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Bio-inspired choline chloride-based deep eutectic solvents as electrolytes for lithium-ion batteries
Solid State Ionics 323 (2018) 44–48
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Bio-inspired choline chloride-based deep eutectic solvents as electrolytes for lithium-ion batteries
T
Luca Milliaa, Valentina Dall'Astaa, Chiara Ferraraa, Vittorio Berbennia, Eliana Quartaronea, ⁎ Filippo Maria Pernab, Vito Capriatib, Piercarlo Mustarellia, a b
Department of Chemistry and INSTM, University of Pavia, Via Taramelli 16, 27100 Pavia, Italy Dipartimento di Farmacia-Scienze del Farmaco, Università di Bari “Aldo Moro”, Consorzio C.I.N.M.P.I.S., Via E. Orabona 4, I-70125 Bari, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-ion batteries Electrolyte Deep eutectic solvents LiPF6
The safety issues of lithium-ion batteries require the substitution of volatile and flammable organic components of the electrolyte. In this context, deep eutectic solvents (DESs) represent an interesting alternative to conventional ionic liquids as green solvents to dissolve common lithium salts. This paper explores two eutectics: i) ethylene glycol/choline chloride (EG/ChCl, 3:1 mol/mol), and ii) L-(+)-lactic acid/choline chloride (LA/ChCl, 2:1 mol/mol). The lithium salts added in both cases were LiN(CF3SO2)2 and LiPF6, both in concentration of either 0.5 M or 1 M. They still retain their liquid properties despite the addition of relatively high molar contents (up to 1.0 M) of lithium salts. The 0.5 M LiPF6/EG:ChCl electrolyte, in particular, displays ionic conductivity of 7.95 mS cm−1 at room temperature, and is thus very promising as a green and cheap electrolyte.
1. Introduction The energy demand of the ever-increasing world population and the issues related to fossil fuel depletion and global warming require concerted efforts to develop sustainable battery systems for electrical energy storage [1]. Lithium-ion batteries are at present the state-of-the-art storage devices. They suffer, however, from several problems including availability and toxicity of active materials, high materials costs, short life cycles, and electrolyte safety at high current rates [2,3]. This last issue requires the replacement of hazardous and volatile organic solvents [4] with safer, cheaper and more sustainable materials. Whereas ionic liquids (ILs) have been a matter of intense research as alternative electrolyte media in recent years [5], deep eutectic solvents (DESs) are opening up new and exciting possibilities as promising next-generation neoteric fluids. DESs are eutectic mixtures easily formed by mixing and gently heating naturally occurring hydrogen-bond donors (e.g., urea, renewable polyols, carbohydrates, carboxylic acids, amines, amides) and hydrogen-bond acceptors (e.g., choline chloride (ChCl), phosphonium salts). Compared with traditional ILs with which they share some physicochemical properties (e.g., negligible vapour pressure, a high thermal stability, nonflammability, recyclability), DESs display attractive advantages such as high biodegradability, low cost (DESs are about ten times cheaper than ILs), and very low toxicity. The solvent properties can be tuned by simply changing the nature and the molar ratio of the components. This last decade has witnessed an exponential increase ⁎
Corresponding author. E-mail address: [email protected] (P. Mustarelli).
https://doi.org/10.1016/j.ssi.2018.05.016 Received 17 July 2017; Received in revised form 10 April 2018; Accepted 16 May 2018 0167-2738/ © 2018 Elsevier B.V. All rights reserved.
of the publications on this topic both in academia and industry [6–12]. In particular, recent breakthroughs in applications of DESs in the fields of organo- and biocatalysis [13–16], organometallic chemistry [17–24], and solar technology [25], have been extremely relevant. Recently, DESs based on N‑methylacetamide (MAc) with different lithium salts (lithium bis(fluoro)sulfonimide (LiN(CF3SO2)2), LiPF6, LiNO3) have been proposed as electrolytes for lithium-ion cells. These systems, however, were liquid at room temperature only for Li molar fraction < 0.35, which limited their ionic conductivity to about 1 mS cm−1 [26]. Geiculescu et al. reported on the usefulness of binary DES electrolytes consisting of mixtures of methanesulfonamide (CH3SO2NH2) and N,N‑dimethyl methanesulfonamide [CH3SO2N (CH3)2] with LiN(FSO2)2 and lithium bis(trifluoromethane)sulfonimide [LiN(CF3SO2)2] (LiTFSI) [27]. The main limitations of this approach were the high viscosity and a relatively low conductivity with respect to LiPF6 in organic carbonate solvents. The transport properties of some of these mixtures were investigated by means of Pulse Field Gradient (PFG) NMR diffusion experiments coupled with molecular dynamics simulations [28]. In this paper, we report on binary DES systems, which still retain their liquid properties despite the addition of a relatively high molar concentration (1 M) of a lithium salt. In particular, we explore two eutectics: i) ethylene glycol/choline chloride (EG/ChCl, 3:1 mol/mol), and ii) L-(+)-lactic acid/choline chloride (LA/ChCl, 2:1 mol/mol) with LITFSI and LiPF6 as the lithium salts, both in concentrations of either
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Fig. 1. DSC thermograms of the DESs-based electrolytes under investigation.
