A Design of Spherical Skein-like Si/C Composite Structure for Large-Volume-Change Lithium Battery Anodes

Abstract

Silicon is an alternative anode material for future energy storage devices because it delivers 10 times greater theoretical (∼4200 mAh/g) specific capacity than that of a traditional graphite anode (∼370 mAh/g). However, Si-based anode materials face significant challenges due to its large volume expansion (~400%) upon lithium insertion. This large volume change causes cracking and pulverization of silicon, which leads to loss in electrical contact and drastic capacity fading. In addition, a continuous growth of solid electrolyte interphase (SEI) due to Si cracking causes low coulombic efficiency, higher resistance to ionic transport. This will eventually result in the exhaustion of the electrolyte and dry-out of the cell. To address this challenge, various strategies have been explored through nanostructured material design.1,2However, synthesis of Si nanostructures usually involves high temperature chemical vapor deposition or complex chemical reactions, therefore their scalability and compatibility with existing battery manufacturing processes remain a challenge. Furthermore, it is hard to apply the conventional binder and electrolyte because of the dramatic volume changes and displacement of Si particles during cycling. Therefore, much labor and time are also being spent to develop new electrolyte and binder proper in silicon based anode. Here, we propose a hierarchical Si/C composite structure that tackles all of the aforementioned problems. Figure 1 shows the conceptual design of the proposed structure that is composed of spherical skein-like CNT matrix, embedded Si particles inside the matrix and protective carbon shell. This structure has several advantages for lithium battery anodes. First, the inner skein-like CNT matrix has lots of pores since CNTs are loosely entangled with each other. As a result, the matrix provides sufficient space that enables Si particles to expand during cycling without deforming/breaking of the outer carbon shell. Secondly, numerous CNTs composing the matrix prevent inactive Si particles by providing the electrical contact to Si particles. Thirdly, the outer carbon shell acts as an electrolyte barrier which prevents the electrolyte from reaching the surface of Si particles inside the shell. This allows the growth of a stable SEI only on the surface of the carbon shell. Lastly, the proposed composite allows the use of the conventional binder and electrolyte because the surface property of the carbon shell is similar to that of the conventional graphite. We produced spherical skein-like Si/CNT composites using a hybridization system that has blades rotating at high speed. Since this equipment is similar in design to the industrial equipment for manufacturing spherical graphite, scaling up for mass production of the Si/CNT cmposites is certainly feasible. Figure 2 shows SEM images of the produced skein-like Si/CNT composites. It is observed that Si particles are uniformly dispersed throughout the whole matrix (Figure 2b). Furthermore, many pores between CNTs are observed from a magnified SEM image of the interior of the composite (Figure 2c). In this presentation, more details and electrochemical characteristics of the composites will be shown. Figure 1Conceptual design of the skein-like CNT matrix and the skein-like Si/CNT composite structure Figure 2 SEM images of the synthesized Si/CNT composites. (a) A low magnification SEM image of the composites, (b) FIB-SEM cross-section image of single Si/CNT composite, and (c) a magnified cross-section SEM image of interior region of the composite. Figure 1