Silicon is considered as a promising anode material to improve the energy density of lithium ion batteries. However, the huge capacity loss in the initial cycles not only consumes the active Li from the cathode, but also shows great influence on the long-term cycle stability for the varied negative/positive capacity ratio (N/P ratio) that may lead to over-charge/discharge, thus hinders silicon from commercial application. Therefore, it's of vital importance to study the N/P ratio to realize the tradeoff between initial coulombic efficiency (ICE) and cycling stability. LiNi0.8Co0.15Al0.05O2||SiO x /Graphite pouch cells with different N/P ratios between 0.85 and 1.8 are designed and prepared in this paper, electrochemical tests prove that a N/P ratio close to unity provide the best balance between energy density and cycle performance. Moreover, capacity decay mechanisms in cathode and anode are carefully studied, which demonstrates that up to 90% of the capacity loss is due to the active Li host sites loss in anode, while the crosstalk between the two electrodes also must be considered.