Agglomeration Mechanism and a Protective Role of Al2O3 for Prolonged Cycle Life of Si Anode in Lithium-Ion Batteries

Si, the high-capacity anode for Li-ion battery (LIB), has intrinsic 300% volume changes limiting its commercial application. The volume change leads to particle pulverization that results in loss of electrical contacts. Various nanostructures are proposed to avoid the pulverization, but the commercialization is still a distant future. Recently, Al2O3 has demonstrated its ability to enhance electrochemical cycling performance. However, a comprehensive mechanistic role of the Al2O3 has not been well-understood. Here, we have combined electrochemical and chemical agitation tests to propose two novel mechanisms: Si agglomeration and a protective role of the Al2O3. LiPF6, the common Li salt of the LIB electrolyte, decomposes and forms HF that etches the native oxide layer then forms a labile Si–H surface. Because of the labile Si–H surfaces, the Si particles agglomerate during the volume changes. The Si agglomeration has a detrimental effect on the cycling performance associated with the loss of electrical contacts. On the other hand, in the presence of the Al2O3, the Al2O3 consumes the HF, protecting the native oxide layer that resists the agglomeration. Thus, the Si particles with Al2O3 are better-dispersed. The Al2O3 allows the better Si dispersion during electrochemical cycles, resulting in improved capacity retention.