Elucidation of the Difference in the Volume Expansion Anisotropy between <i>i</i> - and <i>n</i> -type Silicon Pillars during the Lithiation Process

The anisotropic volume expansion that occurs during the lithiation of crystalline silicon was studied in the mid-2010s. This expansion occurs preferentially in a direction normal to the (110) surface and is related to the emergence of fractures in silicon nanostructures during lithiation/delithiation cycles. However, to the best of our knowledge, the impact of impurities on the lithiation of pristine silicon crystal surfaces remains unexplored. Here, we experimentally demonstrate that Sb-doped silicon crystals undergo an isotropic expansion, which differs from the expansion of the undoped pristine sample. The catalytic role of excess electrons in this process was examined using the first-principles calculation method. We started our calculation with idealized hypothetical models: Li migration in c-Si and Li penetration into (110) and (100) Si surfaces. Then, we transitioned to more realistic models: the amorphous–crystalline interfaces (ACIs) between (110), (100) Si surfaces and the amorphous LixSi (x ∼ 3.6). These interfaces emerge as the lithiation front via a two-phase separation. We included the effect of the Sb dopant by adding an extra electron to the model cells. Regarding Li migration in the bulk material, our calculations indicate that an electron-trapping state emerges near the transition state (TS), which reduces the barrier height under electron-excess conditions. A similar in-gap electronic state emerges for the Li penetration process on the clean (100) surface. In contrast, at the boundary of the amorphous LixSi (x ∼ 3.6) phases and on the silicon (110) surface, an excess electron increases the barrier height for the lithium migration. Our discussion references the band structure of bulk Si and electronic density of states (DOS) observations and is not restricted to the present calculated trajectory. These results explain our experimental findings and the role of excess electrons in the lithiation process, a topic not addressed in previous work.