Synthesis of mesoporous manganese dioxide/expanded graphite composite and its lithium-storage performance

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Synthesis of mesoporous manganese dioxide/expanded graphite composite and its lithium-storage performance

YAN LIN¹ , XIE LIU¹, XIAO-QIN LIU², LIN-BING SUN², FENG CHEN¹, CHENGBAO LIU¹, ZHENGYING WU¹^,∗and ZHIGANG CHEN¹

1Jiangsu Key Laboratory for Environment Functional Materials, School of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, 1 Kerui Road, Suzhou 215009, People’s Republic of China

2State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, People’s Republic of China

∗Author for correspondence (zywu@mail.usts.edu.cn)

MS received 24 April 2019; accepted 10 October 2019

Abstract. A mesoporous manganese dioxide (MnO₂)/expanded graphite (EG) composite was successfully fabricated using mesoporous silica decorated EG (KIT-6/EG) as a hard template. Different amounts of EG were introduced to the synthetic system to adjust the MnO₂:EG mass ratio of the composite. X-ray diffraction, transmission electron microscopy, scanning electron microscopy and nitrogen adsorption–desorption analyses were employed to characterize the structure and morphology of the composite. Results show that the distribution of MnO₂nanoparticles grown on the EG layers decreases gradually with increasing EG content. Moreover, in the presence of excess EG, the specific surface area of the samples dramatically decreases. As the anode electrode of a Li-ion battery (LIB), the composite (MnO₂:EG =34% w/w) exhibits a specific capacity of∼250 mA h g⁻¹at a current density of 200 mA g⁻¹for up to 100 cycles, this capacity is much higher than that of pure MnO₂(∼10 mA h g⁻¹)due to its improved electrical conductivity. The composite also shows good rating performance when the current density is tuned. These results indicate that the composite has potential application as an anode material for next-generation LIBs.

Keywords. Mesoporous MnO₂; KIT-6; expanded graphite; Li-ion battery.

1. Introduction

Given the increased use of non-renewable energy sources, global warming and environmental pollution, it is impera- tive to develop efficient and renewable energy storage devices [1,2]. Lithium-ion batteries (LIBs) are regarded as one of the most promising new-generation energy storage devices that are currently available due to their high energy density and operating voltage, long cycle life and chemical stability [3,4].

The key to these storage devices is the appropriate selection and preparation of the electrode materials [5]. Graphite is commonly used as a commercial anode material, but can- not meet the requirements of new portable electronic devices and electric vehicles due to its low theoretical capability [6]. In recent years, transition metal oxides (TMOs) have been widely used in the research of anode electrode materials because of their prominent properties [7–10]. Among potential oxides, MnO₂ is regarded as an effective alterna- tive because it possesses a rather large theoretical specific capacity of 1230 mA h g⁻¹ and excellent electrochemical stability, it is also inexpensive, naturally abundant and environment friendly [11]. MnO₂ has various crystal structures, such asα-MnO2,β-MnO2,γ-MnO₂ andδ-MnO2 [2,12,13].

Many studies on MnO2 of different synthetic shapes, such

as nanoplatelets [2], nanomeshes [14], nanorods [15,16], nanotubes [17] and nanoflowers [18] have been reported.

The electrical performance of MnO₂depends largely on its structure and morphology. Mesoporous MnO₂ has recently received extensive attention due to its unique catalytic, mag- netic and adsorption properties [12,13]. However, pure TMOs tend to have weak points of low conductivity, which limits the development of their electrical properties [19]. To address this problem, several strategies have been proposed. An effective method to improve the conductivity of TMOs is to combine them with carbon-based materials, such as graphene [20], carbon tubes [17], carbon fibres [21] and graphene oxide [22]. Expanded graphite (EG) is a graphite-derived material with good electrical conductivity and loose porous properties that can effectively alleviate volume expansion during the charge/discharge process [23]. These properties have inspired us to prepare a novel type of anode material.

