Failure mechanisms of lithium metal anode and their advanced characterization technologies

Abstract

Although traditional graphite anodes ensure the cycling stability and safety of lithium-ion batteries, the inherent drawbacks, particularly low theoretical specific capacity (372 mAh·g−1) and Li-free character, of such anodes limit their applications in high energy density battery systems, especially in lithium-sulfur and lithium-air batteries. Lithium metal has been considered as one of the best next-generation anode materials due to its extremely high theoretical specific capacity (3860 mAh·g−1) and low redox potential (−3.04 V vs. the standard hydrogen electrode). The first generation of commercial rechargeable lithium metal batteries were developed by Moli Energy in the late 1980s and were not widely used due to several problems, including low coulombic efficiency, poor cycle stability, and safety hazards. These problems associated with the Li metal anode are mainly caused by lithium dendrite growth, electrode volume changes, and interface instability. During the charge and discharge processes, Li deposition is not uniform across the electrode surface. Due to the low surface energy and high migration energy of Li metal, dendrites are preferentially formed during Li deposition. These dendrites proceed to grow with successive battery cycling, penetrate the separator, and eventually reach the cathode, thereby causing short circuits and thermal runaway. Additionally, the growth of the lithium dendrite is inherently correlated with the reaction interface structure, and dendrite growth results in inhomogeneity of the SEI (solid electrolyte interface) which is inevitably formed on the Li metal surfaces. Moreover, the volume change of lithium metal anodes is of importance, particularly during battery cycling and Li stripping/deposition processes which make the SEI layers considerably unstable. SEI layers usually cannot withstand the mechanical deformation caused by volume changes; such layers continuously break and repair during cycling and consume large amounts of the electrolyte. Additionally, some Li dendrites could break and become wrapped by SEI layers to form electrically isolated “dead” Li, which results in the loss of active Li in the Li metal anode. All these factors are responsible for the failure of Li metal anodes. Herein, recent investigations on the failure mechanisms of lithium metal anodes are reviewed and summarized, including the formation of SEI layers on the surface of Li metal anodes, the behavior and mechanism of lithium dendrite growth, and the mechanism of “dead” lithium formation. Additionally, some advanced characterization techniques for investigating lithium metal anodes are introduced, including in situ tools, cryo-electron microscopy, neutron depth analysis technology, and solid state nuclear magnetic resonance technology. These techniques enable researchers to gain in-depth insights into the failure mechanisms of Li metal anodes.