Mg‐Doped MnO ₂ Cathode with Tailored Oxygen Vacancies for High‐Rate High‐Capacity Calcium Storage

Calcium‐ion batteries are promising candidates for next‐generation energy storage systems due to their high output voltage, abundant calcium resources, and intrinsic safety. Among various cathode materials, MnO ₂ stands out for its low cost, environmental friendliness, and high energy density. However, its practical application is limited by poor conductivity and structural instability arising from the Jahn–Teller distortion of Mn(III)O ₆ octahedra. Herein, Mg‐doped MnO ₂ nanosheets grown on carbon cloth are synthesized via a one‐step hydrothermal method to overcome these limitations. Mg doping introduces oxygen vacancies that alleviate Jahn–Teller distortion by modulating the local electronic environment and further enhances structural stability. Additionally, the enlarged interlayer spacing weakens the electrostatic interactions with Ca ^{2 +} ions, thereby promoting rapid ion diffusion. As a result, the optimized electrode delivers a high reversible specific capacity (458.2 mAh g ⁻¹ at 0.1 A g ⁻¹ ), excellent rate capability (164.2 mAh g ⁻¹ at 1 A g ⁻¹ ), and cycling stability (99.3% capacity retention after 1000 cycles at 1.0 A g ⁻¹ ). Ex situ analyses reveal a cointercalation mechanism involving both H ⁺ and Ca ^{2 +} ions. This work offers a defect‐engineering strategy for developing high‐performance cathode materials.