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Article

Improvement in the Electrochemical Lithium Storage Performance of MgH2

1
School of Materials Science and Engineering and Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, China
2
China-Australia Joint Laboratory for Energy & Environmental Material, South China University of Technology, Guangdong 510641, China
*
Author to whom correspondence should be addressed.
Inorganics 2018, 6(1), 2; https://doi.org/10.3390/inorganics6010002
Submission received: 15 October 2017 / Revised: 11 December 2017 / Accepted: 11 December 2017 / Published: 26 December 2017
(This article belongs to the Special Issue Functional Materials Based on Metal Hydrides)

Abstract

:
Magnesium hydride (MgH2) exhibits great potential for hydrogen and lithium storage. In this work, MgH2-based composites with expanded graphite (EG) and TiO2 were prepared by a plasma-assisted milling process to improve the electrochemical performance of MgH2. The resulting MgH2–TiO2–EG composites showed a remarkable increase in the initial discharge capacity and cycling capacity compared with a pure MgH2 electrode and MgH2–EG composite electrodes with different preparation processes. A stable discharge capacity of 305.5 mAh·g−1 could be achieved after 100 cycles for the 20 h-milled MgH2–TiO2–EG-20 h composite electrode and the reversibility of the conversion reaction of MgH2 could be greatly enhanced. This improvement in cyclic performance is attributed mainly to the composite microstructure by the specific plasma-assisted milling process, and the additives TiO2 and graphite that could effectively ease the volume change during the de-/lithiation process as well as inhibit the particle agglomeration.

Graphical Abstract

1. Introduction

Magnesium hydride (MgH2) has been intensively investigated as a hydrogen and heat energy storage medium [1,2,3,4] because of its high hydrogen storage capacity (7.6 wt % 110 kg/m3), low cost, and abundance. MgH2 is an intrinsically ionic compound. The strong Mg–H bond determines the high hydrogen desorption enthalpy (~74 kJ/mol H2) and the high decomposition temperature (>350 °C) of MgH2. Additionally, MgH2 suffers from sluggish hydrogen sorption kinetics due to the slow hydrogen diffusion in the MgH2 lattice and poor hydrogen dissociation on the surface of Mg [5,6,7,8]. To overcome the thermodynamic and kinetic problems of MgH2, many effective methods including nanostructuring, alloying, catalyzing, and compositing have been developed by ball milling, film deposition, and chemical synthetic processes [9,10,11,12,13,14].
In 2008, MgH2 was first reported as a potential anode material for lithium-ion batteries (LIBs) by Y. Oumellal et al. [15] since it exhibited rather high lithium storage capacity (MgH2 + Li ↔ Mg + LiH, ~2038 mAh·g−1), low work potential (~0.5 V versus Li+/Li), and low voltage hysteresis (<0.2 V). The last point is especially superior over other kinds of conversion reaction materials, such as metal oxides, nitrides, fluorides etc. However, the large volume expansion (>85%) during the conversion reaction of magnesium hydride with lithium resulted in fast capacity fading, which is even more severe than other conversion anode materials as the metal hydride has low conductivity and high activity in the liquid electrolyte. Since then, many efforts have been devoted to improving the electrochemical performance of MgH2 [16,17,18,19,20,21]. As reported by S. Brutti et al. [22], the ball-milled MgH2 sample with the addition of Super P shows a relatively high discharge capacity (~1600 mAh·g−1) and coulombic efficiency (~60%), which are mainly attributed to the reduction in crystallite size and enhancement of the electronic conductivity of MgH2. Furthermore, the addition of metal oxides is also helpful in the conversion reaction of MgH2 with lithium. For example, Kojima et al. [23,24] added the Nb2O5/Al2O3 into the active material MgH2, thus enhancing the coulombic efficiency of all solid-state lithium-ion batteries. However, the improved effect in the coulombic efficiency, the reversibility of the conversion reaction, and the cycling performance of a MgH2 anode is rather limited [25,26,27].
In this work, the expanded graphite (EG) and TiO2 were milled with Mg/MgH2 by dielectric barrier discharge plasma-assisted vibratory milling (P-milling), which is especially advantageous for the preparation of composites containing few-layered graphite [28]. The electrochemical performances of the obtained composites, MgH2–EG and MgH2–TiO2–EG, were compared. Additionally, the new electrode preparation method was also developed to improve the performances of hydride anodes. It was demonstrated that the reversibility of the conversion reaction and the cyclic stability of a MgH2 anode could be greatly enhanced.

