Mechanistic insights into soft shorts in all-solid-state lithium metal batteries using a three-electrode and pressure-monitoring cell

With three-electrode and operando pressure monitoring experiments, the failure mode “soft short” and its partial recovery in all-solid-state lithium metal batteries are elucidated with dynamic evolution of lithium dendrites. All-solid-state lithium metal batteries (ASSLMBs) promise high energy density and enhanced safety. Nevertheless, their performance is hindered by lithium dendrite growth at high current densities, which can induce internal short circuits with abrupt cell voltage drops. However, at intermediate current densities, “soft shorts”, namely partial and transient internal shorts, are more prevalent and difficult to interpret. In such a case, the cell voltage does not collapse to zero but instead fluctuates dynamically and fails to increase further during charge. To elucidate the electro-chemo-mechanical mechanisms underlying this unusual soft short behavior, we investigate the cycling of Li 4 Ti 5 O 12 (LTO)|Li 6 PS 5 Cl (LPSC)|Li in a three-electrode cell configuration equipped with operando pressure monitoring. An in situ lithiated Au/W reference electrode enables independent tracking of the working and counter electrode potentials and their impedance evolution. During galvanostatic cycling, we directly captured the formation of a soft short accompanied by a partial voltage recovery by simultaneously tracking electrode potentials and real-time cell pressure. By correlating pressure transients with the measured Faradaic currents, we reveal for the first time that the onset of a soft short fundamentally decouples the internal electrochemical reactions from the externally imposed current. This hidden current redistribution is further substantiated by impedance spectroscopy, which shows a pronounced drop in ohmic resistance following dendritic bridging, providing direct evidence for the emergence of electronically conductive pathways across the solid electrolyte. Building on these insights, we propose an equivalent circuit model describing the dynamic evolution of soft shorts and introduce quantitative methods to estimate dendrite dimensions, found to range from 10 0 to 10 2 nm. Together, these advances provide the first quantitative link between soft-short electrochemical signatures and the underlying nanoscale morphology of dendritic filaments.