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Journal ofNanoscience and Nanotechnology

Vol. 13, 6199–6202, 2013

Synthesis and Electrochemical Properties ofLi0�33MnO2 Nanorods as Positive Electrode

Material for 3 V Lithium Batteries

Mok-Hwa Kim1�2, Kwang-Bum Kim2, Sun-Min Park1,Chul-Tae Lee3, and Kwang Chul Roh1�∗

1Energy Efficient Materials Team, Korea Institute of Ceramic Engineering and Technology,Seoul 153-801, Republic of Korea

2Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea3Department of Chemical Engineering, Dankook University, Gyeonggi 448-701, Republic of Korea

In this study, one-dimensional Li0�33MnO2 nanorods were synthesized by a solid state reactionusing �-MnO2 as a precursor. �-MnO2 was prepared under different reaction times by a redox pro-cess. The HR-TEM results showed that the diameter and length of the Li0�33MnO2 nanorods are5–20 nm and about 200 nm, respectively. The Li0�33MnO2 nanorods delivered a discharge capac-ity of 157 mA h g−1 at 1 C, and retained 97% of their initial capacity over 30 cycles. Good rateperformance was also observed, with discharge capacities of 201 and 133 mA h g−1 at 0.1 C and2 C, respectively. The morphology of the nanorods could increase their electrochemical properties,resulting in higher capacity and rate performance.

Keywords: Lithiated Manganese Dioxides, Nanorods, Redox Process, Lithium Batteries.

1. INTRODUCTION

3 V positive materials are efficient energy storage devicesfor small size memory back-up sources for various electri-cal devices. Lithiated MnO2, Li0�33MnO2, is such a mate-rials, and it shows a capacity of 140–190 mA h g−1

with a flat potential of 2.9 V versus Li/Li+. Severalresearch works have documented the structural stabilityand the electrochemical properties of Li0�33MnO2. Thiscompound exhibited a tendency for the capacity to fadeduring repeated charge/discharge processes and has shownpoor rate stability.1–3

Recently, nanostructured electrodes have been widelyinvestigated for enhancing the performance of lithiumion batteries. Among them, low-dimension nanostructures,nanorods, are the most attractive morphology. Nanorodmaterials can generally provide high specific surface areas,short ion diffusion pathways and efficient one-dimensionalelectron transport pathways.4–8

This study aims to improve the rate capability ofLi0�33MnO2 by using nanorod morphology in the synthesis.

We report the electrochemical performance ofLi0�33MnO2 nanorods synthesized by heating LiNO3 and�-MnO2 at a low temperature. �-MnO2 was synthesized

∗Author to whom correspondence should be addressed.

under two different reaction times by a redox process.The crystal structures of the materials were studied, andtheir electrochemical properties were investigated. Theinfluences of the precursor reaction time on the morphol-ogy of the Li0�33MnO2 were investigated. The correlationbetween the nanorod morphology and the improvedelectrochemical performance was also discussed.

2. EXPERIMENTAL DETAILS

Nanorods of �-MnO2 were prepared from MnSO4 andMnCO3 using NaClO3 as an oxidizing agent. Stoichiomet-ric amounts of MnSO4, MnCO3, and NaClO3 were dis-solved in distilled water and sulfuric acid was added. Themixture was heated at 80 �C for two different reactiontimes of 1 and 2 h. After the reaction was complete, thedark-brown products were filtered, washed several times,and dried at 60 �C. Li0�33MnO2 nanorods were synthe-sized by a solid state reaction using the prepared �-MnO2

with LiNO3 (Li/Mn molar ratio of 0.33) and preheated at280 �C for 5 h, and finally calcined at 350 �C for 12 h.The crystalline phases were identified by X-ray

diffraction (XRD, D/Max 2500/PC, Rigaku, Japan).Morphological studies were conducted using highresolution-transmission electron microscopy (HR-TEM,

J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 9 1533-4880/2013/13/6199/004 doi:10.1166/jnn.2013.7699 6199

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Synthesis and Electrochemical Properties of Li0�33MnO2 Nanorods as Positive Electrode Material Kim et al.

