Electrochemical properties of LiNi0.8Co0.1Mn0.1O2 synthesized by sol-Gel and co-precipitation methods - Mai Thanh Tung

Vietnam Journal of Chemistry, International Edition, 54(6): 724-729, 2016 DOI: 10.15625/0866-7144.2016-00394 724 Electrochemical properties of LiNi0.8Co0.1Mn0.1O2 synthesized by sol-gel and co-precipitation methods Mai Thanh Tung * , Vu Duc Luong Department of Electrochemistry and Corrosion Protection, School of Chemical Engineering Hanoi University of Science and Technology Received 9 August 2016; Accepted for publication 19 December 2016 Abstract Layered LiNi0.8Co0.1Mn0.1O2 cathode materials have been prepared by sol-gel and co-precipitation methods. The structural, morphological and electrochemical properties of the materials were compared. The XRD patterns show that both the sol–gel and the co-precipitation method formed single phase materials with good layered characteristics. Electrochemical tests indicate that the material prepared by the co-precipitation method has slightly better electrochemical properties, with an initial discharge capacity of 185 mAhg −1 and capacity retention of 96.85 % after 50 cycles at a cycling rate of 1.0 C, as well as better capability at 7 C. The improved performances of the co-precipitation synthesized material may be attributed to the low Li/Ni disorder. Keywords. Lithium ion batteries, Ni-rich material, co-precipitation, LiNi0.8Co0.1Mn0.1O2, co-precipitation, sol-gel method. 1. INTRODUCTION Lithium ion batteries (LIBs) have numerous outstanding features including high energy density, high conversion efficiency, no gaseous exhaust, improved safety and longer cycle life [1, 2]. The research and promotion of cathode materials are most important in the application potential of LIBs. The application of batteries utilizing layered LiCoO2 has been limited by the relatively low specific capacity and high cost of cobalt application in plug-in hybrid vehicles (PHEVs) and all-electric vehicles (EVs) [3- 5]. Recently, the layered structure series material LiNi1−x−yCoxMnyO2 (NCM) has received increased attention [6-9]. High nickel content NCM materials, such as LiNi0.8Co0.1Mn0.1O2, are very attractive cathode materials for lithium-ion batteries in electric and hybrid vehicle applications because of their relatively low cost and high reversible capacity of approximately 200 mAh·g −1 [9-11]. Various methods have been applied to synthesize Ni-rich such as solid state method [12], sol–gel method [13, 14], chloride co-precipitation [15], and co-precipitation method [16-18]. However, the high nickel content NCM materials are more difficult to synthesize in consistent quality due to the difficulty in completely oxidizing the Ni 2+ to Ni 3+ , even in pure O2 atmosphere. Incomplete oxidation will eventually lead to impurities, large cation disorder and lithium deficiency. Furthermore, high nickel cathode materials are not quite stable when exposed to air for a long time, because they can react with CO2 or H2O in air to form Li2CO3 or LiOH [10]. This process can be accelerated as the temperature is elevated [19], which implies that when calcined in air, there is also a possibility of forming some Li2CO3 or LiOH impurities. Co-precipitation method can be classified into two different strategies, namely carbonate co- precipitation method and hydroxide co-precipitation method. The latter one is a more efficient technology and most often used in industry which can easily provide homogeneous precursor [NixMnyCo1−x−y](OH)2 to get ideal homogeneous and high performance LiNixMnyCo1−x−yO2 cathode material with controllable morphology, high tap- density and better process ability. Co-precipitation is a commonly used method to synthesize material with high homogeneity, high tap-density and good stoichiometry. In this work, we report on the synthesis of LiNi0.8Co0.1Mn0.1O2 cathode materials by sol–gel (SG) and co–precipitation (CP) methods. The effects of different preparation methods on the structure, morphology and electrochemical performance of LiNi0.8Co0.1Mn0.1O2 cathode materials are investigated. 