Atomic-Level Understanding toward a High-Capacity and High-Power Silicon Oxide (SiO) Material

Silicon oxide (SiO) has attracted much attention as a promising anode material for Li-ion batteries. The lithiation of SiO results in the formation of active Li-Si alloy cores embedded in an inactive matrix consisting of Li-silicates (Li₂Si₂O₅, Li₆Si₂O₇, and Li₄SiO₄) and Li₂O. The maximum Li content in lithiated SiO (Li_xSiO) is known to be x = 4.4 based on experiments. Our calculations reveal that Li-silicates are dominant over Li₂O among matrix components of the experimental Li_4.4SiO phase. We show that Li_xSiO can become thermodynamically more stable and thus accommodate more Li ions up to x = 5.2 when Li₂O dominates over Li-silicates. The minor portion of Li₂O in the experimental phase is attributed to kinetically difficult transformations of Li-silicates into Li₂O during electrochemical lithiation. The Li₂O subphase can act as a major transport channel for Li ions because the Li diffusivity in Li₂O is calculated to be faster by at least 2 orders of magnitude than in Li-silicates. We suggest that Li₂O is a critical matrix component of lithiated SiO because it maximizes the performance of SiO in terms of both capacity and rate capability.