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Chunjoong Kim, Patrick J. Phillips, Baris Key, Tanghong Yi, Dennis Nordlund,
Young-Sang Yu, Ryan D. Bayliss, Sang-Don Han, Meinan He, Zhengcheng Zhang,
Anthony K. Burrell, Robert F. Klie, and Jordi Cabana*
In a rechargeable battery, energy is stored and released through
reversible redox reactions at both the anode and cathode.
So far, the best performance has been achieved in materials
that undergo intercalation reactions involving Li ions, which
led to the advent of Li-ion batteries.[1,2] These devices dominate the mobile device market due to their high energy density, with applications expanding to automotive vehicles.[3,4]
However, Li-ion batteries are limited in their charge storage
capacity,[5] thus triggering interest in new energy storage concepts.[6,7] Among them, systems based on the intercalation of
multivalent ions are attractive because, while conceptually
similar to those using Li+ ions, they store more charge per
intercalated species, thereby achieving high energy density
with less dramatic structural rearrangements in the host.[9]
For instance, in theory, electrochemical intercalation of Mg2+
into MnO2 to form Mg0.5MnO2 would result in a capacity of
≈270 mAh g−1, higher than any practical candidates for Li-ion
batteries (≈200 mAh g−1).[10–12] When coupled with a Mg metal
anode, the resulting batteries could surpass the current performance barriers of Li-ion technology in terms of specific energy
Dr. C. Kim, Dr. T. Yi, Dr. Y.-S. Yu, Dr. R. D. Bayliss,
Prof. J. Cabana
Department of Chemistry
University of Illinois at Chicago
Chicago, IL 60607, USA
E-mail: [email protected]
Dr. P. J. Phillips, Prof. R. F. Klie
Department of Physics
University of Illinois at Chicago
Chicago, IL 60607, USA
Dr. B. Key, Dr. S.-D. Han, M. He, Dr. Z. Zhang, Dr. A. K. Burrell
Chemical Sciences and Engineering Division
Argonne National Laboratory
Argonne, IL 60439, USA
Dr. D. Nordlund
Stanford Synchrotron Radiation Lightsource
SLAC National Accelerator Laboratory
Menlo Park, CA 94025, USA
Dr. Y.-S. Yu
Advanced Light Source
Lawrence Berkeley National Laboratory
Berkeley, CA 94720, USA
M. He
Mechanical Engineering
Worcester Polytechnic Institute
Worcester, MA 01609, USA
DOI: 10.1002/adma.201500083
Adv. Mater. 2015, 27, 3377–3384
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Direct Observation of Reversible Magnesium Ion
Intercalation into a Spinel Oxide Host
and energy density.[13] In practice, while exciting, this concept
remains to be demonstrated.
Electrochemical intercalation of Mg2+ into several compounds has been claimed,[14–16] yet the clearest evidence of its
feasibility has been provided for Mo6S8.[17,18] This material was
used as the cathode in the only example of a stable Mg battery
available in the literature.[18] However, the low capacity and
potential of intercalation of Mo6S8 compared to existing Li-ion
electrodes resulted in energy densities that were too low to
create a viable alternative. Thus, there is a clear need for the
discovery of hosts that are able to accommodate large amounts
of Mg2+ at a high potential versus Mg2+/Mg0. Oxides with the
spinel structure have long been known as suitable hosts for
batteries based on Li intercalation, largely because of their
structural stability, high potential of reaction, and fast ion diffusion.[19] Indeed, LiMn2O4 is employed in batteries in electric vehicles available to the consumer today.[3] Several studies
have claimed intercalation of Mg2+ into spinel-type Mn2O4 in
aqueous environments,[20,21] yet the mechanism of reaction,
especially as it refers to classical intercalation versus other
competing reactions, and its reversibility have not been ascertained. Given the possible competition of Mn-driven corrosion
reactions at neutral to acidic pH,[22] confirming the successful
intercalation of Mg into the tetrahedral sites of the spinel structure is important. It would justify further efforts to design prototypes containing spinel hosts paired with Mg metal anodes in
compatible nonaqueous electrolytes.
