Preparation of a Mulberry-like MnO Specimen and Its Lithium Property

Abstract

A mulberry-like MnO specimen was prepared using a MnCO₃ sample under nitrogen (N₂) protection at 700 °C (denoted as MnO-700). When the specimen was used in lithium-ion batteries (LIBs) as anode material, the reversible capacity of 702 mAh g⁻¹ was displayed after 120 cycles at a current density 200 mA g⁻¹, and 365 mAh g⁻¹ of discharge capacity was obtained at 1000 mA g⁻¹ at the 200th cycle. Meanwhile, the sample also exhibited an excellent rate capacity (224 mAh g⁻¹ at 2000 mA g⁻¹). The MnO-700 sample displayed a favorable electrochemical performance that may be ascribed to the unique mulberry-like structure of the MnO microparticles, which can provide enough space to satisfy the volume change of the MnO microparticles during lithium cycling, and also lead to more transfer paths for Li⁺ insertion/extraction during charge/discharge processes.

1. Introduction

In recent years, metal oxides (e.g., FeO_x, CoO, NiO, SnO₂, MnO, Co₂SnO₄, ZnFe₂O₄, and CoFe₂O₄) have attracted considerable research interest applied in lithium-ion batteries (LIBs) as anode materials based on their favorable theoretical capacities [1,2,3,4,5,6,7,8,9]. Among the metal oxides, MnO is deemed to be a preferred anode material used in LIBs because of its excellent theoretical capacity, inexpensiveness, the rich manganese (Mn) on earth, and low conversion potential [10,11,12,13]. Nevertheless, there are some disadvantages that hamper pure MnO to be applied in LIBs, including its low electrical conductivity, Mn aggregation, and the volume change effect being big during repeated lithium-cycling processes [14,15].

The use of MnO/carbonaceous composites (e.g., graphene [16,17,18,19,20,21], carbon nanotubes [15], amorphous carbon [22,23,24], and carbon fiber [25]) has shown to be an available approach to settle the disadvantages of pure MnO. The comprehensive electrochemical performances of pure MnO electrodes also can be enhanced via the carbonaceous materials to improve the electrical conductivity and control the volume change during lithium cycling. However, some factors involved in the carbon-doping reactions and the low tap density in the presence of carbon may be unfavorable for practical applications of the MnO/carbonaceous composites. Fortunately, another approach was confirmed to improve the energy-storage capacities of pure MnO, and it consists of adjusting the shapes, sizes, or structures of the MnO particles [26,27,28,29,30,31], etc. Researchers have determined that the MnO microcube, micrometer particles, porous MnO nanoflakes, and two-stage structure MnO microparticle samples resulted in excellent lithium-storage capacities [27,28,29,30,31]. Therefore, it may be possible to design and produce an original structure of MnO that might also provide good lithium-storage capacity.

Herein, we report on a mulberry-like MnO sample derived by annealing of MnCO₃ microparticles, which was fabricated via hydrothermal treatment choosing glycine and KMnO₄ as reagents. When the obtained MnO product was applied in LIBs as an anode material, the electrode displayed an excellent capacity of 703 mAh g⁻¹ after 120 cycles at 200 mA g⁻¹; it also had an initial coulombic efficiency (CE) up to 74.9%. In addition, the sample also exhibited a remarkable rate capacity of about 224 mAh g⁻¹ at a current density of 2000 mA g⁻¹.

2. Experimental

2.1. Fabrication Process of the Mulberry-like MnO

The detailed synthesis process of the mulberry-like MnO microparticles has been described in our previous study [32]. Briefly, 10 m mol KMnO₄ and 43 m mol glycine were added into 60 g deionized water under magnetic stirring for 60 min in a Teflon jar, and then the sealed Teflon jar was enclosed in a stainless-steel autoclave at 180 °C in a vacuum oven with constant temperature for 14 h. The dried MnCO₃ microparticles precursor was collected after several washes with deionized water and kept in an oven to dry at 80 °C for about 12 h. Subsequently, the mulberry-like MnO material was produced by calcination of the MnCO₃ microparticles sample in a protective gas (N₂) at 700 °C for 2 h.

2.2. Material Characterization

A Rigaku D/max 2200VPC X-ray diffractometer (RIGAKU 670, Tokyo, Japan) was used to determine the X-ray diffraction (XRD) data of the sample. Using a Quanta 400F field-emission scanning electron microscope (SEM, Thermo Fisher Scientific, Waltham, MA, USA), we obtained the scanning electron microscopy images of the sample. A JEM-2010HR (JEOL, Tokyo, Japan) was used to conduct transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the sample. A Netzsch 209 F3 TG analyzer (NETZSCH-Gerätebau GmbH, Selb, Germany), range from room temperature to 850 °C, was used to perform the thermogravimetric analysis (TGA) of the material, and the Brunauer–Emmett–Teller (BET) specific surface area of the sample was determined using a Micromeritics ASAP 2460 (Micromeritics Instrument Corporation, Norcross, GA, USA) at −77 °C using nitrogen sorption studies.

