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Accommodating High Transformation Strains in Battery Electrodes via Formation of Nanoscale Intermediate Phases: Operando Investigation of Olivine NaFePO4.

ABSTRACT

Virtually all intercalation compounds exhibit significant changes in unit cell volume as the working ion concentration varies. NaxFePO4 (0<x<1, NFP) olivine, of interest as a cathode for sodium-ion batteries, is a model for topotactic, high strain systems as it exhibits one of the largest discontinuous volume changes (~17% by volume) during its first-order transition between two otherwise isostructural phases. Using synchrotron radiation powder X-ray diffraction (PXD) and pair distribution function (PDF) analysis, we discover a new strain-accommodation mechanism wherein an amorphous phase forms to buffer the large lattice mismatch between primary phases. While the new phase has short-range order only, it is refined with a periodicity closing matching the a and b lattice parameters of one crystalline endmember phase, and the c lattice parameter of the other. Within this amorphous phase form very fine spherical nanocrystallites of the olivine phase. We suggest that amorphous phase formation is a strain-accommodation mechanism that may apply to systems with large transformation strains.