Pyrrolinium-based Ionic Liquid as a Flame Retardant for Binary

Research Article pubs.acs.org/journal/ascecg

Pyrrolinium-based Ionic Liquid as a Flame Retardant for Binary Electrolytes of Lithium Ion Batteries Hyung-Tae Kim,† Jaesik Kang,† Junyoung Mun,†,‡ Seung M. Oh,† Taeeun Yim,*,†,§ and Young Gyu Kim*,† †

School of Chemical and Biological Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea ‡ Department of Energy and Chemical Engineering, College of Engineering, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 406-772, Republic of Korea § Advanced Batteries Research Center, Korea Electronics Technology Institute, 25 Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do 463-816, Republic of Korea ABSTRACT: Ionic liquid (IL)-based electrolytes have attracted much attention to enable safe lithium ion batteries (LIBs) owing to their inflammability and thermal stability as well as their high conductivity and wide electrochemical stability. However, because of the interruption of lithium ion mobility during a charge/discharge process, ILs as the only electrolyte solvent could not be efficient. To improve both the lithium ion mobility and the safety issue of LIBs, we suggest new binary electrolytes that consist of pyrrolinium-based IL and commercial carbonate. The pyrrolinium-based IL is characterized by some task-specific functionalities such as a planar sp2 carbon of double bond, a Li+-binding C−O ether linkage, and no unstable C−H bond, which would provide better physicochemical and electrochemical properties than the known imidazolium- or pyrrolidinium-based ILs. On the basis of the material design, the binary electrolyte systems composed of the pyrrolinium-based IL (N-ethyl-2-methoxypyrrolinium bis(fluorosulfonyl)imide, E(OMe)Pyrl-FSI) and commercial carbonate electrolyte (1.0 M LiPF6 in EC/EMC = 3/7 with 3 wt % VC) were investigated. As a result, the proposed pyrrolinium-based binary electrolyte systems showed the improved properties as an electrolyte for LiFePO4 electrode as follows; (i) the ionic conductivity of some binary electrolyte systems was better than that of the 100 wt % IL or the 100 wt % carbonate-based electrolyte (ii) the significant suppression of the flammability of the binary electrolytes was observed (iii) the comparable cycle life of the binary electrolytes to that of the commercial carbonate electrolyte was maintained even at high rate performance (1 C). KEYWORDS: Pyrrolinium-based ionic liquids, Organic carbonate, Binary electrolytes, Lithium ion batteries, Safety

■

INTRODUCTION

through thermal runaway, which leads to an explosive accident of the cell. It implies that the flammable nature of conventional electrolytes should be improved allowing the LIBs to be more applicable to large-scale electronic devices.10,11 In this respect, ionic liquids (ILs) have been investigated as one promising alternative electrolyte to replace conventional organic electrolytes because the strong ionic bonding nature of ILs allows them to be nonflammable as well as nonvolatile. In addition, wide electrochemical windows of the ILs seem to be well compatible with high-voltage electrode materials, which should be useful for higher energy density of the cell.12−15 Nevertheless, their practical uses are still restricted for current LIBs because their high viscosity seriously disturbs the lithium

Lithium-ion batteries (LIBs) have been considered as one of the most promising energy conversion/storage systems for future large-scale devices such as electric vehicles (EVs) and energy storage systems (ESSs).1,2 However, the relatively low energy density of current LIBs should be overcome for their widespread adoption. In this regard, many attempts have been focused on the development of advanced electrode materials for a large specific capacity as well as high working voltage, which are responsible for great enhancement of overall energy density of the cell.3−9 On the other hand, ensuring a certain level of safety performance is recognized as a challengeable issue with increasing overall energy density of LIBs, which is closely associated with combustible electrolyte components. Conventional carbonate-based electrolytes are highly volatile and flammable. Once the cell is ignited by internal/external short, the combustible electrolytes readily participate in additional combustion reactions as they can act as a fuel. This would result in a rapid rising of the internal temperature © XXXX American Chemical Society

Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: August 30, 2015 Revised: October 20, 2015

A

DOI: 10.1021/acssuschemeng.5b00981 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Synthesis of the Pyrrolinium-based IL and the Structure and Abbreviation of the ILs Used in This Study

■

ion (Li+) migration during a charging/discharging process, resulting in capacity fading of overall electrochemical performances. This limitation should be overcome to facilitate practical adoption of ILs as an electrolyte component for better electrochemical and safety performances of LIBs. To improve the lithium ion mobility in ILs, here we propose a pyrrolinium-based IL,16,17 which consists of some task-specific functionalities such as the sp2 carbon of double bond, the C−O ether linkage, and no unstable C−H bond. First, we expect that the planar sp2 carbon of double bond would be beneficial to enhance physicochemical properties of ILs because the planar structure as shown in imidazolium-based ILs allows rather facile slip between molecules, resulting in higher ionic conductivity as well as lower viscosity.18,19 Second, the additional C−O ether linkage would be advantageous for improving some physicochemical properties together with Li+ migration during electrochemical charging/discharging in that the relatively smaller C−O functional group reduces overall bulkiness of the IL, resulting in lower viscosity and higher ionic conductivity20−22 and the high binding affinity by a Lewisbasic oxygen atom to a Lewis acidic Li+ would enhance the Li+ migration between electrodes, leading to better kinetic performance of the cell.21,22 Third, no acidic C−H proton as in the saturated pyrrolidinium-based ILs would be helpful to increase overall electrochemical stability of pyrrolinium-based ILs. It is well-known that the imidazolium-based ILs suffer from the continuous electrochemical decomposition as a result of the irreversible reduction of the C−H proton at the C-2 position of an imidazolium ring.19,20 On the basis of the material design, we report some binary electrolyte systems that are composed of commercial carbonate-based electrolyte and the pyrrolinium-based IL to improve both the electrochemical and safety performance of electrolytes for LIBs. There have been recently reported a few binary electrolyte systems as an alternative approach to the conventional carbonated-based electrolytes and the noticeable enhancement of nonflammability of the mixed electrolytes was observed. However, those binary electrolyte systems would still have some critical limitations to be employed as an electrolyte for LIBs due to the base ILs used, which were imidazolium- or pyrrolidinium-based ILs. Those binary electrolytes suffer from poor cycling performance or inferior high rate performance, probably because of the poor electrochemical properties of the ILs employed.23−36 Herein, we have also compared the electrochemical properties of the pyrrolinium-based ILs with those of the imidazolium- or pyrrolidinium-based IL.

