Metallurgical investigation of aluminum anode behavior in water-in-salt electrolyte for aqueous aluminum batteries

Although ionic liquid electrolytes (ILs) are environmentally unfriendly, they are the most common electrolyte used in aluminum-ion batteries (AIB). Aqueous electrolytes offer a more sustainable alternative, but problem with oxide passivating barrier on Al surface becomes more profound. Recently, a new sub-class of aqueous electrolytes, water-in-salt (WIS) of (AlCl3·6H2O), has been considered, but experimental validation of the behavior of the Al electrode over cycling is required. This work investigates aluminum/graphitic cells using WIS electrolytes with a mass ratio of salt to water of 4, 8, and 12 and finds that they show similar trends in cycling performance. The degradation observed over cycling has been attributed to the formation of a detrimental solid electrolyte interphase (SEI) layer on the Al surface. It was found that WIS 4 increased Al corrosion, resulting in a slightly higher capacity and longer cycling life. Metallurgical observation showed that the Al matrix has a tendency to initiate corrosion around Al3Fe intermetallic phases in both WIS and ILs. This implies that the presence of Al3Fe particles allows the electrolyte to break the oxide barrier and access the bulk Al. These results suggests that metallurgical treatments are important to enhance the electrochemical performance of AIB.