Destabilization of LiH by Li Insertion into Ge - American Chemical

Mar 1, 2013 - ABSTRACT: Lithium hydride has high hydrogen capacity (12.7 mass %), but could not be considered as practical hydrogen storage media ...
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Destabilization of LiH by Li Insertion into Ge Ankur Jain,*,† Erika Kawasako,‡ Hiroki Miyaoka,§ Tao Ma,∥ Shigehito Isobe,∥ Takayuki Ichikawa,*,†,‡ and Yoshitsugu Kojima†,‡ †

Institute for Advanced Materials Research, ‡Graduate school of Advanced Sciences of Matter, and §Institute for Sustainable Sciences and Development, Hiroshima University, Higashi-Hiroshima 739-8530, Japan ∥ Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8278, Japan

ABSTRACT: Lithium hydride has high hydrogen capacity (12.7 mass %), but could not be considered as practical hydrogen storage media because of being very stable (required 900 °C for 0.1 MPa desorption pressure). Recently, C and Si have been found suitable to reduce the stability of LiH. This motivates us to investigate the properties of other alloys of Li, formed with the other elements. In the present work, Li3.75Ge (Li15Ge4) alloy was synthesized by mechanical milling, which transformed into Li4.2Ge (Li21.1875Ge5) and Li3.5Ge (Li7Ge2) phases during the vacuum heating at 400 °C. Hydrogenation of thus formed alloys at 400 °C under 3 MPa hydrogen pressure during PCI experiment transforms this mixed phase into Li2GeH0.5 (Li4Ge2H) and LiH phase. A remarkable decrease in the desorption temperature (∼300−450 °C) is observed by preparing the above alloy with Ge as observed from TG-DTA-MS experiment. The enthalpy of the reaction has also been calculated using the van’t Hoff plot. The present work concluded with the establishment of a direct relationship between hydrogen storage parameters and electrochemical parameters using the Nernst equation and van’t Hoff equation. A good agreement is found between the values of required potential for lithiation/delithiation as obtained by two methods. LiC6.7 With this proposal applied on LiH/MgH2−Si, Vajo et al. suggested interesting possibilities of destabilization using other elements such as B, C, N, P, and S.6 Since then, there are many reports to destabilize these systems by addition of a third element. In the previous report from our group, it was confirmed that Li intercalated graphite is capable of absorbing/ desorbing hydrogen as per the following reaction8 in the temperature range 200−500 °C:

1. INTRODUCTION Hydrogen energy is considered as one of the solutions of future energy needs. The main obstacle to establish hydrogen economy worldwide is its efficient storage.1 Hydrogen storage in the form of solid-state metal hydride has been proposed as one of the interesting media. Especially light metal hydrides having a high content of hydrogen such as LiH, MgH2 are attractive contenders to fulfill the projected storage capacity of US-DOE and other energy agencies worldwide.2 However, their high stability forms a bottleneck for them to be used as a practical storage media. LiH (12.7 mass % H2) requires 900 °C for 0.1 MPa desorption pressure. This high stability of LiH is due to the presence of a strong ionic bond in contrast to the delocalized metallic bonding presented in transition metal hydride.3,4 The high directionality of this ionic bond generates a high activation barrier for atomic motion, and thus lowers the sorption kinetics and generates many thermodynamic constraints.5 These thermodynamic constraints can be removed by using a third element that forms a new compound with Li upon dehydrogenation of LiH.6 This new compound should have more stability than elemental Li, for example, as in the case of © 2013 American Chemical Society

C6Lix + x /2H 2 ↔ 6C + x LiH

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More recently, our group studied the hydrogen storage properties of destabilized lithium silicon system and proposed the following reaction9 with a lower enthalpy of reaction, that is, −117 and −99 kJ/mol-H2, for the first and second steps of the reaction, respectively: LixSi + x /2H 2 ↔ x LiH + Si

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Received: January 4, 2013 Revised: March 1, 2013 Published: March 1, 2013 5650

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MPa hydrogen into the sample holder and keep it for 8 h. The hydrogen desorption properties of the above hydrogenated samples and LiH−Ge system were measured using a combined TG, DTA (Rigaku, TG 8120), and TDMS (Anelva, M-QA 200 TS) system installed inside an Ar-filled glovebox. The measurements were performed in the temperature range from room temperature to 500 °C at a heating rate of 5 °C/min under He flow. PCIs were measured using Sievert type apparatus (Suzuki Shokan Co. Ltd.) in pressure and temperature ranges of 0.0001−3.0 MPa and 400−500 °C.

