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High electrochemical performance of lithium-rich Li1.2Mn0.54NixCoyO2 cathode materials for lithium-ion batteries
Materials Letters 185 (2016) 100–103
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
High electrochemical performance of lithium-rich Li1.2Mn0.54NixCoyO2 cathode materials for lithium-ion batteries Linsen Zhang n, Huan Wang, Lizhen Wang, Hua Fang, Xiaofeng Li, Haili Gao, Aiqin Zhang, Yanhua Song Henan Provincial Key Laboratory of Surface & Interface Science, School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450002 China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 July 2016 Received in revised form 23 August 2016 Accepted 24 August 2016 Available online 25 August 2016
Li1.2Mn0.54NixCoyO2 (xþy ¼ 0.26, and x:y ¼1:1, 1:2, 2:1, 1:3, 3:1) was synthesized by aerogel with urea as chelating agent. The physical and electrochemical properties of the cathode materials were investigated through X-ray diffraction, scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy analyses as well as and charge/discharge tests. Results showed that samples with different Ni/Co ratios crystallized well and showed a layered structure. The sample, Li1.2Mn0.54Ni0.195Co0.065O2 (x:y ¼3:1), delivered a high discharge capacity of 313 mA h g 1 at 20 mA g 1 and exhibited excellent cycle performance after 50 cycles, with a capacity of 247 mA h g 1 at 100 mA g 1. & 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium-rich Energy storage and conversion Aerogel Sintering Lithium-ion battery Electrochemical performance
1. Introduction Electrochemical storage devices with high specific energy and energy density are highly desired to satisfy the urgent needs in low-emission vehicles, such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in HEVs [1,2]. In recent years, the considerable demand for large-scale safe energy storage systems with high energy conversion rate in EVs has inspired numerous studies to optimize the cathode material [3,4]. LiFePO4 and LiNi1/3Mn1/3Co1/3O2 exhibits their own advantages and disadvantages as conventional cathode materials [5,6]. LiFePO4 shows excellent security performance and a long cycle life, but its energy density is low. LiNi1/3Mn1/3Co1/3O2 features higher energy density, but its security performance is inferior to LiFePO4. Therefore, a material with high energy density and excellent electrochemical performance stability must be developed. Li-rich materials have been widely investigated because of their high theoretical capacity ( Z250 mA h g 1) [7]. Although the high capacity is very attractive, the series of cathode materials drawbacks are mainly not only capacity drop but also voltage fade [8– 10]. Of these materials, the Li1.2Mn0.54NixCoyO2 system exhibits potential and has been increasingly investigated [11,12]. In this n
Corresponding author. E-mail address: [email protected] (L. Zhang).
http://dx.doi.org/10.1016/j.matlet.2016.08.118 0167-577X/& 2016 Elsevier B.V. All rights reserved.
work, Li1.2Mn0.54NixCoyO2 was synthesized by aerogel with urea as chelating agent. The influence of Ni/Co (x/y) on the structure, morphology, and electrochemical properties of Li1.2Mn0.54NixCoyO2 cathode materials was evaluated. Results showed that Li1.2Mn0.54Ni0.195Co0.065O2 (x:y¼ 3:1) exhibits excellent electrochemical performance.
2. Experimental Li1.2Mn0.54NixCoyO2 was prepared by aerogel using urea as chelating agent. Stoichiometric amounts of Li(CH3COO)2 2H2O, Mn (CH3COO)2 4H2O, Ni(CH3COO)2 4H2O, and Co(CH3COO)2 4H2O were dissolved in distilled water to obtain a transparent solution. Urea was dissolved in the solution. The mixture solution was then stirred at 80 °C until it became a purple aerogel. The product was annealed at 500 °C for 10 h, and at 900 °C for 10 h in air atmosphere. The structure of the products was characterized by XRD analysis using a D8-ADVANCE (Bruker, Germany) diffractometer. The morphologies of the sample were investigated with a scanning electron microscope (SEM, JSM-6490). Electrochemical measurements were carried out using CR2016 coin cells. Working electrodes were prepared by mixing 80 wt% active materials, 15 wt% Super P, and 5 wt% polyvinylidene fluorides dissolved in N-methylpyrrolidinone. In a glove box with an
L. Zhang et al. / Materials Letters 185 (2016) 100–103
101
argon atmosphere, test cells were assembled with Li foil as counter electrode, Celgard 2400 as separator, and 1 M LiPF6 solution (1:1 mixture of ethylene carbonate and dimethyl carbonate by volume) as electrolyte. Charge/discharge tests were conducted on NEWER system. Cyclic voltammetry analysis (CV; 2.0–4.8 V, versus Li þ /Li) and electrochemical impedance spectroscopy (EIS) were carried out using an electrochemical workstation (CHI660C). All tests were conducted at room temperature.
