Size-Dependent Lithium Miscibility Gap in Nanoscale Li[sub 1−x]FePO[sub 4]

Meethong, Nonglak; Huang, Hsiao‐Ying Shadow; Carter, W. Craig; Chiang, Yet‐Ming

doi:10.1149/1.2710960

Cited by 435 publications

(460 citation statements)

References 19 publications

Supporting

Mentioning

424

Contrasting

Unclassified

Order By: Relevance

“…13 Meethong et al 15,16 recently highlighted the importance of coherency strains on the rate capabilities of LiFePO 4 crystallites with differing two-phase solubility limits, showing that the nucleation and growth kinetics depend strongly on the misfit strains of the first-order phase transformation in Li x FePO 4 . The observed decrease in the miscibility gap was argued to be due to strain and surface energy and stress.…”

mentioning

confidence: 99%

The Role of Coherency Strains on Phase Stability in Li[sub x]FePO[sub 4]: Needle Crystallites Minimize Coherency Strain and Overpotential

Ven¹,

Garikipati

Kim

et al. 2009

J. Electrochem. Soc.

123

163

View full text Add to dashboard Cite

We investigate the role of coherency strains on the thermodynamics of two-phase coexistence during Li ͑de͒intercalation of Li x FePO 4 . We explicitly account for the anisotropy of the elastic moduli and analytically derive coupled chemical and mechanical equilibrium criteria for two-phase morphologies observed experimentally. Coherent two-phase equilibrium leads to a variable voltage profile of individual crystallites within the two-phase region as the dimensions of the crystallite parallel to the interface depend on the phase fractions of the coexisting phases. With a model free energy for Li The kinetics of intercalation processes in the electrode materials of primary and secondary batteries involves both diffusion and phase transformations.1-6 The first-order phase transformations that occur during the insertion or removal of Li usually result in only slight structural modifications of the host.7-12 As a result, the transformations proceed by the passage of coherent or semicoherent interfaces that separate the new stable phases from the metastable phase. Any difference in lattice parameters between the two phases participating in the phase transformation, however, introduces coherency strains, which affect thermodynamic potentials such as the Li chemical potential and the overall free energy. This, in turn, can modify the composition bounds of any two-phase coexistence regions as well as the voltage profile for cathode compositions where two-phase coexistence occurs and may even alter phase stability qualitatively.Coherency strains are especially important in determining the two-phase equilibrium in the Li x FePO 4 system, as was made evident by the discovery of Chen et al. 13 of a peculiar two-phase morphology within large, chemically delithiated crystallites of Li x FePO 4 . Li diffusion in Li x FePO 4 is restricted to one-dimensional channels parallel to the b-direction of the orthorhombic ͑Pnma, space group 62͒ Li x FePO 4 crystal structure.14 An elementary analysis of phase separation would therefore suggest that the interface separating Li-poor Li FePO 4 from Li-rich Li 1−␦ FePO 4 , where and ␦ are both small, should be perpendicular to the b-direction ͑i.e., parallel to the ac plane͒, as this would allow a uniform extraction ͑insertion͒ of Li ions from ͑in͒ the surfaces perpendicular to the diffusion direction. Transmission electron microscopy ͑TEM͒ observations of chemically oxidized large platelike crystallites, however, have shown that the interfaces separating the Li FePO 4 and Li 1−␦ FePO 4 phases within the crystallites of Li x FePO 4 are perpendicular to the a-direction ͑parallel to the bc plane͒.13 Chen et al. argued that this morphology should minimize the elastic strain energy, allowing the crystal to relax along the a-direction of the orthorhombic cell, which varies by almost 5% between Li-poor Li FePO 4 and Li-rich Li 1−␦ FePO 4 . 13 Meethong et al. 15,16 recently highlighted the importance of coherency strains on the rate capabilities of LiFePO 4 crystallites with differing two-phase solu...

show abstract

mentioning

confidence: 99%

The Role of Coherency Strains on Phase Stability in Li[sub x]FePO[sub 4]: Needle Crystallites Minimize Coherency Strain and Overpotential

