Dimer Model Responsible for the Phase Transformation in Hollow-Spherical Si Anode of Li-ions Batteries with Synchrotron X-Ray Analyses
Shi-Wei Chen / 陳世偉1*, Chih-Hao Lee / 李志浩2, Chih-Hao Lee / 陳燦耀2
1Industrial Application Group, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
2Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan
3Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan
* presenting author:shiwei chen, email:chen.sw@nsrrc.org.tw
With the possible exhaust of fossil oil, effective utility of energy becomes an important issue. Clean energy, rechargeable energy, is thus highly developed. Rechargeable Li-ion batteries especially attract much attention because of its extensive use such as portable electronic devices, powering hybrid electric vehicles (HEV) and zero-emission vehicles (ZEV). The crucial requirement for the real application of Li-ions batteries is the high capacity. Silicon, with ultra-high theoretical capacity and lithium diffusivity, is thus considered as a new anodic material of Li-ions batteries. But, the dramatic volume expansion of silicon, during lithiation, hinders the utility of this material in practice. This issue has not been resolved because the mechsnism of volume expansion is still unknow, the variation in atomically local environment is still controversial. We thus synthesize hollow nano-spheres of silicon to effectively improve the stability of Si anode in battery, meanwhile, explore the structural evolution of silicon anode, during lithiation and de-lithiation, precisely by using synchrotron x-ray techniques. X-ray diffraction spectra indicate the structure of silicon anode transforming from cubic to amorphous structure and the final Li13Si4 orthorhombic structure during lithiation. Complementary X-ray absorption spectra and FDMNES simulation evidence the locally atomic environment of Li13Si4 consists of silicon dimers with layer stacking. Si dimers are existing already in amorphous structure, before the formation of Li13Si4 structure. Si dimers, being a seed of nucleation, can thus facilitate the phase transformation of Si anode and improve the lithiation process. Meanwhile, we also observe the Si-p v.s. Li-s and Si-p v.s. Li-p orbital hybridizations in Li13Si4. That constructs the charge transfer path in anodic materials and completes the circuit of cell. With the exploration of structural evolution in atomic scale and orbital hybridization, this study describes clearly the phase transformation in silicon anode during lithiation that shins a light on the mechanism of volume expansion in silicon material. The presented results could also be the fundamental to design new materials for the use in anodes of advanced batteries.

Keywords: silicon , hollow-spherical particles, synchrotron x-ray analyses, dimer, phase transformation