Journal of Power Sources 297 (2015) 551e555
Contents lists available at ScienceDirect
Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
Cyclic siliconenitrogenesilicon core derived silylamidoemagnesium
compounds for magnesiumebattery electrolytes with improved
oxidation stability
Basab Roy*, Dong Young Kim**, Younhee Lim, Seok-Soo Lee, You-Hwan Son,
Seok-Gwang Doo
Energy Laboratory, Samsung Advanced Institute of Technology, Samsung Electronics Co., 130 Samsung-ro, Suwon, Gyeonggi-do, 443-803, Republic of Korea
h i g h l i g h t s
Use of Cyclic SieNeSi ring core containing magnesium salt is reported.
Use of these salts furnish high oxidation stability (more than 3 V vs Mg/Mg2þ).
Theoretical calculation provides insight into the ionic species present in electrolytes.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2015
Received in revised form
29 July 2015
Accepted 2 August 2015
Available online xxx
Use of a silylamidoemagnesium chloride and a magnesium bis-silylamide where the SieNeSi core is
crafted in a cyclic structure, toward magnesium battery application is demonstrated. Reaction between
prepared silylamidoemagnesium compounds and a Lewis acid in tetrahydrofuran solvent furnishes the
desired electrolytes. Embedding the SieNeSi core into cyclic structure enhances the oxidation potentials
of the ‘in situ’ electrolytes (without any purification) against Mg/Mg2þ compare to the acyclic analogue.
Theoretical calculation provides insight into the ionic species present in the electrolyte systems.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Cyclic
Silylamidoemagnesium
Transmetallation
Anionic complex
Oxidation stability
1. Introduction
Energy materials with high energy density but low cost have
been a priority research focus as next generation energy storage
system [1]. Mg is relatively abundant, cheap, safe, lightweight, and
environmentally friendly. But development of such Mg battery
suffers from lack of high-performance electrolyte systems [2].
Finding a suitable electrolyte with a wide electrochemical window
and good Mg depositionddissolution reversibility is a key issue in
developing high-energy rechargeable Mg batteries.
Magnesium cannot be reversibly deposited from solutions of
* Corresponding author.
** Corresponding author.
E-mail addresses: [email protected] (B. Roy), [email protected]
(D.Y. Kim).
http://dx.doi.org/10.1016/j.jpowsour.2015.08.003
0378-7753/© 2015 Elsevier B.V. All rights reserved.
simple magnesium salts such as Mg(X)2 (X ¼ ClO4 or BF4, PF6) in
conventional organic solvents used in non-aqueous battery electrolyte (e.g., acetonitrile, organic esters or popular carbonate based
solvents used in lithium batteries like dimethyl carbonate, propylene carbonate) as they are reduced to tenacious passivation
films on Mg metal [3,4]. Bivalent magnesium cation cannot penetrate the barrier formed by such organic or inorganic films. Magnesium can only be electrodeposited from ethereal solutions of
Grignard reagents (RMgX, R ¼ organic alkyl or aryl group and
X ¼ halide like Cl or Br) but they suffer from poor anti-oxidation
capabilities [5,6]. Amidomagnesium halides (also known as
Hauser base) in ether solutions, on the other hand, can exert better
anodic stability than that of Grignard/ether solutions due to higher
polarity of magnesiumenitrogen bond compare to magnesiumecarbon bond [7]. Therefore anodic stability of 2.3 V vs. Mg/
Mg2þ was obtained in a solution at 1.2:1 ratio for EtMgCl and
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B. Roy et al. / Journal of Power Sources 297 (2015) 551e555
pyrrolidine [8]. Modifying CeNeC unit of amidomagnesium halide
to SieNeSi unit of hexamethyldisilazide magnesium chloride
(HMDSMgCl), an enhanced oxidation stability of 2.5 V was realized.
The SieNeSi unit is able to delocalize the electron density on the
nitrogen [9].
When organo/amido-magnesium halides are mixed with AlCl3,
a Lewis acid, transmetallation reaction changes anionic Mg complexes to anionic Al complexes which are more resistive toward
oxidation than their predecessors. Using this knowledge, Toyota
researchers mixed HMDSMgCl in THF together with AlCl3 Lewis
acid, in their way to develop a Mg/S battery [10]. The optimum
electrochemical performance of the electrolyte was achieved when
the ratio of HMDSMgCl: AlCl3 was 3:1. Unlike the cases with phenyl
halo-aluminates reported by D. Aurbach et al. [11] the voltage
stability did not improve and was equal to that of HMDSMgCl/THF.
