Regulating Local Mn─O Coordination to Suppress the Intrinsic Anti‐Site Defects in Mn‐Based Phosphate Cathodes

Abstract The Mn‐based phosphate Na _3+2x Mn _1+x Ti _1‐x (PO ₄ ) ₃ cathodes have attracted widespread attention for Na‐ion batteries owing to the abundant Mn resource and high Mn ²⁺ /Mn ³⁺ /Mn ⁴⁺ redox potential. However, they suffer from high voltage hysteresis and low reversible capacity at 2.5–4.2 V (60–80 mAh g−1) due to intrinsic anti‐site defects of Mn ²⁺ occupying Na2 vacancies. Although metal ion substitutions (e.g., V ³⁺ ) can reduce intrinsic anti‐site defects and improve reversible capacity (80–110 mAh g ⁻¹ ), the underlying mechanism remains not well‐understood, limiting further improvement of the reversible capacity. Here, five metal‐ion substitution cathodes: Na _3.4 Mn _1.15 Ti _0.75 M _0.1 (PO ₄ ) ₃ (M = Fe ³⁺ , Al ³⁺ , V ³⁺ , Cr ³⁺ , and Ga ³⁺ ) are systematically studied, among which only two metal ions (V ³⁺ and Cr ³⁺ ) can remarkably reduce intrinsic Mn ²⁺ /Na2 anti‐site defect and voltage hysteresis. The revealed mechanism is that V ³⁺ and Cr ³⁺ substitutions tend to form a MnO ₅ polyhedron with the less Mn─O coordination numbers under the assumption of Mn ²⁺ occupying the Na2 vacancy, compared to the MnO ₆ octahedron in other metal‐ion substitutions (Fe ³⁺ , Al ³⁺ , and Ga ³⁺ ). The formation of MnO ₅ polyhedron indicates an energetically more unfavorable state for Mn ²⁺ /Na2 anti‐site defects. The designed Na _3.5 Mn _1.15 Ti _0.65 V _0.1 Cr _0.1 (PO ₄ ) ₃ cathode exhibits the lowest ratio of intrinsic anti‐site defect (0.28%) and achieves a high reversible capacity (120.7 mAh g ⁻¹ ) and high energy density (431.6 Wh kg ⁻¹ ), with superior cycling stability (93.2% retention after 2000 cycles at 5 C). This work offers valuable insights for designing high‐capacity and high‐energy Mn‐based Na‐ion cathodes.