Chemie
International Edition: DOI: 10.1002/anie.201703168
German Edition:
DOI: 10.1002/ange.201703168
Lithium Ion Batteries
Strain Coupling of Conversion-type Fe3O4 Thin Films for Lithium Ion
Batteries
Sooyeon Hwang, Qingping Meng, Ping-Fan Chen, Kim Kisslinger, Jiajie Cen, Alexander Orlov,
Yimei Zhu, Eric A. Stach, Ying-Hao Chu, and Dong Su*
Abstract: Lithiation/delithiation induces significant stresses
and strains into the electrodes for lithium ion batteries, which
can severely degrade their cycling performance. Moreover, this
electrochemically induced strain can interact with the local
strain existing at solid–solid interfaces. It is not clear how this
interaction affects the lithiation mechanism. The effect of this
coupling on the lithiation kinetics in epitaxial Fe3O4 thin film
on a Nb-doped SrTiO3 substrate is investigated. In situ and
ex situ transmission electron microscopy (TEM) results show
that the lithiation is suppressed by the compressive interfacial
strain. At the interface between the film and substrate, the
existence of LixFe3O4 rock-salt phase during lithiation consequently restrains the film from delamination. 2D phase-field
simulation verifies the effect of strain. This work provides
critical insights of understanding the solid–solid interfaces of
conversion-type electrodes.
Volume expansion and related strain/stress issues are of the
particular importance for the cycling performance of lithium
ion batteries (LIBs).[1] For alloying and conversion reactions,
the volume expansion is more severe than that occurs during
an intercalation reaction,[2] and previous reports have shown
that excessive volumetric changes result in pulverization of
the initial electrode materials, disconnection of active materials from the binder and current collector, and damage of the
established solid–electrolyte interface layer.[3] Moreover,
lithiation-induced stress/strain can also change the driving
force for reactions as well as the rate of reactions.[4] In real
batteries, mechanical stress can be universally existed which
originated from interfaces or neighboring particles. However,
[*] Dr. S. Hwang, Dr. Q. Meng, K. Kisslinger, Dr. Y. Zhu, Dr. E. A. Stach,
Dr. D. Su
Brookhaven National Laboratory
Upton, NY 11973 (USA)
E-mail: dsu@bnl.gov
Dr. P.-F. Chen, Prof. Dr. Y.-H. Chu
Institute of Physics, Academia Sinica
Taipei 11529 (Taiwan)
J. Cen, Prof. Dr. A. Orlov, Dr. D. Su
Department of Materials Science and Engineering
Stonybrook University, Stonybrook, NY 11794 (USA)
Prof. Dr. Y.-H. Chu
Department of Materials Science and Engineering, Department of
Electrophysics, National Chiao Tung University
Hsinchu 30010 (Taiwan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201703168.
Angew. Chem. Int. Ed. 2017, 56, 7813 –7816
the influences of external stress/strain on reaction kinetics are
still not well understood.
Herein, we investigate how pre-existing strain affects the
lithiation reaction with a model system: a Fe3O4 thin film
battery. Magnetite (Fe3O4) is a promising anode material
owing to its cost-effectiveness, non-toxicity, and high energy
density. A formula unit of Fe3O4 can store up to eight Li ions
via a two-step intercalation-conversion reaction, shown in
Equation (1):
Liþ
Liþ
Fe3 O4 K!LiFe3 O4 K!Li2 O þ Fe
ð1Þ
Previous work has confirmed that this two-step reaction
occurs in Fe3O4 nanoparticles.[5] In contrast with unconstrained nanoparticles, nanowires, or nanoplates,[4b,c, 5, 6] the
lithiation behavior of thin films can be different owing to the
interfacial strain originated from the substrate. We take
advantage of in situ (scanning) transmission electron microscopy ((S)TEM), which can provide unique nanoscale information about electrode deformation,[5–7] and directly visualize
the dynamic lithiation process influenced by interfacial strain
in a Fe3O4 thin film grown on a SrTiO3 substrate.
A heteroepitaxial magnetite thin film (60 nm thick) was
deposited onto a (001) Nb-doped SrTiO3 (STO) substrate by
pulsed laser deposition. X-ray diffraction (XRD), selectedarea electron diffraction (SAED), and atomic resolution
high-angle annular dark-field (HAADF)-STEM images demonstrate the epitaxial relationship between the thin film and
the substrate, which can be described as (001)[100]Fe3O4//(001)[100]STO, as shown in Figure 1 and the
Supporting Information, Figures S1 a,b. There are a number
of boundaries (including grain boundaries and twin boundaries) formed between grains as a result of a three-dimensional
island growth (Supporting Information, Figure S1 c). The
lattice parameters of Fe3O4 are measured as 8.301 c inplane (from SAED) and 8.641 c out-of-plane (from XRD).
