Ion-selective membranes with high efficiency and long-term cycling stability at high current densities are critical for the widespread application of vanadium redox flow batteries (VRFBs). In order to improve the selectivity between the protons and vanadium ions, a novel composite membrane consisting of a perfluorinated sulfonic acid ionomer (Nafion) and (2,2,6,6-etramethylpiperidin-1-yl)oxyl radical on silica gel (Si-TEMPO) was developed by a ball-mill method for good dispersion of SiTEMPO in the polymer matrix. When a VRFB is charged, the N-O center dot free radical of Si-TEMPO can be converted into a positively charged N+= O where VO2+ and VO2+ exist, and it effectively reduces the vanadium ion crossover across the membrane due to the Donnan exclusion effect. Therefore, the highest energy efficiency (74.0%) at a current density of 160 mA/cm(2) was obtained from the composite membrane with 6 wt % Si-TEMPO due to the effective ion selectivity during the charging-discharging VRFB operation (69.3% for the recast Nafion). Moreover, it showed excellent discharge capacity retention and high efficiency even after the long-erm VRFB operation of 500 cycles compared to the recast Nafion membrane. These results indicate that the Nafion/Si-TEMPO composite membranes have the potential to enhance the electrochemical performances for VRFB applications.