Development and characterisation of an aqueous aluminium-ion battery

Abstract

Aqueous intercalation batteries, using Earth-abundant electrode materials such as TiO₂, present the possibility of high rate, safe, non-toxic and potentially cheap energy storage. Though the low potential stability window of water hinders energy density, the need for this is reduced for certain applications, such as grid storage, hybridised systems or e-textiles. The present work focused on the development and characterisation of an aqueous aluminium-ion (Al-ion) battery using TiO₂ nanopowders as the negative electrode, copper hexacyanoferrate (CuHCF) as the positive and aqueous AlCl₃ solutions as electrolytes. For the first time, TiO₂ and CuHCF were combined into both a full cell and a 2-cell battery, using aluminium containing aqueous electrolytes, and their electrochemical performance characterised. A proof-of-concept aqueous Al-ion cell was capable of 1750 cycles with energy efficiency remaining between 70-80% when cycled at a 20 C rate. Degradation of the CuHCF electrode resulted in cell failure, though this in turn was shown to be a result of the <100% coulombic efficiency of TiO₂. TiO₂ electrodes were shown to have reversible capacities of approximately 15-20 mA h g^-1 and reasonable cycle lives of up to 5000 cycles. It was found that electrode performance could be improved through both an electrochemical treatment and the vacuum impregnation of electrolyte into electrodes. Both techniques were found to improve TiO₂ rate capability, in addition to improving the stability of capacity and efficiency during initial cycling. The electrochemical treatment consisted of holding the electrode at -1.4 V vs SCE in 1 mol dm^-3 KOH and was found to decrease the IR-drop between charge and discharge, especially at higher currents. Mott-Schottky analysis suggested that the treatment resulted in an increase in the electron charge carrier density, which would be consistent with the introduction of Ti³⁺ and a change in the band gap of TiO₂, expected from the cathodic treatment. The treatment, in conjunction with the use of PVDF as binder (as opposed to Nafion), allowed for a discharge capacity of >30 mA h g^-1 at 1 A g^-1 with coulombic efficiency being nearly 95%. A capacity of 27.5 mA h g^-1 was measured from the same electrode at 4 A g^-1 . The vacuum impregnation technique allowed for greater electrode wetting by releasing trapped air and forcing electrolyte into the pores of the electrode. Greater electrode-electrolyte contact was deemed to be the cause of the improved rate capability, where a capacity of 15 mA h g^-1 could be maintained at the high specific current of 40 A g^-1 (260 mA cm^-2 ) from an electrode with a mass loading of 6.5 mg cm^-2 . This is only a 25% drop in capacity to the 20 mA h g^-1 measured at 1 A g^-1 . The specific currents and cycle lives demonstrated are therefore higher than have previously been reported for TiO₂ in aqueous Al³⁺ -containing electrolyte. Furthermore, the vacuum impregnation technique was also shown to improve the rate capability of CuHCF electrodes.
Additionally, charge storage mechanisms of TiO₂ in 1 mol dm^-3 AlCl₃ were explored via cyclic voltammetry studies, which implied a surface contribution to charge storage capacity. The coulombic efficiency of TiO₂ was also found to decrease with decreasing specific current during constant-current cycling. This had been implicit in the data from previous publications on TiO₂, in aqueous Al³⁺ -containing electrolytes, though not explicitly discussed. Therefore, the nature of this was explored through a systematic study of potential self-discharge mechanisms. Purging electrolytes with N₂ gas improved coulombic efficiency and reduced self-discharge at open circuit potential, while the effect of electrolyte pH and charge redistribution within the electrode were also studied.

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ORCID for Richard Wills: orcid.org/0000-0002-4805-7589

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