Insights into the Electrochemical Performance of NaFePO ₄ and LiFePO ₄ Cathodes in Sulfate-Based Aqueous Electrolytes: A Combined Theoretical–Experimental Approach

Sodium-ion batteries are sustainable alternatives to lithium-based systems. Among olivine-type cathodes, NaFePO4 (NFP) is often considered inferior to LiFePO4 (LFP) due to its metastability and sodium’s higher atomic weight. Here, we present a combined theoretical-experimental study to compare the electronic, structural, and electrochemical properties of NFP/C and LFP/C in sulfate-based aqueous electrolytes. LFP/C was synthesized via a hydrothermal route followed by carbon coating, while NFP/C was obtained through chemical delithiation/sodiation of LFP/C to retain the olivine framework and ensure matched structural and morphological characteristics. Electrochemical analyses show that NFP/C delivers higher specific capacities (100.55 and 54.97 mAh g–1 at 0.1 and 2 C, respectively) and lower charge-transfer resistance than LFP/C (90.25 and 42.69 mAh g–1). This behavior reverses the trend observed in organic media and is attributed to the lower solvation energy of Na+ in water. On the other hand, NFP/C shows lower cycling stability caused by structural expansion. DFT calculations reveal broader Fe 3d states in NFP than in LFP, influencing electrochemical polarization. Analysis of bandgap and charge redistribution at Fe2+/Fe3+ centers further clarifies electron transport. Overall, this work discusses fundamental characteristics of these isostructural systems and the decisive effect of the electrolyte on the performance of the cathode.