The electrolytes of 1 M NaFSI dissolved in various carbonate solvents containing EC, PC and/or DMC have been
investigated to find out the most compatible electrolyte composition with hard carbon (HC) anode in Na-ion batteries
(NIBs). The physical properties including viscosity and conductivity were measured to correlate with the
electrochemical behaviors of these electrolytes. The Na/HC half-cell was used for testing the charge/discharge
performance of prepared electrolytes at room temperature. This best medium for the solvation of NaFSI salt is the
mixture of EC, PC and DMC with the ratio 3:1:1, respectively. Indeed, this electrolyte delivered a highest capacity of
335.6 mAh.g-1, excellent capacity retention of 73.7 % for 100 cycles.
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Cite this paper: Vietnam J. Chem., 2020, 58(5), 643-647 Article
DOI: 10.1002/vjch.202000053
643 Wiley Online Library © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH
Electrochemical performance of hard carbon anode in different
carbonate-based electrolytes
Le Minh Kha
1
, Vo Duy Thanh
2
, Nguyen Van Hoang
1,2
, Le Van Thang
3
, Le My Loan Phung
1,2*
1Department of Physical Chemistry, Faculty of Chemistry, University of Science,
Vietnam National University, 227 Nguyen Van Cu, district 5, Ho Chi Minh City 70000, Viet Nam
2Applied Physical Chemistry, Faculty of Chemistry, University of Science,
Vietnam National University, 227 Nguyen Van Cu, district 5, Ho Chi Minh City 70000, Viet Nam
3Department of Nanomaterials, Faculty of Materials Technology, University of Technology,
Vietnam National University, 268 Ly Thuong Kiet, distrct 10, Ho Chi Minh City 70000, Viet Nam
Submitted April 8, 2020; Accepted April 28, 2020
Abstract
The electrolytes of 1 M NaFSI dissolved in various carbonate solvents containing EC, PC and/or DMC have been
investigated to find out the most compatible electrolyte composition with hard carbon (HC) anode in Na-ion batteries
(NIBs). The physical properties including viscosity and conductivity were measured to correlate with the
electrochemical behaviors of these electrolytes. The Na/HC half-cell was used for testing the charge/discharge
performance of prepared electrolytes at room temperature. This best medium for the solvation of NaFSI salt is the
mixture of EC, PC and DMC with the ratio 3:1:1, respectively. Indeed, this electrolyte delivered a highest capacity of
335.6 mAh.g
-1
, excellent capacity retention of 73.7 % for 100 cycles.
Keywords. Hard carbon anode, NaFSI, carbonate solvent, ionic conductivity, electrochemical performance.
1. INTRODUCTION
Since Li-ion batteries (LIBs) were firstly
commercialized in 1991, LIBs have playing an
important role in modern society as a powerful energy
storage and conversion system. Nowadays, LIBs
present in most of portable electronic devices such as
mobile phones, electronic vehicles, etc. due to its high
volumetric and gravimetric, high durability upon
cycling and low self-discharge,
[1]
However, the
limited lithium sources and its non-uniform on the
Earth’s crust make LIBs hardly spreading in large
scale application.
[2-3]
Thus, future research beyond Li-
ion technology is possibly directing to other alkaline
chemistries-based batteries. Among them, sodium-ion
batteries (NIBs) could be a potential candidate due to
its analogue to previous lithium chemistry. However,
the critical problem coming from the large radius of
Na
+
ion leads to the unstableness in the host structure
and form some unknown phases, so the cycle life still
limits in few hundred cycles.
[4]
To overcome this
problem, many researchers have attempted to
improve the main components of NIBs in which
positive electrode, negative electrode and electrolyte
have been attentively focused.
Electrolyte improvement is an effective way to
achieve a long-cycling performance NIBs. Electrolyte
medium helps Na
+
ion reversibly diffusing from
cathode to anode inside a battery. Some essential
properties having to possess in a basic electrolyte are
chemically, thermally/electrochemically stable, ionic
conductive and electronically insulating.
[2]
Recently,
different research works have been reported on non-
aqueous electrolyte, especially carbonate-based
solvents with various solutes, especially NaClO4,
NaBF6 and sodium trifluoromethanesulfonimide
(NaTFSI) salts.
