Synthesis of Single Crystalline Spinel LiMn2O4 Nanowires for a

Feb 11, 2009 - How to improve the specific power density of the rechargeable lithium ion battery has recently become one of the most attractive topics...
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NANO LETTERS

Synthesis of Single Crystalline Spinel LiMn2O4 Nanowires for a Lithium Ion Battery with High Power Density

2009 Vol. 9, No. 3 1045-1051

Eiji Hosono, Tetsuichi Kudo, Itaru Honma, Hirofumi Matsuda, and Haoshen Zhou* Energy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology, Umezono, 1-1-1, Tsukuba, 305-8568, Japan Received November 10, 2008; Revised Manuscript Received December 22, 2008

ABSTRACT How to improve the specific power density of the rechargeable lithium ion battery has recently become one of the most attractive topics of both scientific and industrial interests. The spinel LiMn2O4 is the most promising candidate as a cathode material because of its low cost and nontoxicity compared with commercial LiCoO2. Moreover, nanostructured electrodes have been widely investigated to satisfy such industrial needs. However, the high-temperature sintering process, which is necessary for high-performance cathode materials based on high-quality crystals, leads the large grain size and aggregation of the nanoparticles which gives poor lithium ion battery performance. So there is still a challenge to synthesize a high-quality single-crystal nanostructured electrode. Among all of the nanostructures, a single crystalline nanowire is the most attractive morphology because the nonwoven fabric morphology constructed by the single crystalline nanowire suppresses the aggregation and grain growth at high temperature, and the potential barrier among the nanosize grains can be ignored. However, the reported single crystalline nanowire is almost the metal oxide with an anisotropic crystal structure because the cubic crystal structure such as LiMn2O4 cannot easily grow in the one-dimentional direction. Here we synthesized high-quality single crystalline cubic spinel LiMn2O4 nanowires based on a novel reaction method using Na0.44MnO2 nanowires as a self-template. These single crystalline spinel LiMn2O4 nanowires show high thermal stability because the nanowire structure is maintained after heating to 800 °C for 12 h and excellent performance at high rate charge-discharge, such as 20 A/g, with both a relative flat charge-discharge plateau and excellent cycle stability.

Introduction. The electric vehicle (EV) using a rechargeable lithium ion battery is seen as one of the ways to solve two great problems of industry: the shortage of oil energy sources and the pollution of the environment.1 However, the specific power density of this kind of battery, which means the capacities at high rate charge-discharge process, is still too low to support the industrial needs. How to improve the specific power density of the rechargeable lithium ion battery has recently become one of the most attractive topics of both scientific and industrial interests. 2-4 Recently, nanostructured electrodes,3-10 including nanocrystal, nanoprous, and nanotube active materials, have been widely investigated to improve the performance of the rechargeable lithium ion battery. Most of the nanostructured electrode materials are synthesized by a low-temperature treatment process such as soft chemical,4 sol-gel,5-10 and hydrothermal methods.8 However, the hightemperature sintering process, which is necessary for highperformance cathode materials based on high-quality crystallinity, such as LiCoO2,11 LiMn2O4,12 and LiFePO4,13 and anodic materials Li4Ti5O12,14 leads to large grain size and aggregation which gives poor of battery performance result* Corresponding author, [email protected]. 10.1021/nl803394v CCC: $40.75 Published on Web 02/11/2009

 2009 American Chemical Society

ing from increased lithium ion diffusion length and decreased effective surface area contact with electrolyte.7 So it is still a challenge to synthesize a high-quality single crystal nanostructured electrode for a high-rate lithium ion battery. Among all of the nanostructures, a single crystalline nanowire is the most attractive morphology because the nonwoven fabric morphology constructed by the single crystalline nanowire suppresses the aggregation and grain growth at high temperature, the potential barrier among the nanosize grains can be ignored to decrease the electronic resistance among the nanosize grains. This has been confirmed in dyesensitized solar cells15 and supercapacitors.16 Moreover, the morphology of a nanowire has the potential not only to increase the filled ratio but also to solve the safety problem regarding the short circuit resulting from the nanoparticle’s penetration through the separator, which is a serious industry problem for the nanocrystal’s practice application in a lithium ion battery. However, only a metal oxide with an anisotropic crystal structure, such as TiO2, ZnO, SnO2, In2O3, ZrO2, and WO3, could be easily synthesized as a single crystalline nanowire until now.8,17,18 The synthesis of a single crystalline nanowire with cubic crystal structure such as LiMn2O4 is difficult because the cubic crystals cannot grow in a onedimentional direction. Up to now, few papers on single

