Probing Alkaline-Earth-Doped Garnet-Type Li7La2.75A0.25Zr1.75M0.25O12 (A = Ca, Sr, Ba; M = Nb, Ta) Electrolytes for All-Solid-State Li Metal Batteries

Solid (ceramic) electrolytes have demonstrated great potential for all-solid-state lithium metal batteries (ASSLMBs), providing better safety and stability over organic liquid electrolyte-based Li-ion batteries. A proper selection of solid electrolytes and an analysis of their compatibility with Li metal are critical for improving battery performance. Here, we present an extensive experimental study realizing the effect of substituting alkaline-earth-metal-doped Li7La2.75A0.25Zr1.75M0.25O12 (A = Ca, Sr, Ba; M = Nb, Ta) garnet-type electrolytes on the ionic and electronic conductivity, Li-ion migration pathways, microstructures, and electrochemical stability properties. We examined the relationship between the bulk and surface properties of garnet solid electrolytes with Li using X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. Among all of the garnets studied, Ba and Ta codoped Li7La2.75Ba0.25Zr1.75Ta0.25O12 showed better chemical stability against Li with an area specific resistance (ASR) of ∼33 Ω cm2 and a critical current density of 0.5 mA cm-2 at 25 °C without any surface coating. At −30 and 50 °C, the cell can cycle up to 50 times with almost negligible voltage fluctuations. This work illustrates that doping could trigger ion-/electron-transport properties and improve the chemical stability of the garnet structure for ASSLMBs.