Sodium Expulsion From Sodium closo-Hydroborates Under Electron Beam-Induced Bias

Electron microscopy invites the opportunity to locally characterize the properties of solid-state electrolytes important for choreographing ion motion within next-generation solid-state batteries. Through the combination of inert gas transfer, cryogenic, time-resolved low-dose analytical electron microscopy, we observe that mixed-anion sodium closo-hydroborate, Na2(B10H10)0.5(B12H12)0.5 (NBH), solid electrolyte suffers from electrostatic charging damage under bias in both transmission electron microscopes and scanning electron microscopes (SEM). During the process of beam damage, crystalline ordering of NBH is lost, while particle morphology remains unchanged. In regions exposed to electron beams at a dose rate of 1-8 e- Å-2 s-1, spectroscopy confirms a vastly reduced sodium signal or even sodium absence from irradiated NBH at total doses less than 80 e- Å-2. Simultaneously, we detect the growth of sodium metal crystals on the unirradiated faces of NBH particles. An SEM experimental study revealed that the maximum damage rate occurs at 2 keV, with increasing energy prolonging the time before sodium is emitted. This work also highlights that cryogenic conditions, while highly effective in mitigating certain types of inelastic beam damage, are not the solution for reducing beam sensitivity. Rather, comprehensive awareness of the underlying physics in competing damage mechanisms is paramount in the selection of operating conditions for battery interface characterization.