Here, the extent and reversibility of the Mg intercalation
into spinel-type Mn2O4 have been examined using characterization tools providing insights across complementary length
and chemical scales. The results provide direct visualization of
the electrochemical intercalation of Mg2+ into the tetrahedral
sites of a spinel oxide host; this magnesiation was found to be
extensive and reversible. The existence of intermediates and
bottlenecks to the completion of the reaction was ascertained.
This work provides conclusive evidence of Mg intercalation into
a spinel oxide framework, identifying a family that deserves
careful consideration in efforts toward the practical development of a multivalent battery system.
In order to facilitate Mg intercalation into the spinel host,
LiMn2O4 was treated in mild acidic media (0.1 M HCl aqueous
solution), as detailed in the Experimental Section. This process
led to the partial delithiation of the material, as confirmed by
X-ray diffraction (XRD, Figure 1a,b). The cubic spinel structure was preserved, but the diffraction peaks shifted towards
higher angles (see LiMn2O4 (LMO) versus acid-treated spinel
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Figure 1. a) XRD patterns of as-received LiMn2O4, acid-treated LixMn2O4 (x ≈ 0.2), and electrodes harvested from a Pt/Mg2+, NO3−/acid-treated
LixMn2O4 electrochemical cell in aqueous solutions. Peaks from cubic and tetragonal spinel structures are labeled as c and t, respectively; b) detailed
view of the patterns surrounding the (111) cubic peak, highlighting the formation of new phases, either tetragonal (t) or intermediate (i); c,d) Representative TEM images of acid-treated LixMn2O4; e) Electrochemical responses of the Pt/Mg2+, NO3−/acid-treated LixMn2O4 cell, with the various
potentials indicated at which the electrodes of a) and b) were harvested.
(p′) in Figure 1a,b), which were attributed to a decrease in the
lattice parameter (from 8.225 to 8.081 Å). Assuming that the
changes are due to the extraction of Li+ ions concurrent with
the oxidation of Mn3+, and considering the lattice parameter
of delithiated spinels in the literature,[23,24] the composition
was estimated to be Li0.2Mn2O4. The possibility of competing
Li+/H+ exchange reactions cannot be discarded, although they
are generally much less extensive in Li1+xMn2−xO4 when x is
close to 0, as is the case here, contrary to what happens close
to x = 1/3.[25,26] The acid treatment induced a small change in
morphology, as observed by transmission electron microscopy
(TEM), from the micrometric particles of parent LiMn2O4
spinel (Figure S1, Supporting Information) to the appearance
of thin nanoflakes in certain parts of the sample (Figure 1c,d).
This morphological change, which has also been reported by
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others,[27] in principle, reduces the lengths for the Mg2+ ions to
diffuse into a particle.
Electrochemical tests were carried out in a three-electrode
beaker cell. The working electrode was the acid-treated spinel
cast on to 304 stainless-steel (SS) foil. A saturated calomel electrode (SCE) and Pt were used as the reference and counter
electrode, respectively. All electrochemical tests were conducted
in 1 M Mg(NO3)2 aqueous solution. Voltages in the electrochemical measurements are referenced by SCE throughout this
manuscript, unless otherwise stated. The cells were galvanostatically charged to 1 V to completely remove residual cations
in the tetrahedral site of the spinel structure, and discharged to
different cut-off voltages (0.1, 0, −0.1, and −0.2 V) at a rate of
C/20 (Figure 1e). Charging resulted in a sustained increase in
potential, with an inflection point at ≈0.7 V, for a total capacity
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(c′ to e′), while the intermediate remained constant. This observation suggests that compositional miscibility exists within the
most reduced state, highlighting the complexity of the reaction mechanism. Measurements on an electrode recharged to
1 V (f′) confirmed the reversibility of these structural changes,
showing the XRD pattern dominated by a cubic lattice close in
size to that obtained after the initial charge (a′). Nonetheless,
a small peak due to remnants of the reduced phase was still
observable at 18.2°, 2θ. This peak was close to the tetragonal
phase obtained after discharge to 0 V (c′); no evidence of the
reduction intermediate could be observed. It is worth noting
that competition with Mn dissolution in aqueous media also
occurred in the tested voltage window,[22] which resulted in
a loss of diffraction intensity and coloration of the otherwise
colorless electrolyte solutions.
