Evaluation of High-Rate Pulsed Vs. Continuous Charging in Lithium-Ion Batteries

Abstract

Many current and future Navy systems require high power lithium-ion batteries (LIB), with recent applications including directed energy weapons and the electromagnetic railgun pushing these requirements to new extremes. LIB with extremely high discharge rate are commercially available, however, high rate charging (or “fast charging”) is significantly more difficult to achieve due to complications occurring at the electrode-electrolyte interfaces as each electrode nears the edge of electrolyte stability. These problems are further exacerbated due to increased Ohmic heating with increasing current density, and can lead to potentially dangerous cell failure events. In this study, pulsed current charging was compared with continuous current charging during extended cycling of commercially available LIB with graphite anode and lithium iron phosphate cathode. Pulsed current charging has been shown in the literature to improve the efficiency and reduce the capacity loss penalties associated with continuous current charging. Charging limits at and above the manufacturers recommended operating limits were implemented. Degradation of cells was evaluated by monitoring cell temperature and impedance and accompanied by cell disassembly to analyze electrode degradation from the macro to the nanoscale using visual inspection, scanning electron microscopy (SEM), X-ray diffraction(XRD), and X-ray photoelectron spectroscopy (XPS). Preliminary results show that in addition to the expected temperature rise associated with high current operation, fast charged cells lose capacity at a faster rate than cells charged at lower rates. Furthermore, signs of an increased solid electrolyte interface (SEI) layer thickness and lithium plating were also observed from cell disassembly.