The overlithiation and delithiation behaviors of sol-gel formed LiNi0.75Co0.25O2 are investigated in this work. As an insertion host material, LiNi0.75Co0.25O2 is cycled in a broad working potential range of 1.35–4.3V. It shows two lithiation platforms at 1.75/3.6V and only one reverse delithiation platform at 3.7V. The long lithiation platform at 1.75V is attributed to the overlithiation behavior of LiNi0.75Co0.25O2. Although overlithiation reaction takes place below 2.0V, LiNi0.75Co0.25O2 can maintain electrochemical reversibility during the initial 20 cycles. To study the overlithiation/delithiation process, the structural evolution of LiNi0.75Co0.25O2 is thoroughly observed by in-situ X-ray diffraction method. It can be found that the electrochemical reaction between LiNi0.75Co0.25O2 and Li in 1.35–4.3V is a reversible process, which is associated with a single-phase transition process above 3.0V and a two-phase transformation below 2.0V. This result not only demonstrates the high structural and electrochemical reversibility of LiNi0.75Co0.25O2 in 1.35–4.3V, but also proves the improved structural stability of LiNi0.75Co0.25O2 via partial replacement of Ni by Co in layered LiNiO2.