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.

In situ diffuse reflectance infrared Fourier‐transformed spectroscopy (DRIFTS) investigations have been made to examine solid‐electrolyte interphase (SEI) formation on lithium‐rich Li1.2Ni0.2Mn0.6O2 (LLNMO) and LiCoO2 cathodes during first‐ and second‐cycle charging and discharging. This DRIFTS technique allows us to clarify SEI formation with different charging voltages. Both cathodes revealed the formation of the same surface species during first‐cycle charging, initially including ethylene carbonate (EC) adsorption, and SEI species, for example, ROCOF, RCOOR, Li2CO3, ROCO2Li, and PFx, are formed above the onset potential, namely 4.0 and 4.5 V for LiCoO2 and LLNMO, respectively. The onset potentials correspond to the upper limit of the reversible redox potential range for transition‐metal couples (e.g. Co3+/Co4+ in LiCoO2 and Ni2+/Ni4+ in LLNMO), which account for the intrinsic instability of these cathode materials. Such results suggest the participation of intermediate reactive oxygen species in SEI formation. SEI species continue to form during the discharge process when the potential is scanned cathodically below 3.6 and 4.0 V for LiCoO2 and LLNMO, respectively. Similar SEI species are also observed during the second cycle charge–discharge over LLNMO, where additional oxidized species such as carboxylate (−COO−) and CO2 are also found during charging. With the exception of PFx, all of the observed SEI species can be attributed to the oxidative decomposition of the organic solvent, EC. Finally, possible reaction mechanisms related to the oxidative decomposition of EC are discussed.

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New electrolytes with good thermal and electrochemical stabilities can improve lithium‐ion batteries for applications at high temperatures and high‐potential working conditions. In this study, the solid–electrolyte interphase (SEI) formed in the presence of the new 5‐cyano‐bis(trifluoroborane)‐trifluoromethyl‐benzimidazole lithium salt is examined on two cathodes, that is, commercial LiCoO2 and Li‐rich Li1.2Ni0.2Mn0.6O2 cathodes, using in situ diffuse reflectance infrared Fourier‐transform spectroscopy (DRIFTS). The SEI species contain no decomposition products related to the new electrolyte, up to a potential of 5.0 V. During the first charging process, SEI formation starts at an onset potential of 4.4 or 4.5 V for LiCoO2 or Li1.2Ni0.2Mn0.6O2, respectively, which is higher than that observed by using commercial LiPF6 electrolyte. SEI species of acid anhydride, alkyl carbonates, and Li2CO3 are identified at and beyond the onset potential, which can be attributed to the decomposition of organic carbonates. Furthermore, SEI formation is relatively insignificant in the second‐cycle charging, indicating the formation of a passivating layer in the first cycle, in contrast to the continuing SEI formation through cycling with LiPF6 electrolyte. The different SEI formed with the new electrolyte can be attributed to the adsorption of the electrolyte anion on the electrode surface. This study demonstrates changes in SEI formation chemistry with the new electrolyte, which appears to be a suitable candidate for transition‐metal‐oxide cathodes at high voltage conditions.