Li-Driven Electrochemical Conversion Reaction of AlH3, LiAlH4, and

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Li-driven Electrochemical Conversion Reaction of AlH, LiAlH, and NaAlH Joseph Anthony Teprovich Jr., Junxian Zhang, Hector Colon-Mercado, Fermin Cuevas, Brent Peters, Scott Greenway, Ragaiy Zidan, and Michel Latroche J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5129595 • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Li-driven Electrochemical Conversion Reaction of AlH3, LiAlH4, and NaAlH4 Joseph A. Teprovich Jr,a Junxian Zhang,b Héctor Colón-Mercado,a Fermín Cuevas,b Brent Peters,a Scott Greenway,c Ragaiy Zidan a and Michel Latroche b* Affiliations: a

Clean Energy Directorate- Savannah River National Laboratory, 301 Gateway Dr., Aiken, SC 29808, USA.

b

Institut de Chimie et des Matériaux Paris Est, ICMPE-CNRS-UPEC, UMR 7182, 2-8 rue Henri Dunant, 94320 Thiais, France. c

Greenway Energy LLC, 189 Gateway Dr., Aiken, SC 29808, USA.

*corresponding author: M. Latroche, Email: [email protected]; Ph.: +33 1 49 78 12 10 Abstract The conversion reaction of AlH3, LiAlH4, and NaAlH4 complex hydrides with lithium has been examined electrochemically. All compounds undergo a conversion reaction in which one equivalent of LiH is formed for each equivalent of hydrogen contained in the hydride material. Decomposition of the hydrides follows different paths depending of the nature of the alkali metal but leads in all cases to pure metallic aluminum. Such very fine and reactive Al particles are able to readily form an alloy with Li at a lower potential. Alternatively, thermal decomposition of alane has been used to produce highly porous aluminum able to react with lithium to form the AlLi alloy directly. Constant current charge/discharge cycling, cyclic voltammetry, and in-operando XRD were utilized to characterize the performance of these materials and to interpret the reaction paths depending of the complex hydride compositions.

Keywords Metal hydride, lithium, conversion reaction, in-operando XRD

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1.

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Introduction

Recently, Aymard et.al demonstrated that some metal hydrides (MHx) can react with lithium to form metal and lithium hydride. Such a reaction could lead to high capacity anodes for use in lithium ion batteries.1,2Indeed, MgH2 has shown good reversibility and has been an active material of choice in several reports.

3–8

The high capacity for MgH2 is achieved through a

conversion reaction in which the magnesium hydride reacts with lithium ions to form LiH and pure magnesium (MgH2 + 2Li+ + 2e- ↔ 2LiH + Mg). However, a recent study of TiH2 as an active anode material suggests that the simple mechanism used to describe the behaviour of MgH2 is not universal to all MHx hydrides. 9 Recent reports have also explored the use of other high gravimetric and volumetric capacity Mg-based hydrides (Mg2NiH4, Mg2CoH6, and Mg2FeH6). These materials demonstrated full conversion into LiH upon the first lithiation but with different reaction paths depending on the transition metal. Moreover, the reversibility of the reaction was not straightforward. 10–12 In this work we explore the Li-driven electrochemical conversion reaction of aluminum based hydrides AlH3, LiAlH4, and NaAlH4. To our knowledge, there has been only one report exploring the reaction of AlH3 with lithium.5 This group demonstrated that ~2.2 equivalents of Li reacted with AlH3 to from LiH. The reverse reaction (reformation of AlH3) was shown to be problematic. Previous work by SRNL has demonstrated that it is possible to electrochemically reform AlH3 from Al in solution.13,14 Interestingly, other alanates (i.e. LiAlH4, NaAlH4) have not been explored even though they could offer higher capacities because of the 4 equivalents of Li that can react with these tetra-alanates during the conversion reaction. 2.

