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Inverted Reactive Spray Processes for Sulphide/Nitride High Surface Area Electrode Coatings

Periodic Reporting for period 2 - ReSuNiCo (Inverted Reactive Spray Processes for Sulphide/Nitride High Surface Area Electrode Coatings)

Reporting period: 2020-07-01 to 2021-12-31

Pure metal sulphides are increasingly important materials for energy storage, electrocatalysis, optoelectronics and batteries. The exploitation of their properties requires new technologies allowing the synthesis of such complex materials with specific size and morphology. While the reactive spray is already a key element for the scalable and economic synthesis of metal oxides, we will fundamentally advance the strength of the reactive spray processes by generating a knowledge-base for sulphide materials through our ReSuNiCo project. To achieve this goal (1) right precursor-solvent combination was selected to identify the reaction mechanism and thermodynamics for specific metal sulfide (2) a fast, safe, versatile efficient high throughput single droplet combustion screening was performed. The thermal decomposition of the metal-organic liquids causes subsequent shell formation followed by micro-explosions. The secondary atomization of the fuel and subsequent fast heat release facilitates the homogenous particle formation. The new pioneering screening method is highly flexible and adaptable to a large variety of reactive liquids with different transition metals (e.g. ferrocene, mesityl copper, zinc naphthenate dissolved in organic liquids) and gas atmospheres that comply with the safety requirements via small volumes, small liquid streams and gas flows. The strong nitrate-ethanol and/or tetrahydrothiophene; mesitylcopper-ethylhexanoaic acid; ferocene-ethanol; interactions in the liquid medium lead to the formation of ZnS, CuS and FeS nanoparticles. The knowledge acquired in this work, helps Identify exothermic precursor-solvent combinations allowing multiple μ-explosions, and confirms single droplet combustion as a promising tool for many other industrially important transition metal sulphides that out-perform the existing state-of-the-art knowledge. Although the particle formation from the single droplet combustion is realized, in-situ process diagnostics helps identify droplet reactions, particle formation pathways and product characteristics. The objectives and work packages of ReSuNiCo reach far beyond the state-of-the-art materials synthesis exploration and calls for new process innovations in reactive spraying technologies, aerosol and gas phase characterizations, process model formulations and particle synthesis. To verify this objective, the phase pure LiMn2O4 particles were synthesized after screening sixteen various Mn-based precursor-solvent combinations. The Mn-precursor with Mn3+ instead of Mn2+ allowed LiMn2O4 in the gas phase caused by the short Mn-oxidation time in the flame. The acquired knowledge shows how the variation in the oxidation states of metal in the precursor have profound effect in the end product. Overall, the investigation shows the high throughput combustion screening is a promising tool towards phase-pure functional and engineered non-oxide particles.
The ERC ReSuNiCo project with core objectives including (1) single droplet reactors and precursor design for sulfides and (2) analysis of the burning droplets, mass transfer, reaction and particle formation was kicked off on January 2019. These objectives are investigated by one PhD student (Ms. Adithya Balakrishnan hired in May 2019) working on the precursor-solvent screening for droplet combustion and the other (Mr. Jan Derk Groeneveld, hired in September 2019) on optical diagnostics of the burning droplets. Mr. Jan Treumann, third PhD student was hired in October 2020 for the execution of the task “Process models and reactor design rules” in WP3. Despite the delays in the experimental set-up (due to pandemic), the work was focused on designing the single droplet reactor, alternative approach for precursor chemistry. While extensive chemical/ laboratory safety, risk protocol and experiments involving H2S were not possible, an alternative pathway to sulphur based precursor-solvent combinations was considered. The highly combustible and exothermic precursor-solvents (from Zn, Cu and Fe precursors) allowed multiple -explosions in the gas phase resulting to crystalline and ultrafine metal sulphide nanoparticles (2-5 nm). While the combustion experiments were performed with the precursor-solvent combinations with inherent S-source, the tasks are in very good alignment with the objectives. From August 2021, the laboratory is operational with H2S gas where all the necessary knowledge of precursor-solvent combustion in such gas atmosphere have been known. In addition, the prototype of the single droplet combustion generator was designed, developed, manufactured and tested in a separate laboratory in presence of O2 atmosphere. While designing the single droplet generator and its components took more time than expected, the prototype is now functional. The single droplet combustion results using sulphide-based precursor and/or solvents was a key for in-depth understanding the combustion behaviour in H2S gas atmosphere. For the alternative chemical reaction, two liquid precursors-one involving metal source and the other with sulfur source were mixed together to obtain precipitation/colloidal free solutions. These precursor-solvent were then screened for the metal sulfide formation. This is in line with the DoA where our work package is based on screening such precursor-solvent combination. Such alternative chemical reaction supports the overall knowledge that we plan to acquire with the reaction using H2S gas. The resources of ReSuNiCo project were largely dedicated to safety installation, laboratory set-up, prototype building, preliminary precursor-solvent combination screening and single droplet combustion to determine droplet reaction and schemes, -explosion kinetics and particle formation. While all the installations are now completed, and that the single droplet reactor is operational, the tasks specified in the work packages will be executed smoothly and rapidly. Although there were problems along the way, we did test the hypotheses and the corresponding objectives were achieved.
Reactive spray technology for designing new sulfide materials provide opportunities for more radical innovation. Based on this hypothesis it was aimed at (1) developing single droplet reactor in order to establish a wide range of novel reaction schemes (2) establishing in-situ process diagnostics identifying chemical kinetics in droplet reactions and particle formation pathways. While these two objectives have been a major focus in the last two years, screening precursor-solvent combinations and successful synthesis of sulfides via single droplet combustion is a proof-of-principle of the project. The presence of oxide contamination in these materials due to challenging thermodynamic reaction kinetics of the s-bearing organic liquids combusted in air will be improved using gaseous H2S atmosphere for such reactions. With our in-house built reactor system and varying changeable gas medium (O2, Ar, N2 and/or H2S) has already demonstrated the possibility of reactive sprays for sulphide syntheses through new reaction schemes and reactor concepts. It is expected that the combustion screening and successful sulphide formation at high temperature will establish the pioneering synthesis process far beyond the existing state-of-the-art with the data published in high impact journals.
Gas phase production of LiMn2O4 using different precursor-solvent combinations of Li and/or Mn.
Schematic representation of the particle formation via μ-explosion during single droplet combustion.
Metal and Sulphur sources reported in the literature for the metal sulphide production.
Schematic of the single droplet reactor and particle tracking during combustion.
The tetrahydrothiophene-zinc nitrate in the liquid phase before and after combustion.