A Quantum Chemical Study on Cathode Active Materials of Organic Radical Molecules in Secondary Battery

摘要

Recently，lithium-ion (Li-ion) batteries have been used as the power supply of smartphones and electric cars owing to the high energy density and high power density as well as long life and environmental friendliness. However，Li-ion batteries with lithium cobalt oxide (LiCoO2) as the cathode-active material have faced serious resource problems because of the high cost of cobalt (Co) atoms in cathode active materials. For the resolution of these problems，in 2011，Y. Morita et al. proposed organic radical-molecules as cathode active materials such as a 6-oxophenalenoxyl (6OPO)，a tri-tert-butylated trioxotriangulene ((t-Bu)3TOT) and a tribrominated trioxotriangulene (Br3TOT). These organic radical-molecules do not contain expensive and heavy Co atoms in these organic radical-molecules，successfully achieving Co-free lithium-ion batteries. Additionally，these small organic molecules have received broad attention due to their innovative physical properties such as multi-stage redox activities (6OPO+2e-?6OPO2-，and R3TOT+4e-?R3TOT4-with R being t-Bu or Br) resulting in a higher capacity and strong one-dimensionalπ―πstacking in crystals leading to a low solubility. However，the mechanism of the multi-stage redox behaviors still remains to be accounted for. To solve this problem，theoretical investigations can greatly aid in explaining the experimentally observed multi-stage redox behaviors.
　　Firstly，the electron state changes were investigated in the process of adding electrons to elucidate the mechanism of the multi-stage redox reactions. As a result，the energy levels of singly occupied molecular orbital (SOMO) and lowest unoccupied molecular orbital (LUMO) were elevated by adding electrons gradually. Secondly，Li adsorption patterns were investigated by using quantum chemical calculation. We elucidated the lowest energy conformation for the Li adsorption patterns by replacing the Li atoms in all possible initial positions and by relaxing the only Li atoms from geometry optimization calculation at the monomer and dimer level. Additionally，Li adsorption patterns were analyzed in the stacked Br3TOT hexamer by using Elongation (ELG) method. It was found that Br3TOT hexamer adsorbing 6 Li atoms by three-times rotated alternative between the oxygen atoms was the most stable at the 1st stage reaction. Thirdly，the interaction energy between organic radical molecules and Li atoms was investigated at each stage reaction. The results show the difficulty of the 5th stage reaction at both dimer and hexamer level，which agrees with the experimental results. Finally，the method to estimate the spin state varied with changes in temperature was developed by considering the medium spin states between the high and low spin states. It is inferred from this method that one-dimensional crystal of Br3TOT maintains the radicals at room temperature and ELG method is possible to configure spin states spatially in the stacked CH3 containing Li atoms.

目录

封面

声明

英文摘要

目录

Chapter 1:Introduction

1.1 Background Research

1.2 Overview of Lithium-ion Battery

1.3 Overview of Organic Tailored Battery

1.4 Organization of Thesis

Chapter 2:Theoretical Backgrounds

2.1 BSSE-corrected Interaction Energy

2.2 Elongation Method

Chapter 3:Electron State Changes of Organic Radical Molecules during Redox Reaction

3.1 Introduction

3.2 Computational Methodology

3.3 Results and Discussion

3.4 Conclusions

Chapter 4:Li Adsorption Patterns at Monomer Level

4.1 Introduction

4.2 Computational Methodology

4.3 Results and Discussion

4.4 Conclusions

Chapter 5:Li Adsorption Patterns at Dimer level

5.1 Introduction

5.2 Computational Methodology

5.3 Results and Discussion

5.4 Conclusions

Chapter 6:Li Adsorption Patterns at Hexamer Level

6.1 Introduction

6.2 Computational Methodology

6.3 Results and Discussion

6.4 Conclusions

Chapter 7:Spin State depending on Temperature Change

7.1 Introduction

7.2 Computational Methodology

7.3 Results and Discussion

7.4 Conclusions

Chapter 8:Spin State depending on Temperature Change in Br3TOT containing Li atoms

8.1 Introduction

8.2 Computational Methodology

8.3 Results and Discussion

8.4 Conclusions and Future Plan

Chapter 9:Conclusions

9.1 Electron State Changes during Multi-stage Redox Reactions

9.2 Li Adsorption Patterns and Li Interaction Energies of Organic Radical Molecules

9.3 Spin State depending on Temperature Change

参考文献

致谢