Extremely Small Pyrrhotite Fe 7 S 8 Nanocrystals with Simultaneous Carbon‐Encapsulation for High‐Performance Na–Ion Batteries

Na/FeS x batteries have remarkable potential applicability due to their high theoretical capacity and cost‐effectiveness. However, realization of high power‐capability and long‐term cyclability remains a major challenge. Herein, ultrafine Fe 7 S 8 @C nanocrystals (NCs) as a promising anode material for a Na–ion battery that addresses the above two issues simultaneously is reported. An Fe 7 S 8 core with quantum size (≈10 nm) overcomes the kinetic and thermodynamic constraints of the Na‐S conversion reaction. In addition, the high degree of interconnection through carbon shells improves the electronic transport along the structure. As a result, the Fe 7 S 8 @C NCs electrode achieves excellent power capability of 550 mA h g −1 (≈79% retention of its theoretical capacity) at a current rate of 2700 mA g −1 . Furthermore, a conformal carbon shell acts as a buffer layer to prevent severe volume change, which provides outstanding cyclability of ≈447 mA h g −1 after 1000 cycles (≈71% retention of the initial charge capacity).