JEOL, JEM-2000EX, Japan). The nitrogen adsorption/desorption isotherms and specific surface area of thematerials were measured with Belsorp-Mini II equipment(Bel Inc., Japan). X-ray photoelectron spectroscopy (XPS,PHI 5000 VersaProbe, ULVAC-PHI, Japan) measurementswere also performed.The electrochemical performances of the samples were

investigated with a two-electrode coin-type cell withlithium foil as the counter electrode. The working elec-trode was composed of 80 wt% active material, 10 wt%carbon black, and 10 wt% polyvinylidene fluoride binder.The electrolyte was a solution of 1.0 M LiPF6 withethylene carbonate/ethyl-methyl carbonate (EC/EMC =1/1 vol%). The galvanostatic charge and discharge cycletests of the cells were carried out at room temperaturebetween 2 and 4 V.

3. RESULTS AND DISCUSSION

Figures 1(b), (d) show the XRD patterns of the Li0�33MnO2

nanorods made with the two different reaction times of the�-MnO2 (Figs. 1(a), (c)). The patterns of the Li0�33MnO2

appear at 2� = 17�6�, 21�6�, 37.0�, 41.7�, 49.3�, 52.8�,53.3�, and 66.1�, which are very different from the featuresof the �-MnO2. The positions of the diffraction peaks andtheir relative intensities agree well with those of previousreports.9�10

Figure 2 shows the morphology of the as-preparedLi0�33MnO2 nanorods. It can be seen that the reaction timeof the precursor has a large effect on the morphology of thenanorods. With the increase in the reaction time, nanorodswith higher aspect ratios were observed. As can be seenfrom Figure 2(a), nanorods with a reaction time of 1 hwere 15–20 nm in diameter and 40–120 nm in length.When the reaction time was prolonged to 2 h (Fig. 2(b)),the nanorods were longer. They had diameters of 5–20 nmand lengths of 150–200 nm.

Fig. 1. XRD patterns of (a) �-MnO2-1 h, (b) Li0�33MnO2 nanorods pre-pared with �-MnO2-1 h precursor, (c) �-MnO2-2 h, and (d) Li0�33MnO2

nanorods prepared with �-MnO2-2 h precursor.

Fig. 2. HR-TEM images for Li0�33MnO2 nanorods with different�-MnO2 reaction times, (a) 1 h (with an inset of low-magnification TEM)and (b) 2 h (with an inset of low-magnification TEM).

Figure 3 shows the nitrogen adsorption–desorptionisotherms of the samples. The samples have a specificsurface area of 34.1 m2 g−1 for Li0�33MnO2-1 h and37.5 m2 g−1 for Li0�33MnO2-2 h, depending on the reactiontime of precursors. The high specific surface area of thesamples could increase the electrode–electrolyte contactarea and thus decrease the current density per unit surfacearea, leading to its high electrochemical properties.1�11

Figure 4 shows the Mn XPS core spectra for the sam-ples. The binding energy was calibrated with reference tothe C1s level of carbon (284.5 eV). There are two peaks inthe Mn 2p core level spectrum, which are ascribed to thespin-orbit splitting of the components Mn 2p3/2 and Mn2p1/2. The major peak for Mn 2p3/2 is centered at 642.2 eVand corresponds to the Mn4+ ions in the compound.12 TheXPS results show that the predominant oxidation states ofMn in the samples are 4. In particular, the Mn 2p3/2 peakposition for Li0�33MnO2-2 h shifts towards higher binding

(a) (b)

Fig. 3. Nitrogen adsorption–desorption isotherms of the samples,(a) Li0�33MnO2-1 h and (b) Li0�33MnO2-2 h.