Herein, a novel MnO₂/EG composite was successfully fabricated using mesoporous KIT-6-decorated EG (KIT-6/EG) as a hard template. The obtained composite has an ordered pore structure, a relatively large specific surface area, and cycling capacity and stability that are notably better than those of pure meso-MnO2. This work presents the potential application of MnO2as an anode material for LIBs.

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2. Experimental

2.1 Materials synthesis

2.1a Preparation of templates KIT-6 and KIT-6/EG:

Exactly 2 g of Pluronic P123 (M_w =5800,EO20PO70EO20) was dissolved in 12 g of 2 M HCl and 57 g of deionized water.

Then, 2 g of butyl alcohol was added to this solution, and the mixture was stirred for 0.5 h. Next, 4.16 g of tetraethyl orthosilicate (TEOS) was added to the above system at 35^◦C, followed by stirring for 24 h. The mixture was placed in a reac- tor for 24 h at 100^◦C, and filtered. The residue was collected and dried at 100^◦C for 24 h. Finally, the powder was calcined at 550^◦C in air to obtain KIT-6 for the template. In a typical reaction, 4.16 g of TEOS precursor is converted into 1.2 g of KIT-6. The synthesis method of KIT-6/EG is similar to that of KIT-6 except that a certain amount of EG is added to the system after the addition of butyl alcohol. The amount of EG was adjusted to obtain KIT-6/EG with different mass ratios.

2.1b Synthesis of MnO₂and MnO₂/EG composites: Four and 0.15 grams of Mn(NO₃)2was dissolved in 25 g of ethanol.

KIT-6 or KIT-6/EG was added to this solution, and the mixture was stirred at room temperature until the reactants turned into a powder. This powder was calcined in a muffle fur- nace at 350^◦C for 5 h. Then, the obtained composites were soaked in 2 M NaOH/ethanol solution to remove the silica.

Finally, the obtained samples were washed several times with ethanol, dried in air, and named MnO2and MnO2/EG. Com- posites with different EG contents were prepared and named MnO₂/EG-23, MnO₂/EG-34, MnO₂/EG-44 and MnO₂/EG- 55, corresponding to EG mass ratios of 23, 34, 44 and 55%, respectively. The detailed formulations of each of the samples are listed in table1.

2.2 Characterizations

X-ray diffraction (XRD, Bruker D8 Advance, Germany) was performed at 40 kV and 40 mA with Cu Kαradiation at 2θ of 0.5–5^◦ (low-angle range) and 10–80^◦ (wide-angle range). The morphologies and structures of the samples were separately examined by a field emission scanning electron microscope (FESEM, S-4800, Japan) and a transmission electron microscope (TEM, JEOL 2100F, Japan). Inductively coupled plasma optical emission spectrometer (ICP–OES,

Optima 8300, Perkin Elmer, USA) was used to determine Mn content in the MnO2/EG composites. Samples were dissolved in concentrated hydrochloride (HCl) aqueous solutions, then expanded graphite was removed by centrifugation and clear solution was obtained for detection. Nitrogen adsorption–

desorption experiments were conducted by using an ASAP 2020 analyzer (Micromeritics, USA). Specific surface areas were calculated according to the Brunauer–Emmett–Teller (BET) method, and the pore size distributions were measured using the Barrett–Joyner–Halenda (BJH) model.

2.3 Electrochemical measurements

Electrochemical experiments were performed on half-cells (CR2032-type) using these obtained materials as electrodes and Li foil as the counter electrode assembled in an Ar- filled glove box. A solution of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (1:1, v/v) was used as the electrolyte. The synthesized material, carbon black (Super P) and polyvinylidene fluoride binder were mixed together at a weight ratio of 7:2:1 and milled for 30 min to prepare the anode. N-methylpyrrolidone was selected as solvent to produce a slurry. The slurry was spread evenly on a cop- per foil (thickness: 100μm) and dried for 24 h in a vacuum at 80^◦C. Galvanostatic charge–discharge tests were conducted by using a multichannel battery test system (LAND CT2001A, China). Cyclic voltammetry (CV) was carried out on an electrochemical workstation (Autolab, PGSTAT302N, Switzerland) at a scan rate of 0.1 mV s⁻¹. The potentio-static electrochemical impedance spectroscopy (PEIS) measure- ment was accomplished using another electrochemical workstation (Bio-Logic, VMP3, France). All tests were performed at 25^◦C.

