Amir Salehi

Graduation Semester and Year

2013

Language

English

Document Type

Thesis

Degree Name

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Fuqiang Liu

Abstract

This thesis explores the ability of Raman spectroscopy to understand the complex chemistry taking place in LiFePO₄ cathodes of Li ion batteries. The performance of Li ion batteries was optimized through electrode fabrication and assembling procedures. Various amounts of Timcal Super P carbon were used to construct a conductive network of C-LiFePO₄ particles and the performance of the cathodes was examined during battery cycling. Raman spectroscopy along with electrochemical characterization such as charge/ discharge curves, electrochemical impedance spectroscopy and Cyclic Voltammetry was employed for detailed investigation of battery performance and aging. It is found that both quantity and quality of the conductive carbon affect the rate performance and cyclic behavior of the cells. The cathodes with 2% additive carbon showed a faster capacity fading during cycling than that with 10% additive carbon due to a quicker degradation of the conductive network as indicated by sp2/sp3 and ID/IG ratios in Raman spectroscopy results. The rate performance of cathodes with 2%, 10% and 20 % carbon was also compared and a better rate performance was found for 2% carbon. It showed a proper electronic network which is mostly provided by carbon coating along with a large pore size of the cathode which facilitates the electrolyte penetration. Furthermore, in situ Raman spectroscopy was employed to probe electrolyte concentration variation at the cathode LiFePO₄ particle surface in an optically transparent lithium ion cell. A Raman laser spot size of 2 μm was applied so that transport dynamics at individual particle surface could be investigated. The variation of Li+ concentration in the LiPF₆/ethylene carbonate (EC) + dimethyl carbonate (DMC) electrolyte was determined, for the first time. This was done by monitoring the C-O stretching vibration signal intensity and the corresponding relationship to EC solvation. The electrolyte concentration at the LiFePO₄ particle surface was found to fluctuate during the battery charge/discharge cycle. Particularly, near the end of battery discharge, it reached to a minimum value which was far less than its initial balanced value (1 mol.dm⁻³).

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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