Mechanistic and Kinetic Study of the Electrochemical Charge and

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MECHANISTIC AND KINETIC STUDY OF THE ELECTROCHEMICAL CHARGE AND DISCHARGE OF LaMgNi BY IN SITU POWDER NEUTRON DIFFRACTION 2

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Michel Latroche, Fermin Cuevas, Wei-Kang Hu, Denis V. Sheptyakov, Roman Denys, and Volodymyr Yartys J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp503226r • Publication Date (Web): 19 May 2014 Downloaded from http://pubs.acs.org on May 26, 2014

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The Journal of Physical Chemistry

Mechanistic and Kinetic Study of the Electrochemical Charge and Discharge of La2MgNi9 by In Situ Powder Neutron Diffraction Michel Latroche1*, Fermìn Cuevas1, Wei-Kang Hu2, Denis Sheptyakov3, Roman V. Denys2 and Volodymyr A. Yartys2,4 1

Institut de Chimie et des Matériaux Paris-Est, UPEC-CNRS, UMR 7182, 2 rue Henri Dunant, Thiais, France 2 Institute for Energy Technology, Department of Energy Systems, Kjeller, Norway 3 SINQ-HRPT, Paul Scherrer Institute, 5232 Villigen, Switzerland 4 Norwegian University of Science and Technology, 7491, Trondheim, Norway Abstract The intermetallic La2MgNi9 has been investigated as negative electrode material for NiMH battery by means of in-situ neutron powder diffraction. This hydride-forming compound exhibits suitable plateau pressures ranging within the practical electrochemical window and leads to significant reversible electrochemical capacities. Charge and discharge of the composite electrode have been performed in beam following various current rates and galvanostatic intermittent titration. From the diffraction data analysis, phase amounts and cell volumes have been extracted allowing to interpret the hydride formation and decomposition. From the evolution of the diffraction line widths, differences are observed between charge and discharge with the possible formation of an intermediate gamma phase on charge. The electrode readily responds to current rate variations and does not show any kinetic limitation in the range C/10 and C/5 (C/n: full capacity C in n hours). This material shows excellent properties regarding electrochemical storage of energy.

Keywords: intermetallic, hydride, NiMH batteries, phase transitions, in operando study

*

Corresponding author : M. Latroche, [email protected], +33 1 49 78 12 10

ACS Paragon Plus Environment

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Introduction Metallic hydrides are very convenient materials for hydrogen storage1. They can store large amount of hydrogen gas at thermodynamic conditions close to ambient pressure and room temperature making them suitable for energy storage. Metallic hydrides have been also developed for electrochemical storage. Indeed, they can react with water to electrochemically exchange one proton and one electron. This allows to design efficient negative electrodes for alkaline secondary batteries (NiMH) which offer better properties that the actual NiCad ones 2-4

.

For two decades, the most developed materials for these negative electrodes were derived from the LaNi5 intermetallic compound. By proper substitutions on either the La site (by other rare earths) or the Ni sites (by other transition or p-metals), and by playing with the stoichiometry (according to the narrow domain of existence of the LaNi5+x compound; 0 0.5 V followed by relaxation periods of one hour.

One can observe in the 3D view of Figure 9 that the diffracted intensities of both phases follow fairly well the different current steps of the GITT. In addition, one can note that the background level is very dependent of the current density. This can be understood if one considers that switching on and off the current involves important changes in the amount of deuterium gas produced in the cell. When the current is off, no gas is produced and the


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quantity of liquid (D2O, NaOD) is larger in the beam causing increased background. Thus, each maximum in the background can be attributed to a relaxation period.

α β β α

35 Ti

me 40 (h )

2

ta the

(°)

45

Figure 9 : 3D view of the NPD pattern evolution as function of time during the GITT discharge cycle (D/7) of the working electrode at 46 mA.g-1. 400

beta alpha

1.0

300

0.8

-1

Q (mAh.g )

Phase amount (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

200

0.4

0.2

100

0.0 34

36

38

40

42

44

46

Time (h)

Figure 10 : Evolution of the phase amounts for the alpha and the beta phases during the in beam discharge obtained by GITT experiments. The current was set to 46 mA.g-1 (D/7) for 1.5 h or Ew > 0.5 V followed by relaxation periods of 1h. For sake of comparison, the evolution of the capacity Q is also shown on the right scale. The initial charge state Q is set at 380 mAh.g-1, a value determined from the PCT curve (Figure 1) at a pressure of 0.1 MPa.