3. Results and discussion 3.1. Characterization XRD patterns of Li1.2Mn0.54NixCoyO2 with Ni/Co ratios are shown in Fig. 1. All the diffraction lines can be indexed as a layered structure based on α-NaFeO2, with an R-3 m space group [13]. The splitting between (006)/(012) and (018)/(110) indicates a highly crystalline layered structure. In addition, c/a ratio is an index for characterization of the layered structure, and high c/a ratio also indicates less cation disorder [14]. The lattice parameters of the samples and values of c/a and I(003)/I(104) are shown in Table 1. The c/a ratios are close to and larger than 4.9, which suggest a highly layered structure. Moreover, the values of I(003)/I(104), as indicators of cation mixture intensity are higher than 1.2 indicating less confusion of Li þ and Ni2 þ [15]. The sample Li1.2Mn0.54Ni0.195Co0.065O2 (x:y ¼3:1) showed the lightest cation mixture with a value of 1.82, which could lead to excellent electrochemical performance. The SEM images of Li1.2Mn0.54NixCoyO2 are shown in Fig. 2. All samples with different Ni/Co ratios exhibit an aspheric morphology with approximately uniform particle size. The small particle size and high calcination temperature led to a certain degree of aggregation. Li1.2Mn0.54NixCoyO2 with x:y ¼ 3:1 displays less aggregation than the other samples. 3.2. Electrochemical performance Fig. 3 shows electrochemical performance of Li1.2Mn0.54NixCoyO2 with different x/y. The initial charge/discharge curves of Li1.2Mn0.54NixCoyO2 at 20 mA g 1 are presented in Fig. 3a. The plateaus at 4.0–4.5 V represent the shift of Co3 þ Co4 þ , Ni2 þ - Ni4 þ [16]. The platform at 4.5 V represents the activation of the Li2MnO3-like region, accompanied by irreversible prolapse of Li2O [17]. The sample Li1.2Mn0.54NixCoyO2 with x: y¼ 3:1 exhibits an excellent initial discharge capacity of 313 mA h g 1, which corresponds to the XRD and SEM results. Fig. 3b presents the cyclic performance curves of different Ni/ Co ratios in Li1.2Mn0.54NixCoyO2 samples and the charge/discharge curves of sample Ni/Co ¼3:1 in inset. The initial discharge capacities of samples with different Ni/Co ratios at 100 mA g 1 corresponded to 198, 149, 21,784, 235, and 272 mA h g 1, and the 50 cycle discharge capacities corresponded to 206, 192, 217, 216, and 246 mA h g 1. The capacities of samples with Ni/Co¼ 1:3 and 1:2 showed certain degrees of growth during the first few cycles, which could be due to the activation of Li2MnO3 [18,19]. When Ni/ Co ¼3:1, the capacity retention rate was 90.59% after 50 cycles. As previously mentioned, high layered structure, less cation mixture, and even uniform particle, exhibit better cycle stability is consistent. Therefore, the sample Ni/Co¼ 3:1 exhibited high charge and discharge specific capacity. Figs. 3c and 3d present the CV curves of Li1.2Mn0.54Ni0.195Co0.065 O2 (x:y¼ 3:1) and Li1.2Mn0.54Ni0.13Co0.13O2 (x:y¼ 1:1) during the first three cycles. The CV curves are similar, indicating that both samples have similar electrochemistry reaction mechanisms. The first cycle has two high potential oxidation peak: one in the
Fig. 1. XRD patterns of Li1.2Mn0.54NixCoyO2 with different Ni/Co ratios.
