(Invited) Mechanisms of Stress Relaxation and Intensification at the Lithium/Solid Electrolyte Interface

Abstract

Under electrochemical cycling, stress intensification and relaxation within small volumes at the lithium/solid-state electrolyte (SSE) interface are thought to be critical factors contributing to mechanical failure of the SSE and subsequent short-circuiting of the device. Recent hypotheses independently proposed by Porz et al.,[1] Herbert et al.[2,3] and Wang et al.[4] consider the relationship between stress intensification and local variations in the current density. These arguments are examined in light of nanoindentation studies performed in high-purity vapor deposited lithium films at indentation depths less than 1 μm.[2, 3] These experimental studies examine how lithium responds to stress and stress gradients at length scales and strain rates relevant to the observed SSE failure. Applying the findings of the nanoindentation studies, we present a mathematically tractable and transparent model that relates a two-dimensional state of stress at the lithium/SSE interface to the average current density, the length scale of an interfacial defect and lithium diffusional creep and/or dislocation plasticity stress relaxation mechanisms. Furthermore, the approach presents a common framework to compare and contrast the most recent hypotheses of SSE stress development.[1-4] Accordingly, the model addresses interfacial defects in the form of local deviations from a flat interface (non-planar) and local variations in the overpotential and/or exchange current density (planar interface). Through this simple elastic perturbation model, we describe a defect length scale that is too large for effective Nabarro-Herring diffusional creep relaxation (diffusion length is too long), but too small for efficient dislocation multiplication (insufficient room to physically operate the source), suggesting the possibility of large, deleterious stresses at the lithium/SSE interface. In this instance, the properties of the SSE may become critical in relieving pressure and mitigating the stress intensification enabling mode I or mixed-mode fracture of the SSE. Such concerns have motivated a nanoindentation creep study of Lipon performed with a diamond Berkovich indenter tip. The indentation study shows that Lipon does have a stress relaxation process with a stress exponent, n, of approximately 20, which is found to be effectively independent of temperature and length scale over the respective ranges of 25 to 60 °C and 400 to 600 nm of indentation depth. The uncommonly high magnitude of n suggests molecular units of Lipon (3 Li+ and a unit of PO4 -3) are capable of local rearrangement by short-range diffusion and, thereby, potentially capable of contributing directly to stress relief at the interface. This suggests Lipon may be less susceptible to failure by fracture. [1] L. Porz, T. Swamy, B.W. Sheldon, D. Rettenwander, T. Frömling, H.L. Thaman, S. Berendts, R. Uecker, W.C. Carter, Y.-M. Chiang, Mechanism of Lithium Metal Penetration through Inorganic Solid Electrolytes, Advanced Energy Materials 7(20) (2017). [2] E.G. Herbert, S.A. Hackney, N.J. Dudney, V. Thole, P.S. Phani, Nanoindentation of high purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow, J. Mater. Res. 33(10) (2018) 1347-1360. [3] E.G. Herbert, S.A. Hackney, N.J. Dudney, V. Thole, P.S. Phani, Nanoindentation of high purity vapor deposited lithium films: A mechanistic rationalization of the transition from diffusion to dislocation-mediated flow, J. Mater. Res. 33(10) (2018) 1361-1368. [4] M. Wang, J. Wolfenstine, J. Sakamoto, Temperature dependent flux balance of the Li/Li7La3Zr2O12 interface, Electrochimica Acta 296 (2019) 842-847.