reduced graphene oxide nanocomposite as an anode electrode for lithium-ion batteries

Author’s Accepted Manuscript Synthesis of MnOx/reduced graphene oxide nanocomposite as an anode electrode for lithiumion batteries Shao-Chieh Weng, Sanjaya Brahma, Chia-Chin Chang, Jow-Lay Huang www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)32401-4 http://dx.doi.org/10.1016/j.ceramint.2016.12.134 CERI14444

To appear in: Ceramics International Received date: 26 October 2016 Revised date: 28 December 2016 Accepted date: 28 December 2016 Cite this article as: Shao-Chieh Weng, Sanjaya Brahma, Chia-Chin Chang and Jow-Lay Huang, Synthesis of MnOx/reduced graphene oxide nanocomposite as an anode electrode for lithium-ion batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.12.134 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of MnOx/reduced graphene oxide nanocomposite as an anode electrode for lithium-ion batteries

Shao-Chieh Weng1, Sanjaya Brahma1, Chia-Chin Chang2, Jow-Lay Huang1, 3, 4, 5* 1

Department of Materials Science and Engineering, National Cheng Kung University,

Tainan 701, Taiwan (R.O.C.) 2

Department of Greenergy, National University of Tainan, Tainan 701, Taiwan (R.O.C.)

3

Department of Chemical and Materials Engineering, National University of Kaohsiung.

Kaohsiung 811, Taiwan (R.O.C) 4

Center for Micro/Nano Science and Technology, National Cheng Kung University,

Tainan 70101, Taiwan (R.O.C) 5

Research Center for Energy Technology and Strategy, National Cheng Kung University,

Tainan 70101, Taiwan (R.O.C)

†

Presenter: Shao-Chieh Weng

*

Corresponding author’s e-mail: [email protected]

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Abstract We report the high performance of the manganese oxide/reduced graphene oxide (MnOx/rGO) nanocomposite as an anode electrode of a lithium-ion battery. The composite is synthesized by a low temperature (83 °C) chemical solution reaction, and shows relatively high specific capacities (660 mAh g-1) after 50 cycles. For MnOx/rGO composites, the cycling stability is increased remarkably as compared to that seen with individual MnOx, and this is due to the synergistic effects of both the components in the composite. The rGO acts as a conductive buffer layer that suppresses the volume change of MnOx, and simultaneously promotes the conductivity of MnOx. The functional groups of graphene oxide facilitate MnOx formation at low temperature, and this retains the MnOx-graphene oxide connection, thus improving the capacity and cycling stability.

Keywords: MnO2; reduced graphene oxide; anode materials; Li-ion batteries

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1. INTRODUCTION Lithium-ion batteries (LIBs) are expected to play an important role as the energy storage systems for electric vehicles/motorcycles and renewable energy applications [1]. Graphite is widely used as an anode in commercially available LIBs, in which graphite intercalates with lithium to form graphite intercalation compounds, (GICs) such as LiC6, which contributes to the highest theoretical capacity of 372 mAh g-1.

However, this

theoretical capacity does not meet the demand for large size battery applications [2-9]. Graphene is a single layer of sp2 carbon atoms arranged in a honeycomb structure, with a very large surface area and high electron mobility (15,000 cm 2V−1s−1) at room temperature. Single-layer graphene has a theoretical lithium storage capacity of 744 mAh g-1, if lithium attaches to both sides of the graphene sheets. The enhanced lithium storage capacity of graphene makes it a suitable alternative to replace the graphite as anode active material of LIB [10-14]. Even though graphene has many advantages, aggregation remains the major problem that hinders its wider application. However, graphene attached with different oxygen-functional groups, such as graphene oxide (GO) and reduced graphene oxide (rGO), would have less aggregation and can serve as a better electrode material with a higher surface area and more stable dispersions in aqueous or organic solvents [15]. Graphene-based materials can be used as a 2D buffer

