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 (I 2 @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-I 2 battery using ionic liquid electrolyte, the proposed aqueous Al-I 2 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 I 2 @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 I 2 and I - , with I 3 - and I 5 - 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-I 2 battery, which shows performance comparable to batteries using liquid electrolyte and can be integrated to power wearable devices as a reliable energy supply.