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Physical Sciences
Research Highlights

August 2015

Better Batteries

Imaging the nanoscale world inside a battery

Rechargeable Li-ion batteries are common in portable electronics and in today’s plug-in hybrid electric vehicles. Current generation cathode materials lose their structural integrity after repeated charge-discharge cycling, resulting in voltage fading and capacity loss. Understanding the actual mechanisms of degradation is needed to design longer lasting and higher performance batteries, but degradation has been extremely difficult to study in detail due to the challenge of imaging and quantifying the distribution of light elements in Li-ion battery electrodes. To examine this degradation process in unprecedented detail, researchers are using a variety of advanced methods to monitor changes in the distribution of elements in fresh cathodes and cathodes at different stages of cycling. The new measurements have revealed an obscure and unexpected capacity-loss mechanism that occurs in some formulations of a new electrode being designed to enhance performance of advanced Li-ion batteries.

Why It Matters: Understanding how cycling affects the nanoscale distribution of elements that make up Li-ion battery cathodes is a critical step toward developing next-generation cathode materials to achieve the highest battery performance. The element lithium, essential to Li-ion batteries, is very difficult to analyze. Current results show the Li distribution after different degrees of cycling, thereby providing information critical to the design of electrodes. The results suggest an important role for atom probe tomography (APT) in accelerating battery development. By offering new insights into cycling-dependent capacity loss, this research could help guide efforts to design Li-ion battery materials with prolonged lifetimes suitable for use in long-range electric vehicles and other demanding energy storage applications.

Methods: Researchers from EMSL, the Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory (PNNL), FEI Company, the Qatar Foundation and Argonne National Laboratory mapped 3-D distribution of Li at the sub-nanometer spatial resolution and correlated it with the distribution of transition metal cations (i.e., Ni and Mn) and O. Researchers used a unique modern microscopy method called APT, a high-resolution 3-D imaging capability, at EMSL, a Department of Energy national scientific user facility.

The researchers found charge-discharge cycling caused an overall loss in Li, in addition to an increase in the segregation of Li, Mn and O in the cathode. These cycling-induced structural changes likely contribute to capacity decay and voltage fading, limiting battery lifetime and performance. These results provided the first evidence for achieving unambiguous quantitative mapping of Li, Ni, Mn and O in two types of advanced Li-ion battery cathode materials before and after cycling. The researchers discovered key insights into the nanoscale mechanism of capacity decay as a function of cycling. As a result, the study will enable optimization of cathode synthesis procedures to achieve the highest battery performance and will aid in the effort to create novel Li-ion materials with prolonged lifetimes. This work also will demonstrate the role APT could have in developing new energy storage technologies for future long-range electric vehicles and applications such as grid energy storage.

Acknowledgments

Contact:
Arun Devaraj

Funding:
Laboratory-Directed Research and Development funds from the Chemical Imaging Initiative at PNNL.


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