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layer for the anisotropic growth of various metals (M)/metal oxide (MO) nanoparticles (NPs), which not only effectively prevent the aggregation and volume expansion of these M/MO NPs, but also enhance the capacity [16, 17]. Hummers’ method is generally used to fabricate graphene oxide nanosheets (GOs) at low cost [18, 19]. Due to the presence of oxygen-containing functional groups on the surface of the GOs, the steric effect facilitates better dispersion of the GOs in solvents and thus extends their range of application. In addition, the oxygen-containing functional groups of GOs can act as active sites to react with transition metal ions for the formation of transition metal oxide (TMO) nanostructures having a uniform distribution over the surface of the GOs. Etacheri, et al. [20] K. T. Lee et al. [21] describe the use of nanoscale TMOS as an anode material which can deal with the mechanical strains produced due to the volume changes caused by Li ion intercalation/deintercalation, and suppress the corrosion of electrodes. Poizot et al. [22] first introduced the nanosized TMOs for LIB applications due to their characteristics of high energy density, high theoretical capacity, long cycle life, low cost, low toxicity, natural abundance and their unique conversion reactions [23, 24]. Electrochemically active transition metal oxides (TMOs), such as CuO [25], Co3O4 [26], SnO2 [27, 28], manganese oxide (MnOx) [29-32] and Fe2O3 [33], have been intensively exploited as potential anode materials because of their high theoretical

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capacity and improved safety. Among these MnOx, manganese dioxide (MnO2) has been widely investigated as a promising anode material due to its highest theoretical capacity of 1232 mAh g−1, low conversion potential, relatively low electrochemical motivation force, natural abundance, and environmental friendliness [24, 34, 35]. Table 1 provides a brief review of the performance of MnO2 and MnO2/graphene (reduced graphene oxide) composites when used as anode materials in LIBs [23, 34-44]. Several studies have reported that manganese dioxide (MnO2) exhibits a number crystallographic polymorphs, such as α-, β-, γ-, δ-type, which are different in that the basic unit [MnO6] octahedra are interlinked in different ways [45]. Meanwhile, α-type MnO2 (α-MnO2) is constructed from double chains of [MnO6] octahedra forming 2  2 tunnels along its c-axis composed of four edge-sharing MnO6 octahedral units. This typical crystallographic structure has sufficient gaps to accommodate these lithium ions intercalation/deintercalation into the 1D tunnel structure of α-MnO2 without destroying it [46, 47]. However, MnO2 anodes suffer from rapid capacity fade during the conversion reaction with lithium ions, due to volume expansion, aggregation, and intrinsically low electronic conductivity. Although there are many reports about MnO2/carbon nanocomposite, the synthesis methods are complicated, time-consuming and are carried out at high temperature (> 100 ºC) [29, 31, 48-50]. Here, we have

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modified a previously described synthesis approach [51] to prepare MnO2 nanoneedles and MnO2/rGO nanocomposite by a simple chemical method at relatively low temperature (83 °C). The prospect of combining α-MnO2 and reduced graphene oxide (rGO) in order to obtain the synergistic effects of their respective advantages is under consideration as a possible strategy for obtaining improved anode materials for high-power LIB applications [52-56].

2. EXPERIMENTAL METHOD 2.1 Synthesis of graphene oxide (GO) In this research, the graphene oxide was synthesized by modifying Hummers’ method, using graphite powder (ALDRICH graphite, 1-2 micron) as the precursor of graphene oxide (GO). Graphite powder (2g) was added into 98 ml of concentrated H2SO4, and stirred for 10 minutes. The 2 g NaNO3 was added into the above mixture and subjected to stirring and cooling in an ice bath for 30 minutes, followed by slowly adding 12 g KMnO4 with the temperature lower than 15 °C and stirring continuously for 30 minutes. The mixture was then heated to 78 °C and this temperature maintained for 2 hours, followed by adding 80 ml deionized water and stirring for 15 minutes in this sequence. After 15 minutes the reaction was terminated by the addition of a large

