Li metal batteries suffer from dendrite formation which causes short circuit of the battery. Therefore, it is important to understand the chemical composition and growth mechanism of dendrites that limit battery efficiency and cycle life. In this study, in situ scanning electron microscopy was employed to monitor the cycling behavior of all-solid Li metal batteries with LiFePO4 cathodes. Chemical analyses of the dendrites were conducted using a windowless energy dispersive spectroscopy detector, which showed that the dendrites are not metallic lithium as universally recognized. Our results revealed the carbide nature of the dendrites with a hollow morphology and hardness greater than that of pure lithium. These carbide-based dendrites were able to perforate through the polymer, which was confirmed by milling the polymer using focused ion beam. It was also shown that applying pressure on the battery can suppress growth of the dendrites.

Dendrite formation, which could cause a battery short circuit, occurs in batteries that contain lithium metal anodes. In order to suppress dendrite growth, the use of electrolytes with a high shear modulus is suggested as an ionic conductive separator in batteries. One promising candidate for this application is Li7La3Zr2O12 (LLZO) because it has excellent mechanical properties and chemical stability. In this work, in situ scanning electron microscopy (SEM) technique was employed to monitor the interface behavior between lithium metal and LLZO electrolyte during cycling with pressure. Using the obtained SEM images, videos were created that show the inhomogeneous dissolution and deposition of lithium, which induce dendrite growth. The energy dispersive spectroscopy analyses of dendrites indicate the presence of Li, C, and O elements. Moreover, the cross-section mapping comparison of the LLZO shows the inhomogeneous distribution of La, Zr, and C after cycling that was caused by lithium loss near the Li electrode and possible side reactions. This work demonstrates the morphological and chemical evolution that occurs during cycling in a symmetrical Li–Li cell that contains LLZO. Although the superior mechanical properties of LLZO make it an excellent electrolyte candidate for batteries, the further improvement of the electrochemical stabilization of the garnet–lithium metal interface is suggested.

Lithium metal is a favorable anode material in all-solid Li-polymer batteries because of its high energy density. However, dendrite formation on lithium metal causes safety concerns. Here we obtain images of the Li-metal anode surface during cycling using in situ scanning electron microscopy. Constructing videos from the images enables us to monitor the failure mechanism of the battery. Our results show the formation of dendrites on the edge of the anode and isles of decomposed lithium bis(trifluoromethanesulfonyl)imide on the grain boundaries. Cycling at high rates results in the opening of the grain boundaries and depletion of lithium in the vicinity of the isles. We also observe changes in the surface morphology of the polymer close to the anode edge. Extrusion of lithium from these regions could be evidence of polymer reduction due to a local increase in temperature and thermal runaway assisting in dendrite formation.