The role of crystal water in the electrochemical properties of sodium manganese hexacyanoferrate cathodes in sodium-ion batteries

Abstract

This paper investigates the hydrated and dehydrated phases of cubic and monoclinic Na₂Mn[Fe(CN)₆], focusing on the impact of crystal water on their electrochemical, structural, and volumetric properties. Interstitial and coordination water impact these materials differently. Removing water increases the coulombic attraction between Na⁺ and N⁻, reducing ∠Mn-N-C and ∠Fe-C-N angles and decreasing volume. The densification effect is more pronounced in the monoclinic sample due to its higher sodium content. The dehydrated cubic sample (PW-DH-C) has better cycling stability than the monoclinic sample (PW-DH-MC). The reduced cycling stability in PW-DH-MC is due to its denser rhombohedral structure, resulting from its higher sodium content, which affects the ∠Mn-N-C and ∠Fe-C-N angles. Dehydration triggers Jahn-Teller distortion in Mn³⁺ ions, inducing reversible rhombohedral-to-tetragonal phase transitions during cycling. After 200 cycles, the capacity retention of the dehydrated cubic sample improves to 59 % when exposed to air and reabsorbing moisture, compared to 50 % in its dehydrated state. Similarly, the dehydrated monoclinic sample shows an increase in retention to 28 %, up from 20 % in its dry condition. Additionally, PW-DH-C experiences lower volumetric changes during cycling, attributed to fewer sodium ions and more Fe(CN)₆⁴⁻ vacancies. These findings highlight water's crucial role in optimizing Na₂Mn[Fe(CN)₆] performance for practical applications.