Detecting voltage shifts and charge storage anomalies by iron nanoparticles in three-electrode cells based on converted iron oxide and lithium iron phosphate

Abstract

Noticeable voltage shifts have been observed in the charge/discharge profiles of a three-electrode cell with a lithium metal reference electrode and having a deeply lithiated iron oxide (Fe/Li2O) negative electrode galvanostatically cycled in a limited potential range against a positive LiFePO4 counterpart. The origin of such shifts has been attributed to charge storage anomalies in the Fe/Li2O nanocomposite due to characteristic reduced Fe nanoparticle sizes. These shifts also affected the extreme points of the voltage profiles of the positive electrode, which was also independently monitored. A combined evaluation of voltage profile slippages with possible changes in internal resistance and/or Li+ inventory loss, including an aimed analysis of current interruptions at the end of each lithiation/de-lithiation half-cycle to monitor the internal resistance and diffusion resistance coefficient of the Fe/Li2O electrode, has enabled a clarification of its altered charge storage. An asymmetric behaviour of the Fe/Li2O electrode during Li+ uptake/release has been revealed, highlighting a progressive, diffusion-controlled-type voltage drift at low potentials vs. Li+/Li, and an unusual tendency to slight oxidation with capacitive variations during the reverse electrochemical processes at higher voltages, instead.