Real-Time Measurement of Stress and Damage Evolution during Initial Lithiation of Crystalline Silicon

Crystalline to amorphous phase transformation during initial lithiation in (100) Si wafers is studied in an electrochemical cell with Li metal as the counter and reference electrode. During initial lithiation, a moving phase boundary advances into the wafer starting from the surface facing the lithium electrode, transforming crystalline Si into amorphous ${\mathrm{Li}}_x\mathrm{Si}$. The resulting biaxial compressive stress in the amorphous layer is measured in situ, and it was observed to be ca. 0.5 GPa. High-resolution TEM images reveal a very sharp crystalline-amorphous phase boundary, with a thickness of $\sim 1\mathrm{nm}$. Upon delithiation, the stress rapidly reverses and becomes tensile, and the amorphous layer begins to deform plastically at around 0.5 GPa. With continued delithiation, the yield stress increases in magnitude, culminating in a sudden fracture of the amorphous layer into microfragments, and the cracks extend into the underlying crystalline Si.

Figure 1

(a) Cross-sectional SEM image of the Si wafer following lithiation at a fixed current density of $12.5\mu \mathrm{A}/{\mathrm{cm}}^2$ for 25 h. The top layer is amorphous ${\mathrm{Li}}_x\mathrm{Si}$ ($a\mathrm{\text{−}}{\mathrm{Li}}_x\mathrm{Si}$), which is separated from the bulk crystalline Si ($c$-Si) by a sharp phase boundary. Note the change in the morphology of the fracture surface between the two phases. (b) High-resolution TEM image of the phase boundary between $c$-Si and $a\mathrm{\text{−}}{\mathrm{Li}}_x\mathrm{Si}$, showing a very sharp phase boundary, the width of which is $\sim 1\mathrm{nm}$. The insets show electron beam diffraction patterns.

Figure 2

Evolution of cell potential (dashed line) and “stress thickness $\sigma h$” (two solid lines corresponding to measurements along two orthogonal directions on the wafer) as a function of time during lithiation and delithiation of a crystalline Si wafer. The potential stays on a plateau during lithiation while a phase boundary moves in the sample, transforming crystalline Si to amorphous lithiated silicon, which is under a state of compressive stress, as indicated by the $\sigma h$ plot. During delithiation, the increase in cell potential is similar to that of an amorphous film.

Figure 3

Evolution of stress in amorphized layer $\sigma$ as a function of time during lithiation and delithiation of a crystalline Si film (two solid lines corresponding to two orthogonal directions on the wafer). During lithiation, the stress is compressive and nearly constant $\sim 0.5\mathrm{GPa}$. During delithiation, the stress rapidly becomes tensile, and the film undergoes plastic deformation in tension starting around 0.5 GPa. The yield stress increases beyond 1.5 GPa, resulting in fragmentation of the amorphous layer, as indicated by a sudden drop in stress.

Figure 4

A network of cracks (a) that forms during delithiation of an amorphized layer. FIB cross section view of the cracks (b) shows that they propagate deeper into the crystalline Si layer, exposing fresh Si surface for solid electrolyte interphase formation during subsequent cycles.