Vol.2 No.1 2009

51/88

Research paper : Basic materials research for the development of ubiquitous-energy devices (M. Kohyama et al.)−48−Synthesiology - English edition Vol.2 No.1 (2009) rare metal Co is desired. Tabuchi et al of AIST developed the high-capacity complex oxide Li2MnO3-LiFeO2 (xLi2MnO3-(1-x)LiFeO2) (capacity > 200 mAh/g) containing Fe and Mn[6]. Li2MnO3 and LiFeO2 that comprise this material are inactive materials that do not absorb or release the Li ions in individual bulk. Superior materials may be developed by clarifying the mechanism of how they are activated to achieve high capacity when they are in complex form.Therefore, we began working on this material using TEM observation under close collaboration with the development group. First, we discovered that each particle of the material had structure with Fe-rich (LiFeO2-like) and Mn-rich (Li2MnO3-like) chemical nano-domain under common oxygen lattice, using the STEM-EELS (scanning transmission electron microscope - electron energy loss spectroscopy) spectrum imaging method (method of quantifying and imaging the element concentration distribution by accumulating EELS data at every scan position of the electron beam[7]) and the nano-beam analysis method (Fig. 2)[8][9]. The nano-scale domain structure without clear grain boundaries or interfaces was a new discovery. It is thought that the difference of lattice constants was small between the two phases and there was mixing at the atomic level in the interface region. In conventional x-ray diffraction observations, it was estimated from average information that the LiFeO2-like and Li2MnO3-like phases might exist, but the existence in the form of chemical nano-domain structure was finding far beyond expectation.To investigate the mechanism whereby chemical nano-domain structure produces high capacity, it is necessary to investigate the absorption and release of Li ions in each phase by charge and discharge. Therefore, we developed a new method to visualize the concentration distribution of Li ions in real space using the STEM-EELS spectrum imaging method. Conventionally, analysis of EELS data for Li was not easy, and quantitative analysis of Li concentration distribution was in fact impossible. If the thickness of the sample was sufficiently thin, the strength of the secondary differential peak strength of the EELS spectrum was deemed to be in proportion with concentration, and therefore, we succeeded in quantitative visualization of Li concentration distribution by devising a data analysis method[10]. This method was first in the world to investigate the real space distribution of Li ions.As shown in Fig. 3, by applying the new method to the positive electrode material in the processes of charge and discharge in a battery cell, it was found that the Li ions are extracted first from the Fe-rich domain in the charge process and then extracted from the Mn-rich domain. Li returned after discharge, but there were unevenness in recovered concentration in certain places in correspondence to decreased capacity[10]. As shown in Fig. 4, it is clear that in nano-domain structure, Fe-rich domain that is inactive Fig. 2 (a) Transmission EM image of a Li2MnO3-LiFeO2 particle as positive electrode materials for high-capacity Li-ion batteries. (b) Concentration distribution of transition-metal elements observed by the STEM-EELS spectrum imaging method. (c) Conceptual diagram of the chemical nano-domain structure.Fig. 3 Concentration distributions of transition-metal elements (upper) and of Li (lower) in Li2MnO3-LiFeO2 particles as positive electrode materials for high-capacity Li-ion batteries, observed during the charge and discharge processes.(i) In a sample before charging, each particle reveals chemical nano-domain structure consisting of Li2MnO3-like (green, blue) and LiFeO2-like (yellow) domains (upper figure), where Li ions are distributed throughout the particle (lower figure).(ii) In a 50 %-charged sample, there is a match between the blue domains for the concentration distribution of Li (lower figure) and the yellow domains for the concentration distribution of transition metal elements (upper figure), which clearly indicates the preferential Li-ion extraction from the LiFeO2-like domains in the early stage of charging.(iii) In a 100 %-charged sample, Li ions are desorbed from the entire region of each particle (lower figure).(iv) In a sample after discharge, Li ions are recovered throughout the particle (lower figure). However, the recovery does not seem to be perfect, because slight decreases of Li-ion concentration are detected locally.Fig. 4 Conceptual diagram of the generation of excellent performance in Li2MnO3-LiFeO2 positive electrode material through the chemical nano-domain structure. (a)(b)(C)10 nm02040608010 nmFe/(Mn+Fe) [at. %]LiFeO2Li2MnO3OLiMnFeLi layersBefore chargeAfter 100 % charge(315 mA h/g)After 50 % charge(150 mA h/g)After 1st cycle(232 mA h/g)(ⅰ)(ⅱ)(ⅳ)(ⅲ)Fe/(Mn+Fe) (at. %)Fe/(Mn+Fe) (at. %)Fe/(Mn+Fe) (at. %)Fe/(Mn+Fe) (at. %)Li map (arb. units)Li map (arb. units)Li map (arb. units)Li map (arb. units)01010101020406080020406080020406080020406080Nano-domain structureEach crystalline phase is inactive for charge and dischargeContact in nano-scale with commonoxygen latticeHigh performance is achievedby utilizing advantages ofLi2MnO3 and LiFeO2 innana-domain structure.advantage : Li rich, high Li diffusiondisadantage : inactive Mn4+ ionsadvantage : active Fe3+ ionsdisadantage : low Li diffusionLi2MnO3LiFeO2

※このページを正しく表示するにはFlashPlayer9以上が必要です