Lithium-rich layered oxides are promising cathode materials for lithium-ion batteries due to their high reversible capacities (more than 250 mAh g−1). Nevertheless, in operation, the oxygen lattice would be transformed into O2 gas with phase transformation. Thus, to suppress O2 gas, it is necessary to pre-generate the oxygen vacancies in the surface of materials. Herein, for facile and scalable pre-generation of oxygen vacancies, pristine Li2MnO3·LiNi1/3Co1/3Mn1/3O2 oxide (PLR-NCM) is treated directly by CO2 gas just at the room temperature. The modified material (MLR-NCM), with rich oxygen vacancies in the surface and no obvious structural change inside, shows a discharge capacity of 321 mAh g−1 based on half-cells at 55 °C and 0.1 C (1.0 C = 250 mA g−1), and 240 mAh g−1 as full cells at the room temperature. In addition, it also exhibits high cycling and rate performance owing to the significantly improved lithium ion and electron diffusion efficiencies. Importantly, by real-time monitoring of structural evolution using in situ XRD technique, we find that the O2 gas release of the modified material is successfully suppressed. This facile method proposed in our work provides a new strategy for greatly improving the performance of lithium-ion batteries.