlying chemistry is absolutely critical. This Perspective highlights the propensity of the transition in the charge-carrier ion species from MV ions to protons in a MV-ion system, especially when water is incorporated into the electrolyte either as a solvent or an additive, directly contradicting the commonly held belief that water improves MV-ion mobility. Recent literature demonstrating this phenomenon is introduced, emphasizing the critical need to employ appropriate analytical techniques to unequivocally determine the nature of charge-carrier ions in MV systems.