Reactivity of Sodium Alanates in Lithium Batteries - The Journal of

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

On the Reactivity of Sodium Alanates in Lithium Batteries Laura Silvestri, Luca Farina, Daniele Meggiolaro, Stefania Panero, Franco Padella, Sergio Brutti, and Priscilla Reale J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10297 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on November 28, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

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

The Journal of Physical Chemistry

On the Reactivity of Sodium Alanates in Lithium Batteries L.Silvestri,aL.Farina,aD.Meggiolaro,a,bS.Paneroa, F.Padella,cS.Bruttib,d* and P.Realec* a

Dipartimento di Chimica, Sapienza Università di Roma, V.le Aldo Moro 5, Roma (Italy) b

c

d

CNR-ISC, U.O.S. Sapienza, Piazzale A. Moro 5, Roma, (Italy)

ENEA Centro Ricerche Casaccia, via Anguillarese 301, Roma (Italy)

Dipartimento di Scienze, Università della Basilicata, V.le dell’Ateneo Lucano 10, Potenza (Italy) *corresponding authors: [email protected]; [email protected]

Abstract Novel chemistries for secondary batteries are investigated worldwide in order to boost the development of next generation rechargeable storage systems and especially of lithium-devices. High capacity anode materials for Li-ion cells are at the center stage of R&D in order to improve the performances. In this view conversion materials are an exciting playground. Among the various proposed class of conversion anodes, metal hydrides are probably the most challenging and promising due to the high theoretical capacities, instability towards the standard carbonate-based electrolytes, large volume variations upon cycling and moderately low working voltages. Among them lightweight hydrides, like alkaline alanates, are an almost unexplored family of materials. In this study we present a fundamental study about the electrochemical conversion reaction of sodium alanates: NaAlH4, Na3AlH6 and Na2LiAlH6. Our goal is to improve the understanding of the basic solid-state electrochemistry that drives the conversion reactions of these materials in lithium cells. Samples have been preparedmechanochemically and characterized by X-ray diffraction (XRD), infrared spectroscopy and transmission electron microscopy. All materials have been assembled in lithium cells with a commercial liquid electrolyte to test their electrochemical activity. The Li incorporation/de-incorporation mechanism for all materials has been explored by in situ XRD and interpreted also in the view of density functional theory thermodynamic calculations. Keywords: Sodium alanates; Lithium batteries; conversion materials; in situ XRD. ACS Paragon Plus Environment


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. Introduction Several metal hydrides proved to be potentially interesting negative electrodes for lithium ion batteries, because,according to lithium conversion reactions occurring at low voltage, they are able to provide high capacity and energy values [1, 2]. Although many aspect of the fundamental chemistry of the conversion of MgH2 are still unclear [3], it still remains the most promising candidate thanks to the reversibility and the small overpotential of the conversion process [4, 5]. Very recently alsolightweight alkaline alanates have been considered and exploredfor application in lithium cells by Latroche and co-workers and by us [6, 7].In fact among hydrides, alane and alanates are very appealing in terms of theoretical capacity values [2]. Alane, i.e. aluminum hydride, can theoretically exchange 3 electrons in a conversion reaction to LiH and metal Al, providing 2680mAhg-1. Unfortunately, it is thermodynamically unstable already at room temperature due to a slow decomposition to Al and H2[8, 9]. In this view a possible exploitation of pure AlH3 in an electrochemical device is likely to be excluded. On the contrary LiAlH4 and NaAlH4 are stable compounds, releasing hydrogen only above 150°C and 180°C, respectively, [10] and thanks to the alkaline metal light weight can still provide very high theoretical capacity: 2119 and 1985 mAhg-1 respectively. Li3AlH6 and Na3AlH6 are even thermally more stable [11]: theirtheoreticalgravimetric capacities approach 1493 and 1576 mAhg-1, respectively.In fact, while LiAlH4 and Li3AlH6 can exchange only 3 electrons to achieve full conversion, NaAlH4 and Na3AlH6 conversion involves 4 and 6 electrons respectively. Therefore capacity values are very close, if not higher for the heavier sodium than for the lighter lithium homologous. In a recent paper we reported our results aboutthe study of the incorporation of lithium alanates, i.e. LiAlH4 and Li3AlH6 as conversion anode in lithium cells [7]. Through a combined experimental and theoretical study, the complex conversion reaction path was described, the partial reversibility highlighted and the reactive hydride/electrolyte interface discussed. Here, we report our recent findings aboutsodium alanates: NaAlH4 and Na3AlH6. Besides these two phases, as discussed by Latroche and co-workers [6], also the mixed alanate, i.e. LiNa2AlH6, ACS Paragon Plus Environment

Page 2 of 32

Page 3 of 32

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


deserves consideration [12] and has been investigated by us in this study. It has a hypothetical specific capacity of 1871 mAhg-1 thanks to the theoretical possibility of exchange 5 electrons upon conversion. Similarly to what done by us for lithium alanates [7] and magnesium hydride [2], mechanochemical treatments have been applied either to activate the commercial NaAlH4 or to synthesize intermediate phases, i.e. Na3AlH6 and LiNa2AlH6.The performances of all synthesized materials have been evaluated in lithium cells by galvanostatic cycling. In order to shed more light in the solid state electrochemistry that drives these conversion reactions, the lithium incorporation/de-incorporation mechanism for NaAlH6has been verifiedexperimentally by in situ Xray diffraction. Our findings complete the reaction scheme proposed recently by Latroche at al. [6]: an additional reaction intermediate has been observed, the reaction reversibility has been provedand the overall mechanism has been discussed in the view of computational thermodynamic predictions obtained by density functional theory (DFT). Going beyond the study of the conversion of NaAlH6, here we also report for the first time a combined electrochemical-XRD analysis of thesolid state reactivity of Na3AlH6and LiNa2AlH6in lithium cells.

2. Experimental details 2.1.

Materials synthesis and experimental techniques

Commercial NaAlH4 (Aldrich, hydrogen storage grade) has been used in this study.This material is deeply studied as reversible H2 storage system, thanks to its high hydrogen weight content (5.5 wt%) and low thermodynamic hydrogen desorption-adsorption temperature [13, 14], especially when titanium doping is induced. Furthermore it is well known to be a strong reducing agent, widely used in organic synthesis. Therefore, safety issues must be taken in serious consideration when handling and conjecturing alanates usage as lithium ion anodes. All manipulations have been therefore performed in a Jacomex Argon filled glove box, with controlled humidity and oxygen content.

