In the case of anodes, silicon is considered to be one of the most promising materials due to its abundance, environmentally benignity, low working potential, and highest theoretical capacity of 4200 mA h g⁻¹ (corresponding to the fully lithiated composition of Li_4.4Si), which is ten times higher than that of traditional graphite anode (∼372 mAh g⁻¹). However, the stress caused by the large volume change (∼400%) during cycling, resulting in electrode pulverization and unstable solid electrolyte interphases (SEI), seriously affects long-time stability and lifetime of Si anode. In recent years, a variety of nanosized Si such as nanoparticles, nanowires, nanotubes and nanoporous networks have demonstrated enhanced mechanical integrity, which can effectively address this issue by accommodating the large volume change. Among them, Si nanoparticles are perceived as a promising candidate for several important reasons.

First, it is found that the surface cracking and particle fracture associated with volume expansion of Si particles can be alleviated when the size of crystal Si nanoparticles is below 150 nm. Second, Si nanoparticles are compatible with the traditional slurry coating manufacturing processes. In addition, Si nanoparticles can serve as elements for advanced structure constructions such as yolk shell and pomegranate, which prevent the instability of solid electrolyte interface (SEI). However, the existing processes for producing Si nanoparticles are still largely limited to a few expensive, complex and energy intensive processes, such as laser ablation of bulk Si and high temperature or high energy pyrolysis of silane/polysilane/halosilane precursors. To enable widespread application, it is desirable to develop scalable, cost- effective, and energy-efficient processes to produce Si nano-particles.

—Zhu et al.

There are two major low grade silicon sources—metallurgical silicon (M-Si, ∼99 wt % Si) and ferrosilicon (F-Si, ∼ 83 wt % Si, ∼ 13 wt % Fe)—available at low cost (∼$1000/ton for metallurgical silicon, and ∼$600/ton for ferrosilicon).

Schematics of fabrications and electrochemical cycling of low grade silicon nanoparticles. (a) Si nanoparticles from metallurgical Si (M-Si) sources. (b) Si nanoparticles from ferrosilicon (F-Si) sources. Credit: ACS, Zhu et al. Click to enlarge.

The researchers from Nanjing University and Soochow University demonstrated that nanosized Si powders, with size around ∼150 nm, smaller than the critical size to alleviate the pulverization during cycling, can be massively produced from these two different low grade Si sources through a one-step, scalable high energy mechanical milling (HEMM).

They found that both M-Si and F-Si nanoparticles maintained good crystal quality after ball milling. Because of excessive iron in ferrosilicon sources, F-Si nanoparticles contain both crystalline Si and FeSi2 after ball milling. Because FeSi₂ is not reactive to Li⁺, it can effectively buffer the volume expansion of Si particles during cycling. On the other hand, the metallurgical Si (M-Si) nanoparticles electrode has higher capacity (∼3200 mAh g⁻¹), in comparison with the 2200 mAh g⁻¹ of F- Si, due to the higher purity of silicon as active material in the source.

Both M-Si and F-Si nanoparticles demonstrated a stable electrochemical performance after carbon modification, with capacities of 1360 and 1205 mAh g⁻¹ after 100 deep electrochemical cycles, respectively.

Resources

• Bin Zhu, Yan Jin, Yingling Tan, Linqi Zong, Yue Hu, Lei Chen, Yanbin Chen, Qiao Zhang, and Jia Zhu (2015) “Scalable Production of Si Nanoparticles Directly from Low Grade Sources for Lithium-Ion Battery Anode” Nano Letters doi: 10.1021/acs.nanolett.5b01698