Porous carbon can be tailored to great effect for electrochemical energy storage. In this study, we propose a novel structured spherical carbon with a macrohollow core and a microporous shell derived from a sustainable biomass, amylose, by a multistep pyrolysis route without chemical etching. This hierarchically porous carbon shows a particle distribution of 2–10 μm and a surface area of 672 m2 g–1. The structure is an effective host of sulfur for lithium–sulfur battery cathodes, which reduces the dissolution of polysulfides in the electrolyte and offers high electrical conductivity during discharge/charge cycling. The hierarchically porous carbon can hold 48 wt % sulfur in its porous structure. The S@C hybrid shows an initial capacity of 1490 mAh g–1 and retains a capacity of 798 mAh g–1 after 200 cycles at a discharge/charge rate of 0.1 C. A capacity of 487 mAh g–1 is obtained at a rate of 3 C. Both a one-step pyrolysis and a chemical-reagent-assisted pyrolysis are also assessed to obtain porous carbon from amylose, but the obtained carbon shows structures inferior for sulfur cathodes. The multistep pyrolysis and the resulting hierarchically porous carbon offer an effective approach to the engineering of biomass for energy storage. The micrometer-sized spherical S@C hybrid with different sizes is also favorable for high-tap density and hence the volumetric density of the batteries, opening up a wide scope for practical applications.Keywords: amylose; biomass material; electrochemical performance; lithium−ion batteries; sulfur cathode;

Lithium sulfide, Li2S, is a promising cathode material for lithium–sulfur batteries (LSBs), with a high theoretical capacity of 1166 mA h g−1. However, it suffers from low cycling stability, low-rate capability and high initial activation potential. In addition, commercially available Li2S is of high cost and of large size, over ten microns, which further exacerbate its shortcomings as a sulfur cathode. Exploring new approaches to fabricate small-sized Li2S of low cost and to achieve Li2S cathodes of high electrochemical performance is highly desired. This work reports a novel mechanochemical method for synthesizing Li2S of high purity and submicron size by ball-milling LiH with sulfur in an Ar atmosphere at room temperature. By further milling the as-synthesized Li2S with polyacrylonitrile (PAN) followed by carbonization of PAN at 1000 °C, a Li2S/C hybrid with nano-sized Li2S embedded in a mesoporous carbon matrix is achieved. The hybrid with Li2S as high as 74 wt% shows a high initial capacity of 971 mA h g−1 at 0.1C and retains a capacity of 570 mA h g−1 after 200 cycles as a cathode material for LSBs. A capacity of 610 mA h g−1 is obtained at 1C. The synthesis method of Li2S is facile, environmentally benign, and of high output and low cost. The present work opens a new route for the scalable fabrication of submicron-sized Li2S and for the development of high performance Li2S-based cathodes.

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.

The utilization of metal borohydride ammoniates as practical hydrogen storage materials is hindered by their unfavorable exothermic dehydrogenation thermodynamics. Here, we report a first successful attempt to tailor the dehydrogenation thermodynamics of magnesium borohydride hexaammoniate (Mg(BH4)2·6NH3) through nanoconfinement into microporous activated carbon (AC). The onset temperature for hydrogen release from the nanoconfined Mg(BH4)2·6NH3 is dramatically decreased to approximately 40 °C, and more encouragingly, hydrogen desorption is endothermic in nature. The relationship between pore size and dehydrogenation behavior is established, and the critical pore size for the endothermic dehydrogenation of the nanoconfined Mg(BH4)2·6NH3 is found to be less than 4 nm. The nanoconfinement effect of carbon scaffolds is believed to be the primary reason for the change in the dehydrogenation pathway caused by incorporating Mg(BH4)2·6NH3 into microporous activated carbon. This finding opens up the possibility to achieve reversible hydrogen storage in metal borohydride ammoniates.

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.

The F-substituted Mg(BH4)2·2NH3 was successfully prepared for the first time by mechanochemically reacting Mg(BH4)2·2NH3 and LiBF4 based on the structural and chemical similarity of [BH4]− and [BF4]− anions. The results indicate that the dehydrogenation properties of Mg(BH4)2·2NH3 are significantly improved by the partial substitution of fluorine for hydrogen. Hydrogen release from the F-substituted Mg(BH4)2·2NH3 is initiated at approximately 70 °C, which is an 80 °C decrease in comparison with the pristine sample. At 150 °C, the 15 mol% F-substituted sample releases ∼5.2 wt% of hydrogen within 40 min. However, only 1.2 wt% of hydrogen could be desorbed from the pristine Mg(BH4)2·2NH3 under identical conditions. Mechanistic investigations reveal that the B–H bonds in Mg(BH4)2·2NH3 are strengthened after F-substitution, which induces more ionised Hδ− in the ammoniate and consequently facilitates the local Hδ+–Hδ− combinations within the Mg(BH4)2·2NH3 molecule. In addition, the F-substitution weakens the Mg–B bonds in Mg(BH4)2·2NH3, which favours the generation of B–N bonds during dehydrogenation. These factors are the most important reasons for the improved dehydrogenation properties of F-substituted Mg(BH4)2·2NH3.

•The ternary alloy Li5AlSi2 was successfully synthesized by HDCR at 500–650 °C.•Li5AlSi2 prepared at 600 °C exhibited the best electrochemical properties.•The maximum discharge capacity of Li5AlSi2 prepared at 600 °C was 1303 mAh/g.•The formation of Li–Si/Li–Al alloys is responsible for the rapid capacity fading.The ternary alloy Li5AlSi2 is successfully synthesized by a hydrogen-driven chemical reaction at 500–650 °C and used as an anode for Li-ion batteries. It is observed that a higher dehydrogenation temperature induces a higher phase purity and a larger particle size and that the Li5AlSi2 prepared at 600 °C exhibits the best electrochemical properties. The Li5AlSi2 prepared at 600 °C delivers a Li-extraction capacity of approximately 849 mAh/g at 100 mA/g via a two-step reaction in the first charge cycle, corresponding to 3.8 mol Li ions. More interestingly, the Li-insertion capacity of the delithiated sample reaches 1303 mAh/g during the subsequent discharge process, much higher than the previous Li-extraction capacity. The capacity retention is determined to be approximately 59% after 25 cycles, which is superior to that of the sample prepared by the conventional melting technique. Structural analyses and CV measurements reveal that the active lithium storage species is converted to the amorphous Li–Si and Li–Al alloys instead of the initial Li5AlSi2 after 1 charge/discharge cycle, which is believed to be the most important reason for the rapid capacity fading upon cycling.

The thermal dehydrogenation process of the KOH-containing Mg(NH2)2–2LiH system was systematically investigated by identifying changes in the structure and composition of its components by XRD and FTIR. During ball milling, the added KOH reacts with Mg(NH2)2 and LiH to produce MgO, KH and Li2K(NH2)3. During the initial heating process (<120 °C), the newly formed KH and Li2K(NH2)3 react with Mg(NH2)2 and LiH to yield MgNH, LiNH2 and Li3K(NH2)4 along with hydrogen release. Raising the temperature to 185 °C results in a reaction between Mg(NH2)2, MgNH and LiH that gives Li2Mg2N3H3 as the product and further releases hydrogen. As the temperature is increased to 220 °C, Li2Mg2N3H3 reacts with LiNH2 and LiH to produce Li2MgN2H2 and H2. Meanwhile, two parallel reactions between Li2Mg2N3H3, Li3K(NH2)4 and LiH also generate additional hydrogen. Specifically, the KH and Li2K(NH2)3, formed in situ during ball milling, serve as reactants in the dehydrogenation reaction of the Mg(NH2)2–2LiH system, which is responsible for the significantly improved thermodynamics and kinetics of hydrogen storage.

