In Situ Transmission Electron Microscopy Analysis of Microstructure Evolution of the Li6PS5cl Solid Electrolyte in Charge/Discharge Process

Abstract

All solid state lithium battery has been attracting attention for its high energy density and safety compared to current lithium ion battery. Here, sulfide-based solid electrolytes (SEs) such as Li6PS5Cl have extremely high lithium ion conductivity, which make them promising candidate for the application. However, their stability against lithium metal anode is under debate. And how does the SE reacts with the Li metal in contact and its effect on the performance is not well understood.In this work, we conducted in situ transmission electron microscopy (TEM) experiments to directly observe the structural changes of Li6PS5Cl (LPSCl) solid electrolyte in contact with lithium metal under bias to clarify its stability against lithium. After contacting them under -1 V of biasing on LPSCl with respect to the lithium metal, the surface of the LPSCl shrunk. Electron diffraction pattern analysis revealed that the initial crystalline structure was lost and LPSCl was amorphized upon contact with lithium. The reaction propagated ~1.5 mm after 4 minutes of biasing, and the structural change was irreversible. In addition, lithium transport through the SE/SE boundary was observed to study the degradation due to the Li concentration gradient at the grain boundaries caused by the slower Li diffusion across the boundaries. When a bias potential was applied across the SE/SE interface between two LPSCl particles, a change in the contrast at the interface was observed. After the two particles were detached, the particle slowly recovered the original contrast. The result indicates there was a shift of Li ions within the particle away from the SE/SE interface due to the driving force from the applied bias. To investigate the structural evolution in the grain boundaries under charge/discharge process, alternating bias potential was applied between the LPSCl particles. The change in the electron diffraction pattern during the alternating bias experiment showed emerging peaks for Li2S, which implies the repeating shift of Li ions within the LPSCl altered the electrolyte structure that may degrade the performance of the SE.Observations in this work revealed that the degradations of the SE may take place at the grain boundaries in addition to the electrochemical interface with lithium metal. The structural change at the grain boundaries may cause reduced ionic conductivity and/or increased electrical conductance which may lead to lithium precipitation inside the SEs that can cause catastrophic failure of the battery due to shorting. Figure caption : In situ TEM observation of lithium shift at SE/SE interface in Li6PS5Cl solid electrolyte under alternating bias potential between -3 and +3 V. Figure 1