School of Metallurgy & Materials Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran
In the energy storage field, lithium-ion batteries were known to be the most important approach for mitigating the environmental impacts of fossil fuels. Cathode materials are the crucial part of a lithium-ion battery, and LiFePO4 cathode material was selected for its high theoretical capacity (170 mAh.g-1), significant cyclic stability, and environmental friendliness. On the contrary, the main downside of LiFePO4 materials is their one-dimensional lithium-ion diffusion channel at the crystallographic direction of . These channels can be blocked by antisite defects, plunging the specific capacity of LiFePO4 materials. Thus, in order to reduce such impacts, having sheet-like morphologies with a significant crystallographic plane of (010) is essential. A great deal of research has been performed using a solvothermal method for the synthesis of LiFePO4 materials, and factors - as precursors, pH of the solution, temperature, time, and additives - were known to have significant roles in the structural as well as electrochemical properties of LiFePO4 materials. Organic additives can act as a soft template for tuning the morphology of particles in solvothermal synthesis. Likewise, additives can interact with the surface of particles to assemble them in the lowest interfacial energy. In this study, different amounts of the amino acids, namely glycine, and glutamic acid, were introduced in the solvothermal synthesis of LiFePO4 materials, and their respective roles in morphology and electrochemical characteristics were investigated. In general, all amino acids have one amine group, one carboxylic acid group, and one extra chain of R group. The only exception of this category is glycine which has no R group. The self-assembled morphology of LiFePO4 particles using glycine was discussed by the formation of peptide bonds. Additionally, having another carboxylic acid group in the molecular structure of glutamic acid sustained a low pH in the solvothermal solution; therefore, the formation of self-assembled morphology could not occur during the synthesis process. Additionally, the specific capacity of the LiFePO4/C materials after the heat treatment was discussed by Rietveld refinement investigations for determining the antisite defects.