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amount of distilled water (200 ml) and 30% H2O2 solution (10 ml), after which the color of the mixture changed to bright yellow. The final step is adding 20 ml HCl into the solution and stirring until no bubbles are generated. GO was then collected by a filter, and the then washed with alcohol and deionized water several times. The GO was finally collected by drying in a vacuum oven at 45 °C overnight. 2.2 Synthesis of manganese dioxide (MnO2) nanoneedle and manganese dioxide /reduced graphene oxide (MnO2/rGO) nanocomposite Graphene oxide (0.066 g) and MnCl2·4H2O (0.27 g) were dispersed in isopropyl alcohol (50 ml), under ultrasonication for 0.5 hour. The mixture was then transferred to a three-neck flask connected to a water-cooled condenser and heated to 83 °C under vigorous stirring. The aqueous KMnO4 solution (0.15 g in 5 ml deionized water (DI)) was added rapidly into the above solution mixture. After refluxing for 4.5 hours, the mixture was allowed to cool to room temperature naturally. For comparison, pure MnO2 was also synthesized chemically without GO via a similar procedure. The precipitate was washed repeatedly with alcohol, DI water, centrifuged at 6000 rpm for 15 minutes, and finally dried in air at 55 °C overnight. 2.3 Characterization of MnO2 nanoneedle and MnO2/rGO nanocomposite The crystallinity and the structure of MnO2 and MnO2/rGO nanocomposite was

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studied

by

X-ray

diffraction

(XRD,

Model

No:

GADDS/D8

DISCOVER

diffractometer) equipped with CuKα (λ = 1.54 Å) as the X-ray radiation source. The surface morphology of the powders was investigated by field emission scanning electron microscopy (FE-SEM, Model: AURIGA-39-50, EHT=5kV). Transmission Electron Microscopy (TEM, JEOL JEM-2100F CS STEM) was used to investigate the detail microstructure and to identify the crystallographic phases of the composite. The powder samples were dispersed in alcohol and dropped on TEM grids for further characterization. Thermogravimetric Analysis (TGA, Perkin Elmer, Pyris 1 TGA) was carried out within 20 °C - 800 °C (heating rate 15 °C per min) in N2 atmosphere. 2.3 Electrochemical analysis Electrochemical measurements were carried out by using CR2032 type coin cells at room temperature. The working electrode was prepared by mixing four kinds of powders: (a) active material 80 wt.%, (b) 10 wt.% of Super P as a conductive additive, (c) 5 wt. % of LiOH, and (d) 5 wt.% of polyacrylic acid (PAA) as a binder of the total electrode mass [57-59]. Polyacrylic acid (PAA) was used as a water-soluble binder for anodes in lithium ion batteries. The PAA based binders were lithiated by titration using aqueous LiOH (Li-PAA) and controlled by adjusting the pH value of the solution [60, 61].

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The four components were mixed with a suitable amount of deionized water to produce a slurry. This was then uniformly loaded on a copper foil with a doctor blade as a current collector. The sample was cut into circular electrodes and dried for 30 min at 70 °C in an electric oven. The cells were assembled in an Ar-filled glove box with lithium foil as the counter electrode, and a solution of 1.0 M LiPF6 dissolved in 1:1 (v/v) EC/ DEC as the electrolyte. Galvanostatic Li+ charge/discharge analysis was carried out using a Wonatech WBCS3000 automatic battery cycler. All electrochemical measurements were conducted in the potential range from 0.002 V to 3 V (vs Li+/Li).

3. RESULTS AND DISCUSSION 3.1 Structural and morphological characterizations of MnO2 nanoneedle and MnO2/rGO nanocomposite Figure 1 shows the XRD patterns of the as-prepared MnO2 nanoneedle and MnO2/rGO nanocomposite. The standard diffraction pattern of MnO2 (JCPDS, card NO.:44-0141) is also provided for comparison with that of the as-prepared MnO2. α-MnO2 generally crystalizes to the pure tetragonal phase with I4/m (87) as the space group. The major diffraction angles at 2θ = 12.78°, 18.11°, 25.71°, 28.84°, 36.69°, 37.52°, 39.01°, 41.97°, 47.37°, 49.86°, 56.37°, 60.27°, 65.11°, 69.71°, 72.71°, and 78.59°, can be

