Solidifying the components of a Li-ion battery consisting of a high energy capacity oxide cathode (e.g. NMC, NCA), a Li metal anode, and a sulfide-based glass ceramic solid electrolyte (SE), is a pathway to overcome the challenges of liquid electrolyte cells, namely dendrite growth and safety concerns without compromising high energy density. Even though the conductivity of a few sulfide solid electrolytes (SSEs) surpasses that of liquid electrolytes, multiple interfacial phenomena at both the cathode and anode interfaces play a crucial role in affecting efficient battery performance. Such phenomena include solid-electrolyte interphase (SEI) formation and mechanical deformation; the SEI forms due to poor chemical and electrochemical stability of SSEs while mechanical deformation arises from volume changes experienced by the cathode during cycling and also the rigid nature of SSEs. In the past several decades, tremendous effort has been made to study the SEI for SSEs, however, the properties of cathodic and anodic SEIs and their effects on long-term All Solid State Lithium Battery (ASSLB) cycling are still not well-understood.
In this study, we applied an argyrodite- (Li6PS5Cl) based Li ion conducting SSE for fabrication of an all solid-state Li-metal battery. Li6PS5Cl was synthesized by mechanical milling followed by heat treatment. It exhibits a room temperature ionic conductivity of 1 mS/cm. An ASSLB was fabricated with this electrolyte, an NCA cathode and a Li metal anode. The as-fabricated ASSLB with the LiNbO3 coating on the cathode exhibits a room temperature capacity of 140 mAh/g at 0.1C with an initial Coulombic efficiency of 68%, compared with 85% for a liquid cell counterpart. There is 10% capacity loss for the initial 20 cycles followed by negligible loss after 90 cycles. Even when replacing the Li metal with a LiIn alloy (0.625 V vs Li/Li+), a similar capacity fade is observed which clearly indicates that the cathode is the dominant contributor to the capacity decay. Although it is not fully understood, part of the irreversible capacity loss arising at the 1st cycle is due to electrochemical decomposition of the SSE at both the cathode and anode interfaces. The SEI was characterized via X-ray photoelectron spectroscopy (XPS) and Raman and the results match with DFT-based calculations. The electronically insulating nature of the SEI prevents further electrochemical decomposition of the SSE within the operating voltage of NCA (2.5 to 4.3 V vs. Li/Li+) after the 1st cycle. Further degradation up to 20 cycles may arise due to volume change of the cathode during cycling and also the poor mechanical property of the SEI. Major cracks are visible in SEM images taken of the cathode composite after the 1st cycle. We will discuss a few mitigation strategies for improving ASSLB.