UC San Diego

All-solid-state batteries (ASSBs) are one of the most promising systems to enable thermally resilient and high energy dense next-generation energy storage. While lithium-ion batteries (LIB) using layered oxide cathodes have made significant advancements, these cathodes are reaching their limits in terms of cost, capacity, and performance. This necessitates the development of cathode alternatives that are safer, high energy dense, and with lower cost, by reducing reliance on critical materials like cobalt and nickel. Pursuing cobalt- and nickel-free chemistries, like LiFePO4 (LFP) and lithium-sulfur (Li-S) in all-solid-state battery (ASSB) architecture is a promising approach to solve some of the current limitations of LIBs. Replacing liquid electrolytes with non-flammable solid-state electrolytes (SSE) can improve both safety and energy density. While SSEs offer many advantages, they often introduce interfacial challenges from resistive solid-solid contact, which can inhibit lithium transport necessary for practical operation. This poses new challenges for LFP and Li-S cathodes, requiring new design strategies due to their unique morphological and material properties.

The morphological features of LFP essential for improved electrochemical performance, are highlighted to elucidate the interfacial challenges when implemented in sulfide based ASSBs. For the first study, the compatibility of LFP with two types of solid-state electrolytes, Li6PS5Cl (LPSCl) and Li2ZrCl6 (LZC), are investigated. Irreversible redox products and interfacial degradation from LPSCl were found to be responsible for unstable performance. This work reveals the intrinsic incompatibility of LFP against sulfide-based SSEs. However, employing the chloride-based electrolyte, LZC, high-rate and stable cycling performance for over a thousand cycles is achieved at room temperature. Although LPSCl was found to be incompatible with LFP, it was found to facilitate beneficial properties when paired with Li-S cathodes. Li-S cathodes can realize some of the highest known energy densities. But similar to LFP, its development in ASSBs has been plagued by interfacial and (chemo)mechanical degradation. In the second study, a scalable synthesis method is introduced to overcome the challenges well known for solid-state Li-S batteries. Facilitating interfacial reactions between sulfur and LPSCl, optimizing the cathode/catholyte microstructure, and tuning the redox behavior of LPSCl was found to improve utilization and stability. As a result, this approach enables high loading sulfur cathodes up to 11 mAh cm-2 with stable operation at room temperature. Several high energy density cell architectures are also proposed and demonstrated. These studies establish new design principles for both LFP and Li-S cathodes in ASSBs, potentially transforming the energy storage landscape by enabling safe, low-cost, and high energy dense storage solutions for a wide range of future applications.