Suppressing passivation layer of Al anode in aqueous electrolytes by complexation of H₂PO₄⁻ to Al³⁺ and an electrochromic Al ion battery

Aqueous aluminum ion batteries (AIBs) are attractive alternatives for post-lithium energy storage systems. However, the short lifespan seriously limits the development of AIBs, arising from the formation of a passivation layer on the Al electrode surface as well as the strong electrostatic interaction between bulky Al³⁺ ions and host materials. Herein, we developed a hybrid electrolyte based on aluminum trifluoromethanesulphonate (Al(TOF)₃) and H₃PO₄ which can effectively alleviate the passivation and exhibit superior stability. A full aqueous AIB based on this electrolyte and polyaniline (PANI) cathode exhibits a record-breaking lifespan of 3850 cycles and superior rate capability. Mechanism study suggests that Al³⁺ ions are bonded to both the H₂PO₄⁻ and TOF⁻ to form complex ions Al(H₂PO₄⁻)_x(TOF⁻)_y⁺(H₂O)_n, which can mitigate the strong charge densities of trivalent Al³⁺ and contribute to fast reaction kinetics. Besides, the ex-situ study reveals that PANI cathode experiences a co-intercalation/deintercalation process of Al(H₂PO₄⁻)_x(TOF⁻)_y⁺(H₂O)_n, TOF⁻, and H⁺ during the charging/discharging process with high reversibility and stability. As a proof of concept, an electrochromic Al//PANI battery is fabricated, which combines both electrochromism and energy storage and delivers a higher coloration efficiency of 84 cm² C⁻¹ at a wavelength of 630 nm. This work presents an effective and facile strategy to developing long-life aqueous Al//PANI batteries and may promote the application of AIBs with electrochromic function.