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

Na₄P₂S_7–6xO_4.62xN_0.92x (NaPSON) glassy solid electrolytes (GSEs) were prepared and tested for their electrochemical properties and processability into thin films. The x = 0.2 composition (NaPSON-2) was found to be highly conducting, non-crystallizable, largely stable against Na-metal, and supports symmetric cell cycling up to >100 μA cm^–2 without shorting. For these reasons, it was processed into thin films drawn to 50 μm and tested in symmetric and asymmetric cells. Measurements of the sodium ion conductivity using symmetric cells demonstrated that the conductivity of NaPSON-2 was unchanged by film forming. Galvanostatic cycling at 5 μA cm^–2 of a 1.3 mm thick disc of NaPSON-2 showed stability over 450 h, while cycling a 50 μm thin film of NaPSON-2 showed a very slow growth in the resistance. Cyclic voltammetry and X-ray photoelectron spectroscopy of the NaPSON-2 thin film GSE revealed that it did not react with Na-metal at its surface, but rather in the bulk of the film, showing S, Na₂S, and Na₃P reaction products. The source of the surface stability was determined to be the preferential segregation of trigonally coordinated nitrogen. These low-cost and easily processed NaPSON GSEs provide a system of materials that could provide for significantly lower cost higher energy density grid-scale batteries.