Fig. 2. Arrhenius plots of DESs-based electrolytes under investigation. Table 1 Parameters of Vogel-Tammann-Fulcher (VTF) fits for each electrolyte together with selected physical properties at T = 25 °C. 0, B and T0 are the pre-exponential factors, pseudo-activation energies and “ideal” glass transition temperatures with R2 > 99%, respectively. σ, Λm, η and Λm.η are ionic conductivity, molar conductivity, dynamic viscosity, and Walden product, respectively. Sample
LA/ChCl LA/ChCl LiPF6 0.5 M LA/ChCl LiPF6 1.0 M LA/ChCl LiTFSI 0.5 M LA/ChCl LiTFSI 1.0 M EG/ChCl EG/ChCl LiPF6 0.5 M EG/ChCl LiPF6 1.0 M EG/ChCl LiTFSI 0.5 M EG/ChCl LiTFSI 1.0 M
VTF σ0 (S cm−1)
B (K−1)
T0 (K)
0.73 0.71 12.21 0.41 0.12 0.57 0.71 4.17 3.23 1.04
791.74 668.20 1536.40 623.93 405.75 594.83 593.23 1107.23 1003.34 725.73
177.92 205.88 149.88 202.14 231.30 168.64 168.43 135.08 144.32 167.26
45
σ (S cm−1)
Λm (S cm2 mol−1)
η (cP)
Λm.η (S cm2 mol−1 P)
1.06 × 10−3 5.41 × 10−4 3.95 × 10−4 6.39 × 10−4 2.99 × 10−4 5.27 × 10−3 7.95 × 10−3 4.76 × 10−3 5.16 × 10−3 4.03 × 10−3
0.29 0.13 0.085 0.15 0.06 1.53 2.02 1.07 1.31 0.91
143.8 298.2 611.1 554.8 668.1 68.5 88.4 94.3 121.7 143.8
0.42 0.39 0.52 0.83 0.40 1.05 1.79 1.01 1.60 1.31
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was controlled by means of an oil bath (to within ± 0.2C). Conductivity was measured over a temperature range from −10 to 90 °C. The electrochemical stability window of the DES-based electrolytes was investigated by means of linear voltammetry scans at 100 mV/s from the cathodic limit to the anodic one. A conventional three-electrode cell was used with glassy carbon as the working electrode, Ag/ Ag+ as the reference electrode and Pt wire as the counter electrode. The reference electrode was obtained by immersing an Ag wire in 0.01 M solution of AgNO3 in CH3CN. NMR spectra were recorded on a 400 MHz Bruker Avance III spectrometer at room temperature under static conditions. Samples were prepared in an Ar-filled glove box (H2O, O2 < 1 ppm) using 4 mm rotors and immediately measured. 1H single pulse spectra were collected using a π/2 pulse of 4.8 μs and recycle delay of 4 s after careful calibration of pulse length and estimation of the delay time. 13C quantitative spectra were acquired under high power decoupling conditions (HPDEC pulse program) with a π/2 pulse of 4.7 μs, recycle delay of 5 s and SPINAL-64 heteronuclear decoupling scheme. 7Li single pulse spectra were obtained using a π/2 pulse of 3 μs and recycle delay of 4 s. Pulse lengths and recycle delays were calibrated to ensure the conditions for quantitative measurements. 1H and 13C spectra were referenced to the signal of adamantane (as a secondary standard with respect to TMS), whereas 7Li data were referenced to the signal of LiCl 1 M solution. For selected spectra, fitting analysis was performed using DmFIT® simulation software [32]. The dynamic viscosity, η, was measured at 20C with a cone-plate rotational viscosimeter (Rheotec) in control-shear-stress mode.
Fig. 3. Walden plot of the investigated electrolytes.
