An Iodine Quantum Dots Based Rechargeable Sodium–Iodine Battery

Rechargeable sodium-iodine batteries represent a promising scalable electrochemical energy storage alternative as sodium and iodine are both low cost and widely abundant elements. Here, the authors demonstrate a rechargeable sodium-iodine battery that employs free-standing iodine quantum dots (IQDs) decorated reduced graphene oxide (IQDs@RGO) as the cathode. Consistent with the density functional theory the authors find the Na+ ions to intercalate into the I unit cell forming stable NaI, and the battery exhibits high capacity, outstanding cycle stability (with a reversible specific capacity of 141 mA h g(-1) after 500 cycles at current density of 100 mA g(-1)), and high rate performance (170, 146, 127, 112, and 95 mA h g(-1) at current densities of 100, 200, 400, 600, and 1000 mA g(-1), respectively). The reversible reactions, I-2/I-3(-) and I-3(-)/I- redox couples on insertion of Na+ ions, are confirmed via in situ Raman spectroscopy. Notably, even after 500 cycles the morphology and structure of the IQDs exhibit no noticeable change implying their use as a stable cathode material for sodium-iodine batteries. Moreover, the IQDs based flexible full-cells also exhibit high capacity and long cycle life (the capacity with 123 mA h g(-1) at current density of 100 mA g(-1) after 100 cycles).