1. Understanding the Enhanced Mg2+
Intercalation
Kinetics Originating from Water Co-Intercalation
Anthony Rock
University of Maryland, Transportation Electrification REU
August 2015
rocka@seattleu.edu
ABSTRACT
This report addresses the need for enhanced conductivity of
magnesium ions in secondary batteries through electrolytic
alteration. Following an overview of chemical processes and
material requirements of magnesium ion batteries,
electrochemical impedance spectroscopy (EIS) data from
multiple electrolytes with and without water are compared and
analyzed. In addition to the varying chemistry of the electrolyte,
this report also discusses the effect of ambient air temperature
and cycling on the electrochemical impedance of magnesium ion
test cells.
EIS data analysis was performed by fitting a modified
Randles’ equivalent circuit to experimental data. The fitted
circuit was then used to identify the electrolytic, charge
transfer, and diffusional impedance changes due to
modifications of the electrolyte. The results indicate that the
addition of water reduces the impedance of magnesium ion
movement by up to one order of magnitude.
INTRODUCTION
A. Why This Research is Important
The extreme pressures on Earth’s natural resources and
environment due to humankind’s consumption and fossil fuel
dependence are no secret. In recent years, it has become
more and more apparent that the need to siphon humans off
fossil fuels and transition to a sustainable energy economy is
paramount. Adding these stresses to increasing consumer
demand for portable electronics and long-range electric
vehicles only increases the pressure to improve existing
battery technology.
In the past few decades, lithium ion batteries have
dominated the consumer and research spotlight. Positively
lithium ion batteries have enabled new eras of technological
development, miniaturization, and sustainability. However,
lithium ion batteries still have significant limitations that
make alternative battery chemistries an important field of
research.
One promising substitute for lithium ion batteries is a
magnesium-ion-based battery. Magnesiumpossesses several
characteristics that make it favorable to lithium as an ion for
intercalation.
First, magnesium’s bivalent character gives associated
batteries a significantly higher volumetric capacity than
lithium-based batteries (however its gravimetric capacity still
remains much lower than that of lithium – see Fig. 1).
Second, magnesium is similar to lithium in size: the ionic
radius of Mg2+ is 0.74 Å, which is very close to Li+’s 0.68 Å
[1, p. 2268]. This property eases the intercalation of
magnesium ions by decreasing the deformation of host
electrodes.
Fig. 1 - Comparisonof theoretical capacities of lithium, graphite, and
magnesium anodes [1, p. 2266]
Third, magnesium is safer than lithium. Magnesium’s
safety can be attributed to its greater stability and higher
melting point [2, p. 16], as well as the absence of dendritic
build up on its electrodes (a historically serious safety
concern of lithium [1, p. 2270]).
Lastly, magnesium’s natural abundance makes it a much
more plentiful resource than lithium. Magnesium is the fifth
most abundant element in the earth’s crust [1, p. 2266],
having a presence multiple orders of magnitude higher than
that of lithium [2, p. 16]. This will become an important cost-
mitigating and environmental sustainability factor as the
demand for more energy storage devices continues to
increase.
Despite all of these positive attributes, magnesium-ion
battereies have been hindered in their development for
several reasons. Although the two-electron charge transfer
associated with magnesium’s bivalency positively increases
its volumetric capacity, the strength of magnesium’s charge
inhibits fast kinetics and the reversibility of intercalation into
compatible electrodes [2, p. 16]. Additionally passivation
films, created by the interaction between electrodes and
atmospheric contaminants and/or the electrolyte, formon the
electrodes in magnesium ion batteries that completely block
the transport of Mg2+ ions into and out of the electrodes,
while such inhibition is not seen with Li ions [1, 2, 3].
The issue of passivation films in particular has made the
discovery of a sutiable electrolyte for magnesium ion
batteries particularly difficult. Thus the research presented in
this paper was performed in an attempt to solve the greatest
bottleneck in magnesium ion battery development. The
following pages detail the process and results of tests of
aqueous electrolytes used in magnesiumbattery cells.
