Sol-gel and solid-state fluorination of lithium cobalt oxide for Li-ion secondary batteries

Abstract

A series of fluorinated LiCoO2 compounds, LiCoO2−xFx with x = 0.05 − 0.65, were synthesised by both a sol-gel and a solid-state method. The sol-gel method utilises LiNO3 and Co(NO3)2.6H2O as Li and Co sources respectively, water and 1-butanol as solvent for the fluoride precursor LiF, and citric acid as the chelating agent. The prepared materials were compared using pXRD, SEM, Raman, TG and DTA analyses. pXRD revealed a secondary phase appearing at x= 0.25 in materials prepared by the sol-gel method and as early as x= 0.08 in materials prepared by the thermal solid-state method. Materials prepared by the thermal solid-state method required higher temperatures and longer times than materials prepared by the sol-gel method. The results showed that more fluorine can be incorporated in the LiCoO2 structure by the sol-gel than the solid-state method. The secondary phase was identified as lithium oxydifluoride, LiCoOF2. Relatively pure material with stoichiometry could be prepared by both methods. The space was determined as either 𝐶2 or 𝑃1 and the unit cell dimensions for the two alternatives are reported. This secondary phase has been reported in the literature, but had not been positively identified at the time of submission of this dissertation. Keywords: fluorinated LiCoO2; LiCoO2−xFx; sol-gel fluorination; solid-state fluorination; lithium oxydifluoride.