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Nano-sized lithium manganese oxide dispersed on carbon nanotubes for energy storage applications
Electrochemistry Communications 11 (2009) 1575–1578
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Nano-sized lithium manganese oxide dispersed on carbon nanotubes for energy storage applications Sang-Bok Ma a, Kyung-Wan Nam b, Won-Sub Yoon c,*, Seong-Min Bak a, Xiao-Qing Yang b, Byung-Won Cho d, Kwang-Bum Kim a,* a
Department of Material Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-gu, Seoul 120-749, Republic of Korea Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, USA c School of Advanced Materials Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea d Battery Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 23 March 2009 Received in revised form 8 May 2009 Accepted 29 May 2009 Available online 23 June 2009 Keywords: Batteries Carbon nanotube Lithium manganese oxide Nanocomposite Nanoparticle
a b s t r a c t Nano-sized lithium manganese oxide (LMO) dispersed on carbon nanotubes (CNT) has been synthesized successfully via a microwave-assisted hydrothermal reaction at 200 °C for 30 min using MnO2-coated CNT and an aqueous LiOH solution. The initial specific capacity is 99.4 mAh/g at a 1.6 C-rate, and is maintained at 99.1 mAh/g even at a 16 C-rate. The initial specific capacity is also maintained up to the 50th cycle to give 97% capacity retention. The LMO/CNT nanocomposite shows excellent power performance and good structural reversibility as an electrode material in energy storage systems, such as lithium-ion batteries and electrochemical capacitors. This synthetic strategy opens a new avenue for the effective and facile synthesis of lithium transition metal oxide/CNT nanocomposite. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries are used widely in portable electronic devices, such as cellular phones, notebook computers and portable recorders owing to their high capacity, high voltage, portability and long cycle life. However, new applications, such as electric and hybrid vehicles, require both high rate capability and high capacity. Nano-sized metal oxides are expected to improve the high rate capability of cathode materials for lithium-ion batteries because of the considerably higher effective interfacial area between the nano-sized metal oxide and electrolyte, and its shorter lithium diffusion length during charge/discharge [1]. Since nano-sized metal oxides tend to agglomerate, good dispersion and improved conductivity of an electrode composed of nano-sized oxides is very important for achieving high rate capability [2]. In this study, carbon nanotubes (CNTs) were used to disperse nano-sized metal oxide and connect the nanoparticles along the one-dimensional conduction path due to their chemical stability, good conductivity and large surface area. Furthermore, the CNTs are strongly entangled, providing a network of open mesopores. * Corresponding authors. Tel.: +82 2 910 4664; fax: +82 2 910 4320 (W.-S. Yoon), tel.: +82 2 2123 2839; fax: +82 2 312 5375 (K.-B. Kim). E-mail addresses: [email protected], [email protected] (W.-S. Yoon), [email protected] (K.-B. Kim). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.05.058
This porous entangled structure of CNTs allows lithium ions to have access readily to reaction sites through the interconnected pore structure. In addition, an increase in contact area between the nano-sized metal oxides and CNTs can improve the electric conductivity of the electrode [3–5]. The successful synthesis of nanolayer MnO2-coated CNT (MnO2/ CNT nanocomposite) indicated the possibility of improving the high rate capability of the cathode material for lithium-ion batteries [6]. This study reports the synthesis of lithium manganese oxide (LMO) nanoparticles dispersed on CNTs (LMO/CNT nanocomposite) via a microwave-assisted hydrothermal treatment of MnO2-coated CNTs in an aqueous LiOH solution. The LMO/CNT nanocomposite showed excellent high rate capability and good structural reversibility for energy storage applications.
