Environmentally Friendly Growth of Well-Developed LiCoO2 Crystals

Aug 27, 2010 - (28-34) Other advantages of flux growth are based on the fact that the crystal can grow in an unconstrained fashion; that is, it can gr...
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DOI: 10.1021/cg100705d

Environmentally Friendly Growth of Well-Developed LiCoO2 Crystals for Lithium-Ion Rechargeable Batteries Using a NaCl Flux

2010, Vol. 10 4471–4475

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Katsuya Teshima,*,† SunHyung Lee,‡ Yusuke Mizuno,† Hikaru Inagaki,† Masato Hozumi,§ Keiichi Kohama,§ Kunio Yubuta, Toetsu Shishido, and Shuji Oishi† †

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Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan, ‡Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan, § Toyota Motor Corporation, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan, and Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Received May 27, 2010; Revised Manuscript Received July 27, 2010

ABSTRACT: High quality LiCoO2 crystals, useful as cathode material for lithium-ion rechargeable batteries, were successfully grown at a holding temperature of 800-1000 °C using the NaCl flux cooling method. The morphology, structure, size uniformity, and crystallinity of the obtained LiCoO2 crystals were obviously dependent on the growth conditions, such as the holding temperature and the starting composition. Well-developed, highly crystalline LiCoO2 crystals were first grown at a holding temperature of 900 °C from a NaCl flux. The grown LiCoO2 crystals had a hexagonal barrel-shaped structure with welldeveloped {001}, {104}, {101}, and {102} faces. On the basis of the powder X-ray diffraction data, the lattice parameters of the crystals were determined as a = 2.816 and c = 14.077 A˚. These values agree approximately with those from the literature (a = 2.816 and c = 14.052 A˚). The average crystal size was about 1.4 μm, which is a relatively small size when compared to previous reports. Transmission electron microscopy images indicate that the LiCoO2 crystals were of very good crystallinity. It was confirmed that the charge and discharge capacities of the lithium-ion rechargeable batteries containing the grown LiCoO2 crystals were 138 and 130 mAh 3 g-1, respectively, values that correspond to the available capacity of 137.5 mAh 3 g-1. The discharge capacity of the grown LiCoO2 crystal is greater at 10 C than that of commercially available crystals.

Introduction Recently, lithium-ion rechargeable batteries (LIBs) have been much studied for use as a power source on hybrid-electric vehicles and electric vehicles because of their high power and highdischarge voltages.1-4 They have also been studied for use in environmentally friendly electric devices. Among LIBs, all-solidstate LIBs using solid-inorganic materials have attracted attention owing to their safety and stability.5-8 Lithium-transition metal oxides, such as LiCoO2,9,10 LiMn2O4,11,12 LiNiO2,13,14 and Li4Ti5O12,15,16 have been extensively studied for use as electrode materials in all-solid-state LIBs. LiCoO2, LiMn2O4, and LiNiO2 materials are usually identified as suitable for use in the cathodes of all-solid-state LIBs that offer a stable operating voltage of approximately 4 V.9,11,17 Additionally, Li4Ti5O12 can be used as an anodic material and can be set with cathodes such as LiMn2O4, LiNiO2, or LiCoO2 to provide a cell with an operating voltage of approximately 2.5 V.16,18 LiCoO2 crystals in particular have attracted attention due to their layered structure because Liþ ions were easily removed from LiCoO2. Layered LiCoO2 is usually cycled to an upper cutoff voltage of 4.2 V versus Li and gives a specific capacity of 140 mAh 3 g-1.19,20 The crystals of LiCoO2 crystallize in the rhombohedral system (space group: R3m) with lattice parameters of a = 2.816 and c = 14.052 A˚.21 There have been many studies on the preparation of LiCoO2 crystals by various synthetic methods such as solidstate reactions,19,22 hydrothermal growth,23-25 and sol-gel method.26,27 These synthetic methods, however, suffer several problems associated with the environment and cost. The solidstate reaction of LiCoO2 has generally been carried out with the conventional solid-state reaction route at high temperature. *Corresponding author. E-mail: [email protected]. r 2010 American Chemical Society

