Letter
pubs.acs.org/NanoLett
Atomic Resolution Structural and Chemical Imaging Revealing the
Sequential Migration of Ni, Co, and Mn upon the Battery Cycling of
Layered Cathode
Pengfei Yan,† Jianming Zheng,‡ Ji-Guang Zhang,‡ and Chongmin Wang*,†
†
Environmental Molecular Sciences Laboratory, ‡Energy and Environment Directorate, Pacific Northwest National Laboratory, 902
Battelle Boulevard, Richland, Washington 99354, United States
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S Supporting Information
*
ABSTRACT: Layered lithium transition metal oxides (LTMO) are
promising candidate cathode materials for next-generation high-energy
density lithium ion battery. The challenge for using this category of
cathode is the capacity and voltage fading, which is believed to be
associated with the layered structure disordering, a process that is
initiated from the surface or solid-electrolyte interface and facilitated
by transition metal (TM) reduction and oxygen vacancy formation.
However, the atomic level dynamic mechanism of such a layered
structure disordering is still not fully clear. In this work, utilizing
atomic resolution electron energy loss spectroscopy (EELS), we map, for the first time at atomic scale, the spatial evolution of Ni,
Co and Mn in a cycled LiNi1/3Mn1/3Co1/3O2 layered cathode. In combination with atomic level structural imaging, we discovered
the direct correlation of TM ions migration behavior with lattice disordering, featuring the residing of TM ions in the tetrahedral
site and a sequential migration of Ni, Co, and Mn upon the increased lattice disordering of the layered structure. This work
highlights that Ni ions, though acting as the dominant redox species in many LTMO, are labile to migrate to cause lattice
disordering upon battery cycling, while the Mn ions are more stable as compared with Ni and Co and can act as pillar to stabilize
layered structure. Direct visualization of the behavior of TM ions during the battery cycling provides insight for designing of
cathode with high structural stability and correspondingly a superior performance.
KEYWORDS: Layered cathode, lithium ion battery, structural degradation, EELS, annular bright field imaging
F
aggregation, lithium depletion, and oxygen loss. Lattice
transformation, from layered to either spinel-like structure or
rock-salt structure, is realized through TM migration, noting
that the oxygen sublattice adopts the same stacking sequence
during the phase transformation. Therefore, suppression of TM
migration is the key to stabilize the layered structure during
cycling. The mobility of TM is mainly determined by its
valence state and ionic radius. When TM is at a high valence
state, the activation energy for migration is high, and
simultaneously the large difference of the ionic radius between
TM and Li+ can significantly suppress a potential Li/TM
interlayer mixing. When TM is reduced to a low valence state,
the energy barrier for migration is decreased and therefore
gains a high mobility, and at the same time, its ionic radius is
increased to close to that of Li+, facilitating the migration of
TM into the Li layer and correspondingly causing lattice
transformation. One of the key questions that needs to be
answered is which species migrates preferentially to the Li
layers. Based on simulation and indirect deduction,18−20 it has
been proposed that, among Ni, Co, and Mn, Ni ions are prone
or rechargeable batteries, pushing toward high energy
density and stable cycling, among other performance
factors, appear to be an insatiable goal. One way to push toward
this goal is to reversibly extract a high fraction of lithium from a
cathode lattice. Layered lithium transition metal oxides
(LTMO), as a category of cathode for lithium ion battery,
appear to be feasible for pushing toward high energy density
and cycling stability due to their high theoretical capacities, rich
redox species, and diversified composition versatility.1−4
Typical trigonal LTMO (LiMO2, M = Co, Mn, Ni, etc.),
such as LiCoO2, LiNi1/3Mn1/3Co1/3O2, and LiNi0.5Mn0.5O2,
have been commercialized. As another category of layered
cathode, Li-rich LTMO cathode can provide an even higher
energy density.5−8 However, the practical capacities of both
trigonal and Li-rich LTMO are much lower than their
theoretical capacities, indicating that the fraction of Li
utilization can be further increased to boost energy density.
