Finding ways to improve the performance and reliability of lithium-ion (Li-ion) batteries is an ongoing scientific and technological quest of major importance. While many techniques exist for studying the behavior of Li-ion batteries at the cell level, and for probing and characterizing the bulk structure and properties of battery materials, these methods still largely miss the complex processes that occur at the microscopic level in Li-ion battery operating conditions. A full understanding of these dynamic processes is crucial for designing and improving new Li-ion batteries.
A team of researchers from several institutions has developed a new strategy for probing Li-ion battery workings at the microscopic level using a combination of coherent multi-crystal diffraction (CMCD) and optical microscopy. Their work appeared in Nature Communications.
The team's experiments focused on sub-particle-scale electrochemical dynamics in local domains of lithium layered oxide NMC cathode materials. With the CMCD performed at the 34-ID-C beamline of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, they were able to observe the dynamics of individual domains in the layered cathodes under charging and discharging.
Operando optical microscopy observations were used to corroborate the CMCD findings. Although operando X-ray diffraction has been used in the past to study lattice evolution under charge, the large beam size of the technique results in a wide footprint from which only bulk-averaged data can be obtained. The present approach, on the other hand, uses a more focused and coherent X-ray beam which can track dynamics in individual grains, offering a much finer and more detailed overall picture.
The greater detail offered by CMCD shows that the entire (de)lithiation process in these cathodes is far more complex than previously understood. Various domains show different state of charge (SOC) within the same particle, even at the same time. Electrochemical reaction rates can also vary widely, and grains can even physically move and rotate in various directions during charging and discharging. The investigators suggest that all of these phenomena can be major factors in the formation of cracks in secondary cathode particles.
Optical microscopy of an exposed cathode cross-section during cycling provided a means of visually correlating the domain dynamics with the CMCD observations. Since a direct correlation between optical intensity of particles with their SOC was established in previous work, the experimenters used it to follow local SOC, and to derive local current density. Nearly 100 separate particles were identified and studied to define the microscopic dynamics with sufficient statistical significance, with several individual particles examined in greater detail.
During a first charge, particles are seen to increase in optical intensity, as expected. Unexpectedly, the investigators did not see the theorized shrinking-core reaction model here, but instead a near-surface point onset with gradual reaction front propagation throughout the entire particle. These phenomena were seen in almost all observed particles. Most particles take about 8 minutes to fully charge, with the entire electrode taking about 15 hours.
However, several other particles took a considerably longer time to fully charge, beginning rapidly with corresponding optical intensity and then a slower increase, split among several different domains each charging at different rates. The researchers noted that this results in stress at domain boundaries with mechanical damage and detachment as different domains experience shrinking, affecting and slowing charging.
The investigators probed further with finite element analysis (FEM) of the interaction of electrochemistry and mechanical damage observed in the CMCD and optical microscopy studies. FEM modeling confirms that the lattice mismatch and structural heterogeneities created by these dynamic phenomena at the sub-particle level greatly degrade lithium ion flow and overall charging characteristics. The cracks, defects, and other microscopic structural deformities in the cathode grain material can directly influence battery reliability and performance.
These experiments provide a deeper and more detailed understanding of the workings of Li-ion batteries at the fundamental level in ways that were previously unavailable. The investigators expect that the new approaches demonstrated in this work will open the way to improved techniques of battery design that will greatly improve the longevity and reliability of these critically important devices. – Mark Wolverton
See: Z. Xue1,2, N. Sharma3, F. Wu1, P. Pianetta2, F. Lin4, L. Li5, K. Zhao3, Y. Liu6, “Asynchronous domain dynamics and equilibration in layered oxide battery cathode,” Nat Commun 14 8394 (December 2023)
Author affiliations: 1Central South University; 2SLAC Nation; 3Purdue University; 4Virginia Tech; 5Argonne National Laboratory; 6University of Texas Austin.
Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. This research used resources of the Advanced Photon Source, US Department of Energy (DOE), Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. The work at the Central South University was sponsored by the National Natural Science Foundation of China (52172264) and Fundamental Research Funds for the Central Universities of Central South University.
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