Ammonium Vanadium Bronze As a Cathode Material for Calcium-Ion Batteries

Calcium-ion battery (CIB) is considered one of the attractive candidates for post-Li-ion batteries. However, few cathode materials intercalate Ca ions reversibly in nonaqueous electrolytes. Herein, we report the discovery of novel ammonium vanadium bronze (NVO) as a host material of CIBs. We synthesized NVO via a facile hydrothermal method. We determined its chemical composition and crystal structure (P-1) from single-crystal X-ray diffraction data for the first time, revealing unprecedented stoichiometry and crystal structure. It comprises V 7 O 16 layers stacking along the c-axis and two NH 4 + ions occupying the interlayer space per formula unit. The galvanostatic discharge/charge cycling demonstrates reversible electrochemical intercalation of Ca 2+ ions into the structure. The discharge capacities are ~98 and 87 mAh g -1 for the first and second cycles, respectively. On charge, NH 4 + ions are de-intercalated. Simultaneously, the ion-exchange reaction occurs: a small amount of Ca 2+ ions are chemically exchanged for NH 4 + ions. On discharge, cation insertion occurs, mainly Ca 2+ ions and partially the extracted ammonium ions. Small amounts of reinserted ammonium ions play an important role in structural stability, enabling better cycle performance. Understanding the newly discovered mechanism of this material is expected to provide general insights into ammonium vanadium bronzes as new cathode materials.