Abstract
The complex reaction mechanism of the lithium–sulfur battery system consists of repetitive dissolution and precipitation of the sulfur-containing species in the positive electrode. In particular, the precipitation of lithium sulfide (Li2S) during discharge has been considered a crucial factor for obtaining a high degree of active material utilization. Here, the influence of electrolyte amount, electrode thickness, applied current, and electrolyte salt on the formation of Li2S is systematically investigated in a series of operando X-ray diffraction experiments. Through a combination of simultaneous diffraction and resistance measurements, the evolution of Li2S is directly correlated to the variation in internal resistance and transport properties inside the positive electrode. The correlation indicates that at different stages the Li2S precipitation both facilitates and impedes the discharge process. This information about the kinetics of Li2S formation offers mechanistic explanations for the strong impact of different electrochemical cell parameters on the cell performance and, thus, directions for holistic optimizations to achieve high sulfur utilization.
Highlights
The analysis is chosen to start at the fifth cycle because it has been observed in previous work that the properties of these Li−S cells stabilize after the first three cycles.[28]
The bottom panel displays the ratio between the change in the normalized intensity and the change in the total charge, which is analogous to the mass per electron value often used in kinetic analyses
A combination of operando X-ray diffraction (XRD) and electrochemical analysis is employed to study how the precipitation of Li2S is affected by the Li−S cell parameters and operating conditions and its effect, in turn, on the electrochemical properties of the cell
Summary
As the positive electrode of a Li−S cell is discharged, elemental sulfur is reduced to lithium polysulfides (Li2Sx, x = 2−8), which are soluble in the commonly used ether-based electrolyte to different degrees.[1] Subsequently, the insoluble reaction product lithium sulfide (Li2S) is formed in the last stages of discharge. Because both elemental sulfur and Li2S are insulating, a conductive matrix, usually made from carbonaceous materials, is necessary to facilitate the electrochemical reactions.[6] the complexity of the multiphase reactions with multiple possible routes is still considered to be the main challenge for elevating the utilization of the active materials in the positive electrode. Besides these issues reducing specific energy, the spontaneous reaction between lithium and dissolved polysulfides compromises the Coulombic efficiency (CE),[8] which is another major challenge facing this system
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