Single-ion conducting polymer electrolytes (SIPEs) are among the most promising candidates for lithium–metal batteries, which commonly suffer from the risk of lithium dendrite growth and continuous electrolyte decomposition at the lithium–electrolyte interface. Herein, we report the introduction of lithium nitrate (LiNO$_3$) as an additive into an SIPE based on a poly(ethylene oxide) backbone and grafted perfluorosulfonate anions. By comparing SIPEs with and without LiNO$_3$ concerning the charge transport, electrochemical stability, reversibility of the lithium stripping/plating process, electrode morphology, and surface composition, it appears that the introduction of LiNO$_3$ yields a thinner, but more robust solid electrolyte interphase, which greatly benefits the eventual performance, Coulombic efficiency, and reversibility of lithium stripping and plating. The advantageous effect of LiNO$_3$ is finally confirmed in Li|SIPE|LFP solid-state battery cells, providing a substantially longer cycle life.