Cobalt Prussian blue analogues (Co-PBA; NaxCo[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 ([FeII(CN)6]4−) and ferricyanide ([FeIII(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).

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in transition metal (M) compounds, the partial substitution of the host transition metal (M h) to guest one (M g) is effective to improve the functionality. To microscopically comprehend the substitution effect, degree of distribution of M g is crucial. Here, we propose that a systematic EXAFS analysis against the M g concentration can reveal the spatial distribution of M g. We chose NaCo 1−x fe x o 2 as a prototypical M compound and investigated the local intermetal distance around the guest Fe [d fe-M (x)] against Fe concentration (x). d fe-M (x) steeply increased with x, reflecting the larger ionic radius of high-spin Fe 3+. The x-dependence of d fe-M (x) was analyzed by an empirical equation, d Fe−M (x) = sxd Fe−Fe + (1 − sx)d Fe−Co , where d fe-fe and d fe-co are the fe-fe and co-fe distances, respectively. The parameter s represents degree of distribution of Fe; s = 1, > 1, < 1 are for random, attractive, and repulsive distribution, respectively. The obtained s value (= 4.8) indicates aggregation tendency of guest Fe.

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