# Unveiling the Formation and Evolution of the Cathode–Electrolyte Interphase in Lithium–Sulfur Batteries

## TL;DR

This study explores how the cathode–electrolyte interphase forms in lithium–sulfur batteries, offering insights to improve battery performance and longevity.

## Contribution

The first real-time investigation of CEI formation in LSBs using in situ FTIR spectroscopy, revealing electrolyte decomposition and reduced species.

## Key findings

- In situ FTIR spectroscopy directly tracked electrolyte species decomposition during CEI formation.
- XAS confirmed sulfur species presence in the CEI after cycling.
- EIS measurements showed structural interface changes at different states of charge.

## Abstract

Lithium–sulfur batteries (LSBs) present high theoretical energy density, but their commercial viability is limited due to poor cyclability, primarily resulting from the shuttle effect. The formation and stability of the cathode–electrolyte interphase (CEI) are crucial for suppressing the shuttle effect and enhancing electrochemical reversibility. However, the CEI formation and evolution mechanism in LSBs remains unexplored. In this context, herein, the first real‐time investigation of the CEI using background‐subtracted in situ Fourier‐transform infrared (FTIR) spectroscopy is presented. The resulting FTIR spectra directly revealed electrolyte species consumption and reduced species’ emergence. Although these findings do not provide detailed compositional information about the CEI, the in situ FTIR results enabled direct tracking of electrolyte species undergoing decomposition associated with the formation and subsequent regeneration of the CEI. The in situ FTIR results are complemented by electrochemical impedance spectroscopy (EIS) measurements at different states of charge (SOCs), which indicate variations associated with structural changes in the interface layers. X‐ray absorption spectroscopy (XAS) at the sulfur K‐edge on post‐cycled electrodes confirmed the presence of sulfur species within the CEI. This work evidences the formation of the CEI, providing new mechanistic insights to support the design of electrodes and electrolytes for long‐term batteries.

By integrating electrochemical impedance spectroscopy (EIS), background‐subtracted in situ Fourier‐transform infrared (FTIR) spectroscopy, and X‐ray absorption spectroscopy (XAS), this work reveals electrolyte decomposition and the reduced species involved in forming the cathode–electrolyte interphase (CEI) in lithium–sulfur batteries (LSBs), offering direct insight into CEI establishment and evolution.