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Inorganics 2014, 2(2), 233-247; doi:10.3390/inorganics2020233
Published: 28 May 2014
Abstract: In this work, LiCoPO4 nanoparticles were synthesized by supercritical fluid method using cobalt nitrate hexahydrate (Co(NO3)2 6H2O) and cobalt acetate tetrahydrate (C4H6CoO4 4H2O) as starting materials. The effect of starting materials on particle morphology, size, and the crystalline phase were investigated. The as-synthesized samples were systematically characterized by XRD, TEM, STEM, EDS, BET, and TG and charge-discharge measurements. In addition, Rietveld refinement analysis was performed. The electrochemical measurements of LiCoPO4 nanoparticles have shown differences in capacities depending on the starting materials used in the synthesis and the results have been discussed in this paper.
In recent years, rechargeable lithium ion batteries have received great attention due to their importance in power sources for electric and hybrid electric vehicles and they offer many more opportunities in electric and electronic domain with higher energy densities than the commercially available rechargeable batteries [1,2]. The crucial factors for the success of new cathode materials depend on the synthesis method, starting materials, particle size, cation order, and other experimental parameters . Since the demonstration of electrochemical properties of LiFePO4 cathode material by Padhi et al.  olivine-type LiMPO4 (M = Fe, Mn, Co and Ni) and Nasicon-type Li3V2(PO4)3 have received great interest owing to their low cost, high reversible capacity, and good stability [5,6,7,8]. The potential of the M3+/M2+ redox couple versus Li/Li+ of LiMPO4 is as follows; 3.5 V for LiFePO4, 4.1 V for LiMnPO4, 5.2–5.4 V for LiNiPO4, and 4.8 V for LiCoPO4. Among these, LiCoPO4 is appealing since it offers both a flat high potential (~4.8 V vs. Li/Li+) and good theoretical capacity (167 mA h/g) . The cyclic and rate performances of LiCoPO4 material is affected by electrolyte degradation which is due to high working voltage, low electronic conductivity, and low intrinsic conductivity. Thus far, the first cycle discharge capacities of 30–140 mA h/g with poor or moderate cyclic performances have been observed for LiCoPO4 synthesized via different synthetic routes. In recent days, effort has been made to improve the cyclic performances by cationic doping and decreasing the particle size. Particularly, LiCoPO4 nanoparticles have shown improved electrochemical performances, which is due to a small Li ion diffusion length. In addition, nanoparticles facilitate fast charge and mass transport because of large surface to volume ratio, as well as enhancing the close contact with the electrolyte and which could speed up the reaction kinetics.
LiCoPO4 cathode material can be synthesized by various methods, such as solid-state reaction [10,11,12,13,14,15,16,17], hydrothermal synthesis [18,19,20], sol–gel method [11,21,22], co-precipitation , optical floating zone method , radio frequency magnetron sputtering , electrostatic spray deposition technique , and microwave heating method . In addition, some approaches have been applied in order to improve the electronic conductivity of the cathode, such as carbon coating , making composite with carbon [10,28]. However, most of the above methods required long duration time to synthesize LiCoPO4 particles, and often it is not easy to control the shape and size of the synthesized particles. Among these methods, hydrothermal and solvothermal process have proved to be beneficial in terms of cost, time and energy savings [29,30]. Very recently, we reported supercritical fluid process for controlled synthesis of plate-like LiCoPO4 nanoparticles within a short reaction time . Supercritical fluid method possess unique properties, such as gas like diffusivity, low viscosity, and the density of the reactants closer to that of liquid, these properties leads to the preparation of high quality materials. In recent days, varieties of electrode materials have been successfully synthesized via supercritical fluid process [32,33,34,35,36,37,38]. Recently, our group has reported supercritical fluid synthesis of nanosize lithium metal phosphates [30,38,39], lithium metal silicates [40,41,42,43,44], and flurophosphates .
Herein, we report supercritical fluid synthesis of LiCoPO4 nanoparticles using two kinds of starting materials, such as cobalt nitrate hexahydrate (Co(NO3)2 6H2O) and cobalt acetate tetrahydrate (C4H6CoO4 4H2O). The reaction was carried out at 400 °C within a short reaction period (7 min). The effect of starting materials on the crystalline structure and particles size were investigated and their electrochemical properties have been characterized.
