Lewis Acid-Induced Reversible Disproportionation of TEMPO Enables Aqueous Aluminum Radical Batteries

Nitroxide radicals, such as 2,2,6,6-tetramethylpiperidyl-1-oxy(TEMPO), are typical organic electrode materials featuring high redoxpotentials and fast electrochemical kinetics and have been widelyused as cathode materials in multivalent metal-ion batteries. However,TEMPO and its derivatives have not been used in emerging rechargeablealuminum-ion batteries (AIBs) due to the known disproportionationand possible degradation of nitroxide radicals in acidic conditions.In this study, the (electro)chemical behavior of TEMPO is examinedin organic and aqueous Lewis acid electrolytes. Through in situ (electro)chemicalcharacterizations and theoretical computation, we reveal for the firsttime an irreversible disproportionation of TEMPO in organic Al(OTf)(3) electrolytes that can be steered to a reversible processwhen switching to an aqueous media. In the latter case, a fast hydrolysisand ligand exchange between [Al(OTf)(3)TEMPO](-) anion and water enable the overall reversible electrochemical redoxreaction of TEMPO. These findings lead to the first design of radicalpolymer aqueous AIBs that are fire-retardant and air-stable, deliveringa stable voltage output of 1.25 V and a capacity of 110 mAh g(-1) over 800 cycles with 0.028% loss per cycle. Thiswork demonstrates the promise of using nonconjugated organic electroactivematerials for cost-effective and safe AIBs that currently rely onconjugated organic molecules.