2. METHODS 2.1. Preparation of materials VJC, 54(6) 2016 Mai Thanh Tung, et al. 725 2.1.1. Sol-gel method (SG) A mixture of LiNO3 (98 %, Sigma-Aldrich), Ni(NO3)2·6H2O (98 %, Sigma Aldrich), Co(NO3)2·6H2O (98 %, Sigma-Aldrich) and Mn(NO3)2·4H2O (97%, Sigma-Aldrich) with a molar ratio of Li:Ni:Co:Mn = 1.05:0.8:0.1:0.1 was dissolved in distilled water. Citric acid (99%, Sigma- Aldrich) was also dissolved in distilled water in a separate container (the molar ratio of metal ions: citric acid = 1:1). The two solutions were mixed together, and the pH of the solution was adjusted to 7.0 by adding NH3·H2O (Sigma-Aldrich). The mixture was stirred for 24h at room temperature. The solution was evaporated under continuous stirring at 80 o C until the viscidity green aquogel was formed. After drying at 120 o C in a drying oven overnight, the xerogel was crushed, subsequently heated at 480 o C for 4 h in oxygen atmospheric to decompose the organic constituents and nitrate components. The sample was then grounded, pelletized and calcined at 800 °C for 16 h under oxygen atmosphere. After being cooled to room temperature, the LiNi0.8Co0.1Mn0.1O2 material was obtained. We denote sample name by SG-NMC 2.1.2. Co-precipitation method (CP) The precursor Ni0.8Co0.1Mn0.1(OH)2 was prepared by co-precipitation method [20]. At first, stoichiometric amounts of NiSO4·6H2O, CoSO4·7H2O, and MnSO4·6H2O were dissolved together in distilled water to get a transparent solution which was pumped into reactor (capacity 1.5 L) under N2 atmosphere. At the sample time, the desired amount of NaOH solution and NH4OH solution as a chelating agent were separately added dropwise to the transition metal sulfate solution. During the reaction process, the pH, temperature and stirring speed were carefully controlled. The spherical Ni0.8Co0.1Mn0.1(OH)2 powders were washed with de-ionized water and dried vacuum at 110 o C for 24 h. Finally the Ni0.8Co0.1Mn0.1(OH)2 precursors were mixed with 5% excess LiOH·H2O and preheated at 480 °C for 5 h and at 800 for 16 h in air atmosphere to obtain the target compound of LiNi0.8Co0.1Mn0.1O2 powders. We denote sample name by CP-NMC. 2.2 Characterizations The crystalline structure of samples was characterized by X-ray diffraction (XRD) measurements using a Rigaku D Max/2000 PC with a CuKα radiation in the 2θ angular range of 10 to 80 o at a scanning rate of 4 o min -1 . The particle morphology and element composition of the powders were observed by using scanning electron microscope (SEM, Hitachi S-4800). 2.3. Electrochemical Characterization Electrochemical cycling of the synthesized materials was performed in coin cells (CR2016) at room temperature. The cathode was prepared by tape casting a mixed slurry onto aluminum foil (battery grade) by doctor blade. The slurry was composed of 80 wt.% active cathode material, 10 wt.% Super-P carbon black, and 10 wt.% polyvinylidene difluoride (PVDF, Kynar, reagent grade) binder dissolved in N-methyl l-2-pyrrolidene (NMP, Sigma-Aldrich, N99%). After drying the tape casted cathodes were dried overnight at 120 °C in a vacuum chamber, and the CR2016 coin cells were assembled in an argon filled glove box (moisture lower than 2 ppm). 1M LiPF6 (Aldrich, ≥ 99.99 %) in 1:1 ethylene carbonate (EC, Sigma, 99 %)/diethyl carbonate (DEC, Aldrich, ≥ 99 %) was used as electrolyte, lithium foil as anode and Celgard 2400 membrane as separator. The cathode electrode loading was about 7 mg cm 2 . After assembly, the cells were allowed to rest for 15 h before electrochemical characterization. For the rate capability test, the cells were charged to 4.3 V with a current density of 0.1, 0.5, 1.0, 3.0, and 7.0 C, then kept at 4.3 V until the current density was below 0.1 C (18 mAg −1 ), followed by discharging at the same rate as the charging rate. Long term cycling was performed at 1.0 C for 50 cycles. Cyclic voltammetry of the electrodes was obtained from test cell with a VMP3 electrochemical workstation (Bio-Logic, France) in the potential range of 3.0-4.3 V at a scanning rate of 0.1 mV·s −1 . 