Unveiling Reaction Mechanisms of Non-Aqueous Zn-Ion Batteries – Zn/LiFePO4 System

Abstract

Zinc-ion batteries (ZIBs) are among the most promising energy storage technologies because of their low cost, safety, and environmental friendliness. In this work, we focus on investigating the electrochemical reaction mechanisms of ZIBs with Lithium iron phosphate (LiFePO4) as an active material in the positive electrode, Zn(OTf)2+LiCl dissolving Tetraethylene glycol dimethyl ether (TEGDME) as the electrolyte, as well as metallic Zinc as the negative electrode. After testing the ZIB cells by galvanostatic cycling, the cells with TEGDME exhibit the best electrochemical performance in terms of long-term stability. The initial specific discharge capacity is 118.3 mAh g-1 and the capacity retention is 91.4% after 100 cycles at a current density of 10 mA g-1. We propose the reaction mechanisms during the charge-discharge processes involve extraction/insertion processes of Li+ into/out of the olivine structure of LiFePO4 as evidenced by X-ray near-edge spectroscopy (XANES). According to X-ray diffraction (XRD) analysis of the discharged and charged LiFePO4 electrodes, the crystal structure changes from LiFePO4 to FePO4 when charging and back into LiFePO4 again when discharging. Besides, XRD and scanning electron microscopy (SEM) confirm that the cycled Zn electrode is hardly present with passivation and/or corrosion. Therefore, this new combination of Zn/LiFePO4 and TEGDME has a potential for ZIB applications.