Synthesis and Structure Stabilization of Disordered Rock Salt Mn/V-Based Oxyfluorides as Cathode Materials for Li-Ion Batteries

The demand for high-performance lithium-ion batteries and thus efficient cathode materials is steadily increasing. In addition to a high energy density and long lifetime, these should also be cost-effective and environmentally benign. Manganesebased materials have particular potential because manganese is available in sufficient quantities and can be supplied at a comparatively low cost. Hence, in this study, manganese-based disordered rock salt oxyfluorides Li(2)Mn(1-x)VxO(2)F (0 <= x <= 0.5) are synthesized as cathode materials for lithium-ion batteries using high-energy mechanochemical ball-milling. The effect of partial vanadium substitution on the sample properties is analyzed, focusing on the electrochemical properties. Furthermore, a heat treatment process for stabilization of the samples is followed, where the morphology and structure of the samples are studied by powder X-ray diffraction and electron microscopy (SEM/TEM). The oxidation states of the transition metals in the synthesized compositions are further investigated using X-ray absorption nearedge structure spectroscopy. The data analysis reveals that the heat treatment resulted in increased symmetry and reduced defects of ball-milled compounds, but it may also affect the local fluorination degree in the structure. However, the results show that this treatment process has a beneficial effect on capacity retention of the formulated electrodes (similar to 81% after 100 cycles), a faster response to the change of cycling rate, and less increase in charge-transfer resistance of the samples during cycling. Such a structural improvement attributed to mitigation of the surface/bulk defects is an additional input to the series of cathode candidates of low temperature stability.

Automated summary

This study synthesizes manganese-based disordered rock salt oxyfluorides as cathode materials for lithium-ion batteries and investigates the effect of partial vanadium substitution on their properties. Heat treatment improves the symmetry and reduces defects, leading to improved capacity retention and response to cycling rate changes in the electrodes.