Diketopyrrolopyrroles Act as Redox Mediators in Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are among the most promising next-generation energy storage technologies, offering gravimetric energy densities greater than those of state-of-the-art lithium-ion batteries. However, widespread commercialization of the technology is hindered by challenges related to the multistep redox chemistry of sulfur. The addition of diketopyrollopyrroles (DPPs) to Li–S battery electrolytes can improve cell performance; however, their mechanism of action remains unclear. Here, we use a range of electrochemical methods to elucidate the mechanism through which DPPs affect the performance of the Li–S batteries. Electrochemical analysis demonstrates the importance of DPP redox potentials in enhancing the redox kinetics of polysulfide reduction, with analytical measurements confirming this is via a redox mediation process in which electrons are shuttled between polysulfide intermediates and the underlying current collector via the DPP molecule. We show that DPP derivatives that are not redox active at the potentials associated with the multi-step interconversion of sulfur and lithium sulfide in Li–S batteries do not show the same effect. Galvanostatic analysis confirms that the enhancement of the kinetics of polysulfide conversion translates to the operation of Li–S batteries, which show an improvement in the discharge capacity after the addition of the additives.