Secondary reactions and solid-electrolyte interface (SEI) formation are crucial aspects for battery lifetime. We show that one part of a natural SEI consists of crystalline NaH, which is formed on the sodium surface when carbonate-based electrolytes are used. Its impact on the electrochemical performance was studied using room-temperature H₂-treated Na anodes and a NaH-Na composite anode. Depending on the preparation conditions, hydrogen was stored on the Na surface in the form of NaOH, enhancing the long-term performance of the cell with a layered Na-oxide cathode, or in the form of NaH, deteriorating the performance in comparison to a reference Na cell. With the help of thermogravimetry coupled with mass spectrometry, we identified an explosion-like thermal decomposition of fatigued Na anodes above approximately 120 °C, but H₂-treated anodes exhibited higher stability of 10-30 °C compared to the reference anode. The composite NaH-Na anode shows a lower electrochemical capacity but no thermally induced explosion. Therefore, for a highly reactive metallic sodium anode, an effective protective layer against liquid electrolyte components is necessary to achieve high capacities and stable long-term operation. This passivation layer must fulfill the requirement of inertness to hydrogen gas to ensure a long lifetime.