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

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.


Introduction
High energy and high density rechargeable batteries are indispensable 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 densities, 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][7][8][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 Mg 2+ 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 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 In eqn (1), extraction/insertion of Mg in a tetrahedral site proceeds with holding the host spinel structure, while a transformation reaction between MgMn 2 O 4 spinel and Mg 2 Mn 2 O 4 rock-salt proceeds via push-out process in eqn (2). 12 Recently, MgMn 2 O 4 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][13][14][15][16][17][18][19][20][21][22][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 Mg 2+ 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 MgMn 2 O 4 has a tetragonal spinel structure due to the Jahn-Teller effect of Mn 3+ ions, 25 while l-MnO 2 and Mg 2 Mn 2 O 4 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 MgMn 2 O 4 is necessary for improving its cathode performance. Feng et al. reported thin lms of cubic-MgMn 2 O 4 spinel obtained by PLD methods exhibited higher cathode performances than those for tetragonal-MgMn 2 O 4 thin lms. 17 Although a cubic MgMn 2 O 4 phase is known as a metastable phase, 26 this phase is only obtained at hightemperature (>950 C) 27,28 or high-pressure (>15.6 GPa) 29 in bulk.
Here we demonstrate high cathode performances of ultrasmall cubic Mg-Mn spinels synthesized via an alcohol reduction 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][31][32] We also reported a high-rate capability of LiMn 2 O 4 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 MnO 4 À in an anhydrous MgCl 2 solution in ethanol within 30 min at room temperature, and its cathode performances were investigated.  36 Since the reduction reaction of MnO 4 À ion proceeds rapidly to form nanoparticles without heating-up, once a meta-stable phase formed, the phase transition to more stable phase hardly proceed. The cubic phase is obtained probably due to the rapid nucleation of MMO particles at room temperature.   (6) a Partial prole relaxation was applied to 400, 440, and 444 reection peaks.

Results and discussion
In our previous work, only todorokite-type Mg-Mn binary oxide (OMS-1) was obtained when a hydrous MgCl 2 $6H 2 O dissolved in 2-propanol was used as a Mg-source. 31 In the solution, Mg 2+ ions were coordinated by water due to their strong Lewis acidic behaviour, and the aqueous complexes were introduced into manganese oxides without dehydration to form OMS-1. In the present work, we found that primary alcohols (e.g., ethanol) have enough solubility of an anhydrous MgCl 2 and reducing ability of permanganates instead of secondary alcohols. In the anhydrous condition, Mg 2+ ions are solvated by ethanol without strong coordination, and are introduced into manganese oxides with desolvation to form a spinel-type Mg-Mn binary oxide. Such difference of products by the amount of water in reaction solutions is also observed in the case of Li-Mn and Co-Mn binary oxides. 31 Next, surface morphology of MMO particles were investigated. Fig. 2a and b show SEM and TEM images of MMO particles, respectively. These micrographs displayed submicron aggregates of approximately 5 nm nanoparticles with a clear lattice fringe spacing of 0.47 nm, which could be attributed to the 111 plane of the cubic spinel structure. The obtained MMO nanoparticles have much smaller crystallites than previously reported MgMn 2 O 4 nanoparticles (>10 nm) synthesized by a sol-gel method. 16,[18][19][20][21][22][23] These ultrasmall particles are likely to have been obtained using this wet-process by suppressing the dissolution-recrystallization at ambient temperature and in an anhydrous solvent, while the sol-gel method contains a calcination step at which crystal growth should proceed. A high Brunauer-Emmett-Teller (BET) surface area of 151 m 2 g À1 was also obtained, validating the formation of ultrasmall nanoparticles with below 10 nm in size. However, these large aggregates formed in the drying process should inhibit both electron conduction and Mg 2+ ion diffusion to degrade electrochemical performances. To suppress such aggregation, graphene was used as an aggregation inhibitor and an electron conducting additive. 30,37 The composite of MMO with graphene (MMO-G) was easily prepared by just adding graphene to the synthetic solution. The Mn K-edge XANES spectrum and the Mg/Mn rate of MMO-G were almost the same as those of MMO, indicating that graphene addition do not affect the structure of MMO (Fig. S1 †). The SEM image and corresponding EDX mappings presented in Fig. 2c-f revealed that MMO particles were wrapped by wrinkled graphene to form larger MMO-G secondary particles. Thus, the aggregation of MMO particles could be suppressed by covering the particles with graphene.
The cathode performances of MMO electrodes with a 0.5 M Mg(ClO 4 ) 2 /CH 3 CN electrolyte were investigated using a threeelectrode cell at 25 C with an Ag/Ag + reference electrode (+2.6 V vs. Mg/Mg 2+ ) and an activated carbon capacitor as a counter electrode (0.0 V vs. Ag/Ag + ). 38 Fig. 3a shows the voltage curves of the MMO and MMO-G electrodes at a current density of 10 mA g À1 . In the MMO electrode, no plateau was observed during discharge/charge and the discharge capacity was 60 mA h g À1 , which was much smaller than that of an ideal one electron reaction per Mn (280 mA h g À1 ). This small capacity is possibly attributed to the slow diffusion of Mg 2+ ions into large aggregates of MMO particles. On the other hand, the MMO-G electrode exhibited gentle slopes and a reversible capacity of more than 200 mA h g À1 . Since the contribution of the electric 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 electrode exhibited a broad reductive peak at À0.2 V and a sharp oxidative peak at 0.3 V, supporting the redox of MMO (Fig. S2 †) (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 (À1.0 V vs. Ag/Ag + ) was approximately in the middle between those of MnO and Mn 2 O 3 , 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 g À1 . Even at a moderate current density, reversible discharge/charge proceeded at room temperature with little 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 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 g À1 (corresponding to a 1.4 C-rate) compared with previous reports (Table S1 †), indicating that the Mg 2+ intercalation/deintercalation proceeded rapidly due to small diffusion paths in the ultrasmall MMO particles covered by graphene prepared by the alcohol reduction method adopted here.

