Practical Development of Si Anodes for High-Energy Lithium Ion Batteries

Department of Energy Engineering (Battery Science and Technology)Since the first commercialization of the lithium ion battery (LIB), LIB has played a significant role as a power source for electric devices. As increasing energy demands with the emergence of the electric vehicle and the energy storage system, improving the energy density of LIB has been recognized as one of the most important issue for battery researchers. Accordingly, the high-capacity materials have been investigated to break the theoretical capacity limit of current LIB chemistry with the carbonaceous anode and the lithium metal oxide cathode. In terms of the anode, silicon (Si) has received great attention because of its low discharge potential and 10 times greater theoretical capacity than the state-of-the-art graphite. However, the intrinsic hurdle of Si anodes, which is the huge volume expansion (300%) during battery operation, retards the application to the practical LIB. Therefore, the concrete strategies for overcoming the challenge are required in order to improve the energy density with utilizing Si anodes. For over twenty years, the nanoengineering has considerably improved the electrochemical performance of the Si anode by alleviating the intensified stress and strain from the volume change. However, there is a significant gap between the nanoengineered Si anode in academic field and the commercial LIB system in terms of the synthesis of Si anode, the battery manufacturing, and the electrochemical cell design. In this regards, to implement the Si anodes in commercial LIBs, several commercial factors such as the scalability, the rational cost, and performance feasibility, should be considered at the beginning of the development. Accordingly, herein, I have covered a comprehensive review about the co-utilization of graphite and Si anodes for commercial LIBs, the development of high-capacity Si anode for commercial high energy LIB, the benchmarking comparison of industrially-developed Si anode, and finally the remaining issues regarding the practical implementation of Si anode. In the Chapter 1, the graphite Specifically, the development of the Si anodes would be presented with physicochemical analysis, and the practical utilization of Si anodes for high-energy LIB would be discussed. Furthermore, in order to compare the performance of the developed Si anodes, benchmarking with industrial samples would be conducted with the electrochemical characterization and the failure mechanism analysis. In chapter 2, the high-capacity Si anodes for commercial high energy LIB is proposed with Fe-Cu-Si composite. FeCuSi is built up with Si nanoparticles and numerous nano-sized metal silicides as a form of a secondary particle. In this design, the micron-sized secondary particle exhibits high tap density which is easy to handle in the battery manufacturing process, and the numerous voids between Si nanoparticles effectively accommodated volume expansion of Si. In addition, the metal silicides such as iron silicide and copper silicide reduced interparticle contact resistance between Si nanoparticles. To investigate the commercial feasibility, the graphite-blended electrode with FeCuSi composite was fabricated under the commercial standard. It exhibited the superior electrochemical performances compared to industrially developed SiOx and FeSi anodes. In chapter 3, the benchmark comparison of industrially developed Si anodes including Si nanolayer-embedded graphite composite, carbon-coated SiOx, Si-containing graphite/carbon composite, has been presented. The benchmarking comparison was performed in graphite-Si blending system (fixed reversible specific capacity of 420 mAh/g) under the industrial electrode density (> 1.6 g/cc), areal capacity (> 3 mAh/cm2), and a small amount of binder (3 wt%). In addition, the one-to-one comparison has included essential items of both material characterization including laser diffraction particle size analysis, BET surface area, tap density, SEM, and HR-TEM, as well as the electrochemical analysis including half-cell and full-cell tests with measuring electrode volume expansion. As a result, the Si nanolayer-embedded graphite composite exhibited a great compatibility with conventional graphite. In chapter 4, I discuss the critical issues of the practical implementation of Si anode in high-energy LIBs. The electrochemical cell design has been systematically presented with proper examples. I emphasize that the influence of the electrochemical cell design on the battery performances when aimed at high volumetric energy density. In addition, based on the electrochemical design, the limit on the electrode swelling of Si anodes in terms of energy density is suggested. Furthermore, the origin of differences in the capacity fading between in the half-cell and in the full-cell is carefully figured out. Finally, I propose the potential future direction regarding with the electrode swelling, the capacity fading, and the feasibility study.clos