They crystallized the electrolyte by slow diffusion of hexane. The
crystal was identified by single crystal X-ray diffraction as
[Mg2(m-Cl)3$6THF](HMDSAlCl3). Re-dissolving the crystallized salt
in THF, an enhanced oxidation stability of 3.2 V vs. Mg/Mg2þ was
observed. Nevertheless, such electro-active species was only obtained via formation of high quality single-crystals, which might be
difficult to produce on a large scale. Such purification process
stands as a bottleneck toward commercial application.
In this work we report preparation and electrochemical investigation of a silylamidoemagnesium chloride and a magnesium bissilylamide where the SieNeSi core is crafted in a cyclic structure of
[2, 2, 5, 5-Tetramethyl-1, 2, 5-azadisilolidine (TMAS)] (Fig. 1).
Heterocyclic 5-membered ring of TMAS anion leads to a less
embedded SieNeSi core structure with a slightly weaker SieNeSi
bond strength, compare to HMDS anion (Fig. 2). This structural
characteristic of TMAS might increase the stabilization of
MgeTMAS complexation, decreasing the probability of existence of
monomeric TMAS anion (vide infra) [12].
The Houser base (TMASMgCl) was prepared by reaction between equimolar 2, 2, 5, 5-Tetramethyl-1, 2, 5-azadisilolidine
(TMASeH) and ethyl magnesium chloride (EtMgCl) in THF. The in
situ prepared TMASMgCl/THF electrolyte exhibited an oxidation
potential of 2.9 V vs Mg/Mg2þ which was higher than that of
HMDSMgCl/THF solution reported earlier. When AlCl3 Lewis acid
was mixed with TMASMgCl in THF, oxidation potentials of in situ
prepared electrolytes were elevated to more than 3 V. Reversible
Mg deposition and dissolution was also obtained from these in situ
electrolyte solutions. This is a major improvement as our system
does not require any purification by crystallization and hence can
be used conveniently for large scale application. During the course
of our work, ZhaoeKarger et al. reported electrochemical performance of in situ generated non-nucleophilic electrolytes from the
reaction between magnesium-bis (hexamethyldisilazide) and AlCl3
in different aprotic solvents [13]. Thereafter, we were curious to
examine the electrochemical behaviour of the TMAS containing
magnesiumebis (cyclicesilylamido) compound together with AlCl3
Lewis acid and extended our effort to synthesize and study the
same. TMASMgCl was conveniently converted to Magnesium-bis
(2, 2, 5, 5-Tetramethyl-1, 2 ,5-azadisilolidine) [(TMAS)2Mg], using
Fig. 1. Structures of cyclic silylamido Houser base (TMASMgCl) and magnesium bis(silylamide) (TMAS)2Mg.
Fig. 2. Optimized structures of HMDS and TMAS at the M062X/6-311þG** level using
Gaussian 09 suite program.
dioxane precipitation method [14]. Electrolytes were synthesized
by drop wise addition of the AlCl3 THF solution, on different basee
acid ratios, to previously prepared THF solutions containing
appropriate amount of (TMAS)2Mg base. Use of (TMAS)2Mg instead
of TMASMgCl with AlCl3 would lead to different equilibrium position in the transmetallation reaction, thus changes in electrochemical properties may be realized.
2. Experimental procedures and calculation methods
2.1. Preparation methods of electrolyte solutions
TMAS-H (98%) was purchased from HANCHEM and was freshly
distilled prior to use. EtMgCl (2 M in THF), Tetrahydrofuran
(anhydrous, 99.9%, inhibitor-free) and AlCl3 (99.99%) was purchased from SigmaeAldrich and was used without further purification. All reactions were performed inside argon-filled glove box
with oxygen and moisture content less than 1 ppm.
2.1.1. Preparation of TMASMgCl in THF and (TMAS)2Mg
For detailed preparation methods see supplementary
information.
2.1.2. Preparation of AlCl3 in THF solution
A dry 250 ml 2-neck double jacket reaction flask connected to a
chiller and fitted with a powder addition funnel was charged with
anhydrous THF (50 mL) and cooled to 0 C. AlCl3 (3.33 g) was added
from the powder addition funnel at a slower rate to avoid instant
temperature hike due an exothermic reaction between THF and
AlCl3. After complete addition, the mixture was allowed to warm
slowly to room temperature finally resulting a clear solution [15].