EXPERIMENTAL SECTION

General. Materials were purchased from commercial suppliers and used without further purification. N-Ethyl-2-pyrrolidone and dimethyl sulfate were distilled under reduced pressure immediately prior to use. All experimental glassware, syringes, and magnetic stirring bars were oven-dried and stored in a desiccator before use. 1H and 13C NMR spectra were obtained in CDCl3 on a Bruker Avance III spectrometer (400 MHz for 1H and 100 MHz for 13C NMR). The 1H NMR spectroscopic data were reported as follows in ppm (δ) from the internal standard (TMS, 0.0 ppm): chemical shift (multiplicity, coupling constant in Hz, integration). The 13C NMR spectra were referenced with the 77.16 resonance of CDCl3. Viscosity measurements were carried out on a Brookfield DV-II+cone/plate viscometer. Ionic conductivity was determined using a TOA-DKK CM-30R benchtop conductivity meter. Elemental analysis was performed on a US/CHNS-932. Synthesis of the Pyrrolinium-based ILs. PMPyrd-FSI and EMIm-FSI (Scheme 1) were synthesized as described in a previous report.14 E(OMe)Pyrl-FSI was synthesized as follows. N-Ethyl-2pyrrolidone (5.0 g, 44.2 mmol) and dimethyl sulfate (6.1 g, 1.1 equiv, 48.6 mmol) were added to a 100 mL RB flask at room temperature under nitrogen atmosphere. The reaction mixture was stirred at room temperature for 12 h. After the reaction was finished, the resulting solution was washed successively with diethyl ether (50 mL × 3) and ethyl acetate (50 mL × 3) to remove the unreacted starting material. After the washed solution (bottom layer) was diluted with dichloromethane (50 mL), the resulting solution was dried over MgSO4, filtered, and concentrated under reduced pressure to afford Nethyl-2-methoxypyrrolinium methyl sulfate (9.94 g, 94%) as a colorless oil. N-Ethyl-2-methoxypyrrolinium methyl sulfate: 1H NMR (400 MHz): δ 1.32 (t, J = 7.4, 3H), 2.44 (m, 2H), 3.37 (t, J = 7.9, 2H) 3.66 (q, J = 7.4, 2H), 3.74 (s, 3H), 4.00 (t, J = 7.9, 2H), 4.38 (s, 3H). 13 C NMR (100 MHz): δ 11.2, 17.1, 29.5, 41.2, 51.9, 54.4, 62.6, 180.3. Anal. Calcd for C8H17NO5S: C, 40.15; H, 7.16; N, 5.85; S, 13.40. Found: C, 40.09; H, 7.08; N, 5.77; S, 13.61. To an aqueous solution of N-ethyl-2-methoxypyrrolinium methyl sulfate (5.0 g, 20.8 mmol in 100 mL of distilled water) in a 250 mL RB flask was added lithium bis(fluorosulfonyl)imide (LiFSI) (4.3 g, 1.1 equiv, 23.0 mmol) at room temperature. After the reaction mixture was stirred for 1 h, the bottom layer containing the desired product was partitioned from the top aqueous layer. The aqueous layer was extracted with CH2Cl2 (100 mL × 2), and the combined organic layers together with the bottom layer were washed with distilled water (150 mL × 3) again. Afterward, the washed organic layers were dried over MgSO4, filtered, and concentrated under reduced pressure to give a colorless oil of N-ethyl-2-methoxypyrrolinium bis(fluorosulfonyl)imide, E(OMe)Pyrl-FSI, in 92% yield (5.93 g). N-Ethyl-2-methoxypyrrolinium bis(fluorosulfonyl)imide, E(OMe)Pyrl-FSI: 1H NMR (400 MHz): δ 1.31 (t, J = 7.5, 3H), 2.42 (m, 2H), 3.23 (t, J = 7.9, 2H) 3.62 (q, J = 7.5, 2H), 3.95 (t, J = 7.9, 2H), 4.32 (s, B


