Identifying the Relaxative Internal Friction Behavior of Mn²⁺/Mn³⁺ Redox in Na₄Fe_1.5Mn_1.5(PO₄)₂(P₂O₇) Cathode for Sodium-Ion Batteries.

Abstract

Fe-Mn-based phosphate material (Na₄Fe_3-<i>x</i>Mn_<i>x</i>(PO₄)₂(P₂O₇)) demonstrates a significantly higher average voltage and energy density compared to Fe-based phosphate material (Na₄Fe₃(PO₄)₂(P₂O₇)). However, its practical application is hindered by issues such as anomalous prolongation of Mn²⁺/Mn³⁺ 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 Mn²⁺/Mn³⁺ redox during the structural evolution of Na₄Fe_1.5Mn_1.5(PO₄)₂(P₂O₇), 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_<i>x</i>O₆] 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 Mn³⁺ under Jahn-Teller distortion. To mitigate adverse effects, typical 2p/3d/4d elements are screened, revealing that Cr³⁺ effectively reduces [MnO₆] distortion by inhibiting Mn³⁺ 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.