Yongfeng Liu

Dielectric relaxations in Ba(Fe1∕2Ta1∕2)O3 giant dielectric constant ceramics

Dielectric relaxations of Ba(Fe1∕2Ta1∕2)O3 ceramics were investigated and discussed over a broad temperature and frequency range. Two dielectric relaxations following Arrhenius law were observed at 153–382 and 440–623K, where there was a giant dielectric constant step between them. The frequency dependent rapid drop of dielectric constant at 153–382K was nearly a Debye relaxation with the intrinsic nature, while the high temperature dielectric relaxation with an extremely high dielectric constant peak and very strong frequency dispersion was attributed to the defect ordering but not a typical relaxor ferroelectric behavior. The O2 annealing almost completely suppressed the dielectric peak and subsequently extended the giant dielectric step, while the low temperature dielectric relaxation and the magnitude of such step were not obviously affected.

A mechanical-force-driven physical vapour deposition approach to fabricating complex hydride nanostructures

Nanoscale hydrides desorb and absorb hydrogen at faster rates and lower temperatures than bulk hydrides because of their high surface areas, abundant grain boundaries and short diffusion distances. No current methods exist for the direct fabrication of nanoscale complex hydrides (for example, alanates, borohydrides) with unique morphologies because of their extremely high reducibility, relatively low thermodynamic stability and complicated elemental composition. Here, we demonstrate a mechanical-force-driven physical vapour deposition procedure for preparing nanoscale complex hydrides without scaffolds or supports. Magnesium alanate nanorods measuring 20-40u2009nm in diameter and lithium borohydride nanobelts measuring 10-40u2009nm in width are successfully synthesised on the basis of the one-dimensional structure of the corresponding organic coordination polymers. The dehydrogenation kinetics of the magnesium alanate nanorods are improved, and the nanorod morphology persists through the dehydrogenation-hydrogenation process. Our findings may facilitate the fabrication of such hydrides with improved hydrogen storage properties for practical applications.

Synergetic effects of in situ formed CaH2 and LiBH4 on hydrogen storage properties of the Li-Mg-N-H system.

Hydrogen storage properties and mechanisms of the Ca(BH(4))(2)-doped Mg(NH(2))(2)-2LiH system are systematically investigated. It is found that a metathesis reaction between Ca(BH(4))(2) and LiH readily occurs to yield CaH(2) and LiBH(4) during ball milling. The Mg(NH(2))(2) -2LiH-0.1Ca(BH(4))(2) composite exhibits optimal hydrogen storage properties as it can reversibly store more than 4.5 wt% of H(2) with an onset temperature of about 90 °C for dehydrogenation and 60 °C for rehydrogenation. Isothermal measurements show that approximately 4.0 wt% of H(2) is rapidly desorbed from the Mg(NH(2))(2) -2LiH-0.1Ca(BH(4))(2) composite within 100 minutes at 140 °C, and rehydrogenation can be completed within 140 minutes at 105 °C and 100 bar H(2). In comparison with the pristine sample, the apparent activation energy and the reaction enthalpy change for dehydrogenation of the Mg(NH(2))(2)-2LiH-0.1Ca(BH(4))(2) composite are decreased by about 16.5% and 28.1%, respectively, and thus are responsible for the lower operating temperature and the faster dehydrogenation/hydrogenation kinetics. The fact that the hydrogen storage performances of the Ca(BH(4))(2)-doped sample are superior to the individually CaH(2)- or LiBH(4)-doped samples suggests that the in situ formed CaH(2) and LiBH(4) provide a synergetic effect on improving the hydrogen storage properties of the Mg(NH(2))(2)-2LiH system.

Tailoring Thermodynamics and Kinetics for Hydrogen Storage in Complex Hydrides towards Applications

Solid-state hydrogen storage using various materials is expected to provide the ultimate solution for safe and efficient on-board storage. Complex hydrides have attracted increasing attention over the past two decades due to their high gravimetric and volumetric hydrogen densities. In this account, we review studies from our lab on tailoring the thermodynamics and kinetics for hydrogen storage in complex hydrides, including metal alanates, borohydrides and amides. By changing the material composition and structure, developing feasible preparation methods, doping high-performance catalysts, optimizing multifunctional additives, creating nanostructures and understanding the interaction mechanisms with hydrogen, the operating temperatures for hydrogen storage in metal amides, alanates and borohydrides are remarkably reduced. This temperature reduction is associated with enhanced reaction kinetics and improved reversibility. The examples discussed in this review are expected to provide new inspiration for the development of complex hydrides with high hydrogen capacity and appropriate thermodynamics and kinetics for hydrogen storage.

