Impact of Fluorination on Phase Stability, Crystal Chemistry, and Capacity of LiCoMnO₄ High Voltage Spinels

Fluorinated LiCoMnO_4-yF_y (y = 0, 0.05, 0.1) spinel electrodes, electrochemically active at 5-5.3 V versus Li/Li⁺, show enhanced phase purity and enhanced capacity with increasing y. We disclose the impact of fluorination on the phase purity and reversible capacity of LiCoMnO₄ via joint Rietveld refinement of neutron and synchrotron powder diffraction data, combined with micro-Raman spectroscopy. It is found that fluorination stabilizes the spinel phase and hinders precipitation of Li₂MnO₃ as a secondary phase, which controls the cation distribution on tetrahedral and octahedral sites in spinel. That is to say, for higher fluorine content the cobalt occupancy at the tetrahedral site in spinel decreases, and the lithium occupancy increases. Accordingly, the number of lithium sites that are available for electrochemical extraction and insertion of lithium ions is raised so that the capacity is increased. Further investigation of the lithium ion diffusion by means of cyclic voltammetry at different scan rates and the application of the Randles-Sevcik equation were carried out to investigate the extent of capacity enhancement due to faster lithium ion diffusion in the high voltage region.