Electrochemical energy storage by aluminum as a lightweight and cheap anode/charge carrier

Various lightweight metals such as Li, Na, Mg, etc. are the basis of promising rechargeable batteries, but aluminium has some unique advantages: (i) the most abundant metal in the Earth's crust, (ii) trivalent charge carrier storing three times more charge with each ion transfer in comparison with Li, (iii) the volumetric capacity of the Al anode is four times higher than that of Li while their gravimetric capacities are comparable, (iv) employing a metallic Al anode does not have a major safety risk as is the case for alkali metals. However, there are serious obstacles to the practical development of Al batteries such as the complicated nature of trivalent Al3+ intercalation into the cathode of Al-ion batteries and corrosion of the metallic Al anode in aqueous electrolytes. Owing to the highly charged nature of small Al3+ ions, the diffusing species are indeed the Al complexed ions, which might be the intercalating ions in the solid-state too. The present manuscript reviews the current status of various aluminum batteries to narrate their unique potentials while highlighting the issues that should be addressed at this stage. Although Al-air batteries have a long history going back to the 1960s, the focus of this manuscript is on Al-ion batteries including Al-sulfur batteries, but other possibilities for electrochemical energy storage by Al charge carriers such as Al redox batteries, Al supercapacitors, etc. will be reviewed too. In the latter case, it seems the pseudocapacitance is more practical than intercalation for the case of Al3+ ions. Furthermore, the application of Al anodes in lithium-ion batteries is briefly described as the anode performance is similar to their application in Al batteries.