Supramolecular approaches to polymeric binders for silicon anodes of lithium-ion batteries

Abstract

Human`s lives in modern society increasingly depend on the electricity with the advent of portable electronic devices, electric vehicles (EV), electric energy storages for intermittent renewable energy sources. The pattern of this energy consumption is aiming at the ubiquitous energy in which we can access the power source without regard to time and place. To this end, it is strongly required to develop high energy density batteries. In order to replace the present graphitic anodes having the low theoretical capacity (372 mAh/$g^{-1}$ for $LiC_6$), various anode materials have been explored. Among them, silicon (Si) active material is regarded as one of the most promising materials due to its high theoretical capacity (4200 mAh/$g^{-1} for $Li_4.4$Si). Nevertheless, the huge volume change of Si during charge/discharge process causes several problems such as delamination, unstable solid electrolyte interphase (SEI) layer, and Si pulverization, resulting in the loss of electric connection and consequent capacity fading. Over recent years, it has turned out that polymeric binders can play a critical role in the aforementioned problems. In this dissertation, we are aiming at establishing the design principle of polymeric binders for Si anodes in lithium ion batteries. We first begin by the functional groups of polymers and move on to the effect of polymer architectures. Then, the superstructures of binders including secondary, tertiary, and quaternary structures are investigated. In Chapter 1, overall backgrounds (the market trend of high-capacity batteries, the (dis)advantages of Si anodes, and the fundamental concepts of batteries) are introduced. In Chapter 2, the important factors in terms of the primary structure of binders are dealt with. It is found that the noncovalent functional groups and branched structures are important for binders. In Chapter 3, it is discussed that the limitation of covalent and noncovalent crosslinking is overcome by the novel functions of superstructures. Subsequently, a new crosslinking - namely, topological crosslinking - is presented. The topological crosslinking based on polyrotaxanes indeed exhibits the commercial-level electrochemical performances: initial Coulombic efficiency and high areal capacity falling in the range of current commercial lithium ion batteries. In Chapter 4, we present the ultimate design principle of polymeric binders for Si anodes by integrating the entire contents.