A Small Molecular Cathode for High‐Performance Calcium Metal Batteries

Abstract Calcium‐ion batteries are emerging as a promising alternative to lithium‐ion batteries by offering potential advantages in cost, affordability, and safety. However, the development of high‐performance calcium batteries has been hindered by the lack of high‐capacity cathodes. Here a small molecular organic cathode, 9,10‐phenanthrenequinone (PQ), is reported which leverages the rapid enolization chemistry of multi‐redox centers (C═O) and the flexible intermolecular structure for high performance. The PQ cathode demonstrates an impressive specific capacity of 250 mAh g −1 at 0.2 C for 200 cycles and superior rate capabilities. The robust performance of PQ cathodes is attributed to the mitigation of small molecule electrode dissolution in the weakly solvating ether electrolytes and the promotion of inter‐and intralayer Ca ion diffusion pathways, which synergistically facilitate the retention of high capacities at elevated current rates. When paired with a Ca metal anode, the full cell achieves remarkable capacities of 214 mAh g −1 at 0.2 C, with an average operating voltage of 2.5 V versus Ca/Ca 2+ , representing the highest performance among CMBs reported to date. The use of organic cathodes in tailored electrolytes with restricted intermediate dissolutions heralds a new era in the development of multivalent metal batteries.