Porous Magnesium Hydride Nanoparticles Uniformly Coated by Mg‐Based Composites toward Advanced Lithium Storage Performance

Magnesium hydride (MgH2) has been recognized as a promising anode material of lithium‐ion batteries (LIBs) owing to its ultrahigh specific capacity. The low conductivity and the structural pulverization induced by large volume expansion, however, has long limited its practical lithium storage performance. Herein, a series of yolk‐shell‐like structures, composed of porous MgH2 nanoparticles (NPs) decorated with Mg‐based composites through in‐situ solid‐gas reaction using MgH2 as both the reactant and the structural template, have been fabricated and uniformly dispersed on electronically conductive graphene. It could not only physically accommodate the volume change of MgH2 owing to the physical protection of Mg‐based composites and the formation of void space inside MgH2 NPs, but also effectively facilitate the transportation of electrons throughout the whole electrode. Particularly, the uniform decoration of ultrathin Mg(BH4)2 as the shell with thermodynamically favorable intercalation of lithium ions and low kinetic barrier for the lithium‐ion diffusion promotes facile transportation of lithium ions into active MgH2, which, coupled with the porous structure constructed by flexible graphene, effectively improves the ion conductivity of the electrode. The synergistic improvement in electronic and ionic conductivity leads to a high reversible capacity of 1651 mAh g−1 at 200 mA g−1 after 380 cycles.


Introduction
The development of rechargeable lithium-ion batteries (LIBs) with high energy density, fast charge/discharge rate, and long cycle life has been rapidly promoted due to the urgent demand of ever-growing markets from portable electronics to grid-scale energy storage. [1]Consequently, tremendous efforts have been devoted to developing high-capacity anode materials for LIBs and among them, conversion-type anodes have been attracting a great deal of attention due to their high theoretical capacities. [2]In comparison with the traditional oxides, sulfides, and phosphides-based conversion-type anodes, the emerging magnesium hydride (MgH 2 ) displays the lowest charge-discharge polarization and more favorable operating potential (0.1-0.5 V vs Li þ /Li 0 ) for lithium storage. [3]More importantly, compared with the transition metal and oxygen, sulfur, or phosphorous, the much lower molecular weights of magnesium and hydrogen of MgH 2 leads to a theoretical specific capacity as high as 2048 mAh g À1 . [4]In spite of these attractive features, little attention has been paid to Mg-based hydrides due to multiple essential issues.The first one is the inherent poor conductivity of MgH 2 , which results in low electrochemical capacity and unsatisfied rate performance.Another challenging issue for MgH 2 -based electrodes is their rapid capacity fading induced by the aggregation and even loss of electrical contact with the current collector resulting from the dramatic volume expansion during the repeated lithiation and delithiation process. [5]5a,b] To solve these issues, our group have developed a hydrogenation-induced solvothermal assembly method to realize the uniform distribution of MgH 2 nanoparticles (NPs) with high loading capacity on the electrically conductive graphene, which also could effectively enhance the electronic DOI: 10.1002/sstr.202300365Magnesium hydride (MgH 2 ) has been recognized as a promising anode material of lithium-ion batteries (LIBs) owing to its ultrahigh specific capacity.The low conductivity and the structural pulverization induced by large volume expansion, however, has long limited its practical lithium storage performance.Herein, a series of yolk-shell-like structures, composed of porous MgH 2 nanoparticles (NPs) decorated with Mg-based composites through in-situ solid-gas reaction using MgH 2 as both the reactant and the structural template, have been fabricated and uniformly dispersed on electronically conductive graphene.It could not only physically accommodate the volume change of MgH 2 owing to the physical protection of Mg-based composites and the formation of void space inside MgH 2 NPs, but also effectively facilitate the transportation of electrons throughout the whole electrode.Particularly, the uniform decoration of ultrathin Mg(BH 4 ) 2 as the shell with thermodynamically favorable intercalation of lithium ions and low kinetic barrier for the lithium-ion diffusion promotes facile transportation of lithium ions into active MgH 2 , which, coupled with the porous structure constructed by flexible graphene, effectively improves the ion conductivity of the electrode.The synergistic improvement in electronic and ionic conductivity leads to a high reversible capacity of 1651 mAh g À1 at 200 mA g À1 after 380 cycles.conductivity of the whole electrode. [6]Subsequently, a core-shell protective strategy via the coating of polythiophene films on MgH 2 NPs has been further developed for physically alleviating the volume change of metal hydrides upon cycling charge and discharge process. [7]Despite these progresses, induced by the slow transfer of lithium ions into active MgH 2 owing to the low lithium-ion conductivity of polythiophene films and the lack of void space for effectively mitigating the volume change of MgH 2 , which results in the serious cracking of the shells upon cycling lithiation and delithiation process, the lithium storage performance of MgH 2 still falls short in systematic rate capabilities and cycling stability for practical applications.Furthermore, the hydrogenation-induced self-assembly process, as well as the subsequent synthesis of polythiophene films, involves the use of massive organic solvent, which has high economical and environmental cost.
