The origin of high electrolyte-electrode interfacial resistances in lithium cells containing garnet type solid electrolytesElectronic supplementary information (ESI) available. See DOI: 10.1039/c4cp02921f

Dense LLZO (Al-substituted Li 7 La 3 Zr 2 O 12 ) pellets were processed in controlled atmospheres to investigate the relationships between the surface chemistry and interfacial behavior in lithium cells. Laser induced breakdown spectroscopy (LIBS), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, synchrotron X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS) studies revealed that Li 2 CO 3 was formed on the surface when LLZO pellets were exposed to air. The distribution and thickness of the Li 2 CO 3 layer were estimated by a combination of bulk and surface sensitive techniques with various probing depths. First-principles thermodynamic calculations confirmed that LLZO has an energetic preference to form Li 2 CO 3 in air. Exposure to air and the subsequent formation of Li 2 CO 3 at the LLZO surface is the source of the high interfacial impedances observed in cells with lithium electrodes. Surface polishing can effectively remove Li 2 CO 3 and dramatically improve the interfacial properties. Polished samples in lithium cells had an area specific resistance (ASR) of only 109 Ω cm 2 for the LLZO/Li interface, the lowest reported value for Al-substituted LLZO. Galvanostatic cycling results obtained from lithium symmetrical cells also suggest that the quality of the LLZO/lithium interface has a significant impact on the device lifetime. High interfacial resistance in Li 7 La 3 Zr 2 O 12 (LLZO)/Li cells is correlated with the presence of Li 2 CO 3 on LLZO surfaces.