DOI: 10.1021/cg900452a
Liquid-Phase Synthesis of Uniformly Nanosized LiMnPO4 Particles and Their Electrochemical Properties for Lithium-Ion Batteries
2009, Vol. 9 4990–4992
Takayuki Doi,*,† Shota Yatomi,‡ Tetsuya Kida,‡ Shigeto Okada,† and Jun-ichi Yamaki† †
Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-koen, Kasuga 816-8580, Japan, and ‡Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga 816-8580, Japan Received April 23, 2009; Revised Manuscript Received July 22, 2009
ABSTRACT: Uniformly nanosized particles of olivine LiMnPO4 were prepared at 280 °C by liquid-phase synthesis using a highboiling-point organic solution, and their electrochemical properties were investigated by charge and discharge measurements for the first time. TEM observation clarified that the resultant LiMnPO4 particles had fairly uniform particle sizes of ca. 7 nm. The discharge curves of the nanosized LiMnPO4 particles showed a monotonous decrease in voltage below 4 V, which was quite different from the clear plateau observed for conventional micrometer-sized LiMnPO4 powder. Thus, uniformly nanosized particles of active materials are readily available for studying the particle-size effect on the electrochemical properties of Li-ion batteries. Introduction The synthesis of inorganic nanocrystals has attracted considerable interest by researchers who hope to achieve dramatic improvements in electronic devices or open new frontiers in nanotechnology. Various kinds of synthetic methods have been developed to produce monodisperse nanocrystals with a controlled size, shape, and composition. Among these methods, wetchemical synthesis is suited to the preparation of uniform nanocrystals because nucleation and subsequent growth processes upon the formation of nanocrystals can be well-controlled in a solution phase. In fact, high-quality nanocrystals of inorganic materials, such as metal, metal alloy, oxide, sulfide, fluoride, and phosphate, have been prepared by liquid-phase synthesis using high-boiling-point organic solutions.1-3 The assembly of monodisperse nanocrystals to form two- and three-dimensional superlattice structures for practical use in information storage, semiconductor, luminescence, laser, and sensors has also been closely studied.4,5 Thus, nanosized particles and their alignment can affect the performance of electronic devices. However, to the best of our knowledge, there have been few studies on monodisperse and uniformly sized nanocrystals of active materials for use in Li-ion batteries. The use of nanosized particles of active materials is needed for Li-ion batteries in hybrid electric vehicles. Rapid charge and discharge reactions are required when Li-ion batteries are used for high power applications. To enhance these charge and discharge reactions, Li-ion diffusion through the active materials must be very fast, and therefore, granulated particles of battery active materials (ca. 5-10 μm) are usually used to shorten the diffusion paths of Li-ion through the active materials in commercial Li-ion batteries. In Li-ion batteries for high power applications, a further decrease in the particle size of active materials is essential for achieving apparent high Li-ion diffusion. In addition, nanosized particles of active materials provide a large surface area, and therefore, the charge transfer resistances at an electrode/electrolyte interface can be effectively decreased. Hence, monodisperse and uniform-sized nanocrystals should be ideal for eliciting high performance in Li-ion batteries. Such particles of active materials are also needed to gain a basic understanding of how particle-size affects electrode performance. Various synthetic methods, such as chemical vapor deposition, pulsed laser ablation, spray pyrolysis, sol-gel, and hydrothermal, *Corresponding author. Telephone: þ81-92-583-7657. Fax: þ81-92-5837791. E-mail address:
[email protected]. pubs.acs.org/crystal
Published on Web 11/02/2009
have been used so far to synthesize nanosized particles.6-8 We previously reported that nanocrystals of spinel Li4/3Ti5/3O4 with a fairly uniform particle size could be prepared by electrospray deposition, and we showed that they could be used to achieve the apparent rapid diffusion of Li-ion.9,10 However, nanosized particles generally have a large distribution in particle size and are strongly agglomerated. Li transition-metal phosphates with an ordered olivine structure have attracted much attention as promising positive-electrode materials for Li-ion batteries due to their high thermal stability compared to conventional Li transition-metal oxides such as LiCoO2. In particular, LiFePO4 and LiMnPO4 are attractive because Fe and Mn are inexpensive, environmentally benign, and less toxic than Co. LiFePO4 shows an acceptably large capacity, while insertion and extraction reactions of Li-ion take place at lower potentials than with LiCoO2.11 On the other hand, LiMnPO4 should be a more ideal substitute for LiCoO2 because the working potentials are very close to those of LiCoO2 and the theoretical energy density is quite large.12 In practice, however, the charge and discharge capacity is very small even at a reasonably low current density. According to the literature, the poor electrochemical performance of LiMnPO4 is likely due to the intrinsically low electronic conductivity and/or slow diffusion of Li-ion through LiMnPO4 particles.13-15 These drawbacks may be overcome by the use of very fine particles of LiMnPO4, which results in a shortening of both the conduction path of electrons and the diffusion path of Li-ions. In this study, we prepared uniformly sized nanocrystals of olivine LiMnPO4 by liquid-phase synthesis using a high-boiling-point organic solution, and we investigated their electrochemical properties by charge and discharge measurements.16 Oleic acid (90%), oleylamine (70%), phosphoric acid (99%), lithium hydroxide monohydrate (95%), manganese nitrate hexahydrate (98%), ethanol (99.5%), and hexane (96%) were used as received without further purification. One millimole each of lithium hydroxide monohydrate and manganese nitrate hexahydrate was added to 30 mmol of oleic acid in a three-neck flask at room temperature. The mixture was heated to 140 at 10 °C min-1 under an Ar flow and kept at this temperature for 1.5 h to form an optically transparent solution; both lithium hydroxide monohydrate and manganese nitrate hexahydrate easily reacted with oleic acid below 140 °C. One millimole of phosphoric acid and 10 mmol of oleylamine were then added to the solution. The mixture was heated to 280 at 10 °C min-1 and kept at that temperature for 1 h under an Ar atmosphere to obtain a turbid solution. After the r 2009 American Chemical Society
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Figure 2. (a) Low- and (b) high-magnification TEM images of LiMnPO4 prepared at 280 °C.
