Anisotropic Elastic Properties of Battery Anodes

Negative battery electrodes based on metals exhibit theoretical energy densities that surpass those of the intercalation-based anodes used in present-day Li-ion batteries. Nevertheless, metal anodes have a propensity to form dendrites during charging; thus, use of interfacial protection schemes or solid electrolytes (SE) may be necessary for these systems to be practical. The efficacy of these schemes are influenced by the interplay of the mechanical properties of the anode with those of the SE. To aid in the design of robust anode/SE interfaces, the present study employs Density Functional Theory calculations to assess the elastic properties of eight candidate anode materials: Li, Na, K, Ca, Mg, Zn, Al, and Si. The elastic constants, Young's modulus, and shear modulus are predicted as a function of temperature within the quasi-harmonic approximation. Anisotropy is assessed by resolving the moduli as a function of crystallographic direction. The alkali metals (Li, Na, and K) are predicted to have the smallest elastic moduli overall, and their moduli decrease with increasing atomic number. Regarding anisotropic behavior, Al and Mg are predicted to exhibit highly isotropic elastic properties, while the alkali metals are highly anisotropic. In the cubic systems, the crystallographic directions exhibiting extrema in the elastic properties are diametrically opposed: under axial loading the stiffest (most compliant) orientation is 〈111〉 (〈100〉), while in shear 〈100〉 (〈111〉) is the stiffest (most compliant). Importantly, the maximum anisotropic shear modulus of some metals is observed to be more than twice as large as their respective polycrystalline values. In these cases the polycrystalline properties may be a poor approximation to the elastic behavior. Accounting for this anisotropy, the resistance to dendrite initiation of several classes of solid electrolytes is discussed.