Improvement of crystal structure and electrical performance of NASICON-type NaTi2(PO4)3 solid electrolyte by doping Cr3+ions

A series of Cr3+-substituted Na1+xTi2-xCrxP3O12 (x=0.1, 0.2, 0.3, 0.4, 0.5, molar fraction) solid electrolytes were prepared by the solid-phase reaction method. The effects of Cr3+ ions on the phase composition, microstructure, and electrochemical ion conductivity of Na-based conductors were studied using X-ray powder diffraction, field emission scanning electron microscopy, and AC impedance measurement. The results show that the main crystal phase of NaTi2(PO4)3 is formed in the solid electrolytes. The substitution of Ti4+ sites by 15 at.% Cr3+ ions contributes to the enhancement of electrical conductivity, which is attributed to the combined effect of suppressing the formation of impurity phases, broadening ion channels, and improving the bonding degree of grains. Na1.3Ti1.7Cr0.3P3O12 electrolyte can obtain the best ionic conductivity of 6.13x10-6 S/cm at room temperature, which is 8 times that of the undoped NaTi2(PO4)3 electrolyte.