A novel electrolyte system without a Grignard reagent for rechargeable magnesium batteries

Article abstract of DOI:10.1039/c2cc35857c

A new phenolate-based magnesium ion conducting electrolyte is prepared. The electrolyte exhibits air insensitive character and excellent electrochemical performances which make it highly promising for advanced rechargeable Mg battery systems.

Full text of DOI:10.1039/c2cc35857c

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COMMUNICATION
A novel electrolyte system without a Grignard reagent for rechargeable
magnesium batteriesw
ab
ab
Fei-fei Wang,z Yong-sheng Guo,z Jun Yang,*ab Yanna Nuliab and Shin-ichi Hiranob
Received 13th August 2012, Accepted 12th September 2012
DOI: 10.1039/c2cc35857c
0
organohaloaluminate salts MgðAlCl4ꢀnRn Rn00 Þ2 (R, R0
=
A new phenolate-based magnesium ion conducting electrolyte is
0
alkyl or aryl groups, n0 + n00 = n) in ethereal solvents. The
optimized composition of the Mg(AlCl2BuEt)2 complex
(called first generation electrolyte)3 and the AlCl3–(PhMgCl)2
complex (the second generation electrolyte)11 in THF solution
exhibited an improved anti-oxidation capability (respectively,
2.4 V and 3.3 V vs. Mg RE (reference electrode)) and reversible
Mg deposition–dissolution characteristics, which greatly promote
the development of rechargeable Mg batteries. Recently, our group
has developed a novel boron based electrolyte system formed by the
reaction of tri(3,5-dimethylphenyl)borane (Mes3B) and PhMgCl in
THF solvent for rechargeable Mg batteries. The electrolyte with
high ionic conductivity, excellent Mg deposition reversibility as well
as the highest anodic potential to date (3.5 V vs. Mg RE) opens the
door to research on high energy rechargeable Mg batteries.12,13
However, all the above reported electrolyte systems have the
common drawback that they are nucleophilic and sensitive to air/
moisture as they contain air sensitive components (such as Grignard
reagent RMgX, or MgR2), which increases the difficulty of
battery production and precludes their potential application in
new Mg/S and Mg/O2 batteries.14 In 2011, Kim et al. reported
the first non-nucleophilic electrolyte based on the crystal of
[Mg2Cl3-6THF][HDMSAlCl3], which was claimed to be com-
patible with sulfur.15 Nevertheless, such electroactive species
was only obtained via formation of high quality single-crystals,
which might be difficult to produce on a large scale for its
practical application.
prepared. The electrolyte exhibits air insensitive character and
excellent electrochemical performances which make it highly
promising for advanced rechargeable Mg battery systems.
Electric vehicles (EVs) have attracted world-wide attention in
recent years. The battery, as the heart of the propulsion systems,
plays a critical role in the cost and performance of EVs.
Lithium-ion packs used today cannot meet all the requirements
for EVs (such as low cost, high energy and power density, high
safety, etc.).1 Therefore, development of new types of green and
safer, less-expensive rechargeable battery systems is critical.
Recently, rechargeable Mg batteries have been recognized
as one of the most promising storage systems for next-genera-
tion high energy rechargeable batteries because Mg possesses
very negative electrode potential (ꢀ2.37 V vs. SHE) and high
theoretical specific capacity (2205 A h Kgꢀ1 or 3830 A h dmꢀ3).
More importantly, Mg is environmentally benign, safe to
handle and of low cost compared to lithium.2–4
However, over the last three decades, in comparison with the
success of rechargeable Li batteries, rechargeable Mg battery system
is still at the primary stage, and only a few reports have been
documented until now.2,3,5,6 In contrast to Li, Mg electrochemistry
suffers from several serious limitations, such as (i) lack of suitable
cathode materials, (ii) limited selectivity of electrolyte systems with
reversible Mg deposition and a wide electrochemical window, which
is the biggest obstacle that constrains the research and development
of rechargeable Mg batteries. Therefore, one of the keys in
developing rechargeable Mg batteries is to find a right electrolyte.