Experimental Section

2.1 Materials All manipulations of the samples were performed in an argon-filled glove box or on a Schlenk line. LiAlH4 and NaAlH4 were purchased from Sigma-Aldrich and α-AlH3 was purchased from ATK. NaAlH4 was first Spex-milled for 1 h (30:1 ball to mass ratio), while LiAlH4 and AlH3 were used as received. The anode material consisted of the metal hydride, acetylene black, and CMC-700k (or CMC-f) typically in a 1:1:1 weight ratio. The CMC-f was synthesized via a previously reported method.15


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2.2 Sample Preparation/Characterization The composites were prepared by hand mixing the components in a mortar and pestle. CMC-f was used with AlH3 and LiAlH4, while un-modified CMC was used with NaAlH4 as a reaction occurs upon the addition of the electrolyte solution to the CMC-f / NaAlH4 mixture. Swagelok type cells were assembled using 7-10 mg of the anode composite, Whatman glass fibre filter

paper

and

1.0M

LiPF6

in

1:1

dimethyl

carbonate:ethylene

carbonate

as

2

separator/electrolyte, and a 1 cm Li foil disk. All cells were assembled/disassembled in an argon filled glovebox. Electrochemical cycling and electrochemical impedance spectroscopy was carried out using a Bio-Logic VMP3 multichannel potentiostat. The electrochemical cycling was carried out by applying a constant current to charge/discharge the cells at a rate of 1 equivalent lithium in 10 hours between 3.0 V and 0.005 V vs. Li+/Li0. Cyclic voltammograms were collected at a cycling rate of 0.1 mV s-1 between 3.0V and 0.005V vs. Li+/Li0. A Bruker DaVinci diffractometer (Cu Kα λ= 0.15418 nm) equipped with a Lynxeye detector was used to carry out the in operando XRD of the electrodes at various stages of the electrochemical reduction. For this purpose, a special electrochemical cell developed by Leriche16 was used. This cell is equipped with a beryllium (Be) window located directly on top of the active material allowing data collection without exposure to air. The discharge was carried out step by step off beam using the VMP3 potentiostat at a rate of one Li per 20 hours. At each step, the cell was turned to open circuit voltage (OCV) and transported to the goniometer for diffraction analysis. The discharge was conducted in five to six steps depending on the sample. Typical relaxation time after each step was 10 h. For AlH3 and NaAlH4, a final charge step was also done. Refinement of the XRD patterns was done by the Rietveld method using TOPAS 4.2 software to determine the relative phase amounts as a function of the lithium content. 3.

Results

3.1 Aluminum Hydride Aluminum hydride has been considered a hydrogen storage material for fuel cell systems because of its high volumetric and gravimetric hydrogen capacities (148 g/L and 10.1 wt% respectively) as well as its favorable desorption kinetics.

17–19

There are many polymorphs of

AlH3 (e.g. α, ά, β, γ), however, the most studied has been the α polymorph because of its stability. α-AlH3 decomposes in a single endothermic reaction during thermal decomposition: 3 ACS Paragon Plus Environment


(1)

2α-AlH3 → 2Al + 3H2

This mechanism suggests that up to 3 equivalents of Li can be transferred and participate in a conversion reaction with aluminum hydride to form 3 equivalents of LiH. Figure 1 shows the constant current cycling performance of a cell operated at a cycling rate of 1 Li/10h (C/30). The first reduction plateau at ~0.75V corresponds to the irreversible solvent reduction/decomposition and the formation of a Solid Electrolyte Interface (SEI) on the conductive carbon additive utilized to prepare the electrode.20 The SEI reaction on the carbon contributes to an irreversible capacity of 0.16 Li. This irreversible capacity is in the range of what is typically observed for graphitic carbons.21

3,0

2

1

2,5 Current / mA

+

Potential / V (vs. Li /Li)

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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2,0

0

-1

-2

-3

1,5

-4 0.0

0.5

1.0

1.5 +

2.0

Potential / V (vs. Li /Li)

