FTIR spectroscopic studies of lithium tetrafluoroborate in propylene carbonate + diethyl carbonate mixtures

https://doi.org/10.1016/j.saa.2013.11.054Get rights and content

Highlights

  • FTIR spectra have been collected and analyzed for LiBF4 in PC, DEC and PC + DEC mixtures.

  • Strong interactions between Li+ and solvent molecules exist.

  • No preferential solvation of Li+ in LiBF4/PC + DEC solutions was detected.

Abstract

FTIR (Fourier transformed infrared) spectra have been collected and analyzed for solutions of lithium tetrafluoroborate in propylene carbonate (PC), diethyl carbonate (DEC), and PC + DEC mixtures. It has been shown that the carbonyl stretch bands of PC and DEC, the ring of PC and the ether oxygen stretch bands of DEC are all very sensitive to the interaction between Li+ and the solvent molecules. New shoulders appear and the original bands split with the addition of LiBF4, indicating that a strong interaction between Li+ and molecules of PC and DEC exists through the oxygen group of Cdouble bondO and ring of PC and both Cdouble bondO oxygen and ether oxygen atoms of DEC. In addition, no preferential solvation of Li+ in LiBF4/PC + DEC solutions was detected.

Introduction

The study of organic electrolyte solutions in lithium ion battery continues to be an active field of research [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. These electrolytes are of current interest because of their potential applications in lithium ion batteries [1], [2]. Electrolyte of lithium ion battery consists of lithium salt (such as LiClO4, LiPF6, and LiBF4), mixture of organic solvents (such as propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC)) and some functional additives. The structure and composition of electrolyte plays an important role in determining the battery performance, such as charge–discharge performance, operating temperature range and service life [1], [2]. Therefore, starting from the perspective of microscopic structural elements angle, knowledge of ion–molecule and intermolecular interactions inside electrolytes is essential for the optimal choice of solvent and electrolyte. It is also very important and meaningful for the research and development of lithium ion battery with high performance.

Vibrational spectroscopy has been proved to be a powerful technique for probing ion–molecule and intermolecular interactions through the changes of frequency, intensity and other bands properties [8]. Such studies [3], [4], [6], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27] could help to identify the action mechanism of lithium salt and solvent inside electrolyte and confirm the factors that affect the general properties and performance of the electrolyte solutions.

As lithium tetrafluoroborate (LiBF4) has a proper thermal and chemical stability, a series of investigations have been carried out to study the ion solvation and association of LiBF4-based electrolytes [28], [22], [23] which have an excellent low temperature battery performance. Xuan et al. [29] studied the ion solvation and association of lithium tetrafluoroborate in acetonitrile by vibrational spectroscopic and density functional methods. Alia and Edwards [30] studied the Raman spectra of LiBF4 in acrylonitrile and found the contact ion pairs and ion dimers. However, there have been no researchers studied the LiBF4/PC + DEC binary electrolyte system. We selected PC (64.40 at 298.15 K [31]) and DEC as the solvents because both of them are liquid at room temperature, and are commonly used in lithium ion batteries [1], [2], [3], [4], [5], [6]. A number of commercial lithium ion batteries have adopted the mixed organic solvents containing PC [32] as their electrolytes. As LiBF4-based electrolytes have an excellent low temperature performance and are widely used in lithium ion battery, the study of LiBF4/PC + DEC is worthy and meaningful for the development of electrolyte with better low temperature performance.

In this paper, in order to capture the nature of ion–molecule and intermolecular interactions inside LiBF4/PC + DEC binary electrolyte system, a systematic FTIR spectroscopic investigations of PC + DEC, LiBF4/PC, LiBF4/DEC and LiBF4/PC + DEC solutions are carried out. In order to separate out various interactions within these solutions, a series of systems with different components and concentrations are systematically studied, ranging from the simple pure components to the final systems of salt-solvent solutions.

Section snippets

Experimental

Lithium tetrafluoroborate (Aladdin, purity >99.99% metal basis) was dried under vacuum for 48 h at 120 °C. PC and DEC (Zhangjiagang Guotai-Huarong New Chemical Materials Co. Ltd., cell grade, purity >99.9%, moisture <20 ppm) were used as purchased. All electrolyte solution manufactures were carried out in nitrogen filled glove box (moisture <0.1 ppm, oxygen <20 ppm). As there is no obvious absorption bands were observed around 3500 cm−1 in the IR spectra, the effect of moisture in the liquid sample

Characteristic peak assignments for PC and DEC

Battisti et al. [4] studied the infrared and Raman spectroscopy of PC doped with various concentrations of lithium perchlorate (LiClO4) and Janz et al. [33] investigated the Raman spectrum of PC. Both of them deeply studied the structure and vibrational spectra of PC, and most of the vibrational peak assignments had been made. Wang et al. [34] investigated the vibrational spectra of DEC, and gave detailed assignments for most of the IR bands. In addition, Xuan et al. [35] studied the IR

Conclusions

The electrolyte solutions of lithium tetrafluoroborate (LiBF4) in PC, DEC and PC + DEC solutions have been systematically investigated by FTIR spectroscopy study. The tentative conclusions can be summarized as follows:

  • (i)

    The dipole–dipole interaction between PC and DEC molecules in PC + DEC mixtures causes the Cdouble bondO stretch of PC shift to the higher wavenumber side with the increasing of volume ratio in PC + DEC solutions.

  • (ii)

    Strong interactions of Li+-PC and Li+-DEC are observed in LiBF4/PC, LiBF4/DEC and LiBF

Acknowledgements

Financial support from the National 863 Program (2013AA110100), “Western light” talent cultivation plan joint scholars Program (Chinese Academy of Sciences) and Open fund of key laboratory of salt lakes resources and chemistry (Chinese Academy of Sciences) are gratefully acknowledged.

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