Gas generation induced by parasitic reactions in lithium-metal batteries (LMB) has been regarded as one of the fundamental barriers to the reversibility of this battery chemistry, which occurs via the complex interplays among electrolytes, cathode, anode, and the decomposition species that travel across the cell. In this work, a novel in situ differential electrochemical mass spectrometry is constructed to differentiate the speciation and source of each gas product generated either during cycling or during storage in the presence of cathode chemistries of varying structure and nickel contents. It unambiguously excludes the trace moisture in electrolyte as the major source of hydrogen and convincingly identifies the layer-structured NCM cathode material as the source of instability that releases active oxygen from the lattice at high voltages when NCM experiences H2 → H3 phase transition, which in turn reacts with carbonate solvents, producing both CO and proton at the cathode side. Such proton in solvated state travels across the cell and becomes the main source for hydrogen generated at the anode side. Mechanisms are proposed to account for these irreversible reactions, and two electrolyte additives based on phosphate structure are adopted to mitigate the gas generation based on the understanding of the above decomposition chemistries.