Regulating Bulk Transport via a Li ₃ P/Li‐Cu Bicontinuous Ionic‐Electronic Conducting Network: A Fundamental Strategy Toward Long‐Cycling Garnet‐Based Solid‐State Lithium Metal Batteries

ABSTRACT Solid‐state lithium metal batteries (SSLMBs) employing garnet electrolytes (e.g., LLZTO) offer high safety and energy density, but their development is hindered by lithium dendrite growth caused by poor interfacial contact and significant volume changes. This work presents a novel composite lithium anode (CLA) designed via in‐situ reaction of molten lithium with red phosphorus and copper, which spontaneously constructs an integrated ion/electron‐conducting network, consisting of a Li 3 P fast‐ion conductor and a three‐dimensional Li‐Cu solid‐solution skeleton. The dual‐network architecture significantly enhances wettability on LLZTO, achieving seamless interfacial contact with an ultralow resistance of 3.2 Ω cm 2 . Crucially, synergistic effect of Li 3 P and LiCu 3D skeleton facilitates rapid ion transport, uniform electron distribution, and desirable mechanical buffering of strain, thereby guiding homogeneous lithium plating/stripping and effectively suppressing dendrite formation. Consequently, the CLA symmetric cell demonstrates a high critical current density of 1.6 mA cm −2 and exceptional long‐term cycling stability over 6400 h. The full cells paired with LiFePO 4 and NCM811 also show excellent cyclability with 93.4% (1 C) and 71.2% (0.5 C) retention after 440 and 300 cycles, respectively. This study introduces a new strategy for constructing stable lithium anodes via in‐situ built dual‐conductive networks, offering a viable solution for high‐performance SSLMBs.