With high energy density, long lifetime, and environmental friendliness, lithium-ion batteries (LIBs) represent one of the most attractive energy storage devices and are playing more and more important roles in modern society. However, the current state-of-the-art LIBs cannot satisfy the ever-increasing demands of electric vehicles and large-scale energy storage.

Up to now, most commercialized LIBs have adopted intercalation reaction-based anode materials, such as graphite and Li₄Ti₅O₁₂. Such anode materials share the common features of outstanding cycle life but limited capacity. The theoretical capacities for graphite and Li₄Ti₅O₁₂ are 372 and 175 mA h g⁻¹, respectively. The relatively low capacity severely limits the energy density of the batteries. As a way to improve the capacity, stability, and the lifetime of the anode materials, an approach of alloying with the lithium has been widely taken. Among the extensively studied alloys of Li-Si (Li_4.4Si: 4212 mAh/g), Li-Sn (Li₁₇Sn₄: 959.5 mAh/g), Li-Sb (Li₃Sb: 660 mAh/g), Si has been considered as the most promising candidate to replace graphite. It is the second most abundant element in the earth's crust, environmentally friendly, and possesses a theoretical capacity. However, its application to LIBs is significantly difficult due to terrible cyclability resulting from severe volume change (~400 %) and excessive structural degradation during charge/discharge. To address these issues, silicon suboxide (SiO_x, 0< x <2) has been proposed as a potential alternative to Si. The SiO_x show a small volume change during cycling when compared to elemental Si. The generated Li₂O and lithium silicates during the first lithiation may buffer the large volume change and lead to improved cycling stability. However, the irreversible formation of Li–O and Li–Si–O phases lead to consumption of lithium as part of the irreversible capacity, which significantly decreases the initial coulombic efficiency (ICE). Furthermore, the SiO_x active material exhibits inferior electron transport kinetics compared with that of Si because of the electrically insulating property of SiO₂, and poor cyclic performance resulting from the unstable solid-electrolyte interphase (SEI) layer formed during lithiation. Recently, considerable efforts have been devoted to tackling the above-mentioned problems and pushing the SiO_x-based anode materials towards practical applications in LIBs. Actually, SiO_x have been blended with graphite (usually with a content of more than 90%) and used in commercialized LIBs.

In the current study, in order to take full advantage of the SiO_x active material, we fabricated a carbon-incorporated/carbon-coated SiO_x ((C-SiO_x)@C) composite containing a significant amount of SiO_x (~90 wt%) by a simple and cost-effective one-pot synthesis method. We also extended the reversibility and capacity improvements beyond the initial cycle by modifying the constitutional structure of the SEI in the (C-SiO_x)@C anode. As a result, the ICE increased significantly from 43.3% to 93.6% and the capacity retention after 500 cycles was improved from 86.7% to 90.4%. Our results suggest that a SiO_x anode electrode that possesses cyclic stability and a reasonable capacity can be prepared through elaborate design of the SEI layer to achieve appropriate physical durability and high Li⁺ diffusivity.