It is well known that highly reversible behaviour of a Mg
electrode could be obtained in organometallic magnesium salt
solutions such as Grignard salts/ethereal solutions (RMgX,
R = alkyl, aryl groups and X = Cl, Br).7–10 However, they
cannot be used for batteries because of their poor anti-oxidation
capability. Breakthroughs were made by Aurbach and co-workers,
who developed a family of electrolyte solutions based on Mg
This communication describes the design and preparation of
a new electrolyte system insensitive to air. In our experiments, we
first synthesized three ROMgCl salts [2-tert-butyl-4-methyl-
phenolate magnesium chloride (BMPMC), phenolate magnesium
chloride (PMC), 2,6-di-tert-butyl-phenolate magnesium chloride
(DBPMC)] via the different phenols (ROH) to react with the
Grignard reagent EtMgCl. Then, the AlCl3–THF solution in
desired concentration was added dropwise to the obtained
ROMgCl–THF solution respectively. The reactions were very
exothermic. The resulting solutions were stirred for 5 hours or
more at room temperature (see experimental procedure details in
the ESIw) to form electrolyte solutions. By varying the electrolyte
concentration (based on the Mg content), we found that the
highest conductivity of the (BMPMC)2AlCl3/THF electrolyte
was 2.56 mS cmꢀ1 at a concentration of 0.5 M (see Fig. S1,
ESIw). Based on this electrolyte concentration, we examined
different (ROMgCl)2–AlCl3/THF electrolyte systems in terms
a School of Chemistry and Chemical Engineering, Shanghai Jiao Tong
University, Shanghai 200240, China. E-mail: yangj723@sjtu.edu.cn;
Fax: +86-21-5474 7667; Tel: +86-21-5474 7667
b Hirano Institute for Materials Innovation, Shanghai Jiao Tong
University, Shanghai 200240, China
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc35857c
z Fei-fei Wang and Yong-sheng Guo contributed equally to this work.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 10763–10765 10763

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Fig. 1 (a) Typical steady-state voltammograms of Pt electrodes in
different electrolytes in which a reversible Mg deposition–dissolution
process is performed: (red) 0.5 M (BMPMC)2–AlCl3/THF, (green)
0.5 M (PMC)2–AlCl3/THF, (blue) (DBPMC)2–AlCl3/THF. (b) Typical
voltammograms of three THF solutions containing BMPMC Lewis
base and AlCl3 Lewis acid at different ratios: (green) 1 : 1, (red) 2 : 1,
Fig. 2 Cyclic voltammograms (10 cycles) of Pt electrodes in
two electrolyte systems exposed to air for 3 hours at a scanning
rate of 50 mV sꢀ1
: (a) (BMPMC)2–AlCl3/THF electrolyte, (b)
(PhMgCl)2–AlCl3/THF electrolyte.
was measured by a cyclic voltammetric method after exposure
to air for several hours. As shown in Fig. 2a, the reversible Mg
deposition–dissolution processes can be maintained, although
the potential polarization is slightly higher. Furthermore, the
anodic decomposition potential and the current density for
Mg deposition–dissolution are still stable after exposure to air.
These results clearly indicate that this electrolyte system is
insensitive to moisture and air. By contrast, the second genera-
tion electrolyte developed by Aurbach is unable to endure air
and no reversible Mg deposition–dissolution process can be
observed after exposure to air for the same time (Fig. 2b). It was
reported that the (PhMgCl)2–AlCl3/THF electrolyte contains
Mg2Cl3+, MgCl2ꢁ4THF, and PhxAlCl4ꢀx (x = 0–4).17 Similarly,
the main species in our electrolyte are Mg2Cl3+, MgCl2ꢁ4THF,
and [(RO)xAlCl4ꢀx]ꢀ. The different air adaptability of the
(BMPMC)2–AlCl3/THF and (PhMgCl)2–AlCl3/THF electro-
lytes is related to the Al–O bond and Al–C bond respectively.
We can conclude that the improved air insensibility of the
BMPMC–AlCl3/THF electrolyte is attributed to the much
stronger Al–O bond than the Al–C bond.18
(blue) 2.5 : 1 ratio. Scanning rate: 50 mV sꢀ1
.
of the possibility of obtaining reversible Mg deposition and higher
anodic stability using a cyclic voltammetric method. Fig. 1a
presents the typical voltammograms of (BMPMC)2–AlCl3/THF,
(PMC)2–AlCl3/THF and (DBPMC)2–AlCl3/THF electrolytes on
a Pt working electrode. Reversible Mg deposition–dissolution
processes could be achieved in the above three electrolyte
solutions. The different anodic stability of the solutions is in
the following order: (BMPMC)2–AlCl3 > (DBPMC)2–AlCl3 >
(PMC)2–AlCl3. In addition, judging from the current density
and distance between oxidation and reduction peaks, the
(BMPMC)2–AlCl3/THF electrolyte solution shows the lowest
overpotential and highest conductivity. Indeed, the ionic
conductivity of 0.5 M (BMPMC)2–AlCl3/THF, (DBPMC)2–
AlCl3/THF and (PMC)2–AlCl3/THF was determined to be, respec-
tively, 2.56, 1.29 and 0.99 mS cmꢀ1 at room temperature, in
accordance with the above results. These primary results indicate
that the different alkyl-substitution on phenoxyl groups of
the conducting complex significantly influences the electro-
chemical performances and the BMPMC–AlCl3 solution exhibits
the best result. Therefore, all further research was conducted
using this complex solution.
