Silicon is one of the most promising anode materials for lithium ion batteries owing to its high theoretical energy capacity. Traditionally, the anodes of batteries are composed of three components, active materials, binders, and conductive additives respectively. However, during cycles of lithiation and delithiation, the large volume expansions occur to cause disconnection between silicon and binders, further resulting in drastic capacity fade.
In this work, a series of conjugated copolymers, comprising cyclopentadithiophene (CPDT), ethylenedioxythiophene (EDOT), fluorene (F), and methylbenzoate (MB), were developed to obtain suitable polymers for silicon binders with both properties of electric and ionic conductivities and mechanical integrities. These polymers were synthesized through the facile synthetic method, Pd-catalyzed direct arylation polycondensation. Reaction conditions were optimized by changing monomer composition ratios, amount of Pd-catalyst, temperatures, reaction time, and solvents. These copolymers were obtained with high molecular weight (Mn = 21200-53000), which was determined by gel permeation chromatography (GPC) eluted with THF compared to polystyrene standards. Methyl ester group (–COOCH3) on MB in these polymers was converted to carboxylic acid (–COOH) via saponification in the presence of KOH aq. followed by treatment with HCl aq. Optical and electrochemical properties of the polymers were measured to estimated bandgap and HOMO and LUMO levels. The polymers exhibited bandgap around 2.0 eV. Impressively, a polymer containing CPDT and MB units showed a quite narrow bandgap 1.57 eV, which had good electric conductivity.
The polymers were used as conducting binders for Si-nanopowder to prepare anode electrodes in lithium ion batteries. In cycle-life tests, the polymers with the COOH functional groups showed great improvement because the COOH groups enhanced ionic conductivity with better adhesion property between the polymers and Si-nanopowder. In particular, the battery fabricated using P[CPDT-MB(COOH)] showed the highest performance with a specific capacity up to 1820 mAh/g at 0.1 C and 1250 mAh/g at 0.5 C for the entire electrode after 100 cycles, and good stability at various rates at 70 cycles. Morphology of the anode electrodes was measured by a field emission scanning electron microscope, showing that some cracks were formed after the cycle test. Ionic conductivities of the polymers were estimated by electrochemical impedance analysis. P[CPDT-MB(COOH)] showed much higher ionic conductivity of 1.58 x 10-10 S cm-1 than that of original polymer P(CPDT-MB) (3.16 x 10-23 S cm-1). These results account for the high cyclic stability of the battery of P[CPDT-MB(COOH)].

Table of contents
Abstract I
Table of contents III
Chapter 1. Introduction and purpose 1
1.1 Introduction 1
1.2 Conjugated polymers 1
1.3 Applications of conjugated polymers 2
1.3.1 Lithium ion batteries 3
1.4 Synthesis methods of conjugated polymers 8
1.4.1 Stille Coupling Reaction 9
1.4.2 Suzuki-Miyaura coupling reaction 11
1.4.3 Direct arylation 14
1.5 Structure-property relationship of conjugated polymers 17
1.6 Aim of this work 21
Chapter 2. Synthesis and characterization of copolymers 22
2.1 Introduction 22
2.2 Synthesis 23
2.2.1 Synthesis of monomers 23
2.2.2 Synthesis of polymers via direct arylation 25
2.2.3 Synthesis of polymers via saponification 35
2.3 Optical and electrochemical properties 39
Chapter 3. Fabrication and characterization of lithium ion batteries using Si-based anodes with conductive polymer binders 48
3.1 Introduction 48
3.2 Fabrication of lithium ion batteries 48
3.2.1 Preparation of electrodes 49
3.2.2 Trimming of electrode sheets 50
3.2.3 Assembly of batteries 50
3.3 Cycle life test of lithium ion batteries 51
3.4 Morphology study 57
3.5 Electrochemical impedance analysis 59
Chapter 4. Conclusions 61
Chapter 5: Experimental section 62
5.1 General Procedures 62
5.2 Synthesis of cyclopentadithiophene (CPDT) 63
5.2.1 Bis(2-iodothiophen-3-yl) methanol (1) 63
5.2.2 Bis(2-iodothiophen-3-yl)methanone (2) 64
5.2.3 Cyclopenta[2,1-b;3,4-b']dithiophen-4-one (3) 64
5.2.4 4H-Cyclopenta[2,1-b;3,4-b']dithiophene (4) 65
5.2.5 4,4-bis(2-ethylhexyl)cyclopenta[2,1-b;3,4-b']dithiophene (5) 65
5.3 Synthesis of polymers 67
5.3.1 P(CPDT-MB) 67
5.3.2 P(EDOT-MB) 67
5.3.3 P(EDOT-CPDT-MB) 68
5.3.4 P(EDOT-F-MB) 68
5.3.5 P(CPDT-F-MB) 68
5.3.6 P(EDOT-CPDT-F-MB) 69
5.3.7 P[CPDT-MB(COOH)] 69
5.3.8 P[EDOT-CPDT-MB(COOH)] 70
5.3.9 P[CPDT-F-MB(COOH)] 71
5.3.10 P[EDOT-CPDT-F-MB(COOH)] 71
References 76

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