  Solid-state electrolytes, by enabling lithium metal anodes, may significantly
increase the energy density of current lithium-ion batteries. However, similar
to their liquid counterparts, these hard and stiff electrolytes can still be
penetrated by soft Li metal, above a critical current density (CCD). The
prevailing method to determine the CCD employs step-wise galvanostatic cycling,
which suffers from inconsistent active interfacial areas due to void formations
after repeated stripping and plating, leaving large variance in the reported
data that preclude precision understandings. Here, we combine a one-way
polarization technique with electrochemical impedance spectroscopy to uncover,
for the first time, the existence of significant polarization dynamics in
ceramic electrolytes. In contrast to the diverging transient current due to
metal penetration, the current peaks we observed suggest a diffusion-limited
mechanism that follows the classic Randles-Sevcik equation for analyzing the
diffusion-limited processes in liquid electrolytes. Our results allow a
rigorous self-consistent analysis to reveal that the CCD is a diffusion-limited
current density, while the system-specific limiting current density for ceramic
electrolytes is still lower than CCD, which suggests that the ion transport
mechanism preceding the dendrite penetration in ceramic electrolytes is
unifiable with that in liquid electrolytes.
