Improving The Rate Capability Of Cathode Material Li₂FeSiO₄by The Introduction Of Nano Conductive Phases

Li2FeSiO4is considered to be one of the most promising cathode materials forlithium-ion batteries due to its theoretical possibilities for reversibly extracting/inserting twolithium-ions from the host structure and thus to increase its instinct capacity to appro ximately330mAh/g. However, Li2FeSiO4possesses a poor electronic conductivity and low lithium iondiffusion coefficient, and always co-exists with a small amount of an insulating glassy SiO2impurity during the calcination process of material preparation. These lead to the poor ratecapability of Li2FeSiO4cathode. In this dissertation, we introduce a second conductive phaseinto the Li2FeSiO4/C system at the nanometer scale to form a composite structure that iscomposed of the insulating Li2FeSiO4nanocrystal and the conductive phase, aiming toimprove the rate capability by the enhancement of electrode reaction kinetics.By controlling the in situ crystallization of SiO2-based multicomponent oxides, a quitesmall amount of the mixed valence Fe7SiO10nanocrystals inter-grow with Li2FeSiO4nanocrystals, thus improving the rate capability of the cathode due to the formedLi2FeSiO4/Fe7SiO10hetero-nanocrystals. The mesoporous Li2FeSiO4/Fe7SiO10hetero-nanocrystal/C nanocomposite is obtained by using P123as the carbon source and thenanostructure direction agent. By carefully deviating the material composition from thestoichiometric ratio and adjusting the calcination atmosphere, the excess FexOyreacts withSiO2phase to produce a conductive phase Fe7SiO10and reduce the SiO2content effectively.The primary phase is Li2FeSiO4indexed to a monoclinic structure with the space group ofP21/n. The discharge capacity of Li2FeSiO4/Fe7SiO10/C cathode is190mAh/g in the firstcycle at0.1C, implying lithiation/insertion of more than one lithium ion. The capacityretention of Li2FeSiO4/Fe7SiO10/C cathode reaches up to58.9%at10C, while that ofLi2FeSiO4/C is only22.5%. The excellent rate capability of Li2FeSiO4/Fe7SiO10/Cnanocomposite is attributed to the enhanced electronic conductivity within theLi2FeSiO4/Fe7SiO10hetero-grain caused by the reduced content of insulating SiO2phase andthe introduction of conductive phase Fe7SiO10. In addition, the carbon nanotubes (CNTs) are introduced into material as conductivenanowires to improve the rate capability of the Li2FeSiO4cathode for the enhancedconductivity between Li2FeSiO4@C nanoparticles. Li2FeSiO4@C/CNTx nanocompositeswith different additive amount of CNTs are obtained through the sol-gel method, by usingP123as a carbon source. The capacity retention of the Li2FeSiO4@C/CNTx (x=2%,4%,8%,12%, wt%vs. pure Li2FeSiO4) nanocomposites are49.41%、58.85%、49.87%、47.75%at10C, respectively, while that of Li2FeSiO4/C is only25.53%. The optimum CNTs additiveamount is4%. The improvement of the rate capability of Li2FeSiO4@C/CNTx owes to1-dimensional CNTs which acts as a conductive bridge between the Li2FeSiO4@C particles,forming a3-D conductive network by the CNTs and amorphous carbon residing aroundLi2FeSiO4nanocrystals.