Mitigating the Effect of High Overpotential during Al Deposition on Aluminium-Graphite Battery Performance

This study investigates the impact of current density on electrode potential during aluminium (Al) dissolution/deposition step from/on an Al foil as well as the charge-discharge behaviour of aluminium-graphite batteries (AGB) in various AlCl3-based electrolytes. Preliminary experiments in a cell with graphite blocking electrodes evidenced higher chemical stability of 1 : 1.5 Urea : AlCl3 electrolyte, followed by 1 : 1.5 TEA : AlCl3 and 1 : 1.5 EMIMCl : AlCl3. In Al−Al symmetric cells, current densities above 1 mA cm−2 led to a notable rise in overpotential up to 100 mV during Al deposition in both TEA : AlCl3 and Urea : AlCl3 electrolytes mostly due to low surface area of native Al foil. Similar trend was observed in AGB full cells, where higher overpotentials during Al deposition caused ‘incomplete’ AlCl₄− intercalation in natural graphite (NG), resulting in capacity fade at current densities in the range between 0.5 and 5 A g−1. By adjusting the upper cut-off voltage (UCV) during charging step as a function of applied current value according to respective electrolyte stability, a significant improvement in specific capacity and energy density was achieved during charging and discharging steps. For instance at 1 A g−1, the specific energy density of AGB increased by 10 % in EMIMCl : AlCl3, 48 % in TEA : AlCl3, and an impressive 250 % in Urea : AlCl3.During long-term cycling post-UCV adjustment, the capacities of AGB increased by 10 %, 13 %, and 27 % for AGBs with EMIMCl : AlCl3, TEA : AlCl3 and Urea : AlCl3, respectively with a negligible capacity fade of less than 1 % for EMIMCl : AlCl3 and TEA : AlCl3, and a 9 % capacity fade for Urea : AlCl3.