  A crucial ingredient in lithium (Li) and sodium (Na)-ion batteries (LIBs and
NIBs) is the electrolytes. The use of Li-metal (Na-metal) as anode in liquid
electrolyte LIBs (NIBs) is constrained by several issues including thermal
runway and flammability, electrolyte leakage, and limited chemical stability.
Considerable effort has been devoted toward the development of solid
electrolytes (SEs) and all-solid-state batteries, which are presumed to
mitigate some of the issues of Li-metal(Na-metal) in contact with flammable
liquid electrolytes. However, most SEs, such as Li$_3$PS$_4$, Li$_6$PS$_5$Cl
and Na$_3$PS$_4$ readily decompose against the highly reducing Li-metal and
Na-metal anodes. Using first-principles calculations we elucidate the stability
of more than 20 solid$||$solid interfaces formed between the decomposition
products of Li$_3$PS$_4$, Li$_6$PS$_5$Cl (and Na$_3$PS$_4$) against the
Li-metal (Na-metal) electrode. We suggest that the work of adhesion needed to
form a hetereogenous interfaces is an important descriptor to quantify the
stability of interfaces. Subsequently, we clarify the atomistic origins of the
resistance to Li-ion transport at interfaces of the Li-metal anode and selected
decomposition products (Li$_3$P, Li$_2$S} and LiCl) of SEs, via a high-fidelity
machine learned potential (MLP). Utilising an MLP enables nano-second-long
molecular dynamics simulations on `large' interface models (here with 8320
atoms), but with similar accuracy to first-principles approaches. Our
simulations demonstrate that the interfaces formed between Li-metal and
argyrodite (e.g., Li$_6$PS$_5$Cl) decomposition products are resistive to
Li-ion transport. The implications of this study are important since binary
compounds are commonly found in the vicinity of Li(Na)-metal upon chemical
and/or electrochemical decomposition of ternary and quaternary SEs.