Scanning transmission electron microscopy (STEM) and
atomic-resolution energy-dispersive X-ray (EDX) spectroscopy
directly probed the presence of Mg in the spinel structure after
reduction to −0.2 V (e′). Annular bright-field (ABF) images
were taken to study the structural evolution in the spinel
phases since light elements (e.g., Mg) can be more readily visualized in ABF. Figure 2a presents an ABF image of the [100]
zone axis orientation, with the atomic positions of Mn, Mg, and
O identified in the figure. The crystallographic information file
(CIF) used for structural identification can be found in the Supporting Information, and was available from the Materials Project website (ICSD ID:16858, MP-32006).[30,31] Notably, in this
[100] zone, atomic columns are unmixed, and the ABF image
clearly shows occupation in the tetrahedral position, imaged as
a dark contrast. By averaging the filtered ABF image, atomic
occupancies in the unit cell could be further visualized as
shown in the inset. The ultimate proof of Mg intercalation was
obtained via atomic-resolution EDX. Multiple EDX line scans
were carried out along the <001> direction, with a representative scan presented in Figure 2a. The line scan shows the spatial distribution of Mn, O, and Mg in the structure (Figure 2b),
where Mn (red), O (green), and Mg (blue) are spatially resolved
with approximately 1 Å resolution. A sequence of Mn-O-MgMg-O-Mn was observed in two consecutive 2 × 2 cells. This
atomic arrangement and distances are consistent with the
structure model of MgMn2O4 based on the CIF. This clear
match between the structural model and atomic distribution
indicates that Mg insertion into the spinel structure occurred
through the galvanostatic process. Additional EDX line scans
from other regions confirmed the reproducibility of these
observations (Figure S3, Supporting Information). In addition,
EDX maps from a large field of view, as shown in Figure 2c,
revealed ≈11 at% Mg that was uniformly distributed (Figure S4,
Supporting Information), ruling out phase segregation or Mgrich surface layers. It should be noted that the insertion of Mg
into the spinel hosts could be dominantly observed in the nanoflakes existing in the sample, where the diffusion lengths were
short, while micrometric particles largely still remained as a
cubic spinel structure.
Results from techniques averaging over a large sample
volume confirmed that magnesiation was extensive, as indicated by XRD. X-ray absorption spectroscopy (XAS) across
the Mn K absorption edge (Figure 3a) was collected from
the acid-treated spinel (p′), and after reduction to −0.2 V (e′).
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of ≈100 mAh g−1. Given the small amount of cations left in
the tetrahedral sites of the spinel structure, this capacity likely
includes contribution from other reactions, such as electrolysis of the aqueous electrolyte.[28] In the following discharge,
cathodic reactions proceeded from ≈0.1 V and a discharge
capacity of ≈190 mAh g−1 was achieved at −0.2 V. Similar electrochemical responses were obtained by cyclic voltammetry
(CV) (Figure S2, Supporting Information) where anodic and
cathodic peaks were observed at 0.8 V and −0.15 V in the acidtreated spinel electrodes, respectively. The average potential, ca.
