Synthesis and electrochemical performance of Na 3− x Li x V 2 (PO 4 ) 3 /C as cathode materials for Li‐ion batteries

Summary Herein, a series of lithium‐sodium hybrid ion Na 3− x Li x V 2 (PO 4 ) 3 /C ( x = 2.5, 2.0, 1.5, 1.0, 0.5) as cathode materials are prepared by sol‐gel method. The lithium storage properties of Na 3− x Li x V 2 (PO 4 ) 3 /C reveal that the lithium sodium ratio has a strong influence on material structure and the electrochemical properties. The multiple voltage platforms are observed at charge and discharge curve for the Na 0.5 Li 2.5 V 2 (PO 4 ) 3 material, as it contains rhombohedral and monoclinic Li 3 V 2 (PO 4 ) 3 phases, whereas a single voltage platform can be observed when x is less than 2, which are mixtures of Na 3 V 2 (PO 4 )3 with rhombohedral structure and Li 3 V 2 (PO 4 ) 3 with rhombohedral structure. Notably, NaLi 2 V 2 (PO 4 ) 3 /C prepared by the calcination at 700°C for 8 hours contains a single rhombohedral phase. And when the charging and discharging voltage at the range 2.5 to 4.5 V and current density at 0.5C, NaLi 2 V 2 (PO 4 ) 3 /C and Na 2.5 Li 0.5 V 2 (PO 4 ) 3 /C are prepared by the pre‐calcination at 400°C for 4 hours and subsequent calcination at 750°C for 8 hours exhibit high first‐cycle specific discharge capacities of 124.8 and 130.0 mAh·g −1 . The capacities are 118.4 and 115.1 mAh·g −1 after 50 cycles, indicating excellent capacity retention.