What if the stuff of soft drink cans could also power our smartphones? Materials researchers today are exploring that very prospect. In many ways, aluminium (Al) shines as a candidate for rechargeable battery parts: it’s abundant, lightweight, highly recyclable, and can potentially store more energy by weight compared to today’s lithium-ion (Li-ion) systems.
However, that potential still faces some technical challenges. “Aluminium battery (Al-battery) research is largely still in its infancy due to the dearth of suitable Al-based electrolytes, particularly non-aqueous ones,” said Zhi Wei Seh, a Senior Principal Scientist II at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
One reason behind that dearth is that most battery research has focused on the electrochemistry of ‘simple’ metal ions, while Al-batteries involve more complex molecules and reactions. Simply swapping out lithium for aluminium in a conventional battery system can lead to poor performance or even safety risks, with some unexpected reactions causing electrodes to collapse for undefined reasons.
To shed more light on Al-battery mechanics, Seh and A*STAR IMRE colleagues launched a joint investigation with A*STAR Institute of High Performance Computing (A*STAR IHPC) Principal Scientist Man-Fai Ng and colleagues, as well as Babak Anasori of Purdue University in the US; Hui Ying Yang of the Singapore University of Technology and Design; and their respective teams. Together, they focused on two deep eutectic solvents (DES) currently in common use as Al-battery electrolytes: namely, urea/AlCl3 and EMImCl/AlCl3.
“DES are a new class of non-aqueous electrolytes that have been indicated to potentially enable room-temperature rechargeable Al-batteries (RABs), making them a worthwhile research target,” said Seh.
Key tools in the team’s study were MXenes: materials structurally similar to carbon-based graphene while containing other transition metals and chemical groups. Seh explained that by using two different MXenes as electrodes, the team could uncover and amplify any unexpected electrochemical side effects that may not have been apparent with graphite or other conventional battery materials.
The team uncovered a range of previously-unreported DES features, including that chloroaluminate (AlCl4−) ions in DES could potentially react with the halogen surface terminations on MXenes, forming unwanted byproducts. They also found that while EMImCl/AlCl3 had a high Coloumbic efficiency—a measure of a battery’s rechargeability—compared to urea/AlCl3, the former was much more irreversible and unstable as an electrolyte.
“From a macro perspective, our work also highlighted the large disparity between Al-battery and existing battery chemistries, particularly when trying to achieve a good room-temperature RAB,” said Seh. “Audacious attempts towards system understanding and manipulation will be needed in this field.”
Seh added that the study marks a first step towards understanding the limitations of irreversibility, side reactions and electrochemical instability in existing DES Al-battery electrolytes. Moving forward, Seh’s team plans to use MXenes for similar studies of unexpected processes or intrinsic characteristics in other battery systems.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and A*STAR Institute of High Performance Computing (A*STAR IHPC).
