Electrical Conductivity and Electrochemical Characteristics of Na3V2(PO4)3-Based NASICON-Type Materials

NASICON-type materials with the compositions Na3V2-x Al (x) (PO4)(3), Na3V2 - x Fe (x) (PO4)(3), Na3 + x V2-x Ni (x) (PO4)(3), and Na3V2 - x Cr (x) (PO4)(3) (x = 0, 0.03, 0.05, and 0.1) have been prepared and characterized by X-ray diffraction analysis, electron microscopy, and impedance spectroscopy. The results demonstrate that the highest electrical conductivity among the samples studied is offered by the material doped with 5% Fe: Na3V1.9Fe0.1(PO4)(3). The activation energy for low-temperature conduction in the doped materials decreases from 84 +/- 2 to 54 +/- 1 kJ/mol and that for high-temperature conduction is similar to 33 kJ/mol. The discharge capacity of Na3V1.9Fe0.1(PO4)(3)/C under typical working conditions of cathodes of sodium ion batteries has been shown to exceed that of Na3V2(PO4)(3)/C. The capacity of the more porous material prepared by the Pechini process (Na3V1.9Fe0.1(PO4)(3)/C-{II}) approaches the theoretical one at a low charge-discharge rate and retains its high level as the charge rate is raised (its discharge capacity was 117.6, 108.8, and 82.6 mAh/g at a discharge rate of 0.1C, 2C, and 8C, respectively).