Operando identification of anion effect on lithium nucleation and growth via in situ transmission electron microscopy.

Lithium metal batteries are considered promising candidates for next-generation energy storage due to the high capacity and low redox potential of lithium negative electrodes. However, dendritic Li growth and unstable solid-electrolyte interphase formation remain critical bottlenecks for practical implementation. While electrolyte anion chemistry critically governs solid-electrolyte interphase formation, nanoscale observations of anion-regulated Li nucleation and growth mechanisms remain limited in tracking dynamic interfacial processes. Here, we employ in situ liquid-phase transmission electron microscopy combined with cryogenic spectroscopy and computational modelling to unravel anion-specific Li nucleation and growth in three distinct electrolytes: LiClO4, LiPF6, and LiTFSI-based electrolytes. Real-time tracking reveals that ClO4- drives dendritic Li growth with organic dominated solid-electrolyte interphase, whereas PF6- stabilizes moss-like Li nucleation through LiF-organic hybrid interphases. Notably, TFSI⁻ forms a bilayer SEI with LiF/Li2CO3-rich inner layers, enabling Li lateral growth and fusion. Molecular dynamics simulations correlate anion-induced interface architectures with Li+ transport and surface potential distributions, demonstrating that TFSI- suppresses dendrites via balanced mechanical confinement and ion-flux regulation. These anion-mediated interface engineering observations offers principles for electrolyte design toward stable lithium metal batteries.