Lithium metal batteries (LMBs) have attracted great intention due to the high energy density [1]. Among various battery technologies, lithium-sulfur (Li-S) batteries are also very unique but important due to its high energy density, low cost and available sources [2]. Although Li-S batteries exhibit high energy density, the cycling life is poor, especially for large-capacity pouch cells [3]. The cycling performance of Li-S batteries is crucially determined by 16-electron complex sulfur reduction reaction (SRR) from S₈ molecules to Li₂S solid, which involves the multiple potential interwoven branches among lithium polysulfide intermediates (LiPS, e.g., S₈, Li₂S₈, Li₂S₆, Li₂S₄ and Li₂S) [4]. The obvious shuttle for soluble LiPS across the cathode and anode leads to the battery capacity fading. Thus, it is necessary to decrease the accumulation of soluble LiPS in the electrolyte through catalysts fastening the key conversion step from high-order polysulfides to insoluble Li₂S₂/Li₂S. Although some effort has been devoted to catalyze SRR, the complex mechanism remains unclear. To address this issue, Duan et al. tried to solve it based on nitrogen, sulfur, dual-doped holey graphene framework (N, S-HGF) electrocatalyst in Nature [5].

In their work, Duan et al. profiled the SRR encompassing the sophisticated 16-electron conversion process from S₈ molecules to Li₂S solid was involved with multiple soluble LiPS intermediates (Fig. 1a). In the CV curve for SRR with N, S-HGF (Fig. 1b), there are two main peaks (one centered at 2.2–2.5 V and another centered at 1.9–2.1 V) during discharge process. Through calculating the charge number by the integrated area in CV, Li₂S₄ is believed as the primary intermediate for separating the two reduction peaks, S₈ + 4Li⁺ + 4e⁻ → 2Li₂S₄ and 2Li₂S₄ + 12Li⁺ + 12e⁻ → 8Li₂S. They further employed in situ Raman spectroscopy technique (Fig. 1c) for probing the specific reaction intermediates in following part.

In situ Raman spectroscopy was used to probe the specific reaction intermediates along a discharge CV scan (Figs. 2a and b). The S₈ signal (469 cm⁻¹) decreased to disappear at ~2.36 V, followed by Li₂S₈ signal at 508 cm⁻¹ at ~2.44 V and the Li₂S₆ signal at 399 cm⁻¹, which is attributed to the electrochemical transformation of Li₂S₈ to Li₂S₄ peak emerged at 501 cm⁻¹. As the potential decreased, Li₂S₄ and Li₂S₆ became the main polysulfide species, with the decrease of the Li₂S₆ peak at 399 cm⁻¹ at ~2.30 V. In the voltage-dependent concentration profile (Fig. 2c), each LiPS derived from the peak area exhibited a similar sequence of concentration evolution for S₈, Li₂S₈, Li₂S₆ and Li₂S₄ with the decrease of the potential. The above analysis validated the SRR molecular pathway: S₈ → Li₂S₈ → 2Li₂S₄ (Li₂S₈ + Li₂S₄ ⇄ 2Li₂S₆) → 8Li₂S.

In summary, Duan et al. successfully established the complex 16-electron SRR network for Li-S batteries. The work not only gives the example on the electrocatalytic approach for studying complex SRR mechanism for Li-S batteries, but also provides useful insights into SRR electrocatalyst design for Li-S batteries. In addition, this research methodology is also expected to study other catalytic reactions in future. This work demonstrates to be perfect from foundation understanding to practical application.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Haixia Wu: Writing – original draft, Project administration, Formal analysis. Kailu Guo: Funding acquisition, Formal analysis.