The high-temperature heating treatments usually cause component loss and an increase in particle size. Additionally, there are several disadvantages in hydrothermal growth techniques such as expensive equipment and high total costs. For sol-gel crystallization, the grown LiCoO2 crystals have a poor crystallinity. Therefore, a simple, environmentally friendly method and mild reaction conditions would be beneficial to synthesize the highly crystalline, well-developed LiCoO2 crystals. Among preparation methods, flux growth is an environmentally friendly, simple, and low-cost method that can produce ternary or multicomponent crystals at temperatures below the melting point of the solute.28-34 Other advantages of flux growth are based on the fact that the crystal can grow in an unconstrained fashion; that is, it can grow free from mechanical or thermal constraints into the solution and therefore develop facets. Recently, preparation methods of LiCoO2 crystals using KNO328 and LiCl-Li2CO329 fluxes have been reported as the simplest method. However, the LiCoO2 crystals obtained by these growth techniques (or these growth conditions) possessed a relatively large size and bulk-like structure (or undeveloped structure). Herein, we report on the growth of LiCoO2 crystals with high crystallinity and well-developed faces by the cooling of a NaCl flux. The melting point of NaCl (801 °C) is relatively low, and NaCl is easily purchased and soluble in warm water. It is abundant in nature and harmless to human beings and the environment. The effects of holding temperature and starting composition (e.g., Li/Co molar ratio) on the morphology, structure, size uniformity, and crystallinity of the LiCoO2 crystals were also studied. In addition, the charge-discharge and current rate characteristics of the LIBs using the grown LiCoO2 crystals as cathodes were investigated. Published on Web 08/27/2010

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Table 1. Growth Conditions of LiCoO2 Crystals from NaCl Flux starting materials run no.

LiOH 3 H2O/g

Co3O4/g

NaCl/g

holding temperature/°C

1 2 3 4 5 6 7

0.869 0.869 0.869 2.019 0.869 0.695 6.952

1.323 1.323 1.323 2.573 1.323 1.323 1.323

18.38 18.38 18.38 16.862 0 18.38 18.38

1000 900 800 900 900 900 900

Experimental Section Lithium cobalt oxide crystals (LiCoO2) were grown using a cooling method with a NaCl flux. Reagent-grade LiOH 3 H2O, Co3O4, and NaCl (Wako Pure Chemical Industries, Ltd.) were used for the growth of LiCoO2 crystals. The typical growth conditions are given in Table 1. The masses of the reagents were kept at approximately 20 g for Run Nos. 1-6 (exc. Run No. 7 = about 26 g). For Run Nos. 1-5, a nonstoichiometric mixture of reagent-grade LiOH 3 H2O and Co3O4 (Li/Co molar ratio = 1.25) powders was used. On the other hand, a stoichiometric mixture (Li/Co molar ratio = 1.00) was used in Run No. 6. Each of the mixtures was placed in 30 cm3 capacity crucibles. After the lids were loosely closed, the crucibles were placed in an electric furnace, heated to 800-1000 °C, and held at this temperature for 5 h. They were then cooled to 500 °C at a rate of 200 °C 3 h-1, which was controlled by a cooling program, and then were allowed to cool to room temperature in the furnace. The crystal products were separated from the remaining flux with warm water. The obtained crystals were imaged using a scanning electron microscopy (SEM, JEOL, JCM-5700) operated at an accelerating voltage of 15 kV. The elemental compositions of the crystals were evaluated using energy-dispersive X-ray spectrometry (EDS, JEOL, JSM-6710F) operating at an acceleration voltage of 15 kV and inductively coupled plasma-optical emission spectrometry (ICP-OES, SII, SPS5510). The phase of the crystals was studied by X-ray diffraction (XRD, RIGAKU, MiniflexII). High-resolution transmission electron microscopy (HRTEM) and electron diffraction measurements were carried out with JEM-2010 and JEM-2000EXII (JEOL) instruments operating at 200 kV to analyze the crystallinity and developed faces of the grown crystals. The morphology was investigated using the TEM, XRD, and interfacial angle data. The electrochemical properties of the obtained crystals as positive electrodes (cathodes) were investigated using coin-type cells. The positive electrodes of the obtained LiCoO2 crystals (80 wt %), acetylene black (15 wt %) and polyvinylidene fluoride (PVDF, 5 wt %) were mixed using 1.23 g of N-methylpyrrolidone (NMP) as solvent. The mixed paste was uniformly cast on aluminum foil of the current collector and dried at 120 °C for 24 h. Lithium metal and polypropylene were used as the negative electrode (anode) and the separator, respectively. Additionally, 1 M LiPF6 in ethylene (EC)/ dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) solution was used as the electrolyte. The coin-type cells were assembled in an argon-filled glovebox that maintained an atmosphere of e1 ppm of H2O and O2. The charge-discharge cycles of the fabricated cells were carried out in the voltage range of 2.4-4.2 V.

