Realization of all-solid-state
batteries combined with metallic
Li/Na is still hindered due to the unstable interface between the
alkali metal and solid electrolytes, especially for highly promising
thiophosphate materials. Artificial and uniform solid-electrolyte
interphases (SEIs), serving as thin ion-conducting films, have been
considered as a strategy to overcome the issues of such reactive interfaces.
Here, we synthesized sulfide-based artificial SEIs (Li
x
S
y
and Na
x
S
y
) on Li and Na by solid/gas
reaction between the alkali metal and S vapor. The synthesized films
are carefully characterized with various chemical/electrochemical
techniques. We show that these artificial SEIs are not beneficial
from an application point of view since they either contribute to
additional resistances (Li) or do not prevent reactions at the alkali
metal/electrolyte interface (Na). We show that Na
x
S
y
is more porous than Li
x
S
y
, supported by (i)
its rough morphology observed by focused ion beam-scanning electron
microscopy, (ii) the rapid decrease of
R
interface
(interfacial resistance) in Na
x
S
y
-covered-Na symmetric cells with liquid electrolyte
upon aging under open-circuit potential, and (iii) the increase of
R
interface
in Na
x
S
y
-covered-Na solid-state symmetric cells
with Na
3
PS
4
electrolyte. The porous SEI allows
the penetration of liquid electrolyte or alkali metal creep through
its pores, resulting in a continuous chemical reaction. Hence, porosity
of SEIs in general should be carefully taken into account in the application
of batteries containing both liquid electrolyte and solid electrolyte.