Silicon has been intensively pursued as the most promising anode material for high-energy-density LIBs because of its high specific capacity (>3500 mAh/g) and abundance. Despite its high capacity, Si suffers from fast capacity fading caused by its large volume change (>300%) during lithiation/delithiation and the serious issues stemming from this volume change, e.g., unstable solid electrolyte interphase (SEI) and disintegration (cracking and crumbling) of the electrode structure. The development of Si-C nanocomposites (e.g., nanowires, nano- tubes, or nanoparticles) has been widely studied.

These nanocomposites proved to be an effective method of improving capacity and cycling stability, since nano-sized Si can alleviate fracture during volume changes and the contact between Si and carbon can maintain electrical contact and improve conductivity of the nanocomposites. However, practical application of nano-sized Si materials in LIBs is difficult. First, achieving a high tap density is important for fabrication of high-energy LIBs for EVs and PHEVs, because it offers a high volumetric energy density. Unfortunately, the tap density of nano-sized materials is generally low, which in turn holds down their volumetric capacity. Furthermore, preparation of nano-sized Si either requires chemical/physical vapor deposition or involves complicated processes, leading to costly, low-yield synthesis that is difficult to scale up to practical levels. To date, the abundance of Si has not been fully capitalized upon due to lack of a low-cost strategy for large-scale synthesis of Si anode materials with superior performance.

—Yi et al.

While micro-sized silicon materials enable higher tap density than nanosized materials, they are more likely to undergo disintegration upon volume change during lithiation/delithiation compared with nanosized materials, resulting in severe capacity fading and also have long ion/electron transport paths that adversely affect high rate capability. A solution that some research teams have suggested would be to develop new materials that combine the advantages of both micro-sized and nano-sized Si materials to improve the cycling performance, rate capability, and energy density of Si anodes.

The Penn State team proposed that micro-sized Si-C composites should have the following features to obtain excellent electrochemical performance:

1. The size of primary Si building blocks should be small enough to avoid building block fracture induced by volume changes during the lithiation/delithiation process.

2. Si and C should be uniformly mixed at the nanoscale to improve the conductivity but also be highly packed to improve the tap density of the micro-sized particles.

3. The carbon and Si building blocks should maintain intimate contact even after the micro-sized Si-C particles are pulverized to prevent loss of electrical contact.

To produce the material, the team started with commercially available micro-sized SiO as the Si source at a gram scale. They heated the SiO to form an Si/SiO₂ composite composed of interconnected Si nanoparticles embedded in an SiO₂ matrix due to the disproportionation of SiO. This was followed by removal of the SiO₂ via an etching route, transforming the Si/SiO₂ composite into porous Si particles. Finally, thermal decomposition of acetylene fills a large portion of the original pores with carbon—creating a micro-sized Si-C composite in which Si and carbon are three-dimensionally interconnected at the nanoscale.

Preparation process from SiO precursor to the Si-C composite. Source: Yi et al. Click to enlarge.

They characterized the electrochemical performance of the Si-C composite as well as the porous Si without the carbon filling at different charge/discharge cycles.

The excellent performance [of the micro-sized Si-C composite] is attributed to the nanoscale size of primary particles and interconnected carbon and Si networks which can maintain internal electrical contact and sustain cycling stability. The synthesis method is low-cost and easy to scale up, and is thus believed to have great potential in practical production of high-performance Si materials for Li-ion batteries.

—Yi et al.

Resources

• Yi, R., Dai, F., Gordin, M. L., Chen, S. and Wang, D. (2012), Micro-sized Si-C Composite with Interconnected Nanoscale Building Blocks as High-Performance Anodes for Practical Application in Lithium-Ion Batteries. Adv. Energy Mater. doi: 10.1002/aenm.201200857