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

TL;DRAbstract

Abstract 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 s

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