X-ray Absorption Spectroscopy and In-Operando Neutron Diffraction Studies on Local Structure Fading Induced Irreversibility in a 18 650 Cell with P2-Na₂/3Fe₁/3Mn₂/3O₂ Cathode in a Long Cycle Test

P2-Na_2/3[Fe_1/3Mn_2/3]O₂ (NFMO) crystal with a maximum capacity of ∼150 mAh was synthesized by a solid-state annealing method and used as a cathode in a sodium ion battery. By combining focused-ion beam section scanning electron microscopy, ex-situ X-ray absorption spectroscopy, X-ray photoemission depth profiling, and in-operando neutron diffraction, we found that Na ion intercalation and extraction distort the local structure in NFMO crystal, resulting in irreversibility of the sodium ion battery (SIB). This reaction pathway is controlled by the transformation kinetics of the Fe sites from octahedral (O_h) to tetragonal (T_d) in the charge and discharge processes. For a SIB operated at 2.0 to 3.8 V, steady kinetics between the Na intercalation and chemical state evolution on the Fe sites enable the homogeneous restructuring in both local and global regimes in NFMO crystal. For a SIB operated at 2.0 to 4.5 V, substantially higher kinetics in the Fe chemical state evolution induce a dramatic lattice expansion. This expansion cracks the interface between the P2 and Na intercalated regions, thereby causing substantial irreversibility of NFMO in a SIB.