Safe sodium on battery using hybrid electrolytes of organic solventpyrrolidinium ionic liquid

DOI: 10.1002/vjch.202000078Safe sodium-ion battery using hybrid electrolytes of organic solvent/pyrrolidinium ionic liquid Phung Quan 1 , Le Thi My Linh 2 , Huynh Thi Kim Tuyen 3 , Ngu

DOI: 10.1002/vjch.202000078

Safe sodium-ion battery using hybrid electrolytes of organic

solvent/pyrrolidinium ionic liquid Phung Quan 1 , Le Thi My Linh 2 , Huynh Thi Kim Tuyen 3 , Nguyen Van Hoang 1,3 , Vo Duy Thanh 3 ,

Tran Van Man 1,3 , Le My Loan Phung 1,3*

1

Department of Physical Chemistry, Faculty of Chemistry, University of Science, Vietnam National University - Ho Chi Minh City, 227 Ly Thuong Kiet, District 5, Ho Chi Minh City 70000, Viet Nam

2

Materials Science and Engineering, Pennsylvania State University, Pennsylvania 16802

3

Applied Physical Chemistry Laboratory (APCLAB), University of Science, Vietnam National University -

Ho Chi Minh City, 227 Ly Thuong Kiet, District 5, Ho Chi Minh City 70000, Viet Nam

Submitted July 2, 2020; Accepted August 11, 2020

Abstract

Ionic liquids (ILs) have been considered as an alternative class of electrolytes compared to conventional carbonate solvents in rechargeable lithium/sodium batteries However, the drawbacks of ILs are their reducing ionic conductivity and their large viscosity Therefore, mixtures of alkyl carbonate solvents with an IL and a sodium bis(trifluoromethane sulfonyl)imide (NaTFSI) have been investigated to develop new electrolytes for sodium-ion batteries In this work, N-Butyl-N-methylpyrrolidinium bis(trifluoro-methanesulfonyl) imide (Py14TFSI) was used as co-solvent mixing with commercial electrolytes based on the carbonate, i.e EC-PC (1:1), EC-DMC (1:1), and EC-PC-DMC (3:1:1) The addition of ionic liquid in the carbonate-based electrolyte solution results in (i) enhancing ionic conductivity to be comparable with a solvent-free IL-based electrolyte, (ii) maintaining the electrochemical stability window, and (iii) IL acted as a retardant rather than a flame-inhibitor based on the self-extinguish time (SET) of the mixed electrolyte mixture when exposed to a free flame All mixed electrolyte systems have been tested in sodium-coin cells versus

Na 0.44 MnO 2 (NMO) and hard carbon (HC) electrodes The cells show good performances in charge/discharge cycling with a retention > 96 % after 30 cycles (∼90 mAh.g -1 for NMO and 180 mAh.g-1 for HC, respectively) demonstrating good interfacial stability and highly stable discharge capacities

Keywords Ionic liquid, Pyr14TFSI, co-solvent, electrolytes, sodium-ion batteries

1 INTRODUCTION

Currently, lithium-ion technology is dominant in the

market from abundant small to medium (e.g

portable electronic devices, power tools etc.) as well

as large-scale applications (e.g electric/hybrid

vehicles, smart grids, electric energy storage from

renewable power sources).[1,2] However, the fast

growth of the lithium-ion batteries market leads to

big concerns about the availability and price rising

of lithium resources.[3-5] Lithium metal and

lithium-based compounds are not worldwide available and

are mainly distributed in some politically unstable

countries

Although large-scale lithium recycling programs

have been planned, the future exhaustion of lithium

could occur.[3,6,7] These concerns have inspired the

battery scientist community to launch a novel

alternative technology with similar characteristics

Low cost, large abundant availability of sodium minerals as well as the feasible use of aluminum as anode current collector[8] have promoted the research

in sodium-based technology as an alternative energy storage system.[9] In addition, sodium- and lithium-chemistry exhibit some similar fundamental features,[4,5] and their redox potential differs by only

300 mV.[8-10] Nevertheless, like lithium-ion batteries, Na-based systems often use alkyl carbonate-based electrolytes, which represent safety issues.[11-14] For instance, uncontrolled internal temperature increase might cause flammability of the volatile organic electrolyte with oxygen originated from the decomposition of the positive electrode,[15,16] leading

to catastrophic events (burning, explosion, rapid cell disassembly) Therefore, many efforts have been devoted to designing highly stable and compatible electrolytes aiming to resolve the drawbacks

