Rapid room-Temperature synthesis of ultrasmall cubic Mg-Mn spinel cathode materials for rechargeable Mg-ion batteries

Article abstract of DOI:10.1039/c9ra08626a

Reducing the particle size of cathode materials is effective to improve the rate capability of Mg-ion batteries. In this study, ultrasmall cubic Mg-Mn spinel oxide nanoparticles approximately 5 nm in size were successfully synthesized via an alcohol reduction process within 30 min at room temperature. Though the particles aggregated to form large secondary particles, the aggregation could be suppressed by covering the particles with graphene. The composite exhibited a specific capacity of 230 mA h g^-1, and could be cycled more than 100 times without any large capacity loss even at a moderate current density with the Mg(ClO₄)₂/CH₃CN electrolyte.

Full text of DOI:10.1039/c9ra08626a

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Rapid room-temperature synthesis of ultrasmall
cubic Mg–Mn spinel cathode materials for
rechargeable Mg-ion batteries†
Cite this: RSC Adv., 2019, 9, 36434
a
b
a
*
Hiroaki Kobayashi,
Kazuya Yamaguchi
and Itaru Honma
Reducing the particle size of cathode materials is effective to improve the rate capability of Mg-ion batteries.
In this study, ultrasmall cubic Mg–Mn spinel oxide nanoparticles approximately 5 nm in size were
successfully synthesized via an alcohol reduction process within 30 min at room temperature. Though
the particles aggregated to form large secondary particles, the aggregation could be suppressed by
covering the particles with graphene. The composite exhibited a specific capacity of 230 mA h gꢀ1, and
could be cycled more than 100 times without any large capacity loss even at a moderate current density
with the Mg(ClO4)2/CH3CN electrolyte.
Received 12th September 2019
Accepted 1st November 2019
DOI: 10.1039/c9ra08626a
rsc.li/rsc-advances
theoretical capacity of 540 mA h gꢀ1 using both Mn3+/Mn4+ and
Mn3+/Mn2+ redox according to the following reactions:
Introduction
High energy and high density rechargeable batteries are indis-
pensable for widespread portable electronic devices and the
spreading of electric vehicles. Currently, Li-ion batteries have
been widely utilized for these high-power applications,1 but to
meet increasing requirements to enhance their energy densi-
ties, it is necessary to develop novel energy storage systems,
such as Li–air,2,3 Li–S,2,4 and multivalent-ion batteries.5
Rechargeable Mg-ion batteries have gained much attention as
promising alternatives to Li-ion batteries due to the high
natural abundance, high volumetric energy density, and no
dendrite formation of the Mg metal anode.6–9 While Mg-ion
batteries are nowadays progressing rapidly, their practical use
is still hampered by problems related to both cathode materials
and electrolytes. One of the crucial problems is the very low rate
capability at the cathodes due to the slow diffusion of Mg2+ ions
in solids.10 Therefore, most reported Mg-ion batteries work at
low current densities or at high temperature. This problem
could be solved using water-based or water-added electrolytes,
but they are incompatible with Mg metal anodes.6
MgMn2O4 % Mn2O4 (l-MnO2) + Mg2+ + 2eꢀ
(1)
(2)
MgMn2O4 + Mg2+ + 2eꢀ % Mg2Mn2O4 (rock-salt)
In eqn (1), extraction/insertion of Mg in a tetrahedral site
proceeds with holding the host spinel structure, while a trans-
formation reaction between MgMn2O4 spinel and Mg2Mn2O4
rock-salt proceeds via push-out process in eqn (2).12 Recently,
MgMn2O4 with particle sizes of 11–200 nm were synthesized by
various processes such as sol–gel and co-precipitation methods,
and their cathode performances were investigated.12–23 Some
reports demonstrated high specic capacities, but so far, there
are no reports where high capacity could be obtained at high
rates with an anhydrous electrolyte under ambient temperature.
Since decreasing the particle size is one of the most effective
ways to enhance the specic capacity and the rate capability for
suppressing the slow Mg2+ diffusion in solids,24 alternative
synthetic methods that enable nanoparticles below 10 nm are
required for achieving a breakthrough in such cathodes.
