Recent Progress in the Fundamental Understanding of Garnet-Based Superionic Conductors and Fabrication of Solid-State Cells for Advanced Lithium Batteries

In recent years, the push to move past conventional Li – ion battery technology has grown. Several advanced battery technologies and cell chemistries have been identified as promising candidates including i) solid-state batteries with Li metal anodes, ii) Li – S, and iii) flow batteries. Although an engineered solution using liquids may be possible for some of these options, a solid electrolyte is an enabling technology for each of these beyond Li – ion alternatives. Li₇La₃Zr₂O₁₂ (LLZO) with the cubic garnet structure is a promising solid electrolyte for advanced battery concepts such as solid state Li-ion batteries. Cubic LLZO is a fast ion conductor and is chemically and electrochemically stable between 0 and 6V vs. Li/Li⁺. This presentation will show recent advances 1) in the understanding of the conduction mechanism and 2) towards fabrication of solid-state device. First, neutron diffraction was used to successfully determine the Li site occupancy for a series of cubic LLZO compositions stabilized by Tantalum. Furthermore, the Li site occupancy has been related to the bulk ionic conductivity and activation energy as measured by Electrochemical Impedance Spectroscopy (EIS) with equivalent circuit modelling as a function of temperature. It was found that the maximum in the conductivity coincides with the cubic-to-tetragonal phase transition. The cubic-to-tetragonal transitional point also represents the maximum in octahedrally coordinated Li site occupancy and shortest Li-Li separation distances. Determination of the carrier concentration from results of the neutron diffraction and conductivity measurements support the earlier suggestion by Ohta et al. that not all the Li-ions are participating in conduction. Second, recent results towards bonding of electrodes to a cubic LLZO membrane will be presented. The ability to form electrochemically active solid/solid interfaces will allow for fundamental investigation of the charge transfer mechanism from one crystalline solid to another. Overall, the purpose of this work is to establish a fundamental knowledge base to assess the feasibility of large-format solid-state batteries.