As reported in the literature,16 different Lewis base–acid
ratio has a strong effect on the electrochemical performance
of the Al-based electrolyte. Fig. 1b compares the typical
voltammograms in three THF solutions with different Lewis
acid–base ratio. The BMPMC–AlCl3 complex solutions at
1 : 1 and 2 : 1 ratios show the highest electrolyte decomposition
potentials (ca. 2.6 V vs. Mg RE), which may be suitable for 2 V
rechargeable Mg battery systems. So far, the discharging
voltage plateau of the available insertion cathodes has not
exceeded 1.6 V vs. Mg RE. When the Lewis base–acid ratio is
set to 2.5 : 1, the electrolyte decomposition potential decreases
to 2.4 V. On the other hand, the ionic conductivities of the
BMPMC–AlCl3 and (BMPMC)2.5–AlCl3 complex solutions
are 1.32 and 1.97 mS cmꢀ1, respectively, significantly smaller
than that of (BMPMC)2–AlCl3 solution. As a result, the
(BMPMC)2–AlCl3/THF electrolyte demonstrates the highest
reversible current peaks and lowest potential polarization for
electrochemical Mg deposition and dissolution.
Constant-current electrodeposition was conducted in the
0.5 M (BMPMC)2–AlCl3/THF electrolyte on a Ni substrate
at 0.1 mA cmꢀ2 for 5 h. The deposit layer was characterized by
X-ray diffraction (XRD) as shown in Fig. 3a. The diffraction
peaks at 32.091, 34.351, 36.611, 47.571, 63.031 and 68.541 (2y)
can be attributed to metallic Mg (JCPDS file 350821) and the
peaks at 44.381 and 51.731 to the Ni substrate. No other peaks
were detected. This result indicates that the deposit layer is
pure Mg metal. The efficient cycling characteristic of the Mg
electrode in an electrolyte system is of special importance for
developing the practical rechargeable Mg battery systems. So
the coulombic efficiency (i.e. the ratio of the charge amount of
The stability of the magnesium ion electrolyte in air is very
important for facilitating the electrolyte and battery processing in
practical application and developing other high energy-density
rechargeable Mg battery systems (such as a Mg/air battery).
The air sensitivity of the (BMPMC)2–AlCl3/THF electrolyte
Fig. 3 (a) XRD pattern of the electrodeposited Mg on a Ni substrate in
0.5 M (BMPMC)2–AlCl3/THF solution. The charge amount is 1.8 C cmꢀ2
.
(b) The cycling efficiencies and the first 10 chronopotentiograms of
Mg deposition and dissolution on Ni in the 0.5 M (BMPMC)2–AlCl3/
THF electrolyte.
c
10764 Chem. Commun., 2012, 48, 10763–10765
This journal is The Royal Society of Chemistry 2012

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good compatibility with the Mo6S8 intercalation cathode and
could be used in practical rechargeable Mg battery systems.
In summary, we have designed and synthesized an air-stable
electrolyte system based on the novel Lewis acid–base complex via a
reaction between AlCl3 and organic Mg salt (ROMgCl). The
(BMPMC)2–AlCl3/THF electrolyte solution shows high ionic
conductivity (2.56 mS cmꢀ1), high reversibility of Mg deposition–
dissolution, and good anodic stability (2.6 V vs. Mg RE). Further-
more, the good compatibility with the Mo6S8 intercalation cathode
confirms that the phenolate based electrolyte could be practically
used in rechargeable Mg battery systems. More importantly, the air
insensitive character of this electrolyte may open up a new approach
to other high energy-density rechargeable Mg battery systems
(e.g. Mg/air battery).
This work is financially supported by National Basic Research
Program of China (2007CB209700).
Fig. 4 Cycling behavior of the rechargeable Mg/Mo6S8 coin-cell
using 0.5 M (BMPMC)2–AlCl3/THF electrolyte solution at a current
rate of 0.05 C at room temperature.
Notes and references
1 J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587–603.
2 T. D. Gregory, R. J. Hoffman and R. C. Winterton,
J. Electrochem. Soc., 1990, 137, 775–780.
3 D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar,
R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi, Nature,
2000, 407, 724–727.
4 Y. N. Nuli, J. Yang, Y. S. Li and J. L. Wang, Chem. Commun.,
2010, 46, 3794–3796.
5 P. Novak, R. Imhof and O. Haas, Electrochim. Acta, 1999, 45,
351–367.