2. Experimental The LMO/CNT nanocomposite was synthesized by a microwaveassisted hydrothermal reaction using a MnO2/CNT nanocomposite and LiOH aqueous solution. MnO2 was first deposited on CNTs (multi-walled CNTs, ILJIN Nanotech, S = 200 m2/g) through a direct redox reaction between the CNTs and permanganate ions. The detailed procedure for preparing the MnO2/CNT nanocomposites is reported elsewhere [6]. Second, 0.3 g of a MnO2/CNT nanocomposite was added to 40 ml of a 0.1 M LiOH aqueous solution, after
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rapidly heating the mixed solution to 200 °C in a microwave-assisted hydrothermal reactor (MARS, CEM Corp.) During synthesis, the solution temperature was maintained at 200 °C for 30 min. The suspension was filtered, washed several times with distilled water, and dried at 100 °C for 24 h in a vacuum oven. The crystalline phase of the LMO/CNT nanocomposite was determined by X-ray diffraction (XRD) using a diffractometer (D/ MAX-IIIC, Rigaku) equipped with a vertical goniometer. The XRD patterns were taken at room temperature in the 2h range of 10° < 2h < 80° at intervals of 0.02°. Transmission electron microscopy (TEM) (JEM2100F, JEOL) was used to observe the morphology of the LMO/CNT nanocomposite powders. The electrochemical properties were determined using a three-electrode electrochemical cell, with two lithium foils as the counter and reference electrodes. The working electrode consisted of a mixture of 67 wt% LMO/CNT nanocomposite, 28 wt% acetylene black as the conducting agent and 5 wt% polyvinylidene fluoride dissolved in N-methylpyrrolidone as the binder. As a slurry, the mixture was coated onto titanium foil and then dried at 100 °C for 12 h. Each working electrode with a 1 1 cm2 area contained 1 mg of the dried slurry. The charge/discharge and CV were carried out at between 3.5 and 4.5 V (vs. Li/Li+) using a potentiostat/galvanostat (VMP2, PRINCETON APPLIED RESEARCH). The electrolyte was 1 M LiClO4 in PC. 3. Results and discussion Fig. 1 shows TEM images of the MnO2/CNT and LMO/CNT nanocomposites. The synthesis mechanism of the MnO2/CNT nanocomposite is described elsewhere [6]. A 10–20 nm nanolayer of MnO2 was coated uniformly onto CNTs by a spontaneous direct redox reaction between the CNTs and permanganate ions in an aqueous solution (Fig. 1a). The LMO/CNT nanocomposite (64 wt% of LMO) was synthesized using a microwave-assisted hydrothermal reaction of MnO2/CNT nanocomposite in an aqueous LiOH solution. LMO nanoparticles, 20–30 nm in diameter, formed along the CNTs in the entangled CNT network (Fig. 1b). The formation of LMO nanoparticles from the MnO2 on CNTs by the microwave-assisted hydrothermal reaction was attributed to the following reactions:
MnO2 þ 2H2 O ! Mn4þ þ 4OH þ
8Mn4þ þ 4Li þ 36OH ! 4LiMn2 O4 þ 18H2 O þ O2
ð1Þ ð2Þ
During the microwave-assisted hydrothermal synthesis of the LMO/CNT nanocomposite, the MnO2 on the CNTs dissolved as aqueous Mn4+ ions [7,8] and reprecipitated preferentially as nanocrystalline LMO nanoparticles on the external surface of the CNTs (Fig. 1b). Microwave-assisted hydrothermal synthesis is a simple and fast method for synthesizing LMO nanoparticles on CNTs [9]. The conductive electrons of the CNTs can be accelerated by absorbing microwaves [10]. These accelerated electrons increase the vibrations of the carbon lattice, resulting in a higher temperature on the CNTs surface than the aqueous media. The LMO particles tend to precipitate heterogeneously on the CNT surfaces within a short period of time because of the selectively-heated CNTs by microwave irradiation. LMO nanoparticles could be prepared in less than 30 min by the microwave-assisted hydrothermal process. In contrast, Jiang et al. reported that the synthesis of LiMn2O4 by a hydrothermal treatment of MnO2 required as much as 1–7 days [8]. Considering the thickness of the MnO2 thin layer on CNTs in Fig. 1a and the size of the LMO nanoparticles in the nanocomposite in Fig. 1b, some CNTs were not coated with LMO in the LMO/CNT nanocomposites. Some exposed CNTs are more likely to provide conduction paths in contact with the LMO-coated CNTs and con-
Fig. 1. TEM images of (a) MnO2/CNT and (b) LMO/CNT nanocomposite synthesized by a microwave-assisted hydrothermal reaction using MnO2/CNT nanocomposite and aqueous LiOH solution.
