The electrochemical properties of layered Li-rich oxide Li1.2Ni0.13Co0.13Mn0.54O2 (LLO) can be enhanced by a facile NaF surface modification. We have clarified by XPS, HR-TEM and HAADF-STEM analyses that the mechanism is the formations of a gradient coating layer Na1-xLixF and a gradient LLO surface Li1.2-xNaxNi0.13Co0.13Mn0.54O2 via a Li+/Na+ exchange reaction. The exchange reaction occurs at the Li-slabs of LLO down to a depth of less than 15 nm. After such a double gradient surface modification, the LLO displays markedly improved cycle life and rate capability as well as suppressed voltage decay. It exhibits a capacity retention of 85% beyond 1000 cycles at 10 C and a voltage decay of only 0.0021 V per cycle at 0.5 C, revealing great application prospects in energy storage materials.