Lithium Plating and Lithiation Heterogeneities in Graphite Anodes after Fast Charging

Charging time and range anxiety are two key barriers to wider electric vehicle (EV) adoption. Reducing the charging time of EVs to be comparable to the refueling time for internal combustion vehicles requires batteries capable of being charged quickly. However, fast charging of conventional lithium-ion batteries with a graphite anode is limited by the degradation reaction of lithium plating as metal on the electrode. Lithium plating can cause significant capacity loss and safety issues. Many EV battery fires have occurred in recent years, and catastrophic failure can come from a single battery cell. While many of these fires came from a short circuit, some fires occurred in stationary EVs that were neither being charged nor driven. Understanding the interactions between plated lithium and graphite during and after fast charging is imperative for developing strategies to mitigate catastrophic failure. The main goal of this dissertation is to shed light on these interactions by using synchrotron hard X-ray microtomography to characterize batteries in three dimensions before and after fast charging./nIncreasing the energy density of batteries is also important for increasing driving range and alleviating range anxiety. Battery chemistries involving lithium metal or silicon are of significant interest due to their high specific capacities. However, both materials experience significant degradation during cycling. Conventional liquid electrolytes are unstable with lithium metal, so solid-like polymer electrolytes are often used instead. However, even in lithium metal batteries with polymer electrolytes, lithium protrusions can nucleate on impurities in the lithium metal and grow through the electrolyte, ultimately causing failure by short circuit. We aim to study the morphology of lithium protrusions in a polymer electrolyte. We also investigate a method to reduce the concentration of impurities in the lithium metal in order to limit the possible nucleation sites for lithium protrusions. Silicon experiences massive volume changes during cycling, leading to large stresses that cause pulverization, loss of electrical contact, and capacity fading. We aim to observe the phase changes and expansion that occurs in silicon during the first charge./nChapter 1 introduces graphite, lithium metal, and silicon anodes, and information about X-ray tomography. Composite electrodes are complex and difficult to image with X-ray tomography. Thus, lithium metal foil, a much simpler electrode, is investigated first in Chapters 2 and 3. Chapter 2 quantifies the relationship between the diameter and height of electrodeposited lithium protrusions through a polymer electrolyte. In Chapter 3, thermally evaporated lithium metal is investigated as a replacement for conventional rolled lithium metal foil. The concentration of impurities in the thermally evaporated lithium is found to be two orders of magnitude lower than that in rolled lithium./nChapters 4 through 6 expand the application of X-ray tomography to fast charged batteries with a graphite anode. Chapter 4 introduces image processing techniques and an in situ cell for conducting high-resolution X-ray microtomography experiments to quantify three-dimensional lithiation heterogeneity and lithium plating. Lithium plating at the surface of the graphite near the electrode-separator interface is found to lead to underlithiated graphite particles underneath, well-removed from the separator. Chapter 5 applies these techniques in operando throughout the rest period immediately after fast charging. Time-resolved measurements of lithium plating and lithiation over time allow for the first measurements of ionic currents within a graphite anode during rest. Large current densities greater than 20 mA cm-2 are measured for more than 20 minutes after current to the battery is shut off. After rest, when a battery is discharged, plated lithium that was not stripped or did not react with electrolyte can become trapped as inactive lithium. Chapter 6 compares measurements of inactive lithium measured by X-ray tomography with those by an ex situ mass spectrometry titration technique on the same electrodes. Good agreement is found between the techniques./nChapter 7 extends the developed X-ray tomography methods to a lithium-ion battery with silicon anode. Increases in the thickness of the electrode that result from the expansion of the silicon particles during lithiation are measured throughout the first charge. Multiple silicon alloy phases are observed within two separate layers that appear near the beginning of charging and merge near the end of charging./nTo make progress towards fast charging and high energy density batteries, a fundamental understanding of the heterogeneities that evolve within electrodes during charging is required. The objective of this dissertation is to increase the depth of understanding of lithium plating and lithiation heterogeneities and the resulting local interactions, enabling the design of safer batteries.

Advisor: Balsara, Nitash P