Xuqing Zhang

A hybrid Si@FeSiy/SiOx anode structure for high performance lithium-ion batteries via ammonia-assisted one-pot synthesis

Synthesised via planetary ball-milling of Si and Fe powders in an ammonia (NH3) environment, a hybrid Si@FeSiy/SiOx structure shows exceptional electrochemical properties for lithium-ion battery anodes, exhibiting a high initial capacity of 1150 mA h g−1 and a retention capacity of 880 mA h g−1 after 150 cycles at 100 mA g−1; and a capacity of 560 mA h g−1 at 4000 mA g−1. These are considerably high for carbon-free micro-/submicro-Si-based anodes. NH3 gradually turns into N2 and H2 during the synthesis, which facilitates the formation of highly conductive FeSiy (y = 1, 2) phases, whereas such phases were not formed in an Ar atmosphere. Milling for 20–40 h leads to partial decomposition of NH3 in the atmosphere, and a hybrid structure of a Si core of mixed nanocrystalline and amorphous Si domains, shelled by a relatively thick SiOx layer with embedded FeSi nanocrystallites. Milling for 60–100 h results in full decomposition of NH3 and a hybrid structure of a much-refined Si-rich core surrounded by a mantle of a relatively low level of SiOx and a higher level of FeSi2. The formation mechanisms of the SiOx and FeSiy phases are explored. The latter structure offers an optimum combination of the high capacity of a nanostructural Si core, relatively high electric conductivity of the FeSiy phase and high structural stability of a SiOx shell accommodating the volume change for high performance electrodes. The synthesis method is new and indispensable for the large-scale production of high-performance Si-based anode materials.

Construction of Nitrogen-Doped Carbon-Coated MoSe2 Microspheres with Enhanced Performance for Lithium Storage

Exploring advanced anode materials with highly reversible capacity have gained great interests for large-scale lithium storage. A facile two-step method is developed to synthesize nitrogen-doped carbon coated MoSe2 microspheres via hydrothermal plus thermal polymerization. The MoSe2 microspheres composed of interconnected nanoflakes are homogeneously coated by a thin nitrogen-doped carbon (N-C) layer. As an anode for lithium ion batteries, the MoSe2 /N-C composite shows better reversibility, smaller polarization, and higher electrochemical reactivity as compared to the unmodified MoSe2 microspheres. The MoSe2 /N-C electrode delivers a high specific capacity of 698u2005mAhu2009g-1 after 100u2005cycles at a current density of 100u2005mAu2009g-1 and good high rate performance (471u2005mAhu2009g-1 at a high current density of 2000u2005mAu2009g-1 ). The improved electrochemical performance is attributed to the conductive N-C coating and hierarchical microsphere structure with fast ion/electron transfer characteristics.

Facile and scalable synthesis of nanosized core–shell Li2S@C composite for high-performance lithium–sulfur batteries

Lithium sulfide (Li2S) is one of the most promising cathode materials for the next-generation advanced Li-ion batteries because of its high theoretical capacity (1167 mA h g−1) and large energy density. However, Li2S suffers from poor rate performance and short cycle life due to its insulating nature and polysulfide shuttle during cycling. In this work, we have proposed a facile and scalable strategy for the synthesis of nanosized Li2S particles via a solution-based method. For further application in Li–S batteries, chemical vapor deposition (CVD) technology is used to successfully deposit a uniform conductive carbon layer on the Li2S particles to obtain a core–shell structured nanocomposite (nano-Li2S@C). The average size of the as-prepared nano-Li2S particles is around 100 nm, and covered by a carbon shell with a thickness of ∼20 nm. These nanoscale Li2S@C particles guarantee a short diffusion distance of lithium ions and the protective carbon layer allows fast electron transport as well as effectively constraining the migration of the soluble polysulfides. As a result, the core–shell structured nano-Li2S@C cathodes show outstanding electrochemical performance with a high initial discharge capacity of 1083.5 mA h g−1 at 0.2C and 766.3 mA h g−1 after 200 cycles with a low decay of 0.15% per cycle. The enhanced electrochemical performance is due to the core-shelled architecture with enhanced electrical conductivity and better suppression effect for the polysulfide shuttle.

Backward Guiding of Terahertz Radiation in Periodic Dielectric Waveguides

The guiding properties of periodic dielectric waveguides (PDWGs) are investigated theoretically at terahertz frequencies for both two- and three-dimensional model systems. It is shown that in a PDWG there may exist several bound modes, and among them often occur backward modes with antiparallel phase velocity and energy flow. The backward guiding behavior of the PDWG is demonstrated by its contra-directional coupling with a conventional dielectric waveguide (CDWG). For the coupler formed by a PDWG and a CDWG, the coupling direction of energy flow is selectable, since the PDWG supports either a forward or a backward mode at different frequencies.

Carbon fiber-incorporated sulfur/carbon ternary cathode for lithium–sulfur batteries with enhanced performance

Smart construction of advanced sulfur cathodes is indispensable for the development of high performance lithium–sulfur (Li–S) batteries. Hence, we report a novel modified sulfur cathode using conductive carbon black as carrier to load sulfur and vapor grown carbon fiber as bridges to connect sulfur/carbon black clusters. The carbon fiber-incorporated sulfur/carbon ternary electrode exhibits superior electrochemical performance with an initial discharge capacity of 1112xa0mAhxa0g−1 at 0.2xa0C (1xa0Cxa0=xa01675xa0mAhxa0g−1) and 758xa0mAhxa0g−1 at 1xa0C. And it maintains much higher Coulombic efficiency and capacity retention after 200xa0cycles than those of the sulfur/carbon binary electrode. Moreover, rate capability of the ternary electrode is enhanced greatly. The improved electrochemical performance is attributed to the addition of carbon fiber, which provides convenient paths for rapid transfer of electrons during the redox reaction of sulfur.

