Sulfide-based all-solid-state batteries (ASSBs) are next-generation batteries, which resolve the safety issues of energy storage systems. Elaborated intimate contact by providing constant external pressure using a customized cell is a way to overcome chemo-mechanical deterioration associated with interfacial issues; however, it is not a practical approach. Here, ASSBs are evaluated by adopting a typical coin-type cell at low pressure (& AP;0.3 MPa) and it is confirmed that cathode deterioration is a more significant factor in lowering capacity retention than contact loss. Sulfide is infused surprisingly along the grain boundary of the cathode, causing gradual lithium deficiency in the cathode active materials by capturing the active lithium, which is revealed by time-of-flight secondary-ion mass spectroscopy using a lithium isotope (Li-6). This study sheds light on the urgency of resolving the depletion of lithium ingredients during cycling rather than surface modification, by investigating the factors that accelerate degradation of the cathode during low-pressure operation of ASSBs.