The cyclic performances of Li-S batteries at 0.2 C are displayed in
Fig. 4b. For PP/PE/PP-assembled Li-S battery, it delivers an initial discharge capacity of 999 mAh/g and retains at 626 mAh/g after 100 cycles. By contrast, the PP/PE/PP@NHCS-assembled Li-S battery shows an initial discharge capacity of 1355 mAh/g and a high capacity of 921 mAh/g after 100 cycles, benefiting from the high utilization of sulfur from the enhanced ionic conductivity of PP/PE/PP@NHCS separators. The corresponding galvanostatic charge/discharge profiles of PP/PE/PP and PP/PE/PP@NHCS assembled Li-S batteries at 0.2 C are shown in
Fig. 4c. Each curve shows one charge plateau and two discharge plateaus, corresponding to the oxidation of Li
2S
2/Li
2S to S
8 and reduction of S
8 to long-chain lithium polysulfides then to Li
2S
2/Li
2S, respectively, which is consistent with the CV results of Li-S batteries (
Figs. 3a and
b). The voltage hysteresis between charge and discharge are 170 and 143 mV for PP/PE/PP-assembled and PP/PE/PP@NHCS-assembled Li-S batteries, respectively, demonstrating the smaller overpotential for PP/PE/PP@NHCS-based configuration. The rate performances and corresponding charge and discharge voltage curves of the Li-S batteries using PP/PE/PP and PP/PE/PP@NHCS separators are shown in
Fig. 4d and Fig. S9 (Supporting information). PP/PE/PP@NHCS-assembled Li-S batteries show discharge specific capacities of 1585, 1023, 877, 793, 750, 684 mAh/g at 0.1, 0.2, 0.5, 0.8, 1 and 2 C, respectively, showing higher specific capacities than those of PP/PE/PP-assembled Li-S batteries at corresponding C-rates. And when current density returns from 2 C to 0.1 C, the discharge specific capacity for PP/PE/PP@NHCS-assembled Li-S batteries recovers to 943.7 mAh/g, which is higher than the PP/PE/PP-assembled Li-S batteries of 511 mAh/g, demonstrating an excellent rate reversibility. The long cyclic performances of Li-S batteries using PP/PE/PP and PP/PE/PP@NHCS separators were further investigated (
Figs. 4e and
f). The PP/PE/PP@NHCS-assembled Li-S batteries deliver excellent cycling stability with high initial discharge capacities of 1002 and 903 mAh/g. And the discharge capacities maintain at 487 and 461 mAh/g with low capacity fading rates of 0.051% and 0.049% per cycle after 1000 cycles at 1 and 2 C, respectively, which is superior to previously reported functional separators (Table S2 in Supporting information). In comparison, the PP/PE/PP-assembled Li-S batteries display inferior cycling stability with discharge capacities of 763 and 633 mAh/g, retaining discharge capacities of 203 and 137 mAh/g and fading rates of 0.073% and 0.078% per cycle after 1000 cycles at 1 and 2 C, respectively. Meanwhile, the corresponding discharge-charge profiles are displayed in Fig. S10 (Supporting information), demonstrating the longer plateau, smaller potential polarization and improved conversion reversibility of PP/PE/PP@NHCS assembled Li-S batteries. Even at a high sulfur loading of 6 mg/cm
2, PP/PE/PP@NHCS-assembled Li-S batteries still exhibit good cycle stability with a capacity of 692 mAh/g at 0.2 C for 100 cycles (
Fig. 4g). The enhanced capacities, reduced polarization and accelerated polysulfide redox reaction kinetics can be ascribed to the adsorption of polysulfides by heteroatoms, the physical barrier and the extended conductive network from the NHCS.