3. RESULTS AND DISCUSSION The morphology of the LiNi0.8Co0.1Mn0.1O2 powders synthesized by the two different methods, investigated by scanning electron microscopy, was shown in Fig. 1. The powders of the CP_NMC sample consist of particles with a diameter of 100-200 nm while that is 10- 20 nm of SG_NMC sample. It seems that the sol-gel sample has smaller primary particle size but is in a serious aggregation. The aggregation makes it more difficult to break up during preparation of the electrode, which would give a smaller total surface area and hence less active surface area in contact with the electrolyte compared to the more porous VJC, 54(6) 2016 Electrochemical properties of LiNi0.8Co0.1Mn0.1O2 726 sol–gel synthesized sample. Figure 1: SEM pictures of the (a) CP_NMC and (b) SG_NMC samples Fig. 2 displayed the X-ray diffraction patterns (XRD) of the LiNi0.8Co0.1Mn0.1O2 powders synthesized via sol–gel and co-precipitation method. All the diffraction peaks can be indexed on the basis of a hexagonal structure of α-NaFeO2-type (space group R-3m) [21], and no impurity phase is detected in the patterns. In the layered structure, a good resolution of the (006)/ (102) and the (108)/(110) reflection pairs is typical of an ideal layered structure [22]. The lattice parameters results and reliability factors are summarized in table 1. The relative intensities of the certain peaks in XRD and the value of c/a demonstrate the crystallization and the level of anti site disordering between Ni 2+ and Li + . The lattice parameter a, c and ratio c/a both of two samples are nearly similar. 10 20 30 40 50 60 70 80 In te n si ty ( a. u ) 2 (degree) SG_NMC CP_NMC Figure 2: The XRD pattern of the SG_NMC and CP_NMC samples Cation mixing is known to deteriorate the electrochemical performance of the layered materials. The intensity ratio of I003/I104 is a sensitive parameter for determining the cation distribution in the lattice of the layered oxide, and a value lower than 1.2 indicates a high degree of cation mixing which is an indication of undesirable cation mixing. From data in table 1, Rw of two samples is greater than 1.2 which shows low Ni 2+ and Li + antisite disordering. It can be provided high electrochemical performance. Table 1: Lattice parameters of LiNi0.8Co0.1Mn0.1O2 prepared with sol-gel and co-precipitation method LNMCO a c c/a Rw SG-NMC 2.8378 14.2019 5.004 1.4018 CP-NMC 2.8753 14.2287 4.9486 1.4131 The electrochemical performances of Li/LiNi0.8Co0.1Mn0.1O2 cells have been investigated. The initial charge - discharge curves of LiNi0.8Co0.1Mn0.1O2 material at a discharge rate of 0.1C (18 mA g -1 ) in the potential range from 3.0 to 4.3 V at 25 o C were shown in Fig.3 (a). The material exhibited one plateau during the first charge, due to the existence of one lithium de-insertion process. This plateau is associated to the delithiation that corresponds to the oxidation of Ni 2+ → Ni4+. The initial discharge capacity of the sample synthesized by the co-precipitation method is 185 mAhg −1 with a columbic efficiency of about 84.09 %. The result is comparable to that of the LiNi0.8Co0.1Mn0.1O2 reported in literature [12], which was also prepared by co-precipitation method. The sample synthesized by the sol–gel method delivered an initial discharge capacity of 182 mAhg −1 with a columbic efficiency of about 86.25 %. Fig. 3 (b) showed the cycling performance of LiNi0.8Co0.1Mn0.1O2 synthesized by the sol–gel and the co-precipitation methods cycled at a current rate