Besides their destabilization property for hydrogen storage materials, these group IV elements are considered as a very important and attractive anode material for Li-ion batteries.10 One can estimate the possibility of destabilization of LiH through their electrochemical behavior and vice versa. Because of the positive potential values (V vs Li/Li+) for the lithiation of these materials,10 these Li absorbing alloys should be more stable than Li; thus these all should be suitable for the destabilization of LiH. Germanium is the next element that belongs to the same group after C and Si. The potential for lithiation of Ge also lies in the positive range, that is, 0.1−0.7 V, as observed in several reports.11−14 Thus we expect the formation of more stabilized Li−Ge compounds in comparison to elemental Li, so Ge should be helpful for the destabilization of LiH through the following reaction: LixGe + x /2H 2 ↔ x LiH + Ge

3. RESULTS AND DISCUSSION Figure 1 shows the XRD pattern of the LiH−Ge system prepared by milling for different duration. Because of the lower

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Recently, Ge has shown its potential as a destabilizing agent for MgH215 with a drastic decrease of dehydrogenation enthalpy from −76 to −14 kJ mol−1 H2. However, there are very few reports on Li−Ge−H systems that exist in the literature,16,17 where only the structure of Li−Ge−H ternary hydride was established using neutron techniques and first principle calculations. Moreover, hydrogen storage properties of Li−Ge system have not been reported so far. In this work, we have chosen Li4.4Ge as a starting material, as it is the highest Li-containing alloy according to the phase diagram.18 A mixture of Li and Ge was mechanically alloyed, and the prepared alloy was used to investigate the hydrogen storage properties. Also, we have started from a mixture of LiH and Ge in the same proportion. This mixture was also ballmilled and used for the understanding of the destabilization pathway.

Figure 1. XRD pattern of a ball-milled mixture of (LiH)4.4−Ge system. Different phases are indexed by symbols (shown in brackets) as follows: LiH no. 01-076-9249 (clover); Ge no. 01-089-3833 (*); GeO2 no. 03-065-8052 (△).

number of electrons in LiH in comparison to Ge, the intensities in XRD peaks corresponding to LiH phases are too weak to observe the LiH phase in XRD spectra. Yet the presence of Ge peaks without any alteration suggests no reaction between LiH and Ge during the milling process even for the 10 h milled sample. This behavior is quite reasonable due to the high stability of LiH, which creates a strong barrier for Ge to diffuse through it. In addition to LiH and Ge, a few small peaks corresponding to GeO2 are also visible, which may be generated during sample handling and milling or may be existing as an impurity in the starting material. The 2 h milled sample was employed for further testing by TG-TDMS as shown in Figure 2, suggesting the destabilization of LiH via a several step reaction. Two minor MS peaks in the region 150−250 °C correspond to a small amount of hydrogen evolution due to O2/H2O impurities. The major step of hydrogen desorption starts at 260 °C, which is much lower than the onset temperature of pure LiH (it is noteworthy here that LiH cannot desorb hydrogen before 600 °C). To identify the reaction pathway of this destabilization process, XRD experiment has been made at the completion point of each step as decided by peaks of MS spectra, that is, at 250, 320, 360, 425, and 500 °C. These XRD results are shown in Figure 3. It is evident from XRD results that the sample keeps its initial phases up to 250 °C; that is, only LiH and Ge phases are visible. As observed from TG-TDMS spectra, the diffusion of lithium into germanium leads to hydrogen desorption as the first step of the reaction, which is completed at 320 °C with the formation of Li4Ge2H and LiGe phases in addition to LiH and