The total cumulated discharge capacity obtained at this rate is 1427 mAh (266 mAh.g-1), a value very close to that measured during the first in situ cycle at C/10. The evolution of the relative amount of each phase (alpha and beta) is shown in Figure 10. Again, the electrode behaves very closely to the previous cycles involving the same transformations.


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In addition, small potential plateaus appear during the OCV periods which correspond to zero current flow for which no phase transformation takes place. However, very small changes in the alpha and beta amounts can be observed during the resting periods (Figure 10). This is consistent with deuterium diffusion from the beta towards the depleted alpha phase on the particle surface, transforming some beta phase into (saturated) alpha phase during the electrode relaxation. The charge and discharge capacities for each in-beam cycle described so far are given in Table 3. 4th cycle (D/10) 1494 mAh (278 mAh⋅g-1) th 5 cycle (D/5) 1332 mAh (248 mAh⋅g-1) 6th cycle (GITT;D/7) 1427 mAh (266 mAh⋅g-1) Table 3 : Discharge capacity at each cycle during in situ measurements of the working electrode.

To get an overview of the electrode behaviour, the evolutions of phase amounts and cell volumes for the alpha and the beta phases during the in-beam charge/discharge cycles at C/10, C/5 and GITT are given and compared to the potential Ew in Figure 11 and Figure 12. D/10

1,0

OCV

C/10

OCV D/5

OCV

C/5

OCV ------------GITT D/7/OCV-----------

0,6

beta alpha 0,8

0,4

0,6

0,2

Ew (V)

Phase amount (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0,4

0,0

0,2

-0,2

0,0 0

6

12

18

24

30

36

42

48

Time (h) Figure 11 : Evolution of the phase amounts for the alpha and the beta phases during the in beam charge/discharge cycles at C/10, C/5 and GITT for the working electrode. For sake of comparison, the evolution of the potential Ew versus the Cd/Cd(OD)2 reference electrode is also shown on the right scale.


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D/10

OCV

C/10

OCV D/5 OCV

C/5

OCV ------------GITT D/7/OCV-----------

0,6

650

3

beta alpha

600

0,2

Ew (V)

0,4

Cell volume (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,0 550 -0,2

500 0

6

12

18

24

30

36

42

48

Time (h) Figure 12 : Evolution of the cell volumes for the alpha and the beta phases during the in beam charge/discharge cycles at C/10, C/5 and GITT for the working electrode. For sake of comparison, the evolution of the potential Ew versus the Cd/Cd(OD)2 reference electrode is also shown on the right scale.

From the precedent data analysis, all measured parameters (phase amounts and cell volumes) have been processed regarding time scale evolution. At this stage, it is worth to look also to the data as a function of the electrochemical charge Q. Once again, one has to be cautious with the values of Q as the electrochemical capacity does not correspond to that of the electrode materials, mainly because of the deuterium gas evolution at the end of each charge. To overcome this difficulty, data can be analyzed by separately considering capacity of charge Qc and capacity of discharge Qd. This has been done in Figure 13 for the cell volume evolution of both phases. Interestingly, a linear behavior is observed for the two phases though the volumes measured during charge are a bit scattered. The beta phase volume increases smoothly as a function of Q following the equation: Vβ=0.101(4)·Qd + 608.5

(1)

whereas that of the alpha phase is nearly constant in the whole range of Qd.


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3

Cell volume (Å )

650

Charge Vα

600

Discharge Vα

Vβ

Vβ

550

500 100

150

200

250

300

350

400

-1

Q (mAh.g )

Figure 13 : Evolution of the cell volumes for the alpha (black square) and the beta (red circle) as a function of the state of charge Qc (full symbols) and Qd (empty symbols). For Qd, the fully charged state was set at 380 mAh.g-1, a value determined from the PCT curve (Figure 1) at a pressure 0.1 MPa.