3862
372
2205
2046
760
3833
0
500
1000
1500
2000
2500
3000
3500
4000
Li Graphite Mg
TheoreticalCapacity mAh/g mAh/cm^3

2. A. Battery Basics
Fig. 2 - Schematic of charge/discharge process in typical lithium-ion cell
[4, p. 10].
Magnesium ion battery technology is based on the same
principles as lithium. This common ground has made the
adaptations in technology and understanding much more
manageable for researchers. Thus to understand the
mechanisms of magnesium ion battery operation, lithium ion
processes will be explored as an equivalent means of
analysis.
At the most fundamental level, batteries store chemical
energy to later be converted into electrical energy. Lithium
ion batteries store this energy through the “rocking-chair” [5,
p. 35.1] method of lithium ion movement between the anode
and cathode. During the discharge and charge processes,
positively charged lithium ions are intercalated—“reversibly
removed or inserted into a host without a significant
structural change to the host” [5, p. 35.4]—into and out of
the anode and cathode materials, while electrons travel in the
same direction to supply an electrical load (see Fig. 2 above).
Maintaining a charge balance epitomizes the key to the
battery charge and discharge processes.
This process, however, is highly dependent on many
internal and external factors (see Fig. 3 below). The
combination of these factors greatly influences the
requirements of the component materials of batteries. A brief
overview of the requirements for cathode materials—similar
to those of anode materials—for magnesium ion batteries is
provided in Table 1.
Fig. 3 - Variables affecting battery kinetics [6, p. 20]
Table 1 - Requirements for Mg-ioncathode materials (quoted from Li-ion
requirements presented in Ref. [5, p. 35.6])
High free energy of reaction/potential difference with
magnesium
Can incorporate large quantities of magnesium (has high
capacity)
Reversibly incorporates magnesium without structural
change
High magnesium ion diffusivity
Good electronic conductivity
Insoluble in the electrolyte
Prepared from inexpensive reagents
Created by low cost synthesis
In light of the processes involved in battery operation and
the requirements of electrode materials discussed, it is quite
apparent what material properties must characterize an
electrolyte for magnesium ion batteries. First, as is true for
electrolytes of any battery chemistry, the electrolyte must
have a high ionic conductivity and no electronic
conductivity. Second, electrolytes must cover a high
proportion of the electrode surface. This characteristic has
made liquid electrolytes highly favorable in the past; and
although solid electrolytes are rising in prevalance,
exclusively liquid electrolytes were used in this work. The
last requirement, specific to magnesium ion batteries, is that
the electrolyte must aid magnesium intercalation. As
previously mentioned, magnesium batteries have been
hindered by passivation films developed on electrode
surfaces due to interactions between the electrode and the
electrolyte [1, 2, 3]. Accordingly any electrolyte studied must
either avoid the development of such a film altogether or aid
in breaking up such films to make electrodes accessible to
magnesium ions.
B. Equivalent Circuits for Modeling Battery Processes
To understand the mechanisms of ion transport in
batteries, it is common practice to model batteries with an
equivalent circuit. The most commonly used circuit in
electrochemistry is the Randles Circuit (pictured in Fig. 4).
As can be seen in the figure (starting on the left), ions begin
their migration by travling through a resistance (RΩ). This
resistance represents the electrolytic impedance to ion
transport—so for all batteries, ideally this value should be
quite small. At the middle node, ions may either travel a
capacitive or faradaic path, hence the subscripts c and f
respectively. This node physically represents the electrode-
electrolyte interface, so the capacitor represents a double-
layer capacitance created by the build up of charge at the
interface, while the faradaic impedance (Zf) resists the
intercalation of ions [6]. Sometimes systems do not exhibit
ideal capacitive behavior, so constant phase elements are
used to account for material imperfections (See Table 2 and
Reference [7] for more details).
Fig. 4 - (a) Randles circuit (b) Division of Zf into its components
[6, p. 376]
Internal
Variables
Electrode
Variables
Material
Surface Area
Geometry
Surface
Condition
Mass
Transfer
Variables
Mode (Diffusion,
Convection, ...)