0.3 V vs. SCE, corresponds to ca. 2.9 V vs. Mg2+/Mg0 (Figure 1e
and Figure S2, Supporting Information), consistent with available computational predictions.[29] Meanwhile, blank stainless
foils did not show any electroactivity within the CV scan range,
indicating that concurrent electrolysis of the electrolyte could
be catalyzed by the delithiated spinel oxide.[28]
XRD, Figure 1a,b, was taken from samples at various stages
of charging/discharging to gain insight into the electrochemical processes. Electrodes were harvested after a charge to 1 V,
as well as after discharges to 0.1, 0, −0.1, and −0.2 V, and after
a recharge to 1 V, as denoted in Figure 1e. Electrodes charged
to 1 V state (a′) showed a shift to higher angles in the XRD data
compared to the pristine state of acid-treated spinels (p′) due
to the complete removal of cations from the tetrahedral sites
to form λ-MnO2. The shift of the XRD peaks is clearly observable in Figure 1b, which provides a zoom into the vicinity of
the (111) peak, yet much smaller than between the as-received
LiMn2O4 and its acid-treated counterpart. This observation
is consistent with a competing process contributing to the
observed specific capacity, such as water electrolysis. When discharged down to 0.1 V (b′), small peak shifts were observed,
together with shoulders to the left of the (111), (311), and (400)
cubic spinel peaks. The shoulders became more prominent
upon further discharging, at the expense of the charged cubic
spinel (b′–d′). Three clearly resolvable peaks were found at
−0.1 V (d′), suggesting the formation of multiple phases. Thus,
the electrochemical reaction proceeded through a multiphase
transition. The presence of these phases was more obvious
when a higher negative discharge cut-off voltage, −0.2 V (e′),
was applied. The cubic spinel peak (marked as c with indices)
was still present, indicating an incomplete reduction, but a new
set of peaks (marked as t with indices) was very apparent. The t
set of peaks could be indexed with a tetragonally distorted spinel
lattice, whose clearest signature is the superstructure peaks
between 19° and 35°, 2θ. The structural distortion from cubic
to tetragonal phases resulted from the Jahn-Teller distortion
due to the presence of significant amounts of Mn3+, indicating
that Mn4+ was successfully reduced. The position of the t set of
peaks matched the pattern of MgMn2O4 (JCPDS card number:
72-1336, a = 5.75, c = 9.38 Å). This result is consistent with the
observations by Sinha and Munichandraiah,[21] although in this
study the sample remained rather crystalline during the reaction. In turn, the third set of peaks, indicated as i, appeared to
be due to an intermediate phase between the cubic-to-tetragonal
transition. Other than a peak at 18.7°, 2θ, which could be due
to the (101) or (111) reflection in a tetragonal or cubic spinel
lattice, respectively, no other peaks ascribable to this intermediate phase could be distinguished. It is worth noting that there
was a continuous shift in the t peaks between 0 and −0.2 V
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Figure 2. a) ABF STEM image of the acid-treated spinel reduced to −0.2 V vs. SCE (sample e′) aligned along a [100] zone axis. Atomic positions of
Mn (red), O (green), and Mg (blue) are clearly identified in the raw image, while the inset, a filtered and averaged/cross-correlated subimage, further
highlights the atomic occupancies; b) an EDX line scan along the [001] direction (yellow line in a), confirming the presence of Mg, as well as the consecutive atomic arrangements of Mn-O-Mg-Mg-O-Mn, in accordance with the MgMn2O4 structure; c) an integrated EDX spectrum obtained from a
large field of view confirms the existence of ≈11 at% Mg in the discharged spinel host.
Other compounds with the spinel structure were collected as
standards for various oxidation states: Mn3O4 (Mn2+, Mn3+),
MgMn2O4 (Mn3+), LiMn2O4 (Mn3+, Mn4+), and Mn2O4 (Mn4+).