Figure 1. (a) The XRD patterns of single crystalline Na0.44MnO2 nanowires, which agrees with JCPDS (No.27-0750) of Na0.44MnO2. (b) The FE-SEM images of unbundled single crystalline Na0.44MnO2 nanowires with a diameter of about several 10s of nanometers and a length of about several 100s of micrometers. (c) The high-resolution transparent electron microscopy (HRTEM) images of the single crystalline Na0.44MnO2 nanowires. (d) The ED patterns of the single crystalline Na0.44MnO2 nanowires. According to HRTEM and ED, the growth direction is [001] and the wall plane’s direction is [100].

crystalline nanowire19-21 and nanorod22 structures with cubic crystal structure are reported. Here we designed a strategic synthesis process to synthesize novel single crystalline spinel LiMn2O4 nanowires without any bundle-like structure based on a novel reaction method using Na0.44MnO2 nanowires as a self-template. We have chosen the spinel LiMn2O4 because it is the most promising cathode material based on low cost, large deposits, and nontoxicity. These single crystalline spinel LiMn2O4 nanowires have excellent thermal stability for a hightemperature sintering process, a relative flat charge-discharge potential plateau, and a stable cycle performance even at high rate cycles. The single crystalline spinel LiMn2O4 nanowires display the reversible capacities of 108, 102, and 88 mA h/g even at remarkable fast charge-discharge rates of 5, 10, and 20 A/g, respectively. Experimental Section. The commercial Mn3O4 powders were dispersed into the NaOH aqueous solution of 5 mol/ dm3. It was placed in the Teflon-lined autoclave. The autoclave was heated at 205 °C for 4 days. After that, the synthesized Na0.44MnO223 was washed repeatedly by deionized water. The washed Na0.44MnO2 was dried at room temperature under vacuum conditions. The sodium/lithium ion exchange and addition of more lithium were performed in molten salts of LiNO3 (88 mol %) and LiCl (12 mol %) at 450 °C for 1 h. The sample was washed repeatedly in deionized water and dried at room temperature under vacuum conditions. Finally, the sample was sintered at 800 °C for 1 h. The crystal structure was identified using X-ray diffraction (XRD) analysis with a Bruker axs D8 Advance using Cu KR radiation. The morphology was observed by a field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) using a Carl Zeiss Gemini Supra and a JEOL JEM-2010F, respectively. 1046

Electrochemical measurements were performed by a threeelectrode cell. The LiMn2O4 nanowires were mixed and ground with 5 wt % Teflon powder and 45 wt % acetylene black. The mixture was spread and pressed on the SUS-304 mesh as a working electrode. The reference and counter electrodes were prepared by lithium metals on SUS-304 mesh. A 1 mol dm-3 concentration of LiClO4 in EC/DEC was used as the electrolyte. Cell assembly was carried out in a glovebox under an argon atmosphere. The weight in specific capacity (mA h/g) and current rate are calculated only for active materials. Results and Discussion. The single crystalline LiMn2O4 nanowire is synthesized by a novel reaction using long single crystalline Na4Mn9O18 (Na0.44MnO2) nanowire as a selftemplate,23 which is synthesized by a simple hydrothermal process. The resulting Na0.44MnO2 is a single crystalline with ultralong length nanowire in a tunnel structure.23 The morphology of a single crystalline Na0.44MnO2 nanowire is confirmed by the XRD patterns, field-emission scanning electron microscopy (FE-SEM), and transparent electron microscopy (TEM) images (Figure 1). In Figure 1a, the XRD pattern of single crystalline Na0.44MnO2 nanowires agrees with JCPDS (No.27-0750) of Na4Mn9O18. The FE-SEM image in Figure 1b shows the unbundled single crystalline Na0.44MnO2 nanowires with a diameter of several 10s of nanometers and a length of several 100s of micrometers. Panels c-e of Figures 1 indicate the HRTEM images and electron diffraction (ED) patterns of the single crystalline Na0.44MnO2 nanowire, respectively. According to HRTEM and ED, the growth direction is [001] and the wall plane’s direction is [100]. The single crystalline Na0.44MnO2 was then performed in molten salts. In this process, the Li0.44MnO2 and the Li2MnO3 phases have both been confirmed in the XRD patterns as Nano Lett., Vol. 9, No. 3, 2009

