Study On Cathode Na₃V₂?PO₄?₃/C Composites For Sodium Ion Batteries Synthesized By Ultrasonic-assisted Solution Combustion

Na₃V₂?PO₄?₃ with NASICON frame structure has become one of cathode alternatives for NIBs because of its high ion mobility,high voltage platform and high energy storage capacity.Solution combustion synthesis is a technique for preparing nanocrystalline/fine grain materials.However,Na₃V₂?PO₄?₃ fabricated by traditional solution combustion synthesis has the disadvantages of serious particle agglomeration and difficulty in controlling the morphology and structure.Ultrasonic can accelerate and control chemical reaction and improve the microstructure of the synthesized materials.Herein,we combine ultrasonic technology with solution combustion synthesis to explore the effects of ultrasound on the synthesis of Na₃V₂?PO₄?₃/C and Na₃V₂?PO₄?₃/C/G.And the effects of key parameters on the structure,morphology and electrochemical properties of Na₃V₂?PO₄?₃/C and Na₃V₂?PO₄?₃/C/G are studied.The results show that ultrasonic-assisted solution combustion synthesis can prepare three-dimensional porous Na₃V₂?PO₄?₃/C composites,in which the diameter of pores is about 1?m.The porous composites have a high specific capacity of 118 mAhg^-1at 0.1 C,good cycling stability of 93.8%of the initial capacity after 120 cycles at 0.2 C and good rate performance of the specific capacity being 85.3 mAhg^-11 at 2 C.The analysis of the products taken from different stages of ultrasonic-assisted solution combustion shows that the formation process of Na₃V₂?PO₄?₃ is not changed by ultrasound,but ultrasound can introduce a large amount of cavitation bubbles in the precursor solution.The mechanical effect of ultrasonic and shock wave and liquid micro-circulation that generate when cavitation bubble collapse occurs can lead to a strong mechanical agitation,which is helpful to eliminate the large bubbles resulting from the decomposition of nitrates and citric acid.Thus,a homogeneous mesoporous honeycomb structure is obtained.Na₃V₂?PO₄?₃/C/G composites are successfully prepared by ultrasonic-assisted solution combustion synthesis.The surface of Na₃V₂?PO₄?₃ particles is first coated with an amorphous carbon layer,then coated with a graphene layer.With the increasement of graphene content,the material is transformed from three-dimensional reticular structure to flake structure.By optimizing the graphene content?GO=5 wt%?and C₆H₈O₇ content??=0.8?,Na₃V₂?PO₄?₃/C/G composites with good electrochemical properties can be obtained.The Na₃V₂?PO₄?₃/C/G delivers an initial discharge capacity of 120.7 mAhg^-11 at 0.5 C and117.5 mAhg^-11 at 1 C.After 300 cycles at 1 C,it can retain 79.3%of the initial capacity.In the rate of 10 C,it can deliver a discharge capacity as high as 100.1 mAhg^-1.