The purpose of this work is the study of the charge–discharge mechanism in the all-solid-state lithium/composite-polymer-electrolyte/pyrite battery operating at 120°C. Effects on the charge–discharge mechanism of particle size, type of binder and cathode preparation method of the pyrite-based cathode are addressed. Analysis of the experimental XRD, XPS and electrochemical data suggests that creation of sulfur vacancies in pyrite suppresses a sudden jump of charge voltage, which is associated with slow mass transport of iron(II) cations through the Li2FeS2 phase. We believe that our experimental findings show considerable promise of creating sulfur-deficient pyrite structures for cathodes to be used in high-energy-density all-solid-state lithium batteries. The nature and exact composition of a 1.2–1.3V discharge plateau is still unclear. To clarify the composition of this low-voltage phase synchrotron X-ray absorption measurements were performed on a series of cells cycled more than 100 times. The results will be presented in a forthcoming publication.