Strategies for improving cycling life of Li-S batteries via polysulfide interaction and elimination
Authors:   , Pai Rahul Nagesh Neeta , Kalra Vibha 
Published: April 2022

Rechargeable lithium-ion batteries have transformed the world of portable electronics and consumer devices today, but their specific energy and cycle life remain insufficient for many emerging, modern-day applications such as electric vehicles and grid energy storage. Lithium-sulfur batteries represent a very promising technology for these applications because their theoretical specific energy is about 6 times of lithium-ion batteries. However, the challenges of Sulfur and Li2S cathodes include: (1) the low electronic conductivity of S and Li2S and (2) the formation of intermediate lithium polysulfide species which dissolve into the electrolyte during cycling. In this thesis I will present my work on developing strategies for improving cycle life of lithium-sulfur batteries. First, we will focus on development of conductive metal oxides as host materials with improved active sites for polysulfide binding via Lewis acid base interaction. We chose vanadium monoxide (VO) based carbon nanofibers due to the high conductivity of VO and its active sites for polysulfide binding. The host is developed utilizing carbon nanofiber-based architecture to improve the interaction sites. This study involved electrochemical performance evaluation at low and high sulfur loading along with fundamental analysis to understand the interaction between host (VO) and polysulfides. Second, we present a facile strategy to utilize a new class of 2D materials, MXenes, made by etching A layer in MAX (ceramic, M=metal, A= Aluminum, X= carbon/nitrogen) phases. We for the first time modify MXene surface hydrophobic and use in Li-S batteries via room temperature surface treatment. This method has various advantages compared to conventional methods which usually employ high temperature treatments wherein MXene surface may oxidize. In addition, hydrophobic MXenes can be easily delaminated in sulfur dissolved CS2 solution and my easy evaporation of CS2 we can have conformal coating of sulfur on MXene nanosheets. The advantages of MXene lie in its metal like conductivity to quickly utilize polysulfides and reduce them to Li2S, this rendered us towards improvement on discharge capacity and cycling performance. Upon further analysis we observe interaction of MXenes with LiPs to form a lewis acid-base binding along with formation of complexes such as thiosulfate and polythionate. While we were developing strategies for binding polysulfides we knew that whenever polysulfides are formed the problem with capacity fade is going to persist. Hence the ideal way to improve cycling is by eliminating polysulfides. So, we tried to confine sulfur molecules in between MXene sheets, the conductive sheets will help for faster reaction kinetics during the redox processes. Furthermore, confining sulfur can further help in utilization of carbonate electrolytes, traditionally used in ion battery industry. The cathode was tested in all alkali metal anode batteries (Li/Na/K). The electrode demonstrated stable performance for 500 cycles with ~700 mAh.g-1 capacity. However, the sulfur loading was low ~15% due to limitations of confinement. Simultaneously, while we were working on improvement in sulfur loading on CNFs, we came across development of completely novel sulfur allotrope. This novel sulfur allotrope which is gamma-monoclinic sulfur is not stable at room temperature (stable between 95- 116.5 °C). The sulfur allotrope when tested in carbonate electrolyte ran with single oxidation/reduction peak suggesting digression from polysulfide formation route. The interesting aspect here is the sulfur is in non-confined state and we were the first ones to report it. The non-confinement approach opens another arena to load as high as sulfur as required, currently we were able to reach ~45-50%. The postmortem characterization results confirmed the complete conversion of sulfur to metal-sulfide and vice versa. We have tried to understand the underlaying conversion mechanism via electrochemical impedance spectroscopy. Nonetheless, the battery performed for well over 4000 cycles in carbonate electrolyte. This cathode not only exhibited high-rate capability of 40C but also functioning at high sulfur loading of 5 mg.cm-2, both of which represent unprecedented levels of performance. These works pave the way for the future development of high-performance and long-lasting rechargeable sulfur batteries.