As the energy demand has grown exceptionally in the past several decades, the need to develop battery materials with higher energy and power density is on the table. To further push the capacity limit of next-generation battery material, a deeper insight into the reaction mechanisms and degradation processes is required. Operando characterization fills in this gap perfectly. It traces the behavior of active materials while the battery is running in its native environment, making it an excellent method to understand the origin of electrodes capacity and the cause of the deprivation. The broad purpose of this dissertation work is to uncover the reaction mechanisms and degradation processes of the battery material, including both electrode and electrolytes, using operando methods such as the high-energy synchrotron X-ray techniques and differential electrochemical mass spectrometry (DEMS). Synchrotron X-ray diffraction and X-ray absorption spectroscopy have made it possible to study the atomic structure transformations and electrochemical reactions of the electrode materials in a non-destructive way. DEMS combines the electrochemical cell with a mass spectrometer, enabling concurrent data collection from both electrochemistry and corresponding gas evolution reactions. Three projects were discussed herein to demonstrate the importance of operando characterization methods. First, the reaction pathways and activation reaction of Cu2S anode in two electrolyte systems were investigated using operando X-ray and electrochemical techniques. Second, the interaction between the CoS2 host and sulfur cathode in the lithium-sulfur battery was revealed by X-ray absorption spectroscopy. Finally, a coin cell-based DEMS cell was developed, allowing operando gas evolution analysis of lithium-ion batteries at room temperature and elevated temperature. These studies provide valuable insights into the degradation pathways of battery materials, facilitating the rational design of next-generation high-performance battery materials.