Unfortunately, many previous studies have indicated that
increasing Li utilization will aggravate the degradation of the
cell, especially on the cathode side.9−12
One of the well-known degradation mechanisms occurred on
LTMO is the phase transformation which has been widely
observed at the particle surface.13−17 Intensive studies reveal
that such a phase transformation is initiated by a local chemistry
change, including transition metal (TM) reduction and
© 2017 American Chemical Society
Received: April 12, 2017
Revised: May 5, 2017
Published: May 9, 2017
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Figure 1. Electrochemical data of NMC333 using half-cell configuration. (a) Cycling performance and (b, c) the corresponding charge/discharge
voltage profile evolutions of NMC333 during cycling at C/10 in the voltage range of (b) 2.7−4.2 V and (c) 2.7−4.8 V.
Figure 2. STEM-HAADF images of the surface layer of NMC333 cathode. (a) Pristine sample without cycling. (b) After 100 cycles with a high
cutoff voltage of 4.2 V. (c) After 100 cycles with a high cutoff voltage of 4.8 V. (d) Enlarged area from the region marked with red box in panel c. The
insets in panel d are the structural models illustrating lattice transformation from surface into bulk, where transition metal ions, Li ions, and oxygen
ions are denoted by blue, green, and red balls, respectively. The yellow dashed lines denote the surface reconstruction layer. Fast Fourier
transformation from the three different regions in panel d are shown at the bottom, indicating structure transformation. Blue arrows indicate extra
spots, and dashed black frames highlight unit cell changes due to lattice disordering.
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Figure 3. STEM-HAADF and STEM-ABF images simultaneously captured on NMC333 cathode following 100 cycles with a high cutoff voltage of
4.8 V. (a, b) From a region with the well-preserved layered structure. (e, f) From disordered region. (i, j) From heavily disordered region. Enlarged
images from panels b, f, and j are shown in panels c, g, and k, respectively. Panels d and h are the corresponding crystal model for panels c and g. (l)
Illustrates tetrahedral site TM ion and lattice distortion in panel k. The scale bars are 1 nm.
to migrate to initialize the lattice transformation. However,
direct evidence for this claim is still lacking. Based on high
energy electron irradiation, Lu et al. noticed that Ni ions, as
contrasted with Mn ion, are prone to migrate into Li-layer in
the Li-rich LTMO.21 Essentially, migration behavior of Ni, Co,
and Mn during the electrochemical cycling is still not
experimentally addressed.
In this work, we captured atomic resolution elemental maps
on a well-known LiNi1/3Mn1/3Co1/3O2 (NMC333) layered
cathode by virtue of electron energy loss spectroscopy (EELS)
in scanning transmission electron microscopy (STEM),
indicating a sequential migration of Ni, Co, and Mn to Li
layer, which correspondingly contributes to the lattice transformation.
The electrochemical properties of the battery with the
NMC333 cathodes were tested using a coin cell configuration
with Li metal as an anode and cycled at the rate of 0.1 C at
room temperature with the high-cutoff voltages of 4.2 and 4.8
V. Figure 1 shows the electrochemical performance of the cells
at different high-cutoff voltages after 100 cycles demonstrating
that NMC333 charged to 4.8 V shows much faster capacity
decay and voltage fading during cycling, while the cell charged
to 4.2 V shows negligible performance decay. Although the
initial capacity is higher when charged to 4.8 V (∼220 mA h/g),
after 100 cycles, both the discharge capacity and the averaged
discharge voltage are lower than that of charged to 4.2 V,
indicating a high voltage cycling leads to poor cyclability in
terms of both capacity retention and voltage fading.
Structurally, our previous investigation has indicated that
intragranular cracking is one of the major features for the
high voltage cycled sample, which directly contributes to the
fast degradation of the cathode.10 In this work, we focus on
probing the atomic level process that contributes to the
degradation mechanism of cathode particles when cycled at
different voltages.
