Is Nitrogen Present in Li₃N·P₂S₅ Solid Electrolytes Produced by Ball Milling?

Nitrogenous solid electrolytes such as lithium phosphorus oxynitride (LiPON) have effectual interfacial compatibility with lithium metal; in part, this has enabled the development of thin-film solid-state batteries with excellent long-term cycling performance. However, most known nitrogen-containing solid electrolytes lack the ionic conductivity required for high-power/high-capacity batteries; therefore, the development of new nitrogenous solid electrolytes with increased ionic conductivity is highly desirable. The mechanical milling of lithium nitride (Li₃N) with phosphorus pentasulfide (P₂S₅) has previously been reported to produce amorphous lithium-ion conductors, but the composition of these materials and the reactions occurring during the milling processes were hitherto undetermined. Here, we show that mechanochemically milled Li₃N·P₂S₅ solid electrolytes contain less nitrogen than expected as N₂ gas is released during an early stage of the ball milling process. Li₃N·P₂S₅ solid electrolytes are mixtures composed of multiple lithium thiophosphates, lithium sulfide, and red phosphorus. We show that amorphous Li₃PS₄ is responsible for the ionic conductivity of Li₃N·P₂S₅ electrolytes produced by ball milling.