The intelligent design of electrocatalytic sulfur host materials is an essential step to achieving high-performance lithium−sulfur (Li–S) batteries. Herein, we report double-shelled ZnS@CoS nanocages as sulfur hosts to electrocatalytically enhance sulfur cathode kinetics and suppress the shuttle effect of polysulfides. Combining theoretical calculations and experimental tests, we reveal that the built-in electric field formed at the ZnS@CoS heterojunction is uniquely suited to promoting sulfur cathode electrochemistry; the double-shelled ZnS@CoS nanocages facilitate Li transport while at the same time enabling the immobilization and rapid electrocatalytic conversion of key polysulfides. Consequently, Li–S cells prepared from ZnS@CoS as sulfur host exhibit a high specific capacity (e.g., 1443 mA h g at the current rate of 0.1 C) and an impressive cycling stability (e.g., an initial specific capacity of 801 mA h g achieved at 2 C and 507 mA h g retained after 650 cycles). Our new concept of designing a heterojunction to promote sulfur cathodic reactions can be potentially extended to other contemporary energy-storage systems.