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 (LixSiO) 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 Li4.4SiO phase. We show that LixSiO 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.