2. Results and Discussion

The MgH2–EG composite was prepared by the hydrgogenation treatment of the milled mixture (denoted as Mg–EG) of Mg powder and expandable graphite. Figure 1a shows the XRD patterns of the as-milled and hydrogenated Mg–EG composites. After milling for 10 h, the hexagonal Mg remains the major phase of the composite, and the weak MgO peak is due to the slight oxidation during sample transfer. It is stated that the weak MgF2 diffractions indicate the reaction of Mg with the electrode bar composed of polytetrafluoroethylene, which was also reported in previous work [20]. After hydrogenation, all the Mg diffraction peaks disappear, indicating complete hydrogenation of the Mg powder. The rutile-type α-MgH2 phase is indexed by strong and sharp diffraction peaks, implying grain growth of MgH2 due to the hydrogenation treatment.
Figure 1b shows the galvanostatic discharge/charge curves of the MgH2–EG electrode at different cycles. In the first discharge profile, the potential drops rapidly from the initial open circuit potential (OCP) to 0.27 V, and then increases to 0.33 V, followed by a well-defined potential plateau assigning to the conversion reaction of MgH2 with lithium. The slight polarization of the MgH2–EG composite is owing to the kinetic limitation caused by the poor electronic conductivity of MgH2 and the weak electronic contact between the active material and the nickel foam [29]. After that, the potential gradually drops further to 0.10 V with another plateau, which is attributed to the alloying of Mg with Li. Upon charging, two potential plateaus ranging from 0.10 V to 0.21 V and from 0.21 V to 0.60 V are assigned to the de-alloying reaction and the reverse conversion reaction of Mg/LiH, respectively. The first discharge capacity, amounting to 717.4 mAh·g−1, is much less than its theoretical capacity (1704.8 mAh·g−1 = 2038 mAh·g−1 × 80 wt % (MgH2) + 372 mAh·g−1 × 20 wt % (graphite)). This result reflects the loss of active Mg during milling, as well as the kinetic limitation of coarsening the MgH2 electrode. In addition, in the initial charge process, the MgH2–EG electrode shows a total charge capacity of 320.6 mAh·g−1, corresponding to an initial coulombic efficiency (ICE) of 44.7%. This low ICE value also indicates the incomplete reversible formation of MgH2 in the delithiation process.
The cycling performance of MgH2–EG electrode is shown in Figure 1c. The rapid capacity fading in the initial several cycles may be attributed to the pulverization of active material leading to the loss of electronic contact between the active material and the nickel foam. After 50 cycles, a capacity of only 48.1 mAh·g−1 is maintained in the cell; this value is even less than the capacity (~70 mAh·g−1) contributed by the graphite component. In addition, according to the discharge profile at the 10th cycle (Figure 1b), the plateau corresponding to the conversion reaction of MgH2 with lithium is invisible. Further, the differential capacity plots (dQ/dV) of different cycles are compared in Figure 1d. The peak centered at 0.61 V, which is assigned to the reverse conversion reaction of Mg with LiH, disappears after 10 cycles. This result further confirms the poor conversion reversibility and cycling performance of the MgH2–EG electrode.
To improve the electrochemical performances, especially the cycling stability of a MgH2 electrode, TiO2 was added to the MgH2–EG composite to accommodate the large volume variation. In addition, Mg was replaced by MgH2 as the starting milling material in order to avoid grain growth during the hydrogenation treatment. The XRD patterns of as-milled MgH2–TiO2–EG composite with different milling times are shown in Figure 2a. The MgH2 peaks for the 20 h-milled composite (denoted as MgH2–TiO2–EG-20 h) show a relative broadening effect compared to that of the 10 h-milled composite (denoted as MgH2–TiO2–EG-10 h), implying a finer grain size of the MgH2 by longer