The influence of high C rate pulsed discharge on lithium-ion battery cell degradation

Over the past decade, lithium-ion battery (LIB) technology has advanced beyond the scope of simple consumer electronic devices. Nowadays, LIBS of advanced, high power chemistries are being used as a prime power source for many large scale applications, such as electric automobiles and pulsed power platforms, which require a higher standard of performance. LIB degradation mechanisms have been studied thoroughly for decades in nominal rate cycling applications. The common mechanisms that attribute to cell capacity fade and failure are well documented in these cases. However, recent studies into cells cycled at rates above five times their nominal capacity rate show that some degradation mechanisms become more favorable than others. In some extreme cases, cell failure can occur abruptly during ultrahigh rate pulsed discharged cells as a result of accelerated degradation phenomenon occurring at the surface of the cell electrode materials. In the work presented here, lithium iron phosphate (LFP) cells have been cycled at 15C with a pulsed discharge profile and the results show unique capacity fade when compared to previously published studies. An abrupt decrease in the usable capacity fade occurs within forty cycles of high rate operation. Electrochemical impedance spectroscopy (EIS) shows that there is a separation in the EIS spectrum over the lifetime of the cell indicating a dominant failure mechanism which is responsible for failure. Electron micrographs of the electrodes surfaces show large SEI buildup on the surface of the graphite anode. However, the buildup is uneven across the length of the roll with the bulk of the growth occurring on the material near the cell core. Areas of graphite anode near the edge of the cell show far less growth and contain small areas that are relatively unaffected by solid-electrolyte interphase (SEI) layer buildup. Overall, the evidence further indicates that the previously presented studies on LIB degradation mechanisms is insufficient for characterizing degradation in cells cycled at ultra-high rates with a pulsed discharge profile. These mechanisms must be understood in order to fully understand how these cells will behave and perform when they are used at high pulsed rates in future, compact, high energy, and high pulsed power systems.