Copyright © 2020 American Chemical Society. The practical application of Li-S batteries is being obstructed by severe safety concerns due to lithium dendrites. The Li-ion/sulfur batteries, which use Li2S as the cathode and can be coupled with different anodes (e.g., Si or Sn), can eliminate the safety issues related to lithium metal. To improve the performance of Li2S cathode for Li-ion/sulfur batteries, CoS2-decorated hollow carbon spheres (HCS) are first explored as a conductive matrix for the Li2S cathode. Hollow carbon spheres (HCS) possesses a strong tendency to physically absorb and trap high-order lithium polysulfides, while the CoS2 can chemically bond with low-order lithium polysulfides. Moreover, CoS2 has a catalytic effect that can reduce the energy barrier in the first charge. In situ synchrotron X-ray diffraction has clarified the catalytic mechanism of CoS2 toward barrier reduction. CoS2 can boost the electrochemical reactions from Li2S to polysulfide and act as a redox mediator, lowering the overpotential of Li2S in the first charge process, resulting in less electrolyte decomposition, stable cycling performance, and higher capacity. The data show that CoS2-decorated hollow carbon spheres have a higher initial specific capacity and better capacity retention, with specific capacity of 831 mA h g-1 and capacity retention of 79.5% after 100 cycles, which is much better than the performance of hollow carbon spheres (Li2S-HCS) alone. The full cell core-shell Si@C||Li2S-HCS/CoS2 shows a specific capacity of 650 mA h g-1 and a capacity retention of 65% after 50 cycles at an average voltage of 1.6 V with low electrolyte to sulfur (E/S) and anode to cathode (A/C) ratio.