Protic ionic liquids as electrolytes for lithium-ion batteries

Protic ionic liquids as electrolytes for lithium-ion batteries

Electrochemistry Communications 31 (2013) 39–41 Contents lists available at SciVerse ScienceDirect Electrochemistry Communications journal homepage:...

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Electrochemistry Communications 31 (2013) 39–41

Contents lists available at SciVerse ScienceDirect

Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

Protic ionic liquids as electrolytes for lithium-ion batteries S. Menne a, J. Pires b, M. Anouti b, A. Balducci a,⁎ a b

Westfälische Wilhelms-Universität, Institut für Physikalische Chemie-MEET, Corrensstr. 28/30, 48149 Münster, Germany Université François Rabelais, Laboratoire PCMB (EA 4244), équipe (CIME), Parc de Grandmont, 37200 Tours, France

a r t i c l e

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Article history: Received 4 February 2013 Received in revised form 24 February 2013 Accepted 25 February 2013 Available online 6 March 2013 Keywords: Protic ionic liquids Lithium-ion batteries LFP LTO

a b s t r a c t In this work we report for the first time about the use of protic ionic liquids (PILs) as electrolyte for lithium-ion batteries. The electrolyte 1 M LiTFSI in Et3NHTFSI displays a conductivity comparable to that of aprotic ionic liquids, and electrochemical stability window large enough to allow the realization of LIBs containing LFP as cathode and LTO as anode. The use of this PIL as electrolyte in LIBs allows the realization of devices able to deliver good capacity and promising cycling stability. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Lithium-ion batteries (LIBs) are nowadays considered one of the most important energy storage devices and in the last years they have been introduced in an increasing number of applications [1]. Currently, LIBs contain electrolytic solution based on mixtures of organic carbonates (e.g. ethylene carbonate, EC, propylene carbonate PC, etc.) and lithium hexafluorophosphate (LiPF6) as lithium salt [1]. These electrolytes display high conductivity and they allow the realization of high performance batteries. However, due to the flammability of the solvent their use poses serious safety risks and strongly reduces the battery operative temperature range [2]. With the aim to improve the safety of LIBs, in the last years a lot of research has been focused on the development of alternative, advanced electrolyte components [1]. Among the electrolytes proposed so far, those based on aprotic ionic liquids (ILs) appear to be very promising since their introduction can improve both, the safety as well as the operative temperature range of use of LIBs [3]. Nevertheless, the performance of IL-based LIBs still needs to be improved in order to be fully competitive with those of the conventional electrolytes. Moreover, also the cost of ILs might represent an obstacle for the introduction of these electrolytes in LIBs. Protic ionic liquids (PILs) are a subset of ILs, they display all typical (and beneficial) properties of IL, but they have the advantage of being easier to synthesize and cheaper compared to aprotic ionic liquids. For these reasons, they can be certainly considered as interesting candidate for the realization of safer LIBs. So far, PILs have been proposed as electrolyte for fuel cells [4] and supercapacitors [5–7]; however, to ⁎ Corresponding author. Tel.: +49 2518336083; fax: +49 2518336084. E-mail address: [email protected] (A. Balducci). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.02.026

the best of our knowledge, no studies have been dedicated to the use of PILs as electrolytes for LIBs. In a recent work we showed that electrolytes containing PILs can be conveniently used in combination with lithium iron phosphate (LFP) electrodes, and that the use of these electrolytes allows good performance in terms of both, electrode capacity and electrode stability [8]. Considering these results, using an anodic material compatible with these types of electrolytes, the introduction of PILs in LIBs could be therefore possible. In this paper we report for the first time the use of PILs as electrolytes for LIBs. The investigated electrolytes consisted of a solution containing the PIL triethylammonium bis(tetrafluoromethylsulfonyl) amide (Et3NHTFSI) and the lithium salt lithium bis(tetrafluoromethylsulfonyl)amide (LiTFSI) (1 M). Initially, the conductivity and the electrochemical stability of the electrolyte were considered. Afterwards, the electrolyte 1 M LiTFSI in Et3NHTFSI was used in a lithium-ion battery containing lithium titanate (Li4Ti5O12, LTO) as anode and LFP as cathode. 2. Experimental The triethylammonium bis(tetrafluoromethylsulfonyl)amide (Et3NHTFSI) (Fig. 1) was synthesized using a procedure previously described in the literature [6]. The electrolyte used for all electrochemical tests was a solution of Et3NHTFSI containing 1.0 M LiTFSI as lithium salt. In the following pages the electrolytes will be indicated as 1 M LiTFSI in Et3NHTFSI. The water content of the prepared electrolytes was below 10 ppm as measured by automated Karl–Fischer Titration. The conductivity and the electrochemical stability window (ESW) of the electrolyte were determined as reported in reference [8].

