Electrochemical Stability of Electrode/Electrolyte Interfaces in Metal Borohydride Solid Electrolytes

Meeting the promise of all-solid-state batteries will require solid electrolytes that have high metal ion conductivity, low electronic conductivity, are electrochemically stable at both low and high potential, and have sufficiently high critical current density to avoid metal dendrite penetration. Metal borohydrides comprise a relatively new class of compounds that is being explored for its potential as a solid electrolyte material. Here, we examine the electrochemical thermodynamic stability of sodium amide borohydride (Na 2 NH 2 BH 4 , or NNB) over the potential range 1.5 to 5 V vs. Na/Na + , using electrodynamic methods, X-ray photoelectron spectroscopy (XPS) characterization of the interfacial reaction products 1 , and electron microscopy characterization of microstructure. We observe the formation of a CEI layer that passivates the NNB electrolyte up to 5V, while the electrolyte is stable without needing additional passivation down to potentials as low as 1.5V. The onset potentials of the CEI formation reactions agree with those predicted by DFT 2 and suggest that the resulting CEI is composed of closo-borohydrates, which are a type of metal borohydride with a cage-like structure that holds promise for high sodium ion conductivities 3 . XPS measurements of atomic compositions in the CEI layer show the formation of these closo-borohydrates and, coupled with charge transfer calculations, predict that the passivating CEI layer is thin (tens of nm). Impedance spectroscopy is performed to characterize the contribution of the CEI layer to the total cell resistance. Several formation cycling methods are explored to show that with appropriate cycling, the passivating layers contribute