Figure 2. (a) XRD pattern of the sample after the molten salt treatment of Na0.44MnO2 nanowires at 450 °C for 1 h. The main phase agrees with Li0.44MnO2, and the second phase is similar to Li2MnO3 with ICDD (No. 01-084-1634). This sample is called the Li0.44MnO2 + Li2MnO3 nanowire. (b) FE-SEM images of the Li0.44MnO2 + Li2MnO3 nanowire. (c) and (d) The HRTEM images of the Li0.44MnO2 + Li2MnO3 nanowire. (d) The ED patterns of the Li0.44MnO2 + Li2MnO3 nanowire, which clearly show the diffraction patterns from the planes of (1j10), (020), and (110) of Li2MnO3 and (200) of Li0.44MnO2.

shown in Figure 2a. The main phase agrees with Li0.44MnO2,24,25 and the second phase is similar to Li2MnO3 with ICDD (No. 01-084-1634). This sample is called as the Li0.44MnO2 + Li2MnO3 nanowire. Panels b, c and d, and e of Figure 2 show the FE-SEM image, TEM images, and ED pattern images of the Li0.44MnO2 + Li2MnO3 nanowires, respectively. These images of Li0.44MnO2 + Li2MnO3 nanowires indicate that the single crystalline nanowire morphology of Na0.44MnO2 is maintained after the molten salts treatment. The ED pattern shows the diffraction patterns from the planes of (1j10), (020), and (110) of Li2MnO3 and (200) of Li0.44MnO2. The Li2MnO3 takes an important role in forming pure spinel crystalline LiMn2O4. The formation reaction from Na0.44MnO2 to Li0.44MnO2 + Li2MnO3 nanowire can be explained as follows. At first the Na+ ion in Na0.44MnO2 is replaced by the Li+ ion to form a tunnel structure Li0.44MnO2 nanowire retaining the morphology of a single crystalline ultralong length nanowire, which can be written as 0.44Li+ + Na0.44MnO2 w Li0.44MnO2 + 0.44Na+

(1)

then some of the resulting Li0.44MnO2 is oxidized to form Li2MnO3, which can be expressed as 2Li0.44MnO2 + 3.12LiNO3 w 2Li2MnO3 + 3.12NO + 2.12O2 v (2)

The resulting sample, which is Li0.44MnO2 + Li2MnO3 nanowire, is sintered at a high temperature of 800 °C for 1 h to synthesize the high-quality single crystalline cubic spinel LiMn2O4 nanowire. The XRD patterns and FE-SEM images of LiMn2O4 nanowires after heating at 800 °C for 1 h are shown in panels a and b of Figure 3, respectively, Nano Lett., Vol. 9, No. 3, 2009

The XRD pattern agrees very well with the cubic spinal LiMn2O4 JCPDS (No. 35-0782). The spinel LiMn2O4 shows the single crystalline nanowire with a high aspect ratio of about several 100s and the diameter of around 50-100 nm. The TEM image and the ED patterns of the LiMn2O4 nanowire are shown in Figure 4 a-c and Figure 4 d, respectively. These images show that the synthesized LiMn2O4 nanowire is a single crystalline nanowire. The lattice of the spinel was 8.23 Å, according to the XRD in Figure 3, which points out a Li-rich spinel phase as Li1.07Mn1.93O4 (a linear interpolation of the lattice constants from Li1.00Mn2.00O4 with 8.248 Å to Li1.333Mn1.667O4 with 8.16 Å).26,27 The reaction mechanism from Li0.44MnO2 + Li2MnO3 nanowire to LiMn2O4 nanowire can be expressed as 25Li0.44MnO2 w 25Li0.44Mn0.88O1.76 + Mn3O4 + O2 w 11LiMn2O4 + Mn3O4 + O2 (3) Li2MnO3 + Mn3O4 + 1 ⁄ 2O2 w 2LiMn2O4

