  The electrochemical gating technique is a powerful tool to tune the
\textit{surface} electronic conduction properties of various materials by means
of pure charge doping, but its efficiency is thought to be hampered in
materials with a good electronic screening. We show that, if applied to a
metallic superconductor (NbN thin films), this approach allows observing
reversible enhancements or suppressions of the \emph{bulk} superconducting
transition temperature, which vary with the thickness of the films. These
results are interpreted in terms of proximity effect, and indicate that the
effective screening length depends on the induced charge density, becoming much
larger than that predicted by standard screening theory at very high electric
fields.