ACS Paragon Plus Environment


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

Sodium alanate has been used as purchasedand after a mechanochemical activation treatmentof 15h by high-energy ball milling. Ball milling has been further used to promote the intimate mixing of the activated alanate samples with the conductive carbon Super P for different times. Alanate to Super P weight ratio was 5:3. Samples have been labeled as BxDy, where x indicates the time of mechanochemical activation, while y indicates the following milling time with Super P (BxD0 indicates that Super P was not high energy milled but added by hand grinding in mortar, while Bx indicate that Super P has not yet been added and the sample is pure alanate). Na3AlH6and LiNa2AlH6have been synthesized by ball milling stoichiometric amount of NaAlH4, NaH and LiH, for the latter, for 15h. All hydrides have been purchased from Sigma Aldrich and used as received. As synthesized samples havebeen labeled S15, whilesamples added with Super P by hand grinding in mortar in the same ratio as above described, have been labeled S15D0. All ball milling have been performed by means of a M400 Spex Shaker, in stainless steel jars, with 10mm diameter stainless steel balls and a powder to balls weight ratio 1:20.Milling sessions have been carried out at room temperature intermittently for 15 minutes followed by 15 minutes rest to avoid the thermal deterioration of the samples. Jars have been always carefully filled, sealed and opened in the Ar-filled glove box in order to prevent any contact of the samples with air. The effect of milling on phase purity has been evaluated by x-ray powder diffraction and infrared spectroscopy. NaAlH4_Bx XRD patterns have been collected in borosilicate capillary sample holder in a RigakuUltima+ Diffractometer, equipped with Cu Kα source in a theta-theta Bragg-Brentano geometry. Na3AlH6_S15 and LiNa2AlH6_S15 XRD pattern have been acquired in Phillips X-pert Pro theta-theta diffractometer by using ahome-made sealed planar holder equipped with a Kapton window, in order to protect samples from air.Experimental patterns have been compared with reference pattern ICSD 154907, 154909, 152893for NaAlH4,α-Na3AlH6and LiNa2AlH6respectively. FTIR spectra have been acquired by a JascoFTIR-300 apparatus. All spectra have been recorded in the wavenumber range between 2000 and 400 cm-1 at room temperature in transmission mode. The ACS Paragon Plus Environment

Page 4 of 32

Page 5 of 32

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


samples, as fine powder, have been mixed with CsI in a ratio of 1:100 mg (powder to CsI respectively) and then pressed in pellets by an Pike die set an hand-press. FTIR assignment has been done by comparing with literature data[15, 16]. Particles morphology has been observed by transmission electron microscopy using a FEI Tecnaicryo-TEM instrument. In order to prevent sample damaging due to the interaction with the electron beam, TEM experiments have been carried out at 80 keV in cryo-condition by cooling the temperature of the sample holder to liquid nitrogen. TEM investigation has been carried out only at low and intermediate magnification due to the quick and destructive sample-electron beam interaction observed at high magnification as well as in all the experimental condition where the use of a concentrated beam is required (e.g. dark field or diffraction modes). Thus extreme care has been paid in the study of alanates by bright field TEM in order to prevent instrumental artifacts due to sample deterioration. On passing it is important to mention that possible contamination upon milling due to iron release from the jars and balls have been checked and excluded by using an XEDS EDAX microanalysis system coupled to an ESEM XL30 FEI apparatus. For electrochemical tests, electrodes have been prepared by adding PVdF Kynar 2801 to the mixture of alanate and Super P, in order to get an “active material / SuperP / polymer” weight ratio of 5/3/2. The final mixture has been pressed on 10mm diameterCu disks with a mild pressure, in order to obtain electrodes with around 1-2mg/cm2 active material. For Potentiodynamic Cycling with Galvanostatic Acceleration (PCGA) tests, three electrodes electrochemical cells have been assembled by facing the working electrode to a lithium metal foil counter through a Whatmann borosilicate fiber separator swollen with a 1M solution of LiPF6 in ethylene carbonate – dimethyl carbonate (EC:DMC) 1:1 mixture (LP30, MerkSelectipur). Lithium reference electrode is placed perpendicularly to the assembly, soaked by an electrolyte excess. In order to collect potential profiles negligibly affected by overpotentials, thus under quasithermodynamic control, PCGA testshave been carried out by means of a Biologic VSPpotentiostat,



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

in the potential range of 2.5-0.01V, with 10mV steps and a cutoff current equal to C/20, calculated as the current to deliver in 20h the full theoretical conversion capacity. In order to collect in situ diffraction experiments, free-standing pellets 7-10mgcm-1 have been prepared and used to assemble two electrode lithium cells with a beryllium window positive current collector. Cells have been placed in the focus of a simultaneous 120° angular dispersion X-ray diffractometer Italstructure (curved PSD detector from INEL) equipped with a Cu Kα1 source, and pattern have been collected in continuous every 20 minutes while the electrode was discharged and charged under galvanostatic conditions at C/20.

2.2.Computational details. The prediction of the expected potential vs Li of the possible conversion reactions involving NaAlH4, LiNa2AlH6and α-Na3AlH6 phases has been carried out by DFT in generalized-gradientapproximation (GGA-PBE) [17] projector augmented wave (PAW) potentials [18] and planewaves following the simple approach used by us in recent papers [2, 7]. Besides the tetragonal NaAlH4,[19, 20]the cubic LiNa2AlH6[21, 22]and the monoclinic α-Na3AlH6structures [23, 11]also cubic NaH, LiH, Li and Al lattices have been computed [24, 25]. A kinetic energy cutoff converged to 500 eV on plane waves has been converged and used for all calculations. For each phase the number of k-points on equally spaced meshes in the Brillouin zone (BZ) have been set in order to obtain an accuracy of 1 meV/atom on the total energy. The equilibrium structure of the phases has been found relaxing both the cell and ionic positions until a convergence on the forces of 0.001 eV/Å has been achieved. The energetics at 0 K have been evaluated for each reaction starting from the DFT total energy of each phase by simple stoichiometric calculations. In the evaluation of the reactions energetics, corrections due to the zeropoint energy (ZPE) of the involved phases have been neglected. It is important to underline that ZPE are necessary to correctly evaluate the thermodynamics of the hydride phases where


Page 6 of 32

Page 7 of 32

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


lightweight elements may imply moderate vibrational contributions to the 0K energy functions. However ZPEs are expected to play a minor role in the evaluation of the energetics of all the reaction considered. In fact the absence of any gaseous molecule among all the involved species is expected to lead to a partial compensation of the ZPEs between solid crystalline reagents and products. The cell voltage ∆V associated to each conversion reaction has been calculated according to the general relation ∆V = - ∆rE/nF, where ∆rE is the total energy of reaction as obtained by the DFT calculations, n the number of exchanged electrons in the reaction and F the Faraday constant.

3. Results and discussion Mechanochemical treatments have been applied either to activate commercial NaAlH4 (Sigma Aldrich) and to synthesize α-Na3AlH6and LiNa2AlH6. In general mechanochemical methods are used for either synthesis or activation, and, again, they are very common and effective strategies for improving the properties of hydrogen storage materials. Mechanochemical treatments are complex processes that result in a number of different positive effects: (a) the breaking of passivation layers on the material surface; (b) the high level of elastic shear and other stresses induced by milling; (c) the particle size reduction; (d) the induced formation of stacking fault disorder; (e) the fragmentation of bulks into layered nanocrystals; (f) the increase of the atomic disorder. Mechanochemical pretreatments proved to be essential to activate MgH2[2] and TiH2[26, 27]efficient in the Li3AlH6 synthesis but, conversely, any ball milling activation of LiAlH4 revealed to be detrimental due to the promoted hydrogen desorption [7].

3.1.Mechanochemical activation of NaAlH4 The sodium alanate under investigation has been previously characterized in terms of structure and morphology. It is pure NaAlH4, crystallized according to the tetragonal space group I41/a, [20]with crystallites average dimension of 110nm as demonstrated either by XRD peaks width analysis and ACS Paragon Plus Environment


by TEM, see figure 1a and its inset. Sub-micrometric prismatic particles with well-defined boundaries either at low and high enlargements characterize the morphology of this pristine sample.

c

B15D5

Al

Al

b 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

Page 8 of 32

B15

a B0

25

30

35

40

45

50

55

60

2θ

Figure 1. XRD pattern and TEM images of (a) B0 pristine sample; (b) sample B15, milled 15h alone ; (c) alanate B15D5, milled 15h+5h with Super P.

