The spectrum of two-dimensional (2D) and layered materials 'beyond graphene' offers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (∼5.0-6.0 eV), has clearly established that 2D nitrides are key to advancing 2D devices. A gap, however, remains between the theoretical prediction of 2D nitrides 'beyond hBN' and experimental realization of such structures. Here we demonstrate the synthesis of 2D gallium nitride (GaN) via a migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene. We theoretically predict and experimentally validate that the atomic structure of 2D GaN grown via MEEG is notably different from reported theory. Moreover, we establish that graphene plays a critical role in stabilizing the direct-bandgap (nearly 5.0 eV), 2D buckled structure. Our results provide a foundation for discovery and stabilization of 2D nitrides that are difficult to prepare via traditional synthesis.
A strong correlation between host tortuosity and cycling reversibility of a hosted Li-metal anode is revealed for the first time. High tortuosity leads to preferential top-surface Li deposition based on locally enhanced current density and concentration gradient. This top-surface accumulated Li blocks inward ion transport and invalidates the internal electrode, further aggravating the uneven current distribution and non-uniform plating and stripping. Decreased electrode tortuosity can significantly improve the anodic Coulombic efficiency, uniformity of Li-metal stripping and plating, and cycling stability of the rGO host.
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 energiesthose 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.
Lithium (Li) metal has long been considered the “holy grail” of battery anode chemistry but is plagued by low efficiency and poor safety due to its high chemical reactivity and large volume fluctuation, respectively. Here we introduce a new host of wrinkled graphene cage (WGC) for Li metal. Different from recently reported amorphous carbon spheres, WGC show highly improved mechanical stability, better Li ion conductivity, and excellent solid electrolyte interphase (SEI) for continuous robust Li metal protection. At low areal capacities, Li metal is preferentially deposited inside the graphene cage. Cryogenic electron microscopy characterization shows that a uniform and stable SEI forms on the WGC surface that can shield the Li metal from direct exposure to electrolyte. With increased areal capacities, Li metal is plated densely and homogeneously into the outer pore spaces between graphene cages with no dendrite growth or volume change. As a result, a high Coulombic efficiency (CE) of ∼98.0% was achieved under 0.5 mA/cm2 and 1–10 mAh/cm2 in commercial carbonate electrolytes, and a CE of 99.1% was realized with high-concentration electrolytes under 0.5 mA/cm2 and 3 mAh/cm2. Full cells using WGC electrodes with prestored Li paired with Li iron phosphate showed greatly improved cycle lifetime. With 10 mAh/cm2 Li metal deposition, the WGC/Li composite anode was able to provide a high specific capacity of ∼2785 mAh/g. With its roll-to-roll compatible fabrication procedure, WGC serves as a highly promising material for the practical realization of Li metal anodes in next-generation high energy density secondary batteries.
As a promising candidate for efficient, scalable, and inexpensive solar cells, hybrid organic-inorganic halide perovskites have attracted tremendous research attention. However, their structural degradation during environmental exposure (e.g., UV light, moisture), which limits the commercialization of hybrid perovskites, is poorly understood. Using cryoelectron microscopy, we stabilize these electron beam-sensitive materials for atomic-scale observation and reveal the nanoscale morphology and structure by freezing these materials during the degradation (e.g., UV illumination, moisture exposure) process.
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