Fracture of One-Dimensional Crystalline Nanostructures during Lithiation

4 Fracture of One-Dimensional Crystalline Nanostructures during Lithiation 

As discussed earlier in this chapter, the mechanical degradation and fracture of Si structures during cycling is a major source of capacity decay in Si electrodes. Therefore, it is important to understand the conditions that result in fracture of individual nanostructures, particularly the critical diameters needed for fracture to occur. Ex situ SEM experiments of crystalline Si nanopillars with diameters ranging from 140 nm to 390 nm have revealed that fracture occurs at the surface of the pillars during lithiation for pillars with initial diameters between 240 nm and 360 nm. Example of SEM images of fractured pillars with three different axial orientations are shown in Fig. 1.20. 

Finally, based on statistical analysis, it was found that the critical diameter for fracture of Si nanopillars upon lithiation was between 240 nm and 360 nm. Below this size, very few pillars fractured, while 90-95% of pillars larger than 360 nm fractured. This can be rationalized since fracture depends on both the mechanical stresses present and a characteristic length scale for smaller pillars, the elastic energy present during deformation is not large enough to drive crack extension. This is important because most NWs used in battery electrodes have diameters that are less than 200 nm, resulting in resistance to fracture and decrepitation. 

---