We report the high-temperature failure behaviours and mechanisms of K-based additives in the Mg(NH2)2–2LiH hydrogen storage system. The onset of hydrogen release from a Mg(NH2)2–2LiH–0.08KF sample is approximately 80 °C; this is a 50 °C reduction in comparison with the pristine Mg(NH2)2–2LiH. However, the positive effects of K-based additives disappear when the hydrogen release and uptake of the KF-added Mg(NH2)2–2LiH samples are performed at higher temperatures (>200 °C). The change in the crystal structure of the dehydrogenation product, the enlargement in the grain and particle sizes of the dehydrogenation/hydrogenation products, and the increase in the inhomogeneous degree of mixing and distribution of K-based additives should be the three most important reasons for the increased operating temperature during the follow-up cycles. In particular, the ability of K-based additives to lower the operating temperature for hydrogen storage in the Mg(NH2)2–2LiH system can be sufficiently recovered after ball milling. Therefore, the failure of K-based additives after high-temperature treatment is only phenomenological instead of being natural. Strictly limiting the dehydrogenation/hydrogenation of the K-added Mg(NH2)2–2LiH system at lower temperatures is critical for maintaining the superior effect of K-based additives.

Lithium halides were introduced into the LiNH2–MgH2 system by ball milling the corresponding chemicals under 50 bar of H2 to decrease the dehydrogenation temperature and enhance the dehydrogenation kinetics. The results show that the LiNH2–MgH2–0.05LiBr sample exhibited optimal hydrogen storage performance. The onset dehydrogenation temperature of the LiNH2–MgH2–0.05LiBr sample was only 120 °C, which represents a 55 °C reduction with respect to that of the pristine LiNH2–MgH2 sample. The dehydrogenation rate of the LiNH2–MgH2 sample at 210 °C was increased threefold upon addition of LiBr, which is attributed to the reduction in the dehydrogenation activation energy. Moreover, the addition of LiBr could significantly suppress ammonia emission during the dehydrogenation process of the LiNH2–MgH2 sample. Structural examinations reveal that the added LiBr could react with LiNH2 to form Li7(NH2)6Br during the dehydrogenation process. The in situ-formed Li7(NH2)6Br not only weakens the N–H bond but also promotes the migration of Li+, consequently improving the dehydrogenation kinetics of the LiNH2–MgH2 sample.

•Amorphous Si (a-Si) powder with SiOx surface layer introduced is prepared by ball-milling.•Carbon coating and floc-like carbon are introduced in situ in the a-Si@SiOx particles.•The a-Si@SiOx/C composite with 8 wt.% C provides superior electrochemical performance.•The method is facile in large-scale production of Si-based anode material for LIBs.Amorphous-Si@SiOx/C composites with amorphous Si particles as core and coated with a double layer of SiOx and carbon are prepared by ball-milling crystal micron-sized silicon powders and carbonization of the citric acid intruded in the ball-milled Si. Different ratios of Si to citric acid are used in order to optimize the electrochemical performance. It is found that SiOx exists naturally at the surfaces of raw Si particles and its content increases to ca. 24 wt.% after ball-milling. With an optimized Si to citric acid weight ratio of 1/2.5, corresponding to 8.4 wt.% C in the composite, a thin carbon layer is coated on the surfaces of a-Si@SiOx particles, moreover, floc-like carbon also forms and connects the carbon coated a-Si@SiOx particles. The composite provides a capacity of 1450 mA h g−1 after 100 cycles at a current density of 100 mA g1, and a capacity of 1230 mA h g−1 after 100 cycles at 500 mA g1 as anode material for lithium-ion batteries. Effects of ball-milling and the addition of citric acid on the microstructure and electrochemical properties of the composites are revealed and the mechanism of the improvement in electrochemical properties is discussed.

We demonstrate the synthesis, crystal structure and thermal decomposition behavior of a novel ammonia-stabilized mixed-cation borohydride where the NH3 groups enable the coexistence of Li and Mg cations as an “assistant”. Li2Mg(BH4)4·6NH3, which is comprised of orderly arranged Mg[NH3]62+ ammine complexes and Li2[BH4]42− complex anions, was synthesized by the mechanochemical reaction between Mg(BH4)2·6NH3 and LiBH4. This novel compound crystallizes in a tetragonal P43212 (No. 96) structure with lattice parameters a = b = 10.7656(8) Å and c = 13.843(1) Å with very short dihydrogen bonds, which determine a very low onset temperature of 80 °C for hydrogen release and are also responsible for the nucleation of Li2Mg(BH4)4·3NH3 as a decomposition intermediate. Mechanistic investigations on the thermal decomposition showed that the Hδ+–Hδ− combination in the ammonia-stabilized mixed-cation borohydride was significantly enhanced due to the strengthened Mg–N bonds. Upon heating, 11.02 moles of H2 (equivalent to 11.1 wt%) and 3.07 moles of NH3 are evolved from one mole of Li2Mg(BH4)4·6NH3 with a three-step reaction. The insights into the formation mechanism of ammonia-stabilized mixed-cation borohydride and the role played by NH3 group are very useful as a guideline for the design and synthesis of novel B–N-based materials with high hydrogen content.

Potassium hydride (KH) was directly added to a Mg(NH2)2–2LiH system to improve the hydrogen storage properties; the corresponding mechanisms were elucidated. The Mg(NH2)2–2LiH–0.08KH composite displays optimized hydrogen-storage properties, reversibly storing approximately 5.2 wt% hydrogen through a two-stage reaction and a dehydrogenation onset at 70 °C. The 0.08KH-added sample fully dehydrogenated at 130 °C begins to absorb hydrogen at 50 °C, and takes up approximately 5.1 wt% of hydrogen at 140 °C. Adding KH significantly enhances the de-/hydrogenation kinetic properties; however, an overly rapid hydrogenation rate enlarges the particle size and raises the dehydrogenation temperature. A cycling evaluation reveals that the KH-added Mg(NH2)2–2LiH system possesses good reversible hydrogen storage abilities, although the operational temperatures for de-/hydrogenation increase during cycling. Detailed mechanistic investigations indicate that adding KH catalytically decreases the activation energy of the first dehydrogenation step and reduces the enthalpy of desorption during the second dehydrogenation step as a reactant, significantly improving the hydrogen storage properties of Mg(NH2)2–2LiH.

•A reactive composite of LiBH4-xMg(OH)2 was prepared.•Combining LiBH4 with Mg(OH)2 remarkably reduces the dehydrogenation temperature.•The LiBH4-0.3Mg(OH)2 composite possesses optimal dehydrogenation properties.•A total of 9.6 wt% H2 is released from LiBH4-0.3Mg(OH)2 when initiated at 100 °C.•The dehydrogenated LiBH4-0.3Mg(OH)2 takes up 4.7 wt% H2.The dehydrogenation/hydrogenation properties of LiBH4-xMg(OH)2 were systematically investigated. The results show that the LiBH4-0.3Mg(OH)2 composite possesses optimal dehydrogenation properties: approximately 9.6 wt% of hydrogen is released via a stepwise reaction with an onset temperature of 100 °C. In the range of 100–250 °C, a chemical reaction between LiBH4 and Mg(OH)2 first occurs to give rise to the generation of LiMgBO3, MgO and H2. From 250 to 390 °C, the newly developed LiMgBO3 reacts with LiBH4 to form MgO, Li3BO3, LiH, B2O3 and Li2B12H12 with hydrogen release. From 390 to 450 °C, the decomposition of LiBH4 and Li2B12H12 proceeds to release additional hydrogen and to form LiH and B. A further hydrogenation experiment indicates that the dehydrogenated LiBH4-0.3Mg(OH)2 sample can take up 4.7 wt% of hydrogen at 450 °C and 100 bar of hydrogen with good cycling stability, which is superior to the pristine LiBH4.

