Lithium Containing Fluorides and Oxyfluorides as Solid Electrolytes

Research on all solid-state batteries has increased as lithium-ion based chemistries approach theoretical capacities with conventional graphite anodes. Aside from the safety-related improvements that come with the removal of traditional liquid electrolytes, all solid-state batteries offer advantages related to cost, operating conditions, and energy density. Most of the materials that show high lithium-ion conductivity are oxide- or sulfide-based and have vastly different diffusion conditions, operating windows, and stabilities against lithium metal. As a result, exploring other chemistries—as has been done recently with halides in the argyrodite structure—is paramount to the understanding and improving the properties of solid electrolytes.<br/><br/>We chose to explore fluoride and oxyfluoride based compositions with the motivation that fluoride substitution should widen the stability window as well as provide a passivating layer of LiF in the SEI when used against lithium metal. This will work to combat the deposition of lithium metal that can occur along the grain boundaries in oxide based solid electrolytes leading to dendrite formation and cell failure. Exploring compounds in this phase space is accompanied by many challenges associated with synthetic methods as well as sample preparation and instrumental setup for various characterization methods. Conventional solid-state synthetic methods for oxides and sulfides are not applicable to fluorides due to their volatility and reactivity with several common vessels. Additionally, fluoride materials tend to be less stable at elevated temperatures which makes sintering of samples to raise the density challenging.<br/><br/>We will present our work on the examination of new lithium containing fluoride and oxyfluoride materials for use as solid electrolytes beginning with a gallium-based fluoride garnet. We have developed new synthetic methods for fluoride-based phases using custom high-temperature and high-pressure autoclaves. Cold-isostatic pressing was used to produce pellets of near theoretical density without the need for high-temperature sintering. We examine the effects of substitution of different redox-inactive metal centers through X-ray diffraction and electrochemical impedance spectroscopy. Additionally, through computational methods we investigate the activation energy of lithium ion hopping in these phases and compare these values to those extracted from Arrhenius plots derived from conductivity measurements.