Researchers are using various approaches using nanostructured silicon materials to address one of the main challenges to using silicon in a Li-ion battery anode—the considerable volume expansion of up to ≈300% during electrochemical lithiation, leading to cracking and pulverization of the Si, ultimately causing rapid and serious loss of capacity.

In addition to using nanostructures, the enhancement of mechanical stability by applying nanocomposites is also important for anode materials, and Si-based nanocomposites coated with carbon-related materials and metals have also been investigated. The amorphous carbon and metal coating acts as a buffering material to minimize the mechanical stress induced by the huge volume change in the Si; it also acts as an electron conductor.

In this study, we used chemical vapor deposition (CVD) to synthesize Si-metal nanocomposite structures. The most notable structural and compositional features can be summarized as follows. The structure looks like an aggregation of nanoparticles with internal cavities. Furthermore the Si-metal nanocomposite structures are connected with Ge nanostructures, which play roles to make more cavities and to introduce metals such as iron (Fe) from the stainless steel substrate during the growth of the Si-metal nanocomposite structures. By introducing Fe into Si-metal nano-composite structure, we could obtain higher electrical conductivity and enhance the mechanical strength. On the other hand, theoretical capacity of Ge is about 1620 mAH/g and lower than Si (4200 mAH/g). Considering this point, we formed thick Si-metal nanostructure layers to minimize the reaction between Li ions and Ge nanostructures. Finally the new nanomaterials showed higher capacity than commercially available graphite anode materials and stable cyclic property compared with pure Si materials.

—Fukata et al.

Charge (red), discharge capacity (blue), coulombic efficiency (black) versus cycle number for a Si-Fe based nanocomposite structure sample. Fukata et al. Click to enlarge.

The joint research groups formed one-dimensional germanium (Ge) nanowires on metal substrates and then created nanostructured Si-metal composites using the nanowires as a base material layer. The Si-metal nanocomposite structures were directly formed on stainless steel substrates by chemical vapor deposition (CVD).

The formed nanostructured material is characterized by numerous cavities existing inside aggregated nanoparticles of about several tens of nanometers to a hundred nanometers. There also are larger cavities present between the Si-metal composites and the Ge nanostructures.

Another characteristic is that the material consists of not only pure Si but also metal atoms (mainly iron) that are spontaneously provided from the substrate via the underlying Ge nanostructures and incorporated into the growing Si material, forming silicon-metal composites.

The new material is capable of increasing both the capacity and life of Li-ion rechargeable battery anodes. The research groups attained these features by creating internal cavities in the material, which act as buffer space to absorb stress generated by the expansion of pure Si, and by regulating the composition of Si and metal elements in the Si-based nanostructure.

Resources

• Naoki Fukata, Masanori Mitome, Yoshio Bando, Wenzhuo Wu, Zhong Lin Wang (2016) “Lithium ion battery anodes using Si-Fe based nanocomposite structures” Nano Energy, Volume 26, Pages 37-42 doi: 10.1016/j.nanoen.2016.05.007