NASICON is one of the most promising sodium solid electrolytes that can enable the assembly of cheaper and safer sodium all-solid-state batteries. In this study, we perform a combined experimental and computational investigation into the effects of aliovalent doping in NASICON on both bulk and grain boundary (secondary phase) ionic conductivity. Our results show that the dopants with low solid solubility limits in NASICON lead to the formation of a conducting (less insulating) secondary phase, thereby improving the grain boundary conductivity measured by electrochemical impedance spectroscopy (including grain-boundary, secondary-phase, and other microstructural contributions) that is otherwise hindered by the poorly-conducting secondary phases in undoped NASICON. This is accompanied by a change in the Si/P ratio in the primary NASICON bulk phase, thereby transforming monoclinic NASICON to rhombohedral NASICON. Consequently, we have synthesized NASICON chemistries with significantly improved and optimized total ionic conductivity of 2.7 mS/cm. More importantly, this study has achieved an understanding of the underlying mechanisms of improved conductivities via doping (differing from the common wisdom) and further suggests a new general direction to improve the ionic conductivity of solid electrolytes via simultaneously optimizing the primary bulk phase and the microstructure (including grain boundary segregation and secondary phases). (C) 2017 Elsevier B.V. All rights reserved.