AbstractThe application of Li‐rich layered oxides is hindered by their dramatic capacity and voltage decay on cycling. This work comprehensively studies the mechanistic behaviour of cobalt‐free Li_1.2Ni_0.2Mn_0.6O₂ and demonstrates the positive impact of two‐phase Ru doping. A mechanistic transition from the monoclinic to the hexagonal behaviour is found for the structural evolution of Li_1.2Ni_0.2Mn_0.6O_2, and the improvement mechanism of Ru doping is understood using the combination of in operando and post‐mortem synchrotron analyses. The two‐phase Ru doping improves the structural reversibility in the first cycle and restrains structural degradation during cycling by stabilizing oxygen (O²⁻) redox and reducing Mn reduction, thus enabling high structural stability, an extraordinarily stable voltage (decay rate <0.45 mV per cycle), and a high capacity‐retention rate during long‐term cycling. The understanding of the structure‐function relationship of Li_1.2Ni_0.2Mn_0.6O₂ sheds light on the selective doping strategy and rational materials design for better‐performance Li‐rich layered oxides.