Five electrolytes based on Ce_0.8Nd_0.2O_1.9 with or without 1000 ppm SiO₂ (NDC and NDCSi) are synthesised via a sol–gel process. Composite electrolytes are prepared by mixing NDC and NDCSi sols with 10 wt% precalcined La_0.9Sr_0.1Ga_0.9Mg_0.1O_3−δ (LSGM) powders. The structure and ionic conductivity of these compounds are investigated by X-ray diffraction, field-emission scanning electron microscopy, and alternating current impedance spectroscopy. The result indicated that the LSGM additives can significantly promoted the grain boundary conductivity of the composite electrolytes. The total conductivity of the composite electrolytes significantly increased with increasing grain boundary conductivity. This paper thereby proposes LSGM as new scavenger materials able to mitigate the harmful effects of SiO₂ impurities on grain boundary conduction in NDCSi. Indeed, these additives substantially increase both the grain boundary and the total conductivity of NDCSi.