An evaluation of the thermal stability of cubic Li 7La 3Zr 2O 12 (LLZO) and implications for LLZO as a solid-state lithium ion conducting electrolyte

Abstract

Work described in this dissertation focuses on the properties and thermal stability of cubic lithium lanthanum zirconate (c-L i7La 3Zr 2O 12 or c-LLZO), a lithium ion conducting ceramic generally regarded as a leading electrolyte candidate for all solid-state lithium-ion batteries. Experiments were carried out to characterize c-LLZO's thermal stability and identify mechanisms responsible for thermal induced degradation under conditions commonly used to sinter dense c-LLZO electrolytes. At room temperature the tetragonal phase of LLZO (t-LLZO) is thermodynamically stable but with a 10-fold smaller ion conductivity compared to c-LLZO. Different dopants are commonly used to stabilize c-LLZO under ambient conditions. Work described in this thesis examined c-LLZO where the cubic phase was stabilized by aluminum (Al 3+) substituting for lithium or either tantalum (Ta 5+) or niobium (Nb 5+) substituting for zirconium. Material stability for each doped sample was quantified using vibrational Raman spectroscopy (including spatially resolved Raman mapping), thermal gravimetric analysis (TGA), and X-ray diffraction (XRD). Results showed that heating Al-doped c-LLZO to 1000 °C leads to lithium volatilization and formation of a lithium deficient secondary phase, lanthanum zirconate (La 2Zr 2O 7 or LZO). A characteristic radial pattern showing the extent of LZO formation implies that lithium diffusion is much faster than Li 2O formation and volatilization. Ta-doped and Nb-doped c-LLZO are less thermally stable than Al-doped c-LLZO. These zirconium substituted materials lost more lithium when sintered and formed both LZO and La 2O 3. XRD experiments quantified the amount of LZO formed upon decomposition. After heating to 1000 °C with a 1-hour dwell at temperature, Al-, Ta- and Nb-doped samples showed 1.7, 13.1 and 17.9 % LZO content, respectively. A final study tested the role of post-sintering thermal treatment as a means of removing unwanted secondary materials and residues from the surface of c-LLZO. These last experiments compare the efficacy of thermal heating compared to mechanically polishing sintered c-LLZO samples in preparation for use as battery electrolytes. Electrochemical impedance spectroscopy measurements show a 1.7-fold increase in conductivity for samples that were thermally treated. Discoveries reported in this thesis should inform and guide processing strategies so that c-LLZO can be used effectively in new solid-state batteries.