Influence of Iron Sulfide Nanoparticle Sizes in Solid-State Batteries**

Given the inherent performance limitations of intercalation-based lithium-ion batteries, solid-state conversion batteries are promising systems for future energy storage. A high specific capacity and natural abundancy make iron disulfide (FeS2) a promising cathode-active material. In this work, FeS2 nanoparticles were prepared solvothermally. By adjusting the synthesis conditions, samples with average particle diameters between 10 nm and 35 nm were synthesized. The electrochemical performance was evaluated in solid-state cells with a Li-argyrodite solid electrolyte. While the reduction of FeS2 was found to be irreversible in the initial discharge, a stable cycling of the reduced species was observed subsequently. A positive effect of smaller particle dimensions on FeS2 utilization was identified, which can be attributed to a higher interfacial contact area and shortened diffusion pathways inside the FeS2 particles. These results highlight the general importance of morphological design to exploit the promising theoretical capacity of conversion electrodes in solid-state batteries.