Interestingly, mixing ionic liquids (ILs) with

aprotic organic solvents to form hybrid electrolytes

have been proposed in the literature.[17-20] For

example, a hybrid electrolyte comprising 1M LiPF6

in ethylene carbonate and diethyl carbonate mixed

bis(trifluoromethanesulfonyl) imide ionic liquid is

possible to improve the safety without

compromising performances.[17] Moreover, N-alkyl

pyrrolidinium (Py) and piperidinium (Pp) cations

combined with imide anion have exhibited some

interesting properties Indeed, viscosities are close to

those involved by imidazolium ILs and good

conductivity values are reached.[21] Comparing to the

quaternary ammonium ILs (N111xILs) and

piperidinium ILs, PyILs is as stable in oxidation as

PpILs with the HOMO values showing in table 1

Furthermore, Py14TFSI + LiTFSI electrolyte showed

remarkable performance in terms of efficiency and

rate-capability for using in lithium cell using the

alloying Sn–C nanocomposite negative and

LiFePO4 positive electrodes.[22] Full-cell using ILs

electrolyte delivered a maximum reversible capacity

of about 160 mA h.g-1 (versus cathode weight) at a

working voltage of about 3 V corresponding to an

estimated practical energy density of about 160

Wh.kg-1 prolonged over 2000 cycles without

declined signs and satisfactory rate capability This

high performance and the high safety provided by

the IL-electrolyte make this cell chemistry feasible

for application in new-generation electric and

electronic devices.[22] Wongittharom et al.[23]

demonstrated the Na/NaFePO4 cell with a sodium

bis(trifluoromethanesulfonyl)imide

(NaTFSI)-incorporated Py14TFSI ionic liquid (IL) electrolyte

operating in the voltage of ∼3 V The relationship

between cell performance and NaTFSI concentration

(0.1-1.0 M) at 25 and 50 °C is investigated At 50

°C, the highest capacity of 125 mAh.g−1 (at 0.05 C)

was found for NaFePO4 in a 0.5 M

NaTFSI-incorporated IL electrolyte; moreover, the cell could

retain 65% of this capacity when the

charge-discharge rate increased to 1C Py14TFSI could be

used as a co-solvent with conventional carbonate

solvents in mixed electrolytes to enhance the thermal

and oxidation potential stability The previous

studies indicated that electrolytes contain 20-30

%wt of IL give the best balance between viscosity

and ionic conductivity.[24-26]

Herein, we report the characterization of hybrid

electrolytes prepared by a large addition of an ionic

liquid, Py14TFSI, to binary solvents containing a

sodium salt dissolved in carbonate-based

(combination of ethylene carbonate (EC), dimethyl

carbonate (DMC), and propylene carbonate (PC))

solutions, i.e 1M NaTFSI in EC-DMC (1:1 %wt)

and EC-PC (1:1 %wt) The performance of sodium half-cells using Hard carbon (HC) and NaMn0.44MnO2 (NMO) was tested with these new electrolytes Our results demonstrate good stability and highly stable discharge capacity of the battery based on these electrolytes

Table 1: Physical properties of different ionic liquids

using a variety of cations combined with

TFSI- anion Ionic

liquid

HOMO value (eV)1

Eanodic

(V)2

Viscosity,

20 oC (mPa.s)

N1114TFSI -0.453 5.6 168

Pp14TFSI -0.462 5.3 210

1 Value from DFT calculation

2

Oxidation potential determined from cyclic voltammetry

in a three-electrode cell

2 MATERIALS AND METHODS

2.1 Preparation of ionic liquid-based electrolytes and electrodes

Py14TFSI and NaTFSI were bought from Sigma-Aldrich (≥ 99 %), stored in a controlled argon-filled glovebox having a humidity content below 5 ppm to avoid any contamination Other chemical reagents including EC, PC, and DMC were also purchased from Sigma-Aldrich (≥ 98 %) Ionic liquid-based electrolytes were obtained by mixing different amounts (10-40 %wt.) of Py14TFSI to both the carbonate-based solutions EC-PC (1:1), EC-DMC (1:1), and 1M NaTFSI These mixtures were vigorously stirred with a magnetic paddle for at least