Research Article

ACS Sustainable Chemistry & Engineering 3H). 13C NMR (100 MHz): δ 11.1, 16.9, 29.3, 41.5, 51.9, 62.7, 179.7. Anal. Calcd for C7H14F2N2O5S2: C, 27.27; H, 4.58; N, 9.09; S, 20.80. Found: C, 27.15; H, 4.55; N, 9.05; S, 20.98. Preparation of Binary Electrolytes. For the preparation of binary electrolytes, each electrolyte component, the IL (E(OMe)PyrlFSI with 1.0 M LiTFSI) and commercial carbonate (EC/EMC = 3/7 with 1.0 M LiPF6 and 3% VC), was mixed in different ratios (20, 40, 60, and 80 wt %). Then, the resulting mixture was stirred for 10 min at room temperature to yield a homogeneous solution of the binary electrolytes. Note that we chose lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) instead of LiFSI as a lithium salt for the electrolyte solution, because (1) LiTFSI has a similar chemical structure to LiFSI (compatibility with FSI-based ILs), (2) it is more resistive to the corrosion of a current collector, and (3) it is cheaper than LiFSI and commercially available.14,37−41 The total weight of each electrolyte prepared was fixed to 1 g (100 wt %) in this work. Flammability Test. Method A:23,26,28−30,35,36 Flammability tests were performed with a Kovea TKT-9607 gas torch by directly exposing the flame of a gas torch to each binary electrolyte on a Petri dish. After exposure, the flame was removed and each electrolyte on a Petri dish was taken a picture after 10 and 20 s, respectively. The flame-retarding properties of the binary electrolytes were evaluated by measuring the self-extinguishing time (SET) normalized by the liquid mass.25,27,28,36 Method B:25 A glass fiber (2 cm × 4 cm) soaked with each binary electrolyte was exposed to the flame of a candle for 3 s. Then, the flame was removed and the burning time of the glass fiber was measured. Lithium Ion Transference Number. The lithium transference number was determined by a dc polarization combined with impedance spectroscopy following the technique proposed by Bruce et al.42 The method consists of applying a small dc pulse to a symmetrical Li/electrolyte/Li cell and measuring the initial (I0) and the steady-state (Iss) current that flow through the cell. The cell was also monitored by impedance spectroscopy to measure the initial (R0) and the final (Rss) resistance of the two Li interfaces. Under these circumstances, the lithium transference number, tLi+, is given by t Li + = Iss(ΔV − R 0I0)/I0(ΔV − R ssIss)

Figure 1. Viscosity of the binary electrolytes at 25 °C (blue) and 60 °C (red).

(1)

Electrochemical Performance. For the preparation of a LiFePO4-containing composite electrode, a mixture of carbon-coated LiFePO4 powder, Super-P (as a carbon additive for conductivity enhancement), and poly(vinylidenefluoride) (PVdF, as a binder) were mixed in a 8:1:1 wt % ratio, respectively, which was finely dispersed in N-methyl-2-pyrrolidone (NMP). The resulting slurry was coated on a piece of an Al current collector, which was dried in a vacuum oven at 120 °C. The 2032 coin-type half cells were fabricated with the composite electrode, each binary electrolyte, a glass fiber as a separator, and Li foil as a counter and reference electrode. Galvanostatic charge−discharge cycling was conducted using a WBCS3000 cycler, and the charge−discharge tests on the cells were performed at 2.5−3.9 V of cutoff voltage. The current density was 1 C at 25 and 60 °C, and the cells were fully charged and discharged by a constant current (CC) mode. Linear sweep voltammetry was performed with a CHI660A electrochemical workstation for the electrochemical stability window measurement at a scan rate of 10 mV s−1 with a glassy carbon electrode (7.07 × 10−2 cm2) as a working electrode. The working electrode was polished before every measurement. A lithium electrode was used as both a counter electrode and a reference electrode. Note that the cutoff density of current was fixed at 1 mA cm−2 according to the previous literature.14,43−45 The surface morphology of the cycled LiFePO4 cathode was observed by scanning electron microscopy (SEM) with a Quanta 3D FEG (FEI) after the 50th charge−discharge cycle.

Figure 2. Ionic conductivity of the binary electrolytes at 25 °C (blue) and 60 °C (red).

electrolytes seemed to be proportional to the composition of the IL and increased with an increase of the IL composition (Figure 1). As expected, a decrease in viscosity was observed at higher temperature (60 °C). The ionic conductivity of the binary systems seemed to be dependent on the IL composition (Figure 2). It showed the maximum value with 40 wt % of the IL (10.8 mS cm−1 at 25 °C, 16.2 mS cm−1 at 60 °C), and then gradually decreased as the IL content increased in the binary electrolyte systems. This trade-off effect seems to be attributed to the increased viscosity of the binary electrolytes with an increase of the IL.46,47 That is, the increase of the IL contents not only contributes to enhancement of overall ionic conductivity up to an optimal point but also results in an increase of viscosity of the electrolyte. Therefore, the mobility of ionic species seemed to be hindered once the IL contents was over 40 wt %. This behavior could be explained by an inverse relationship between ionic conductivity and viscosity, i.e., the increase in the ionic conductivity by adding more ionic liquids results in the increase of the viscosity, which would decrease an overall mobility of the lithium ions in electrolyte after an optimal point.29,32,48−50 It is interesting to note that the pyrrolinium-based IL showed higher ionic conductivity (5.3 mS

■

RESULTS AND DISCUSSION Physicochemical Properties. The viscosity and ionic conductivity of the binary electrolytes are shown in Figures 1 and 2 and summarized in Table 1. The viscosity of the binary C


Research Article


Table 1. Viscosity and Ionic Conductivity of Binary Electrolytes, PMPyrd-FSI, and EMIm-FSI at 25 and 60 °C electrolytes with lithium salta viscosity (η, cP)

electrolytes without lithium salt

ionic conductivity (σ, mS cm−1)

ionic conductivity (σ, mS cm−1)

viscosity (η, cP)

electrolytesb

25 °C

60 °C

25 °C

60 °C

25 °C

60 °C

25 °C

60 °C

E 00 E 20 E 40 E 60 E 80 E 100 PMPyrd-FSI EMIm-FSI

3.6 4.9 8.0 14.0 25.0 52.0 59.0 30.0

1.7 2.6 3.8 4.7 6.5 16.0 23.0 17.0

8.9 10.7 10.8 9.0 6.9 5.3 4.7 10.3

9.5 13.6 16.2 15.5 13.3 12.1 9.5 17.1

32.0 40.0c 19.0d

13.0 19.0 10.0

13.0 8.8c 15.8d

18.4 13.2 19.7

a

E 00: a carbonate-only electrolyte, EC/EMC = 3/7 (V/V) with 1.0 M LiPF6 and 3 wt % VC. E 100: an IL-only electrolyte, E(OMe)Pyrl-FSI with 1.0 M LiTFSI. E 20, E 40, E 60, and E 80 are mixtures of E 00 and E 100 in different ratios, and contain 20, 40, 60, and 80 wt % of E 100 in each binary electrolyte, respectively. bThe data were measured with 1.0 M lithium salt; both PMPyrd-FSI and EMIm-FSI contain 1.0 M LiTFSI. cThe reported viscosity and ionic conductivity of PMPyrd-FSI without lithium salt are 40 cP and 8.2 mS cm−1 at 25 °C, respectively.14 dThe reported viscosity and ionic conductivity of the EMIm-FSI without lithium salt are 18 cP and 15.4 mS cm−1 at 25 °C, respectively.14

