Insights into the Energy Storage Differences of Zinc and Calcium Ions with Layered Vanadium Oxide as a Model Material

Multivalent ion batteries (e.g., Zn2+, Ca2+) are gaining great attention owing to their potentially high capacity, cheap cost, and good safety. However, significant disparities exist in achieved capacity, voltage, and kinetic performance within zinc and calcium ion electrolytes. Herein, the electrochemical and kinetic properties of Zn2+ and Ca2+ in aqueous electrolytes are investigated using a single-crystal V2O5 (V2O5 & BULL;Pyridine, PVO) model material with stable large interlayer spacing. It is found that the discharge-specific capacity in 1 m Zn(ClO4)2 aqueous solution is 247.3 mAh g-1 at 0.3 A g-1, which is remarkably higher than 158.4 mAh g-1 in 1 m Ca(ClO4)2. Mechanistic studies show that in aqueous ZIB, H+ is intercalated first, followed by the generation of Zn4(OH)7ClO4, and finally H+ and Zn2+ are co-intercalated. But in aqueous CIB, H+ dominates the intercalation process. It is found that six times more Zn2+ than Ca2+ is intercalated into PVO, owing to its smaller radii and its relative higher intercalation potential (Zn2+ @-0.34 V, Ca2+ @-0.65 V vs.. Ag/AgCl), giving it a higher specific capacity. Furthermore, density functional theory calculations demonstrate lower intercalation energies for Ca2+ (-6.67 eV) compared to Zn2+ (-1.85 eV), explaining the lower intercalation potential of Ca2+. Multivalent ion batteries, exemplified by Zn2+ and Ca2+ systems, hold potential for large-scale energy storage. This study employs PVO (V2O5 & BULL;Pyridine) as a model material, examining electrochemical and kinetic distinctions of Zn2+ and Ca2+. Notably, Zn2+ displays higher specific capacity than Ca2+. Detailed analyses encompass intercalation content, potentials, proton co-insertion, and first-principle calculations. These findings illuminate cation-specific energy storage mechanisms, guiding diverse aqueous ion battery cathode designs.image