Despite silicon’s well-known advantages as a high energy capacity anode for Li-ion batteries (abundance, environmental friendliness, low operation potential and ultrahigh theoretical capacity), commercial applications have been hampered by equally well-known disadvantages (including poor Li⁺ electron transport and huge volume variation causing structural collapse).

Many researchers have focused on preparing silicon nanomaterials to address the problems; one of the more effective has been core-shell Si@C and yolk-shell Si@void@C nanostructures. (E.g., earlier post.)

As comprehensive modifications, core-shell Si@C and yolk-shell Si@void@C nanostructures are the most effective. As for the core-shell Si@C structure, the compact carbon shell can not only effectively improve the conductivity but also inhibit the electrolyte corrosion on the Si surface to a large extent. … Nevertheless, the core-shell structure does not have enough space to buffer the vast volume expansions of alloying Si and as a result the integrity of the structure would be difficult to keep and the compact carbon shell would be broken. In contrast, yolk-shell Si@void@C structures have been attracting much more attention since the void space provided can better manage the volume expansions of Si-Li alloying and maintain the structure integrity.

… Note that, the yolk-shell structure has two non-ignorable problems: (i) insufficient electronic contact between Si yolks and hollow carbon shells and (ii) the corrosion of Si yolks from outside electrolytes. … Herein, we reported the observation of enhanced conductivity and suppressed surface corrosion and passivation of Si cores by adopting core-shell Si@C nanoparticles as the yolks instead, i.e., preparing core-shell yolk-shell Si@C@void@C nanostructure. Besides, the introduction of the inner carbon shell provided an effective protection of the Si cores from extra corrosion when etching the SiO₂ intermediate layer to obtain the anticipated thicknesses of void space. The obtained Si@C@void@C demonstrated a remarkably improved cycling stability and rate performance (with a reversible capacity of 1366 mAh g^-1 at 500 mA g^-1 and a capacity retention of ~60% at 4000 mA g^-1). This work provides a novel clue to develop advanced Si- or Sn-based anode materials for energy storage.

—Xie et al.

Schematic diagram for the preparation of traditional yolk-shell Si@void@C (A) and novel core-shell yolk-shell Si@C@void@C (B). Xie et al. Click to enlarge.

The thicknesses of the inner carbon, void, and outer carbon shells of the as-prepared core-shell yolk-shell Si@C@void@C nanohybrids were approximately 10, 50, and 10 nm, respectively.

The team found that the extra carbon shell could not only decrease the electrical resistance between Si yolks and hollow carbon shells but also effectively protect the Si yolks from electrolyte corrosion—i.e., the formation of Li₂SiF₆ nanolayers on the Si surface. This resulted in the enhanced electrochemical performance.

The researchers noted that while the carbon shell effectively inhibited electrolyte corrosion during the short-term (12 h), the electrolyte penetrated the carbon shell and reacted with Si to form Li₂SiF₆ layers under long immersion of 120 h.

As a result, they are working on Si yolks with advanced surface modification and specific Si-based electrolytes for the next-generation yolk-shell Si@void@C materials.

Resources

• Jian Xie, Liang Tong, Liwei Su, Yawei Xu, Lianbang Wang, Yuanhao Wang (2017) “Core-shell yolk-shell Si@C@Void@C nanohybrids as advanced lithium ion battery anodes with good electronic conductivity and corrosion resistance,” Journal of Power Sources, Volume 342, Pages 529-536 doi: 10.1016/j.jpowsour.2016.12.094