Mesocrystallinely stabilized lithium storage in high-entropy oxides

Abstract

High-entropy oxides (HEOs) have received growing recognition as an anode candidate for lithium-ion batteries, primarily attributed to their decent lithium storage capabilities and high cycling durability. However, the underlying lithium storage mechanism of HEOs remains ambiguous, particularly the origins for their high structural stability, necessitating more comprehensive investigations. In this research, the working mechanisms of one representative HEO anode, the rock salt-structured Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O, are explored via state-of-the-art in-situ characterizations. Findings point to an interesting mesocrystal-stabilized lithium-ion storage mechanism responsible for maintaining the structural stability of HEOs during cycling, where, upon lithiation, Mg2+ remains electrochemically inactive within the oxygen lattice to stabilize the overall oxide framework. Co and Zn can be reversibly reduced/oxidized upon (de)lithiation, contributing to the electrochemical capacity; while for Cu and Ni, once reduced to metallic state under a relatively high current density, could not be re-oxidized but interconnect to form an electron-conductive network through the HEO body, contributing for the decent lithium-storage performance. Such feature depends on the applied current density, i.e. when decreasing the current, Ni regains its redox capability upon cycling with only Cu0 sustaining the conductive metallic network. This work is expected to serve as a benchmark for structurally and compositionally designing the next-generation high-entropy electrode materials for lithium storage.