Lithium/Sulfur Batteries upon Cycling: Application of Electrochemical Impedance Spectroscopy and in Situ X-Ray Diffraction

Lithium/Sulfur batteries, due to their high theoretical values of gravimetric (2500 Wh kg^-1) and volumetric (2800 Wh L^-1) energy densities, became one of the most popular candidates for next-generation energy storage system. However, the practical discharge capacity and cycle life of Li/S cells are still below expectation despite of many efforts which has been done during last two decades. Therefore, it is necessary to better understand the working mechanism of this system, in order to help to improve the electrochemical performances. Several different techniques can be used to analyze the components of Li/S batteries. Nevertheless, very often characterization methods are applied via an ex situ methodology, where post treatments of the samples are required. Ideal case is, when there is a possibility to “observe” the battery under operating conditions, via an in situ methods.  

X-ray diffraction (XRD) has proven to be a powerful technique to follow the lithium insertion mechanism in Li-ion batteries. It is also well known that structural and morphological changes occur inside the Li/S cell upon cycling, since the red-ox reaction is accompanied by phase transformation of active material (solid/liquid phases). Here we present synchrotron based, operando and in situ XRD studies performed on few independent cells. The Li/S batteries were monitored during several complete cycles and at two different rates (C/20 and C/8). Special pouch cell design allowed us to monitor the evolution of complete cell as well as each electrode separately. Our results show that all elemental sulfur present at the beginning in the electrode is getting reduced into soluble lithium polysulfides during initial discharge and the moment of sulfur disappearance corresponds nicely to the end of first discharge plateau. Well defined peaks of crystalline lithium sulfide (Li₂S) start to appear just at the beginning of lower discharge plateau and reach the maximum intensity at the very end of discharge (1.5V). During following charge crystalline Li₂S got oxidized back to the soluble polysulfides, which are further oxidized back to the elemental sulfur (end of charge, 3V). Moreover, the crystal structure of sulfur formed after recrystallization was found to be different from the orthorombic α-sulfur used in electrode preparation. Here, for the first time, we report appearance of another sulfur allotrope in Li/S system: monoclinic β-S. 

Another powerful technique which can be used to investigate the physical and electrochemical processes appearing inside the battery upon cycling is Electrochemical Impedance Spectroscopy. By applying EIS in Li/S system, important information concerning: electrolyte viscosity evolution, electrode morphology and passivation layer formation (Li₂S and/or S₈) can be obtained. Here we present the impedance of complete Li/S cell as a function of both: state of charge and cycle number. In order to separate the contribution of each electrode or phase (liquid polysulfides) to the impedance spectrum of complete two-electrode Li/S battery, symmetrical cells at different state of charge were also prepared.