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The cold sintering process (CSP) and Bi2O3-activated liquid-phase sintering (LPS) are combined to densify Mg-doped NASICON (Na3.256Mg0.128Zr1.872Si2PO12) to achieve high densities and conductivities at reduced temperatures. As an example, a cold-sintered specimen with the addition of 1.1 wt % Bi2O3 sintering additive achieved a high conductivity of 0.91 mS/cm (with ∼96% relative density) after annealing at 1000 °C; this conductivity is >70% higher than that of a cold-sintered specimen without adding the Bi2O3 sintering additive, and it is >700% of the conductivity of a dry-pressed counterpart with the same amount of Bi2O3 added, all of which are subjected to the same heating profile. The highest conductivity achieved in this study via combining CSP and Bi2O3-activated LSP is  > 1.5 mS/cm. This study suggests an opportunity to combine the new CSP with the traditional LPS to sinter solid electrolytes to achieve high densities and conductivities at reduced temperatures. This combined CSP-LPS approach can be extended to a broad range of other materials to fabricate the “thermally fragile” solid electrolytes or solid-state battery systems, where reducing the processing temperature is often desirable. Keywords: Cold sintering process, Liquid-phase sintering, NASICON, Solid electrolytes, Solid-state sodium-ion battery