Conductive Li 2 S‐NbSe 2 Cathode Material Capable of Bidirectional Self‐Activation for High‐Performance All‐Solid‐State Lithium Metal Batteries

All‐solid‐state lithium metal batteries (ASSLBs) offer an alternative route to safe and high energy density power sources. Sulfur‐based cathodes with high theoretical specific capacity and low cost are crucial for advancing ASSLBs. However, the electronic insulation and sluggish kinetics of sulfide materials like Li 2 S severely limit battery performance. Herein, the strategy is proposed that a highly electronic conductive Li 2 S‐NbSe 2 material enhances the electrochemical performance through bidirectional self‐activation. The chemical interaction between Li 2 S and NbSe 2 induces NbSe 2‐x S x and Li 2 Se derivatization, serving as the basis for activation. The carrier transport properties, chemical evolution and self‐activation mechanism of Li 2 S‐NbSe 2 during the oxidation–reduction process are revealed. The chemical activation of NbSe 2‐x S x is explored to accelerate the electrochemical processes by modifying the conductivity of sulfur species and the conversion pathways without insulating Li 2 S and S aggregation. Therefore, ASSLBs using Li 2 S‐NbSe 2 as cathode active material achieve an electrode‐level energy density of 394 Wh kg −1 and a power density of 524 W kg −1 at 1 C (4.35 mA cm −2 ) and 25 °C. The capacity retention after 100 cycles at 0.5 C is ≈99.3% with almost no degradation. This work provides new options and insights for the rational design and development of chalcogenide cathode active materials for high‐performance ASSLBs.