The control of voltage window has been considered as a universal strategy in improving the cycling stability of cathode materials, which is supposed and explained by avoiding side reactions. To address the conjecture, the detailed structure evolution of Na0.76V6O15 nanorods is investigated with different electrochemical reaction voltage windows. High time resolution in situ X-ray diffraction, ex situ X-ray photoelectron spectroscopy, ex situ Raman spectroscopy, and transmission electron microscopy demonstrate the amorphization of Na0.76V6O15 nanorods and formation of ionized oxygen in nanorods, leading to the increased polarization voltage and fast capacity fading. The amorphization and diffusion of ionized oxygen in nanorods is controlled by optimizing the voltage window, resulting in the great increase of capacity retention from 26% to 80%. It is demonstrated that controlling the voltage window and corresponding ionized oxygen diffusion can mitigate the fast capacity fading to achieve long lasting lithium ion batteries.