Hongyuan Yi

Charge transfer at solid-liquid interfaces plays a critical role in various energy-storage systems¹, particularly under dynamically varying reactant concentrations. Deciphering these intricate reaction pathways remains a substantial challenge, notably in lithium-sulfur (Li-S) batteries, in which achieving high energy density requires efficient conversion of highly concentrated lithium polysulfides (LiPSs)^2,3. However, the mechanisms governing lithium sulfide (Li₂S) deposition and dissolution under lean electrolyte conditions remain poorly understood. Here, using in situ liquid-cell electron microscopy, we directly visualize concentration-driven phase segregation at the electrode-electrolyte interface. Within these high-concentration interfacial layers (HCILs), competitive surface and solution dictate the charge-transfer dynamics and ultimately govern Li₂S deposition at different phase boundaries. Density functional theory (DFT) calculations reveal that the aggregation of LiPSs alters molecular geometry, electronic properties and orbital hybridization, collectively facilitating charge transfer through highly concentrated LiPSs clusters. Guided by these insights, we design optimized electrodes that balance interfacial reaction pathways, enabling fast charging (4 C, 26.8 mA cm^-2) and achieving high energy densities exceeding 400 Wh kg^-1. These findings provide mechanistic understanding of interfacial reactions under practical working conditions and offer a design strategy to advance Li-S batteries.

In pursuing cheap and effective oxygen reduction catalysts, the Fe/N/C system emerges as a promising candidate. Nevertheless, the structural transformations of starting materials into Fe- and N-doped carbon catalysts remains poorly characterized under pyrolytic conditions. Here, we explore the evolution of Fe species and track the formation of Fe–N ₄ site development by employing diverse in-situ diagnostic techniques. In-situ heating microscopy reveals the initial formation of FeO _x nanoparticles and subsequent internal migration within the carbon matrix, which stops once FeO _x is fully reduced. The migration and decomposition of nanoparticles then leads to carbon layer reconstruction. Experimental and theoretical analysis reveals size-dependent behavior of FeO _x where nanoparticles below 7 nm readily release Fe atoms to form Fe–N ₄ while nanoparticles with sizes >10 nm tend to coalesce and impede Fe–N ₄ site formation. The work visualizes the pyrolysis process of Fe/N/C materials, providing theoretical guidance for the rational design of catalysts.