Defects and Surface Reactions Boost Batteries

February 13, 2018

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Atomic-scale processes are critical to improving the performance of lithium-ion batteries. How lithium atoms move around is one important aspect. Combining x-ray spectroscopic imaging and phase-field modeling, the lithium transport dynamics in a cathode (olivine lithium iron phosphate) are now better understood. Lithium diffusion results in a lithium-deficient phase (blue) that grows at the expense of a lithium-rich phase (red). The presence of the resulting anti-site defects (a disorder in the lithium and iron atom sites in the crystal structure) enables lithium transport over much more surface area than in a defect-free crystal.

The Science

The science behind the operation of a lithium-ion battery is extraordinarily complex. In batteries, electric charge is carried by positive lithium ions. During charging or discharging of the battery, the lithium ions are shuttled between two electrodes. To understand this process, researchers examined lithium in a single crystal of olivine lithium iron phosphate, an important material in electrodes. The results showed the critical role that crystal defects play in the operation of the battery. The defects were iron atoms occupying lithium sites. The scientists suggested that a few defects in the electrode material may actually improve the battery performance by enhancing the overall speed at which lithium travels.

The Impact

The study offers insights into how tiny structural changes evolve in common lithium-ion battery electrodes. This new knowledge provides a basis to update design rules for lithium-ion batteries. Specifically, design rules should include the particle shape, defects, and surface reactions. The result will be designed cathode materials and better batteries.

Summary

Olivine lithium iron phosphate (LiFePO4) is an important electrode material used for lithium-ion batteries. Single crystalline micro-particles of the electrode material have been used as a model system for studying lithium intercalation (migration, uptake, and release of lithium during battery charging and use) and phase transformation mechanisms for commercial battery electrodes. Using operando transmission x-ray microscopy, researchers came to a surprising conclusion about lithium transport and phase boundary migration. The researchers confirmed that the presence of more than 3 percent anti-site defects in the electrode is sufficient to change lithium transport from the 1-D diffusivity predicted for perfect LiFePO4 crystals, to 2-D behavior. In this material, an anti-site defect occurs when an iron atom occupies a lithium site. Because the electrode material commonly used in commercial batteries has many defects, the conclusion from this study suggests that practical, defect-containing LiFePO4 has a much larger active surface area. This will lead to more intercalation of lithium during charge and discharge than previously thought. Corroborated by phase-field simulations, the operando observations reveal a new hybrid-mode phase transformation mechanism. In this mechanism, the phase boundary exhibits a fast surface-reaction-limited propagation along particle surfaces and a slow diffusion-limited propagation perpendicular to the surface. This study offers new insights on how defects and particle morphology should be tailored to improve battery electrode performance. The results will have broad implications for the use of other lithium intercalation materials as electrodes for batteries.

Contact

Ming Tang
Rice University
mt20@rice.edu

Song Jin
University of Wisconsin, Madison
jin@chem.wisc.edu

Linsen Li
University of Wisconsin, Madison
linsen@mit.edu

 

Funding

Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (theory, part of material characterization including operando transmission x-ray microscopy); National Science Foundation (synthesis and characterization); University of Wisconsin-Madison (x-ray absorption near edge structure spectroscopy); and Vilas Research Travel (travel). This work utilized the National Synchrotron Light Source, Advanced Photon Source, and National Energy Research Scientific Computing Center, all DOE Office of Science user facilities. Additional computational resources were provided by Texas Advanced Computing Center, National Science Foundation, and Rice University.

Publications

L. Hong, L. Li, Y.K. Chen-Wiegart, J. Wang, K. Xiang, L. Gan, W. Li, F. Meng, F. Wang, J. Wang, Y.M. Chiang, S. Jin, and M. Tang, "Two-dimensional lithium diffusion behavior and probable hybrid phase transformation kinetics in olivine lithium iron phosphateExternal link." Nature Communication 8, 114 (2017). [DOI: 10.1038/s41467-017-01315-8]

Related Links

Rice University news release: Microscopic defects make batteries betterExternal link

EurekAlert! public release: Microscopic defects make batteries betterExternal link

Science Daily news: Microscopic defects make lithium-ion batteries betterExternal link

United Press International, Inc. science news: Microscopic defects improve performance of lithium-ion batteriesExternal link

Futurity article: Defects actually give lithium-ion batteries a boostExternal link

Press Release Point post: Defects actually give lithium-ion batteries a boostExternal link

Global News Connect article: Microscopic defects make batteries betterExternal link

Highlight Categories

Program: ASCR, BES, MSE

Performer/Facility: University, DOE Laboratory, SC User Facilities, ASCR User Facilities, NERSC, BES User Facilities, NSLS-II

Additional: Collaborations, Non-DOE Interagency Collaboration