•The crystal structure of Li2Mg(NH)2 is associated with the gas back pressure.•Hydrogen storage properties of Mg(NH2)2–2LiH depend on the gas back pressure.•Mechanism of hydrogen storage property depended on gas back pressure is discussed.The ternary imide Li2Mg(NH)2 is considered to be one of the most promising on-board hydrogen storage materials due to its high reversible hydrogen capacity of 5.86 wt%, favorable thermodynamic properties and good cycling stability. In this work, Li2Mg(NH)2 was synthesized by dynamically dehydrogenating a mixture of Mg(NH2)2–2LiH up to 280 °C under different gas (Ar and H2) and pressures (0–9.0 bar). The crystal structure of Li2Mg(NH)2 was found to depend on the gas back pressure in the dehydrogenation process. The crystal structure of Li2Mg(NH)2 and the dehydrogenation/rehydrogenation properties of the Mg(NH2)2–2LiH system strongly depend on the gas back pressure in the dehydrogenation process due to the effect of the pressure on the dehydrogenation kinetics. This study provides a new approach for improving the hydrogen storage properties of the amide–hydride systems.

A ternary hydrogen storage system, of superior cyclic stability and high capacity, was developed from a mixture of Ca(BH4)2, LiBH4 and MgH2 in molar ratios of 1:2:2. Investigation on both non-isothermal and isothermal hydrogen desorption/absorption properties shows that the hydrogen desorption starts from 320 °C and completes at 370 °C under a heating rate of 2 °C min−1, releasing ca. 8.1 wt% H2. The finishing temperature of desorption is much lower and the capacity much higher than any of the two-hydride mixtures in the ternary system. In particular, hydrogenation of the ternary system initiates at an extremely low temperature of ca. 75 °C and the onset dehydrogenation temperature is significantly reduced by 90 °C after the initial dehydrogenation/hydrogenation cycle, which is ascribed to the formation of an active dual-cation hydride of CaMgH3.72 for dehydrogenation in the hydrogenation process. There is ca. 7.6 wt% H2 absorbed at 350 °C and 90 bar H2 for 18 h for the system post-dehydrogenated at 370 °C for 30 min, demonstrating a reversibility of over 94%. The capacity seems to fade mainly in the initial few cycles and stabilizes after further cycling. The reversibility is as high as 97% and a dehydrogenation capacity of ca. 6.2 wt% H2 at the 10th cycle. Favourable kinetics and thermodynamics of hydrogen desorption/absorption are achieved, which are responsible for the low completion temperature and the superior cycling performance. Mechanisms of the improved dehydrogenation/hydrogenation properties including the cyclic behaviour of the system are also proposed in relation to microstructural analyses.

In this paper, we report a KOH-doped Mg(NH2)2–2LiH system with low operating temperatures and good cycling stability. The Mg(NH2)2–2LiH–0.07KOH sample can reversibly desorb/absorb ∼4.92 wt% hydrogen with a starting and peak dehydrogenation temperature of ∼75 °C and ∼120 °C, respectively, the lowest in the current Mg(NH2)2–2LiH system studied. Moreover, the cycling stability of de-/hydrogenation is also remarkably improved by KOH doping as the average capacity degradation of the Mg(NH2)2–2LiH–0.07KOH system is of only 0.002 wt% per cycle within 30 cycles. Detailed structural investigations reveal that during ball milling, the doped KOH can react with Mg(NH2)2 and LiH to convert to MgO, KH and Li2K(NH2)3, which work together to provide the synergistic effects of thermodynamics and kinetics on hydrogen desorption and absorption of the Mg(NH2)2–2LiH system upon heating, consequently inducing a significant improvement in hydrogen storage properties.

A Mg(BH4)2-added Mg(NH2)2–2LiH system was prepared by ball milling the corresponding chemicals. The hydrogen storage properties of the Mg(NH2)2–2LiH–xMg(BH4)2 (x = 0, 0.1, 0.2, 0.3) samples and the role played by Mg(BH4)2 were systematically investigated. The results show that the onset and peak temperatures for hydrogen desorption from the Mg(BH4)2-added Mg(NH2)2–2LiH sample shifted to lower temperatures. In particular, the Mg(NH2)2–2LiH–0.1Mg(BH4)2 sample could reversibly absorb ∼4.5 wt% of hydrogen in the temperature range of 120–150 °C, which is superior to the pristine sample. During ball milling, a metathesis reaction between Mg(BH4)2 and LiH readily occurred to form LiBH4 and MgH2 and subsequently, the newly formed MgH2 reacted with Mg(NH2)2 to generate MgNH. Upon heating, the presence of LiBH4 not only decreased the recrystallization temperature of Mg(NH2)2 but also reacted with LiNH2 to form the Li4(BH4)(NH2)3 intermediate, which weakens the N–H bonding and enhances the ion conductivity. Meanwhile, MgNH may act as the nucleation center for the dehydrogenation product of Li2MgN2H2 due to the structural similarity. Thus, the in situ formed LiBH4 and MgNH provide a synergetic effect to improve the hydrogen storage performances of the Mg(NH2)2–2LiH system.

•CoO was first introduced into the Li–B–N–H system as a catalyst precursor.•The onset dehydrogenation temperature decreased by 120 °C after adding CoO.•Metallic Co was identified to be the active catalytic species.•CoO-added samples exhibited partial reversibility for hydrogen storage.Cobalt monoxide (CoO) was introduced into the Li–B–N–H system as a catalyst precursor, and the hydrogen desorption behavior of the LiBH4–2LiNH2–xCoO (x = 0–0.20) composites was investigated. It was observed that the majority of hydrogen desorption from the CoO-added sample occurred simultaneously with the melting of α-Li4BN3H10. Moreover, the 0.05CoO-added sample exhibited optimized dehydrogenation properties, desorbing 9.9 wt% hydrogen completely with an onset temperature of 100 °C and exhibiting a decrease of more than 120 °C in the onset dehydrogenation temperature with respect to that of the additive-free sample. The activation energy of hydrogen desorption for the 0.05CoO-added sample was reduced by 30%. XAFS measurements showed that the CoO additive was first reduced chemically to metallic Co during the initial stage of thermal dehydrogenation, and the newly produced metallic Co acted as the catalytic active species in favor of the creation of B–N bonding. More importantly, approximately 1.1 wt% of hydrogen could be recharged into the fully dehydrogenated 0.05CoO-added sample at 350 °C and a hydrogen pressure of 110 atm, which represents much better performance than that exhibited by the pristine sample.

The hydrogen storage properties and mechanisms of the Ca(BH4)2-added 2LiNH2–MgH2 system were systematically investigated. The results showed that the addition of Ca(BH4)2 pronouncedly improved hydrogen storage properties of the 2LiNH2–MgH2 system. The onset temperature for dehydrogenation of the 2LiNH2–MgH2–0.3Ca(BH4)2 sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH2–MgH2–0.1Ca(BH4)2 sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH4)2 and LiNH2 firstly occurred to convert to Ca(NH2)2 and LiBH4, and then, the newly developed LiBH4 reacted with LiNH2 to form Li4(BH4)(NH2)3. Upon heating, the in situ formed Ca(NH2)2 and Li4(BH4)(NH2)3 work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH4)2-added 2LiNH2–MgH2 system.Highlights► Hydrogen storage properties of 2LiNH2–MgH2 are improved by doping Ca(BH4)2. ► Hydrogen desorption from the 0.3Ca(BH4)2-doped sample starts at 80 °C. ► Ca(BH4)2 reacts with LiNH2 to form Ca(NH2)2 and LiBH4 during ball milling. ► The newly formed Ca(NH2)2 and LiBH4 provide the synergetic effects.

Nanocrystalline Mg2Si with high purity, homogeneous composition and uniform particle size was successfully synthesized by a facile hydrogen-driven chemical reaction at below 500 °C. The as-prepared Mg2Si exhibited an improved Li-ion storage ability as anode material for Li-ion batteries as it delivered an initial capacity of about 1095 mAh/g at 100 mA/g. The discharge capacity stayed at 406 mAh/g after 60 cycles, which is much better than that of the commercial Mg2Si anode and previously reported Mg2Si anode synthesized by mechanical alloying. The proof-of-concept synthesis technique presented in this work should be valuable for the preparation of other intermetallic compounds with difference of melting point between the constituent elements.Highlights► Nanocrystalline Mg2Si was synthesized via a facile hydrogen-driven reaction. ► The as-prepared Mg2Si exhibits a significantly improved Li-ion storage ability. ► The as-prepared Mg2Si delivers an initial discharge capacity of 1095 mAh/g. ► The first Coulombic efficiency is about 92%. ► More than 37% of the reversible capacity is retained after 60 cycles.

