Identifying the Relaxative Internal Friction Behavior of Mn ²⁺ /Mn ³⁺ Redox in Na ₄ Fe _1.5 Mn _1.5 (PO ₄ ) ₂ (P ₂ O ₇ ) Cathode for Sodium-Ion Batteries

Fe–Mn-based phosphate material (Na 4 Fe 3– x Mn x (PO 4 ) 2 (P 2 O 7 )) demonstrates a significantly higher average voltage and energy density compared to Fe-based phosphate material (Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 )). However, its practical application is hindered by issues such as anomalous prolongation of Mn 2+ /Mn 3+ deintercalation platform and poor cycling stability, and the failure mechanisms of Fe–Mn-based phosphate material are still shrouded in mystery. This research uncovers the relaxative internal friction behavior of Mn 2+ /Mn 3+ redox during the structural evolution of Na 4 Fe 1.5 Mn 1.5 (PO 4 ) 2 (P 2 O 7 ), highlighting its dual nature. The three Mn sites within the lattice exhibit distinct coordination environments, reactivities, and resistances to Jahn–Teller distortion, leading to relaxative internal friction during sodium extraction. The distortion of [Mn x O 6 ] octahedra facilitates Na + diffusion but also results in lattice mismatch and voltage hysteresis, causing rapid electrode degradation. Additionally, this study identifies a connection between relaxative internal friction and the orbital electron behavior of Mn 3+ under Jahn–Teller distortion. To mitigate adverse effects, typical 2p/3d/4d elements are screened, revealing that Cr 3+ effectively reduces [MnO 6 ] distortion by inhibiting Mn 3+ orbital splitting, thus decreasing voltage hysteresis and enhancing cycling stability. Furthermore, targeted defect engineering is employed to eliminate impurities and improve Na + migration. These findings provide valuable insights and strategies for the practical application of Fe–Mn-based phosphate cathode materials.