Mechanochemical activation has been carried on using a two step milling protocol: a first simple high energy ball milling of the bare alanate for 15h (sample B15) and a following milling with Super P alternatively in agate mortar or by high energy ball milling for 5h, producing samples B15D0 and B15D5 respectively. XRD and FTIR spectroscopy have been used to verify the effect of


Page 9 of 32

these activation treatments in the bulk and on the surface of the particles, while TEM has given information about the morphology modifications. Figure 1b and 1c reports the XRD collected before and after the most significant mechanochemical activation treatments. The background halo observed is due to the glass capillary used as holder in the diffraction experiments. As expected, upon milling peaks width broad and their intensity decreases. The analysis of peaks width allows to estimate a progressive decrease of the average crystalline domains size from 110 nm in the case of the B0 pristine alanate to 30 nm evaluated for B15D5 sample. Furthermore, it is worth noting that while pattern of sample B15 confirms the purity of the alanate phase after the first activation step, on the contrary the pattern of sample B15D5 reveal the presence of a small amount of metal Al. Al-H strch NaAlH4

Absorbance / 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


Al-H strch Na3AlH6

Na-Al-H bend Na3AlH6

Na-Al-H bend NaAlH4

1000

600

B15D5

B15

B0

2000

1800

1600

1400

1200

800

400

-1

Wavenumber / cm

Figure 2. FTIR spectra of NaAlH4 samples as a function of the mechanochemical treatment.

FTIR spectra as a function of the mechanochemical activation show complementary information, see figures 2. Sample B0 spectrum is characterized by the Al-H stretching modes centered at 1675, 1372 (weak) and a shoulder around 1770cm-1 and two clear Na-Al-H bending modes at 904 and 727 cm-1 plus a 4 other convolute modes developing between in the 1022-630cm-1 region. On the contrary, in the B15 spectra very small AlH63- stretching and corresponding H-Al-H bending signals can be observed. Indeed these signals become important after milling with Super P, evidencing a significant conversion to α-Na3AlH6 at least on the surface. ACS Paragon Plus Environment


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

The transmission electron microscopy confirms the particle size reduction upon milling but more interestingly severe morphological modifications are evidenced after milling with Super P. The insets of figure 1b and 1c report the image of the B15 and the B15D5 samples. During the TEM experiment a fast, unavoidable and, in some cases, massive interaction between the electrons beam and the samples has been observed. This interaction has produced a rapid desorption of molecular hydrogen thus damaging on the original morphology, in particular on mechanochemically activated samples. In fact, although after 15 h of milling the alanate still presents a rather regular shape slowly interacting with the e-beam in the TEM microscope, samples produced by milling with Super P, i.e. sample B15D5, are constituted by an amorphous composite matrix, rapidly damaged by the e-beam: the pristine alanate morphology is not recognizable anymore and, interestingly, no traces of the characteristic spherical morphology of Super P can be highlighted [2]. The experimental results show that (i) the high energy ball milling of the bare alanate has as only consequence the decrease of the crystallite average dimension; (ii) the mechanochemical treatment performed in addition to Super P, instead, likely promotes a certain hydrogen desorption according to the reaction: NaAlH4 = 1/3 α-Na3AlH6 + 2/3 Al + H2 Al presence is proved by XRD while α-Na3AlH6 by FTIR spectroscopy. The last one is probably too poorly crystalline to be observed by XRD. Hydrogen desorption is catalyzed by the intimate carbon dispersion and likely occurs thanks to the local overheating developing upon ball milling. Any contamination due to iron release from the jar and the balls has been excluded by energy dispersive spectroscopy (EDS) microanalysis for all samples.

3.2.Synthesis of α-Na3AlH6 and LiNa2AlH6


Page 10 of 32

Page 11 of 32

Mechanochemistry has been used to synthesize α-Na3AlH6 from stoichiometric amount of NaAlH4 and NaH, and LiNa2AlH6 from NaAlH4, NaH and LiH in 1:1:1 mole ratio. Milling has last 15h and the obtainment of the desired phases has been verified by both XRD and FTIR spectroscopy, see figures3a-c. The XRD analysis suggests the formation of almost pure α-Na3AlH6 and LiNa2AlH6 phases with apparent lack of contamination from the pristine NaAlH4 or other phases and the obtainment of very small crystallites, 30±10nm in case of LiNa2AlH6 and 36±10nm for α-Na3AlH6. The FTIR analysis (figure 3c) confirms the formation of almost pure α-Na3AlH6 with minor traces of the pristine NaAlH4. In fact the FTIR spectra of the Na3AlH6S15 sample shows two bands at 1432 and 1293 cm-1 due to the AlH63- stretching modes and a composite broad band centered at 840 cm-1 (with possible separated features at 910, 872, 837 and 796 cm-1) due to the AlH63- stretching modes [16], partially overlapped with smaller lines at 1772, 1682 cm-1 and at 725 cm-1 due to the vibration modes of the AlH4- units [15].

Al-H strch

* 20

25

30

35

40

45

50

55

60

Na3AlH6

b

H-Al-H bend

c

LiNa2AlH6

65

Absorbance / a.u.

*

a 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


d

LiNa2AlH6 s15

Na3AlH6 s15

e NaAlH4

*

pristine

* 20

25

30

35

40

45

2θ

50

55

60

65 2000

1800

1600

1400

1200

1000

800

600

400

-1

Wavenumber / cm

Figure 3. XRD pattern of LiNa2AlH6_S15 (a) and Na3AlH6_S15 (b); FTIR spectra (c) of the hexahydrides alanates in comparison; TEM images collected for the LiNa2AlH6_S15 (d) and Na3AlH6_S15 (e) samples.

Turning to the FTIR spectra of the LiNa2AlH6S15 sample, it shows features similar to the Na3AlH6 S15 one: two well separated composite broad bands centered at 1410 and 792 cm-1 likely due to the stretching and bending modes of the AlH63- units, respectively. Also in this case possible traces of



the pristine NaAlH4 phase can be also highlighted by the weak bands at 1767 and 1660 cm-1. Apparently, as far as we know, this is the first ever reported FTIR spectrum of the LiNa2AlH6 phase. From the morphological point the mechanochemical synthesis of the two samples results in a rather similar morphology as shown in the TEM micrographs reported in the figures 3d-e. A homogeneous morphology is observed for both the α-Na3AlH6 s15 and the LiNa2AlH6 s15 samples constituted by large micrometric round-shaped particles with smooth surfaces and low phase contrast on the particle edges.

3.3.Electrochemical performances The alanates mixed to Super P were further added with PVdF and used to realize electrodes in the shape of pellets. Latroche et al. [6] proved the reduction of NaAlH4in lithium cells through a twostep conversion reaction path involving the intermediate phase LiNa2AlH6and the final formation of Na, Al and LiH. Furthermore an apparently unavoidable lithium alloying into metal aluminum occurs at low voltages competing with the conversion reaction.

-1

-1

specific capacity / mAh g 0

300

600

900

1200

specific capacity / mAh g 1500

1800

0

2.5

300

600

900

1200

1500

1800

2.5

B0D0 B15D0

B15D0 B15D5

2.0

Potential / V vs Li

2.0

Potential / V vs 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

Page 12 of 32

1.5 1.0

a

0.5

1.5 1.0

b 0.5

0.0

0.0 0

1

2

3

4

0

1

x equivalents

2

3

4

x equivalents

Figure 4. Voltage profiles upon first PCGA cycle of the NaAlH4_BxDy samples. (a) comparison between the pristine sample and the 15h milled alanate, with a simple Super P hand-grinding treatment, (b) effect of high energy ball milling with Super P on the B15 sample.


