Challenges at the Lithium Anode and Composite Cathode in All-Solid-State Batteries

Solid-state batteries using a lithium metal anode and a ceramic electrolyte promise to improve cell energy densities. However, cycling such cells at practical current densities and practical stack pressures often leads to cell failure. Much attention has been devoted to understanding the formation of voids on discharge (stripping lithium) and to dendrite penetration on charging (plating lithium). Both play an important role in the failure and will be discussed. Recently, we have considered contouring the Li/electrolyte interface as a strategy to mitigate dendrite formation. While a contoured interface can provide a higher surface area, inhomogeneous pressures and plating currents can limit improvements in the critical current density for dendrite formation.
The composite cathode presents at least as great a challenge as the lithium anode. Highly conducting solid electrolytes are required in the cathode to achieve high capacities at practical rates. Sulphide based solid electrolytes can do so but they suffer from low oxidative stability. This generally requires that the cathode active material (e.g., NMC811) be coated, to avoid deleterious oxidative decomposition reactions with the solid electrolyte, but these coatings add cost and complexity to manufacture. The reactions occurring in the composite cathode at the various interfaces with the cathode active material and the carbon’s requirement for good electronic transport will be considered.