Hard carbons are among the most promising materials for alkali-ion metal anodes. These materials have a highly complex structure and understanding the metal storage and migration within these structures is of utmost importance for the development of next-generation battery technologies. The effect of different carbon structural motifs on Li, Na, and K storage and diffusion are probed using density functional theory based on experimental characterizations of hard carbon samples. Two carbon structural models-the planar graphitic layer model and the cylindrical pore model-are constructed guided by small-angle X-ray scattering and transmission electron microscopy characterization. The planar graphitic layers with interlayer distance <6.5 Å are beneficial for metal storage, but do not have significant contribution to rapid metal diffusion. Fast diffusion is shown to take place in planar graphitic layers with interlayer distance >6.5 Å, when the graphitic layer separation becomes so wide that there is negligible interaction between the two graphitic layers. The cylindrical pore model, reflecting the curved morphology, does not increase metal storage, but significantly lowers the metal migration barriers. Hence, the curved carbon morphologies are shown to have great importance for battery cycling. These findings provide an atomic-scale picture of the metal storage and diffusion in these materials.

Amorphous aluminum oxide Al 2 O 3 (a-Al 2 O 3 ) layers grown by various deposition techniques contain a significant density of negative charges. In spite of several experimental and theoretical studies, the origin of these charges still remains unclear. We report the results of extensive Density Functional Theory (DFT) calculations of native defects -O and Al vacancies and interstitials, as well as H interstitial centers -in different charge states in both crystalline α-Al 2 O 3 and in a-Al 2 O 3 . The results demonstrate that both the charging process and the energy distribution of traps responsible for negative charging of a-Al 2 O 3 films [M. B. Zahid et al., IEEE Trans. Electron Devices 57, 2907 (2010)] can be understood assuming that the negatively charged O i and V Al defects are nearly compensated by the positively charged H i , V O and Al i defects in as prepared samples. Following electron injection, the states of Al i , V O or H i in the band gap become occupied by electrons and sample becomes negatively charged. The optical excitation energies from these states into the oxide conduction band agree with the results of exhaustive photo-depopulation spectroscopy (EPDS) measurements [M. B. Zahid et al., IEEE Trans. Electron Devices 57, 2907 (2010)]. This new understanding of the origin of negative charging of a-Al 2 O 3 films is important for further development of nanoelectronic devices and solar cells.

Hard carbon anodes have shown significant promise for next‐generation battery technologies. These nanoporous carbon materials are highly complex and vary in structure depending on synthesis method, precursors, and pyrolysis temperature. Structurally, hard carbons are shown to consist of disordered planar and curved motifs, which have a dramatic impact on anode performance. Here, the impact of position on defect formation energy is explored through density functional theory simulations, employing a mixed planar bulk and curved surface model. At defect sites close to the surface, a dramatic decrease (≥50%) in defect formation energy is observed for all defects except the nitrogen substitutional defect. These results confirm the experimentally observed enhanced defect concentration at surfaces. Previous studies have shown that defects have a marked impact on metal storage. This work explores the interplay between position and defect type for lithium, sodium, and potassium adsorption. Regardless of defect location, it is found that the energetic contributions to the metal adsorption energies are principally dictated by the defect type and carbon interlayer distance.

The microstructure of hard carbons can be designed to maximize their performance as anodes for sodium‐ion batteries. However, the nature of the electrolyte is also decisive in the capacity and long‐term stability. Here, hard carbons with a tailored bimodal pore network of internal micropores interconnected through mesopores are studied as sodium‐ion battery anodes. The evolution of their solid electrolyte interphase (SEI) is analyzed in three different electrolytes (NaPF6 in an ether‐based solvent, and NaPF6 or NaClO4 in a carbonate‐based system). Combining experiments with density functional theory calculations, it is proposed that formation of the SEI is mainly controlled by the decomposition of the salt anion. This process occurs through the intermediate functionalization of the carbon surface by the decomposed anion fragments. It is suggested that the innermost SEI sub‐layer governs the performance and long‐term stability of the anode. While the presence of a fluorine‐containing salt appears to have a determining role in the SEI stability, the electrochemical decomposition of carbonate‐based solvents is detrimental for the long‐term stability as the interfacial resistance increases. In contrast, the ether‐based system enables stable long‐term cycling as the interphase remains almost intact once the first fluorine‐rich SEI layer is formed.

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