High voltage high energy cathode materials for Li-ion battery are highly needed. High nickel content NMC (e.g. NMC811) is believed to be a promising candidate. When raised the upper cutoff voltage of NMC811 cathode to 4.5 V vs. Li⁺/Li, the cathode materials could deliver a capacity above 215 mAh g^-1, which is very attractive to the electric vehicle application. However, raising the upper cutoff voltage will also greatly accelerate the capacity fade of the full cell. Of the various causes proposed by the battery community, a large volume of gases generated at high voltage is believed to be one of the major issues. In this study, we constructed full pouch cells composed of graphite anode, NMC811 cathode and standard GEN2 electrolyte which is 1.2M LiPF₆ in EC/EMC (3/7 by weight). We also constructed anode and cathode symmetric pouch cells to investigate the gas evolution and crosstalk at specific sides. Using Archimedes’ Principle, we measured the generated gas volume along cycles within the pouch cells with different cutoff voltages. We found out that the full cells using 4.2 V cutoff voltage barely generated gases along the course of 100 cycles while the ones using 4.4 V cutoff voltage generated a large amount of gases in the first 20 cycles, but the gases would be gradually consumed in the subsequent cycles. By using GC-MS, we determined the main gas species generated in the full cell to be carbon dioxide and alkanes. Moreover, the relative amount of carbon dioxide decreased along cycles while the amount of alkanes would increase. In cathode symmetric cells, CO₂ remained the major component after 100 cycles and its signal was much stronger than in full cells. Interestingly, some fluorinated alkanes not seen in the full cells were also identified in cathode symmetric cells, suggesting potential involvement and loss of LiPF₆ from electrolyte due to the parasitic side reactions on the cathode. In anode symmetric cells, hydrocarbons attributed to electrolyte decomposition were the main products. Unlike CO₂ and fluorinated alkanes formed on the cathode, these species were relatively stable and remained in the full cell after extensive cycling. It was also shown that gas crossover to the anode leads to the depletion of gaseous products during cycling, which could stabilize the cell cycle performance. Although some inactive surface reconstruction layer was formed on the cathode at high voltage operation, impedance buildup and active lithium inventory loss contributed to the significant capacity loss in the full cell. Our study revealed the important role of crossover effect in stabilizing Ni-rich layered oxide cathode, and provided a better understanding on capacity fading to take more effective measures for improving the full cell cycle life.