Local structure and conversion chemistry of high-entropy oxides as Li-ion anodes

High-entropy oxides (HEO) enable stabilization of numerous transition metals in a single solid solution phase, and provide a new platform for electrode design, overcoming the rapid degradation of simple metal oxide conversion anodes for Li-ion batteries (LIBs). Their promising electrochemical performance with high capacity and long cycling stability was first demonstrated for the (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O composition. In this study, we use advanced characterization methodologies to assess individual contribution of each metal in this composition to the electrochemical reaction mechanism on charge and discharge. The change in local structure at the endpoints of the initial conversion reaction and through three hundred cycles is analyzed by ex situ extended x-ray absorption fine structure (EXAFS) for Co, Ni, Cu, and Zn K-edges. The number of near neighbors and bond distances extracted from the EXAFS elucidates the electrochemical activity of each atomic species and describes a new disordered state consisting of very small, isolated metallic nanoparticles in an oxide matrix, capable of storing remarkable amounts of Li for hundreds of cycles.