Consolidation of composite cathodes with NCM and sulfide solid-state electrolytes by hot pressing for all-solid-state Li metal batteries

In this study, hot pressing was evaluated as a method of cell fabrication to increase the energy density of next-generation all-solid-state batteries with NCM active material and sulfide solid-state electrolyte. Hot pressing involves consolidating glassy sulfide electrolyte by the application of pressure at a temperature above the electrolyte's glass transition temperature. Typically, cell stacks are formed at room temperature and retain 15–30% porosity that limits cell energy density. On the other hand, the porosity of hot-pressed cell stacks is reduced to less than 10%. The electrochemical function of hot-pressed cathode composites was assessed as a function of active material and solid-state electrolyte compositions. Specifically, LiNi0.85Co0.10Mn0.05O2 and LiNi0.6Co0.2Mn0.2O2 were studied in combination with either glassy Li7P3S11, glassy Li3PS4, or β-Li3PS4 solid-state electrolytes. Cathode composites composed of LiNi0.6Co0.2Mn0.2O2 and Li3PS4 maintained the best function after hot pressing at 200 °C and 370 MPa for 10 min. It was found that LiNi0.6Co0.2Mn0.2O2's resistance to microcracking and the inherent stability of Li3PS4's fully de-networked local structure are critical to maintain good electrochemical function after hot pressing. The results of this study show that hot pressing reduces porosity in the cathode composite and confirms the feasibility of cathode support of consolidated, reinforced glass separators.