Page 13 of 32

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


Here potentiodynamic cycles with galvanostatic acceleration versus Li counter and reference electrodes in LP30 electrolyte have been carried out for all prepared NaAlH4 samples,as shown in figures4a-b, in order to measure their electrochemical fingerprint under quasi-thermodynamic conditions and verify the effect of milling on the lithium loading ability of these materials. The electrochemical response of sodium alanate after the first step of mechanochemical activation is shown in the figure 4a. B0D0 and B15D0 samples have almost the same potential profile upon discharge and charge. Discharge develops inthree plateaus around 0.41, 0.26 and 0.17V vs Li respectively, for an overall capacity of 1700-1800 mAhg-1, i.e. more than 3.5 lithium equivalents over the theoretical 4. Recharge efficiency is lower than 30%, and only one lithium equivalent is given back, through a two steps process evolving around 0.40 and 0.47 V vs Li. Latroche et al. were unable to discriminate all these processes, and found a sloping reduction plateau below 0.3V vs. Li. this difference may arise from the slightly different discharge conditions. Nevertheless the conversion mechanism proposed by Latroche et al. [6]may allow a first interpretation of our measured potential profiles. However we postpone the discussion of the reaction mechanism and the comparison with Latroche results in the following section where our in operando XRD data are also shown and discussed. Here we only discuss that the very short decomposition plateau centered around 0.84V vs. Li is likely due to the electrolyte decomposition on the Super P surface [28]. Apparently the mechanochemical activation doesn’t affect the reaction sequence but widen the length of the conversion plateau thus suggesting an improved electrochemical activity of the sodium alanate. In fact the amount of lithium loading occurred above 0.25V vs. Li increases from 1.28 Li eq for sample B0D0 to approximately 2 Li eq for sample B15D0. An additional ball milling with SuperP carbon has a dramatic beneficial effect of the reversibility of the conversion process as shown in the figure 4b.Apparently the conversion reversibility is drastically enhanced and a much greater capacity is delivered back on charge (2.5eq), with a coulombic efficiency close to 70%. On the other hand the voltage profile is strongly modified. In fact the electrochemical process at about 0.8V vs. Li due to the electrolyte decomposition does not ACS Paragon Plus Environment


occur. This effect is observed for the cycling of all samples ball milled with SuperP carbon independently from the milling time.Upon reduction, sample B15D5 exhibits short plateaus at 0.62, 0.37, 0.24 and finally 0.18V vs. Li. Also the charge profile is modified and in addition to the process occurring at about 0.46V vs. Li, there is a sloping oxidation around0.8V vs Li. The observed differences could be the effect of either the nucleation of the Na3AlH6phase on the surface of the NaAlH4 particles (see previous section) and Al upon milling with carbon, and the creation of a rather efficient carbon-hydride composite, also bearing in mind the lack of the morphological evidences of Super P particles in the TEM observations of the BxDy samples. It is worth nothing that the Super P contribution to the total exchanged capacity is completely negligible in the case of bare Super P added by mortar grinding, while it has been experimentally estimated as maximum 7% in the case of Super P ball milled 5h.

2.5

Na3AlH6_S15D0 LiNa2AlH6_S15D0

2.0

Potential / V vs 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

Page 14 of 32

NaAlH4_B15D0 1.5 1.0 0.5 0.0 0

1

2

3

4

5

6

x equivalents

Figure 5. Voltage profiles upon first PCGA cycle of the Na3AlH6_S15 (a) and LiNa2AlH6_S15 samples in comparison to NaAlH4_B15. All samples have been hand-ground with Super P.

The

potentiodynamic

cycles

with

galvanostatic

acceleration

of

Na3AlH6_S15D0

and

LiNa2AlH6_S15D0 are shown in the figure 5. The Na3AlH6_S15D0 sample shows poor performances: only 2.65 lithium equivalents over 6 have been exchanged upon discharge and only


Page 15 of 32

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


0.77 during the following recharge. Reduction occurs in two badly resolved slopes centered approximately at 0.86 and 0.15V vs Li, whereas in the following oxidation a very short plateau around 0.4V vs Li can be tentatively identified. On the contrary, the LiNa2AlH6_S15D0 sample is able to deliver all the theoretical capacity, i.e. 1872 mAhg-1 corresponding to 5 lithium equivalents, in the first reduction, through a very long plateau at 0.19V vs Li and a final slope to 0 V vs. Li. Upon oxidation nearly 2 lithium equivalents are returned, in two short sloping plateaus occurring at 0.38 and 0.49V vs Li. Generally speaking the experimental reversibility of the sodium alanates reduction appears to be poor. However, considering the large volumetric variations associated to conversion processes and eventually to Li alloying to metal aluminum or sodium, the poor reversibility could be ascribed to technological problems and not to the intrinsic irreversibility of the processes. In fact the expected conversionleads to a huge variation of particles volume (see as an example ref. [2]), enough to justify a mechanical disintegration and failure of the electrode [29]. As a confirmation, the improved reversibility of sample B15D5 must be taken in account. Far from being optimized, the increased coulombic efficiency can be considered a consequence of the improved distribution between the alanate and the carbon additive that likely buffers the volume variations and prevent the loss of electronic contact between particles even after the large volume variations occurred upon full conversion.

3.4.Mechanism comprehension: DFT study In the Table 1 the optimized ground state crystal structures of the computed hydrides are reported. Besides these hydrides, DFT calculations have also been performed on simple Li (bcc), Na (bcc) and Al (fcc) phases in order to predict the conversion potentials vs Li. The calculated lattice parameters of all phases are in good agreement with experiments. NaH and LiH hydrides crystallize in a simple fcc lattice (s.g. Fm-3m) with lattice constants of 4.004 Å and 4.829 Å respectively. NaAlH4 belongs to the I41/a (88) space group and presents a tetragonal lattice ACS Paragon Plus Environment


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

Page 16 of 32

with four unit formula per cell and lattice constants of 4,994 Å and 11.089 Å. Within the structure the anionic sublattice is organized in AlH4- clusters where the Al ions are tetrahedrically coordinated by four H atoms at a bond length value of 1.64 Å.

Table 1. Optimized ground state structures for the computed hydrides. Experimental data are reported in parenthesis for comparison. Phase

Space group and lattice constants

Atoms and Wickoff positions

x/a

y/b

z/c

LiH

Fm-3m (225) a= 4.004Å (4.090Å)[30]

Li 4a H 4b

0 0.5

0 0.5

0 0.5

NaH

Fm-3m (225) a= 4.829Å (4.910Å)[31]

Na 4a H 4b

0 0,5

0 0,5

0 0,5

NaAlH4

I41/a (88) a=4.994Å c=11.089Å (4.980Å, 11.148Å)[32]

Na 4b Al 4b H 16f

0 0 0.764

0.25 0.25 0.109

0.625 0.125 0.044

α Na3AlH6

P21 /n (14) a=5.356Å b=5.546Å c=7.711Å β=89.91° (5.402Å, 5.507Å, 7.725Å, 89.49°)[32]

Na 2b Na 4e Al 2a H 4e H 4e H 4e

0 -0.010 0 0.101 0.230 0.162

0 0.454 0 0.0478 0.329 0.266

0.5 0.253 0 0.216 0.544 0.936

LiNa2AlH6

Fm-3m (225) a=7.345Å (a=7.385Å)[21]

Na 8c Li 4b Al 4a H 24e

0.25 0.5 0 0.238

0.25 0.5 0 0

0.25 0.5 0 0

The α polymorph of Na3AlH6 has a monoclinic structure with 2 formula units per cell and predicted lattice constants of 5.356 Å ,5.546Å, 7.711 Å with the angle β=89.91°. In the lattice the Li ions are coordinated by six H ions in octahedral structures at a distance of 1.77 Å as well as the Na ions, octahedrically coordinated at a distance of 2.28 Å. The LiNa2AlH6 phase presents a simple fcc lattice with 4 formula units per cell and a calculated lattice constant of 7.345 Å. In the cell both the Li and Al atoms are coordinated by six H atoms at the vertex of an octahedron at a distance of 1.93 Å and 1.75 Å respectively.


