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<inf>1.8</inf>(MnCrAlZnIn)<inf>0.2</inf>(PO<inf>4</inf>)(MoO<inf>4</inf>)<inf>2</inf> (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.