2. EXPERIMENTAL SECTION LiH−Ge and Li−Ge mixture/alloy were prepared using Fritsch P7 ball milling apparatus started from LiH (95%, Sigma Aldrich), Ge (99.999%, Sigma Aldrich), and Li (≥99%, Sigma Aldrich) with a 4.4:1 molar ratio for both systems. The milling was performed at 370 rpm for 2 h with 30 min rest after 1 h under 0.1 MPa Ar atmosphere using 20 balls (SUJ-2, 7 mm in diameter). All of the handling of materials was done in a high purity Ar (99.9999%) filled glovebox (Miwa MFG, MP-P60W). The structural characterization was made by powder X-ray diffraction technique (Rigaku, RINT-2100) using Cu Kα radiation. To avoid direct interaction with air and moisture, the XRD sample was covered by polimide sheet (Kapton, Dupont−Toray Co. Ltd.). In addition to XRD, transmission electron microscopy (TEM) observation was taken out for better understanding of the reaction mechanism using 200 kV TEM (JEOL JEM-2010). The sample was dispersed on a Mo TEM grid, and then transferred into a TEM chamber. Plastic bag method19 was used to protect the sample from oxidation. To investigate the thermal behavior of the mixture/alloy, differential scanning calorimetry (TA Instruments, Q10PDSC) was used under 0.15 MPa Ar flow and 3.0 MPa H2 pressure with a heating rate of 5 °C/min up to 500 °C. The hydrogenation of Li−Ge sample was carried out by two methods: (1) Fill 3.0 MPa H2 in the sample holder and then heat the sample to 400 °C at a heating rate of 10 °C/min and keep it for 8 h; and (2) heat the sample first to 400 °C at a heating rate 10 °C/min under vacuum and then introduce 3.0 5651

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in temperature to 425 °C (third step) resulted in a mixed phase of Li7Ge2, Li21.1875Ge5, and LiH. The final step occurred in the temperature range 425−500 °C, where complete hydrogen from the LiH−Ge system is desorbed, and the sample acquires final phases as Li21.1875Ge5, Li9Ge4, and Li7Ge2. To examine the reversibility of the above reaction, we hydrogenate the final product under 400 °C and 3 MPa of H2, but surprisingly the initial starting material could not be achieved. The final hydrogenated product was able to be indexed by Li4Ge2H and LiH phases. This behavior is different from that observed in case of Li−Si system, where a complete reversibility could be achieved.6,9 So our expected reaction (eq 3) is not true. The explanation of this irreversibility is given later in this Article. Thus, to explore this different and unexpected behavior, we started our observations of the hydrogenating process from the dehydrogenated state, that is, Li−Ge system. We tried to prepare Li4.4Ge alloy by ball milling under the same conditions as for LiH−Ge. The XRD pattern of as-prepared product is shown in Figure 4. It is clear from the diffractogram that the

Figure 2. TG-DTA-MS spectra of a 2 h milled (LiH)4.4−Ge system; the slope in DTA spectra corresponds to the baseline. MS signal shown in the figure corresponds to H2 evolution.

Figure 4. XRD profile of as-milled and heat-treated Li15Ge4 system. Different phases are indexed by symbols (shown in brackets) as follows: Li15Ge4 no. 01-089-2584 (*); Li7Ge2 no. 01-080-0531 (△); Li21.1875Ge5 no. 01-070-7635 (●).