3. Results and discussion Fig. 1 shows the DSC behaviours of the two DESs with LiTFSI (0.5 M) and LiPF6 (1.0 M). All the electrolytes are in the liquid state over the temperature range from −50 to at least 100 °C, and no degradation phenomena were observed in this interval. ESs with LiPF6 showed exothermic features around 130 °C, regardless of the salt concentration; this is likely due to LiPF6 degradation [33]. Fig. 2 shows the Arrhenius plots of the ionic conductivity for all the investigated electrolytes. In all cases, the temperature dependence of the conductivity exhibits the Vogel-Tammann-Fulcher (VTF) behaviour typical of supercooled liquids (Eq. 1), in agreement with the DSC results reported in Fig. 1. Here σ0 is a pre-exponential factor related to the carrier concentration, B represents a pseudo-activation energy for the ion diffusion, and T0 is a parameter which can be interpreted as the ideal glass transition temperature, which lies 20–50 K below the glasstransition temperature, Tg, as determined by standard DSC measurements. The conductivity values, and the parameters of the VTF fits, are reported in Table 1, together with other relevant physico-chemical quantities.
Fig. 4. Electrochemical stability window of EG/ChCl with LiTFSI and LiPF6 0.5 M vs. a glassy carbon electrode.
0.5 M or 1.0 M. 2. Experimental details DES components were purchased at the highest purity available from Alfa Aesar, and were used without further purification: L(+)-lactic acid (LA): 90% aqueous solution; choline chloride (ChCl): 98 + %; ethylene glycol (EG): 99%. The employed DESs (EG/ChCl 3:1 mol/mol [11,29,30] and LA/ChCl 2:1 mol/mol [12,31]) were prepared by gentle heating under stirring at 70 °C for 15 min the corresponding individual components in the appropriate molar ratio until a clear solution was obtained. All prepared DESs were fluid at room temperature. The water content of the EG/ChCl (3:1 mol/mol) eutectic mixture was found to be < 0.1 wt% soon after its preparation using the Karl Fischer titration method [29]. LiPF6 (> 99,99%) and LiTFSI (> 99,00%) were purchased from Sigma-Aldrich and used without any further purification. The electrolyte solutions were prepared in an argon-filled glove-box (MBraun, < 1 ppm O2 and H2O). Differential scanning calorimetry (DSC) measurements were carried out using a DSC Q2000 (TA Instruments). For each composition, ~6 mg was weighed inside the glove-box and sealed into Al pans to perform the thermal treatment. A heating ramp of 10 °C/min was used. Conductivity values were obtained by electrochemical impedance spectroscopy using a Solartron 1255 Frequency Response Analyzer combined with a SI 1287 electrochemical interface. The temperature
σ (T ) = σ0 ∙e
−
B T − T0
(1)
The conductivity of EG/ChCl, which is in good agreement with values in the literature [11], is higher than that of LA/ChCl. The conductivity values of the systems based on EG/ChCl are higher than those based on LA/ChCl, and better values are obtained in all cases for 0.5 M compositions. Besides the conductivity, Table 1 reports the viscosity values of the pure DESs, and all the electrolyte mixtures. Generally speaking, the systems based on EG/ChCl are less viscous than the ones based on LA/ChCl. The addition of salts produces an increase in viscosity, with LiPF6 < LiTFSI and 0.5 M < 1 M. For pure DES, values obtained for EG/ChCl and LA/ChCl are in good agreement with data reported in the literature ([11,31] respectively). More information can be obtained from the Walden plot, which is a log-log plot of molar conductivity, Λm, against reciprocal dynamic (or shear) viscosity, η. This plot was recently employed for the analysis of the ionic transport of ionic liquids [34-36]. Based on their position on 46
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Fig. 5. 13C HPDEC NMR spectra for pure EG/ChCl (a and a’), the EG/ChCl LiPF6 0.5 M solution as prepared (b and b’), and for the cycled EG/ChCl LiPF6 0.5 M solution (c and c’).