To overcome these shortcomings, herein, we report a facile solvent-free strategy to fabricate yolk-shell-like structure, i.e., porous MgH 2 NPs coated with Mg-based composites (including MgS, Mg(NH 2 ) 2 , and Mg(BH 4 ) 2 ) using an in-situ solid-gas reaction strategy. [8]First, by virtue of the favorable adsorption of dibutyl-magnesium on graphene, a solvent-free strategy using the self-decomposition of dibutyl-magnesium was adopted to fabricate homogeneous MgH 2 NPs with an average particle size of 50 nm uniformly distributed on graphene, which would effectively contribute to the enhancement of electronic conductivity of the whole electrode. [9]Subsequently, taking advantage of the inherent high chemical reactivity of MgH 2 NPs, a homogeneous shell composed of Mg-based composites are in-situ built on the surface of MgH 2 NPs via a series of solid-gas reactions using MgH 2 as both the reactant and the structural template.Interestingly, due to the Kirkendall effect upon the uniform formation of Mg-based composites, the original MgH 2 NPs inside break up into smaller clusters, leading to the formation of porous structure with void spaces under the physical protection of Mgbased composites, which essentially accommodates the volume expansion of MgH 2 NPs during cycling charge-discharge process.More interestingly, it is theoretically verified that, among all these Mg-based composites, the intercalation of Li ions into Mg(BH 4 ) 2 is thermodynamically favorable with low kinetic barrier for Li ion diffusion inside of Mg(BH 4 ) 2 , which promotes facile transportation of Li ions into active MgH 2 , contributing to its most effective effect on improving the lithium storage performance of MgH 2 .Taking advantage of the synergistic improvement on electronical and ionic conductivity, the as-formed MgH 2 @Mg(BH 4 ) 2 /G (MHG@MBH) exhibits a stable capacity as high as 1651 mAh g À1 at 200 mA g À1 after 380 cycles.

Results and Discussion
The preparation process of porous MgH 2 NPs decorated with Mg-based composites is schematically illustrated in Figure 1.First, graphene-supported MgH 2 NPs were fabricated through a solvent-free hydrogenation-induced decomposition of (C 4 H 9 ) 2 Mg according to our previous work. [8]Taking advantage of the favorable adsorption energy between (C 4 H 9 ) 2 Mg and GNs, the thus-formed MgH 2 NPs with an average size of 50 nm could be firmly anchored on the GNs with well distribution (Figure S1 and S2, Supporting Information), which was subsequently adopted as the nanoreactor to support in-situ solid-gas reaction between MgH 2 NPs and the precursors (i.e., B 2 H 6 , NH 3 , and S, respectively).The graphene with flexible and porous structure could not only act as the structural support to inhibit the aggregation and growth of MgH 2 NPs, but also provide facile pathways for the transportation of gases to facilitate the uniform reaction with MgH 2 NPs.Based on this methodology, a series of Mgbased composites, denoted as MHG@MgX (X = BH 4 , S, NH 2 ), are in-situ coated on the surface of MgH 2 NPs distributed on graphene, including MgH 2 @Mg(BH 4 ) 2 /G (denoted as MHG@MBH), MgH 2 @MgS/G (denoted as MHG@MS), and MgH 2 @Mg(NH 2 ) 2 /G (denoted as MHG@MNH) (Figure S3-S5, Supporting Information).As shown in Figure 2a,b, the morphology of MgH 2 NPs and their homogeneous distribution on graphene could be well preserved after the in-situ solid-gas reaction owing to the presence of graphene as the structural support.Specifically, after the reaction between MgH 2 NPs and B 2 H 6 , a uniform shell of Mg(BH 4 ) 2 with an average thickness of only 3 nm was coved on the surface of MgH 2 NPs and it could be easily controlled by tuning the reaction time and the amount of reactants (Figure 2e,f ).Moreover, characteristic lattice fringes of 2.13 and 2.86 Å, corresponding to the ( 122) and ( 021) planes of Mg(BH 4 ) 2 , respectively, could be clearly observed on the surface of MgH 2 NPs, indicating the successful formation of Mg(BH 4 ) 2 resulting from the reaction between MgH 2 NPs and B 2 H 6 .More interestingly, scanning transion electron microsopy (STEM) images (Figure 2c,d) validate that, accompanied with the formation of Mg(BH 4 ) 2 , original MgH 2 NPs break up into several smaller nanoparticles, which could be attributed to the Kirkendall effect induced by the different diffusion rates of Mg and B-containing species during the solid-gas reaction, leading to the simultaneous formation of Mg(BH 4 ) 2 as the shell