A significant improvement of hydrogen storage properties was achieved by introducing MgH2 into the 6LiBH4–CaH2 system. It was found that ∼8.0 wt% of hydrogen could be reversibly stored in a 6LiBH4–CaH2–3MgH2 composite below 400 °C and 100 bar of hydrogen pressure with a stepwise reaction, which is superior to the pristine 6LiBH4–CaH2 and LiBH4 samples. Upon dehydriding, MgH2 first decomposed to convert to Mg and liberate hydrogen with an on-set temperature of ∼290 °C. Subsequently, LiBH4 reacted with CaH2 to form CaB6 and LiH in addition to further hydrogen release. Hydrogen desorption from the 6LiBH4–CaH2–3MgH2 composite finished at ∼430 °C in non-isothermal model, a 160 °C reduction relative to the 6LiBH4–CaH2 sample. JMA analyses revealed that hydrogen desorption was a diffusion-controlled reaction rather than an interface reaction-controlled process. The newly produced Mg of the first-step dehydrogenation possibly acts as the heterogeneous nucleation center of the resultant products of the second-step dehydrogenation, which diminishes the energy barrier and facilitates nucleation and growth, consequently reducing the operating temperature and improving the kinetics of hydrogen storage.

Hydrogen storage properties and mechanisms of the combined Mg(BH4)2–NaAlH4 system were investigated systematically. It was found that during ball milling, the Mg(BH4)2–xNaAlH4 combination converted readily to the mixture of NaBH4 and Mg(AlH4)2 with a metathesis reaction. The post-milled samples exhibited an apparent discrepancy in the hydrogen desorption behavior with respect to the pristine Mg(BH4)2 and NaAlH4. Approximately 9.1 wt% of hydrogen was released from the Mg(BH4)2–2NaAlH4 composite milled for 24 h with an onset temperature of 101 °C, which is lowered by 105 and 139 °C than that of NaAlH4 and Mg(BH4)2, respectively. At initial heating stage, Mg(AlH4)2 decomposed first to produce MgH2 and Al with hydrogen release. Further elevating operation temperatures gave rise to the reaction between MgH2 and Al and the self-decomposition of MgH2 to release more hydrogen and form the Al0.9Mg0.1 solid solution and Mg. Finally, NaBH4 reacted with Mg and partial Al0.9Mg0.1 to liberate all of hydrogen and yield the resultant products of MgAlB4, Al3Mg2 and Na. The dehydrogenated sample could take up ∼6.5 wt% of hydrogen at 400 °C and 100 atm of hydrogen pressure through a more complicated reaction process. The hydrogenated products consisted of NaBH4, MgH2 and Al, indicating that the presence of Mg(AlH4)2 is significantly favorable for reversible hydrogen storage in NaBH4 at moderate temperature and hydrogen pressure.Highlights► A reactive composite system of Mg(BH4)2–NaAlH4 is designed and prepared. ► A metathesis reaction occurs readily between Mg(BH4)2 and NaAlH4 upon ball milling. ► The post-milled Mg(BH4)2–xNaAlH4 samples exhibit a three-step dehydrogenation. ► 6.5 wt% of hydrogen is stored reversibly in the post-milled Mg(BH4)2–2NaAlH4 sample.

Mg(AlH4)2 submicron rods with 96.1% purity have been successfully synthesized in a modified mechanochemical reaction process followed by Soxhlet extraction. ∼9.0 wt% of hydrogen is released from the as-prepared Mg(AlH4)2 at 125–440 °C through a stepwise reaction. Upon dehydriding, Mg(AlH4)2 decomposes first to generate MgH2 and Al. Subsequently, the newly produced MgH2 reacts with Al to form a Al0.9Mg0.1 solid solution. Finally, further reaction between the Al0.9Mg0.1 solid solution and the remaining MgH2 gives rise to the formation of Al3Mg2. The first step dehydrogenation is a diffusion-controlled reaction with an apparent activation energy of ∼123.0 kJ/mol. Therefore, increasing the mobility of the species involved in Mg(AlH4)2 will be very helpful for improving its dehydrogenation kinetics.Highlights► Mg(AlH4)2 submicron rods with high purity are successfully synthesized. ► ∼9.0 wt% of hydrogen is released from Mg(AlH4)2 with a three-step reaction. ► The resultant product is composed of Al3Mg2 and Al0.9Mg0.1. ► A diffusion-controlled kinetics is determined for the 1st step dehydrogenation.

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.Keywords: cycling stability; hydrogen-driven reaction; Li-ion batteries; Si; volume change;

Hydrogen storage alloys are of particular interest as a novel group in functional materials owing to their potential and practical applications in Ni/MH rechargeable batteries. This review is devoted to the specific alloy families developed for high-energy and high-power Ni/MH batteries in the last decades, especially for EV, HEV and PHEV applications. The scope of the work encompasses principles of Ni/MH batteries, electrochemical hydrogen storage thermodynamics and kinetics, prerequisites for hydrogen storage electrode alloys and recent advances in hydrogen storage electrode alloys. Rare earth AB5-type alloys, Ti- and Zr-based AB2-type alloys, Mg-based amorphous/nanocrystalline alloys, rare earth-Mg–Ni-based alloys and Ti–V-based alloys are highlighted. Additionally, the challenges met in developing advanced hydrogen storage alloys for Ni/MH rechargeable batteries are pointed out and some research directions are suggested.

A synergetic effect of K, Ti and F together on improving the reversible hydrogen storage properties of NaAlH4 is found by intruding K2TiF6 as catalyst precursor. Around 4.4 wt% of hydrogen can be released from the NaAlH4–0.025 K2TiF6 sample within 40 min at 140 °C.

Hydrogen storage performances of a Li2NH-xMgNH combination system (x = 0, 0.5, 1 and 2) are investigated for the first time. It is found that the hydrogenated samples with MgNH exhibit a significant reduction in the dehydrogenation temperatures. Mechanistic investigations reveal that there is a strong dependence of the hydrogen storage reaction process on the molar ratio between MgNH and Li2NH. As a consequence, tuning of thermodynamics is achieved for hydrogen storage in the Li2NH-xMgNH system by changing the reaction routes, which is ascertained to be the primary reason for the reduction in the operating temperature for hydrogen desorption. Specifically, it is found that under 105 atm hydrogen (140–280 °C) 5.6 wt% hydrogen is reversibly stored in the Li2NH-0.5MgNH combination system, which is greater than in the well-investigated Mg(NH2)2-2LiH system.

A dramatic enhancement in the dehydrogenation/hydrogenation kinetics of Na2LiAlH6 was achieved by adding 5 wt% TiF4. The starting temperature for dehydrogenation of the sample with TiF4 added was lowered by 50 °C with respect to the pristine sample. In-depth investigations revealed that the addition of TiF4 to Na2LiAlH6 not only enhanced the dehydrogenation kinetics, but also altered the dehydrogenation thermodynamics. The apparent activation energy and the desorption enthalpy change of the Na2LiAlH6 with 5 wt% TiF4 added were calculated to be ca. 143.6 ± 2.7 kJ mol−1 and 57.3 ± 2.3 kJ mol−1 of H2 by using Kissinger's approach and the van't Hoff equation, both lower than those of the pristine sample, respectively. We firmly believe that both Ti and F play critical roles in the dehydrogenation/hydrogenation process of the Na2LiAlH6 sample with added TiF4 due to the formation of Ti–Al clusters, the substitution of F− for H− and the presence of LiF. All these are responsible for the significant improvement of hydrogen storage performances of the Na2LiAlH6 sample with the addition of TiF4. This understanding provides us with insights into the dehydrogenation kinetic mechanisms of Na2LiAlH6 used for hydrogen storage and directs the optimization of high-performance catalysts for the hydrogen storage reaction of the alanates.