3. Results and discussion

3.1 Structure and morphology of the mesoporous MnO2/EG composites

Figure1a shows the XRD patterns of KIT-6, pristine MnO₂, EG and the MnO₂/EG composite (MnO₂:EG= 34% w/w).

The wide-angle XRD patterns of MnO₂sample reveal obvious diffraction peaks at 2θ=28.7, 37.3, 42.8, 56.7, 59.4, 64.8 and 72.3^◦, which can easily be identified as the (110), (101),

Table 1. The amount of reactants used in this work.

Sample

MnO₂/EG (%) P123 (g) HCl (g) H₂O (g) TEOS (g) BuOH (g) Mn(NO₃)2(g) EtOH (ml) EG (g)

55 2 19 57 4.16 2 4.15 25 1.2

44 2 19 57 4.16 2 4.15 25 0.8

34 2 19 57 4.16 2 4.15 25 0.52

23 2 19 57 4.16 2 4.15 25 0.3

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10 ˩m

a b

c d

10 20 30 40 50 60 70 80

EG MnO₂ Mn₂O₃

MnO₂

1 2 3

(332)

MnO₂/EG

(211)

2Theta (degree)

Relativeintensity(a.u.)

KIT-6

(004)

(002) (301)(002)(220)(211)(111)(101)

2 Theta (degree)

Relativeintensity(a.u.) (110)MnO₂/EG

EG x0.3

Figure 1. (a) Wide-angle XRD patterns of MnO₂, EG and MnO₂/EG composite (inset shows low-angle XRD patterns of KIT-6 and the MnO₂/EG composite). (b) SEM images of the MnO₂/EG composite. (c,d) TEM images of the MnO₂/EG composite at different magnifications.

(111), (211), (220), (002) and (301) planes of the narrowest (1×1) channels ofβ-MnO₂(space group:P42/mnm), such a pattern corresponds to that of pyrolusite with a rutile structure [24]. Moreover, two weak peaks corresponding to Mn₂O₃ can also be observed in the MnO₂sample [8]. Similar strong peaks of β-MnO2 can be observed from XRD patterns of the MnO₂/EG composite (figure1a). In addition, diffraction peaks at 2θ=26.5 and 54.6^◦are also found in the MnO2/EG sample, which are corresponding to the (002) and (004) crystal planes of EG [25]. In the low-angle XRD patterns of KIT-6 and MnO2/EG (inset of figure 1a), characteristic diffraction peaks at 2θ =0.88 and 1.68^◦, corresponding to the (211) and (332) planes of theIa3d cubic phase, are observed in both samples, thus indicating that the MnO2/EG sample preserves the 3D-ordered mesoporous structure of the KIT-6 [11,24,26].

The intensity and shape of the peaks for MnO2/EG sample are not as defined as those of the template KIT-6, probably, because Mn species form a metal oxide phase only at certain regions of the template.

The morphology and detailed structure of the MnO₂/EG composites are investigated by SEM and TEM. The SEM

image in figure 1b reveals that the MnO2particles grow uni- formly on the surface of the graphite sheets. Figure1c further shows that the mesoporous MnO₂ particles (see the region in yellow square) are assembled on the graphite layers (see the region in red circle). TEM image at high magnification (figure1d) reveals that pores with a size of<10 nm are clear and well-ordered. Cubic symmetry of (311) can be clearly observed, this characteristic is typical of mesoporous materials withIa3dcubic phase, which is consistent with the XRD results [27,28]. The mesopores of the MnO2/EG composite are well preserved, and the sample remains uniform in structure after template removal.

