Mg doping in MnO₂ introduces abundant oxygen vacancies and expands interlayer spacing, effectively suppressing Jahn–Teller distortion and facilitating rapid Ca²⁺ diffusion, enabling high-rate and long-cycle calcium-ion 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, MnO₂ 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)O₆ octahedra. Herein, Mg-doped MnO₂ 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.