Figure 3. Images of the flammability tests of E 00, E 20, E 40, E 60, E 80, E 100. (a) Method A: direct flame combustion test at contact for 3 s, after 10 s and after 20 s. (b) Method B: indirect flame combustion test at noncontact for 3 s and after removing the flame.

cm−1 with 1.0 M LiTFSI, 13.0 mS cm−1 without LiTFSI) than the pyrrolidinium-based IL (4.7 mS cm−1 with 1.0 M LiTFSI, 8.8 mS cm−1 without LiTFSI),14,51 but lower than the imidazolium-based IL (10.3 mS cm−1 with 1.0 M LiTFSI, 15.8 mS cm−1 without LiTFSI) at 25 °C (Table 1).14 The similar trend was also observed at 60 °C. This would mean that the chemical structure of the cation of the IL plays an

important role to enhance its physicochemical properties and the planar double bond in the pyrrolinium cation seems to contribute to the increase of the ionic conductivity by decreasing the viscosity for not only the IL itself but also the binary electrolyte systems. Flammability Test. To verify the flame-retarding effect of the pyrrolinium-based binary electrolytes, two kinds of D


Research Article

ACS Sustainable Chemistry & Engineering Table 2. Flammability Tests (Direct and Indirect Flame) and SET Values binary electrolyte E E E E E E

00 20 40 60 80 100

method A (direct flame)a

method B (indirect flame)b

SET (s g−1)

O O O O X X

O O X X X X

103 45 30 5

a,b O means the ignition of the electrolyte continues when the flame is removed, whereas the X means that there is no ignition at all.

Figure 5. Cycle performance of the Li/LiFePO4 cells in six different electrolytes at 25 °C; E 00 (black), E 20 (red), E 40 (blue), E 60 (green), E 80 (orange), and E 100 (pink).

Figure 6. Cycle performance of the Li/LiFePO4 cells in eight different electrolytes at 60 °C; E 00 (black), E 20 (red), E 40 (blue), E 60 (green), E 80 (orange), E 100 (pink), PMPyrd-FSI (purple), and EMIm-FSI (khaki).

Table 3. Initial Discharge Capacity and Retention Ratio of the Pyrrolinium-based Binary Electrolytes at 25 and 60 °C 25 °C

Figure 4. Electrochemical window of E 00 (black), E 20 (red), E 40 (blue), E 60 (green), E 80 (orange), E 100 (pink), PMPyrd-FSI (purple), and EMIm-FSI (khaki); (a) cathodic limit (0.60−2.00 V vs Li/Li+) and (b) anodic limit (3.00−5.25 V vs Li/Li+).

binary electrolytes E E E E E E

flammability tests were performed as shown in Figure 3a.b (see theExperimental Section for details). In a direct combustion test (Figure 3a), the flammability of each binary electrolyte depended mainly on the IL compositions and the binary electrolyte comprising 80 wt % or more of the IL did not ignite at all, whereas the binary electrolytes with or less than 60 wt % of the IL were combustible at the initial stage of ignition. However, the combustion feature seemed to be different after removal of the flame and the fire was gradually extinguished in the binary electrolytes with or more than 60 wt % of the IL.

00 20 40 60 80 100

initial discharge capacity (mA h g−1) 145.3 150.4 148.8 148.6 148.4 147.3

60 °C

retention ratio at 50th cycle (%)

initial discharge capacitya (mA h g−1)

retention ratio at 50th cycleb (%)

99.4 98.5 98.0 98.8 99.0 99.4

147.3 148.4 151.2 152.4 152.6 151.3

99.4 99.2 98.1 99.0 98.5 98.9

a

The initial discharge capacities of EMIm-FSI and PMPyrd-FSI at 60 °C were 154.5 and 134.9 mA h g−1, respectively. bThe retention ratios at the 50th cycle of EMIm-FSI and PMPyrd-FSI at 60 °C were 89.4% and 99.9%, respectively.

Similar results were observed in the other flammability test, which employed an indirect combustion method as shown in E


