On the potential use of carbon-free mesoporous precursors of LiFePO4 for lithium-ion batteries electrode

• First report on totally carbon-free mesoporous “lithium iron phosphate”.
• Mesoporous “lithium iron phosphate” shows ordered pores.
• Influence of carbon-free precursors mesoporosity on the LiFePO4 electrodes.
• Reactivity and lithium ion diffusivity enhanced by mesoporosity.

This paper describes the preparation, characterization and electrochemical study of various iron and lithium iron phosphates. The pristine iron phosphate samples obtained were amorphous materials with various textural properties, in terms of average pore size, pore order and specific surface area. Some solids were mesoporous, featuring high surface area, narrow pore size distribution and high pore volume. Lithium iron phosphates were obtained by chemical lithiation of these iron phosphates, using LiI dissolved in acetonitrile. These materials were amorphous and electrochemically inactive against lithium. Following heating at 550 °C, the materials crystallized and showed disparate electrochemical performance in lithium half-cells. This can be ascribed not only to the presence in the samples of impurities of different nature (and inherent to the synthesis method), but also to different textural and morphological properties. In fact, the best electrochemical performance was that of crystallized LiFePO4 obtained from the mesoporous iron phosphate and consisting of rounded nanoparticles with high surface area and adequate porosity. As a result, a better electrolyte impregnation in the porous structures is highly expected. Furthermore, enhancement of the lithium ion diffusion inside the network is observed. This justifies the potential use of carbon-free mesoporous precursors of LiFePO4. However, introduction of a conductive carbon coating onto the phosphate particles and/or removal of the impurities prove necessary for good capacity retention upon cycling.