2. Results and Discussion
2.1. One Pot Synthesis
Supercritical fluid process is a one pot synthesis process for LiCoPO4 nanoparticles as shown in Figure 1. Two kinds of precursor solution were used for the synthesis of LiCoPO4 nanoparticles. For the synthesis, 5 ml of precursor solution was fed into the supercritical reactor and heated at 400 °C for 7 min to obtain well-crystallized LiCoPO4 nanoparticles.
2.2. X-ray Powder Diffraction Analysis
The XRD patterns of as-synthesized LiCoPO4 particles at 400 °C for 7 min using Co(NO3)2 6H2O and C4H6CoO4 4H2O as starting materials via supercritical fluid process are shown in Figure 2.
The observed diffraction peaks of both LiCoPO4 particles are well matched with JCPDS file (# 00-085-0002), and belongs to orthorhombic crystal system with Pnma space group. The XRD patterns are in agreement with the reported pattern [23,31,46,47,48,49]. The calculated cell parameters and the Rietveld reliability factors for LiCoPO4 particles synthesized using Co(NO3)2 6H2O and C4H6CoO4 4H2O are a = 10.222Å, b = 5.9273, c = 4.7088, RWP = 29.42%, RI = 13.19% and a = 10.2046Å, b = 5.9183, c = 4.6977, RWP = 27.63%, RI = 16.22%, respectively. The cell parameters are almost consistent with the reported values [50,51,52]. The Rietveld refinement results obtained from XRD pattern are shown in Table 1, the results showed almost same structural parameters for LiCoPO4 nanoparticles synthesized using Co(NO3)2 6H2O and C4H6CoO4 4H2O. The experimental, calculated and observed XRD pattern are shown in Figure S1(a,b) (Supplementary information), which show no significant changes and impurity phases. From the XRD results, it is confirmed that single phase of LiCoPO4 was synthesized using Co(NO3)2 6H2O (Figure 1(a)) and C4H6CoO4 4H2O (Figure 1(b)) as starting materials via supercritical fluid process. The XRD pattern did not shows much difference except variations in the peak intensity. Due to higher solubility of Co(NO3)2 6H2O in water-ethanol, the particles are well crystallized, so that the peak intensity of LiCoPO4 particles synthesized using Co(NO3)2 6H2O is higher than that of LiCoPO4 particles synthesized using C4H6CoO4 4H2O. The model of LiCoPO4 crystal structure is shown in Figure 2(c), which shows the arrangement of cobalt octahedral and phosphor tetrahedral linked with oxygen either by corner sharing or edge sharing. It is also proved that, regardless of using different source materials in the present synthesis, supercritical fluid process is capable of producing high quality LiCoPO4 nanoparticles.
