  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.
