In Situ Li Growth Mechanism at Initial Stage by Liquid Phase Transmission Electron Microscopy

High energy density and cycling stability of Lithium-ion batteries(LIBs) are receiving great attention as LIBs have been essential goods in our life from portable devices to Electric Vehicles(EVs). Lithium metal anode material is one of the most promising options for next generation battery system. Although it has many advantages such as high theoretical capacity, low density and lowest standard electrochemical redox potential, irregular Li deposition on the anode is the biggest obstacle for practical application. Therefore, It is important to understand the dynamics of Li nucleation and growth in the early stages, because it has a decisive influence on uneven surface formation that determines the performance and safety of Lithium Metal Batteries(LMBs). In this aspect, the initial stage of Li plating in Ethylene carbonate/Diethyl carbonate (EC/DEC, 1M LiPF₆) and 1,3-Dioxolane/Dimethoxyethane (DOL/DME, 1M LiTFSI) are investigated by in situ liquid phase transmission electron microscopy. The formation of large Li particles accompanied by low nucleation density and rapid growth is observed in DOL/DME electrolyte, whereas small-sized Li particles are formed with features of high nucleation density and slow growth rate in EC/DEC electrolyte, and the difference in initial nucleation and growth is closely related to SEI structures determining its mechanical and kinetic property. Heterogeneously distributed ionic species in the organic-rich outer layer from EC/DEC induce structural instability and uneven Li⁺ flux, which leads to cracks on the SEI and low kinetics, revealing many nucleation sites and slow growth, respectively. In contrast, flexible components such as amorphous alkoxide and poly(DOL) and uniform multi-layered Li₂O on the surface in the SEI from DOL/DME improve structural stability and ionic conductivity, which facilitates less nucleation sites and fast growth rate. This results are expected to shed light on the understanding of Li dendrite growth in LMBs and rational design of the next-generation battery.