Addressing the voltage and energy fading of Al-air batteries to enable seasonal/annual energy storage

Abstract

Al-air batteries are promising candidates for seasonal and annual energy storage. However, severe voltage decay upon discharge limits their practical specific energy. Herein, we first explore the effect of different Al(OH)4− concentrations in alkaline electrolytes on the electrochemical oxidation of Al metal anodes (AMAs). Simulation analysis on the electrochemical impedance spectra of AMAs reveals that the formation of Al(OH)4− reduces the OH− concentration and negatively affects the reaction kinetics of AMAs, which is responsible for increased potentials of AMAs and the consequent voltage decay of Al-air batteries. Subsequently, a seeded precipitation process taking advantage of the lower solubility of Al(OH)4− at 20 °C than at 50 °C is proposed to recover the voltage decay of Al-air batteries. Inductively coupled plasma atomic emission spectroscopy demonstrates that more than 70 wt % of Al(OH)4− in the electrolyte can be removed via this process. Raman spectra and ionic conductivity tests of the electrolyte, together with X-ray diffraction of the precipitate, reveal that the removed Al(OH)4− is converted into insoluble Al(OH)3 with release of OH−. Making use of the precipitation process, Al-air prototypes of Ah-level delivering 3.95 kWh kgAl−1 at 50 mA cm−2 and 3.52 kWh kgAl−1 at 100 mA cm−2 are demonstrated.