24 hours to form a homogeneous solution

The anode/cathode electrodes were prepared by doctor-blade coating on the aluminum substrate of a slurry formed of 80%wt active material (home-made NMO or HC KUREHA, Japan), 5%wt PVDF

6020 (Solvay Solef) binder and 10%wt acetylene black (Timcal, Swiss) The electrode films were all dried at 80 oC in a vacuum oven overnight then were punched in 15 mm diameter round discs The electrode discs were dried under vacuum overnight

at 100 oC and directly transferred into an Ar-filled glove box for cell assembly

2.2 Characterization techniques

Density functional theory (DFT) calculations of the HOMO value (based on geometry optimization and frequency computation) were carried out with the

GAUSSIAN 03 software package with a basic set of

B3LYP/6-311++G(2d,p)

Thermal stability of mixed electrolytes was

characterized by thermogravimetric analysis (TGA)

measurements using TGA Q500 V20.10 Build A

few milligrams of the sample were heated from the

room temperature up to 600 °C at 10 °C.min-1 with

nitrogen flow

Flammability tests were performed to measure

the thermal stability of hybrid electrolytes A fixed

weight of electrolytes was impregned into a glass

fiber filter that was exposed for 5 seconds by a

burner staying 15 cm far away The time required to

extinguish the flame was recorded and normalized

against liquid mass to evaluate the self-extinguish

time (SET) in s.g-1.[18]

The ionic conductivity of mixed electrolytes was

calculated from AC impedance spectroscopy method

using an HP 4192A impedance analyzer in the

frequency range from 5 to 13 MHz The

conductivity test cell with platinized platinum

blocking electrodes was dipped in the electrolyte

solution and calibrated by 0.01 M KCl at 25 oC to

determine the cell constant The ionic conductivity

measurements were performed in the temperature

range from 10 to 60 oC The cell should be kept at a

constant temperature for at least 1 hour to reach

thermal equilibration

The electrical conductivity data taken at different

temperatures were fitted using Vogel–Tamman–

Fulcher (VTF) equation to obtain activation energy

(Eq 1) It is common for researchers to utilize the

(VTF) equation to separate the effects of charge

carrier concentration, often related to the pre-factor,

A, and segmental motion, related to the activation

energy, Ea, on overall conductivity, σ, at a given

temperature T.[27]

(

( )) (1)

T0 in this equation is referred to as the Vogel

temperature, equal to the glass transition in ideal

glasses,[28] but typically taken as 50 °C below the

glass transition temperature in several electrolytes

Cyclic voltammetry (CV) measurements were

performed at the scan rate of 1 mV.s-1 recorded on

MGP2 Biologic Instrument to assess the stability of

the electrolytes over oxidation and reduction

Measurements were carried out by using a standard

three-electrode cell The counter electrode was a Pt

wire and the working electrode was a Pt

micro-electrode with a diameter of 25 μm The reference

electrode was a silver wire embed in a solution of

AgNO3 10 mM in acetonitrile + 0.1 M

tetrabutylammonium perchlorate (TBAP)

The coin cells for galvanostatic tests were assembled by coupling a sodium metal foil with NMO or HC that separated by a Celgard separator soaked by the prepared electrolytes

The sodium half-cells were cycled at the current constant C/10 in the potential range 0.04-2.4 V and 2-4 V for HC and NMO, respectively, using an MGP2 Biologic Battery Test System The performance of the cells was evaluated in terms of specific capacity, charge/discharge efficiency, and cycle life

3 RESULTS AND DISCUSSION

3.1 Thermal and conduction properties of electrolytes containing various amount of ionic liquid Py14TFSI

Figure 1 shows the TGA curves of all considered

electrolytes As indicated in Fig 1, there was almost

no weight loss for pure Py14TFSI-based electrolyte

up to 360 oC; confirming the excellent thermal stability of the ionic liquid In contrast, the significant weight loss of the carbonate solvent-based electrolytes, due to vigorous evaporation, was approximately 85 % per initial content at 180 oC for all cases.[29,30] The addition of Py14TFSI into the electrolyte shifted the solvent evaporation temperatures to higher values The increase in EC,

