The structural stability of transition metal oxides for lithium rechargeable cells

This research considered the crystal structural or phase stability of lithium-cobalt and lithium-manganese oxides used as positive electrodes in lithium rechargeable cells. Using previous experimental observations, first principle calculations reported in the literature and a novel method of internal energy calculation developed here, a hypothesis of the relative crystal structural stability of the layered and spinel-type structures was proposed. This hypothesis was examined using experiments that utilize electrochemical and microstructural studies.; This work focused on the following oxides: low-temperature LiCoO{dollar}sb2{dollar} (LT-LiCoO{dollar}sb2{dollar}) prepared at 400{dollar}spcirc{dollar}C, novel LiMnO{dollar}sb2{dollar} synthesized by an ion-exchange process, and standard spinel Li (Mn{dollar}rmsb2rbrack Osb4{dollar} and lithium-stabilized {dollar}rm Lisb{lcub}1.05{rcub}lbrack Mnsb{lcub}1.95{rcub}rbrack Osb4.{dollar} Electron diffraction analysis found that the poor electrochemical properties of LT-LiCoO{dollar}sb2{dollar} electrodes were attributed to the intermediate cation distribution. The presence of the intermediate cation distribution in LT-LiCoO{dollar}sb2{dollar} products suggested the structural instability of the lithiated-spinel structure and prolonged heat-treating of LT-LiCoO{dollar}sb2{dollar} products showed that the layered structure of LiCoO{dollar}sb2{dollar} was thermodynamic stable and the lithiated-spinel structure was metastable at 400{dollar}spcirc{dollar}C. On the other hand, electron diffraction analyses of electrochemically cycled Li{dollar}sb{lcub}rm x{rcub}{lcub}rm MnO{rcub}sb2 (0< x <1){dollar} electrodes showed that the layered LiMnO{dollar}sb2{dollar} structure was not stable to repeated lithium intercalation/de-intercalation and transformed to a spinel-type structure. These experimental observations were in agreement with the proposed internal energy calculation results of the layered and the lithiated-spinel structures of LiCoO{dollar}sb2{dollar} and LiMnO{dollar}sb2{dollar}. The presence of the lithiated-spinel {dollar}rm Lisb2lbrack Mnsb2rbrack Osb4{dollar} structure in the {dollar}rm Lisb{lcub}x{rcub}lbrack Mnsb2rbrack Osb4{dollar} electrodes that showed capacity fade upon cycling was found to be more pronounced than that in the lithium-stabilized {dollar}rm Lisb{lcub}1+x{rcub}lbrack Mnsb2rbrack Osb4{dollar} electrodes that showed improved capacity retention during cycling. Therefore, the presence of the lithiated-spinel {dollar}rm Lisb2lbrack Mnsb2rbrack Osb4{dollar} structure contributed to the capacity fade observed for the 4 V {dollar}rm Li/Lisb4lbrack Mnsb2rbrack Osb4{dollar} cells.