Failure Modes in Sulfide-Based All Solid-State Batteries (ASSB) Investigated By Operando SEM

Studies on solid state batteries (ASSB) which began back in 190’s has seen a tremendous growing interest for multiple reasons such as the safety and the ability to use the metallic anodes for enhancing the gravimetric energy density. However, it exists different issues linked with poor performance of the batteries, which could be the results of interfacial challenges such as poor ion transport, dendrite formation, electrochemical degradation and chemo-mechanical degradation[1]. Among the potential parameters impacting the cycling process, the influence of the particle size of solid electrolyte is not well studied. In this study, we focused on sulfide-based SEs particle size distribution and its impact on electrochemical performance by investigating from the morphological point of view via operando SEM. A batch of Argyrodite (Li6PS5Cl (LPSCl)) was sieved to obtain two batches of particles range (0,5-20µm and 50-150µm). The AASBs were prepared using the two particles size distribution while keeping the anode (lithium metal), cathode (MA: NMC) composite formation and the fabrication process constant. The different morphological and chemical evolutions were monitored in real time by scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX) to obtain a complete of the different modification throughout the battery components and their interfaces. To do so, a home-made electrochemical cell was developed to perform in-situ and operando cycling in the SEM. From the two particles sized distribution, the one with the large particles have better electrochemical performance. From morphological point of view at the cathode composite electrode, the formation of a cathode electrolyte interface is visible in the different batteries but is more developed in the case of small sized particles. In the solid electrolyte, the type of cracks differs with relatively strait feature for small particles and sinuous one for large one. At the anodic interface, a lost in contact between the SE separator and the lithium is observed and increased with the cycling process with a higher impact for the small particles (Figure 1.a and b). By investigating further, the formation of dendrites with different morphologies are visible at the anodic interfaces (Figure1.c). We categorized the observed changes into three modes: (i) electrical failure by the formation of lithium dendrites of different morphologies following the ES going up to short circuit in the case of small particles, (ii) mechanical failure by the formation of cracks in the electrolyte whose shape and propagation strongly depend on the distribution particles size and (iii) electrochemical failure with the formation of solid electrolyte interphases on the surface of the active material. The main obstacles for the use of lithium metal are related to the propagation of lithium dendrites and thus to the mechanical instability of solid electrolytes.