Silicon oxide (SiO), inheriting the high-capacity characteristic of silicon-based materials but possessing superior cycling stability, is a promising anode material for next-generation Li-ion batteries. SiO is typically applied in combination with graphite (Gr), but the limited cycling durability of the SiO/Gr composites curtails large-scale applications. In this work, this limited durability is demonstrated in part related to the presence of a bidirectional diffusion at the SiO/Gr interface, which is driven by their intrinsic working potential differences and the concentration gradients. When Li on the Li-rich surface of SiO is captured by Gr, the SiO surface shrinks, hindering further lithiation. The use of soft carbon (SC) instead of Gr can prevent such instability is further demonstrated. The higher working potential of SC avoids bidirectional diffusion and surface compression thus allowing further lithiation. In this scenario, the evolution of the Li concentration gradient in SiO conforms to its spontaneous lithiation process, benefiting the electrochemical performance. These results highlight the focus on the working potential of carbon as a strategy for rational optimization of SiO/C composites toward improved battery performance.