In addition, the MgMn2O4 has a tetragonal spinel structure
due to the Jahn–Teller effect of Mn3+ ions,25 while l-MnO2 and
Mg2Mn2O4 rock-salt are cubic phase. The above redox reactions
should exhibit large polarizations because of the less reversible
tetragonal–cubic phase transitions, hence a suppression of the
lattice distortion of MgMn2O4 is necessary for improving its
cathode performance. Feng et al. reported thin lms of cubic-
MgMn2O4 spinel obtained by PLD methods exhibited higher
cathode performances than those for tetragonal-MgMn2O4 thin
lms.17 Although a cubic MgMn2O4 phase is known as a meta-
stable phase,26 this phase is only obtained at high-
temperature (>950 ꢁC)27,28 or high-pressure (>15.6 GPa)29 in bulk.
Among various cathode candidates for Mg-ion batteries,
spinel oxides have a high redox potential and a relatively low
diffusivity.11 Especially in the common electrochemical window
of electrolytes (<3.5 V vs. Mg/Mg2+), MgMn2O4 can exhibit a high
aInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai, Miyagi, 980-8577, Japan. E-mail: h.kobayashi@
tohoku.ac.jp
bDepartment of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-
1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra08626a
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Here we demonstrate high cathode performances of ultra-
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Table 1 Results of EXAFS analysis of MMOa
small cubic Mg–Mn spinels synthesized via an alcohol reduc-
tion process. The process is commonly applied for synthesizing
metal nanoparticles as a wet-process, and recently our group
applied it for various Mn oxides and showed high catalytic
activity for aerobic oxidation reactions.30–32 We also reported
a high-rate capability of LiMn2O4 nanospinels in a Li-ion
battery.30 In this study, an Mg–Mn spinel oxide (MMO) with
approximately 5 nm was prepared by reduction of MnO4ꢀ in an
anhydrous MgCl2 solution in ethanol within 30 min at room
temperature, and its cathode performances were investigated.
Coordination
number
Debye–Waller
factor/A
2
˚
˚
Distance/A
1.981(7)
2.88(1)
First coordination
of Mn–O
Second coordination
of Mn–Mn
1.0(2)
0.003(1)
0.005(2)
0.7(2)
a
R ¼ 1.97%.
ꢀ
Mg0.81(3)Mn2O4 with the Fd3m space group by the Rietveld
renement (Table 2), though the error of rened parameters
should become large with such broad XRD patterns. According
to the result, and cation-mixing between Mg and Mn is not
observed34 and only vacancy in Mg-site formed.35 The estimated
formula is consistent with the ICP elemental analyses, and
Results and discussion
Fig. 1a shows synchrotron powder XRD patterns of MMO,
MgMn2O4 spinel prepared by a sol–gel method,19 and LiMn2O4
spinel. Peaks of MMO were very broad, indicating the formation
of nanocrystals as described later. The XRD pattern of MMO was
similar to that of cubic LiMn2O4 spinel rather than that of
tetragonal MgMn2O4 spinel. In the pseudo radial distribution
functions (p-RDF) obtained by a Fourier transform (FT) of k2-
weighted Mn K-edge EXAFS spectra (Fig. 1b), MMO and
˚
estimated bond length of Mn–O and Mn–Mn are 1.988(5) A and
˚
2.878(1) A, respectively, supporting the result of EXAFS analysis.
The estimated structure is closely similar to the meta-stable
cubic-MgMn2O4 phase17 or MgyMn2O4 solid solution phase
between ꢀl-MnO2 and MgMn2O4.36 Since the reduction reaction
of MnO4 ion proceeds rapidly to form nanoparticles without
heating-up, once a meta-stable phase formed, the phase tran-
sition to more stable phase hardly proceed. The cubic phase is
obtained probably due to the rapid nucleation of MMO particles
at room temperature.