The energy threshold, E0, of each spectrum was estimated
from the first derivative in the region of the main edge onset
(1s → 3p transition, see the detailed fitting method in Supporting Information). The values for the standards followed
linear correlation between energy threshold and formal valence
state (Figure 3b).[32,33] The value for the acid-treated spinel oxide
(p′) was consistent with an oxidation state closer to Mn2O4
than LiMn2O4, in agreement with the composition, Li0.2Mn2O4,
derived from the XRD pattern. Meanwhile, the discharged electrode revealed a significant edge shift toward lower energy by
reduction of Mn4+ to Mn3+. Mn in the discharged electrode was
found to be more reduced than LiMn2O4 (average Mn3.5+). Comparison of the Mn L-edge XAS data for (p′) and (e′) confirmed
the reduction of Mn from 4+ to 3+, without appearance of any
low energy features (e.g., at 640.5 eV in the Mn L3 edge) associated with Mn2+, as indicated by the guideline for the Mn3O4
standard (Figure 3c).[34,35] Similar conclusions were reached
from data collected across the O K-edge (Figure 3d). The prepeaks below 535 eV, as marked with triangles, result from transitions from O 1s core states to 2p states hybridized with Mn 3d
orbitals,[36] of which splitting is induced by the ligand field and
exchange energy. While only two pre-peaks are observed in (p′)
due to the high content of octahedral Mn4+ ions, a splitting into
three peaks is evident as shown in (e′) and Mn3O4. This splitting is attributed to the transitions to eg-up, t2g-down, and egdown states for Mn3+,[37] which supports the reduction of Mn4+
to Mn3+ by Mg intercalation.
25Mg NMR further confirmed the incorporation of Mg2+
into the spinel lattice. In NMR spectroscopy, the presence of
paramagnetic centers (e.g., Mn4+ and Mn3+) in the cationic
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environment should result in large shifts via Fermi contacts, as
extensively demonstrated for Li,[38,39] which provides high selectivity between species inside the electrode material and diamagnetic compounds, such as MgO, that may form on the surface.
However, the low natural abundance (10%), highly quadrupolar nuclear spin (5/2) and very low gyromagnetic ratio (i.e.,
30.6 MHz Larmor frequency relative to 1H = 500 MHz) of the
NMR active Mg isotope (25Mg) poses a limitation in solid samples.[40,41] As a result, to the best of our knowledge, no studies
could be found in the literature studying Mg in lattices with paramagnetic centers. In order to minimize spectrometer ringing
at low radiofrequencies, which makes observation of broad lineshapes with fast relaxation times extremely challenging, a relatively moderate magnetic field strength (11.7 T) and moderate
MAS rates (20–24 kHz) were used here. Figure 4a shows the
25
Mg magic angle spinning (MAS) NMR of MgMn2O4 directly
made by solid-state synthesis, used as a standard to establish
expected shift ranges for Mg cations in Mn spinel hosts. The
data were collected using a conventional rotor-synchronized
spin-echo sequence (see experimental details in the Supporting
Information). Two broad 25Mg resonance(s) shifted by Fermicontacts were observed, centered at 2980 ppm and 2850 ppm
(Figure 4a, spin lattice relaxation time: T1 ≈ 300 ms). A small
sharp diamagnetic resonance at 26 ppm was also observed, possibly due to the presence of unreacted MgO.[40] These results
confirm that Mg in a Mn spinel lattice should be easily distinguishable from diamagnetic impurities by NMR. Attempts
were made to fit the spectra with a unique site at isotropic
chemical shift (δiso) of 3091 ppm with a quadrupolar coupling
constant of 5.4 MHz reasonably (Figure S5, Supporting Information). The fact that some intensity remains unexplained
suggests the existence of more than one environment, even in
small amounts. While an extensive discussion of the structural
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Figure 3. a) Mn K-edge X-ray absorption spectra of acid-treated spinel oxide (sample p′) and after reduction to −0.2 V vs SCE (sample e′), compared
to representative standards; b) formal valence state of Mn in the samples interpolated from the position of the Mn K-edge onset in the spectra of
standards of known states; c) Mn L- and d) O K-edge XAS of the same two samples, compared with a Mn3O4 standard.
implications will be reported elsewhere, this conclusion is consistent with Mg in both octahedral and tetrahedral sites, in an
inverted structure that is expected when the oxide is made at
high temperature.[42] Inversion proceeds by the following defect
chemical equilibrium:
•
×
×
×
Mg Mg
+ 2Mn Mn
↔ Mg ′Mn + Mn Mg
+ Mn Mn
which implies the formation of Mn2+ and Mn4+ in the tetrahedral and octahedral sites, respectively, at the expense of Mn3+.