1,0

0,5

0,0 0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

x (Li) Fig 1. Constant current cycling of the AlH3 based anode material at a rate of 1Li/10h. Soliddischarge and dash- charge. Inset shows the cyclic voltammogram of the first cycle at a scan rate of 0.1mV/s. The plateau observed between 0.6 V and 0.5 V is attributed to the reaction of Li+ with AlH3 to form LiH through a conversion reaction. This plateau corresponds to approximately 1.62 Li indicating that approximately 54 % of the active material participated in the reaction. The second plateau from 0.3 V to 0.18 V is believed to be the lithiation of the dehydrogenated Al and 4 ACS Paragon Plus Environment

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provides a capacity of 0.37 Li. This indicates that 62% of the dehydrogenated AlH3 (i.e. Al) gets lithiated. The capacity of the third plateau (below 0.18 V) is ~ 1 Li, and is attributed to the lithiation of the conductive carbon additive and the formation of amorphous AlLi phases. While it is tempting to attribute the additional capacity to the formation of a Al4Li9 phase, studies on aluminum thin films have shown that this behavior results from the continuous exposure of fresh and unreacted Al surfaces to the electrolyte induced by the swelling of the electrode.22 Upon charging, the plateau attributed to the delithiation of the aluminum is observed at ~0.5V.

22

The

delithiation step shows a capacity of 0.67 Li, indicating that indeed 44 % more Al was lithiated during the last discharge plateau. Following the delithiation of aluminum, the reformation of AlH3 at ~0.8V from LiH and Al produced during the previous discharge step is observed. This results in a plateau with a capacity of 0.67 Li which is approximately 32% of the AlH3 that originally reacted in the first discharge plateau.

The reformation of AlH3, through the

rehydrogenation of aluminum, is significantly reduced in subsequent cycles. On the second cycle only two plateaus are observed (Figure S1; Supporting Information). As in the first cycle, the plateau at 0.4 V is attributed to the reaction of AlH3 to form LiH. On the second cycle only 0.48 Li is reacted towards the LiH formation.

The second plateau at

approximately 0.15 V is attributed to the lithiation of Al and shows a capacity contribution of 0.76 Li. On the charge cycle, the delithiation of Al shows 0.67 Li indicating that 88% release of Li by the Al. The plateau corresponding to the reformation of AlH3 shows a capacity of 0.36 Li which corresponds to a 75 % recovery of the material that was reacted in the second cycle. By the 10th cycle only the Al lithiation/delithiation plateaus are observed. The capacity of the plateaus are 0.51 Li and 0.44 Li for the discharging and charging cycle, respectively, which corresponds to an 88 % recovery. As shown in the inset in Figure 1, cyclic voltammograms in the same potential window as the constant current cycling show similar reactions as compared to the differential capacity plots for the constant current experiments. During the first cycle, the cathodic scan is dominated by the formation of the SEI starting at approximately 0.8 V followed by a broad peak corresponding to the formation of LiH and the lithiation of Al. On the anodic scan, the delithiation of Al at 0.5 V and the rehydrogenation the Al at 0.8 V are observed. As the material is cycled, the reaction peaks decrease and the curves are dominated by the lithiation/delithiation of Al (Figure S1; Supporting Information). 5 ACS Paragon Plus Environment


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3.2 Lithium Aluminum Hydride Lithium aluminum hydride (LiAlH4) is a complex metal hydride with a 3 step decomposition mechanism during thermal desorptions. (2)

3LiAlH4 → Li3AlH6 + 2Al + 3H2

(3)

2Li3AlH6 → 6LiH + 2Al + 3H2

(4)

2LiH → 2Li + H2 In this system we would expect the LiAlH4 and Li3AlH6 to participate in the conversion

reaction with Li+ to produce LiH during the discharge cycle. In this case, the alkali hydride LiH produced during the LiAlH4 decomposition process would not participate in any further conversion reaction with Li+. Figure 2 shows the constant current cycling and the cyclic voltammogram of the LiAlH4 based anode. During the galvanostatic cycling experiments, a sloping plateau commencing at ~0.76V contributes 0.91 Li to the capacity. This plateau can be attributed to the combination of reactions (2) and (3) and the formation of the SEI layer. The second plateau at 0.27V yields an additional capacity of 1.54 Li, and can be attributed to the lithiation of Al as it is freed from the hydrogen.

