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.

Electronic structures of hole-doped transition metal cyanides, Na 0:84Àx Co[Fe(CN) 6 ] 0:71 . 3.8H 2 O (NCF71), Na 0:72Àx Ni[Fe(CN) 6 ] 0:68 . 5.1H 2 O (NNF68) and Na 1:60Àx Co[Fe(CN) 6 ] 0:90 . 2.9H 2 O (NCF90),were investigated by means of the x-ray absorption spectroscopy and the valence differential spectroscopy. The x-ray absorption spectroscopy revealed that the holes are introduced on the Fe, Fe, and Co sites for the NCF71, NNF68 and NCF90 films, respectively. Owning to the valence differential spectroscopy, we unambiguously assigned the spectral components to the respective optical transitions. We further found that an ab initio band calculation based on the local density approximation with the onsite Columbic repulsion (LDA+U) semi-quantitatively explains the optical transitions.

Manganese hexacyanoferrate, La x Mn[Fe(CN)6] y zH2O, is a promising candidate for cathode materials for lithium ion secondary batteries, because the compound exhibits an ideal two-electron reaction without structural phase transition at y =0.83. We found that its capacity (Q) increases with Fe concentration (y) from Q = 115 mAh/g at y = 0.83 to 130 mAh/g at 0.87, and 143 mAh/g at 0.93. We further investigated the structural properties of La x Mn[Fe(CN)6] y zH2O against the Li concentration (x). In the low-x region, the y = 0.87 compound exhibits a phase separation into the two cubic phases, whereas the y = 0.93 compound separates into the cubic and tetragonal phases. We ascribed these phase separations to the Jahn–Teller (JT) instability as well as the smaller ionic radius of Mn3+.

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Prussian blue analogies (PBAs) are promising cathode materials for lithium ion (LIB) and sodium ion (SIB) secondary batteries, reflecting their covalent and nanoporous host structure. With use of synchrotron-radiation (SR) X-ray source, we investigated the structural and electronic responses of the host framework of PBAs against Li+and Na+intercalation by means of the X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS). The structural investigation reveals a robust nature of the host framework against Li+and Na+intercalation, which is advantageous for the stability and lifetime of the batteries. The spectroscopic investigation identifies the redox processes in respective plateaus in the discharge curves. We further compare these characteristics with those of the conventional cathode materials, such as, LiCoO2, LiFePO4, and LiMn2O4.

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Manganese and cobalt hexacyanoferrates are promising candidates for cathode materials for lithium ion secondary batteries (LIBs), because such compounds exhibit high capacity (?130-150 mA h/g) and good discharge rate properties. Here, we investigated electrochemical, structural, and electronic properties of solid solutions of Mn and Co hexacyanoferrates, Li x Mn 1%y Co y [Fe(CN) 6 ] z wH 2 O, against Li concentration (x). In all the compounds, we observed two plateaus at 3.8-4.0 V (plateau I) and 3.2-3.4 V (plateau II). Ex situ X-ray absorption spectroscopy (XAS) revealed that plateau I (II) is ascribed to the reduction of Co 3+ (Fe 3+ ) at y : y c (= 0.33) while plateau I (II) is ascribed to the reduction of Fe 3+ (Co 3+ and Mn 3+ ) at y ; y c . The lattice constant (a Fe ) at the Fe redox plateau discontinuously decreases from ?10.5 Å at y : y c and ?10.0 Å at y ; y c . We will discuss the lattice effects on the electrochemical and electronic properties.