Structural and Morphological Control of Spherical NFM333 Precursors for Na-Ion Batteries: A Comparative Study of Ammonia- and Citrate-Based Coprecipitation Routes

Morphology control is a key design parameter for sodium-ion layered oxide cathodes, yet achieving uniform precursors with balanced stoichiometry is more challenging than in lithium-ion systems due to the broader range of transition-metal chemistries involved. These complexities highlight the need for systematic comparisons of coprecipitation routes tailored to sodium-ion compositions. Here, we examine how ammonia- and citrate-based coprecipitation methods shape the morphology and composition of equimolar Ni–Fe–Mn hydroxide and oxyhydroxide precursors. We investigate how pH, ligand concentration, and temperature jointly influence precipitation onset, particle shape, and metal incorporation. In the ammonia system, precipitation proceeds readily between pH 10.5–11.5, with pH ≈ 11.0 yielding the most uniform morphology and target Ni:Fe:Mn ≈ 1:1:1 stoichiometry. Higher ammonia levels improve morphology but above ∼1.0 M begin to delay Ni incorporation and introduce phase separation. In contrast, the citrate system shows delayed precipitation (threshold pH ≈ 11.8) but forms dense granular microspheres with narrow size distributions across 0.1–0.6 M citrate, with Fe enrichment emerging at higher ligand concentrations. X-ray diffraction reveals β -Ni(OH) 2 -type hydroxides for ammonia-derived precursors and δ -FeOOH-type oxyhydroxides for citrate-derived ones. Together, these results provide practical guidance for tailoring precursor morphology and composition in Fe- and Mn-rich sodium-ion cathode materials.