The same figure can also be drawn for the phase amounts though it is only significant for the discharge. This evolution is shown in Figure 14 for which two domains can be clearly identified. From 100 to 310 mAh.g-1, a clear two phase transition domain can be seen. Then, for Qd larger than 310 mAh.g-1, the beta solid solution domain starts and extends up to the upper charge state (380 mAh.g-1). Interestingly, for the alpha phase, an instant formation of the electrochemically stable solid solution is observed. As it is not possible to electrochemically decompose it, the domain of the alpha solid solution with a variable content of H/D is effectively inexistent. It is also worth to note that a fully charge state (i.e. 100% beta) is never reached whereas the full discharge state (i.e. 100% alpha) is almost completed. 1,0

0,8

Phase amount (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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beta 0,6

0,4

alpha 0,2

0,0 100

150

200

250

300

350

-1

Qd (mAh.g )

Figure 14 : Evolution of the phase amounts for the alpha (black square) and the beta (red circle) phases as a function of the state of discharge Qd. The fully charged state was set at 380 mAh.g-1, a value determined from the PCT curve (Figure 1) at a pressure 0.1 MPa.


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Discussion The intermetallic La2MgNi9 absorbs readily and reversibly hydrogen by solid-gas route and, thanks to its suitable equilibrium pressures, the same compound can be used as negative electrode in alkaline medium. Indeed, after few activation cycles, an excellent correlation between the capacities obtained by solid gas and electrochemical measurements is observed. Using neutron diffraction data analysis, we observed that the structural properties of the charged electrode material are comparable to that published by Denys et al. 19 for the hydride loaded under gas pressure. Starting from this charged state, the electrochemical cycling behavior has been investigated in beam at different rates to follow the mechanism of the reversible charge-discharge process. At first glance, one can consider that the electrode material behaves like a classic one following an intermetallic to hydride transformation through the sequences alpha, alpha to beta and beta formation, a typical behavior for a two-phase reaction. This is indeed the case during the discharge according to the variations of the phase amounts, the cell volumes and the line diffraction half widths. All these parameters agree well with a two-phase behavior. This is however not so obvious for the charge process. Despite the fact that a two-phase transformation seems to take place in both processes, important increase of the half width during charge involves strong overlapping between the alpha and beta diffraction peaks. This unexpected behavior can be attributed to large concentration gradient in the H concentration or heavy constraints during the charge process. However, this is not supported neither by the small hysteresis between the absorption and desorption branches nor by the relatively flat plateaus of the PCT curves. A better hypothesis might be the formation of an intermediate gamma phase as previously observed in the LaNi5-type system

6,8,11

.

Similarly, the gamma phase was observed only during the charge process. This out of equilibrium phase was related to constraint relaxation at the alpha to beta interface and plays a key role in the cycle life by decreasing the decrepitation process responsible of high corrosion rate in alkaline medium. As the PuNi3-type structure is built from the stacking of LaNi5 and MgNi2 sub-units, such a mechanism may also take place in this system. A better characterization of this intermediate phase could be obtained in the future by performing in beam GITT during charge with longer relaxation times assuming that the gamma phase amount will persist at the phase interfaces due to kinetic barriers. To our knowledge, the


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formation of this intermediate gamma phase was never reported before in these stacking structures.

From Figure 13, it is observed that the cell volume for the beta phase is strongly dependant of the state of charge and follows a linear dependence. From Eq. 1, one can derive that this corresponds to 3.26 Å3/D atom, a value in agreement with previously reported ones ranging between 2.5 and 3.5 Å3/ at.H20,21. On the contrary, the volume of the alpha phase remains almost constant at all states of charge. Evolution of the phase amounts as a function of the state of discharge allows to clearly discriminate between beta solid solution, beta to alpha transformation and alpha solid solution domains (Figure 14). This latter domain is very small in the range of the electrochemical state of charge studied in the present work which explains the little variation of the cell volume of the alpha phase.

In

other

words,

the

reversible

capacity

is

mainly

obtained

from

hydrogen

absorption/desorption in the beta solid solution and at the alpha to beta transformation but not in the alpha solid solution. This statement agrees with the shape of the PCT curve (Figure 1) for which the hydrogen trapped into the alpha solid solution is difficult to extract at low pressure, i.e. out of the practical electrochemical window.

