High-Rate Aqueous Aluminum-Ion Batteries Enabled by Confined Iodine Conversion Chemistry

Most reported cathode materials for rechargeable aqueous Al metal batteries are based on an intercalative-type chemistry mechanism. Herein, iodine embedded in MOF-derived N-doped microporous carbon polyhedrons (I₂@ZIF-8-C) is proposed to be a conversion-type cathode material for aqueous aluminum-ion batteries based on “water-in-salt” electrolytes. Compared with the conventional Al–I₂ battery using ionic liquid electrolyte, the proposed aqueous Al–I₂ battery delivers much enhanced electrochemical performance in terms of specific capacity and voltage plateaus. Benefitting from the confined liquid–solid conversion of iodine in hierarchical N-doped microporous carbon polyhedrons and enhanced reaction kinetics of aqueous electrolytes, the I₂@ZIF-8-C electrode delivers high reversibility, superior specific capacity (≈219.8 mAh g⁻¹ at 2 A g⁻¹), and high rate performance (≈102.6 mAh g⁻¹ at 8 A g⁻¹). The reversible reaction between I₂ and I⁻, with I₃⁻ and I₅⁻ as intermediates, is confirmed via ex situ Raman spectra and X-ray photoelectron spectroscopy. Furthermore, solid-state hydrogel electrolyte is employed to fabricate a flexible Al–I₂ battery, which shows performance comparable to batteries using liquid electrolyte and can be integrated to power wearable devices as a reliable energy supply.