In situ IR spectroscopy during oxidation process of cobalt Prussian blue analogues

Cobalt Prussian blue analogues (Co-PBA; Na(x)Co[Fe(CN)(6)](y)), consisting of cyano-bridged transition metal network, –Fe–CN–Co–NC–Fe–, are promising cathode materials for Na-ion secondary batteries. In the oxidation process, oxidization of Fe and/or Co are compensated by Na(+) deintercalation. Here, we investigated the oxidization process of three Co-PBAs by means of in situ infrared absorption (IR) spectroscopy. With use of an empirical rule of the frequencies of the CN(−) stretching mode in ferrocyanide ([Fe(II)(CN)(6)](4−)) and ferricyanide ([Fe(III)(CN)(6)](3−)), the oxidation processes of Co-PBAs were determined against the Fe concentration (y) and temperature (T). We will discuss the interrelation between the oxidation processes and Fe concentration (y).