Understanding ultrafast rechargeable Al/graphite battery by visualizing phase separation

Al/graphite batteries (ABs) using ionic liquid electrolytes exhibit exceptionally fast charging and cycling stability. However, the mechanisms underlying their high rate capabilities remains elusive. In this study, in situ optical microscopy is employed to investigate the intercalation dynamics of single-flake graphite in ABs. Observations reveal that surface reaction limitations, rather than AlCl₄⁻ mass transfer, primarily govern performance in the graphite cathode. During charging under varying current densities, the ABs display distinct phase separation behaviour with an intercalation wave morphology, indicating that surface reactions restrict the intercalation process. This finding explains the ultrafast recharge capability of ABs, where active sites in graphite become nearly fully intercalated with AlCl₄⁻ at high current densities. Additionally, slight rate performance loss occurs due to increasing ohmic and charge transfer polarisation (η_ohm and η_ct) at higher current densities. To address this limitation, we propose increasing the cut-off voltage as a straightforward and effective method to mitigate these polarization effects. This study offers valuable insights into the electrochemical behaviour of rechargeable secondary ion batteries by visualising their phase separation.