Similar spectra were obtained for 1.0 M salt concentration (not shown). The mixture of the two components did not lead to reaction products, as the only observed signals can be attributed to the pure EG and ChCl species. The EG unit generated a single signal detected at 63.8 ppm, in good agreement with data reported in the literature for this molecule in different solvents [39]. The other signals in the region 50–70 ppm can be attributed to the ChCl carbon atoms [40]. The assignment of the different signals is easily made, as the molar ratio between the two components is known, and the conditions for acquisition of quantitative data have been carefully controlled. The integrals obtained from deconvolution analysis (not shown) are in very good agreement with the nominal composition. The addition of lithium salts to the eutectic mixture leads to very small changes in the 13C NMR spectra, about 0.2 ppm in the de-shielded direction for all the peaks, which is consistent with no significant specific interactions between salt ions and DES components. There is only a non-localized de-shielding effect compatible with the addition of ionic species. This is somehow expected because of the extended hydrogen-bonding network typical of DESs [6,41]. 13 C NMR data acquired after the cyclic voltammetry seem to confirm the good electrochemical stability of the electrolyte solutions, as no relevant changes were observed in the 13C NMR spectra. These results were confirmed by 7Li NMR spectra acquired on the same samples with both the salts (see Fig. S2, ESI). These spectra contained a single and non-structured peak at 0.35 ppm, consistent with the presence of a mobile Li+ species in the liquid system. Also, the 7Li peak is not affected by electrochemical tests, as demonstrated by its position, shape and line width, which remained substantially unchanged after cycling. We understand that electrode size, electrolyte volume, imposed current and number of cycles are very important parameters in affecting the concentration of decomposition products in the sample. In this sense, these NMR results provided preliminary evidence of the system stability, and more thorough investigations following galvanostatic cycling,
the Walden plot relative to a line of slope unity (“ideal”), which is typical of aqueous KCl solutions, ionic liquids may be classified into three categories. The first one, “good liquids”, is for those near to the ideal line. The ionic liquids below the ideal line (i.e., those with ionic mobility lower than that expected from the viscosity) are labelled “poor liquids”. Finally, ionic liquids above the ideal line are referred to as “superionic liquids”. Fig. 3 shows the Walden plot of the two DES-salt systems. The LA/ChCl-based electrolytes are poor liquids, whereas the EG/ChCl ones are good or even superionic liquids. The best performance is indeed offered by EG/ChCl LiPF6 0.5 M. This is likely due to good salt dissociation [37] produced by the favourable dielectric properties of the EG/ChCl DES, in agreement with the conductivity results. In the following, we focus our attention on the EG/ChCl eutectic mixture as a possible electrolyte for in lithium batteries, and we present some preliminary results. We first checked the stability of the lithium/ electrolyte interface by simply inserting a foil of Li metal into the electrolye, inside the dry box. Gas bubbles were generated in a matter of minutes, and the Li metal was rapidly corroded. The same phenomenon was observed with systems containing LiTFSI. Despite the high reactivity of this electrolyte with metallic lithium, we can still envisage its use in lithium-ion cells with carbon-based anodes. Fig. 4 shows the preliminary electrochemical stability windows of the EG/ ChCl eutectic mixture with the two salts (0.5 M), performed against glassy carbon as the working electrode. We obtained 3.5 and 3.8 V with LiPF6 and LIFTSI, respectively, which are close to the value of 3.61 V reported for the DES mixture [38]. NMR provided more detailed insights into the interactions between the electrolyte components. Fig. 5 reports the 13C NMR quantitative spectra of pure EG/ChCl eutectic mixture (a and a’), and of EG/ChCl LiPF6 0.5 M, before (b and b’) and after (c and c’) the electrochemical stability measurements. The corresponding figure for EG/ChCl LITFSI 0.5 M is reported in the ESI (Fig. S1).
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also at high current rates, are needed to fully assess the electrolyte behaviour.
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4. Conclusions The electrochemical properties of the EG/ChCl eutectic mixture with a lithium salt (LiPF6) as a liquid electrolyte have been investigated. Data indicate a very good ionic conductivity and a reasonable thermal and electrochemical stability, at least at a preliminary level. In addition, since the costs of EG and ChCl are about US$1 per kg and US$0.5 per kg, respectively, the investigated DES is also cheap, and constitutes a very interesting basis for electrolytes to be used in lithiumion batteries. Considering the high compositional flexibility of DESs, a new generation of environmentally friendly and low-cost lithium-ion devices utilizing them can be expected to appear in the near future, enabling greater deployment of renewable energy technologies and reduction of global carbon emissions. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2018.05.016. References [1] D. Larcher, J.-M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19. [2] C.P. Grey, J.M. Tarascon, Sustainability and in situ monitoring in battery development, Nat. Mater. 