tribute to the electrical conductivity of the LMO/CNT nanocomposite electrode (Fig. 1b), which can lead to improved high rate capability of the LMO/CNT nanocomposite electrode. Fig. 2 shows the XRD patterns of the CNTs, MnO2/CNT [6,11] and LMO/CNT nanocomposites as well as spinel LiMn2O4 (JCPDS No. 35-0782) as a reference. The main XRD peaks (Fig. 2c) could be indexed to spinel LiMn2O4, by considering the standard data of JCPDS No. 35-0782 (Fig. 2d). This confirmed the successful synthesis of the LMO/CNT nanocomposite from the MnO2/CNT nanocomposite (Fig. 2b) by a microwave-assisted hydrothermal reaction. The unit cell parameter and volume of the spinel LMO nanoparticles calculated from the XRD data were 8.166 Å and 544.5 Å3, the unit cell parameter and volume of the reference were 8.247 Å and 560.9 Å3 of JCPDS No. 35-0782, respectively. The Li/Mn ratio of the spinel LMO nanoparticle was 0.74, as determined by inductively-coupled plasma (ICP) spectroscopy, which is believed to be the main reason for the smaller unit cell volume of the LMO nanoparticles compared to that of the standard data
S.-B. Ma et al. / Electrochemistry Communications 11 (2009) 1575–1578
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Fig. 3. (a) Discharge profiles at a different C-rate and (b) normalized specific capacity of the LMO/CNT nanocomposite during cycling between 3.5 and 4.5 V (vs. Li/Li+).
Fig. 2. XRD patterns of (a) CNT; (b) MnO2/CNT nanocomposite; (c) LMO/CNT nanocomposite; and (d) spinel LiMn2O4 (JCPDS No. 35-0782).
of JCPDS No. 35-0782. The structure of spinel LiMn2O4 has the space group Fd3m, which consists of close-packed oxygen where lithium and manganese ions occupy the tetrahedral 8a and octahedral 16d sites, respectively [12–14]. When excess lithium ions occupy some of the octahedral 16d sites, Li1 + xMn2xO4 becomes a lithium-rich spinel and the unit cell parameter a0 decreases with increasing lithium and Mn4+ content in spinel Li1 + xMn2xO4 due to the smaller ionic radii of Mn4+ (0.53 Å) than Mn3+ (0.65 Å) [14,15]. Fig. 3a shows the discharge curves of the LMO/CNT nanocomposite. Charge and discharge were performed between 3.5 and 4.5 V (vs. Li/Li+) at a 1.6 and 16 C-rate, respectively. The discharge curves of the LMO/CNT nanocomposites showed the typical discharge behavior of spinel LiMn2O4. The initial specific capacity based on the LMO weight was 99.4 mAh/g at 1.6 C-rate. A specific capacity of 99.1 mAh/g was achieved even at 16 C-rate, which suggests excellent high rate capability of the LMO/CNT nanocomposite. Lithium-rich LMO is known to be good at high rate capability. In this study, we could further improve its high rate capability of Li-rich LMO through the synthesis of LMO nanoparticle/CNT nanocomposite with the following features: (1) the shorter diffusion length of the nano-sized LMO; (2) the well-dispersed LMO nanoparticles on CNTs; (3) the large interfacial area between the LMO and electrolytic solution; (4) the high electrolyte-accessibility due to the interconnected pores of the entangled CNTs; (5) the high electrical conductivity of CNT. Cyclic voltammetry (CV) was used to determine the capacity fading of the LMO/CNT nanocomposite electrode. The electrode potential was scanned at a scan rate of 10 mV/s between 3.5 and 4.5 V
(vs. Li/Li+) in the both anodic and cathodic directions. Fig. 3b shows the cyclability of the LMO/CNT nanocomposite electrode. The specific capacity was 107 mAh/g based on the LMO weight in the first cycle, which was maintained at 104 mAh/g at the 50th cycle to give capacity retention of 97%. In previous reports, lithium-rich spinel compounds with a smaller cell volume usually showed good rechargeability due to the higher oxidation state of manganese ions, which can reduce the Jahn–Teller effect [16,17]. In addition, well-dispersed LMO nanoparticles in the LMO/CNT nanocomposite can better accommodate the strain of Li ion insertion/extraction even at a high potential scan rate. 4. Conclusion The LMO/CNT nanocomposite was synthesized by a microwaveassisted hydrothermal reaction at 200 °C for 30 min using MnO2/ CNT nanocomposite and 0.1 M LiOH. The MnO2-coated uniformly on CNT transformed to well-dispersed LMO nanoparticles on CNT. The initial specific capacity based on the LMO weight was 99.4 mAh/g at 1.6 C-rate, and was maintained at 99.1 mAh/g even at 16 C-rate, indicating excellent high rate capability. The initial specific capacity was 107 mAh/g at a scan rate of 10 mV/s, which was maintained to 104 mAh/g up to the 50th cycle. The LMO/ CNT nanocomposite showed excellent power performance and good structural reversibility as an electrode material in energy storage systems, such as lithium-ion batteries and electrochemical capacitors, due to the following: the shorter diffusion length of the nano-sized LMO well-dispersed on the CNTs; the high interfacial area between the LMO and electrolytic solution; the interconnected pores in the three-dimensional entangled structure of CNTs; and the good electrical conductivity. This synthetic strategy opens a new avenue for the effective and facile synthesis of lithium transition metal oxide/CNT nanocomposites.
