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.

All experimental data within the article and its Supplementary Information will be made available upon reasonable request to the authors.

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.

Present address: Sandia National Laboratories, Livermore, CA, USA

Department of Materials Science & Engineering, Stanford University, Stanford, CA, USA

Yiyang Li, Kipil Lim, Haitao D. Deng, Jongwoo Lim, Peter M. Attia, Sang Chul Lee, Norman Jin, Jihyun Hong, Martin Z. Bazant & William C. Chueh

Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

Department of Applied Physics, Stanford University, Stanford, CA, USA

Department of Chemistry, Stanford University, Stanford, CA, USA

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Slovenia

Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA, USA

SUNCAT Interfacial Science and Catalysis, Stanford University, Stanford, CA, USA

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.

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Supplementary Figures 1–17, Supplementary Table 1, Supplementary References

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