Iron Phosphates as Cathodes of Lithium-Ion Batteries
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0973-BB05-01
Iron Phosphates as Cathodes of Lithium-Ion Batteries Shijun Wang and M. Stanley Whittingham Materials Science, State University of New York at Binghamton, Binghamton, NY, 13902
ABSTRACT This study focusses on optimizing the parameters of the hydrothermal synthesis to produce iron phosphates for lithium ion batteries, with an emphasis on pure LiFePO4 with the olivine structure and compounds containing a higher iron:phosphate ratio. Lithium iron phosphate (LiFePO4) is a promising cathode candidate for lithium ion batteries due to its high theoretical capacity, environmentally benign and the low cost of starting materials. Well crystallized LiFePO4 can be successfully synthesized at temperatures above 150 °C. The addition of a reducing agent, such as hydrazine, is essential to minimize the oxidation of ferrous (Fe2+) to ferric (Fe3+) in the final compound. The morphology of LiFePO4 is highly dependent on the pH of the initial solution. This study also investigated the lipscombite iron phosphates of formula Fe1.33PO4OH. This compound has a log-like structure formed by Fe-O octahedral chains. The chains are partially occupied by the Fe3+ sites, and these iron atoms and some of the vacancies can be substituted by other cations. Most of the protons can be ion-exchanged for lithium, and the electrochemical capacity is much increased. INTRODUCTION Lithium iron phosphate (LiFePO4) has been widely investigated due to the low cost, environmentally benign nature and high capacity. It can be synthesized by different methods [13], and the conventionally synthesis for this material involves high temperatures. Hydrothermal method is a low energy cost synthesis because of the low temperature. Therefore, it can take best advantage of the low cost starting material for LiFePO4. Our research group first reported that the synthesis temperature can be as low as 120 °C by hydrothermal method [1]. However, the disorder of Fe on Li sites is about 7% [4], which prevents a high capacity. Since Li diffusion in the olivine structure is along the one-dimensional tunnels, any Fe on the Li site will reduce the diffusivity of Li. An alternative structure for iron phosphates, lipscombite with two-dimensional tunnels, gives an excellent electrochemical cycling [5]. The lipscombite name was originally applied by Gheith to name the tetragonal compounds with a general formula of Fe2-yPO4OH [6]. Among these compounds, Fe1.33PO4OH (y = 2/3, abbreviated as Fe-2/3-T) was successfully synthesized above 500 °C [7]. We have renewed the hydrothermal process of LiFePO4 by investigating the synthesis temperature, addition of reducing agent, and the pH. The reducing agent, hydrazine, was introduced to prevent the formation of ferric from ferrous in aqueous solutions. For the tetragonal lipscombite of Fe-2/3-T, we report here the hydrothermal synthesis, the thermal and electrochemical behavior, as well as the properties of the substituted compounds.
EXPERIMENT Preparation of LiFePO4 15 ml 1M (~0.015 mol H3PO4) phosphoric acid was first diluted into
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