Huan Ye

An Advanced Selenium–Carbon Cathode for Rechargeable Lithium–Selenium Batteries†

The rapidly developing market for mobile electronics and hybrid electric vehicles (HEVs) has prompted the urgent need for batteries with high energy density, long cycle life, high efficiency, and low cost. Recently, rechargeable lithium-sulfur (Li–S) batteries have attracted considerable attention because of their high theoretical gravimetric (volumetric) energy density of 2570 Wh kg 1 (2200 Whl ), and low cost. However, the use of S as cathode material for Li–S batteries suffers from two major issues. One is the insulating nature of S, which results in low active-material utilization and limited rate capability. The other is the formation of electrolytesoluble polysulfides; these polysulfide intermediates, which are generated in the discharge/charge process, dissolve in the electrolyte and migrate to the Li anode, a process known as the shuttle effect. Consequently, the S cathode suffers a significant loss of S during cycling, resulting in a rapid capacity decrease. Many strategies have been used to address these problems, such as the impregnation of S into various conductive porous matrixes, surface coating of S, and the use of suitable electrolytes and additives. Although remarkable improvements have been achieved, the application of Li–S batteries is still hindered by the intrinsic drawbacks of S. Therefore, it is of great importance to explore and develop new high-energy cathode materials with improved electronic conductivity and cycling stability, to cover the shortfalls of S and provide alternative choices for practical applications. From this perspective, selenium, an element belonging to the same group in the periodic table as sulfur, is a prospective candidate for cathode materials. Although Se has a lower theoretical gravimetric capacity (675 mAhg ) than S (1675 mAh g ), its higher density (ca. 2.5 times that of S) offsets the deficiency and provides a high theoretical volumetric capacity density (3253 mAh cm ), comparable to that of S (3467 mAh cm ). It has been reported that Li–Se batteries deliver a high output voltage, so Li–Se batteries are also expected to have a high volumetric energy density. It is known that for applications in portable devices and HEVs, volumetric energy density is more important than gravimetric energy density because of the limited battery packing space. Moreover, the electronic conductivity of Se (1 10 3 Sm ) is considerably higher than that of S (5 10 28 Sm ), which suggests that Se could have higher utilization rate, better electrochemical activity, and faster electrochemical reaction with Li. Therefore, the advantages of Se promise an attractive alternative cathode material for building high-energy batteries for specific applications, including consumer electronics and transportation. However, at present, research on Li–Se batteries is still at a very early stage. Recently, Abouimrane et al. conducted pioneering work on the use of Se as a cathode material. The results show that, even bulk Se has an active material utilization of ca. 45% upon cycling, which is not commonly observed in Li–S batteries with a bulk S cathode. This suggests that Se cathode has a much better activity and a weaker shuttle effect than S. Nevertheless, bulk Se cannot completely deliver the theoretical capacity. Moreover, given the weak interaction between bulk Se and the conductive substrate, the polyselenide species generated during the Li uptake/release process cannot be effectively restrained on the cathode side. Thus, the shuttle effect of Se is not eliminated, which deteriorates the cycling performance of the Se cathode. To address these issues, encapsulation of Se molecules into a conductive porous carbon matrix may greatly improve the electrochemical performance of Se. However, this assumption has not yet been demonstrated, and the mechanism of the electrochemical reaction between Se molecules and Li remains unclear to date. Herein, we report a Se composite cathode material, in which Se is confined as cyclic Se8 molecules in the mesopores of an ordered mesoporous carbon (CMK-3) matrix. When assembled into Li–Se batteries with the water-soluble binder sodium alginate (SA), the Se/CMK-3 composite exhibits novel electrochemical behavior with a single plateau in the discharge/charge process. Data from ex situ Raman and X-ray diffraction (XRD) analysis suggest that this behavior is due to the conversion of cyclic Se8 molecules into chain-like Sen molecules in the carbon channels. Given the high electrochemical activity of the chain-like Sen molecules and the strong interaction between them and the carbon mesopores, this Se cathode shows a high capacity that approaches the theoretical value of Se, and exhibits favorable capacity retention upon cycling. The Se/CMK-3 composite was synthesized through a facile melt-diffusion process from a ball-milled mixture of [*] C.-P. Yang, S. Xin, Dr. Y.-X. Yin, H. Ye, J. Zhang, Prof. Y.-G. Guo CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS) Beijing 100190 (P. R. China) E-mail: ygguo@iccas.ac.cn

Insight into the effect of boron doping on sulfur/carbon cathode in lithium-sulfur batteries.