Given the uneven magnesiation observed by XRD, a signal
enhancement pulse sequence, the quadrupolar Carr-PurcellMeiboom-Gill (qCPMG), was used for a scaled-up version
of the most reduced sample (e′).[41] The advantage of this
technique is that it uses a train of π pulses yielding a series
of echo pulses in the time domain, which are subsequently
Fourier-transformed to produce a manifold of sharp spikelets
in the frequency domain. The separation between the spikelets is constant (46.6 ppm in frequency domain) and correlated
with the spacing between the echoes in time domain. Two
groups of resonances were observed; they spanned from 100 to
−120 ppm (centered at −30 ppm, T1 ≈ 400 ms) and from 3100 to
2880 ppm, respectively. Based on the data from the MgMn2O4
standard, the latter was assigned to Mg ions in the paramagnetic spinel lattice. This resonance(s) might represent a broader
Adv. Mater. 2015, 27, 3377–3384
and distinct NMR lineshape that, at this point, cannot be accurately determined due to the limited signal to noise. The origin
of the resonance set at low frequency is unclear. Although not
observed by XRD, they could reflect the existence of degradation in products from the electrochemical reaction, such as
MgO (δiso = 26 ppm),[40] Mg(OH)2 (δiso = 12.5 ppm, Figure 4b),
and residual Mg(NO3)2 (δiso = 0.9 ppm, Figure 4c) from the
electrolyte. However, it must be noted that lithium NMR shifts
at low frequencies have been observed for Mn+3-containg
oxides such as orthorombic LiMnO2 (36 ppm) and the tetragonal spinel Li2Mn2O4 (110 ppm).[43] It is plausible that part of
the signal intensity observed within 100 to −120 ppm range is
also due to Mg2+ in environments richer in Mn3+ than in the
MgMn2O4 standard, especially considering the relatively short
T1 relaxation times observed and the absence of Mn2+ signals
noted from the Mn L-edge XAS data above.
The results in this report clearly prove that Mg2+ can be
reversibly and extensively intercalated into the tetrahedral
sites of a spinel oxide, in this case Mn2O4. This intercalation
was achieved in aqueous electrolyte cells. The location of Mg
was confirmed by STEM/EDX, and the XRD data were found
to be consistent with the formation of a structure very close to
MgMn2O4. These results indicate that water does not cointercalate in the structure, as would be expected from the small space
available in the 3D spinel structure. In turn, XAS and NMR
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into Mn2O4. STEM-EDX supported this
conclusion (Figure S6e,f, Supporting Information). The degree of intercalation appeared to
be considerably lower than in aqueous electrolytes, but it was found to be highly reversible. The lower levels of Mg2+ intercalation and
higher voltage hysteresis suggest considerable
kinetic barriers in non-aqueous electrolytes,
which must be addressed in future work. This
report provides motivation for such efforts.
These results are clearly different from
reports on α-MnO2 after magnesiation,[44]
where self-limited surface conversion to (Mg,
Mn)O was observed instead of bulk intercalation, as in spinel hosts. While the cells used
here do not yet constitute a practical system
for high energy Mg batteries, confirmation of
the ability of spinels to intercalate Mg electrochemically clearly identifies an important
thread to be followed in subsequent studies.
The formation of MgMn2O4 corresponds to a
theoretical capacity of 270 mAh g−1, and the
reaction occurs at 2.9 V versus Mg2+/Mg0.
Coupled with Mg metal, the resulting battery
would have a theoretical energy density of
783 Wh kg−1, surpassing all the commercial
technologies available today. The challenge
for this technology lies on developing suitable nonaqueous electrolyte systems, coupled with the design of particles of Mn2O4
(or MgMn2O4) that maximize the utilization
of the system. This work highlights that fully
functional, high voltage, high capacity oxide
cathodes for multivalent batteries could be
forthcoming, which would bring this technology several steps closer to fulfilling the
promise of the concept.