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assigned to the (110), (200), (220), (310), (400), (211), (330), (301), (510), (411), (600), (521), (002), (541), (312), and (332) crystal phases, respectively. All the diffraction peaks of as-prepared MnO2 agree well with the standard pattern of α-MnO2 (JCPDS, card NO.:44-0141). The absence of any other peak from either the precursor or from the impurities confirms that the α-type MnO2 crystalline phase is produced by a facile chemical method. Figure 2 show FESEM images of the MnO2 nanoneedle (Fig. 2a) and MnO2/rGO nanocomposite (Fig. 2b). The typical diameter and length of the as-prepared nanoneedles are estimated to be 20±2 nm and 480±40 nm, respectively. The morphology of the MnO2/rGO nanocomposite is fiber-like, but the shape of the MnO2 nanoneedles (Fig. 2b) in the composite is identical to that seen with the pure MnO2 nanoneedles (Fig. 2a). The morphology of the as-prepared MnO2 remains almost identical even when the reaction time is increased, in contrast to a previous report [51]. Figure 3 shows TEM images of MnO2 nanoneedles (left panel) and MnO2/rGO nanocomposite (right panel) for direct comparison. Figure 3a shows the bright field TEM image of MnO2 nanoneedles, and it is obvious that these are largely attached to or overlapped with each other (inset of Fig. 3a). For example, three nanoneedles are attached to each other at the bottom of Fig. 3a (marked by a black square). Each MnO2

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nanoneedle is a single crystal, as shown by the visible bright spots (Fig. 3c) in the nano-beam electron diffraction pattern (NBED), and the planes (bright spots) match well with the XRD patterns of α-MnO2. The TEM image of the MnO2/rGO nanocomposite (Fig. 3b) demonstrates that the MnO2 nanoneedles are uniformly distributed on the surface of the reduced graphene oxide sheet (Fig. 3a). The size of the rGO sheet is estimated to be 4 μm2. The NBED of rGO shown in Fig. 3d confirms the crystallinity of the rGO. The HRTEM image (Fig. 3e) of the MnO2 nanoneedle shows clear, distinct atomic planes that confirm its high crystallinity, but the crystal orientation is different for individual needles, which indicates that there is a large variation in the orientation distribution of MnO2 in the matrix. The d-spacing 2.39 Å and 6.79 Å, corresponds to the (211) and (110) planes (Fig. 3c), and this agrees well with the interplanar spacing (2.395 Å and 6.919 Å) obtained from the standard XRD pattern. Similarly, the d-spacing of MnO2 obtained from the MnO2/rGO composite (Fig. 3f) is 2.36 Å and 2.13 Å, which corresponds to (211) and (301) and agrees well with the interplanar spacing of 2.395 Å and 2.151 Å obtained from the standard XRD pattern. Thermogravimetric analysis (TGA) is important to identify the ratio of composites containing carbon materials. As such, the amount of the carbon was calculated based on the results of an in situ TGA test. The TGA results for the MnO2 nanoneedles (black line)

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and MnO2/rGO nanocomposite (red line) are shown in Fig. 4. The mass loss of nanoneedles and nanocomposite are about 2.3 % and 7 % from room temperature to 180 °C. This may be attributed to the loss of water molecules existing both on the surface and in the materials [62, 63]. TGA analysis shows that the weight loss of the nanoneedles and nanocomposite are about 4.8 % and 23 %, respectively in the 210–460 °C and 460–580 °C temperature ranges. The weight loss between 210–460 °C is attributed to the thermolysis of graphene oxide to generate carbon monoxide (CO) or carbon dioxide (CO2) [64-67]. The major weight loss that occurred between 460–580 °C is mainly due to the loss of oxygen, and this results in the phase transformation of MnO2 to Mn2O3 [68]. The weight loss is minimal after 600 °C. The TGA results for the nanocomposite verify that the ratio of MnO2 to graphene oxide is 7 to 3. The electrochemical performance of the as-prepared MnO2 nanoneedle and MnO2/rGO nanocomposite anodes are evaluated in a half-cell utilizing a metallic lithium film as the counter electrode and reference electrode. Figures 5a-b show the galvanostatic charge–discharge curves for MnO2 nanoneedle and MnO2/rGO nanocomposite electrodes between 0.002 and 3.0 V (vs. Li/Li+) at a current density of 123 mA g-1. The initial charge-discharge capacities are 688.4 mAh g-1, 665.5 mAh g-1 for MnO2 nanoneedle and 1100.4 mAh g-1 and 855.2 mAh g-1 for MnO2/rGO

