Solid-state batteries replace liquid electrolytes with solid ionic conductors — materials through which lithium ions can move without the fire risk and degradation of organic liquids. Sulfide glasses like Li₃PS₄ are among the best candidates: their ionic conductivities approach those of liquid electrolytes at room temperature. The standard explanation is that polyhedral rotations in the glass network — the PS₄ tetrahedra rocking and tilting — create transient pathways for lithium ions to hop between sites.
Manohar, Muñoz-García, and Pavone (arXiv 2602.22989, February 2026) simulate Li₃PS₄ glass with molecular dynamics and find a different mechanism. Lithium ions near isolated sulfur species — sulfur atoms bonded to only one or three phosphorus neighbors rather than sitting inside a complete PS₄ tetrahedron — move significantly faster than those near the intact polyhedral units. The atomic displacements are up to 1.7 times larger near these isolated sulfur sites.
The mechanism is geometric. An isolated sulfur atom presents a smaller coordination shell to a passing lithium ion than a full PS₄ tetrahedron does. The steric barrier is lower. The lithium ion can slide past without waiting for the tetrahedron to rotate out of the way. The polyhedral rotation mechanism is not wrong — it exists — but it is slower than the direct pathway through the gaps left by incomplete coordination.
The implication for materials design is counterintuitive. The best ionic conductor is not the most structurally perfect glass — it is the one with the most defects of the right kind. Isolated sulfur species are structural imperfections. They represent incomplete polymerization of the glass network. They are precisely what conventional design strategies would try to eliminate.
The fast path runs through the broken part of the structure.