Cell modification strategies for high-energy lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have drawn tremendous interest in the next-generation energy-storage field due to the high theoretical capacity (1675 mA h g⁻¹) and low cost of the eco-friendly sulfur. Nevertheless, the practical realization of Li-S technology is still challenging. The major bottleneck lies in the insulating nature of sulfur and its redox products, together with the shuttling of intermediate polysulfides (Li₂S [subscript x], 4 ≤ x ≤ 8) during cycling, leading to low active-material utilization and fast capacity fade. This dissertation focuses on the development of advanced cell configurations with novel modification strategies to improve the electrochemical performance of Li-S cells. First, a multi-layer-coated separator is established to suppress the polysulfide migration. The functional coating films act as net-like filters to intercept the diffusing polysulfides by both physical and chemical interactions, contributing to enhanced cycling stability and capacity retention. Second, a new sulfur cathode configuration with a poached-egg-shaped architecture is proposed to improve the cyclability of Li-S cells. The carbon shell not only achieves an effective physical encapsulation of the "sulfur yolk" to localize active material, but also serves as interlinked electron pathways to favor the active-material reactivation, greatly enhancing the electrochemical utilization and reversibility. Third, in addition to the physical polysulfide-entrapment, the chemical adsorbent is also introduced into the sulfur cathode substrate. By coupling the sulfiphilic metal compounds (e.g., NiS₂ and SnS₂) with a conductive carbon framework to construct a hybrid sulfur host, the polysulfide adsorptivity is significantly improved due to the physical confinement and chemical anchoring, further limiting the active-material loss and polysulfide diffusion. Fourth, another novel cathode design with electrocatalyst incorporation is presented to enhance the rate capability and cycle life of Li-S cells. The electrocatalysts (e.g., Ni and B₄C) function as efficient redox mediators to accelerate the reaction kinetics of polysulfide transformation, leading to highly promoted active-material utilization and rate performance. Finally, an advanced Li-metal host is also designed with a three-dimensional lithiophilic architecture. The lithiophilic seeds (e.g., Mo₂N) substantially lower the Li nucleation overpotential, thus spatially guiding the uniform Li deposition in the conductive matrix and suppressing the Li-dendrite formation as well as Li anode degradation.