Abstract:
Lithium-sulfur batteries are among the most attractive candidates for next generation battery systems, offering both high theoretical capacities and thus high energy densities (1675 mAh/g and 3518 Wh/kg, respectively), despite a relatively low operating voltage (~2.1 V).
However, they suffer from numerous severe challenges such as the so-called internal polysulfide shuttle: The sulfur active material dissolves in the course of the electrochemical reactions and the polysulfide intermediates can diffuse to and directly react with the metallic lithium anode. This severely lowers the Coulombic charge efficiency of the cells and results in a loss of active material.
Moreover, the electrolyte is slowly consumed in reduction reactions continuously taking place on the anode surface. These side reactions lead to the formation and growth of surface layers (SEI). However, they are not stable upon cycling, as lithium is deposited in mossy and dendritic structures. The growth of lithium dendrites between the electrodes can lead to short-circuits and poses severe safety issues.
It will be shown that an addition of excess amounts of electrolyte results in a considerable improvement of the cycle life (on the expense of the energy density of the cells under consideration), showing that the drying out of the cells due to electrolyte decomposition is one of the major reasons for cell failure upon long-term cycling. In addition, the degradation of the electrolyte solvents leads to gas formation, which can be monitored by means of differential electrochemical mass spectrometry (DEMS) and ex situ gas analysis.
However, also the sulfur active material content exerts a strong influence on the overall cell performance and its cycle life, which is discussed in this presentation.
Given that the major failure mechanisms involve reactions with the metallic lithium, it can be expected that only protected anodes have the potential to extend the cycle life of lithium-sulfur batteries far enough to become commercially of interest.
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