Understanding lithium, sodium, and potassium storage mechanisms in silicon oxycarbide

The use of silicon oxycarbides (SiOCs) as anode materials in lithium-ion batteries and sodium-ion batteries has risen considerably in recent years. However, the amorphous and complex structures of SiOCs that contains C-rich and O-rich SiOC phases make it difficult to clarify Li⁺- and Na⁺-ion storage mechanisms experimentally. This study uncovers the Li⁺-, Na⁺-, and K⁺-ion storage mechanisms in both the O-rich SiO_1.5C_0.5 and C-rich SiO_0.5C_1.5 structures using the density functional theory. The ions inserted at the initial discharge process fill the microvoids in the SiOCs. A further ion insertion causes Si–O and Si–C bond cleavage, and thus results in the formation of a large-size free volume, which is favorable for subsequent ion insertion. The reasons for the high Li⁺-ion storage capacity as compared to Na⁺-ion are less severe volume expansion, more favorable formation of Li-rich Si compounds and Li–Si alloys. The theoretical K⁺-ion storage capacities in the O-rich SiO_1.5C_0.5 and C-rich SiO_0.5C_1.5 phases are much lower (335 and 186 mAh g⁻¹, respectively) than those of Li⁺-ion (519 and 681 mAh g⁻¹, respectively) and Na⁺-ion storages (335 and 681 mAh g⁻¹, respectively). The huge structural instability caused by the repulsive interaction between the K⁺ ions results in the low storage capacity.