Lithium metal negative electrodes are often used as counter electrodes while testing other electrochemically active materials, and are considered to be equivalent, independently of their thickness, supplier and production processes used. Here, we clearly demonstrate, using Electrochemical Impedance spectroscopy (EIS) that it is not the case, as well as the often-used symmetric cells are actually not so symmetric, when EIS spectra are disentangled using Thee-electrode cells.