Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 40–45
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Li+-molecule interactions of lithium tetrafluoroborate in propylene carbonate + N,N-dimethylformamide mixtures: An FTIR spectroscopic study Binbin Zhang a,b, Yuan Zhou a,⇑, Xiang Li a,⇑, Jingying Wang c, Gang Li a,b, Qiang Yun a,b, Xiufang Wang a a Key Laboratory of Salt Lake Resources and Chemistry of Chinese Academy of Sciences, Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China b University of Chinese Academy of Sciences, Beijing 100049, China c School of Environmental Science & Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
+
Strong Li -molecule interactions are
observed in LiBF4-PC/DMF/PC + DMF solutions. + Li -molecule interaction occurs through the O atom of C@O group. There is no or very weak interaction between PC and DMF molecules. + Li are preferentially solvated with DMF in LiBF4–PC + DMF solutions.
a r t i c l e
i n f o
Article history: Received 28 November 2013 Received in revised form 30 December 2013 Accepted 3 January 2014 Available online 10 January 2014 Keywords: Li+-molecule interaction Lithium tetrafluoroborate Propylene carbonate (PC) N,N-dimethylformamide (DMF) FTIR spectroscopy Preferential solvation
a b s t r a c t FTIR (Fourier transformed infrared) spectra have been collected and analyzed for solutions of lithium tetrafluoroborate in propylene carbonate (PC), N,N-dimethylformamide (DMF), and PC + DMF mixtures. The band splitting and symmetric ring deformation for PC and O@CAN deformation for DMF suggest that there is a strong interaction between lithium cations and solvent molecules. The solvent molecules have been assigned to two types, the free and complexed molecules. By a comparison of the intensity for the corresponding bands, it has been concluded that Li+ cations are preferentially solvated by DMF molecules in the LiBF4/PC + DMF solutions. This has been explained by the difference in values of donor number (DN). Ó 2014 Elsevier B.V. All rights reserved.
Introduction Lithium ion batteries have been widely applied to portable electronic devices in the recent decades due to their high performance. A typical lithium ion battery system is composed of a cathode, an ⇑ Corresponding authors. Tel.: +86 971 6330483; fax: +86 971 6306002. E-mail addresses: [email protected] (Y. Zhou), [email protected] (X. Li). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2014.01.001
anode and a nonaqueous organic electrolyte. 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 electrolyte property and battery performance, such as electrochemical performance, operating temperature range and service life [1,2]. The study of electrolyte
B. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 40–45
solutions in lithium ion battery continues to be an active field of research [1–10]. These electrolytes are of current interest because of their potential applications in lithium ion batteries [1,2]. Therefore, starting from the perspective of microscopic structural elements angle, knowledge of ion solvation and ion association (ion–molecule and ion–ion 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 or quantum chemistry calculation has been proved to be powerful techniques for probing ion–molecule through the changes of frequency, intensity, bands properties and calculation results [8]. Such studies [3,4,6,8,11–27] 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 [22,23,28] 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 + DMF binary electrolyte system. We selected PC (64.40 at 298.15 K [31]) and DMF as the solvents because both of them are liquid at room temperature, and are commonly used in lithium ion batteries [1–6]. The co-solvent chosen is DMF. This is, in part, due to its importance in analytical and inorganic chemistry; and in part because PC + DMF mixtures have potential use in lithium ion battery [32,33]. A number of commercial lithium ion batteries have adopted the mixed organic solvents containing PC [32,33] 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 + DMF is worthy and meaningful for the development of electrolyte with better low temperature performance. In this paper, in order to capture the nature of Li+-molecule interactions inside LiBF4/PC + DMF binary electrolyte system, a systematic FTIR spectroscopic investigations of LiBF4/PC, LiBF4/DMF, PC + DMF and LiBF4/PC + DMF solutions are carried out. In order to separate out various interactions within these complexed 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.
