Access Type

Open Access Dissertation

Date of Award

January 2014

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics and Astronomy

First Advisor

Ratna Naik

Second Advisor

Abbas Nazri

Abstract

Lithium-ion batteries are the power source of choice for portable electronics, power tools and electric-based transportation. This outstanding commercial success has spawned great international interest in applying this technology to systems that demand higher power, such as the electric component of hybrid, extended range, and electric vehicles. This would require new electrode materials that are less expensive, more energetic, and more environmentally friendly than the present ones. Of particular interest is the olivine-structured LiFePO4 cathode developed by Goodenough and co-workers, which offers several appealing features, such as a high, flat voltage profile and relatively high theoretical specific capacity (170 mAhg−1), combined with low cost and low toxicity. However, the intrinsically poor electronic and ionic conductivities of LiFePO4 limit the delivery of high specific capacity at high discharge rates. Several strategies have been devised to overcome the inherent limitations of LiFePO4.

Carbon coating is one of the remedies to improve the performance of LiFePO4. We studied the effect of carbon coating on the performance of LiFePO4. First, we synthesized carbon-coated LiFePO4 samples with different amount of surfactant, lauric acid that acts as carbon source. We were able to show that an optimized amount of carbon results in greatly improved room-temperature electrochemical performance. On the other hand, because the electrochemical properties are strongly dependent on the quality of deposited carbon, we did also study the effect of carbon sources: lauric acid, myristic acid and oleic acid. We successfully showed that the proper carbon sources and carbon content played a key role on improving the initial charge-discharge capacity of the LiFePO4/C cathode. In addition, we did also shed the light of the positive impact of excess of Li on the electrochemical performance of C-LiFePO4.

Knowing that Fe-site doping is considered to be an effective way to improve the rate performance of LiFePO4, we were able to show that composing LiFePO4 with both carbon-networks and tuning electronic conductivity by metal doping (1mol % In3+) is effective in achieving better electrochemical properties, especially at high rates of charge-discharge. We were also interesting to examine the effect of the partial substitution of Fe2+ with Mn2+. We synthesized LiMn0.2Fe0.8PO4 material and perform in-depth characterizations to gain knowledge on physical and electrochemical properties of this class of battery materials. That project looks promising since that new materials will operate at higher potential (3.4V, 4.1V) than LiFePO4.

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