Optimized Thin Film Processing of Sodium Mixed Oxy-Sulfide-Nitride Glassy Solid Electrolytes for All-Solid-State Batteries

Na4P2S7-6x O4.62x N0.92x (NaPSON)glassy solid electrolytes (GSEs) were prepared and testedfor their electrochemical properties and processability into thinfilms. The x = 0.2 composition (NaPSON-2) was foundto be highly conducting, non-crystallizable, largely stable againstNa-metal, and supports symmetric cell cycling up to >100 mu Acm(-2) without shorting. For these reasons, it wasprocessed into thin films drawn to 50 mu m and tested in symmetricand asymmetric cells. Measurements of the sodium ion conductivityusing symmetric cells demonstrated that the conductivity of NaPSON-2was unchanged by film forming. Galvanostatic cycling at 5 mu Acm(-2) of a 1.3 mm thick disc of NaPSON-2 showed stabilityover 450 h, while cycling a 50 mu m thin film of NaPSON-2 showeda very slow growth in the resistance. Cyclic voltammetry and X-rayphotoelectron spectroscopy of the NaPSON-2 thin film GSE revealedthat it did not react with Na-metal at its surface, but rather inthe bulk of the film, showing S, Na2S, and Na3P reaction products. The source of the surface stability was determinedto be the preferential segregation of trigonally coordinated nitrogen.These low-cost and easily processed NaPSON GSEs provide a system ofmaterials that could provide for significantly lower cost higher energydensity grid-scale batteries.