Continuous‐Flow Synthesis of Nanostructured Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Cathode With Superior Rate Capability for Fast‐Charging Sodium‐Ion Batteries

Current synthetic methods for nanostructured Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) with favorable fast-charging ability mainly rely on batch reactors that still suffer from limitations such as small-scale production, manual operating requirements, and challenges in batch-to-batch reproducibility. Herein, two editions of continuous-flow millifluidic synthetic devices are rationally designed for scalable and controllable production of NFPP nanostructures in high-boiling solvents. Merely 8 min of reaction time in the first-edition helical coil reactor achieves the optimal balance between yield and electrochemical performance of the resulting NFPP cathode. The as-synthesized NFPP/C-8 cathode delivers an initial capacity of 89.6 mAh g^-1 that maintains 88.5% over 2000 cycles at a high rate of 20 C. An optimized multi-turn pancake coil reactor that accommodates up to 389.5 mL of solution is further designed, resulting in 116.8 g h^-1 of throughput that reaches a 46-fold increase compared to that of a conventional batch reactor, finally producing ≈120 g of NFPP-LS in this work. Impressively, the full cell matching NFPP-LS cathode and hard carbon anode offers a reversible capacity of 70.7 mAh g^-1 with 77.5% of retention after 200 cycles, even at 20 C, ranking the leading state among the current polyanion-based full batteries.