Unprotected organic cations – the dilemma of highly Li salt-concentrated ionic liquid electrolytes

Highly Li salt-concentrated electrolytes have been widely studied to uniquely vary their bulk and interface properties arising from their distinctive physicochemical properties and coordination structures. Similar strategies in the realm of ionic liquid electrolytes have also received significant attention owing to the improvement in their functionality. Nevertheless, compared with the broad study on anion behavior, the impact of cation behavior on interfacial processes has been largely overlooked. This study elucidates that the perceptibly weakened interaction between cations and anions facilitates the emergence of “unprotected” cations in highly Li salt-concentrated ionic liquid electrolytes, thereby serving as the primary driver for the decomposition of organic cations during the initial charge. This decomposition behavior manifests in the form of substantial irreversible capacities and inferior initial Coulombic efficiency during the charging of graphite negative electrodes, resulting in considerable electrolyte consumption and detrimental consequences for energy density optimization in a full cell. The veracious cation behavior is substantiated by examining the coordination environment of ionic liquid across varying Li concentrations, which reveals intricate ionic interactions between organic cations and anions. In addition, anionic species with high Lewis basicity were introduced to reinforce the ionic interactions involving organic cations, and the increment in the initial Coulombic efficiency is confirmed, which verifies the equal significance of the coordination environment centering unprotected organic cations in ionic liquid electrolytes.