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Acknowledgements This work was supported by Korea Science and Engineering Foundation (KOSEF) through the National Research Lab. Program funded by the Ministry of Education, Science and Technology (No. R0A-2007-000-10042-0). This work at Kookmin University was supported by research program 2008 of Kookmin University in Korea. The work at BNL was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, under the program of ‘‘Hybrid and Electric Systems”, of the US Department of Energy under Contract No. DEAC0298CH10886. References [1] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W.V. Schalkwijk, Nat. Mater. 4 (2005) 366. [2] C. Jiang, M. Ichihara, I. Honma, H. Zhou, Electrochim. Acta 52 (2007) 6470.
[3] K.W. Nam, E.S. Lee, J.H. Kim, Y.H. Lee, K.B. Kim, J. Electrochem. Soc. 152 (2005) A2123. [4] I.H. Kim, J.H. Kim, K.B. Kim, Electrochem. Solid-State Lett. 8 (2005) A369. [5] S.B. Ma, K.W. Nam, W.S. Yoon, X.Q. Yang, K.Y. Ahn, K.H. Oh, K.B. Kim, J. Power Sources 178 (2008) 483. [6] S.B. Ma, K.Y. Ahn, E.S. Lee, K.H. Oh, K.B. Kim, Carbon 45 (2007) 375. [7] Q. Feng, H. Kanoh, Y. Miyai, K. Ooi, Chem. Mater. 7 (1995) 1226. [8] C.H. Jiang, S.X. Dou, H.K. Liu, M. Ichihara, H.S. Zhou, J. Power Sources 172 (2007) 410. [9] S. Komarneni, R. Roy, Q.H. Li, Mater. Res. Bull. 27 (1992) 1393. [10] M. Tsuji, M. kubokawa, R. Yano, N. Miyamae, T. Tsuji, M.S. Jun, S. Hong, S. Lim, S. Yoon, I. Mochida, Langmuir 23 (2008) 387. [11] C. Julien, M. Massot, R. Baddour-Hadjean, S. Franger, S. Bach, J.P. PereiraRamos, Solid State Ionics 159 (2003) 345. [12] M.H. Rossouw, A. de Kock, L.A. de Picciotto, M.M. Thackeray, W.I.F. David, R.M. Ibberson, Mater. Res. Bull. 25 (1990) 173. [13] A. de Kock, M.H. Rossouw, L.A. de Picciotto, M.M. Thackeray, W.I.F. David, R.M. Ibberson, Mater. Res. Bull. 25 (1990) 657. [14] Y. Xia, M. Yoshio, J. Electrochem. Soc. 144 (1997) 4186. [15] J. Cho, M.M. Thackeray, J. Electrochem. Soc. 146 (1999) 3577. [16] M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2558. [17] Y. Xia, M. Yoshio, J. Electrochem. Soc. 143 (1996) 825.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Nano-sized lithium manganese oxide dispersed on carbon nanotubes for energy storage applications Sang-Bok Ma a, Kyung-Wan Nam b, Won-Sub Yoon c,*, Seong-Min Bak a, Xiao-Qing Yang b, Byung-Won Cho d, Kwang-Bum Kim a,* a
Department of Material Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-gu, Seoul 120-749, Republic of Korea Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, USA c School of Advanced Materials Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea d Battery Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 23 March 2009 Received in revised form 8 May 2009 Accepted 29 May 2009 Available online 23 June 2009 Keywords: Batteries Carbon nanotube Lithium manganese oxide Nanocomposite Nanoparticle
a b s t r a c t Nano-sized lithium manganese oxide (LMO) dispersed on carbon nanotubes (CNT) has been synthesized successfully via a microwave-assisted hydrothermal reaction at 200 °C for 30 min using MnO2-coated CNT and an aqueous LiOH solution. The initial specific capacity is 99.4 mAh/g at a 1.6 C-rate, and is maintained at 99.1 mAh/g even at a 16 C-rate. The initial specific capacity is also maintained up to the 50th cycle to give 97% capacity retention. The LMO/CNT nanocomposite shows excellent power performance and good structural reversibility as an electrode material in energy storage systems, such as lithium-ion batteries and electrochemical capacitors. This synthetic strategy opens a new avenue for the effective and facile synthesis of lithium transition metal oxide/CNT nanocomposite. