One of the biggest obstacles holding solid-state batteries back is the difficulty of achieving continuous contact surfaces between the electrodes and the separator. This leads to current constriction and a higher overvoltage within the Cell. Partial contact loss can already be present after the cell is manufactured, due to preexisting small pores, surface contamination, small material defects, and depressions within the materials. It can, however, also occur during battery usage. One of the most common phenomena observed is the formation of pores between a lithium anode and an inorganic solid electrolyte separator. Generally, it is agreed that this is due to high current densities during battery discharging.

This work aims to provide a detailed explanation of how this process operates. This is first done through FEM simulations with Comsol Multiphysics. A simplified geometry is used to simulate the current density distribution on the lithium separator boundary for three distinct phases. An uneven, but completely connected contact surface, a small existing pore, and a small remaining contact surface on an isolated plane. Additionally, tests are conducted using solid-state half-cells in a coin cell format. Two different-sized lithium electrodes are melted onto a separator made of dense LLZO. Current pulses of 1 h each are applied in both directions. The voltage response to different current densities is observed.

The main findings of this work include that preexisting surface defects will lead to uneven current distribution and, consequently, uneven lithium stripping, acting as starting points for future pore formation. Self-diffusion within the lithium, and pressure can mitigate uneven current densities to an extent. Contact loss, therefore, does not occur at low current densities. A smaller actual contact surface, due to existing pores, will result in higher local current densities, which will further accelerate pore formation. If contact surfaces become too small and local current densities become too high during subsequent plating, dendrites will start to form. This will ultimately lead to short circuits, destroying the cell.