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.