Figure 3. XRD profile of a 2 h milled (LiH)4.4−Ge system after heating to different temperatures. Different phases are indexed by symbols (shown in brackets) as follows: LiH no. 01-076-9249 (#); Ge no. 01-089-3833 (*); Li4Ge2H no. 01-076-3500 (clover); LiGe no. 01080-0483 (○); Li14.10Si6 no. 03-065-2234 (□); Li7Ge2 no. 01-080-0531 (△); Li21.1875Ge5 no. 01-070-7635 (|); Li9Ge4 no. 01-073-6200 (●).

prepared alloy is Li15Ge4 rather than Li22Ge5. We further milled for a prolonged time, for 5 and 10 h, but could not succeed in preparing the desired phase, and the final product was Li15Ge4 in each case. Remaining Li could not be observed by XRD as the peaks corresponding to Li should be quite weak or overlapped with the peaks of the main phase. To investigate the thermodynamic transformation and hydrogenation process, DSC measurements were performed on the Li15Ge4 system under 0.1 MPa Ar flow and 3.0 MPa H2

Ge. The reaction of Li diffusion becomes more pronounced with rising temperature and reaches completion in the second step of the process of hydrogen evolution in TG spectra. This step generates some unknown phase according to the existing Li−Ge phase diagram and ICDD database. It could be fitted with a structure identical to that of Li14.10Si6 phase crystallized in trigonal structure having space group R3̅m. Further increase 5652

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Figure 5. DSC curve of as-milled Li15Ge4 under (a) 0.1 MPa Ar flow; and (b) 3.0 MPa H2 pressure.

pressure, respectively, as shown in Figure 5a and b. We can see an endothermic peak at around 175−180 °C in Figure 5a, which may be attributed to the melting of unreacted fraction of Li presented in as-prepared sample. On further heating to 500 °C, the DSC curve indicates that an exothermic reaction starts at around 300 °C, which can be suggested as melted Li diffused into the Li15Ge4 phase; this diffusion continued until 460 °C with continue phase transition/new phase formation. This phase transition is found to be irreversible and could not follow the same path of onward reaction. A sharp exothermic peak around 460 °C is observed, which must be due to the crystallization of the formed phases during the earlier reaction. To confirm this, XRD measurement is carried out after DSC measurement, and the results are shown in Figure 4 in combination with the XRD plot of a heated sample at 500 °C under vacuum for 1 h. It is observed that both diffractograms are very similar and can be fitted by Li21.1875Ge5 and Li7Ge2. These are the same phases as observed as final products of LiH−Ge sample after hydrogen desorption. Li15Ge4 under H2 atmosphere shows exothermic peaks at around 300 °C that correspond to hydrogen absorption, while two endothermic peaks at 452 and 484 °C correspond to the hydrogen desorption process. Although an additional small exothermic peak also appears at around 350 °C, it is not confirmed with this experiment whether it is an intrinsic reaction or it is a kinetic effect due to different particle size. From this DSC result, the hydrogenation temperature was chosen to be 400 °C to ensure complete hydrogenation of the Li15Ge4 system. The hydrogenation treatment was performed in two ways: (i) The sample was heated first to 400 °C at the heating rate of 10 °C/ min under vacuum and then exposed to 3.0 MPa hydrogen by keeping it for 8 h; this will be called sample A; and (ii) the sample was exposed to 3.0 MPa hydrogen first, then start heating to 400 °C at the heating rate of 10 °C/min and keep at this temperature for 8 h; this will be called sample B. The pressure is maintained at 3.0 MPa during heating of the system by releasing the pressure manually. XRD spectra after both treatments are shown in Figure 6. XRD of sample A confirms the formation of Li4Ge2H and LiH as a final product, while the XRD profile of sample B is found to be entirely different, indicating that this spectra could not be fitted by any of the known ICDD database structures. It means that the reaction atmosphere has its own impact on the reaction pathway. However, after desorbing sample B under

Figure 6. XRD profile of the hydrogenated Li15Ge4 system. Phases are indexed by symbols (shown in brackets) as follows: Li14.10Si6 no. 01076-3500 (*); unknown phase (?).

vacuum at 500 °C for 1 h, rehydrogenation intended this sample to follow the same reaction as that of sample A; this sample will be called sample C. So the final product of sample C is also Li4Ge2H and LiH. This difference in reaction pathway is clearly visible in the TG-TDMS spectra, shown in Figure 7. Sample A shows a clear three step desorption with a total weight loss of 2.5 mass %, which is consistent with DSC results. However, like DSC it is not very clear if it is intrinsic reaction or different kinetics. The TG-TDMS profile of sample B is shown in Figure 7b, which is entirely different from that of sample A. The desorption peaks corresponding to the first step and second steps of the reaction were positioned at 415 and 455 °C, respectively, in comparison to 360 and 420 °C observed for sample A. Moreover, the capacity loss for sample B is more than 2.5 mass % (as observed for sample A), which 5653

