Cobalt-Free O2-Type Lithium-Rich Layered Oxides

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[Li 1/4 Mn 3/4 ]O 2 −Li[Ni 1/3 Mn 2/3 ]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.