The cathode foils were disassembled from Li/NMC333 halfcells, and cross-sectional TEM specimens were prepared using
focused ion beam (FIB) lift-out technique (Figure S1) as
detailed in a previous publication.10 STEM high-angle annular
dark field (HAADF) imaging reveals that the surface of the
NMC333 particle was modified after 100 cycles. As shown in
Figure 2, cycling introduces a surface layer, which shows a
bright contrast under STEM-HAADF imaging due to the TM
aggregation and formation of solid electrolyte interphase.13−15,22 Figure S2 and Figure 2d show the lattice images
from the surface layer of both pristine and cycled samples.
Figure 2 clearly demonstrates that cycling of the battery at 4.8
V can introduce a thicker surface reconstruction layer of ∼25
nm as compared with that of ∼2 nm for cycling at 4.2 V. In
Figure 2c and d, we can identify three zones from surface into
bulk, which are highlighted by purple and yellow dashed lines.
Beyond the purple dashed line is the outmost surface layer,
which is heavily disordered, and the layered structure features
can be hardly seen. In between the purple and yellow dashed
lines, the layered structure is mildly disordered, still maintaining
a layered feature. Beyond the yellow dashed line toward the
bulk lattice, the layered structure is well-preserved. Such a
gradual structure evolution from bulk lattice to surface that
featuring a layered to disordered layer and then to rock salt is
also evidenced by the fast Fourier transformation images
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Figure 4. Atomic resolution STEM-EELS mapping. (a) From the pristine NMC333 without cycling. (b−d) From NMC333 after 100 cycles with a
high cutoff voltage of 4.8 V. (b) From the well-preserved layered region. (c) From the disordered region. (d) From the heavily disordered region.
The dashed blue frame at bottom right highlights TM maps with disordered structure. The white arrows in row b indicates a correspondence
between the STEM-HAADF image and EELS map for identifying the Ni that migrates to the Li layer. The scale bars in HAADF images are 0.5 nm.
(shown in Figure 2d). It is interesting to note that, for the case
of Li-rich LTMO, such a surface reconstruction layer is more
likely to be the spinel structure.15
The structural details of the surface layer were further
revealed by using a combination of HAADF and annular bright
field (ABF) imaging. As shown in Figure 3, the HAADF and
ABF images are collected simultaneously from the wellpreserved layered region (Figure 3a and b), the moderately
disordered region (Figure 3e and f), and the heavily disordered
region (Figure 3i and j). Both HAADF and ADF series indicate
the TM ions in the Li layer are gradually increased, causing
disordering of the layered structure. As shown in Figure 3k, the
magnified image from Figure 3j clearly shows TM-ions sitting
at tetrahedral sites (highlighted by a pink arrow) as well as
severe lattice distortion (highlighted by a blue arrow). The
lattice distortion is associated with the reduction of the TM
cation and formation of oxygen vacancies for which the former
can rise Jahn−Teller distortion, while the later can cause TM−
O debonding. This observation for the first time provides direct
evidence that TM-ions reside at tetrahedral sites in NMC
layered cathode, which indicates that the degradation
mechanism of trigonal LTMO is similar to that of Li-rich
LTMO for which trapping of TM ions at tetrahedral ions are
believed to be the origin of voltage decay.9
To address the key question as which TM ions initiates the
disordering of layered structure, we use atomic resolution
STEM-EELS mapping to directly correlate the lattice structure
with the spatial distribution of Ni, Co, and Mn. Figure 4a was
taken from the pristine NMC333 (collected under 200 kV
imaging electron beam). For the cycled samples, we used 80 kV
imaging electron beam to minimize the beam damage on the
cycled samples (Figure 4b−d). As evidenced by Figures S3 and
S4, following the EEL mapping, the lattice shows no
appreciable change, indicating that the sample damage during
the STEM-EELS mapping is well-mitigated with the 80 keV.
Details on EELS acquisition and data processing are provided
in the Supporting Information.