and the porous structure of MgH 2 inside. [10]The void spaces between different MgH 2 crystals inside of the shell provide extra spaces for accommodating the volume expansion of MgH 2 NPs during the cycling charge-discharge process.In addition, abundant spaces are present between individual MHG@MBH NPs uniformly distributed on graphene, which could further alleviate the volume change and the aggregation of electroactive MgH 2 NPs and, coupled with the porous structure constructed by the flexible graphene, effectively facilitate the rapid diffusion of lithium ions through the whole electrode.The yolk-shell-like structure of MHG@MBH could be further verified by the elemental line-scan profile (Figure 2g), which reveals that the content of B belonging to Mg(BH 4 ) 2 in the fringe of MgH 2 NPs is lower than that in the center area, indicating the formation of Mg(BH 4 ) 2 on the surface of MgH 2 NPs.Furthermore, the elemental mapping images of Mg, B, and C provide additional evidences for the uniform distribution of elemental B from Mg(BH 4 ) 2 on the surface of MgH 2 NPs and the yolk-shell-like structure of MgH 2 @Mg(BH 4 ) 2 throughout the graphene (Figure 2h-l), which could largely promote the electronic conductivity of the whole electrodes.
Upon the adopting of MgH 2 /G as the nanoreactor to react with NH 3 and S, comparable yolk-shell-like structure, composed of porous MgH 2 NPs as the core and MgS and Mg(NH 2 ) 2 as the shell, could be observed for the as-synthesized MHG@MS and MHG@MNH, respectively.As illustrated in Figure S6 and S7, Supporting Information, the uniform decoration of MgS and Mg(NH 2 ) 2 shells on the surface of MgH 2 NPs homogeneously distributed on graphene could be clearly verified by the corresponding HRTEM images, the elemental line-scan profile and the elemental mapping results of MHG@MS and MHG@MNH, respectively.X-ray diffraction (XRD) patterns (Figure S8, Supporting Information) demonstrate that, after the in-situ reaction, all the diffraction peaks of MgH 2 become weaker in comparison with pure MHG, which could be attributed to the consumption of MgH 2 NPs due to the formation of Mg-containing species as the shells.In particular, the formation of Mg(BH 4 ) 2 in MHG@MBH and MgS in MHG@MS could be directly demonstrated by the presence of their characteristic peaks in the respective XRD patterns while no detectable diffraction peaks of Mg(NH 2 ) 2 could be observed for the MHG@MNH composite possibly due to its poor crystallinity (Figure S8, Supporting Information).Therefore, Fourier-transform infrared (FTIR) spectra (Figure S9, Supporting Information) and Raman spectra (Figure S10, Supporting Information) were subsequently conducted to characterize the chemical composition of the thusformed Mg-based composites.It could be observed that, after the solid-gas reaction, the characteristic peaks of B─H bonds of Mg(BH 4 ) 2 , N─H bonds of Mg(NH 2 ) 2 , and Mg─S bonds of MgS are clearly detected by both FTIR and Raman spectra of MHG@MBH, MHG@MNH, and MHG@MS, respectively, which provides further evidence to the formation of Mg(BH 4 ) 2 , Mg(NH 2 ) 2 , and MgS.Moreover, two broad peaks at around 1338 and 1600 cm À1 observed in the Raman spectra of all the as-synthesized composites could be indexed to the typical D and G bands of graphene. [11]o evaluate the effect of in-situ decoration of Mg-based composites as the shells on improving the lithium storage performance of MgH 2 , the charge-discharge performance of the as-synthesized MHG@MBH, MHG@MS, and MHG@MNH electrodes was first investigated at a current density of 200 mA g À1 , with pure MHG included for comparison.As shown in Figure 3a, although an initial charge capacity of 1298 mAh g À1 could be achieved for pure MHG, which is much higher than bulk MgH 2 or the nanoconfined MgH 2 NPs reported previously due to the structural support role of graphene with excellent electrical conductivity, [7] the reversible capacity rapidly faded to only 350 mAh g À1 after 30 cycles of lithiation and delithiation process induced by the large volume change.It should be noted that all specific capacities in this work are calculated based on the weight of MgH 2 .In strong contrast, after in-situ coating of Mg-based composites as the shell, both the specific capacity and the cycling stability of MHG electrode are significantly improved.Among them, the MHG@MBH electrode exhibits highest specific capacity with an initial charge capacity of 1454.6 mAh g À1 , much higher