(4)

The total reaction can be written as 25Li0.44MnO2 + Li2MnO3 w 25Li0.44Mn0.88O1.76 + Mn3O4 + Li2MnO3 + O2 w 11LiMn2O4 + Mn3O4 + Li2MnO3 + O2 w 13LiMn2O4 + 1 ⁄ 2O2 (5)

The Mn3O4, which is an intermediate product, has been confirmed in the experimental process as follows. Figure 5a shows the XRD patterns of the sample after molten salts treatment of the Na0.44MnO2 nanowires at 300 °C for 10 h. Only single phase Li0.44MnO2 could be found. Figure 5b indicates the XRD pattern of the sample after heat treatment of the above pure Li0.44MnO2 nanowires at 800 °C for 1 h. The main phase agrees with LiMn2O4, and the second phase 1047

Figure 3. (a) XRD pattern of the synthesized single crystalline spinel LiMn2O4 nanowires, which agrees very well with cubic spinal LiMn2O4 JCPDS (No. 35-0782). (b) SEM images of the synthesized single crystalline spinel LiMn2O4 nanowires. According to the lattice of 8.23 Å, the synthesized single crystalline spinel LiMn2O4 nanowire is a Li-rich spinel phase as Li1.07Mn1.93O4.

Figure 5. (a) XRD patterns of the sample after molten salts treatment of the Na0.44MnO2 nanowires at 300 °C for 10 h. Only single phase Li0.44MnO2 could be found. (b) XRD patterns of the sample after heat treatment of the above Li0.44MnO2 nanowires at 800 °C for 1 h. The main phase agrees with LiMn2O4, and the second phase is similar to Mn3O4 with JCPDS (No.24-0734).

which is 1/25 ratio of Li0.44MnO2, takes the key role in synthesis of pure spinal LiMn2O4. The CV curves were measured for the confirmation of the crystalinity of the synthesized LiMn2O4 nanowire. The CV curves at scan rate 0.1 mV/s of single crystalline LiMn2O4 and two other commercial spinel LiMn2O4 electrode samples by Honjyo Chemical and Mitsui Metal are shown in Figure 6a. The lithium ion’s insertion and extraction can be expressed as Figure 4. (a) TEM images of the synthesized single crystalline spinel LiMn2O4 nanowires. (b, c) High-resolution TEM images of the single crystalline spinel LiMn2O4 nanowires. (d) ED patterns of the single crystalline spinel LiMn2O4 nanowires. The spinel LiMn2O4 is the single crystalline nanowire with a high aspect ratio of about several 100s and diameter of around 50-100 nm. According to the TEM image and the ED, the nanowire’s growth direction is [011] and the wall plane of the nanowire is [100]. The morphology of the top head of the nanowires as shown in Figure 4b is in a pyramidal structure that can be commonly observed in cubic spinel single crystalline LiMn2O4 with bipyramidal heads of octahedron shape (see Figure S1b and Figure S1d in Supporting Information).

is similar to Mn3O4 with JCPDS (No.24-0734). It points out that Mn3O4 is the intermediate product of eq 3. So, pure cubic spinel LiMn2O4 cannot be obtained from pure tunnel structure Li0.44MnO2. However, Li2MnO3 can react with Mn3O4 to form LiMn2O4 according to eq 4. Here, a little Li2MnO3, 1048

LiMn2O4 S Li1-xMn2O4 + xLi+ + xe-

(6)

In the charge curve, the first peak located at 4.05 V (Li/ Li+) means an extraction of Li ions from half of the tetrahedral sites with Li-Li interaction;12,28 the second peak at 4.15 V (Li/Li+) means an extraction of Li ions from the other half of the tetrahedral sites without Li-Li interaction.12,28 The potential drop with about 100 mV between these two extraction processes is from the repulsion between Li ions.28 So in the discharge curve, two peaks also correspond to the two insertion processes, respectively. The CV curve by single crystalline LiMn2O4 nanowires shows two sharp peaks which indicate that the single crystalline LiMn2O4 nanowire has very good crystallinity quality after a hightemperature sintering process. This is also confirmed by the charge-discharge curves at the 0.1 A/g as shown in Figure 6b. The single crystalline LiMn2O4 nanowire clearly shows Nano Lett., Vol. 9, No. 3, 2009

