  Solid electrolytes are considered a potentially enabling component in
rechargeable batteries that use lithium metal as the negative electrode, and
thereby can safely access higher energy density than available with today's
lithium ion batteries. To do so, the solid electrolyte must be able to suppress
morphological instabilities that lead to poor coulombic efficiency and, in the
worst case, internal short circuits. In this work, lithium electrodeposition
experiments were performed using single-crystal Li6La3ZrTaO12 garnet as solid
electrolyte layers to investigate the factors that determine whether lithium
penetration occurs through brittle inorganic solid electrolytes. In these
single crystals, grain boundaries are excluded as possible paths for lithium
metal propagation. However, Vickers microindentation was used to introduce
sharp surface flaws of known size. Using operando optical microscopy, it was
found that lithium metal penetration sometimes initiates at these controlled
surface defects, and when multiple indents of varying size were present,
propagates preferentially from the largest defect. However, a second class of
flaws was found to be equally or more important. At the perimeter of surface
current collectors, an enhanced electrodeposition current density causes
lithium metal filled cracks to initiate and grow to penetration, even when the
large Vickers defects are in close proximity. Modeling the electric field
concentration for the experimental configurations, it was shown that a factor
of 5 enhancement in field can readily occur within 10 micrometers of current
collector discontinuities, which we interpret as the origin of
electrochemomechanical stresses leading to failure. Such field amplification
may determine the sites where supercritical surface defects dominate lithium
metal propagation during electrodeposition, overriding the presence of larger
defects elsewhere.
