Intraparticular Heterogeneity Limits Capacity in Lithium–Sulfur Batteries With Carbonate Electrolyte

ABSTRACT The formation of a stable cathode‐electrolyte interphase (CEI) is critical for the performance of lithium–sulfur (Li–S) batteries with carbonate‐based electrolytes, as it suppresses parasitic polysulfide reactions and enables solid‐state sulfur conversion. In nanoporous carbon hosts, the CEI together with nanopore confinement plays a key role in capacity retention and long‐term cycling. Yet, its spatial formation, stability, and contribution to electrochemical performance remain poorly understood, partly due to challenges in characterization caused by beam and air sensitivity. Here, we employ cryogenic transmission electron microscopy (cryo‐TEM) with electron energy loss spectroscopy and energy‐dispersive X‐ray spectroscopy, X‐ray photoelectron spectroscopy and electrochemical testing together with galvanostatic intermittent titration technique measurements to elucidate how carbon particle size affects CEI formation and electrochemical performance. We find that the CEI is not a uniform surface film but extends heterogeneously into the particle bulk. Mass transport during the first discharge dictates CEI development, and larger particles suffer from inactive regions due to the preferential CEI formation only in the outer regions of the particles. During extended cycling, charge transfer resistance at confined CEI/active material/carbon interfaces emerges as the dominant performance‐limiting factor. These findings show that particle size controls CEI formation during initial discharge, offering guidance for designing carbon hosts from nano‐ to micrometer length scales in Li–S battery cathodes.