Influence of mixing organic silicon compounds into 1 M (M: mol L−1) LiPF6-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) mixed solvent electrolytes on charge–discharge cycling efficiencies of lithium metal negative electrodes is examined. As organic silicon compounds, polyether-modified siloxanes with polyethylene oxide chains, chlorotrimethylsilane, tetraethoxysilane, cis-tetra [isobutyl (dimethylsiloxy)] cyclotetrasiloxane and cage-type silsesquioxane are investigated. Charge–discharge cycling tests of lithium are galvanostatically carried out using stainless steel working electrodes. Charge–discharge cycling efficiencies of lithium tend to improve by mixing organic silicon compounds. A cage-type silsesquioxane, octaphenyloctasilsesquioxane (Ph8T8) exhibits the highest cycling efficiency of approximately 80% with small mixing amount of 0.02 M Ph8T8. Mechanism of enhancement of lithium cycling efficiencies by mixing organic silicon compounds is considered to be due to the suppression of excess reduction of LiPF6-EC/EMC by lithium and the growth of surface film on lithium.