Lithium dendrites are protruding growths that form via ionic flow into and out of the lithium metal throughout the use cycle of a battery. Many more changes occur in the cell during this process, including increase of pressure and temperature, which have been shown to alter the mechanics of the materials inside the battery. The increase of pressure deforms the lithium metal electrode, which can ultimately enhance dendritic plating of lithium and destruction of the electrolyte. Within the last decade, theory on threshold mechanical stiffness ratios between lithium and an electrolyte inside battery cells has been widely accepted as the preventative measure against lithium dendrite propagation. This theory assumes that materials behave as linear elastic. However, recent work in electrochemical-mechanical modeling of solid state batteries has begun to employ not only elastic properties but also plastic and viscoelastic material properties. Complete confirmation of the elastic-viscoplastic modeling work will greatly impact solid state battery research and development. This work expands on the characterization and verification of predicted lithium mechanical behavior.

Recent characterization work has shown that, in an inert environment, polycrystalline lithium deforms primarily though creep at room temperature and at a common battery cell pressure. Specifically, lithium tensile specimen deformed more than ten percent strain at room temperature and 0.43 MPa. Modeling work has derived analytical solutions for this elastic-viscoplastic behavior of lithium, which not only line up with the characterization experiments but also predict more temperature, strain rate, and large deformation behavior. This study validates the areas in this model which lack overlapping experimental data. Furthermore, it shows that this behavior does not significantly change during exposure to conventional cell-assembly environments.

Specifically, this study conducts the same multiple-temperature strain ramp experiments with the same lithium specimen used in prior work with the help of a Dynamic Mechanical Analysis (DMA) apparatus in a commercial-grade dry room. This supports that even with variation across lots of lithium metal from the same suppler and between an inert and a dry air environment, the major mode of deformation in lithium is creep, and the activation energy does not change. Lithium metal in batteries is often exposed to this environment due to high volume manufacturing in standard dry rooms. It is crucial to realize the minimal effect of this environment on the mechanics. Next, the same experimental setup is employed for further analysis on large deformation at room temperature at multiple strain rates. This validates the predictions made by recent models and enables the next steps to be sought after to develop dendrite prevention strategies in solid state batteries.


Elastic-Viscoplastic Mechanics of Lithium in a Standard Dry Room

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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