Grain Boundary Density Governs the Discharge Behavior of Mg Anodes in Biodegradable Batteries

Biodegradable magnesium (Mg) alloys have entered clinical use, yet their development as self‐powered absorbable batteries is hindered by poor anode performance, including high self‐corrosion, passivation, and nonuniform dissolution. The role of grain boundary density in governing discharge behavior remains unclear. Here, we investigate a Mg‐2Zn alloy with a controlled grain‐size gradient (1–60 μm) and nearly unchanged texture. Higher grain boundary density promotes uniform matrix dissolution, accelerates passive‐film exfoliation, lowers interfacial polarization, and raises discharge voltage. These boundaries also serve as Mg 2+ diffusion channels, reducing concentration polarization and enhancing utilization efficiency, particularly at high current densities. Grain boundary density is thus a key parameter governing discharge stability, plateau voltage, and anode efficiency. This work offers theoretical guidance for designing high‐performance biodegradable Mg‐air batteries and practical insights for self‐powered resorbable medical implants.