Oxocarbon Salts for Fast Rechargeable Batteries

Oxocarbon salts (M 2 (CO) n ) prepared through one‐pot proton exchange reactions with different metal ions (M=Li, Na, K) and frameworks ( n =4, 5, 6) have been rationally designed and used as electrodes in rechargeable Li, Na, and K‐ion batteries. The results show that M 2 (CO) 5 /M 2 (CO) 6 salts can insert two or four metal ions reversibly, while M 2 (CO) 4 shows less electrochemical activity. Especially, we discover that the K 2 C 6 O 6 electrode enables ultrafast potassium‐ion insertion/extraction with 212 mA h g −1 at 0.2 C and 164 mA h g −1 at 10 C. This behavior can be ascribed to the natural semiconductor property of K 2 C 6 O 6 with a narrow band gap close to 0.9 eV, the high ionic conductivity of the K‐ion electrolyte, and the facilitated K‐ion diffusion process. Moreover, a first example of a K‐ion battery with a rocking‐chair reaction mechanism of K 2 C 6 O 6 as cathode and K 4 C 6 O 6 as anode is introduced, displaying an operation voltage of 1.1 V and an energy density of 35 Wh kg −1 . This work provides an interesting strategy for constructing rapid K‐ion batteries with renewable and abundant potassium materials.