Evolution of Electrochemical Interface and Weakening Mechanisms of Li₆PS₅Cl Solid State Electrolyte—Space Charge Region and Acceleration of Lithium Penetration

All-solid-state lithium battery has been developed as a next generation energy storage device because of its potential to exceed current Li-ion battery technology by enabling high voltage cathode and lithium metal anode. However, there are several challenges that need to be overcome for before the practical implementation of the technology to real-world applications. These include lithium penetration, electrolyte/electrode cracking, stability of solid electrolyte, and low ionic conductivity across grain boundaries, to name a few. In particular, the lithium penetration can cause serious safety hazard and unexpected failure of the battery. Lithium deposition at the electrode electrolyte interface induces stress on the solid electrolyte. And at the same time, electrochemical reaction induces change in the mechanical property of the solid electrolyte in addition to the reduction in the ionic conductivity. This complex interaction of electrochemistry and mechanics is responsible for the lithium penetration. Moreover, a slow ionic conductance across the grain boundaries creates a lithium concentration gradient near the boundary forming space-charge regions within the solid electrolyte. The lithium excess or deficiency may decrease the mechanical strength of the electrolyte further promoting the lithium penetration. Understanding the evolution and behavior of electrochemical interfaces during the charge/discharge process is a key to identify root cause of the failure and performance degradation of the all-solid-state lithium batteries.<br/>In this work, in situ transmission electron microscopy (TEM) and ab initio simulation are performed to study the electrochemomechanics of the lithium penetration in Li₆PS₅Cl (LPSCl) solid electrolyte typically used in all-solid-state lithium battery. Here, conductive atomic force microscopy (AFM) cantilever was integrated in the in situ TEM experiment to evaluate the mechanical force associated with the lithium penetration during the real-time observation of the process. It was shown that lithium can penetrate the solid electrolyte even with a minute interfacial force with lithium metal in contact. Ab initio modeling showed that this may be caused by the reduction in the mechanical property when the solid electrolyte is electrochemically reduced or oxidized at the interface with lithium. Additionally, a sign of a lithium concentration gradient and a space charge region was observed at the grain boundary of the solid electrolyte by in situ TEM analysis, and ab initio simulation showed a significant reduction in the mechanical strength of the solid electrolyte compared with electrochemically reacted LPSCl. This may contribute to formation of internal cracks in the solid electrolyte accelerating the lithium penetration.