Electro-Chemo-Mechanical Evaluation of Garnet Surface Treatments

Solid state batteries (SSBs) are predicted to be a step-change improvement over their liquid predecessors due to enhanced performance over a wider temperature range, safety, and theoretical compatibility with lithium metal anodes¹. However, significant challenges must be overcome before SSBs can be commercially realized. One such issue is the interfacial stability between lithium metal and the solid electrolyte (SE). For instance, Li_6.6La₃Zr_1.6Ta_0.4O₁₂ (LLZO) has been widely studied as a SE which is predicted to be stable with lithium metal²; however, the surface of LLZO is often contaminated during sample preparation resulting in high interfacial resistance and diminished cell performance. This interfacial resistance is often attributed to a hydroxide and carbonate contamination layer on the surface that leads to poor lithium wetting³. Mechanical polishing, heating⁴, and acid treatment⁵ have all been investigated to reduce the interfacial resistance and restore a pristine LLZO surface. Here, we present a holistic study on the electro-chemo-mechanical impact of each of these treatments to elucidate both the primary sources of contamination and the optimal surface treatment methods; furthermore, while many studies have investigated individual treatments or contaminants, we additionally examine the highly detrimental impacts of surface lithium inventory (SLI) changes during surface treatments.<br/><br/>To address this challenge, we use synchrotron ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to directly quantify the evolution of carbonate formation during controlled gas exposure and subsequent contaminant removal during heating steps. We also monitor the lithium peak intensity to track how the SLI changes due to lithium uptake during contamination formation and lithium loss during heating.<br/><br/>We examine the impact of mechanical polishing on LLZO performance compared to pristine LLZO surfaces prepared via fracturing or Argon ion milling. Scanning electron microscopy (SEM) and lab-scale XPS indicate significant morphological and chemical changes are introduced during polishing that impact all downstream treatments and LLZO behavior in subsequent characterization techniques; the polished surface exhibits altered performance compared to pristine surfaces. This level of surface sensitivity is often overlooked yet is essential for accurate SE characterization.<br/><br/>Finally, to further demonstrate the importance of SLI, we immerse contaminated LLZO pellets in 1M HCl for various times to remove the surface contamination layer. We then examine the surfaces via SEM and lab-scale XPS to monitor the morphology and chemical structure of the surface, respectively. These results reveal stark morphological changes and vastly decreased SLI with increasing acid exposure time. Using electrochemical impedance spectroscopy studies on the same samples, we corroborate the variations in SLI which are manifested in significant bulk impedance changes in the collected spectra that have not yet been reported in literature. We explore additional solvent exposure experiments to understand the impact of protic and aprotic solvents on surface morphology and SLI. We make use of our unique inert atmosphere in-<i>situ</i> atomic force microscopy setup, outfitted with a potentiostat-enabled conductive probe, to investigate the LLZO surface. This enables us to perform local lithium plating experiments to probe inhomogeneous deposition behavior and correlate results to surface treatments. Taken together, this fleet of experiments uniquely uncovers the electro-chemo-mechanical behavior of LLZO during both contaminant formation and removal.<br/><br/>References:<br/>1. Janek, J., Zeier, W. Nature Energy, 2016, 1, 16141<br/>2. Hofstetter, K <i>et al.</i> J. Power Sources, 2018, 390, 297-312<br/>3. Sharafi, A. <i>et al.</i> J. Mater. Chem. A, 2017, 5, 13475-13487<br/>4. Sharafi, A. <i>et al.</i> Chem. Mater. 2017, 29, 18, 7961–7968<br/>5. Huo, H. <i>et al.</i> Nano Energy. 2019, 61, 119-125