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the same reaction path as sample A. This must be due to the difference in the initial hydrogenation treatment of samples. To clarify the doubts remaining from TG-TDMS spectra, PCI experiment is performed in the pressure and temperature ranges of 0.0001−3.0 MPa and 400−500 °C, respectively. These PCI curves are shown in Figure 8. Two plateaus that are observed from PCI curves at all temperatures revealed two major steps of reactions. However, plateaus are not very flat, and the possibility of multistep reaction during hydrogen absorption/desorption processes cannot be denied. However, the two major steps can be identified as follows: first step is between 0.001 and 0.01 MPa, and second one is between 0.2 and 3.0 MPa corresponding to 1.5 and 1.0 mass % hydrogen absorption, respectively. The plateaus for all of the temperatures show hysteresis as found in the case of the Li−Si system. The heterogeneous solid-state reaction and volume change are responsible for this effect as discussed in our previous report.9 To identify the major steps, XRD and TEM measurements are performed at each step of absorption/desorption processes of PCI at 500 °C. XRD spectra of all of the samples after each step are shown in Figure 9. After first step absorption corresponding to a total hydrogen quantity as 1.5 mass %, sample changes to Li14.10Ge6 and LiH phases, which convert to Li4Ge2H and LiH as a final product upon addition of 1 mass % H2 in the second step. In the desorption process, the sample follows the same path as absorption and changes to Li14.10Ge6 and LiH as an intermediate step before going to its final dehydrogenated state, that is, Li21.1875Ge5 and Li7Ge2. It is noteworthy here that further evacuation for a long time gives rise to an additional phase, that is, Li9Ge4, in small amount. To confirm this reaction, in situ TEM experiments (not shown here) starting from the hydrogenated state were performed with heating to 390 °C for the first step reaction and to 440 °C for the second step reaction as decided by the TG-TDMS experiment. Unfortunately, we could not follow the same reaction as observed by other experiments as the sample decomposes to its final stage, that is, Li21.1875Ge5 and Li7Ge2 phases only at 390 °C. This must be due to high energy supplied through heating and electron beam at the same time. Some diffraction patterns corresponding to Li2O and Ge were also visible, which is enough to consider this high energy effect. No further changes were observed upon heating to 440 °C, which should be the required temperature for the second step reaction. So we decided to observe different samples after each step separately. The results are shown in Figure 10. Figure 10a showed the TEM bright field images of the sample in the fully hydrogenated state. Circles mark the position of selective aperture for TEM analysis. The respective diffraction patterns from these apertures are also shown in the same figure indicated by corresponding numbers. At positions 1 and 4, we found the diffraction pattern corresponding to Li4Ge2H, while at positions 2 and 3 the diffraction pattern could be indexed as LiH. Because LiH is very easy to decompose under electron beam and decomposed Li gets oxidized immediately, we found some traces of Li2O as shown for position 5. Figure 10b shows the bright field images of first step desorbed sample at different spots with their corresponding diffraction pattern. The single crystal pattern of Li14.10Ge6 phase is observed in the images. However, we could not observe LiH for this sample, which must be decomposed as stated above, and thus formed Li2O, which is clearly visible in the diffraction patterns. Figure 10c shows the TEM images of completely desorbed sample at 500 °C. It was very hard to distinguish the different

Figure 7. TG-DTA-MS spectra of hydrogenated Li15Ge4: (a) sample A; (b) sample B; (c) sample C. MS signal shown in the figure corresponds to H2 evolution.