Figure 4. a) 25Mg MAS qCPMG NMR spectrum of the spinel electrode discharged to −0.2 V vs
SCE (sample e′, red) and spin-echo spectrum of an MgMn2O4 standard made by direct solidstate synthesis (black). The spectra were collected at 11.7 T with spinning speeds of 20 kHz and
24 kHz, respectively. The asterisk (*) and pound (#) indicate spinning sidebands and carrier frequency, respectively. 25Mg MAS NMR spin-echo spectra of b) Mg(OH)2 and c) Mg(NO3)2.6H2O,
spectra were collected at 11.7 T with spinning speeds of 10 kHz and 20 kHz respectively.
confirmed the formation of Mn3+, without presence of Mn2+ that
could be linked to corrosion processes, and the existence of Mg2+
in close proximity to paramagnetic centers. The fact that water
does not directly participate in the reaction is a crucial observation.
It implies that the same reaction should be thermodynamically
possible in non-aqueous electrolytes. To verify this hypothesis, a
fully delithiated Mn2O4 electrode was assembled in a 3-electrode
cell with Mg(TFSI)2 in diglyme (G2) or propylene carbonate (PC)
electrolytes (see Supporting Information for details). Although the
cells showed extremely high voltage hysteresis, XRD signals on
a reduced sample (Figure S6a–d, Supporting Information) were
reminiscent of those obtained in aqueous electrolytes (Figure 1),
with weak superstructure peaks corresponding to a tetragonally
distorted spinel oxide. They confirm that some Mg2+ intercalated
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Experimental Section
Preparation of Acid-Treated LixMn2O4 (x ≈ 0.20):
6 g LiMn2O4 (Nippon Denko Co. Ltd.) was added to
a 600 mL 0.1 M hydrochloric acid solution (258148,
Sigma–Aldrich) at room temperature. The solution
was stirred overnight and the powder was collected
by filtering the solution with filter paper (1001-185, Whatman), followed
by thorough washing with deionized water and drying at 60 °C.
Electrochemical Measurement: The working electrodes were prepared
by mixing the acid-treated spinel oxide, carbon black (Denka), and
polyvinylidene difluoride (PVDF) (Kynar) in N-methylpyrrolidone (NMP)
(270458, Sigma–Aldrich) (80:10:10 wt%), which were then cast on a
25-µm-thick stainless-steel foil (6304-1, TBI Inc.) using a doctor blade
to have a loading level of ≈6 mg cm−2 followed by drying under vacuum
at 110 °C overnight. Dried electrodes were punched with 1/2″ diameter
and tested in a three-electrode beaker cell with a SCE and a platinum
piece as a reference and a counter electrode, respectively. The electrolyte
was composed of 1 M Mg(NO3)2 (237175, Sigma–Aldrich) in water. CV
and galvanostatic measurements were performed on a Biologic VMP3
galvanostat/potentiostat at room temperature. CV was carried out
between −0.8 and 1 V with a sweep rate of 50 mV min−1. Galvanostatic
tests were performed with a charge cut-off voltage of 1 V and various
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported as part of the Joint Center for Energy Storage
Research (JCESR), an Energy Innovation Hub funded by the US
Department of Energy (DOE), Office of Science, Basic Energy Sciences
(BES). Y.S.Y. acknowledges the joint financial support of an Advanced
Adv. Mater. 2015, 27, 3377–3384
Light Source fellowship, covered with funds from the Office of Science,
BES, under contract DE-AC02-05CH11231, and as part of the NorthEast
Center for Chemical Energy Storage (NECCES), an Energy Frontier
Research Center funded by the US DOE, Office of Science, BES under
Award # DE-SC0012583. Use of the Stanford Synchrotron Radiation
Lightsource, SLAC National Accelerator Laboratory, is supported by
the US DOE, Office of Science, BES under Contract No. DE-AC0276SF00515. The UIC JEOL JEM-ARM 200CF is supported by an MRI-R^2
grant from the National Science Foundation (Grant No. DMR-0959470).