6 Y. Liang, R. Feng, S. Yang, H. Ma, J. Liang and J. Chen, Adv.
Mater., 2011, 23, 640–643.
7 C. Liebenow, Z. Yang and P. Lobitz, Electrochem. Commun., 2000,
2, 641–645.
8 C. Liebenow, Z. Yang and P. Lobitz, Bull. Electrochem., 1999, 15,
424–426.
9 D. Aurbach, M. Moshkovich, A. Schechter and R. Turgeman,
Electrochem. Solid-State Lett., 2000, 3, 31–34.
Mg dissolution to that of Mg deposition) of the Mg deposition–
dissolution process in the (BMPMC)2–AlCl3/THF electrolyte
solution was evaluated by a galvanostatic cycling experiment. It
can be observed from Fig. 3b that the coulombic efficiency
increases gradually with the progressive cycle number. After
50 cycles the coulombic efficiency reaches about 99%. The high
ionic conductivity, outstanding electrochemical reversibility,
good anodic stability and air insensitivity make the (BMPMC)2–
AlCl3/THF electrolyte solution highly promising for use in
practical rechargeable Mg batteries.
In order to verify the compatibility of the (BMPMC)2–AlCl3/
THF electrolyte solution with a magnesium ion intercalation
cathode, we have synthesized the Mo6S8 Chevrel phase cathode
material, which can intercalate Mg ions reversibly,19 using the
molten salt method according to the literature.20
10 Y. S. Guo, J. Yang, Y. N. NuLi and J. L. Wang, Electrochem.
Commun., 2010, 12, 1671–1673.
The 2016-type coin cell was constructed using the 0.5 M
(BMPMC)2–AlCl3/THF solution as the electrolyte, a Mg disc
as a negative electrode, and Mo6S8 as a positive electrode. This
cell was cycled at a current rate of 0.05 C (6.4 mA gꢀ1) with the
discharge and charge voltage limits of 0.5 V and 1.7 V vs. Mg
RE. As shown in Fig. 4, the discharge voltage shows two distinct
voltage plateaus (at 1.1 V and 0.9 V vs. Mg RE), which are
in accordance with Aurbach’s results.3 The average discharge
capacity at room temperature was ca. 100 mA h gꢀ1 calculated
based on the weight of the cathode’s active mass. The electro-
chemical performance at elevated temperatures of 50 1C and
65 1C was further evaluated at the same current rate (see Fig. S2,
ESIw). The cell shows not only good rechargeability, but also
higher specific capacity. The discharge capacity of 120 mA h gꢀ1
at 65 1C is close to the theoretic value of Mo6S8 material. This
enhancement is likely related to the faster interfacial reactions
and higher diffusion rate of Mg ions at elevated temperatures.
These results confirm that the phenolate based electrolytes have
11 O. Mizrahi, N. Amir, E. Pollak, O. Chusid, V. Marks, H. Gottlieb,
L. Larush, E. Zinigrad and D. Aurbach, J. Electrochem. Soc.,
2008, 155, A103–A109.
12 Y. S. Guo, F. Zhang, J. Yang and F. F. Wang, Electrochem.
Commun., 2012, 18, 24–27.
13 Y. S. Guo, F. Zhang, J. Yang, F. F. Wang, Y. N. NuLi and
S. I. Hirano, Energy Environ. Sci., 2012, 5, 9100–9106.
14 J. Muldoon, C. B. Bucur, A. G. Oliver, T. Sugimoto, M. Matsui,
H. S. Kim, G. D. Allred, J. Zajicek and Y. Kotani, Energy Environ.
Sci., 2012, 5, 5941.
15 H. S. Kim, T. S. Arthur, G. D. Allred, J. Zajicek, J. G. Newman,
A. E. Rodnyansky, A. G. Oliver, W. C. Boggess and J. Muldoon,
Nat. Commun., 2011, 2, 427.
16 D. Aurbach, H. Gizbar, A. Schechter, O. Chusid, H. E. Gottlieb,
Y. Gofer and I. Goldberg, J. Electrochem. Soc., 2002, 149, A115–A121.
17 D. Aurbach, N. Pour, Y. Gofer and D. T. Major, J. Am. Chem.
Soc., 2011, 133, 6270–6278.
18 A. R. Barron, Polyhedron, 1995, 14, 3197–3207.
19 M. D. Levi, E. Lancry, H. Gizbar, Z. Lu, E. Levi, Y. Gofer and
D. Aurbach, J. Electrochem. Soc., 2004, 151, A1044.
20 E. Lancry, E. Levi, A. Mitelman, S. Malovany and D. Aurbach,
J. Solid State Chem., 2006, 179, 1879–1882.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 10763–10765 10765