Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST)
○Yasuhiko Takahashi Junji Awaka Norihito Kijima Akimoto Junji

In the past few years, the lithium transition metal oxides have been extensively studied as cathode materials for rechargeable lithium-ion batteries, since these can be reversibly deintercalated and reintercalated by lithium ions at high potential. Recently, special attention has been paid to the spinel-type lithium manganates, LiMn₂O₄, due to its economical and environmental advantages.
LiMn₂O₄ presents a first order structural transition around room temperature. Above the transition temperature, it has the cubic spinel structure with Fd-3m space group. The crystal structure of the low temperature form has been investigated with the orthorhombic symmetry and Fddd space group by using the neutron powder diffraction method. However, the mechanism of structural transition has not been clarified yet. Recently, we have succeeded in the synthesis of LiMn₂O₄ single crystals by a flux method [1], and revealed the precise crystal and electronic structures of the cubic LiMn₂O₄ [2]. In this presentation, we demonstrate the precise crystal structure determination of low temperature form [3] and electron density distribution of LiMn₂O₄ by single-crystal X-ray diffraction method and Maximum Entropy Method using program PRIMA. In the electron density analysis of orthorhombic LiMn₂O₄ experimentally, some different tendency could be observed for Mn³⁺-O₆ and Mn⁴⁺-O₆ octahedral bonding natures which results were comparable the result of bond valence sum results [3]. These tendencies were comparison to the theoretical DFT calculations using program Wien2K and similar results were confirmed in this study.
[1] J. Akimoto et al., Chem. Mater. 12 (2000) 3246.
[2] Y. Takahashi et al., J. Phys. Soc. Jpn. 72 (2003) 1483.
[3] J Akimoto et al., Solid State Ionics 172 (2004) 491.