Cooperative Lithium-Ion Insertion Mechanisms in Cathode Materials for Battery Applications

Sammanfattning: Understanding lithium-ion insertion/extraction mechanisms in battery electrode materials is of crucial importance in developing new materials with better cycling performance. In this thesis, these mechanisms are probed for two different potential cathode materials by a combination of electrochemical and single-crystal X-ray diffraction studies. The materials investigated are V6O13 and cubic LiMn2O4 spinel.Single-crystal X-ray diffraction studies of lithiated phases in the LixV6O13 system (x=2/3 and 1) exhibit superlattice phenomena and an underlying Li+ ion insertion mechanism which involves the stepwise addition of Li+ ions into a two-dimensional array of chemically equivalent sites. Each successive stage in the insertion process is accompanied by a rearrangement of the Li+ ions together with an electron redistribution associated with the reduction of specific V-atoms in the structure. This results in the formation of electrochemically active sheets in the structure. A similar mechanism occurs in the LiMn2O4 delithiation process, whereby lithium is extracted in a layered arrangement, with the Mn atoms forming charge-ordered Mn3+/Mn4+ layers.Lithium-ion insertion/extraction processes in transition-metal oxides would thus seem to occur through an ordered two-dimensional arrangement of lithium ions extending throughout the structure. The lithium ions and the host structure rearrange cooperatively to form superlattices through lithium and transition-metal ion charge-ordering. A picture begins to emerge of a universal two-dimensional lithium-ion insertion/extraction mechanism analogous to the familiar staging sequence in graphite.