Results and Discussion Idiomorphic crystals of LiCoO2 for use as cathode material in LIBs were successfully grown at 900 °C using the NaCl flux cooling method. Typical LiCoO2 crystals are depicted in Figure 1 (Run No. 2). It was found that the grown LiCoO2 crystals at 900 °C were relatively uniform and poorly aggregated, as depicted in Figure 1a. Additionally, it was found from the high magnification SEM images (Figure 1b,c) that the grown crystals had a hexagonal barrel shape with welldeveloped faces, and their surface was relatively flat. The average size of the crystals at 900 °C is about 1.4 μm, which is relatively small when compared with previous reports.

Figure 1. (a) Low and (b and c) high magnification SEM of typical LiCoO2 crystals grown at a holding temperature of 900 °C (Run No. 2) and (d) the schematic drawing of a LiCoO2 crystal with {001}, {104}, {101}, and {102} faces.

Figure 2. Low and high magnification SEM images of the LiCoO2 crystals grown at a holding temperature of 1000 (a and b, Run No. 1) and 800 °C (c and d, Run No. 3).

Figure 1d depicts the schematic drawing of the LiCoO2 crystal with {001}, {104}, {101}, and {102} faces using data from the literature.21 This schematic corresponded exactly with the structure of the LiCoO2 crystals grown at 900 °C (Figure 1c). Sodium and chlorine atoms from the NaCl flux were not detected in the EDS analysis (Supporting Information 1). In addition, the amounts of Na and Cl atoms in the products were much less than 0.1 ppm by ICP-OES analysis (Supporting Information 2; Na), which means that extremely low amounts of Na and Cl atoms were left in the grown crystals. Therefore, the crystals grown from the NaCl flux were of high quality, well developed, and highly crystalline. The morphology, structure, size uniformity, and crystallinity of the obtained LiCoO2 crystals were dependent on the various preparation conditions, such as the holding temperature and starting composition. First, the impact of the holding temperature on the growth of LiCoO2 crystals was investigated. Figure 2 depicts the low and high magnification SEM images of the grown LiCoO2 crystals at a holding temperature

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Figure 3. XRD profiles for pulverized crystallites grown at a holding temperature of (a) 1000 (Run No. 1), (b) 900 (Run No. 2), (c) 800 (Run No. 3), and (d) LiCoO2 ICDD PDF.21

of 1000 (Figure 2a,b, Run No. 1) and 800 °C (Figure 2c,d, Run No. 3), respectively. The crystals grown at 1000 °C had poor size uniformity and high aggregation compared with the crystals grown at 900 °C as depicted in Figure 2a. The crystals grown had a large bulk-like appearance (Figure 2a) and welldeveloped structure (Figure 2b). On the other hand, the grown crystals at 800 °C had a poorly developed spherical structure with high size uniformity, as depicted in Figure 2c,d. Figure 3 shows XRD profiles of data for the pulverized crystallites grown at holding temperatures of 1000 (Figure 3a, Run No. 1), 900 (Figure 3b, Run No. 2), 800 (Figure 3c, Run No. 3), and LiCoO2 ICDD PDF21 (Figure 3d), respectively. The crystals grown at 800-900 °C were identified as highly crystalline and homogeneous by their powder XRD patterns using data from the literature (Figure 3d) because the diffraction patterns of the crystals attributed to byproducts were not observed in Figure 3b,c. Additionally, on the basis of the powder XRD data, the lattice parameters of the LiCoO2 crystals at 900 °C were determined as a = 2.816 and c = 14.077 A˚. These values agree approximately with those (a = 2.816 and c = 14.052 A˚) from the literature.21 On the other hand, the crystals grown at 1000 °C consist of LiCoO2, Co3O4, and CoO because diffraction patterns attributed to LiCoO2, Co3O4, and CoO (marked peaks) were observed in Figure 3a. At 1000 °C, the concentration of Li ions was decreased in the solution because the boiling point of the LiOH powders was about 925 °C. Therefore, unreacted Co3O4 remained in solution, and some formed CoO crystals. Figure 4a,b depicts the bright field TEM image and the corresponding selected area electron diffraction (SAED) pattern of the LiCoO2 crystals grown in Run No. 2. The LiCoO2 crystals were found to have a relatively high crystallinity because the highly ordered SAED spots were clearly observed. The bright field TEM image showed that the crystals had a well-formed shape. Furthermore, the lattice image obtained from a LiCoO2 crystal, taken with the incident beam along the [111] direction, is depicted in Figure 4c. The crystals were of very good crystallinity because no defects were observed in this image. From these results of SEM, XRD, and TEM, we concluded that the highly crystalline, well-developed,

Figure 4. (a) Bright field TEM micrograph, (b) selected area electron diffraction pattern, and (c) lattice image of LiCoO2 crystals grown at 900 °C (Run No. 2).

