Electrochemical performance of hard carbon anode in different carbonate-Based electrolytes

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.