Synthesis, structural and electrochemical properties of Ni-Rich material prepared by a sol-gel method - Mai Thanh Tung

Vietnam Journal of Chemistry, International Edition, 54(6): 760-764, 2016 DOI: 10.15625/0866-7144.2016-00400 760 Synthesis, structural and electrochemical properties of Ni-rich material prepared by a sol-gel method Mai Thanh Tung * , Vu Duc Luong Department of Electrochemistry and Corrosion Protection, School of Chemical Engineering Hanoi University of Science and Technology, Hanoi, Vietnam Received 26 July 2016; Accepted for publication 19 December 2016 Abstract We report on a novel synthetic method of sol-gel processing to prepare Ni-rich cathode materials. We also studied and reported on the electrochemical properties of the resultant products. XRD revealed that a single phase Ni-rich powder can be synthesized by sol-gel processing. The Ni-rich material obtained has a high electrochemical capacity and good cycle ability. These results indicate that sol-gel processing is a promising method for preparing Ni-rich cathode materials. The sample prepared under the optimal conditions had a well ordered hexagonal layered structure. The charge–discharge tests showed that the initial capacities of the sample were 220.456 and 185.937 mAhg-1at the discharge rate of 0.1 C between 3.0 and 4.3 V, respectively. The capacity retention ratio was 81.36 % at 1 C after 50 cycles. Keywords. Lithium-ion battery, cathode material, LiNi0.8Co0.1Mn0.1O2, sol-gel. 1. INTRODUCTION Lithium cobalt oxide (LiCoO2), initially introduced in 1980, has been one of the most widely used positive electrode material in commercial lithium-ion batteries due to its high working voltage, reasonable cycle-life (300-500 cycles), and its easy preparation [1-3]. However, its high cost, toxicity, and the thermal instability of LixCoO2 phases limit its further use in newly developed multifunctional portable devices and electric vehicle systems [4]. LiNiO2 is one of the most attractive next- generation cathode-material candidates for lithium- ion batteries (LIBs) because its reversible capacity is higher and its cost is lower than those of LiCoO2 [2,3,5,6]. However, LiNiO2 suffers from an intrinsic poor thermal stability in its fully charged state and a poor cycle life, both of which are related to the chemical and structural instability of tetravalent nickel. Comparatively, Ni-rich layered LiNi1-x-yMnxCoyO2, wherein the composition of Ni is dominant over the Co and Mn, is a promising material because of a lower cost, less toxicity, an improved thermal stability, a sound cycling stability, and safety [7]. In these materials, layered LiNi0.8Co0.1Mn0.1O2 has been intensively studied as a potential positive active electrode for application in plug-in hybrid electric vehicles (P-HEVs) [8-11]. The LiNi0.8Co0.1Mn0.1O2 solid solution was initially synthesized by the conventional method such as solid-state method. However this method involves very high temperatures for a prolonged period of time with intermediated grinding, this led to problems with poor stoichiometry control, non- homogeneity. In recent years, low temperature wet chemistry methods of synthesizing cathode active materials have gained importance because they do not have above problems and produce material with homogeneous distribution and high surface area. In addition, this technique makes possible the synthesis of nanosized particles, which has two advantages: it increases the effective surface area of the powder with the electrolyte and it increases the lithium intercalation efficiency by reducing the electron path inside the material that has a poor electronic conductivity [12]. In the case of Ni-rich cathode material, several low temperature methods like sol- gel and combustion methods have been reported [13, 14]. In this work, we report the sol-gel synthesis Ni- rich LiNi0.8Co0.1Mn0.1O2 material. The effects of varying each initial condition on the structure, morphology, and electrochemical performances of the Ni-rich cathode material were investigated and the details are discussed in this paper. VJC, 54(6) 2016 Mai Thanh Tung, et al. 761 2. EXPERIMENTAL 2.1. Material synthesis The cathode material LiNi0.8Co0.1Mn0.1O2 was synthesized by sol–gel method. Stoichiometric amounts of lithium acetate, nickel acetate, manganese acetate and cobalt acetate in a cationic ratio Li:Ni:Co:Mn = 1.05:0.8:0.1:0.1 (with 5 % excess of lithium source) were dissolved in distilled water and mixed with an aqueous solution of acid acetic. The mixture was stirred for 24 h 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 at800 °C for 16 h under oxygen atmosphere. After being cooled to room temperature, the LiNi0.8Co0.1Mn0.1O2 material was obtained. 