ABSTRACT
Due to its high theoretical specific capacity, the lithium/sulfur battery is one of the most promising candidates for replacing current lithium-ion batteries. In this work, we investigate both chemical and morphological changes in the electrodes during cycling, by coupling operando spatially resolved X-ray diffraction and absorption tomography to characterize Li/S cells under real working conditions. By combining these tools, the state of the active material in the entire cell was correlated with its electrochemical behavior, leading to a deeper understanding of the performance limiting degradation phenomena in Li/S batteries. Highly heterogeneous behavior of lithium stripping/plating was observed in the anode, while the evolution of sulfur distribution in the cathode depth was followed during cycling.
ABSTRACT
In order to improve the electrochemical performances of lithium-sulfur batteries, it is crucial to understand profoundly their working mechanism and the limitation factors. This communication presents synchrotron-based in situ XRD studies of structural modifications occurring inside the cell upon cycling, since the active material changes constantly its form between solid and liquid phases.
ABSTRACT
The lithium/sulfur battery is a promising electrochemical system that has a high theoretical capacity of 1675 mAh g(-1), but its discharge mechanism is well-known to be a complex multistep process. As the active material dissolves during cycling, this discharge mechanism was investigated through the electrolyte characterization. Using high-performance liquid chromatography, UV-visible absorption, and electron spin resonance spectroscopies, we investigated the electrolyte composition at different discharge potentials in a TEGDME-based electrolyte. In this study, we propose a possible mechanism for sulfur reduction consisting of three steps. Long polysulfide chains are produced during the first reduction step (2.4-2.2 V vs Li(+)/Li), such as S(8)(2-) and S(6)(2-), as evidenced by UV and HPLC data. The S(3)(â¢-) radical can also be found in solution because of a disproportionation reaction. S(4)(2-) is produced during the second reduction step (2.15-2.1 V vs Li(+)/Li), thus pointing out the gradual decrease of the polysulfide chain lengths. Finally, short polysulfide species, such as S(3)(2-), S(2)(2-), and S(2-), are produced at the end of the reduction process, i.e., between 2.1 and 1.9 V vs Li(+)/Li. The precipitation of the poorly soluble and insulating short polysulfide compounds was evidenced, thus leading to the positive electrode passivation and explaining the early end of discharge.