than that of both MHG@MS (1186.4mAh g À1 ) and MHG@MNH (1195 mAh g À1 ).More importantly, a reversible capacity of 1629.4 mAh g À1 could be maintained for the MHG@MBH electrode even after 380 cycles with a Coulombic efficiency of approximately 99.1%, while this value is gradually decreased to be around 561 and 523 mAh g À1 for MHG@MS and MHG@MNH electrodes after only 200 cycles, respectively, which is comparable with pure MHG electrode.It could be noted that the reversible capacity of the MHG@MBH electrode is gradually increased, which could be possibly induced by the continuous infiltration of the electrolyte into the electrode during cycling charge and discharge process.On one hand, upon the continuous infiltration of the electrolyte, the layer spacing between the graphene becomes larger, and hence more reaction sites of the working MgH 2 anode would be exposed for electrochemical reaction.Meanwhile, a larger layer spacing also facilitates the facile transfer of Li ions within the electrode toward enhanced electrochemical reaction of MgH 2 anode. [12]On the other hand, after the uniform coating of ultrathin Mg(BH 4 ) 2 shell, MHG@MBH anode has a more stable structure than MHG@MNH and MHG@MgS anode, enabling it to maintain the continuous progress of activation.These results demonstrate that the uniform coating of Mg-based composites, especially Mg(BH 4 ) 2 , could effectively enhance the cycling stability of MgH 2 and more importantly, considering their comparable structures, Mg(BH 4 ) 2 could play an important role in unlocking the lithium performance of MgH 2 .Hence, to understand the role of Mg-based composites during the lithium storage process, the morphology of various electrodes after 100 cycles at 200 mA g À1 was systematically investigated using SEM, TEM measurement and the corresponding elemental mapping (Figure 3b-i).It reveals that, after 100 cycles, MgH 2 NPs of pure MHG exhibit serious aggregation and collapse due to the large volume change of MgH 2 and the lack of the physical protection upon cycling process, which corresponds well with its fast decay of specific capacities.By comparison, under the identical condition, the homogeneous distribution of MgH 2 NPs on graphene could be preserved to a large extent in MHG electrodes by virtue of the coating with Mg-based composites due to the formation of yolk-shell-like structures, which confirms that the coating of Mgbased composites could alleviate the volume change of MgH 2 electrode upon cycling lithiation and delithiation process.In comparison with MHG@MBH and MHG@MNH (Figure S11 and S12, Supporting Information), the trend toward aggregation and particle growth is more obvious for MgH 2 NPs after coating with MgS (Figure S13, Supporting Information).Additionally, obvious cracks on the surface of MHG@MS electrode could be observed with a large thickness expansion rate of 64% after 100 cycles of charge-discharge process (Figure S14, Supporting Information).In strong contrast, (Figure 3h), the MHG@MBH electrode maintains its flat surface with few visible cracks and the thickness expansion rate is limited to be only 3% even after 100 cycles.In order to unravel the reason behind this phenomenon, the electrochemical performances of pure Mg(BH 4 ) 2 , MgS, and Mg(NH 2 ) 2 as electrodes were tested.Only neglectable electrochemical capacity could be observed for Mg(BH 4 ) 2 and Mg(NH 2 ) 2 electrodes under the adopted experimental condition (Figure S15 and S16, Supporting Information).Moreover, the characteristic peaks of Mg(BH 4 ) 2 and Mg(NH 2 ) 2 in FTIR spectra are well preserved after the first discharge process (Figure S17, Supporting Information), which further demonstrates that they are almost inactive upon the electrochemical reaction.By comparison, a specific capacity of about 400 mAh g À1 could still be observed after 100 cycles for bulk MgS electrode (Figure S18, Supporting Information) and the corresponding CV result (Figure S19, Supporting Information) exhibit the electrochemical reaction process between lithium and MgS, [13] which leads to the rupture and collapse of the shell upon charge-discharge cycling process and hence the fast decay of specific capacities upon cycling.Meanwhile, the shell composed of Mg(BH 4 ) 2 and/or Mg(NH 2 ) 2 , which is electrochemically inactive at room temperature, could generally maintain their stable structure during repeated lithiation and delithiation process, resulting in a superior cycling stability than MHG@MS.
