Solid electrolyte-based solid-state batteries have been touted as one of the most promising next generation batteries with high energy densities and enhanced thermal safety [1,2]. Lithium-rich garnets, LLZO (Lithium Lanthanum Zirconium Oxide have both high room temperature ionic conductivity and suitable electrochemical stability making them an ideal solid electrolyte choice for solid-state batteries [3]. Undoped LLZO crystallises in a tetragonal lattice, I4₁/acd at room temperature and has a relatively low ionic conductivity (~ 10^-2-10^-3 mScm^-1). Doping the LLZO lattice with multivalent atoms such as Al and Ga, results in the stabilisation of highly conducting cubic phase, Ia3d (or I43d) at RT wherein ionic conductivities > 0.1 mScm^-1 have been reported for both Al and Ga-doped LLZO [4]. Although numerous studies exist on characterizing the electrochemical properties, structure, and lithium diffusion in Al- and Ga-LLZO, the local structure and the site occupancy of dopants in these compounds remain under dispute. A range of ionic conductivities have been reported for LLZO having similar composition and the underlying factors behind these observations are also unclear.

Two broad ²⁷Al or ^69,71Ga resonances are often observed with chemical shifts consistent with tetrahedrally coordinated Al/Ga in the magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of both Al- and Ga-LLZO, which have been assigned to either Al and/or Ga occupying 24d and 96h/48g sites in the LLZO lattice or the different Al/Ga configurations that arise from different arrangements of Li around these dopants [5-8]. In this study, we unambiguously show that the side products γ-LiAlO₂ and LiGaO₂ lead to the high frequency resonances observed by NMR spectroscopy and that both Al and Ga only occupy the 24d site in the LLZO lattice. Furthermore, it will be shown that the excess Li often used during synthesis leads to the formation of these side products by consuming the Al/Ga dopants and leads to the tetragonal phase formation commonly observed in the literature, even after careful mixing of precursors. These side-products are found to remain even after sintering, thereby controlling the Al/Ga content in the LLZO lattice and substantially influencing the lithium-ion conductivity in LLZO [9]. Finally, necessary conditions to achieve > 1 mScm^-1 conductivity will be shown experimentally.