application range of the battery materials from energy storage to energy conversion. The exploration of high-α material with different sign is quite effective to fabricate a high-η thermocell.Among the battery materials, PBAs, whose chemical formula are Li x M[Fe(CN) 6 ] y and Na x M[Fe(CN) 6 ] y (M is transition metal), are promising candidates for the cathode materials in lithium-ion/sodium-ion secondary batteries. [8] For example, thin film of Li 1.6 Co[Fe(CN) 6 ] 0.9 shows a high capacity of 132 mAh/g with a good cyclability. Most of PBAs have the face-centered cubic (fcc) (Fm3m; Z = 4) or trigonal (R3m; Z = 3) structures. [9] They consist of three-dimensional (3D) junglegym-type host framework and guest Li + /Na + ions and H 2 O molecules, which are accommodated in nanopores of the framework. The framework contains considerable [Fe(CN) 6 ] vacancies (10-30 %). The discharge curves of CoÀ and Mn-PBA show characteristic plateaus, whose redox reactions are well assigned by means of X-ray absorption spectroscopy. [10,11] Recently, the α values are reported in Li x Co[Fe(CN) 6 ] 0.71 and Figure 2. Discharge curves of (a) Na x Co[Fe(CN) 6 ] 0.71 (NCF71), (b) Na x Co[Fe (CN) 6 ] 0.9 (NCF90) and (c) Na x Mn[Fe(CN) 6 ] 0.83 (NCF83) films measured at 0.1 C. For convenience of explanation, we defined plateaus I, II, II, IV, and V.

The chemical substitution of a transition metal (M) is an effective method to improve the functionality of a material, such as its electrochemical, magnetic, and dielectric properties. The substitution, however, causes local lattice distortion because the difference in the ionic radius (r) modifies the local interatomic distances. Here, we systematically investigated the local structures in the pure (x = 0.0) and mixed (x = 0.05 or 0.1) layered oxides, Na(M1−xM′x)O2 (M and M′ are the majority and minority transition metals, respectively), by means of extended X-ray absorption fine structure (EXAFS) analysis. We found that the local interatomic distance (dM-O) around the minority element approaches that around the majority element to reduces the local lattice distortion. We further found that the valence of the minority Mn changes so that its ionic radius approaches that of the majority M.

O3‐NaFe1/2Co1/2O2 shows initial capacity of 160 mAh/g and an average operating voltage of 3.1 V (vs. Na) with good cyclability, and is a promising candidate of the cathode materials for sodium‐ion secondary batteries (SIBs). Here, we found that the cyclability of the slowly‐cooled sample is much worse than that of quenched one, even though the former sample keeps the O3‐type structure. The energy dispersive X‐ray spectroscopy (EDX) images suggest that the slow‐cooled sample (Nax Fe1–yCoy O2) is inhomogeneous in the Fe concentration (1 – y), perhaps triggered by the Na deficiency (1 – x). We ascribed the poor cyclability in the slowly‐cooled sample to the concentration inhomogeneity (Δy). The Δy is further responsible for the fluctuation of the lattice constants (a and c), as revealed by the Williamson–Hall plot. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Layered cobalt oxides are promising cathode materials for sodium ion secondary batteries (SIBs). By combined study of the X-ray absorption spectroscopy (XAS) around the O K-edge and ab initio calculation, we investigated the electronic state of the NaxCoO2 with different oxidization state, i.e, in O3-Na0.91CoO2 (CoO2-0.91) and P2-Na0.66CoO2 (CoO2-0.66). The O K-edge spectra in the pre-edge (529-536 eV) region shows significant change with oxidization of NaxCoO2. In O3-Na0.91CoO2, the spectra shows an intense band (B band) at 531 eV. In P2-Na0.66CoO2, the spectral weight of the B band increases and a new band (A band) appears at 530 eV. These spectral changes are qualitatively reproduced by the calculated partial density of states (pDOSs) of O3-NaCoO2 and P2-Na1/2CoO2. These results indicate that the electrons are partially removed from the O 2p state with oxidization of NaxCoO2.

The 3d transition metal (TM) oxides with layered structure, NaxMO2 (M = Mn, Co), are promising cathode materials of sodium ion secondary batteries (SIBs). Here, we investigated the electronic structure of M of four layered oxides (Na0.91CoO2, Na0.66CoO2, Na1.00MnO2, and Na0.54MnO2) by means of high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES), which utilizes the 1s core-hole lifetime-broadening-reduction. The high energy-resolved spectroscopy revealed a shoulder structure in the pre-edge regions of the Co K-edge spectra in Na0.91CoO2. The structure is ascribed to the transition to the Co 3d/4p state via slight hybridization with the Na 3s state.