Howard Wang, PhD, State University of New York at Binghamton
In this project, time-resolved small angle neutron scattering (trSANS) has been used to monitor the structural evolution of the electrode materials in rechargeable batteries in operando. The battery assembly consists of graphite composite vs. Li electrodes for studying lithiation and delithiation processes in the electrode. SANS spectra were collected in real time over 10 lithiation/delithiation cycles. The variation of the scattering intensity upon charge/discharge is small compared to the overall intensity. A dominant scattering feature in the Q-range is the ~Q-4 power law behavior, which is characteristic Porod scattering law due mostly to interfacial contributions.
The variation of integrated SANS intensity upon cyclic charge/discharge rises and falls synchronizing with charge states. This is due to the contrast variation induced by lithiation/delithiation. As Li intercalates graphite particles, the overall SLD decreases, reducing the contrast with the matrix, hence decreasing the scattering intensity, and vice versa with de-intercalation. However, as the charge transfer becomes shallower at higher cycling rates, the amplitude of the intensity variation becomes even larger. This is contradictory to the prediction that the contrast variation is the only source of the scattering intensity change. The excess scattering could result from new surfaces created due to fracturing of graphite particles.
To examine the morphology of graphite particles upon lithiation/delithiation cycling, coin cells have been fabricated with the same construction and cycling history as the SANS cell, but been stopped at various times. Graphite particles were washed using DMC to remove the residual electrolyte and SEI and examined using field-emission SEM. Compared to the smooth surface in as-received graphite, all of the cycled particles have the conspicuous cracks on the surfaces. Minor cracks already appeared on graphite particle only after one cycle of lithiation/delithiation. As cycling goes on, particularly at high rates, more fractures have been generated, confirming the increase of surface areas due to fracturing.
To quantify the evolution of the new surface area responsible for the excess scattering from graphite particles, we use a model based on the Porod’s scattering law for a two-phase system with sharp interfaces. We assume that Li in graphite are uniformly distributed among all graphite particles as well as within individual particles. The fitting of SANS data are assessed by comparing two models of graphite surface area variation. Model calculation based on normal elastic expansion and recovery greatly under-estimate the Porod coefficient. On the other hand, by assuming the increasing the surface area to twice as much as the orginal, SANS data can be globally fit remarkably well. This quantity has been independently measured electrochemically using SEI formation and irreversible capacity loss from the first lithiation.
In summary, in situ SANS has been used to study the compositional and structural variation of graphite particles in a battery half-cell during their lithiation/delithation cycles. SANS data show that the variation of scattering intensity fully synchronizes with the lithiation/delithation cycles, decreasing upon lithiation and increasing upon delithiation. The observation is consistent with the variation of the neutron SLD of graphite due to Li intercalation hence the change the scattering contrast with the surroundings. Two aspects are particularly interesting from this measurement, one is the observation of SEI formation, when SANS intensity remain invariant; the other is the stress-induced fracturing of graphite particles, resulting in the creation of new surface formation therefore enhanced scattering. The irreversible process is partly responsible for the permanent capacity loss of LIBs for the consumption of mobile Li. Using the Porod’s law, we have quantitatively analyzed the lithiation state and the total area of fracture surfaces of graphite particles. SANS could be a useful tool to address the capacity fading of LIBs by quantifying the total surface areas of electrode particles in situ of battery operation.