Table 1 XRD performance parameters of Li1.2Mn0.54NixCoyO2. Ni/Co
c (Å)
a (Å)
c/a
I(003)/I(104)
1:3 1:2 1:1 2:1 3:1
14.21 14.20 14.05 14.21 14.17
2.84 2.84 2.83 2.85 2.83
4.99 4.99 4.96 4.98 4.99
1.52 1.58 1.65 1.52 1.82
vicinity of 4.1 V, which could be attributed to the reduction of Ni4 þ and Co4 þ [20]; and another close to 4.7 V, which is associated with the reduction of Mn4 þ triggered by electrochemical activation of Li2MnO3 and decomposition of the electrolyte [17]. The CV curve features of the next cycle significantly differed from those in the first cycle. The strongest oxidation peak at 4.7 V gradually weakened and disappeared during the third cycle. The reduction peak in the first cycle occurred mainly in the vicinity of 3.75 V, which corresponds to Ni3 þ , Ni4 þ , and Co4 þ reductions. During the second and third cycles, the emergence of Mn4 þ /Mn3 þ reduction peak near 3.25 V could be due to Li2MnO3 decomposition; this process produced Mn4 þ , which was electrochemically active during the first cycle and demonstrated capacity growth. Fig. 3d reveals that the CV curves of the Li1.2Mn0.54Ni0.195Co0.065O2 electrode have a larger area than those of Li1.2Mn0.54Ni0.13Co0.13O2 electrodes. Thus, Li1.2Mn0.54Ni0.195Co0.065O2 exhibits a larger capacity. Meanwhile, the CV curves of Li1.2Mn0.54Ni0.195Co0.065O2 show better consistency, indicating that the sample of Li1.2Mn0.54Ni0.195Co0.065O2 is electrochemically stable. Fig. 3e displays the EIS curves of Li1.2Mn0.54NixCoyO2 after 50 cycles at 100 mA g 1 and equivalent circuit with parameters. The Nyquist plots contain a semicircle at high frequency, a second semicircle in the middle frequency, and a straight line in the low frequency. The intercept at high frequency is ohmic resistance (Rs). The first semicircle is related to Li þ diffusion through the SEI layer (R1), which is indicated by the first semicircle. The second semicircle is related to charge transfer process occurring at the electrode-electrolyte interface (R2), and the straight line is attributed to Warburg impedance (W) [21,22]. Although the ohmic resistance and Warburg impedance are close, the values of R1 and R2 the sample with Ni/Co ¼3:1 were the smallest. Therefore, the sample with Ni/Co¼ 3:1 showed low electrode polarization and a fast electrode/interfacial reaction, which lead to satisfactory good charge/discharge performance.
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Fig. 2. SEM images of Li1.2Mn0.54NixCoyO2 with different x and y (a, b, c, d, e referring to samples of x:y ¼1:1, 1:2 , 1:1, 2:1, 3:1).
Fig. 3. Electrochemical tests of the Li1.2Mn0.54NixCoyO2: (a) first charge/discharge profiles at 20 mA g 1; (b) cycling performance at 100 mA g 1 and inset is the charge/ discharge curves of sample Ni/Co ¼ 3:1; (c) and (d) CV curves; and (e) EIS. curves of Li1.2Mn0.54NixCoyO2 after 50 cycles at 100 mA g 1 and inset is equivalent circuit with parameters.
4. Conclusions A Li-rich cathode material, namely, Li1.2Mn0.54NixCoyO2, was synthesized by aerogel with urea as chelating agent. The results showed Li1.2Mn0.54Ni0.195Co0.065O2 (x:y¼3:1) had high layered
structure, less cation mixture, and even uniform particle. Li1.2Mn0.54Ni0.195Co0.065O2 exhibited excellent rate capability and cycle performance after 50 cycles, with a capacity of 247 mA h g 1 at 100 mA g 1.
L. Zhang et al. / Materials Letters 185 (2016) 100–103
Acknowledgements This work is supported by foundation of Henan educational committee (No. 13A530366), foundation for young teachers of 2012 Henan Province colleges and universities (ggjs116), foundation for young teachers of Zhengzhou University of Light Industry (2011XGGJS005), and scientific research foundation of Zhengzhou University of Light Industry in 2015 (No. 2015XJJZ036).
References [1] [2] [3] [4] [5] [6] [7] [8]
J.B. Goodenough, K.S. Park, J. Am. Chem. Soc. 135 (2013) 1167–1176. B. Diouf, R. Pode, Renew. Energy 76 (2015) 375–380. B. Zakeri, S. Syri, Renew. Sustain. Energy Rev. 42 (2015) 569–596. X. Luo, J. Wang, M. Dooner, J. Clarke, Appl. Energy 137 (2015) 511–536. J. Wang, X. Sun, Energy Environ. Sci. 8 (2015) 1110–1138. F. Wu, M. Wang, Y. Su, L. Bao, S. Chen, J. Power Sources 195 (2010) 2362–2367. L. Li, K.S. Lee, L. Lu, Funct. Mater. Lett. 07 (2014) 1430002. B.H. Song, M.O. Lai, Z.W. Liu, H.W. Liu, L. Lu, J. Mater. Chem. A 1 (2013) 9954–9965.
