The partial electronic conductivity of lithium halides and their role in SEI formation in solid-state batteries – Hebb-Wagner-type measurements

In the context of solid-state batteries (SSBs), minimizing the partial electronic conductivity ( σ el ) of solid electrolytes is essential to minimize self-discharge and degradation processes, ensuring the viability of SSBs. Thus, a precise determination of σ el is of high interest as it varies significantly across the operational range of practical SSBs. Here, ion-blocking Hebb-Wagner polarization measurements are a well-established technique for assessing the partial conductivity of minority charge carriers in solid ion conductors ( i.e. , σ el ). However, measuring σ el of lithium-ion conducting solid electrolytes often yields inaccurate results due to electrochemical reduction reactions at low (reference) potentials. This study provides a comprehensive framework encompassing fundamental derivations, accurate testing protocols, and comprehensive data analysis for determining σ el of lithium-ion conducting solid electrolytes. The partial ionic and electronic conductivities of lithium halides (Li X , with X = Cl, Br, or I) – components of typical interphases in SSBs – were systematically evaluated over a range of the lithium activity. Pristine Hebb-Wagner cells ( i.e. , reservoir-free SSBs) exhibit “self-polarization”, which we discuss as part of the formal treatment, and cause the material to be probed in the extrinsic region. Our experimental findings offer systematic insight into the electronic properties of lithium halides, extrapolated to room temperature, revealing conductivities in the order of 10 −9 S cm −1 or lower within the tested activity range. These findings deepen the understanding of the role of Li X in solid electrolyte interphase formation affecting the performance and stability of SSBs.