[5-7]
Sodium bis(fluorosulfonyl)imide
(NaFSI) is also alternative among these solutes with
high conductivity and less aluminum corrosive than
NaTFSI. However, very few studies about diluted 1
M NaFSI dissolved in carbonate-based solvents has
been reported while NaFSI salt showed effectively in
high concentration (HCE) or localized high
concentrated electrolyte (LCHE) using
dimethoxyethane (DME) solvent within some
diluents.
[8]
Hence, in this work, hard carbon anode
was tested the compatibility with NaFSI dissolved in
carbonate-based electrolytes in searching the
optimized composition for long-cycling performance.
Vietnam Journal of Chemistry Le Loan My Phung et al.
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 644
2. MATERIALS AND METHODS
2.1. Preparation of electrolytes and their physical
properties testing
All the electrolytes were prepared by dissolving
sodium bis(fluorosulfonyl)imide (NaFSI, Sigma-
Aldrich, USA) in ethylene carbonate (EC, Acros,
France), propylene carbonate (PC, Acros, France)
and/or dimethyl carbonate (DMC, Acros, France)
with 1 M salt concentration. All the preparation steps
were done in the argon-controlled glovebox
(Jacomex, France) with the concentration of O2 and
H2O less than 10 ppm. After that, the electrolytes
were used for conductivity and viscosity testing.
Electrolyte viscosity was carried out with a
Brookfield DV2+ ProViscometer equipped with a
circulating bath for precise temperature control. The
measured temperature is ranged from 30 to 60
o
C for
10
o
C/step.
The ionic conductivity of carbonate-based
electrolytes was measured at range of temperature 20-
60
o
C with dip-typed glass cell with two Pt electrodes
fixed at a constant distance on a Bio-Logic MCS 10
(France) fully integrated multi-channel conductivity
spectroscopy. The cell was initially calibrated by
using 0.1 M KCl solution at 25
o
C to determine the
cell constant (K). The obtained resistance was used to
calculate the conductivity of the electrolytes
determined by the following equation:
L K
RS R
(1)
where κ is the specific conductivity (S.cm-1), K is the
cell constant (cm
-1
), R is the resistance of solution
(Ω), S is the surface area of Pt electrode; L is the
distance between two electrodes.
2.2. Coin cell assembly and electrochemical
testing
Hard carbon (Kureha, Japan, 9 µm size) electrode
was prepared by the mixing of HC, super P carbon
(Timcal, Switzerland) and poly(vinylidene fluoride)
(PVDF, Sigma-Aldrich) with the ratio of 90: 5: 5 in
N-methylpyrrolidone (NMP, Acros, France) solvent
using a ball milling machine (MTI, USA) for 1.5 h.
The mixture was coated directly onto aluminum foil
using MSK-AFA-III Automatic thick film coater
(MTI, USA) before drying in vacuum oven EQ-6020-
FP (MTI, USA) at 110
o
C for 10 h. Then, the thin film
was punched into ground shape 12.7 mm diameter to
match the required dimensions of a CR2032 coin cell
kit (MTI, USA). The HC mass loading was roughly 2
mg/cm
2
. Thick Na foil and a glassy fiber membrane
were used as the counter electrode and separator
impregnated by the as-prepared carbonate electrolyte,
respectively. The coin cell assembly was performed
in glovebox. The cells were then charged and
discharged at the voltage range between 0.05 and 2 V
at a different Galvanostatic rate of C/5, C/10, C/2, C,
2C and C/10, respectively. All the testing data was
recorded on LANHE battery tester (China).
