Mg Substitution Clarifies the Reaction Mechanism of Olivine LiFePO4

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g Substitution Clari M es the Reaction Mechanism of Olivine LiFePO 4

Fredrick Omenya ,Bohua Wen ,Jin Fang ,Ruibo Zhang ,Qi Wang ,Natasha A. Chernova ,Joe Schneider-Haefner ,Frederic Cosandey ,and M. Stanley Whittingham* [ 1–3 ] The solu-phases, LiFePO 4 and FePO 4.Understanding the reaction mechanism of olivine compounds as electrode bility depends on the particle size; smaller

particles exhibit higher solubility limits materials for lithium lithium-ion batteries have has received much atten-[ 4,5 ] One of the compared with larger ones. tion recently. The question whether olivine LiFePO4 undergoes two-phase or

attractive properties of this compound is

non-nonequilibrium single-phase reaction during electrochemical processes

the fast electrochemical response despite

has taken center stage in the understanding of the faster reaction kinetics an insulating nature and 1D lithium inser-observed in this material. Here, the lithiation/delithiation mechanism of Mg tion/extraction, which is not well eluci-dated by the classical two-phase reaction. Mg-substituted LiFePO4 using high high-resolution X-ray diffraction(XRD),

Many mechanisms have been proposed to transmission electron microscopy(TEM), and electrochemical measurements

understand the lithiation–delithiation pro-is reported. Ex situ partially (de)lithiated olivine- LiMg0.2Fe0.8PO4 show the [ 1]domino-cas-cesses, such as core–shell, existence of stable equilibrium intermediate phases as characterized by [ 6] and nonequilibrium single-phase cade,

[ 7] In the domino-cascade the presence of more than two phases and broadness of diffraction peaks. transformation.

Electron energy loss spectroscopy pro les across individual nanoparticles model, the existence of a very rapid lithi-ation/delithiation front compared with further con rm uniform lithiation with a constant Fe–L3 energy measured

the initial nucleation has been proposed. across each nanoparticle, suggestive of solid solution behavior in individual

According to this model, at any given

particles. In addition, a continuous shift in the diffraction peak position is

time, partially delithiated small particles

observed even in the “two-phase” region in the ex situ electrochemical (de)in equilibrium state exist as either fully lithiated electrodes. lithiated (iFePO ) or fully delithiated 4

(FePO ). In departure to the nucleation 4

[ 7]argue two-phase mechanism, Malik et al.

that the nucleation energy required to initiate the two-phase 1. Introduction

transformation is too high, and inconsistent with fast kinetics

LiFePO 4 has emerged as a major viable lithium-ion battery observed in LiFePO . They predicted, based on rst-principle 4 cathode material for large-scale applications. Despite the suc-calculations, a kinetically favored nonequilibrium solid solution

cess in the improvement of the electrochemical performance path. At equilibrium, however, the sample is expected to relax by using nanosized particles, carbon coating, and substitu-to two phases, Li-rich and Li-poor, either in different particles as

[ 6]tion, the reaction mechanism of LiFePO in the case of small particles demonstrated by Delmas et al., 4 is still under active

research. The lithiation/delithiation of LiFePO or within the same particle for large particles as demonstrated 4 has been pro-[ 8] Various experiments have established that the posed to occur through a two-phase reaction mechanism with

by Chen et al.

the existence of a narrow solid solution toward the end member core–shell was not applicable at particle level but at an agglom-[ 6,8,9 ] erate scale.

The experiments to con rm the nonequilibrium phases are very challenging and few experiments have shown the possi-Dr. F. Omenya, B. Wen, J. Fang, Dr. R. Zhang,

Dr. Q. Wang, Dr. N. A. Chernova, Prof. M. S. Whittingham bility of such mechanism because of the sequential transfor-[ 10 ] ionic transport [ 11 ] between particles, and inhomo-NorthEast Center for Chemical Energy Storage mation, Binghamton University [ 12 ]geneous electrode reaction in the cell. Recently, using in Binghamton ,NY 13902–6000 ,USA [ 13 ]were situ X-ray diffraction (XRD), Ogumi and co-workers E-mail: stanwhit@gmail.com able to observe a metastable phase of LiFePO 4 at high current Dr. Q. Wang

densities. However, upon cell relaxation, the metastable phase Brookhaven National Laboratory

Upton ,NY 11973 ,USA was reported to disappear leaving only the near-end member

[ 13 ] J. Schneider-Haefner, Prof. F. Cosandey phases.

Department of Materials Science and Engineering Previously, we investigated aliovalent substitution in LiFePO 4;Rutgers University

our results showed that vanadium substitution: improves the Piscataway ,NJ 08854 ,USA rate capability of LiFePO , lowers the solid solution forma-4

[ 14–16 ]tion temperature, and increases the solid solution range. DOI: 10.1002/aenm.201401204

FULL PAPER

Adv. Energy Mater. 2014, 1401204© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.com

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