The metastable O2-type cobalt-free lithium-rich layered oxide Li0.84Ni0.14Mn0.72O2 was successfully prepared by a new all solid-state ion-exchange reaction from the P2-type sodium layered oxide precursor Na0.7[Li0.14Ni0.14Mn0.72]O2 using lithium chloride at moderate temperature. The particular oxygen stacking in the resulting O2-type structure is assumed to suppress the detrimental layer-to-spinel phase transition usually observed upon cycling in conventional O3-type lithium-rich layered oxides due to the irreversible migration of transition metal cations, causing substantial voltage decay and capacity fading. Despite the existence of stacking faults originating from the P2-to-O2 topotactic reaction during the Na+-to-Li+ exchange, evidenced by X-ray diffraction simulation and high-resolution microcopy, the electrochemical tests conducted on the faulted O2-type positive electrode material revealed a greatly improved reversible (de)intercalation mechanism along with high specific capacity values. An operando X-ray diffraction study indicated that there are only small structural changes upon cycling and that they are stable and reversible. Moreover, operando X-ray absorption spectroscopy experiments showed that a large part of the capacity relies on the oxygen redox which is also reversible upon cycling.Read less <