An Exploration of Sulfur Redox in Lithium Battery Cathodes

Secondary Li-ion batteries have enabled a world of portable electronics and electrification of personal and commercial transportation. However, the charge storage capacity of conventional intercalation cathodes is reaching the theoretical limit set by the stoichiometry of Li in the fully lithiated structure. Increasing the Li:transition metal ratio and consequently involving structural anions in the charge compensation, a mechanism termed anion redox, is a viable method to improve storage capacities. Although anion redox has recently become the front-runner as a next-generation storage mechanism, the concept has been around for quite some time. In this perspective, we explore the contribution of anions in charge compensation mechanisms ranging from intercalation to conversion and the hybrid mechanisms between. We focus our attention on the redox of S because the voltage required to reach S redox lies within the electrolyte stability window, which removes the convoluting factors caused by the side reactions that plague the oxides. We highlight examples of S redox in cathode materials exhibiting varying degrees of anion involvement with a particular focus on the structural effects. We call attention to those with intermediate anion contribution to redox and the hybrid intercalation- and conversion-type structural mechanism at play that takes advantage of the positives of both mechanistic types to increase storage capacity while maintaining good reversibility. The hybrid mechanisms often invoke the formation of persulfides, and so a survey of binary and ternary materials containing persulfide moieties is presented to provide context for materials that show thermodynamically stable persulfide moieties.