Pressure‐Induced Vapor Synthesis of Carbon‐Encapsulated SiO x /C Composite Spheres with Optimized Composition for Long‐Life, High‐Rate, and High‐Areal‐Capacity Lithium‐Ion Battery Anodes

SiO x exhibits a high specific capacity as anode materials for lithium‐ion batteries. However, it is still a challenge to achieve long life, high rate performance, and high areal capacity for the SiO x materials. Herein, carbon‐encapsulated SiO x /C composite spheres are prepared by a high pressure caused by heating SiO x /C spheres in pure dimethylformamide in a sealed vessel. The SiO x /C spheres with different compositions are synthesized by pyrolysis of different liquid siloxanes and hexamethyldisilane in a sealed vessel and subsequent heat treatment. The electrochemical performances are strongly dependent on the composition of the SiO x /C spheres. The specific capacity and initial coulombic efficiency (ICE) greatly increase, and cyclability slightly decreases with a decrease in the x value from 1.28 to 0.59. However, the capacity, ICE, and cyclability greatly decrease with a further decrease in the x value because of the formation of SiC. The carbon‐encapsulated SiO 0.59 /C spheres exhibit a high reversible capacity (1561.3 mAh g −1 ), an outstanding cyclability (0.0332% capacity loss per cycle during 500 cycles at 1 A g −1 ), a good rate performance (245.3 mAh g −1 at 10 A g −1 ), and a high areal capacity (4.08 mAh cm −2 ). Formation of the spherical morphology and carbon‐encapsulating layers is a result of the high vapor pressure generated at a high temperature.