Page 17 of 32

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


Starting from the calculated DFT energies of the phases, the voltage vs Li of the main conversion reactions of NaH, NaAlH4, Na3AlH6 and LiNa2AlH6 have been determined according to the equation 1. Results are reported in the Table 2. Concerning the NaAlH4 phase, four different reduction paths have been considered (e.g. Reactions R1-R4). The calculated voltages of the partial reduction of NaAlH4 to form Na3AlH6 or Na2LiAlH6 phases are very similar, 0.70 V and 0.73 V, respectively, nevertheless the formation of Na2LiAlH6 is slightly more favoured. The directconversions of NaAlH4 to give NaH (R3) or Na (R4) without formation of other intermediates are predicted to occur, as expected, at an electrochemical potential lower than the two reactions (R1) and (R2).This picture suggests a thermodynamically-driven multistep conversion for the NaAlH4 phase. In particular reactions (R2) is likely the firststepof the overall process.

Table 2. DFT Calculated voltages vs Li of the electrochemical conversion processes involving the NaAlH4, Na3AlH6 and LiNa2AlH6 phases. Conversion Reaction

Electrochemical potentials (V vs. Li)

(R1)

3 NaAlH4 + 6 Li = Na3AlH6 + 6 LiH + 2 Al

0.70

(R2)

2 NaAlH4 + 3 Li = LiNa2AlH6 + 2 LiH + Al

0.73

(R3)

NaAlH4 + 3 Li = NaH + Al + 3 LiH

0,68

(R4)

NaAlH4 + 4 Li = Na + Al + 4 LiH

0.62

(R5)

Na3AlH6+ Li = LiNa2AlH6+ Na

0.41

(R6)

Na3AlH6 + 3 Li = 3 NaH + Al + 3 LiH

0.61

(R7)

Na3AlH6 + 6 Li = 3 Na + Al + 6 LiH

0.53

(R8)

3/2 LiNa2AlH6 + 3/2 Li = 3 LiH + 1/2 Al + Na3AlH6

0.66

(R9)

LiNa2AlH6 + 3 Li = 2 NaH + Al + 4 LiH

0.64

(R10) LiNa2AlH6 + 5 Li = 2 Na + Al + 6 LiH

0.56

(R11) NaH + Li = Na + LiH

0.43



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

Page 18 of 32

Turning to Na3AlH6, the reaction thermodynamics of the sodium hexahydride suggests a favoured path through a direct conversion reaction (R5-R6) rather than the formation of the intermediate LiNa2AlH6 (R4). Between the two overall conversions, as expected the partial reaction to give NaH is thermodynamically more favoured in comparison to the deeper conversion to Na metal. The mixed Li-Na hexahydride is predicted to undergo to a conversion reaction (R8) to Na3AlH6 as intermediate product phase. In the view of the discussed partial conversion for the three sodium alanates, and also considering the predicted thermodynamics of conversion of NaH (R11), it is possible to suggest a complex fourstep reaction mechanism for the electrochemical lithium incorporation in NaAlH4-based electrodes. The sequence (R2)-(R8)-(R5)-(R11) is the whole reaction mechanism predicted from our ab initio thermodynamic evaluations on bulks. This reaction sequence occurs starting from NaAlH4 by forming subsequently LiNa2AlH6, Na3AlH6 and NaH as reaction intermediates before leading to a complete conversion to a mixture of metallic aluminium and sodium with lithium hydride. Partial identical (R8)-(R5)-(R11) and (R5)-(R11) reaction sequences are predicted to occur starting from LiNa2AlH6 and Na3AlH6 hexahydrides. It is interesting to observe that our DFT predictions imply that upon charge, starting from the fully reduced Na(s)+Al(s)+4LiH(s) mixture, the reverse mechanism may differ due to the possible occurrence

of

the

Na(s)

stripping

reaction.

Experimentally

the

reduction

reaction

Na(s)→Na+(solv)+e- occurs at 0.33 V vs Li [33]. Our DFT calculations predict for the same reaction a redox potential of 0.41 V vsLi obtained by computing the energetics at 0K of the process Li(s)+Na+→ Na(s)+Li+ in the gas phase corrected for the Na+ and Li+ solvation energies in organic carbonates [34-35]. Therefore our thermodynamic evaluations suggest that, once reduced to metallic Na(s), upon charge the electrode is expected to strip sodium instead of convert back through the reverse R11 reaction. This complex asymmetric reactivity may also be complicated by the unavoidably different charge transfer kinetics of the R11 conversion reaction compared sodium


Page 19 of 32

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


stripping. However the final net effect of this charge reaction mechanism may be the Na-depletion of the electrode and therefore the irreversible loss of capacity. On passing it should be mentioned that this analysis do not consider the possible occurrence of thermodynamic overpotentials due to a specific surface reactivity upon lithium incorporation. In fact thermodynamic overvoltagesin conversion reactions may play an important role, not only by driving the nanomorphology but also limiting/promoting specific reaction paths, as already shown in the case of oxides [36], phosphides [37]and, recently by us, for magnesium hydrides [38].

3.5.Mechanism comprehension: experimental in situ XRD measurements In order to demonstrate the electrochemical reactions mechanisms, in situ XRD have been carried out on NaAlH4 best performing sample, i.e. B15D5, and on the Na3AlH6_S15D0 and LiNa2AlH6_S15D0.The resolution of the collected patterns is rather poor due to several reasons: the poor crystallinity of the ball milled powders, the low intensity of the radiation emerging from the cell associated to the low scattering factors of the light-weight elements, the incident and scattering beam attenuation due to the beryllium window. Nevertheless the measurements are useful to a qualitative evaluation of the developing processes and identification of the involved phases. Figure 6 reports the XRD pattern evolution of NaAlH4_B15D5 upon lithium incorporationin a galvanostatic discharge. As previously observed, the pattern collected in the open circuit condition shows clearly that the sample contains some aluminum due to the milling with carbon that induces superficial hydrogen desorption. Upon discharge, a complex reaction mechanism can be outlined involving different intermediate phases. The NaAlH4 peaks monotonically decrease to 50% and 20% of the original intensity after incorporation of 1 and 2 lithium equivalents, respectively. After the incorporation of 2.75 Li eq the NaAlH4 diffraction intensity reduces to less than 10% of the original value and does not change thereafter. In parallel with the fading of the NaAlH4 peaks, diffraction lines assigned to Al appear



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

Page 20 of 32

and continuously increase, in addition to LiNa2AlH6 lines which instead reach a maximum intensity after the incorporation of 1.75-2 lithium equivalents. In fact, at deeper states of discharge also LiNa2AlH6 peaks start decreasing, at the advantage of the nucleation of metallic sodium. The Na peak at 29.3° appears after the incorporation of approximately 2.25 Li eq and its intensity rises monotonically down to full discharge. Besides these sharp diffraction lines also small peaks attributable to the Na3AlH6 appears after the incorporation of 1.5 lithium equivalents, respectively. LiH is not observed probably for the too low scattering factors of both lithium and hydrogen. No signature of NaH can be found in the whole sequence. At the end discharge cutoff potential, i.e.10mV, the alanates reduction is not complete. NaAlH4 has almost disappeared, while Na3AlH6 and LiNa2AlH6 signals are present in addition to diffraction lines of metallic aluminum and sodium.

a - NaAlH4

b c

e

d - Na3AlH6

b - Al e - Na c - LiNa2AlH6 f - LiAl b

d

c

f

x=3.62 x=3 x=2

x=1 a

28

a

30

32

a

34

36

38

a

b

b

40

42

44

46

48

x=0, OCV

50

2θ Figure 6. XRD pattern evolution of a NaAlH4_B15D5 electrode upon galvanostatic discharge in lithium cell.

