Separator design variables and recommended characterization methods for viable lithium–sulfur batteries

The lithium–sulfur (Li–S) battery is at the forefront of technologies that can outperform lithium-ion in at least one index of performance, provided that solutions to poor cycle-life can be devised. One key component of the Li–S battery is the separator, because it holds tremendous promise for improving cycle-life by mitigating the well-known polysulfide shuttle, enabling lean electrolyte configurations, and restricting solid electrolyte interphase growth at the Li-metal anode. However, in response to the advent of the “functional separator” for Li–S batteries, severe misinterpretations of progress have been made due to the often incomplete presentation of the performance and characterization criteria for these new components. Accordingly, there is an urgent need to look critically at what has been achieved in Li–S separator research, with the aim of reconciling actual progress against claimed improvements. This review advocates the best practices for reporting the performance of Li–S separators and proposes guidelines on measurements with respect to key properties. It is believed that the adoption of these measurement practices, testing, and reporting styles will enable more accurate determination of separator performance and properties, which in turn can allow for more meaningful comparisons between various approaches, as well as facilitating the transition of laboratory concepts to practical designs.