Microstructure evolution and capacity: Comparison between 2090-T8 aluminium alloy and pure aluminium anodes

Abstract

The paper offers a contribution towards solving the problem of the fast capacity decay of pure aluminium anodes for lithium batteries. To this aim, an analysis of the compounds that form during the electrochemical reactions which occur during the working of the anode has been carried out. The phase transformations during lithium insertion and lithium extraction have been studied and the electrochemical behaviour of pure aluminium and 2090-T8 aluminium-copper-lithium alloy powders have been compared. An electrode has been developed with the active material of the powder, without employing a binder, carbon or additives in the electrolyte, in order to follow the lithiation phenomenon by means of X-ray diffraction as closely as possible. It has been found that, at low current density, the rapid capacity decay of pure aluminium is prevalently due to the incomplete reversibility of the lithiated phase, Li9Al4, during the delithiathion of the anode. Lithium extraction has been found to always be incomplete during anode discharge. Partially reversible Li9Al4 is also present in the 2090-T8 aluminium alloy, together with fully reversible crystals of the Al5CuLi3 phase. In this case, the anode capacity fade after the second galvanostatic cycle is modest. As far as the pure aluminium anode is concerned, thick compound layers, made up of oxides and phosphates on the electrode surface, hinder Li diffusion in the host metal to a great extent, at a high current density, thus abating the anode capacity. However, no surface problems have been encountered for the 2090-T8 alloy, and a high capacity was maintained, after the second galvanostatic cycle, due to the presence of the partially reversible Al4Li9 and Al2Li3 crystals, together with the fully reversible Al6CuLi3 quasicrystal. Therefore, one way of overcoming the low capacity of a pure aluminium anode is to adopt an Al6CuLi3 quasicrystal, or its crystal approximant Al5CuLi3, which may be present in the 2090-T8 alloy because of the previous presence of copper and lithium.