High volumetric energy density Li-S batteries enabled by dense sulfur monolith cathodes with ultra-small-sized sulfur immobilizers

Abstract It is extremely challenging to achieve a high-volumetric-energy-density lithium-sulfur batteries (LSBs) for sulfur/carbon cathodes due to their low volumetric density and many invalid pores. Herein, such a dense sulfur monolith cathode with a high volumetric energy density of 2067 Wh L-1 and a large volumetric capacity of 980 mAh cm-3 (of the total volume of cathode) is demonstrated, where sulfur/TiO2 quantum dots (QDs, ∼3.6 nm) are anchored on nitrogen-rich graphene (S/TiO2-QDs/N-G) and sulfur cathode is densified to a volumetric density of 1.6 g cm-3 with a conductivity of ∼270 S m-1 by drying the graphene hybrid hydrogel. Such high-volumetric capacity is one of the best value in all reported sulfur/carbon cathodes. Besides, the monolith cathode gives a superior long-term cycling stability with only 0.06% capacity decay per cycle for 500 cycles at 0.5 A g-1. More importantly, under a low electrolyte/sulfur ratio of 4.2 μL mg-1, the thick and dense monolith cathode with sulfur loading of 5.5 mg cm-2 displays an areal capacity of 4.92 mAh cm-2 at 0.1 A g-1. Furthermore, the size effect of TiO2-QDs (anchored on N-G) on electrochemical performances of LSBs and its adsorption mechanism for polysulfides demonstrate the significantly enhancement in discharge capacity and cycling stability of dense monolith cathode derives from the strong capture ability of ultra-small-sized TiO2-QDs for polysulfides. The highly dense sulfur cathode with ultra-small sized sulfur immobilizers (e.g. metal oxides, sulfides, or phosphides) provides a new strategy for designing high-volumetric-energy-density LSBs with superior long-term cycling stability even in lean electrolyte condition.