We report that a solid‐state battery architecture enables the reversible, four electron storage of fully utilized solvothermally synthesized cubic‐FeS2 (pyrite). With a sulfide based glass electrolyte we successfully confine electro‐active species and permit the safe use of a lithium metal anode. These FeS2/Li solid‐state cells deliver a theoretical specific capacity of 894 mAh g−1 at 60 °C. We find that nanoparticles of orthorhombic‐FeS2 (marcasite) are generated upon recharge at 30–60 °C which explains a coincident change in rate kinetics.

Multiple lines of recent research indicate that iron pyrite (FeS(2)) requires a {100}-terminated crystal morphology in order to maintain semiconducting properties. Additionally, the large absorption coefficient of pyrite allows for the near complete absorption of above band gap radiation in <50 nm layers. However, to our knowledge <50 nm pyrite nanocubes have yet to be isolated. Herein, we demonstrate the synthesis of ~37 nm phase pure pyrite nanocubes by manipulating the sulfur chemical potential and ligand environment of the system. Ultraviolet-visible (UV-vis) absorption spectroscopy gives a signal of resonant light scattering indicating strong electronic coupling between nanocubes, which may allow for nanocube films with superior electron mobility. The absorption spectroscopies of cubic and irregular nanocrystals are contrasted and compared with recent theoretical work in order to investigate the effect of shape on electronic properties. Specifically, nanocubes have been found to have absorption characteristics closer to theory as compared to irregular nanocrystals, especially for UV radiation: 250-350 nm. Pyrite nanocubes display an indirect band gap at ~1.1 eV in addition to two direct transitions at ~1.9 and ~3.0 eV, correlating well to theoretical values.

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