Flavanthrone: From Vat Dye to Organic Cathode Material for Rechargeable Lithium Batteries

Organic cathode materials (OCMs) hold significant promise as alternatives to transition-metal-based inorganic counterparts for constructing sustainable and efficient rechargeable batteries. However, it is a huge challenge to overcome the dissolution problem at an affordable cost, especially for small-molecule organic cathode materials (SMOCMs). Herein, we investigated a commercially available vat dye, namely, flavanthrone (FVT), directly as a novel SMOCM for rechargeable lithium batteries. It possesses a low solubility benefiting from the extensive aromatic system and a high theoretical capacity of 262 mAh g^–1 based on the four electroactive C═O/C═N groups. The four-electron redox reaction can be nearly fully utilized within a voltage window of 0.8–3.5 V, exhibiting two distinct and stable potential plateaus at 2.50 and 0.95 V versus Li⁺/Li. Within an optimal voltage window of 1.5–3.5 V and in an optimal electrolyte of 1 M LiTFSI/G4 [LiTFSI, lithium bis(trifluoromethanesulfonyl)imide; G4, tetraethylene glycol dimethyl ether], FVT effectively executes a two-electron redox reaction, displaying a discharge voltage of 2.5 V, a reversible capacity of 131 mAh g^–1, and a high capacity retention of 95% after 100 cycles. Furthermore, the electrochemical redox and capacity fading mechanisms as well as the influence of varying voltage windows and electrolytes have been thoroughly elucidated, providing important insights that guide the rational design of SMOCMs’ structures and test conditions.