Metathesis Reaction-Induced Significant Improvement in Hydrogen Storage Properties of the KF-Added Mg(NH₂)₂–2LiH System

The hydrogen storage properties and mechanisms of the Mg­(NH₂)₂–2LiH system with potassium halides (KF, KCl, KBr, and KI) were investigated and discussed. The results show that the KF-added sample exhibits superior hydrogen storage properties as ∼5.0 wt % of hydrogen can be reversibly stored in the 0.08KF-added sample via a two-stage reaction with an onset dehydrogenation temperature of 80 °C. However, hydrogen storage behaviors of the samples with KCl, KBr, and KI remain almost unchanged. The fact that KF can readily react with LiH to convert to KH and LiF due to the favorable thermodynamics during ball milling should be the primary reason for its significant effects, as the presence of KH provides a synergetic thermodynamic and kinetic destabilization in the hydrogen storage reaction of the Mg­(NH₂)₂–2LiH system by declining the activation energy of the first-step dehydrogenation as a catalyst and reducing the desorption enthalpy change of the second step as a reactant. The understanding on the role played by KF sheds light on how to further decrease the operating temperature and enhance the hydrogen storage kinetics of the metal–N–H system

Heating Rate-Dependent Dehydrogenation in the Thermal Decomposition Process of Mg(BH₄)₂·6NH₃

The detailed mechanism of thermal decomposition of Mg­(BH₄)₂·6NH₃ synthesized via a mechanochemical reaction between Mg­(BH₄)₂ and NH₃ at room temperature was investigated for the first time. A six-step decomposition process, which involves several parallel and interrelated reactions, was elucidated through a series of structural examinations and property evaluations. First, the thermal decomposition of Mg­(BH₄)₂·6NH₃ evolves 3 equiv of NH₃ and forms Mg­(BH₄)₂·3NH₃. Subsequently, Mg­(BH₄)₂·3NH₃ decomposes to release an additional 1 equiv of NH₃ and 3 equiv of H₂ to produce the [MgNBHNH₃]­[BH₄] polymer. And then, [MgNBHNH₃]­[BH₄] further desorbs 3 equiv of H₂ through a three-step reaction to give rise to the formation of the polymer intermediates of [MgNBHNH₂]­[BH₄], MgNBHNH₂BH₂, and MgNBNHBH, respectively. Finally, an additional 1 equiv of H₂ is liberated from MgNBNHBH to yield Mg and BN as the resultant solid products. In total, about 7 equiv of H₂ and 4 equiv of NH₃ are released together from Mg­(BH₄)₂·6NH₃ upon heating. Moreover, there is a strong dependence of the gas compositions released from Mg­(BH₄)₂·6NH₃ on the heating rate because the decomposition reaction of Mg­(BH₄)₂·3NH₃ is sensitive to the heating rate, as the faster heating rate induces a lower ammonia evolution. The finding in this work provides us with insights into the dehydrogenation mechanisms of the metal borohydride ammoniates as hydrogen storage media

Reaction Pathways for Hydrogen Uptake of the Li–Mg–N-Based Hydrogen Storage System

Hydrogen storage properties and mechanisms of the Li₃N–<i>x</i>Mg₃N₂ (<i>x</i> = 0, 0.25, 0.5, 1.0) composites were investigated in this paper. It was found that the Li₃N–0.25Mg₃N₂ composite exhibited optimal hydrogen storage performances as it can store reversibly ∼8.4 wt % hydrogen with an onset temperature of 125 °C for dehydrogenation. Upon absorbing hydrogen, Li₃N converted to Li₂NH and LiH first and was further hydrogenated to generate LiNH₂. The newly developed LiNH₂ then reacted with Mg₃N₂ under hydrogen pressure to produce Li₂Mg₂N₃H₃ and MgNH. Finally, Li₂Mg₂N₃H₃ and MgNH along with LiNH₂ further reacted with hydrogen to form the resultant products of Mg­(NH₂)₂ and LiH. More Mg₃N₂ in the Li₃N–<i>x</i>Mg₃N₂ composites retarded Li₃N to react with H₂ at the beginning of hydrogenation due to the baffle effect but facilitated the hydrogenation of Mg₃N₂ at the second-stage hydrogenation because of the decreased particle size and the frequent contact of the constituent species