of 1 C between 3.0 and 4.3 V. The CP_NMC sample showed the highest specific discharge capacity. The cell delivers a capacity of 165.232 mAhg -1 at 1 st cycle and 160.035 mAhg -1 at 50 th cycle, while capacity of 161.395 mAhg -1 at 1 st cycle and 131.729 mAhg -1 at 50 th cycle of SG_NMC sample, respectively. The samples synthesized by the sol–gel method show capacity retention of 87.83 % and 81.61 % for 30 and 50 cycles, respectively. The sample synthesized by co-precipitation shows higher capacity retention of 98.15 %, and 96.85% during the same cycling period. From the results of initial charge–discharge, rate capability and cycling performance, we found that the sample synthesized by the co-precipitation method displayed slightly higher initial discharge capacity, improved capacity retention and better rate capability at high discharge rates compared to the sample synthesized by the sol-gel method. The 1 µm 200 nm 1 µm 5 µm a b VJC, 54(6) 2016 Mai Thanh Tung, et al. 727 better electrochemical performance of the co- precipitation synthesized material might be attributed to the lower Li/Ni disorder, the latter providing better contact between the electrolyte and the active material. The fact that the sol-gel sample has a higher Li/Ni disorder leads to lower Li diffusivity, which will drive down its capacity at high rates. 0 30 60 90 120 150 180 210 240 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 P o te n ti al ( V v s. L i+ / L i) Discharge capacity (mAhg -1 ) CP_NMC SG_NMC (a) Discharge capacity (mAhg -1 ) 0 5 10 15 20 25 30 35 40 45 50 0 20 40 60 80 100 120 140 160 180 SG_NMC CP_NMC D is ch ar g e ca p ac it y / m A h g -1 Cycle number (b) 3 6 9 12 15 18 40 60 80 100 120 140 160 180 200 220 240 0.1C 3C 7C 0.5C 1C D is ch ar g e ca p ac it y / m A h g -1 Cycle number SG_NMC CP_NMC 0.1C (c) a Figure 3: Electrochemical properties of samples in a voltage window of 3.0-4.3 V at 25 o C: (a) Initial charge – discharge capacity at a rate of 0.1C and (b) discharge capacity vs cycle number at a rate of 1C and (c) rate capability at various C-rates Table 2: Specific discharge capacity at various C-rates of SG_NMC and CP_NMC samples LNMCO 0.1C 0.5C 1C 3C 7C SG-NMC 182.59 168.72 160.17 144.88 110.60 CP-NMC 185.79 174.12 167.36 154.85 140.62 Fig.3 (c) showed the rate performance of the samples with various current densities between potential limits of 3.0-4.3 V. The cell is charged at a current density of 92.5 mAg -1 (0.5C) before each discharge test. The cells are first cycled at 0.1 C (18.5 mAg −1 ) and then at 0.5 C (92.5 mAg −1 ), 1 C (185 mAg −1 ), 3 C (555.5 mAg −1 ) and 7 C (1285 mAg −1 ) for every discharge cycles. From 0.1C to 1C, the two materials show similar electrochemical performances. However, when the discharge current rate is increased to 7 C, the sample synthesized by the co-precipitation method shows more stable and higher capacity compared to the sample synthesized by the sol-gel method which was shown more detail in Table 2. For instance, the discharge capacity of the CP_NMC sample is about 167 and 140 mAhg −1 at 1 C and 7 C, while the discharge capacity is 160 and 110 mAh·g −1 of SG_NMC sample, respectively. When the discharge current rate goes back to 0.1 C, the discharge capacity of the two samples showed much difference, the capacity of CP_NMC sample returned to approximately 188 mAh·g −1 higher than that of SG_NMC sample. The difference in the discharge capacity between the sol–gel and the co- precipitated samples at high C-rates can possibly be explained by the faster charge transfer in the co- precipitation synthesized sample due to higher density and higher porosity. 