A Si3N4/graphene composite is designed as an interlayer for a carbon-nanotubes/sulfur cathode to improve electrochemical performance of lithium-sulfur batteries. In the interlayer, Si3N4 nanoparticles suppress the migration of the dissolved polysulfides and graphene sheets construct a 3-dimensional charge-transfer network. The carbon-nanotubes/sulfur@Si3N4/graphene cathode delivers an initial discharge capacity of 1334.7 mAh g−1 at 0.1 C and retains a capacity as high as 745.8 mAh g−1 after 200 cycles, with a capacity fade ratio of 0.22% per cycle. The cathode shows good cycling life, delivering a discharge capacity of 413.3 mAh g−1 for 1 C after 1000 cycles. According to the results of density functional theory calculation, the anchoring of the Si3N4/graphene interlayer to lithium polysulfide can be attributed to a coefficient chemical binding of Li-N and Si-S bonds generating from electronic conjugation effect between the Si3N4 supercell surface and the polysulfides. Generally, the improvement in electrochemical performance originates from the enhancements in Li+ diffusion coefficient and charge transfer, and from the restraining of the shuttle effect of the dissolved lithium polysulfide as a result of the Si3N4/graphene interlayer.