Lithium manganese oxides for rechargeable lithium batteries

Abstract

Lithium manganese spinel compounds HT-LiMn 2O 4 (HT means high temperature) synthesized at > 700 °C have high capacity in the 4 V range (Li x Mn 2O 4, x ⩽ 1), However, lithium manganese oxides LT-LiMn 2O 4 (LT means low temperature) synthesized at temperatures lower than 400 °C, resemble to the spinel structure and tend to have a reduced capacity in the 4 V range. We investigated the factor of potential difference between the 4 V type and the 3 V type by using 7Li nuclear magnetic resonance (NMR) and electron spin resonance (ESR) measurements. It was found by using 7Li-NMR that there were two kinds of spectrum in these materials; in the 4 V type, the spectrum having about 530 ppm of chemical shift with spinning side band was mainly observed. On the other hand, in the 3 V type, the broad spectrum having about 760 ppm of chemical shift without spinning side band was mainly observed. There are two different lithium sites in the manganese spinel compound (space group of Fd3m), one is at the 8 a site located far from the manganese ion and the other one is at the 16 c site located close to the manganese ion. It is presumed that the spectrum having 530 ppm of chemical shift with spinning side band is corresponding to the 8 a site, and the broad spectrum having 760 ppm of chemical shift without spinning side band is corresponding to the 16 c site, with taking into consideration the influence of paramagnetic character of manganese on the lithium site which is closely related to the distance from the manganese ion.