Cathode property of na₂C_xO_x [x = 4, 5, and 6] and K₂C₆O₆ for sodium-ion batteries

Sodium-ion batteries now become a strong alternative candidate for lithium-ion batteries due to abundant Na resources, and organic electrode active materials for sodium-ion batteries also have possibility to bring additional cost reduction. Although many organic-based active materials have been researched as cathode materials for lithium-ion batteries, few works have been conducted for sodium-ion batteries. In this study, the cathode properties of several organic compounds of sodium rhodizonate analogs, Na₂C_xO_x [x = 4 and 5] and K₂C₆O₆ were newly investigated. Among them, it was revealed that Na₂C₅O₅ and Na₂C₆O₆ deliver large first discharge capacity close to 200 mAh g^-1 and exhibit reversible sodium insertion/de-insertion of 1.35 and 1.25Na, respectively, with good cycle performance.