16 (2017) 45. [3] E. Quartarone, P. Mustarelli, Electrolytes for solid-state lithium rechargeable batteries: recent advances and perspectives, Chem. Soc. Rev. 40 (2011) 2525. [4] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104 (2004) 4303. [5] I. Osada, H. de Vries, B. Scrosati, S. Passerini, Ionic-liquid-based polymer electrolytes for battery applications, Angew. Chem. Int. Ed. 55 (2016) 500. [6] Y. Gu, F. Jérôme, Bio-based solvents: an emerging generation of fluids for the design of eco-efficient processes in catalysis and organic chemistry, Chem. Soc. Rev. 42 (2013) 9550. [7] M. Francisco, A. van den Bruinhorst, M.C. Kroon, Low-transition-temperature mixtures (LTTMs): a new generation of designer solvents, Angew. Chem. Int. Ed. 52 (2013) 3074. [8] E.L. Smith, A.P. Abbott, K.S. Ryder, Deep eutectic solvents (Dess) and their applications, Chem. Rev. 114 (2014) 11060. [9] P. Liu, J.-W. Hao, L.-P. Mo, Z.-H. Zhang, Recent advances in the application of deep eutectic solvents as sustainable media as well as catalysts in organic reactions, RSC Adv. 5 (2015) 48675. [10] D.A. Alonso, A. Baeza, R. Chinchilla, G. Guillena, I.M. Pastor, D.J. Ramón, Deep eutectic solvents: the organic reaction medium of the century, Eur. J. Org. Chem. (2016) 612. [11] Q. Zhang, K. De Oliveira Vigier, S. Royer, F. Jérôme, Deep eutectic solvents: syntheses, properties and applications, Chem. Soc. Rev. 41 (2012) 7108. [12] M. Francisco, A. van den Bruinhorst, M.C. Kroon, New natural and renewable low transition temperature mixtures (LTTMs): screening as solvents for lignocellulosic biomass processing, Green Chem. 14 (2012) 2153. [13] C.R. Muller, I. Meiners, P. Domínguez de María, Highly enantioselective tandem enzyme-organocatalyst crossed aldol reactions with acetaldehyde in deep-eutecticsolvents, RSC Adv. 4 (2014) 46097. [14] E. Massolo, S. Palmieri, M. Benaglia, V. Capriati, F.M. Perna, Stereoselective organocatalysed reactions in deep eutectic solvents: highly tunable and biorenewable reaction media for sustainable organic synthesis, Green Chem. 18 (2016) 792. [15] D. Brenna, E. Massolo, A. Puglisi, S. Rossi, G. Celentano, M. Benaglia, V. Capriati, Towards the development of continuous, organocatalytic, and stereoselective reactions in deep eutectic solvents, Beilstein J. Org. Chem. 12 (2016) 2620. [16] P. Vitale, V.M. Abbinante, F.M. Perna, A. Salomone, C. Cardellicchio, V. Capriati, Unveiling the hidden performance of whole cells in the asymmetric bioreduction of aryl-containing ketones in aqueous deep eutectic solvents, Adv. Synth. Catal. 359 (2017) 1049. [17] V. Mallardo, R. Rizzi, F.C. Sassone, R. Mansueto, F.M. Perna, A. Salomone, V. Capriati, Regioselective desymmetrization of diaryltetrahydrofurans via directed ortho-lithiation: an unexpected help from green chemistry, Chem. Commun. 50
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Bio-inspired choline chloride-based deep eutectic solvents as electrolytes for lithium-ion batteries
T
Luca Milliaa, Valentina Dall'Astaa, Chiara Ferraraa, Vittorio Berbennia, Eliana Quartaronea, ⁎ Filippo Maria Pernab, Vito Capriatib, Piercarlo Mustarellia, a b
Department of Chemistry and INSTM, University of Pavia, Via Taramelli 16, 27100 Pavia, Italy Dipartimento di Farmacia-Scienze del Farmaco, Università di Bari “Aldo Moro”, Consorzio C.I.N.M.P.I.S., Via E. Orabona 4, I-70125 Bari, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-ion batteries Electrolyte Deep eutectic solvents LiPF6
The safety issues of lithium-ion batteries require the substitution of volatile and flammable organic components of the electrolyte. In this context, deep eutectic solvents (DESs) represent an interesting alternative to conventional ionic liquids as green solvents to dissolve common lithium salts. This paper explores two eutectics: i) ethylene glycol/choline chloride (EG/ChCl, 3:1 mol/mol), and ii) L-(+)-lactic acid/choline chloride (LA/ChCl, 2:1 mol/mol). The lithium salts added in both cases were LiN(CF3SO2)2 and LiPF6, both in concentration of either 0.5 M or 1 M. They still retain their liquid properties despite the addition of relatively high molar contents (up to 1.0 M) of lithium salts. The 0.5 M LiPF6/EG:ChCl electrolyte, in particular, displays ionic conductivity of 7.95 mS cm−1 at room temperature, and is thus very promising as a green and cheap electrolyte.
1. Introduction The energy demand of the ever-increasing world population and the issues related to fossil fuel depletion and global warming require concerted efforts to develop sustainable battery systems for electrical energy storage [1]. Lithium-ion batteries are at present the state-of-the-art storage devices. They suffer, however, from several problems including availability and toxicity of active materials, high materials costs, short life cycles, and electrolyte safety at high current rates [2,3]. This last issue requires the replacement of hazardous and volatile organic solvents [4] with safer, cheaper and more sustainable materials. Whereas ionic liquids (ILs) have been a matter of intense research as alternative electrolyte media in recent years [5], deep eutectic solvents (DESs) are opening up new and exciting possibilities as promising next-generation neoteric fluids. DESs are eutectic mixtures easily formed by mixing and gently heating naturally occurring hydrogen-bond donors (e.g., urea, renewable polyols, carbohydrates, carboxylic acids, amines, amides) and hydrogen-bond acceptors (e.g., choline chloride (ChCl), phosphonium salts). Compared with traditional ILs with which they share some physicochemical properties (e.g., negligible vapour pressure, a high thermal stability, nonflammability, recyclability), DESs display attractive advantages such as high biodegradability, low cost (DESs are about ten times cheaper than ILs), and very low toxicity. The solvent properties can be tuned by simply changing the nature and the molar ratio of the components. This last decade has witnessed an exponential increase ⁎
Corresponding author. E-mail address: [email protected] (P. Mustarelli).