The higher than normal capacity for the formation of AlLi is a result of the

continuous decomposition reactions (2) and (3). Finally, the third plateau (1. As expected, the cell parameter of Al remains nearly constant during the reaction but strikingly, the cell parameter of AlLi raises from 6.36 to 6.38 Å beyond x=1. This is attributed to Li enrichment of the alloy which accepts a small solid solution range23. It is worth noting that despite the depth of discharge, no other alloy other than AlLi is observed in the crystalline form.

a

2,5

(x, E)

(0, 2.1)

2,0 +

E (V vs Li /Li)

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


1,5 (0.34, 1.12)

1,0 (0.64, 0.61) (1.34, 0.65)

(2.44, 0.38) (3.1,0.22)

0,5 0,0 0

1

2

3

x (Li)



b

Intensity (a.u.)

x=3.10

x=2.44

LiAlH4 Li3AlH6

x=1.34

Al LiAl LiH Be

x=0.64 x=0.34 x=0

20

25

30

35

40

45

50

55

2θ (°) c 100

Phase abundance (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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LiAlH4

80

LiH 60 40

Li3AlH6

LiAl

20

Al 0 0

1

2

3

x (Li)

Fig 6. In-operando XRD analysis as a function of x(Li) for the first discharge for LiAlH4; discharge curve with x and E (a); evolution of the diffraction patterns (b); evolution of the phase amounts (c). Dashed lines are guides for the eye.

For LiAlH4, the discharge curve (Fig 6a) was performed through six galvanostatic steps up to 3.1 Li (x(Li)= 0, 0.34, 0.64, 1.34, 2.44 and 3.1). Diffraction patterns for each Li content are shown in Fig 6b and data analysis in Fig 6c. The reaction begins with the decomposition of LiAlH4 into Li3AlH6, LiH and Al. Again, LiH and Al diffraction lines are difficult to resolve however, the formation of the hexa-alanate increases rapidly to 1.4 Li and then begins to decrease. During the second part of the discharge (between 1.4 and 2.5 Li), Li3AlH6, decomposes


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into Al and LiH. Therefore, the electrochemical reactions that describe the conversion reaction of lithium alanate can be summarized as (10)

2LiAlH4 + 3Li → Li3AlH6 + Al + 2LiH

(11)

Li3AlH6 + 3Li → Al + 6LiH

which are equivalent to the solid gas decomposition steps of LiAlH4 described by reactions 3 and 4, respectively. Thus, the reaction of four Li atoms per initial LiAlH4 hydride is expected for the overall conversion reaction, including reaction 9. a 2,7

(x, E)

1,8

+

E (V vs Li /Li)

(0, 2.5)

(0.74, 0.72)

0,9

(1.5, 0.50)

(2.6, 0.32)

(3.73, 0.17)

0,0 0

1

2

3

4

x (Li)

b x=3.34 (charge)

Intensity (a.u.)

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


NaAlH4

x=3.73

LiNa2AlH6

x=2.6

Al LiAl LiH Na Be

x=1.5 x=0.75 x=0

20

30

40

50

2θ (°)



c 100

Phase abundance (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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NaAlH4

80

LiH

60

40

LiNa2AlH6

20

Al

LiAl

Na

0 0

1

2

3

x (Li) Fig 7. In-operando XRD analysis as a function of x(Li) for the first discharge (and first charge) for NaAlH4; discharge curve with x and E (a); evolution of the diffraction patterns (b); evolution of the phase amounts (c). Dashed lines are guides to the eye.