Four typical mapping results are shown in Figure 4 for which
the oxygen maps are presented and set as references to ensure
our mapping results with high spatial resolution (the oxygen
columns can be well-resolved in all series, indicating the spatial
resolution is better than 2 Å). For the pristine sample (Figure
4a), the maps of Mn, Co, and Ni clearly reveal that Ni, Co, and
Mn reside at the TM layer and the lattice maintains a wellordered layered structure. However, following the battery
cycling, elemental distribution exhibits disordering, and such a
disordering is different for Ni, Co, and Mn, which in turn
depends on the degree of the overall structural disordering of
the lattice as shown in Figure 4b−d. Moreover, in Figure 4b, for
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the slightly structural disordered region, although the layered
structure is presented as evidenced by the corresponding
HAADF image, the Ni map shows the Ni distribution is
deviated from a layered feature, indicating substantial Ni has
migrated from the TM layer into the Li layer. Clear evidence is
highlighted by the arrows in Figure 4b, where Ni is sitting in the
Li layer and leads to the bright contrast under HAADF image.
In contrast, Co and Mn maps show a well-preserved layered
structure as indicated in Figure 4b. This set of maps captured
on the slightly structural disordered region indicate that it is the
Ni ions, rather than the Mn or Co ions, that preferentially
migrate into the Li layer to contribute the lattice disordering. In
other words, Ni ions disorder first upon battery cycling. For the
moderate structural disordered region as judged from the
HAADF image shown in Figure 4c that is captured from a
region that is adjacent to the region of Figure 4b, the elemental
maps clearly indicate at this region both Ni and Co
distributions are significantly disordered, while the Mn
distribution is only slightly disordered. For the heavily
structural disordered region as evidenced by the HAADF
image shown in Figure 4d, the corresponding maps of Ni, Co,
and Mn indicate all of the TM elements are heavily mixed and
lost their original layered arrangement. The map series shown
in Figure 4 indicates that, among Ni, Co, and Mn, the Mn ions
are the most stable ions in keeping the layered structure.
Though the regions we chose in Figure 4b−d are spatially
different, the increasing degree of lattice structural disordering
actually reflect the temporal evolution of layered structure
disordering with progression of the battery cycling. Therefore,
what we have observed on the dependence of the spatial
distribution of Ni, Co, and Mn on the degree of the lattice
disorder actually reflects different propensity of migrating from
TM layer to Li layer for Ni, Co, and Mn, featuring a sequential
migration behaviors of Ni, Co, and Mn in accordance with the
lattice disordering of layered structure.
The structural and chemical evolution as described above is
also consistently supported by the correspondingly electronic
structure evolution as indicated by the EELS spectra shown in
Figure 5, which are captured from the four mapping regions in
Figure 4. In Figure 5, spectra i−iv correspond to regions a−d in
Figure 4. In Figure 5a, these EELS spectra show two significant
features with the progression of the structural lattice disordering. One is the oxygen prepeak, which is gradually decreased
from i to iv as denoted by the arrows. The oxygen prepeak is
associated with the transition of electrons from the 1s core state
to unoccupied 2p states hybridized with 3d states in transition
metals.23 Therefore, a gradual suppression of oxygen prepeak
indicates the formation of oxygen vacancy and the reduction of
TM that coordinates with the oxygen.24 The other trend is the
evolution of the Mn L3/L2 ratio, which gradually increases as
shown in Figure 5a, indicating Mn reduction is more significant
in the heavily disordered regions. The EELS L-edges of Co and
Ni are shown in Figure 5b. Clearly, the chemical shift on the L
edge of Co can be noticed as marked in iii and iv of Figure 5b,
indicating the reduction of Co, which is consistent with the
disordering of Co as shown in Figure 4c and d. The chemical
shift for the L-edge of Ni can also be noticed as indicated in ii,
iii, and iv in Figure 5b. Combined the elemental mapping
(Figure 4) with EELS valence state analysis (Figure 5), it is
clearly indicated that lattice disordering (TM migration) always
associates with the reduction of TM ions. Therefore, our EELS
analysis corroborates the argument that the surface phase
transformation is facilitated by oxygen loss and TM reduction.
