Synthesis and characterization of novel fluoride and oxide cathodes for rechargeable batteries

Abstract

Developing new cathode materials is key to improving the energy density of rechargeable batteries and enabling new applications of energy storage. In this thesis, two families of materials were explored as candidate cathode materials: the dirutile and rutile polymorphs of LiMnF4, and layered lithium-excess . . . Dirutile LiMnF4 was identified from high-throughput computation as a promising conversion cathode. The dirutile polymorph was synthesized through a new low temperature route, and the rutile polymorph was discovered upon mechanical milling. With simple synthesis and electrode preparation methods, both dirutile and rutile polymorphs of LiMnF4 showed electrochemical activity. Electron diffraction confirmed both polymorphs to convert upon lithiation along different reaction paths. As with other fluorides, specific capacity was strongly linked with processing conditions. The layered lithium-excess . . . compounds were designed from recent understanding of diffusion channels in lithium-excess materials. Increasing lithium content was found to improve both discharge capacity and capacity retention. Structural studies revealed a complex nanostructure pattern of Li-Sb and Ni-Sb ordering where the interface between these domains formed the correct local configuration for good lithium mobility. The < 1nm Li-Sb stripe domains enable percolation of the low barrier lithium diffusion channels at lower lithium excess levels. The redox mechanisms of the lithium-excess . . . materials were then studied as a function of lithium content and rate. . . . surprisingly exhibited higher discharge capacities at faster rates, and traversed distinct voltage curves at different rates. Characterization of redox processes confirmed nickel redox and oxygen loss, with oxygen redox proposed to account for the balance of the capacity. Finally, irreversible nickel migration is suggested as an explanation for the rate-dependent voltage curve features.