Surface
Concentrations
Adsorption
Electrolytic
Variables
Concentrations
of Electroactive
Species
Concentrations
of Other Species
Solvent
External
Variables
Environment
al Variables
Temperature
Pressure
Local Chemical
Contaminants
Electrical
Variables
Potential
Current
Rate of
(Dis)charge
Duration of
(Dis)charge

3. In addition to the electrolytic resistance (RΩ), two other
components of the Randles circuit are of particular interest to
electrolyte research. Fig. 4 (b) details both of them, which—
from left to right—represent the charge transfer resistance
(Rct) and Warburg impedance (Zw). The charge transfer
resistance models the impedance to the initial intercalation of
ions while the Warburg impedance models diffusional
impedance restricting ion movement once the ions have been
inserted into the host electrode [6]. Both of these impedance
values are highly influenced by the properties of the
electrolyte. The technique used to measure these impedance
values in this research will be detailed in the next section.
C. Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) measures
the total impedance of an electrical system through
sinusoidal perturbations of the system and subsequently
measuring the system’s response at various frequencies.
After outputing either voltage or current stimuli at a specific
frequency, a data acquisition system will read the current or
voltage response and then calculate the impedance of the
systemusing the ubiquitous 𝑉 = 𝐼𝑍.
Given the nature of electrochemical cells, both real and
imaginary components of impedance exist. Table 2 lists
elements commonly used in EIS tests along with their
corresponding impedance values (where 𝑗 = √−1). Most
often EIS data is plotted with real and imaginary components
of impedance on separate axes via a Nyquist plot. Coupling
the Nyquist plot of impedance data with knowledge of the
physical processes involved in the Randles circuit allows for
simplified data fitting and subsequent analysis of
electrochemical behavior. (For additional, more detailed
information regarding the elements in Table 2 along with
their impact on EIS data, readers are encouraged to explore
References [6, 7, 8].)
Table 2 - Common elements usedin electrochemical circuits of batteries
Element Name Circuit Symbol Impedance (Z)
Resistor R
Capacitor
𝑗
𝜔𝐶
⁄
Constant Phase
Element (CPE)
1
𝑄(𝑗𝜔) 𝑛⁄
Warburg Element
(Open Circuit
Terminus)
𝜎𝜔−1 2⁄
− 𝑗𝜎𝜔−1 2⁄
EXPERIMENTATION & RESULTS
A. Experimental Methods
Three-electrode flooded cells were used in the
electrochemical tests. Ag/Ag+ was used as the
reference electrode and active carbon (AC) was used
as the counter electrode. MgClO4 salt was baked in a
glove box to remove any absorbed moisture at 80°C
and propylene carbonate (PC) solvent was dried with
a molecular sieve in a glove box. Nonaqueous,
organic electrolyte, denoted as PC electrolyte, was
made by dissolving 0.1 M Mg(ClO4)2 into PC
solvent. An aqueous electrolyte, denoted as PC-H2O
electrolyte, was made by adding 0.6 M water as a
solute to the aforementioned PC electrolyte outside
the glove box, as previous work suggested that a 6:1
H2O-Mg ratio shows the strongest kinetics
enhancement [9]. EIS tests were performed with
Gamry Interface 1000. Potentiostat EIS tests were
conducted at open circuit potential before discharge
(0.1V for PC) or after full discharge in the frequency
range of 106 Hz to 0.01 Hz with AC signal amplitude
of 10-3 V [10].
B. Results
Fig. 5 - Nyquist plot of aqueous andnonaqueous electrolytes'EIS results
after two charge-discharge cycles [10]
The results obtained in Fig. 5 were fitted to an equivalent
circuit using a non-linear least-square fit. The calculated
charge transfer resistance values (proportional to the
diameter of the circles between 100 kHz and 5 Hz in Fig. 5
[6, p. 386]) were 3080 Ω and 426 Ω for the nonaqueous (PC)
and aqueous (PC-6H2O) electrolytes, respectively [10].
DISCUSSION & FUTURE WORK
The results presented above directly affirm the notion
hinted by some researchers that the presence of water affects
magnesium intercalation kinetics [1, 2, 9, 11]. The fact that
an order of magnitude difference in charge transfer resistance
exists between aqueous and nonaqueous electrolytic systems
is extremely promising for the future of magnesium ion
battery systems.