Ultrafine crystalline SnO2 particles (2–3 nm) dispersed carbon matrix composites are prepared by a sol–gel method. Citric acid and hydrous SnCl4 are used as the starting constituents. The effect of the calcination temperatures on the structure and electrochemical properties of the composites has been studied. Structure analyses show that ultrafine SnO2 particles form and disperse in the disordered carbon matrix in the calcination temperature range of 500–800 °C, forming SnO2/C composites, and the carbon content shows only a slight increase from 35.8 wt.% to 39.1 wt.% with the temperature. Nano-Sn particles form when the calcination temperature is increased to 900 °C, forming a SnO2/Sn/C composite, and the carbon content is increased to 49.3 wt.%. Electrochemical testing shows that the composite anodes provide high reversible cycle stability after several initial cycles, maintaining capacities of 380–400 mAh g−1 beyond 70 cycles for the calcination temperature of 600–800 °C. The effect of the structure feature of the ultrafine size of SnO2 and the disordered carbon matrix on the lithium insertion and extraction process, especially on the reversible behavior of the lithium ion reaction during cycling, is discussed.

Carbon coating and iron phosphides of high electron conductivity were introduced into the LiFePO4 materials which were derived via a sol–gel method in order to improve the high discharge rate performance. The start constituents were FeC2O4·2H2O, LiOH·H2O, NH4H2PO4 and ethylene glycol. Effects of the calcination temperature and the ethylene glycol on the structure and the electrochemical performance of the LiFePO4 materials were investigated. Structure analyses showed that the addition of ethylene glycol caused an obvious decrease in the particle size of LiFePO4. Calcination temperature and ethylene glycol jointly affected the formation of iron phosphides. Combining the electrochemical testing, it was found that, at low discharge rate, small particle size and high content of LiFePO4 were much important for the capacity rather than the iron phosphides, and relative high content of Fe2P (e.g. 8 wt.%) even worsened the capacity. However, with the increase of the discharge rate, the high electron conductive iron phosphides turned to play important role on the capacity. Fe2P effectively increased both the reaction and diffusion kinetics and hence enhanced the utilization efficiency of the LiFePO4 at high discharge rate. Combining relative small particle size, even 2 wt.% of iron phosphides could improve the high rate performance of LiFePO4/C significantly.

The reaction details of hydrogen storage process of a ternary imide Li2Mg2N3H3 are elucidated for the first time. It is found that 1 mole of Li2Mg2N3H3 converts to a mixture of Mg(NH2)2–2LiH–MgNH on taking up 2 moles of H2, and that the presence of MgNH in the Mg(NH2)2–2LiH system not only alters the dehydrogenation thermodynamics but also improves the dehydrogenation kinetics.

Complex hydrides and Metal–N–H-based materials have attracted considerable attention due to their high hydrogen content. In this paper, a novel amide–hydride combined system was prepared by ball milling a mixture of Na2LiAlH6–Mg(NH2)2 in a molar ratio of 1:1.5. The hydrogen storage performances of the Na2LiAlH6–1.5Mg(NH2)2 system were systematically investigated by a series of dehydrogenation/hydrogenation evaluation and structural analyses. It was found that a total of ∼5.08 wt% of hydrogen, equivalent to 8.65 moles of H atoms, was desorbed from the Na2LiAlH6–1.5Mg(NH2)2 combined system. In-depth investigations revealed that the variable milling treatments resulted in the different dehydrogenation reaction pathways due to the combination of Al and N caused by the energetic milling. Hydrogen uptake experiment indicated that only ∼4 moles of H atoms could be reversibly stored in the Na2LiAlH6–1.5Mg(NH2)2 system perhaps due to the formation of AlN and Mg3N2 after dehydrogenation.

Hydrogen-storage properties and mechanisms of a novel Li–Al–N ternary system were systematically investigated by a series of performance evaluation and structural examinations. It is found that ca. 5.2 wt% of hydrogen is reversibly stored in a Li3N–AlN (1:1) system, and the hydrogenated product is composed of LiNH2, LiH, and AlN. A stepwise reaction is ascertained for the dehydrogenation of the hydrogenated Li3N–AlN sample, and AlN is found reacting only in the second step to form the final product Li3AlN2. The calculation of the reaction enthalpy change indicates that the two-step dehydrogenation reaction is more thermodynamically favorable than any one-step reaction. Further investigations exhibit that the presence of AlN in the LiNH2–2LiH system enhances the kinetics of its first-step dehydrogenation with a 10% reduction in the activation energy due likely to the higher diffusivity of lithium and hydrogen within AlN.

Na2LiAlH6 was synthesized by ball-milling a mixture of NaH and LiAlH4 at a molar ratio of 2:1. NaH and LiAlH4 were readily converted in the initial ball-milling process to LiH and NaAlH4, which subsequently reacted with the remaining excessive NaH to form Na2LiAlH6. The thermodynamic and kinetic mechanisms of dehydrogenation of Na2LiAlH6 were systematically elucidated. Approximately 6.7 wt % of hydrogen was found stored reversibly in Na2LiAlH6 through a few sequential reactions. An enthalpy change of 63.8 kJ/mol-H2 and an apparent activation energy of about 173 kJ/mol were determined for the first-step hydrogen storage reaction, indicating the reaction was thermodynamically relatively stable with a high kinetic barrier for the decomposition of Na2LiAlH6. In depth kinetic investigations showed that the first-step dehydrogenation reaction of Na2LiAlH6 could be well interpreted with a nucleation and growth model, and its reaction rate was controlled by the diffusion of substance. The dehydrogenation mechanism developed in this work can be helpful for further efforts on the improvement of the hydrogenation/dehydrogenation performances of Na2LiAlH6.

A novel LiNH2−MgH2 hydrogen storage system with a molar ratio of 1:1, which can desorb 6.1 wt % hydrogen or 3 equiv of H atoms during first ball milling and subsequent heating, was investigated. During ball milling, the NH2 group in LiNH2 and H atoms in MgH2 switched with each other, leading to the formation of Mg(NH2)2 and LiH, and then the newly formed Mg(NH2)2 and LiH and the remaining MgH2 reacted continuously to desorb hydrogen by absorbing heat at an elevated temperature. After heating to 390 °C, the final product was identified to be Mg3N2, Li2MgN2H2, and LiH by means of XRD and IR characterization. Thermodynamic examination shows that the desorption heat effect is around 45.9 kJ/mol of H2.

Carbon coated LiFePO4 (LiFePO4/C) with different contents of high electron conductive iron phosphide phase was synthesized by an aqueous sol–gel method in a reductive sintering atmosphere. Different synthesis parameters were used for adjusting the microstructure and phase compositions of the products. The effects of the carbon coating and iron phosphides on the electrochemical properties of the LiFePO4/C electrodes were studied by means of testing the discharge capacities at rates of 0.1–5C (1C = 170 mAh g−1) and analyzing the CV curves. The results show that carbon coating in a content of 1.5 wt.% derived from the carbon source of ethylene glycol greatly decreases the particle size of LiFePO4 in one order in the specific surface area, and significantly improves the rate capability of LiFePO4. The effect of the content of FeP on the capacity of the carbon coated LiFePO4 was different at different discharge rates. Increasing the content of FeP from 1.2 to 3.7 wt.% slightly decreases the capacity of LiFePO4/C at low discharge rate (0.1C and 1C), but obviously increases the capacity of LiFePO4/C when the discharge rate is increased to 5C. For the carbon free sample, even it also has 1.8 wt.% FeP, it still possesses poor capacity due to the large particle size of LiFePO4 and the lack of conductivity. And too much iron phosphides lowers the discharge capacity of the electrode since they are inert for the deinsertion/insertion of lithium ion.

