Toward Understanding the Reactivity of Garnet-Type Solid Electrolytes with H₂O/CO₂ in a Glovebox Using X-ray Photoelectron Spectroscopy and Electrochemical Methods

Abstract

Chemical stability of garnet-type lithium ion conductors is one of the critical issues in their application in all-solid-state batteries. Here, we conducted quantitative analysis of impurity layers on the garnet-type solid electrolytes, Li_6.5La_3-<i>x</i>AE_<i>x</i>Zr_1.5-<i>x</i>Ta_0.5+<i>x</i>O₁₂ (<i>x</i> = 0 and 0.1; AE = Ca, Sr, and Ba), by means of X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Two complimentary XPS techniques were employed: (i) background analyses by Tougaard's method and (ii) relative intensity analyses of La 3d/La 4d spectra to determine the surface chemical composition. XPS revealed that even after cleaning by annealing and polishing, the surface is covered by LiOH- and Li₂CO₃-based compounds with a thickness of 4-6 nm within 30 min as a result of the reaction with traces of H₂O (<0.5 ppm) and CO₂ (<5 ppm) in an Ar-filled glovebox. The sensitivity to H₂O and CO₂ depends on the basicity of dopants. Ba-doped solid electrolytes exhibited the thickest impurity layers compared to Sr- and Ca-doped compounds. A surface cleaning process, consisting of annealing and polishing, effectively reduces the charge-transfer resistance to 10-15 Ω cm² because of negligible impurity layers. Highest short-circuit tolerance is obtained for a 700 °C annealed specimen (critical current density: 0.5 mA cm^-2), which is possibly due to the strengthened grain boundaries by Li₂CO₃ among grains around its melting point.