Stack Pressure and Critical Current Density in Li-metal Cells: The Role of Mechanical Deformation

Abstract

Abstract Lithium-ion batteries constructed with an oxide electrolyte are a subset of solid-state batteries. The ceramics are hard and brittle which poses contact issues related to the metal-electrolyte interface. Recent experiments have shown that the electrochemical current of lithium across the interface can have significant spatial variability. For example, high local flux can nucleate voids at the interface, causing the cell to fail at low values of critical current density. Application of stack pressure compensates for the spatially non-uniform electrochemical current with mechanical currents of lithium, such that the sum of the mechanical and electrical currents, across the entire interface becomes uniform. Two scenarios for compensating mechanical currents are presented. In one case, called Type I, compression creep normal to the interface plane produces greater flow of lithium into high conductivity regions and less so to the low conductivity regimes, thereby equalizing the sum of electrical (with a negative sign) and mechanical (with a positive sign) currents. In Type II, stack pressure causes lithium to be transported by diffusion, parallel to the interface from regions of low, to regions of high electrochemical current to ensure that the lithium interface moves like a rigid body. The time dependent deformation in the lithium layer is matched with the current density to derive equations for the critical current density as a function of the stack pressure. Experimental data, though limited, when compared to these predictions shows better agreement with the Type II mechanism.