This indicates that the upper part of thin film is strain-free but
the Fe3O4 film close to the interface between Fe3O4 and STO
has a compressive strain as high as 7.5 % induced by the
lattice mismatch with the substrate.[8]
We then used a “dry-cell” in situ TEM technique[4b,c, 5–7] to
investigate how this misfit strain affects the kinetics of lithium
insertion. Figure 2 a shows a low magnification HAADFSTEM image of the whole sample, and Figure 2 b presents
a representation of the open cell configuration used in this
study. Figure 2 c presents a time series of ADF-STEM images
obtained during in situ lithiation of the Fe3O4 epitaxial thin
film. The red-colored area (false color) indicates the surface
area which undergoes the conversion reaction, as it proceeds.
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Figure 1. a) XRD and b) SAED patterns, c) atomic resolution HAADFSTEM image, and d) atomic model of the Fe3O4 thin film on the
SrTiO3 substrate. Scale bars: c) 1 nm; inset 0.5 nm.
Figure 2. a) HAADF image and b) illustration of a dry electrochemical
cell inside a TEM. c) ADF-STEM image series captured in real time.
d) HRTEM image and e) corresponding phase map. Insets of (e) are
FFT results acquired from the designated areas in (d). f) HRTEM
image after the conversion reaction. i)–iv) FFT results obtained from
designated areas. ADF-STEM image of thin film and substrate
g) before and (h) after the in situ lithiation. Scale bars: a) 0.5 mm,
c) 10 nm, d) 5 nm, f) 2 nm, g) 200 nm, h) 200 nm.
Raw images and a video from this experiment are presented
in the Supporting Information, Figure S2 and Movie S1,
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respectively. The lithiation initiates at the upper area in
Figure 2 c. As lithiation proceeds, crevices develop at the
Fe3O4 grain boundaries, and subsequently the conversion
reaction initiates. Because the conversion reaction is accompanied by volume expansion,[2e, 5, 7h] the cracks are filled in.
Since the thickness of FIB-processed sample is comparable
over the entire Fe3O4 thin film (Supporting Information,
Figure S3), the lithiation process is expected to occur
uniformly; however, the degree of lithiation is inhomogeneous. Specifically, lithiation reaction is incomplete only near
the interface. Figure 2 d,e presents a high-resolution TEM
(HRTEM) image and a corresponding phase map in the
middle of lithiation process. The phase map in Figure 2 e
shows the spinel Fe3O4 phase with blue and the LixFe3O4 rocksalt phase with yellow (the phase map is created based on the
fast Fourier transform (FFT) patterns shown in the insets of
Figure 2 e). Previous work has demonstrated that rock-salt
LixFe3O4 is formed as a result of Li intercalation into Fe3O4
after partial lithiation.[5, 9] Both spinel and rock-salt structures
are found in the vicinity of the STO substrate, whereas above
this area considerable changes in morphology (evolution of
fine nanoparticles) are observed. This indicates that the
conversion reaction has occurred in the thin film except at the
region close to the substrate. We can identify that the Fe3O4
thin film has transformed into the composite of Fe and Li2O
by SAED and elemental mapping (Supporting Information,
Figure S4). However, the HRTEM image of Figure 2 f
demonstrates that the LixFe3O4 nanoparticles coexist with
the Fe nanoparticles in the region near the Fe3O4/STO
interface. Interestingly, a self-limiting lithiation was reported
in Si nanowires where the compressive strain from lithiatiated
part retards the reaction. We deduce that the external
compressive strain generally plays a negative role in the
lithiation of both conversion and alloying reaction.[10] Additionally, we can observe the evolution of strains at the
substrate induced by the expansion of the thin film. Figures 2 g,h compare ADF-STEM images before and after the
in situ experiments. After lithiation, the film cause the
development of an interfacial tensile strain into the substrate,
evident from the strain-induced bend contours shown in
Figure 2 h.
Ex situ electrochemical tests of Fe3O4/STO were performed with Swagelok type Li-half cells. The Fe3O4 thin film
battery underwent three discharge and charge cycles, as
shown in Figure 3 a. In case of conventional LIBs, the
electrolyte is impregnated into a porous composite of active
materials, binder, and conducting agent; thus, electrochemical
reactions occur over the electrode relatively simultaneously.
On the other hand, only the surface of Fe3O4 film is in contact
with electrolyte in thin film batteries; therefore, we use much
longer testing time (ca. 10 times) to insure a full discharge.
Scanning electron microscopy (SEM) image of the Fe3O4 thin
film after 3 cycles to 0.12 V (Figure 3 b) shows that the thin
film has undergone a “mud-crack” fracture mode.[11] To verify
this mechanical failure of discharged Fe3O4 thin film, extra
in situ lithiation of the sample was performed after the stage
shown in Figure 2 c. Figure 3 c presents a time sequence of
ADF-STEM images of the sample during additional lithium
insertion. Raw images and a video are available in the
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Scheme 1. Summary of the lithiation process under the compressive
strain at the interface between Fe3O4 thin film and STO substrate.
