The Contact Interface Engineering of All‐Sulfide‐Based Solid State Batteries via Infiltrating Dissoluble Sulfide Electrolyte

All‐solid‐state lithium batteries (ASSLBs) based on sulfide solid electrolytes (SEs) are one of the most promising strategies for next‐generation energy storage systems and electronic devices. However, the poor chemical/electrochemical stability of sulfide SEs with oxide cathode materials and high interfacial impedance, particularly due to physical contact failure, are the major limiting factors to the development of sulfide SEs in ASSLBs. Herein, the composite cathode of MOF‐derived Fe 7 S 8 @C and Li 6 PS 5 Br fabricated by an infiltration method (IN–Fe 7 S 8 ) with dissoluble sulfide electrolyte (dissoluble SE) is reported. Dissoluble SE can easily infiltrate the porous sheet‐type Fe 7 S 8 @C cathode to homogeneously contact with Fe 7 S 8 nanoparticles that are embedded in the surrounding carbon matrixes and form a fast ionic transport network. Benefiting from applying dissoluble SE and Fe 7 S 8 @C, the IN‐Fe 7 S 8 ‐based cells displayed a reversible capacity of 510 mAh g −1 after 180 cycles at 0.045 mA cm −2 at 30 °C. This work demonstrates a novel and practical method for the development of high‐performance all‐sulfide‐based solid state batteries.