Tuning Sodium Source Chemistry for Enhanced Structural Homogeneity and Kinetic Properties of Sol–Gel Layered Cathode in Sodium-Ion Batteries

The sol–gel technique offers a versatile route for synthesizing electrode materials with reduced and uniform grain sizes, enabling the development of high-performance cathodes for next-generation sodium-ion batteries (SIBs). Achieving high-performance layered oxide cathodes requires control over chemical homogeneity and phase composition. Herein, we investigate the feasibility of tuning the structural integrity, electrochemical behavior, and Na + transport kinetics of NaFe 0.5 Co 0.5 O 2 using different external sodium sources (Na 2 CO 3 and NaOH) combined with sol–gel-derivated transition metal oxide precursor. Characterization using X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy reveals distinct surface constituents and morphological differences between the samples obtained by conventional and modified sol–gel routes. Electrochemical measurements further confirm that the morphological modulation enhances surface reaction kinetics, leading to an improved high-rate capability. Owing to the high melting point of sodium carbonate, a homogeneous surface morphology can be achieved, effectively suppressing side reactions that lead to impurity phase formation and capacity fading. The carbonate-derived modified sol–gel sample delivers an initial discharge capacity of 145 mAh g –1 at C/10 and exhibits capacity retention exceeding 90% over 100 cycles at C/3. A full cell assembled with the above cathode material and bare hard carbon anode achieves a capacity of 114.9 mAh g –1 at an average working potential of 2.8 V at C/10. This study highlights the use of external sodium sources as a facile, tunable, and scalable pathway toward the rational design of advanced cathode materials for SIBs.