For NaAlH4, galvanostatic charging was performed in five steps: x(Li) = 0, 0.74, 1.5, 2.6 and 3.73 (Fig 7a). XRD patterns were collected at each relaxation period (Fig 7b). XRD data analysis is shown in Fig 7c. At the beginning of the reaction, NaAlH4 starts to decompose into LiH, Al, and the bi-alkali hexa-alanate phase LiNa2AlH6 instead of the expected Na3AlH6 phase because of its relative stability (reaction 5). The amount of LiNa2AlH6 increases up to 2 Li and then begins to decrease. The first reaction step can then be written according to reaction 12. Above 2 Li, the bi-alkali hexa-alanate phase decomposes into pure sodium, aluminium, and lithium hydride following reaction 13. (12)

NaAlH4 + (3/2)Li → (1/2)LiNa2AlH6 + (1/2)Al + LiH

(13)

LiNa2AlH6 + 5Li → 2Na + Al + 6LiH Finally, the Al produced by decomposition of the alanates reacts with lithium to form

AlLi (reaction 9). Reactions 9, 12, and 13 correspond to the reaction of five equivalents of Li per NaAlH4 compound. After the last step, the sample was tentatively recharged and the pattern is shown on top of Fig 7b. Diffraction analysis leads to two possible mechanisms. The first is the decomposition of AlLi into the elements Al and Li (reverse reaction 9) however, new lines belonging to the 16 ACS Paragon Plus Environment

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hydride NaH are clearly observed in the pattern. This means that a reaction takes place between sodium and lithium hydride leading to equation 14. This is convenient with the equilibrium potential of NaH, which lies at approximately 0.38 V above Lio/Li+. (14)

LiH + Na → Li + NaH 3.5 Attempt to Improve AlH3 Cycling Based on the information from the electrochemical and in-situ XRD studies indicating

that the interaction of lithium with aluminum occurs in these conversion reactions, we revisited the AlH3 based material because of its simple one-step decomposition mechanism (to avoid the intermediates encountered with LiAlH4 and NaAlH4). In these experiments, we constrained the cycling window to potentials where the lithiation of aluminum does not occur (> +0.29V) 22,24–26. We found that limiting the potential window eliminated the lithiation of aluminum, because there was no plateau observed for delithiation during the charging cycle (~ +0.46V). However, the capacity of the cell did fade rapidly and there were no plateaus associated with the formation/decomposition of LiH after the 2nd cycle (Figs. S4 and S5; Supporting Information). This is consistent with the findings observed during the full charge/discharge cycles (3.0V to 0.005V) with the majority of the capacity observed for the cell as a result of the lithiation of Al beyond the 3rd cycle. We attribute this rapid capacity fade to the large volume expansion/contraction observed for metal hydrides during dehydrogenation. This will result in the segregation of Al from LiH in the anode and inhibit the reformation of AlH3 during the charging step resulting in the observed capacity fade. 4.

Discussion

From both electrochemical and structural measurements, it is clearly demonstrated that AlH3, LiAlH4, and NaAlH4 react with lithium showing significant discharge capacities of their H contents. The reaction is almost fully completed for all hydrides though some material remains unreacted as a result of losses of electrical contacts. Electrochemical measurements indicate that the overall reaction is accomplished at ~ 75% (x ~ 3 over 4 for AlH3 and LiAlH4, and x ~ 4 over 5 for NaAlH4). Structural measurements indicate that 5 to 12 wt% of the pristine material remains unreacted. It is worth noting that the working electrodes were prepared by hand mixing of commercial powders without any particle size optimisation. However, the reversibility is poor upon cycling and capacity fade is observed after the first cycle. To understand this behaviour, we 17 ACS Paragon Plus Environment


