A new method developed for studying battery failures points to a potential next step in extending lithium-ion battery lifetime and capacity.

Using a novel X-ray technique—coherent X-ray diffractive imaging (CXDI)—at the US Department of Energy’s Advanced Photon Source (APS) at Argonne National Laboratory, researchers have revealed surprising dynamics in the nanomechanics of operating batteries.

Their findings suggest a way to mitigate battery failures by minimizing the generation of elastic energy. A paper on their work is published in the ACS journal Nano Letters.

Lithium ion batteries are ubiquitous in mobile devices, increasingly used in transportation, and promising candidates for renewable energy integration into the electrical grid, provided the degradation of electrochemical performance upon use can be understood, mitigated, and ideally eliminated. Central to degradation mechanisms in nanostructured electrodes, which are increasingly used in batteries due to their enhanced functionality, are the nanomechanics of lithium ions, which remains insufficiently characterized at the single particle level under operando conditions.

In particular, nano-structured spinel materials such as disordered LiNi_0.5Mn_1.5O₄ (LNMO) are appealing as high voltage, high capacity, environmentally friendly, and low cost cathodes for use in numerous markets. However, capacity loss due to degradation is limiting its current use. Important degradation processes, including active material cracking, disconnection, and impedance increase can be understood in terms of strain evolution at the single particle level. Strain needs to be imagined in situ under operando conditions in order to provide insight into real processes and mechanisms.

… Elastic energy is useful in describing structural two-phase coexistence in battery materials, which is key to understanding degradation due to damage induced by the lattice mismatch. The strain generated during, for example, the cubic-tetragonal phase transformation in LiM₂O₄ causes irreversible damage, including defect nucleation, which leads to large capacity fade. Structural transformations can be understood by mapping the elastic energy landscape, that is, the barrier height and width between the two energy minima. This two-state formalism is ubiquitous and very successful in describing diverse phenomena including formation of ferromagnetic and ferroelectric domains, spinodal decomposition, early universe scenarios, and simple molecules. Applied to batteries, it could suggest avenues to mitigate phase transformation induced damage.

Using CXDI, a team of researchers for the APS, the University of California-San Diego, SLAC National Accelerator Laboratory and the Center for Free-Electron Laser Science mapped the three-dimensional strain in individual nanoparticles within the electrodes of operating coin cell batteries.

Bragg Coherent Diffraction imaging (Bragg-CDI) is a technique that only uses the coherent part of the beam. It can pick out single nanocrystals based on their crystal structure, and map out the evolution of strain inside the nanostructure as the whole battery is cycled, explained Ross Harder, co-author and X-ray physicist at the APS.

The high brilliance of the APS at high photon energies is a necessary requirement to pursue this kind of research on individual nanoparticles inside their intact matrix. The APS Upgrade with will allow us look at nanoscale systems of this complex nature with orders of magnitude increased speed, sensitivity and resolution.

They focused coherent X-rays on an in situ coin cell; the signal scattered by an individual LNMO particle was recorded at the detector.

Top: Isosurface projections of compressive (blue) and tensile (red) strain evolution on the shell and core as the battery underwent the first discharge at a C/2 rate. The nanoparticle shell and core both show inhomogeneous strain during discharge. Images are labeled by their respective lattice constant values and open circuit voltages. The highest lattice strain occurs immediately prior to the phase transformation.Bottom: Interior strain distribution on selected cross sections at positions shown by the leftmost figure. Credit: ACS, Ulvestad et al. Click to enlarge.

In the paper, the team reported evidence that the history of charge cycles alters the patterns of strain in single particles of the electrode material.

This discovery was only made possible by the ability to use extend the use of CXDI to study batteries cycling under read operating conditions. The APS is one of the few places where this research can be done.

This new approach will help to reveal fundamental processes underlying the transfer of electrical charge, insight that could help to guide the design of economical batteries with longer useful lives.

We studied strain evolution in situ at the single particle level under operando conditions during (dis)charging using CXDI. We discovered a surprisingly rich set of phenomena related to strain formation and propagation, coherency strain and striping, and the evolution of the elastic energy landscape with 40 nm spatial resolution and 0.5 femtojoule energy resolution. Going beyond traditional imaging, we used the strain mapping to determine key material properties, including the minimum size for two-phase coexistence and the interfacial energy, and we mapped the asymmetric energy barrier to the structural phase transformation. This approach unlocks a new, powerful way to conduct in situ studies under operando conditions of nanomechanics in many electrochemical energy storage systems at the single particle level.

This work was funded by the DOE Office of Science and a UC San Diego Chancellor’s Interdisciplinary Collaborators Award. The APS is a DOE Office of Science User Facility at Argonne National Laboratory.

Hyung-Man Cho and Jong Woo Kim, graduate students in materials science and engineering at UC-Davis, Jörg Maser and Ross Harder of Argonne National Laboratory and Jesse Clark of SLAC National Accelerator Laboratory contributed to this work, which was directed by UC-Davis Shirley Meng, professor of nanoengineering and Oleg Shpyrko professor of physics.