<p>The widespread application of Li-rich manganese-based layered oxides (LROs), distinguished by their high energy density and low cost, is significantly constrained by voltage decay. The presently unresolved mechanism underlying this phenomenon impedes the development of effective countermeasures. Extensive research has established that voltage decay originates from irreversible oxygen release and structural evolution, but the relationship between these two factors remains incompletely characterized. This study investigates the impact of oxygen (O) redox and transition metal (TM) redox on voltage decay using Ni/Co-free Mn-full Li-rich layered oxides (MFLROs), which mitigates the confounding effects of concurrent Ni/Co redox and O redox. Electrochemical analysis demonstrates that stabilizing the O redox through Ti doping significantly inhibits voltage decay. Critically, multiple ex/in situ measurements and first-principles calculations reveal irreversible oxygen release as the dominant factor driving voltage decay. This insight establishes a foundational framework for addressing voltage decay in future research. </p> Graphical abstract <p></p>