Mn-based layered oxide cathodes with anionic redox activity are promising candidates for sodium-ion batteries (SIBs) owing to their high operating voltage and energy density. However, the inevitable Mn3+ generation after cycles results in severe Jahn–Teller distortion, damaging the long cycling stability of the cathodes. Herein, we present a metal-to-metal charge transfer (MMCT) mechanism that acts at the end of discharge to inhibit the generation of Mn3+, thereby improving the structural sustainability of the electrode. A P2-Na0.69Li0.23Ru0.17Mn0.6O2 cathode is selected as a model. The doping of highly electronegative Ru regulates the covalency of asymmetrical Mn–O–Ru, ensuring the electron transfer between Mn3+ and Ru4+, which reduces Mn3+ content and mitigates the Jahn–Teller distortion. As a result, the electrode shows a robust structural stability with a minimal unit cell volume change of only 0.2%. Moreover, the cathode exhibits excellent electrochemical performance, maintaining capacity retention of over 90% after 900 cycles at 2–4.3 V. The findings provide fresh insights for inhibiting the Jahn–Teller effect, offering a new paradigm for the application of Mn-based layered cathodes with high energy density for sustainable SIBs.