Figure 5. EELS spectra acquired from the regions in Figure 4. (a) O
K-edges and Mn L-edges; and (b) Co L-edges and Ni L-edges. Spectra
labeled as i−iv correspond to the regions a−d in Figure 4, respectively.
As marked in panel a, with the gradual increase of the lattice
disordering, the prepeak on the oxygen K-edge gradually decrease,
indicating formation of oxygen vacancies; while s and the L3/L2 ratio
on Mn-L edge gradually increases, indicating the reduction of Mn. As
indicated by the black dashed line in panel b, the chemical shift of of Ledge for both Ni and Co indicates the reduction of Ni and Co.
In pristine LTMO, previous studies have shown that Ni ions,
due to a similar ionic radius to Li+, can have slight interlayer
mixing with Li ions, while Li/Co and Li/Mn interlayer mixing
is negligible.25−28 After high voltage cycling, oxygen vacancies
are introduced into the surface layer either through oxygen gas
evolution and/or cathode/electrolyte side reactions. Oxygen
vacancy on one hand can lead to the reduction of the adjacent
TM to a lower valence states, and on the other hand it may
facilitate the migration of the reduced TM.24 The present
atomic resolution structural and chemical imaging reveal that,
upon battery cycling, the disordering of layered structure is not
a process of random hopping of all TM ions to the Li layer;
rather, it is a process with strong preference of one species over
the others. For the case of coexistence of Ni, Co, and Mn, the
Ni ion is observed to be the most labile one to migrate during
the battery cycling, followed by Co and Mn. Therefore, in
terms of lattice stability, among Ni, Co, and Mn, the Co ion
shows better stability than Ni in countering layered structure
disordering, and the Mn ion is the most stable one among all of
these three, indicating Mn ions play a key role in stabilizing
layered structure. On the other hand, suppressing oxygen anion
loss is also very crucial for stabilizing the layered structure,6,29,30
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because the absence of oxygen vacancy can maintain a high
valence state for the TM cations and thus immobile.
In summary, we investigated the surface layer structural and
chemical evolution of NMC333 following the high-cutoff
voltages cycling at 4.2 and 4.8 V and found 4.8 V cycling can
introduce a much thicker surface phase transformation layer.
STEM-ABF imaging directly reveals the residing of TM-ions at
the tetrahedral site in the heavily disordered region. Atomicresolution STEM-EELS mapping captured at the regions with
different levels of structural disordering in the cycled sample
indicates a sequential migration behaviors of Ni, Co, and Mn in
facilitating the disordered layered structure, suggesting that Ni
ion is the most labile one and Mn-ion is the most stable one in
terms of the tendency of migrating to Li layers. The present
observations provide unprecedented insights on the lattice
degradation mechanism of layered LTMO at atomic level,
shedding new light on the material selection and composition
optimization for NMC-based LTMO in countering lattice
degradation and correspondingly mitigating capacity and
voltage fading.
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.nanolett.7b01546.
Snapshot images during FIB lift-out, STEM-HAADF
images, and EELS experimental details (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: Chongmin.wang@pnnl.gov.
ORCID
Ji-Guang Zhang: 0000-0001-7343-4609
Chongmin Wang: 0000-0003-3327-0958
Author Contributions
P.Y. and J.M.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work is supported by the Assistant Secretary for Energy
Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No.
DE-AC02-05CH11231, Subcontract No. 18769 and No.
6951379 under the Advanced Battery Materials Research
(BMR) program. The microscopic analysis in this work was
conducted in the William R. Wiley Environmental Molecular
Sciences Laboratory (EMSL), a national scientific user facility
sponsored by DOE’s Office of Biological and Environmental
Research and located at PNNL. PNNL is operated by Battelle
for the Department of Energy under Contract DE-AC0576RLO1830.
■
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