|Table 1. Rietveld refinement results of XRD pattern of LiCoPO4 synthesized using Co(NO3)2 6H2O and C4H6CoO4 4H2O.|
|Formula Crystal |
|LiCoPO4 (Using Co(NO3)2 6H2O) |
|LiCoPO4 (Using C4H6CoO4 4H2O) |
2.3. TEM and HRTEM Analysis
The as-synthesized particles morphology, size and their crystalline nature are observed by TEM and HRTEM analysis and the results are shown in Figure 3(a–d) and Figure 3(e–h). The particles synthesized at 400 °C for 7 min using Co(NO3)2 6H2O (Figure 3(a–d)) and C4H6CoO4 4H2O (Figure 3(e–h)) as starting materials via supercritical fluid process show mixed type morphologies, such as sphere, plate, and rod. The particles size are ranging from 20–50 nm in diameter for as-synthesized LiCoPO4 particles using Co(NO3)2 6H2O (Figure 3(a,b)) and C4H6CoO4 4H2O (Figure 3(e,f)) as starting materials. The as-synthesized particles are well distributed and softly agglomerated. The ED pattern and HRTEM images of as-synthesized particles using Co(NO3)2 6H2O and C4H6CoO4 4H2O are shown in Figure 3(c,d,g,h), respectively. The SAED images of both samples (Figure 3(c,g)) are exhibited dot like pattern, which clearly shows the single crystalline nature of LiCoPO4 particles. However, as-synthesized particles using Co(NO3)2 6H2O (Figure 4(c)) show weaker diffraction than the SAED pattern of as-synthesized particle using C4H6CoO4 4H2O (Figure 3(g)). The HRTEM images show well resolved lattice fringes exhibiting interplanar spacing along  and  axis as shown in Figure 3(d,h). The interplanar spacing along a axis show 1.02 nm which is consistent with the unit cell parameter of olivine structured LiCoPO4 along a-axis (a = 10.02 A°). LiCoPO4 nanoparticles synthesized using two kinds of starting materials within a short reaction time via supercritical fluid process show well crystalline nanosize particles. In order to investigate effect of temperature and time on the particles formation, synthesis was carried out using Co(NO3)2 6H2O and C4H6CoO4 4H2O as starting materials and the results are shown in Table 2. When the reaction temperature was 300 °C, sphere like particles were observed at 7, 10, and 15 min of reaction time with particles size ranging from 10–30 nm, the XRD pattern of this particles show nearly amorphous phase and particles were covered with lot of carbonaceous materials. When the reaction temperature was increased to 350 °C for about 7, 10, and 15 min of reaction time, the obtained particles were sphere and rod like particles with 20–70 nm in diameter. The XRD of these particles show mixed phase of LiCoPO4, and Li3PO4 phase. Single phase LiCoPO4 particles were obtained only at 400 °C for about 7, 10, and 15 min of reaction time, which were the optimum experimental conditions for the formation of LiCoPO4 under supercritical fluid conditions.
|Table 2. Synthesis conditions for LiCoPO4 particles using Co(NO3)2 6H2O and C4H6CoO4 4H2O as starting materials.|
|Reaction Time |
|Morphology||Particle size |
|300||7,10 &15||Sphere||10-30||nearly amorphous|
|350||7,10 &15||Sphere & rods||10-30||LiCoPO4, Li3PO4|
|400||7,10 &15||Sphere, plates & rods||10-30||LiCoPO4|
2.4. Elemental Mapping
The as-synthesized particles using Co(NO3)2 6H2O and C4H6CoO4 4H2O as starting materials were subjected to STEM analysis to verify the purity of LiCoPO4 nanoparticles. Figure 4 shows the elemental mapping of both the particles.
The uniform distribution of oxygen, phosphor and cobalt elements can be clearly observed in the STEM images. This result indicated that the as-synthesized particles have homogeneous elemental distribution and high purity without any impurities. The EDS spectrum of the as-synthesized using Co(NO3)2 6H2O and C4H6CoO4 4H2O as starting materials are shown in Figure 5. The particles exhibited characteristic peaks of Co, P, and O elements in LiCoPO4 nanoparticles. The peak below 0.3 eV is corresponds to the carbon peak, the intensity of C peak is higher for the LiCoPO4 nanoparticles synthesized using C4H6CoO4 4H2O as starting material due to the formation of carbon after dissociation reaction of C4H6CoO4 4H2O at supercritical fluid conditions. However, presence of carbon in these cathode materials is beneficial in order improve the conductivity of the LiCoPO4 nanoparticles during electrochemical reactions [46,47].
2.5. BET and TG-Analysis
The surface area of the as-synthesized particles using Co(NO3)2 6H2O and C4H6CoO4 4H2O are analysed by BET analysis. The surface area of 14.3 m2/g for LiCoPO4 nanoparticles synthesized using Co(NO3)2 6H2O and 16.5 m2/g for LiCoPO4 particles synthesized using C4H6CoO4 4H2O were observed. Generally, nanosize particles with higher surface area are beneficial for enhanced electrochemical reactions.