looks further increasing with temperature rise. The capacity of sample B is found to be decreased after 1 cycle of desorption/ absorption (i.e., sample C), which can be explained on the basis of XRD results. XRD results revealed some traces of Li14.10Ge6 in sample C, which means that the sample could not reach its full hydrogen content during absorption. This justifies the low capacity loss observed in TG results even if sample C followed 5654

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Figure 8. Pressure−composition isotherms for the Li−Ge−H system. Inset shows the van’t Hoff plot for the first and second steps of the absorption reaction.

we could confirm the initial phases are Li21.1875Ge5 and Li7Ge2, while an additional phase Li9Ge4 appeared at a later stage of observation. This makes it clear that vacuum heating/electron beam during TEM for a long time can form an additional phase Li9Ge4 in addition to the main phases of final product, that is, Li21.1875Ge5 and Li7Ge2. Thus, on the basis of all of the above results, that is, TG-TDMS, PCI, XRD, and TEM, we can speculate the reversible reaction as follows: Li3.75Ge → 0.6Li4.4Ge + 0.4Li3.5Ge

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0.6Li4.4Ge + 0.4Li3.5Ge + 0.8H 2 ↔ Li 2.35Ge + 1.6LiH (5)

Li 2.35Ge + 1.6LiH + 0.5H 2 ↔ 0.5Li4Ge2H + 2.1LiH (6)

To estimate the thermodynamic parameters, enthalpy and entropy of reaction, the van’t Hoff plot was drawn for the first and second steps of the reaction as shown in the inset of Figure 8. The parameters were found as ΔH = −143.313 ± 5.31 kJ/ mol-H2 and ΔS = 155.39 ± 7.00 J/K·mol-H2 for the first step absorption reaction 5 and ΔH = −79.021 ± 6.17 kJ/mol-H2 and ΔS = 117.453 ± 6.17 J/K·mol-H2 for the second step absorption reaction 6. These values are much lower than the enthalpy of reaction 2LiH → 2Li + H2, that is, 181 kJ/mol H2 which is due to the formation of more stable Li−Ge intermetallic alloys. The larger value of ΔS in comparison to the enthalpy of gaseous hydrogen holds the same reason as discussed in detail for the Li−Si system.9 On comparing the results on the Li−Ge−H system presented in this Article to those previously published on the Li−Si−H system by our group,9 we found that both of the systems follow different reaction paths to reach Li4Ge2H, LiH and LiH, and Si as the end products. The ternary hydride systems Li−Ge−H and Li−Si−H were recently discovered, and the formation of Li4Si2H is found to be more complex in comparison to Li4Ge2H as shown by Wu et al.16 It needs an excess amount of Si to be formed, although the reason for this

Figure 9. XRD profile at different steps of pressure−composition absorption/desorption isotherm at 500 °C. Different phases are indexed by symbols (shown in brackets) as follows: Li4Ge2H no. 01076-3500 (●); Li14.10Si6 no. 03-065-2234 (*); Li7Ge2 no. 01-080-0531 (△); Li21.1875Ge5 no. 01-070-7635 (clover); Li9Ge4 no. 01-073-6200 (□).

phases existing in this sample by the contrast in the bright field images. Also, this sample is quite sensitive to the electron beam so the diffraction pattern kept changing during observation. Still 5655

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Figure 10. TEM images of (a) fully hydrogenated, (b) first step desorbed, and (c) completely desorbed Li−Ge−H system.