To exploit the high energy density of lithium-sulfur batteries, porous carbon materials have been widely used as the host materials of the S cathode. Current studies about carbon hosts are more frequently focused on the design of carbon structures rather than modification of its properties. In this study, we use boron-doped porous carbon materials as the host material of the S cathode to get an insightful investigation of the effect of B dopant on the S/C cathode. Powder electronic conductivity shows that the B-doped carbon materials exhibit higher conductivity than the pure analogous porous carbon. Moreover, by X-ray photoelectron spectroscopy, we prove that doping with B leads to a positively polarized surface of carbon substrates and allows chemisorption of S and its polysulfides. Thus, the B-doped carbons can ensure a more stable S/C cathode with satisfactory conductivity, which is demonstrated by the electrochemical performance evaluation. The S/B-doped carbon cathode was found to deliver much higher initial capacity (1300 mA h g(-1) at 0.25 C), improved cyclic stability, and rate capability when compared with the cathode based on pure porous carbon. Electrochemical impedance spectra also indicate the low resistance of the S/B-doped C cathode and the chemisorption of polysulfide anions because of the presence of B. These features of B doping can play the positive role in the electrochemical performance of S cathodes and help to build better Li-S batteries.

Ab initio elastic constants for the lonsdaleite phases of C, Si and Ge

The elastic constants of lonsdaleite C, Si and Ge are calculated by using the plane-wave pseudopotential method in the scheme of density functional theory and the local density approximation. For comparison, the elastic constants of the cubic diamond phases of these elements, zincblende SiC and 6H-SiC, are also calculated.

Tuning the porous structure of carbon hosts for loading sulfur toward long lifespan cathode materials for Li–S batteries

As a crucial component, carbon substrates with appropriate porous structures are highly desired in developing sulfur–carbon cathodes for Li–S batteries with superior performance. Here we show that the electrochemical performance of the sulfur–carbon cathode can be easily adjusted by tuning the pore structure of the carbon substrate. With potassium hydroxide as the activation agent, a series of micro-/mesoporous carbon hosts have been prepared via chemical activation of hydrothermal carbon precursors. The pore structure of the carbon host can be easily controlled by adjusting the activation concentration of KOH, and is found to be directly related to the battery performance of sulfur loaded inside. An optimized pore structure is yielded at a KOH concentration of 1 M, at which the sulfur–carbon cathode shows a high specific capacity, favourable rate capabilities and a long cycle life of 800 cycles at 1 C. The impressive electrochemical performances benefit from the advanced micro-/mesoporous carbon spheres with a large percentage of micropores, moderate activation and surface area.

Graphitized Carbon Fibers as Multifunctional 3D Current Collectors for High Areal Capacity Li Anodes

The Li metal anode has long been considered as one of the most ideal anodes due to its high energy density. However, safety concerns, low efficiency, and huge volume change are severe hurdles to the practical application of Li metal anodes, especially in the case of high areal capacity. Here it is shown that that graphitized carbon fibers (GCF) electrode can serve as a multifunctional 3D current collector to enhance the Li storage capacity. The GCF electrode can store a huge amount of Li via intercalation and electrodeposition reactions. The as-obtained anode can deliver an areal capacity as high as 8 mA h cm-2 and exhibits no obvious dendritic formation. In addition, the enlarged surface area and porous framework of the GCF electrode result in lower local current density and mitigate high volume change during cycling. Thus, the Li composite anode displays low voltage hysteresis, high plating/stripping efficiency, and long lifespan. The multifunctional 3D current collector promisingly provides a new strategy for promoting the cycling lifespan of high areal capacity Li anodes.

