Title Page
Abstract
Abstract (Korean)
Contents
Nomenclature 20
1. Introduction 22
1.1. Battery Electrodes 26
1.2. Tab-Electrode Welding Method 30
1.2.1. Ultrasonic Welding 31
1.2.2. Laser Welding 33
1.3. Welding Defects and Quality Assessment 34
1.3.1. Welding Defects 35
1.3.2. Weld Quality Evaluation 37
1.4. Overview of Thesis 41
2. Welding Defect Observation 44
2.1. SEM Observation 46
2.1.1. Crack 47
2.1.2. Delamination 48
2.2. Classification of Defects according to Weld Strength 49
2.3. Summary 57
3. Tensile Test 59
3.1. Experimental Setup 60
3.1.1. Test Specimen Alignment 61
3.2. Method 1: Testing on Full Specimen 63
3.2.1. Fracture Progress 64
3.2.2. Tensile Property of Al and Cu Sheet 72
3.2.3. Discussion 74
3.3. Method 2: Testing on Divided Specimen 84
3.3.1. Discussion 85
3.4. Comparison between Full and Divided 95
3.5. Summary 102
4. T-Peel Test 104
4.1. Experiment Setup 105
4.2. Peel Test for One and the other Sheets 106
4.3. Peel Test for an Equal Number of Sheets 109
4.3.1. Fracture Progress 110
4.3.2. Discussion 116
4.4. Summary 124
5. Fatigue Test 126
5.1. Theoretical Background 127
5.1.1. Fatigue Theory 127
5.1.2. Centrifugal Force 130
5.2. Fatigue Test Method Using Centrifugal Force 133
5.2.1. Concept of Test Equipment 136
5.2.2. Experiment Setup 142
5.2.3. Discussion 143
5.3. Summary 150
6. Correlation Studies 152
6.1. Tensile Strength VS Welding Defect 153
6.2. Tensile Strength VS Fatigue Strength 162
6.3. Summary 166
7. Conclusions and Future Work 167
7.1. Conclusions 167
7.2. Future Work 171
References 172
Table 2-1. Details of the tab-electrode welding specimen 45
Table 2-2. Distribution of the number and size of crack by specimen type 51
Table 2-3. Distribution of the number and size of delamination by specimen type 52
Table 3-1. Maximum tensile force and stress for each welded specimen 78
Table 5-1. Fatigue limit for each weld intensity 145
Table 6-1. Tensile strength and defect information of ultrasonic cathode specimens 155
Table 6-2. Tensile strength and defect information of ultrasonic anode specimens 156
Table 6-3. Tensile strength and defect information of laser cathode specimens 157
Table 6-4. Tensile strength and defect information of laser anode specimens 158
Table 6-5. Tensile strength and fatigue limit by polarity of ultrasonic welded specimens 164
Table 6-6. Tensile strength and fatigue limit by polarity of laser welded specimens 165
Figure 1-1. Type of battery cell a) cylindrical, b) prismatic, c) pouch 23
Figure 1-2. Structure of battery cell, module and pack 23
Figure 1-3. Components of a pouch type battery 24
Figure 1-4. Voltage and capacity ranges for each material of anode and cathode of battery cell 27
Figure 1-5. Schematic diagram of the ultrasonic welding process 32
Figure 1-6. Schematic diagram of the laser welding process 33
Figure 2-1. Welded specimens by process conditions 44
Figure 2-2. Schematic diagram of weld specimen a) ultrasonic welding specimen, b) laser welding specimen 45
Figure 2-3. Cutting position of welding specimen for SEM observation 46
Figure 2-4. Crack shape observed in weld cross-section 47
Figure 2-5. Delamination shape observed in the weld cross section 48
Figure 2-6. Number of defects in ultrasonic specimens by weld strength a) cathode, b) anode 53
Figure 2-7. Number of defects in laser specimens by weld strength a) cathode, b) anode 54
Figure 2-8. Defect size x number of defects in ultrasonic specimens by welding strength a) cathode, b) anode 55
Figure 2-9. Defect size x number of defects in laser specimens by welding strength a) cathode, b) anode 56
Figure 3-1. Schematic diagram of tensile test 60
Figure 3-2. Full specimen for Method 1 62
Figure 3-3. Divided specimen for Method 2 a) Left, b) Middle, c) Right 62
Figure 3-4. The process of fracture of specimen due to tensile load 66
Figure 3-5. Fracture surface of weld specimen destroyed by tensile test : Separation of the weld nugget 67
Figure 3-6. Fracture surface of weld specimen destroyed by tensile test : Electrode dislodgment 68
Figure 3-7. Load position of tab-electrode welding part when tensile load is applied 69
Figure 3-8. Tab and electrode cross section loaded by tensile load a) isometric view, b) side view 70
Figure 3-9. Observation of fracture surfaces of tensile specimens using SEM 71
Figure 3-10. Tensile test results for a single sheet a) welding width: 42 mm, b) welding width: 30.8 mm 73
Figure 3-11. Main fracture area of welded specimen 74
Figure 3-12. Low intensity specimen of cathode/anode tested by Method 1 a) ultrasonic welded specimen b) laser welded specimen 79
Figure 3-13. Medium intensity specimen of cathode/anode tested by Method 1 a) ultrasonic welded specimen b) laser welded specimen 80
Figure 3-14. High intensity specimen of cathode/anode tested by Method 1 a) ultrasonic welded specimen b) laser welded specimen 81
Figure 3-15. Low intensity of ultrasonic specimen tested by Method 2 a) cathode specimen, b) anode specimen 87
