<p>Solid-state electrolytes with high ionic conductivity are crucial for high-capacity and safer electrochemical devices such as all-solid-state batteries. Interface effects in ion conductor-insulator nanocomposites have recently emerged as an attractive approach to improve the often poor ionic conductivity of solid electrolytes, a major limitation to their applications. The origin of profound increases in ionic conductivity at ion conductor-insulator interfaces is not yet well understood. Herein, we show that in oxide-confined nitrates (LiNO₃, NaNO_3, and NaNO₂) model systems, the increased ion conductivity is highly influenced by the acidity of the metal oxides. By confining these ionic compounds in the mesopores of oxides with different acidity, their ionic conductivity increased by over three orders of magnitude depending on the oxide scaffold used. Remarkably, the conductivity scales with the density or the strength of the acid sites of the metal oxides. This new insight will aid the development of novel high-conducting nanocomposite solid electrolytes for next-generation batteries.</p>