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 Fe²⁺/Fe³⁺ redox-couple. A major roadblock in such cathodes stems from the voltage hysteresis at higher current rates. Herein, a NASICON-type NaFe_2-xIn_x(PO₄)(MoO₄)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.