Tailored\nSynthesis\nof High-Performance Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> for Na-Ion Batteries through\nthe Sol–Gel Process

Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> is a typical\ncathode material for sodium-ion batteries and is usually prepared\nby the sol–gel method. However, Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> is far from the level of tailored preparation\ndue to the lack of understanding of the structure evolution during\nthe sol–gel process. Here, we revealed that the initially formed\nvanadyl citrate is cross-linked by intermolecular hydrogen bonds,\nforming large polymers. Therefore, the crystal size of the precursor\nas well as the final Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> can be controlled by adjusting the pH value of the solution.\nAs a result, the optimized NVP-7 material exhibited a specific discharge\ncapacity of 59.8 mAh g<sup>–1</sup> at 50 C and can still retain\n82.9% of the initial capacity after 500 cycles at 1 C. This study\nprovides a theoretical basis for controllable preparation of NVP materials\nby the sol–gel method.