To investigate the effect of EG on the morphology and structure of the material, MnO2/EG composites synthesized with different EG mass ratios (23, 34, 44 and 55%) were examined by SEM. As shown in figure2a, agglomerations of MnO₂ on the EG surface are severe and accompanied by the formation of large spheroidal particles when the EG content is low (MnO₂/EG-23). However, when the EG content is increased to 55% (MnO₂/EG-55), the MnO₂particles nonuni- formly grown on carbon layers of EG are particularly small.

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Figure 2. SEM images of (a) MnO₂/EG-23, (b) MnO₂/EG-34, (c) MnO₂/EG-44 and (d) MnO₂/EG-55.

Under the appropriate EG content (figure2b and c), orderly and uniform MnO2particles on the carbon sheets of EG can be observed.

To determine whether different contents of EG influence the pore structure of the samples, nitrogen adsorption–desorption measurements are carried out. Figure 3 shows the N₂ adsorption–desorption isotherms and pore-size distributions of EG and MnO₂/EG composites with different EG contents.

As shown in figure 3a, the nitrogen adsorption–desorption isotherms of the sample MnO₂/EG-23 and MnO2/EG-34 show type IV adsorption isotherms with H3-type hysteresis loops, indicating the presence of mesopores with a relatively narrow pore size distribution [11,29]. The remarkable increase in adsorption of MnO2/EG-34 at around p/p0 = 0.45 indicates an increase in mesoporous adsorption, consistent with the corresponding pore size distributions in figure3b.

Moreover, no adsorption limitation is observed at relatively high partial pressures (p/p0 = 0.99), which means other macropores, besides mesopores, are present in the material.

These macropores may be originated from EG in the composite. With the higher EG contents, the N₂ adsorption for MnO₂/EG-44 and MnO₂/EG-55 are decreased. At the same time, the hysteresis loops for these two samples are deformed, indicating non-uniform mesopores and the wide pore-size distributions (figure3a and b). The pore-size distribution of MnO2/EG-34 calculated by the BJH model is 4.2 nm, which is in good accordance with the TEM results (figure1d), and

contributes to the capacitance of the composite when applied to a LIB. Pore size of the MnO2/EG-23 is 3.5 nm, which is a bit smaller than that of MnO2/EG-34 (figure3b).

BET specific surface areas and pore volumes of the MnO2/EG composites are measured to be 6–45 m²g⁻¹ and 0.025–0.118 cm³g⁻¹, respectively (table2). It should also be pointed out that surface area of MnO₂/EG-34 is the largest in four samples, which is probably due to the appropriate MnO₂ (EG) content in it. MnO₂/EG-55 and MnO2/EG-44 with the relatively low MnO₂ (high EG) content have small surface areas. With the higher MnO2content, MnO2/EG-34 has the larger surface area than those of MnO2/EG-55 and MnO2/EG-44. However, too much MnO2 in the composite may obscure some pores of EG, so surface area of MnO2/EG-23 is a bit lower than that of MnO2/EG-34.

3.2 Electrochemical performance of the meso-MnO2/EG composites

Figure4portrays the CV curves of the MnO2/EG composites during the first three cycles at a scan rate of 0.1 mV s⁻¹within the voltage window 0.01–3.0 V. The CV results of the four materials are generally similar, and the peak positions are slightly shifted. In the first cathodic scan, a cathodic peak at 0.7 V is observed, this peak disappears in the following cycles, probably due to the reduction of MnO₂to Mn with the synchronous formation of Li₂O (equation (1)), as well as the

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2 10

EG

dV/dLog(D)(cm3 g-1 )

Pore diameter (nm)

MnO₂/EG-34 MnO₂/EG-23

MnO₂/EG-44 MnO₂/EG-55

0.0 0.2 0.4 0.6 0.8 1.0

EG MnO₂/EG-34 MnO₂/EG-23

MnO2/EG-55 MnO₂/EG-44

Relative pressure (p/p₀)

Quantity adsorbed (cm3 g-1STP) a b

Figure 3. (a) Nitrogen adsorption–desorption isotherms and (b) pore-size distributions of MnO₂/EG composites with different EG contents.