2.3. Electrochemical Measurements

The working electrodes were fabricated by painting the homogeneous mixture onto copper foil with mass ratio of 75:10:15 (MnO:super-P:PVDF) using a certain amount of NMP solution, and then we placed the painted copper foil in an oven at 120 °C for 12 h. The CR2032-type coin half-cells were assembled in a glove box using the Li foil as the counter electrode, a Celgard 2400 film (Celgard, Charlotte, NC, USA) as the separator, 1 mol LiPF₆ in ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate (EC/EMC/DMC, 1:1:1 vol) used as electrolyte (about 80 μL in each cell), and the as-prepared MnO as working electrodes (~2.2 mg MnO in each electrode). The galvanostatic discharge/charge performances of the batteries were recorded using a battery test system (Neware), with a range of 0.01~3.0 V (vs. Li/Li⁺). A CHI660E electrochemical workstation (CH Instruments, Austin, TX, USA) was used to test the cyclic voltammetry (CV) of the CR2032-type coin half-cells with 0.1 mV/s scan rate.

3. Results and Discussion

Figure 1a displays the MnCO₃ precursor particles, which are about 1–3 μm in length and have a fusiform-like structure. The XRD pattern of the MnCO₃ specimen with six characteristic peaks is shown in Figure 1b, which are assigned to the rhombohedral phase corresponding to JCPDS card No. 44-1472 [33]. Figure 1c shows the TGA curve of the MnCO₃; there was nearly no weight loss below 200 °C (see the position of the purple circle), implying there is no adsorbate for the MnCO₃ particles’ smooth surface. Afterward, the MnCO₃ decomposed entirely in the temperature range from 200 °C to ca. 450 °C, and the pure MnO was obtained at temperatures above 500 °C. The weight loss was 36.3% during this process; this proportion is generally in agreement with the result of Equation (1), which yielded ~38.2%.

$${\mathrm{MnCO}}_3\to \mathrm{MnO}+{\mathrm{CO}}_2$$

(1)

Figure 2a shows the mulberry-like shape of the MnO microparticles derived at a 700 °C heat temperature (denoted as MnO-700). As shown in Figure 2b, the magnified image confirmed that the MnO microparticles had a mulberry-like appearance (a photo of a mulberry is shown in the upper left corner of the image) and some holes were observed on the surface of the MnO microparticles (yellow arrows). The samples derived at calcination temperatures of 500 °C and 600 °C (denoted as MnO-500 and MnO-600, respectively) also exhibited a similar morphology as the MnO-700 sample (as shown in Supplementary Figures S1 and S2, respectively). The five characteristic diffraction peaks at 34.9°, 40.5°, 58.7°, 70.1°, and 73.8°, respectively, in Figure 2c, correspond to the (111), (200), (220), (311), and (222) planes of the cubic MnO (JCPDS Card No. 07-0230) [21,34]. The MnO-500 and MnO-600 materials have the same characteristic diffraction peaks as the MnO-700 sample in Supplementary Figure S3. The TEM image provides further evidence of the MnO microparticle with a mulberry-like structure (Figure 2d), and the HRTEM image demonstrated that the MnO-700 sample has a good crystal structure (Figure 2e). The lattice fringe of the MnO-700 microparticle has an approximately 0.29 nm interplanar spacing, which can be ascribed to the cubic MnO (111) plane. The BET result of the MnO-700 microparticle is shown in Figure 2g, and the sample shows a type-IV isotherm from the nitrogen adsorption/desorption curves. The BET surface area of the sample is 5.33 m²/g based on the fitting result, and the pore size distribution ranged from 2 to 5 nm (Figure 2f, insert).

