A method for applying a protective layer on at least part of an exposed surface of a substrate, wherein the exposed surface comprises an alkali metal or an alkali metal alloy, the method comprising:
- (i) activating a gas by means of a plasma discharge, to obtain an activated gas,
- (ii) contacting the exposed surface with the activated gas, wherein a protective layer is formed on at least part of the exposed surface,
- wherein the gas comprises a nitrogen-comprising compound in an amount, such that the protective layer comprises at least 60 mol % of a corresponding alkali metal nitrate, as measured by X-ray photoelectron spectroscopy.