Dynamic Volume Change of Li2S-Based Active Material and the Influence of Stacking Pressure on Capacity in All-Solid-State Batteries

All-solid-state batteries are considered as a reasonable further development of conventional lithium-ion batteries. While the same active materials may be used, solid electrolytes may offer higher safety than liquid electrolytes and enable the reversible operation of the lithium metal anode. Also, solid-state lithium/sulfur (Li/S) batteries are being investigated due to their high theoretical specific energy. Li2S-based composite positive electrodes have been demonstrated to achieve their theoretical capacity, high rate performance, and good cycling stability under high stacking pressures. However, during charging, Li2S is converted to sulfur and shrinks by 45%, and the resulting loss of contact between the composite particles leads to mechanical degradation during cycling. To better understand the correlation between the charge-discharge capacities and stack pressure during cycling in high-capacity Li2S-based active materials, the dynamic volume change of the positive electrode layer is measured. The volume change of the Li2S-based composite positive electrode is observed by using in situ scanning electron microscopy. Furthermore, by using Li4Ti5O12, which is well-known to undergo almost no volume change during cycling, as the counter electrode, charge-discharge capacities were found to depend on the initial volume change of the composite positive electrode. Finally, candidates for the negative electrode in full-cell applications of all-solid-state Li/S batteries are discussed. This study represents a major step toward mitigating mechanical deterioration in all-solid-state Li/S batteries stemming from the volumetric expansion and contraction of sulfur-active materials.