Nanostructured birnessite exhibits high specific capacitance and nearly ideal capacitive behaviour in aqueous electrolytes, rendering it an important electrode material for low-cost, high-power ...energy storage devices. The mechanism of electrochemical capacitance in birnessite has been described as both Faradaic (involving redox) and non-Faradaic (involving only electrostatic interactions). To clarify the capacitive mechanism, we characterized birnessite's response to applied potential using ex situ X-ray diffraction, electrochemical quartz crystal microbalance, in situ Raman spectroscopy and operando atomic force microscope dilatometry to provide a holistic understanding of its structural, gravimetric and mechanical responses. These observations are supported by atomic-scale simulations using density functional theory for the cation-intercalated structure of birnessite, ReaxFF reactive force field-based molecular dynamics and ReaxFF-based grand canonical Monte Carlo simulations on the dynamics at the birnessite-water-electrolyte interface. We show that capacitive charge storage in birnessite is governed by interlayer cation intercalation. We conclude that the intercalation appears capacitive due to the presence of nanoconfined interlayer structural water, which mediates the interaction between the intercalated cation and the birnessite host and leads to minimal structural changes.
The cold sintering process (CSP) densifies ceramics at much lower temperatures than conventional sintering processes. Several ceramics and composite systems have been successfully densified under ...cold sintering. For the grain growth kinetics of zinc oxide, reduced activation energies are shown, and yet the mechanism behind this growth is unknown. Herein, we investigate these mechanisms in more detail with experiments and ReaxFF molecular dynamics simulations. We investigated the recrystallization of zinc cations under various acidic conditions and found that their adsorption to the surface can be a rate‐limiting factor for cold sintering. Our studies show that surface hydroxylation in CSP does not inhibit crystallization; in contrast, by creating a surface complex, it creates an orders of magnitude acceleration in surface diffusion, and in turn, accelerates recrystallization.
In the cold sintering process, the presence of water enhances the surface diffusion of ions at low temperatures and results in extraordinary grain growth. This accelerated surface diffusion mechanism has been revealed using ReaxFF simulations to model the ZnO/water/acetic acid interface.
► The structure and growth of SEI composed of EC, DMC, and EC+DMC are quantitatively studied for the first time using molecular dynamics simulations. ► Our simulations show that the SEI is rich in ...inorganic salts and organic salts near the anode and the electrolyte interface, respectively. ► The formation potential for SEI is found to be close to 1.0V vs. Li/Li+ in agreement with experiments. ► Mechanically, the SEI is found be to be stiffer than Li metal, but somewhat softer compared to the graphite anode.
We have studied the formation and growth of solid-electrolyte interphase (SEI) for the case of ethylene carbonate (EC), dimethyl carbonate (DMC) and mixtures of these electrolytes using molecular dynamics simulations. We have considered SEI growth on both Li metal surfaces and using a simulation framework that allows us to vary the Li surface density on the anode surface. Using our simulations we have obtained the detailed structure and distribution of different constituents in the SEI as a function of the distance from the anode surfaces. We find that SEI films formed in the presence of EC are rich in Li2CO3 and Li2O, while LiOCH3 is the primary constituent of DMC films. We find that dilithium ethylene dicarbonate, LiEDC, is formed in the presence of EC at low Li surface densities, but it quickly decomposes to inorganic salts during subsequent growth in Li rich environments. The surface films formed in our simulations have a multilayer structure with regions rich in inorganic and organic salts located near the anode surface and the electrolyte interface, respectively, in agreement with depth profiling experiments. Our computed formation potentials 1.0V vs. Li/Li+ is also in excellent accord with experimental measurements. We have also calculated the elastic stiffness of the SEI films; we find that they are significantly stiffer than Li metal, but are somewhat more compliant compared to the graphite anode.
Developing strategies for atomic-scale controlled synthesis of new two-dimensional (2D) functional materials will directly impact their applications. Here, using in situ aberration-corrected scanning ...transmission electron microscopy, we obtain direct insight into the homoepitaxial Frank-van der Merwe atomic layer growth mechanism of TiC single adlayers synthesized on surfaces of Ti
C
MXene substrates with the substrate being the source material. Activated by thermal exposure and electron-beam irradiation, hexagonal TiC single adlayers form on defunctionalized surfaces of Ti
C
MXene at temperatures above 500 °C, generating new 2D materials Ti
C
and Ti
C
. The growth mechanism for a single TiC adlayer and the energies that govern atom migration and diffusion are elucidated by comprehensive density functional theory and force-bias Monte Carlo/molecular dynamics simulations. This work could lead to the development of bottom-up synthesis methods using substrates terminated with similar hexagonal-metal surfaces, for controllable synthesis of larger-scale and higher quality single-layer transition metal carbides.
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