  Lithium thiophosphates (LPS) with the composition
(Li$_2$S)$_x$(P$_2$S$_5$)$_{1-x}$ are among the most promising prospective
electrolyte materials for solid-state batteries (SSBs), owing to their
superionic conductivity at room temperature ($>10^{-3}$ S cm$^{-1}$), soft
mechanical properties, and low grain boundary resistance. Several glass-ceramic
(gc) LPS with different compositions and good Li conductivity have been
previously reported, but the relationship between composition, atomic
structure, stability, and Li conductivity remains unclear due to the challenges
in characterizing non-crystalline phases in experiments or simulations. Here,
we mapped the LPS phase diagram by combining first principles and artificial
intelligence (AI) methods, integrating density functional theory, artificial
neural network potentials, genetic-algorithm sampling, and ab initio molecular
dynamics simulations. By means of an unsupervised structure-similarity
analysis, the glassy/ceramic phases were correlated with the local structural
motifs in the known LPS crystal structures, showing that the energetically most
favorable Li environment varies with the composition. Based on the discovered
trends in the LPS phase diagram, we propose a candidate solid-state electrolyte
composition, (Li$_{2}$S)$_{x}$(P$_{2}$S$_{5}$)$_{1-x}$ ($x\sim{}0.725$), that
exhibits high ionic conductivity ($>10^{-2}$ S cm$^{-1}$) in our simulations,
thereby demonstrating a general design strategy for amorphous or glassy/ceramic
solid electrolytes with enhanced conductivity and stability.
