Fig. 4: Electrocatalytic effect of CaS and its rejuvenation during cell operation.

a HAADF-STEM image and corresponding element mapping of the CNF cathode after discharge of the Li-S/CNF cell to 2.1 V, scale bar: 200 nm. b Rate capability test for the Li–S coin cells (E/S ratio of 4 μL mg⁻¹) with the control electrolyte and Ca electrolyte. c PITT measurements for the step-wise discharge from 2.4 V to 1.8 V for the Li–S cells (E/S ratio of 4 μL mg⁻¹) with the control and Ca electrolyte. d DFT calculations of the adsorption strength on CaS or graphene surfaces for various LiPSs. Insets show the optimized configurations of LiPSs adsorbed on CaS. The yellow, green, and purple balls indicate the S, Ca, and Li atoms, respectively. e DFT calculations of the free energy diagram during the discharge process from S₈ to Li₂S on the surface of CaS and graphene. f ICP-MS analysis of the cathode and electrolyte at various states of charge. Error bars denote the standard deviation from measurements for cathodes obtained from independent coin cells. The data are presented as mean values ± standard deviation. g Linear sweep voltammetry profiles of the carbon paper electrode, Li₂S-loaded carbon paper electrode, and CaS-loaded carbon paper electrode.