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have investigated the discharge reaction in-operando and, to some extent, the charge process for AlH3 and NaAlH4. It is observed that a conversion reaction exists for all compounds but with significant differences in the reaction route depending on the starting compounds. In addition, an alloying reaction between Al and Li is also observed at low potential. For AlH3, which corresponds to the simplest chemical hydride studied here, the reaction occurs in two different steps. The first step corresponds to the conversion reaction between AlH3 and Li, forming LiH and pure Al according to reaction 8. The expected potential for this reaction vs. Li+/Li° can be calculated using the Nerst law (Ecal = -∆rG/xF) in which the free energy ∆rG is determined from the formation energy difference between the reactant and the product (Table 1). The calculated potential is 0.97 V (Table 2), a value close to the experimental relaxation potential of 1.07 V measured at x = 0.25. This process leads to pure and chemically reactive Al, which forms very fine particles as it is supported by the broad width of the diffraction peaks observed for this element (Fig 4b). Such fine Al particles can then react with Li to form AlLi at Ecal = 0.36 V, which is supported by XRD that shows the progressive disappearance of Al to the profit of the AlLi alloy. To confirm this, thermally desorbed alane was prepared to study the behaviour of such highly porous aluminium. The relaxation potential at the second step (0.36 V) of the discharge curve (Fig. 5a) and the XRD analysis (Fig. 5b-d) endorse the alloying reaction and the formation of only the AlLi phase. Table 1: Free energy of formation of relevant compounds identified in this study Compound

∆Gf° (kJ mol-1)

LiH AlH3 NaH LiAlH4 Li3AlH6 NaAlH4 Na2LiAlH6 AlLi

-69.96 69.6 -33.52 -35.9* -190.5* -39.43 -158.2* -35

Reference 27 27 27 28 28 29 30 31

*Estimated from the free energy of their decomposition reaction stated in the given reference and the free energy of formation of the corresponding decomposition products.


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Table 2: Calculated free energy of reaction and electrochemical potentials for the reactions investigated in this work. Last column displays experimental potentials at selected Li contents. Reaction

Reactant

Product

(8) (9) (10) (11) (12) (13) (14)

AlH3 Al LiAlH4 Li3AlH6 NaAlH4 LiNa2AlH6 NaH

Al + 3LiH AlLi (1/3)Li3AlH6 + (2/3)Al + 2LiH Al + 6LiH (1/2)LiNa2AlH6 + (1/2)Al + LiH 2Na + Al + 6LiH LiH + Na

x ∆ rG (kJ mol-1) (Li) -279 -35 -167 -229 -110 -262 -36.4

Reaction of Li with Al has been studied in the past

3 1 2 3 1.5 5 1

22,26

Ecal (V vs. Li+/Li) 0.97 0.36 0.87 0.79 0.76 0.54 0.38

(x, Eexp) (Li,V) (0.25, 1.07) (2.5, 0.39) (0.34, 1.12) (1.34, 0.65) (0.74, 0.72) (1.5, 0.50) -

and large capacities were

expected through the formation of three possible alloys AlLi, Al2Li3, and Al4Li9. The lower-Li containing alloy AlLi exhibits a theoretical capacity of 993 mAhg-1 making this material suitable as a negative electrode material. Hamon et al. 22 studied Al thin films with different thickness as negative electrodes and observed a potential plateau at 0.26 V versus Li under dynamic conditions for the formation of AlLi. This potential value is very similar to that observed here for the second part of the discharge and the charge curves which show potential plateaus at 0.19 and 0.46 V, respectively. It is also consistent with the discharge curve of the thermally desorbed alane. It is worth noting that Hamon et al.

22

could not confirm the formation of AlLi from

diffraction data (attributed to amorphous state), whereas in our case we clearly observed a crystalline material using either AlH3 or Al powders as initial reactants (Fig. 4b and Fig. 5b). Moreover, a cell parameter variation of the AlLi phase could be measured (Fig. 5d). According to Kishio and Brittain (32), the solid solution domain for AlLi ranges from 48 to 55 at.% Li. In this composition domain, they observed a cell parameter ranging from 6.3612 to 6.3902 Å respectively, in fairly good agreement with our measurements. For the following charge, the data analysis shows that the hydride amounts (for both AlH3 and LiH) remain nearly constant whereas the AlLi transforms into pure Al. This leads to a reversibility of 0.8 Li, close to one Li expected for the full conversion of AlLi. Therefore, the reaction path can be described as the conversion reaction of AlH3 into LiH and Al followed by



