Ni‐rich layered oxides (LiNi1–xMxO2; M = Co, Mn, …) are appealing alternatives to conventional LiCoO2 as cathodes in Li‐ion batteries for automobile and other large‐scale applications due to their high theoretical capacity and low cost. However, preparing stoichiometric LiNi1–xMxO2 with ordered layer structure and high reversible capacity, has proven difficult due to cation mixing in octahedral sites. Herein, in situ studies of synthesis reactions and the associated structural ordering in preparing LiNiO2 and the Co‐substituted variant, LiNi0.8Co0.2O2, are made, to gain insights into synthetic control of the structure and electrochemical properties of Ni‐rich layered oxides. Results from this study indicate a direct transformation of the intermediate from the rock salt structure into hexagonal phase, and during the process, Co substitution facilities the nucleation of a Co‐rich layered phase at low temperatures and subsequent growth and stabilization of solid solution Li(Ni, Co)O2 upon further heat treatment. Optimal conditions are identified from the in situ studies and utilized to obtain stoichiometric LiNi0.8Co0.2O2 that exhibits high capacity (up to 200 mA h g−1 ) with excellent retention. The findings shed light on designing high performance Ni‐rich layered oxide cathodes through synthetic control of the structural ordering in the materials.

batteries crucially relies on electrochemical characteristics of electrode materials, i.e., anode and cathode materials. [ 1 ] Various alternative anode materials have recently been developed, including silicon-based composites, [ 2,3 ] nanoscale transition metal oxides, [ 4,5 ] titanium-based materials, [ 6,7 ] and graphene-based sulfi de, [ 8 ] etc. These materials have demonstrated excellent rate capability and specifi c capacities several times higher than conventional graphite anodes. Since capacities of cathode materials are usually much lower than those of anodes, the cathode is considered as the limiting factor for lithium ion batteries. To a great extent, development of newgeneration lithium ion batteries is limited by the low energy density, low operating voltage, and poor rate capability of cathode materials. [ 9 ] Recently, the emerging Li-excess layered oxides have attracted a great deal of research efforts due to their high capacities. These cathode materials can be cycled over a broad voltage range between 2.0 and 4.8 V versus Li/Li + and deliver specifi c capacities higher than 250 mAh g −1 . They also offer many other advantages such as low cost, environmental benignity, and safety. [ 10 ] A representative example is Li-excess ternary manganese-nickel-cobalt oxide, composed

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