The carbon content in the as-synthesized LiCoPO4 nanoparticles using Co(NO3)2 6H2O and C4H6CoO4 4H2O are analyzed by TG analysis. The weight loss of 16 wt% for LiCoPO4 nanoparticles synthesized using Co(NO3)2 6H2O and 18 wt% for LiCoPO4 particles synthesized using C4H6CoO4 4H2O were observed as show in Figure 6. The observed weight loss is the weight of carbon present in the as-synthesized particles. This carbon could be amorphous material and is not conductive, which is not useful in improving the electronic conductivity of LiCoPO4 electrode materials.
2.6. Electrochemical Performance
The electrochemical performances of as-synthesized particles using Co(NO3)2 6H2O and C4H6CoO4 4H2O were measured by galvanostatic charge-discharge method. Figure 7 shows the discharge profiles and cyclic performance (Figure 7(c)) of LiCoPO4 nanoparticles synthesized using Co(NO3)2 6H2O (Figure 7(a)) and C4H6CoO4 4H2O (Figure 7 (b)) at 400 °C for 7 min via supercritical fluid process. The discharge measurement was carried out at 0.1C rate for LiCoPO4 nanoparticles, which show a flat discharge curve around 4.75 V. The charge capacities of 150 and 165 mAhg−1 for first and second cycle was observed for LiCoPO4 nanoparticles synthesized using Co(NO3)2 6H2O as starting material. The charge capacities of 140 mA h g−1 and 152 mA h g−1 for the first and second cycle were observed for LiCoPO4 nanoparticles, which were synthesized using C4H6CoO4 4H2O as starting material. The discharge capacities of 98 mA h g−1 and 115 mA h g−1 (Figure 7 (a)) for the first cycle and 62 mA h g−1 and 101 mA h g−1 (Figure 7(b)) for the second cycles were observed. The origin of discharge capacity of 20 mA h g−1 around 2.5–3 V with slope like profile is might be due to the effect of particle size because of high surface area than the bulk particles, which could be responsible for secondary reactions. However, the observed discharge capacities are reasonable and even higher than that of few recently reported discharge capacities of LiCoPO4 particles synthesized by different routes [20,22].
LiCoPO4 nanoparticles synthesized using C4H6CoO4 4H2O show the highest discharge capacities, which might be due to the presence of carbon residue derived from decomposition of C4H6CoO4 4H2O at the supercritical fluid reaction. Probably, this carbon was coated onto the surface of LiCoPO4 nanoparticles, which results in better electronic mobility. However, reasonable discharge capacities observed for both the LiCoPO4 nanoparticles, which are due to the small particle size, good crystallinity, and due to the nature of starting materials used in the synthesis. However, poor cyclic performance was observed for LiCoPO4 nanoparticles (Figure 7 (c)), which is mainly due to the instability of electrolyte at 5 V, and due to the side reactions between electrolyte and electrode and results in the formation a solid-electrolyte interface (SEI) which cause the electrolyte oxidation and lithium loss . Usually, LiCoPO4 cathodes suffered from poor cycling stability . Another possible reason may be due to the antisite defects, where Li+ site is occupied by Co atom upon charge process and which block the Li+ during discharge process, so deficiency of lithium is occurred during electrochemical reaction . Further, characterization is necessary to investigate on antisite defects in olivine structured cathode materials, for that ADF/ABF analysis might be very useful to locate the cobalt atoms in lithium site.
3. Experimental Section
3.1. Synthesis of LiCoPO4 Nanoparticles
LiCoPO4 nanoparticles were synthesized using two kinds of starting materials such as cobalt (II) nitrate hexahydrate (Co(NO3)2 6H2O) and cobalt (II) acetate tetra hydrate (C4H6CoO4 4H2O). In a typical synthesis, cobalt (II) nitrate hexahydrate or cobalt (II) acetate tetra hydrate (Wako, Japan), orthophosphoric acid (o-H3PO4: Wako, Japan) and lithium hydroxide monohydrate (LiOH H2O: Wako, Japan) were used in 1:1:1 molar ratio. Ascorbic acid (Wako, Japan) was used as reducing agent. The above starting materials were dissolved in mixed solvents of water and ethanol (1:1 volume ratio), the solution was stirred for 15–20 min and then 0.5 g of ascorbic acid was added to the solution. Later, the solution mixture (5 mL) was transferred to batch reactors (4 reactors, each 10 mL volume) followed by heating at 400 °C for 7 min, then reactors were quenched in cold water, the products were recovered through repeated washings using ethanol and water.
