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{sup +}-(MEP-7)/A (A = C{sub 60}, C{sub 70}, C{sub 60}F{sub x}, and C{sub 70}F{sub x}, MEP-7 = a polyphosphazene derivative). In the cyclic voltammograms of C{sub 60} and C{sub 70}, the first half-wave potential of C{sub 70} is found to be 0.2 V more positive than that of C{sub 60}, suggesting that the electron affinity of C{sub 70} is slightly larger than that of C{sub 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{single_bond}F bonds by X-ray photoelectron spectroscopy. From the discharge curves of C{sub 60}F{sub x} and C{sub 70}F{sub x}, a high utility of 90% is obtained. Changes in 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.