Iron Phosphates as Cathodes of Lithium-Ion Batteries

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 Fe_1.33PO₄OH. 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.