Much attention has been payed to lithium metal silicate (Li₂MSiO₄, M=Fe and Mn) as a promising cathode of large-scale lithium ion battery which can be used in electric vehicles and renewable energy generation systems. The Li₂MSiO₄, however, has a disadvantage in electronic conduction: their electrical conductivities are below 10^-14 S/cm, which are much lower than those of LiCoO₂. In addition, the diffusivity of lithium ion in the Li₂MSiO₄ is low compared with that in LiCoO₂. Two approaches have been proposed for improvement in the conduction properties of Li₂MSiO₄: one is addition of conductive carbon, and the other reduction of particle size. Although various processes including use of expensive graphene and carbon nano-tube have been reported to obtain high capacity Li₂MSiO₄/carbon composite fine powders, it is important to develop cost-effective processes for broad commercial applications of the lithium ion batteries.

The authors have applied spray-frozen/freeze-drying (SF/FD) process to preparation of Li₂MSiO₄/carbon composite fine powders. The SF/FD is a practical process based on combination of spray-drying and freeze-drying processes, leading to ceramic fine powders with large specific surface area in high purity. In this study, Li₂FeSiO₄/carbon composite powder was prepared by SF/FD of solutions containing carbon sources followed by heat-treatment in flowing argon. Indian ink and glucose was added as the carbon sources. High capacity of 255 mAh/g was obtained at an initial discharge for the as-prepared Li₂FeSiO₄/carbon composite cathode powders even though any conductive additives were not added to the composite powders thus prepared. The Li₂FeSiO₄ showed high value of 173 mAh/g even after ten cycles of charge/discharge. This result indicates that most of Li₂FeSiO₄ particles are connected with carbon network from the starting carbon sources. The capacity at an initial discharge was increased to 323 mAh/g by adding an amount of Ketjenblack to the composite powder. The value became 238 mAh/g after ten cycles. TEM observations revealed that most of Li₂FeSiO₄ particles had size of 30-50 nm and the nano-Li₂FeSiO₄ particles were homogeneously complexed with colloidal carbons from Indian ink. In addition, it was observed that the nano-Li₂FeSiO₄ particles were coated with carbon layer with about 2 nm in thickness. Thus, homogeneous composites of Li₂FeSiO₄ and carbon nano-particles were successfully prepared by SF/FD of the solutions containing two kinds of carbon sources, leading to high cathode performance. These results clearly indicate that the practical SF/FD process can be applied for preparation of high capacity Li₂FeSiO₄/carbon composite cathode powder.