3. RESULTS AND DISCUSSION
3.1. Physical and chemical properties of
electrolytes
Figure 1(a) displayed dependence of viscosity on the
temperature of carbonate-based electrolytes
containing 1 M NaFSI salt. All electrolytes consist of
EC in the composition because of its high dielectric
constant (ɛ = 89.78, 25 oC). However, the high
viscosity related to its high melting point is
inconvenient for using as mono-solvent based
electrolyte. To deal with this problem, some
carbonates as co-solvents with low viscosity (Table 1)
were added into EC to decrease its high viscosity. The
binary electrolyte marked EC: PC (1:1) obviously has
the highest viscosity because of high value of both EC
and PC containing in their mixture. The larger
amount of EC and PC in the electrolytes, the higher
viscosity of electrolyte was obtained. On the contrary,
DMC co-solvent induces a decrease of viscosity of
mixture. Therefore, the viscosity decreases in the
order EC:PC:DMC (3:1:1) (60 % EC) > EC:DMC
(1:1) (50 % EC) > EC:PC:DMC (1:1:1) (33.3 % EC)
> EC: PC: DMC (1:1:3) (20 % EC) corresponding to
the decrease amount of EC added. At 25
o
C, the
lowest value is 2.77 cP for EC:PC:DMC (1:1:3) and
the highest one is 6.08 cP for EC: PC (1:1). In
comparing with NaTFSI salt, NaFSI salt based-
electrolytes have higher conductivity despite of their
higher viscosity
[5]
due to the higher dissociation
degree of NaFSI salt.
[9]
As the temperature increases,
the viscosity of these electrolyte decreases
significantly due to the facile movement of solvent
molecules and decrease of intermolecular force inside
the liquid medium. In other words, viscosity influence
strongly on the ionic conductivity at low
temperatures, but it tends to have a negligible impact
at high temperature amongst other factors
contributing to the conductivity.
Temperature dependence conductivity is shown in
Figure 1(b). Expectedly, EC: PC (1:1) electrolyte
display a lowest conductivity (7.19 mS.cm
-1
, 25
o
C)
corresponding to its highest viscosity. In the binary
electrolyte containing DMC, particularly, EC:DMC
(1:1) with a half amount of DMC possesses the
Vietnam Journal of Chemistry Electrochemical performance of hard carbon
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 645
highest conductivity at all temperature range. This
suitable combination could be effective, whereas high
dielectric constant EC naturally facilitates the
dissociation of the salt and low viscosity solvent
DMC helps to improve the ionic mobility. Therefore,
it is very important to compromise these two factors
to achieve the electrolyte with optimized ion
conductivity. Regarding ternary electrolytes, the
variation amounts of EC, PC and DMC showed a
slight difference in the conductivity. At low
temperature, the viscosity still had a significant
impact on conductivity, the conductivity of these
ternary electrolytes decrease in the order:
EC:PC:DMC (1:1:3) > EC:PC:DMC (1:1:1) >
EC:PC:DMC (3:1:1), which corresponds to the
increase of EC amount. Indeed, the lower EC content
conducts the lower viscosity of electrolyte.
Figure 1: (a) Viscosity and (b) conductivity varies
with temperature of the electrolytes based on 1 M
NaFSI salt in various carbonate solvents at 25
o
C
At high temperature, the evolution of ionic
conductivity is completely opposite to the above
observation due to the strong impact of dielectric
constant leading to the high dissociation of NaFSI
salt.
Table 1: Viscosity () and dielectric constant () of
some carbonate solvents at 25
o
C
[2]
Solvents Formula (cP)
EC
O
O
O
## 89.78
PC
O
O
O
2.53 64.92
DMC
O
O O
0.59 3.107
Figure 2 summarizes the viscosity and
conductivity of five electrolytes containing 1 M
NaFSI at 25
o
C. Even though the electrolyte EC:DMC
(1:1) exhibited the best conductivity, its poor thermal
stability and high flammability due to high amount of
volatile DMC solvent risking fire/explosion hazards
when short circuit thermal runaway happens, setting
bottleneck in battery design and safety engineering in
large scale.
[7]
Next, the electrolyte EC:PC (1:1)
showed a lowest performance because of highest
viscosity and lowest conductivity, which is unsuitable
for battery application. Finally, the ternary
electrolytes with relatively similar properties are
promising for testing the electrochemical
performances in HC/Na cell.
Figure 2: Viscosity and conductivity of the
electrolytes based on 1 M NaFSI salt in various
carbonate solvents at 25
o
C
Vietnam Journal of Chemistry Le Loan My Phung et al.
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 646
3.2. Electrochemical performance of HC/Na cell
Figure 3 (a) displayed typically the initial voltage
profile of HC/Na cell cycled in various electrolytes.
An initial linear potential decay observed at a high
voltage is related to Na
+
ion insertion process in the
graphene layers of HC. A long plateau at lower
potential (< 0.2 V) is associated with the adsorption
of Na
+
ion into the HC pores.
