Miaofang Chi, Oak Ridge National Laboratory
Electron Microscopy for All-Solid-State Batteries—Addressing Challenges at Atomic Scale
Written by Aashutosh Mistry
Battery science has seen a fundamental shift in recent years toward the use of solid electrolytes. Conventional liquid electrolytes cause a conformal contact at the electrode–electrolyte interface, which switches to intrinsically nonconformal solid–solid contact in the newer setting. However, the issues with the solid electrolytes extend beyond this geometrical nonconformity, and in order to investigate the fundamental origins of electrochemical limitations the state-of-the-art characterization tools are insufficient since they were designed explicitly for solid–liquid electrochemical contact. Thus, there is an urgent requirement to invent (or modify) a host of characterization techniques for this new class of electrochemistry.
Miaofang Chi and her colleagues at Oak Ridge National Laboratory have been advocating the use of in situ functional imaging—electron-microscopy techniques posing requisite scientific merit. She emphasized that the primary bottleneck in solid-state electrochemistry is the myriad set of interfacial phenomena, namely, instability, impurity, elemental diffusion, and space charge, for instance. In one of the examples, the researchers explored the grain boundary resistivity for two different solid electrolytes (LLTO and LLZO, lithium lanthanum titanium oxide and lithium lanthanum zirconium oxide) and found that the LLZO grain boundaries exhibit similar chemical composition as the grains, while LLTO grain boundaries are lithium-deficient which explains a greater grain boundary resistance in LLTO. Another puzzling observation had been the relative stability of Li–LiPON (another solid electrolyte) interface. The researchers’ in situ observation of Li and LiPON contact showed that a chemical reaction takes place upon contact between the two solids which results in a new interphase between the two. This interphase is stable over the electrochemical operation and is the root cause for extended cycling. Upon elemental mapping using EELS (electron energy-loss spectroscopy), it was revealed that this interphase represents the decomposition of LiPON into Li3N, Li2O, and Li3P. The developed techniques are suitable for different types of 4D (space and time) in situ investigations of solid–solid electrochemistry.