PC, DMC evaporation temperature deduced from the variable temperature TGA experiments support the interaction between solvents and Py14TFSI.[24,25]

At 100 oC, the mixtures containing 10 %, 20 %,

30 %, and 40 % of Py14TFSI displayed weight losses

of 2.6, 1.8, 1.4 and 0.9 %, respectively A weight-loss corresponding to 5 % was reached at significantly higher temperatures for the mixtures (130.2, 136.1, 138.0, 143.5 oC for the 10, 20, 30 and

40 % IL mixture, respectively) with respect to the EC-PC-based electrolytes, for which this weight loss was already reached at 80 oC

For the EC-DMC-based samples, the first weight loss commensurate with insoluble DMC evaporation

in the complexes, the second thermal process begins near 150 oC and finishes near 250 oC proportionated with solvents evaporation in the ternary systems and the third degradation starts near 400 oC and finishes approximately 500 oC Thermal decomposition of the Py14TFSI component of the mixtures appears to shift to a higher temperature (at least for these variable-temperature measurements)

According to Fig 1(c), a similar result was found for EC-PC-DMC-based electrolytes with two steps of weight loss observed The first process

related to solvents evaporation starts at near 100 oC

and finishes at near 200 oC The second process

proportionated with a decomposition of IL was in

the range of 400 – 500 oC The separated addition of

EC or PC in pure IL does not affect much in the complexes The range of temperature for solvents evaporation and IL degradation in the mixtures was the same with the electrolyte only IL contained

Figure 1: TGA diagrams of ionic quid Py14TFSI mixed with (a) EC-DMC 1:1, (b) EC-PC 1:1, (c) 30 %wt

EC or PC along with 1M NaTFSI, (d) evolutions of weight loss versus temperature

Fig 2 (a-d) shows the glass fiber mat after the flame

switched off Table 2 reports the occurrence of

ignition (each sample was tested 6 times) and the

average value (ca 10% error) of the

self-extinguishing time of the mixed electrolytes

containing organic solvents and Py14TFSI

Like EMITFSI, under exposure to the burner,

Py14TFSI produced only small flare-ups that

promptly extinguished once the burner switched off,

thus it was not considered as ignition Ignition

occurrence in table 1 indicates the flame inhibition

effect induced by the addition of IL: all 6 samples

containing 10 %wt of IL ignited, but only 1 sample

out of 6 containing 40 %wt did The lower amount

of Py14TFSI needed to observe the flame-inhibition

effect was 20 wt%., whereas at 40 wt% the

tendency to ignite was significantly reduced In

contrast, the SET values showed an opposite trend: the larger IL content in the sample, the higher the

Figure 2: Glass fiber mats after flammability tests of

the electrolytes: (a) IL pure, (b-d) 20 %wt PY14TFSI + 1M NaTFSI amalgamated in solvent solutions EC-PC, EC-DMC, EC-PC-DMC, respectively (e)

IL during exposure to flame EC-DMC (1:1) + 10

%wt IL +1M NaTFSI

time needs for the flame to extinguish (normalized

against liquid mass) The samples with 10 %

Py14TFSI ignited with a SET value of 74.7 s.g-1 and

67.3 s.g-1) for the electrolytes mixed with EC-PC

and EC-DMC, respectively, which were higher than

that (50.7 s.g-1 of pure EC-PC and 63.6 s.g-1 of pure

EC-DMC)

The results can be assumed that ignited solvent

vapors triggered the combustion of IL The

oxidizing flame completely burned the solvent vapors without leaving a layer of carbon as the reducing yellow flame of a lighter does By contrast, the samples soaked with the solutions containing

Py14TFSI in different percentages displayed carbonaceous deposit due to slow, oxygen-poor combustion of the IL triggered by the organic electrolyte: the more IL in the mixture, the more carbon was formed

Table 2: The mean values of the Self-Extinguishing Time (SET) of several mixtures of IL and binary-solvent

systems with 1M NaTFSI in all samples

Electrolytes Ignition

occurrence

SET (s.g-1) Electrolytes

Ignition occurrence

SET (s.g-1)

