Understanding surface reactivity, local structure, and lithium-metal dendrite initiation in garnet solid electrolytes
Solid electrolyte-based lithium-ion batteries can enable long lasting and safe energy storage devices with high energy densities. Out of the many solid electrolytes explored to date, doped Li₇La₃Zr₂O₁₂ (LLZO) garnets have high room temperature ionic conductivity and wide electrochemical stability making them promising candidates for commercial applications. Despite receiving much attention, the surface reactivity, regeneration, local structure of dopants in doped LLZO and the dendrite formation have not been systematically characterized and understood. This thesis elucidates the importance of the gas atmosphere and temperature for regeneration by tracking the surface of LLZO with near ambient pressure x-ray photoelectron spectroscopy and grazing incidence x-ray diffraction. From these results, a standardised protocol for handling LLZO is presented and a method to achieve a lithium metal - LLZO interface with low resistance is devised. The local structure of dopants in LLZO is clarified using ²⁷Al and ⁷¹Ga magic angle spinning magnetic resonance (MAS NMR) spectroscopy on LLZO and model compounds. The side-products on the surface of the grains and/or grain boundaries are demonstrated to give rise to multiple peaks in the MAS NMR spectra and it is shown that dopants occupy a single site in the LLZO lattice. The factors that give rise to these side-products is discussed and it is shown that the distribution of dopants in these side-products and LLZO significantly influences the ionic conductivity of LLZO. When LLZO is cycled with lithium metal in a symmetrical cell configuration (Li - LLZO - Li), continuous stripping and plating results in the formation of lithium metal filaments (dendrites) which initiate on the cathode, propagate through the solid electrolyte, and short-circuit the cell. By careful testing of symmetric cells under different cyclic current loading conditions, the critical current density (ICCD) at which the dendrites initiate, is shown to be protocol dependent. Unidirectional current experiments are shown to be a better way of estimating ICCD in solid electrolytes. Finally, the factors that determine the ICCD in solid electrolytes are identified by using an Onsager formalism to model the various non-equilibrium processes involved in lithium metal plating at the lithium metal electrode - electrolyte interface and possible solutions to increase ICCD in solid electrolytes are proposed.
Royal Society (RP/R1/180147)