Analysis of the Lithium Storage Mechanism in the SiO_x/C Composite Based on the Performance Variation Applied to a Lithium-Ion Battery

The SiO_x/C composite, as a form of silicon-based materials, has been considered as an attractive alternative anode for next-generation lithium-ion batteries. The porous SiO_0.71C_1.95N_0.47 anode material exhibiting robust Si–O skeletons wrapped by carbon layers is successfully prepared and delivers an initial capacity of over 1700 mAh g^–1 with an initial coulombic efficiency of 69.4% and favorable cycle life. Both Si (2p) X-ray photoelectron spectroscopy (XPS) and ²⁹Si nuclear magnetic resonance (NMR) demonstrate the existence of SiO₄ and SiO₃C units as main lithium storage sites in the original state. The XPS curve moved toward the direction of the binding energy decreasing with NMR spectra shifting to a high field after the first lithiation process. The massive capacity loss during the first discharge and charge cycle results from the formation of irreversible Li silicate (Li₂SiO₄). The fluctuation of the charge and discharge capacity, including a persistent decline during the first 30 cycles and a continuous elevation in the following 400 cycles, could be attributed to the participated degree of reversible Li silicate (Li₂SiO₃ and Li₂Si₂O₅) in the delithiation process. The Si–O skeletons are gradually corroded and ultimately destroyed in the final 400 cycles, leading to the sharp drop of the cycling performance of the half-cell.