In order to more fully understand the nature of these
results, further experimentation must occur. Cyclic
voltammetry tests are currently being performed to ensure
aqueous electrolytes have adequate cycling performance to
justify further research (albeit the sizeable impact of water in
these results warrants further research anyway). Additionally
it is essentialthat electrolytes other than propylene carbonate
are tested to understand which systems are most compatible
with water and thus may be enhanced. Lastly, although the
charge transfer resistance of the magnesium ion system
significantly decreased in the tests detailed above, more
experiments/analysis must be performed to understand how
the addition of water affects the diffusional properties of
magnesium ions.
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
ZReal
(k)
PC @ 29C
PC-6H2O @ 25C
-ZImag
(k)
2.52 Hz
5.01 Hz
100 kHz

4. ACKNOWLEDGMENTS
This report was based on the research performed in the
paper of the same name, see reference [10]. The sincerest
thanks must be given to my graduate mentor, Tao Gao, for
his immense contributions of knowledge and time, without
which this work would not be possible. I would also like to
thank Dr. Chunsheng Wang for accomodating me and
incorporating me into his research group.
This work has been supported through the National
Science Foundation grant number EEC 1263063, REU Site:
Summer Engineering Research Experiences in
Transportation Electrification, which is gratefully
acknowledged.
The staff and faculty of the Transportation Electrification
REU program also deserve recognition. Ms. Michelle
Wilson, the program coordinator, made this research a
possibility through her diligent coordination efforts.
Dr. Alireza Khaligh enriched all students’ experiences
through his passion, discourse, and wisdom. Thank you both
for all that you have done to make this opportunity possible.
REFERENCES
[1] H. D. Yoo, I. Shterenberg, Y. Gofer, G.
Gershinsky, N. Pour and D. Aurbach, "Mg
rechargeable batteries: an on-going challenge,"
Energy & Environmental Science, vol. 6, no. 8, pp.
2245-2550, 2013.
[2] M. Huie, D. Bock, E. Takeuchi, A. Marschilok and
K. Takeuchi, "Cathode materials for magnesium and
magnesium-ion based batteries," Coordination
Chemistry Reviews, vol. 287, pp. 15-27, 2015.
[3] Z. Lu, A. Schechter, M. Moshkovich and D.
Aurbach, "On the electrochemical behavior of
magnesium electrodes in polar aprotic electrolyte
solutions," Journal of Electroanalytical Chemistry,
vol. 466, pp. 203-217, 1999.
[4] G.-A. Nazri and G. Pistoia, Eds., Lithium
Batteries: Science and Technology, New York:
Springer Science+Business Media, LLC, 2003.
[5] D. Linden and T. B. Reddy, Eds., Handbook of
Batteries, 3rd ed., New York, New York: The
McGraw-Hill Companies, Inc., 2002.
[6] A. J. Bard and L. R. Faulkner, Electrochemical
Methods: Fundamentals and Applications, 2nd ed.,
New York: John Wiley & Sons, Inc., 2001.
[7] M. S. Abouzari, F. Berkemeier, G. Schmitz and D.
Wilmer, "On the physical interpretation of constant
phase elements," Solid State Ionics, vol. 180, pp.
922-927, June 2009.
[8] S. Taylor and E. Gileadi, "Physical Interpretation
of the Warburg Impedance," Corrosion Science, vol.
51, no. 9, pp. 664-671, September 1995.
[9] J. Song, M. Noked, E. Gillette, J. Duay and G.
Rubloff, "Activation of a MnO2 cathode by water-
stimulated Mg2+ insertion for a magnesium ion
battery," Physical Chemistry Chemical Physics, vol.
17, no. 7, pp. 5256-5264, 2015.
[10] T. Gao, "Understanding the Enhanced Mg2+
Intercalation Kinetics Originating from Water Co-
Intercalation," Unpublished.
[11] P. Novak, W. Scheifele, F. Joho and O. Haas,
"Electrochemical Insertion of Magnesium into
Hydrated Vanadium Bronzes," Journal of the
Electrochemical Society, vol. 142, no. 8, pp. 2544-
2550, August 1995.