Enhanced strength and temperature dependence of mechanical properties of Li at small scales and its implications for Li metal anodes
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 (SPEs) 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 limited because of its extreme reactivity, which renders sample preparation, transfer, microstructure characterization, and mechanical testing extremely challenging. We conduct nanomechanical experiments in an in situ scanning electron microscope and show that micrometer-sized Li attains extremely high strengths of 105 MPa at room temperature and of 35 MPa at 90 °C. We demonstrate that single-crystalline Li exhibits a power-law size effect at the micrometer and submicrometer length scales, with the strengthening exponent of −0.68 at room temperature and of −1.00 at 90 °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.
Additional Information© 2017 National Academy of Sciences. Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved November 28, 2016 (received for review September 22, 2016). Published online before print December 19, 2016, doi: 10.1073/pnas.1615733114 We thank Dr. Chi Ma for EBSD assistance and useful discussions. We gratefully acknowledge the financial support of the US Department of Energy through J.R.G.'s Early Career Grant DE-SC0006599. This work was supported by National Science Foundation CAREER Award CBET-1554273 (to V.V.). Author contributions: C.X., Z.A., V.V., and J.R.G. designed research; C.X. and Z.A. performed research; A.A. contributed new reagents/analytic tools; C.X., Z.A., V.V., and J.R.G. analyzed data; and C.X., Z.A., V.V., and J.R.G. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1615733114/-/DCSupplemental.
Published - PNAS-2017-Xu-57-61.pdf
Submitted - 1606.05826.pdf
Supplemental Material - pnas.201615733SI.pdf