The Rice lab of materials scientist Ming Tang has analyzed nano- and micro-scale interactions within lithium iron phosphate cathodes through modeling and imaging offered by the transmission X-ray microscopy capabilities at Brookhaven National Laboratory and Argonne National Laboratory. Their paper, published in the journal ACS Energy Letters, supports theories Tang and his colleagues formed several years ago that foresaw how lithium travels in the dynamic environment inside a typical commercial cathode.

As an important battery cathode material, reaction distribution in lithium iron phosphate (LiFePO₄) has been extensively studied in dispersed particle systems, but remains poorly understood for mesoscopic agglomerates (or secondary particles) that are used in most commercial batteries. Herein, we apply three-dimensional X-ray spectroscopic imaging to characterize the two-phase structure in LiFePO4 secondary particles during electrochemical cycling.

(De)lithiated domains are found to not form the commonly assumed core–shell structure but develop highly anisotropic filamentary morphology that is rate independent and symmetric between charging and discharging. Phase-field simulations elucidate that the observed 1D phase growth behavior is not caused by the 1D lithium diffusivity of LiFePO4 but the elastic interaction between primary particles, which gives rise to stronger reaction heterogeneity than dispersed nanoparticles. As a result, uniform lithium (de)intercalation does not occur on the secondary particle surface even at high cycling rates.

—Wang et al.

A phase map of an agglomerated particle in a common lithium iron phosphate (LFP) battery electrode shows the charge distribution as it goes from 4% to 86%. FP refers to iron phosphate. Rice University scientists found that the FP phase spreads nonuniformly on an aggregate surface upon charging, rather than the expected even spread of lithium over the surface. The scale bar is 10 microns. (Credit: Mesoscale Materials Science Group/Rice University)

Batteries have a lot of particle aggregates that soak up and give up lithium, and we wanted to know what happens on their surfaces, how uniform the reaction is. In general, we always want a more uniform reaction so we can charge the battery faster.

—Ming Tang

In images taken at Brookhaven’s powerful X-ray synchrotron, the researchers saw some regions inside the cathode were better at absorption than others. The ability to look at single or aggregated particles in 3D showed that rather than reacting over their entire surfaces, lithium favored particular regions over others.

This is very different from conventional wisdom. The most interesting observation is that these reaction regions are shaped like one-dimensional filaments lying across the surface of these aggregated particles. It was kind of weird, but it matched what we saw in our models.

—Ming Tang

A study by Rice University materials scientists suggests that lithium batteries would benefit from more porous secondary (agglomerated) particles with better-aligned crystallites that don’t limit lithium distribution. The scientists studied 3D transmission X-ray images of cycled battery electrodes to analyze the phase change between lithium iron phosphate (blue) and iron phosphate (red) on the surface of particle agglomerates that make up the electrodes. (Credit: Mesoscale Materials Science Group/Rice University)

Tang said the lithium filaments looked something like thick nanotubes and were several hundred nanometers wide and several microns long. He said stress between misaligned crystallites in the particle agglomerates prevents lithium from being uniformly inserted into or extracted from the aggregate surface because that will generate too large an energy penalty. Instead, lithium is forced to flow into or out of the aggregates at “hot spots” that develop the filament shape.

Because the lithium can’t go into the cathode uniformly, it slows down the intercalation mechanics, Tang explained.

What our study offers is some potential ways to help make lithium insertion or extraction more uniform on these aggregates or individual particles. Introducing some porosity in the particle agglomerates might sacrifice some energy density, but at the same time would allow lithium to go in more uniformly. That could allow you to get more energy at a given charge/discharge rate.

Another thought is if we can somehow align the orientation of these small particles so their maximum expansion is perpendicular to each other, they’ll better accommodate lithium intercalation.

We don’t have enough experience in synthesis to know how to make that happen. What we’re providing is bait. Let’s see if somebody bites.

—Ming Tang

The Department of Energy, Basic Energy Sciences (DE-SC0019111) and the National Science Foundation (CMMI-1929949) supported the research.

Resources

• Fan Wang, Kaiqi Yang, Mingyuan Ge, Jiajun Wang, Jun Wang, Xianghui Xiao, Wah-Keat Lee, Linsen Li, and Ming Tang (2022) “Reaction Heterogeneity in LiFePO₄ Agglomerates and the Role of Intercalation-Induced Stress” ACS Energy Letters 0, 7 doi: 10.1021/acsenergylett.2c00226