2.2. Material characterization The crystal structures of the LiNi0.8Co0.1Mn0.1O2 material was characterized by an X-ray diffraction (XRD) measurement for which a Rigaku D Max/2000 PC with a CuKα radiation in the 2θ range of 10 o to 80 o was used at a scanning rate of 4 o min -1 . The particle morphology and elemental composition of the powders were analyzed using SEM, whereby the Hitachi S-4800 was equipped with an energy dispersive spectroscopy (EDS, OXFORD 7593-H) capability. 2.3. Electrochemical measurements The electrochemical performances of the LiNi0.8Co0.1Mn0.1O2 material was measured by using the CR2016 coin-type cells. To ensure a high electronic conductivity, 80:10:10 (wt.%) mixture of active material, polyvinylidenedifluoride (PVdF) as a binder and super P carbon as a conduction material respectively was grinded in a mortar. About 3 mg of the mixture were pressed with 90 MPa (5 t at ∅ 13 mm) on aluminium mesh and dried for 24 h in a vacuum oven at 110 °C. Lithium metal was used as a anode electrode, and a microporous-polyethylene separator was inserted between the cathode and the counter electrode and 1molL −1 LiPF6 in an ethylene carbonate (EC) and dimethyl carbonate (DMC) mixture in the ratio EC:DMC (1:1) as electrolyte. All of the coin-type cells were prepared in an Ar-filled glove box in which the oxygen and moisture contents were controlled below 2.0 ppm. The cells were galvano statically charged and discharged at 25 °C in the voltage range 3-4.3 V. 3. RESULTS AND DISCUSSION Figure 1 displayed the XRD patterns of the LiNi0.8Co0.1Mn0.1O2 cathode material. The XRD patterns of the sample showed sharp and clear doublet-peak splits of (006)/(102) and (108)/(110), indicating that samples comprising a well-ordered crystalline structure were formed. All reflections are indexed in agreement with the rhombohedral α-NaFeO2 structure R3m and distinct impurity phases were not found in any of this pattern [15]. The unit-cell parameters were estimated and the results are summarized in table 1. Generally, the c/a value is employed to examine a layered material, for example, the c/a value of the material with an ideal cubic close layered structure is over 4.899 [16]. The refinement converged with lattice parameters a = 2.8378 Å, c = 14.2019 Å and c/a = 5.004. This large value of c/a gives evidence of a well-ordered rhombohedral structure [15]. Table 1: The lattice parameters and the intensity ratios of the (003)/(104) and [(006) + (102)]/(101) for all samples Sample name a c c/a Rw Rf LiNi0.8Co0.1Mn0.1O2 2.8378 14.2019 5.004 1.4018 0.5833 It has been reported that the intensity ratio of I(003)/I(104) reflects the cation mixing degree of the layered structure. In general, the value of I(003)/I(104) over 1.2 is an indication of small cation mixing [17, 18]. From table 1, the I003/I104 ratio of the LiNi0.8Co0.1Mn0.1O2 sample is 1.4 which is greater than 1.2, indicating a low cation disorder between the Li + and Ni 2+ of the LiNi0.8Co0.1Mn0.1O2 samples. The SEM picture of the LiNi0.8Co0.1Mn0.1O2 sample was shown in Fig. 2. The as-prepared LiNi0.8Co0.1Mn0.1O2 powder has well developed regular particles of quasi-spherical shape with a diameter distribution in the range 500-1000 nm, similar to previously reported SEM images [19]. The fast hydrolysis reaction causes micelles to appear rapidly. These rapidly produced micelles are very VJC, 54(6) 2016 Synthesis, structural and electrochemical properties 762 easy to coagulate each other upon their appearance and to form gel quickly. So the micelles do not have enough time to grow separately but reunite together easily. This can be the reason why LiNi0.8Co0.1Mn0.1O2 powder has a small particle size but serious agglomeration. 10 20 30 40 50 60 70 80 1 1 6 0 2 11 1 3 1 1 0 0 1 8 1 0 7 0 1 5 1 0 2 0 0 6 1 0 4 1 0 1 0 0 3 In te n si ty ( a. u .) 2 (degree) 2 (degree) Figure 1: XRD patterns of the LiNi0.8Co0.1Mn0.1O2 sample Figure 2: SEM images of as-prepared LiNi0.8Co0.1Mn0.1O2 powder The electrochemical performance of LiNi0.8Co0.1Mn0.1O2 layered material was examined at 25 °C by charge/discharge and cyclic voltammetry (CV) studies. Fig. 3a displayed the charge–discharge curves as measured during galvanostatic cycling with potential limitation of LiNi0.8Co0.1Mn0.1O2 material at 0.1 C-rate was applied between 3 and 4.3 V vs. Li/Li + . The first charge and discharge capacities of the LiNi0.8Co0.1Mn0.1O2 material are about 220.456 and 185.937 mAhg -1 .The cycling stabilities of the samples at a constant current density of 185.0 mAg -1 (1C rate) between 3.0 V and 4.3 V were also investigated and the results are shown in Fig. 3 (b). The first cycle discharge capacity was 161.395 mAhg -1 and that of the 50 th cycles was 131.729 mAhg -1 with 81.36 % of capacity retention. Rate capability is one of the most important electrochemical-performance measures of the LIBs that are required for high-power devices such as electric hybrid vehicles and power tools. The