Material Design of Advanced Graphite/Silicon Anodes for High-energy Lithium-ion Batteries

Abstract

The application range of lithium-ion batteries (LIBs) has been broaden from portable devices to electric vehicles and grid storage system. To achieve these targets, it is of particular interest for LIBs to achieve higher energy density as much as possible. As a result, material paradigm has been changed from intercalation-based materials into alloying-type, conversion-type materials and so on. Graphite has been utilized as a major anode material in commercial LIBs from 1991 due to the high electrochemical stability, high electrical conductivity, and low cost. However, as it is confronted with intrinsic capacitive limit (372 mAh g‒1 at LiC6), the high capacity anode materials such as silicon (Si), germanium (Ge), tin (Sn) have been considered as alternative to resolve the limitation of graphite. Among them, Si has gained the greatest attention because of the 10-times higher theoretical specific capacity (3579 mAh g‒1 at Li15Si4) than that of graphite, and its abundance in the earth. However, its high Li storage nature of the high capacity anode materials upon charging/discharging leads to the serious problems with causing to the mechanical failure as form of crack or pulverization, the side reaction with liquid electrolyte, thereby deteriorating battery performance. To address this problematic issue, numerous strategies have been suggested such as nanochemistry. When Si size become decrease to the nanoscale, the mechanical failure could be substantially mitigated. Furthermore, introducing amorphous Si can also successfully alleviate the mechanical fracture compared to the crystalline Si. Also, incorporating void spaces leads to the golden age for research for Si-based anodes by allowing Si volume expansion without breaking outermost shell, which achieve the outstanding electrochemical performance even several hundreds of repeating cycles. Unfortunately, huge volume variation of Si has hindered the good battery performance under industrial electrode condition including high areal capacity, high electrode density, and high proportion of active materials in electrodes, which is regarded as the major reasons to hinder the practical application of Si. Alternatively, graphite/Si composite could be the reasonable system for practical implementation of Si with synergetic effects of both high electrochemical stability of graphite and high capacity of Si. Since the first report of graphite/Si composite at 1998 by C. S. Wang and co-workers, graphite/Si composite has been explosively studied with several attractive approaches including controlling synthetic methods, introducing of inactive phases of Si, or surface engineering. Despite these great efforts, graphite/Si composite has still shown the unsatisfactorily electrochemical performance. In this dissertation, I start with short review of the LIBs in chapter 1 with the history, basic working principle upon charging and discharging, 4 major components (cathode, anode, electrolyte, and separator) in LIBs. Then, I introduced my research results related with material concepts, electrochemical results such as half- and full-cell, and analysis data. Especially, I have categorized the intrinsic limitation of graphite/Si composite into the 3 topics (surface, structural stability, and solid electrolyte interphase (SEI)), and tried to overcome the problems with innovative solution. In chapter 2, I have introduced pitch carbon, the ingredient of tar or petroleum, as a cheap carbon coating sources for graphite/Si composite. With in situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analysis, I have revealed the excellent mechanical strength of pitch under enormous internal and external pressure, compared to the other carbonaceous materials. With various physicochemical characterization, I have confirmed the origin of good mechanical properties, which are due to the formation of long-ranged graphitic ordering upon carbonization process. To unveil the effectiveness of pitch on graphite/Si composite, I have applied to the pitch coating on the Si nanolayer-embedded graphite, and demonstrated its power in battery performance, electrode swelling under industrial electrode conditions. In chapter 3, I have implanted the conformal high-capacity containers (CHC) within graphite/Si composite. With in situ SEM analysis, I have observed the effect of CHC, where the huge Si expansion can be efficiently accommodated with preserving structural integrity of graphite/Si composite. Considering the structural collapse of graphite/Si composite become serious under large amounts of Si in composite, this is very effective approach in high-capacity graphite/Si composite. The chemomechanical calculation supports the effects of CHC in maintain the structural integrity of composite. With repeating half- and full-cell cycling, composite with CHC could lead to better cycling performance than that of composite without CHC. In chapter 4, to resolve the intrinsic problem of natively formed SEI, I have introduced artificial SEI on graphite/Si composite with self-reconstruction. When I coated artificial SEI precursors with electrical shorting, I have observed that the artificial SEI precursors are self-rearranged, and formed the uniform artificial SEI layer. With these artificial SEI layer, I have enhanced the full-cell performance under industrial electrode condition. In addition, artificial SEI layer can effectively maintain the amounts of cyclable Li upon repeating cycling, thereby giving rise to the stabilization of cathode material better. Through this dissertation, I carefully expect that three innovative strategies for graphite/Si composite will advance its practical implementation for high-energy LIBs.