Research Article


binary electrolyte with 60 wt % of the IL took only 5 s, whereas the binary electrolyte with 40 wt % of the IL showed the much longer SET value (30 s g−1). It suggests that the binary electrolyte with about 60 wt % of the pyrrolinium-based IL would be optimal when both the physicochemical properties (viscosity and ionic conductivity) and the safety performance of the binary electrolyte are considered. Electrochemical Performances. To investigate electrochemical compatibility of the binary electrolytes systems, their electrochemical windows were examined by linear sweep voltammetry (LSV) as shown in Figure 4. In the LSV results for cathodic polarization, none of suggested binary electrolytes (from E 00 to E 100) showed significant currents within the effective current value (≤1 mA cm−2) regardless of the IL contents (Figure 4a). This implies that all of the binary electrolytes have a similar cathodic stability in the typical potential range for cathodic polarization. A noticeable point in the LSV results is that the cation structure of IL significantly affects overall cathodic stability. Note that all of the electrolytes composed of the pyrrolinium-based IL were quite stable under cathodic polarization. However, the imidazolium-based IL seriously suffered from continuous decomposition at 0.68 V (vs Li/Li+), which is highly associated with the cation structure of the IL.14,52−56 In practice, several previous studies demonstrated that existence of an acidic proton at the C-2 position of the imidazolium cation accelerated electrochemical reduction under cathodic polarization, resulting in poor compatibility for conventional lithium-ion batteries (LIBs).57−59 These cathodic polarization results mean that the pyrrolinium cation is effective to improve the cathodic stability by removing the reactive C−H proton in the modified structure. In contrast to the cathodic stability, there was different electrochemical behavior in anodic stability with the binary electrolytes and the wider electrochemical stability was observed as increasing the IL contents (Figure 4b). First, the anodic limits seemed to be more dependent on the IL contents and the electrochemical potential for initiation of electrolyte decomposition was gradually increased as the amount of the pyrrolinium-based IL in each binary electrolyte increased. For example, the decomposition of the carbonate-only electrolyte (E 00) started at around 5.1 V (vs Li/Li+), whereas some binary electrolytes with or more than 40 wt % of the pyrroliniumbased IL were stable over 5.2 V. In addition, further decomposition of the binary electrolytes (E 40 to E 100) seemed to be well-suppressed, whereas continuous electrolyte decomposition was observed with the carbonate-only electrolyte (E 00). Interestingly, they also showed an improved anodic stability compared to the known IL electrolytes such as the pyrrolidinium- and imidazolium-based ILs. According to the results of the anodic polarization, we presumed that the enhanced anodic stability could to be attributed the formation of a protective layer due to the unique structure of the pyrrolinium cation, which seems in line with the previous results in literature.20−22,32,56,60 Therefore, we suppose that the pyrrolinium-based IL allows the binary electrolytes to be more applicable to LIBs as they do not compromise their remarkable nonflammability with the electrochemical stability. The electrochemical properties of the binary electrolytes were investigated and the results are shown in Figures 5 and 6 and summarized in Table 3. At 25 °C, all of the cells containing the binary electrolyte showed similar discharge specific capacity at around 150 mA h g−1, which is almost identical to the

Figure 7. Galvanostatic charge−discharge potential profiles of the LiFePO4/Li+ cell in five different electrolytes at 60 °C at (a) the 1st cycle, and (b) the 50th cycle (E 00 (black), E 60 (green), E 100 (pink), PMPyrd-FSI (purple), and EMIm-FSI (khaki)).

Table 4. Lithium Ion Transference Number (tLi+) of the ILs and Commercial Carbonatea electolyte

carbonate

E(OMe)Pyrl-FSI

PMPyrd-FSI

EMIm-FSI

tLi+

0.29

0.17

0.14 (0.113)b

0.08

a

All of the electrolytes are with 1.0 M of LiTFSI except the carbonateonly electrolyte with 1.0 M LiPF6. bThe value in parentheses is the reported lithium transference number of PMPyrd-FSI in the literature.51

Figure 3b. In contrast to the highly flammable behavior of the electrolyte with or less than 20 wt % of the IL, the higher contents of the IL made the binary electrolytes nonflammable probably because the pyrrolinium-based IL suppressed the flammable nature of the carbonate-based electrolytes. This is well consistent with the self-extinguishing time (SET) results in Table 2, which indicates the fire-extinguishing performance of each electrolyte. Addition of the pyrrolinium-based IL seemed effective to suppress the flammability of the carbonates-based electrolytes and the flammability was negligible in the binary electrolytes composed of 80 wt % or more of the IL and the SET values were greatly reduced with an increase of the IL contents. Note also that the time to extinguish a flame in the F


Research Article


Figure 8. SEM images of the LiFePO4 cathode in each binary electrolyte after the 50th cycle between 2.5 and 3.9 V (vs Li/Li+) at 60 °C; (a) E 00, (b) E 20, (c) E 40, (d) E 60, (e) E 80, and (f) E 100.

safety issues in LIBs as well as to overcome the disturbance of lithium ion mobility. The task-specific compound, pyrroliniumbased IL (E(OMe)Pyrl-FSI), was synthesized efficiently and the corresponding binary electrolyte systems were prepared by changing the ratios between the pyrrolinium-based IL and commercial carbonate. The newly prepared binary electrolytes showed significant fire-retarding results and enhanced physicochemical properties as well as comparable electrochemical performance to the commercial carbonate electrolyte. Among the prepared binary electrolyte systems, those with 40 and 60 wt % of the IL are considered as an optimal range because they have much improved fire-retarding characteristics as well as large ionic conductivity values (16.2 mS cm−1 for E 40, 15.5 mS cm−1 for E 60 at 60 °C). Especially, the inflammable nature of the IL has a drastic decreasing effect on the SET values for the binary electrolytes with 60 wt % or more of the IL. In terms of the electrochemical stability, the initial discharge capacity and retention ratio of the pyrrolinium-based binary electrolytes are nearly identical with that of the commercial carbonate as an only electrolyte solvent. Furthermore, the electrochemical performance of the pyrrolinium-IL only electrolyte at 1 C rate exhibits the outstanding stability with the LiFePO4 cathode than that of the pyrrolidinium- or imidazolium-IL only electrolyte. Moreover, the SEM analysis of the morphologies of the active material (LiFePO4) in each binary electrolyte (the 50th cycle at 60 °C) provided another proof that the binary electrolytes were stable during the galvanostatic charge−discharge process under harsh conditions. It seems to confirm that the binary electrolytes prepared from the pyrrolinium-based IL and carbonate solvent have a noticeable synergistic effect on both the fire-retarding characteristics and electrochemical properties.

