PNNL

Abstract

Cation-disordered rocksalt (DRX) oxides are promising candidates for next-generation cathode materials for lithium-ion batteries. It has been reported that partial fluorination of DRX oxides enhances their cyclability. However, three outstanding questions need to be answered: the lattice position of fluorine, the concentration of fluorine incorporated into the DRX lattices, and the spatial distribution of fluorine within the DRX lattices. Here, we use electron-based imaging and spectroscopic techniques, including atom location by channeling-enhanced microanalysis, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy and integrated differential phase contrast imaging in a scanning transmission electron microscope to gain atomic level insights into the DRX materials with the nominal composition of Li1.2Mn0.7Ti0.1O1.7F0.3 and Li1.15Ni0.45Ti0.3Mo0.1O1.85F0.15. We directly unveil that fluorine substitutes oxygen in the DRX lattices, the O:F ratio as expressed by the term of O+F = 2 is in the range from 1.92:0.08 to 1.82:0.18. Spatially, fluorine is distributed in close proximity to the Li-rich regions but distinct from lithium fluoride phase. Additionally, we observe that incorporation of fluorine atoms in the DRX lattice induces larger variation in cation-anion separation. These observations provide insight into guided design of oxyfluoride DRX cathodes for high performance batteries.