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries are used widely in portable electronic devices, such as cellular phones, notebook computers and portable recorders owing to their high capacity, high voltage, portability and long cycle life. However, new applications, such as electric and hybrid vehicles, require both high rate capability and high capacity. Nano-sized metal oxides are expected to improve the high rate capability of cathode materials for lithium-ion batteries because of the considerably higher effective interfacial area between the nano-sized metal oxide and electrolyte, and its shorter lithium diffusion length during charge/discharge [1]. Since nano-sized metal oxides tend to agglomerate, good dispersion and improved conductivity of an electrode composed of nano-sized oxides is very important for achieving high rate capability [2]. In this study, carbon nanotubes (CNTs) were used to disperse nano-sized metal oxide and connect the nanoparticles along the one-dimensional conduction path due to their chemical stability, good conductivity and large surface area. Furthermore, the CNTs are strongly entangled, providing a network of open mesopores. * Corresponding authors. Tel.: +82 2 910 4664; fax: +82 2 910 4320 (W.-S. Yoon), tel.: +82 2 2123 2839; fax: +82 2 312 5375 (K.-B. Kim). E-mail addresses: [email protected], [email protected] (W.-S. Yoon), [email protected] (K.-B. Kim). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.05.058
This porous entangled structure of CNTs allows lithium ions to have access readily to reaction sites through the interconnected pore structure. In addition, an increase in contact area between the nano-sized metal oxides and CNTs can improve the electric conductivity of the electrode [3–5]. The successful synthesis of nanolayer MnO2-coated CNT (MnO2/ CNT nanocomposite) indicated the possibility of improving the high rate capability of the cathode material for lithium-ion batteries [6]. This study reports the synthesis of lithium manganese oxide (LMO) nanoparticles dispersed on CNTs (LMO/CNT nanocomposite) via a microwave-assisted hydrothermal treatment of MnO2-coated CNTs in an aqueous LiOH solution. The LMO/CNT nanocomposite showed excellent high rate capability and good structural reversibility for energy storage applications.
2. Experimental The LMO/CNT nanocomposite was synthesized by a microwaveassisted hydrothermal reaction using a MnO2/CNT nanocomposite and LiOH aqueous solution. MnO2 was first deposited on CNTs (multi-walled CNTs, ILJIN Nanotech, S = 200 m2/g) through a direct redox reaction between the CNTs and permanganate ions. The detailed procedure for preparing the MnO2/CNT nanocomposites is reported elsewhere [6]. Second, 0.3 g of a MnO2/CNT nanocomposite was added to 40 ml of a 0.1 M LiOH aqueous solution, after
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rapidly heating the mixed solution to 200 °C in a microwave-assisted hydrothermal reactor (MARS, CEM Corp.) During synthesis, the solution temperature was maintained at 200 °C for 30 min. The suspension was filtered, washed several times with distilled water, and dried at 100 °C for 24 h in a vacuum oven. The crystalline phase of the LMO/CNT nanocomposite was determined by X-ray diffraction (XRD) using a diffractometer (D/ MAX-IIIC, Rigaku) equipped with a vertical goniometer. The XRD patterns were taken at room temperature in the 2h range of 10° < 2h < 80° at intervals of 0.02°. Transmission electron microscopy (TEM) (JEM2100F, JEOL) was used to observe the morphology of the LMO/CNT nanocomposite powders. The electrochemical properties were determined using a three-electrode electrochemical cell, with two lithium foils as the counter and reference electrodes. The working electrode consisted of a mixture of 67 wt% LMO/CNT nanocomposite, 28 wt% acetylene black as the conducting agent and 5 wt% polyvinylidene fluoride dissolved in N-methylpyrrolidone as the binder. As a slurry, the mixture was coated onto titanium foil and then dried at 100 °C for 12 h. Each working electrode with a 1 1 cm2 area contained 