The electrical behaviors of the melilite-like solid solutions Na2Co(P2−xAsx)O7 (x = 0.5; 1): effect of the P/As substitution on structure and electrical properties

The melilite-like Na2CoP2O7 diphosphate, due to its structural features, is a promising material for sodium-ion battery cathodes. Aiming at improving the conduction properties of this phase, the P5+ ions were partially substituted by As5+ ones, and two Na2Co(P2−xAsx)O7 (x = 0.5 and x = 1) solid solutions were synthesized. The purity of the obtained solid solutions was confirmed by powder X-ray diffraction, and their electrical properties were investigated using complex impedance spectroscopy. The effects of the P/As substitution on sintering temperature, relative density and ionic conductivity were studied. The mean particle size was reduced by ball milling. The sintering temperature decreased from 680 to 640 °C from the pure phosphate to the arsenic richest solid solution, respectively, and the relative density of the pellets obtained after 120-min ball milling of the three samples showed an increase from 90 to 94%. The conductivity of the samples with the same relative density 90% was compared. The main effect of the substitution is a strong increase in the electrical conductivity, by about 104, and a significant decrease in the activation energy. Further studies involving simulations will be carried out to correlate these observations with the structural features of the substituted phases. The obtained results may be significant for overall sodium-ion battery performance.