A comparison of core–shell Si/C and embedded structure Si/C composites as negative materials for lithium-ion batteries

Silicon materials have attracted wide attention as negative materials due to exceptional gravimetric capacity and abundance. The strategy of using nano-silicon materials as structural units to construct nano/micro-structured silicon-based negative materials for lithium-ion batteries has come into sight in recent years. In order to provide guidance for the material structure design of micro-sized silicon-based negative materials in practical application, in this work, two commercialized nano/micro-structured silicon-based negative materials with a specific capacity of about 650 mAh·g^(-1) were investigated and compared in the aspects of material microstructure, electrochemical performance of half cells, and electrode morphological evolution during cycling. The cycling performance(with capacity retention ratio of about 17% higher after 100 cycles) and electrode structure maintenance of the embedded structure Si/C material are superior to those of core–shell Si/C material. This research can provide guidance on design and application of nano/micro-structured silicon-based negative materials.

Photo-Catalyzed Rare Earth Materials with Ability to Translate Free Radicals into Negative Ions

Materials with function of producing negative ions effection,containing valency-variable rare earth elements and semiconductor oxide,were fabricated.Free radicals produced by the materials were tested.The result shows that the materials can produce quite a few free radicals as·O^-_2 no matter whether they are under illumination of ultraviolet radiation or under visible light radiation,or under no light radiation, demonstrating semiconductor oxide can be catalysed under the visible light radiation.At the same time the result shows there is direct relation between the number of free radicals and of the negative ion produced by the materials,which meant that during photo-catalyzed and redox process of valency-variable rare earth elements free radicals translate into negative ions. A circular model is presented involving circulating change of valency-variable rare earth elements and water and oxygen absorbed on the surface of materials under the condition of photocatalysis.

Enhancing fly ash utilization in backfill materials treated with CO_(2)carbonation under ambient conditions

The environmental concerns resulting from coal-fired power generation that produces large amounts of CO_(2)and fly ash are of great interest.To mitigate,this study aims to develop a novel carbonated CO_(2)-fly ash-based backfill(CFBF)material under ambient conditions.The performance of CFBF was investigated for different fly ash-cement ratios and compared with non-CO_(2)reacted samples.The fresh CFBF slurry conformed to the Herschel-Bulkley model with shear thinning characteristics.After carbonation,the yield stress of the fresh slurry increased significantly by lowering fly ash ratio due to gel formation.The setting times were accelerated,resulting in approximately 40.6%of increased early strength.The final strength decreased when incorporating a lower fly ash ratio(50%and 60%),which was related to the existing heterogeneous pores caused by rapid fluid loss.The strength increased with fly ash content above 70%because additional C-S(A)-H and silica gels were characterized to precipitate on the grain surface,so the binding between particles increased.The C-S(A)-H gel was developed through the pozzolanic reaction,where CaCO_(3)was the prerequisite calcium source obtained in the CO_(2)-fly ash reaction.Furthermore,the maximum CO_(2)uptake efficiency was 1.39 mg-CO_(2)/g-CFBF.The CFBF material is feasible to co-dispose CO_(2)and fly ash in the mine goaf as negative carbon backfill materials,and simultaneously mitigates the strata movement and water lost in post-subsurface mining.

Origin of Negative Imaginary Part of Effective Permittivity of Passive Materials

The anti-resonant phenomenon of effective electromagnetic parameters of metamaterials has aroused controversy due to negative imaginary permittivity or permeability. It is experimentally found that the negative imaginary permittivity can occur for the natural passive materials near the Fabry Perot resonances. We reveal the nature of negative imaginary permittivity, which is correlated with the magnetoelectric coupling. The anti-resonance of permittivity is a non-inherent feature for passive materials, while it can be inherent for devices or metamaterials. Our finding validates that the negative imaginary part of effective permittivity does not contradict the second law of thermodynamics for metamaterials owing to the magnetoelectric coupling.

Toward High Capacity and Stable SnO_(2)Hollow Nanosphere Electrode Materials:A Case Study of Ni-substituted Modification

To develop the urgent requirement for high-rate electrodes in next-generation lithium-ion batteries,SnO_(2)-based negative materials have been spotlighted as potential alternatives.However,the intrinsic problems,such as conspicuous volume variation and unremarkable conductivity,make the rate capability behave badly at a high-current density.Here,to solve these issues,this work demonstrate a new and facile strategy for synergistically enhancing their cyclic stability by combining the advantages of Ni doping and the fabrication of hollow nanosphere.Specifically,the incorporation of Ni^(2+)ions into the tetragonal rutile-type SnO_(2)shellsimproves the charge transfer kinetics effectively,leading to an excellent cycling stability.In addition,the growth of surface grains on the hollow nanospheres are restrained after Ni doping,which also reduces theunexpected polarization of negative electrodes.As a result,the as-prepared Ni doped electrode delivers a remarkable reversible capacity of 712 mAh g^(-1)at 0.1 A g^(-1)and exhibits outstanding capacity of 340 mAh g^(-1)at 1.6 A g^(-1),about 2.58 times higher than that of the pure SnO_(2)hollow sample.

