SOLID-STATE LITHIUM CELLS BASED ON FLUORINATED FULLERENE CATHODES

The electrochemical characteristics of fullerenes and fluorinated full erenes have been investigated by cyclic voltammetry and galvanostatic discharge using solid-state lithium cells Li/Li+-(MEP 7)/A (A = C-60, C-70, C60Fx, and C70Fx, MEP-7 = a polyphosphazene derivative). In the cyclic voltammograms of C-60 and C-70, the first half-wave potential o f C-70 is found to be 0.2 V more positive than that of C-60, suggestin g that the electron affinity of C-70 is slightly larger than that of C -60. For the fluorinated fullerenes, cyclic voltammetry gives one irre versible reduction peak at a high potential, which has been verified t o be due to the reduction of the C-F bonds by x-ray photoelectron spec troscopy. From the discharge curves of C60Fx and C70Fx, a high utility of 90% is obtained. Changes is electronic structure of the cathode ma terials upon discharge are examined by x-ray photoelectron spectroscop y and open-circuit voltage dependence on cathode utilities, and the di scharge reaction mechanism is deduced. The open-circuit voltage depend ence reveals that the electronic structure of fluorinated fullerenes c hanges continuously as a homogeneous electrochemical reduction proceed s.