More on the performance of LiFePO₄ electrodes-The effect of synthesis route, solution composition, aging, and temperature

Three types of olivine compounds were prepared by three different routes: sol-gel, solid-state, and hydrothermal syntheses, showing the expected structure and containing a small amount of carbon and iron phosphide for maintaining sufficient intrinsic electrical conductivity. These materials were tested in LiClO₄, and in dry and wet LiPF₆ solutions in mixtures of ethylene and dimethyl carbonate (EC-DMC, 1:1) at 30 and 60 °C. Iron dissolution from these materials, upon storage in the three solutions at these two temperatures, was measured by ICP. Aged electrodes were measured by XRD and SEM: the electrochemical performance of the three types of olivine compounds in the three solutions and at the two temperatures was measured by voltammetry and impedance spectroscopy. The behavior of pristine and aged electrodes was systematically compared. It was found that there is a strong correlation between the rate of iron dissolution and the performance of these systems in terms of high capacity, low capacity fading, and low and stable impedance upon aging. The material prepared by sol-gel synthesis demonstrated a low iron dissolution rate, even in a corrosive solution such as wet LiPF₆ solutions, and high performance, even in these solutions. When the solutions contain no acidic contaminants, all the compounds demonstrated negligible iron dissolution rates, even at 60 °C, and a good electrochemical performance. The electrochemical comparison described herein shows a pronounced impact of the solution composition on the electrodes' impedance, due to the unique surface chemistry developed in each solution.