Electrode-level coexistence with an iron-based polyanion stabilizes O3-type sodium layered cathodes for ultralong-life pouch cells

O3-type layered oxides such as NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) offer the highest capacity among sodium-ion cathodes, but coupled cycling, air and processing instabilities, together with the cost burden of nickel, keep them from practical cells, and material-level modifications address only part of them at added cost. Here we show that they can be addressed at the electrode-material level, through the coexistence of the layered oxide with a structurally robust iron-based polyanion, Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP), blended at a 7:3 mass ratio (denoted NP). Combining air-storage and post-mortem XRD, XPS, FTIR and FIB cross-sectional EDS with DRT/EIS kinetics, we identify the failure of O3-NFM as a coupled structural–interfacial process, in which anisotropic lattice strain nucleates particle cracks, exposing fresh surface and driving side reactions, and the coexisting NFPP interrupts this loop by diluting the reactive layered surface and taking up part of the cycling current, while retaining the discrete phase identities of NFM and NFPP. The composite electrode shows about four times less cycling-induced (003) broadening, a more slowly growing interphase, better air stability and a six-fold longer slurry pot life. In industrial pouch full cells, it sustains more than 10,000 cycles at 70% capacity retention, against about 2000 cycles for the NFM-only baseline, with better high-rate retention and lower gas evolution, at the cost of a slightly lower specific capacity. Because NP is a physical mixture of a scalable NFM and a commercially available NFPP, these gains arise from electrode-level stabilization and provide a design basis for other layered/polyanion combinations in practical sodium-ion batteries.