Multimodal Characterization of Degradation Mechanisms in Lithium-Ion Batteries from Extreme Fast Charging

Abstract

Electric vehicles (EVs) powered by Li-ion batteries (LIBs) continue to gain prominence in comparison to traditional gasoline-powered vehicles. A major and immediate area of improvement for current LIBs used in EVs is to decrease the recharging time, in order to make it comparable to traditional refueling times. To this end, a sub-15 minute recharging time has been identified as an immediate target to achieve, called extreme fast charging (XFC) [1]. However, XFC of LIB is associated with safety and performance issues. Safety issues during XFC cycling can result from irreversible Li plating on the anode, which can lead to a short between the electrodes [2]. Two major pathways for capacity loss in LIBs over cycling are (i) depletion of the recyclable lithium inventory in the cell and (ii) the loss of sites on the active material (electrodes) [3]. Thus, an in-depth of understanding of the mechanisms of irreversible capacity loss including characterization of individual lithium loss mechanisms and the extent of electrode degradation, as well as the conditions that cause it hold the key to designing the next generation of XFC-capable LIBs.In this work, we employ a combination of sub-mm-scale in-situ high-energy x-ray diffraction (XRD) and ex-situ mass spectrometry titration (MST) to quantify the various loss mechanisms in LIBs from XFC cycling. XRD is used to furnish local information on the spatial heterogeneity of loss of lithium inventory (through mechanisms such as irreversible Li plating) and loss of active electrode material, as well as the contribution of such mechanisms to the global cell performance. MST is used to quantify the exact amounts of loss of lithium inventory over the entire cell (such as plated Li, Li-carbonate and Li-acetylide species). The end-of-life characterizations are conductedafter 450 XFC cycles, on single layer pouch cells (3 mAh/cm2 specific capacity, with graphite anode and NMC cathode), where the charging rate and protocol are systematically varied (4C to 9C). The experiments reveal that irreversible lithium plating and electrolyte oxidation/reduction reactions at either electrode are the primary contributions to XFC capacity fade. Dead lithiated graphite species that are co-located with the irreversibly plated lithium constitute a minor component of the capacity fade. Additionally, even though a loss of active electrode material is seen in the cells, the cell capacity is completely dictated by the loss of recyclable lithium ions. Such anapproach emphasizes the importance of multimodal, multiscale characterizations in understanding degradation pathways in LIBs, in order to design the next generation of XFC capable batteries.