Flexible Structural Changes of the Oxocarbon Salt K2C6O6 during Potassium Ion Insertion: An In-depth First-principles Study

Abstract Inorganic-organic crystal materials such as K2C6O6 have received intensive interest as electrode materials for metal ion batteries. In order to gain deep understanding of the structural, electronic and kinetic properties of K2C6O6 during electrochemical processes, the K ion storage properties of K2C6O6 are comprehensively studied by first-principles calculations. The insertion of K begins with a two-phase transition from K2C6O6 to K3C6O6, followed by a homogeneous reaction from K3C6O6 to K4C6O6. The space group of the material successively changes with the potassiation process, which provides flexible structures for K ion storage. The material shows a first voltage plateau at 2.76 V, followed by stepwise voltage decrease from 1.06 V to 0.74 V, providing a theoretical capacity of 218 mA∙h∙g−1. Bader charge analysis demonstrates that C is the redox center of the electrode reactions and O acts as a charge transfer medium between C and K. The flexible structures provides feasible pathways for K ions, which allows fast K ion migration under low energy barriers. The calculated K ion diffusion coefficients between 7.25×10−9 and 9.34×10−8 cm2/s indicate that K2C6O6 can realize excellent rate capability in K ion batteries.