Role of NH4+/H3O+ on the Na+ storage performance for aqueous-synthesized Na3(VOPO4)2F cathode materials and their removal

The aqueous preparation of Na₃(VOPO₄)₂F cathode material with low cost and good structural stability has attracted extensive attention for advancing sodium-ion batteries (SIBs). However, the inclusive heterogeneous cations incorporated into the material lattice, dominated by coordination chemistry, are always overlooked. Herein, the embroiled NH₄⁺/H₃O⁺ cations in the Na₃(VOPO₄)₂F lattice have been first disclosed during aqueous co-precipitation. It involves the electrostatic interactions between hydrogen protons (NH₄⁺/H₃O⁺) and electronegative oxygen atoms (V=O and V–O–P groups), which induces the terrible Na⁺-storage performance, as demonstrated by multiple characterizations. Followingly, the very-facile operation, i.e. heat treatment, has been raised to remove NH₄⁺/H₃O⁺ cations and then achieved high-performance Na₃(VOPO₄)₂F. Therefore, the Na₃(VOPO₄)₂F||Na cell contributes to the significantly improved discharge capacity (129.7 mAh g⁻¹) and voltage plateau from 3.63 to 3.87 V (vs. Na/Na⁺) at 0.2 C. The ultrahigh capacity retentions of 93.7% and 76.7% after 1000 and 3500 cycles at 1 and 20 C rates under 25 °C are harvested, respectively, as well as high/low-temperature performances and rate capability. Eventually, the as-assembled Na₃(VOPO₄)₂F||hard carbon full-cell delivers excellent long-term cycling stability over 1000 cycles with 97.5% retention at 3 C. These emphasize the high-efficacy synthesis of Na₃(VOPO₄)₂F and provide insights into the aqueous co-precipitation for the development of materials used in SIBs.