Cost–Effective Synthesis and Electrochemical Evaluation of Triphylite NaFePO4 as a High–Performance Cathode Material for Sodium–Ion Batteries

Abstract

Sodium iron phosphate (NaFePO₄) has emerged as a promising cathode material for sodium-ion batteries (SIBs) due to its cost-effectiveness, environmental sustainability, and structural similarity to the well-established lithium iron phosphate (LiFePO₄) used in commercial lithium-ion batteries. The triphylite phase of NaFePO₄ offers a theoretical capacity of 154 mAh/g, making it an attractive candidate for large-scale energy storage applications. This study presents a scalable and economical synthesis route for producing triphylite NaFePO₄ through a two-step conversion process involving chemical delithiation of commercial LiFePO₄ followed by sodiation. Structural and morphological characterizations using X-ray diffraction (XRD), attenuated total reflectance (ATR) spectroscopy, and field-emission scanning electron microscopy (FESEM) confirmed the successful formation of phase-pure triphylite NaFePO₄ with an average crystallite size of 25 nm and flake-like morphology (~60 nm thickness). Electrochemical performance evaluation in half-cell configurations demonstrated a reversible capacity of 42 mAh/g after 100 cycles at 100 mA/g, with 91% capacity retention and near-100% Coulombic efficiency. Rate capability tests revealed stable performance across varying current densities (50–2000 mA/g), with capacity recovery to 91% upon returning to 50 mA/g. The low charge transfer resistance and structural stability of NaFePO₄ underscore its suitability for SIB applications. This work highlights a facile, scalable synthesis method that leverages existing LiFePO₄ infrastructure, offering a viable pathway for commercialization. The findings contribute to advancing sustainable and cost-efficient cathode materials for next-generation sodium-ion batteries.