Table 2. N₂physisorption results of the MnO₂/EG composites.

Sample MnO₂* (wt.%) S_BET(m²g⁻¹) V_p(cm³g⁻¹) D_p(nm)

EG — 20 0.11 —

MnO₂/EG-55 21.95 5.7 0.025 —

MnO₂/EG-44 21.96 8.4 0.030 —

MnO₂/EG-34 24.44 45 0.118 4.2

MnO₂/EG-23 26.82 30 0.079 3.5

*The real Mn content in the composite was detected by ICP-OES.

irreversible formation of a solid electrolyte interphase layer [14]. Another obvious cathodic peak at 0.1 V is attributed to the insertion of Li into graphene (EG) (equation (2)) [30]. In the anodic scan, a peak appears at 1.15 V, likely due to the oxidation of Mn to MnO2. Oxidation peaks<0.6 V are also detected in meso-MnO2/EG and attributed to the extraction of Li from expanded graphite. In the next two cycles, the CV curves of MnO2/EG-23 and MnO2/EG-34 nearly overlap, indicating redox reactions [31] that are more reversible than those of MnO2/EG-44 and MnO2/EG-55.

MnO₂+4Li⁺+4e⁻↔Mn+2Li₂O, (1)

C+xLi⁺+xe⁻↔LixC. (2)

The long-term cycling performance (discharge capacity) of pure MnO2 and MnO2/EG composites at a current density of 200 mA g⁻¹ with activation of the first five circles at 50 mA g⁻¹ is shown in figure 5. The specific capacity of pure MnO₂ is about 10 mA h g⁻¹, much lower than that of the MnO₂/EG composites (150–250 mA h g⁻¹). The specific capacity of MnO₂/EG-44 is similar to that of MnO₂/EG-34. While MnO2/EG-34 has a higher specific capacity in the first 80 laps, the specific capacity of MnO2/EG-44 exceeds that of MnO2/EG-34 in the last 20 laps. The capacities of MnO2/EG-23 and MnO2/EG-55 are

∼150 and∼175 mA h g⁻¹, respectively, which indicates that too much or too little EG is not conducive to Li storage. Stable performance and good coulombic efficiency are observed for up to 100 cycles.

The high coulombic efficiency of the investigated electrodes indicates highly reversible Li-ion insertion–extraction in MnO2/EG composite electrodes. This finding may be attributed to the pore volume of the composites, which enables the storage and retention of Li ions, and their structure, which allows active substances to fit into pores resulting from volume expansion during cycling.

Figure 6 shows the rate performance of pure MnO₂ and MnO₂/EG composite electrodes in the current density range of 50–2000 mA g⁻¹. All the composites with different MnO₂/EG mass ratios show improved rate per- formance in comparison with that of pure porous MnO₂. Among the composites tested, MnO2/EG-34 presents the best capacity (∼400 mA hg⁻¹) at a low current density (50 mA g⁻¹), by comparison, MnO2/EG-44 shows a capacity higher than those of the other composites at high current densities (200–2000 mA g⁻¹). When the current density is returned to 50 mA g⁻¹, a high capacity of∼300 mA hg⁻¹is quickly recorded for MnO2/EG-34. This result may be due to increased Li-ion accessibility and the conductivity of the mesoporous composite structure during cycling [3].