Stopband Phenomena in the Passband of Left-Handed Metamaterials

Stopband phenomena are reported in the passband of left-handed metamaterials. The samples with linear defect are designed by removing one layer of split ring resonators （SRRs）. It is shown that the left-handed transmission peaks have a distinct transform with the relative deviation of the SRRs centre from the wire centre 8, from a single left-handed peak, double left-handed peaks with different magnitude to no transmission peak, i.e. left-handed properties of metamaterials disappear. Numerical simulation shows that the change of 8 makes the effective permeability shift at a frequency range, where stopband occurs. It is thought that the stopband in left-handed passband is due to the symmetry breaking between SRRs and wires in the metamaterials.

Effect of Different Plasmonic Nano-Inclusion on Double Negative-Semiconductor Photonic Crystal in Visible Region:Gain Assistance and All-Angle Negative Refraction

A gain assisted double negative- Metallo-semiconductor photonic crystal （DN-MSPC） for visible light with effect of different plasmonic （Al, Ag, Au, Cu） nanorod inclusion, is presented. Negative real values of both permeability （μ） and permittivity （ε） with extremely low imaginary values for visible light is obtained by applying Coupled dipole approximation. All-Angle negative refraction is obtained by applying surface plasmon polariton excitation （SPPE） in DN-MSPC operating in a dispersion regime with anti-parallel refracted wave vector and Poynting vector. Index matched to the incident light and compensated losses due the gain assistance leads the light amplification in the designed structure. Furthermore, extremely high left-handed transmission efficiency （〉99%） is also investigated. Demonstration of near and far-field resonance patterns reveal the nano-photonic device applications potential i.e. highly directional optical nanoantenna, filter, etc.

Influence of particle structure on electrochemical character of composite graphite

The natural graphite has been used as the anode material for Lithium-Ion batteries, because of its low cost, chemical stability and excellent reversibility for Li+ insertion. However, the slow diffusion rate of lithium ion and poor compatibility with electrolyte solutions make it difficult to use in some conditions. In order to solve these problems, an epoxy-coke/graphite composite has been manufactured. The particle of composite carbonaceous material coated on non-graphitizable (hard) carbon matrix. Due to the disordered structure, the diffusion rate of lithium species in the non-graphitzable carbon is remarkably fast and less anisotropic. The process for preparing a composite carbon powder provides a promising new anode material with superior electrochemical properties for Li-ion batteries. The unique structure of epoxy-coke/graphite composite electrodes results in much better kinetics, also better recharge ability and initial charge/discharge efficiency.

The phase transition, hygroscopicity, and thermal expansion properties of Yb_(2-x)Al_xMo_3O_(12)

Materials with the formula Yb2-xAlxMo3O12 （x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, and 1.8） were synthesized and their structures, phase transitions, and hygroscopicity investigated using X-ray powder diffraction, Raman spectroscopy, and thermal analysis. It is shown that Yb2-xAlxMo3012 solid solutions crystallize in a single monoclinic phase for 1.7 〈 x 〈 2.0 and in a single orthorhombic phase for 0.0 〈 x 〈 0,4, and exhibit the characteristics of both monoclinic and orthorhombic structures outside these compositional ranges. The monoclinic to orthorhonlbic phase transition temperature of A12Mo3012 can be reduced by partial substitution of A13＋ by Yb3＋, and the Yb2-zAlxMo3012 （0.0 〈 x 〈 2.0） materials are hydrated at room temperature and contain two kinds of water species. One of these interacts strongly with and hinders the motions of the polyhedra, while the other does not. The partial substitution of A13＋ for Yb3＋ in Yb2Mo3012 decreases its hygroscopicity, and the linear thermal expansion coefficients after complete removal of water species are measured to be -9.1 x 10-6/K, -5.5 x 10-6/K, 5.74 x 10-6/K, and 9.5 x 10 6/K for Ybl.sAlo.2（MoO4）3, Yb1.6Alo.4（MoO4）3, Ybo.4All.6（Mo04）3, and Ybo.2Al1.8（MoO4）3, respectively.

