Toward Ab initio Simulation of Operando Raman Spectroscopy: Application to Sulfur/Carbon Copolymer Cathodes in Li-S Batteries

Abstract

Sulfur/carbon copolymers have emerged as a promising alternative for conventional crystalline sulfur cathodes for lithium-sulfur batteries. Among these, sulfur–n–1,3– diisopropenylbenzene (S/DIB) copolymers, which present a network of DIB molecules interconnected via sulfur chains, have particularly shown a good performance and, there- fore, have been under intensive experimental and theoretical investigations. However, their structural complexity and flexibility have hindered a clear understanding of their structural evolution during redox reactions at an atomistic level. Here, by performing state-of-the-art finite-temperature ab initio Raman spectroscopy simulations, we inves- tigate the spectral fingerprints of S/DIB copolymers during consecutive reactions with lithium. We discuss in detail Raman spectral changes in particular frequency ranges which are common in S/DIB copolymers having short sulfur chains and those consisting of longer ones. We also highlight those distinctive spectroscopic fingerprints specific to local S/DIB structures containing only short or long sulfur chains. This distinction could serve to help distinguish between them experimentally. Our theoretically predicted results are in a good agreement with experimental Raman measurements on coin cells at different discharge stages. This work represents, for the first time, an attempt to compute operando Raman spectra using quantum-chemical calculations and provides a guideline for Raman spectral changes of arbitrary electrodes during the discharge.