Performance Enhancement of a Sulfur/Carbon Cathode by Polydopamine as an Efficient Shell for High‐Performance Lithium–Sulfur Batteries

Lithium-sulfur batteries (LSBs) are considered to be among the most promising next-generation high-energy batteries. It is a consensus that improving the conductivity of sulfur cathodes and impeding the dissolution of lithium polysulfides are two key accesses to high-performance LSBs. Herein we report a sulfur/carbon black (S/C) cathode modified by self-polymerized polydopamine (pDA) with the assistance of polymerization treatment. The pDA acts as a novel and effective shell on the S/C cathode to stop the shuttle effect of polysulfides. By the synergistic effect of enhanced conductivity and multiple blocking effect for polysulfides, the S/C@pDA electrode exhibits improved electrochemical performances including large specific capacity (1135u2005mAhu2009g-1 at 0.2u2005C), high rate capability (533u2005mAhu2009g-1 at 5u2005C) and long cyclic life (965u2005mAhu2009g-1 after 200u2005cycles). Our smart design strategy may promote the development of high-performance LSBs.

Metal-Embedded Porous Graphitic Carbon Fibers Fabricated from Bamboo Sticks as a Novel Cathode for Lithium–Sulfur Batteries

Lithium-sulfur batteries (LSBs) are deemed to be among the most prospective next-generation advanced high-energy batteries. Advanced cathode materials fabricated from biological carbon are becoming more popular due to their unique properties. Inspired by the fibrous structure of bamboo, herein we put forward a smart strategy to convert bamboo sticks for barbecue into uniform bamboo carbon fibers (BCF) via a simple hydrothermal treatment proceeded in alkaline solution. Then NiCl2 is used to etch the fibers through a heat treatment to achieve Ni-embedded porous graphitic carbon fibers (PGCF/Ni) for LSBs. The designed PGCF/Ni/S electrode exhibits improved electrochemical performances including high initial capacity (1198 mAh g-1 at 0.2 C), prolonged cycling life (1030 mAh g-1 at 0.2 C after 200 cycles), and improved rate capability. The excellent properties are attributed to the synergistic effect of 3D porous graphitic carbon fibers with highly conductive Ni nanoparticles embedded.

Hierarchical MoS2/Carbon Composite Microspheres as Advanced Anodes for Lithium/Sodium-Ion Batteries

It is crucial to design advanced electrodes with large Li/Na-ion storage capacities for the development of next-generation battery systems. Herein, hierarchical MoS2 /C composite microspheres were constructed by facile template-free self-assembly sulfurization plus post-carbonization. Cross-linked MoS2 nanosheets and outer carbon layer are organically combined together to form composite microspheres with diameters of 400-500u2005nm. Due to enhanced electrical conductivity and good structural stability, the MoS2 /C composite microspheres exhibit substantially improved Li/Na-ion storage performance. Compared to unmodified MoS2 , MoS2 /C composite microspheres deliver higher Li/Na-ion storage capacity (Li+ : 1017u2005mAu2009hu2009g-1 at 100u2005mAu2009g-1 and Na+ : 531u2005mAu2009hu2009g-1 at 100u2005mAu2009g-1 ), as well as better rate capability (Li+ : 434u2005mAu2009hu2009g-1 at 1u2005Ag-1 and Na+ : 102u2005mAu2009hu2009g-1 at 1u2005Ag-1 ) and capacity retention (Li+ : 902u2005mAu2009hu2009g-1 after 200u2005cycles and Na+ : 342u2005mAu2009hu2009g-1 over 100u2005cycles). The superior Li/Na-ion storage performance is mainly attributed to the unique porous microsphere architecture with increased electrode/electrolyte interfaces and more diffusion paths for Li/Na ion insertion. Additionally, the carbon coating can not only improve the electronic conductivity, but also suppress the shuttle effect of polysulfides.

A 3D conductive network with high loading Li2S@C for high performance lithium–sulfur batteries

Construction of novel cathodes with a high loading of active material and excellent confinement effect for polysulfides is indispensable and vital for the realization of high-energy and commercially viable lithium–sulfur batteries. In this study, we demonstrated a facile strategy for the fabrication of an integrated lithium sulfide (Li2S) cathode by incorporating lithium sulfide (Li2S)/carbon black (CB) into the carbon felt (CF) with a 3D conductive network and further modifying it by an outer amorphous carbon shell (CF–CB–Li2S@C) via facile liquid solution-evaporation plus chemical vapor deposition technologies. The CF with abundant macroporous channels provides enough reaction sites to load and stabilize a high amount of active materials. The inter-connected conductive network and efficient carbon shell not only provide efficient electron transport and guarantee high active material utilization, but also form a durable protective shield for suppressing polysulfide dissolution. As a result, the CF–CB–Li2S@C cathode with a high loading of 7 mg cm−2 demonstrates an initial discharge capacity of 943.7 mA h g−1 (6.60 mA h cm−2) at 0.1C. Importantly, it still maintains a capacity of 567.5 mA h g−1 (3.97 mA h cm−2) at 1C after 200 cycles, corresponding to a low fading rate of 0.12% per cycle.