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 -2 -1 0 1 2 3 4 CP_NMC SG_NMC I (m A ) E (V) E(V) Figure 4: Cyclic voltammetry of 2 samples SG_NMC and CP_NMC at 0.1 mV s-1 in a voltage range 3.0-4.3 V Fig. 4 shows the CV curves of LiNi0.8Co0.1Mn0.1O2 synthesized via the sol–gel and the co-precipitation method for the initial two cycles in the potential range of 3.0-4.3 V at a scanning rate of 0.1 mVs −1 . During charge–discharge, layered cathode materials with high nickel content such as LiNiO2 and LiNi0.8Co0.15Al0.05O2, often exhibit four different phases (one monoclinic phase, M, and three hexagonal phases, H1, H2, and H3) [23]. P o te n ti al ( V v s. L i+ /L i) D is ch ar g e ca p ac it y /m A h g -1 I (m A ) D is ch ar g e ca p ac it y /m A h g -1 VJC, 54(6) 2016 Electrochemical properties of LiNi0.8Co0.1Mn0.1O2 728 Table 3: Redox peak of cyclic voltammetry analysis of CP_NMC and SG_NMC electrodes Sample Peak Oxidation (V) Reduction (V) Polarization (V) SG_NMC 1 st cycle 4.032 3.653 0.379 2 nd cycle 3.866 3.611 0.255 CP_NMC 1 st cycle 3.936 3.703 0.233 2 nd cycle 3.831 3.699 0.132 As shown in Fig. 4, the curves of the two samples both show characteristic of layered oxide cathodes with a couple of significant redox peaks corresponding to Ni 2+ /Ni 4+ reaction. However, the activation peak disappears in the second cycle and a broad peak appears at about 3.85 V, which is the main anodic peak for the newly formed oxide, containing the oxidation of Ni and Co element. The Ni 2+ /Ni 4+ reaction contributes mainly to the charge/discharge capacity of NMC and the difference of their peak voltages (ΔEp) reflects the polarization for lithium insertion or extraction. Through comparing the oxidation peaks of the SG_NMC and CP_NMC samples (table 3), it is clear that the polarization of CP_NMC sample is smaller than that of SG_NMC, implying that the kinetics behavior has been improved. The improvement herein can be ascribed to the CP_NMC which is good for the extraction of lithium during charge. It helps to understand why the material is often facing rapid capacity loss during cycling as shown in Fig. 3(b). 4. CONCLUSION In conclusion, we reported high capacity nickel- based LiNi0.8Co0.1Mn0.1O2 cathode materials synthesized by sol–gel and co-precipitation methods. The effects of different synthesis methods on the crystal structure, morphology, and electrochemical properties of the materials have been investigated. The two samples prepared by the different methods show good layered characteristics and high crystallinity. The electrochemical study showed that the sample prepared by the co-precipitation method has better electrochemical properties, with a higher initial discharge capacity of 185 mAhg −1 and capacity retention of 96.85 % after 50 cycles at a cycling rate of 1.0 C, as well as better capability at 7 C. The better electrochemical properties of the co- precipitation sample may be ascribed to the lower cation (Li/Ni) disorder. The LiNi0.8Co0.1Mn0.1O2 material is attractive as a positive electrode material for high power energy lithium-ion batteries, and this work shows that low cost and environmentally friendly co-precipitation synthesis is a very promising method to achieve high performance cathodes. Acknowledgement. This work was supported by the Nippon Sheet Glass Foundation for Materials Science and Engineering. REFERENCES 1. K. Ozawa, Lithium-Ion Rechargeable Batteries with LiCoO2 and Carbon Electrodes: The LiCoO2/C System, Solid State Ionics, 69, 212-221 (1994) 2. T. Ohzuku, A. Ueda, M. Nagayama, Y. Iwakoshi, and H. Komori, Comparative Study of LiCoO2, LiNi1/2Co1/2O2 and LiNiO2 for 4 Volt Secondary Lithium Cells, Electrochim. Acta, 38, 1159-1167 (1993) 3. K. Mizushima, P. C. Jones, P. J. Wiseman, and J. B. Goodenough. LixCoO2 (0<x<1): A New Cathode Material for Batteries of High Energy Density, Mater. Res. Bull., 15, 783-789 (1980) 4. J. B. Goodenough and Y. Kim. Challenges for Rechargeable Li Batteries, Chem. Mater., 22, 587- 603 (2010) 5. B. L. Ellis, K. T. Lee, and L. F. Nazar. Positive Electrode Materials for Li-Ion and Li-Batteries, Chem. Mater., 22, 691-714 (2010) 6. D. Abraham et al. Surface Changes on LiNi0.8Co0.2O2 Particles during Testing of High-Power Lithium-Ion Cells, Electrochem. commun., 4, 620-625 (2002) 7. H. Bang, D.