A thin film of sodium manganese ferrocyanide, Na1.32Mn[Fe(CN)6]0.83·3.5H2O, exhibits discharge capacity (= 109 mA h g(-1)) and discharge voltage (3.4 V in average) at 0.5 C against Na in aprotic solvent. The ex situ XRD experiments reveal that the host framework remains cubic without showing any structural phase transition during the charge process. The discharge property is discernible up to 40 C.
We investigated structural and electrochemical properties of thin film electrodes of cobalt hexacyanoferrate, NaxCo[Fe(CN)6]0.902.9H2O, against x. The compound exhibits a high capacity of 135 mAh/g and an average operating voltage of 3.6 V against Na, with a good cyclability. The discharge curve exhibits two plateaus at ≈3.8 and ≈3.4 V, which are ascribed to the reduction processes of Fe3+ and Co3+, respectively. The ex situ X-ray diffraction (XRD) profiles reveal the robust nature of the host framework against Na+ intercalation/deintercalation. Thus, cobalt hexacyanoferrate is a promising candidate for the cathode material of sodium-ion secondary battery (SIB).
A new class of electrode materials are being intensively investigated to achieve a low-cost Li+ secondary battery (LIB) with a high discharge rate. Here, we investigated the discharge rate in thin film electrodes of the Prussian blue analogues (Li,Na)4y-2M[Fe(CN)6]yzH2O (M =Ni, Co, Mn, and Cd). Except for the Co compound, the capacities at 100 C exceed 60% of those at 1 C. We further investigated the electronic and crystal structures of Prussian blue analogues against Li+ concentration (x). On the basis of these data, we will discuss the origin of the rapid Li+ intercalation.
Prussian blue analogues with jungle-gym-type structure are promising candidates for cathode materials of the lithium-ion secondary battery (LIB). Here, we investigated the structural, electronic, and electrochemical properties of cobalt hexacyanoferrate, Li
x
Co[Fe(CN)6]0.902.9H2O, against Li concentration (x). The capacity (= 139 mAh/g) of the thin-film electrode was close to the ideal value (= 132 mAh/g) for the two-electron reaction. The discharge curve exhibits three plateaus, i.e., plateaus I, II, and III. The material exhibits a first-order phase transition accompanied by significant volume expansion by 7% at the boundary between plateaus II and III. Ex situ X-ray absorption spectroscopy (XAS) indicates that the discharge processes of plateaus I, II, and III are ascribed to the reduction processes of Fe3+, Co3+, and Fe3+, respectively. The rate (r) and cycle (n) dependence of the electrode performance will be discussed in terms of the reduction processes.
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]0.902.9H2O (denoted as NCF90) are ascribed to the reduction processes of Fe3+ and Co3+, respectively. On the other hand, 3.6 and 3.2 V plateaus of Na
x
Mn[Fe(CN)6]0.833.5H2O (NMF83) are ascribed to the reduction processes of Mn3+ and Fe3+, 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.
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