Nucleation and Growth Mechanism of Lithium Metal Electroplating

Understanding the mechanism of Li nucleation and growth is essential for providing long cycle life and safe lithium ion batteries or lithium metal batteries. However, no quantitative report on Li metal deposition is available, to the best of our knowledge. We propose a model for quantitatively understanding the Li nucleation and growth mechanism associated with the solid-electrolyte interphase (SEI) formation, which we name the Li-SEI model. The current transients at various overpotentials initiate the nucleation and growth of Li metal on bare Cu foil. The Li-SEI model considering a three-dimensional diffusion-controlled instantaneous process (J(3D-DC)) with the simultaneous reduction of electrolyte decomposition (J(SEI)) due to the SEI fracture is employed for investigating the Li nucleation and growth mechanism. The individual contributions of experimental and theoretical transient states, i.e., the fundamental kinetic values of diffusion coefficient (D), rate of nucleation (N-0), and rate constant of electrolyte decomposition (k(SEI)), can be determined from the Li-SEI model. Interestingly, J(SEI) increases with time, indicating that the current contributing from the electrolyte decomposition increases with time due to the SEI fracture upon Li deposition. Meanwhile, the k(SEI) increases with overpotential, indicating the SEI fracture is more serious at higher overpotential or higher growth rate. The k(SEI) is smaller in the electrolyte with fluoroethylene carbonate (FEC) additive, indicating that FEC additive can significantly suppress the SEI fracture during Li metal deposition. This proposed model opens a new way to quantitatively understand the Li nucleation and growth mechanism and electrolyte decomposition on various substrates or in different electrolytes.