Researchers have synthesised highly crystalline pyrite FeS₂at low temperatures and utilized them for fabricating electrochemical energy storage devices such as batteries and high energy density supercapacitors (SCs).

Transition metal sulfides (TMS) are an important class of inorganic materials and find applications in diverse fields including electrochemical energy storage devices such as batteries and supercapacitors (SCs). Solid-state synthetic methods are used to generate metal sulfides from the corresponding metal salts or their equivalent oxides usually by by annealing at high temperatures.

However, the experiments carried out by Ms. Savithri Vishwanathan, under the supervision of Dr. H. S. S. Ramakrishna Matte at Centre for Nano and Soft Matter Sciences, Bengaluru, an autonomous institute under Department of Science & Technology (DST) demonstrated the low-temperature synthesis of crystalline pyrite FeS₂ through a solid-state synthesis route. They have utilized a metastable oxyhydroxide (FeOOH) precursor for this process.

The team reported stabilising this intermediate oxyhydroxide and utilizing it as a precursor for sulfidation, in the presence of H₂S gas, for the first time in their paper published in the journal Chemical Communications

Using a metastable precursor helped in lowering the annealing temperature, as FeOOH converted into pyrite FeS₂ with fairly good crystallinity at a low temperature. This synthetic route of obtaining sulfides from their corresponding metastable oxyhydroxides can be extended to other transition metals to obtain crystalline materials in an energy intensive way.

Electrodes for high-energy density SCs were fabricated from the as-synthesized FeS₂, resulting in superior performance in the presence of organic and ionic-liquid (IL)-based electrolytes. This could be attributed to the improved conductivity as a result of good crystallinity of the material as well as the significantly enhanced wettability of the FeS₂ electrode in the presence of the organic and IL-based electrolytes. The FeS₂ electrode exhibited high energy and power densities, clearly highlighting the role of the synthetic procedure employed for enhancing electrochemical properties.

Link to published paper: DOI: 10.1039/D3CC02153J

Figure. Low-temperature synthesis of crystalline pyrite-FeS₂from FeOOH precursor and its application in high energy density SCs.