Li
6PS
5Cl is a highly wanted sulfide-solid-electrolyte (SSE) for developing allsolid-state lithium batteries, due to its high ionic conductivity, good processability and abundant compositional elements. However, its cyclability is poor because of harmful side reactions at the Li
6PS
5Cl/Li interface and growth of lithium dendrites inside Li
6PS
5Cl phase. Herein, we report a simple interface-engineering remedy to boost the electrochemical performance of Li
6PS
5Cl, by coating its surface with a Li-compatible electrolyte Li
3OCl having low electronic conductivity. The obtained Li
6PS
5Cl@Li
3OCl core@shell structure exhibits a synergistic effect. Consequently, compared with the bare Li
6PS
5Cl, this composite electrolyte exhibits great performance improvements: 1) In Li|electrolyte|Li symmetric cells, the critical current density at 30℃ gets increased from 0.6 mA/cm
2 to 1.6 mA/cm
2, and the lifetime gets prolonged from 320 h to 1400 h mA/cm
2; 2) In Li|electrolyte|NCM721 full cells running at 30°C, the cycling capacity at 0.2 C (or 0.5 C) gets enhanced by 20% (or from unfeasible to be feasible) for 100 cycles and the rate capability reaches up to 2 C from 0.2 C; and in full cells running at 60°C, the cycling capacity is increased by 7% at 0.2 C and the rate capability is enhanced to 3.0 C from 0.5 C. The experimental studies and theoretical computations show that the performance enhancements are due to the confined electron penetration and suppressed lithium dendrites growth at the Li
6PS
5Cl@Li
3OCl interface.