Simple cathode design for Li-S batteries: cell performance and mechanistic insights by in operando X-ray diffraction

Rechargeable batteries have been receiving increasing attention over the past several years, particularly with regard to the accelerated development of electric vehicles, but also for their potential in grid storage applications. Among the broad range of cathode active materials, elemental sulfur has the highest theoretical specific capacity, thereby making it one of the most promising positive electrode materials these days. In the present work, we show that already a simple cathode design (cathodes with a non-optimized composite microstructure) provides good electrochemical performance both in coin and pouch cells with sulfur loadings of 2 mg cm(-2). Our research data demonstrate that (1) specific capacities of 1000 mA h g(-1) can be achieved over 60 cycles at room temperature while the cyclability at elevated temperatures (here, l > 40 degrees C) is poor, (2) the discharge is the kinetically rate-limiting process, (3) the major fraction of active sulfur in the electrode is lost during the formation cycle at C/50 and (4) the Li-S cells suffer from drying-out due to continuous electrolyte decomposition on the lithium metal anode. In addition, in operando X-ray diffraction shows Li2S formation (grain size of <10 nm) on discharge and the appearance of single phase beta-sulfur in the sub-100 nm size range - rather than the thermodynamically stable orthorhombic polymorph (alpha-sulfur) - by the end of the charge cycle.