A mixed polyanionic Na4Fe3(PO4)2(P2O7)/C nanocomposite is synthesized via a sol-gel route. The phosphate raw material is transformed to the mixed phosphate-pyrophosphate with high phase purity via a self-condensation reaction at 500 °C. Na4Fe3(PO4)2(P2O7)/C can deliver an initial capacity of 110 mAh g−1 at 0.05C with the average discharge voltage approaching 3.1 V. The nanocomposite shows excellent rate capability because of the presence of an in-situ formed 3-D network of carbon. At 10 C rate, the nanocomposite delivers a discharge capacity of 78 mAh g−1 at 25 mAh °C and 82 mAh g−1 at 55 °C. The nanocomposite has a good long-term cycling stability, retaining 89% of the initial discharge capacity after 300 cycles. In-situ XRD results demonstrate that the sodium insertion/extraction process in Na4Fe3(PO4)2(P2O7) is an imperfect solid-solution reaction with an obvious local lattice distortion instead of an ideal solid-solution reaction. Using a solid-state NMR technique, it is further found that the sodium extraction from the Na1, Na3, and Na4 sites causes an obvious change in local structure. However, the local structure of Na2 remains unchanged, which may aid the stability of the host structure.