In Situ Transmission Electron Microscopy Investigation of All Solid-state Sodium Batteries

All solid-state batteries (ASSBs) utilizing metal anodes such as lithium and sodium hold great promise for achieving high energy and power density, surpassing the safety limitations associated with liquid-electrolyte counterparts. However, the development of commercially viable ASSBs operating at room temperature remains limited. This is primarily due to the sluggish kinetics and solid-solid interfacial issues that impede the performance of batteries. Among the various interfacial challenges, the growth of dendritic structures leading to cell failure is a persistent problem that cannot be mitigated solely by the initially anticipated high elastic modulus of solid electrolytes (SEs) for ASSBs. Despite significant progress in understanding the filamentary growth mechanism in lithium metal based ASSBs using inorganic SEs, the understanding of sodium ASSBs remains far from complete.
To gain insights into the microstructural influences on sodium filament growth and Na+ ion transport, polycrystalline Na-β′′-alumina SE was employed as a model material due to its outstanding stability with Na metal. In this work, in situ biasing transmission electron microscopy (TEM) measurements were conducted to realize the cathodic sodium deposition at the interface between the Na-β′′-alumina and the electrode, as well as grain boundaries (GBs) within Na-β′′-alumina TEM lamellas. ... mehrBased on orientation analysis and composition distribution, the layered crystal structure induces anisotropic Na+ ion transport under the electric field, significantly facilitating the blockade of Na+ ion transport at some GBs and consequently influencing the position of Na filament growth. Furthermore, the microstructural evolution of the Au interlayer, which is believed to protect against dendrite growth, was explored during the inhomogeneous sodium deposition using the same in situ biasing TEM setup. Notably, while Na-Au alloy particle forms by cathodic sodium deposition, Na-Au interdiffusion occurs at the interface, rather than solely sodium diffusion along the Au interlayer. Sodium diffusion along the Au interlayer leads to alloy formation, while the diffusion of Au towards the sodium deposition site may result in the redistribution of the Au interlayer. Additionally, the Au interlayer exhibits distinct behavior under different conditions, e.g. different bias voltages and layer morphology including the Au interlayer thickness and gap between Au particles.
In addition to investigating interfacial issues in sodium metal based ASSBs, the study on the influence of scanning electron microscopy (SEM) imaging and focused ion beam (FIB) processing on the SEs was conducted to ensure the reliable preparation of TEM samples for in situ TEM measurements. The irradiation damage mechanism and the corresponding solution were understood during this investigation.