In this work, the effects of Co substitution for Ni on the microstructures and electrochemical properties of Ti0.8Zr0.2V2.7Mn0.5Cr0.6Ni1.25−xCoxFe0.2 (x = 0.00–0.25) alloys were investigated systematically by XRD, SEM and electrochemical measurements. The structural investigations revealed that the main phases of all of the alloys were the C14 Laves phase in a three-dimensional network and the V-based solid solution phase with a dendritic structure. The lattice parameters and unit cell volumes of the two phases gradually increased with the increase of Co concentration. The relative abundance of the C14 Laves phase slightly increased from 47.3% to 49.6%, accordingly that of the V-based solid solution phase decreased, with the increase of x from 0.00 to 0.25. The crystal grain of the V-based solid solution phase was obviously refined after Co substitution. The electrochemical investigations showed that the proper substitution of Co for Ni improved the cycling durability of the alloy electrodes mainly due to the suppression of both the pulverization of the alloy particles and the dissolution of the main hydrogen absorbing elements (V and Ti) into the KOH solution. The cycling stability of the alloy electrode with x = 0.1 was 79.8% after 200 cycles. However, the maximum discharge capacity (Cmax) was decreased from 340.5 to 305.6 mAh g−1, and the high rate dischargeability (HRD) gradually decreased from 66.8% to 55.0% with increasing x from 0.00 to 0.25.

The electrochemical kinetic performance of the Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni1.75Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni1.75 hydrogen storage alloy electrode with particle size ranging from 100 mesh to below 500 mesh (∼139–27μm) has been systematically investigated during cycling. Results show that when the electrode discharges at the low current density as 100–200 mA/g, the alloy electrode shows good high-rate dischargeability (HRD) even after 100 cycles, and the particle size only slightly affects the HRD value. While the discharge current density increases to 300–500 mA/g, the HRD of the alloy electrode is obviously affected by the particle size. In other words, the effect of the particle size on the electrochemical performance becomes larger with increasing the discharge current density, especially for the post-cycled electrodes. For the post-5 and 15-times cycled electrode, the HRD of the alloy electrode increases first and then decreases with decreasing the particle size. On the contrary, the HRD of the post-110 times cycled alloy electrode first decreases when the particle size decreases from ∼139∼139 to 59μm and then increases with further decreasing the particle size to ∼27μm. Further, EIS and micro-polarization investigations are in good agreement with the HRD behavior. Combined with SEM observations, it can be believed that it is the real size of the alloy particles, which was minimized by pulverization during cycling, rather than the original particle size, more effectively affects the electrochemical kinetics of the alloy electrode. In other words, not only the original particle size of the electrode alloy but also the pulverization and corrosion caused by the charge/discharge cycling affect the kinetic properties of the alloy electrode.

In this work, the microstructures and electrochemical properties of the La0.7Mg0.3Ni2.45-xCrxCo0.75Mn0.1Al0.2La0.7Mg0.3Ni2.45-xCrxCo0.75Mn0.1Al0.2 (x=0.00x=0.00–0.20) hydrogen storage alloys were investigated systematically for the purpose of improving the cycling stability of this type alloy. X-ray powder diffraction (XRD) analysis showed that all of the alloys mainly consisted of an (La,Mg)Ni3Ni3 phase with PuNi3PuNi3-type structure and an LaNi5LaNi5 phase with CaCu5CaCu5-type structure. The abundance of the (La,Mg)Ni3Ni3 phase in the alloys decreased from 69.6% to 42.9%, accordingly that of the LaNi5LaNi5 phase increased with increasing xx value. With the increase of xx, the lattice parameters and the unit cell volumes of the two phases increased gradually due to the larger atom radius of Cr (1.85A˚) than that of the Ni (1.62A˚). Electrochemical studies showed that the cycling stability (C100/CmaxC100/Cmax) of the alloy electrodes firstly increased from 66.2% (x=0.00)(x=0.00) to 70.6% (x=0.10)(x=0.10), and then decreased to 62.8% (x=0.20)(x=0.20) with the increase of xx value. However, the maximum discharge capacity (Cmax)(Cmax) of these alloy electrodes decreased from 369.7 (x=0.00)(x=0.00) to 311.5 mA h/g (x=0.20)(x=0.20), and the high rate dischargeability (HRD) also showed a decreasing tendency with increasing Cr content. Further, the electrochemical impedance spectra, the linear polarization, the anodic polarization and the potential-step measurements revealed that the decrease of the HRD of this type alloy electrodes can be ascribed to the decrease of both the charge-transfer rate on the surface of the alloy electrodes and diffusion rate of the H atom in the bulk of the alloys with increasing xx.

To improve the electrochemical kinetic performances of the Ti-V-based hydrogen storage alloys, the Pd element was introduced into the Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni1.0 alloy, and the structure and electrochemical properties of the Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni1.0Pdx(x=0–0.2)(x=0–0.2) alloys were systematically investigated by X-ray diffraction (XRD), galvanostatic charge/discharge, high rate discharge (HRD), electrochemical impedance spectroscopy (EIS), linear polarization and potentiostatic step discharge measurements. The structural investigations show that all of the alloys consist of a C14 Laves phase and a V-based solid solution (VSS) phase. The increase of Pd content reduces the abundance of the C14 Laves phase, accordingly increases that of the V-based solid solution phase. Electrochemical measurements indicate that the activation property of the alloy electrodes is obviously improved after adding Pd due to its good electrocatalytic activity. With increasing Pd content, the maximum discharge capacity of the alloy electrodes first increases from 331 mAh/g (x=0x=0) to 357 mAh/g (x=0.15x=0.15) and then decreases to 338 mAh/g (x=0.2x=0.2), meanwhile, the cycling stability of the alloy electrodes is markedly improved. The HRD results reveal that the proper addition of Pd can effectively improve the electrochemical kinetic performances of the alloy electrodes. Further kinetic parameter measurements show that the charge-transfer reaction resistance of the alloy surface first decreases when x increases from 0 to 0.1, and then increases with x   further increasing to 0.2, and the exchange current density and the hydrogen diffusion coefficient first increases and then decreases, indicating the charge-transfer reaction rate and the hydrogen diffusion rate were first accelerated and then slowed down due to the change of the lattice parameters, the crystallite size, the phase abundance and the electrocatalytic activity of the alloys with increasing Pd content. The optimum composition was found to be x=0.1x=0.1 in this work.

The capacity degradation mechanism of the Ti−V-based hydrogen storage electrode alloys was systematically investigated from the viewpoint of intrinsic/extrinsic degradation for the first time. The oxidation/corrosion of the active components and the appearance of the irreversible hydrogen were found to be the two dominating intrinsic factors for the cycling capacity degradation of the Ti−V-based electrode alloys, rather than the segregation and dissolution of the main hydrogen absorbing element V, the loss of which had some effect on the degradation of its discharge capacity but not dominating. The extrinsic degradation of the electrode alloys was mainly caused by the pulverization of the alloy particles, the increase of the contact resistance between particles, and the reaction resistance on surfaces during charge/discharge cycling. In the meantime the pulverization of the alloy particles accelerates the oxidation/corrosion of the active components, which further promotes the formation of the oxide layer and decreases the reaction speed. The decay of the apparent discharge capacity of the Ti−V-based electrode alloys is jointly affected by these factors.

A Mg(BH4)2-added Mg(NH2)2–2LiH system was prepared by ball milling the corresponding chemicals. The hydrogen storage properties of the Mg(NH2)2–2LiH–xMg(BH4)2 (x = 0, 0.1, 0.2, 0.3) samples and the role played by Mg(BH4)2 were systematically investigated. The results show that the onset and peak temperatures for hydrogen desorption from the Mg(BH4)2-added Mg(NH2)2–2LiH sample shifted to lower temperatures. In particular, the Mg(NH2)2–2LiH–0.1Mg(BH4)2 sample could reversibly absorb ∼4.5 wt% of hydrogen in the temperature range of 120–150 °C, which is superior to the pristine sample. During ball milling, a metathesis reaction between Mg(BH4)2 and LiH readily occurred to form LiBH4 and MgH2 and subsequently, the newly formed MgH2 reacted with Mg(NH2)2 to generate MgNH. Upon heating, the presence of LiBH4 not only decreased the recrystallization temperature of Mg(NH2)2 but also reacted with LiNH2 to form the Li4(BH4)(NH2)3 intermediate, which weakens the N–H bonding and enhances the ion conductivity. Meanwhile, MgNH may act as the nucleation center for the dehydrogenation product of Li2MgN2H2 due to the structural similarity. Thus, the in situ formed LiBH4 and MgNH provide a synergetic effect to improve the hydrogen storage performances of the Mg(NH2)2–2LiH system.