Figure 3-16. Medium intensity of ultrasonic specimen tested by Method 2 a) cathode specimen, b) anode specimen 88
Figure 3-17. High intensity of ultrasonic specimen tested by Method 2 a) cathode specimen, b) anode specimen 89
Figure 3-18. Low intensity of laser specimen tested by Method 2 a) cathode specimen, b) anode specimen 90
Figure 3-19. Medium intensity of laser specimen tested by Method 2 a) cathode specimen, b) anode specimen 91
Figure 3-20. High intensity of laser specimen tested by Method 2 a) cathode specimen, b) anode specimen 92
Figure 3-21. Average tensile strength of ultrasonic cathode specimens according to the tensile test method for each weld intensity a) Low, b) Medium, c) High 98
Figure 3-22. Average tensile strength of ultrasonic anode specimens according to the tensile test method for each weld intensity a) Low, b) Medium, c) High 99
Figure 3-23. Average tensile strength of laser cathode specimens according to the tensile test method for each weld intensity a) Low, b) Medium, c) High 100
Figure 3-24. Average tensile strength of laser anode specimens according to the tensile test method for each weld intensity a) Low, b) Medium, c) High 101
Figure 4-1. Schematic diagram of peel test 105
Figure 4-2. A schematic diagram of a peel test of a single sheet 106
Figure 4-3. Peel test result of single sheet 108
Figure 4-4. Fracture shape of single sheet after peel test 108
Figure 4-5. A schematic diagram of the peel test for the same number of sheets 109
Figure 4-6. The process of fracture of specimen due to peel load 112
Figure 4-7. Fracture surface of weld specimen destroyed by peel test : Separation of the weld nugget 113
Figure 4-8. Fracture surface of weld specimen destroyed by peel test : Electrode dislodgment 114
Figure 4-9. Observation of fracture surfaces of peel specimens using SEM 115
Figure 4-10. Peel strength according to the position of the specimen for each weld intensity of the ultrasonic cathode specimen a) Low, b) Medium, c) High 118
Figure 4-11. Peel strength according to the position of the specimen for each weld intensity of the ultrasonic anode specimen a) Low, b) Medium, c) High 119
Figure 4-12. Peel strength according to the position of the specimen for each weld intensity of the laser cathode specimen a) Low, b) Medium, c) High 120
Figure 4-13. Peel strength according to the position of the specimen for each weld intensity of the laser anode specimen a) Low, b) Medium, c) High 121
Figure 5-1. Fatigue curves according to stress combinations a) Fully reversed cycle, b) Repeated cycle, c) Fluctuation cycle 128
Figure 5-2. SN curve and fatigue limit according to material properties 129
Figure 5-3. a) Position and velocity vectors of particles in constant velocity circular motion, b) triangle of velocity vector 131
Figure 5-4. Centripetal force and centrifugal force of particles in constant velocity circular motion 132
Figure 5-5. Schematic diagram of fatigue test equipment using resonance 134
Figure 5-6. Fatigue test of dog-bone specimen using centrifugal force 134
Figure 5-7. Test equipment using centrifugal force of motor 135
Figure 5-8. Schematic diagram of fatigue test equipment 136
Figure 5-9. 3D model of fatigue test jig 136
Figure 5-10. Stress distribution of an object subject to a force 138
Figure 5-11. Overview of fatigue testing using centrifugal force 140
Figure 5-12. Cycle curve of fatigue test using centrifugal force 140
Figure 5-13. Fracture surface of weld specimen destroyed by fatigue test : Separation of the weld nugget 145
Figure 5-14. Fracture surface of weld specimen destroyed by fatigue test : Electrode dislodgment 145
Figure 5-15. SN curve by weld intensity of ultrasonic cathode specimen 146
Figure 5-16. SN curve by weld intensity of ultrasonic anode specimen 146
Figure 5-17. SN curve by weld intensity of laser cathode specimen 147
Figure 5-18. SN curve by weld intensity of laser anode specimen 147
Figure 6-1. Correlation between defects and tensile strength of ultrasonic cathode specimens a) S x n vs Tensile strength, b) n vs Tensile strength[이미지참조] 155
Figure 6-2. Correlation between defects and tensile strength of ultrasonic anode specimens a) S x n vs Tensile strength, b) n vs Tensile strength[이미지참조] 156
Figure 6-3. Correlation between defects and tensile strength of laser cathode specimens a) S x n vs Tensile strength, b) n vs Tensile strength[이미지참조] 157
Figure 6-4. Correlation between defects and tensile strength of laser anode specimens a) S x n vs Tensile strength, b) n vs Tensile strength[이미지참조] 158
Figure 6-5. The shape of the crack defect present in the weld cross-section a) isometric view, b) side view, c) front view 160
Figure 6-6. The shape of the delamination defect present in the weld cross-section a) isometric view, b) side view, c) front view 160
Figure 6-7. Correlation between fatigue limit and tensile strength of ultrasonic welded specimens 164
Figure 6-8. Correlation between fatigue limit and tensile strength of laser welded specimens 165