In Situ Electrochemical Mapping of Lithium–Sulfur Battery Interfaces Using AFM–SECM

Although lithium-sulfur (Li-S) batteries are explored extensively, several features of the lithium polysulfides (LiPS) redox mechanism at the electrode/electrolyte interface still remain unclear. Though various in situ and ex situ characterization techniques have been deployed in recent years, many spatial aspects related to the local electrochemical phenomena of the Li-S electrode are not elucidated. Herein, we introduce the atomic-force-microscopy-based scanning electrochemical microscopy (AFM-SECM) technique to study the Li-S interfacial redox reactions at nanoscale spatial resolution in real time. In situ electrochemical and alternating current (AC) phase mappings of Li2S particle during oxidation directly distinguished the presence of both conducting and insulating regions within itself. During charging, the conducting part undergoes dissolution, whereas the insulating part, predominantly Li2S, chemically/electrochemically reacts with intermediate LiPS. At higher oxidation potentials, as-reacted LiPS turns into insulating products, which accumulate over cycling, resulting in reduction of active material utilization and ultimately leading to capacity fade. The interdependence of the topography and electrochemical oxidative behavior of Li2S on the carbon surface by AFM-SECM reveals the Li2S morphology-activity relationship and provides new insights into the capacity fading mechanism in Li-S batteries.