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the alloying reaction of Al with Li. The lithiation/delithiation of aluminium is the primary contributor to the reversible capacity beyond the first cycle however, it should be noted that the formation of very small and non-oxidized Al particles allows a nearly complete (80%) reaction between AlLi and Li. This is in agreement with the results of Lindsay

26

that reported better

electrochemical performance for very fine particles obtained by the ball milling of Fe2Al5. We attribute the additional capacity observed in the charging cycle with the reformation of AlH3 (amorphous), which is consistent with our electrochemical measurements, even though it was not directly observed during the in-situ XRD study and is worthy of further investigation. It is believed that the particle size of the electrochemically reformed AlH3 is significantly smaller than the particle size of the starting AlH3. This smaller particle size results in an unstable material that will decompose at or near room temperature. For LiAlH4, the reaction path is somewhat different though comparable to AlH3. The main difference occurs with the formation of the hexa-alanate, Li3AlH6, as an intermediate phase. This is not surprising because the decomposition of LiAlH4 by the solid gas route follows the same path33 indicating that the hexa-alanate is a stable phase forming either chemically or electrochemically. The calculated equilibrium potential for the transformation of LiAlH4 into Li3AlH6 is Ecal = 0.87 V, meanwhile experimentally we obtain Eexp = 1.12 V (Table 2). Further discharge leads to decomposition of the hexa-alanate into Al and LiH, theoretically at Ecal = 0.79 V, and then the reaction proceeds with the formation of AlLi at Ecal = 0.36 V as was observed for AlH3. Such three-step behaviour should lead to three different plateaus in the discharge curve according to the different reaction enthalpies for the lithium alanate decomposition28. This is not obvious from the current electrochemical data (Fig. 6a) that shows a continuous decrease of the potential in the range 0.5-3.1 Li. Recording data at a slower rate could potentially allow for better resolution of each step. NaAlH4 exhibits the most complex behaviour from the three materials. As for LiAlH4, the formation of a hexa-alanate could be foreseen according to the solid gas behaviour. 34 However, in the present case, we observe the formation of LiNa2AlH6 instead of mono-alkali alanate Na3AlH6. The calculated equilibrium potential for the formation of LiNa2AlH6 is Ecal = 0.76 V, in close agreement with the observed relaxation potential at x(Li) = 0.74, Eexp = 0.72 V (Table 2). Such bialkali hexa-alanate has already been reported in the past33 and was found to be stable by DFT calculations

35

. It can be prepared by ball milling of NaAlH4, LiH, and NaH and 20 ACS Paragon Plus Environment

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decomposes above 160°C into LiH, NaH, and Al. 33 In the present case, a slightly different route is observed as the bialkali hexa-alanate seems to decompose into Na, LiH, and Al below 0.50 V without formation of the sodium hydride. However, during the following charge, diffraction lines belonging to NaH appear according to reaction 14. This indicates that there is a competition between the decomposition of AlLi (reverse reaction 9) and the formation of NaH (reaction 14). Indeed, both reactions occur at very close potentials (Ecal = 0.36 and 0.38 V for reactions 9 and 14, respectively), which may promote both of them. 5.

Conclusion

We have looked at the electrochemical behavior of alanates AlH3, LiAlH4¸and NaAlH4 in a lithium ion batteries environment. All aluminium-based hydrides react electrochemically with lithium ions through a conversion reaction to form LiH and Al as final products. Additional discharge leads to the alloying reaction between Al and lithium to form the compound AlLi leading to increased capacities. Despite similar end products, reaction paths for Al-based hydrides are very different depending on the hydride composition; AlH3 reacts directly with Li following a one-step reaction, whereas LiAlH4 and NaAlH4 convert through the formation of hexa-alanates leading to a two-step process. For the sodium alanate, the unexpected formation of the bialkali hexa-alanate, LiNa2AlH6, is reported. For all materials, the reactions led to the emergence of pristine and highly reactive very fine aluminium particles believed to enhance the alloying reaction between Li and Al, and to provide reversibility for this latter element. However, reversibility of the reaction to form the pristine hydrides remains challenging.