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nanocomposite electrodes. The initial discharge capacity of MnO2/rGO nanocomposite increased due to the presence of reduced graphene oxide, which enhanced the conductivity. The presence of MnO2 suppresses the aggregation of reduced graphene oxide and contributes to the higher capacity. The irreversible capacities were 3.3 % for the MnO2 nanoneedles and 22.3 % for the nanocomposite. According to previous studies [24, 35], the 22.3 % irreversible capacity of the MnO2/rGO nanocomposite was due to (i) the irreversible reaction between Li ions and residual oxygen groups on reduced graphene oxide during the charge-discharge processes, (ii) the irreversible redox reaction of MnO2 nanoneedles and Li ions, and (iii) the formation of a solid-electrolyte interface (SEI) layer caused by the electrolyte decomposition on the surface of the electrode. The cyclic performance of the as-prepared MnO2 nanoneedle and MnO2/rGO nanocomposite electrodes at a current density of 123 mA g-1 in the voltage range of 0.002 to 3.0 V is shown in Fig. 5c and Fig. 5d, respectively. Interestingly, the coulomb efficiency of the as-prepared MnO2 nanoneedle electrodes in the first charge/discharge cycle are as high as 96.7 %, which is the highest value that has yet been reported [23, 34, 37-40]. This enhancement in coulomb efficiency can be ascribed to the structure of α-type MnO2 which is constructed from double chains of [MnO6] octahedra in the form

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of 2  2 tunnels along its c-axis composed of four edge-sharing MnO6 octahedral units. This crystallographic structure possesses sufficient gaps to accommodate the lithium ions intercalation/deintercalation into the 1D tunnel structure of α-MnO2 without destroying it. The coulomb efficiency in the first charge/discharge cycle of the as-prepared MnO2/rGO nanocomposite electrodes is ~77.7 %, which is much lower than that found for the MnO2 nanoneedle. After the 20th cycle, no obvious capacity fading is observed for the nanocomposite, which indicates that a combination of MnO2 with reduced graphene oxide is an effective way to enhance the capacity and reduce its fading [1]. According to the explanations presented in Longo et al. and Fang et al. [69, 70], the sloped region of the plateaus of MnO2 may be divided into two stages with a turning point at ~1.5 V. There are four plateaus at ~2.0, ~1.25, ~0.75, and ~0.4 V, respectively. The voltage plateau at around 0.4 V can be attributed to the conversion reactions of the MnO2 nanoneedles to Mn metal with Li2O formation [29, 34, 71-74]. Moreover, the voltage plateau at around 0.75 V can be assigned to the decomposition of electrolyte and the deposition of the SEI layer, and this plateau vanished in the following cycles. The voltage plateaus at ~2.0 and 1.25 V are mainly from the reactions between Li ions and MnO2 to form LixMnO2 (x = 1, and 2). The conversion reactions between MnO2

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nanoneedle, MnO2/rGO nanocomposite and Li ions can be expressed by the following four equations: 𝑀𝑛𝑂2 + 𝐿𝑖 + + 𝑒 − ↔ 𝐿𝑖𝑀𝑛𝑂2

(1)

𝐿𝑖𝑀𝑛𝑂2 + 𝐿𝑖 + + 𝑒 − → 𝐿𝑖2 𝑀𝑛𝑂2

(2)

𝐿𝑖2 𝑀𝑛𝑂2 + 2𝐿𝑖 + 2𝑒 − → 𝑀𝑛 + 𝐿𝑖2 𝑂

(3)

𝑀𝑛 + x𝐿𝑖2 𝑂 ⟷ 𝑀𝑛𝑂𝑥 ( 0 < 𝑥 < 1 ) + 2𝑥𝐿𝑖 + + 2𝑥𝑒 −

(4)

Notably, it is worth mentioning that, for the electrodes with pure MnO2 and MnO2/rGO nanocomposite, an appreciable reversible capacity can still be maintained at 547.8 mAh g-1 and 660.9 mAh g-1 up to 40 and 50 cycles, respectively. This can be verified by the fact that combining MnO2 nanoneedles with rGO can not only maintain the intactness of the structure and accommodate the volume change upon lithiation/delithiation, but also enhance the cyclic stability and performance of LIBs.