difference in not clear yet. In our previous work, the Li−Si system undergoes through first step reaction product as Li14.10Si6 and LiH, so an excess amount of Si in this case could not be available; thus the reaction goes through the other intermediate phases of the Li−Si binary phase diagram, which are much easier to be formed in comparison to Li4Si2H and ended with LiH and Si as a final product. In the case of the Li− Ge system, the reaction goes through the Li14.10Ge6 phase (same as the Li−Si system) and then converts to its hydrogenated state easily as it does not need any excess amount of Ge. Once Li4Ge2H is formed, it is difficult to continue further phase changes (even up to 5.0 MPa H2). It is suggested that Li4Ge2H has strong ionic bonds between Li and H in the range of 1.88−2.09 Å, which is comparable to that in pure LiH (2.031 Å).16,20 Another unique feature of Li4Ge2H is its unusual structure having an interstitial Li6 octahedral hydrogen site and the presence of long-range Ge−Ge chains with bond distance 2.53 Å, which is much higher than the isolated Ge−Ge units found in any binary Li−Ge compounds.16,17 These two properties make Li4Ge2H highly stable and thus stopped any further conversion, thus completing the reaction with the final products of Li4Ge2H and LiH in place of LiH and Si in the case of the Li−Si−H system. The final objective of this work is to correlate the hydrogenation properties of the Li−Ge system with the electrochemical properties of lithium ion batteries having Ge as an anode material. This can be done using the Nernst equation and van’t Hoff equation. The enthalpy for hydrogen absorption reaction can be written as:

ΔHabsorb = 2 × [ΔH °(LiH) + {ΔH °(M) − ΔH °(MLix)} /x]

(7)

The value of potential required for lithiation/delithiation in electrochemical reaction can be written as: E° = −[ΔH °(MLix) − ΔH °(M) − T {ΔS°(MLix) − ΔS°(M)}]/xF

(8)

where the term {ΔS°(MLix) − ΔS°(M)} should be a small contribution and so can be neglected, and then eq 8 will become: E° = −[ΔH °(MLix) − ΔH °(M)]/x F

(9)

Combining eqs 7 and 9, we get E° ≈ [ΔHabsorb/2 − ΔH °(LiH)]/F

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This equation can serve as a direct relation between enthalpy of hydride formation and the required potential for lithiation. The values for potential for the first and second steps of the reaction are found to be 0.197 and 0.527 V for the Li−Ge system in comparison to 0.331 and 0.425 V for the Li−Si system as calculated from the enthalpy values reported in our previous paper.9 These values are in close agreement with the reported values by electrochemical method in the literature.10−14 Thus one can estimate the electrochemical behavior by using hydrogenation properties and vice versa in this easy way as shown above. 5656

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(11) Yoon, S.; Park, C. M.; Sohn, H. J. Electrochemical characterizations of germanium and carbon-coated germanium composite anode for lithium-ion batteries. Electrochem. Solid-State Lett. 2008, 11, A42−A45. (12) St. John, M. R.; Furgala, A. J.; Sammells, A. F. Thermodynamic studies of Li-Ge alloys: Application to negative electrodes for molten salt batteries. J. Electrochem. Soc. 1982, 129, 246−250. (13) Graetz, J.; Ahn, C. C.; Yazami, R.; Fultz, B. Nanocrystalline and thin film germanium electrodes with high lithium capacity and high rate capabilities. J. Electrochem. Soc. 2004, 151, A698−A702. (14) Lee, H.; Kim, H.; Doo, S. G.; Cho, J. Synthesis and optimization of nanoparticle Ge confined in a carbon matrix for lithium battery anode material. J. Electrochem. Soc. 2007, 154, A343−A346. (15) Walker, G. S.; Abbas, M.; Grant, D. M.; Udeh, C. Destabilisation of magnesium hydride by germanium as a new potential multicomponenthydrogen storage system. Chem. Commun. 2011, 47, 8001−-8003. (16) Wu, H.; Hartman, M. R.; Udovic, T. J.; Rush, J. J.; Zhou, W.; Bowman, R. C., Jr.; Vajo, J. J. Structure of the novel ternary hydrides Li4Tt2D (Tt = Si and Ge). Acta Crystallogr. 2007, B63, 63−68. (17) Wu, H.; Zhou, W.; Udovic, T. J.; Rush, J. J.; Yildirim, T.; Hartman, M. R.; Bowman, R. C., Jr.; Vajo, J. J. Neutron vibrational spectroscopy and first-principles calculations of the ternary hydrides Li4Si2H(D) and Li4Ge2H(D): Electronic structure and lattice. Phys. Rev. B 2007, 76, 224301. (18) Sangster, J.; Pelton, A. D. The Ge-Li (germanium-lithium) system. J. Phase Equilib. 1997, 18, 289−294. (19) Yao, H.; Isobe, S.; Wang, Y.; Hashimoto, N.; Ohnuki, S. Plastic bag method for active sample loading into transmission electron microscope. J. Electron Microsc. 2011, 60, 375−378. (20) Calder, R. S.; Cochran, W.; Griffiths, D.; Lowde, R. D. An X-ray and neutron diffraction analysis of lithium hydride. J. Phys. Chem. Solids 1962, 23, 621−632.

