Liquid structures of Li/Mg/Cs-TFSA (TFSA: bis(trifluoromethylsulfonyl)amide, N(SO₂CF₃)₂⁻) molten-salt systems have been investigated by Raman spectroscopy and high-energy X-ray diffraction. The Raman spectroscopic measurements indicate that the cis-conformer of the TFSA anion is more favorable than the trans-conformer in the molten salts containing Li⁺ and/or Mg²⁺, whereas the latter is dominant for a pure Cs[TFSA] molten salt. The present reverse Monte Carlo (RMC) structural modelling suggests that the coordination number of the anion around Mg, N_Mg-anion, decreased in Li_0.1Mg_0.1Cs_0.8[TFSA]_1.1 molten salt compared to that in Mg_0.1Cs_0.9[TFSA]_1.1. The Mg-anion distance under the coexistence of Li⁺ is found to be about 10–15% longer than that without Li⁺ at around N_Mg-anion ∼ 1.5, which means the increase of free volume around Mg cations in the Li-contained molten salt. Thus, the electrodeposition of Mg in alkali-TFSA molten salts would be facilitated under the coexistence of Li cations.