A Hexaanionic Carboxyphenolate Framework for High Energy Alkali Cation Storage

Abstract

The development of high-performance alkali-rich organic positive electrode materials remains a significant challenge. Recent advances across various classes of organic compounds have led the achievement of either high capacity or high voltage improvements. However, the quest for materials that simultaneously offer both high capacity and voltage, and thus high-energy density remains challenging. Herein, we report a new organic redox system, 2,2',5,5'-tetrahydroxy-4,4'-biphenyl dicarboxylic acid (H6THBPD), designed for high-performance alkali metal-ion storage. The prepared Li6, Na6, and K6THBPD positive electrodes reveal a high degree of redox utilization, with nearly four electrons (4e-) per formula unit attained, resulting in specific capacities of 313, 244, and 200 mAh/g, for Li-, Na-, and K-ion storage, respectively. A high redox potential of 2.85 V vs. Li+/Li, 2.4 V vs. Na+/Na, and 2.5 V vs. K+/K is observed, attributed to the combined electron-withdrawing effect, and reduced conjugation, which destabilize charge localization and thereby increase the redox potential. These metrics position the developed A6THBPD phases among the best in alkali containing form positive electrode materials reported to date. A prototype Li-ion full-cell using a graphite anode is assembled and shown to provide an output voltage of 2.7 V, with competitive cycling performances. This study expands the potential applications of monovalent cations in conjugated carboxyphenolate organic chemistry.