Improvement of charge/discharge properties of oligoether electrolytes by zwitterions with an attached cyano group for use in lithium-ion secondary batteries
Introduction
Polymer electrolytes have been vigorously studied for battery applications over the last three decades [1] because they are superior to conventional organic liquid electrolytes in terms of leakage, volatility, and processability. Since poly(ethylene oxide) (PEO) can solubilize alkali metal salts, as a result of the dipole moment of the ether oxygen, and transport ions along the main chain, it is a good candidate for use as a solid polymer electrolyte [2]. Some problems must be addressed before PEO can be utilized as a promising electrolyte material. PEO exhibits a low ionic conductivity (below 10−4 S cm−1) at room temperature due to the crystallinity. It also has a cation transference number below 0.4 because of interaction between the cation and ether segments [3], [4]. Several researchers attempted to solve these problems by evaluating various PEO derivatives. For example, building comb-like anionic structures and attaching them to the PEO chain is effective for improving ionic conductivity [5], [6] and lithium ion transference number [7], [8].
Additives have been actively studied to improve the properties of the PEO matrix. For example, plasticizers [9], [10] and inorganic fillers [11], [12] have been evaluated as additives that can improve ionic conductivity, lithium ion transference number, and interfacial resistance between the electrode and electrolyte. In recent years, ionic liquids (ILs) have been studied as plasticizers for PEO matrixes [13]. Although the addition of ILs to PEO matrixes improves ionic conductivity, the lithium ion transference number is still low owing to the migration of IL itself along the potential gradient [14].
On the other hand, zwitterions, which have a positive and negative charge in the same molecule, are one of potential electrolyte material additives that can help transport ions effectively [15], [16], [17]. Some electrochemical properties of electrolyte materials have been improved by the addition of zwitterions. Tiyapiboonchaiya et al. reported that ionic conductivity was improved by adding zwitterions to a polymer gel electrolyte [18]. Byrne et al. also reported that ionic conductivity was improved by the addition of zwitterions and inorganic nanofillers [19]. The addition of zwitterions to ILs led to an increase in the diffusion coefficient of lithium ions, the formation of solid electrolyte interphases (SEIs) on electrodes, and the improvement of coulombic efficiencies for the lithium plating-striping reaction [20].
Recently, we found that the oxidation limit of poly(ethylene glycol)dimethyl ether (PEGDME)/lithium bis(fluorosulfonyl)amide (LiFSA) was raised to 5 V by adding a small amount of a zwitterion with an oligoether on the side chain [21]. In general, high-voltage active cathode materials such as LiCoO2 and LiNi1/3Mn1/3Co1/3O2 [22], [23], [24] cannot be applied to lithium-ion batteries with PEO-based electrolytes because the oxidation limit of the PEO chains is ∼4 V [25], [26], [27]. Zwitterions are interesting alternative additives for PEO-based electrolytes. In this study, we focused on zwitterions with cyano groups that showed a dissociation effect for lithium salts due to its interaction with cations [28]. It is also known that the introduction of a cyano group into an IL structure leads to the formation of SEIs on electrodes [29]. According to the knowledge, zwitterions with cyano groups would improve the electrochemical and charge-discharge properties of oligoethers. We synthesized a novel zwitterion with a cyano group on the side chain, and added the zwitterion to an oligoether, poly(ethylene glycol) dimethyl ether (PEGDME), matrix as model compounds for polymer electrolyte to investigate the effect of the zwitterion and cyano group on the electrochemical properties of PEGDME-based electrolytes.
Section snippets
Materials
Poly(ethylene glycol)dimethyl ether (PEGDME) (Mn = 1000 g mol−1) and 1-(2-hydroxyethyl)imidazole (97.0%) were purchased from Sigma-Aldrich. PEGDME was dried at 60 °C in vacuo before use. Potassium hydroxide (86.0%) and acrylonitrile (99.0%) were purchased from Kanto Chemical Co., Inc. Propanesultone (>99.0%) was purchased from Tokyo Chemical Industry Co., Ltd. and was distilled in vacuo before use. Dehydrated acetonitrile was purchased from Wako Pure Chemical Industries, Ltd.
Thermal properties
CZ showed a Tg at 0 °C and a Tm at 107 °C, which were lower than other zwitterions with alkyl group and methoxy ethyl group on the side chains. The Tg and Tm values of zwitterions with a butyl group (Bimps) were 11 °C and 170 °C, respectively. The Tg and Tm values of zwitterions with methoxyethyl group (OE1imps) were 7 °C and 175 °C, respectively [31]. It is difficult to lower the Tg and Tm of zwitterions [32]. The introduction of a cyano group with ether oxygen was effective in lowering the Tg and Tm
Conclusions
A new zwitterion containing a cyano group, CZ, was synthesized. The introduction of the cyano group with ether oxygen was effective in lowering the Tg and Tm of zwitterions. The addition of CZ to PEGDME/LiTFSA resulted in an increase in viscosity and a decrease in ionic conductivity, lithium transference number, and limit current density. One reason for these changes was the interaction between the cyano groups and lithium ions. The electrochemical stability of PEGDME/LiTFSA/CZ was over 5 V vs
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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