(Invited) Understanding the Factors Affecting the Performance of Li-Metal/Garnet Interfaces with Surface Analysis Techniques

Abstract

Garnet-type electrolytes have received great attention in the development of the next generation of solid state batteries based on Li-metal anodes because of their adequate Li conductivity (greater than 1 mS cm-1 at room temperature)1–3 and stability against Li metal4. However, their applicability has been hindered due to major issues related to stability and degradation phenomena that need understanding and addressing. The wide-ranging literature and lack of consensus with respect to the performance of garnets, in terms of Li conductivity (mS-10mS reported), polarisation resistance with Li metal (kW-W) and critical current density for dendrite formation (mA-mA) can be correlated to a lack of consistency in controlling the processing conditions for the material. This has a knock-on effect on the microstructure, local chemical composition of bulk, surfaces and grain boundaries and can also induce degradation phenomena such as segregation of secondary phases or moisture reactivity. All these parameters have a direct impact on the Li-ion dynamics in these systems, leading to significant implications on the cell performance in terms of power density and cycle life. In this work, we use a combination of 6Li-isotopic labelling, surface analysis techniques and electrochemistry to understand factors limiting the performance of garnet electrolytes when using Li metal as an anode. Three main effects will be discussed here; first, we analyse the effect of Li-H exchange due to moisture reactivity on the Li mobility at the interface with Li metal and in the bulk of the electrolyte5; second, we study the effect of local chemical inhomogeneity on the critical current density for dendrite formation6 and finally, we analyse possible ways to reveal Li-ion mobilities within grain, grain boundaries and interfaces using 6Li isotopic labelling and in situ techniques. Our main results highlight the complexity of these systems and propose new in situ techniques to gain a deeper understanding of the processes taking place. In this regard, we introduce a novel and beyond state of the art dual secondary ion mass spectrometer (SIMS) aimed at innovative 3D chemical and microstructural analysis with in operando capabilities. (1) Bernuy-lopez, C. et al. Chem. Mater. 2014, 26, 3610–3617. (2) El Shinawi, H.; Janek, J. J. Power Sources 2013, 225, 13–19. (3) Rettenwander, D et al. Inorg. Chem. 2014, 53 (12), 6264–6269. (4) Thangadurai V., D. et al. J. Phys. Chem. Lett., 2015, 6, 292–299. (5) Brugge R. et al. Chem. Mater. 2018, 30, 3704 (6) Pesci F. et al. J. Mat. Chem A. 2018, 6, 19817