rate performance of LiNi0.8Co0.1Mn0.1O2 prepared was investigated between 3.0 and 4.3 V. The initial discharge curves of LiNi0.8Co0.1Mn0.1O2 at different charge–discharge rates (0.1 C/0.1 C, 0.5 C/0.5 C, 0.5 C/1 C, 0.5 C/3 C, 0.5 C/4 C, 0.5 C/5 C, 0.5 C/7 C) are shown in Fig. 4(a). It can be seen that the profiles were similar to each other except the faster voltage drop with capacity fading at the higher C-rates, which can be attributed to the increased polarization of the electrodes at high current densities. The polarization increases with increasing current rate as a result of the reduced discharge time for lithium on intercalation into the crystal lattice, as only the surfaces of active materials participate in the reaction [16]. As shown in Fig (4b), for 0.1C, 0.5C, 1C, 3C, 4C, 5C, and 7C, the discharge capacities are 182 mAhg −1 , 168 mAhg −1 , 160 mAhg −1 , 144 mAhg −1 , 137 mAhg −1 , 127 mAhg −1 , and 110 mAhg −1 . The curves demonstrated good rate capability when the C-rates increased from 0.1 C to 7 C, and excellent cycling performance was observed at each C-rate for three cycles. The similar phenomena were found when the C-rates decreased from 7 C to 0.1 C. The main reason for the capacity fading may result from 0 30 60 90 120 150 180 210 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 ) (a) Discharge capacity (mAhg -1 ) 0 5 10 15 20 25 30 35 40 45 50 50 60 70 80 90 100 110 120 130 140 150 160 170 D is ch ar g e ca p ac it y ( m A h g -1 ) Cycle number le number Figure 3: Electrochemical properties of samples in the voltage range of 3.0 V to 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 1C. 1 µm 200 nm In te n si ty ( a. u .) 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 ) VJC, 54(6) 2016 Mai Thanh Tung, et al. 763 the accumulation of lattice defects especially at a high charge and discharge rate, as well as the occurrence of irreversible structural phase transition which led to no enough sites for lithium ion intercalation. 4 8 12 16 20 24 80 90 100 110 120 130 140 150 160 170 180 190 200 5C 4C 3C 1C 0.5C 7C 0.1C D is ch ar g e ca p ac it y ( m A h g -1 ) Cycle number 0.1C (a) Cycle number 0 30 60 90 120 150 180 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 7C 5C 4C 3C 1C 0.5C P o te n ti al ( V v s. L i+ / L i) Discharge capacity (mAhg -1 ) 0.1C (b) Discharge capacity (mAhg -1 ) Figure 4: Rate capability of the LiNi0.8Co0.1Mn0.1O2 from 18.5 mAg -1 to 129.5 mAg -1 with potential limits of 3.0 V to 4.3 V (vs. Li + /Li) at room temperature: (a) specific discharge capacity, and (b) normalized capacity retention rate vs. 0.1C at different C rates The cyclic voltammetry was carried out for LiNi0.8Co0.1Mn0.1O2 to evaluate the reaction progress during charge–discharge experiment. Fig. 5 shows the cyclic voltammetry curves of LiNi0.8Co0.1Mn0.1O2 electrode for initial three cycles. The profiles of the curves are similar except the positive scan for the first cycle, which can be attributed to the cation mixing. It's known that the cation mixing results in obvious irreversible capacity in the initial cycle, which corresponds to significantly reduced peak area in later cycle in the cyclic voltammetric curves. According to the literature [10, 20, 21] the peaks in the cyclic voltammetric curve demonstrate the phase transition along with lithium insertion and extraction. When two phases were coexisted, one peak can be observed. As seen from the Fig. 5, three couples of peaks were found during the charge–discharge process in the second and third cycle. It has been reported that the three peaks occurred in the positive scan correspond to the transition of hexagonal phase (H1) to monoclinic phase (M), monoclinic phase (M) to hexagonal phase (H2), hexagonal phase (H2) to hexagonal phase (H3), respectively. Generally, phase transitions may result in capacity fading due to the irreversible change of the structure. In our work, the sample synthesized at the optimal conditions exhibited excellent cycling performance, as confirmed by the almost overlapping cyclic voltammetric curves during discharge process. 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 I (m A ) E (V) E (V) Figure 5: Cyclic voltammetry of LNMC sample with 0.1 mVs-1 in a range of 3.0-4.3 V 4. CONCLUSION In summary, we have successfully synthesized highly crystalline layered LiNi0.8Co0.1Mn0.1O2 cathode material by conventional sol–gel method. The obtained product develops a well-ordered rhombohedral structure with high c/a and low cation mixing. The structure and morphology of this sample was investigated as a function of the cycling in the voltage range 3.0-4.3 V. In addition, cyclic voltammetric tests demonstrated that there are three reversible phase transitions involved in the charge– discharge process Acknowledgement. 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