Synthesis and Characterization of Apatite-Type Lanthanum Silicate Electrolyte Codoped With Cu and Nb

This study successfully synthesized Cu and Nb codoped apatite-type lanthanum silicate solid electrolyte materials, La_8.83Cu_0.5Si_6-xNb_xO_{25.75 + 0.5x} (x = 0.0, 0.1, 0.2, 0.3, 0.4), using the urea-nitrate combustion method. The crystal structure and elemental valence states of the materials were systematically analyzed by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The results indicated that all samples maintained a pure phase with the P6₃/m structure. Building upon the partial substitution of La³⁺ by Cu⁺, Nb⁵⁺ successfully substituted for part of Si⁴⁺, forming [Si(Nb)O₄] tetrahedra. Scanning electron microscopy (SEM) and density tests showed that appropriate Nb doping (x = 0.2) significantly enhanced the densification of the material, achieving a relative density of 83.2%. Electrochemical impedance spectroscopy revealed that at 600°C, La_8.83Cu_0.5Si_5.8Nb_0.2O_25.85 exhibited the highest conductivity (2.29 × 10^-3 S/cm) and the lowest activation energy (0.858 eV). This conductivity is approximately 8.5 times higher than that of the unmodified pure La_9.33Si₆O₂₆ (2.71 × 10^-4 S/cm) and about 2.2 times higher than that of the singly Cu-doped La_8.83Cu_0.5Si₆O_25.75 (1.02 × 10^-3 S/cm). Structural defect analysis further revealed that Cu²⁺ substitution for La³⁺ introduces oxygen vacancies, while Nb⁵⁺ substitution for Si⁴⁺ introduces interstitial oxygen. Their synergistic effect forms an "oxygen vacancy-interstitial oxygen synergistic conduction" mechanism, significantly enhancing the oxygen ion migration capability.