Figure 6. (a) The Cyclic voltammetry (CV) curves (0.1 mV/s) of the electrode by using the single crystalline spinel LiMn2O4 nanowires, commercial LiMn2O4 by Honjyo Chemical, and commercial LiMn2O4 with 0.5 wt % of Mg by Mitsui Metal. Only the curve of single crystalline spinel LiMn2O4 nanowires indicates the sharp peaks of CV. It shows the very high crystallinity of the LiMn2O4 nanowires. (b) The charge-discharge curves at 0.1 A/g for our sample, commercial LiMn2O4 by Honjyo Chemical, and commercial LiMn2O4 by Mitsui Metal. The charge-discharge curve at 0.1 A/g of single crystalline LiMn2O4 nanowires shows the twostep flat plateaus based on the two different lithium ion’s insertion (or extraction) processes.

Figure 7. SEM images of the synthesized single crystalline spinel LiMn2O4 nanowires obtained by heating at 800 °C for 12 h. The nanowire structure is maintained without deformation although the nanowires suffered high temperatures (to 800 °C) and long heating times (for 12 h).

Figure 9. (a) The discharge curves of the single crystalline spinel LiMn2O4 nanowires and commercial LiMn2O4 by Honjyo Chemical, Mitsui Metal, and Aldrich at the second cycle at various rates of 0.1, 5, 10, and 20 A/g. (b) The charge-discharge curves at the second cycle at a high rate of 5 A/g. (c) Cycle performance at a high rate of 5 A/g. (d) The relationships between discharge capacity and current density of the single crystalline spinel LiMn2O4 nanowires at the second cycle. In (b) and (c), red solid and dotted lines, green solid and dotted lines, blue solid and dotted lines, and black solid and dotted lines represent the results from the single crystalline spinel LiMn2O4 nanowires and commercial LiMn2O4 samples by Honjyo Chemical, Mitsui Metal, and Aldrich, respectively. In (d), red circles, green boxes, blue triangles, and black boxes are the discharge capacities in various rates of the single crystalline spinel LiMn2O4 nanowires and commercial LiMn2O4 samples by Honjyo Chemical, Mitsui Metal, and Aldrich, respectively.

the two-step flat plateaus based on the two different insertion (or extraction) processes. The capacities at 0.1 A/g of single crystalline LiMn2O4 nanowires are 118 mA h/g, which agrees with the theoretical value of Li1.07Mn1.93O4.29 In the paper of the synthesis of single crystalline LiMn2O4 nanobelts,21 the lithium ion battery property was not shown. To obtained excellent battery performance, the surface of

Figure 8. The model scheme of the cubic structure with facet plane such as synthesized nanowires. LiMn2O4 with cubic spinel structure is very suitable for a high rate lithium ion battery because lithium ions can easily diffuse thorough every plane based on the cubic structure The one-dimensinal structure by cubic structure is similar to the synthesized nanowire structure. Nano Lett., Vol. 9, No. 3, 2009

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Figure 10. The TEM images of the single crystalline LiMn2O4 nanowire after 100 cycles at 5A/g. (a) Low magnification image. (b, c) High-resolution images. The inset image in (c) is the electron diffraction pattern. The very clear lattice image and the electron diffraction pattern indicate that the single crystalline nature is maintained after 100 cycles at high current density.

the active materials is very important. The surface of our single crystalline nanowires is a very pure and clean surface based on the facet plane by the novel reaction process. It is considered that the facet plane of the single crystalline nanowire is much clear than the surface of the LiMn2O4 nanorods.22 So we think that the nanowires are suitable for the high performance of the lithium ion batteries as follows. Many reports on the nanomaterials are not conducted at high temperature heating due to the decomposition of the nanostructure by the high temperature heating. So the surface of the materials may have many defects and dangling bonds. Our group reported the synthesis of spinel LiMn2O4 nanoparticles.30 However, the nanoparticles indicate the broader CV peaks and lower capacities than those of LiMn2O4 nanowires. It is caused by the low crystalinity. We think that LiMn2O4 nanoparticles can be easily synthesized. However, the nanoparticles cannot indicate the sharp CV peaks and large capacities based on good crystallinity. For the fabrication of high-quality single crystalline LiMn2O4, the nanowire structure is very important because high 1050

