Mechanics of Lithium Metal at Nanoscale by EBSD Coupled Nanoindentation

The fracture of ceramic solid electrolytes, driven by the plating of lithium within cracks, has been identified as one of the fundamental issues to resolve to successfully develop solid-state batteries[1]. Understanding the mechanics of lithium at the nanoscale is therefore essential. In this work, the elastic and plastic properties of lithium are measured by Berkovich nanoindentation within an enclosed argon glovebox system. Lithium is particularly challenging to study due to its high reactivity and softness, preventing the use of traditional surface preparation methods. Using a novel surface preparation methodology, lithium metal samples are characterised by electron backscattered diffraction (EBSD) before and after indentation to allow the dependence of the mechanical properties on crystallographic orientation to be measured, and the stiffness tensor components, moduli and poisons ratio to be independently determined using a method first proposed by Vlassak and Nix[2]. The measured stiffness tensor components are C_11 = 13.3 GPa, C12 = 11.2 GPa and C44 = 8.8 GPa. These values agree with earlier acoustic works on lithium single crystals within 2%[3]. Hardness measurements show anisotropy according to Schmid’s law, with a clear size effect which may explain observed lithium filament propagation. The experiments were repeated across lithium magnesium solid solution alloys from 5 to 20 atomic percent to investigate the change in elastic and plastic properties with solute content.<br/><br/>[1] E. Kazyak, R. Garcia-Mendez, W.S. LePage, A. Sharafi, A.L. Davis, A.J. Sanchez, K.H. Chen, C. Haslam, J. Sakamoto, N.P. Dasgupta, Li Penetration in Ceramic Solid Electrolytes: Operando Microscopy Analysis of Morphology, Propagation, and Reversibility, Matter. 2 (2020) 1025–1048. https://doi.org/10.1016/j.matt.2020.02.008.<br/>[2] J.J. Vlassak, W.D. Nix, Measuring the Elastic Properties of Materials By Means of Indentation, J. Mech. Phys. Solids. 42 (1994) 1223–1245.<br/>[3] T. Slotwinski, J. Trivisonno, Temperature dependence of the elastic constants of single crystal lithium, J. Phys. Chem. Solids. 30 (1969) 1276–1278. https://doi.org/10.1016/0022-3697(69)90386-2.