Insights into the Interconnection of the Electrodes and Electrolyte Species in Lithium-Sulfur Batteries Using Spatially Resolved Operando X-ray Absorption Spectroscopy and X-ray Fluorescence Mapping

The lithium–sulfur (Li–S) battery chemistry has attracted great interest in the last decade because of its outstanding theoretical gravimetric energy density compared to the state-of-the-art lithium-ion battery technology. However, practically achieved energy density is still far below the theoretical value, even in small laboratory-scale batteries. The problems seen in laboratory-scale batteries will inevitably increase during scale-up to large application-format cells, as the electrolyte to active material (AM) ratio will need to be reduced in these cells to achieve high gravimetric energy density on cell-level basis. Our study shows the unique possibility of X-ray fluorescence (XRF) mapping to visualize the spatial distribution of the AM inside operating Li–S batteries in all cell components [working electrode (WE), separator, and counter electrode (CE)]. Through a combination of operando XRF mapping and X-ray absorption spectroscopy, we show that unless self-discharge is efficiently prevented, the AM can completely dissolve and distribute throughout the cell stack within a time frame of 2 h, causing poor capacity retention. Using a polysulfide diffusion barrier between the WE and the CE, we successfully suppress these processes and thereby establish a tool for examining the sealed cathode electrode compartment, enabling sophisticated studies for future optimization of the WE processes

Understanding the Charging Mechanism of Lithium-Sulfur Batteries Using Spatially Resolved Operando X-Ray Absorption Spectroscopy

Replacement of conventional cars with battery electric vehicles (BEVs) offers an opportunity to significantly reduce future carbon dioxide emissions. One possible way to facilitate widespread acceptance of BEVs is to replace the lithium-ion batteries used in existing BEVs with a lithium-sulfur battery, which operates using a cheap and abundant raw material with a high specific energy density. These significant theoretical advantages of lithium-sulfur batteries over the lithium-ion technology have generated a lot of interest in the system, but the development of practical prototypes, which could be successfully incorporated into BEVs, remains slow. To accelerate the development of improved lithium-sulfur batteries, our work focuses on the mechanistic understanding of the processes occurring inside the battery. In particular, we study the mechanism of the charging process and obtain spatially resolved information about both solution and solid phase intermediates in two locations of an operating Li2S-Li battery: the cathode and the separator. These measurements were made possible through the combination of a spectro-electrochemical cell developed in our laboratory and synchrotron based operando X-ray absorption spectroscopy measurements. Using the generated data, we identify a charging mechanism in a standard DOL-DME based electrolyte, which is consistent with both the first and subsequent charging processes