A Low Cost and High Capacity Synthesis of Lithium Iron Phosphate Using a Pulse Combustion Reactor System

We have developed a reactor for continuous synthesis of various metal oxide materials in oxidizing or reducing atmosphere. The capacity of the current pilot reactor version is 2 L of precursor per hour. In the case of synthesis of lithium iron phosphate this corresponds to 100 grams per hour and the procedure is scalable to 100 kilograms per hour. The apparatus is comprised of a pulse combustion type burner, which operates at a frequency of 150 – 240 Hz and a temperature above 1000 °C, a two fluid nozzle for spraying the precursors, and a reactor pipe, where the reaction takes place. The oxidizing and reducing atmosphere is controlled by setting a fuel rich or fuel lean burning. When working with reducing atmosphere the spraying gas is nitrogen, otherwise air or oxygen can be used. The particles are cooled down with air to about 150 °C and collected with an electrostatic precipitator.

Here we demonstrate the functioning of the reactor on the example of lithium iron phosphate. The two precursors used consisted of iron nitrate and lithium carbonate, dissolved in nitric acid. The phosphorus source in one case was ammonium dihydrogen phosphate and triethyl phosphate in the other. As the fuel we used either urea combined with sucrose or glycine. All compounds were dissolved in deionized water. When the precursor is heated up, the water evaporates, and the remaining nitrates and organics react. This process is equivalent to solution combustion synthesis. However, changing the composition of the precursor strongly affects the morphology of the produced materials, which means that the reaction in the precursor is as important as the conditions in the reactor.

By spraying such precursors into the hot zone of the reactor, which is held at about 800 °C, the formed droplets are each its own microreactor. The water from the droplets evaporates very fast because of the high temperature and good heat transfer in pulse combustion burners; the temperature in the reactor drops to about 700 °C. This leads to a precipitation of precursor salts and hinders the coalescence of droplets. After the salts reach a high enough temperature, the reaction starts so that a lot of gaseous products are produced. In the case of urea precursor, porous spheres with a diameter of 3 – 15 μm are formed; in the case of glycine precursor agglomerates of particles from 50 to 300 nm are formed. The particles in both cases are carbon coated, either due to carbonization of sucrose or an excess of glycine fuel.

As synthesized lithium iron phosphate is partly amorphous, mainly because of the very short residence time at a temperature above 500 °C. Heat treatment in an Ar atmosphere at 700 °C for 3 hours yielded phase pure lithium iron phosphate. The as prepared and heat treated materials have been investigated using XRD, SEM, CHNS elemental analysis and ICP. The electrochemical behavior of as prepared and thermally treated materials has been investigated in a lithium half cell.