Single-Crystal NMC811 Cathodes in Sulfide-Based All-Solid-State Batteries: Composition and Pressure Effects

Abstract

This thesis investigates cathode design and pressure-dependent cell architecture for sulfide-based all-solid-state batteries (ASSBs), with the primary objective of optimizing composite cathode composition and understanding how stack pressure and cell geometry govern interfacial contact loss and capacity reversibility. A nickel-rich layered oxide cathode, LiNi0.8Mn0.1Co0.1O2 (NMC811), was chosen for its high theoretical specific capacity along with argyrodite-type solid electrolyte Li6PS5Cl (LPSCl). Four single-crystal (SC) NMC811:LPSCl:VGCF cathode formulations were evaluated in PEEK cells under high stack pressure, alongside a comparison between SC and polycrystalline (PC) NMC811 using the optimized composite. Composite formulations spanning 70–90% cathode active material (CAM) fraction were systematically evaluated to probe the tradeoff between ionic and electronic percolation. Among the formulations studied, 70:30:3 gave the highest discharge capacity, indicating an optimal balance between ionic and electronic percolation, while reducing LPSCl content to 10 wt% led to severe performance loss from insufficient ionic percolation. SC-NMC811 delivered approximately 75% higher initial discharge capacity than PC-NMC811, partially attributable to shorter Li+ diffusion length in smaller SC particles. However, SC-NMC showed accelerated capacity fade and incomplete recovery after rate testing, consistent with progressive cathode active material–solid electrolyte contact loss caused by anisotropic mechanical strain of the SC particles at the interface. To assess practical low-pressure operation, SC-NMC811 composites were tested in a silicon-anode pouch cell format, representing a more realistic ASSB cell geometry. Capacity recovery improved substantially under reduced pressure, reaching near-complete recovery in larger pouch cells compared with limited recovery in rigid plunger cells. This improvement is attributed to reduced pressure-driven edge constraints, while the small remaining irreversible loss is assigned to pressure-independent interfacial degradation between NMC811 and LPSCl. These results show that SC-NMC811 offers a higher capacity ceiling in sulfide ASSBs, but requires careful optimization of composite design and mechanical stack pressure conditions. Recovery capacity is proposed as a practical diagnostic for surviving electrochemically active electrode area. Future work should focus on decoupling mechanical and chemical pathways, as well as extending these design principles to pouch-cell architectures and other scalable cell architectures operating under low stack pressure.