Enhanced Lithium Storage Capacity of a Tetralithium 1,2,4,5-Benzenetetracarboxylate (Li4C10H2O8) Salt Through Crystal Structure Transformation

Abstract

Because of their low price, design flexibility, and sustainability, organic-based electrode materials are considered one of the most promising next-generation alternatives to inorganic materials in Li-ion batteries. However, a clear understanding of the changes in the molecular crystal structure during Li-ion insertion/extraction and its relationship to excess capacity (over theoretical capacity) is still lacking. Herein, the tetralithium 1,2,4,5-benzenetetracarboxylate (Li4C10H2O8, Li4BTC) salt was prepared using a simple ion-exchange reaction at room temperature and under solvothermal conditions (100 °C). The solvothermally synthesized salt (Li4BTC-S) exhibited a well-ordered nanosheet morphology, whereas the room-temperature salt (Li4BTC-R) was comprised of irregularly shaped particles. During the cycling of Li4BTC-S, molecular rearrangement occurred to reduce the stress caused by repeated Li-ion insertion/extraction, resulting in a change in the crystal structure from triclinic to monoclinic and an increased free volume. This contributed to an increase in the reversible capacity to 1016 mAh g-1 during the initial 25 cycles at 0.1 A g-1, and finally the capacity stabilized at ca. 600 mAh g-1 after 100 cycles, which is much higher than its theoretical capacity (234 mAh g-1). Compared with Li4BTC-R, Li4BTC-S delivered a higher reversible capacity of 190 mAh g-1 at a high current density of 2 A g-1, with an excellent long-term cyclability of up to 1000 cycles, which was attributed to the straight free volume columns and the low-charge-transfer limitation.