It is interesting to note that, although bearing the comparable structure, the electrochemical performance of MHG@MBH, particularly the specific capacity, is much better than that of MHG@MNH.Hence, in addition to the structural effect, the impact of the coating material, i.e., Mg(BH 4 ) 2 and Mg(NH 2 ) 2 , on improving the electrochemical performance of MgH 2 was further investigated by adopting DFT calculations.In order to initiate the electrochemical reaction of MgH 2 with a yolk-shell-like structure, Li þ ions should be able to intercalate into the coating layers first, followed by their diffusion toward the active materials.Therefore, the Li þ insertion behavior into MgH 2 , Mg(BH 4 ) 2 , Figure 3. a) Cycling performance of MHG@MBH, MHG@MS, MHG@MNH, and MHG electrodes at 200 mA g À1 .b,c) STEM images of MHG@MBH in the charged state after 100 cycles at 200 mA g À1 .d) Elemental mapping images of MHG@MBH in the charged state after 100 cycles.e-g) TEM images of MHG in the charged state after 100 cycles at 200 mA g À1 .Top-view, cross-sectional SEM images, and the corresponding elemental mapping of h) MHG@MBH, and i) MHG electrode before and after 100 cycles at 200 mA g À1 .and Mg(NH 2 ) 2 was subsequently examined.As shown in Figure 4a-c and S20, Supporting Information, the intercalation behavior of Li þ into each compound could be evaluated quantitatively by the formation energy (E f ).After structural optimization, the E f for the lithium ions intercalation into MgH 2 is calculated to be 0.16 eV.In strong contrast, the E f for the Li-inserted Mg(NH 2 ) 2 (À1.91 and À1.45 eV) and Mg(BH 4 ) 2 (À0.87,À0.79 and À0.61 eV) are verified to be thermodynamically favorable, indicating the facile lithium insertion into both Mg(NH 2 ) 2 and Mg(BH 4 ) 2 , which could effectively reduce the energy required for lithium ions diffusion through the coating layers and hence promote the lithium storage performance of MgH 2 .
Subsequently, the diffusion barrier of Li þ inside Mg(BH 4 ) 2 and Mg(NH 2 ) 2 were calculated using the climbing image nudged elastic band (CI-NEB) method [14] and the optimized pathway of Li þ ions diffusion between two equivalent sites and corresponding energy profiles were plotted in Figure 4dÀf.It could be clearly observed that the diffusion barrier of Li þ in Mg(BH 4 ) 2 is calculated to be only 0.3 eV, while this value approaches 0.5 eV for Mg(NH 2 ) 2 , which provides direct evidence to the faster ion-diffusion performance inside Mg(BH 4 ) 2 compared with Mg(NH 2 ) 2 , kinetically promoting the lithium storage performance of MgH 2 .Therefore, in comparison with Mg(NH 2 ) 2 , the building of Mg(BH 4 ) 2 as the shell could not only physically alleviate the volume change of MgH 2 during cycling charge and discharge process, but also effectively facilitate the intercalation and diffusion of lithium ions into MgH 2 , leading to superior electrochemical performance of MgH 2 with a higher specific capacity.
