This work presents a different approach to preparing polymer electrolytes having borate ester groups for lithium ion batteries. The polymers were synthesized by reaction between poly(ethylene glycol) methyl ether (PEGME) and BH3-THF complex. Molecular weight of PEGMEs was changed with different chain lengths. Then the polymer electrolytes comprising boron were prepared by doping of the matrices with CF3SO3Li at various molar ratios with respect to EO to Li and they are abbreviated as PEGMEX-B-. The identification of the PEGME-borate esters was carried out by FTIR and 1H NMR spectroscopy. Thermal properties of these electrolytes were investigated via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The ionic conductivity of these novel polymer electrolytes was studied by dielectric-impedance spectroscopy. Lithium ion conductivity of these electrolytes was changed by the length of PEGME as well as the doping ratios. They exhibit approximate conductivities of 10−4 S·cm−1 at 30°C and 10−3 S·cm−1 at 100°C.

1. Introduction

Lithium ion batteries have been widely used as energy storage devices due to their high volumetric (W h L−1) and gravimetric energy (W h kg−1) density and high cycle life [1, 2], but their safety problems should be solved, especially for the high energy density applications such as portable phones and electric vehicles [13]. These drawbacks of lithium ion batteries are mainly caused by the use of organic liquid electrolytes due to high volatility, leakage, or flammability [4]. Solid polymer electrolyte seems to be an alternative to solve the safety problem, but the low ionic conductivity at room temperature is not enough to cover the required performance. The other alternative is aqueous electrolyte lithium ion batteries which promise higher safety due to nonflammability or toxicity of water [5, 6]. Narrow electrochemical stability window of water (1.23 V) as a consequence of low energy density and instability of electrode materials prevents its applications. Although water in salt electrolyte system seems to be promising due to increased working voltage, this technology is still premature and the salts that are used are too expensive for any application [7, 8]. Gel polymer electrolytes (GPEs) has higher ionic conductivity than solid polymer electrolyte, better stability than liquid electrolyte, and higher operating voltage than aqueous electrolytes yielding a different method to solve the problems of those energy storage systems [911].

Generally, GPE systems have been produced by immersing the polymer in a polar solvent where it dissolves the salt and enhances ionic conductivity. GPEs at lithium anode/electrolyte interface inhibit dendrite formation on lithium anode. Previously, GPEs based on poly(vinylidene fluoride) (PVdF) [12], poly(vinylidene fluoride cohexafluoropropylene) P(VdF-co-HFP) [5, 13], poly(methyl methacrylate) (PMMA) [14, 15], and poly(acrylonitrile) (PAN) [16] have been reported by several groups.

Another method of synthesis of GPEs was the direct production of PEG branched macromolecules that bear flexible side chains for ion transfer. For example, polymer boron containing branched chain systems were synthesized from boron trioxide, poly(ethylene glycol) (PEG), and methoxy end-capped PEG [17]. They have inserted a Lewis acid in the branched structure of a poly(ethylene oxide) (PEO) and then the transfer of species between the electrolyte and the electrode is improved as a result of ion-pairing in the electrolyte [18, 19]. Lewis acid boron and Lewis-base oxygen interact with both cations and anions in the electrolytes and contribute to the ion-pairing in the electrolytes [2023]. Borane or borate derivatives can also improve the thermal stability of the electrolytes [24]. Lithium batteries or ultracapacitors include these kinds of stable branched electrolytes either directly or imbibing into porous nanofibers that have been produced via electrospinning.

In the present work, a more practical and innovative method was proposed for the synthesis of high ionic conductivity and stable boron containing PEG branched electrolytes for faster charge-discharging applications. Branched PEG-borate esters were produced with different lengths. To have high ionic conductivity, the branched materials were doped with CF3SO3Li at several stoichiometric ratios. After drying the samples, they were characterized by Fourier transformed infrared (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and AC impedance spectroscopy.

2. Experimental

2.1. Materials

BH3/THF complex and PEGME (average Mn = 350, 550, and 750 g/mol) were purchased from Aldrich. Dimethylsulfoxide (DMSO, analytical reagent) was received from Merck. CF3SO3Li 97% was bought from Alfa Aesar and stored in a dry atmosphere.