Advanced Se–C nanocomposites: a bifunctional electrode material for both Li–Se and Li-ion batteries

A bifunctional nanostructured electrode material of a selenium/micro–mesoporous carbon sphere nanocomposite (Se/MPCS) is reported. With the unusual chain-like Sen molecules hosted in a MPCS substrate, the Se/MPCS nanocomposite presents impressive electrochemical performances in metallic Li secondary batteries, i.e. Li–Se batteries. Furthermore, a new Li-ion full battery with remarkable properties is constructed by coupling the Se/MPCS anode with the traditional layered cathode. The new design of the as-assembled lithium-ion cell with the Se/MPCS anode promises good security and high capacity as well as a long lifespan.

Ab initio investigation of the pressure dependences of phonon and dielectric properties for III-V semiconductors

Theoretical results of a first-principles plane-wave pseudopotential study on the phonon and dielectric properties for the nitrides, phosphides, and arsenides of Al, Ga, and In under hydrostatic pressure are presented. The pressure dependences of the dielectric constant, phonon frequencies at the Gamma point, polarity, and localized and non-localized effective charges are calculated. We found that the dielectric constant, dynamic effective charge, and polarity decrease, while the localized effective charge and optical phonon frequencies increase with pressure for all these III-V phases. The distinctive behaviours as regards lattice dynamics of these compounds are explained on the basis of the non-localized to localized charge transference inside the crystal under pressure.

Core-shell meso/microporous carbon host for sulfur loading toward applications in lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries belong to one of the promising technologies for high-energy-density rechargeable batteries. However, sulfur cathodes suffer from inherent problems of its poor electronic conductivity and the shuttling of highly dissoluble lithium polysulfides generated during the cycles. Loading sulfur into porous carbons has been proved to be an effective approach to alleviate these issues. Mesoporous and microporous carbons have been widely used for sulfur accommodation, but mesoporous carbons have poor sulfur confinement, whereas microporous carbons are impeded by low sulfur loading rates. Here, a core-shell carbon, combining both the merits of mesoporous carbon with large pore volume and microporous carbon with effective sulfur confinement, was prepared by coating the mesoporous CMK-3 with a microporous carbon (MPC) shell and served as the carbon host (CMK-3@MPC) to accommodate sulfur. After sulfur infusion, the as-obtained S/(CMK-3@MPC) cathode delivered a high initial capacity of up to 1422 mAh·g−1 and sustained 654 niAh·g−1 reversible specific capacity after 36 cycles at 0.1 C. The good performance is ascribed to the unique core-shell structure of the CMK-3@MPC matrix, in which sulfur can be effectively confined within the meso/microporous carbon host, thus achieving simultaneously high electrochemical utilization.

First-principles study on the lonsdaleite phases of C, Si and Ge

Crystalline C, Si and Ge in a lonsdaleite (hexagonal diamond) structure are studied by plane-wave pseudopotential calculations in the scheme of density-functional theory and the local density approximation. The same calculations with generalized gradient corrections and also for the cubic diamond phases of these elements are also performed for comparison. Our results show that the bulk moduli are quite similar between the diamond and lonsdaleite polytypes of these elements. The theoretical bulk modulus of lonsdaleite C is 0.2-0.3% higher than diamond. It is expected to replace diamond as the hardest material in the world. The LDA result shows lonsdaleite Ge as a semimetal for its zero band gap at its Gamma point. Considering the exchange-correlation energy correction, it is estimated that lonsdaleite Ge is a semiconductor with a small direct band gap.

Atomic and electronic structures of the lattice mismatched metal-ceramic interface

This research purposes to investigate the atomic and electronic structures of the Al/TiC(001) interface with lattice misfit using the ab initio pseudopotential approach. A detailed analysis of the relaxed atomic structure reveals that the atoms over the initial unfavourable sites relax to the favourable sites along the lateral plane. The properties of the semicoherent interface can be taken as averages over the different coherent sites. In addition, the interface atoms in relatively favourable regions are dragged near to the interface, while those in unfavourable regions are pushed away from the interface. Therefore, a large war-ping near the interface is made perpendicular to the lateral plane. The calculated adhesions explain the different wetting results from the viewpoint of structural transition. The subsequent analysis of electronic properties demonstrates that adhesions dominate mainly via the strong Al-C covalent bond.