6200 J. Nanosci. Nanotechnol. 13, 6199–6202, 2013

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Kim et al. Synthesis and Electrochemical Properties of Li0�33MnO2 Nanorods as Positive Electrode Material

Fig. 4. The XPS spectra of Mn 2p for samples.

energies, owing to the (+4) oxidation state of the Mncations.13

Figure 5 shows voltage profiles of the rate capabilitiesof the samples at different charge–discharge rates. The firstdischarge capacities of Li0�33MnO2-1 h and Li0�33MnO2-2 hare 189 and 201 mA h g−1, respectively. The discharge

(a)

(b)

Fig. 5. Voltage profiles of samples at different current rates between 2and 4 V; (a) Li0�33MnO2-1 h and (b) Li0�33MnO2-2 h.

Fig. 6. Cycle life performance of samples at a 1 C charge–dischargerate.

capacity of the Li0�33MnO2-1 h nanorods decreases to146, 123, 105, 80, and 61 mA h g−1 as the current rateincreases to 0.5, 1, 2, 5, and 10 C, whereas the dischargecapacity of the Li0�33 MnO2-2 h nanorods is 182, 156,133, 105, and 83 mA h g−1 at these same current rates.The Li0�33MnO2-2 h nanorods exhibit higher initial dis-charge capacity and rate capability than Li0�33MnO2-1 h.The BET surface area of the Li0�33MnO2-1 h nanorods islower than that of the Li0�33MnO2-2 h nanorods, and thus,the reduced electrolyte contact area leads to a decreasein the capacity decay at a higher C rate. This advan-tage results from the one-dimensional efficient electrontransport of the Li0�33MnO2-2 h along the length of eachrod. In addition, the small diameters the Li0�33MnO2-2 hnanorods contribute to improving the rate capability owingto the short lithium ion diffusion distance.14�15 The ini-tial discharge capacity of the nanorods synthesized inthis study was higher than that of previously synthesizedLi0�33MnO2 (194 mA h g−1� and Li0�33MnO2 nanorods(199 mA h g−1�.2�4 In addition, our present results weresuperior to He et al.’s work with respect to the rods’rate capability. Apparently, the nanorod morphology ofLi0�33MnO2 benefits from the shorter lithium diffusion pathand larger contact area between active material and elec-trolyte, which promises higher discharge capacity.Figure 6 shows the cycling behaviors of the samples for

30 cycles. The initial discharge capacity of the Li0�33MnO2-2 h nanorods is 157 mA h g−1, which is higher than that ofLi0�33MnO2-1 h (128 mA h g−1�. Li0�33MnO2-2 h has a highcapacity retention of 97% after 30 cycles, compared with93% for Li0�33MnO2-1 h. This is because the oxidationstate of Mn in Li0�33MnO2 indicates 4+ with the increasein the reaction time of the precursor, as revealed by XPS.16

4. CONCLUSION

In summary, �-MnO2 nanorods were used to success-fully synthesize Li0�33MnO2 nanorods at 350

�C. Morphol-ogy control of the Li0�33MnO2 nanorods was demonstrated

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using precursors with 1-h and 2-h reaction times. Nanorodswith a precursor reaction time of 1 h had diameters of15–20 nm and lengths of 40–120 nm. When the reac-tion time was increased to 2 h, the nanorods obtained haddiameters of 5–20 nm and lengths of 150–200 nm. TheLi0�33MnO2-2 h nanorods exhibited a discharge capacityof 157 mA h g−1 with capacity retention of 97% over30 cycles. They also showed good rate capability. Theimproved electrochemical performance comes from thenanorods, which decreased the diffusion lengths of boththe lithium and the electrons.

Acknowledgment: This work was supported by theAdvanced Technology Center program (No. 10035919)under the Ministry of Knowledge Economy, Republic ofKorea.

References and Notes

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3. M. Yoshio, H. Nakamura, and Y. Xia, Electrochim. Acta 45, 273(1999).

4. P. He, J.-Y. Luo, X.-H. Yang, and Y.-Y. Xia, Electrochim. Acta54, 7345 (2009).

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16. M. Yoshio, H. Noguchi, T. Miyashita, H. Nakamura, and A. Kozawa,J. Power Sources 54, 483 (1995).

Received: 6 October 2012. Accepted: 12 January 2013.

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