Figure 1. XRD patterns of LiMnPO4 particles prepared at 280 °C.
resultant solution was allowed to cool to room temperature, it was poured into an excess amount of ethanol, and this mixture was centrifuged to give deposits. The deposits were washed several times in a mixture of ethanol and hexane. The products were characterized by X-ray diffraction (XRD, RINT2100, Rigaku). Typical working conditions were 50 kV and 300 mA with a scanning speed of 0.25o min-1. A sol of the resultant particles was dropped on carbon-coated copper grids so that their morphology and microstructure could be observed by transmission electron microscopy (TEM, JEM2100F, Jeol). Electrochemical properties were studied by charge and discharge measurements using a coin-type cell. The test electrode was prepared from a mixture of the resultant particles (70 wt %), acetylene black (25 wt %), and a poly(vinylidenefluoride) binder (KF#7305, Kureha) (5 wt %) dispersed in 1-methyl-2-pyrrolidinone. The slurry was applied to a current collector consisting of a Ni foil and then dried at 120 °C for 12 h in a vacuum oven. A Li-foil and a polypropylene film (Celgard 3501) were used as the counter electrode and separator, respectively. The electrolyte used was 1 mol dm-3 LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v, Tomiyama Chemical). Unless otherwise stated, the charge/discharge cycles were carried out at a constant current rate of 0.01 C, which corresponds to the current density (ca. 0.2 μA cm-2) so that each charge and discharge process should theoretically be completed in 100 h, with a relaxation period of 1 h at the end of each discharge/ charge measurement. The charging and discharging processes ended at 3.0 and 4.5 V, respectively. Cell assembly was conducted under an Ar atmosphere with a dew point below -70 °C. Figure 1 shows the XRD patterns of the resultant particles. All the peaks were identified as olivine LiMnPO4 (space group: Pnma). The LiMnPO4 particles were easily dispersed in hexane to form a transparent sol, which indicates that very small-sized and highly dispersive particles were obtained. Figure 2 shows typical (a) low- and (b) high-magnification TEM images of LiMnPO4 particles. Fairly uniform particles were observed, and their size was ca. 7 nm, although some were larger, with a particle size of around 10 nm. The particles were highly dispersed with regard to each other. The nanosized LiMnPO4 particles gave a FT-IR spectrum very similar to that of conventional micrometersized LiMnPO4 powder, except for two peaks at around 1450 and 1550 cm-1 (see Supporting Information, Figure 1S). Based on the literature, these two bands indicate that oleic acid molecules are attached to the LiMnPO4 particles.17 Hence, the LiMnPO4 particles could be dispersed by the effect of steric hindrance between oleic acid molecules, which is in good agreement with the TEM observations, as shown in Figure 2. Figure 3 shows charge (extraction of Liþ from LiMnPO4) and discharge (insertion of Liþ into MnPO4) curves of the LiMnPO4 particles.
Figure 3. Charge and discharge curves of LiMnPO4 particles in the initial three cycles.