Next, we wanted to understand the electrochemical performances of the MnO-700 microparticle specimen. The first three cyclic voltammetry (CV) analyses of the MnO-700 electrode was conducted with scan rate of 0.1 mV s⁻¹ (Figure 3a). A sharp reduction peak of about 0.4 V can be seen in the first cathodic sweep, indicating the initial reduction action of the MnO particles to Mn (equivalent equation is MnO + 2Li⁺ + 2e⁻ → Mn + Li₂O), and the solid electrolyte interface (SEI) layer of the MnO sample also formed [34]. Another oxidation peak was observed at 1.25 V in the anodic sweep, suggesting the oxidation of Mn⁰ to Mn²⁺ [30]. Subsequently, the reduction peaks showed some deviation in the 2nd and 3rd cycles, which explained the instability of the electrode and may have been the result of the Mn aggregation or the formation of defects in the reduction reaction [21,35,36]. However, the oxidation peaks of the 2nd and 3rd curve overlapped at ~1.35 V [37,38]. Figure 3b displayed the cycling performance of the MnO-700 electrode at 200 mA g⁻¹. It was found that the discharge capacity decreased to 562 mAh g⁻¹ (0.81 Ah/cm²) after 20 cycles and then enhanced to 702 mAh g⁻¹ (1.01 Ah/cm²) after 120 cycles; the change can be explained the Mn²⁺ being transformed into a higher oxidation state (Mn³⁺ or Mn⁴⁺) under the charge/discharge reaction [32,35]. Meanwhile, this MnO-700 sample demonstrates a competitive reversible capacity when compared to MnO/carbonaceous composites [21,32,39] and pure MnO with different morphologies [27,31]. Moreover, the reversible capacity of the MnO-700 was significantly higher than the MnO-500 (274 mAh g⁻¹) and MnO-600 (482 mAh g⁻¹) specimens at the same current density as shown in Supplementary Figure S4, which suggests that the crystal structure of MnO was improved by higher temperature for lithium storage performance increased [28]. Meanwhile, we noticed that the first CE of the MnO-700 sample was high, ~74.9%, and then increased to 99.2% after the 10th cycle. In addition, the MnO-700 electrode exhibited a stable discharge capacity of 365 mAh g⁻¹ after 200 cycles at a current density of 1000 mA g⁻¹ (Figure 3c), which is relatively equal to the commercial graphite theoretical capacity, and the CE was nearly 99% after the 5th cycle; this indicates that the MnO-700 sample can be used as an anode candidate instead of commercial graphite for fast lithium storage. The MnO-700 delivered excellent rate performance with an average capacity of 695, 590, 469, 379, and 224 mAh g⁻¹ at different current densities (100, 200, 500, 1000 and 2000 mA g⁻¹, respectively), as shown in Figure 3d. Remarkably, the MnO-700 electrode returned to 738 mAh g⁻¹ reversible capacity after being at 100 mA g⁻¹, which indicated that the mulberry-like MnO-700 microparticle material has excellent structural stability after high current cycling. Meanwhile, comparison with different shapes of pure MnO and MnO carbon-based composites in the literature (

Table 1. Comparison of the cycling performances of the MnO-700 electrode in this work with different shapes of pure MnO and MnO carbon-based composites in the literature.

Sample Name	Current Density (mA g−1)	Capacity Retention (mAh g−1)	Cycle Number (Times)	References
Mulberry-like MnO	200	702	120	This work
Nano MnO	141.1	581.5	50	[40]
MnO cubes	200	615.9	10	[27]
MnO-nanoparticle	151.2	428	100	[41]
star-like MnO	500	844.8	200	[42]
MnO micrometer particles	100	747	100	[28]
MnO particles	95	650	150	[43]
u-MnO/C	152	723	80	[44]
MnO/C composite	200	664	100	[45]
MnO/RGO composite	300	544	60	[46]
MnO/rGO	100	846	110	[47]
MnO/natural graphite	152	497	120	[48]
MnO/Carbon fibers	400	680	100	[49]

), the MnO-700 electrode also displayed competitive lithium storage performances.

Figure 4a displayed that the MnO-700 material exhibits a low discharge specific capacity of 22 mAh g⁻¹ at 200 mA g⁻¹ after 500 cycles as an anode material used in sodium-ion batteries (SIBs). Meanwhile, the electrode exhibits a good rate performance range from 100 to 2000 mA g⁻¹, and also maintained a low 12 mAh g⁻¹ at 2000 mA g⁻¹ (Figure 4b). Apparently, the reversible capacity absolute value is not very high for the MnO-700 specimen applied in SIBs, which may be ascribed to the Na⁺ radius beings larger than the Li⁺ radius [50], and the pure MnO material has low electrical conductivity.

We concluded that the excellent electrochemical performances of the MnO-700 sample could be attributed to its unique mulberry-like structure, and the special two-stage structure may provide more spaces to suppress the volume expansion of the MnO microparticles during the lithiation and de-lithiation cyclic process [28,29], which is beneficial for postponing the pulverization and collapse of the MnO-700 electrode during the charge and discharge processes. Moreover, the holes on the surface of the microparticles also can shorten the lithium ions’ (Li⁺) transfer paths [27], and also be beneficial for the electrolyte infiltration into the MnO electrodes, so that the wettability of the electrolytes between the MnO material is improved.

4. Conclusions

In summary, mulberry-like MnO-700 microparticles were obtained using the fusiform MnCO₃ precursor microparticles via calcination treatment. When the MnO-700 was applied in LIBs as the anode material, the electrodes delivered a high discharge capacity of 703 mAh g⁻¹ (1.01 Ah/cm²) at a current density of 200 mA g⁻¹ after 120 cycles, and a stable capacity of 224 mAh g⁻¹ (0.32 Ah/cm²) at a current density of 2000 mA g⁻¹. The excellent lithium storage capacity can be ascribed to the novel mulberry-like structure of the MnO particles that buffered the large volume expansion and also shortened the transfer paths of the Li⁺ between the lithiation and de-lithiation cyclic process. Therefore, this research may stimulate scholars to develop oxides or composites with novel structures as anodes for high-performance LIBs.