˚
LiMn2O4 had a single peak at 2.48 A attributed to isotropic Mn–
˚
˚
Mn pairs, whereas MgMn2O4 had two peaks at 2.36 A and 2.76 A
attributed to anisotropic Mn–Mn pairs.33 The EXAFS spectrum
of MMO was well tted using the rst coordination shell con-
sisting of single Mn–O paths (1.891(7) A) and the second coor-
dination shell consisting of single Mn–Mn paths (2.88(1) A),
˚
˚
suggesting that no Jahn–Teller distortion occurred in MMO to
form a cubic phase (Table 1). From the result of ICP elemental
analyses of Mg/Mn ¼ 0.40(3) (mol molꢀ1), the formula of MMO
can be described as Mg0.80(6)Mn2O4 or (Mg0.86(4)Mn0.14(4))Mn2O4
Table 2 Results of the Rietveld refinement
with the average valence state of Mn3.20(6)+ or Mn2.93(7)+
,
Space
Lattice
Occupancy (g) and
˚
Phase
group parameters (A) Atom Site atomic coordination
respectively. In the former structure, vacancies in cation sites
form, while the latter structure is a solid-solution of MgMn2O4
and Mn3O4. Mn K-edge XANES spectra shown in Fig. 1c sup-
ported the former structure. The Mn K-edge energy was higher
than that of MgMn2O4 and was similar to that of LiMn2O4,
supporting a higher valence state of Mn in MMO than trivalent.
The XRD pattern in Fig. 1a was successfully tted to
MMOa Fd3m
a ¼ 8.141(4)
Mg
Mn
O
8b
g ¼ 0.81(3)
ꢀ
16c g ¼ 1.00(1)
32e x ¼ 0.2440(6)
a
Partial prole relaxation was applied to 400, 440, and 444 reection
peaks. Rwp ¼ 2.81%, Rp ¼ 2.31%, Re ¼ 1.65%, S ¼ 1.70.
Fig. 1 (a) Synchrotron XRD patterns with the fitting curve by the Rietveld refinement and the schematic illustration for cubic Mg0.81Mn2O4, (b) p-
RDF obtained from the FT of k2-weighted Mn K-edge EXAFS spectra with the fitting curve by the first and second coordination shell analysis, and
(c) Mn K-edge XANES spectra.
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In our previous work, only todorokite-type Mg–Mn binary nanoparticles with below 10 nm in size. However, these large
oxide (OMS-1) was obtained when a hydrous MgCl2$6H2O dis- aggregates formed in the drying process should inhibit both
solved in 2-propanol was used as a Mg-source.31 In the solution, electron conduction and Mg2+ ion diffusion to degrade elec-
Mg2+ ions were coordinated by water due to their strong Lewis trochemical performances. To suppress such aggregation, gra-
acidic behaviour, and the aqueous complexes were introduced phene was used as an aggregation inhibitor and an electron
into manganese oxides without dehydration to form OMS-1. In conducting additive.30,37 The composite of MMO with graphene
the present work, we found that primary alcohols (e.g., ethanol) (MMO–G) was easily prepared by just adding graphene to the
have enough solubility of an anhydrous MgCl2 and reducing synthetic solution. The Mn K-edge XANES spectrum and the
ability of permanganates instead of secondary alcohols. In the Mg/Mn rate of MMO–G were almost the same as those of MMO,
anhydrous condition, Mg2+ ions are solvated by ethanol without indicating that graphene addition do not affect the structure of
strong coordination, and are introduced into manganese oxides MMO (Fig. S1†). The SEM image and corresponding EDX
with desolvation to form a spinel-type Mg–Mn binary oxide. mappings presented in Fig. 2c–f revealed that MMO particles
Such difference of products by the amount of water in reaction were wrapped by wrinkled graphene to form larger MMO–G
solutions is also observed in the case of Li–Mn and Co–Mn secondary particles. Thus, the aggregation of MMO particles
binary oxides.31
could be suppressed by covering the particles with graphene.