specific capacity of LiFePO4 (Figure 5). In addition, all of the binary electrolytes with the pyrrolinium-based IL showed stable galvanostatic charge−discharge processes that the cells cycled with the binary electrolytes still had high specific capacities over 145 mA h g−1 (more than 98.0% of specific capacity retention) at the end of cycling at both 25 and 60 °C, regardless of the pyrrolinium-based IL contents (Figures 5 and 6). Note also that the pyrrolinium-based binary electrolytes exhibited an improved electrochemical performance compared to the imidazolium- and pyrrolidinium-based ILs (Figure 7). The cell cycled with the pyrrolinium-based IL only electrolyte showed a higher initial specific capacity than the pyrrolidiniumcontaining cell (151.3 mA h g−1 (E 100) vs 134.9 mA h g−1 (PMPyrd-FSI) at the 50th cycle at 60 °C, see footnote of Table 3) and exhibited a better cycling performance than the imidazolium-containing cell (98.9% (E 100) vs 89.4% (EMIm-FSI) at the 50th cycle at 60 °C, see footnote of Table 3). It could be attributed to the task-specific structure of the pyrrolinium cation characterized by the sp2 carbon of double bond (lower viscosity), the C−O ether linkage (higher ionic conductivity), and excluding an unstable C−H bond (wider electrochemical window). In practice, the lithium ion transference number (tLi+) of electrolytes, which is responsible for the migrating rate of Li+ ions, was mainly dependent on the cation structure of the ILs and the multifunctionalized E(OMe)Pyrl-FSI showed higher tLi+ than either PMPyrdFSI51 or EMIm-FSI (Table 4). It implies that the pyrrolinium cation plays an important role to enhance the overall electrochemical performance of the cell as well as the safety issues in LIBs. Further SEM analysis provided an informative clue to estimate effectiveness of the binary electrolytes. The surface morphologies of the LiFePO4 electrodes cycled between 2.5 and 3.9 V (vs Li/Li+) were almost similar to each other without any evidence of the electrolyte decomposition (Figure 8). It seems to confirm that the proposed approach with the binary electrolytes based on the combination of the pyrrolinium-based IL with the carbonate solvent would be effective to achieve an excellent safety performance of the cell together with a good cycling performance without any trade-off effect.

■

AUTHOR INFORMATION

Corresponding Authors

*T. Yim. E-mail: [email protected]. Tel:+82-31-789-7394. Fax: +82-31-789-7499. *Y. G. Kim. E-mail: [email protected]. Tel:+82-2-880-8347.

■

CONCLUSIONS We have developed new binary electrolytes that consist of the pyrrolinium-based IL and commercial carbonate to address the

Notes

The authors declare no competing financial interest. G


Research Article


■

(22) Jin, Y.; Fang, S.; Chai, M.; Yang, L.; Tachibana, K.; Hirano, S. Properties and application of ether-functionalized trialkylimidazolium ionic liquid electrolytes for lithium battery. J. Power Sources 2013, 226, 210−218. (23) Guerfi, A.; Dontigny, M.; Charest, P.; Petitclerc, M.; Lagacé, M.; Vijh, A.; Zaghib, K. 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. (24) Lane, G. H.; Best, A. S.; MacFarlane, D. R.; Forsyth, M.; Bayley, P. M.; Hollenkamp, A. F. The electrochemistry of lithium in ionic liquid/organic diluent mixtures. Electrochim. Acta 2010, 55, 8947− 8952. (25) Arbizzani, C.; Gabrielli, G.; Mastragostino, M. Thermal stability and flammability of electrolytes for lithium-ion batteries. J. Power Sources 2011, 196, 4801−4805. (26) Kühnel, R.; Böckenfeld, N.; Passerini, S.; Winter, M.; Balducci, A. Mixtures of ionic liquid and organic carbonate as electrolyte with improved safety and performance for rechargeable lithium batteries. Electrochim. Acta 2011, 56, 4092−4099. (27) Kim, K.; Cho, Y.; Shin, H. 1-Ethyl-1-methyl piperidinium bis(trifluoromethanesulfonyl)imide as a co-solvent in Li-ion batteries. J. Power Sources 2013, 225, 113−118. (28) Lombardo, L.; Brutti, S.; Navarra, M. A.; Panero, S.; Reale, P. Mixtures of ionic liquid − Alkylcarbonates as electrolytes for safe lithium-ion batteries. J. Power Sources 2013, 227, 8−14. (29) Quinzeni, I.; Ferrari, S.; Quartarone, E.; Tomasi, C.; Fagnoni, M.; Mustarelli, P. Li-doped mixtures of alkoxy-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide and organic carbonates as safe liquid electrolytes for lithium batteries. J. Power Sources 2013, 237, 204−209. (30) Li, H.; Pang, J.; Yin, Y.; Zhuang, W.; Wang, H.; Zhai, C.; Lu, S. Application of a nonflammable electrolyte containing Pp13TFSI ionic liquid for lithium-ion batteries using the high capacity cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O2. RSC Adv. 2013, 3, 13907−13914. (31) Forgie, J. C.; Khakani, S. E.; MacNeil, D. D.; Rochefort, D. Electrochemical characterisation of a lithium-ion battery electrolyte based on mixtures of carbonates with a ferrocene-functionalised imidazolium electroactive ionic liquid. Phys. Chem. Chem. Phys. 2013, 15, 7713−7721. (32) DiLeo, R. A.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. Battery Electrolytes Based on Unsaturated Ring Ionic Liquids: Conductivity and Electrochemical Stability. J. Electrochem. Soc. 2013, 160, A1399−A1405. (33) Menne, S.; Kühnel, R.; Balducci, A. The influence of the electrochemical and thermal stability of mixtures of ionic liquid and organic carbonate on the performance of high power lithium-ion batteries. Electrochim. Acta 2013, 90, 641−648. (34) Kühnel, R.; Balducci, A. Lithium Ion Transport and Solvation in N-Butyl-N-methylpyrrolidinium Bis(trifluoromethanesulfonyl)imide− Propylene Carbonate Mixtures. J. Phys. Chem. C 2014, 118, 5742− 5748. (35) Montanino, M.; Moreno, M.; Carewska, M.; Maresca, G.; Simonetti, E.; Presti, R. L.; Alessandrini, F.; Appetecchi, G. B. Mixed organic compound-ionic liquid electrolytes for lithium battery electrolyte systems. J. Power Sources 2014, 269, 608−615. (36) Wilken, S.; Xiong, S.; Scheers, J.; Jacobsson, P.; Johansson, P. Ionic liquids in lithium battery electrolytes: Composition versus safety and physical properties. J. Power Sources 2015, 275, 935−942. (37) Park, K.; Yu, S.; Lee, C.; Lee, H. Comparative study on lithium borates as corrosion inhibitors of aluminum current collector in lithium bis(fluorosulfonyl)imide electrolytes. J. Power Sources 2015, 296, 197−203. (38) Kerner, M.; Plylahan, N.; Scheers, J.; Johansson, P. Ionic liquid based lithium battery electrolytes: fundamental benefits of utilising both TFSI and FSI anions? Phys. Chem. Chem. Phys. 2015, 17, 19569− 19581. (39) Cho, E.; Mun, J.; Chae, O. B.; Kwon, O. M.; Kim, H.; Ryu, J. H.; Kim, Y. G.; Oh, S. M. Corrosion/passivation of aluminum current

