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Li-rich manganese-based (LRM) cathode materials are known as one of the most promising cathode materials for new-generation lithium-ion batteries. At present, exploring the complex voltage decay mechanism of LRM is the main task to promote its commercialization. Herein, the structural evolution and transition metal valence state change of LRM during different reaction stages under different temperatures are discussed, and the mechanism of voltage decay is finally determined based on the electrochemical properties. The results show the evolution of irreversible thermodynamic structure is the fundamental cause leading to voltage decay of LRM cathode, and it worsens with increasing temperature. The early activation of inert Mn, multiple phase transitions, migration of transition metals to the surface, anisotropy of internal valence states caused by partial valence failure and severe interfacial reactions are all strong proofs of the above views. In summary, the reason for voltage decay is revealed by investigating the sensitivity of the LRM cathode materials to temperature. This work not only provides strong evidence for the mechanism of the voltage decay, but also points out the direction to modification design for achieving future commercialization of LRM cathode materials.