Phase Separation of a Hexacyanoferrate-Bridged Coordination Framework under Electrochemical Na-ion Insertion

Phase
separation and transformation induced by electrochemical
ion insertion are key processes in achieving efficient energy storage.
Exploration of novel insertion electrode materials/reactions is particularly
important to unravel the atomic/molecular-level mechanism and improve
the electrochemical properties. Here, we report the unconventional
phase separation of a cyanide-bridged coordination polymer, Eu­[Fe­(CN)6]·4H2O, under electrochemical Na-ion insertion.
Detailed structural analyses performed during the electrochemical
reaction revealed that, in contrast to conventional electrochemical
phase separation induced by the elastic interaction between nearest
neighbors, the phase separation of NaxEu­[Fe­(CN)6]·4H2O is due to a long-range
interaction, namely, cooperative rotation ordering of hexacyanoferrates.
Kolmogorov-Johnson-Mehl-Avrami analysis showed that the activation
energy for the phase boundary migration in NaxEu­[Fe­(CN)6]·4H2O is lower than that
in other conventional electrode materials such as Li1–xFePO4.