Electrochemical and Structural Characteristics of Type I and Type II Silicon Clathrate Anodes

Lithium-ion batteries are one of the most promising energy storage devices due to their high energy density, long cycle life, high voltage, and excellent rate capability. Si is a promising anode material due to its extremely large theoretical capacity of 3579 mAh/g at room temperature. However, the practical use of Si anodes is hindered by the structural failure upon lithium insertion/extraction processes and the low intrinsic electric conductivity.

In our group, we have investigated several different silicon clathrate materials, which have cage-like structures, as anode materials of Li-ion battery. The work to be presented here will discuss the electrochemical and structure characteristics of both type-I and type-II silicon clathrate anodes.

Type-I clathrates, which are synthesized using thermal annealing and arc-melting methods, are made of two pentagonal dodecahedra (Si₂₀) cages and six tet-rakaidecahedra (Si₂₄ cages) per unit cell and crystallize in the Pm3n space group. Results show that ternary type-I clathrates have the ability to allow lithium insertion and removal without a large volume change or pulverization. Type-II clathrates, which are synthesized by thermal decomposition of the Zintl compound, are made of sixteen pentagonal dodecahedra plus eight hexakaidecahedra (Si₂₈ cages) per unit cell and crystallize in the Fd3m space group. Results show that upon electrochemical lithiation the type-II clathrates become amorphous and transform into c-Li₁₅Si₄ at low potentials, then remain an amorphous silicide after delithiation. Ex-situ X-ray diffraction, X-ray photoelectron spectroscopy and nuclear magnetic resonance are also employed to understand the structural changes upon lithiation, delithiation and prolonged cycling.