Solid State Enabled Reversible Four Electron Storage

We report that a solid‐state battery architecture enables the reversible, four electron storage of fully utilized solvothermally synthesized cubic‐FeS 2 (pyrite). With a sulfide based glass electrolyte we successfully confine electro‐active species and permit the safe use of a lithium metal anode. These FeS 2 /Li solid‐state cells deliver a theoretical specific capacity of 894 mAh g −1 at 60 °C. We find that nanoparticles of orthorhombic‐FeS 2 (marcasite) are generated upon recharge at 30–60 °C which explains a coincident change in rate kinetics.