Understanding and modifying strategies for lithium metal anode solid electrolyte interphase

The lithium metal anode (LMA) is considered the most promising anode material due to its high specific capacity and low electrode potential.The energy density is doubled by replacing the graphite anode in a lithium-ion battery with LMA. However, the formation of an unstable solid electrolyte interphase (SEI) and dendrite growth of LMA limit the practical applications of lithium metal batteries. A well-formed SEI can effectively passivate the lithium metal surface, preventing further reactions between LMA and electrolyte. However, the mechanical stability of the SEI formed in a general electrolyte is poor. The SEI ruptures due to the violent expansion and contraction of the LMA, resulting in continuous electrolyte consumption. Meanwhile, the inhomogeneous deposition of lithium metal can cause severe dendrite growth. Lithium dendrites have a high specific surface area, which increases active lithium and electrolyte consumption. Once the lithium dendrites pierce the separator, safety issues arise. Furthermore, the inhomogeneous stripping behavior of lithium metal can cause partially active lithium to lose its conductive connection with the substrate, resulting in a loss of reversible capacity. This results in lower coulombic efficiency and shorter cycle life for LMA. During the platting/stripping process, lithium ions first pass through the SEI. Therefore, the composition and structural distribution of the SEI have a decisive effect on the deposition/dissolution behavior of lithium. Understanding the composition and structure of SEI is crucial for solving the historical challenges of LMA. In recent years, researchers have made significant progress towords understanding SEI through advanced characterization and analysis tools, such as cryo-electron microscopy, titration gas chromatography, atomic force microscopy, and nuclear magnetic resonance. However, the conventional experimental methods developed so far have considerable spatial and temporal resolution limitations. It is difficult to understand the interfacial reaction mechanism and the effect of interfacial composition on ion transport using a single experimental method. A combination of experiments and theoretical calculations, such as density functional theory, can simulate the interfacial reaction and help understand the mechanism of interface formation. Molecular dynamics simulation can be used to predict the interfacial lithium-ion transport paths. The experimental process can be simplified using machine learning to screen the experimental design. In the future, the close integration of theoretical calculations and experiments will be a major driving force for developing high energy density lithium metal batteries. This review summarizes the characterization techniques and regulation strategies of SEI, such as electrolyte optimization design, artificial SEI, and current collector design. Among them, we focused on the electrolyte composition design, the most direct and efficient means. By changing the composition and ratio of the electrolyte, the interfacial reaction process can be directly affected to form a stable SEI, which inhibits the continuous consumption of active lithium and the growth of dendrites. Simultaneously, we introduced the current state of interphase research in lithium metal pouch cells. This review has a certain guiding significance for LMA toward practical applications.