Herein, a systematic surface modification approach via double gas (SO2 and NH3) treatment at elevated temperatures is taken to achieve a stable electrochemical performance for Li and Mn-rich NCM cathode materials 0.33Li2MnO3•0.67LiNi0.4Co0.2Mn0.4O2 (HE-NCM). Partial surface reduction of Mn+4 and the formation of a modified interface comprising Li-ions conductive nano-sized Li2SO4/Li2SO3 phases are established. Prolonged cycling performance in Li coin-cells demonstrates significantly improved capacity retention (~2.2 times higher than the untreated sample) for the double-gas-treated cathodes after 400 cycles at a 1.0 C rate. Stable discharge potential and lower voltage hysteresis during cycling were also achieved. The comparative electrochemical studies in full-pouch cells [vs. Graphite anode] also demonstrates considerably stabilized electrochemical behavior for the double-gas-treated HE-NCM than the untreated sample. Lower gases (O2, CO2, and H2) evolution in the first charge-discharge cycle and better thermal stability are indeed two crucial achievements of this treatment. The electrodes` post-cycling investigation established integral morphological integrity of the gas-treated cathode materials and lower transition metals (TMs) dissolution from the active cathode. The positive effects of double-gas treatment are related to the modified surface, lessening the undesirable side reactions at the electro-electrolyte interphase with the battery solutions.