Li(metal)–sulfur (Li–S) systems are among the rechargeable batteries of the highest possible energy density due to the high capacity of both electrodes. The surface chemistry developed on Li electrodes in electrolyte solutions for Li–S batteries was rigorously studied using Fourier transform infrared and X-ray photoelectron spectroscopies. A special methodology was developed for handling the highly reactive Li samples. It was possible to analyze the contribution of solvents such as 1-3 dioxolane, the electrolyte LiN(SnormalO2CnormalF3)2 , polysulfide (normalLi2normalSn) , and LiNnormalO3 additives to protective surface films that are formed on the Li electrodes. The role of LiNnormalO3 as a critical component whose presence in solutions prevents a shuttle mechanism that limits the capacity of the sulfur electrodes is discussed and explained herein.

In this work, structural and morphological changes in composite sulfur electrodes were studied due to their cycling in rechargeable Li–S cells produced by Sion Power Inc. Composite sulfur cathodes, comprising initially elemental sulfur and carbon, undergo pronounced structural and morphological changes during discharge–charge cycles due to the complicated redox behavior of sulfur in nonaqueous electrolyte solutions that contain Li ions. Nevertheless, Li–S cells can demonstrate prolonged cycling. To advance this technology, it is highly important to understand the evolution of the structure and morphology of sulfur cathodes as cycling proceeds. High resolution scanning and tunneling microscopy, scanning probe microscopy, and Raman spectroscopy were used in conjunction with the electrochemical measurements. A special methodology for slicing composite sulfur electrodes and their cross sectioning and depth profiling was developed. The gradual changes in the structure of sulfur cathodes due to cycling is described and discussed herein. Important phenomena include changes in the surface electrical conductivity of sulfur electrodes and pronounced morphological changes due to the irreversibility of the sulfur redox reactions. Based on the observations presented in this work, it may be possible to outline guidelines for improving Li–S battery technology and extending its cycle life.

The primary mechanisms limiting lithium sulfur (Li-S) cell cycle life and thermal stability are discussed. Two major cycle life limiting mechanisms are identified: development of rough surface morphology on the metallic lithium anode with cycling; and depletion of lithium and electrolyte components through chemical reaction. The approach taken here to mitigate these problems, by employing physical protection, including multi-functional membrane assemblies and non-isotropic pressure is presented. Sulfur utilization of 92%, at C/5 discharge rates, increased cycle life and elimination of thermal runaway in 300 mAh Li-S cells was achieved.

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