Hence, the lithium storage performance of the as-prepared MHG@MBH was systematically investigated in detail via galvanostatic charge-discharge test using coin-type half-cells with the Li-metal as the counter electrode.As shown in Figure 5b, the galvanostatic charge-discharge profiles of MHG@MBH in the 0.01À3 V range from the first to 15th cycle at 200 mA g À1 is comparable with that of MHG, due to the electrochemically inative nature of Mg(BH 4 ) 2 under the adopted experimental condition.3d] Subsequently, the potential gradually drops down to around 0.25 V with a relatively flat plateau between 0.18 and 0.25 V, corresponding to the reduction of MgH 2 to Mg accompanied by the formation of LiH.Additionally, the plateau below 0.18 V, approaching the cut-off potential, could be ascribed to the formation of Li-Mg alloy.After being discharged to 0.01 V, HRTEM results demonstrate that the d-spacings of the crystalline nanodomains dispersed on graphene in MHG@MBH were measured to be 0.25 and 0.26 nm, which could be indexed to the (200) planes of LiH and the (002) planes of Li x Mg (x ≦ 1.5) (Figure S21, Supporting Information), respectively, elucidating the complete lithiation process with the formation of LiH and Li-Mg alloys accompanied by the disappearance of MgH 2 .Upon the reversible charge process, two plateaus observed at around 0.25 and 0.6 V could be attributed to the extraction of Li þ ions from Li x Mg alloy and the oxidation of Mg into MgH 2 , respectively, as verified by the HRTEM results of fully charged sample.These results validate that the reversible lithiation and delithiation process of MgH 2 in MHG@MBH is derived from the conversion reaction between MgH 2 and Li x Mg alloys and LiH.The MHG@MBH electrode delivers a high initial discharge capacity of 3700 mAh g À1 , slightly higher than the theoretical capacity of MgH 2 (2243 mAh g À1 based on the conversion reaction to form LiH and the alloying reaction to form hcp-type Li x Mg alloys), which could be attributed to the irreversible formation of the SEI layer on the surface and the possible formation of amorphous solid solution of Li-Mg alloys.In the subsequent cycles, the discharge and charge curves of MHG@MBH electrodes are almost overlapping, indicating its good reversibility owing to the protection of Mg(BH 4 ) 2 as the coating shell.By comparison, although MHG electrode exhibits comparable plateaus for charge and discharge curves within the same potential range, the specific capacities decreased rapidly upon cycling (Figure S22, Supporting Information).Besides, the MHG@MBH electrode illustrates superior rate performance than pure MHG electrode (Figure 5c).In the term of pure MHG, the average reversible specific capacity rapidly fades from 649 to 436 mAh g À1 , when the current density gradually increases from 100 to 1000 mA g À1 .In strong contrast, the reversible capacities of the MHG@MBH electrode could reach 1708, 1450, 1143, 1062, and 890 mAh g À1 at 100, 200, 400, 800, and 1000 mA g À1 , respectively.More importantly, when the current density is turned back to 100 mA g À1 , a specific capacity of up to 1588.6 mAh g À1 could still be recovered, corresponding to 93% of the initial reversible specific capacity, which corroborates the strong tolerance of MHG@MBH toward fast lithiation/delithiation process due to the protection of Mg(BH 4 ) 2 with low lithiumion diffusion barriers.
To further evaluate the long-term lithium storage performance, the cycling retention of MHG@MBH anode was investigated at high current density (Figure 5d).Under the current density of 1 A g À1 , the MHG@MBH electrode delivers a high reversible capacity of 2005 mAh g À1 for the first cycle and still maintains a reversible capacity of 1247 mAh g À1 after 300 cycles, corresponding to a capacity retention of 96% compared to the specific capacity of the 4th cycle.By comparison, the reversible specific capacities of MHG electrode rapidly drop to 235 mAh g À1 after only 23 cycles and remained unchanged after prolonging to 300 cycles under identical conditions.Upon increasing the current density to 2 A g À1 , a reversible capacity of 1032 mAh g À1 could still be achieved for the MHG@MBH electrode after 300 cycles, corresponding to a capacity retention of 85% compared to the specific capacity of the 10th cycle.More importantly, the coulumbic efficiencies (CEs) of MHG@MBH could be preserved at a level over 98.6% through the entire cycling process, which provides further evidence to the excellent reversibility induced by the formation of yolk-shell-like structure homogeneously distributed on graphene with an excellent electrical and ionic conductivity.To illustrate the effects of the Mg(BH 4 ) 