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

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 Ca 2+ , thus contributing to large capacity storage of Ca 2+ in layered vanadium oxides (Ca 0.25 V 2 O 5 · n H 2 O, CVO). Meanwhile, the acetonitrile component noticeably suppresses the dissolution of vanadium species during repeated Ca 2+ -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 Ca 2+ 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.