Evolution of Pore Volume During Stripping of Lithium Metal in Solid-State Batteries Observed with Operando Dilatometry.

Lithium metal solid-state batteries are prone to pore formation at the interface between the metal anode and the solid electrolyte during discharge, when the applied current density exceeds the lithium vacancy diffusion rate in the metal. This leads to contact loss and eventually battery failure. To better understand the mechanisms of pore formation at the anode interface, operando information on the total pore volume is essential. In this study, a dedicated dilatometric measurement setup for operando tracking of pore volume is presented and validated. The working principle of this method is demonstrated in a case study on symmetric lithium-garnet cells. The combination of the dilatometric data with galvanostatic electrochemical impedance spectroscopy (GEIS) allows for a systematic study of pore formation in any metal solid-state battery. To quantify this process, the vacancy injection ratio is introduced, as a measure of the fraction of lithium atom sites that remain empty during stripping. These experimental results reveal that, under the conditions applied, pore formation begins immediately upon stripping, even before becoming detectable by GEIS. Furthermore, it is shown that pores initially grow laterally near the surface before deepening at later stages. Based on these findings, a pore formation and growth mechanism is proposed.