Ultra‐Low‐Strain Calcium and Magnesium Ion Storage Enabled by Tunnel‐Structured MoO ₃ Positive Electrode

ABSTRACT Rechargeable divalent batteries employing Ca or Mg metal negative electrodes have attracted considerable interest due to their low cost and potentially high energy density. However, the development of high‐energy Ca and Mg batteries remains limited by the lack of oxide positive electrodes capable of reversibly accommodating divalent ions at room temperature. Here, we demonstrate a new positive electrode material, a nano‐sized hexagonal tunnel‐structured MoO ₃ (nano‐ h ‐MoO ₃ ), as a structurally robust host for both Ca ²⁺ and Mg ²⁺ storage, exhibiting markedly improved reversibility and capacities. Comprehensive structural analyses, supported by computational modeling, reveal a unique charge–discharge mechanism in which divalent‐ion (de)insertion occurs through reversible modulation of host metal–oxygen bond lengths while retaining an intact host framework, resulting in minimal lattice expansion (<2%). This structurally resilient tunnel‐oxide design provides a promising pathway for developing high‐energy, practical divalent metal battery systems.