2.2. Synthesis of the Graft Copolymer Electrolytes

The PEG-borate esters were produced via the method as presented in Figure 1, by the reaction of poly(ethylene glycol) methyl ether, CH3O(CH2CH2O)H, of various molecular weights , 550, and 750 g/mol, and borane tetrahydrofuran complex solution, BH3/THF. Firstly, PEGME was added to DMSO, and then BH3/THF was dropwise admixed into the solution. The resulting solution was stirred under N2 atmosphere and refluxed at 100°C for 24 h. After that DMSO was removed by rotary evaporator at 80°C, under vacuum. Finally, transparent pale yellowish liquid polymer (for  g/mol) and gels (for  g/mol and  g/mol) were obtained. All the materials were kept in a Ar-filled glove box (moisture content below 1 ppm).

All the electrolytes were prepared by doping of branched polymers with lithium trifluoromethanesulfonate (CF3SO3Li, 97%) at several stoichiometric ratios. For example, the prepared PEGME-borate esters and lithium trifluoromethanesulfonate (CF3SO3Li, 97%) were transferred into a flask and stirred for about 4 h. The concentration of CF3SO3Li was adjusted so that the molar ratio of lithium atoms to ether oxygen atoms in the PEGME-borate ester is [EO] : [Li] = 25, 50, 75, and 100. The final homogeneous viscous solutions were casted on Teflon plates.

2.3. Characterizations

A Bruker Alpha-P in ATR system was used to record the IR spectra within the range of 4000–400 cm−1 with a resolution 4 cm−1. Prior to Fourier transform infrared (FTIR) spectra measurements, materials were dried under vacuum and stored in a glove box.

Thermogravimetric analysis (TGA) polymer electrolytes were examined with a Perkin Elmer Pyris 1. The samples (~5 mg) were heated from RT to 750°C under inert atmosphere at a heating rate of 10°C min−1. Differential scanning calorimetry (DSC) (Perkin Elmer Pyris 1 instrument) was used to study glass transition temperature and melting temperatures. The temperature of the samples was reduced from RT to −50°C, kept isothermally for 10 min, and then heated up to 150°C. Then the samples were cooled to −50°C and reheated to 180°C. All thermograms were recorded at a rate of 10°C under inert gas flow. All DSC experiments were done in duplicate and the thermograms shown refer to the final heating.

Novocontrol dielectric-impedance analyzer was used to study alternating current (AC) conductivities of the samples in the frequency range from 0.1 Hz to 3 MHz with respect to temperature. The samples were poured into the specially designed vacuum sealed liquid cell with parallel plate electrodes with a diameter of 20 mm and a thickness of approximately 4.5 mm and their conductivities were measured under an inert atmosphere.

3. Result and Discussion

3.1. NMR Study

The 1H NMR spectrum of the final boron comprising branched polymer, PEGME550-B, is presented in Figure 2. The chemical shift at 3.26 ppm (a) clearly indicates the methoxy group at terminal position. The prominent peak at 3.42 and 3.53 ppm (b) can be attributed to ethylene oxide in the polymer backbone: esteric methyl groups, which were shifted to the downfield at 4.55 ppm (c) due to deshielding effect, which is a result of interactions between the boron and the ethylene oxide units.

3.2. FTIR Study

The FTIR spectrum of the branched polymer electrolyte PEGME350-B- is given in Figure 3. All the samples show a peak located at 3400 cm−1 which corresponds to the absorption of hydroxyl units. It is clearly shown that disappearance of this peak illustrates highest transformation of the hydroxyl groups into borate esters [21]. In addition, strong bands at 2910–2870 cm−1 due to C-H stretching of the polymer electrolytes and weaker peaks located around 667 cm−1 belong to B-O units [25, 26]. The peaks at ~1029 and ~1350 were characteristic band of B-O-C bond and also antisymmetric stretching vibration of B-O [17]. This confirms the reaction of BH3 with hydroxyl units of PEGs. The broadening of the C-O-C peaks near 1150 cm−1, especially at higher doping ratios, indicates a higher degree of interactions between Li+ and EO [27]. The characteristic absorptions of the pure LiCF3SO3 are located at 1266 and 1033 cm−1, corresponding to asymmetric SO3 and symmetric SO3 vibrations of LiCF3SO3, respectively [28]. Consequently, the FTIR spectra reveal that borate ester polymer electrolytes were successfully synthesized.