temperature heating can conduct to nanowires such as in other traditional cathode materials. The surface of the nanowires shows a very flat surface (facet). So this material is not similar to other nanomaterials. Although the image is the LiMn2O4 nanowire after heating at 800 °C for 12 h, the nanowire structure is maintained without deformation as shown in Figure 7. LiMn2O4 with cubic spinel structure is very suitable for a high rate lithium ion battery because lithium ions can easily diffuse thorough every plane based on the cubic structure as shown in Figure 8. The one-dimensinal cubic structure in Figure 8 is similar to the synthesized nanowire structure. Figure 9a shows the properties of lithium ion’s discharge curves at various rates of 0.1, 5, 10, and 20 A/g of the single crystalline LiMn2O4 nanowires, and three other commercial samples by Honjyo Chemical, Mitsui Metal, and the Aldrich Co. The SEM images of the commercial samples show their particle sizes in a range from several hundred nanometers to several micrometers (see Figure S1 in Supporting Information). The discharge capacities of single crystalline spinel Nano Lett., Vol. 9, No. 3, 2009

LiMn2O4 at rates of 0.1, 5, 10, and 20 A/g are 118, 108, 102, and 88 mA h/g, respectively. It indicates that above 90%, 85%, and 75% capacities at 0.1 A/g can be retained even at remarkable high charge-discharge rates of 5, 10, and 20 A/g, respectively, The large potential drops in the charge-discharge curves of single crystalline spinel LiMn2O4 nanowires result from the iR drops of the lead line and cell structure of a three-electrode cell at high rate chargedischarge. The charge-discharge curves at a constant current density of 5 A/g are shown in Figure 9b. It seems to be the best high rate charge-discharge result not only for spinal LiMn2O4 but also for other cathode and anode electrode active materials such as LiCoO2,31 LiFePO4,32 and Li4Ti5O12.33 Moreover, the discharge properties at high rates of 5, 10, and 20 A/g still have both a relative flat charge-discharge plateau and excellent cycle stability. The maintained relative flat plateau in the high rate of 5 A/g is also interesting, though such a phenomenon has recently been only observed in the high-rate charge-discharge of nanoporous metal Sn.34 The commercial samples show a much smaller capacity compared with that of single crystalline LiMn2O4 nanowires, and a nonflat plateau with a capacitor-like charge-discharge curve at high rates, which cannot maintain a constant output voltage. The cycle performance of single crystalline LiMn2O4 nanowires at a rate of 5 A/g is shown in Figure 9c. After 100 cycles at a 5 A/g rate, the large capacity of around 100 mA h/g is still maintained. The relationship between discharge capacities and rates of single crystalline LiMn2O4 nanowires, commercial samples are shown in Figure 9d. Single crystalline LiMn2O4 nanowire maintains the high capacity despite increasing the current density. On the other hand, the capacity of three kinds of commercial samples is decreased with an increase in the current density. Finally, we confirmed the structure of the nanowires after the 100th charge/discharge cycle at 5 A/g as shown in Figure 10. We can see a very clear lattice image. The electron diffraction patterns indicate that the single crystalline nature is maintained after 100 cycles at high current density. In summary, single crystalline LiMn2O4 nanowires are fabricated by a novel reaction using Na0.44MnO2 nanowires. These single crystalline LiMn2O4 nanowires have not only excellent thermal stability for a high-temperature sintering process but also a charge-discharge reversible stability even for high rate cycles. Moreover, the spinel LiMn2O4 nanowires display the reversible large capacities even at remarkable high charge-discharge rates. These large capacities at a high rate come from the nanowire morphology and the high quality of the single crystal, which decreased both the lithium and electron’s diffusion lengths. Moreover, the nanowire morphology also has a future in industry for improving the electrodefilled ratio and safety in a practice lithium ion battery. So, these spinel LiMn2O4 nanowires could be used to satisfy the industrial needs in electric vehicles (EV), hybrid electric vehicles (HEV), and other mobile or portable electric devices. Acknowledgment. Authors acknowledge Mr. M. Ichihara for help with TEM observation. Nano Lett., Vol. 9, No. 3, 2009

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