3.2. Material Characterization
The crystalline phase of the samples were characterized using powder X-ray diffraction (XRD; Rigaku RINV-2200, 40 kV and 30 mA) with CuKα radiation (λ = 1.5406 Å). The XRD patterns were analyzed by the Rietveld method using the program RIETAN-2000 . The particles morphology, purity and elemental distribution was observed by transmission electron microscopy (JEOL TEM-2100F), elemental mapping and energy dispersive spectroscopy (EDS, JEM-2010F at 200 KeV). Selected area electron diffraction (SAED) and lattice fringes observation done by using high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2100F, 200 kV). The surface area was measured using a Brunauer–Emmet–Teller (BET, NOVA 4200e) surface area and pore size analyzer. Thermo-gravimetric (TG) analysis was performed in air using a TG-DTA 2000S system to ascertain the exact carbon content in the LiCoPO4 cathode material; the temperature was limited to 650 °C in order to avoid the oxidation of LiCoPO4.
3.3. Electrochemical Performance
The electrochemical performance of LiCoPO4 was investigated using coin-type cells (CR2032). The working electrodes is composed of 83 wt% LiCoPO4 mixed with 10 wt.% acetylene black and 7 wt.% PTFE (poly(tetrafluoroethylene)) as a binder. The electrode paste was spread uniformly and vacuum dried for 12 h at 160 °C. Later, cathode was punched into circular discs and cut into wafers (7 mm in diameter, 0.025 mm in thickness, 3–5 mg). The tested cell was assembled inside an argon-filled glove box. For electrochemical measurements, the cell is composed of lithium metal counter, reference electrodes and a LiCoPO4 positive electrode. 1 M LiPF6 in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) with 1:1 in volume ratio (Tomiyama Pure Chemical Co., Ltd., Tokyo, Japan) was used as the electrolyte. The galvanostatic charge-discharge cycling was performed between 2.5 and 5.1V versus Li+/Li on multi-channel battery testers (Hokuto Denko, Japan). Current densities and specific capacities were calculated on the basis of the weight of LiCoPO4 cathode used in the electrode.
LiCoPO4 nanoparticles were successfully synthesized using cobalt (II) nitrate hexahydrate and cobalt (II) acetate tetrahydrate at 400 °C for 7 min via supercritical fluid process. The as-synthesized particles showed 20–50 nm in diameter with mixed type morphology observed by TEM analysis. The starting materials did not affect much on particles size and morphology of LiCoPO4 particles. The phase purity of as-synthesized particles was confirmed by XRD, Rietveld analysis, STEM, and EDS analysis. Electron diffraction analysis and high-resolution image shows that, the particles are single crystalline and exhibits well-aligned crystal planes without any dislocations or defects. The experimental results show that, supercritical fluid process can synthesize single crystalline LiCoPO4 nanoparticles using different kind of starting materials. Further, application of as-synthesized LiCoPO4 nanoparticles as cathode part to Li-ion battery is tested by galvanostatic charge-discharge methods. LiCoPO4 nanoparticles synthesized using cobalt (II) nitrate hexahydrate and cobalt (II) acetate tetra hydrate show reasonable electrochemical performances. The highest discharge capacity of 115 mA h g−1 for the first cycle is observed for the nanoparticles synthesized using cobalt (II) acetate tetrahydrate. However, poor cyclic performance was observed for these nanoparticles, conductive carbon coating and protection of interface would require improving the cyclic performance.
One of the authors (Murukanahally Kempaiah Devaraju) would like to acknowledge the financial support by Japan Society for Promotion of Science, Japan for their fellowship to carry out the research work.
M.K.D. and I.H. conceived and designed this work. The synthesis of the material and characterization was carried out by M.K.D. and the paper was written by M.K.D., H.H performed Rietveld analysis. T.T. and Q.D.T. participated in discussion of the results.
Conflicts of Interest
The authors declare no conflict of interest.
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