Structure and Lithium Ion Dynamics in Fluoride Doped Cubic LLZO Garnet for Li Solid State Battery Applications

The lithium-stuffed garnet LLZO (Li₇La₃Zr₂O₁₂), when suitably doped, is a promising candidate material for use as a solid state electrolyte within advanced Li-ion batteries. It possesses the thermal and mechanical stability of many inorganic ceramics, while exhibiting the high Li+ ionic conductivities often associated with conventional liquid electrolytes, making it an ideal component for large scale energy storage. However, only the high temperature cubic phase has any meaningful Li-ion conductivity. Typically the formation of this phase is achieved through cation doping (e.g. Al³+ on the Li site) to lower the Li content and so disrupt Li ordering. However, Li site doping, in particular, may potentially lead to some disruption of the Li ion conduction pathways and sub-optimal ionic conductivities. Consequently, other novel doping strategies involving the anion site is gaining traction, such as F- for O²- as an alternative strategy to lower the Li content without directly blocking the lithium diffusion pathways. Classical potentials-based simulations have been employed to simulate the incorporation of fluoride anions into LLZO. Low incorporation energies have been calculated suggesting fluoride anions are stable on the oxygen sites with a compensating lithium ion vacancy defect. Molecular dynamics (MD) calculations suggest a definitive phase transition to the more desirable cubic phase of LLZO when doped with fluoride at temperatures significantly lower than the tetragonal-cubic phase transition found for pure LLZO. Concurrently, the lithium ion transport properties are shown to improve in the fluoride doped samples particularly at low temperatures due to the stabilisation of the cubic phase, suggesting anion doping of garnet systems may be a compelling alternative route to optimise the ionic conductivity.