(TMASMgCl)m(AlCl3)n in THF and (TMAS)2Mgez(AlCl3) in THF
solutions were prepared separately, by dropwise addition of AlCl3/
THF solution in appropriate ratio into the TMASMgCl/THF and
(TMAS)2Mg in/THF solutions at 15 C over 10 min. The solutions
were then warmed to 25 C over a 30 min period and stirred further
for additional 6 h.
2.2. Electrochemical measurement procedures and apparatus
All the measurements were performed at room temperature
(25 ± 2 C). The electrochemical measurements were performed
using a Solartron model 1286 electrochemical interface controlled
by the Corrware program (Scribner, Inc.). Cyclic voltammograms
(CVs) were conducted in three-electrode cells inside the argonfilled glove box. The working electrode was a Pt or Au QCM electrode (ICM incorporated, Electrode Diameter: 0.201'', Electrode
Material:100 Å Ti, 1000 Å Pt/Au, Mounted & Bonded, electrode
area ¼ 0.2 cm2). A piece of freshly polished Mg strip and Mg wire,
were used as the counter and the reference electrode respectively.
B. Roy et al. / Journal of Power Sources 297 (2015) 551e555
2.3. Theoretical calculation to determine electrochemical window
We performed ab initio calculations using the Gaussian 09 suite
of programs [16]. The molecular geometries were optimized using
density functional theory with the M062X functional [17] and the
6-311þG** basis set. Frequency analysis was performed to confirm
the structures as minima. To evaluate the energies in the condensed
phase, single-point energy calculations were performed using selfconsistent reaction field theory with the isodensity surface polarized model. The dielectric constant of THF (7.58) was used for the
condensed phase calculations.
Transmetallation reaction changes the species which are most
susceptible to oxidation, from anionic Mg complexes to anionic Al
complexes (SI Fig. 1). HOMO energy levels of transmetallation
products [TMASnAlClm] depend on the ratio of n/m.
To evaluate oxidation potential of an electrolyte against Mg/
Mg2þ (Vox_Mg), it is imperative that Vox_Mg of all anionic species
present in the electrolyte should to be calculated. Because, electrolyte oxidation occurs by losing electrons from the HOMO of
anions or solvents present in the electrolyte. Oxidation potentials
are calculated from the equation: Vox_Mg ¼ (IE1 þ DGred/2)/F where
F is the Faraday constant (1 eV/V), IE1 is the first ionization free
energies, and DGred is the Mg/Mg2þ reduction free energies. We
used linear relationship between HOMO energies at the MO62X/6311þG** level and IE1 values in the THF phase in order to evaluate
IE1 values [10]. It is to be noted that Mg-deposition is influenced by
the microstructures of regions interacting with adsorbed species.
The deposition paths via Mg-Cl complexes are known to be dominant in the RMgCl þ AlCl3/THF system [18]. In an initial single
electron reduction process, MgeCls on the Mg surface would be
formed (SI Fig. 2). This intermediate species leads to breaking of the
MgeCl bond forming a reduced Mg atom on the Mg metal surface
via a second electron transfer. Neutral MgCl2 or cationic MgClþ aid
the process releasing energetically stable anionic MgCl
3 or neutral
MgCl2 complexes in solution [19].
3. Results and discussions
A typical cyclic voltammogram of 0.25 mol/L TMASMgCl in THF
on platinum electrode is shown in Fig. 3. The voltammogram presents a typical overpotential-driven nucleation/growth current
loop, indicating that electrodeposition of magnesium on the electrode requires overpotential to initiate the nucleation and subsequent growth of magnesium [20].
The electrolyte decomposition potentials, (defined as the potential in which iox > 20 mA/cm2) is determined by a linear sweep
Fig. 3. Typical cyclic voltammogram on Pt electrode in 0.25 mol/L TMASMgCl/THF at
20 mV/s; Inset- Linear sweep voltammogram of 0.25 mol/L TMASMgCl/THF at 10 mV/s.
553
voltammetry at a slower scan rate of 10 mV/s (Fig. 3 inset) [21]. It is
observed that TMASMgCl in which the SieNeSi core is a part of a
cyclic structure exhibits an anodic stability of 2.9 V.