The cathode performances of MMO electrodes with a 0.5 M
Next, surface morphology of MMO particles were investi-
gated. Fig. 2a and b show SEM and TEM images of MMO Mg(ClO4)2/CH3CN electrolyte were investigated using a three-
particles, respectively. These micrographs displayed submicron electrode cell at 25 ꢁC with an Ag/Ag+ reference electrode
aggregates of approximately 5 nm nanoparticles with a clear (+2.6 V vs. Mg/Mg2+) and an activated carbon capacitor as
lattice fringe spacing of 0.47 nm, which could be attributed to a counter electrode (0.0 V vs. Ag/Ag+).38 Fig. 3a shows the voltage
the 111 plane of the cubic spinel structure. The obtained MMO curves of the MMO and MMO–G electrodes at a current density
nanoparticles have much smaller crystallites than previously of 10 mA gꢀ1. In the MMO electrode, no plateau was observed
reported MgMn2O4 nanoparticles (>10 nm) synthesized by during discharge/charge and the discharge capacity was
a sol–gel method.16,18–23 These ultrasmall particles are likely to 60 mA h gꢀ1, which was much smaller than that of an ideal one
have been obtained using this wet-process by suppressing the electron reaction per Mn (280 mA h gꢀ1). This small capacity is
dissolution–recrystallization at ambient temperature and in an possibly attributed to the slow diffusion of Mg2+ ions into large
anhydrous solvent, while the sol–gel method contains a calci- aggregates of MMO particles. On the other hand, the MMO–G
nation step at which crystal growth should proceed. A high electrode exhibited gentle slopes and a reversible capacity of
Brunauer–Emmett–Teller (BET) surface area of 151 m2 gꢀ1 was more than 200 mA h gꢀ1. Since the contribution of the electric
also obtained, validating the formation of ultrasmall double-layer capacitor (EDLC) of the graphene (dotted lines in
Fig. 3a) is negligible, the enhancement of the specic capacity
derives from the increase in the amount of the redox-active
MMO particles. Moreover, dQ/dV curves of the MMO–G elec-
trode exhibited a broad reductive peak at ꢀ0.2 V and a sharp
oxidative peak at 0.3 V, supporting the redox of MMO (Fig. S2†).
The obtained discharge capacity of 230 mA h gꢀ1 corresponds to
0.83 electrons transferred per Mn, indicating that the average
valence state of Mn changed from +3.19 to +2.36 and the
discharge reaction can be expressed by the following equation:
Mg0.81Mn2O4 + 0.83Mg2+ + 1.66eꢀ / Mg1.64Mn2O4
(3)
Fig. 3b shows Mn K-edge XANES spectra of the MMO–G
electrodes. The Mn K-edge was shied to lower energies during
discharge, and almost recovered aer re-charge, indicating the
reversible redox reaction of Mn. The Mn K-edge position at the
full discharge (ꢀ1.0 V vs. Ag/Ag+) was approximately in the
middle between those of MnO and Mn2O3, supporting eqn (3).
According to the equation, it is indicated that both eqn (1) and
(2) proceeded with a partial formation of rock-salt phase during
the discharge.
A rate capability test of the MMO–G electrode was also
carried out using the three-electrode cell. Fig. 3c shows voltage
curves of an MMO–G electrode at a current density of
190 mA gꢀ1. Even at a moderate current density, reversible
discharge/charge proceeded at room temperature with little
Fig. 2 (a) A SEM and (b) a TEM image of MMO particles. (c) A SEM
image and (d–f) EDX mappings of MMO–G composites.
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Fig. 3 (a) Voltage curves of MMO and MMO–G cathodes, (b) Mn K-edge XANES spectra of MMO–G cathodes. (c) Voltage curves of MMO–G
cathode at 190 mA gꢀ1 using a 3-electrode cell or (d) a coin-type cell. (e) Cyclability and (f) rate-capability tests of MMO–G cathode using a coin-
type cell.
change in the voltage curves. However, the reversible capacity
gradually decreased with the number of cycles, probably due to
the dissolution of Mn to the electrolyte or the exfoliation of the
Experimental section
Synthesis of Mg–Mn binary oxides
electrode from the current collector during discharge/charge.
To reduce the amount of electrolyte and add a conning
pressure on the electrode for suppressing the dissolution and
exfoliation, a cycle test with a 2032 coin-type cell was investigated.
Though the voltage curves with the coin cell slightly reected the
EDLC behaviour of the activated carbon counter electrode, the
cell was well cycled more than 100 times without any large
capacity loss (Fig. 3d and e). The Mn K-edge position of the
electrode aer 10th charge was almost the same as that aer rst
charge, suggesting the redox of Mn proceeds at least 10th cycle
without decomposition of MMO (Fig. S3†). Furthermore, the cell
exhibited a relatively high rate capability (Fig. 3f) even at
380 mA gꢀ1 (corresponding to a 1.4 C-rate) compared with
previous reports (Table S1†), indicating that the Mg2+
intercalation/deintercalation proceeded rapidly due to small
diffusion paths in the ultrasmall MMO particles covered by gra-
phene prepared by the alcohol reduction method adopted here.
