Many battery electrodes contain ensembles of nanoparticles that phase-separate on (de)intercalation. In such electrodes, the fraction of actively intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports of the active particle population in the phase-separating electrode lithium iron phosphate (LiFePO4; LFP) vary widely, ranging from near 0% (particle-by-particle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 LFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in LFP, and such a phenomenon probably extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phase-separating battery electrodes.
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The research at Stanford was supported by the Samsung Advanced Institute of Technology Global Research Outreach Program, and by startup funding from Stanford School of Engineering and Precourt Institute for Energy. Support for the research at MIT was provided by the Samsung-MIT Program for Materials Design in Energy Applications. F.E.G. and N.C.B. were supported by the Office of Basic Energy Sciences, Division of Materials and Engineering Sciences, US Department of Energy, under contract DE-AC04-94AL85000. J.D.S. and K.R.F. were supported by US Department of Energy through the Sandia Laboratory Directed Research and Development program under contract DE-AC04-94AL85000. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231. Beam line 184.108.40.206 at the Advanced Light Source was funded through a donation by the King Abdullah University of Science and Technology. Y.L. was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-114747. We acknowledge M. Homer of Sandia and J. Perrino of Stanford for ultramicrotoming. We thank J. Nelson Weker of the Stanford Synchrotron Radiation Lightsource for insightful discussions.
The authors declare no competing financial interests.
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Li, Y., El Gabaly, F., Ferguson, T. et al. Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes. Nature Mater 13, 1149–1156 (2014). https://doi.org/10.1038/nmat4084
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