Co doping induced high initial utilization boosting Li storage performance of ZnFe2O4

Abstract

Spinel zinc ferrite (ZnFe₂O₄) exhibits large theoretical capacity based on the lithiation reaction involving multi-electron transfer, which is irreversible and yield ZnO and Fe₂O₃ after the first recharging process. However, the poor intrinsic electrical conductivity of ZnFe₂O₄ make them difficult to be fully utilized, which leads to lower specific capacity compared with the theoretical value. In this work, metal cation (Co) doping is conducted by a simple hydrothermal method to enhance the utilization of ZnFe₂O₄ during the first cycle. Density functional theory calculations revealed that Co doping could alter the electronic structure of ZnFe₂O₄, resulting in larger adsorption energy of lithium ions. Meanwhile, Co doping introduces new electron state near the Fermi level, contributing to lower migration energy barrier of lithium ions and faster charge transfer. Benefiting from the improved lithiation reaction kinetics, as indicted by electrochemical impedance spectroscopy (EIS), Co doped ZnFe₂O₄ exhibits much larger discharge/charge capacity during the first cycle, suggesting its higher utilization. The large capacity could also be maintained in subsequent cycles. Specifically, Co doped ZnFe₂O₄(Co-ZFO-5) exhibits a specific capacity of 839 mAh g⁻¹ after 200 cycles at 0.5 A g⁻¹, and a specific capacity of 510 mAh g⁻¹ after 500 cycles at 1.0 A g⁻¹. This work provides a fundamental understanding to the intrinsic structure-function relationship of transition metal oxide anode.