1 mg of the dried slurry. The charge/discharge and CV were carried out at between 3.5 and 4.5 V (vs. Li/Li+) using a potentiostat/galvanostat (VMP2, PRINCETON APPLIED RESEARCH). The electrolyte was 1 M LiClO4 in PC. 3. Results and discussion Fig. 1 shows TEM images of the MnO2/CNT and LMO/CNT nanocomposites. The synthesis mechanism of the MnO2/CNT nanocomposite is described elsewhere [6]. A 10–20 nm nanolayer of MnO2 was coated uniformly onto CNTs by a spontaneous direct redox reaction between the CNTs and permanganate ions in an aqueous solution (Fig. 1a). The LMO/CNT nanocomposite (64 wt% of LMO) was synthesized using a microwave-assisted hydrothermal reaction of MnO2/CNT nanocomposite in an aqueous LiOH solution. LMO nanoparticles, 20–30 nm in diameter, formed along the CNTs in the entangled CNT network (Fig. 1b). The formation of LMO nanoparticles from the MnO2 on CNTs by the microwave-assisted hydrothermal reaction was attributed to the following reactions:
MnO2 þ 2H2 O ! Mn4þ þ 4OH þ
8Mn4þ þ 4Li þ 36OH ! 4LiMn2 O4 þ 18H2 O þ O2
ð1Þ ð2Þ
During the microwave-assisted hydrothermal synthesis of the LMO/CNT nanocomposite, the MnO2 on the CNTs dissolved as aqueous Mn4+ ions [7,8] and reprecipitated preferentially as nanocrystalline LMO nanoparticles on the external surface of the CNTs (Fig. 1b). Microwave-assisted hydrothermal synthesis is a simple and fast method for synthesizing LMO nanoparticles on CNTs [9]. The conductive electrons of the CNTs can be accelerated by absorbing microwaves [10]. These accelerated electrons increase the vibrations of the carbon lattice, resulting in a higher temperature on the CNTs surface than the aqueous media. The LMO particles tend to precipitate heterogeneously on the CNT surfaces within a short period of time because of the selectively-heated CNTs by microwave irradiation. LMO nanoparticles could be prepared in less than 30 min by the microwave-assisted hydrothermal process. In contrast, Jiang et al. reported that the synthesis of LiMn2O4 by a hydrothermal treatment of MnO2 required as much as 1–7 days [8]. Considering the thickness of the MnO2 thin layer on CNTs in Fig. 1a and the size of the LMO nanoparticles in the nanocomposite in Fig. 1b, some CNTs were not coated with LMO in the LMO/CNT nanocomposites. Some exposed CNTs are more likely to provide conduction paths in contact with the LMO-coated CNTs and con-
Fig. 1. TEM images of (a) MnO2/CNT and (b) LMO/CNT nanocomposite synthesized by a microwave-assisted hydrothermal reaction using MnO2/CNT nanocomposite and aqueous LiOH solution.
tribute to the electrical conductivity of the LMO/CNT nanocomposite electrode (Fig. 1b), which can lead to improved high rate capability of the LMO/CNT nanocomposite electrode. Fig. 2 shows the XRD patterns of the CNTs, MnO2/CNT [6,11] and LMO/CNT nanocomposites as well as spinel LiMn2O4 (JCPDS No. 35-0782) as a reference. The main XRD peaks (Fig. 2c) could be indexed to spinel LiMn2O4, by considering the standard data of JCPDS No. 35-0782 (Fig. 2d). This confirmed the successful synthesis of the LMO/CNT nanocomposite from the MnO2/CNT nanocomposite (Fig. 2b) by a microwave-assisted hydrothermal reaction. The unit cell parameter and volume of the spinel LMO nanoparticles calculated from the XRD data were 8.166 Å and 544.5 Å3, the unit cell parameter and volume of the reference were 8.247 Å and 560.9 Å3 of JCPDS No. 35-0782, respectively. The Li/Mn ratio of the spinel LMO nanoparticle was 0.74, as determined by inductively-coupled plasma (ICP) spectroscopy, which is believed to be the main reason for the smaller unit cell volume of the LMO nanoparticles compared to that of the standard data
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Fig. 3. (a) Discharge profiles at a different C-rate and (b) normalized specific capacity of the LMO/CNT nanocomposite during cycling between 3.5 and 4.5 V (vs. Li/Li+).
