Trade-off Effect of Fe in Earth-abundant P2-Type Sodium Cathode Materials: Transition Metal Dissolution and Oxygen Redox Mechanism

Abstract

P2-Na_x[Li_yMn_1−y−z]O₂ (TM: Ni, Cu, Co, Fe) exhibits anionic redox reactions, and among these, the cost-effective Fe substitution has attracted significant attention to aid reasonable a promising material. However, we find that the dissolution and deposition of Fe present in the TM layer can cause detrimental effects, structural disintegration that affects capacity fading. Operando X-ray diffraction shows that the P2 phase of Na_0.6[Li_0.15Fe_0.15Mn_0.7]O₂ is maintained during de/sodiation processes, and X-ray absorption analysis reveals the redox activity of Fe³⁺/Fe⁴⁺, Mn³⁺/Mn⁴⁺, and O²⁻/(O₂)ⁿ⁻redox pairs. Mössbauer spectroscopy provides insights into the behavior of Fe during de/sodiation, particularly in the two-electron oxidation process observed during charging, where the Fe³⁺/Fe⁴⁺ redox reaction simultaneously influences the oxidation of lattice oxygen, thereby aiding overall charge compensation during desodiation. These findings clarify the connection between this process and the redox activity of lattice oxygen. Additionally, ⁷Li NMR is employed to analyze the migration of Li from the transition-metal layer to the Na layer, elucidating the anionic-redox-reaction mechanism. Notably, X-ray photoelectron spectroscopy and inductively coupled plasma–atomic emission spectroscopy analyses demonstrate Fe dissolution and subsequent deposition on the surface of anode, leading to capacity degradation and poor electrochemical performance. These findings underscore the significant impact of Fe dissolution and deposition on the performance of Na_0.6[Li_0.15Fe_0.15Mn_0.7]O₂, highlighting the challenges associated with Fe doping in cathode materials for sodium-ion batteries.