4. CONCLUSIONS The Li−Ge−H ternary system has been explored in detail in terms of its structural, morphological, and thermodynamical properties. The hydrogenation properties of this system have also been established with a reaction pathway systematically. Li15Ge4 phase could be formed by ball milling. After the first step of the reaction of hydrogen absorption, an unknown phase appeared, which does not exist in the Li−Ge phase diagram but could be identified as an alloy having the same structure as Li14.10Si6. After completion of hydrogenation up to 3 MPa pressure, the material could not be converted to LiH and Ge as observed in the case of Si substitution, but it follows a different pathway with concluding Li4Ge2H and LiH phases. The enthalpies of formation for the first and second steps of the reaction are found to be 143 and 79 kJ/mol-H2, which are much lower than that of pure LiH (182 kJ/molH2). The calculated values of electrochemical voltage plateaus from H2 storage properties are in good agreement with the values reported through the direct electrochemical method.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +81-(0)82-424-5744. E-mail: ankurjainankur@sify. com (A.J.); [email protected] (T.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by JSPS under “Postdoctoral Fellowship Programme for International Researchers (P11068)” and KAKENHI (21686068) of the Grant-in-Aid for Young Scientists (A). We gratefully acknowledge Mr. Toru Kimura for his help in this work.

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REFERENCES

(1) Züttel, A. Hydrogen storage methods. Naturwissenschaften 2004, 91, 157−172. (2) Satyapal, S.; Petrovic, J.; Read, C.; Thomas, G.; Ordaz, G. The U.S. Department of Energy’s National Hydrogen Storage Project: Progress towards meeting hydrogen-powered vehicle requirements. Catal. Today 2007, 120, 246−256. (3) Wiberg, E.; Amberger, E. Hydrides of the Elements of Main Groups I−IV; Elsevier: Amsterdam, 1971. (4) Miwa, K.; Ohba, N.; Towata, S.; Nakamori, Y.; Orimo, S. I. Firstprinciples study on lithium borohydride LiBH4. Phys. Rev. B 2004, 69, 245120. (5) Vajo, J. J.; Olson, G. L. Hydrogen storage in destabilized chemical systems. Scr. Mater. 2007, 56, 829−834. (6) Vajo, J. J.; Mertens, F.; Ahn, C. C.; Bowman, R. C., Jr.; Fultz, B. Altering hydrogen storage properties by hydride destabilization through alloy formation: LiH and MgH2 destabilized with Si. J. Phys. Chem. B 2004, 108, 13977−13983. (7) Reynier, Y. F.; Yazami, R.; Fultz, B. Thermodynamics of lithium intercalation into graphites and disordered. J. Electrochem. Soc. 2004, 151, A422−A426. (8) Miyaoka, H.; Ishida, W.; Ichikawa, T.; Kojima, Y. Synthesis and characterization of lithium−carbon compounds for hydrogen storage. J. Alloys Compd. 2011, 509, 719−723. (9) Doi, K.; Hino, S.; Miyaoka, H.; Ichikawa, T.; Kojima, Y. Hydrogen storage properties of lithium silicon alloy synthesized by mechanical alloying. J. Power Sources 2011, 196, 504−507. (10) Park, C. M.; Kim, J. H.; Kim, H.; Sohn, H. J. Li-alloy based anode materials for Li secondary batteries. Chem. Soc. Rev. 2010, 39, 3115−3141. 5657

dx.doi.org/10.1021/jp400133t | J. Phys. Chem. C 2013, 117, 5650−5657