According to Meng, the widespread commercialization of lithium-ion batteries at the end of the 20th century played a 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.
Thin vs. thick electrodes
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 — with 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, co-first author of the paper that presents the new findings. “Degradation is actually much worse in thicker, higher-energy electrodes, which has been a struggle for the field.”
Zhang pointed out that it’s also harder to quantitatively study thick electrodes. The tools that previously worked to study thin electrodes can’t capture the structures of larger, denser materials.
In the new work, Meng, Zhang and collaborators from Thermo Fisher Scientific turned to 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 charged ions and electrons to assemble an ultra-high-resolution picture of a material’s three-dimensional structure.
The researchers used the imaging approach to collect data on a brand-new cathode as well as one that had been charged and depleted 15 times. With the data from the electron microscopy experiments, the team built computational models illustrating the process of degradation in the batteries.
This combination of nanoscale resolution experimental data and modelling allowed them to determine how the cathode degrades.
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 CBD — a porous grid of fluoropolymer and carbon atoms that holds the active materials of an electrode together contribute and helps conduct electricity through the battery. Previous research has not characterized how the CBD degrades during battery use, but the new work suggested that the weakening of contacts between the CBD and active materials of the cathode directly to the decline in the 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,” Zhang said.
Batteries of the future
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 active materials, making batteries last longer — a hypothesis that engineers can now follow up with physical experiments.
The group is now using the same approach to study even thicker cathodes, as well as carrying out additional modelling on how to slow electrode degradation.