Redox Reactions in Prussian Blue Analogue Films with Fast Na⁺ Intercalation

Electronic state of cobalt and manganese Prussian blue analogues were systematically investigated against Na + intercalation by means of the infrared and X-ray absorption spectroscopies. The spectroscopies revealed that 3.8 and 3.4 V plateaus in the discharge curve of Na x Co[Fe(CN) 6 ] 0.90 2.9H 2 O (denoted as NCF90) are ascribed to the reduction processes of Fe 3+ and Co 3+ , respectively. On the other hand, 3.6 and 3.2 V plateaus of Na x Mn[Fe(CN) 6 ] 0.83 3.5H 2 O (NMF83) are ascribed to the reduction processes of Mn 3+ and Fe 3+ , respectively. We found that the film electrodes with a thickness of 1.1 µm exhibit fast Na + intercalations: the discharge capacity at 60 C of the NCF90 (NFM83) film was 90% (45%) of the open-circuit-voltage (OCV) value. Thus, Prussian blue analogue films are promising cathode candidates for sodium-ion secondary batteries.