Self‐Adaptive Gradient Interphase Enables Durable Solid‐State Lithium Metal Batteries

Garnet‐type solid‐state electrolytes (SSEs) enable safe, high‐energy solid‐state lithium metal batteries (SSLMBs) but suffer from poor interfacial contact and lithium dendrite penetration. Here, we report a machine learning ‐guided interfacial strategy that constructs a self‐adaptive graded interphase from a B 105 M 40 composite interlayer (BiOCl: Mg 3 N 2 = 105:40). The interphase features a lithiophilic LiMg phase at the bottom to ensure intimate Li/LLZTO contact, a porous Li 3 Bi phase to accommodate lithium plating/stripping, and Li 3 N/Li 3 OCl phases that enable fast ionic transport and dendrite suppression. The self‐adaptive gradient interphase maintains stable contact with lithium metal during cycling. As a result, B 105 M 40 ‐Li||LLZTO||B 105 M 40 ‐Li symmetric cells exhibit a high critical current density of 15 mA cm −2 and stable cycling over 5000 h at 14 mA cm −2 . Full cells retain 93.7% capacity after 1000 cycles with LiFePO 4 cathodes at 2.0 C, while those paired with high‐voltage LiNbO 3 @LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathodes exhibit cycling for over 1000 cycles at 1.0 C. Moreover, the strategy is applicable to sulfide SSEs, offering a universal interfacial design for long‐life SSLMBs.