Impact of Secondary Phases on the Electrochemical Performance of Li6.4La3Zr1.4Ta0.6O12 Garnet Solid Electrolytes through Grain Boundary Engineering

Fast-conducting Li₇La₃Zr₂O₁₂ (LLZO)-type garnet solid-state electrolytes face the considerable challenge of deleterious metallic dendrite formation during operation, with suggestions that this behavior may be linked to electronic conductivity effects. To examine in detail how electronic conductivity effects at the grain boundary can affect the electronic properties of cubic LLZO-type garnets, we report a family of Ta-doped LLZO garnets with a Mn-containing secondary phase, which is spatially selective toward the grain boundaries. The inclusion of this targeted grain boundary phase, whose composition is revealed as La₄LiMnO₈, alters the ionic and local electronic conductivities of the final composite, resulting in improvements in the observed critical current densities. We find that the critical current density before short-circuiting is highly dependent on this secondary phase, increasing with increasing content up to a maximum of 0.30 mA cm^–2. X-ray absorption spectroscopy and X-ray diffraction computed tomography studies complement these findings, revealing that a darkening of the composite electrolyte post cycling is accompanied by Mn reduction and a reduction in the phase fraction of La₄LiMnO₈. Guided by electrochemical characterization and finite element analysis, we highlight the critical role of grain boundaries in bulk garnet degradation pathways and evidence how spatially targeted secondary phases, introduced during initial synthesis, can impact electrochemical performance in LLZO-type garnets.