It is worth noting that in the early stage of the discharge, very low intensity lines relative to the LiAl alloy can be detected: they appear after only 0.75 Li eq and soon stop increasing. It is possible that ACS Paragon Plus Environment

Page 21 of 32

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


the alloying process involves only the pristine superficial aluminum produced during the mechanochemical treatment, and not that displaced in the conversion reactions. This latter aluminum is probably embedded in a complex matrix made of alanates of various composition and LiH, which prevent its reaction thermodynamically less convenient then alanates conversion. Apparently a multistep conversion process occurs upon discharging NaAlH4 electrodes. However, before going more in details in the discussion of the overall reaction mechanism, in order to support the identification ofthe reactions involved in the NaAlH4 discharge, also in situ XRD experiments of Na3AlH6 and LiNa2AlH6phases have beenrecorded upon electrochemical lithium incorporation.

Na

a

b

Na

Al Al

Na3AlH6

l LiNa2AlH4 28

29

30

31

32

33

34

35

36

37

38

39

40

28

29

30

31

32

33

2θ

34 2θ

35

36

37

38

39

40

Figure 7. XRD pattern evolution of LiNa2AlH6_S15D0 (a) and Na3AlH6_S15D0 (b) electrodes upon galvanostatic discharge in lithium cells

Figures 7a and b report a selected enlargement of the pattern sequence collected during the Na3AlH6 and LiNa2AlH6 discharges. While the pristine alanates peaks intensity decrease monotonically, only metallic sodium lines grow. Apparently both reactions occur through an overall conversion mechanism to metals and LiH, without the formation of the intermediate NaH phase or the mutual interconversion between the two hexahydrides. Surprisinglynor Al or LiAl signals can be noticed. On the other hand the ratio between Al and Na is smaller in both Na3AlH6 and LiNa2AlH6compared to NaAlH4 and consequently the amount of metallic Al produced in the various conversion is ACS Paragon Plus Environment


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

Page 22 of 32

expected to be different. The complete absence of Al reflection observed in the figures 7a-b also suggests that it may be produced in a poorly crystalline or nanometric form. In summary, the conversion reactions observed by in situ XRD starting from both the sodium hexahydrides are: LiNa2AlH6 + 5 Li = 2 Na + Al + 6 LiH

(R7)


(R10)

From the electrochemical point of view, both these reactions occur below 0.3 V vs. Li (see figure 4). The corresponding experimental equilibrium potentials are therefore 200-300 mV smaller in comparison to all our thermodynamic evaluations for the possible conversion reactions of both hexahydrides (see previous section). In fact our predictions, summarized in the Table 2, range between 0.66-0.53 V and 0.61-0.41 V vs. Li for LiNa2AlH6and the Na3AlH6phases, respectively. It is likely that thermodynamic overpotentials,originated by the different possible interphase and surface micro-mechanism,play a crucial role in determining the reaction sequence [36-38]. In fact reactions (R7) and (R11) are not the most favored process from the thermodynamic point of view: our calculations predicted a multi-step process for both hexahydride following the (R8)-(R5)-(R11) and the (R5)-(R11) sequences starting from LiNa2AlH6 or Na3AlH6, respectively. As already mentioned the occurrence of thermodynamic overpotentials have been observed and discussed for different class of conversion materials (e.g. phosphides, oxides and hydrides) in order to rationalize the large voltage hysteresis typically observed between electrochemical lithium incorporation and de-incorporation. These overpotentials largely vary being their estimates in the range 150-400 V depending on ionicity of the chemical bond in the conversion material [1].Apparently, in the two cases of the sodium hexahydrides, these thermodynamic overvoltages are large enough to reverse the expected reaction sequence and promote the occurrence of the direct nucleation of metallic sodium besides the formation of other intermediates, especially NaH. A more detailed ab initio study of the surface reactivity of the various hydrides may clarify this point:


Page 23 of 32

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


however such study is beyond the scope of this communication and also of the adopted computational approach. Turning back to the conversion mechanism of the NaAlH4 it is possible to propose a whole reaction sequence in the view of the observed reactivity of the hexahydrides. The collected XRD evolution of the NaAlH4 phase upon discharge suggeststhe occurrence of at least three reactions: 2 NaAlH4 + 3 Li = LiNa2AlH6 + 2 LiH + Al

(R2)

3 NaAlH4 + 6 Li = Na3AlH6 + 6 LiH + 2 Al

(R1)

LiNa2AlH6 + 5 Li = 2 Na + Al + 6 LiH

(R10)

One may also not exclude the occurrence of the following two other reactions especially in the last stage of the discharge: NaAlH4 + 4 Li = Na + Al + 4 LiH

(R4)


(R7)

This reaction sequence describes a mechanism different from that reported by Latroche et al. [6]. In fact our proposed mechanismalso involves the nucleation of the Na3AlH6 as intermediate phase besides the mixed LiNa2AlH6 hexahydride. However the diffraction lines that highlight the formation of this second intermediate phase are weak and, once appeared, they do not vary much upon further lithium incorporation. This maysuggest that the conversion of NaAlH4 to Na3AlH6 is a competitive process, possibly affected by severe kinetic limitation,rather than a separate step of a multi-reaction conversion sequence. Also in this case the measured electrochemical equilibrium potentials are approximately 200-300 mV smaller in comparison with all thermodynamic predictions. Therefore the here proposed reaction mechanism is plausible from the thermodynamic point of view but implies the occurrence of thermodynamic overpotentials. In the first part of the electrochemical lithium incorporation, the thermodynamicoverpotentials decrease the equilibrium potential of the reaction (R2) without altering the overall reaction sequence expected on the basis of the thermodynamics. However, besides (R2), also reaction (R1) ACS Paragon Plus Environment


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

occurs, although thermodynamically unfavorable: it is likely activated by a competitive surface reactivity with comparable thermodynamic overpotentials. Once converted to the LiNa2AlH6 phase (reaction R2), the conversion process occurs directly nucleating metallic Na (R7) whereas the thermodynamics predicts the formation of NaH as intermediate phase (R8). Thus the experimental in situ XRD highlightsin the second part of the discharge an alteration of the expected thermodynamic reaction sequence,likely driven by thermodynamic overpotentials,in perfect agreement with the discharge of the pure LiNa2AlH6 hexahydride (see above).

