Freestanding Ammonium Vanadate Composite Cathodes with Lattice Self‐Regulation and Ion Exchange for Long‐Lasting Ca‐Ion Batteries

Calcium-ion batteries (CIBs) have emerged as a promising alternative for electrochemical energy storage. The lack of high-performance cathode materials severely limits the development of CIBs. Vanadium oxides are particularly attractive as cathode materials for CIBs, and preinsertion chemistry is often used to improve their calcium storage performance. However, the room temperature cycling lifespan of vanadium oxides in organic electrolytes still falls short of 1000 cycles. Here, based on preinsertion chemistry, the cycling life of vanadium oxides is further improved by integrated electrode and electrolyte engineering. Utilizing a tailored Ca electrolyte, the constructed freestanding (NH₄)₂V₆O₁₆·1.35H₂O@graphene oxide@carbon nanotube (NHVO-H@GO@CNT) composite cathode achieves a 305 mAh g^-1 high capacity and 10 000 cycles record-long life. Additionally, for the first time, a Ca-ion hybrid capacitor full cell is assembled and delivers a capacity of 62.8 mAh g^-1. The calcium storage mechanism of NHVO-H@GO@CNT based on a two-phase reaction and the exchange of NH₄ ⁺ and Ca²⁺ during cycling are revealed. The lattice self-regulation of V─O layers is observed and the layered vanadium oxides with Ca²⁺ pillars formed by ion exchange exhibit higher capacity. This work provides novel strategies to enhance the calcium storage performance of vanadium oxides via integrated structural design of electrodes and electrolyte modification.