Figure 5. SEM images of the LiCoO2 crystals grown at a Li-Co concentration of (a) 17 (Run No. 4) and (b) 100 mol % (no NaCl, Run No. 5). XRD profiles for the pulverized crystallites grown at (c) 10 and (d) 100 mol %, and (e) LiCoO2 ICDD PDF.21

homogeneous LiCoO2 crystals were successfully grown at 900 °C by the NaCl flux cooling method. The effect of the NaCl flux on the growth of LiCoO2 crystals was investigated. Figure 5 depicts the SEM images of the LiCoO2 crystals grown at a Li-Co concentration [i.e., (LiOH 3 H2O þ Co3O4)/(LiOH 3 H2O þ Co3O4 þ NaCl) molar ratio] of about 17 mol % (Figure 5a, Run No. 4) and 100 mol % (no NaCl, Figure 5b, Run No. 5), respectively. The grown

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Figure 6. Low and high magnification SEM images of the grown LiCoO2 crystals at a Li/Co molar ratio of 1 (a and b, Run No. 6) and 10 (c and d, Run No. 7).

crystals at a Li-Co concentration of about 8 mol % are depicted in Figure 1. It was found that the size uniformity of the LiCoO2 crystals deteriorated and that the average size increased in proportion to the Li-Co concentration. The crystals grown at low concentration were well-developed. The average sizes of the crystals grown at 17 and 100 mol % were 3.4 and 15.2 μm, respectively. The LiCoO2 crystals grown at 17 mol % had a well-developed and a variously sized, bulk-like structure, as depicted in Figure 5a. In contrast, the crystals grown at 100 mol % (no NaCl) had a large bulklike structure, and their surface was relatively rough, as depicted in Figure 5b. From these SEM images, there are indications that the NaCl impacted the formation of uniquely sized, well-developed LiCoO2 crystals. Figure 5c-e shows the XRD profiles of the crystals grown at Li-Co concentrations of 17 and 100 mol %, and LiCoO2 ICDD PDF,21 respectively. The crystals grown at the Li-Co concentration of 17 and 100 mol % were identified as highly crystalline, homogeneous LiCoO2 crystals by their powder XRD patterns using data from the literature (Figure 5e) because the diffraction patterns of the crystals attributed to byproducts were not observed in Figure 5c,d. From the XRD results, it is confirmed that the NaCl do not impact the crystallinity of the LiCoO2 crystals grown at 900 °C. The effect of the stoichiometric ratio of Li/Co in the starting composition on the structures of the grown LiCoO2 crystals was confirmed as depicted in Figure 6. Figure 6 shows the low and high magnification SEM images of the LiCoO2 crystals grown at a Li/Co molar ratio of 1 (Figure 6a,b, Run No. 6) and 10 (Figure 6c,d, Run No. 7), respectively. The crystals grown with a Li/Co molar ratio of 1.25 are depicted in Figure 1. The size of crystals grown with the Li/Co molar ratio of 1 were poorly uniform, and the crystals were aggregates of sphere structures and the bulk-like structure, as depicted in Figure 6a,b. On the other hand, the crystals grown at the Li/Co molar ratio of 10 had well-developed hexagonal plate-like, bulk-like, and spherical structures as depicted in Figure 6c,d. Figure 7 depicts the XRD profiles of the data for the pulverized crystallites grown with a Li/Co molar ratio of 1 (Figure 7a, Run No. 6), 10 (Figure 7b, Run No. 7) and LiCoO2 ICDD PDF21 (Figure 7c), respectively. The crystals grown with a Li/Co molar ratio of 10 were identified as highly

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Figure 7. XRD profiles for the pulverized crystallites grown at a Li/Co molar ratio of (a) 1 (Run No. 6), (b) 10 (Run No. 7), and (c) LiCoO2 ICDD PDF.21