https://doi.org/10.1016/j.ssi.2018.05.016 Received 17 July 2017; Received in revised form 10 April 2018; Accepted 16 May 2018 0167-2738/ © 2018 Elsevier B.V. All rights reserved.
of the publications on this topic both in academia and industry [6–12]. In particular, recent breakthroughs in applications of DESs in the fields of organo- and biocatalysis [13–16], organometallic chemistry [17–24], and solar technology [25], have been extremely relevant. Recently, DESs based on N‑methylacetamide (MAc) with different lithium salts (lithium bis(fluoro)sulfonimide (LiN(CF3SO2)2), LiPF6, LiNO3) have been proposed as electrolytes for lithium-ion cells. These systems, however, were liquid at room temperature only for Li molar fraction < 0.35, which limited their ionic conductivity to about 1 mS cm−1 [26]. Geiculescu et al. reported on the usefulness of binary DES electrolytes consisting of mixtures of methanesulfonamide (CH3SO2NH2) and N,N‑dimethyl methanesulfonamide [CH3SO2N (CH3)2] with LiN(FSO2)2 and lithium bis(trifluoromethane)sulfonimide [LiN(CF3SO2)2] (LiTFSI) [27]. The main limitations of this approach were the high viscosity and a relatively low conductivity with respect to LiPF6 in organic carbonate solvents. The transport properties of some of these mixtures were investigated by means of Pulse Field Gradient (PFG) NMR diffusion experiments coupled with molecular dynamics simulations [28]. In this paper, we report on binary DES systems, which still retain their liquid properties despite the addition of a relatively high molar concentration (1 M) of a lithium salt. In particular, we explore two eutectics: i) ethylene glycol/choline chloride (EG/ChCl, 3:1 mol/mol), and ii) L-(+)-lactic acid/choline chloride (LA/ChCl, 2:1 mol/mol) with LITFSI and LiPF6 as the lithium salts, both in concentrations of either
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Fig. 1. DSC thermograms of the DESs-based electrolytes under investigation.
Fig. 2. Arrhenius plots of DESs-based electrolytes under investigation. Table 1 Parameters of Vogel-Tammann-Fulcher (VTF) fits for each electrolyte together with selected physical properties at T = 25 °C. 0, B and T0 are the pre-exponential factors, pseudo-activation energies and “ideal” glass transition temperatures with R2 > 99%, respectively. σ, Λm, η and Λm.η are ionic conductivity, molar conductivity, dynamic viscosity, and Walden product, respectively. Sample
LA/ChCl LA/ChCl LiPF6 0.5 M LA/ChCl LiPF6 1.0 M LA/ChCl LiTFSI 0.5 M LA/ChCl LiTFSI 1.0 M EG/ChCl EG/ChCl LiPF6 0.5 M EG/ChCl LiPF6 1.0 M EG/ChCl LiTFSI 0.5 M EG/ChCl LiTFSI 1.0 M
VTF σ0 (S cm−1)
B (K−1)
T0 (K)
0.73 0.71 12.21 0.41 0.12 0.57 0.71 4.17 3.23 1.04
791.74 668.20 1536.40 623.93 405.75 594.83 593.23 1107.23 1003.34 725.73
177.92 205.88 149.88 202.14 231.30 168.64 168.43 135.08 144.32 167.26
45
σ (S cm−1)
Λm (S cm2 mol−1)
η (cP)
Λm.η (S cm2 mol−1 P)
1.06 × 10−3 5.41 × 10−4 3.95 × 10−4 6.39 × 10−4 2.99 × 10−4 5.27 × 10−3 7.95 × 10−3 4.76 × 10−3 5.16 × 10−3 4.03 × 10−3
0.29 0.13 0.085 0.15 0.06 1.53 2.02 1.07 1.31 0.91
143.8 298.2 611.1 554.8 668.1 68.5 88.4 94.3 121.7 143.8
0.42 0.39 0.52 0.83 0.40 1.05 1.79 1.01 1.60 1.31
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was controlled by means of an oil bath (to within ± 0.2C). Conductivity was measured over a temperature range from −10 to 90 °C. The electrochemical stability window of the DES-based electrolytes was investigated by means of linear voltammetry scans at 100 mV/s from the cathodic limit to the anodic one. A conventional three-electrode cell was used with glassy carbon as the working electrode, Ag/ Ag+ as the reference electrode and Pt wire as the counter electrode. The reference electrode was obtained by immersing an Ag wire in 0.01 M solution of AgNO3 in CH3CN. NMR spectra were recorded on a 400 MHz Bruker Avance III spectrometer at room temperature under static conditions. Samples were prepared in an Ar-filled glove box (H2O, O2 < 1 ppm) using 4 mm rotors and immediately measured. 1H single pulse spectra were collected using a π/2 pulse of 4.8 μs and recycle delay of 4 s after careful calibration of pulse length and estimation of the delay time. 13C quantitative spectra were acquired under high power decoupling conditions (HPDEC pulse program) with a π/2 pulse of 4.7 μs, recycle delay of 5 s and SPINAL-64 heteronuclear decoupling scheme. 7Li single pulse spectra were obtained using a π/2 pulse of 3 μs and recycle delay of 4 s. Pulse lengths and recycle delays were calibrated to ensure the conditions for quantitative measurements. 1H and 13C spectra were referenced to the signal of adamantane (as a secondary standard with respect to TMS), whereas 7Li data were referenced to the signal of LiCl 1 M solution. For selected spectra, fitting analysis was performed using DmFIT® simulation software [32]. The dynamic viscosity, η, was measured at 20C with a cone-plate rotational viscosimeter (Rheotec) in control-shear-stress mode.
