O3-type lithium-rich transition-metal oxides have been attracting interest because of their large specific capacity achieved by additional oxygen-redox reactions. However, their practical application is precluded in part by continuous capacity and voltage fading upon cycling, which mainly originates from layer-to-spinel phase transformation in an O3-type oxide-ion array. In this work, as an attempt to suppress these degradations, we report O2-type cobalt-free lithium-rich layered oxides (Li[Li1/4Mn3/4]O-2-Li[Ni1/3Mn2/3]O-2 solid solution) which deliver large capacities of 220-250 mAh/g without substantial capacity or voltage fading upon cycling. Our study demonstrates that controlling the O2/O3 polymorphism is a promising approach toward designing large capacity positive electrode materials. (C) 2018 The Electrochemical Society.