Unveiling complex lithiation/delithiation mechanism in AgNbO3 model perovskite using operando X-ray absorption spectroscopy

Abstract

In AgNbO₃ perovskite structure, electrochemical activation is speculated during the first lithiation cycle enabling the material to reversibly store Li⁺ by the contributions of both Ag and Nb cation. However, the origin of electrochemically induced structural activation and understanding of cations involvement in complex Li⁺ storage mechanism is still elusive. Herein, operando synchrotron X-ray absorption spectroscopy (XAS) was applied to clarify this mechanism under different cycling conditions. Ag K-edge XAS measurements during first lithiation revealed a gradual Ag⁺ to Ag⁰ reduction starting at a relatively high potential of 1.0 V vs Li⁺/Li, thus creating vacancies in the lattice for Li⁺ insertion and inducing a crystalline-to-amorphous structural transition. Below 0.3 V vs Li⁺/Li, metallic Ag forms multiple intermetallic Li-Ag alloys, resulting in lithium-rich Li₉Ag at the end of lithiation. Simultaneously, Nb K-edge XAS measurements indicate an irreversible Nb⁵⁺ to Nb³⁺ reduction with formation of metastable phases during first lithiation. Upon extended cycling at high current densities, intermediate phases sustain reversible Li⁺ storage through Nb-redox activity and Li-Ag (de)alloying reactions, facilitating fast charging capability. This study will help in designing new conversion-alloying type negative electrodes for fast-charging batteries.