Figure 3. a) Charge–discharge profile of Fe3O4/STO thin film battery
for 3 cycles. b) Plan-view SEM image of Fe3O4 film after 3 cycles.
c) Time series of ADF-STEM image with further Li insertion after the
conversion reaction. Red-colored area indicates that the conversion
reaction has occurred. d) HREM image after the film delamination. i)–
iv) FFT results obtained from designated areas. Scale bars: c) 10 nm,
d) 2 nm.
Supporting Information, Figure S5 and Movie S2. We
observed that the film delamination initiated at the top of
the thin film and proceeded along the diffusion path of the Li
ions. HRTEM analysis (Figure 3 d) confirms that the transition from intermediate LixFe3O4 rock-salt phase to the
composite of Fe and Li2O had occurred in the delaminated
thin film, which implies that the complete conversion reaction
of Fe3O4 bring about substantial diffusion-induced strain and
engender mechanical and contact failure for Fe3O4 thin film
batteries.
Scheme 1 summarizes the real-time observations of
compressive-strained Fe3O4 thin film during lithiation.
The effect of pre-existing strain on the kinetics of
lithiation is also investigated with phase field simulation.
We develop our model based on the Butler–Volmer and
Cahn–Hilliard equations (see the Supporting Information for
details).[7h, 12] Figure 4 a shows a model for the simulation,
where planes I, II, III are cross-section planes of the Fe3O4
thin film parallel with the substrate. We preset the interfacial
strain at plane I, II, and III as 7.5 %, 3.75 %, and 0,
respectively. Plane IV is cross-section of Fe3O4 normal to
the substrate to show the reaction nature with strain gradient.
Figures 4 b–d and the Supporting Information, Movies S3–S5
show the microstructural evolution during lithiation at planes
Angew. Chem. Int. Ed. 2017, 56, 7813 –7816
Figure 4. a) Representation of the planes where the simulation are
performed. b)–d) The degree of lithiation with a function of time at I,
II, III planes, which have strain of b) 7.5 %, c) 3.75 %, and d) 0,
respectively. e) The average concentrations of Li ion with simulation
time in the three cross-sections of planes I to III. f) The concentration
profile of Li = 0.8 ion on the cross section IV as a function of reaction
time.
I, II, and III, respectively, and demonstrate that the Li-ion
insertion becomes slow with increasing compressive strain. A
faster surface diffusion is also considered in simulations.
Figure 4 e presents the average Li concentration of planes I,
II, and III as a function of reaction time. We speculate that Li
diffusion along the surface is dominant initially; but, once the
Li concentration at the surface becomes saturated, the
kinetics of lithium diffusion are strongly dependent on the
strain energy of the coupling between the interfacial strain
and lithiation-induced strain.[12e] At the highest compressive
strain (plane I), the rate of Li insertion is the slowest and it
takes much more time to reach the saturated Li concentration
on plane I than on plane II or plane III. Figure 4 f shows the Li
profile in function of time (see also the Supporting Information, Movie S6). The upper x-axis (with the cross-hatch)
indicates the compressive strained part of the film, where the
lithiation is always slower than at any other area of thin film.
These profiles are in excellent agreement with the in situ
STEM results in Figure 2 c. It shows that compressive strain
coupling component indeed decelerates lithiation by increasing the energy required for Li-ion insertion into the lattice.
The elastic energy resulting from the coupling of lithiation-
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Communications
induced strain and interfacial strain is the main factor to
determine the reaction rate. Thus, it is possible to tune the
diffusion rate by controlling external strain with lower
lithiation rate and the formation of LixFe3O4 buffer layer at
the interface.
In summary, we have investigated the lithiation dynamics
in an epitaxial Fe3O4 thin film grown on SrTiO3 substrate
using in situ STEM and phase-field simulation. The preexisting compressive strain from the substrate plays an
important role in impeding the Li insertion due to the elastic
energy from the coupling between the electrochemically
induced stress and interfacial strain. The thin film delamination is suggested to be controllable by the lithiation level,
despite the severe volume expansion in the fully-reacted part
of the film; the strained part of the film can be a buffer against
mechanical failure.
Acknowledgements
We acknowledge support of the Center for Functional
Nanomaterials, Brookhaven National Laboratory, which is
supported by the U.S. Department of Energy (DOE), Office
of Basic Energy Sciences, under Contract No. DE-SC00112704. Q.M. and Y.Z. were supported by DOE/BES,
Division of Materials Science and Engineering, under Contract No. DE-SC-0012704.
Conflict of interest
The authors declare no conflict of interest.
Keywords: conversion electrodes · lithiation · magnetite ·
strain · thin films
How to cite: Angew. Chem. Int. Ed. 2017, 56, 7813 – 7816
Angew. Chem. 2017, 129, 7921 – 7924
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Manuscript received: March 27, 2017
Accepted manuscript online: May 9, 2017
Version of record online: May 29, 2017
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