Fast Pulse Combustion Process for Producing Lithium-Ion Cathode Materials
Authors: Krizan Gregor , Krizan Janez , Dominko Robert , Gaberscek Miran 
Published: February 2020

With the use of a fast pulse combustion process in a reactor we are able to produce LiFePO4, albeit with the use of an additional annealing step[1]. The role of latter is to introduce appropriate ordering into the structure. Disorder in the crystal structure of LiFePO4 has been identified by many groups[2–4], especially for materials synthesized via reactions without a high-temperature sintering step in an inert or reducing atmosphere, or using fast reactions on the order of minutes[5,6]. The most favorable intrinsic defect is the Li-Fe ‘anti-site’ pair in which the Li ion (on the M1 site) and the Fe ion (on the M2 site) are interchanged[2]. In this report we show that such a disordered structure can be obtained even with synthesis times shorter than 2 seconds; at such short times one usually obtains amorphous materials[7]. The disorder will be observed through Mossbauer analysis and Rietveld refinement of XRD data. The galvanostatic cycling behavior of such disordered LiFePO4 is shown together with the annealed version on the attached graph. We were able to cycle the former up to a current density of 340 mA/g (corresponds to 2C).
Another group of interesting cathode materials is the lithium nickel manganese cobalt oxides. Using different ratios between the three transition metals different compositions can be obtained, some showing a very high capacity of 220 mAh/g when charged to 4.7 V[8,9]. With our method we are able to produce a wide range of compositions, also lithium rich ones. This is possible because of the very fast reaction from solutions of metal salts, where the mixing proceeds on the molecular level. The fast reaction is the reason that no phase separation or grouping of an element can occur, leading to a uniform composition across the whole material. Simultaneously the method also allows decoration of the particles with a carbon coating, which could help improve the capacity retention[9].
The synthesis of present cathode materials was carried out in a pulse combustion reactor[1], which uses the pulsating flow of flue gases from the combustion chamber to supply the necessary energy for the reaction. The pulsating flow is a result of pulsating fuel and air ignition. It has been shown previously that this kind of burning has a better heat transfer rate than similar turbulent flows[10]. The flue gases enter the reactor pipe that has a two fluid nozzle at the point of flue gas entry. The nozzle is used to produce droplets of precursor in the hot zone of the reactor. The solvent from the droplets evaporates at a very high rate causing formation of agglomerated nano powders, which are then fully or partly crystallized after passing through the heated reactor pipe. The desired temperature in the reactor depends on the product. The dried precursor undergoes a reaction, similar to that in the solution combustion synthesis[11] (SCS), because the precursor can be composed of nitrates and a carbohydrate, acting as a fuel. Another possible precursor type are flammable precursors, such as those used in liquid feed flame spray pyrolysis[7,12](LF-FSP), however, they are usually not used in production of cathode materials with this reactor.
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Figure 1