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

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Abstract
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 Ni 3+ 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.

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