Lithium additive driven phase modulation in thick, low tortuosity sulfur cathodes for lean Li-S batteries

Abstract

The accessibility of sulfur and its reactivity in a lean electrolyte volume holds promise for the practical realization of a high energy density lithium-sulfur battery. However, the thick electrode and increased sulfur mass result in low wettability and highly tortuous paths, which impede electron and ion transport. Herein, a novel approach to modulating the phase of conventional polyvinylidene difluoride (PVDF) binders using lithium (Li) salt-based additives on the cathode is reported. The Li additives involving anions F-, OH-, NO3- , and TFSI- are rationally studded with PVDF binder to construct thick sulfur cathodes. The Li additive-based electrode demonstrates an increased wettability even in a low electrolyte volume and an amicable tortuous pathway through smooth surfaces. The increased sulfur reactivity at a low electrolyte volume is found to be due to the modulated alpha phase of PVDF and the ion donation properties of the electrode during the cycling. As a result, the LiF-based sulfur cathodes achieve a high sulfur reactivity of 56.4 % with a specific capacity of 945 mAh g- 1 at an electrolyte-to-sulfur ratio of 8 mu L mg- 1. The amount of reactive sulfur is 35 % higher than the pristine sulfur electrodes. Moreover, the Li-S pouch cell constructed with a LiF additive-based electrode of 200 mu m thickness and 4.0 mg cm- 2 sulfur loading is successfully demonstrated even at an electrolyte volume of 8 mu L mg-1. The Li additive-based approach will pave the way for the development of ultrathick electrodes capable of operating at an even lower electrolyte volume.