High Rate Performance of Na₃V_2-xCu_x(PO₄)₃/C Cathodes for Sodium Ion Batteries

The Na 3 V 2-x Cu x (PO 4 ) 3 /C (x = 0, 0.01, 0.03, 0.05 and 0.07) cathodes have been prepared by ballmilling and spray-drying assisted high-temperature solid-state reaction. The effects of Cu doping on Na 3 V 2 (PO 4 ) 3 /C are characterized by XRD, SEM, XPS, EPR and electrochemical measurements. The results show that the Na 3 V 1.95 Cu 0.05 (PO 4 ) 3 /C delivers the highest discharge capacity of 90.6 mAh g −1 at 50 C and superior cyclic retention of 88.2% after 2000 cycles at 20 C, which could be mainly attributed to the high electronic conductivity (7.2 × 10 −2 S cm −1 ) and Na ion diffusion coefficient (1.695 × 10 −12 cm 2 s −1 ). The introduction of Cu 2+ into the Na 3 V 2 (PO 4 ) 3 /C lattice produces oxygen vacancies with accompanying conduction electrons, which lead to the enhanced electronic conductivity and Na ion diffusion coefficient. Besides, inactive Cu 2+ acts as a pillar to reduce the negative effects caused by the volume shrinking/swelling during the Na + extraction/insertion, resulting in increased cycle stability of the Cu-doped Na 3 V 2 (PO 4 ) 3 system.