Oxocarbon Salts for Fast Rechargeable Batteries

Oxocarbon salts (M2(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 M2(CO)5/M2(CO)6 salts can insert two or four metal ions reversibly, while M2(CO)4 shows less electrochemical activity. Especially, we discover that the K2C6O6 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 K2C6O6 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 K2C6O6 as cathode and K4C6O6 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.