No abstract

Due to their unique boundary conditions, nanowire heterostructures may exhibit defect-free interfaces even for systems with large lattice mismatch. Heteroepitaxial material integration is limited by lattice mismatches in planar systems, but we use a variational approach to show that nanowire heterostructures are more effective at relieving mismatch strain coherently. This is an equilibrium model based on the Matthews critical thickness in which the lattice mismatch strain is shared by the nanowire overlayer and underlayer, and could as well be partially accomodated by the introduction of a pair of misfit dislocations. The model is highly portable to other nanowire material systems and can be used to estimate critical feature sizes. We find that the critical radius of this system is roughly an order of magnitude larger than the critical thickness of the corresponding thin film/substrate material system. Finite element analysis is used to assess some aspects of the model; in particular, to show that the variational approach describes well the decay of the strain energy density away from the interface.

Hard carbon is the leading candidate anode for commercialization of Na-ion batteries. Hard carbon has a unique local atomic structure, which is composed of nanodomains of layered rumpled sheets that have short-range local order resembling graphene within each layer but complete disorder along the caxis between layers. A primary challenge holding back the development of Na-ion batteries is that a complete understanding of the structure-capacity correlations of Na-ion storage in hard carbon has remained elusive. This article presents two key discoveries: first that characteristics of hard carbon's structure can be modified systematically by heteroatom doping, and second, that these structural changes greatly affect Na-ion storage properties, which reveals the mechanisms for Na storage in hard carbon. Specifically, via P or S doping, the interlayer spacing is dilated, which extends the low-voltage plateau capacity, while increasing the defect concentrations with P or B doping leads to higher sloping sodiation capacity. Our combined experimental studies and first principles calculations reveal that it is the Na-ion-defect binding that corresponds to the sloping capacity, while the Na intercalation between graphenic layers causes the low-potential plateau capacity. The new understanding provides a new set of guiding principles to optimize hard carbon for Na-ion battery applications.

During the operation of an NH 4 + -ion battery electrode of a bi-layered V 2 O 5 , the charge carrier of NH 4 + migrates through the electrode's lattice in a fashion akin to monkey-bar walking, very different from spherical metal ions. Computation studies reveal that there is a charge transfer from the VO layers to the NH 4 + ions via a robust H bonding, where such H bonding has been revealed by characterization. Interestingly, the results point to a correlation between the H bonding and some rarely seen strong pseudocapacitance.

Graphite is the commercial anode for lithium-ion batteries; however, it fails to extend its success to sodium-ion batteries. Recently, we demonstrated that a low-cost amorphous carbonsoft carbon exhibits remarkable rate performance and stable cycling life of Na-ion storage. However, its Na-ion storage mechanism has remained elusive, which has plagued further development of such carbon anodes. Here, we remedy this shortfall by presenting the results from an integrated set of experimental and computational studies that, for the first time, reveal the storage mechanism for soft carbon. We find that sodium ions intercalate into graphenic layers, leading to an irreversible quasi-plateau at ∼0.5 V versus Na+/Na as well as an irreversible expansion seen by in situ transmission electron microscopy (TEM) and X-ray diffraction (XRD). Such a high-potential plateau is correlated to the defective local structure inside the turbostratic stacking of soft carbon and the associated high-binding energies with Na ions, suggesting a trapping mechanism. On the other hand, soft carbon exhibits long sloping regions above and below the quasi-plateau during the first sodiation, where the sloping regions present highly reversible behavior. It is attributed to the more defects contained by soft carbon revealed by neutron total scattering and the associated pair distribution function studies.