Elucidating the Charge Storage Mechanism of MnO2 in Aqueous Aluminum Batteries

Abstract

The rapid development of renewable energy technologies entails the demand for high-performance energy storage devices for effective on-demand utilization. The sustainable future of the commonly used lithium-ion battery is critically debatable, as a result of the imminent exhaustion of the Li and Co resources and their geopolitically restricted abundance. To this end, rechargeable aluminum-ion batteries (AIBs) are gaining considerable attention owing to the favorable characteristics of aluminum (Al) metal, including high natural abundance, low cost, ease of handling in an ambient environment as well as a high theoretical gravimetric and volumetric capacity (2981 mAh g−1 and 8046 mAh cm−3, respectively).Existing chloroaluminate ionic liquid electrolyte employed in AIBs serve as (i) a medium for ion transportation, and concurrently, (ii) as an active material known as liquid anode/anolyte. This fundamentally limits the achievable capacity of chloroaluminate IL-based AIBs at the cell level. Moreover, drawbacks including ambient sensitivity, high cost, toxicity, high corrosivity, and low conductivity limit the non-aqueous AIBs for large-scale application. To address these limitations, research attention has therefore been shifted towards aqueous electrolytes lately, owing to their stability in ambient environment, cost-effectiveness, and high ionic conductivity. Transitional metal oxide-based materials of high theoretical capacity were being explored in the aqueous aluminum electrolytes with appreciable electrochemical performance attained. However, the underlying charge storage mechanism involved remains equivocal.In this work, we comprehensively investigate the electrochemical performance and intercalation mechanism of alpha manganese dioxide (α-MnO2) electrode in aqueous aluminum trifluoromethanesulfonate (Al(OTF)3) electrolyte through various electrochemical techniques and extensive spectroscopic characterizations, and an alternative charge storage mechanism is proposed. With a combination of the electrochemical and the spectroscopic analysis, we observe that H3O+ is the dominant intercalation species and H3O+ intercalation/de-intercalation contributes to the reversible capacity of α-MnO2 over cycles in aqueous aluminum electrolytes, while a small amount of Al3+ could also intercalate into α-MnO2. The formation of the surface complex during discharge is observed, which may contain Al3+, OH-, and OTF-. After charge, this surface complex would dissolve. Lastly, we offer recommendations on prospective electrode and electrolyte design using the insights obtained from this study. We expect that our findings will shed light on achieving high-performance aluminum-based energy storage devices.