4. CONCLUSIONS In summary, in this work α-MnO2 nanoneedle and α-MnO2/rGO nanocomposite were successfully fabricated via a facile route and their electrochemical properties were investigated for use in LIBs. α-MnO2/rGO nanocomposite not only maintained a reversible capacity of 660.9 mAh g-1 after 50 cycles at a current density of 123 mA g-1,

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which is higher than the reversible capacity of 547.8 mAh g-1 after 40 cycles of α-MnO2 nanoneedle, but also had steady cyclic performance. In particular, the α-MnO2 nanoneedle produced in this work have a very high coulomb efficiency of 96.7 % in the first cycle, which rarely occurs. The investigation of the electrochemical characteristics of the α-MnO2/rGO nanocomposite showed enhanced specific capacity and cyclic stability compared to that of α-MnO2 nanoneedle. The enhancement in the electrochemical properties of α-MnO2/rGO nanocomposite can be attributed to the synergistic effects of combining α-MnO2 with rGO. The results demonstrate that α-MnO2/rGO nanocomposite is one of the promising candidates for use as an anode material in next-generation LIBs.

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28

Table 1. Physical properties and electrochemical Li cycling data of MnO2 and MnO2/graphene nanocomposite. Fig. 1 XRD patterns of (a) MnO2/rGO, (b) MnO2 and (c) α-MnO2 PDF #44 - 0141. Fig. 2 FESEM images of as-prepared (a) MnO2 nanoneedles and (b) MnO2/rGO nanocomposite. Fig. 3. Bright field TEM images of (a) MnO2 nanoneedles, (b) MnO2/rGO nanocomposite. Nano beam electron diffraction (NBED) of (c) MnO2 nanoneedles, (d) rGO nanocomposite. HRTEM images of (e) MnO2 nanoneedles, (f) MnO2 nanoneedles from MnO2/rGO nanocomposite. Fig. 4. TGA analysis of α-MnO2 (black line) and α-MnO2/rGO nanocomposite (red line). Fig. 5. Charge/discharge curve of (a) α-MnO2 and (b) α-MnO2/rGO nanocomposite. Capacity vs. cycle number plots of (c) α-MnO2 and (d) α-MnO2/rGO nanocomposite.

Fig. 1 XRD patterns of (a) MnO2/rGO, (b) MnO2 and (c) α-MnO2 PDF #44 - 0141.

29

Fig. 2 FESEM images of as-prepared (a) MnO2 nanoneedles and (b) MnO2/rGO nanocomposites.

30

Fig. 3. Bright field TEM images of (a) MnO2 nanoneedles, (b) MnO2/rGO nanocomposite. Nano beam electron diffraction (NBED) of (c) MnO2 nanoneedles, (d) rGO nanocomposite. HRTEM images of (e) MnO2 nanoneedles, (f) MnO2 nanoneedles from MnO2/rGO nanocomposite.

31

Fig. 4. TGA analysis of α-MnO2 (black line) and α-MnO2/rGO nanocomposite (red line).

Fig. 5. Charge/discharge curve of (a) α-MnO2 and (b) α-MnO2/rGO nanocomposite. Capacity vs. cycle number plots of (c) α-MnO2 and (d) α-MnO2/rGO nanocomposite.

