In this study, the physical properties differences were investigated and the performance and degradation mechanisms of the GrSi electrode caused by the density were analyzed through separation analysis of Gr and Si.
As the density increased, the pore size and volume present in the electrode decreased, but the electronic conductivity, adhesion, and cohesion improved due to an increase in contact with the components of an electrode. As a result of analyzing the cycle performance through electrochemical analysis, there were three degradation mechanisms in the GrSi electrode. The ① degradation is a rapid capacity reduction that occurred at the beginning of the cycle. The cause of ① degradation is the density effect on contact maintenance between the components. It lasted longer at higher density because low porosity induces a high expansion rate and components hard to return to their original position. Therefore, the ① degradation regards as the process in which components in the electrode find their position to work.
The ② degradation shows a similar capacity reduction slope regardless of density, and the slope changes depending on the Si content. Thus, the ② degradation is the process in which repairing broken part of the SEI. Lastly, the ③ degradation that occurred only in d1.2 reduces capacity rapidly. The main cause of ③ degradation is overpotential. It is caused by thick SEI and electrolyte exhaustion.
This degradation is the process in which occurrence of cell failure.
Therefore, the low density that has high initial electrode porosity can complete the ① degradation quickly, thereby increasing the utilization of the active material. It causes a continuous SEI reaction and a thick SEI layer, finally, electrolyte exhaustion occurred. On the contrary, high density like d1.65 this phenomenon was hard to find because particle disconnection in the electrode reduces active material utilization during the cycle. The decline of active material utilization reduces SEI regeneration. As a result, the increases in RSEI are small than d1.2 and the CE converges more rapidly than other densities.
Therefore, when a GrSi electrode is manufactured at a high density, the capacity of Si can be maximized only when the ① degradation is mitigated. It can be achieved by controlling the absolute expansion of the electrode by introducing pores into the Si particles. In addition, a conductive material, such as CNT, which makes a long electrical conduction pathway in electrodes can resolve the disconnection between particles generated at a high density to enhance cycle life with maintaining high performance.
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Figure 31. Three degradations occurred during cycle test at d1.2 and the degradation mechanism schematics
①
②
③
Pristine Expansion Contraction
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Acknowledgement
First of all, I would like to express my deepest thanks to Professor Kyeong-Min Jeong, who led my master’s course and made this work possible. His advice and instructions gave me enlightenment and improved my work further. Besides my advisor, I would like to thank my thesis committee Professor Yunseok Choi and Professor Dong-Hwa Seo who offered helpful comments and encouragement.
Also, I would like to thank the ECheSL members who have been through a lot together with me for two years.
Finally, I am sincerely grateful to my family, especially my parents for their caring and sacrifices in educating and supporting my past, present, and future.