Developments in electrode materials and electrolytes for aluminium–air batteries

Aluminium-air cells are high-energy density (< 400 W h kg-1), primary batteries first developed in the 1960s. The review shows how the performance of the battery is influenced by the choice of materials, including the type of aluminium alloy, oxygen reduction catalyst and electrolyte type. Two continuing issues with these batteries are (a) the parasitic corrosion of the aluminium, at open-circuit and under discharge, due to the reduction of water on the anode surface and (b) the passive hydroxide layer that forms on the aluminium surface in alkaline solutions, which inhibits dissolution and shifts its potential to more positive values. One method to overcome these two issues is the use of super-pure (99.999 wt%) aluminium alloyed with trace amounts of ‘activating’ elements such as Mg, Sn, In and Ga, to either inhibit corrosion or break down the passive hydroxide layer. Since the manufacture of high-purity aluminium alloys is expensive an alternative approach is to add solution phase inhibitors or additives directly to the electrolyte. The effectiveness of alloying elements, in binary and ternary alloys, and the effectiveness of different electrolyte additives are evaluated. Novel methods to overcome the self-corrosion problem include using anionic membranes and gel electrolytes or identifying alternative solvents, such as alcohols or ionic liquids, to replace aqueous solutions. The air cathode side of the battery is also considered. Future opportunities and directions for the development of aluminium-air cells are highlighted.