Creating a cost effective, high energy density, long cycle life transportation battery based on the lithium – sulfur electrochemical couple requires overcoming a series of scientific and technical challenges (1). These challenges include reducing the electrolyte volume fraction within the cell, increasing the sulfur loading in the cathode, reducing sulfur loss from the cathode and the subsequent redox shuttle of lithium polysulfides, and reducing excess lithium in the anode, all while maintaining the necessary kinetics to support facile charge transport. The Joint Center for Energy Storage Research (JCESR) is redesigning the lithium-sulfur battery using a combination of novel materials concepts to address these challenges. An example is the use of electrolytes that are sparingly solvating of the polysulfide reaction intermediates and that constrain sulfur at the cathode, reducing the quantity of electrolyte necessary within the cell (2).

Focus will be placed on the design of a select class of sparingly solvating electrolytes, their structure-function relationship, and their impact on cell performance. Solvent-in-salt electrolytes, loosely referred to as solvates, represent a class of electrolytes in which the saturated lithium polysufide concentration can be tailored to greatly reduce redox shuttling between cathode and anode. Polysulfide solubility restriction has been demonstrated to redirect sulfur reduction from a solution mediated to a semi-solid state path. A number of solvates are shown to exhibit this behavior. Experimental methods are developed to describe the extent of the solvate regime (solvent-to-salt ratio) in these electrolytes using NMR, Raman spectroscopy, and conductivity measurements. Solvates are found to function below target compositions expected for the standard coordination of the lithium cation, raising questions as to details of the local structure and impact on kinetics. Experimental data is complemented by molecular dynamics simulations to gain insight into electrolyte structure at low solvent levels. The benefit of a semi-solid reaction pathway is the possibility of using the electrolyte in concert with the cathode structure to regulate how lithium sulfide is redistributed. The challenge of integrating both electrolyte and cathode design to take advantage of the unique properties of these electrolytes will be discussed. 

The author acknowledges the contribution of the Joint Center for Energy Storage Research Li-S team in this presentation. This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science. Sandia is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. DOE’s NNSA under contract DE-AC04-94AL85000.

1. Eroglu, D.; Zavadil, K. R.; Gallagher, K. G., J Electrochem Soc 2015, 162, A982-A990.

2. Cheng, L.; Curtiss, L. A.; Zavadil, K. R.; Gewirth, A. A.; Shao, Y.; Gallagher, K. G. ACS Energy Lett. 2016, 1, 503−509.