Excellent cycle stability of P2 Na0.67Co0.25Mn0.705V0.045O2@ NaV6O15 composite cathode for sodium ion battery

Abstract Na 0.67 Co 0.25 Mn 0.705 V 0.045 O 2 @NaV 6 O 15 composite cathode for sodium-ion batteries was prepared by solid state reaction via V-doping in order to improve the cycle stability of P2-type materials. The results confirm that V-ions are successfully doped into Na Co Mn O lattice, and the layered P2-type structure with P6 3 /mmc space group remains unchanged after V-doping. The composite Na 0.67 Co 0.25 Mn 0.705 V 0.045 O 2 @NaV 6 O 15 cathode presents the maximum discharge capacity of 122.84 mAh g −1 within 2.0–4.0 V at a current density of 20 mA g −1 with an excellent retention of 88.35% after 100 cycles. The retention of the composite is 62.39% higher than that of Na 0.67 Co 0.25 Mn 0.75 O 2 oxide, which might be attributed to a-axis expansion and the stronger bond energy of V O than that of Mn O and Co O. The composite cathode also shows superior electrochemical dynamic properties, which is probably ascribed to c-axis expansion and the increase of the electronic conductivity and disorder of the transition metal in layers by supervalent V-ions doping. The composite technique through element doping is a promising strategy for the development of P2-type oxide electrodes for sodium ion batteries.