Single‐Phase Solid‐Solution Reaction Facilitated Sodium‐Ion Storage in Indium‐Substituted Monoclinic Sodium‐Iron Phosphomolybdate Cathodes

Abstract Despite being a compelling alternative to the existing lithium‐ion battery technology, the unavailability of cathodes with high energy density and capacity poses a key challenge toward the wider adaption of sodium‐ion batteries (NIB). In this regard, iron‐rich NASICONs have triggered significant attention owing to a greater abundance of Fe and higher operating voltages of Fe ²⁺ /Fe ³⁺ redox‐couple. A major roadblock in such cathodes stems from the voltage hysteresis at higher current rates. Herein, a NASICON‐type NaFe _2‐x In _x (PO ₄ )(MoO ₄ ) ₂ (NFIPM) cathode is reported that shows a stable single‐phase solid‐solution reaction with significantly attenuated overpotential. Indium is strategically incorporated at the iron sites, expanding the lattice space to facilitate enhanced sodium‐ion diffusion and also reducing the energy bandgap of NFIPM. Magnetic susceptibility (M‐T) and Electron Paramagnetic Resonance (EPR) measurements reveal an increased spin state of iron following indium substitution. First principle calculations also confirm the lowering of the Na ⁺ migration energy barrier post indium doping. The optimized NFIPM10 shows a specific capacity of 111.85 mAh g ⁻¹ at 0.1 C with remarkable cycling stability of up to 800 cycles at 2C. In situ X‐ray diffraction confirms reversible structural stability of NFIPM during (de)sodiation, emphasizing the role of strategic doping in enhancing sodium‐ion storage.