Application of electrohydrodynamic instability theory to a planar liquid‐lithium surface suggests that liquid lithium may provide a large‐area ion source for intense ion‐beam diodes. Such a source could uniformly produce ions on a nanosecond time scale if the applied electric field exceeds 10 MV/cm. Wavelengths and growth times of the most rapidly growing instability have been calculated to first order as a function of applied electric field. Results have been interpreted in the context of an ion source for a light ion‐beam driver for inertial confinement fusion.