[10]
From Figure 3(b), it
is noticeable that the electrolyte EC:PC:DMC (3:1:1)
exhibited the highest discharge capacity of 335.6
mAh.g
-1
at the C/5 rate despite it has a moderate ionic
conductivity value. Therefore, the ternary electrolytes
with a high concentration of EC exhibited better
discharge capacity and longer cycle life when cycling
in HC/Na cell. This result is coherent with previous
reports whereas the ternary electrolyte containing the
largest amount of EC was an optimal electrolyte with
the highest discharge value and a good Coulombic
efficiency.
[5]
EC is an important component
enhancing the capacity due to its highest electric
constant as aforementioned. The long cycle life is
obviously related to the SEI layer, which effectively
prevent the electrolyte from further reduction. It could
be inferred from some previous works that the main
compositions of the SEI layer are Na2CO3 coming
from EC and NaF coming from NaFSI as found in
LIBs.
[11,12]
In contrast to EC:PC:DMC (3:1:1), the
electrolyte EC:DMC (1:1) even with the highest
conductivity demonstrated unexpectedly a low
discharge capacity of 144.1 mAh.g
-1
at the C/5 rate
and the gradual decrease of capacity upon the
consecutive cycles. Similarly, EC:DMC (1:1) with 1
M NaClO4 as solute also performed a poor discharge
capacity, cycle life and Coulombic efficiency as
well.
[7]
This result can be explained by the unstable
characteristic of SEI layer forming at the electrode-
electrolyte interphase, which couldn’t prevent the
continuous electrolyte reduction to form the non-
conductive compounds and penalize consequently the
cycling performance of batteries. As earlier reported,
the initial SEI layer was dissolved and stabilized
again during some subsequent cycles to prevent the
electrolyte further reduced. For other electrolytes, SEI
layer was fast stabilized after few cycles and enough
thick to passivate HC anode away from the side
reactions with the electrolyte. Hence, the battery
performance consisting of discharge value and
capacity retention are stabilized even for long-cycling
test EC:PC:DMC (3:1:1), EC:PC:DMC (1:1:3) and
EC:PC:DMC (1:1:1).
The capability test was also performed to test high
rate performance of hard carbon in various
electrolytes. Expectedly, the capacity of five
electrolytes drops rapidly when the charge/discharge
rate increased to 2 C because the charge transfer
reaction is limited by the Na
+
ion diffusion from the
electrolyte bulk to the electrode surface. The highest
capacity of 284.7 mAhg
-1
was obtained for
EC:PC:DMC (3:1:1) at 2 C rate. In addition, all
electrolyte exhibited Coulombic efficiency reaches
nearly 100 % (Figure 3(c)) confirmed that the
oxidation/reduction reaction related to Na
+
ion
insertion mechanism is totally reversible.
Figure 3: (a) Initial voltage profiles, (b) Discharge capacity and (c) Coulombic efficiency versus cycle
number of HC/Na half-cells using five carbonate-based electrolytes with 1 M NaFSI
Vietnam Journal of Chemistry Electrochemical performance of hard carbon
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 647
4. CONCLUSIONS
The physical properties of NaFSI salt dissolved in
different carbonate-based solvents and their effects on
the electrochemical performance in HC/Na half-cell
have been investigated successfully. The electrolyte
EC:DMC (1:1) has good conductivity and low
viscosity but it unanticipated performs low discharge
capacity of 144.1 mAh.g
-1
. The electrolyte
EC:PC:DMC (3:1:1) is the optimal electrolyte with
the highest capacity and long cycle life despite its
moderate conductivity. The presence of EC is
essential for the effective formation of the SEI layer
on hard carbon surface. The Coulombic efficiency of
these electrolytes almost reached to 100 % confirms
that the charge/discharge of hard carbon is totally
reversibly in carbonate-based electrolytes. Further
study should focus on the characterization of SEI
formation (structure and composition) on the surface
of the hard carbon electrode to explore mechanism of
initial electrolyte reduction.
Acknowledgement. This research is supported by
Viet Nam National University through the research
grant number: B2020-18-06.
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Corresponding author: Le My Loan Phung
University of Science, VNU-HCM
227, Nguyen Van Cu, district 5, Ho Chi Minh City 70000, Viet Nam
E-mail: lmlphung@hcmus.edu.vn.