Conclusions
Cubic spinel-type Mg-Mn oxide nanoparticles with sizes of approximately 5 nm and their composites with graphene as cover were successfully synthesized at room temperature and exhibited a moderate rate cathode performance at room temperature. Both downsizing the primary particles of active materials and suppressing the aggregation of particles are important for further development of cathodes in Mg-ion batteries. Especially in MgMn 2 O 4 cathode, utilization of metastable cubic phase is important. The alcohol reduction process presented in this letter is a facile method to synthesize ultrasmall cathode materials.

Synthesis of Mg-Mn binary oxides
A precursor n-Bu 4 NMnO 4 was synthesized according to a reported procedure. 30

Material characterization
Synchrotron powder X-ray diffraction (XRD) patterns with a wavelength of l ¼ 0.775Å were collected at the BL5S2 beamline of the Aichi Synchrotron Radiation Center. Samples were charged into Lindeman glass capillaries of 0.5 mm diameter and measured with a rotating stage and a PILATUS 100K detector. The Rietveld renement was performed using the RIETAN-FP program. 39 Mn K-edge X-ray absorption spectroscopy (XAS) was carried out using the transmission method at the BL11S2 beamline of the Aichi Synchrotron Radiation Center. Samples were sealed in an Al-laminated packaging lm and attached to a sample holder with Mn foil. Energy calibrations 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. Xray absorption near edge structure (XANES) and extended X-ray absorption ne structure (EXAFS) were analyzed by the Athena and Artemis programs. 40 For the EXAFS analysis, the k-range of the FT was 3-14Å À1 with a Hanning window of 1Å À1 , and the radial distance range of the inverse FT was 1-3Å. 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 obtained using JSM-7800F (JEOL) and EM-002B (Topcon), respectively. Brunauer-Emmett-Teller (BET) surface areas were measured by N 2 adsorption at 77 K using BELSORP-mini (MicrotracBEL).

Electrochemical measurements
MMO and MMO-G were mixed with acetylene black (AB; Denka Black, FX-35, Denka Co., Ltd.) and polytetrauoroethylene (PTFE; Teon, 6-J, DuPont-Mitsui Fluorochemicals Co., Ltd.) at a weight ratio of 60/30/10 and 80/10/10, respectively. In the MMO-G cathode, MMO/graphene/AB/PTFE ¼ 68/ 12/10/10 by weight. These mixtures were cut into 8 mm diameter disks of typically 2.5 mg and pressed on an Al mesh current collector to serve as cathodes. For the anodes, a nanoporous activated carbon (Maxsorb®, MSC-30, Kansai Coke and Chemicals Co., Ltd.) was mixed with AB and PTFE at a weight ratio of 8/1/1 and typically 15 mg of the mixture was pressed on a SUS 304 stainless steel mesh current collector. The electrodes were dried at 160 C under vacuum and introduced into an Ar-lled glove box. For the electrolyte solution, 0.5 M Mg(ClO 4 ) 2 (Sigma-Aldrich) dissolved in acetonitrile (Kanto Chemical Co., Inc.) was prepared and stored over molecular sieves. The cathode, the anode, and the electrolyte were assembled in a three-electrode cell (EC Frontier Co., Ltd.) with an Ag/Ag + reference electrode or a 2032 coin-type cell (Hohsen Corp.) with a glass-ber separator (GA-55, Toyo Roshi Kaisha, Ltd.). The amount of the electrolyte was 2 mL with the three-electrode cell and 0.1 mL with the coin-type cell, respectively. For the reference electrode, a double junction reference electrode was used with internal 0.01 M AgNO 3 + 0.1 M n-Bu 4 NClO 4 solution in acetonitrile separated by porous glasses. The aforementioned cell preparations were conducted in an Ar-lled glove box. Charge/discharge tests were carried out at 25 C in constantcurrent (CC) mode using a multi-channel potentiostat system (VMP3, Bio-Logic Science Instruments) or a battery test system (HJ-1001SD8, Hokuto Denko Corp.). The specic capacity and the current density were calculated on the basis of the weight of MMO in the electrode. Caution: anhydrous perchlorate salts are potentially explosive and should be handled with appropriate care.

Conflicts of interest
There are no conicts to declare.