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

Abstract

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 V<inf>2</inf>O<inf>5</inf> (V<inf>2</inf>O<inf>5</inf>·Pyridine, PVO) model material with stable large interlayer spacing. It is found that the discharge-specific capacity in 1 m Zn(ClO<inf>4</inf>)<inf>2</inf> 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(ClO<inf>4</inf>)<inf>2</inf>. Mechanistic studies show that in aqueous ZIB, H⁺ is intercalated first, followed by the generation of Zn<inf>4</inf>(OH)<inf>7</inf>ClO<inf>4</inf>, 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⁺.