Ion-Exchange Mechanism of Layered Transition-Metal Oxides: Case Study of LiNi_0.5Mn_0.5O₂

An ion-exchange process can be an effective route to synthesize new quasi-equilibrium phases with a desired crystal structure. Important layered-type battery materials, such as LiMnO2 and LiNi(0.5)Mn(0.5)O2, can be obtained through this method from a sodium-containing parent structure, and they often show electrochemical properties remarkably distinct from those of their solid-state synthesized equivalents. However, while ion exchange is generally believed to occur via a simple topotactic reaction, the detailed phase transformation mechanism during the process is not yet fully understood. For the case of layered LiNi(0.5)Mn(0.5)O2, we show through ex situ X-ray diffraction (XRD) that the ion-exchange process consists of several sequential phase transformations. By a study of the intermediate phase, it is shown that the residual sodium ions in the final structure may greatly affect the electrochemical (de)lithiation mechanism.