<p>Reliable access to electricity from renewable sources such as wind and solar underpins the green economy, and energy storage is an essential component of this system. Among various energy storage technologies, lithium–sulfur (Li–S) batteries have attracted interest due to their multielectron redox reactions, high theoretical energy density (2,500 Wh kg^–1) and cost-effectiveness. However, major challenges in realizing their potential lie in the formation of soluble liquid intermediates and uncontrolled precipitation of solid sulfur species, impairing long-term cycling stability. In this Review, we discuss the development of semi-liquid Li–S batteries with soluble sulfur species as cathode active materials (catholytes), which can resolve the irreversible solid–liquid–solid phase transitions that affect conventional Li–S systems. In both static and redox-flow configurations, the attainable energy density of semi-liquid Li–S batteries is governed primarily by the solubility and utilization of active sulfur species in non-aqueous catholytes. Rational optimization of the catholyte composition, electrode substrates and cell architectures is therefore required to balance sulfur utilization, mass transport and device-level constraints. Continued advances in improving sulfur solubility in catholytes and reducing system complexity and cost will be critical for translating semi-liquid Li–S batteries into scalable technologies for large-scale energy storage.</p>