The Lithium-Sulfur (Li-S) battery with metal sulfide additives attracts more interests in recent years due to its potential to provide higher energy density, to increase electric capacity, and to improve the electrical conductivity of the insulated sulfur electrode. This multi-functional design on cathode showed initial promise; however, complicated interactions at the system level are accompanied by some detrimental side effects. The metal sulfide additives that promote a chemical conversion as the reaction mechanism, e.g., CuS and FeS₂, to increase the theoretical capacity of the Li-S system can cause undesired parasitic reactions, such as the dissolution of the additive in the electrolyte. Studying such complex reactions presents a challenge because it requires experimental methods that can track the chemical and structural evolution of the system during an electrochemical process. To address the fundamental mechanisms in these systems, structural and chemical evolution of CuS additive in Li-S battery was conducted via operando multi-modal synchrotron techniques in this work. Transformation of phase and chemical state in S-CuS hybrid electrode during cell discharge/charge was investigated by X-ray Powder Diffraction and X-ray Absorption Spectroscopy. The Cu ion dissolution and its migration from cathode to anode during battery reaction were monitored by X-ray Fluorescence Microscopy. With the combination of operando multi-modal synchrotron approach, a new reaction mechanism was understood and proposed in this study.