Role of Polymer‐Iodine Complexes on Solid‐Liquid Polysulfide Phase Transitions and Rate Capability of Lithium Sulfur Batteries

Abstract Lithium–sulfur (Li–S) batteries are considered as a viable technology offering energy‐dense electrochemical energy storage systems. However, the inherently slow reaction kinetics manifested in the slow charge and discharge characteristics constrain their real‐world applications. Here, it is reported that polyiodide species held within a complex polar network of polyvinylpyrrolidone (PVP) accelerate the rate‐limiting solid‐liquid phase transitions both in the reduction and oxidation steps during battery cycling. Density functional theory calculations support a mechanism in which a combination of enhanced binding of polysulfides and additional energy states in the PVP‐iodine‐polysulfide complexes accelerates the reaction pathways mediated by inter‐valance polyiodide reactions within the working voltage of Li–S batteries. These studies show that PVP‐iodine (PVP‐I) complexes enhance the rate capability of cells with practical sulfur loadings delivering a high areal capacity of ≈7 mAh cm −2 at the practical 0.5C rate. This advantage is demonstrated in one of the highest‐rate pouches reported in Li–S literature, attaining energy densities of 215 and 156 Wh kg at 0.1C and 0.3C, respectively. The results demonstrate a subtle but powerful shift in the design of molecular binder systems, which have functional roles above and beyond the role of simply holding the active materials together.