Elastic-Viscoplastic Mechanics of Lithium in a Standard Dry Room

Abstract

In electrochemical-mechanical modeling of solid-state batteries, there is a lack of understanding of the mechanical parameters and mode of deformation of lithium metal. Understanding these characteristics is crucial for predicting the propagation of lithium dendrites through the electrolyte — a key element of battery safety. Past theories have assumed linear elastic as well as elastic-plastic deformation of lithium. However, recent experiments show that the primary mode of deformation is creep. This study replicates the temperature dependent mechanical experiments but inside an industrial dry room, where battery cells are manufactured at high volume. Furthermore, this work conducts time dependent studies — also inside the dry room — to gain insight of the large deformation theories of lithium metal. The results confirm the activation energy, which dictates the creep mechanism, is correlated to core diffusion rather than lattice diffusion.

Creep and Anisotropy of Free-Standing Lithium Metal Foils in an Industrial Dry Room

“…The other option is creep by lattice diffusion, which has a reported activation energy value of 50 kJ/mol [3]. The values of the activation energy obtained in the dry room environment in this study for 0.10 mm thick lithium as well as in previous work for 0.75 mm foil is significantly smaller than reported value for activation through lattice diffusion [3,4]. Furthermore, the activation energy values are not a function of crystallographic orientation in the plane of the foil, as the activation energies are comparable between MD and TD in the 0.10 mm lithium foil.…”

“…[4], the activation energy (37.6 ± 7.9 kJ/mol) nearly matches the previously recorded value (37.0 ± 6.0 kJ/mol) [3]. Combining these results with prior mechanical data at various temperatures, the power law creep exponents and activation energies that are both similar and significantly below those of lattice diffusion (50 kJ/mol) lead us to the conclusion that the dry room environment in these experiments does not play a major role in the bulk time and temperature-dependent mechanics of the lithium foil [4]. Further supporting this claim, SEM was done on the 0.10 mm lithium foil that was sitting in the dry room for 1 day as well as the same foil that was brought outside of the dry room.…”

“…The setup and procedure described in Ref. [4] were used in this study for the temperaturedependent experiments. The 0.10 mm lithium foil was cut into rectangular strips 10 mm wide and were tensile tested at a strain rate of 3 × 10 −5 s −1 at the temperatures 273 K, 298 K, 348 K, and 398 K. Lithium strips were cut to have a tensile gage length of 25 mm.…”

“…However, the limitations of the preservation of pure lithium's mechanics in dry air were not determined. Reference [4] showed that 0.75 mm lithium foil's creep mechanism did not change in a dry room environment following mechanical tests at several temperatures. This work aims to determine the difference, if any, between the viscoplastic measurements of 0.10 mm lithium foil and the environment, including a directional study on 0.75 mm and 0.10 mm.…”

“…The first subsection delivers the mechanics of both 0.75 and 0.10 mm thick lithium metal at different strain rates and in both orthogonal directions, obtaining the power law creep exponent, m, per the orientation of the material. The creep activation energy of the 0.75 mm foil in the dry room environment is compared with previously calculated values [3,4]. The second subsection investigates the temperature-dependent mechanics of the 0.10 mm foil in its two orthogonal directions, calculating corresponding activation energies per orientation and per power law creep exponents measured in this study.…”