The electrochemical profiles of three kinds of Li-ion intercalated compounds, LiMn2O4, LiCo1/3Ni1/3Mn1/3O2, and LiCoO2, used as positive electrodes for hybrid aqueous electrochemical supercapacitors in combination with activated carbon (AC) negative electrode were studied in a 1 M Li2SO4 solution. The effects of pH in the electrolyte solution on the stability of Li-ion intercalation reaction, the evolution potential of oxygen, and the specific capacity of both the Li-ion intercalated compound and AC electrode materials were intensively investigated by cycling voltammetry and charge/discharge tests in 1 M Li2SO4 solutions with various pH values from pH 7 to 13. LiMn2O4 shows stable Li-ion intercalation in the solution over pH 7, LiCo1/3Ni1/3Mn1/3O2 over pH 11, and LiCoO2 over pH 9. Under optimal conditions, the specific energy, rate capability, and cycling performance of hybrid cells based on the three kinds of positive electrodes and AC negative electrode were compared. There was no significant difference in the specific energy among the three hybrid cells, but the hybrid cell based on the LiMn2O4 showed good cycling life and rate capability. LiCoO2 also had good rate capability but with poor cycling performance, and LiCo1/3Ni1/3Mn1/3O2 showed good cycling life but with poor rate capability. (c) 2006 The Electrochemical Society.