Balancing Reactivity and Ion Transport in Mg Alloy Anodes via Secondary-Phase Engineering

Solid-state batteries are limited by an interfacial trade-off: interfacial reactions are necessary to establish ion-transport pathways, yet excessive reactions lead to passivation and polarization. Here, we establish a descriptor-guided framework for heterogeneous metal anodes by integrating interfacial energy, phase-to-phase Volta potential difference, and microstructural percolation. This framework identifies Mg2Sn as the optimal secondary phase, with an interfacial formation energy of −0.55 J m–2 and a moderately positive phase-to-phase Volta potential difference that together enable controlled interfacial reactivity. A percolated Mg2Sn network together with continuous α-Mg pathways delivers a stripping current more than 400 times that of pure Mg and stable stripping/plating for over 1300 h at 0.1 mA cm–2, representing a record cycling performance for solid-state Mg metal anodes. This descriptor set provides a general framework for secondary-phase engineering in solid-state batteries.