The safety, affordability, and impressive electrochemical performance of many Zn-ion batteries (ZIBs) has recently triggered an overwhelming literature surge. As is typical for a new area, initial enthusiasm and high expectations have now been replaced by a more measured period of research that reaches deep into the underlying factors controlling electrochemical properties. Rather than battery metrics, this review focuses on fundamental aspects of the chemistry of ZIBs that are the least understood and on which there has been progress over the last few years. We provide guidance for future research regarding (1) the significant challenge of proton/Zn 2+ co-intercalation in aqueous media, (2) limitations to conversion chemistry that often accompanies ZIB electrochemistry, (3) positive aspects of facile Zn 2+ (de)intercalation in nonaqueous electrolytes and organic cathode materials, (4) the desolvation penalty at electrode-electrolyte interfaces, (5) solutions for controlling Zn dendritic growth, and (6) suggested electrochemistry protocols for the field.

Interest has rekindled in reversible calcium plating and stripping, renewing hopes for the development of Ca-ion batteries. However, the development of an electrolyte that operates at room temperature and is stable to oxidation at practical potentials remains a significant barrier. Here we report the synthesis and crystal structure of a new fluorinated alkoxyborate Ca(B(Ohfip) 4 ) 2 •4DME salt. Reversible plating and the dissolution of calcium from pure solutions of this salt in dimethoxyethane are demonstrated at 25 °C with capacities of 1 mAh cm −2 at a rate of 0.5 mA cm −2 over 30−40 cycles, with an anodic stability of >4.1 V vs Ca/Ca 2+ (and up to 4.9 V in dimethyltriflamide). The dominant product is calcium, accompanied by CaF 2 that forms by the reduction of the fluorinated anion. Whereas the cathodic stability requires improvement, this work shows that facile calcium plating and stripping at room temperature can be achieved using bulk electrodes.

Calcium-ion batteries (CIBs) are under investigation as next-generation energy storage devices due to their theoretically high operating potentials and lower costs tied to the high natural abundance of calcium. However, the development of CIBs has been limited by the lack of available positive electrode materials. Here, for the first time, we report two functional polyanionic phosphate materials as high-voltage cathodes for CIBs at room temperature. NaV2(PO4)3 electrodes were found to reversibly intercalate 0.6 mol of Ca2+ (81 mA h g–1) near 3.2 V (vs Ca2+/Ca) with stable cycling performance at a current density of 3.5 mA g–1. The olivine framework material FePO4 reversibly intercalates 0.2 mol of Ca2+ (72 mA h g–1) near 2.9 V (vs Ca2+/Ca) at a current density of 7.5 mA g–1 in the first cycle. Structural, electronic, and compositional changes are consistent with reversible Ca2+ intercalation into these two materials.