Received: January 7, 2015
Revised: March 7, 2015
Published online: April 17, 2015
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discharge cut-off voltages (0.1, 0, −0.1, and −0.2 V). The rate, C/n,
was defined as the current density to achieve theoretical capacity,
C = 270 mAh g−1, in n hours assuming the reaction of MgMn2O4 ↔
Mg2+ + Mn2O4. Post-mortem samples were collected from the various
(dis)charge states and rinsed thoroughly with deionized water to remove
Mg residuals from the electrolyte.
X-Ray Diffraction: Powder X-ray diffraction was performed on a Bruker
D8 Advance using Cu Kα (λavg = 1.5418 Å) radiation. Scan rates were
0.04 ° s−1 from 10° to 80° (2θ). The XRD patterns of harvested electrodes
were aligned by linearly shifting them based on the position of the
(220) peak of stainless steel (JCPDS card number: 33–397, austenitic
Fe0.70Cr0.19Ni0.11), used as internal standard.
Conventional and Scanning Transmission Electron Microscopy and
Energy-Dispersive X-Ray Spectroscopy: TEM images were obtained using a
JEM 3010 (JEOL) operated at 300 kV. STEM images and EDX data were
acquired on an aberration-corrected JEM-ARM200CF (JEOL) operated
at 200 kV, with a 28 mrad semiconvergence angle. Collection angles for
ABF mode was set to 14-28 mrad. The ARM200CF is equipped with an
Oxford X-Max 100TLE windowless silicon drift EDX detector.[45] EDX data
were collected from a fully reduced electrode at 200 kV.
X-Ray Absorption: Mn K-edge XAS spectra were collected for the
samples of interest and relevant standard powders with different valence
state of Mn (Mn2O4, LiMn2O4, MgMn2O4, and Mn3O4). LiMn2O4
and Mn3O4 (377473, Sigma–Aldrich) were used as received from a
commercial supplier. Spectra were measured in transmission mode at
beamline 4-1 at the Stanford Synchrotron Radiation Lightsource (SSRL,
Menlo Park, CA) using a 20-pole wiggler source with a liquid nitrogen
cooled Si (220) double-crystal monochromator (energy resolution of
ΔE/E = 10−4).
XAS measurements at the Mn L- and O K-edges were performed
at the 31-pole wiggler beamline 10-1 at SSRL with a ring current of
500 mA, operating the spherical grating monochromator (SGM) with
the 1000 L mm−1 grating set to an intermediate resolution (≈0.3 eV),
providing ≈1011 ph s−1 in a 1 mm2 beam spot.
Solid-State NMR: 25Mg MAS NMR experiments were performed at
11.7 Tesla (500 MHz) on a Bruker Avance III spectrometer operating
at a Larmor frequency of 30.64 MHz, using a 3.2 mm MAS probe. The
spectrum of a fully reduced sample (collected at −0.2 V) was acquired
at a spinning speed of 20 kHz (νr) with a rotor-synchronized qCPMG
pulse sequence (90°-τ-180°-τ-(180-τ)17), where τ = 1/νr.[41] A calibrated
π/2 pulse width of 3 µs was used with pulse recycle delays of 1 s.
The spectrum was collected for approximately 7 d at 283 ± 0.1 K. The
spectrum of an MgMn2O4 standard was acquired at a spinning speed
of 20 kHz with a rotor-synchronized spin-echo experiment (90°-τ180°-τ) where τ = 1/νr. A calibrated π/2 pulse width of 3 µs was used
with pulse recycle delays of 0.2 s. The spectrum was collected for 1 d
at 283 ± 0.1 K. The spectra for Mg(NO3)2.6H2O and Mg(OH)2 were
acquired at spinning speeds of 20 and 10 kHz, respectively, with a rotor
synchronized spin-echo experiment (90°-τ-180°-τ) where τ = 1/νr. Again,
a calibrated π/2 pulse width of 3 µs was used with pulse recycle delays
of 1 s. All chemical shifts were referenced by 5 M MgCl2 at 0 ppm.
Fitting of MgMn2O4, Mg(NO3)2.6H2O, and Mg(OH)2 NMR spectra can
be found in Figure S5 and S7 (Supporting Information).
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