Dual Functionalities of Rb Cation in Lean Electrolyte Lithium Sulfur Batteries

•Rubidium cation reduces to rubidium sulfide (Rb2S) at both anode and cathode surfaces.•Rb2S acts as a catalyst in the cathode part and reinforces the SEI layer at the anode surface.•Postmortem analysis substantiates Rb2S assisted SEI layer restrains the heterogenous Li deposition.•The incorporation of rubidium cation into the electrolyte facilitates the efficient operation of lithium-sulfur batteries, even in extremely lean electrolyte conditions.