Researchers from the Laboratory of Hierarchically Structured Materials at the Skoltech Engineering Center and the Center for Energy Technologies (Skoltech is part of VEB.RF group), together with colleagues from Lomonosov Moscow State University and the IDEA Research Center, developed a unique method for studying the mechanical degradation of lithium-ion batteries directly during operation. Using a modified cell and a flat diamond nanoindenter tip, the scientists were able, for the first time, to capture in real time how chemical changes inside the battery lead to abrupt embrittlement of its components. The findings revealing the hidden mechanisms of battery aging were published in the journal Measurement.

Mechanical degradation of nickel-rich cathode materials (NMC811) remains a key obstacle to creating durable and safe batteries for portable electronics and electric vehicles. During repeated charging and discharging, material particles change their volume and shape, which leads to microcracks and gradual battery failure. Previously, engineers had to disassemble and dry batteries for analysis, but this approach greatly distorted the real picture and created artificial defects, preventing researchers from capturing the true properties of materials in the aggressive environment of a liquid electrolyte.

“We were the first to carry out this kind of testing and were able to quantify changes in the mechanical properties of the cathode material in real time during electrochemical cycling,” notes first author Evgeny Statnik, senior research scientist at the Laboratory of Hierarchically Structured Materials at the Skoltech Engineering Center. “While the hardness and elastic modulus of the cathode remained essentially unchanged throughout the charge-discharge cycle, the work of plastic deformation dropped sharply — by almost a third — during the transition to the high-voltage H3 phase. However, the cathode material did not become softer — it lost its ability to deform plastically under mechanical load, which sharply increased the risk of particle cracking and fragmentation of the entire structure when operating above 4.2 volts.”

In the experiments, the researchers used a flat tip indenter instead of a standard sharp pyramidal tip. This engineering solution made it possible to distribute the load evenly over the surface of the porous electrode and obtain averaged mesoscale data that incorporates the influence of the polymer binder, carbon additives, and the liquid electrolyte. The method helped track the mechanical property changes across the four key stages of phase transformations in the cathode.

“We managed to overcome the fundamental sealing limitation of modern batteries by creating an original design for a CR2032 coin cell with a special access window,” says co-author Artem Abakumov, director of the Skoltech Energy Center and a distinguished professor at Skoltech. “This made it possible to simultaneously monitor the battery’s electrical parameters and perform nanoindentation — literally ‘feeling out’ the mechanical response of the composite cathode with a diamond probe right as electrochemical reactions proceed.”

“The developed experimental platform represents an extension of the operando research paradigm for complex electrochemical and mechanical processes in lithium batteries,” concludes co-author Alexander Korsunsky, Professor at the Skoltech Engineering Center and Head of the Laboratory of Hierarchically Structured Materials. “The results of the new experiments provide us with reliable reference parameters that are critically important for calibrating and validating multiscale computational models. Accurate quantitative data for the loss of cathode plasticity will allow engineers to make more reliable predictions of internal stress evolution and crack propagation. Ultimately, this may serve as a new diagnostic basis for optimizing safe operating windows and will help manufacturers purposely design mechanically robust high-energy-density batteries.”

The study was supported by a grant from the Russian Science Foundation.