Safe sodium-ion battery using hybrid electrolytes of organic solvent/pyrrolidinium ionic liquid

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.

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Cite this paper: Vietnam J. Chem., 2021, 59(1), 17-26 Article DOI: 10.1002/vjch.202000078 17 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH 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 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. 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 Vietnam Journal of Chemistry Le My Loan Phung et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 18 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 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 o C (mPa.s) N1114TFSI -0.453 5.6 168 Py14TFSI -0.467 5.2 92 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 o C in a vacuum oven overnight then were punched in 15 mm diameter round discs. The electrode discs were dried under vacuum overnight at 100 o C 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 Vietnam Journal of Chemistry Safe sodium-ion battery using hybrid electrolytes © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 19 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 o C to determine the cell constant. The ionic conductivity measurements were performed in the temperature range from 10 to 60 o C. 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 o C; 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 o C 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 o C, 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 o C 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 o C. 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 o C and finishes near 250 o C proportionated with solvents evaporation in the ternary systems and the third degradation starts near 400 o C and finishes approximately 500 o C. 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 Vietnam Journal of Chemistry Le My Loan Phung et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 20 related to solvents evaporation starts at near 100 o C and finishes at near 200 o C. 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 Vietnam Journal of Chemistry Safe sodium-ion battery using hybrid electrolytes © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 21 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 ) EC-PC 6/6 50.7 EC-DMC 6/6 63.6 EC-PC + 10 %wt. IL 6/6 74.7 EC-DMC + 10 %wt. IL 6/6 67.3 EC-PC + 20 %wt. IL 6/6 87.7 EC-DMC + 20 %wt. IL 5/6 80.9 EC-PC + 30 %wt. IL 4/6 104.9 EC-DMC + 30 %wt. IL 4/6 103.3 EC-PC + 40 %wt. IL 3/6 113.7 EC-DMC + 40 %wt. IL 3/6 116.0 IL + 30 %wt. EC 2/6 61.2 EC-PC-DMC + 20 %wt.IL 6/6 71.9 IL + 30 %wt. PC 2/6 94.0 Py14TFSI pure 0/6 - 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 ) Py14TFSI + 1M NaTFSI 5.6 1.412 EC-PC (1:1 in vol%) + 1M NaTFSI 13.2 - EC-PC + 10 %wt. IL + 1M NaTFSI 10.8 1.249 EC-PC + 20 %wt. IL + 1M NaTFSI 10.3 1.270 EC-PC + 30 %wt. IL + 1M NaTFSI
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