Lithium-sulfur batteries represent a promising next-generation energy storage technology due to their high theoretical energy density of 2600 Wh/kg, significantly surpassing conventional lithium-ion systems. The electrochemical performance of these batteries hinges on the complex multi-electron redox chemistry of sulfur, which undergoes a series of phase transformations and solution-mediated reactions during cycling.
The sulfur reduction mechanism proceeds through a sequence of stepwise reactions, beginning with the ring-opening reduction of cyclic octasulfur (S₈) and culminating in the formation of lithium sulfide (Li₂S). Upon discharge, the initial reduction of S₈ yields higher-order lithium polysulfides (Li₂Sₙ, where 4 ≤ n ≤ 8), which are soluble in common organic electrolytes. These long-chain polysulfides further reduce to shorter-chain intermediates (Li₂S₂ and Li₂S₃) before final conversion to insoluble Li₂S. The reverse process occurs during charging, where Li₂S is oxidized back to S₈ through the same polysulfide intermediates.
Spectroscopic techniques have been instrumental in elucidating these reaction pathways. In situ UV-Vis spectroscopy reveals distinct absorption peaks corresponding to S₈²⁻ (420 nm), S₄²⁻ (420–500 nm), and S₃⁻ (617 nm), confirming the presence of soluble polysulfide species during cycling. Raman spectroscopy provides further evidence of S–S bond stretching modes in polysulfides, while X-ray absorption near-edge structure (XANES) spectroscopy tracks the evolution of sulfur’s oxidation state. Nuclear magnetic resonance (NMR) studies have identified Li₂S₄ and Li₂S₆ as dominant intermediates in conventional ether-based electrolytes.
The electrolyte composition critically influences the sulfur redox process. Ether-based solvents such as 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) facilitate polysulfide solubility, enabling efficient charge transfer but also contributing to the shuttle effect—a parasitic reaction where soluble polysulfides migrate between electrodes, reducing Coulombic efficiency. Alternative electrolytes, including carbonate-based or highly concentrated systems, can suppress polysulfide dissolution but may lead to passivation due to Li₂S deposition. Recent studies demonstrate that localized high-concentration electrolytes (LHCEs) with fluorinated co-solvents improve sulfur utilization while mitigating shuttle effects.
Recent discoveries have uncovered alternative sulfur reduction pathways that deviate from the conventional stepwise mechanism. In some systems, sulfur can undergo direct solid-state conversion to Li₂S without forming soluble polysulfides, particularly in confined sulfur cathodes or with tailored electrocatalysts. For example, sulfur embedded in microporous carbon hosts exhibits a two-electron reduction pathway, bypassing the typical polysulfide intermediates. Additionally, certain transition metal compounds, such as cobalt-doped graphene, catalyze the conversion of Li₂S₄ directly to Li₂S₂, altering the reaction kinetics.
The formation of Li₂S is a critical bottleneck due to its insulating nature and sluggish nucleation kinetics. Studies using operando X-ray diffraction (XRD) reveal that Li₂S crystallization follows a two-step process: initial amorphous Li₂S forms at higher potentials (2.1 V vs. Li/Li⁺), followed by crystallization at lower potentials (1.7 V). The overpotential required for Li₂S deposition is highly sensitive to electrolyte additives, with LiNO₃ and P₂S₅ shown to reduce nucleation barriers.
Polysulfide shuttle mitigation remains a key challenge. Strategies such as polysulfide-trapping interlayers, redox mediators, and sulfur host materials with strong adsorption sites (e.g., polar metal oxides or nitrides) have demonstrated improved cycling stability. Recent work on single-atom catalysts reveals that atomically dispersed metal sites can selectively bind polysulfides while enhancing their conversion kinetics.
Understanding the multi-electron sulfur redox mechanism is essential for optimizing lithium-sulfur battery performance. Advances in operando characterization techniques continue to refine the reaction pathway models, while novel electrolyte formulations and electrode designs offer pathways to overcome existing limitations. The interplay between sulfur chemistry, electrolyte engineering, and interfacial phenomena will dictate the future development of this high-energy-density battery system.