Interfacial Stabilization of Li 1.3 Al 0.3 Ti 1.7 (PO ₄ ) ₃ Solid Electrolytes Enabled by a Sulfone‐Based Crystalline Organic Interlayer

NASICON‐structured Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) solid electrolyte (SE) exhibits satisfactory ionic conductivity and robust mechanical strength, but it suffers from interfacial instability against lithium metal anodes due to Ti 4+ reduction, which limits its application in solid‐state lithium metal batteries (SSLMBs). Herein, we developed a sulfone‐based crystalline organic interlayer (COI) composed of dimethyl sulfone and lithium bis(trifluoromethanesulfonyl)imide at an optimized molar ratio of 8:2, and applied it to LATP pellet surfaces via a melt‐casting process. The standalone COI exhibits a room‐temperature ionic conductivity of 0.67 mS cm −1 . The resulting LATP@COI composite shows an ionic conductivity of 0.78 mS cm −1 and a critical current density of 1.5 mA cm −2 . Li|LATP@COI|Li symmetric cells deliver stable cycling for over 700 h at 0.1 mA cm −2 . Full cells using LiFePO 4 and LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode with the lithium metal anode retain 93.1% and 92.3% of their initial capacities after 200 cycles, respectively, while maintaining excellent rate capability up to 2 C and operable discharge down to −40 °C. This COI provides a practical interfacial‐engineering strategy to overcome the intrinsic limitations of LATP SEs for SSLMBs.