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

A series of Cr³⁺-substituted Na_1+xTi_2−xCr_xP₃O₁₂ (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. Na_1.3Ti_1.7Cr_0.3P₃O₁₂ electrolyte can obtain the best ionic conductivity of 6.13×10⁻⁶ S/cm at room temperature, which is 8 times that of the undoped NaTi₂(PO₄)₃ electrolyte.