Speakers

​​​​​​Metal anodes promise high energy density for next-generation rechargeable batteries, yet their long-term stability remains poorly understood. While prior efforts have largely focused on enabling cycling reversibility, practical devices spend most of their lifetime at rest, where degradation during calendar aging can dominate capacity loss. Here, we systematically investigate the calendar aging of Li, Na, Al, and Mg anodes under electrolytes that enable >98.5% Coulombic efficiency during cycling. Despite their comparable cycling stability, these metals diverge sharply under calendar aging: Li undergoes continuous solid electrolyte interphase thickening, Na and Al suffer localized pitting driven by both chemical and galvanic corrosion, and Mg uniquely retains its capacity with only ~0.4% loss after 14 days. Using advanced microscopy and impedance analyses, we discover that Mg’s stability arises from a reversible interfacial resistance layer that forms during rest to kinetically suppress corrosion and is fully removed upon cycling. This discovery reframes resistance buildup (typically associated with degradation) as a protective mechanism, and establishes dynamic, reversible interphases as a new design principle for mitigating calendar aging in reactive metal anodes​.