Haiping Jia, Xiaolin Li, and Ji-Guang Zhang

Pacific Northwest National Laboratory, Richland, WA, United States.

With the increasing demand on high energy Li-ion batteries (LIBs) for electric vehicles and other large scale energy storage applications, silicon (Si) based composite material has been regarded as one of the most promising candidate to replace the state-of-the-art graphite anodes and can further boost the specific energy of LIBs to be more than 300 Wh/kg. Silicon has the highest practical capacity of 3,579 mAh g^-1 and a relatively low lithiation potential of 0.2 V vs. Li/Li⁺. However, fast capacity fade still largely limits the practical application of Si based anode. A large volume change during lithiation and delithiation processes lead to pulverization and subsequent loss of electrical contact, continuous consumption of electrolyte, repeated breaking/formation of the solid electrolyte interphase (SEI) and an increase in the overall resistance. There has been significant effort to understand and mitigate the capacity fade in Si-based anodes exploiting nanostructured electrodes, surface coatings, additives and novel binders. These advances have paved the way for practical application of Si-based anodes for Li-ion battery applications. Although significant progress has been made, most nanostructured Si materials have to be prepared by high-cost processes (such as chemical vapor deposition) that are difficult to scale up. Therefore, there is an increasing need to prepare Si microparticles (SiMP) with high conductivity and stable structures.

In this work, we developed a low-cost and scalable synthesis processes to make desired micronmeter scale SiO₂/MWNT spheres. The highly spherical SiO₂/MWNT macrobeads ranging from 500 nm to 10 µm in diameter were employed as template and silicon precursor (Figure 1a, b). The as-prepared material basically retains the original morphology of template and demonstrates unique hierarchical structure (Figure 1c, d). The fine primary silicon particles can shorten the Li⁺ transport route, and avoid the pulverization of SiMP. The obtained silicon composite delivers a reversible capacity of 1290 mAh g^-1 at 1C and 87% capacity can be retained after 300 cycles. We attribute this promising cycling stability to the well-designed hierarchical structure, which provides: 1) enough void space to accommodate the volume change so that the swelling of the composite can be minimized; 2) good mechanical structure and electronic conductivity from the MWNT matrix. More details on the synthesis process and electrochemical performance of SiMP will be reported in this report.

Figure 1. (a, b) SEM images of microsphere SiO₂/MWNT; (c, d) SEM images of the obtained Si/MWNT.