The overlithiation and delithiation behaviors of sol-gel formed LiNi0.75C0.25O2 are investigated in this work. As an insertion host material, LiNi0.75C0.25O2 is cycled in a broad working potential range of 1.35-4.3 V. It shows two lithiation platforms at 1.75/3.6 V and only one reverse delithiation platform at 3.7 V. The long lithiation platform at 1.75 V is attributed to the overlithiation behavior of LiNi0.75C0.25O2. Although overlithiation reaction takes place below 2.0 V, LiNi0.75C0.25O2 can maintain electrochemical reversibility during the initial 20 cycles. To study the overlithiation/delithiation process, the structural evolution of LiNi0.75C0.25O2 is thoroughly observed by in-situ X-ray diffraction method. It can be found that the electrochemical reaction between LiNi0.75C0.25O2 and Li in 1.35-4.3 V is a reversible process, which is associated with a single-phase transition process above 3.0 V and a two-phase transformation below 2.0 V. This result not only demonstrates the high structural and electrochemical reversibility of LiNi0.75C0.25O2 in 1.35-4.3 V, but also proves the improved structural stability of LiNi0.75C0.25O2 via partial replacement of Ni by Co in layered LiNiO2. (C) 2015 Elsevier Ltd. All rights reserved.