Bilayered Mg_0.25V₂O₅·H₂O as a Stable Cathode for Rechargeable Ca-Ion Batteries

The calcium-ion battery is an emerging energy storage system that has attracted considerable attention recently. However, the absence of high-performance cathode materials is one of the main challenges for the development of calcium-ion batteries. Herein, a bilayered Mg0.25V2O5·H2O as a stable cathode for rechargeable calcium-ion batteries is identified. Remarkably, an unexpected stable structure of the material for Ca2+ storage is demonstrated. It is found that the interlayer spacing shows only a tiny variation of ∼0.09 Å during Ca2+ insertion/extraction, which results in its outstanding cycling stability (capacity retention of 86.9% after 500 cycles) for Ca2+ storage. On the basis of in situ/ex situ experimental characterizations and ab initio simulation, the origin of such superior structural stability is revealed. This ultrastable cathode together with the understanding lays a strong foundation for developing high-performance calcium-ion batteries.