Enhanced electrical conductivity of P-doped apatite-type lanthanum silicate solid electrolytes and analysis of the mechanism

Mono-doped lanthanum silicate solid electrolyte ceramics with phosphorus was successfully prepared by using the urea-nitrate combustion method at 600 °C. The crystal structure and morphology were analyzed by scanning electron microscope (SEM), infrared spectrometer (IR), and X-ray diffractometer (XRD), and the results showed that the successful doping of phosphorus into the lanthanum silicate electrolyte did not destroy the unique crystal structure of the space group P63/m, and the best-sintered morphology of lanthanum silicate was achieved at 1500 °C. Meanwhile, the impedance test was conducted at 600 °C using electrochemical impedance spectroscopy (EIS), and the experimental data showed that with the increase of phosphorus doping, the conductivity of La9.33Si6-xPxO26+x/2 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8) first increased and then decreased. Also, at x = 0.4, the conductivity of the electrolyte reached a maximum of 7.27 mS/ cm (800 °C), which is because when phosphorus enters the Si site in the lanthanum silicate crystal structure, interstitial oxygen is generated in the 63 channel of the structure to maintain the electrovalent balance, and the increase in the concentration of interstitial oxygen increases the conductivity of the electrolyte. Therefore, an enhanced conductivity mechanism with defects in interstitial oxygen concentration is proposed.