3.3. Thermal Properties

The thermal stability of polymer electrolytes is a critical issue for application in rechargeable lithium polymer batteries. The thermal stabilities of these novel electrolytes were studied by TGA at various Li-salt.

Figure 4 shows the TGA results of the polymer electrolytes PEGME550-B-. As seen in the figure, the onset of degradation temperature of LiCF3SO3 introduced PEGME-borate esters is approximately 180°C and degradation occurs via two steps. It was previously mentioned that this behavior may be due to the weakness of the C-O bond caused by a reduction in electronic density by Li/EO [29, 30]. All the samples can be said to be thermally stable up to 180°C which is far higher than the operating temperature of lithium ion batteries.

DSC was used to investigate the thermal transitions of the polymer electrolytes with various Li-salt contents. Figure 5 shows the melting endotherms of polymer electrolyte systems. Clearly, for all samples, there is no definite within the given temperature range. The polymer electrolytes of PEGME550-B- ( = 25, 50, 75, and 100) showed respective melting endotherms at 13.50, 16.40, 13.15, and 19.80°C.

3.4. Ionic Conductivity

Frequency dependence of the ac conductivity () of PEGME350 B-25 within 20–100°C is given in Figure 7. It can be observed that the ac conductivity versus frequency for various temperatures exhibits almost linear increasing at low frequencies which are assigned to the polarization of the electrodes; then frequency independent plateau regions which belong to are well developed at higher frequencies ( Hz).

The versus inverse temperature variations of the polymer electrolytes with different molecular weight of PEGME and doping ratios of LiCF3SO3 are shown in Figure 6. The bend in the curve has been observed in ion conducting polymer electrolytes and has been explained by the free volume concept [30, 31], which can be described by the Vogel–Tamman–Fulcher (VTF) equation (1). To have better insight into the temperature dependence of , the conductivity data have been fitted to the equation.where (S/cm) is the conductivity at infinite temperature, is the Vogel activation energy (eV), is the Boltzmann constant, is the absolute temperature, and is the Vogel temperature which is the onset of polymer segmental relaxations (°C).

Figure 7 shows the temperature dependence of ionic conductivity for the polymer electrolytes. The ionic conductivity increases with decreasing molecular weight of the starting PEGME.

Over the whole temperature range, the sample containing PEGME-borate ester whose molecular weight is = 350 shows the highest ionic conductivity, and the values of the ionic conductivity were 1.5 × 10−4 S cm−1 at 30°C and 1.7 × 10−3 S·cm−1 at 100°C. It is thought that the PEG-borate ester ( = 350) has the highest ability to dissolve and transfer Li ions which may be due to the shorter EO chain length. At higher chain lengths, it is clearly seen that there is almost no significant increase of the conductivity. In particular, PEGME 750-25 seems to be of optimum composition and shows slightly higher conductivity at higher temperatures.

4. Conclusions

This study presents a new approach to preparing polymer electrolytes having borate ester groups for lithium ion batteries. The PEGME-borate esters were synthesized by the reaction of poly(ethylene glycol) methyl ether and borane tetrahydrofuran complex solution, BH3/THF. Then the electrolytes were successfully produced by doping of the polymer matrix with CF3SO3Li at different ratios which are abbreviated as PEGMEX-B-. NMR, FTIR, TGA, and dielectric-impedance spectroscopy methods were used for the characterization of these electrolytes. The FTIR and NMR spectra reveal that borate ester polymer electrolytes were successfully synthesized. TGA demonstrated that the samples are thermally stable up to 200°C. The conductivity results illustrated that the lithium ion conductivity of branched polymer electrolytes depends on chain length of PEGME as well as the CF3SO3Li content. The polymer electrolyte PEGME350-B-25 yielded a promising ionic conductivity of 1.5 × 10−4 S cm−1 at 30°C and 1.7 × 10−3 S·cm−1 at 100°C. The curved lines at conductivity versus 1000/, especially at higher salt content, are typical for VTF behavior, indicating a positive contribution of segmental relaxation to the ion motion. These materials are nonvolatile, have promising ionic conductivity, and can be suggested for use in energy storage systems.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.