Mechanism of bulky imidazolium cation storage in dual graphite batteries: A spectroscopic and theoretical investigation

Dual graphite batteries (DGBs) employ graphite as an intercalation host and store energy using the redox reactions of ionic liquid electrolyte-derived ions with graphite electrodes. As bulky (e.g., imidazolium-based) cation intercalation into the negative graphite electrode may cause its deterioration and thus aggravate self-discharge behaviour, a deep mechanistic understanding of such intercalation is required to enhance the DGB performance; however, the related studies remain scarce. Herein, the intercalation of 1,2-dimethyl-3-propylimidazolium (DMPI+) cations into the negative graphite electrode is probed by in situ X-ray diffraction and in situ Raman spectroscopy. The results show that these cations are stored in the shallow surface layer of bulk graphite without dramatic expansion into the deep graphite lattice, in stark contrast to the intercalation of AlCl4− counter-anions into the positive graphite electrode. Further analysis reveals that DMPI+ storage kinetics can be described as intercalation pseudocapacitance dominated, while first-principles calculations show that intercalation is thermodynamically more favourable than surface adsorption. Although only the DMPI+ cation is studied herein, the obtained insights are expected to be extendable to other bulky cations (e.g., pyrrolidinium or piperidinium). In addition, this study deepens our understanding of factors influencing the performance of DGBs with ionic liquid electrolytes and promotes the further development of energy storage systems with greater power and energy densities.