First-Principles Studies of the Lithiation and Delithiation Paths in Si Anodes in Li-Ion Batteries

Understanding the lithiation of Si and the resulting formation of amorphous lithium silicides (a-Li_xSi) has been the subject of numerous studies due to the importance of silicon anodes for next-generation, high-energy-density lithium batteries. Experimental studies have shown that a-Li_xSi with 2 ≤ x ≤ 3.75 appears as a prominent phase in the phase transformations of Si during lithiation and delithiation, but computational studies have yet to elucidate why. In this work, first-principles molecular dynamics computations were performed to simulate the various immediate steps associated with lithiation and delithiation of Si anodes in a Li-ion half-cell battery. The energy of formation and unit cell volume for relevant Li_xSi phases and Si have been computed for both crystalline and amorphous states. The first-principles results are utilized to construct lithiation and delithiation pathways and compute the corresponding changes in the voltage, capacity, and cell volume of the anode. These results indicate that multiple pathways are possible during delithiation of c-Li_15/4Si and a two-phase delithiation field is energetically favored to occur between a-Li_15/4Si and a-Li_9/4Si due to a large amorphization energy for Li₃Si and a site-dependent amorphization energy for Li_9/4Si. In addition, large tensile stresses are generated during the removal of Li atoms from the 48e sites in the Li_15/4Si crystalline lattice. These hydrostatic tensile stresses can cause changes to the open-circuit potential, can lead to the fracture of Si or Li₁₅Si₄, and may be responsible for the change in the lithiation process from a ledge mechanism to a two-phase mechanism at x ≈ 2.25.