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Fluid-enhanced surface diffusion controls intraparticle phase transformations


Phase transformations driven by compositional change require mass flux across a phase boundary. In some anisotropic solids, however, the phase boundary moves along a non-conductive crystallographic direction. One such material is LiXFePO4, an electrode for lithium-ion batteries. With poor bulk ionic transport along the direction of phase separation, it is unclear how lithium migrates during phase transformations. Here, we show that lithium migrates along the solid/liquid interface without leaving the particle, whereby charge carriers do not cross the double layer. X-ray diffraction and microscopy experiments as well as ab initio molecular dynamics simulations show that organic solvent and water molecules promote this surface ion diffusion, effectively rendering LiXFePO4 a three-dimensional lithium-ion conductor. Phase-field simulations capture the effects of surface diffusion on phase transformation. Lowering surface diffusivity is crucial towards supressing phase separation. This work establishes fluid-enhanced surface diffusion as a key dial for tuning phase transformation in anisotropic solids.

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This experimental work at Stanford and SLAC was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under contract DE-AC02-76SF00515. Phase-field theoretical work at MIT and Stanford was supported by the Toyota Research Institute through D3BATT: Center for Data-Driven Design of Li-Ion Batteries. The Advanced Light Source and the Stanford Synchrotron Radiation Lightsource are supported by the DOE Office of Basic Energy Sciences under contracts DE-AC02-05CH11231 and DE-AC02-76SF00515. M.S.I. and H.C. acknowledge support from the EPSRC (grant EP/K016288) and the Archer HPC facilities through the Materials Chemistry Consortium (EP/L000202). Y.L. and P.M.A. were supported by the NSF Graduate Research Fellowship under grant DGE-114747. K.L. was supported by the Kwanjeong Education Foundation Fellowship. M.Z.B. was supported by the Global Climate and Energy Project at Stanford University and the DOE Office of Basic Energy Sciences through the SUNCAT Center for Interface Science and Catalysis. Part of this work was conducted the Stanford Nano Shared Facilities. We thank W. D. Nix (Stanford) for insightful discussions on metallurgy and mechanical properties and R. B. Smith (MIT) for assistance with the phase-field model. We also thank A. L. D. Kilcoyne (Berkeley) and D. Shaprio (Berkeley) for assistance with synchrotron measurements.

Author information

Y.L. conceived and designed the project, analysed the experimental data and performed the phase-field simulations. H.C. and M.S.I. conducted the molecular dynamics simulations. Y.L., K.L. and J.H. conducted diffraction. Y.L., J.L., P.M.A., N.J., W.E.G. and Y.S.Y. collected the X-ray microscopy images. S.C.L. performed transmission electron microscopy. D.F., Y.L. and M.Z.B. designed and executed the linear stability analysis. H.D.D., J.M. and M.G. quantified the resistance increase during relaxation. M.S.I. supervised the molecular dynamics simulations. M.Z.B. supervised the phase-field simulations and linear stability analysis. W.C.C. supervised the experimental components of the work. All authors contributed to writing the text.

Competing interests

The authors declare no competing interests.

Correspondence to M. Saiful Islam or Martin Z. Bazant or William C. Chueh.

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Fig. 1: Crystallographic directions, phase separation and in-plane lithium migration in a Li0.5FePO4 platelet particle.
Fig. 2: Tracking solid solution Li0.5FePO4 electrodes during phase separation.
Fig. 3: Atomic geometries at the LiFePO4 (010)/EC and LiFePO4 (010)/H2O interfaces from ab initio MD simulations indicating Li-ion migration at the surface.
Fig. 4: The distribution of lithium in ionically connected particles shows that phase separation occurs primarily via surface diffusion.
Fig. 5: Origin of phase separation and solid solution from single-particle simulations.