Uncovering the interplay of Competing Distortions in the Prussian Blue Analogue K2Cu[Fe(CN)6]


Illustation of charged and charging batteries

We report on the synthesis, crystal structure, thermal response and the electrochemical behaviour of the Prussian Blue analogue (PBA) K2Cu[Fe(CN)6].

From a structural perspective, this is the most complex PBA yet characterised; its triclinic crystal structure results from an interplay of co-operative Jahn-Teller order, octahedral tilts, and a collective 'slide' distortion involving K-ion displacements.

These different distortions give rise to two crystallographically distinct K-ion channels with different mobilities.  Variable-temperature X-ray powder diffraction measurements show that K-ion slides are the lowest-energy distortion mechanism at play, as they are the only distortion to be switched off with increasing temperature.

Electrochemically, the materials operate as a K-ion cathode with a high operating voltage and an improved initial capacity relative to higher-vacancy PBA alternatives.  On charging, K-ions are selectively removed from a single K-ion channel type, and the slide distortions are again switched on and off accordingly.

We discuss the functional importance of various aspects of structural complexity in this system, placing our discussion in the context of other related PBAs.


Read the full paper online at ACS Chemistry Materials.