(404c) Rechargeable Aluminum-Graphite Batteries: Elucidation of Ionic Transport and Intercalation Reaction Limitations Via Electrochemical Analyses

Recently, novel rechargeable aluminum-graphite batteries using AlCl₃:[EMIm][Cl] ionic liquid electrolytes have been demonstrated, which store charge when chloroaluminate anions (e.g., AlCl₄^-) electrochemically intercalate into graphitic electrodes. However, technological development of this emerging battery chemistry is limited by incomplete scientific understanding of its charge storage mechanism, particularly since the intercalation of molecular chloroaluminate anions differs greatly compared to the intercalation of atomic cations (e.g., Li⁺, Na⁺, Mg²⁺, etc.). While graphite electrodes have worked very well as an intercalation host for monatomic Li­⁺ cations in Li-ion batteries, they present a greater challenge for Al-graphite batteries, which involve intercalation of sterically-bulky, AlCl₄^- molecular anions with ionic diameters (~5.28 Å) that are larger than the interlayer spacing between adjacent graphene layers (~3.35 Å).

Here, we study the transport behavior of chloroaluminate ions into graphite by comparing the electrochemical behavior at various cycling conditions of aluminum batteries containing natural graphite (NG), synthetic graphite (SG) and pyrolytic graphite (PG) electrodes. Variable-rate cyclic voltammetry (CV) revealed that the electrochemical reaction is controlled by a mixture of diffusion and other processes (e.g., kinetic or non-Nernstian). Analysis of how the CV peak current scales with varying CV scan rate (i.e., i_p ~ v^b) suggests that the intercalation processes exhibit different extents of diffusion-limitation at different cell voltages. These potential-dependent transport regimes vary with graphite type and are correlated with surface area and population of edge sites. Our measurements revealed a remarkable departure from lithium cation intercalation into graphite, which have been shown to exhibit a classical diffusion-limited scaling over similar scan rates. This analysis enables a quantitative comparison of how mass transport affects the rate of electrochemical intercalation of chloroaluminate species among the three graphite types, which correlates with their performance in rechargeable aluminum-graphite batteries.