Organic positive-electrode material utilizing both an anion and cation: a benzoquinone-tetrathiafulvalene triad molecule, Q-TTF-Q, for rechargeable Li, Na, and K batteries

Abstract

This study highlights the design concept of a positive electrode material which can accommodate both cations and anions during the charge/discharge process for realizing high energy density rechargeable batteries. Fusing an electron-acceptor (N type) and donor (P type) would endow such properties. In addition, unlike rigid inorganic frameworks, flexible organic compounds have the potential to adopt a variety of ion species. We examined the electrochemical properties of a fused triad molecule incorporating N-type benzoquinones and a P-type tetrathiafulvalene (Q-TTF-Q). Preliminary electrochemical measurements revealed Q-TTF-Q to be electrochemically active, exhibiting a high capacity at a decent voltage, comparable to reported organic cathode materials not only for the current lithium ion technology but also for the emerging sodium and potassium-ion batteries. For instance, Q-TTF-Q has a capacity of 236 mA h g⁻¹ with the decent average potential of 2.8 V vs. Li⁺/Li, translating to the high energy density of 661 mW h g⁻¹. A carrier ion analysis based on energy dispersive X-ray spectrometry (EDX) proved that Q-TTF-Q indeed stores both anions and cations during the charge/discharge processes. The approach of fusing P- and N-type organic moieties, as described in this study, provides a rational design strategy to further develop high-energy-density organic electrode materials.