Solid-state lithium cells based on fluorinated fullerene cathodes

The electrochemical characteristics of fullerenes and fluorinated fullerenes 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 of C-70 is found to be 0.2 V more positive than that of C-60, suggesting that the electron affinity of C-70 is slightly larger than that of C-60. For the fluorinated fullerenes, cyclic voltammetry gives one irreversible reduction peak at a high potential, which has been verified to be due to the reduction of the C-F bonds by x-ray photoelectron spectroscopy. From the discharge curves of C60Fx and C70Fx, a high utility of 90% is obtained. Changes is electronic structure of the cathode materials upon discharge are examined by x-ray photoelectron spectroscopy and open-circuit voltage dependence on cathode utilities, and the discharge reaction mechanism is deduced. The open-circuit voltage dependence reveals that the electronic structure of fluorinated fullerenes changes continuously as a homogeneous electrochemical reduction proceeds.