Fig. 3. Walden plot of the investigated electrolytes.
3. Results and discussion Fig. 1 shows the DSC behaviours of the two DESs with LiTFSI (0.5 M) and LiPF6 (1.0 M). All the electrolytes are in the liquid state over the temperature range from −50 to at least 100 °C, and no degradation phenomena were observed in this interval. ESs with LiPF6 showed exothermic features around 130 °C, regardless of the salt concentration; this is likely due to LiPF6 degradation [33]. Fig. 2 shows the Arrhenius plots of the ionic conductivity for all the investigated electrolytes. In all cases, the temperature dependence of the conductivity exhibits the Vogel-Tammann-Fulcher (VTF) behaviour typical of supercooled liquids (Eq. 1), in agreement with the DSC results reported in Fig. 1. Here σ0 is a pre-exponential factor related to the carrier concentration, B represents a pseudo-activation energy for the ion diffusion, and T0 is a parameter which can be interpreted as the ideal glass transition temperature, which lies 20–50 K below the glasstransition temperature, Tg, as determined by standard DSC measurements. The conductivity values, and the parameters of the VTF fits, are reported in Table 1, together with other relevant physico-chemical quantities.
Fig. 4. Electrochemical stability window of EG/ChCl with LiTFSI and LiPF6 0.5 M vs. a glassy carbon electrode.
0.5 M or 1.0 M. 2. Experimental details DES components were purchased at the highest purity available from Alfa Aesar, and were used without further purification: L(+)-lactic acid (LA): 90% aqueous solution; choline chloride (ChCl): 98 + %; ethylene glycol (EG): 99%. The employed DESs (EG/ChCl 3:1 mol/mol [11,29,30] and LA/ChCl 2:1 mol/mol [12,31]) were prepared by gentle heating under stirring at 70 °C for 15 min the corresponding individual components in the appropriate molar ratio until a clear solution was obtained. All prepared DESs were fluid at room temperature. The water content of the EG/ChCl (3:1 mol/mol) eutectic mixture was found to be < 0.1 wt% soon after its preparation using the Karl Fischer titration method [29]. LiPF6 (> 99,99%) and LiTFSI (> 99,00%) were purchased from Sigma-Aldrich and used without any further purification. The electrolyte solutions were prepared in an argon-filled glove-box (MBraun, < 1 ppm O2 and H2O). Differential scanning calorimetry (DSC) measurements were carried out using a DSC Q2000 (TA Instruments). For each composition, ~6 mg was weighed inside the glove-box and sealed into Al pans to perform the thermal treatment. A heating ramp of 10 °C/min was used. Conductivity values were obtained by electrochemical impedance spectroscopy using a Solartron 1255 Frequency Response Analyzer combined with a SI 1287 electrochemical interface. The temperature
σ (T ) = σ0 ∙e
−
B T − T0
(1)
The conductivity of EG/ChCl, which is in good agreement with values in the literature [11], is higher than that of LA/ChCl. The conductivity values of the systems based on EG/ChCl are higher than those based on LA/ChCl, and better values are obtained in all cases for 0.5 M compositions. Besides the conductivity, Table 1 reports the viscosity values of the pure DESs, and all the electrolyte mixtures. Generally speaking, the systems based on EG/ChCl are less viscous than the ones based on LA/ChCl. The addition of salts produces an increase in viscosity, with LiPF6 < LiTFSI and 0.5 M < 1 M. For pure DES, values obtained for EG/ChCl and LA/ChCl are in good agreement with data reported in the literature ([11,31] respectively). More information can be obtained from the Walden plot, which is a log-log plot of molar conductivity, Λm, against reciprocal dynamic (or shear) viscosity, η. This plot was recently employed for the analysis of the ionic transport of ionic liquids [34-36]. Based on their position on 46
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Fig. 5. 13C HPDEC NMR spectra for pure EG/ChCl (a and a’), the EG/ChCl LiPF6 0.5 M solution as prepared (b and b’), and for the cycled EG/ChCl LiPF6 0.5 M solution (c and c’).