32

Table 1. Physical properties and electrochemical Li cycling data of MnO2 and MnO2/graphene nanocomposites. Authors

Kim SJ et

Morphology

MnO2 nanowires

al. Chen J et

MnO2 nanorods

MnO2

Current

Reversible

Voltage

Capacity

morphology/size

rate

capacity of 1st

range

retention after

Diameters: 10 to

–1

30 nm

g

Diameter: 100 nm

100 mA

Length: 10 μm

al.

123 mA

g

–1

cycle (mAh

n cycles

g–1)

(cycling range)

1573 mAh g –

0.01 to

300 mAh g –1

1

3.0 V

(n=2-100)

1119.2 mAh g

0.01 to

1074.8 mAh g –

3.0 V

1

–1

Ref.

[36]

[23]

(n=2-100) 0.02 to

602.1 mAh g –1

1

3.3 V

(n=2-20)

100 mA

1271 mAh g –

0.01 to

626 mAh g –1

g –1

1

3.3 V

(n=2-100)

Zhao J et

nanoporous γ- MnO2

MnO2 hollow.

100 mA

1300 mAh g

al.

hollow microspheres

Diameter :2.5 μm

g –1

–

[37]

and a 500 nm thick shell Li J et al.

Mesoporous γ- MnO2

Pore size: 4–50 nm

microcrystal

Li B et al.

α- MnO2 hollow

MnO2 nanorods

270 mA

746.0 mAh g

0.01 to

481 mAh g –1

urchins

Diameters: 30 nm

g –1

–1

2.0 V

(n=2-40)

Diameter: 12-18

85 mA

600 mAh g –

0.01 to

170 mAh g –1

nm

g –1

1

2.0 V

(n=2-50)

0.01

105 mAh g –1

and 3.0

(n=2-6)

[38]

[39]

Lengths: 200-300 nm. Wu M-S et

γ- MnO2

al.

(100 °C

[34]

annealing) Li L et al.

MnO2

Needle shaped structure

100 mA g

265 mAh g –1

–1

[40]

V This work

α-MnO2 nanoneedles

α- MnO2

123 mA

665.5 mAh g

0.002 to

547.8 mAh g –1

Diameter: 15~20

g –1

–1

3.0 V

(n=2-40)

123 mA

1215 mAh g –

0.01 to

1100 mAh g –1

-

nm Length: 450-550 nm Kim SJ. et

MnO2/rGO

Diameters: 10 to

33

[36]

al.

nanocomposites

30 nm

g –1

1

3.0 V

(n=2-100)

Jiang Y et

MnO2-nanorods/rGO

-

1.0 A g-1

1945.8 mAh

0.01 to

1635.3 mAh g –

3.0 V

1

al.

nanocomposites

g

–1

[41]

(n=2-450) Zhang Y et

Graphene/α-MnO2

α- MnO2 nanowire

60 mA

~1150 mAh g

0.01 to

998 mAh g –1

al.

nanocomposites

Diameter: 40–50

g –1

–1

3.0 V

(n=2-30)

746 mAh g –1

0.01 to

752 mAh g –1

3.0 V

(n=2-65)

726.5 mAh g

0.01 to

575 mAh g –1

–1

3.0 V,

(n=2-20)

753 mAh g –1

0.01 to

470 mAh g –1

3.0 V

(n=2-5)

0.01 V

495 mAh g –1

to 3.0 V

(n=2-40)

855.2 mAh g –

0.002 to

660.9 mAh g –1

1

3.0 V

(n=2-50)

[42]

nm,Length: 5–10 μm Wen K et

MnO2-graphene

MnO2

100 mA

al.

composite

nanoparticles

g –1

Xing L et al.

α- MnO2/graphene

α- MnO2

0.1 C

nanocomposites

nanosheets

MnO2 –GNRs (MG)

-

Chen J et

100 mA

al. Yu A et al.

g graphene- MnO2

MnO2 nanotubes

nanotube (NT) thin

Diameters: 70 to

film composites

80 nm.Lengths: 1

–1

100 mA g

686 mAh g –1

–1

[43]

[44]

[23]

[35]

μm and This work

α-MnO2/rGO nanocomposites

α- MnO2 nanoneedels

123 mA g

Diameter: 15~20 nm Length: 450-550 nm

34

–1

-