A synergetic effect of K, Ti and F together on improving the reversible hydrogen storage properties of NaAlH4 is found by intruding K2TiF6 as catalyst precursor. Around 4.4 wt% of hydrogen can be released from the NaAlH4–0.025 K2TiF6 sample within 40 min at 140 °C.

Potassium hydride (KH) was directly added to a Mg(NH2)2–2LiH system to improve the hydrogen storage properties; the corresponding mechanisms were elucidated. The Mg(NH2)2–2LiH–0.08KH composite displays optimized hydrogen-storage properties, reversibly storing approximately 5.2 wt% hydrogen through a two-stage reaction and a dehydrogenation onset at 70 °C. The 0.08KH-added sample fully dehydrogenated at 130 °C begins to absorb hydrogen at 50 °C, and takes up approximately 5.1 wt% of hydrogen at 140 °C. Adding KH significantly enhances the de-/hydrogenation kinetic properties; however, an overly rapid hydrogenation rate enlarges the particle size and raises the dehydrogenation temperature. A cycling evaluation reveals that the KH-added Mg(NH2)2–2LiH system possesses good reversible hydrogen storage abilities, although the operational temperatures for de-/hydrogenation increase during cycling. Detailed mechanistic investigations indicate that adding KH catalytically decreases the activation energy of the first dehydrogenation step and reduces the enthalpy of desorption during the second dehydrogenation step as a reactant, significantly improving the hydrogen storage properties of Mg(NH2)2–2LiH.

We demonstrate the synthesis, crystal structure and thermal decomposition behavior of a novel ammonia-stabilized mixed-cation borohydride where the NH3 groups enable the coexistence of Li and Mg cations as an “assistant”. Li2Mg(BH4)4·6NH3, which is comprised of orderly arranged Mg[NH3]62+ ammine complexes and Li2[BH4]42− complex anions, was synthesized by the mechanochemical reaction between Mg(BH4)2·6NH3 and LiBH4. This novel compound crystallizes in a tetragonal P43212 (No. 96) structure with lattice parameters a = b = 10.7656(8) Å and c = 13.843(1) Å with very short dihydrogen bonds, which determine a very low onset temperature of 80 °C for hydrogen release and are also responsible for the nucleation of Li2Mg(BH4)4·3NH3 as a decomposition intermediate. Mechanistic investigations on the thermal decomposition showed that the Hδ+–Hδ− combination in the ammonia-stabilized mixed-cation borohydride was significantly enhanced due to the strengthened Mg–N bonds. Upon heating, 11.02 moles of H2 (equivalent to 11.1 wt%) and 3.07 moles of NH3 are evolved from one mole of Li2Mg(BH4)4·6NH3 with a three-step reaction. The insights into the formation mechanism of ammonia-stabilized mixed-cation borohydride and the role played by NH3 group are very useful as a guideline for the design and synthesis of novel B–N-based materials with high hydrogen content.

The reaction details of hydrogen storage process of a ternary imide Li2Mg2N3H3 are elucidated for the first time. It is found that 1 mole of Li2Mg2N3H3 converts to a mixture of Mg(NH2)2–2LiH–MgNH on taking up 2 moles of H2, and that the presence of MgNH in the Mg(NH2)2–2LiH system not only alters the dehydrogenation thermodynamics but also improves the dehydrogenation kinetics.

Hydrogen storage alloys are of particular interest as a novel group in functional materials owing to their potential and practical applications in Ni/MH rechargeable batteries. This review is devoted to the specific alloy families developed for high-energy and high-power Ni/MH batteries in the last decades, especially for EV, HEV and PHEV applications. The scope of the work encompasses principles of Ni/MH batteries, electrochemical hydrogen storage thermodynamics and kinetics, prerequisites for hydrogen storage electrode alloys and recent advances in hydrogen storage electrode alloys. Rare earth AB5-type alloys, Ti- and Zr-based AB2-type alloys, Mg-based amorphous/nanocrystalline alloys, rare earth-Mg–Ni-based alloys and Ti–V-based alloys are highlighted. Additionally, the challenges met in developing advanced hydrogen storage alloys for Ni/MH rechargeable batteries are pointed out and some research directions are suggested.

The utilization of metal borohydride ammoniates as practical hydrogen storage materials is hindered by their unfavorable exothermic dehydrogenation thermodynamics. Here, we report a first successful attempt to tailor the dehydrogenation thermodynamics of magnesium borohydride hexaammoniate (Mg(BH4)2·6NH3) through nanoconfinement into microporous activated carbon (AC). The onset temperature for hydrogen release from the nanoconfined Mg(BH4)2·6NH3 is dramatically decreased to approximately 40 °C, and more encouragingly, hydrogen desorption is endothermic in nature. The relationship between pore size and dehydrogenation behavior is established, and the critical pore size for the endothermic dehydrogenation of the nanoconfined Mg(BH4)2·6NH3 is found to be less than 4 nm. The nanoconfinement effect of carbon scaffolds is believed to be the primary reason for the change in the dehydrogenation pathway caused by incorporating Mg(BH4)2·6NH3 into microporous activated carbon. This finding opens up the possibility to achieve reversible hydrogen storage in metal borohydride ammoniates.

The F-substituted Mg(BH4)2·2NH3 was successfully prepared for the first time by mechanochemically reacting Mg(BH4)2·2NH3 and LiBF4 based on the structural and chemical similarity of [BH4]− and [BF4]− anions. The results indicate that the dehydrogenation properties of Mg(BH4)2·2NH3 are significantly improved by the partial substitution of fluorine for hydrogen. Hydrogen release from the F-substituted Mg(BH4)2·2NH3 is initiated at approximately 70 °C, which is an 80 °C decrease in comparison with the pristine sample. At 150 °C, the 15 mol% F-substituted sample releases ∼5.2 wt% of hydrogen within 40 min. However, only 1.2 wt% of hydrogen could be desorbed from the pristine Mg(BH4)2·2NH3 under identical conditions. Mechanistic investigations reveal that the B–H bonds in Mg(BH4)2·2NH3 are strengthened after F-substitution, which induces more ionised Hδ− in the ammoniate and consequently facilitates the local Hδ+–Hδ− combinations within the Mg(BH4)2·2NH3 molecule. In addition, the F-substitution weakens the Mg–B bonds in Mg(BH4)2·2NH3, which favours the generation of B–N bonds during dehydrogenation. These factors are the most important reasons for the improved dehydrogenation properties of F-substituted Mg(BH4)2·2NH3.

Lithium sulfide, Li2S, is a promising cathode material for lithium–sulfur batteries (LSBs), with a high theoretical capacity of 1166 mA h g−1. However, it suffers from low cycling stability, low-rate capability and high initial activation potential. In addition, commercially available Li2S is of high cost and of large size, over ten microns, which further exacerbate its shortcomings as a sulfur cathode. Exploring new approaches to fabricate small-sized Li2S of low cost and to achieve Li2S cathodes of high electrochemical performance is highly desired. This work reports a novel mechanochemical method for synthesizing Li2S of high purity and submicron size by ball-milling LiH with sulfur in an Ar atmosphere at room temperature. By further milling the as-synthesized Li2S with polyacrylonitrile (PAN) followed by carbonization of PAN at 1000 °C, a Li2S/C hybrid with nano-sized Li2S embedded in a mesoporous carbon matrix is achieved. The hybrid with Li2S as high as 74 wt% shows a high initial capacity of 971 mA h g−1 at 0.1C and retains a capacity of 570 mA h g−1 after 200 cycles as a cathode material for LSBs. A capacity of 610 mA h g−1 is obtained at 1C. The synthesis method of Li2S is facile, environmentally benign, and of high output and low cost. The present work opens a new route for the scalable fabrication of submicron-sized Li2S and for the development of high performance Li2S-based cathodes.