The hybrid electrolytes based on binary systems

EC-DMC or EC-PC with Py14TFSI are less volatile

than the pure conventional electrolyte, an effect that

was more evident the more IL was added

Nevertheless, the mixtures containing Py14TFSI

easily ignite because the presence of the organic

solvent continued to burn with SET values

proportional to the amount of IL, which acted as a

retardant rather than a flame-inhibitor The fact that

the mixtures with high amounts of ionic liquids are

more difficult to ignite and burn for a longer time

once they are ignited is worth noting especially for

the overall estimation of the safety behavior of IL-based mixed electrolytes

Ionic conductivity and density of ILs based electrolyte was also evaluated at room temperature (table 3) Py14TFSI has the lowest conductivity compared to the carbonated-based electrolyte due to its high viscosity The increase of Py14TFSI addition

in conventional electrolytes lowered their ionic conductivity because of the viscosity increase Thus, the two aspects (viscosity and conductivity) should

be comprised to obtain the favorable performance of the ILs-based electrolytes

Table 3: Density and ionic conductivity of complex electrolytes based on Py14TFSI at 30 °C

Electrolytes Conductivity (mS.cm-1) Density (g.cm-3)

Figure 3: The dependence of conductivity on the temperature of electrolyte mixtures

a) EC-DMC (1:1) + %wt IL, b) EC-PC (1:1) + %wt IL, c) EC-PC-DMC (3:1:1) + %wt IL,

d) IL as the main solvent NaTFSI was dissolved in all samples at a concentration of 1 M

In figure 3, the evolution of ionic conductivity

against temperature range 20-60 oC for all

investigated electrolytes are reported Ionic liquids

normally display a relatively high viscosity, which

continuously increases with the addition of Na-salt

When the organic solvent was added to the complex

solutions, the conductivity of mixed electrolytes is

increased because the viscosity decreased

significantly compared to the pure ionic liquid The tendency of conductivity is also explained by the dilution of Na-salt into solutions However, the temperature also has a considerable contribution to viscosity and conductivity values The viscosity was decreased along with the rising of ionic rate versus the temperature, thus, leading to an enhancement in conductivity

Table 4: The activation energy of the mixed electrolytes: EC-PC (1:1) or EC-DMC (1:1) + x %wt Py14TFSI

Electrolytes Ea (J.mol-1) Electrolytes Ea (J.mol-1)

The fitting results with VTF equation of ionic

conductivity in a range of ~25 to ~60 oC help to

explain the conduction mechanism of the complex electrolytes As expected, the ionic conductivity of

the electrolytes drops at decreasing temperature and

shows the VTF behavior normally observed in

amorphous ionic conductors Table 4 shows that the

activation energy of IL decreased along with an

increase in the amount of EC-PC or EC-DMC This

is because dilution of solutions partially holds up

ionic bond slightly, which is easy to them extract

and become non-electrical charged particles

3.2 Electrochemical window of ionic liquid

py14TFSI as an electrolyte for sodium-ion battery

Voltammograms for each electrolyte (with a

platinum working electrode) is shown in Fig 4(a-b)

The oxidation stability limit of this IL is

approximately 6.1 V (vs Na/Na+) corresponds to the

irreversible oxidation of Py14

+

cation, while the cathodic limiting current at 0.9 V (vs Na+/Na)

indicated the reduction of TFSI anion and with the

lack of a passive layer formation.[31,32] This limit is

much higher than might be expected as anion

contains an oxalate group This confirms the

Figure 4: Cyclic voltammetry curves of (a)

Py14TFSI used as co-solvent in binary system

EC-PC and 1M NaTFSI; (b) pure Py14TFSI at a scan

rate of 1 mV.s-1

feasibility of Py14TFSI for the application of sodium secondary batteries CV exhibits a pair of redox peaks around 1 V (vs Na+/Na) in the presence of NaTFSI, which corresponds to the deposition and dissolution of Na on and from the copper substrate, respectively These pieces of evidence demonstrate that NaTFSI-Py14TFSI binary electrolyte was cathodically stable towards sodium metal, with an electrochemical window of ca 0-6.1 V (vs Na+/Na)

