Impedance spectroscopic study on interfacial ion transfers in cyanide-bridged coordination polymer electrode with organic electrolyte

Interfacial charge transfer is a fundamental issue in both science and technology of the batteries. In this report, interfacial alkali-ion transfer between the cyanide-bridged coordination polymer (Prussian blue analogue, PBA) electrode and organic electrolytes was investigated. Electrochemical impedance spectroscopy (EIS) suggested that alkali-ion transfer at the K _0.1Mn[Fe(CN) ₆] _0.7·3.6H ₂O (MnFe-PBA) electrode-electrolyte interface involves two processes. One process could be interpreted as the ion transfer between the Outer Helmholtz Plane (OHP) and Inner Helmholtz Plane (IHP) including the solvation/desolvation process, the other could be interpreted as that between the IHP and electrode, including ad-ion diffusion on the electrode surface. Temperature dependence of the charge transfer resistances gave the activation energy for each process. The activation energy for Li-ion transfer between the OHP and IHP in propylene carbonate (PC) electrolyte is almost constant at the composition range of 0.1 < x < 0.6 in Li _xMnFe-PBA, which is comparable to that in ethylene carbonate (EC)-diethyl carbonate (DEC) electrolyte. In contrast, the activation energy for Li-ion transfer between the IHP and electrode depends largely on the Li-ion concentration in the PBA electrode. However, the averaged value for Li-ion transfer is higher than that for Na-ion transfer. This result indicated that Li-ion on the PBA surface diffuses with higher potential barrier than Na-ion. Furthermore, the effect of the interfacial charge transfer resistance was evaluated by the high charge/discharge rate experiments.