Experimental Lithium tetrafluoroborate (Aladdin, purity >99.99% metal basis) was dried under vacuum for 48 h at 120 °C. PC (Zhangjiagang Guotai-Huarong New Chemical Materials Co. Ltd., cell grade, purity >99.9%, moisture 99.8%, moisture
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Li+-molecule interactions of lithium tetrafluoroborate in propylene carbonate + N,N-dimethylformamide mixtures: An FTIR spectroscopic study Binbin Zhang a,b, Yuan Zhou a,⇑, Xiang Li a,⇑, Jingying Wang c, Gang Li a,b, Qiang Yun a,b, Xiufang Wang a a Key Laboratory of Salt Lake Resources and Chemistry of Chinese Academy of Sciences, Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China b University of Chinese Academy of Sciences, Beijing 100049, China c School of Environmental Science & Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
+
Strong Li -molecule interactions are
observed in LiBF4-PC/DMF/PC + DMF solutions. + Li -molecule interaction occurs through the O atom of C@O group. There is no or very weak interaction between PC and DMF molecules. + Li are preferentially solvated with DMF in LiBF4–PC + DMF solutions.
a r t i c l e
i n f o
Article history: Received 28 November 2013 Received in revised form 30 December 2013 Accepted 3 January 2014 Available online 10 January 2014 Keywords: Li+-molecule interaction Lithium tetrafluoroborate Propylene carbonate (PC) N,N-dimethylformamide (DMF) FTIR spectroscopy Preferential solvation
a b s t r a c t FTIR (Fourier transformed infrared) spectra have been collected and analyzed for solutions of lithium tetrafluoroborate in propylene carbonate (PC), N,N-dimethylformamide (DMF), and PC + DMF mixtures. The band splitting and symmetric ring deformation for PC and O@CAN deformation for DMF suggest that there is a strong interaction between lithium cations and solvent molecules. The solvent molecules have been assigned to two types, the free and complexed molecules. By a comparison of the intensity for the corresponding bands, it has been concluded that Li+ cations are preferentially solvated by DMF molecules in the LiBF4/PC + DMF solutions. This has been explained by the difference in values of donor number (DN). Ó 2014 Elsevier B.V. All rights reserved.
Introduction Lithium ion batteries have been widely applied to portable electronic devices in the recent decades due to their high performance. A typical lithium ion battery system is composed of a cathode, an ⇑ Corresponding authors. Tel.: +86 971 6330483; fax: +86 971 6306002. E-mail addresses: [email protected] (Y. Zhou), [email protected] (X. Li). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2014.01.001
anode and a nonaqueous organic electrolyte. 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 electrolyte property and battery performance, such as electrochemical performance, operating temperature range and service life [1,2]. The study of electrolyte
B. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 40–45
solutions in lithium ion battery continues to be an active field of research [1–10]. These electrolytes are of current interest because of their potential applications in lithium ion batteries [1,2]. Therefore, starting from the perspective of microscopic structural elements angle, knowledge of ion solvation and ion association (ion–molecule and ion–ion 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 or quantum chemistry calculation has been proved to be powerful techniques for probing ion–molecule through the changes of frequency, intensity, bands properties and calculation results [8]. Such studies [3,4,6,8,11–27] 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 [22,23,28] 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 + DMF binary electrolyte system. We selected PC (64.40 at 298.15 K [31]) and DMF as the solvents because both of them are liquid at room temperature, and are commonly used in lithium ion batteries [1–6]. The co-solvent chosen is DMF. This is, in part, due to its importance in analytical and inorganic chemistry; and in part because PC + DMF mixtures have potential use in lithium ion battery [32,33]. A number of commercial lithium ion batteries have adopted the mixed organic solvents containing PC [32,33] 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 + DMF is worthy and meaningful for the development of electrolyte with better low temperature performance. In this paper, in order to capture the nature of Li+-molecule interactions inside LiBF4/PC + DMF binary electrolyte system, a systematic FTIR spectroscopic investigations of LiBF4/PC, LiBF4/DMF, PC + DMF and LiBF4/PC + DMF solutions are carried out. In order to separate out various interactions within these complexed 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.
Experimental Lithium tetrafluoroborate (Aladdin, purity >99.99% metal basis) was dried under vacuum for 48 h at 120 °C. PC (Zhangjiagang Guotai-Huarong New Chemical Materials Co. Ltd., cell grade, purity >99.9%, moisture 99.8%, moisture