To enhance the electrochemical performance of TMASMgCl/
THF electrolyte, we investigated its reactivity with a Lewis acid,
AlCl3. Cyclic voltammograms of each of the prepared electrolyte
(0.25 mol/L in THF), with various bases to acid ratios, on platinum
working electrode are represented in Fig. 4. We found that
anodic stability of 3.2 V and 3.3 V could be achieved using a base
to acid ratio of 2:1 [2(TMASMgCl)e(AlCl3)] and 3:1
[3(TMASMgCl)e(AlCl3)] respectively (Fig. 4, Inset A).
Complexation with Lewis acid increases the amount of ionic
species in the solution; consequently more than [10] fold increase
in current density is also observed. Addition of Lewis acid also
reduced the overpotential for magnesium deposition (Fig. 4 Inset
B). The magnesium deposition e dissolution cycling efficiencies for
the electrolytes were calculated based on the ratio between the
integration of the magnesium deposition peaks and the integration
of magnesium dissolution peaks in the cyclic voltammograms (CVs)
[21]. The magnesium deposition e dissolution cycling efficiency for
the base (TMASMgCl/THF) was calculated to be 51% which was
improved further to 66% and 69% upon addition of AlCl3 Lewis acid
in 2:1 and 3:1 base to acid ratio, respectively.
The most susceptible components toward oxidation in
TMASMgCl electrolytes are anionic [TMASnMgClm] species, for
which calculated Vox_Mg values are 4.9, 3.1, 3.0, and 2.7 V for n: m
ratios of 0:3, 1:2, 2:1 respectively. Similar species in
TMASMgCleAlCl3 electrolytes are transmetallated anionic
[TMASnAlClm]e species for which calculated Vox_Mg values 5.2, 3.4,
3.3, and 3.0 V for n:m ratios of 0:4, 1:3, 2:2, and 3:1 respectively
(Fig. 5A).
Although the Vox_Mg of TMAS is lower than those of TMASeMg
anionic complexes, it rarely exists in the TMASMgCl electrolyte as a
free monomer [12]. Therefore out of all equilibrated anionic
[TMASnMgClm]e species present in TMASMgCl/THF solution, the
most susceptible species to oxidation is TMAS3Mge (Vox_Mg 2.7 V;
experimental anodic stability of TMASMgCl/THF is 2.9 V). So,
anionic TMAS3Mge can determine the oxidation limit of
TMASMgCl/THF electrolyte.
3TMASMgCl / (TMAS)3Mge þ [Mg2(m-Cl)3]þ (Schlenk equilibrium) [22].
Fig. 4. Typical cyclic voltammogram on Pt electrode in 0.25 mol/L each of TMASMgCl/
THF (Black); [2(TMASMgCl)e(AlCl3)]/THF (Red); [3(TMASMgCl)e(AlCl3)]/THF (Blue),
Scan rate for all cyclic voltammograms are 20 mV/s; Inset A- Linear sweep voltammogram (scan rate 10 mV/s) in 0.25 mol/L each of [(TMASMgCl)e(AlCl3)]/THF (Black);
[2(TMASMgCl)e(AlCl3)]/THF (Red) and [3(TMASMgCl)e(AlCl3)]/THF (Blue); Inset BExpansion of voltammograms between 1.25 V and 0.25 V. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
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B. Roy et al. / Journal of Power Sources 297 (2015) 551e555
Fig. 6. Typical cyclic voltammogram on Au electrode in 0.25 mol/L each of
[2(TMASMgCl)e(AlCl3)]/THF
(Black);
[(TMAS)2MgeAlCl3]/THF
(Red);
[(TMAS)2Mge2AlCl3] (Blue), Scan rate for all cyclic voltammograms are 20 mV/s, Inset
- Linear sweep voltammogram (Au working electrode, scan rate 10 mV/s) in 0.25 mol/L
each of 2(TMASMgCl)e(AlCl3)/THF (Black); [(TMAS)2MgeAlCl3]/THF (Red);
[(TMAS)2Mge2AlCl3] (Blue). (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
Fig. 5. (A) Calculated Vox_Mg values of Cle, TMASe, [TMASnMgClm]e and
[TMASnAlClm]e anions; (B) Representative HOMOs of TMASe, [TMASnMgClm]e and
[TMASnAlClm]e species.
In TMASMgCleAlCl3/THF mixtures, the experimental oxidation
potentials
varied
depending
on
the
mixing
ratio.
