Insight into the Interfacial Process and Mechanism in Lithium-Sulfur Batteries: An In Situ AFM Study

Lithium–sulfur (Li–S) batteries are highly appealing for large‐scale energy storage. However, performance deterioration issues remain, which are highly related to interfacial properties. Herein, we present a direct visualization of the interfacial structure and dynamics of the Li–S discharge/charge processes at the nanoscale. In situ atomic force microscopy and ex situ spectroscopic methods directly distinguish the morphology and growth processes of insoluble products Li2S2 and Li2S. The monitored interfacial dynamics show that Li2S2 nanoparticle nuclei begin to grow at 2 V followed by a fast deposition of lamellar Li2S at 1.83 V on discharge. Upon charging, only Li2S depletes from the interface, leaving some Li2S2 undissolved, which accumulates during cycling. The galvanostatic precipitation of Li2S2 and/or Li2S is correlated to current rates and affects the specific capacity. These findings reveal a straightforward structure–reactivity correlation and performance fading mechanism in Li–S batteries. Why do batteries fail? To study the performance fading of Li–S batteries, the nanoscale processes at a highly oriented pyrolytic graphite (HOPG) cathode/polysulfide‐electrolyte interface were directly observed by using in situ AFM and spectroscopic methods. The morphology and growth processes of insoluble products Li2S2 and Li2S were monitored and provided evidence for a mechanism for capacity fading.