2 as the shell on improving the cycling performance of MgH 2 electrode, the electrochemical impedance spectroscopy (EIS) of MHG@MBH and MHG electrodes was performed (Figure 5e).All EIS spectra are composed of a compressed semicircle in the high-to-medium frequency region, corresponding to the charge transfer resistance (R ct ), and an inclined straight line in the low-frequency region, corresponding to the typical Warburg behavior related to the Li ions diffusion within the electrode. [15]According to the fitting results based on the equivalent circuit model (Figure S23, Supporting Information), the R ct of MHG@MBH electrode extracted from the high-frequency range are calculated to be only 65.8 Ω before cycling, which is much lower than that of the pure MHG (169.1 Ω).This could be mainly derived from the synergistic effects of the homogeneous distribution of MgH 2 on the graphene with high electronic conductivity and the rapid lithium-ion diffusion inside of the Mg(BH 4 ) 2 , which effectively facilitates both electron and lithium-ion transfer throughout the whole electrode and hence decreases the apparent resistance of the electrode.Upon the proceeding of cycling lithiation and delithiation to 200 cycles, the charge transfer resistance for MHG@MBH electrode exhibits a slight decrease to only 50.2 Ω owing to the activation process during cycling charge and discharge process, while this value is still as high as 120.6 Ω for pure MHG electrode.To further illustrate the effect of ultrathin Mg(BH 4 ) 2 shell, EIS measurement of MHG@MNH and MHG@MgS, respectively, were conducted.As shown in Figure S24, Supporting Information, the R ct of MHG@MNH in the high-frequency region is similar to that of MHG@MBH at the initial state, indicating their comparable electrochemical reaction resistance before cycling.However, the slope of the inclined straight line of MHG@MBH in the low-frequency region is significantly larger, which demonstrates that the diffusion rate of Li ions within the MHG@MBH electrode is faster.In addition, unlike MHG@MBH and MHG@MNH anode, the EIS spectra of MHG@MgS anode is composed of two compressed semicircles, which could be attributed to the formation of SEI films with larger resistance that limits the electrochemical performance of the thus-assembled battery.

Conclusion
In summary, facile synthesis of porous MgH 2 NPs with void spaces decorated with a series of Mg-based composites uniformly distributed on graphene has been achieved via the in-situ solidgas reaction using MgH 2 NPs as both the reactants and the structural template.Particularly, the formation of yolk-shell-like structure of MgH 2 @Mg(BH 4 ) 2 on graphene could not only physically accommodate the volume change of MgH 2 , but also significantly facilitate the transportation of electrons throughout the whole electrode.More importantly, the uniform decoration of ultrathin Mg(BH 4 ) 2 as the shell with thermodynamically favorable intercalation of lithium ions and low kinetic barrier for the lithium-ion diffusion contributes to the facile transportation of lithium ions into active MgH 2 , which, coupled with the porous structure constructed by flexible graphene, effectively improves the ion conductivity of the electrode.The synergistic improvement on electronic and ionic conductivity leads to outstanding lithium storage properties, including high specific capacity, superior rate capability, and long cycle life.Therefore, this work offers a new method for developing metal-hydride-based anodes with superior lithium storage performance and this novel strategy could be extended to the fabrication of various types of advanced electrode materials.

Figure 4 .
Figure 4.The intercalation behavior of Li þ into each compound is described quantitatively via the Li-inserted stable structure and the corresponding formation energy (E f ) when Li þ inserted into a) MgH 2 , b) Mg(NH 2 ) 2 , and c) Mg(BH 4 ) 2 .Diffusion paths of Li þ inside d) Mg(NH 2 ) 2 , and e) Mg(BH 4 ) 2 between two equivalent sites and f ) the corresponding energy profiles (the orange line corresponds to the Mg(BH 4 ) 2 , and the blue line corresponds to the Mg(NH 2 ) 2 .Orange, green, turquoise, blue and pink spheres are Mg, Li, B, N and H, respectively.

Figure 5 .
Figure 5. Electrochemical performance in half cell.a) Schematic illustration of the structural advantages of MBH@MBH as lithium storage materials.b)Galvanostatic charge-discharge profiles of MHG@MBH at 100 mA g À1 at selected cycles.c) Rate performance of MHG@MBH and MHG electrode.d) Cycling performance of MHG@MBH and MHG at 1 and 2 A g À1 .e) Nyquist plots of MHG@MBH and MHG electrodes in the discharged state before and after 200 cycles.