Elucidation of Capacity Degradation for Graphite in Sulfide-Based All-Solid-State Lithium Batteries: A Void Formation Mechanism

Capacity degradation mechanism of graphite in sulfide-based all-solid-state lithium batteries has been investigated by adopting three different press conditions for cell assembling. Reversible redox reaction and stage transition of graphite were recognized from cyclic voltammetry and X-ray diffraction measurement. Initial charge and discharge capacity of graphite in the thus obtained three cells were ca. 310 and 270 mAh g(-1), respectively, indicating that the press conditions did not affect the initial charge and discharge capacity of graphite. After 50 cycles, the cell pressed at the highest pressure maintained over 95% of the capacity of that obtained at the 10th cycle. The capacity of the cell pressed at the lowest pressure, on the other hand, faded near 90%. The increase of resistance assigned to bulk solid electrolyte during cycling was detected by the impedance spectroscopy, and the cell pressed at the lowest pressure exhibited the largest resistance increase among all of the cells. Cross-sectional scanning electron microscope observation revealed formation of the voids in the bulk solid electrolyte, especially in the cell pressed at the lowest pressure, after 50 cycles, which resulted in the increase of the resistance for bulk solid electrolyte and capacity degradation of graphite.