Synergy of inside doped metals–Outside coated graphene to enhance hydrogen storage in magnesium-based alloys

Grain growth of magnesium(Mg)and its hydride is one of the main reasons for kinetic and capacity degradation during the hydrogen absorption and desorption cycles.To solve this problem,herein we propose a novel method involving synergistic effect of inside embedded metals and outside coated graphene to limit the growth of Mg and its hydride grains.The graphene coated Mg-Y-Al alloys were selected as a model system for demonstrating this positive effect where the Mg_(91)Y_(3)Al_(6)alloy was first prepared by rapidly solidified method and then high-pressure milled with 5 wt%graphene upon 5 MPa hydrogen gas for obtaining in-situ formed YAl_(2)and YH_(3)embedded in the MgH_(2)matrix with graphene shell(denoted as MgH_(2)-Y-Al@GR).In comparison to pure MgH_(2),the obtained MgH_(2)-Y-Al@GR composites deliver much better kinetics and more stable cyclic performance.For instance,the MgH_(2)-Y-Al@GR can release about 6.1 wt%H_(2)within 30 min at 300℃ but pure MgH_(2)only desorbs∼1.5 wt%H_(2).The activation energy for desorption of MgH_(2)-Y-Al@GR samples is calculated to be 75.3±9.1 kJ/mol that is much lower than approximately 160 kJ/mol for pure MgH_(2).Moreover,its capacity retention is promoted from∼57%of pure MgH_(2)to∼84%after 50th cycles without obvious particle agglomeration and grain growth.The synergistic effect of outside graphene coating with inside embedded metals which could provide a huge number of active sites for catalysis as well as inhibit the grain growth of Mg and its hydride is believed to be responsible for these.

Anion-Regulated Weakly Solvating Electrolytes for High-Voltage Lithium Metal Batteries

Development of advanced high-voltage electrolytes is key to achieving high-energy-density lithium metal batteries(LMBs).Weakly solvating electrolytes(WSE)can produce unique anion-driven interphasial chemistry via altering the solvating power of the solvent,but it is difficult to dissolve the majority of Li salts and fail to cycle at a cut-off voltage above 4.5 V.Herein,we present a new-type WSE that is regulated by the anion rather than the solvent,and the first realize stable cycling of dimethoxyethane(DME)at 4.6 V without the use of the“solvent-in-salt”strategy.The relationships between the degree of dissociation of salts,the solvation structure of electrolytes,and the electrochemical performance of LMBs were systematically investigated.We found that LiBF_(4),which has the lowest degree of dissociation,can construct an anion-rich inner solvation shell,resulting in anion-derived anode/cathode interphases.Thanks to such unusual solvation structure and interphasial chemistry,the Li-LiCoO_(2)full cell with LiBF_(4)-based WSE could deliver excellent rate performance(115 mAh g^(-1)at 10 C)and outstanding cycling stability even under practical conditions,including high loading(10.7 mg cm^(-2)),thin Li(50μm),and limited electrolyte(1.2μL mg^(-1)).

YC_(x)F_(y)nanosheets-supported Ni nanoparticles as a high-efficient catalyst for hydrogen desorption of MgH_(2)

Magnesium hydride(MgH_(2))has been considered as a promising hydrogen storage material,but the pressing issues including sluggish kinetics and poor cyclic stability hampered its practical applications.Herein,a high-efficient catalyst comprising of YC_(x)F_(y)nanosheets-supported Ni nanoparticles(Ni_(30)/YC_(x)F_(y))was designed and constructed aiming to resolve the abovementioned restrictions facing MgH_(2).After hybridizing with Ni_(30)/YC_(x)F_(y),the as-achieved MgH_(2)–10 wt.%Ni_(30)/YC_(x)F_(y)composite exhibits superior hydrogen desorption kinetics with an activation energy of 80.9 kJ·mol^(−1)and a high capacity retention of 97.6%after 50 cycles.It is confirmed that the in situ formed Mg_(2)NiH_(4)and YH3 catalytic phases accelerate the hydrogen desorption kinetics,while the dispersed MgF_(2)and carbon species prevent the crystallite growth,particle aggregation,and catalyst redispersion,contributing an excellent cyclic stability.This work provides a new strategy to synthesize efficient catalysts for hydrogen desorption of MgH_(2).

Growth kinetics of MgH2 nanocrystallites prepared by ball milling

MgH2 is one of promising hydrogen storage materials due to its high hydrogen capacity of 7.6 wt%.However,MgH2 nanocrystallites easilygrow up during hydrogen absorption-desorption cycling,leading to deterioration of hydrogen storage properties.To clarify the growth kinetics of MgH2 crystallites,the growth characteristics of MgH2 nanocrystallites are investigated in this work.The growth exponents of MgH2 nanocrystallites in pure MgH2 and MgH2-10 wt% Pr3 Al11 samples are evaluated to be n=5 and n=6,respectively.Meanwhile,their activation energies for crystallite growth are also determined to be109.2 and 144.2 kJ/mol,respectively.The increase of growth exponent and rise of activation energy for crystallite growth in MgH2-10 wt% Pr3 Al11 composite are ascribed to the presence of nano-sized Pr3 Al11phase.

Structural features of 18R-type long-period stacking ordered phase in Mg85Zn6Y9 alloy and the Ni doping effect on its hydrogen storage properties

Magnesium-based alloys with 18R-type long-period stacking ordered(LPSO)structures have attracted wide attention for structural and functional applications.To understand hydrogen storage properties of 18R phase,the Mg_(85)Zn_(6)Y_(9)alloy with 94 wt.%of 18R-type LPSO phase is prepared in this work.The 18R phase has a layered structure where Y-Zn-Mg and Mg layers alternately stack along the c-axis.In the Y-Zn-Mg layers,Y,Zn and partial Mg sites are co-occupied by Y and Mg,Zn and Mg,and Mg and Zn/Y atoms,respectively.Thus the 18R phase is easily decomposed intoα-MgH_(2),γ-MgH_(2),YH_(2),YH_(3),C14-type Laves phase MgZn_(2)and minor CsCl-type Y(Mg,Zn)during ball milling under hydrogen atmosphere.Af-ter further hydrogen absorption-desorption cycling,Y(Mg,Zn)disappears gradually and C14 phase trans-forms into C15-type Laves phase.By contrast,the Mg_(85)Zn_(6)Y_(9)alloy has better hydrogen storage kinetics and cycle durability than pure Mg because of the catalytic effect of YH_(2)/YH_(3)on hydrogen absorption-desorption and inhibition role of Laves phase in Mg crystallite growth.Moreover,the introduction of Ni into Mg_(85)Zn_(6)Y_(9)sample leads to a further decrease in activation energy of hydrogen desorption from 106.39 to 96.78 kJ mol−1 due to the formation of Mg_(2)Ni.This work not only provides new insights into structural features and hydrogen storage characteristics of 18R phase but offers an effective method for improving hydrogen storage properties.