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Infrared Spectroscopy of linear Carbonate conformer in Nonaqueous Electrolytes

Infrared Spectroscopy of linear Carbonate conformer in Nonaqueous Electrolytes

  • 김민주 (대학원 화학과 물리화학전공)

초록/요약

Lithium Ion Batteries(LIBs) play an essential roles in alternative energy storage device. Especially, because the use of environmentally friendly fuel has become an important issue in recent years, LIBs has replaced the petroleum energy. Accordingly, efforts about supplementing LIBs properties have proceeded. For this reason, studies on interaction between Molecules in battery system and dynamics of Li^+ movement have been actively conducted. Charging and discharging of LIBs is carried out Li^+ movement between the cathode and anode. Therefore, research on the electrolyte used to transport lithium is very crucial. There are a variety of electrolytes according to battery system. The most basic electrolyte used is carbonate mixed solvent. There are two types of carbonate, cyclic carbonate and linear carbonate. The cyclic carbonates are so-called co-solvent, which is known to be used directly dissolving Li^+. So, it has been the main research that investigating cyclic carbonate - Li^+ interaction, linear carbonates are secondary since it has been utilized such as reducing viscosity and melting point. Nevertheless, linear carbonate also reaction with Li^+. Spectroscopically, we can see amount of linear carbonate molecules coordinating Li^+, in the binary system. One NMR research find out Linear carbonate interaction with Li^+ in the secondary solvation shell. As a result, we cannot ignore the role of linear carbonate in electrolytes. Linear carbonates are assorted by length of methyl group, and two molecules are used by electrolytes – dimethyl carbonate and diethyl carbonate. Those molecules have no difference on properties. If those are used by single solvent electrolyte, however, ionic conductivity of the electrolytes has a large gap about 1.5~ 2 times. It seems to have a cause on molecular-weight. In this article, we inspected the solvation number of those solvent. DMC has higher Li^+ coordinated number than DEC. It means DMC dissolves more Li^+ and hinders Li^+ from associating with count anion than DEC. we also learned DMC has less CIP using FTIR spectroscopy. According to the dihedral angle of ester ether bond, linear carbonates have stable conformer called cis-cis and cis-trans conformer. Cis-trans conformer has high dielectric constant like cyclic carbonate. And we studied cis-cis & cis-trans ratio by FTIR analysis, DFT calculation, and classical molecular dynamics. All data told that DMC has more cis-cis conformer than DEC and this is the reason of ionic conductivity difference between the linear carbonates.

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목차

Abstract

Contents

1. Introduction 1
1.1. Fundamental principles of LIB 4
1.2. Components of LIB & usability 5
1.3. Interaction between solvent molecules in Electrolytes 7
1.4. Properties of linear carbonate 9

2. Experimental section 11
2.1. Sample preparation 12
2.2. FTIR spectroscopic analysis 13
2.3. DFT calculation analysis 14
2.4. Classical MD calculation analysis 16

3. Result and discussion 17
3.1. Solvation structure analysis 18
3.1.1. Free/ Li coordinated C=O, OCO vibration mode 20
3.1.2. Solvent separated ion pair & contact ion pair 26
3.2. Conformer of linear carbonate 29
3.2.1. Rotational barrier and conformer properties 29
3.2.2. Comparing conformer ratio of DMC/DEC 31
3.3. Li diffusion coefficient effected by conformer species 35

4. Conclusion 37

Appendix 40
A1. Fitting quality of OCO, C=O vibration 41
A2. Fitting quality of P-F vibration 43
A3. Fitting quality of conformer assigned peak 44

References 46

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