PME researchers collected data on how different components of a thick lithium-ion battery electrode evolve after successive cycles (one snapshot of the microscopy data is shown on the left). They then used this data to create a computational model (right) illustrating the degradation of the electrode and pointing toward how to improve the lifespan of the batteries. Image courtesy of the Laboratory for Energy Storage and Conversion.From the moment you first use it, a new lithium-ion battery is degrading. After a few hundred charge cycles, you’ll notice – your phone, laptop or electric car battery will start to wear out more quickly. Eventually, it will stop holding a charge at all.
Researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) have now used a combination of high-powered electron microscopy and computational modeling to understand, at an atomic level, exactly what occurs when lithium-ion batteries degrade. Their research points toward one approach to designing longer-lasting lithium-ion batteries – by focusing on an oft-ignored structural component called the carbon binder domain (CBD).
“To tackle many of the world’s energy storage and conversion challenges over coming decades, we need to keep innovating and improving batteries,” said Shirley Meng, who led the research, which is reported in a paper in Joule. “This work is one step toward more efficient and sustainable battery technology.”
The widespread commercialization of lithium-ion batteries at the end of the twentieth century played a central role in the advent of lightweight, rechargeable electronics. Lithium is the lightest metal and has a high energy-density-to-weight ratio. When a lithium-ion battery is charged, lithium ions move from a positively charged cathode to a negatively charged anode. To release energy, those ions flow back from the anode to the cathode.
Throughout charging cycles, the active materials of the cathode and anode expand and contract, accumulating ‘particle cracks’ and other physical damage. Over time, this makes lithium-ion batteries work less well.
Researchers have previously characterized the particle cracking and degradation that occurs in small, thin electrodes for lithium-ion batteries. However, thicker, more energy-dense electrodes are now being developed for larger batteries — used in applications such as electric cars, trucks and airplanes.
“The kinetics of a thick electrode are quite different from those of a thin electrode,” said project scientist Minghao Zhang of the University of California, San Diego, a co-first author of the paper. “Degradation is actually much worse in thicker, higher-energy electrodes, which has been a struggle for the field.”
It’s also harder to quantitatively study thick electrodes, Zhang pointed out. The tools that previously worked to study thin electrodes can’t capture the structures of larger, denser materials.
In this study, Meng, Zhang and collaborators from Thermo Fisher Scientific turned to a technique called plasma focused ion beam-scanning electron microscopy (PFIB-SEM) to visualize the changes that occur inside a thick lithium-ion battery cathode. PFIB-SEM uses focused rays of charged ions and electrons to assemble an ultra-high-resolution picture of a material’s three-dimensional structure.
The researchers used this imaging approach to collect data on a brand new cathode, as well as one that had been charged and depleted 15 times. Using the data from these electron microscopy experiments, the team built computational models illustrating the process of degradation in the batteries.
“This combination of nanoscale-resolution experimental data and modeling is what allowed us to determine how the cathode degrades,” said PME postdoctoral research fellow Mehdi Chouchane, a co-first author of the paper. “Without the modeling, it would have been very hard to prove what was happening.”
The researchers discovered that variation between areas of the battery encouraged many of the structural changes. Electrolyte corrosion occurred more frequently with a thin layer at the surface of the cathode. This top layer therefore developed a thicker resistive layer, which led the bottom layer to expand and contract more than other parts of the cathode, leading to faster degradation.
The model also pointed toward the importance of the CBD – a porous grid of fluoropolymer and carbon atoms that holds the active materials of an electrode together and helps conduct electricity through the battery. Previous research had not characterized how the CBD degrades during battery use, but this new work suggests that the weakening of contacts between the CBD and the active materials of the cathode can lead directly to the decline in performance of lithium-ion batteries over time.
“This change was even more obvious than the cracking of the active material, which is what many researchers have focused on in the past,” said Zhang.
With their model of the cathode, Meng’s group studied how tweaks to the electrode design might impact its degradation. They showed that changing the CBD structure network could help prevent the worsening of contacts between the CBD and the active materials, making batteries last longer – a hypothesis that engineers can follow up with physical experiments.
The group is now using the same approach to study even thicker cathodes, as well as carrying out additional modeling on how to slow electrode degradation. “This study develops a methodology of how to design electrodes to enhance future battery performance,” said Zhao Liu, senior manager for battery market development at Thermo Fisher Scientific, who contributed to the research.
This story is adapted from material from the University of Chicago, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.