3.3 Battery tests with different electrode materials and ionic liquid-based electrolytes

The galvanostatic charge-discharge curves of the HC/EC-DMC (1:1) + x %wt IL + 1 M NaTFSI/Na cell, cycled 30 times between 0 and 2.5 V are shown

in figure 5 The HC cell demonstrated a single charge-discharge plateau at 0.8 and 1.1 V versus Na/Na+, which reflects the low cell resistance that remains stable during cycling During the first charging process at 0.1 C-rate, the voltage increased regularly to 2.5 V from the open-circuit voltage, gradually increased to the cut-off voltage and a high capacity of 485 mAh.g-1 for both electrolytes containing 10 and 20 %wt IL The irreversible capacity loss between the first charge and discharge reaction was less than 20 mAh.g-1 giving almost 93

% Colombic efficiency for HC The cell shows stable cycling performance at 0.1C over the 30 cycles made in this work One can see that, except for the initial three cycles, the cell cycled with an initial capacity above 130 mAh.g-1 for all investigated electrolytes and retained excellent capacity retention of 96 % at room temperature The good cycle ability of electrolyte mixtures could be suggested by the diffusion of Py14TFSI from electrolytes to the composite cathode to form a stable ternary blend with the PVdF binder in the electrode The good cycling performance of the cell

is promising and reflects a combination of thermal and electrochemical stability of the ionic liquid electrolyte and excellent conduction properties The charge/discharge behavior at a 0.1C-rate and

at room temperature of a HC/EC-PC (1:1) + x %wt

IL + 1M NaTFSI/Na cell during the 30 cycles is shown in figure 6(a) The highest first charge and discharge capacities are 370 and 400 mAh.g-1, respectively These values were reached at EC-PC adulterated with 20 %wt IL, confirming the high Coulombic efficiency of the redox process However, during the first charge/discharge cycle, the plateaus are shorter than in the following cycles The apparent increase in performance may be attributed

to a decrease in internal resistance during the first completed cycle, thereby decreasing the ohmic loss

in the battery The mechanism behind such a

decrease is presently unknown Usually, the

formation of the SEI layer on the electrode surface involves an irreversible capacity fade

Figure 5: (a) Initial discharge curve of half-cell (-) Na | EC-DMC (1:1) + x %wt IL + 1M NaTFSI | HC, (b)

Discharge capacity as a function of cycle number at C/5 The discharge capacity as a function of the cycle

number of the cells cycled at room temperature

under 0.1C-rates is presented in figure 6(b) The cell

shows a stable cycling performance over the 50

cycles followed in this work After 30 cycles, a discharge capacity of 140 mAh.g-1 was obtained for the virtual cells, thus the capacity retention is 96 %

Figure 6: (a) Initial discharge curve of half-cell (-) Na | EC-PC (1:1) + x %wt IL + 1M NaTFSI | HC,

(b) Discharge capacity as a function of cycle number at C/5 Similarly, the cycling performance was conducted

on some IL-based electrolytes for NMO electrode

material during 30 cycles and displayed in figure 7

Among those investigated, EC-PC- based mixtures

containing 10 wt.% of IL appeared the best choice,

showing capacity values in sodium metal cells

comparable The origin of this superior behavior is

rooted in the fundamental chemistry that drives the

changes of the conductivities, variation of the

stability windows, and ability to sustain

galvanostatic cycling, and it is beyond the scope of

this paper After 30 cycles, the Coulomb efficiency was approximately 95 % and the capacity retention around 90 mAh.g-1

The cycling performance of SIBs with IL used as co-solvent with EC-PC (1:1) and EC-PC-DMC (3:1:1) for anode material HC at a rate of C/10 was shown in figure 8 As the amounts of IL in the binary electrolytes increase, the irreversible capacity

is also observed in the first cycle of discharge due to the formation of the SEI layer on the anode materials In the following cycles, the discharge

Figure 7: (a) Initial discharge curve of half-cell (-)

Na | EC-PC (1:1) + x %wt IL + 1M NaTFSI | NMO,

(b) Discharge capacity as a function of cycle number

at C/10

Figure 8: (a) Discharge capacity as a function of

cycle number at C/10 of half-cell (-) Na | EC-PC

(1:1) + x %wt IL + 1M NaTFSI | HC with additive

FEC was used capacity will be stabilized However, with increasing

of ILs amount in EC-PC mixture, the 1st irreversible

increase significantly and the battery lost completely performance in following cycles, especially, up to 40