-H. Kim, Y. C. Bae, J. Prakash, and Y.-K. Sun. Effects of Metal Ions on the Structural and Thermal Stabilities of LiNixCoyO2 (x+y ≤0.5) Studied by In Situ High Temperature XRD, J. Electrochem. Soc., 155, A952 (2008) 8. I. Belharouak, Y.-K. Sun, J. Liu, and K. Amine. Li(Ni1/3Co1/3Mn1/3)O2 as a Suitable Cathode for High Power Applications, J. Power Sources, 123, 247-252 (2003) 9. I. Belharouak, W. Lu, D. Vissers, and K. Amine. Safety Characteristics of Li(Ni0.8Co0.15Al0.05)O2 and Li(Ni1/3Co1/3Mn1/3)O2, Electrochem. Commun., 8, 329-335 (2006) 10. J. Eom, M. G. Kim, and J. Cho. Storage Characteristics of LiNi0.8Co0.1+xMn0.1-xO2 (x=0, 0.03, and 0.06) Cathode Materials for Lithium Batteries, J. Electrochem. Soc., 155, A239 (2008) 11. K. K. Cheralathan et al. Preparation of Spherical LiNi0.8Co0.15Mn0.05O2 Lithium-Ion Cathode Material by Continuous Co-Precipitation, J. Power Sources, 195, 1486-1494 (2010) 12. J. J. Saavedra-Arias et al. Synthesis and VJC, 54(6) 2016 Mai Thanh Tung, et al. 729 Electrochemical Properties of LiNi0.8Co0.1Mn0.1O2 Cathode Material: Ex Situ Structural Analysis by Raman Scattering and X-Ray Diffraction at Various Stages of Charge–discharge Process, J. Power Sources, 183, 761-765 (2008). 13. G. T. K. Fey, V. Subramanian, and C. Z. Lu. Tartaric Acid-Assisted Sol-Gel Synthesis of LiNi0.8Co0.2O2 and Its Electrochemical Properties as a Cathode Material for Lithium Batteries, Solid State Ionics, 152-153, 83-90 (2002). 14. G. T. Fey, R. Shiu, V. Subramanian, J. Chen, and C. Chen. LiNi0.8Co0.2O2 Cathode Materials Synthesized by the Maleic Acid Assisted Sol–gel Method for Lithium Batteries, J. Power Sources, 103, 265-272 (2002). 15. L. I. Ling-jun, L. I. Xin-hai, and W. Zhi-xing. Synthesis of LiNi0.8Co0.1Mn0.1O2 cathode material by chloride co-precipitation method, 8-11, T. Nonferrous Met. Soc. China (2009). 16. C. Fu et al. Nickel-Rich Layered Microspheres Cathodes: Lithium/Nickel Disordering and Electrochemical Performance, ACS Appl. Mater. Interfaces, 6, 15822-15831 (2014). 17. L. Liang et al. Co–precipitation Synthesis of Ni0.6Co0.2Mn0.2(OH)2 Precursor and Characterization of LiNi0.6Co0.2Mn0.2O2 Cathode Material for Secondary Lithium Batteries, Electrochim. Acta, 130, 82-89 (2014). 18. Y. Zhang, H. Cao, J. Zhang, and B. Xia. Synthesis of LiNi0.6Co0.2Mn0.2O2 Cathode Material by a Carbonate Co-Precipitation Method and Its Electrochemical Characterization, Solid State Ionics, 177, 3303-3307 (2006) 19. K.-S. Lee, S.-T. Myung, K. Amine, H. Yashiro, and Y.-K. Sun. Structural and Electrochemical Properties of Layered LiNi1-2xCoxMnxO2 (x=0.1–0.3) Positive Electrode Materials for Li-Ion Batteries, J. Electrochem. Soc., 154, A971 (2007). 20. D.-L. Vu and J. Lee. Properties of LiNi0.8Co0.1Mn0.1O2 as a High Energy Cathode Material for Lithium-Ion Batteries, Korean J. Chem. Eng., 32, 1-13 (2015). 21. H. Lu et al. High Capacity LiNi0.8Co0.1Mn0.1O2 Synthesized by Sol–gel and Co-Precipitation Methods as Cathode Materials for Lithium-Ion Batteries, Solid State Ionics, 249-250, 105-111 (2013). 22. J. J. Saavedra-Arias et al. A Combined First- Principles Computational/experimental Study on LiNi0.66Co0.17Mn0.17O2 as a Potential Layered Cathode Material, J. Power Sources, 211, 12-18 (2012). 23. B. Huang, X. Li, Z. Wang, H. Guo, and X. Xiong, Synthesis of Mg-Doped Li(Ni0.8Co0.15Al0.05)O2 Oxide and Its Electrochemical Behavior in High-Voltage Lithium-Ion Batteries, Ceram. Int., 40, 13223-13230 (2014). Corresponding author: Mai Thanh Tung Department of Electrochemistry and Corrosion Protection School of Chemical Engineering Hanoi University of Science and Technology, Hanoi No. 1, Dai Co Viet, Hai Ba Trung, Hanoi E-mail: tung.maithanh@hust.edu.vn.