Dynamic spatial progression of isolated lithium during battery operations.

The increasing demand for next-generation energy storage systems necessitates the development of high-performance lithium batteries1-3. Unfortunately, current Li anodes exhibit rapid capacity decay and a short cycle life4-6, owing to the continuous generation of solid electrolyte interface7,8 and isolated Li (i-Li)9-11. The formation of i-Li during the nonuniform dissolution of Li dendrites12 leads to a substantial capacity loss in lithium batteries under most testing conditions13. Because i-Li loses electrical connection with the current collector, it has been considered electrochemically inactive or 'dead' in batteries14,15. Contradicting this commonly accepted presumption, here we show that i-Li is highly responsive to battery operations, owing to its dynamic polarization to the electric field in the electrolyte. Simultaneous Li deposition and dissolution occurs on two ends of the i-Li, leading to its spatial progression toward the cathode (anode) during charge (discharge). Revealed by our simulation results, the progression rate of i-Li is mainly affected by its length, orientation and the applied current density. Moreover, we successfully demonstrate the recovery of i-Li in Cu-Li cells with >100% Coulombic efficiency and realize LiNi0.5Mn0.3Co0.2O2 (NMC)-Li full cells with extended cycle life.

Lithium metal stripping beneath the solid electrolyte interphase.

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.