Enhanced strength and temperature dependence of mechanical properties of Li at small length scales and its implications for Li metal anodes

TL;DR

This study reveals that micron-sized lithium exhibits exceptionally high strength and anisotropic elastic properties at small scales and elevated temperatures, providing insights into dendrite suppression challenges in lithium metal batteries.

Contribution

It provides the first nano-mechanical measurements of lithium's strength and elastic properties at small scales and different temperatures, highlighting size effects and anisotropy.

Findings

Micron-sized Li attains strengths of 105 MPa at room temperature and 35 MPa at 90°C.

Li exhibits a power-law size-effect with a strengthening exponent of -0.68 at room temperature.

Elastic and shear moduli of Li are highly anisotropic, varying by a factor of ~4.

Abstract

Most next-generation Li-ion battery chemistries require a functioning lithium metal (Li) anode. However, its application in secondary batteries has been inhibited because of uncontrollable dendrite growth during cycling. Mechanical suppression of dendrite growth through solid polymer electrolytes (SPE) or through robust separators has shown the most potential for alleviating this problem. Studies of the mechanical behavior of Li at any length scale and temperature are virtually non-existent because of its extreme reactivity, which renders sample preparation, transfer, microstructure characterization and mechanical testing prohibitively challenging. We conduct nano-mechanical experiments in an in-situ Scanning Electron Microscope and show that micron-sized Li attains extremely high strengths of 105 MPa at room temperature and of 35MPa at 90$^\circ$C. We demonstrate that single crystalline Li exhibits a power-law size-effect at the micron- and sub-micron length scales, with the strengthening exponent of -0.68 at room temperature and of -1.00 at 90$^\circ$C. We also report the elastic and shear moduli as a function of crystallographic orientation gleaned from experiments and first principles calculations, which show a high level of anisotropy up to the melting point, where the elastic and shear moduli vary by a factor of ~4 between the stiffest and most compliant orientations. The emergence of such high strengths in small-scale Li and sensitivity of this metal's stiffness to crystallographic orientation help explain why the existing methods of dendrite suppression have been mainly unsuccessful and have significant implications for practical design of future-generation batteries.