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3. Results and discussion

Fig. 1. Chemical structure of the protic ionic triethylammonium bis(tetrafluoromethylsulfonyl)amide (Et3NHTFSI).

The LFP and the LTO electrodes used for the realization of the LIBs investigated in this work were prepared using a procedure identical to that reported in reference [3]. The geometric area of the electrodes was 1.13 cm 2. The mass loading of LFP electrodes was 1 mg cm −2, while that of the LTO electrodes was 3.1 mg cm −2. All the electrochemical tests were carried out in three-electrode Swagelok® cells at room temperature (RT). The cells were assembled in a dry box (MBraun) with oxygen and water contents lower than 1 ppm. As separator, a stack of three non-woven fleeces (FS2226, Freudenberg, Germany) drenched with 80 μL of electrolyte was used. The electrochemical measurements were carried out using a MACCOR Series 4000 battery tester. Constant current cycling (CC) was carried out at RT using current densities ranging from 0.0172 A g −1 to 1.72 A g −1. The C-rate was calculated on the base of the theoretical capacity of LFP [8]. The LIBs investigated in this work were all cathode limited. Therefore, all values of capacity reported in the next pages are always referred to the LFP cathode.

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Fig. 2a shows the variation of the ionic conductivity of the electrolyte 1 M LiTFSI in Et3NHTFSI in the temperature range from − 20 °C to 60 °C. As shown in the figure, the electrolyte displays conductivity of 1.85 mS cm −1 and 5.46 mS cm −1 at 20 °C and 60 °C, respectively. These values are comparable to those displayed by many aprotic ILs already used in LIBs [2,9]. It is important to note that these values of conductivities are significantly lower (nearly three times lower) than those already reported in the literature for Et3NHTFSI [6]. As mentioned in the experimental section, the considered electrolyte displayed water content lower than 10 ppm. This value is among the lowest so far reported for PIL, and it is two orders of magnitude lower than that of the PIL considered in ref. [6]. Taking into account this difference, it is therefore reasonable to suppose that the lower water content of 1 M LiTFSI in Et3NHTFSI is responsible for the lower conductivities observed for this PIL in our investigation. Fig. 2b shows the electrochemical stability window (ESW) of 1 M LiTFSI in Et3NHTFSI at 20 °C. As shown, the electrolyte displayed an overall electrochemical stability window of about 4 V, which was comprised in the voltage range from 1.5 V to 5.6 V vs. Li/Li +. The cathodic limit showed by this electrolyte was significantly higher (vs. Li/Li +) than that normally observed for aprotic ionic liquids, and it was mainly limited by the deprotonation of the cation [6]. On the contrary, the anodic limit was comparable to those of other electrolytes containing the anion TFSI −. Considering these results, 1 M LiTFSI in Et3NHTFSI appears to display an electrochemical stability window large enough to be used as electrolyte for LIBs. Nevertheless,

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Voltage / V vs. Li/Li+ Fig. 2. (a) Ionic conductivity of the electrolyte 1 M LiTFSI in Et3NHTFSI in the temperature range between −40 °C and 60 °C; (b) electrochemical stability window of the same electrolyte at 20 °C.

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Cycle number Fig. 3. (a) Charge–discharge voltage profiles of a LFP cathode and LTO anode and (b) evolution of the discharge capacity vs. cycle number for a LIBs containing 1 M LiTFSI in Et3NHTFSI as electrolyte. The tests were carried out at RT at 1C rate.

S. Menne et al. / Electrochemistry Communications 31 (2013) 39–41

Moreover, the stability of the LIB seems to indicate that 1 M LiTFSI in Et3NHTFSI can be safely used in combination with LFP and LTO based electrodes. Fig. 4 shows the capacity displayed by the investigated LIBs during charge–discharge tests carried out at 0.1C and 10C (for comparison also the test at 1C is included in the figure). As shown, during the test at 0.1C and 10C the LIBs delivered a capacity of 115 mA h g −1 and ca. 30 mA h g −1, respectively. These values of capacity are comparable with those obtained in similar test conditions with aprotic ionic liquid-based electrolytes [9]. Clearly, the performance of the LIBs needs to be improved. Nevertheless, the results of this test can be seen as a proof of concept about the use of PILs as electrolyte in LIBs.