Negative thermal expansion: Mechanisms and materials

Negative thermal expansion(NTE)of materials is an intriguing phenomenon challenging the concept of traditional lattice dynamics and of importance for a variety of applications.Progresses in this field develop markedly and update continuously our knowledge on the NTE behavior of materials.In this article,we review the most recent understandings on the underlying mechanisms(anharmonic phonon vibration,magnetovolume effect,ferroelectrorestriction and charge transfer)of thermal shrinkage and the development of NTE materials under each mechanism from both the theoretical and experimental aspects.Besides the low frequency optical phonons which are usually accepted as the origins of NTE in framework structures,NTE driven by acoustic phonons and the interplay between anisotropic elasticity and phonons are stressed.Based on the data documented,some problems affecting applications of NTE materials are discussed and strategies for discovering and design novel framework structured NET materials are also presented.

Electrolytic silicon/graphite composite from SiO_(2)/graphite porous electrode in molten salts as a negative electrode material for lithium-ion batteries

Nano-silicon(nano-Si)and its composites have been regarded as the most promising negative electrode materials for producing the next-generation Li-ion batteries(LIBs),due to their ultrahigh theoretical capacity.However,the commercial applications of nano Si-based negative electrode materials are constrained by the low cycling stability and high costs.The molten salt electrolysis of SiO_(2)is proven to be suitable to produce nano-Si with the advantages of in-situ microstructure control possibilities,cheap affordability and scale-up process capability.Therefore,an economical approach for electrolysis,with a SiO_(2)/graphite porous electrode as cathode,is adopted to produce nano-Si/graphite composite negative electrode materials(SGNM)in this study.The electrolytic product of the optimized porous electrode is taken as the negative electrode materials for LIBs,and it offers a capacity of 733.2 mAh·g^(-1)and an initial coulombic efficiency of 86.8%in a coin-type cell.Moreover,the capacity of the SGNM retained 74.1%of the initial discharging capacity after 50 cycles at 0.2C,which is significantly higher than that of the simple mixture of silicon and graphite obtained from the formation of silicon carbide(SiC)between nano-Si and graphite particles.Notably,this new approach can be applied to a large-scale production.

o-ZrW_(1.6)Mo_(0.4)O_8:A Novel Orthorhombic Intermediate Phase Formed During the Synthesis of the Negative Thermal Expansion Cubic ZrW_(1.6)Mo_(0.4)O_8 Material by the Precursor Dehydration Route

The mechanism of the precursor dehydration route was revealed for the synthesis of NTE c-ZrW_ 1.6 Mo_ 0.4 O_8. The hydrate precursor was dehydrated at 473 K and transformed to a NTE cubic compound above 800 K. A novel intermediate phase o-ZrW_ 1.6 Mo_ 0.4 O_8 occurs between the temperature range of 573—800 K. The XRD pattern of novel intermediate was refined with the structural model of LT-ZrMo_2O_8 by using Rietveld method. The residuals are R_ wp =7.80% and R_p=5.79%. The space group is Pmn2_1 and the lattice parameters are a=0.5917(4) nm, b=0.7273(4) nm, c=0.9148(6) nm, and Z=2.

Realization of absolute negative refraction index by a photonic crystal using anisotropic dielectric material

A method to realize absolute negative refraction index -1 with a two-dimensional （2D） photonic crystal is presented by introducing dielectric anisotropy in the photonic crystal material. The band structures of E-polarization mode and H-polarization mode can be adjusted by changing the parameters of materials. Thus the two modes with different polarizations have the same negative refraction index -1 for the same frequency. The results are demonstrated by numerical simulation based on the finite-difference time-domain （FDTD） method.

Novel Biomass-derived Hollow Carbons as Anode Materials for Lithium-ion Batteries

A novel hollow carbon derived from biomass lotus-root has been prepared by a one-step carbonization method.The carbon anode obtained at 900℃ showed the best electrochemical performance,corresponding to a high specific capacity of 445 mA·h/g at 0.1 C,as well as excellent cycling stability after 500 cycles.Further investigation exhibits that the lithium storage of hollow carbon involves Li^(+) adsorption in the defect sites and Li^(+) insertion.The results showed that the intrinsic structure of lotus root can inspire us to prepare biomass carbon with a hollow structure as an excellent anode for lithium-ion batteries.

