Binder-free carbon monolith cathode material for operando investigation of high performance lithium-sulfur batteries with X-ray radiography

Lithium sulfur batteries are considered to hold great potential for the next generation of high energy density portable electronic devices or vehicles, however, they are hindered by their poor cycling stability. Recently, more attention have been paid on the mechanism study, trying to reveal and solve the major existing issues, furthermore, to guide and improve the design of new lithium sulfur cells with high performance. Here, a novel route is developed to synthesize light-weight and mechanically stable reduced graphene oxide (rGO) rGO/carbon monoliths for the application in lithium sulfur (Li/S) cells. This cathode material is characterized by a hierarchical pore structure with a BET surface area of 1029 m²/g. The monolithic material exhibits a remarkably low electrochemical overpotential during the cyclic charging and discharging. These electrochemical properties qualify this binder-free monolith to be used as model cathode material for a multidimensional operando analysis. This means a simultaneous performance of cyclic charge/discharge, electrochemical impedance spectroscopy (EIS) and X-ray radiography, which in turn allows new insights into processes on the macroscopic length scale. First, it is observed that surplus electrolyte is soaked towards the circular hole lithium anode during discharging. The ring observed shrinks and expands periodically during serial charge and discharge processes at different C rates (0.1 C and 0.5 C). We assume a concentration gradient and properties change of electrolyte driven polysulfides diffusion as the underlying process for this observation. Second, sulfur dendrites in centimeter length were found, however, can disappear very quickly while discharge despite the insulating nature of sulfur. Third, another macroscopic mechanism is the occurrence of a reaction front in form of a ring of high X-ray transmittance that quickly propagates from the edge of the hole lithium anode to the center of the carbon monolith at the end of each discharge step (the maximum front movement speed is 0.8 µm/s). This process is correlated with the results of the time-resolved EIS and it is assumed that a local increase in Li⁺ ion concentration in the electrolyte is involved to obtain the final discharge product Li₂S.