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.2

-0.9 -0.6 -0.3 0.0 0.3

Current (mA)

Voltage Vs Li⁺/Li (V)

1 st 1.15V

0.5V

0.1V 0.75V

MnO₂/EG-34

2 nd 0.3V

3 rd 0.17V

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-2.0 -1.5 -1.0 -0.5 0.0 0.5

1 st 2 nd 3 rd

Current (mA)

Voltage (V) vs. Li/Li⁺

1.2V

0.7V 0.2V

0.4V 0.32V 0.29V

0.13V

MnO₂/EG-55

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5

1 st 2 nd 3 rd

Current (mA)

Voltage (V) vs. Li/Li⁺

0.2V 1.2V

0.3V

0.27V

0.12V

MnO₂/EG-44

0.7V

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-0.15 -0.12 -0.09 -0.06 -0.03 0.00 0.03

Current (mA)

Voltage Vs Li+/Li (V)

1 st 0.7V

1.5V

0.3V

MnO₂/EG-23

2 nd 0.45V 1.06V

0.2V

3 rd

a b

c d

Figure 4. Cyclic voltammograms of the MnO₂/EG composites during the first three cycles at a scan rate of 0.1 mV s⁻¹.

0 20 40 60 80 100

0 100 200 300 400 500 600 700 800

50 200 mA g^-1

MnO₂/EG-23 MnO2/EG-34 MnO₂/EG-44 MnO₂/EG-55 MnO₂

Capacity (mA h g-1 )

Cycle Number

Figure 5. Cycling performance of the pure MnO₂and MnO₂/EG electrodes at a current density of 200 mA g⁻¹with activation of the first five circles.

Potentio-static electrochemical impedance spectroscopy (PEIS) tests for MnO₂ and MnO₂/EG-34 electrodes before cycling were performed to understand the electrical conductivity. In the equivalent circuit model, R₁ represents

0 10 20 30 40 50 60

0 100 200 300 400 500 600 700

800 MnO₂/EG-23

MnO₂/EG-34 MnO₂/EG-44 MnO₂/EG-55 MnO₂

Capacity (mA h g-1 )

Cycle Number

50

200 500

1000 2000 50

Figure 6. Rate performance of the pure MnO₂ and the MnO₂/EG electrodes at current densities ranging from 50–2000 mA g⁻¹.

the internal resistance of the electrolyte. R_f and CPE₁ are resistance and capacitance, respectively, which are related to the SEI films formed on the electrode.R_ctand CPE₂refer to charge transfer resistance and double layer capacitance. Z_w

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0 200 400 0

200 400 600 800 1000 1200 1400

0 50 100 150

0 100 200

Z'' (Ohm)

Z' (Ohm)

MnO₂ MnO₂/EG-34

Z'' (Ohm)

Z' (Ohm)

Figure 7. Nyquist plots of MnO₂ and MnO₂/EG-34 electrodes with the equivalent circuit model inset.

is the Warburg impedance corresponding to the diffusion of Li⁺into electrode [32,33]. TheRctof the MnO2/EG electrode (73.4)is obviously lower than that of the MnO2electrode (103.3), indicating that the composite has the higher conductivity than pure MnO2. This is because of the presence of carbonaceous EG in the composite can offer much faster electron-transfer channels than that of the pristine MnO2

(figure7).

4. Conclusion

In summary, a unique mesoporous MnO2/EG composite was successfully synthesized using KIT-6/EG as a hard template for LIB applications. The presence of carbonaceous EG and the mesoporous structure in the material allows the efficient transport of Li ions and electrons. As a consequence, the composite exhibits excellent cycling capacity of about 250 mA h g⁻¹ at a current density of 200 mA g⁻¹ after 100 cycles with good stability, this capacity is much higher than that of pure MnO₂. Mesoporous MnO₂/EG materials may have great potential use as anodes in the LIB field.

Acknowledgements

This work was supported by the Natural Science Found- ation of Jiangsu Province-Outstanding Youth Project (BK20180103) and the Science and Technology Development Project of Suzhou (SYG201818). Financial supports from Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment, the Open Projects of the Inter- national Joint Laboratory of Chinese Education Ministry on Resource Chemistry (A-2017-002) and the State Key Labora- tory of Materials-Oriented Chemical Engineering (KL17-06)

and the Jiangsu Innovation Project for Graduate Education (KYCX17_2064) are also gratefully acknowledged.

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