Phase transition and thermal expansion property of Cr_(2-x)Zr_(0.5x)Mg_(0.5x)Mo_3O_(12) solid solution

Compounds with the formula Cr2-xZr0.5xMg0.5xMo3O12（x = 0.0, 0.3, 0.5, 0.9, 1.3, 1.5, 1.7, 1.9） are synthesized, and the effects of Zr4＋ and Mg2＋ co-incorporation on the phase transition, thermal expansion, and Raman mode are investigated. It is found that Cr2-xZr0.5xMg0.5xMo3O12 crystallize into monoclinic structures for x 〈 1.3 and orthorhombic structures for x _〉 1.5 at room temperature. The phase transition temperature from a monoclinic to an orthorhombic structure of Cr2Mo3O12 can be reduced by the partial substitution of （ZrMg）6＋ for Cr3＋. The overall linear thermal expansion coefficient decreases with the increase of the （ZrMg）6＋ content in an orthorhombic structure sample. The co-incorporation of Zr4＋ and Mg2＋ in the lattice results in the occurrence of new Raman modes and the hardening of the symmetric vibrational modes, which are attributed to the MoO4 tetrahedra sharing comers with ZrO6/MgO6 octahedra and to the strengthening of Mo-O bonds due to less electronegativities of Zr4＋ and Mg2＋ than Cr3＋, respectively.

Near-zero thermal expansion inβ-CuZnV_(2)O_(7)in a large temperature range

We report a new type of near-zero thermal expansion materialβ-CuZnV_(2)O_(7)in a large temperature range from 173 K to 673 K.It belongs to a monoclinic structure(C2/c space group)in the whole temperature range.No structural phase transition is observed at atmospheric pressure based on the x-ray diffraction and Raman experiment.The high-pressure Raman experiment demonstrates that two structural phase transitions exist at 0.94 GPa and 6.53 GPa,respectively.The mechanism of negative thermal expansion inβ-CuZnV_(2)O_(7)is interpreted by the variations of the angles between atoms intuitively and the phonon anharmonicity intrinsically resorting to the negative Grüneisen parameter.

Energy flow in negative index materials

Prom Maxwell's equations, we compute the speed and the direction of propagation of active power refracted from air into a negative index material. We prove, both analytically and numerically that the power may refract positively even if phase fronts refract negatively. Considerations on the usage of ray optics in problems involving negative index materials are drawn.

Effects of Annealing Temperature on Microstructure and Electrochemical Properties of Perovskite-type Oxide LaFeO3 as Negative Electrode for Metal Hydride/Nickel（MH/Ni） Batteries

We reported the effects of annealing temperatures on microstructure and electrochemical properties of perovskite-type oxide LaFeO3 prepared by stearic acid combustion method. X-Ray diffraction（XRD） patterns show that the annealed LaFeO3 powder has orthorhombic structure. Scanning electron microscopy（SEM） and transmission electron microscopy（TEM） images show the presence of homogeneously dispersed, less aggregated, and small crystals（30--40 nm） at annealing temperatures of 500 and 600 ℃. However, as the annealing temperature was increased to 700 and 800 ℃, the crystals began to combine with each other and grew into further larger crystals（90--100 nm）. The electrochemical performance of the annealed oxides was measured at 60 ℃ using chronopotentiometry, potentiodynamic polarization, and cyclic voltammetry. As the annealing temperature increased, the discharge capacity and anti-corrosion ability of the oxide electrode first increased and then decreased, reaching the optimum values at 600 ℃, with a maximum discharge capacity of 563 mA-h/g. The better electrochemical performance of LaFeO3 annealed at 600℃ could be ascribed to their smaller and more homogeneous crysals.

表面氧化构建PPy@VNO/NG核壳结构作为长寿命超级电容器负极材料

高容量、长寿命负极材料的开发是限制超级电容器发展的重要因素.氮化钒(VN)作为一种新兴超级电容器负极材料,具有较高的理论比容量(1340 F g^(-1))和优异的电导率(1.6×10^(6)S m^(-1)),然而其循环性能较差,容量衰减迅速.研究表明,在水系电解液中VN充放电过程中不可逆氧化形成的水溶性钒氧离子是其容量衰退的主要原因.本文利用表面氧化辅助策略原位构建了Ppy@VNO二元核壳纳米结构,并将其复合到氮掺杂石墨烯(NG)所筑的多孔导电网络骨架中.该材料不仅兼具VNO高容量和Ppy导电的优点,而且表面氧化过程进一步优化了VN的价态组合,有效改善了其电化学储能性能.该材料作为超级电容器负极材料实现了650 F g^(-1)的较高容量以及万次循环后容量的高稳定性.分析表明价态优化后的VN能够维持不同价态V的含量,证明了在充放电循环过程中V可在不同价态下进行可逆转换,从而使得其保持优异的稳定性.