milling time. The SEM observation shown in Figure 2c,d also displays smaller particle size (~10 μm) for the MgH2–TiO2–EG-20 h composite. Actually, the composite particles consist of nanosized primary particles according to the magnified SEM images (Figure 2c,d). With regard to the graphite after P-milling for 20 h, the graphite peak around 26.6° disappears in Figure 2a, implying the formation of a disordered structure of the graphite. It is believed that the graphite could be effectively exfoliated to few-layer graphene (FLG) nanosheets due to the synergic effect of the plasma heating and the impact stress from the milling balls [28,30,31]. Additionally, the hydrogen desorption kinetic curves (Figure 2b) show that both samples could release ~3.4 wt % H2 within 15 min, corresponding to the actual MgH2 content of ~44.3 wt % in the MgH2–TiO2–EG composite.
The galvanostatic charge/discharge curves of the MgH2–TiO2–EG composite with different milling times are compared in Figure 3a,b. In the first discharge profile, the MgH2–TiO2–EG-10 h composite electrode delivers a total discharge capacity of ~1224.6 mAh·g−1, which is very close to its theoretical capacity (1193.9 mAh·g−1 = 2038 mAh·g−1 × 50 wt % (MgH2) + 335 mAh·g−1 × 30 wt % (TiO2) + 372 mAh·g−1 × 20 wt % (graphite)), and this result is also much higher than that of the MgH2–EG electrode mentioned above. The ICE for the MgH2–TiO2–EG-10 h composite electrode is 46.4%, which is a little higher than that for the MgH2–EG electrode. As seen in Figure 3c, the MgH2–TiO2–EG-10 h composite electrode exhibits a discharge capacity of 179.1 mAh·g−1 at the 100th cycle, with a capacity retention of 33%. Compared with the MgH2–TiO2–EG-10 h electrode, the MgH2–TiO2–EG-20 h electrode shows a similar initial discharge capacity (~1218.6 mAh·g−1) and ICE (48.1%) but possessing a much higher cycling capacity of 305.4 mAh·g−1 after 100 cycles and a capacity retention of ~31%. As also shown in Figure 3c, while the MgH2–TiO2–EG-10 h composite electrode experiences rapid capacity fading within the first several cycles, the MgH2–TiO2–EG-20 h electrode delivers more stable capacity within 10 cycles, and it also shows higher coulombic efficiency throughout the cycling. Additionally, as shown in Figure 3d, the distinct anodic peak in the differential capacity plots (dQ/dV) at the 100th cycle clearly demonstrates the reversible formation of MgH2, which indicates that the enhanced cyclic stability of the MgH2–TiO2–EG-20 h composite electrode is due to the enhanced conversion reaction reversibility of MgH2.
XRD analysis was performed to the change of phase structure of the MgH2–TiO2–EG-20 h electrode after cycling, but the result (not shown here) shows no diffractions and implies poor crystallinity of the active materials. SEM observation was also carried out to investigate the microstructural evolution of the composite electrodes, and the results are shown in Figure 4. Before cycling, the electrode surface of both the MgH2–TiO2–EG-10 h and the MgH2–TiO2–EG-20 h electrodes are composed of irregular particles with sizes less than 20 μm (Figure 4a,c), and there is no remarkable morphological difference between them. After 100 cycles, the surface morphology of both electrodes experiences obvious particle coarsening, which is due to the lithiation and delithiation of the active material, which causes repeated powder pulverization and agglomeration. This result also explains the capacity loss of the electrodes during cycling. Further, it is also shown that the particle coarsening effect for the MgH2–TiO2–EG-10 h electrode is more serious than for the MgH2–TiO2–EG-20 h electrode and the large voids between coarse particles are clearly observed. This microstructural difference indicates that large-volume changes are better accommodated by the TiO2 and graphite additives with finer microstructure and by longer P-milling time, which help to maintain the structural integrity of the electrode.

