The electrodeposition of metallic lithium is a major cause of failure in lithium batteries. The 3D microstructure of electrodeposited lithium 'moss' in liquid electrolytes has been characterised at sub-micron resolution for the first time. Using synchrotron X-ray phase contrast imaging we distinguish mossy metallic lithium microstructures from high surface area lithium salt formations by their contrasting X-ray attenuation.