Hybrid aqueous energy storage cells using activated carbon and lithium-intercalated compounds I. The C/LiMn2O4 system

A hybrid aqueous electrochemical supercapacitor technology is presented in which activated carbon was used as a negative electrode and a lithium-ion intercalated compound LiMn2O4 as a positive electrode in a neutral Li2SO4 aqueous electrolyte. The charge/discharge process is associated with the transfer of a Li ion between two electrodes. It is quite different from the electrolyte behavior in conventional electrochemical double-layer supercapacitors and other reported hybrid supercapacitors, where the cations and anions separate and the electrolyte is consumed. By optimization of the positive/negative electrode mass load ratio, operating voltage window, and pH of the electrolyte solution, the cell exhibits a sloping voltage profile from 0.8 to 1.8 V and delivers an estimated specific energy of ca. 35 Wh/kg based on the total weight of the active electrode materials. The cell exhibits excellent cycling performance with less than 5% capacity loss over 20,000 cycles at 10 C charge/discharge rate. (c) 2006 The Electrochemical Society.