Electrochemical activation of hard carbon electrodes through sodium?ion for high?capacity lithium?ion batteries

다국어 초록 (Multilingual Abstract)

Currently, graphite is commercialized as an anode in lithium–ion batteries (LIBs). However, due to performance degradation issues in high–rate and low-temperature charging/discharging, hard carbon's distinctive internal structure advantages are gaining attention in specific fields. Nevertheless, its lower specific capacity compared to graphite remains a barrier to commercialization. Although various physical methods have been studied to modify the internal structure of hard carbon, these approaches require high energy input or involve complex steps, making uniform modification difficult. In this study, utilizing hard carbon's structural characteristics, we modified its internal structure through Na pre–cycling to increase Li storage sites. Through Raman and XRD analyses after Na pre–cycling, we confirmed that while the internal structure remained stable during Li charge/discharge, Na storage in internal pores led to irreversible amorphization. When applied to Li–ion half cells, this treatment resulted in approximately 37% higher charging capacity and 24% higher discharge capacity compared to untreated samples. To further investigate this phenomenon, we conducted SAXS analysis with different Na pre–cycling cut–off voltages, revealing that lower cut–off voltages resulted in increased Na filling of internal pores, leading to greater pore expansion. Cycle performance evaluation at 0.3 C demonstrated a discharge capacity about 30 mAh g–1 higher than the untreated sample even after 500 cycles, with dQ/dV analysis showing enhanced peaks around 0.1–0.2 V. These results indicate that Na electrochemical activation modified the hard carbon's internal structure, enabling Li storage in previously inaccessible pores and thereby increasing available capacity. This finding is particularly significant as it demonstrates the possibility of structural modification at the electrode level rather than the powder stage.

목차 (Table of Contents)

• CHAPTER 1. Introduction 1
• CHAPTER 2. Experimental methods 11
• 2.1 Material preparation 11
• 2.2 Electrode fabrication 11
• 2.3 Material characterization 12
• 2.3.1 Field Emission Scanning Electron Microscope (FE-SEM) 12
• 2.3.2 Transmission Electron Microscope (TEM) 12
• 2.3.3 Brunauer-Emmett-Teller analysis (BET) 13
• 2.3.4 Raman spectroscopy 13
• 2.3.5 X-ray diffraction analysis (XRD) 14
• 2.3.6 Small-Angle X-ray Scattering analysis (SAXS) 14
• 2.4 Electrochemical measurement 15
• 2.4.1 Cell assembly 15
• 2.4.2 Electrode pre-cycling 15
• 2.4.3 Cell disassembly and reassembly 16
• 2.4.4 Sequential electrochemical test 16
• 2.4.5 Preparation of Na pre–cycling electrodes with different cut–off voltages 16
• 2.4.6 Cycling test 17
• 2.4.7 Rate capability test 17
• CHAPTER 3. Results and discussion 18
• CHAPTER 4. Conclusions 44
• 국문 초록 46
• References 49