Investigation of Chemical and Thermal Stability of Li7−xLa3Zr2−xTaxO12 Garnet Type Solid‐State Electrolyte to Assemble Self‐Standing Li‐based All Solid‐State Battery

All solid‐state batteries (ASSB) are the next generation of safe and high‐energy‐density energy storage technology. Their development is currently impeded by the stability issues of solid‐state electrolyte (SSE) limiting the creation of interfaces of quality high enough to ensure efficient transfer of charges. The garnet Li7−xLa3Zr2−xTaxO12 (LLZO:Ta) is one of the most appealing oxide‐based SSE but its high sensitivity to moist air engenders difficulties in designing process to densify and assemble components of the ASSB. Based on a careful investigation of LLZO:Ta thermal and chemical stability, a heat treatment is designed and shown to fully reverse the protonation process. The investigation of spark plasma sintering protocols confirms that pretreated LLZO:Ta can be densified at a temperature as low as 850 °C or with a short duration (few seconds) at 900 °C. The characterization of obtained ceramics shows conductivities close to the bulk properties and confirms the absence of influence of grain boundaries. The study of composite electrodes shows that LLZO:Ta is suitable to act both as SSE separator and ionic percolator in positive electrode, the selection of active material remains the main issue to target self‐standing Li‐based ASSB. The elucidation of the reaction mechanism between Li7−xLa3Zr2−xTaxO12 and air allows proposing a simple process to reverse it. The study of LLZO:Ta stability shows that while it can be used as solid‐state electrolyte separator and ionic additive in the catholyte, the low stability of selected active material is the main issue to be solved to obtain self‐standing all solid state batteries.