Multi‐Ion Doping Controlled CEI Formation in Structurally‐Stable High‐Energy Monoclinic‐Phase NASICON Cathodes for Sodium‐Ion Batteries

Overcoming the energy density limitations of sodium‐ion batteries (NIBs) requires innovative strategies to optimize cathode materials. While entropy‐engineering through multi‐ion doping has shown promise, previous efforts in polyanion‐type cathodes are confined to conventional (pyro)phosphate‐based systems. Here, it is reported for the first time a entropy‐engineered NASICON‐type cathode, NaFe 1.8 (MnCrAlZnIn) 0.2 (PO 4 )(MoO 4 ) 2 (NFM'PM20), stabilized in a rare monoclinic P2/c phase via solid‐state reaction. This entropy design enables robust cathode‐electrolyte interphase (CEI) formation, mitigates lattice strain, and reduces the bandgap, collectively facilitating reversible 2.6 Na⁺ storage with an exceptional energy density of 315.62 Wh kg −1 . The NFM'PM20 cathode demonstrates outstanding cycling stability (92.2% capacity retention after 500 cycles at 5C) and ultra‐long cycle life exceeding 2000 cycles. Mechanistic investigations via in situ X‐ray diffraction confirm a strain‐accommodating solid‐solution reaction mechanism with minimal volume change (≈4.5%). At the same time, electron paramagnetic resonance and magnetic susceptibility measurements demonstrate enhanced Fe spin‐states, which improve electrontransport. Ex‐situ transmission electron microscope images reveal a thin and stable CEI layer. Density functional theory calculations elucidate the atomic‐scale advantages, including optimized Na⁺ migration pathways with 0.45 eV lower diffusion barriers and enhanced interfacial charge transfer kinetics. The NFM'PM20 cathode represents a transformative advancement for developing practical high‐energy‐density NIBs.