Insights into Kinetics-Driven Lattice Modulation of Transition Metal-Based Hexacyanoferrate Positrodes for Enhanced Rechargeable Sodium-Ion Pouch Batteries: A Study on Sodium Nickel Hexacyanoferrate

This work presents a comprehensive study on how precursor mixing kinetics influence nucleation behavior-driven lattice modulation in both pristine/dehydrated transition metal-based hexacyanoferrate and alter the electrochemical performance of sodium-ion battery positrodes using sodium nickel hexacyanoferrate (Ni-HCF) as a representative system. By iterating the addition sequence of nickel/ferrocyanide precursors, three unique monoclinic Ni-HCFs are synthesized: simultaneous mixed, ferrocyanide-limited mixed, and nickel-ion-limited mixed. Surprisingly, the simultaneous mixed sample (Ni-HCF (BA)) shows the highest sodium content (1.42 Na per formula unit), lowest CN⁻ vacancy (Γ = 0.15), and superior crystallinity. After dehydration, all samples maintain their structural framework, although the emergence of Ni³⁺ species is observed via XPS and XANES, indicating water-absence mediated local lattice distortion. Electrochemical assessments using half-pouch cells establish that Ni-HCF (BA) achieves the greatest capacity (73.74 mAh∙g⁻¹), minimizes voltage hysteresis (≈0.35 V), and exhibits exceptional cycling stability across 2500 cycles at 1.0 C, surpassing even its dehydrated form. Combined with a hard carbon negatrode, the resulting full pouch cell attains an energy density of 144.2 Wh∙kg⁻¹ (2871.3 W·kg⁻¹) with an 83.71% retention after 500 cycles (1.0 C). Collectively, this investigation elucidates the role of kinetics-induced lattice modulation in engineering defect-tolerant, high-performance transition metal-based hexacyanoferrate positrodes for sodium-ion pouch cells.