Operando Visualization of Two-Dimensional Li Diffusion Behavior and Hybrid Phase Transformation Kinetics in Olivine Lithium Iron Phosphate (LiFePO4)

Olivine lithium iron phosphate (LiFePO 4 ) is a technologically important electrode material for lithium-ion batteries and a model system for studying electrochemically-driven phase transformations. Despite extensive studies, many aspects of the phase transformation and Li transport in LiFePO 4 are still not well understood. Here we combine operando hard X-ray spectroscopic imaging and phase-field modeling to elucidate the delithiation dynamics of single-crystal LiFePO 4 microrods with long-axis along the [010] direction. Our study reveals two-dimensional Li diffusivity in microsized LiFePO 4 particles containing ~3% Li-Fe antisite defects and provides direct evidence for the previously predicted surface-reaction-limited phase boundary migration mechanism for the first time. Further, a new hybrid mode of phase growth is discovered, in which phase boundary movement is controlled by surface reaction or Li diffusion in different crystallographic directions. These findings uncover the rich phase transformation behaviors in LiFePO 4 and intercalation compounds in general and can help the design of better electrodes. Acknowledgments This research was supported by U.S. National Science Foundation grant DMR-1106184 and DMR-1508558 for the synthesis and characterization of the material, and the UW-Madison WEI Seed Grant for the TXM-XANES studies (to S.J.). Material characterization is also partially supported by U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Contract No. DE-SC0002626 (to Y.M.C.). Theoretical modeling and analysis (L.H. and M.T.) are supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-SC0014435. The operando TXM-XANES experiments were performed at beamline X8c, National Synchrotron Light Source I, Brookhaven National Laboratory, which are supported by the U.S. DOE, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-98CH10886. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. DOE, Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. L.L also thanks Vilas Research Travel Awards for partially supporting the travel to the synchrotron facilities. The authors thank Olaf J. Borkiewicz and Kamila M. Wiaderek for helping to collect powder X-ray diffraction data of LiFePO 4 reported in this work. We acknowledge supercomputing allocations provided by the Texas Advanced Computing Center (TACC) at The University of Texas, and also the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The research also uses HPC resources supported in part by the Big-Data Private-Cloud Research Cyberinfrastructure MRI-award funded by NSF under grant CNS-1338099 and by Rice University. Figure 1