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Tantalum‐doped lithium lanthanum zirconate garnet (Li_7−xLa₃Zr_2−xTa_xO₁₂[LLZTO]) has received interest as a solid electrolyte for solid‐state lithium batteries due to its good electrochemical properties and ionic conductivity. However, the source of discrepancies for reported values of ionic conductivity in nominally or nearly equivalent compositions of LLZTO is not completely clear. Herein, synthesis‐related factors that may contribute to the differences in performance of garnet electrolytes are systematically characterized. The conductivity of samples with composition Li_6.4La₃Zr_1.4Ta_0.6O₁₂prepared by various methods including solid‐state reaction (SSR), combustion, and molten salt synthesis is compared. Varying levels of elemental inhomogeneity, comprising a variation in Ta and Zr content on the level of individual LLZTO particles, are identified. The elemental inhomogeneity is found to be largely preserved even after high‐temperature sintering and correlated with reduced ionic conductivity. It is shown that the various synthesis and processing‐related variables in each of the preparation methods play a role in these compositional variations, and that even LLZTO synthesized via conventional, high‐temperature SSR can exhibit substantial variability in local composition. However, by improving reagent mixing and using LLZTO powder with low agglomeration and small particle size distribution, the compositional uniformity, and hence, ionic conductivity, of sintered garnet electrolytes can be improved. 

The guest‐free, type‐II Si clathrate (Si₁₃₆) is an open cage polymorph of Si with structural features amenable to electrochemical Li storage. However, the detailed mechanism for reversible Li insertion and migration within the vacant cages of Si₁₃₆is not established. Herein, X‐ray characterization and density functional theory (DFT) calculations are used to understand the structural origin of electrochemical Li insertion into the type‐II clathrate structure. At low Li content, instead of alloying with Si, topotactic Li insertion into the empty cages occurs at ≈0.3 V versus Li/Li⁺with a capacity of ≈231 mAh g⁻¹(corresponding to composition Li₃₂Si₁₃₆). A synchrotron powder X‐ray diffraction analysis of electrodes after lithiation shows evidence of Li occupation within the Si₂₀and Si₂₈cages and a volume expansion of 0.22%, which is corroborated by DFT calculations. Nudged elastic band calculations suggest a low barrier (0.2 eV) for Li migration through interconnected Si₂₈cages, whereas there is a higher barrier for Li migration into Si₂₀cages (2.0 eV). However, if Li is present in a neighboring cage, a cooperative migration pathway with a barrier of 0.65 eV is possible. The results show that the type‐II Si clathrate displays unique electrochemical properties for potential applications as Li‐ion battery anodes.