Redox-Active HalideCatholytes for Enhanced EnergyDensity in Solid-State Sodium Batteries

Sodium-based all-solid-state batteries (ASSBs) are a promising technology for grid-scale energy storage applications due to their theoretically low cost and high energy density. However, state-of-the-art ASSB cathodes are often in the form of heterogeneous composites with 20–40% inactive solid catholyte, which undermines the energy density benefits of the solid-state format. Furthermore, solid catholytes are often comprised of cost-prohibitive, rare metals. Here, we demonstrate a new class of redox-active solid electrolytes (RASEs) as an effective method for enhancing the cathode-level energy density. ASSBs utilizing RASEs exhibit a 31–79% increase in cathode-level energy density, depending on the cathode composite composition, when compared to standard, redox-inactive solid electrolytes. Furthermore, solid electrolyte compositing through a mechanochemical method is investigated as a promising method for reducing the material cost and improving ionic conductivity for a NaNb_1–<i>x</i>Al_<i>x</i>Cl_6–2x RASE system. In the best case, NaNb_0.5Al_0.5Cl₅ exhibits an order-of-magnitude increase in conductivity over the baseline NaNbCl₆.