Figure 8. XRD patterns collected on a NaAlH4_B15D5 electrode during the galvanostatic first recharge process in lithium cell

In the Figure 8 the in situ XRD results collected upon NaAlH4_B15D5 re-charge are shown. As suggested by the electrochemical data, see figure 7b, the NaAlH4_B15D5 conversion is partially


Page 24 of 32

Page 25 of 32

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


reversible. Metallic Al and Na as well as the Na3AlH6 and LiNa2AlH6 hexahydride peaks progressively decrease in intensity, whereas NaAlH4 diffraction lines grow. At the end of the charge step NaAlH4 diffraction lines recover around 45% of their original intensity and in parallel the hexahydride phases signature disappear, while the aluminum fingerprint is still marked. Metallic Al is present in the pristine NaAlH4_B15D5 as an impurity produced during the intensive mechanochemical activation treatment (see previous section). Nevertheless the (111) and (200) peaks at the end of the complete discharge-charge cycle are largely more intense then in pristine state. This indicates that part of the aluminum particles produced in the alanate conversion is apparently unable to be oxidized upon charge giving back the NaAlH4 structure. This evidence also matches well with the irreversible capacity observed in the PCGA discharge-charge test and the residual lithium equivalents kept into the electrode at the end of the first cycle (see figure 4b). Indeed sodium stripping upon charge could be an intrinsic cause of irreversibility.

4. Conclusions The study of the alanates as negative electrodes in lithium ion batteries is just at the beginning. The electrochemical activity of NaAlH4 has been confirmed and the effect of mechanochemical activation treatments outlined. Na3AlH6 and LiNa2AlH6 have been successfully prepared by high energy ball milling and their electrochemical conversion in lithium cell disclosed for the first time. The NaAlH4conversion reaction mechanism has been found to involve as intermediates both the hexhydridesLiNa2AlH6 and Na3AlH6, while their conversion proceed directly to metals and LiH. This reaction mechanism partially follows our thermodynamic predictions from ab initio calculations, thus suggesting the occurrence of thermodynamic overpotentials due to different possible interphase and surface micro-mechanisms. Our proposed mechanism differs from that illustrated by Latroche et al. [6] as we have also highlighted the nucleation of the Na3AlH6 as intermediate phase besides the mixed LiNa2AlH6 hexahydride.



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

The conversion reaction has been proven to be reversible from electrochemical and the electrode composition point of views. The XRD in situ demonstration of the conversion process reversibility is an outstanding result never reported before for hydride conversion process in lithium cells, and may focus even more interest in NaAlH4 as negative electrode in lithium ion batteries. Our optimized NaAlH4 sample is capable to give a discharge capacity value close to the theoretical one (1985mAhg-1) with a coulombic efficiency in the whole discharge-charge cycle of around 70%. Inefficiency can be the effect of the theoretical concurrent sodium stripping reaction upon charge and consequence electrode’s sodium depletion. The only way to prevent this would be avoiding the deep electrode discharge. Finally inefficiency can be also ascribed to possible technological issues related to electrode pulverization [39, 40] likely due to the large volume expansion experienced upon conversion. In order to overcome the mechanical vulnerability due to particles volume variations, we are presently studying the alanates confinement in carbonaceous matrices following the successful strategy already proven by Oumellal et al. for MgH2[5]. In factit is likely that an improved electrode morphology may improve the cyclability and sodium alanates could become candidates as new high energy, low cost and sustainable negative electrodes for lithium ion batteries.

Acknowledgments This study was carried in the framework of the FIRB 2010 Futuro in Ricerca project “Idruri come anodi ad altacapacità per batterielitio-ione” (RBFR10ZWMO), supported by Italian Minister for University and Research. The authors would like to thank the ELETTRA synchrotron radiation facility (MCX beamiline) for the support of this study through the beam-time grants 20140458 and 20150410. The authors would like to acknowledge Simona Forgia for her contribution in experiments realization.


Page 26 of 32

Page 27 of 32

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


References [1]

Oumellal Y., Rougier A. ,Nazri G.A., Tarascon J.-M. , Aymard L., Metal Hydrides for Lithium-ionBatteries. Nature Materials 2008, 7, 916-921.

[2]

Brutti S., Mulas G., Piciollo E., Panero S., Reale P., MagnesiumHydrideas a High Capacity Negative Electrode for LithiumIonBatteries. Journal of Materials Chemistry, 2012, 22, 1453114537.

[3]

Meggiolaro, D., Gigli, G., Paolone, A., Vitucci, F., Brutti, S., Incorporation of Lithium by MgH2: An AbInitioStudy. J.Phys.Chem.C. 2013, 117, 22467-22477.

[4]

Oumellal, Y., Courty, M., Rougier, A., Nazri, G.A., Aymard, L., Electrochemical Reactivity of Magnesium Hydride Toward Lithium: New Synthesis Route of Nano-Particles Suitable for Hydrogen Storage. International Journal of Hydrogen Energy, 2014, 39, 5852-5857.

[5]

Oumellal Y., Zlotea C., Bastide S., Cachet-Vivier C., Léonel E., Sengmany S., Leroy E., Aymard L., Bonnet J.-P., Latroche M., Bottom-up Preparation of MgH• Nanoparticles with Enhanced Cycle Life Stability During Electrochemical Conversion in Li-ion Batteries. Nanoscale, 2014, 6, 14459-14466.

[6]

Teprovich J.A., Zhang J., Colón-Mercado H., Cuevas F., Peters B., Greenway S., Zidan R., Latroche M., Li-driven Electrochemical Conversion Reaction of AlH3, LiAlH4, and NaAlH4. Journal of Physical Chemistry C, 2015, 119, 4666-4674.

[7]

Silvestri L., Forgia S., Farina L., Meggiolaro D., Panero S., La Barbera A., Brutti S., Reale P., Lithium Alanates as Negative Electrodes in Lithium-Ion Batteries. ChemElectroChem, 2015, 2, 877-886.

[8]

Eisenreich N., Keßler A., Koleczko A., Weiser V., On the Kinetics of AlH3 Decomposition and the Subsequent Al Oxidation. International Journal of Hydrogen Energy International Journal of Hydrogen Energy, 2014, 39, 6286-6294.

[9]

Graetz J., Reilly J.J., Thermodynamics of the α, β and γ Polymorphs of AlH3.Journal of Alloys and Compounds, 2006, 424, 262-265. ACS Paragon Plus Environment


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

Page 28 of 32

[10] Dilts J.A., Ashby E.C., A Study of the Thermal Decomposition of Complex Metal Hydrides. Inorganic Chemistry, 1972, 11, 1230-1236. [11] Jeloaica L., Zhang J., Cuevas F., Latroche M., Raybaud P., ThermodynamicProperties of Trialkali (Li, Na, K) Hexa-alanates: A Combined DFT and ExperimentalStudy.J. Phys. Chem. C, 2008, 112, 18598–18607. [12] Huot J., Boily S., Guther V., Schulz R., J. Synthesis of Na3AlH6 and Na2LiAlH6 by MechanicalAlloying. Alloys Compounds, 1999, 383, 304–306. [13] Orimo S., NakamoriY.,Eliseo J.R., Zuttel A., Jensen C.M., ComplexHydrides for Hydrogen Storage.Chem. Rev., 2007, 107, 4111−4132. [14] Jain I.P., Jain P., Jain A., NovelHydrogen

Storage

Materials: A Review of

LightweightComplexHydrides.Journal of Alloys and Compounds, 2010, 503, 303–339. [15] Gomes S., Renaudin G., Hagemann H., Yvon K., Sulic, M.P., Jensen C.M., Effects of Milling, Doping and Cycling of NaAlH4 Studied by VibrationalSpectroscopy and XrayDiffraction.J. Alloys Compounds, 2005, 390, 305-313. [16] Bureau J.-C., Z.Amri Z., Claudy P., Létoffé J.-M., Comparative Study of Lithium and SodiumHexahydrido- and Hexadeuterido-Aluminates. I - IR and RamanSpectra of Li3 - and Na3AlH6 and Li3AlD6.Mat. Res. Bull., 1989, 24, 23-31. [17] Kawamura T., Okada S., Yamaki J., DecompositionReaction of LiPF6-Based Electrolytes for LithiumIonCells.J. Power Sources, 2006, 126, 547-554. [18] Perdew J., Burke K., Ernzerhof M., GeneralizedGradientApproximation Made Simple.Phys. Rev. Lett., 1996, 77, 3865-3868. [19] Peles A., Alford J.A., Ma Z., Yang L., Chou M.Y., First-PrincipleStudy of NaAlH4 and Na3AlH6ComplexHydrides.Phys Rev. B., 2004, 70, 165105-165112. [20] Belskii K.V., Bulychev B.M., Golubeva, A.V., A RepeatedDetermination of the NaAlH4Structure.ZhurnalNeorganicheskoiKhimii., 1983, 28 (10), 1528-1529.


