  As typical solid-state electrolytes (SSEs),
{Na}$_{1+x}${Zr}$_2${Si}$_{x}${P}$_{3-x}${O}$_{12}$ NASICONs provide an ideal
platform for solid-state batteries (SSBs) that display higher safety and
accommodate higher energy densities. The critical points for achieving SSBs
with higher efficiencies are to improve essentially the ionic conductivity and
to reduce largely the interfacial resistance between SSEs and cathode
materials, which would necessitate extremely high level of craftsmanship and
high-pressure equipment. An alternative to higher-performance and lower-cost
SSBs is additive manufacturing. Here, we report on an effective additive,
MgF$_2$, which was used in synthesizing NASICONs, resulting in SSEs with fewer
defects and higher performance. With an addition of mere 1 wt$\%$ MgF$_2$
additive, the total room-temperature ionic conductivity of the NASICON
electrolyte reaches up to 2.03 mS cm$^{-1}$, improved up to $\sim$ 181.3$\%$,
with an activation energy of 0.277 eV. Meanwhile, the stability of the Na
plating/stripping behavior in symmetric cells increases from 236 to 654 h. We
tried to reveal the microscopic origins of the higher ionic conductivity of
MgF$_2$-doped NASICONs by comprehensive in-house characterizations. Our study
discovers a novel MgF$_2$ additive and provides an efficient way to prepare
higher-performance SSEs, making it possible to fabricate lower-cost SSBs in
industries.
