Cite this paper: Vietnam J Chem., 2021, 59(1), 17-26 Article DOI: 10.1002/vjch.202000078 Safe sodium-ion battery using hybrid electrolytes of organic solvent/pyrrolidinium ionic liquid Phung Quan1, Le Thi My Linh2, Huynh Thi Kim Tuyen3, Nguyen Van Hoang1,3, Vo Duy Thanh3, Tran Van Man1,3, Le My Loan Phung1,3* 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 Materials Science and Engineering, Pennsylvania State University, Pennsylvania 16802 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, NButyl-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 Na0.44MnO2 (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 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 lithiumbased 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 lithiumchemistry 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 17 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Vietnam Journal of Chemistry 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 with 1-ethyl-3-methylimidazolium 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 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 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 NaTFSIincorporated IL electrolyte; moreover, the cell could retain 65% of this capacity when the chargedischarge 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) Le My Loan Phung et al 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 N1114TFSI Py14TFSI Pp14TFSI HOMO value (eV)1 -0.453 -0.467 -0.462 Eanodic (V)2 5.6 5.2 5.3 Viscosity, 20 oC (mPa.s) 168 92 210 Value from DFT calculation 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 SigmaAldrich (≥ 99 %), stored in a controlled argon-filled glovebox having a humidity content below 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 (homemade 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 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 18 Safe sodium-ion battery using hybrid electrolytes… Vietnam Journal of Chemistry 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 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 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 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 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 microelectrode 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 RESULTS AND DISCUSSION 3.1 Thermal and conduction properties of electrolytes containing various amount of ionic liquid Py14TFSI Figure 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 solventbased 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 weightloss corresponding to % 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 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 19 Vietnam Journal of Chemistry 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 Le My Loan Phung et al 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 (a-d) shows the glass fiber mat after the flame switched off Table reports the occurrence of ignition (each sample was tested times) and the average value (ca 10% error) of the selfextinguishing 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 indicates the flame inhibition effect induced by the addition of IL: all samples containing 10 %wt of IL ignited, but only sample out of 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 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 20 Safe sodium-ion battery using hybrid electrolytes… Vietnam Journal of Chemistry 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 EC-PC EC-PC + 10 %wt IL EC-PC + 20 %wt IL EC-PC + 30 %wt IL EC-PC + 40 %wt IL IL + 30 %wt EC IL + 30 %wt PC Ignition occurrence 6/6 6/6 6/6 4/6 3/6 2/6 2/6 SET (s.g-1) 50.7 74.7 87.7 104.9 113.7 61.2 94.0 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 Electrolytes EC-DMC EC-DMC + 10 %wt IL EC-DMC + 20 %wt IL EC-DMC + 30 %wt IL EC-DMC + 40 %wt IL EC-PC-DMC + 20 %wt.IL Py14TFSI pure Ignition occurrence 6/6 6/6 5/6 4/6 3/6 6/6 0/6 SET (s.g-1) 63.6 67.3 80.9 103.3 116.0 71.9 - the overall estimation of the safety behavior of ILbased 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 Py14TFSI + 1M NaTFSI EC-PC (1:1 in vol%) + 1M NaTFSI EC-PC + 10 %wt IL + 1M NaTFSI EC-PC + 20 %wt IL + 1M NaTFSI EC-PC + 30 %wt IL + 1M NaTFSI EC-PC + 40 %wt IL + 1M NaTFSI EC-DMC (1:1 in vol%) + 1M NaTFSI EC-DMC + 10 %wt IL+ 1M NaTFSI EC-DMC + 20 %wt IL+ 1M NaTFSI EC-DMC + 30 %wt IL+ 1M NaTFSI EC-DMC + 40 %wt IL+ 1M NaTFSI EC-PC-DMC (3:1:1 in vol%) + 1M NaTFSI EC-PC-DMC + 20 %wt IL+ 1M NaTFSI EC-PC-DMC + 40 %wt IL+ 1M NaTFSI IL + 30 %wt EC + 1M NaTFSI IL + 30 %wt PC + 1M NaTFSI Conductivity (mS.cm-1) 5.6 13.2 10.8 10.3 8.90 8.10 15.1 12.6 11.6 11.2 10.1 13.9 12.2 10.9 11.2 10.0 Density (g.cm-3) 1.412 1.249 1.270 1.293 1.310 1.189 1.209 1.217 1.253 1.263 1.297 1.370 1.320 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 21 Vietnam Journal of Chemistry Le Loan My Phung et al 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 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 EC-PC EC-PC + 10 %wt IL EC-PC + 20 %wt IL EC-PC + 30 %wt IL EC-PC + 40 %wt IL EC-DMC EC-DMC + 10 %wt IL Ea (J.mol-1) 1760 2295 2559 2726 2959 1890 2112 The fitting results with VTF equation of ionic conductivity in a range of ~25 to ~60 oC help to Electrolytes EC-DMC + 20%wt IL EC-DMC + 30 %wt IL EC-DMC + 40 %wt IL EC-PC-DMC EC-PC-DMC + 20 %wt IL EC-PC-DMC + 40 %wt IL IL + 30 %wt EC IL + 30 %wt PC Ea (J.mol-1) 2303 2702 2968 1882 2542 2849 2959 2976 explain the conduction mechanism of the complex electrolytes As expected, the ionic conductivity of © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 22 Vietnam Journal of Chemistry the electrolytes drops at decreasing temperature and shows the VTF behavior normally observed in amorphous ionic conductors Table 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 mV.s-1 Safe sodium-ion battery using hybrid electrolytes… feasibility of Py14TFSI for the application of sodium secondary batteries CV exhibits a pair of redox peaks around 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 + M NaTFSI/Na cell, cycled 30 times between and 2.5 V are shown in figure 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 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 23 Vietnam Journal of Chemistry in the battery The mechanism behind such a decrease is presently unknown Usually, the Le My Loan Phung et al 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 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 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 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 24 Vietnam Journal of Chemistry Safe sodium-ion battery using hybrid electrolytes… 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 + %wt FEC + 1M NaTFSI CONCLUSIONS 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 A systematic study of ion liquid Py14TFSI basedelectrolyte 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 + %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 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by a large addition of an ionic liquid, Py14TFSI, to binary solvents containing a sodium salt dissolved in carbonate-based (combination of ethylene carbonate... Py14TFSI at a scan rate of mV.s-1 Safe sodium- ion battery using hybrid electrolytes? ?? feasibility of Py14TFSI for the application of sodium secondary batteries CV exhibits a pair of redox peaks around... ammonium ionic liquids, Ionics, 2012, 18, 817-827 G F Elia, U Ulissi, S Jeong, S Passerini, J Hassoun Exceptional long-life performance of lithium-ion batteries using ionic liquid- based electrolytes,