Effect of zwitterions on electrochemical properties of oligoether-based electrolytes

doi:10.1016/j.electacta.2014.12.067

Electrochimica Acta

Volume 175, 1 September 2015, Pages 209-213

https://doi.org/10.1016/j.electacta.2014.12.067 Get rights and content

Abstract

Solid polymer electrolytes show great potential in electrochemical devices. Poly(ethylene oxide) (PEO) has been studied as a matrix for solid polymer electrolytes because it has relatively high ionic conductivity. In order to investigate the effect of zwitterions on the electrochemical properties of poly(ethylene glycol) dimethyl ether (G5)/lithium bis(fluorosulfonyl) amide (LiFSA) electrolytes, a liquid zwitterion (ImZ2) was added to the G5-based electrolytes. In this study, G5, which is a small oligomer, was used as a model compound for PEO matrices. The thermal properties, ionic conductivity, and electrochemical stability of the electrolytes with ImZ2 were evaluated. The thermal stabilities of all the G5-based electrolytes with ImZ2 were above 150 °C, and the ionic conductivity values were in the range of 0.8–3.0 mS cm⁻¹ at room temperature. When the electrolytes contained less than 5.5 wt% ImZ2, the ionic conductivity values were almost the same as that of the electrolyte without ImZ2. The electrochemical properties were improved with the incorporation of ImZ2. The anodic limit of the electrolyte with 5.5 wt% ImZ2 was 5.3 V vs. Li/Li⁺, which was over 1 V higher than that of G5/LiFSA.

Introduction

Polymer electrolytes have been investigated for their potential application in electrochemical devices, particularly in secondary lithium-ion batteries. Solid polymer electrolytes have several advantages, such as no leakage of electrolytes and no internal shorting. Poly(ethylene oxide) (PEO)/lithium salt complexes have been extensively studied as polymer electrolytes [1], [2], [3], [4], [5], [6], [7] because PEO has a high polarity, which promotes the dissociation of lithium salts, and high mobility, which can promote the transport of dissociated ions. However, the conductivity of these complexes is mainly due to the migration of anionic species; the lithium-ion transference number (t_Li+) is generally low, with values below 0.4. These low values may result in a concentration polarization that limits the rate (power) of batteries [8]. In addition, the anodic limit of pristine PEO is about 4.0 V. It is difficult to use electrodes with high charge voltages, such as LiCoO₂ and LiNi_1/3Mn_1/3Co_1/3O₂ (NMC), as cathode materials [9], [10]. In order to improve the electrochemical stability of these materials, many kinds of additives have been investigated. For example, PEO-based electrolytes exhibited an anodic stability of up to 4.5 V with addition of low molecular weight poly(ethylene glycol) dimethyl ether (PEGDME) [11], ceramic fillers [12], [13], or PEG-borate ester [14]. Although the electrochemical stability of PEO-based electrolytes was improved with these additives, the ionic conductivity values were below 10⁻⁴ S cm⁻¹ at room temperature. It is still difficult to achieve high ionic conductivity and good electrochemical stability simultaneously because the additives cause an increase in the viscosity of matrices.

It has been reported that the addition of zwitterions promotes the dissociation of lithium salts and facilitates the transport of lithium ions in ionic-liquid-based electrolytes and polymer electrolytes [15]. In addition to enhancing the conduction of lithium ions, battery cycle performances were improved by the formation of a solid electrolyte interface (SEI) film on the surface of the electrodes [16], [17], [18]. We previously reported that an imidazolium-based zwitterion containing two oxyethylene units (ImZ2) can be synthesized as a colorless liquid at room temperature [19], and that lithium bis(fluorosulfonyl) amide (LiFSA) mixed with a small amount of the liquid zwitterion exhibited high ionic conductivity and t_Li+ values [20].

In order to improve the lithium transference number and oxidation resistance at around 4.0 V for PEO, ImZ2 was added to poly(ethylene glycol) dimethyl ether (G5)/LiFSA electrolytes. In this study, we investigated the effects of ImZ2 on the ionic conductivity and electrochemical stability of G5/LiFSA electrolytes.

Section snippets

Materials

3-(1-(2-Methoxyethoxyethyl)-1H-imidazol-3-ium-3-yl) propane-1-sulfonate (ImZ2) was synthesized according to the published procedure [19]. LiFSA (Piotrek, 99.0%) was used as received. Poly(ethylene glycol) dimethyl ether (G5) (Aldrich, M_n = 250) was dried in vacuo before use. The chemical structures of G5, LiFSA, and ImZ2 are shown in Fig. 1. G5 was mixed with LiFSA in an argon filled glove box. The molar ratio of ethylene oxide (EO) units to Li⁺ was fixed at 20 for the G5/LiFSA electrolytes [21].

Thermal properties

TGA measurements were carried out for G5/LiFSA and G5/LiFSA/ImZ2(x). Watanabe and co-workers reported that complexes of glyme and lithium bis(trifluoromethylsulfonyl) amide (LiTFSA) are thermally stable up to 200 °C [24]. Although the onset thermal decomposition temperature (T_d) values of G5/LiFSA and G5/LiFSA/ImZ2(x) were about 150 °C, G5/LiFSA and G5/LiFSA/ImZ2(x) exhibited sufficient thermal stability (> 100 °C) for use as electrolytes in lithium-ion batteries. These lower T_d values could be

Conclusion

G5-based electrolytes (G5/LiFSA and G5/LiFSA/ImZ2(x)) were prepared and the effects of ImZ2 on the thermal and electrochemical properties were investigated. When the added amount of ImZ2 was below 5.5 wt%, the ionic conductivity values of the G5/LiFSA electrolytes were maintained above 10⁻³ S cm⁻¹ at room temperature. The electrochemical stability of the G5-based electrolytes was improved by the incorporation of ImZ2. G5/LiFSA/ImZ2(5.5) showed an electrochemical window in the potential range of

Acknowledgement

This study was supported by a Grant-in-Aid for Scientific Research (C) (No. 26410140) from Japan Society for the Promotion of Science (JSPS).

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Effect of zwitterions on electrochemical properties of oligoether-based electrolytes

Abstract

Introduction

Section snippets

Materials

Thermal properties

Conclusion

Acknowledgement

Electrochim. Acta

J. Power Sources

Electrochim. Acta

J. Power Sources

Electrochim. Acta

Electrochim. Acta

J. Power Sources

Eur. Polym. J.

J. Power Sources

Electrochim. Acta

J. Power Sources

Solid State Ionics

Electrochem. Commun.

J. Power Sources

Electrochim. Acta

Int. J. Electrochem. Sci.

J. Power Sources

J. Power Sources