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Nano Lett. 2015 Jul 8;15(7):4282-8. doi: 10.1021/acs.nanolett.5b01314. Epub 2015 Jun 18.

Dependence on Crystal Size of the Nanoscale Chemical Phase Distribution and Fracture in LixFePO₄.

Author information

1
†Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, United States.
2
‡Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
3
§Department of NanoEngineering, University of California, San Diego, La Jolla, California 92121, United States.
4
∥Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
5
⊥Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
6
#Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
7
∇Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States.

Abstract

The performance of battery electrode materials is strongly affected by inefficiencies in utilization kinetics and cycle life as well as size effects. Observations of phase transformations in these materials with high chemical and spatial resolution can elucidate the relationship between chemical processes and mechanical degradation. Soft X-ray ptychographic microscopy combined with X-ray absorption spectroscopy and electron microscopy creates a powerful suite of tools that we use to assess the chemical and morphological changes in lithium iron phosphate (LiFePO4) micro- and nanocrystals that occur upon delithiation. All sizes of partly delithiated crystals were found to contain two phases with a complex correlation between crystallographic orientation and phase distribution. However, the lattice mismatch between LiFePO4 and FePO4 led to severe fracturing on microcrystals, whereas no mechanical damage was observed in nanoplates, indicating that mechanics are a principal driver in the outstanding electrode performance of LiFePO4 nanoparticles. These results demonstrate the importance of engineering the active electrode material in next generation electrical energy storage systems, which will achieve theoretical limits of energy density and extended stability. This work establishes soft X-ray ptychographic chemical imaging as an essential tool to build comprehensive relationships between mechanics and chemistry that guide this engineering design.

KEYWORDS:

High resolution chemical imaging; LiFePO4; battery materials; chemo-mechanical coupling; redox phase transformations

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