Morphology and electrical properties of conductive carbon coatings for cathode materials

Many interesting cathode materials, such as LiFePO4, LiMnPO4, LiFeBO3 or the recently discovered Li2FeSiO4 and Li2MnSiO4, exhibit extremely low electronic conductivity (<10−9 S cm−1). A very efficient way for improving the electronic transport in such materials is supposed to be the preparation of carbon coatings around individual active particles. Despite the increasing number of reports on preparation of various carbon coatings, neither the formation mechanism nor the detailed coating properties have been explained satisfactorily. The present paper is an attempt to find a clear correlation between the synthesis parameters, the resulting coating morphology and, finally, its electrical properties. As a substrate material for deposition of coatings, more or less monodisperse TiO2 particles in various sizes were used. As a carbon precursor, citrate was used because it had given excellent results in our previous investigation of the LiFePO4 system. It is shown that citrate precursor delivers pretty good conductivity (ca. 30 S cm−1) after a 10 h heat treatment at 700 °C or higher. The conductivity percolation threshold can be reached already at 1.5 vol.% of carbon, while the plateau conductivity of the whole composite is about 0.1 S cm−1. At that level, the carbon phase is supposed to form a well-distributed 3D electrical network within the composite.