%wt IL The stable performance is only maintained within a good value of capacity when using FEC additive due to stabilizing SEI layer to prevent the electrolyte reduced or oxidized of electrolyte so far after the 1st discharge cycle The best capacity of approximated 160 mAh.g-1 was gained by the cell using EC-PC-DMC ternary solvents + 40 %wt IL +

2 %wt FEC + 1M NaTFSI

4 CONCLUSIONS

A systematic study of ion liquid Py14TFSI based-electrolyte was conducted in three solvent mixtures: EC-PC (1:1), EC-PC-DMC (3:1:1), EC-DMC (1:1)

In the mixtures of Py14TFSI used as co-solvent, good conductivity values are obtained with Py14TFSI; conductivities slightly similar to those obtained with

Py14fTFSI, Pp14TFSI, and Pp13TFSI The addition of ILs amount increased the thermal stability and oxidation limitation potential of electrolytes, however, a significant decrease of ionic conductivity

at high IL concentration was observed due to the increase of viscosity Cycling performance was tested for electrode materials of hard carbon and

Na0.44MnO2 The first discharge capacity was related

to the formation of the SEI layer and this value climbed up with an increase of ILs content in mixtures The stable discharge capacity of half-cell using EC-PC (1:1) + 20 %wt EC + 2 %wt FEC was obtained This approach indicated a similar “good” ionization The SET values also show Py14TFSI good flammable resistance, which acted as a retardant in the mixtures

Acknowledgments The authors acknowledge

funding from University of Science through the research grant number: T2018-15

REFERENCES

1 B E Conway, Electrochemical Supercapacitors, Kluwer Academic/Plenum Publishers, New York,

1999

2 J M Tarascon, M Armand Issues and challenges

facing rechargeable lithium batteries, Nature, 2001,

414, 359-367

3 V Palomares, P Serras, I Villaluenga, K B Hueso,

J Carretero-González, T Rojo Na-ion batteries, recent advances and present challenges to become

