We present in situ environmental transmission electron microscopy (ETEM) observation of metallic lithium nucleation, growth and shrinkage in a liquid confining cell, where protrusions are seen to grow from their roots or tips, depending on the overpotential. The rate of solidelectrolyte interface (SEI) formation affects root vs. tip growth mode, with the former akin to intermittent volcanic eruptions, giving kinked segments of nearly constant diameter. Upon delithiation, root-grown whiskers are highly unstable, because the segmental shrinkage rate depends on Li + transport across SEI, which is the greatest around the latest grown segment with the thinnest SEI, and therefore the near-root segment often dissolves first and the rest of the dendrite then loses electrical contact. These electrically isolated dead lithium branches are also easily swept away into the electrolyte to become "nano-lithium flotsam" because the hollowedout SEI tube is very brittle. Our observations are consistent with SEI-obstructed growth by two competing mechanisms; tip growth of dense Eden-like clusters and root growth of whiskers, resulting from the voltage-dependent competition between lithium electrodeposition and SEI formation reactions. Similar phenomena could occur whenever chemical deposition/dissolution competes with irreversible side reactions that form a passivating layer on the evolving surface.

Carbon nanotubes have been proposed as promising hydrogen storage materials for the automotive industry. By theoretical analyses and total-energy density functional theory calculations, we show that contribution from physisorption in nanotubes, though significant at liquid nitrogen temperature, should be negligible at room temperature; contribution from chemisorption has a theoretical upper limit of 7.7 wt %, but could be difficult to utilize in practice due to slow kinetics. The metallicity of carbon nanotube is lost at full hydrogen coverage, and we find strong covalent C-H bonding that would slow down the H 2 recombination kinetics during desorption. When compared to other pure carbon nanostructures, we find no rational reason yet why carbon nanotubes should be superior in either binding energies or adsorption/desorption kinetics.

Liquid-cell in situ transmission electron microscopy (TEM) observations of the charge/discharge reactions of nonaqueous Li-oxygen battery cathode were performed with ∼5 nm spatial resolution. The discharging reaction occurred at the interface between the electrolyte and the reaction product, whereas in charging, the reactant was decomposed at the contact with the gold current collector, indicating that the lithium ion diffusivity/electronic conductivity is the limiting factor in discharging/charging, respectively, which is a root cause for the asymmetry in discharging/charging overpotential. Detachments of lithium oxide particles from the current collector into the liquid electrolyte are frequently seen when the cell was discharged at high overpotentials, with loss of active materials into liquid electrolyte ("flotsam") under minute liquid flow agitation, as the lithium peroxide dendritic trees are shown to be fragile mechanically and electrically. Our result implies that enhancing the binding force between the reaction products and the current collector to maintain robust electronic conduction is a key for improving the battery performance. This work demonstrated for the first time the in situ TEM observation of a three-phase-reaction involving gold electrode, lithium oxides, DMSO electrolyte and lithium salt, and O2 gas. The technique described in this work is not limited to Li-oxygen battery but also can be potentially used in other applications involving gas/liquid/solid electrochemical reactions.

Although Li-ion battery is one of the most widely used energy storage devices, there have been extensive efforts to push its limit to meet the ever increasing demands to increase its energy density for applications such as electric vehicles, portable electronics, and grid storages. Here, Lithium metal anode plays a key role in the next generation energy storage devices, ultimately removing the Li metal anode in the initial state with the anode-free configuration. However, there are major challenges that need to be overcome. These include low Coulombic efficiency and formation of dendrites. In this work, we adopted gallium-based liquid metal (LM) as a coating layer on a copper current collector to uniformly deposit Li to prevent the dendrite formation and inmprove the cycle efficiency. The LM coating effectively improved the cycle performance in the anode-free configuration combined with Li(Ni,Co,Mn)O2 cathode. The effect of the LM coating was confirmed by in situ transmission electron microscopy and optical microscopy observations. LM reduced the charge/discharge overpotentials with its high affinity with lithium. It also contributed to decompose the dendritic lithium in the discharge process reducing the dead lithium disconnected from the current collector.

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