Figure 8. Voltage-capacity profiles of the lithium-ion rechargeable batteries with (a) the LiCoO2 crystals grown at 900 °C (Run No. 2) with a current rate of 0.1 C; (b) the LiCoO2 crystals grown at 900 °C and commercially available LiCoO2 powders with a current rate of 10 C.

crystalline, homogeneous LiCoO2 crystal by their powder XRD patterns using data from the literature (Figure 7c) because the diffraction patterns of the crystals attributed to byproducts were not observed in Figure 7b. On the other hand, the crystals grown at the Li/Co molar ratio of 1 coexist with Co3O4 and CoO because diffraction patterns attributed to LiCoO2, Co3O4, and CoO (marked peaks) were observed in Figure 7a. The unreacted Co3O4 powders remained in solution, and some of the remaining Co ions formed CoO crystals

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because there was an insufficient amount of Li ions to react with all Co ions at the Li/Co molar ratio of 1. From these results, we confirmed that the Li/Co ratio in the starting composition affects the structure and crystallinity of the LiCoO2 crystals grown. The charge-discharge and current rate characteristics of LIBs using the grown LiCoO2 crystals as cathodes were investigated using a galvanostatic cycling measurement at 1 cycle. Figure 8a depicts the representative voltage-capacity profiles of the LIBs with single, well-developed, highly crystalline LiCoO2 crystals grown at 900 °C (Run No. 2) measured with a current rate of 0.1 C and a voltage range from 2.4 to 4.2 V. The charge and discharge capacity of the LIBs with the grown LiCoO2 crystals were 138 and 130 mAh 3 g-1, respectively, which corresponded to an available capacity of 137.5 mAh 3 g-1. Therefore, we concluded that the LiCoO2 crystals grown at 900 °C could be used as the cathodes of LIBs. Figure 8b depicts the voltage-capacity profiles of the LIBs with the LiCoO2 crystals grown at 900 °C and commercially available LiCoO2 crystals with a current rate of 10 C and a voltage range from 2.4 to 4.2 V. The discharge capacities of LIBs with the grown and commercially available LiCoO2 crystals as the cathode are 127 and 117 mAh 3 g-1, respectively, indicating that the grown LiCoO2 crystal has a capacity 10 C higher than that of commercially available crystals. Additionally, it was found that the capacity of LiCoO2 crystals at 10 C has a loss of 7% compared with that at 0.1 C. We conclude that the LiCoO2 crystals grown by the cooling of NaCl flux show a good charge-discharge characteristic for LIBs. Therefore, NaCl can be used as an environmentally friendly method for the growth of LiCoO2 crystals. Conclusions Well-developed, high-quality LiCoO2 crystals for use as cathodic material in LIBs were successfully grown at 900 °C using the NaCl flux cooling method. The LiCoO2 crystals grown were homogeneous, uniform in size, and hexagonal barrel-shaped in structure with well-developed {001}, {104}, {101}, and {102} faces. TEM images first indicated that the grown LiCoO2 crystals were of very good crystallinity without defect. The average size of the crystals grown was about 1.4 μm. Additionally, the morphology, structure, size uniformity, and crystallinity of the obtained LiCoO2 crystals were dependent on the growth conditions such as the holding temperature and starting composition. It was confirmed that the charge and discharge capacities of the LIBs with the LiCoO2 crystals grown were 138 and 130 mAh 3 g-1, respectively, which corresponded to the available capacity of 137.5 mAh 3 g-1. The grown LiCoO2 crystals have a capacity 10 C higher than that of commercially available crystals. Finally, in our growth technique of LiCoO2 crystals using a NaCl flux, environmental damages (e.g., by exhausting greenhouse gases) and product costs are thought to be greatly reduced. In the future, it will become more important to develop various functional materials with the use of environmentally friendly techniques. Acknowledgment. This research was partially supported by a Grant-in-Aid (Nos. 20350093 and 21686063) from the Ministry of Education, Culture, Sports, Science and Technology (Japan). A part of this work was supported by the Steel Industry Foundation for the Advancement of Environmental Protection Technology (2007-2008). This research was partially

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performed under the interuniversity cooperative research program of the Advanced Research Center of Metallic Glasses, Institute for Materials Research, Tohoku University. Supporting Information Available: EDS analysis of well-developed LiCoO2 crystals; Na spectra (ICP-OES) of well-developed LiCoO2 crystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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