Improved hydrogen storage performance of Ca(BH4)2: a synergetic effect of porous morphology and in situ formed TiO2

A porous Ca(BH4)2-based hydride, CaB2H7, with nano-TiO2 introduced in situ, was successfully synthesized via mixing Ca(BH4)2 with Ti(OEt)4 followed by heat treatment. The effects of the porous structure and introduction of TiO2 on both the non-isothermal and isothermal hydrogen desorption–absorption properties of the porous system were systematically investigated. The results show significant improvements on both the kinetics and thermodynamics of hydrogen desorption–absorption of the porous CaB2H7–0.1TiO2 system, compared with the dense Ca(BH4)2. The desorption peak temperature is reduced by more than 50 °C and sorption capacity of ca. 5 wt% H2 is rapidly achieved below 300 °C. The porous structure was retained in the dehydrogenated products, and rapid hydrogen absorption, approximately 80% of the desorption capacity, is obtained upon heating the product, post-dehydrogenated at 300 °C for 1 h, to 350 °C at 90 bar H2. External addition of nano-TiO2 also enhances the hydrogen storage properties of Ca(BH4)2, but to a lesser extent, compared with the synergetic effect of the porous structure and the in situ formed nano-TiO2. In addition, desorption–absorption mechanisms of the porous CaB2H7–0.1TiO2 system are also proposed.

Synthesis and Thermal Decomposition Behaviors of Magnesium Borohydride Ammoniates with Controllable Composition as Hydrogen Storage Materials

An ammonia-redistribution strategy for synthesizing metal borohydride ammoniates with controllable coordination number of NH(3) was proposed, and a series of magnesium borohydride ammoniates were easily synthesized by a mechanochemical reaction between Mg(BH(4))(2) and its hexaammoniate. A strong dependence of the dehydrogenation temperature and purity of the released hydrogen upon heating on the coordination number of NH(3) was elaborated for Mg(BH(4))(2)·xNH(3) owing to the change in the molar ratio of H(δ+) and H(δ-), the charge distribution on H(δ+) and H(δ-), and the strength of the coordinate bond N:→Mg(2+). The monoammoniate of magnesium borohydride (Mg(BH(4))(2)·NH(3)) was obtained for the first time. It can release 6.5% pure hydrogen within 50 minutes at 180 °C.

Chemical Preinsertion of Lithium: An Approach to Improve the Intrinsic Capacity Retention of Bulk Si Anodes for Li-ion Batteries

Silicon represents one of the most promising anodes for next-generation Li-ion batteries due to its very high capacity and low electrochemical potential. However, the extremely poor cycling stability caused by the huge volume change during charge/discharge prevents it from the commercial use. In this work, we propose a strategy to decrease the intrinsic volume change of bulk Si-based anodes by preinsertion Li into Si with a chemical reaction. Amorphous Li12Si7 was successfully synthesized by a hydrogen-driven reaction between LiH and Si associated with subsequent energetic ball milling. The as-prepared amorphous Li12Si7 anode exhibits significantly improved lithium storage ability as ∼70.7% of the initial charge capacity is retained after 20 cycles. This finding opens up the possibility to develop bulk Si-based anodes with high capacity, long cycling life and low fabrication cost for Li-ion batteries.

Achieving ambient temperature hydrogen storage in ultrafine nanocrystalline TiO2@C-doped NaAlH4

Sodium alanate (NaAlH4) has attracted tremendous interest as a prototypical high-density complex hydride for on-board hydrogen storage. However, poor reversibility and slow kinetics limit its practical application. In this paper, we propose a novel strategy for the preparation of an ultrafine nanocrystalline TiO2@C-doped NaAlH4 system by first calcining the furfuryl alcohol-filled MIL-125(Ti) at 900 °C and then ball milling with NaAlH4 followed by a low-temperature activation process at 150 °C under 100 bar H2. The as-prepared NaAlH4-9 wt% TiO2@C sample releases hydrogen starting from 63 °C and re-absorbs starting from 31 °C, which are reduced by 114 °C and 54 °C relative to those of pristine NaAlH4, respectively. At 140 °C, approximately 4.2 wt% of hydrogen is released within 10 min, representing the fastest dehydrogenation kinetics of any presently known NaAlH4 system. More importantly, the dehydrogenated sample can be fully hydrogenated under 100 bar H2 even at temperatures as low as 50 °C, thus achieving ambient-temperature hydrogen storage. The synergetic effect of the Al–Ti active species and carbon contributes to the significantly reduced operating temperatures and enhanced kinetics.

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.

Role of Co3O4 in improving the hydrogen storage properties of a LiBH4–2LiNH2 composite

Adding a small amount of Co3O4 significantly reduces the operating temperatures of dehydrogenation and improves the hydrogen storage reversibility of the LiBH4–2LiNH2 system. The LiBH4–2LiNH2–0.05/3Co3O4 composite desorbs ∼9.9 wt% hydrogen by a four-step reaction with a 96 °C reduction in the midpoint temperature with respect to the pristine sample. The first and third steps of the dehydrogenation of the 0.05/3Co3O4-added sample are endothermic in nature, which is different from the pristine sample. Upon thermal dehydrogenation, the Co3O4 additive undergoes a series of chemical transformations and finally converts to the metallic Co, which is responsible for the improved thermodynamics and kinetics of the Co3O4-added sample. More importantly, 1.7 wt% of hydrogen is recharged into the 0.05/3Co3O4-added system under 110 bar hydrogen at 220 °C, which is superior to the pristine system.