|Mapping Particle Charges in Battery Electrodes|
The deceivingly simple appearance of batteries masks their chemical complexity. A typical lithium-ion battery in a cell phone consists of trillions of particles. When a lithium-ion battery is charged or discharged lithium ions move from one electrode to another, filling and unfilling individual, variably-sized battery particles. The rates of these processes determine how much power a battery can deliver. Despite the technological innovations and widespread use of batteries, the mechanism behind charging and discharging particles remains largely a mystery, partly because it is difficult to visualize the motion of lithium ions for a significant number of battery particles at nanoscale resolution.
Researchers from Sandia National Laboratories and Berkeley Lab used synchrotron-based scanning transmission x-ray microscopy (STXM) at ALS Beamlines 5.3.2 and 11.0.2 to probe the charging and discharging dynamics of lithium iron phosphate (LFP), a promising positive battery electrode. By tracking the movements of lithium ions, one can decipher the process that ultimately limits the rate of battery charge and discharge. Previous work focused on either the behavior of a single battery particle or the spatially averaged behavior of the entire battery electrode. STXM gives a microscopic “lithium map” with a resolution down to 10 nm for hundreds of battery particles at a time. This unique visualization of lithium distribution across multiple length scales has generated significant insights into lithium-ion battery charging and discharging.
Researchers first charged commercial-grade battery cells to 50% full in 30 minutes, mimicking real world conditions. Then, the battery cell was immediately disassembled and the liquid electrolyte removed to lock in the lithium distribution. Next the battery was sliced into thin pieces about 500 nm thick using an ultramicrotome. Finally, the samples were brought to the ALS, where Fe L-edge spectromicroscopy was performed to determine the nanoscale distribution of oxidation states. In the charged, or de-lithiated state, the material adopts an Fe3+PO4 composition, whereas in the discharged, or lithiated state, the cathode material adopts a LiFe2+PO4 composition. Therefore, the local Fe oxidation state can be used to directly determine the local lithium content.
STXM couldn’t quite resolve individual particles, especially those smaller than 100 nm; complementary transmission electron microscopy (TEM) was performed on the same regions to discern one particle from another.
Researchers analyzed and quantified the local state-of-charge of approximately 500 individual LFP particles over nearly the entire thickness of the porous electrode. Using the STXM lithium maps and the high-resolution TEM images, researchers found that LFP battery particles do not charge simultaneously. Instead, the locked-in lithium distribution indicates that only about 2% of the battery particles were actively undergoing charging. The rest of the particles were either already charged or were yet to be charged.
This simple observation has a significant implication: The battery particles actively undergoing charging (2%) carry all of the electrical current. In other words, the local charging current is about 50 times higher than the overall charging current. The researchers propose that such behavior is due to the high nucleation barrier that limits the rate at which LFP becomes de-lithiated (charged).
These results confirm a mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating phase transformation. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation. Moving forward, the researchers will apply this powerful combination of techniques to study other battery chemistries.
Research conducted by: W.C. Chueh, F. El Gabaly, J.D. Sugar, N.C. Bartelt, A.H. McDaniel, K.F. McCarty, K.R. Fenton, and K.R. Zavadil (Sandia National Laboratories), T. Tyliszczak (ALS), and W. Lai (Michigan State Univ.).
Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES); Sandia Truman Fellowship in National Security Science and Engineering; and Michigan State University. Operation of the ALS is supported by DOE BES.
Publication about this research: W.C. Chueh, F. El Gabaly, J.D. Sugar, N.C. Bartelt, A.H. McDaniel, K.R. Fenton, K.R. Zavadil, T. Tyliszczak, W. Lai, and K.F. McCarty, “Intercalation Pathway in Many-Particle LiFePO4 Electrode Revealed by Nanoscale State-of-Charge Mapping” Nano Lett. 13, 866 (2013).
ALS Science Highlight #276