Ven¹,

Garikipati

Kim

et al. 2009

J. Electrochem. Soc.

123

163

View full text Add to dashboard Cite

show abstract

“…Generally, there have been two major approaches to interpret the property improvement for LiFePO 4 using nano-scaling induced electron/ion transport: the first is called the "domino-cascade" model, which describes a fast anisotropic lithium insertion/extraction action coupled to the LiFePO 4 -FePO 4 interface movement through an electron polaronic hopping mechanism in nano-crystallites, [5] and the second focuses on the surface effect with carbon coating and miscibility gap size reduction as a result of nano-scaling. [4,7] Conventionally, the iron ion has been viewed as Fe …”

mentioning

confidence: 99%

“…[1] LiFePO 4 is a band insulator in bulk crystal form in agreement with the calculated large band gap, [2] however, the fast electron/ion charge/discharge process has been shown to be possible only when the particle size is reduced to below ∼100 nm. [3] While significant progress has been made to demonstrate the impact of nanoscaling on defect chemistry, electrochemical behavior, electron-ion conductivity, lithium ion diffusion, and the charge/discharge rate in general, [4][5][6] the fundamental question of "why nano-scaling matters?" remains.…”

mentioning

confidence: 99%

Finite-size effect of antiferromagnetic transition and electronic structure in LiFePO4

Shu

Chou

2012

Phys. Rev. B

View full text Add to dashboard Cite

The finite-size effect on the antiferromagnetic (AF) transition and electronic configuration of iron has been observed in LiFePO4. Determination of the scaling behavior of the AF transition temperature (TN ) vs. the particle size dimension (L) in the critical regime, 1 −∼ L −1 , reveals that the activation nature of the AF ordering strongly depends on the surface energy. In addition, the effective magnetic moment that reflects the electronic configuration of iron in LiFePO4 is found sensitive to the particle size. An alternative structural view based on the polyatomic ion groups of PO 3− 4 is proposed. LiFePO 4 has been considered as an ideal cathode electrode material with a workable redox potential of 3.45 V and a high theoretical capacity of 170 mAh/g.[1] LiFePO 4 is a band insulator in bulk crystal form in agreement with the calculated large band gap,[2] however, the fast electron/ion charge/discharge process has been shown to be possible only when the particle size is reduced to below ∼100 nm.[3] While significant progress has been made to demonstrate the impact of nanoscaling on defect chemistry, electrochemical behavior, electron-ion conductivity, lithium ion diffusion, and the charge/discharge rate in general,[4-6] the fundamental question of "why nano-scaling matters?" remains. Generally, there have been two major approaches to interpret the property improvement for LiFePO 4 using nano-scaling induced electron/ion transport: the first is called the "domino-cascade" model, which describes a fast anisotropic lithium insertion/extraction action coupled to the LiFePO 4 -FePO 4 interface movement through an electron polaronic hopping mechanism in nano-crystallites, [5] and the second focuses on the surface effect with carbon coating and miscibility gap size reduction as a result of nano-scaling. [4,7] Conventionally, the iron ion has been viewed as Fe 2+with oxygen ligands to form the octahedral crystal electric field (CEF) of e g -t 2g splitting.[8] However, the magnitude of the effective magnetic moment (µ ef f ) obtained from the Curie-Weiss law fitting was inconsistent and spread in the range of µ ef f ∼4.7-5.5 µ B , which has been interpreted that Fe 2+ must be in the high-spin state (HS) as t 4 2g e 2 g of S=2 with a spin-only value of µ ef f = 4.9µ B , and the inconsistent moment above 4.9 µ B has often been described by incomplete orbital-quenching, i.e., the theoretically calculated value from the Lande g-factor with J=L+S (S=2, L=2) without orbital quenching should be µ ef f =6.7µ B .[9, 10] Considering a typical iron ion in the crystal field composed of oxygen ligands with high electronegativity, the HS choice of d 6 implies an unexpectedly small crystal field splitting between e g − t 2g , unless the influence of the nearby PO 3− 4 phosphate groups are included. In addition, within the explanation of in-

show abstract

“…19 In addition, some reports revealed that the solid-solution and two-phase reactions could occur simultaneously. 20,40,43 According to thermodynamics anticipation, there are two solid-solution regions outside the miscibility gap at a finite temperature in a binary system. The lithium-rich and lithiumpoor end member phases were usually expressed as Li 1-α FePO 4 and Li β FePO 4 , notwithstanding the differences in the α and β values dependent on the morphology and dimension of LiFePO 4 /FePO 4 particle size, charge/discharge rates and temperature.…”