ACKNOWLEDGMENTS This work was supported by the BK 21 Plus Program, Institute of Chemical Processes, Agency for Defense Development, and Samsung Electro-Mechanics Co., Ltd.. We thank Nippon Shokubai Co., Ltd. for generous supply of LiFSI.

■

REFERENCES

(1) Lu, L.; Han, X.; Li, J.; Hua, J.; Ouyang, M. A review on the key issues for lithium-ion battery management in electric vehicles. J. Power Sources 2013, 226, 272−288. (2) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the development of advanced Li-ion batteries: a review. Energy Environ. Sci. 2011, 4, 3243−3262. (3) Fergus, J. W. Recent developments in cathode materials for lithium ion batteries. J. Power Sources 2010, 195, 939−954. (4) Chen, J. Recent Progress in Advanced materials for lithium ion batteries. Materials 2013, 6, 156−183. (5) Julien, C. M.; Mauger, A.; Zaghib, K.; Groult, H. Comparative Issues of Cathode Materials for Li-Ion Batteries. Inorganics 2014, 2, 132−154. (6) Croguennec, L.; Palacin, M. R. Recent Achievements on Inorganic Electrode Materials for Lithium-Ion Batteries. J. Am. Chem. Soc. 2015, 137, 3140−3156. (7) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for LiIon Batteries. Chem. Rev. 2014, 114, 11444−11502. (8) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587−603. (9) Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271−4301. (10) Scrosati, B.; Garche, J. Lithium batteries: Status, prospects and future. J. Power Sources 2010, 195, 2419−2430. (11) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (12) Lewandowski, A.; Świderska-Mocek, A. Ionic liquids as electrolytes for Li-ion batteriesAn overview of electrochemical studies. J. Power Sources 2009, 194, 601−609. (13) Galiński, M.; Lewandowski, A.; Stępniak, I. Ionic liquids as electrolytes. Electrochim. Acta 2006, 51, 5567−5580. (14) Matsumoto, H.; Sakaebe, H.; Tatsumi, K.; Kikuta, M.; Ishiko, E.; Kono, M. Fast cycling of Li/LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI]−. J. Power Sources 2006, 160, 1308−1313. (15) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater. 2009, 8, 621−629. (16) Kim, H.; Lee, H.; Kwon, O. M.; Yim, T.; Mun, J.; Oh, S. M.; Kim, G. J.; Kim, Y. G. Pyrrolinium-based Ionic Liquids with Ether Substituents as Electrolytes for Lithium Ion Batteries. Presented at the 2013 MRS Spring Meeting & Exhibit, San Francisco, USA, April 1−5; Paper VV3.04. (17) Chen, Z.; Xi, H.; Lim, K. H.; Lee, J. Distillable Ionic Liquids: Reversible Amide O Alkylation. Angew. Chem., Int. Ed. 2013, 52, 13392−13396. (18) Min, G.; Yim, T.; Lee, H. Y.; Huh, D. H.; Lee, E.; Mun, J.; Oh, S. M.; Kim, Y. G. Synthesis and Properties of Ionic Liquids: Imidazolium Tetrafluoroborates with Unsaturated Side Chains. Bull. Korean Chem. Soc. 2006, 27, 847−852. (19) Min, G.; Yim, T.; Lee, H. Y.; Kim, H.; Mun, J.; Kim, S.; Oh, S. M.; Kim, Y. G. Synthesis and Physicochemical Properties of Ionic Liquids: 1-Alkenyl-2,3-dimethylimidazolium Tetrafluoroborates. Bull. Korean Chem. Soc. 2007, 28, 1562−1566. (20) Yim, T.; Choi, C. Y.; Mun, J.; Oh, S. M.; Kim, Y. G. Synthesis and Properties of Acyclic Ammonium-based Ionic Liquids with Allyl Substituents as Electrolytes. Molecules 2009, 14, 1840−1851. (21) Fang, S.; Zhang, Z.; Jin, Y.; Yang, L.; Hirano, S.; Tachibana, K.; Katayama, S. New functionalized ionic liquids based on pyrrolidinium and piperidinium cations with two ether groups as electrolytes for lithium battery. J. Power Sources 2011, 196, 5637−5644. H