low cost energy storage systems, Energy Environ

Sci., 2012, 5, 5884-5901

4 J M Tarascon Is lithium the new gold?, Nat Chem.,

2010, 2, 510

5 H Kawamoto, W Tamaki Trends in supply of

lithium resources and demand of the resources for

automobiles, Sci Technol Trends Q Rev., 2011, 39,

51-64

6 F Risacher, B Fritz Origin of salts and brine

evolution of Bolivian and Chilean salars, Aquat

Geochem., 2009, 15, 123-157

7 A Yaksic, J E Tilton Using the cumulative

availability curve to assess the threat of mineral

depletion: the case of lithium, Resour Pol., 2009, 34,

185-194

8 S W Kim, D H Seo, X Ma, G Ceder, K Kang

Electrode materials for rechargeable sodium-ion

batteries: potential alternatives to current lithium-ion

batteries, Adv Eng Mater., 2012, 2, 710-721

9 M D Slater, D Kim, E Lee, C S Johnson

Sodium-ion batteries, Adv Funct Mater., 2013, 23, 947-958

10 B L Ellis, L F Nazar Sodium and sodium-ion

energy storage batteries, Curr Opin Solid State

Mater Sci., 2012, 16, 168-177

11 R Spotnitz, J Franklin Abuse behavior of

high-power, lithium-ion cells, J Power Sources, 2003,

113, 81-100

12 H Yang, S Amiruddin, H J Bang, Y K Sun, J

Prakash A review of Li-ion cell chemistries and their

potential use as hybrid electric vehicles, J Ind Eng

Chem., 2006, 12, 12-38

13 D P Abraham, E P Roth, R Kostecky, K

McCarthy, S MacLaren, D H Doughty Diagnostic

examination of thermally abused high-power

lithium-ion cells, J Power Sources, 2006, 161, 648-657

14 T M Bandhauer, S Garimella, T F Fuller A

critical review of thermal issues in lithium-ion

batteries, J Electrochem Soc., 2011, 158, R1-R25

15 Q Wang, P Ping, X Zhao, G Chu, J Sun, C Chen

Thermal runaway caused fire and explosion of

lithium ion battery, J Power Sources, 2012, 208,

210-224

16 H F Xiang, H Wang, C H Chen, X W Ge, S

Guo, J H Sun, W Q Hu Thermal stability of

LiPF6-based electrolyte and effect of contact with

various delithiated cathodes of Li-ion batteries, J

Power.Sources, 2009, 191, 575-581

17 A Guerfi, M Dontigny, P Charest, M Petitclerc, M

Lagacé, A Vijh, K Zaghib Improved electrolytes

for Li-ion batteries: Mixtures of ionic liquid and

organic electrolyte with enhanced safety and

electrochemical performance, J Power Sources,

2010, 195, 845-852

18 C Arbizzani, G Gabrielli, M Mastragostino

Thermal stability and flammability of electrolytes for

lithium-ion batteries, J Power Sources, 2011, 196,

4801-4805

19 P M Bayley, G H Lane, N M Rocher, B R Clare,

A S Best, D M MacFarlane, M Forsyth Transport

properties of ionic liquid electrolytes with organic

diluents, Phys Chem Chem Phys., 2009, 11,

7202-7208

20 T Sato, T Maruo, S Marukane, K Takagi Ionic

liquids containing carbonate solvent as electrolytes

for lithium ion cells, J Power Sources, 2004, 138,

253-261

21 M L P Le, F Alloin, P Strobel, J C Leprêtre, L Cointeaux, Carlos Pérez del Valle Electrolyte based

on fluorinated cyclic quaternary ammonium ionic

liquids, Ionics, 2012, 18, 817-827

22 G F Elia, U Ulissi, S Jeong, S Passerini, J Hassoun Exceptional long-life performance of lithium-ion batteries using ionic liquid-based

electrolytes, Energy Environ Sci., 2016, 9,

3210-3220

23 N Wongittharom, T -C Lee, C -H Wang, Y -C Wang, J K Chang Electrochemical performance of Na/NaFePO4 sodium-ion batteries with ionic liquid

electrolytes, J Mater Chem A, 2014, 2, 5655-5661

24 M L P Le, L Cointeaux, P Strobel, J C Leprêtre,

P Judenstein, F Alloin Influence of solvent addition

on the properties of ionic liquid, J Phys Chem C,

2012, 116, 7712-7718

25 T D Vo, H V Nguyen, Q D Nguyen, Q Phung, V

M Tran, P L M Le Carbonate solvents and ionic liquid mixtures as an electrolyte to improve cell

safety in sodium-ion batteries, J Chemistry, 2019,

Article ID 7980204

26 M L P Le, F Alloin, P Strobel, J C Leprêtre, L Cointeaux, Carlos Pérez del Valle, P Judeinstein Structure-Properties relationships of lithium

electrolytes based on ionic liquid, J Phys Chem B,

2010, 114(2), 894-903

27 K M Diederichsen, H G Buss, B D McCloskey The compensation effect in the Vogel-Tammann-Fulcher (VTF) equation for polymer-based

electrolytes, Macromolecules, 2017, 50, 3831-3840

28 G Y Gu, S Bouvier, C Wu, R Laura, M Rzeznik,

K M Abraham 2-Methoxyethyl (methyl)

carbonate-based electrolytes for Li-ion batteries, Electrochimica

Acta, 2000, 45, 3127-3139

29 T Kawamura, S Okada, J I Yamaki Decomposition reaction of LiPF6-based electrolytes

for lithium ion cells, J Power Sources, 2006, 156,

547-554

30 X Kang Nonaqueous liquid electrolytes for

Lithium-based rechargeable batteries, Chem Rev., 2004, 104,

4303-4418

31 V Borgel, E Markevich, D Aurbach, G Semrau, M Schmidt On the application of ionic liquids for

rechargeable Li batteries: High voltage systems, J

Power Sources, 2009, 189, 331-336

32 P C Howlett, E I Izgorodina, M Forsyth, D R MacFarlane Electrochemistry at negative potentials

in bis(trifluoromethanesulfonyl)-amide ionic liquis,

Phys Chem., 2006, 220, 1483-1498

Corresponding author: Le My Loan Phung

University of Science, VNU-HCM, 227 Nguyen Van Cu, Dist 5, Ho Chi Minh City 70000, Viet Nam E-mail: lmlphung@hcmus.edu.vn