Abstract
Lithium metal was left unused for a long time in conventional liquid-based batteries, with a fear of catching fire for safety reasons. However, with advancements in solid electrolytes, these were considered the holy grail in the development of Li metal-based all-solid-state batteries (ASSBs) alongside NMC which is considered the next-generation cathode material. Unfortunately, Li metal faces tremendous challenges when brought into contact with many currently considered promising solid electrolytes (SEs). The associated challenges are electrical, electro-(chemical) and mechanical instabilities. Surprisingly, if one studies the literature thoroughly, it seems that batteries based on SEs show much faster dendrite formation than liquid electrolytes. For this particular study, thanks to in situ and operando SEM, we studied these sulfide-based SEs, viz. β-Li3PS4 (LPS) and Li6PS5Cl (LPSCl). We highlight the key differences in failure modes of two sulfide-based SEs, by keeping the anode (Li metal) and cathode (NMC111) constant. For both the SEs, initially, electro-chemo-mechanical stress is induced, due to volume expansion of active materials and plating during cycling. However, in the case of LPS, this leads to electrical failure causing a short circuit, whereas in the case of LPSCl it leads to further mechanical damage-causing delamination of the cathode. Thus, the porous structure of SEs heavily determines the mechanical behaviour of the ASSB, the distribution of the induced electro-chemo-mechanical stress and in turn, the mechanism of ASSB failure. The other striking observations are: (1) cracks travel ahead of the dendrites (2) cracks grow and propagate at a much faster rate in the case of β-LPS than in LPSCl (3) higher surface roughness of LPSCl gives rise to many different morphologies of Li deposition/dendrites (4) the cathode electrolyte interface (CEI) forms and grows at a much faster rate in the case of LPS than in LPSCl. We demonstrate that differences in cycling performance and mode of failure are due to the differences in SEs. We also highlight that the key limitation in implementing Li metal as an anode in ASSBs is the dendrite formation and mechanical instability. These findings again emphasize the importance of coating active materials, the introduction of a buffer layer between the SE/Li metal interface, coating of SE, or the need for using composite/hybrid or bilayer solid electrolytes at least.