Acknowledgements J.A.T., H.C-M., B.P., and R.Z. would like to thank the U.S. DOE Vehicle Technologies Program for Funding. We would also like to thank Mr. Joseph Wheeler (SRNL) for his assistance with the laboratory operations. References (1) Larcher, D.; Beattie, S.; Morcrette, M.; Edström, K.; Jumas, J. C.; Tarascon, J. M. Recent Findings and Prospects in the Field of Pure Metals as Negative Electrodes for Li-Ion Batteries. J. Mater. Chem. 2007, 17, 3759–3772. (2) Oumellal, Y.; Rougier, A.; Nazri, G. A.; Tarascon, J.-M.; Aymard, L. Metal Hydrides for Lithium-Ion Batteries. Nat. Mater. 2008, 7, 916–921. 21 ACS Paragon Plus Environment


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(3)

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Figure caption Fig 1. Constant current cycling of the AlH3 based anode material at a rate of 1Li/10h. Soliddischarge and dash- charge. Inset shows the cyclic voltammogram of the first cycle at a scan rate of 0.1mV/s. Fig 2. Constant current cycling of the LiAlH4 based anode material at a rate of 1Li/10h. Soliddischarge and dash- charge. Inset shows the cyclic voltammogram of the first cycle at a scan rate of 0.1mV/s. Fig 3. Constant current cycling of the NaAlH4 based anode material at a rate of 1Li/10h. Soliddischarge and dash- charge. Inset shows the cyclic voltammogram of the first cycle at a scan rate of 0.1mV/s. Fig 4. In-operando XRD analysis as a function of x(Li) for the first discharge (and first charge) for AlH3; discharge curve with x and E (a); evolution of the diffraction patterns (b); evolution of the phase amounts (c) Dashed lines are guides to the eye. Fig 5. In-operando XRD analysis as a function of x(Li) for the first discharge for Al obtained by thermal decomposition of alane ; discharge curve with x and E (a); evolution of the diffraction patterns (b); evolution of the phase amounts (c); evolution of the cell parameters (d). Dashed lines are guides to the eye. Fig 6. In-operando XRD analysis as a function of x(Li) for the first discharge for LiAlH4; discharge curve with x and E (a); evolution of the diffraction patterns (b); evolution of the phase amounts (c). Dashed lines are guides to the eye. Fig. 7 In-operando XRD analysis as a function of x(Li) for the first discharge (and first charge) for NaAlH4; discharge curve with x and E (a); evolution of the diffraction patterns (b); evolution of the phase amounts (c). Dashed lines are guides to the eye. Supporting information Fig S1. Constant current cycling of the AlH3 based anode material at a rate of 1Li/10h. Inset shows the cyclic voltammogram for the corresponding cycles (scan rate of 0.1 mV/s). Black- 1st cycle, Red- 2nd cycle, and Green- 10th cycle. Fig S2. Constant current cycling of the LiAlH4 based anode material at a rate of 1Li/10h. Inset shows the cyclic voltammogram for the corresponding cycles (scan rate of 0.1 mV/s). Black- 1st cycle, Red- 2nd cycle, and Green- 10th cycle. Fig S3. Constant current cycling of the NaAlH4 based anode material at a rate of 1Li/10h. Inset shows the cyclic voltammogram for the corresponding cycles (scan rate of 0.1 mV/s). Black- 1st cycle, Red- 2nd cycle, and Green- 10th cycle. Fig S4. Constrained cycling of the AlH3 based anode material between 3.0-0.3V at a cycling rate of 1Li/10hr. Fig S5. Constrained cycling of the AlH3 based anode material with a 8 hr discharge time limit at a cycling rate of 1Li/10hr. 25 ACS Paragon Plus Environment


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Table caption Table 1: Free energy of formation of relevant compounds identified in this study. Table 2: Calculated free energy of reaction and electrochemical potentials for the reactions investigated in this work. Last column displays experimental potentials at selected Li contents. Table of Content


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Li driven AlH

Li-Driven Electrochemical Conversion Reaction of AlH3, LiAlH4, and

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