REVERSIBLE ELECTROCHEMICAL INSERTION OF LITHIUM IN FINE-GRAINED POLYCRYSTALLINE THIN-FILMS OF MIXED-VALENCY METAL-OXIDES - APPLICATION TO LIXFE2O3 THIN-FILM ELECTRODES PREPARED BY THE SOL-GEL PROCESS

A new family of insertion-compound electrodes able to sustain long-term Li+ electrochemical cyclability in fine-grained polycrystalline film form is presented. The major requirement is that the electrode materials have to be amorphous or polycrystalline with a grain size as small as possible. Indeed, by minimizing the size of the crystallites the formation of defect bonds is favoured, particularly at the crystallite surface, acting as reversible (de)grafting sites of Li+. Related to that the cation-anion bonding is weakened not only in the grain boundary region but also within the grain close to its surface; therefore the electrochemical insertion of Li+ can also occur through easy bonding rearrangement. These concepts are used to describe the electrochemical Li+ (de)insertion of LixFe2O3 thin fam electrodes prepared by the sol-gel process.