Finally, it is interesting to note that the electrode material readily responds to the electrochemical solicitations. Typical example is shown in Figure 10 where the diffraction peak intensities follow very closely the current variation imposed by the GITT technique. Indeed, good kinetics are obtained at all rates for the electrode though the maximum rate was limited to only C/5 in this experiment.

Conclusions

The electrochemical behavior of a composite electrode made of La2MgNi9 has been thoroughly investigated using deuterated samples by in situ neutron powder diffraction at different charge/discharge rates. From the data analysis, combining diffraction and PCT measurements, the mechanisms and the kinetic of the electrode have been determined. A fairly good agreement is observed between the solid-gas and electrochemical capacities. The electrode material works by following a hydride to intermetallic transformation through a beta solid solution domain and a beta to alpha transformation with low capacity attributed to the


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alpha solid solution domain. During charge, heavy line broadening is however observed and might be related to the formation of an intermediate gamma phase as previously observed in the LaNi5 systems. The electrochemical reaction easily follows the current variations at all rates indicating no kinetic limitation of hydrogen exchange in the materials between C/10 and C/5. However, for the studied bulky electrode, part of the reversible capacity is lost because of (a) formation of electrochemically stable alpha H solid solution which contains up to 1 at. H/f.u. and (b) incompleteness of the conversion of the metal hydride anode electrode alloy into the beta hydride phase during the charging process. Thus, care should be taken to achieve efficient performance of the metal hydride electrodes on their scaling up.

Acknowledgments This work was financially supported by the project NOVEL MAGnesium based nanomaterials for advanced rechargeable batteries (NOVELMAG) in the frame of the ERA.Net RUS FP7 Programme # 225.

Supplementary Information Available Crystallographic data taken from (13) for La2MgNi9 (Rm; N°166) and from (19) for La2MgNi9D13 (Rm; N°166) are available as Supplementary Information. This information is available free of charge via the Internet at http://pubs.acs.org.


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Figure caption Figure 1 : Pressure-Composition-Temperature (PCT) isotherm curve for La2MgNi9 during a full absorption/desorption cycle at 20°C. The dotted lines stand for the electrochemical window (between 0.1 and 0.001 MPa) and define about 310 mAh⋅g-1 of reversible capacity (data from 13) whereas the total capacity at 0.1 MPa reaches 380 mAh⋅g-1. Figure 2 : Refined neutron powder diffraction pattern (measured (dots), calculated (solid line)) for the electrode at initial charged state. Crystallographic structures are taken from Denys and Yartys 13,19. Vertical bars correspond to diffraction line positions for each phase: La2MgNi9 (alpha) and deuterated La2MgNi9D13 (beta) phases. Nickel lines (heavily textured) arise from the current collector of the working electrode and from the counter electrodes. Background around 30° comes from the silica cell and the NaOD/D2O electrolyte. One can also note an extra peak at 2θ = 17.5° that is attributed to PTFE. Figure 3 : Evolution of the potential Ew versus the Cd/Cd(OD)2 reference electrode during the first in beam charge-discharge (C/10) of the electrode at a current rate of 33 mA.g-1. Figure 4 : 3D view of the NPD pattern evolution as function of time during the first cycle (D/10+C/10) of the working electrode at a current rate of 33 mA.g-1. Figure 5 : Comparison between the alpha and beta phase amounts and the capacity Q during the in beam charge/discharge cycle at D/10+C/10 for the working electrode. The initial charge state Q at t=0 is set at 380 mAh.g-1, a value determined from the PCT curve (Figure 1) at a pressure of 0.1 MPa. Figure 6 : Evolution of the cell volumes for the alpha and the beta phases during the in beam charge/discharge cycle at C/10 for the working electrode. For sake of comparison, the evolution of the potential Ew versus the Cd/Cd(OD)2 reference electrode is also shown on the right scale. Figure 7 : Evolution of the half width of the diffraction peaks for the beta and alpha phases during the in beam charge/discharge cycle at D/10+C/10 for the working electrode. For sake of comparison, the evolution of the capacity Q is also shown on the right scale. The initial charge state Q at t=0 is set at 380 mAh.g-1, a value determined from the PCT curve (Figure 1) at a pressure of 0.1 MPa. Figure 8 : Evolution of the phase amounts for the alpha and the beta phases during the in beam charge/discharge cycle at C/5 for the working electrode. For sake of comparison, the evolution of the potential Ew versus the Cd/Cd(OD)2 reference electrode is also shown on the right scale. Figure 9 : 3D view of the NPD pattern evolution as function of time during the GITT discharge cycle (D/7) of the working electrode at 46 mA.g-1. Figure 10 : Evolution of the phase amounts for the alpha and the beta phases during the in beam discharge obtained by GITT experiments. The current was set to 46 mA.g-1 (D/7) for 1.5 h or Ew > 0.5 V followed by relaxation periods of 1h. For sake of comparison, the evolution of the capacity Q is also shown on the right scale. The initial charge state Q is set at 380 mAh.g-1, a value determined from the PCT curve (Figure 1) at a pressure of 0.1 MPa. Figure 11 : Evolution of the phase amounts for the alpha and the beta phases during the in beam charge/discharge cycles at C/10, C/5 and GITT for the working electrode. For sake of comparison, the evolution of the potential Ew versus the Cd/Cd(OD)2 reference electrode is also shown on the right scale. Figure 12 : Evolution of the cell volumes for the alpha and the beta phases during the in beam charge/discharge cycles at C/10, C/5 and GITT for the working electrode. For sake of comparison, the evolution of the potential Ew versus the Cd/Cd(OD)2 reference electrode is also shown on the right scale. Figure 13 : Evolution of the cell volumes for the alpha (black square) and the beta (red circle) as a function of the state of charge Qc (full symbols) and Qd (empty symbols). For Qd, the