Research Article

ACS Sustainable Chemistry & Engineering collector in bis(fluorosulfonyl)imide-based ionic liquid for lithium-ion batteries. Electrochem. Commun. 2012, 22, 1−3. (40) Zhang, L.; Chai, L.; Zhang, L.; Shen, M.; Zhang, X.; Battaglia, V. S.; Stephenson, T.; Zheng, H. Synergistic effect between lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis-xalato borate (LiBOB) salts in LiPF6-based electrolyte for high-performance Li-ion batteries. Electrochim. Acta 2014, 127, 39−44. (41) Evans, T.; Olson, J.; Bhat, V.; Lee, S. Corrosion of stainless steel battery components by bis(fluorosulfonyl)imide based ionic liquid electrolytes. J. Power Sources 2014, 269, 616−620. (42) Bruce, P. G.; Evans, J.; Vincent, C. A. Conductivity and Transference Number Measurements on Polymer Electrolytes. Solid State Ionics 1988, 28−30, 918−922. (43) De Vos, N.; Maton, C.; Stevens, C. V. Electrochemical Stability of Ionic Liquids: General Influences and Degradation Mechanisms. ChemElectroChem 2014, 1, 1258−1270. (44) O’Mahony, A. M.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. Effect of Water on the Electrochemical Window and Potential Limits of Room-Temperature Ionic Liquids. J. Chem. Eng. Data 2008, 53, 2884−2891. (45) Koch, V. R.; Dominey, A.; Nanjundiah, C.; Ondrechen, M. J. The Intrinsic Anodic Stabilily of Several Anions Comprising SolventFree Ionic liquids. J. Electrochem. Soc. 1996, 143, 798−803. (46) Han, S.; Kim, J.; Kim, D. Cycling Performances of Lithium-Air Cells Assembled with Mixed Electrolytes of Ionic Liquid and Diethylene Glycol Diethyl Ether. J. Electrochem. Soc. 2015, 162, A3103−A3109. (47) Liao, C.; Guo, B.; Sun, X.; Dai, S. Synergistic Effects of Mixing Sulfone and Ionic Liquid as Safe Electrolytes for Lithium Sulfur Batteries. ChemSusChem 2015, 8, 353−360. (48) Fox, E. T.; Paillard, E.; Borodin, O.; Henderson, W. A. Physicochemical Properties of Binary Ionic Liquid−Aprotic Solvent Electrolyte Mixtures. J. Phys. Chem. C 2013, 117, 78−84. (49) Ding, M. S.; Xu, K.; Zhang, S. S.; Amine, K.; Henriksen, G. L.; Jow, T. R. Change of Conductivity with Salt Content, Solvent Composition, and Temperature for Electrolytes of LiPF6 in Ethylene Carbonate-Ethyl Methyl Carbonate. J. Electrochem. Soc. 2001, 148, A1196−A1204. (50) Chen, H. P.; Fergus, J. W.; Jang, B. Z. The Effect of Ethylene Carbonate and Salt Concentration on the Conductivity of Propylene Carbonate|Lithium Perchlorate Electrolytes. J. Electrochem. Soc. 2000, 147, 399−406. (51) Yoon, H.; Howlett, P. C.; Best, A. S.; Forsyth, M.; MacFarlane, D. R. Fast Charge/Discharge of Li Metal Batteries Using an Ionic Liquid Electrolyte. J. Electrochem. Soc. 2013, 160, A1629−A1637. (52) Yim, T.; Kwon, M.; Mun, J.; Lee, K. T. Room Temperature Ionic Liquid-based Electrolytes as an Alternative to Carbonate-based Electrolytes. Isr. J. Chem. 2015, 55, 586−598. (53) Wang, Z.; Cai, Y.; Wang, Z.; Chen, S.; Lu, X.; Zhang, S. Vinylfunctionalized imidazolium ionic liquids as new electrolyte additives for high-voltage Li-ion batteries. J. Solid State Electrochem. 2013, 17, 2839−2848. (54) Shamsipur, M.; Beigi, A. A. M.; Teymouri, M.; Pourmortazavi, S. M.; Irandoust, M. Physical and electrochemical properties of ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. J. Mol. Liq. 2010, 157, 43− 50. (55) Ferrari, S.; Quartarone, E.; Tomasi, C.; Ravelli, D.; Protti, S.; Fagnoni, M.; Mustarelli, P. Alkoxy substituted imidazolium-based ionic liquids as electrolytes for lithium batteries. J. Power Sources 2013, 235, 142−147. (56) Wang, M.; Shan, Z.; Tian, J.; Yang, K.; Liu, X.; Liu, H.; Zhu, K. Mixtures of unsaturated imidazolium based ionic liquid and organic carbonate as electrolyte for Li-ion batteries. Electrochim. Acta 2013, 95, 301−307. (57) Lane, G. H. Electrochemical reduction mechanisms and stabilities of some cation types used in ionic liquids and other organic salts. Electrochim. Acta 2012, 83, 513−528.

(58) Budi, A.; Basile, A.; Opletal, G.; Hollenkamp, A. F.; Best, A. S.; Rees, R. J.; Bhatt, A. I.; O’Mullane, A. P.; Russo, S. P. Study of the Initial Stage of Solid Electrolyte Interphase Formation upon Chemical Reaction of Lithium Metal and N-Methyl-N-Propyl-PyrrolidiniumBis(Fluorosulfonyl)Imide. J. Phys. Chem. C 2012, 116, 19789−19797. (59) Xiao, L.; Johnson, K. E. Electrochemistry of 1-Butyl-3-methyl1H-imidazolium Tetrafluoroborate Ionic Liquid. J. Electrochem. Soc. 2003, 150, E307−E311. (60) Mun, J.; Kim, S.; Yim, T.; Ryu, J. H.; Kim, Y. G.; Oh, S. M. Comparative Study on Surface Films from Ionic Liquids Containing Saturated and Unsaturated Substituent for LiCoO2. J. Electrochem. Soc. 2010, 157, A136−A141.

I


Pyrrolinium-based Ionic Liquid as a Flame Retardant for Binary

Recommend Documents