3. Experimental

3.1. Materials Preparation

To synthesize the MgH2–EG composite, 1.48 g Mg powder (99.9% purity, ~50 μm) and 0.4 g expandable graphite (EG) with a mass ratio of 80:20 were handled in a steel vial. The expandable graphite (99.9% purity, 100 mesh) was preheated at 1000 °C and held for 90 s under air atmosphere to obtain the worm-like expandable graphite. The handling process was operated in an argon-filled glovebox with an O2 and H2O content of less than 1 ppm to minimize the contamination. The milling was carried out on a dielectric barrier discharge plasma-assisted vibratory miller with the ball to powder weight ratio of 50:1; the details of plasma-assisted milling (P-milling) have been described in previous work [30,31]. After ball milling for 10 h, the as-prepared Mg–EG sample was hydrogenated at 450 °C under 6 MPa H2 for 6 h.
To synthesize the MgH2–TiO2–EG composite, 2 g mixture of MgH2 powder (hydrogen-storage grade), TiO2 powder (99.0% purity, ≥325 mesh), and the worm-like EG with a weight ratio of 5:3:2 were handled in a steel vial and milled with the same parameters for 10 h and 20 h and denoted as MgH2–TiO2–EG-10 h and MgH2–TiO2–EG-20 h, respectively.

3.2. Material Characterization

X-ray diffraction (XRD, Empyrean diffractometer, PANAlytical Inc., Almelo, The Netherland) with Cu Kα radiation was used to characterize the phase structure of the samples. The microstructure was observed by using a scanning electron microscope (SEM, Carl Zeiss Supra 40, Oberkochen, Germany). To determine the hydrogen content of the as-prepared MgH2–TiO2–EG sample, the desorption kinetics were measured at 400 °C using a Sievert-type automatic apparatus.

3.3. Electrochemical Measurement

The electrochemical properties of the active materials were measured using coin-type half-cells (CR2016) assembled in an Ar-filled glovebox. For preparation of the MgH2–EG electrode, the active material was cold pressed directly on the nickel foam with the pressure of 20 MPa. For preparation of the MgH2–TiO2–EG electrode, the active material was first mixed with the conductive agent (Super-P) and the binder (polyvinylidene fluoride (PVdF)) in a mass ratio of 8:1:1 and then dissolved in solvent (N-methyl-2-pyrrolidinone (NMP)) to make a slurry with the appropriate viscosity. The slurry was then manually spread onto a Cu foil in the glovebox filled with Ar and dried in a vacuum oven at 80 °C for 12 h. The loading of the active material was ~1.0 mg·cm−2. The cell used Li foil as the counter and reference electrode and a Celgrad 2400 membrane as the separator. The electrolyte was 1 M LiPF6 in ethylene carbonate and diethyl carbonate (1:1 by volume) with 10 wt % fluoroethylene carbonate (FEC).
The galvanostatic charge/discharge tests were performed in a voltage range of 0.01 V to 2.0 V (vs. Li/Li+) at the current density of 100 mA·g−1 using a Land test system (Wuhan, China) at a constant temperature (30 °C).

4. Conclusions

In summary, the electrochemical lithium storage properties of MgH2 were greatly improved by compositing with graphite and TiO2 via the discharge plasma milling process. The resulting MgH2–TiO2–EG composites show a remarkable increase in the initial discharge capacity and cycling capacity compared to pure MgH2 and MgH2–EG composite electrodes with different preparation processes. The 20 h-milled MgH2–TiO2–EG-20 h composite delivered a stable discharge capacity of 305.5 mAh·g−1 even after 100 cycles, and the reversible conversion reaction of MgH2 has been greatly enhanced. This work demonstrates the potential of the MgH2–TiO2 graphite composite by plasma-milled milling for electrochemical applications. The next goal is to obtain a higher cyclic capacity by suppressing the fast capacity fading within the initial discharge/charge cycle, and further elevate the reversible conversion reaction of MgH2. It is also stated that the possible hydrogen release from hydride materials during discharge/charging should be avoided and given more attention.

