Transition-metal vs. rare-earth NaFePO4 hybrid nanocomposites: accelerating kinetics for emerging sodium energy storage devices

Abstract

Emerging energy storage demands materials that are safe, cheap, and high-performing. While sodium-iron phosphate (NaFePO4/NFP) perfectly fits the bill for next-gen batteries, its commercial reality is stalled by a major bottleneck: poor conductivity and slow ion kinetics. We bypass this limitation by engineering nickel (Ni-NFP) and yttrium (Y-NFP) hybrid nanocomposites using a scalable sol-gel approach. By directly contrasting transition-metal and rare-earth composites, this study delivers a blueprint for tuning crystal structure and unlocking elite electrochemical performance. Comprehensive structural and morphological characterization, including XRD, Raman, FTIR, FESEM, and EDX, confirmed successful element incorporation into the host lattice. Morphologically, pristine NaFePO4 displayed highly aggregated particles with surface fractures (∼50.89 nm). In contrast, Ni-composite yielded crack-free, flake-like particles (∼ 71.49 nm) with minimized surface defects, while Y-composite promoted compact, densely packed grains (∼ 51.75 nm), signifying bolstered structural integrity. Crystallographic analysis confirmed that pristine NaFePO4 adopts the thermodynamically stable maricite framework (Pnma). Incorporation of Ni2+ induced significant lattice expansion (+3.86% unit-cell volume expansion) within the maricite host, whereas Y3+ doping resulted in a distorted maricite lattice with a 1.36% volume contraction. Electrochemical evaluations demonstrate that the unique chemistry of each element dictates redox activity, charge-transfer kinetics, and long-term cycling pathways. Electrochemically active Ni participates directly in electronic transport and redox behavior, boosting electronic conductivity to deliver improved sodium diffusion coefficient (6.23×10−9 cm2 s−1), enhanced dual-redox activity (Fe2+\Fe3+ and Ni2+\Ni3+), and a high initial voltametric capacity of 93.63 mAh g−1. Conversely, electrochemically inactive Y3+ primarily functions as a robust structural stabilizer. Although Y-NFP exhibited a lower initial diffusion coefficient (1.75×10−10 cm2 s−1), its structurally fortified, compact lattice provided exceptional resistance against degradation during extended operation.