The commercialized electrode materials for lithium-ion batteries are limited to a few highly crystalline ones (for instance, graphite and LiCoO2). Due to the highly crystalline nature of these materials, the Li+ storage sites are confined only to the crystallographic sites. It is known that the structural defects in non-graphitic carbons can serve as additional Li+ storage sites, but little attention has been paid to the possible Li+ storage at defect sites in metal oxide electrodes.
The oxides of early transition metal elements (for instance, MoO2, TiO2, and V2O5) are lithiated by the insertion reaction rather than the conversion reaction, due to relatively strong metal-oxygen bond, when they are applied for the negative electrode . In the insertion-type lithiation reaction, Li+ ions are inserted into crystallographic sites without structural disintegration. The crystallographic sites are well-defined in highly crystalline metal oxides but their number is limited to give a limited Li+ storage capacity. Obviously, this structural constraint is opened if Li+ ions are hosted by any structural defects populated on surface or bulk of metal oxides. This work demonstrates that the structural defects (vacancies, void spaces and others), which are highly populated in the amorphous early transition metal oxides (MoO2, TiO2, and V2O5), do serve as Li+ storage sites to give an enlarged Li+ storage capacity.
Three amorphous metal oxide electrodes outperform the crystalline counterparts with respect to Li+ storage capacity and rate capability. For instance, the amorphous MoO2 (a-MoO2) electrode is lithiated up to 4.0 Li/Mo, whereas the crystalline one (c-MoO2) only 1.0 Li/Mo (Fig. 1a). The absence of Li2O in the lithiated electrode and retention of Mo-O bonds (EXAFS data in Fig. 1b) illustrate that a-MoO2 is not lithiated by the conversion reaction but by the addition reaction, and Li+ ions are inserted into structural defects. The higher rate capability for a-MoO2 is likely due to the higher mobility of inserted lithium species, which can be evidenced by the peak sharpening with an increase in temperature in the variable-temperature magic-angle spinning 7Li nuclear magnetic resonance spectra .
Na+ hosting materials should have more opened Na+ storage sites and ion conduction paths because Na+ ions are larger in size than Li+ ions . Hence, amorphous metal oxides seem to be a prospective Na+ host since they carry the more opened structural defects. This work confirms this premise. The three amorphous metal oxide electrodes exhibit a reversible Na+ insertion/extraction (Fig. 2), which is not observed with the crystalline counterparts.
Research Activities :
Fundamental phenomena involved in batteries and capacitors
Honors and Awards :
- 2010-present: outside director of LG Chem Ltd.
- 2004-2008: Head of next generation battery project (MKE)
- 2000: Citation Award from ISI (Institute of Science Information)