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

Synthesis of a Nanosized Carbon-Supported Ni Composite and Its Remarkable Catalysis for Hydrogen Desorption from the LiBH₄–2LiNH₂ System

A nanosized Ni@C composite (Ni particles: < 10 nm) was successfully synthesized by casting a Ni-based metal–organic framework MOF-74­(Ni) filled with furfuryl alcohol at 700 °C. The resulting Ni@C composite exhibits remarkable catalytic activity for reducing the operating temperature of hydrogen release from the LiBH₄–2LiNH₂ system. The LiBH₄–2LiNH₂–10 wt % Ni@C sample releases approximately 10 wt % of hydrogen at 135–250 °C, and the end temperature for hydrogen release is reduced by 110 °C in comparison to that of the pristine sample. During dehydrogenation, nanosized Ni remains almost unchanged and only works as a catalyst to reduce kinetic barriers of hydrogen release from the LiBH₄–2LiNH₂ system, which is responsible for lowered dehydrogenation temperatures of the Ni@C-containing sample. More importantly, the dehydrogenated Ni@C-containing sample presents a loose porous morphology that slightly improves its rehydrogenation properties

Improved Hydrogen Storage Properties of LiBH₄ Destabilized by in Situ Formation of MgH₂ and LaH₃

A reactive composite of LiBH₄–<i>x</i>La₂Mg₁₇ was successfully prepared by means of mechanochemical reaction under 40 bar of H₂. It was found that MgH₂ and LaH₃ were readily formed in situ during high-pressure ball milling, and a strong dependency of hydrogen storage performance of the LiBH₄–<i>x</i>La₂Mg₁₇ composites on the content of La₂Mg₁₇ was observed. The as-prepared LiBH₄–0.083La₂Mg₁₇ composite under 40 bar of H₂ exhibits superior hydrogen storage properties as ∼6.8 wt % of hydrogen can be reversibly desorbed and absorbed below 400 °C. It was also purposed that the self-decomposition of MgH₂ first occurred to convert into Mg with hydrogen release upon dehydrogenation and subsequently catalyzed the reaction of LiBH₄ and LaH₃ to liberate additional hydrogen along with the formation of LaB₆ and LiH. The in situ formed MgH₂ and LaH₃ provide a synergetic thermodynamic and kinetic destabilization on the de/hydrogenation of LiBH₄, which is responsible for the distinct reduction in the operating temperatures of the as-prepared LiBH₄–<i>x</i>La₂Mg₁₇ composites

Reaction-Ball-Milling-Driven Surface Coating Strategy to Suppress Pulverization of Microparticle Si Anodes

In this work, we report a novel reaction-ball-milling surface coating strategy to suppress the pulverization of microparticle Si anodes upon lithiation/delithiation. By energetically milling the partially prelithiated microparticle Si in a CO₂ atmosphere, a multicomponent amorphous layer composed of SiO_<i>x</i>, C, SiC, and Li₂SiO₃ is successfully coated on the surface of Si microparticles. The coating level strongly depends on the milling reaction duration, and the 12 h milled prelithiated Si microparticles (BM12h) under a pressure of 3 bar of CO₂ exhibit a good conformal coating with 1.006 g cm^–3 of tap density. The presence of SiC remarkably enhances the mechanical properties of the SiO_<i>x</i>/C coating matrix with an approximately 4-fold increase in the elastic modulus and the hardness values, which effectively alleviates the global volume expansion of the Si microparticles upon lithiation. Simultaneously, the existence of Li₂SiO₃ insures the Li-ion conductivity of the coating layer. Moreover, the SEI film formed on the electrode surface maintains relatively stable upon cycling due to the remarkably suppressed crack and pulverization of particles. These processes work together to allow the BM12h sample to offer much better cycling stability, as its reversible capacity remains at 1439 mAh g^–1 at 100 mA g^–1 after 100 cycles, which is nearly 4 times that of the pristine Si microparticles (381 mAh g^–1). This work opens up new opportunities for the practical applications of micrometer-scale Si anodes

