[Image above] Example of a lithium-ion battery assembly. Researchers led by ACerS Fellow Ricardo Castro provided new insight into why these batteries fail. Credit: Ricardo Castro
Lithium-ion batteries are the core of many of today’s energy, mobility, and electronic systems. Yet, as much as we rely on Li-ion batteries, this technology is a fickle beast. Low temperatures can degrade charging efficiency, while high temperatures can cause swelling. Charging too quickly can affect battery health as well, and the list goes on.
To enhance the reliability of Li-ion battery technology, researchers are exploring alternatives for key components, such as swapping ceramic films for polymeric separators and testing different electrolyte materials. But numerous studies are also focused on understanding the fundamental science behind Li-ion batteries (such as here and here), as this knowledge enables researchers to develop more targeted and effective solutions.
In a recent open-access paper, researchers led by ACerS Fellow Ricardo Castro, professor of materials science and engineering at Lehigh University, investigated the thermodynamic changes that occur during battery cycling to understand how it affects cathode stability.
During battery cycling, cathode structures are delithiated, i.e., lithium ions are removed and transferred to the other electrode. This process can cause local deviations in the cathode’s stoichiometry, which according to density functional theory, can impact the cathode’s surface energy and its potential for interaction with the electrolyte.
However, direct thermodynamic measurements to corroborate the predictions are unavailable, mainly because these surface energies are so small that it is practically impossible to quantify them. To overcome this challenge, the researchers pioneered a water adsorption microcalorimetry method that uses water molecules as nondestructive probes to determine the slight energy changes.
In an email, Castro explains that this method is comparable to sticking gum on a chair. “How much it sticks to the chair has a relationship with the energy of the surface. Likewise, our method helps us measure how much energy it takes for water to be absorbed on the surface of the cathode on a molecular level,” he says.
Using this method, the authors determined that mild delithiation of the LiCoO2 cathode causes surface energy reduction, which negatively affects the adhesion between adjacent grains. In other words, the cathode is structurally stressed during every cycle, which degrades its stability over time.
After confirming this behavior, the authors considered how the cathode might be stabilized to prevent degradation. They explored using lanthanum as a dopant in the cathode because it has previously been reported that lanthanum causes improved structural stability during cycling.
They discovered that the lanthanum-doped stoichiometric cathode (LiCoO2) had a reduced surface energy, but more importantly, it showed no surface energy variation after delithiation (Li0.57CoO2). This finding “implies that La3+ doping serves as a thermodynamic buffer that reduces cycling-induced thermodynamic stresses,” the authors write.
Plus, the lowered surface energy brought other advantages that positively impact battery performance, for example, by causing the cathode to experience less coarsening and dissolution, “likely due to the enhanced bond strengths,” the authors add.
Illustration showing how lanthanum-doped battery cathodes experience reduced thermodynamic stress between grains during cycling, thus mitigating degradation mechanisms. Credit: Dahl et al., The Journal of Physical Chemistry C (CC BY-NC-ND 4.0)
Castro says besides their general applicability, these findings are “especially important for nanocathodes, which, while enabling faster charge–discharge cycles, are prone to instability due to their increased surface area.”
The open-access paper, published in The Journal of Physical Chemistry C, is “Enhanced thermodynamic stability of delithiated nano-LiCoO2 by lanthanum doping” (DOI: 10.1021/acs.jpcc.4c03415).
