Showing papers by "Penghao Xiao published in 2015"
TL;DR: The newly discovered dehydrated Na2-δMnHFC phase exhibits superior electrochemical performance compared to other reported Na-ion cathode materials and delivers at 3.5 V a reversible capacity in a sodium half cell and 140 mAh g(-1) in a full cell with a hard-carbon anode.
Abstract: Sodium is globally available, which makes a sodium-ion rechargeable battery preferable to a lithium-ion battery for large-scale storage of electrical energy, provided a host cathode for Na can be found that provides the necessary capacity, voltage, and cycle life at the prescribed charge/discharge rate. Low-cost hexacyanometallates are promising cathodes because of their ease of synthesis and rigid open framework that enables fast Na+ insertion and extraction. Here we report an intriguing effect of interstitial H2O on the structure and electrochemical properties of sodium manganese(II) hexacyanoferrates(II) with the nominal composition Na2MnFe(CN)6·zH2O (Na2−δMnHFC). The newly discovered dehydrated Na2−δMnHFC phase exhibits superior electrochemical performance compared to other reported Na-ion cathode materials; it delivers at 3.5 V a reversible capacity of 150 mAh g–1 in a sodium half cell and 140 mAh g–1 in a full cell with a hard-carbon anode. At a charge/discharge rate of 20 C, the half-cell capacity ...
580 citations
TL;DR: In this article, the phase transition of NaxFeMn(CN)6 is explained in terms of a competition between Coulomb attraction, Pauli repulsion, and d−π covalent bonding.
Abstract: The Prussian Blue analog, NaxFeMn(CN)6, is a potential new cathode material for Na-ion batteries. During Na intercalation, the dehydrated material exhibits a monoclinic to rhombohedral phase transition, while the hydrated material remains in the monoclinic phase. With density functional theory calculations, the phase transition is explained in terms of a competition between Coulomb attraction, Pauli repulsion, and d–π covalent bonding. The interstitial Na cations have a strong Coulomb attraction to the N anions in the host material, which tend to bend the Mn–N bonds and reduce the volume of the structure. The presence of lattice H2O enhances the Pauli repulsion so that the volume reduction is suppressed. The calculated volume change, as it depends upon the presence of lattice H2O, is consistent with experimental measurements. Additionally, a new LiFeMn(CN)6 phase is predicted where MnN6 octahedra decompose into LiN4 and MnN4 edge-sharing tetrahedra.
82 citations
TL;DR: In this article, a combined experimental and theoretical investigation of Ge-substitution effects on the band structures and thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6−yGey (y = 0, 0.1, and 0.2) synthesized by solid state reaction was conducted.
Abstract: The optimized thermoelectric figure of merit (ZT) of Mg2Si0.4Sn0.6 peaks at about 750 K because its relatively narrow band gap results in pronounced bipolar transport at higher temperatures. To suppress the bipolar transport, we have conducted a combined experimental and theoretical investigation of Ge-substitution effects on the band structures and thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6−yGey (y = 0, 0.1, and 0.2) synthesized by solid state reaction. The measured transport properties of these compositions can be interpreted by a triple-parabolic-band model based on first-principle calculation of band structures. The results show that the bipolar conduction in the temperature range up to 800 K was effectively suppressed by Ge substitution that widens the band gap. As a side effect, Ge substitution induces the separation of two otherwise converged conduction bands in Mg2Si0.4Sn0.6, leading to reduced thermoelectric performance at low temperatures. The result of these two competing effects is th...
56 citations
TL;DR: This work mitigates the dimensionality scaling problem by constructing bias potentials that are based upon the distance to the boundary of the reactant basin that are demonstrated to scale qualitatively better with dimensionality than the existing methods.
Abstract: An effective way to accelerate rare events in molecular dynamics simulations is to apply a bias potential which destabilizes minima without biasing the transitions between stable states. This approach, called hyperdynamics, is limited by our ability to construct general bias potentials without having to understand the reaction mechanisms available to the system, a priori. Current bias potentials are typically constructed in terms of a metric which quantifies the distance that a trajectory deviates from the reactant state minimum. Such metrics include detection of negative curvatures of the potential, an energy increase, or deviations in bond lengths from the minimum. When one of these properties exceeds a critical value, the bias potentials are constructed to approach zero. A problem common to each of these schemes is that their effectiveness decreases rapidly with system size. We attribute this problem to a diminishing volume defined by the metrics around a reactant minimum as compared to the total volume of the reactant state basin. In this work, we mitigate the dimensionality scaling problem by constructing bias potentials that are based upon the distance to the boundary of the reactant basin. This distance is quantified in two ways: (i) by following the minimum mode direction to the reactant boundary and (ii) by training a machine learning algorithm to give an analytic expression for the boundary to which the distance can be calculated. Both of these ridge-based bias potentials are demonstrated to scale qualitatively better with dimensionality than the existing methods. We attribute this improvement to a greater filling fraction of the reactant state using the ridge-based bias potentials as compared to the standard potentials.
7 citations