Fig. 2. XRD patterns of (a) CNT; (b) MnO2/CNT nanocomposite; (c) LMO/CNT nanocomposite; and (d) spinel LiMn2O4 (JCPDS No. 35-0782).
of JCPDS No. 35-0782. The structure of spinel LiMn2O4 has the space group Fd3m, which consists of close-packed oxygen where lithium and manganese ions occupy the tetrahedral 8a and octahedral 16d sites, respectively [12–14]. When excess lithium ions occupy some of the octahedral 16d sites, Li1 + xMn2xO4 becomes a lithium-rich spinel and the unit cell parameter a0 decreases with increasing lithium and Mn4+ content in spinel Li1 + xMn2xO4 due to the smaller ionic radii of Mn4+ (0.53 Å) than Mn3+ (0.65 Å) [14,15]. Fig. 3a shows the discharge curves of the LMO/CNT nanocomposite. Charge and discharge were performed between 3.5 and 4.5 V (vs. Li/Li+) at a 1.6 and 16 C-rate, respectively. The discharge curves of the LMO/CNT nanocomposites showed the typical discharge behavior of spinel LiMn2O4. The initial specific capacity based on the LMO weight was 99.4 mAh/g at 1.6 C-rate. A specific capacity of 99.1 mAh/g was achieved even at 16 C-rate, which suggests excellent high rate capability of the LMO/CNT nanocomposite. Lithium-rich LMO is known to be good at high rate capability. In this study, we could further improve its high rate capability of Li-rich LMO through the synthesis of LMO nanoparticle/CNT nanocomposite with the following features: (1) the shorter diffusion length of the nano-sized LMO; (2) the well-dispersed LMO nanoparticles on CNTs; (3) the large interfacial area between the LMO and electrolytic solution; (4) the high electrolyte-accessibility due to the interconnected pores of the entangled CNTs; (5) the high electrical conductivity of CNT. Cyclic voltammetry (CV) was used to determine the capacity fading of the LMO/CNT nanocomposite electrode. The electrode potential was scanned at a scan rate of 10 mV/s between 3.5 and 4.5 V
(vs. Li/Li+) in the both anodic and cathodic directions. Fig. 3b shows the cyclability of the LMO/CNT nanocomposite electrode. The specific capacity was 107 mAh/g based on the LMO weight in the first cycle, which was maintained at 104 mAh/g at the 50th cycle to give capacity retention of 97%. In previous reports, lithium-rich spinel compounds with a smaller cell volume usually showed good rechargeability due to the higher oxidation state of manganese ions, which can reduce the Jahn–Teller effect [16,17]. In addition, well-dispersed LMO nanoparticles in the LMO/CNT nanocomposite can better accommodate the strain of Li ion insertion/extraction even at a high potential scan rate. 4. Conclusion The LMO/CNT nanocomposite was synthesized by a microwaveassisted hydrothermal reaction at 200 °C for 30 min using MnO2/ CNT nanocomposite and 0.1 M LiOH. The MnO2-coated uniformly on CNT transformed to well-dispersed LMO nanoparticles on CNT. The initial specific capacity based on the LMO weight was 99.4 mAh/g at 1.6 C-rate, and was maintained at 99.1 mAh/g even at 16 C-rate, indicating excellent high rate capability. The initial specific capacity was 107 mAh/g at a scan rate of 10 mV/s, which was maintained to 104 mAh/g up to the 50th cycle. The LMO/ CNT nanocomposite showed excellent power performance and good structural reversibility as an electrode material in energy storage systems, such as lithium-ion batteries and electrochemical capacitors, due to the following: the shorter diffusion length of the nano-sized LMO well-dispersed on the CNTs; the high interfacial area between the LMO and electrolytic solution; the interconnected pores in the three-dimensional entangled structure of CNTs; and the good electrical conductivity. This synthetic strategy opens a new avenue for the effective and facile synthesis of lithium transition metal oxide/CNT nanocomposites.
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Acknowledgements This work was supported by Korea Science and Engineering Foundation (KOSEF) through the National Research Lab. Program funded by the Ministry of Education, Science and Technology (No. R0A-2007-000-10042-0). This work at Kookmin University was supported by research program 2008 of Kookmin University in Korea. The work at BNL was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, under the program of ‘‘Hybrid and Electric Systems”, of the US Department of Energy under Contract No. DEAC0298CH10886. References [1] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W.V. Schalkwijk, Nat. Mater. 4 (2005) 366. [2] C. Jiang, M. Ichihara, I. Honma, H. Zhou, Electrochim. Acta 52 (2007) 6470.
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