NCM (Ni, Co, Mn Oxide) cathode powder was treated by the gas containing fluorine that is activated by the DBD (Dielectric Barrier Discharge) plasma. Up to 80% of the residual impurities (LiOH, Li₂CO₃) in NCM powder were removed and a thin film was newly formed on each powder surface at the same time. The major component of the film was identified as fluorine atom by the XPS and SEM-EDX analysis results and the DSC analysis showed that the treatment enhanced the thermal stability of the cathode powder. At the early stage of the treatment, a lot of film seeds were formed on the powder surface. As the treatment time went on, the seeds grew to a complete film and was getting thicker. After the 20 minutes of excessive treatment, the film surface was getting rougher and the thermal stability was getting worse at high temperature range. The reduced charge/discharge capacities of the treated cathode powder was less than about 1.6%, regardless of the treatment time.