Short communicationIonic conductance behavior of polymeric gel electrolyte containing ionic liquid mixed with magnesium salt
Introduction
Magnesium (Mg) is one of the ideal materials as the negative electrode of practical batteries because of its low electrochemical equivalence (12.15 g eq−1) and low electrode potential (−2.3 V versus SHE). Despite of the practical use of primary and reserve batteries [1], development of rechargeable Mg batteries has been retarded mainly due to the irreversibility of the Mg negative electrode and the lack of proper Mg2+-conducting nonaqueous electrolyte [2]. Thus, the development of Mg2+-ion conductor, preferably at solid state, has been a key issue to realize the rechargeable Mg batteries.
On the analogy of the Li+-ion conductor, there have been some publications on the polymeric complexes of poly(ethylene oxide) (PEO) with inorganic Mg salts [3], [4], [5]. High conductivities ranging from 10−6 to 10−4 S cm−1 were reported for PEO–Mg salt complexes in the temperature range of 80–100 °C, but the transport number of Mg2+ was found to be very low in those systems [6]. Polymeric gel systems swollen with such aprotic solvents as propylene carbonate (PC) [7], [8] and gel electrolytes based on PEO-Grignard [9] have also been proposed as Mg2+-conductors with high ionic mobility. We also obtained high ionic conductivity for the gels containing Mg salts with mixed alkyl carbonates, ethylene carbonate (EC) and dimethyl carbonate (DMC), as a plasticizer [10], where we chose a cross-linked polymethacrylate with repeated ethylene oxide (EO) units as the solid polymer matrix reported previously [11]. Such a network polymer with polar functionalities dissolves a wide variety of inorganic salts of multivalent cations with high concentrations even at ambient temperature. However, those low molecular weight organic plasticizers would cause serious troubles when the gel will be applied to practical battery systems at a higher temperature range than their boiling or flash points.
Recently, air- and water-stable organic ionic liquids (also known as room temperature molten salts) have been proposed as non-volatile polar media in variety of electrochemical systems. Especially, much attention has been directed to the combinations of imidazolium cations and specific organic anions [12], [13], due to their unique characteristics of thermal stability, non-flammability and extremely high ionic conductivity. These ionic liquids can be used for polymeric gel electrolyte as not only the charge carrier but also the plasticizer [14], [15], [16], [17], [18].
In this work, we have applied the ionic liquid to polymeric solid (gel) electrolyte that conducts Mg2+ ion at ambient temperature. The system consists of poly(ethylene oxide)-modified poly(metahcrylate) (PEO-PMA) as the polymer matrix and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI), dissolving Mg salt, Mg[(CF3SO2)2N]2. The ionic conductance behavior of the resulting gel was investigated by ac impedance measurements in an ambient temperature range. The mobile species in the polymeric gel system was also examined by a dc polarization method using blocking and non-blocking electrodes.
Section snippets
Materials
Poly(ethylene glycol) monomethacrylate (PEM) and poly(ethylene glycol) dimethacrylate (PED) (Shin-Nakamura Chemical, Scheme 1(a)) were used as prepolymers for the polymer matrix formation [19]. The prepolymers, PEM and PED, were dehydrated by molecular sieves prior to use, and then kept in a glove box filled with dry Ar. Reagent grade of 1-ethyl-3-methylimidazolium bromide (EMIBr; Acros Organics) was used as received. Salts of bis(trifluoromethylsulfonyl)imide, LiTFSI and Mg(TFSI)2, were also
Thermal properties
The polymeric gel was obtained as a self-standing transparent film with sufficient mechanical strength, as shown in Fig. 1. The thermal properties of the polymer matrix generally determine the ion-transport behavior of the resulting polymeric gel electrolyte. Fig. 2 shows typical DSC profiles of the polymeric systems. As shown in curve a, phase transition behavior was observed at about −70 °C (203 K) for the polymer matrix, PEO-PMA. This is attributed to the glass transition of the repeated EO
Conclusions
Polymeric gel electrolytes consisting of a PEO-PMA matrix with EMITFSI dissolving Mg(TFSI)2 were synthesized, and their ionic conductance behavior has been investigated as solid electrolyte for rechargeable Mg batteries. The results are summarized as follows:
- 1.
The polymeric gel electrolyte is flexible and self-standing with proper mechanical strength.
- 2.
The ionic conductivity depended on the content of the ionic liquid containing Mg(TFSI)2. The highest conductivity of the gel examined was 1.1 × 10−4 S
Acknowledgements
This work was financially supported by the Grants-in-Aid for Scientific Research (Nos. 14380226 and 15550164) from JSPS.
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