In this paper, we report a KOH-doped Mg(NH2)2–2LiH system with low operating temperatures and good cycling stability. The Mg(NH2)2–2LiH–0.07KOH sample can reversibly desorb/absorb ∼4.92 wt% hydrogen with a starting and peak dehydrogenation temperature of ∼75 °C and ∼120 °C, respectively, the lowest in the current Mg(NH2)2–2LiH system studied. Moreover, the cycling stability of de-/hydrogenation is also remarkably improved by KOH doping as the average capacity degradation of the Mg(NH2)2–2LiH–0.07KOH system is of only 0.002 wt% per cycle within 30 cycles. Detailed structural investigations reveal that during ball milling, the doped KOH can react with Mg(NH2)2 and LiH to convert to MgO, KH and Li2K(NH2)3, which work together to provide the synergistic effects of thermodynamics and kinetics on hydrogen desorption and absorption of the Mg(NH2)2–2LiH system upon heating, consequently inducing a significant improvement in hydrogen storage properties.

Lithium halides were introduced into the LiNH2–MgH2 system by ball milling the corresponding chemicals under 50 bar of H2 to decrease the dehydrogenation temperature and enhance the dehydrogenation kinetics. The results show that the LiNH2–MgH2–0.05LiBr sample exhibited optimal hydrogen storage performance. The onset dehydrogenation temperature of the LiNH2–MgH2–0.05LiBr sample was only 120 °C, which represents a 55 °C reduction with respect to that of the pristine LiNH2–MgH2 sample. The dehydrogenation rate of the LiNH2–MgH2 sample at 210 °C was increased threefold upon addition of LiBr, which is attributed to the reduction in the dehydrogenation activation energy. Moreover, the addition of LiBr could significantly suppress ammonia emission during the dehydrogenation process of the LiNH2–MgH2 sample. Structural examinations reveal that the added LiBr could react with LiNH2 to form Li7(NH2)6Br during the dehydrogenation process. The in situ-formed Li7(NH2)6Br not only weakens the N–H bond but also promotes the migration of Li+, consequently improving the dehydrogenation kinetics of the LiNH2–MgH2 sample.

We report the high-temperature failure behaviours and mechanisms of K-based additives in the Mg(NH2)2–2LiH hydrogen storage system. The onset of hydrogen release from a Mg(NH2)2–2LiH–0.08KF sample is approximately 80 °C; this is a 50 °C reduction in comparison with the pristine Mg(NH2)2–2LiH. However, the positive effects of K-based additives disappear when the hydrogen release and uptake of the KF-added Mg(NH2)2–2LiH samples are performed at higher temperatures (>200 °C). The change in the crystal structure of the dehydrogenation product, the enlargement in the grain and particle sizes of the dehydrogenation/hydrogenation products, and the increase in the inhomogeneous degree of mixing and distribution of K-based additives should be the three most important reasons for the increased operating temperature during the follow-up cycles. In particular, the ability of K-based additives to lower the operating temperature for hydrogen storage in the Mg(NH2)2–2LiH system can be sufficiently recovered after ball milling. Therefore, the failure of K-based additives after high-temperature treatment is only phenomenological instead of being natural. Strictly limiting the dehydrogenation/hydrogenation of the K-added Mg(NH2)2–2LiH system at lower temperatures is critical for maintaining the superior effect of K-based additives.

Hydrogen storage performances of a Li2NH-xMgNH combination system (x = 0, 0.5, 1 and 2) are investigated for the first time. It is found that the hydrogenated samples with MgNH exhibit a significant reduction in the dehydrogenation temperatures. Mechanistic investigations reveal that there is a strong dependence of the hydrogen storage reaction process on the molar ratio between MgNH and Li2NH. As a consequence, tuning of thermodynamics is achieved for hydrogen storage in the Li2NH-xMgNH system by changing the reaction routes, which is ascertained to be the primary reason for the reduction in the operating temperature for hydrogen desorption. Specifically, it is found that under 105 atm hydrogen (140–280 °C) 5.6 wt% hydrogen is reversibly stored in the Li2NH-0.5MgNH combination system, which is greater than in the well-investigated Mg(NH2)2-2LiH system.

A ternary hydrogen storage system, of superior cyclic stability and high capacity, was developed from a mixture of Ca(BH4)2, LiBH4 and MgH2 in molar ratios of 1:2:2. Investigation on both non-isothermal and isothermal hydrogen desorption/absorption properties shows that the hydrogen desorption starts from 320 °C and completes at 370 °C under a heating rate of 2 °C min−1, releasing ca. 8.1 wt% H2. The finishing temperature of desorption is much lower and the capacity much higher than any of the two-hydride mixtures in the ternary system. In particular, hydrogenation of the ternary system initiates at an extremely low temperature of ca. 75 °C and the onset dehydrogenation temperature is significantly reduced by 90 °C after the initial dehydrogenation/hydrogenation cycle, which is ascribed to the formation of an active dual-cation hydride of CaMgH3.72 for dehydrogenation in the hydrogenation process. There is ca. 7.6 wt% H2 absorbed at 350 °C and 90 bar H2 for 18 h for the system post-dehydrogenated at 370 °C for 30 min, demonstrating a reversibility of over 94%. The capacity seems to fade mainly in the initial few cycles and stabilizes after further cycling. The reversibility is as high as 97% and a dehydrogenation capacity of ca. 6.2 wt% H2 at the 10th cycle. Favourable kinetics and thermodynamics of hydrogen desorption/absorption are achieved, which are responsible for the low completion temperature and the superior cycling performance. Mechanisms of the improved dehydrogenation/hydrogenation properties including the cyclic behaviour of the system are also proposed in relation to microstructural analyses.

The thermal dehydrogenation process of the KOH-containing Mg(NH2)2–2LiH system was systematically investigated by identifying changes in the structure and composition of its components by XRD and FTIR. During ball milling, the added KOH reacts with Mg(NH2)2 and LiH to produce MgO, KH and Li2K(NH2)3. During the initial heating process (<120 °C), the newly formed KH and Li2K(NH2)3 react with Mg(NH2)2 and LiH to yield MgNH, LiNH2 and Li3K(NH2)4 along with hydrogen release. Raising the temperature to 185 °C results in a reaction between Mg(NH2)2, MgNH and LiH that gives Li2Mg2N3H3 as the product and further releases hydrogen. As the temperature is increased to 220 °C, Li2Mg2N3H3 reacts with LiNH2 and LiH to produce Li2MgN2H2 and H2. Meanwhile, two parallel reactions between Li2Mg2N3H3, Li3K(NH2)4 and LiH also generate additional hydrogen. Specifically, the KH and Li2K(NH2)3, formed in situ during ball milling, serve as reactants in the dehydrogenation reaction of the Mg(NH2)2–2LiH system, which is responsible for the significantly improved thermodynamics and kinetics of hydrogen storage.

A significant improvement of hydrogen storage properties was achieved by introducing MgH2 into the 6LiBH4–CaH2 system. It was found that ∼8.0 wt% of hydrogen could be reversibly stored in a 6LiBH4–CaH2–3MgH2 composite below 400 °C and 100 bar of hydrogen pressure with a stepwise reaction, which is superior to the pristine 6LiBH4–CaH2 and LiBH4 samples. Upon dehydriding, MgH2 first decomposed to convert to Mg and liberate hydrogen with an on-set temperature of ∼290 °C. Subsequently, LiBH4 reacted with CaH2 to form CaB6 and LiH in addition to further hydrogen release. Hydrogen desorption from the 6LiBH4–CaH2–3MgH2 composite finished at ∼430 °C in non-isothermal model, a 160 °C reduction relative to the 6LiBH4–CaH2 sample. JMA analyses revealed that hydrogen desorption was a diffusion-controlled reaction rather than an interface reaction-controlled process. The newly produced Mg of the first-step dehydrogenation possibly acts as the heterogeneous nucleation center of the resultant products of the second-step dehydrogenation, which diminishes the energy barrier and facilitates nucleation and growth, consequently reducing the operating temperature and improving the kinetics of hydrogen storage.

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.

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.