Alloy Phase-Formation-Driven Lithium Deposition Pathways in Magnesium-Based Breathing Anodes for Long-Lasting Anode-Free All-Solid-State Batteries

Mg particle-based “breathing” anodes provide a volumetrically adaptive host for all-solid-state batteries (ASSBs), yet structurally biased Li accumulation limits the long-term stability. Here, we demonstrate that the alloying mode of a secondary metal─solid-solution (Ag) versus intermetallic formation (Sn) decisively regulates Li-flux directionality and deposition pathways in Mg-based anodes. Ag dissolves continuously into the Li–Mg matrix, relaxing concentration gradients and redirecting Li growth toward the solid–electrolyte interface, which undermines the beneficial breathing behavior. In contrast, Sn forms immobile, stoichiometric Li22Sn5 domains that serve as internal nucleation anchors, guiding Li uniformly into the anode interior. As a result, the Sn-modified breathing anode exhibits markedly homogeneous volumetric expansion and significantly improves cycling stability, sustaining >500 cycles with an average Coulombic efficiency of 99.7% and 56.4% capacity retention, compared to about 350 cycles for pristine Mg. These results establish solid-solubility engineering as an effective strategy for enabling structurally resilient and volumetrically adaptive anode-free ASSBs.