Synthesis and characterization of lithium-ion cathode materials in the system (1-x-y)LiNi1/3Mn1/3Co1/3O2 ∙ xLi2MnO3 ∙ yLiCoO2

Abstract

Considering various technologies for storing energy the usage of lithium (Li) - ion batteries still stands as one of the most promising options, especially for the on-going huge demand for electric and plug-in hybrid vehicles. The main limiting factor in the performance of a Li-ion battery is the cathode material. The current cathode material that is being used in the present market is LiCoO2 cathode which is effective but is expensive and toxic. The objective of this thesis is to find a cathode material which is advantageous and a probable replacement for LiCoO2. Based on the previously studied work on ternary solid solutions and its advantages, this system (1-x-y)LiNi1/3Mn1/3Co1/3O2•xLi2MnO3•yLiCoO2 was chosen. This was made using a ternary composition diagram which is a combination of LiNi1/3Mn1/3Co1/3O2, Li2MnO3 and LiCoO2 materials. Points inside the ternary diagram were chosen in an arrangement conducive to mathematical modeling and compositions of the cathodes were processed accordingly. All the 28 samples in the system were synthesized using the sol-gel method, each sample was characterized using X-ray diffraction (XRD) scans and electrochemical testing was performed by using an Arbin BT2000 battery testing system with MITS Pro Arbin software. The XRD results showed that all the samples established a α-NaFeO2 structure and a space group of R3m with varying degrees of purity in the crystal structure. The compounds in this system showed fairly consistent charge/ discharge curves during the electrochemical testing with initial discharge capacities varying from 122mAh/g to 240mAh/g and high capacity compositions were found in the region of high Li2MnO3 content. The highest capacity found was Li1.222Mn0.5Ni.056Co0.222O2 composition with a discharge capacity of 242mAh/gin the voltage range 4.6 - 2V at a C/15 rate. A statistical based analysis was carried out using JMP version 7.0.1 with the design of experiments (DOE) procedure for a mixture design to find variations in capacity and cyclability trends over the composition triangle. Inductively couple plasma (ICP) analysis was carried out on 4 samples which were found to be most promising basing on the statistical analysis. ICP results confirmed the formation of optimum compositions for the specified synthesis conditions. All these 4 samples were synthesized and tested again to confirm consistent performance across repeat samples. XPS (X-ray photoelectron spectroscopy) was conducted for these samples to determine the chemical environment of transition metals. The composition with the highest capacity was found to be Li1.222Mn0.5Ni0.056Co0.222O2 electrode. Its high capacity is attributed to the Ni+2/Ni+4 and Co+3/Co+4 red-ox couple and the Li-ion extraction form Li2MnO3 at a voltage >4.5V which results in an extended discharge profile. This cathode composition is very promising because it shows high capacity and good cyclability, and would be much cheaper than conventional LiCoO2 cathodes since it has more manganese which is less expensive than cobalt.