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due to the limited cathodic stability of this electrolyte, only anodic materials in which the insertion–extraction process of lithium occurs at potential higher than 1.5 V vs. Li/Li + can be considered as suitable for use in 1 M LiTFSI in Et3NHTFSI. Taking this point into account, we therefore decided to investigate the use of 1 M LiTFSI in Et3NHTFSI as electrolyte in LIBs containing LFP as cathode and LTO as anode. LFP was selected because our previous study showed that this cathodic material can be used in electrolyte containing PILs [8]. LTO was selected because in this anodic material the lithium insertion–extraction process takes place at a potential above (but very close) to 1.5 V vs. Li/Li +, which is the lowest potential that can be safely used in the investigated electrolyte. For this reason, in order to use LTO-based electrodes in combination with 1 M LiTFSI in Et3NHTFSI, it is important to reduce as much as possible the risk of having this electrode working outside the electrochemical stability of the electrolyte. In order to reduce the abovementioned risk, we decided to realize a cathode limited LIB. Fig. 3a shows the voltage profiles of the LFP cathode and LTO anode in the electrolyte 1 M LiTFSI in Et3NHTFSI during charge–discharge tests carried out at 1C. As shown in the figure, the LFP electrode displayed the typical voltage profile, and the plateau corresponding to the lithium insertion–extraction was visible at around 3.45 V vs. Li/Li +. On the other hand, since the LIBs were cathode limited, the LTO anode was subject to a more limited voltage excursion, which occurred inside the electrochemical stability of the electrolyte. As shown in Fig. 3b, during the charge–discharge test carried out at RT and 1C the LIBs delivered a specific capacity of about 60 mA h g −1, stable for 300 cycles. This value of capacity is clearly lower than those observed in conventional electrolytes (ca. 150 mA h g −1 in the electrolyte 1 M LiTFSI in PC, [3]). Most likely, the higher viscosity and the reduced lithium mobility in 1 M LiTFSI in Et3NHTFSI compared to the conventional electrolytes are the reasons of this lower capacity. Nevertheless, it is important to note that the capacity showed by the LIBs is comparable with those obtained with aprotic ionic liquid-based electrolytes [2,9].

In this manuscript we reported for the first time about the use PILs as electrolyte for LIBs. PILs display all beneficial properties of ILs, they are easy to synthesize and cheaper than aprotic liquids. The electrolyte 1 M LiTFSI in Et3NHTFSI displays a conductivity comparable to that of aprotic ionic liquids, and electrochemical stability window large enough to allow the realization of LIBs containing LFP as cathode and LTO as anode. This PIL-based electrolyte allows the realization of devices able to deliver capacities comparable with those possible with aprotic ionic liquid-based electrolyte. The performance of the investigated LIBs certainly needs to be improved. Nevertheless, the results of this work clearly show that PIL can be successfully introduced in LIBs. Considering the beneficial properties (and the cost) of these IL, such introduction could represent an important contribution for the realization of safer and environmentally friendly LIBs. Acknowledgments The authors wish to thank the University of Münster and the Ministry of Innovation, Science and Research of North Rhine-Westphalia (MIWF) within the project “Superkondensatoren und Lithium-Ionen-HybridSuperkondensatoren auf der Basis ionischer Flüssigkeiten” and the Conseil Régional within the project Sup'Caplipe for the financial support. References [1] B. Scrosati, J. Garche, Journal of Power Sources 195 (2010) 2419. [2] A. Balducci, S.S. Jeong, G.T. Kim, S. Passerini, M. Winter, M. Schmuck, G.B. Appetecchi, R. Marcilla, D. Mecerreyes, V. Barsukov, V. Khomenko, I. Cantero, I. De Meatza, M. Holzapfel, N. Tran, Journal of Power Sources 196 (2011) 9719. [3] S. Menne, R.S. Kühnel, A. Balducci, Electrochimica Acta 90 (2013) 641. [4] H. Matsuoka, H. Nakamoto, M.A.B.H. Susan, M. Watanabe, Electrochimica Acta 50 (2005) 4015. [5] L. Mayrand-Provencher, S. Lin, D. Lazzerini, D. Rochefort, Journal of Power Sources 195 (2010) 5114. [6] L. Timperman, P. Skowron, A. Boisset, H. Galiano, D. Lemordant, E. Frackowiak, F. Beguin, M. Anouti, Physical Chemistry Chemical Physics 14 (2012) 8199. [7] L. Timperman, H. Galiano, D. Lemordant, M. Anouti, Electrochemistry Communications 13 (2011) 1112. [8] N. Böckenfeld, M. Willeke, J. Pires, M. Anouti, A. Balducci, Journal of the Electrochemical Society 160 (2013) A559. [9] G.T. Kim, S.S. Jeong, M. Joost, E. Rocca, M. Winter, S. Passerini, A. Balducci, Journal of Power Sources 196 (2011) 2187.