3(TMASMgCl)e(AlCl3)/THF electrolyte elicited highest oxidation
potential of 3.3 V, which is isoenergetic to the calculated Vox_Mg of
[TMASmAlCln]e species (For m ¼ 1, n ¼ 3 and m ¼ 2, n ¼ 2 are 3.4
and 3.3 V respectively). Tripositive aluminum cation has higher
electron withdrawing power than bivalent magnesium cation
which is also reflected in the calculated HOMOs (Fig. 5B). Therefore
TMASAlCl
3 or ðTMASÞ2 AlCl2 anionic species which are formed by
transmetallation reactions, might regulate the oxidation limit of
TMASMgCl/THF electrolyte.
þ
TMASMgCl þ AlCl3 / TMASAlCl
3 þ MgCl
TMASAlCl
3 þ TMASMgCl / ðTMASÞ2 AlCl2 þ MgCl2
MgCl2 þ MgClþ / [Mg2(m-Cl)3]þ
We also examined the electrochemical performances of
(TMAS)2Mg derived electrolytes mixed with AlCl3 in different Lewis
acidebase ratios. Fig. 6 represents the typical voltammograms of in
situ generated THF solutions with the base and acid ratio of 1: 1 and
1: 2 (denoted as [(TMAS)2MgeAlCl3] and [(TMAS)2Mge2AlCl3],
respectively) on Au working electrode. For convenient comparison,
cyclic voltammogram of [2(TMASMgCl)eAlCl3]/THF electrolyte
against Au working electrode was also recorded [23].
We observed that both [2(TMASMgCl)eAlCl3]/THF and
[(TMAS)2MgeAlCl3]/THF exhibited anodic stability of 2.7 V whereas
the same for [(TMAS)2Mge2AlCl3] was recorded to be 2.9 V against
Au working electrode (Fig. 6 inset). Current density of
[2(TMASMgCl)eAlCl3]/THF electrolyte was comparable to the current density recorded with [(TMAS)2Mge2AlCl3] where the former
has a base to acid ratio of 2:1 but later one is composed of 1:2 base
to acid ratio. Stability of by-products or the anions of magnesium
species are responsible for the electrochemical stability of the in
situ generated electrolyte [24]. Mixing TMAS2Mg with AlCl3 in THF
solution results in equilibrium distribution of neutral as well as
ionic species through transmetallation reactions d
2(TMAS)2Mg þ 4AlCl3 / [TMASAlCl3]e þ 3TMASAlCl2
þ [Mg2(m-Cl)3]þ
2[TMASAlCl3]e þ (TMAS)2Mg / 2[(TMAS)2AlCl2]e þ MgCl2
TMASAlCl2 þ (TMAS)2Mg / [(TMAS)3AlCl]e þ MgClþ
Five consecutive cyclic voltammograms of [(TMAS)2Mge2AlCl3]
where magnesium depositionedissolution cycling where efficiency
each cycle was recorded to be more than 90% is represented in
Fig. 7.
4. Conclusions
In this work we demonstrated preparation and electrochemical
stability of a silylamidoemagnesium chloride (TMASMgCl) where
the SieNeSi core is crafted in a cyclic structure of 2, 2, 5, 5Tetramethyl-1, 2, 5-azadisilolidine. Embedding the SieNeSi core
into cyclic structure enhanced the oxidative stability compare to
the acyclic analogue (HMDSMgCl). Addition of AlCl3 Lewis acid
imparts transmetallation reaction and the resulting anionic
aluminium complexes, being more oxidation resistant, increases
Fig. 7. Cyclic voltammograms of 0.25 mol/L [(TMAS)2Mge2AlCl3]/THF on Au electrode
at 20 mV/s scan rate; Inset: plot of cycling efficiency vs number of cycles calculated
from the cyclic voltammograms.
B. Roy et al. / Journal of Power Sources 297 (2015) 551e555
the electrochemical oxidation window to 3.3 V vs Mg/Mg2þ. This
electrolyte can be stored over months under inert condition (SI
Fig. 3). We also prepared and tested the corresponding magnesium bis-silylamide compound (TMAS)2Mg. It is to be noted that
bisamide magnesiums are more stable at room temperature than
Grignard compounds and it is practically easy to handle. Importantly, none of our electrolyte preparations require any purification
step, so facilitates large scale production process. Theoretical
calculation provided insight into the ionic species present in the
electrolyte system.
Acknowledgement
The authors thank members of Energy Laboratory at SAIT for
useful comments and discussions. SAIT provided funding, all resources and supports to carry out the research.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jpowsour.2015.08.003.
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