High nickel content layered oxide LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) is a promising cathode material with a high theoretical capacity of 200 mAh g⁻¹ for use in high energy density lithium-ion batteries. However, its surface can easily get passivated by LiOH and Li₂CO₃ due to its surface residual Li₂O being reacting with ambient moisture and CO₂. Herein, NMC811 was treated in a 3.0 M solution of lithium bis(fluorosulfonyl)imide (LiFSI) in dimethyl carbonate (DMC) at 60 °C for 8 h, 16 h and 24 h, respectively, resulting in coating of the NMC811 surface with LiF due to the basic nature of those residual lithium salts that react with LiFSI. The facile treatment of NMC811 in LiFSI/DMC not only improves the cycling stability but also enhances the capacity of the Li/NMC811 cells, mainly because of the thinning of the Li₂CO₃ layer as suggested by cyclic voltammograms and impedance analyses. This method opens a new avenue for activation of passivated NMC811 particles for practical battery applications.