Nano to macro-scale elastic and plastic characteristics of calcium metal and implications for rechargeable battery applications

Abstract

Multivalent metals (Ca, Mg, Al, etc.) are promising for anodes of rechargeable batteries owing to their high theoretical capacity that stems from multiple electron transfer per redox center. Among these multivalent metals, calcium exhibits a low electrochemical potential of −2.87 V (relative to the standard hydrogen electrode) and is the fifth most abundant element in the earth’s crust. In addition to resolving electrochemical issues, prior to practical use it remains critical to fully understand calcium’s mechanical properties to mitigate any potential degradation and failure mechanisms. To this end, we have conducted mechanical testing of Ca at the nano- and macro-scale through nanoindentation and bulk compression testing. Nanoindentation tests indicate an elastic modulus that ranges from 25.2 to 21.7 GPa and a hardness that ranges from 0.88 to 0.46 GPa as the indentation depth increases from 0.25 to 10 μm. Bulk compression tests show a yield strength of 107 ± 4.6 MPa (average ± standard deviation). These tests demonstrate a minimal sensitivity of calcium’s mechanical properties to strain rate or “size effects”, which differs from previous studies of alkali metal anodes, likely stemming from Ca’s relatively high melting point compared to the alkali metals. We conclude the manuscript by discussing the implications of these measured mechanical properties in the context of energy storage applications.