Mechanistic Insights into Soft Shorts in All-Solid-State Lithium Metal Batteries using a Three-Electrode and Pressure-Monitoring Cell

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 behavior, we investigate the cycling of Li4Ti5O12 (LTO)|Li6PS5Cl (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 observed the occurrence of a soft short followed by a partial voltage recovery, while simultaneous monitoring electrode potentials and real-time cell pressure. Correlating the pressure changes with the Faradaic currents reveals that, once a soft short forms, the actual electrochemical reactions deviate substantially from the externally applied current. Impedance analysis further indicates a marked reduction in ohmic resistance after dendritic bridging, confirming the establishment of electronic pathways across the solid electrolyte. Building on these insights, we propose an equivalent circuit model describing the dynamic evolution of soft shorts and introduce a two quantitative methods to estimate dendrite dimensions, found to range from 100 to 102 of nanometers.