Operando FTIR study on water additive in lithium-sulfur batteries to mitigate shuttle effect

Abstract

Additives in the electrolytes of Li-S batteries aim to increase overall capacity, improve Li+ ion conductivity, enhance cyclability, and mitigate the shuttle effect, which is one of the major issues of this system. Here, the use of water as an additive in the commonly used electrolyte, 1.0 M LiTFSI/1.0% (w/w) LiNO3 and a 1:1 mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) was investigated. We used Co2Mn0.5Al0.5O4 (CMA) as an electrocatalyst anchored on an activated carbon (AC) electrode with added sulfur via a melt-diffusion process. The structural analysis of CMA via Rietveld refinement showed interatomic spaces that can promote ionic conductivity, facilitating Li+ ion migration. Electrochemical tests determined 1600 ppm as the optimal water concentration, significantly reducing the shuttle effect. Post-mortem XPS analysis focused on the lithium metal anode revealed the formation of Li2O layers in dry samples and LiOH in wet samples. Better capacity was observed in wet samples, which can be attributed to the superior ionic conductivity of LiOH at the electrode/electrolyte interface, surpassing that of Li2O by 12 times. Finally, Operando FTIR experiments provided real-time insights into electrolyte degradation and SEI formation, elucidating the activity mechanisms of water and Li2CO3 over the cycles. This work presents results that could aid future advancements in Li-S battery technology, offering possibilities to mitigate its challenges with inexpensive and scalable additives.