Highly Reversible Intercalation of Calcium Ions in Layered Vanadium Compounds Enabled by Acetonitrile-Water Hybrid Electrolyte

Abstract

Currently, the development of calcium-ion batteries (CIBs) is still in its infancy and greatly plagued by the absence of satisfactory cathode materials and compatible electrolytes. Herein, an acetonitrile-water hybrid electrolyte is first developed in CIB chemistry, in which, the strong lubricating and shielding effect of water solvent significantly boosts the swift transport of bulky Ca²⁺, thus contributing to large capacity storage of Ca²⁺ in layered vanadium oxides (Ca<inf>0.25</inf>V<inf>2</inf>O<inf>5</inf>·nH<inf>2</inf>O, CVO). Meanwhile, the acetonitrile component noticeably suppresses the dissolution of vanadium species during repeated Ca²⁺-ion uptake/release, endowing the CVO cathode with a robust cycle life. More importantly, spectral characterization and molecular dynamics simulation confirm that the water molecules are well stabilized by the mutual hydrogen bonding with acetonitrile molecules (O-H···N), endowing the aqueous hybrid electrolyte with high electrochemical stability. By using this aqueous hybrid electrolyte, the CVO electrode shows a high specific discharge capacity of 158.2 mAh g^-1 at 0.2 A g^-1, an appealing capacity of 104.6 mAh g^-1 at a high rate of 5 A g^-1, and a capacity retention of 95% after 2000 cycles at 1.0 A g^-1, which is a record-high performance for CIBs reported so far. A mechanistic study exemplifies the reversible extraction of Ca²⁺ from the gap of VO polyhedral layers, which are accompanied by the reversible V-O and V-V skeleton change as well as reversible variation of layer spacing. This work constitutes a major advance in developing high-performance Ca-ion batteries.