The Role of Ionic Liquid Electrolyte in an Aluminum-Graphite Electrochemical Cell

Using first-principles calculations and molecular dynamics simulation, we study the working mechanism in an aluminum-graphite electrochemical cell, which was recently reported to exhibit attractive performance. We exclude the possibility of Al3+ cation intercalation into graphite as in standard Li-ion batteries. Instead, we show that the AlCl4- anion intercalation mechanism is thermodynamically feasible. By including the ionic liquid electrolyte in the overall redox reaction, we are able to reproduce the high voltage observed in experiment. The active involvement of electrolyte in the reaction suggests that the evaluation of energy density needs to take the electrolyte into consideration. Our proposed structural model is consistent with the new peaks appearing in X-ray diffraction from the intercalation compound. The high rate capability is explained by the ultralow diffusion barriers of the AlCl4 intercalant. With the clarified working mechanism, it becomes clear that the high voltage of the Al-graphite cell is a result of the thermodynamic instability of the AlCl4-intercalated graphite.