Strategies to Mitigate the Challenges of Corrosive Electrolytes in Rechargeable Aluminum Batteries

Rechargeable aluminum batteries (RABs) hold great promise as an energy storage system.
Aluminum (Al), abundant in Earth’s crust, offers advantages over lithium (Li) in terms of
environmental impact and safety. Nevertheless, gaining the full potential of RABs requires
addressing challenges related to electrolyte chemistry. The reversible Al plating and stripping
process faces obstacles due to various factors, and the existing electrolytes that facilitate Al
plating and stripping are highly corrosive. This thesis focuses on tackling two critical
challenges hindering the RABs development: (i) struggle to achieve reversible Al plating and
stripping from non-corrosive electrolytes and (ii) mitigating the corrosivity of chloroaluminateionic liquid electrolytes through the application of corrosion inhibitors.
The feasibility of Al plating on a platinum (Pt) electrode was explored using
trifluoromethanesulfonate (Al(OTf)3)/N-methylacetamide (NMA)/urea (ANU-20000)
electrolyte as a non-corrosive alternative composition to chloroaluminate-based ionic liquid
electrolytes. Cyclic voltammetry measurements (CV) indicate redox activity on Pt, but further
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analysis reveals no meaningful Al plating. Instead, hydrogen evolution reaction (HER)
dominates the observed current, confirmed by operando microscopy and gas chromatography
(GC) measurements. The residual water in the electrolyte promotes the HER. While further
drying the electrolyte, up to approximately 300 ppm of H2O (ANU-300), reduced the HER
current, Al plating remained elusive under the tested conditions. The concurrent
adsorption/desorption of Al(OH)2+ and Al3+ ions on the Pt surface during CV measurements
was proposed according to electrochemical quartz crystal microbalance (EQCM) studies.
Unfortunately, no successful Al plating was achieved even in the ANU-300 electrolyte. Yet,
valuable insights were gained into the side reactions occurring under the experimental
conditions. Identifying these reactions, particularly those potentially influenced by residual
water (in the supposedly dried electrolyte), is crucial for the ongoing research into alternative
electrolytes for RABs.
Another crucial aspect was mitigating the corrosivity of the chloroaluminate ionic liquid
electrolytes using corrosion inhibitors. The highly corrosive nature of the chloroaluminate ionic
liquid electrolytes limits compatible current collector materials. Existing options are often
excessively dense or expensive for practical applications. The effectiveness of 1-Ethyl-3-
methylimidazolium tetrafluoroborate (EMImBF4), 1-Ethyl-3-methylimidazoliumXXIII
trifluoromethanesulfonate (EMImOTf), and 1-Butyl-1-methylpyrrolidinium chloride (Py1.4Cl)
inhibitors in mitigating the corrosion of AISI 316L stainless steel as potential current collectors
in the chloroaluminate ionic liquid electrolyte compose of AlCl3 and EMImCl with the molar
ration of 1.5:1 was investigated. Tafel plots and X-ray photoelectron spectroscopy (XPS)
analysis revealed that these inhibitors offered a degree of corrosion protection at the opencircuit voltage (OCV). The adsorption and reduction of inhibitor cations (EMIm+ and Py+) on
the AISI 316L surface can be the reason for the limited corrosion rate. Moreover, XPS analysis
revealed the least amount of iron chloride formation on the AISI 316L surface, indicating the
most effective corrosion mitigation by EMImOTf at OCV and EMImBF4 at higher potentials.
However, the presence of inhibitors affected Al plating and stripping behavior as evaluated by
CV. While all electrolytes exhibited an increase in Al plating and stripping current density after
cycling, the initial cycles were negatively affected by the inhibitor adsorption on the surface of
Al. This highlights a trade-off between corrosion protection and achieving efficient Al
plating/stripping. Notably, the electrolyte containing EMImOTf demonstrated the least impact
on Al plating/stripping compared to other inhibitor-containing electrolytes.
Overall, this study explores the detrimental role of side reactions in Al plating within the AlCl3-
free electrolyte. Identifying these reactions is crucial for the development of efficient
electrolytes. Secondly, an approach to mitigate the corrosivity of chloroaluminate ionic liquid
electrolytes by incorporating inhibitors provides valuable insights into the inhibitor’s behavior
on Al and AISI 316L surfaces. It revealed an unexpected consequence: the inhibitor negatively
impacts Al plating and stripping at the negative electrode. These findings offer a deeper
understanding of the challenges of applying the corrosion inhibitor in the electrolytes for RAB
technologies.