Similar spectra were obtained for 1.0 M salt concentration (not shown). The mixture of the two components did not lead to reaction products, as the only observed signals can be attributed to the pure EG and ChCl species. The EG unit generated a single signal detected at 63.8 ppm, in good agreement with data reported in the literature for this molecule in different solvents [39]. The other signals in the region 50–70 ppm can be attributed to the ChCl carbon atoms [40]. The assignment of the different signals is easily made, as the molar ratio between the two components is known, and the conditions for acquisition of quantitative data have been carefully controlled. The integrals obtained from deconvolution analysis (not shown) are in very good agreement with the nominal composition. The addition of lithium salts to the eutectic mixture leads to very small changes in the 13C NMR spectra, about 0.2 ppm in the de-shielded direction for all the peaks, which is consistent with no significant specific interactions between salt ions and DES components. There is only a non-localized de-shielding effect compatible with the addition of ionic species. This is somehow expected because of the extended hydrogen-bonding network typical of DESs [6,41]. 13 C NMR data acquired after the cyclic voltammetry seem to confirm the good electrochemical stability of the electrolyte solutions, as no relevant changes were observed in the 13C NMR spectra. These results were confirmed by 7Li NMR spectra acquired on the same samples with both the salts (see Fig. S2, ESI). These spectra contained a single and non-structured peak at 0.35 ppm, consistent with the presence of a mobile Li+ species in the liquid system. Also, the 7Li peak is not affected by electrochemical tests, as demonstrated by its position, shape and line width, which remained substantially unchanged after cycling. We understand that electrode size, electrolyte volume, imposed current and number of cycles are very important parameters in affecting the concentration of decomposition products in the sample. In this sense, these NMR results provided preliminary evidence of the system stability, and more thorough investigations following galvanostatic cycling,
the Walden plot relative to a line of slope unity (“ideal”), which is typical of aqueous KCl solutions, ionic liquids may be classified into three categories. The first one, “good liquids”, is for those near to the ideal line. The ionic liquids below the ideal line (i.e., those with ionic mobility lower than that expected from the viscosity) are labelled “poor liquids”. Finally, ionic liquids above the ideal line are referred to as “superionic liquids”. Fig. 3 shows the Walden plot of the two DES-salt systems. The LA/ChCl-based electrolytes are poor liquids, whereas the EG/ChCl ones are good or even superionic liquids. The best performance is indeed offered by EG/ChCl LiPF6 0.5 M. This is likely due to good salt dissociation [37] produced by the favourable dielectric properties of the EG/ChCl DES, in agreement with the conductivity results. In the following, we focus our attention on the EG/ChCl eutectic mixture as a possible electrolyte for in lithium batteries, and we present some preliminary results. We first checked the stability of the lithium/ electrolyte interface by simply inserting a foil of Li metal into the electrolye, inside the dry box. Gas bubbles were generated in a matter of minutes, and the Li metal was rapidly corroded. The same phenomenon was observed with systems containing LiTFSI. Despite the high reactivity of this electrolyte with metallic lithium, we can still envisage its use in lithium-ion cells with carbon-based anodes. Fig. 4 shows the preliminary electrochemical stability windows of the EG/ ChCl eutectic mixture with the two salts (0.5 M), performed against glassy carbon as the working electrode. We obtained 3.5 and 3.8 V with LiPF6 and LIFTSI, respectively, which are close to the value of 3.61 V reported for the DES mixture [38]. NMR provided more detailed insights into the interactions between the electrolyte components. Fig. 5 reports the 13C NMR quantitative spectra of pure EG/ChCl eutectic mixture (a and a’), and of EG/ChCl LiPF6 0.5 M, before (b and b’) and after (c and c’) the electrochemical stability measurements. The corresponding figure for EG/ChCl LITFSI 0.5 M is reported in the ESI (Fig. S1).
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also at high current rates, are needed to fully assess the electrolyte behaviour.
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