The solid electrolyte interphase (SEI) forms on all lithium battery anodes during operation and dictates their performance. Using cryoelectron microscopy, we stabilize these reactive materials for atomic-scale observation and correlate their nanostructure with battery performance. By imaging at various stages of battery operation, we reveal that the distribution of crystalline domains within the SEI is critical for the uniform transport of lithium ions. This establishes the important role that the SEI nanostructure plays in determining the performance of a battery.

The stability of lithium batteries is tied to the physicochemical properties of the solid-electrolyte interphase (SEI). Owing to the difficulty in characterizing this sensitive interphase, the nanoscale distribution of SEI components is poorly understood. Here, we use cryogenic scanning transmission electron microscopy (cryo-STEM) to map the spatial distribution of SEI components across the metallic Li anode. We reveal that LiF, an SEI component widely believed to play an important role in battery passivation, is absent within the compact SEI film (∼15 nm); instead, LiF particles (100–400 nm) precipitate across the electrode surface. We term this larger length scale as the indirect SEI regime. On the basis of these observations, we conclude that LiF cannot be a dominant contribution to anode passivation nor does it influence Li+ transport across the compact SEI film. We refine the traditional SEI structure derived from ensemble-averaged characterizations and nuance the role of SEI components on battery performance.

Preservation of cycling behavior Understanding the changes in interfaces between electrode and electrolyte during battery cycling, including the formation of the solid-electrolyte interphase (SEI), is key to the development of longer lasting batteries. Z. Zhang et al . adapt a thin-film vitrification method to ensure the preservation of liquid electrolyte so that the samples taken for analysis using microscopy and spectroscopy better reflect the state of the battery during operation. A key finding is that the SEI is in a swollen state, in contrast to current belief that it only contained solid inorganic species and polymers. The extent of swelling can affect transport through the SEI, which thickens with time, and thus might also decrease the amount of free electrolyte available for battery cycling. —MSL

Lithium stripping is a crucial process coupled with lithium deposition during the cycling of Li metal batteries. Lithium deposition has been widely studied, whereas stripping as a subsurface process has rarely been investigated. Here we reveal the fundamental mechanism of stripping on lithium by visualizing the interface between stripped lithium and the solid electrolyte interphase (SEI). We observed nanovoids formed between lithium and the SEI layer after stripping, which are attributed to the accumulation of lithium metal vacancies. High-rate dissolution of lithium causes vigorous growth and subsequent aggregation of voids, followed by the collapse of the SEI layer, i.e., pitting. We systematically measured the lithium polarization behavior during stripping and find that the lithium cation diffusion through the SEI layer is the rate-determining step. Nonuniform sites on typical lithium surfaces, such as grain boundaries and slip lines, greatly accelerated the local dissolution of lithium. The deeper understanding of this buried interface stripping process provides beneficial clues for future lithium anode and electrolyte design.

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Fully understanding the mechanism of lithium metal deposition is critical for the development of rechargeable lithium battery anodes. The heterogeneous electron transfer kinetics are an important aspect of lithium electrodeposition, but they have been difficult to measure and understand. Here, we use transient voltammetry with ultramicroelectrodes to explicitly investigate the electron transfer kinetics of lithium electrodeposition. The results deviate from the Butler−Volmer model of electrode kinetics; instead, a Marcus model accurately describes the electron transfer. Measuring the kinetics in a series of electrolytes shows the mechanism of lithium deposition under electron transfer control is consistent with the general framework of Marcus theory. Comparison of the transient voltammetry results to electrochemical impedance spectra provides a strategy for understanding how the interplay of the electron transfer and mass transport resistances affect the morphology of lithium.

The electrolyte plays a critical role in lithium-ion batteries, as it impacts almost every facet of a battery’s performance. However, our understanding of the electrolyte, especially solvation of Li+, lags behind its significance. In this work, we introduce a potentiometric technique to probe the relative solvation energy of Li+ in battery electrolytes. By measuring open circuit potential in a cell with symmetric electrodes and asymmetric electrolytes, we quantitatively characterize the effects of concentration, anions, and solvents on solvation energy across varied electrolytes. Using the technique, we establish a correlation between cell potential (E cell) and cyclability of high-performance electrolytes for lithium metal anodes, where we find that solvents with more negative cell potentials and positive solvation energiesthose weakly binding to Li+lead to improved cycling stability. Cryogenic electron microscopy reveals that weaker solvation leads to an anion-derived solid-electrolyte interphase that stabilizes cycling. Using the potentiometric measurement for characterizing electrolytes, we establish a correlation that can guide the engineering of effective electrolytes for the lithium metal anode.