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  • Savignac2019_Manuscript_V02

    Rights statement: This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acs.jpcc.9b09279

    Accepted author manuscript, 348 KB, PDF document

    Embargo ends: 18/03/21

    Available under license: CC BY-NC: Creative Commons Attribution-NonCommercial 4.0 International License

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Chemically Prepared Li0.6FePO4 Solid Solution as a Vehicle for Studying Phase Separation Kinetics in Li-ion Battery Materials

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Published
<mark>Journal publication date</mark>9/04/2020
<mark>Journal</mark>The Journal of Physical Chemistry C
Issue number14
Volume124
Number of pages7
Pages (from-to)7608-7614
Publication StatusPublished
Early online date18/03/20
<mark>Original language</mark>English

Abstract

The commercial success of LiFePO4 in high-power Li-ion batteries is strongly related to its unique ultrahigh-rate charge/discharge performance that permits full charge in less than a minute. Since Li1–xFePO4 (0.05 ≤ x ≤ 0.95) separates into two phases with poor electronic and ionic conduction, this raises questions regarding the structural dynamics of phase separation. In this paper, the transformation of metastable solid solution Li0.6FePO4 into a phase-separated material is studied by analysis of the local and bulk structure. 6Li MAS NMR is used to probe the immediate environment where proximity to Fe3+ results in a significant shift in resonance frequency. Conversely, time-resolved X-ray diffraction (XRD) measurements reveal the transformation kinetics at the unit cell scale. The XRD showed no preferential relaxation along the a, b, and c crystal axes, consistent with the absence of a phase boundary perpendicular to the fast diffusion b axis. Key to the analysis is the preparation of the solid solution, which yields phase-pure samples exhibiting no evidence of the thermodynamically stable LiFePO4 or FePO4 phases. Long-term measurement indicated that after 263 days under an argon atmosphere these samples still exhibited a solid solution fraction > 40%. However, in the presence of an electrolyte, phase separation is significantly more rapid. The results presented support Li et al. model [Nat. Mater.2018, 17, 915], where vehicular lithium transport at the surface determines the rate of phase separation and offers a methodology for studying high-energy-density LiMPO4 systems (M = transition metal) that currently are limited by poor high-rate performance.

Bibliographic note

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acs.jpcc.9b09279