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.