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Register a new userA new porous metallic silicon dicarbide for highly efficient Li-ion battery anode identified by targeted structure search
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Extensive efforts have been devoted to C-Si compound materials for improving the limited specific capacity of graphite anode and avoiding the huge volume change of Si anode in Li-ion battery, but not much progress has been made during the past decades. Here, for the first time we apply the targeted structure search by using Li in desired quantity as chemical template to regulate the bonding between C and Si, which makes searching more feasible for us to find a new stable phase of C2Si (labelled as T-C2Si) that can better fit the XRD data of silicon dicarbide synthesized before. Different from the conventional semiconducting silicon carbides, T-C2Si is not only metallic with high intrinsic conductivity for electrons transport, but also porous with regularly distributed channels in suitable size for Li ions experiencing a low energy barrier. T-C2Si exhibits a high specific capacity of 515 mA/g, a high average open-circuit voltage of 1.14 eV, and a low volume change of 1.6%. These parameters meet the requirements of an ideal anode material with high performance for electric vehicles. Moreover, our targeted search strategy guarantees the resulting anode material with a desirable specific capacity and a small volume change during charging /discharging, and it can be used to find new geometric configurations for other materials.
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Amorphous polymer derived silicon oxycarbide (SiOC) is an attractive candidate for Lithium ion battery anodes, as an alternative to graphite, which is limited to a theoretical capacity of 372 mAh/g. However, SiOC tends to exhibit poor transport properties and cycling performance as a result of sparsely distributed carbon clusters and inefficient active sites. To overcome these limitations, we designed and fabricated a layered graphene SiOC heterostructure by solvent assisted infiltration of a polymeric precursor into a modified three dimensional graphene aerogel skeleton. The use of a high melting point solvent facilitated the precursors freeze drying, which following pyrolysis yielded SiOC as a layer supported on the surface of nitrogen doped reduced graphene oxide aerogels. The fabrication method employed here modifies the composition and microstructure of the SiOC phase. Among the studied materials, the highest levels of performance were obtained for a sample of moderate SiOC content, in which the graphene network constituted 19.8 wt % of the system. In these materials, a stable reversible charge capacity of 751 mAh/g was achieved at low charge rates. At high charge rates of 1480 mA/g, the capacity retention was ca. 95% (352 mAh/g) after 1000 consecutive cycles. At all rates, Coulombic efficiencies greater than 99% were maintained following the first cycle. Performance across all indicators was majorly improved in the graphene aerogel SiOC nanocomposites, compared with unsupported SiOC. The performance was attributed to mechanisms across multiple length scales. The presence of oxygen rich SiO4-xCx tetrahedral units and a continuous free carbon network within the SiOC provides sites for reversible lithiation, while high ionic and electronic transport is provided by the layered graphene SiOC heterostructure.
The 3D ternary Li_4Ti_5O_12, the Li+-based battery anode, presents the unusual lattice symmetry (a triclinic crystal), band structure, charge density, and density of states, under the first-principles calculations. It belongs to a large direct-gap semiconductor of E_g^d~ 2,98 eV. The atom-dominated valence and conduction bands, the spatial charge distribution and the atom- and orbital-decomposed van Hove singularities are available in the delicate identifications of multi-orbital hybridizations in Li-O and Ti-O bonds. The extremely non-uniform chemical environment, which induce the very complicated hopping integrals, directly arise from the large bonding fluctuations and the highly anisotropic configurations. Also, the developed theoretical framework is very useful for fully understanding the cathodes and electrolytes of oxide compounds.
We performed density functional theory (DFT) calculations for a bi-layered heterostructure combining a graphene layer with a MoS2 layer with and without intercalated Li atoms. Our calculations demonstrate the importance of the van der Waals (vdW) interaction, which is crucial for forming stable bonding between the layers. Our DFT calculation correctly reproduces the linear dispersion, or Dirac cone, feature at the Fermi energy for the isolated graphene monolayer and the band gap for the MoS2 monolayer. For the combined graphene/MoS2 bi-layer, we observe interesting electronic structure and density of states (DOS) characteristics near the Fermi energy, showing both the gap like features of the MoS2 layer and in-gap states with linear dispersion contributed mostly by the graphene layer. Our calculated total density of states (DOS) in this vdW heterostructure reveals that the graphene layer significantly contributes to pinning the Fermi energy at the center of the band gap of MoS2. We also find that intercalating Li ions in between the layers of the graphene/MoS2 heterostructure enhances the binding energy through orbital hybridizations between cations (Li adatoms) and anions (graphene and MoS2 monolayers). Moreover, we calculate the dielectric function of the Li intercalated graphene/MoS2 heterostructure, the imaginary component of which can be directly compared with experimental measurements of optical conductivity in order to validate our theoretical prediction. We observe sharp features in the imaginary component of the dielectric function, which shows the presence of a Drude peak in the optical conductivity, and therefore metallicity in the lithiated graphene/MoS2 heterostructure.
We report a comprehensive study on the electrochemical performance of four different Transition Metal Oxides encapsulated inside carbon nanotubes (CNT). Irrespective of the type of oxide-encapsulate, all these samples exhibit superior cyclic stability as compared to the bare-oxide. Innovative use of camphor during sample synthesis enables precise control over the morphology of these self-organized carbon nanotube structures, which in turn appears to play a crucial role in the magnitude of the specific capacity. A comparative evaluation of the electrochemical data on different samples bring forward interesting inferences pertaining to the morphology, filling fraction of the oxide-encapsulate, and the presence of oxide nano-particles adhering outside the filled CNT. Our results provides useful pointers towards the optimization of critical parameters, thus paving the way for using these synthetically encapsulated and self-organized carbon nanotube structures as anode materials for Li-ion batteries, and possibly other electrochemical applications.
Thermoelectric device is a promising next-generation energy solution owing to its capability to transform waste heat into useful electric energy, which can be realized in materials with high elec- tric conductivities and low thermal conductivities. A recently synthesized silicon allotrope of Si$_{24}$ features highly anisotropic crystal structure with nanometre-sized regular pores. Here, based on first-principles study without any empirical parameter, we show that the slightly doped Si$_{24}$ can pro- vide an order-of-magnitude enhanced thermoelectric figure of merit at room temperature, compared with the cubic diamond phase of silicon. We ascribe the enhancement to the intrinsic nanostructure formed by the nanopore array, which effectively hinders heat conduction while electric conductivity is maintained. This can be a viable option to enhance the thermoelectric figure of merit without further forming an extrinsic nanostructure. In addition, we propose a practical strategy to further diminish the thermal conductivity without affecting electric conductivity by confining rattling guest atoms in the pores.
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