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fully charged state was set at 380 mAh.g-1, a value determined from the PCT curve (Figure 1) at a pressure 0.1 MPa. Figure 14 : Evolution of the phase amounts for the alpha (black square) and the beta (red circle) phases as a function of the state of discharge Qd. The fully charged state was set at 380 mAh.g-1, a value determined from the PCT curve (Figure 1) at a pressure 0.1 MPa.


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Table caption Table 1 : Discharge capacities at each cycle during activation of the composite electrode. Table 2 : Cumulated discharge capacities obtained from GITT experiment. The current was set at 46 mA.g-1 (D/7) for one and a half hour or Ew > 0.5 V followed by relaxation periods of one hour. Table 3 : Discharge capacity at each cycle during in situ measurements of the working electrode.


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of the Co-free La2MgNi9 Anode for Ni-Metal Hydride Batteries. Electrochim. Acta. 2013, 96, 27-33. Nwakwuo, C. C.; Holm, T.; Denys, R. V.; Hu, W. K.; Maehlen, J. P.; Solberg, J. K.; Yartys, V. A., Effect of Magnesium Content and Quenching Rate on the Phase Structure and Composition of Rapidly Solidified La2MgNi9 Metal Hydride Battery Electrode Alloy. J. Alloys Compd. 2013, 555, 201-208. Rodríguez-Carvajal, J., Fullprof: a Program for Rietveld Refinement and Pattern Matching Analysis. Physica B. 1993, 192, 55-69. Denys, R. V.; Yartys, V. A.; Webb, C. J., Hydrogen in La2MgNi9D13: The Role of Magnesium. Inorg. Chem. 2012, 51, (7), 4231-4238. Dorogova, M.; Hirata, T.; Filipek, S. M., Hydrogen-Induced Volume Changes in ZrCr2 and Pseudo-Binary Compounds of ZrCr2, ZrMn2 and ZrV2. Phys. Status Solidi A. 2003, 198, 38-42. Yartys, V. A.; Burnasheva, V. V.; Semenenko, K. N., Structural Chemistry of Hydrides of Intermetallic Compounds. Russ. Chem. Rev. 1983, 52, 529-562.


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3D view of the ND pattern evolution during the first discharge/charge cycle of La2MgNi9. 196x160mm (96 x 96 DPI)


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Mechanistic and Kinetic Study of the Electrochemical Charge and

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