Page 29 of 32

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


[21] Brinks H.W., Hauback B.C., Jensen, C.M., Zidan R., Synthesis and Crystal Structure of Na2LiAlD6.Journal of Alloys and Compounds, 2005, 392, 27-30. [22] Opalka S.M., Løvvik O.M., Brinks H.W., Saxe P.W., Hauback, B.C., IntegratedExperimentalTheoreticalInvestigation of the Na-Li-Al-H System. Inorg. Chem., 2007, 46, 1401-1409. [23] Hauback

B.C.,

Brinks

H.W.,

Jensen

C.M.,

Murphy

K.,

Maeland

A.J.,

NeutronDiffractionStructureDetermination of NaAlD4. J.Alloys Compd., 2003, 358, 142-145. [24] Van Setten M.J., Popa V.A., de Wijs G.A., Brocks G., Electronic Structure and Optical Properties of Lightweight Metal Hydrides. Phys Rev. B., 2007, 75, 035204-035207. [25] Donohue J. The Structure of Elements. New York : Wiley, 1974. [26] Oumellal, Y., Zaïdi, W., Bonnet, J.-P., Cuevas, F., Latroche, M., Zhang, J., Bobet, J.-L., Rougier, A., Aymard, L., Reactivity of TiH2Hydride with LithiumIon: Evidence for a New Conversion Mechanism. International Journal of Hydrogen Energy., 2012, 37, 7831-7835. [27] Vitucci F.M., Paolone A., Brutti S., Munaò D., Silvestri L., Panero S., Reale P., H2 Thermal Desorption

and

Hydride

Conversion

Reactions

in

LiCells

of

TiH2/C

AmorphousNanocomposites. J.AlloysCompd, 2015, 465, S239-S241. [28] Verma P., Marie P., Novak P., A Review of the Fatures and Analyses of the Solid ElectrolyteInterphase in Li-ionBatteries. Electrochimica Acta, 2010,55,6332-6341. [29] Besenhard J.O., Yang J., Winter M., Will Advanced Lithium-AlloyAnodesHave a Chance in Lithium-ionBatteries? Journal of Power Sources, 1997, 68, 87-90. [30] Stephens D.R., Lilley E.M., J. Compressions of IsotopicLithiumHydrides. Appl. Phys., 1968, 39, 177-180. [31] Martins J.L., Equations of State of AlkaliHydridesat High Pressures. Phys.Rev B., 1990, 41, 7883-7886. [32] Ozolins

V.,

Majzoub

E.H.,

Udovic

T.J.,

Electronic

Structure

and

RietveldRefinementParameters of Ti-dopedSodium Alanates. J Alloys Compd. 2004, 375, 110. ACS Paragon Plus Environment


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

[33] Hongyan Kang, Yongchang Liu, Kangzhe Cao, Yan Zhao, Lifang Jiao, Yijing Wang, Huatang Yuan. Update on anode materials for Na-ion batteries. J. Mater. Chem. A, 2015, 3, 17899 [34] Daniel M. Seo, Stefanie Reininger, Mary Kutcher, Kaitlin Redmond, William B. Euler, Brett L. Lucht. Role of Mixed Solvation and Ion Pairing in the Solution Structure of Lithium Ion Battery Electrolytes. J. Phys. Chem. C 2015, 119, 14038−14046 [35] Mehdi Shakourian-Fard, Ganesh Kamath, Kassiopeia Smith, HuiXiong, Subramanian K. R. S. Sankaranarayanan. Trends in Na-Ion Solvation with Alkyl-Carbonate Electrolytes for Sodium-Ion Batteries: Insights from First-Principles Calculations. J. Phys. Chem. C 2015, 119, 22747−22759 [36] Dalverny A.L., Filhol J.S., Doublet, M.L., Interface Electrochemistry in Conversion Materials for Li-ionBatteries. J. Mater. Chem., 2011, 21, 10134-10142. [37] Khatib R., Dalverny A.L., Sauban M., Gaberscek M., Doublet M.L., Origin of the Voltage Hysteresis in the CoP Conversion Material for Li-ionBatteries. J. Phys. Chem. C,. 2013, 117, 837-849. [38] Meggiolaro D., Gigli G., Paolone A., Reale P., DoubletM.L.,Brutti, S., Origin of the Voltage Hysteresis of MgH2Electrodes in LithiumBatteries. J.Phys.Chem.C. 2015, 119, 17044-17052. [39] W.-J., Zhang., A Review of the Electrochemical Performance of AlloyAnodes for LithiumionBatteries. Journal of Power Sources, 2011, 196, 13-24. [40] Nazri G.A., Pistoia G. Lithium Batteries. s.l. : Springer, 2003.B


Page 30 of 32

Page 31 of 32

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


Tables Table 1. Optimized ground state structures for the computed hydrides. Experimental data are reported in parenthesis for comparison. Table 2. DFT Calculated voltages vs Li of the electrochemical conversion processes involving the NaAlH4, Na3AlH6 and LiNa2AlH6 phases.

Captions Figure 1. XRD pattern and TEM images of (a) B0 pristine sample; (b) sample B15, milled 15h alone ; (c) alanate B15D5, milled 15h+5h with Super P. Figure 2. FTIR spectra of NaAlH4 samples as a function of the mechanochemical treatment. Figure 3. XRD pattern of LiNa2AlH6_S15 (a) and Na3AlH6_S15 (b); FTIR spectra (c) of the hexahydrides alanates in comparison; TEM images collected for the LiNa2AlH6_S15 (d) and Na3AlH6_S15 (e) samples. Figure 4. Voltage profiles upon first PCGA cycle of the NaAlH4_BxDy samples. (a) comparison between the pristine sample and the 15h milled alanate, with a simple Super P hand-grinding treatment, (b) effect of high energy ball milling with Super P on the B15 sample. Figure 5. Voltage profiles upon first PCGA cycle of the Na3AlH6_S15 (a) and LiNa2AlH6_S15 samples in comparison to NaAlH4_B15. All samples have been hand-ground with Super P. Figure 6. XRD pattern evolution of a NaAlH4_B15D5 electrode upon galvanostatic discharge in lithium cell. Figure 7. XRD pattern evolution of LiNa2AlH6_S15D0 (a) and Na3AlH6_S15D0 (b) electrodes upon galvanostatic discharge in lithium cells Figure 8. XRD patterns collected on a NaAlH4_B15D5 electrode during the galvanostatic first recharge process in lithium cell



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

Table of content


Page 32 of 32

Reactivity of Sodium Alanates in Lithium Batteries - The Journal of

Recommend Documents