| Research Abstract |
Li-ion secondary batteries have widely been utilized as consumers' electronic devices such as cellular phones, personal computers, etc. because of their high electrical density. A phosphate system, LiMPO_4 (M = Fe,Ni,Co) with the olivine-related structure, has recently been intensively studied as a candidate for positive electrodes. In the Olivine-type phosphate, it has been generally recognized that PO_4 polyanion units form their valence and conduction bands far from the Fermi level where an electronic exchange mainly occurs because of its closed shell electronic configuration. As a result, the transition-metal valence electrons tend to be isolated from those of polyanions, leading to the fact that the electronic exchange arising from Li removal/uptake mainly occurs at transition metal ions. The isolated transition-metal electronic structure would be the reason for the observed high voltage properties, and it simultaneously leads to the low electronic conductivities, causing technica
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l difficulties in fabrication of batteries. Thus, an understanding of their unique electronic structure is important to apply to rechargeable lithium-ion batteries.
Co,P, and 0 K-edge XAS measurements were carried out using synchrotron radiation at beam lines BL-7C,BL-11B, and BL-11A. In Co K-edge XANES around the threshold energy for electrochemically prepared samples, Li_<1-x>,CoPO_4, the threshold energy of LiCoPO_4 is close to that of a divalent reference sample, CoCO_3, the Co ions could be almost divalent in LiCoPO_4. The threshold energy gradually shifted to higher energy with charge reaction, indicating that Co ions are oxidized for the charge compensation over an entire region of the charge reaction (from x = 0.0 to 1.0). A absorption peak in P K-edge XANES gradually shifted with electrochemical Li removal, resulting an 0.2 eV shift at a composition of x = 1.0. This indicates that phosphate ions also contribute to the variation of the electronic structure and become less negative with charging. The results of 0 K-edge XANES of Li_<1-x>CoPO_4 indicate Ssarp absorption peaks appeared at the lower energy region. Since they are caused by the transition to hybridized unoccupied orbitals between the Co 3d and 0 2p orbitals, peak A is attributed to an absorption associated with the Co 3d-0 2p hybridized orbital. Peak A gradually shifted to lower energy at small x values (0 5_x approx. 0.5). The intensity of peak A increased with composition x, indicating an increase of holes at oxygen sites. Namely, the observed peak shift and the intensity increase of peak A indicate a decrease in d electron donation from the transition metal ions to hybridized orbitals. In addition, only a little variation was observed at the 0 K-edge XAS in the region of x 【approximately equal】0.5 which is a one-to-one correspondence to that observed in the Co K-edge XANES. Therefore, the oxide ions only contribute to charge compensation process at an early stage of lithium extraction, while the cobalt ions contribute to it at a later stage of the reaction.
The analysis of XANES for Li_<1-x>CoPO_4 shows that two effects would be responsible for the modification of the electronic structure of PO_4 polyanion, (1) the hybridization effect between the Co 3d and 0 2p orbitals, and (2) the polarization effect induced by strong ionic character of Li ions. The variation of the Co-, P-and 0 K-edge XANES reveals the important role of the hybridization between the Co and 0 ions and of the polarization in Li-O-P sequences. Less
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