Root-growing, containable mossy lithium changes to tip-growing, short-causing dendritic lithium at “Sand's capacity”, which is set by electrolyte diffusion limitation.

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Recent experiments on nanoscale materials, including nanowires, nanopillars, nanoparticles, nanolayers, and nanocrystals, have revealed a host of "ultra-strength" phenomena, defined by stresses in the material generally rising up to a significant fraction of the ideal strength-the highest achievable strength of a defect-free crystal. This article presents an overview of the strength-controlling deformation mechanisms and related mechanics models in ultra-strength nanoscale materials. The critical role of the activation volume is highlighted in understanding the deformation mechanisms, as well as the size, temperature, and strain rate dependence of ultra strength.

Topologically protected fermionic quasiparticles appear in metals, where band degeneracies occur at the Fermi level, dictated by the band structure topology. While in some metals these quasiparticles are direct analogues of elementary fermionic particles of the relativistic quantum field theory, other metals can have symmetries that give rise to quasiparticles, fundamentally different from those known in high-energy physics. Here, we report on a new type of topological quasiparticles-triple point fermions-realized in metals with symmorphic crystal structure, which host crossings of three bands in the vicinity of the Fermi level protected by point group symmetries. We find two topologically different types of triple point fermions, both distinct from any other topological quasiparticles reported to date. We provide examples of existing materials that host triple point fermions of both types and discuss a variety of physical phenomena associated with these quasiparticles, such as the occurrence of topological surface Fermi arcs, transport anomalies, and topological Lifshitz transitions.

Lithium metal has gravimetric capacity ∼10× that of graphite which incentivizes rechargeable Li metal batteries (RLMB) development. A key factor that limits practical use of RLMB is morphological instability of Li metal anode upon electrodeposition, reflected by the uncontrolled area growth of solid-electrolyte interphase that traps cyclable Li, quantified by the Coulombic inefficiency (CI). Here we show that CI decreases approximately exponentially with increasing donatable fluorine concentration of the electrolyte. By using up to 7 m of Li bis(fluorosulfonyl)imide in fluoroethylene carbonate, where both the solvent and the salt donate F, we can significantly suppress anode porosity and improve the Coulombic efficiency to 99.64%. The electrolyte demonstrates excellent compatibility with 5-V LiNiMnO cathode and Al current collector beyond 5 V. As a result, an RLMB full cell with only 1.4× excess lithium as the anode was demonstrated to cycle above 130 times, at industrially significant loading of 1.83 mAh/cm and 0.36 C. This is attributed to the formation of a protective LiF nanolayer, which has a wide bandgap, high surface energy, and small Burgers vector, making it ductile at room temperature and less likely to rupture in electrodeposition.

Retaining the high energy density of rechargeable lithium ion batteries depends critically on the cycle stability of microstructures in electrode materials. We report the reversible formation of nanoporosity in individual germanium nanowires during lithiation-delithiation cycling by in situ transmission electron microscopy. Upon lithium insertion, the initial crystalline Ge underwent a two-step phase transformation process: forming the intermediate amorphous Li(x)Ge and final crystalline Li(15)Ge(4) phases. Nanopores developed only during delithiation, involving the aggregation of vacancies produced by lithium extraction, similar to the formation of porous metals in dealloying. A delithiation front was observed to separate a dense nanowire segment of crystalline Li(15)Ge(4) with a porous spongelike segment composed of interconnected ligaments of amorphous Ge. This front sweeps along the wire with a logarithmic time law. Intriguingly, the porous nanowires exhibited fast lithiation/delithiation rates and excellent mechanical robustness, attributed to the high rate of lithium diffusion and the porous network structure for facile stress relaxation, respectively. These results suggest that Ge, which can develop a reversible nanoporous network structure, is a promising anode material for lithium ion batteries with superior energy capacity, rate performance, and cycle stability.

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.