Achieving Long Cycle Life of Ta-Doped Manganese-Based Oxide Through Complementary Polyhedral Distortion

The sodium-ion cathode material Na_0.44MnO₂ with a unique S-shaped tunnel has emerged as a promising candidate for large-scale energy storage due to its exceptional air stability, low cost and better cycling stability than the layered structure. However, practical implementation faces significant challenges from sluggish Na⁺ migration kinetics and severe Jahn-Teller distortion of Mn³⁺ ions, which induce structural degradation during cycling, ultimately leads to poor long-term cycle performance at high rate. In this study, computational results reveal that 1% Ta doping at Mn sites induces complementary polyhedral distortion in adjacent [Mn^IIIO₆] octahedron. That is one adjacent Mn─O bond length elongates while the other shortens, which contributes to the maintain structural stability. Concurrently, the distortion indices of both two of 5-coordinate [Mn^IIIO₅] polyhedra in the S-shaped tunnel significantly decrease, thereby weakening the Jahn-Teller effect. Furthermore, the Ta doping reduces particle size to shortening Na⁺ diffusion paths, which facilitates Na⁺ diffusion (diffusion coefficient: 10^-12 to 10^-9 cm² s^-1). Electrochemical characterization reveals that NMTO-1 maintains 98.6% capacity retention after 400 cycles at 3C and 84.53% after 3000 cycles at 20C due to enhanced structural stability and improved Na⁺ diffusion kinetics. These results demonstrate Ta doping as an effective strategy for developing high-performance Na_0.44MnO₂ cathodes.