Section: Resultsmentioning

confidence: 99%

“…However, the intermediate phases within miscibility gap had been not yet clarified to be whether the fully lithiated (LiFePO 4 ) /delithiated (FePO 4 ) phases or the two separated phases with definite partial lithium occupancy and partial lithium defect in Li x FePO 4 . 20,40,45 Apparently, the mechanism for the lithiation/delithiation processes still remain controversial.…”

Section: Resultsmentioning

confidence: 99%

Dynamic Lithium Intercalation/Deintercalation in 18650 Lithium Ion Battery by Time-Resolved High Energy Synchrotron X-Ray Diffraction

Liu

Abouimrane

et al. 2015

J. Electrochem. Soc.

View full text Add to dashboard Cite

A time-resolved in situ high energy synchrotron X-ray diffraction (HESXRD) technique is employed to study the lithiation/delithiation of cathode/anode in a commercial 18650 battery under real working condition (current rate is 4 C). The phases and their changes in both the cathode and anode are identified simultaneously. For the anode component, during the charge process, as well as the Li x C 6 phase, a lithium-rich phase close to LiC 6 phase and a series of intermediate phases between the Li 0.5 C 6 and LiC 6 phases are observed. A distinct lithium intercalation/deintercalation mechanism is proposed for the cathode. The transforms of LiFePO 4 into the FePO 4 consists three periods with different components of phases, i.e., LiFePO 4 + lithium-deficient solid solution phases (period I), FePO 4 + LiFePO 4 phases (period II), and FePO 4 + lithium-rich solid solution phases (period III). The changes in both the andode and cathode during the discharge process are just inversed to those occurrs during the charge process. The present work indicates that dynamic lithiation/delithaition process under real working condition is different from those at the thermodynamic state, and the in situ HESXRD is one of the most promising technique to monitor such kind of dynamic lithium behavior. In the past twenty years, lithium ion battery experienced a fast development due to its large energy density, high voltage and excellent cycle performance.1-2 While various Li ion battery systems are commercialized, the understanding of the lithiation/delithiation process mechanism occurred in electrode material is recognized to be crucial in developing new batteries and thus is attracting more and more attentions. To study the structure changes in electrode materials during lithium intercalation/deintercalation, various of in situ techniques, such as X-ray diffraction (XRD), 3-5 X-ray absorption, 6-7 Raman spectroscopy, 8 nuclear magnetic resonance [9][10] and transmission electron microscopy, 11 have been employed. However, these techniques possess a weak ability of penetration and can be only utilized in the characterizations of some special designed batteries.12-14 Besides, most of these techniques are based on stepscan mode, by which as long as several minutes are usually required to collect the structure information. Therefore, a small current density has to be applied in order to monitor effectively the structure change in electrode materials. The thus-obtained mechanisms disclosed only the lithiation/delithiation process under thermodynamic equilibrium status. [15][16] In addition, in the graphite/LiFePO 4 battery system, while the intercalation/deintercalation mechanism on the anode (graphite/Li x C 6 ) has been well established, that on the cathode (FePO 4 /LiFePO 4 ) is still under debate. [17][18][19] It was usually reported in the literature that the lithiation/delithiation of FePO 4 /LiFePO 4 involved either a two-phase reaction [16][17] or a solidsolution reaction. [20][21][22] However, notwithstanding the differences in thes...

show abstract

Size-Dependent Lithium Miscibility Gap in Nanoscale Li[sub 1−x]FePO[sub 4]

Cited by 435 publications

References 19 publications

The Role of Coherency Strains on Phase Stability in Li[sub x]FePO[sub 4]: Needle Crystallites Minimize Coherency Strain and Overpotential

The Role of Coherency Strains on Phase Stability in Li[sub x]FePO[sub 4]: Needle Crystallites Minimize Coherency Strain and Overpotential

Finite-size effect of antiferromagnetic transition and electronic structure in LiFePO4

Dynamic Lithium Intercalation/Deintercalation in 18650 Lithium Ion Battery by Time-Resolved High Energy Synchrotron X-Ray Diffraction

Contact Info

Product

Resources

About