Acknowledgments

We acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 51471070, U1601212), the Fund for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51621001), and the Natural Science Foundation of Guangdong Province (2016A030312011).

Author Contributions

Shuo Yang: materials preparation, electrode preparation and electrochemical tests. Hui Wang: data analysis and writing of paper. Jiangwen Liu, Liuzhang Ouyang, Min Zhu: discussion on the research plan and experimental results.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) XRD patterns of the as-milled and hydrogenated Mg–EG composites; (b) discharge/charge curves of MgH2–EG at a current rate of 100 mA·g−1; (c) cycling performance of MgH2–EG electrode; (d) differential capacity plots (dQ/dV) of the MgH2–EG electrode at different cycles.
Figure 1. (a) XRD patterns of the as-milled and hydrogenated Mg–EG composites; (b) discharge/charge curves of MgH2–EG at a current rate of 100 mA·g−1; (c) cycling performance of MgH2–EG electrode; (d) differential capacity plots (dQ/dV) of the MgH2–EG electrode at different cycles.
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Figure 2. (a) XRD patterns of as-milled MgH2–TiO2–EG-10 h and MgH2–TiO2–EG-20 h composite; (b) desorption kinetic plots of MgH2–TiO2–EG-10 h and MgH2–TiO2–EG-20 h composite measured at 400 °C; and typical SEM images of (c) MgH2–TiO2–EG-10 h composite and (d) MgH2–TiO2–EG-20 h composite.
Figure 2. (a) XRD patterns of as-milled MgH2–TiO2–EG-10 h and MgH2–TiO2–EG-20 h composite; (b) desorption kinetic plots of MgH2–TiO2–EG-10 h and MgH2–TiO2–EG-20 h composite measured at 400 °C; and typical SEM images of (c) MgH2–TiO2–EG-10 h composite and (d) MgH2–TiO2–EG-20 h composite.
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Figure 3. Discharge/charge curves of (a) MgH2–TiO2–EG-10 h and (b) MgH2–TiO2–EG-20 h electrode at a current rate of 100 mA·g−1; (c) cycling performance of MgH2–TiO2–EG electrodes; (d) differential capacity plots (dQ/dV) of MgH2–TiO2–EG-20 h electrode at different cycles.
Figure 3. Discharge/charge curves of (a) MgH2–TiO2–EG-10 h and (b) MgH2–TiO2–EG-20 h electrode at a current rate of 100 mA·g−1; (c) cycling performance of MgH2–TiO2–EG electrodes; (d) differential capacity plots (dQ/dV) of MgH2–TiO2–EG-20 h electrode at different cycles.
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Figure 4. SEM surface morphological evolution of (a,b) the MgH2–TiO2–EG-10 h electrode and (c,d) the MgH2–TiO2–EG-20 h electrode.
Figure 4. SEM surface morphological evolution of (a,b) the MgH2–TiO2–EG-10 h electrode and (c,d) the MgH2–TiO2–EG-20 h electrode.
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Yang, S.; Wang, H.; Ouyang, L.; Liu, J.; Zhu, M. Improvement in the Electrochemical Lithium Storage Performance of MgH2. Inorganics 2018, 6, 2. https://doi.org/10.3390/inorganics6010002

AMA Style

Yang S, Wang H, Ouyang L, Liu J, Zhu M. Improvement in the Electrochemical Lithium Storage Performance of MgH2. Inorganics. 2018; 6(1):2. https://doi.org/10.3390/inorganics6010002

Chicago/Turabian Style

Yang, Shuo, Hui Wang, Liuzhang Ouyang, Jiangwen Liu, and Min Zhu. 2018. "Improvement in the Electrochemical Lithium Storage Performance of MgH2" Inorganics 6, no. 1: 2. https://doi.org/10.3390/inorganics6010002

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