A Dilatometric Study of Graphite Electrodes during Cycling with X-ray Computed Tomography

H Michael,J S Weaving,R Jervis,F Iacoviello,D J L Brett,T M M Heenan,A Llewellyn,P R Shearing

doi:10.1149/1945-7111/abd648

H Michael, J S Weaving + Show 6 more

Open Access

https://doi.org/10.1149/1945-7111/abd648

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Abstract

Graphite is the most commonly used anode material in commercial lithium-ion batteries (LiBs). Understanding the mechanisms driving the dimensional changes of graphite can pave the way to methods for inhibiting degradation pathways and possibly predict electrochemical performance loss. In this study, correlative microscopy tools were used alongside electrochemical dilatometry (ECD) to provide new insights into the dimensional changes during galvanostatic cycling. X-ray computed tomography (CT) provided a morphological perspective of the cycled electrode so that the effects of dilation and contraction on effective diffusivity and electrode pore phase volume fraction could be examined. During the first cycle, the graphite electrode underwent thickness changes close to 9% after lithiation and, moreover, it did not return to its initial thickness after subsequent delithiation. The irreversible dilation increased over subsequent cycles. It is suggested the primary reason for this dilation is electrode delamination. This is supported by the finding that the electrode porosity remained mostly unchanged during cycling, as revealed by X-ray CT.

Highlights

Phase transitions that occur in graphite during Li+ intercalation, solid electrolyte interphase (SEI) formation and degradation mechanisms can be tracked using X-ray Diffraction (XRD), and the associated volume changes monitored with electrochemical dilatometry (ECD)
Electrochemical dilatometry: single cycle characterisation.— Fig. 2 shows the voltage profile and the simultaneous dilation/ contraction recorded for graphite during its first discharge/charge cycle
Dimensional changes of graphite electrodes have been investigated using physical and electrochemical techniques to explore the effect that continuous dilation and contraction has on the overall electrode performance

Summary

OPEN ACCESS

A Dilatometric Study of Graphite Electrodes during Cycling with X-ray Computed Tomography. X-ray CT is useful for the inspection of cracks, particle expansion and material composition that contribute to electrode dilation/contraction.[16,17,18] The present study uses correlative ECD, scanning electron microscopy (SEM) and X-ray CT to elucidate the mechanisms driving the dilation of graphite electrodes in LiBs with a view to inhibiting this degradation pathway and possibly predicting its impact on capacity fade. The cell was held at open circuit for several hours prior to cycling, to allow for baseline stabilization This initial rest period helped to discern charging induced dimensional changes from the initial creep, which can be induced during charging as a result of the mechanical properties of the working electrode being altered. Scanning electron microscopy (SEM).—SEM imaging was performed using an EVO MA 10 microscope (ZEISS, UK) in order to examine morphological changes of pristine and tested graphite electrodes.

Results and Discussion

Conclusions

Graphite electrode