The initial charge capacity was only 30 (mA h) g-1. This value is much smaller than the theoretical capacity of 171 (mA h) g-1 even though the charge and discharge rates were relatively slow. The initial discharge capacity was also small, at about 6 (mA h) g-1. In addition, the discharge curve showed a monotonous decrease in voltage, which is quite different from that obtained for micrometer-sized LiMnPO4 powder: LiMnPO4 powder usually gives a clear plateau at around 4 V in charge and discharge curves.12 An oleic acid molecule consists of 18 carbon atoms and is very long. Hence, oleic acid attached to LiMnPO4 particles may prevent electron and/or Li-ion transfer at LiMnPO4. This assumption can be examined by the removal of oleic acid by heat treatment. FTIR spectra of the heat-treated LiMnPO4 particles suggest that oleic acid molecules attached to the LiMnPO4 particles could be removed by heat treatment in Ar (see Supporting Information, Figure 2S). In Raman spectra, two broad peaks appeared at around 1360 and 1600 cm-1 after heat treatment in Ar (see Supporting Information, Figure 3S), which indicates that carbonaceous materials with low crystallinity were formed from oleic acid by heat treatment in Ar. This seems to be favorable for improving the charge and discharge performance of LiMnPO4, since the carbonaceous materials should serve as conductive additives in a LiMnPO4 particle electrode. Figure 4 shows a typical TEM image of the LiMnPO4 particles after heat treatment in Ar. Rods with a length of 100-250 nm and a width of 40-125 nm, but no small particles, were observed. These results indicate that the LiMnPO4 nanocrystals connected to form rods during heat treatment in Ar. Based on the literature, such rodlike particles of LiMnPO4 can be produced by liquid-phase synthesis in a hydrothermal method; LiMnPO4 particles preferably grow in the [101] direction.18 Interestingly, preferential orientation was observed for LiMnPO4 particles even after heat treatment in this
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Figure 4. TEM image of LiMnPO4 particles after heat treatment in Ar at 500 °C for 1 h.
Doi et al. studying the particle-size effect on the electrochemical properties of Li-ion batteries. In summary, nanocrystals of olivine LiMnPO4 were prepared by liquid-phase synthesis using high-boiling-point organic solution. The LiMnPO4 particles had a fairly uniform particle size of around 7 nm, while some were larger. The LiMnPO4 particles were highly dispersed by the effect of steric hindrance between attached oleic acid molecules. Charge and discharge measurements gave very small initial capacities compared to the theoretical capacity of 171 (mA h) g-1 even at reasonably slow charge/ discharge rates. The discharge curve showed a monotonous decrease in voltage, while micrometer-sized LiMnPO4 powder gave a clear plateau at around 4.0 V. This behavior appears to be characteristic of the insertion and extraction of Li-ions in nanosized LiMnPO4 particles. The present results may promote intensive studies on the particle-size effect in battery reactions with the use of uniformly nanosized particles with a well-defined particle size. The present liquid-phase synthetic methods are very useful for preparing such nanosized particles of active materials. Acknowledgment. The authors wish to thank Takeshi Tanaka at the Analytical Center of the Institute for Materials Chemistry and Engineering, Kyushu University, for his help with the TEM observation. This work was supported by the “Project of Development of High-performance Battery System for Nextgeneration Vehicles” from the New Energy and Industrial Technology Development Organization (NEDO). Supporting Information Available: FT-IR spectra of as-prepared LiMnPO4 particles (Figure S1) and FT-IR spectra (Figure S2) and Raman spectra (Figure S3) of heat-treated LiMnPO4 particles. This material is available free of charge via the Internet at http:// pubs.acs.org.
References
Figure 5. Charge and discharge curves of LiMnPO4 particles after heat treatment in Ar at 500 °C for 1 h.
work. Figure 5 shows charge and discharge curves of the LiMnPO4 particles after heat treatment in Ar. The initial charge and discharge capacities were 118 and 65 (mA h) g-1, respectively. These values are larger than those obtained with as-prepared LiMnPO4 particles but still smaller than the theoretical capacity of 171 (mA h) g-1. The charge and discharge capacities were determined using weights of samples including carbonaceous materials. In the discharge curve, a short plateau newly appeared at about 4.0 V with heat treatment. However, a monotonous decrease in voltage was still seen over most of the discharge process even after the removal of oleic acid. Therefore, this behavior may not be caused by the oleic acid molecules attached to LiMnPO4 particles but rather is a characteristic of the insertion and extraction of Li-ions in nanosized LiMnPO4 particles. Micron-sized LiMnPO4 powder usually gives a clear plateau at around 4 V in charge and discharge curves, which is explained by two-phase equilibrium reactions via a first-order transition between LiMnPO4 and MnPO4. On the other hand, the two-phase equilibrium reactions are not likely to occur at nanosized LiMnPO4 particles any longer; a continuous phase transition would occur between LiMnPO4 and MnPO4, and therefore, a continuous change in voltage should be observed in charge and discharge curves, as shown in Figures 3 and 5. These results were obtained with the use of uniformly nanosized particles. Thus, uniformly nanosized particles of active materials are useful for
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