From Inorganic to Organic Iodine: Stabilization of I⁺ Enabling High-Energy Lithium-Iodine Battery

Organic materials have been considered a class of promising cathodes for metal-ion batteries because of their sustainability in preparation and source. However, organic batteries with high energy density and application potential require high discharge voltage, multielectron transfer, and long cycling performance. Here, we report an exceptional lithium-iodine (Li//I₂) battery, in which the organic iodine (BPD-HI) cathode formed by the Lewis acid-base coordination between hydroiodic acid (HI) and 4,4'-bipyridine (BPD) allows 2e^- transfer via the I^-/I⁰ and I⁰/I⁺ redox couples. The I⁺ stabilized by BPD exhibits a high discharge voltage plateau at ∼3.4 V. Remarkably, from inorganic to organic iodine, it realizes a 2-fold increase in the achieved capacity, up to ∼400 mA h gI^-1 (Theor. 422 mA h gI^-1 and 245.6 mA h g^-1 based on the mass of BPD-HI), and an over 2-fold energy density, reaching 1160 W h kgI^-1 (Theor. 1324 W h kgI^-1). More importantly, a capacity retention rate of 85% over 850 cycles is attained for the Li//BPD-HI battery at a current density of 2 A gI^-1. This facile strategy enables positively charged I⁺ to be electrochemically active in a rechargeable lithium battery. The new redox chemistry discovered provides new insights for developing organic batteries with high energy density.