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Identifying the Relaxative Internal Friction Behavior of Mn2+/Mn3+ Redox in Na4Fe1.5Mn1.5(PO4)2(P2O7) Cathode for
Wenbin Fei1, Yulei Sui1, Yuxuan Liu1
1School of Iron and Steel, Soochow University, Suzhou 215000, P. R. China.
Abstract:
Fe-Mn-based phosphate material (Na4Fe3-xMnx(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 Na4Fe1.5Mn1.5(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 [MnxO6] 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.
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