Ionic conductivity and mechanical properties of the solid electrolyte interphase in lithium metal batteries

With the fullness of time, metallic lithium (Li) as an anode could become highly promising for high-energy-density batteries. Theoretically, using Li metal as the negative electrode can result in higher theoretical capacity and lower oxidation voltage and density than in current commercially available batteries. During the charge/discharge process, however, metallic Li shows unavoidable drawbacks, such as dendritic growth, causing capacity degradation and a solid electrolyte interphase (SEI) layer derived from the side reactions between the Li metal anode and the electrolyte, resulting in depletion of the electrolyte. The formation of a suitable SEI is crucial to avoid the side reactions at the interface by circumventing direct contact. Unavoidable dendritic growth at the Li metal anode can be controlled by its ionic conductivity. Furthermore, the SEI is also required as a mechanical reinforcement for withstanding the volume change and suppressing dendritic growth in the Li metal anode. A limiting factor due to complex SEI formation must be considered from the perspectives of chemical and mechanical properties. To further enhance the cycling performance of Li metal batteries, an in-depth understanding of the SEI needs to be achieved to clarify these issues. In this mini review, we focus on the SEI, which consists of various deposited components, and discuss its ionic conductivity and mechanical strength for applications in electric vehicles.