Stability Enhancement in Na_0.67Fe_0.2Mn_0.8O₂Positive Electrodes via Spherical Coprecipitated Hydroxide Precursor Synthesis for Na-Ion Batteries

Mn/Fe-based layered transition metal oxides (LTMOs) are promising positive electrode materials for sodium-ion batteries (SIBs) due to their high abundance, low cost, and stable price. At the commercial scale, the fabrication of these materials commonly employs coprecipitation of hydroxide precursors, which enables for the scalable synthesis of uniform, dense particles with tunable morphology. However, the commonly used chelating agent (ammonia) forms unstable complexes with Fe²⁺ ions, resulting in uncontrollable particle morphology and poor electrochemical properties. Here, three chelation strategies (no chelation, ammonia, and oxalate) for Fe/Mn-based hydroxides are evaluated. It was found that oxalate chelation produced uniform, dense spherical hydroxide particles, while particles via ammonia/no chelate routes exhibited no morphological control. The LTMOs synthesized from the oxalate-chelated hydroxide precursor formed uniform spherical particles, while the other two LTMOs showed greater variation in particle morphology. The oxalate-chelated LTMO electrode exhibited increased cycling stability due to reduced parasitic reactions with the electrolyte, as characterized by static leakage current measurements and electrochemical impedance spectroscopy.