Superior Dehydrogenation/Hydrogenation Kinetics and Long-Term Cycling Performance of K and Rb Cocatalyzed Mg(NH₂)₂‑2LiH system

The coaddition of KH and RbH significantly improves the hydrogen storage properties of the Mg­(NH₂)₂-2LiH system. An Mg­(NH₂)₂-2LiH-0.04KH-0.04RbH composite was able to reversibly store 5.2 wt % H₂ when the dehydrogenation operates at 130 °C and the hydrogenation operates at 120 °C. The isothermal dehydrogenation rate at 130 °C was approximately 43 times that of a pristine sample. During ball-milling, KH reacts with RbH to form a K­(Rb)H solid solution. Upon heating, RbH first separates from the K­(Rb)H solid solution and participates in the first step of dehydrogenation reaction, and then the remaining KH participates in the second dehydrogenation reaction. The presence of RbH and KH provide synergetic effects, which improve the thermodynamics and kinetics of hydrogen storage in the Mg­(NH₂)₂-2LiH system. In particular, more than 93% of the hydrogen storage capacity (4.4 wt %) remains after cycling a sample with 0.04 mol of KH and RbH for 50 cycles, indicating notably better cycling stability compared with any presently known Li–Mg–N–H systems

Facile Synthesis and Superior Catalytic Activity of Nano-TiN@N–C for Hydrogen Storage in NaAlH₄

Herein, we synthesize successfully ultrafine TiN nanoparticles (<3 nm in size) embedded in N-doped carbon nanorods (nano-TiN@N–C) by a facile one-step calcination process. The prepared nano-TiN@N–C exhibits superior catalytic activity for hydrogen storage in NaAlH₄. Adding 7 wt % nano-TiN@N–C induces more than 100 °C reduction in the onset dehydrogenation temperature of NaAlH₄. Approximately 4.9 wt % H₂ is rapidly released from the 7 wt % nano-TiN@N–C-containing NaAlH₄ at 140 °C within 60 min, and the dehydrogenation product is completely hydrogenated at 100 °C within 15 min under 100 bar of hydrogen, exhibiting significantly improved desorption/absorption kinetics. No capacity loss is observed for the nano-TiN@N–C-containing sample within 25 de-/hydrogenation cycles because nano-TiN functions as an active catalyst instead of a precursor. A severe structural distortion with extended bond lengths and reduced bond strengths for Al–H bonding when the [AlH₄]⁻ group adsorbs on the TiN cluster is demonstrated for the first time by density functional theory calculations, which well-explains the reduced de-/hydrogenation temperatures of the nano-TiN@N–C-containing NaAlH₄. These findings provide new insights into designing and synthesizing high-performance catalysts for hydrogen storage in complex hydrides

A Metal–Organic Framework with Open Metal Sites for Enhanced Confinement of Sulfur and Lithium–Sulfur Battery of Long Cycling Life

The lithium–sulfur battery has a very high theoretical capacity and specific energy density, yet its applications have been obstructed by fast capacity fading and low Coulombic efficiency due to the dissolution of polysulfides. Herein we utilize HKUST-1 as the host material to trap sulfur and thus to diminish the dissolution problem. A large amount of sulfur (40 wt %) has been incorporated in HKUST-1 pore metrics to achieve HKUST-1⊃S composite whose structure has been established by both single and powder X-ray diffraction studies. The strong confinement of HKUST-1 for sulfur attributed to the suitable pore spaces and open Cu²⁺ sites has enabled the resulting Li–S⊂HKSUT-1 battery to show excellent performance with a capacity of about 500 mAh/g after 170 cycles