A precursor n-Bu4NMnO4 was synthesized according to a re-
ported procedure.30 In brief, an aqueous n-Bu4NBr solution was
slowly added to an aqueous KMnO4 solution under vigorous
stirring, and the mixed solution was subsequently stirred for 1 h
to obtain n-Bu4NMnO4. A solution of n-Bu4NMnO4 in acetoni-
trile (2.5 M, 1 mL) was slowly added to an anhydrous MgCl2
solution in ethanol (0.1 M, 100 mL) under vigorous stirring to
form a brown colloidal solution. The solution was stirred for
30 min followed by addition of deionized water to form
precipitates of the Mg–Mn binary oxide (MMO). The precipi-
tates were collected by membrane ltration, washed with water
and ethanol, and dried at 120 ꢁC. A composite of MMO with
graphene (MMO–G) was similarly obtained by dispersing 50 mg
of graphene nanopowder (G-10, EM Japan Co., Ltd.) in the
MgCl2 solution before adding the n-Bu4NMnO4 solution.
ꢀ
Caution: n-Bu4NMnO4 can react violently with itself (the MnO4
anion can oxidize the n-Bu4N+ cation) and possibly catch re,
hence it should be handled with appropriate care and stored
under appropriate conditions (e.g. refrigerated conditions).
Conclusions
Cubic spinel-type Mg–Mn oxide nanoparticles with sizes of
approximately 5 nm and their composites with graphene as
Material characterization
cover were successfully synthesized at room temperature and Synchrotron powder X-ray diffraction (XRD) patterns with
˚
exhibited a moderate rate cathode performance at room a wavelength of l ¼ 0.775 A were collected at the BL5S2 beam-
temperature. Both downsizing the primary particles of active line of the Aichi Synchrotron Radiation Center. Samples were
materials and suppressing the aggregation of particles are charged into Lindeman glass capillaries of 0.5 mm diameter
important for further development of cathodes in Mg-ion and measured with a rotating stage and a PILATUS 100K
batteries. Especially in MgMn2O4 cathode, utilization of meta- detector. The Rietveld renement was performed using the
stable cubic phase is important. The alcohol reduction RIETAN-FP program.39 Mn K-edge X-ray absorption spectros-
process presented in this letter is a facile method to synthesize copy (XAS) was carried out using the transmission method at
ultrasmall cathode materials.
the BL11S2 beamline of the Aichi Synchrotron Radiation
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Center. Samples were sealed in an Al-laminated packaging lm
and attached to a sample holder with Mn foil. Energy calibra-
tions were carried out using the rst peak of Mn foil (6539 eV) in
a derivative spectrum. Electrode samples were washed with
acetonitrile and dried in an Ar-lled glove box before sealing. X-
ray absorption near edge structure (XANES) and extended X-ray
absorption ne structure (EXAFS) were analyzed by the Athena
and Artemis progrꢀam1 s.40 For the EXAFS analysis, the k-range of
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We thank Prof. Takashi Kyotani and Dr Yasuto Hoshikawa for
their supports about measurements of BET surface area. Parts
of this work were supported by JSPS Grant-in-Aid for Research
Activity Start-up (Grant No. 17H06515) and ALCA-SPRING
(Advanced Low Carbon Technology Research and Develop-
ment Program-Specially Promoted Research for Innovative Next
Generation Batteries) from Japan Science and Technology
Agency (JST).
the FT was 3–14 A with a Hanning window of 1 Aꢀ1, and the
˚
˚
˚
radial distance range of the inverse FT was 1–3 A. Elemental
analyses were performed using inductively coupled plasma
atomic emission spectroscopy (ICP-AES) on an Optima 3300XL
(PerkinElmer) and a CHN analyzer (Micro Corder JM10, J-
Science Lab Co., Ltd.). Scanning electron microscopy (SEM)
and Transmission electron microscopy (TEM) images were ob-
tained using JSM-7800F (JEOL) and EM-002B (Topcon), respec-
tively. Brunauer–Emmett–Teller (BET) surface areas were
measured by N2 adsorption at 77 K using BELSORP-mini
(MicrotracBEL).
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RSC Adv., 2019, 9, 36434–36439 | 36439