Here, we elucidate the mechanisms of phase transition of the Na3V2(PO4)3 -Na1V2(PO4)3 system including the intermediate Na2V2(PO4)3 phase, utilizing V 4+/3+ redox couple. We also investigate the Na3V2(PO4)3 -Na4V2(PO4)3 system involving the V 3+/2+ redox couple. The intermediate Na2V2(PO4)3 phase was detected by operando X-ray synchrotron diffraction as well as predicted by theoretical calculations. Here, we report for the first time on the crystal structure of the Na2V2(PO4)3 phase, compared in detail with those of NaxV2(PO4)3 (x = 1, 3, 4). The intermediate Na2V2(PO4)3 phase appears as reducing the lattice mismatch between Na3V2(PO4)3 and Na1V2(PO4)3 phases, and as the Na (1) and Na(2) sites split into several positions with different Na + distributions, including vacancies. This study is highly relevant to better understand the behavior of fast-rate electrode materials that display transient phases, such as the intermediate LixFePO4 phases (x = 0.6-0.75) detected during nonequilibrium battery operations. Experimental Synthesis: The carbon-coated Na3V2(PO4)3 was prepared by a sol-gel-assisted solid-state reaction. Na2CO3 (Sigma-Aldrich, 99.5 %), C10H14O5V (Sigma-Aldrich, 97 %), NH4H2PO4 (Sigma-Aldrich, 98.5 %), and citric acid (Alfa Aesar, 99 %) in a molar ratio of 1.5:2:3:2, were dissolved in deionized water and ethanol (50/50 by volume). The solution was stirred overnight at 80 °C in an oil bath before being dried in oven. The obtained powder was ground and pre-heated at 400 °C for 4 h in Ar. The resulting powder was then mixed in a mortar and annealed at 700 °C for 10 h under Ar/H2 (95/5) atmosphere.