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[9] B.H. Song, C.F. Zhou, Y. Chen, Z.W. Liu, M.O. Lai, J.M. Xue, L. Lu, RSC Adv. 4 (2014) 44244–44252. [10] J. Deng, H. Liu, J. Pan, C.Y. Chung, Q. Yao, Z. Wang, H. Zhou, Funct. Mater. Lett. 07 (2014) 1450061. [11] W.C. West, J. Soler, B.V. Ratnakumar, J. Power Sources 204 (2012) 200–204. [12] D. Ma, P. Zhang, Y. Li, X. Ren, Sci. Rep. 5 (2015) 11257. [13] W. ZhenYao, L. Biao, M. Jin, X. DingGuo, Electrochim. Acta 117 (2014) 285–291. [14] L.F. Jiao, M. Zhang, H.T. Yuan, M. Zhao, J. Guo, W. Wang, X. Di Zhou, Y.M. Wang, J. Power Sources 167 (2007) 178–184. [15] L. Zhang, K. Jin, L. Wang, Y. Zhang, X. Li, Y. Song, J. Alloy. Compd. 638 (2015) 298–304. [16] S.J. Shi, Z.R. Lou, T.F. Xia, X.L. Wang, C.D. Gu, J.P. Tu, J. Power Sources 257 (2014) 198–204. [17] A.R. Armstrong, M. Holzapfel, P. Novák, C.S. Johnson, S.-H. Kang, M. M. Thackeray, P.G. Bruce, J. Am. Chem. Soc. 128 (2006) 8694–8698. [18] S. Park, S. Kang, C. Johnson, K. Amine, M. Thackeray, Electrochem. Commun. 9 (2007) 262–268. [19] S.K. Martha, J. Nanda, G.M. Veith, N.J. Dudney, J. Power Sources 199 (2012) 220–226. [20] Z. Lu, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A815–A822. [21] M. Bettge, Y. Li, B. Sankaran, N.D. Rago, T. Spila, R.T. Haasch, I. Petrov, D. P. Abraham, J. Power Sources 233 (2013) 346–357. [22] D. Dees, E. Gunen, D. Abraham, A. Jansen, J. Prakash, J. Electrochem. Soc. 152 (2005) A1409–A1417.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
High electrochemical performance of lithium-rich Li1.2Mn0.54NixCoyO2 cathode materials for lithium-ion batteries Linsen Zhang n, Huan Wang, Lizhen Wang, Hua Fang, Xiaofeng Li, Haili Gao, Aiqin Zhang, Yanhua Song Henan Provincial Key Laboratory of Surface & Interface Science, School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450002 China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 July 2016 Received in revised form 23 August 2016 Accepted 24 August 2016 Available online 25 August 2016
Li1.2Mn0.54NixCoyO2 (xþy ¼ 0.26, and x:y ¼1:1, 1:2, 2:1, 1:3, 3:1) was synthesized by aerogel with urea as chelating agent. The physical and electrochemical properties of the cathode materials were investigated through X-ray diffraction, scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy analyses as well as and charge/discharge tests. Results showed that samples with different Ni/Co ratios crystallized well and showed a layered structure. The sample, Li1.2Mn0.54Ni0.195Co0.065O2 (x:y ¼3:1), delivered a high discharge capacity of 313 mA h g 1 at 20 mA g 1 and exhibited excellent cycle performance after 50 cycles, with a capacity of 247 mA h g 1 at 100 mA g 1. & 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium-rich Energy storage and conversion Aerogel Sintering Lithium-ion battery Electrochemical performance
1. Introduction Electrochemical storage devices with high specific energy and energy density are highly desired to satisfy the urgent needs in low-emission vehicles, such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in HEVs [1,2]. In recent years, the considerable demand for large-scale safe energy storage systems with high energy conversion rate in EVs has inspired numerous studies to optimize the cathode material [3,4]. LiFePO4 and LiNi1/3Mn1/3Co1/3O2 exhibits their own advantages and disadvantages as conventional cathode materials [5,6]. LiFePO4 shows excellent security performance and a long cycle life, but its energy density is low. LiNi1/3Mn1/3Co1/3O2 features higher energy density, but its security performance is inferior to LiFePO4. Therefore, a material with high energy density and excellent electrochemical performance stability must be developed. Li-rich materials have been widely investigated because of their high theoretical capacity ( Z250 mA h g 1) [7]. Although the high capacity is very attractive, the series of cathode materials drawbacks are mainly not only capacity drop but also voltage fade [8– 10]. Of these materials, the Li1.2Mn0.54NixCoyO2 system exhibits potential and has been increasingly investigated [11,12]. In this n
Corresponding author. E-mail address: [email protected] (L. Zhang).
http://dx.doi.org/10.1016/j.matlet.2016.08.118 0167-577X/& 2016 Elsevier B.V. All rights reserved.
work, Li1.2Mn0.54NixCoyO2 was synthesized by aerogel with urea as chelating agent. The influence of Ni/Co (x/y) on the structure, morphology, and electrochemical properties of Li1.2Mn0.54NixCoyO2 cathode materials was evaluated. Results showed that Li1.2Mn0.54Ni0.195Co0.065O2 (x:y¼ 3:1) exhibits excellent electrochemical performance.
2. Experimental Li1.2Mn0.54NixCoyO2 was prepared by aerogel using urea as chelating agent. Stoichiometric amounts of Li(CH3COO)2 2H2O, Mn (CH3COO)2 4H2O, Ni(CH3COO)2 4H2O, and Co(CH3COO)2 4H2O were dissolved in distilled water to obtain a transparent solution. Urea was dissolved in the solution. The mixture solution was then stirred at 80 °C until it became a purple aerogel. The product was annealed at 500 °C for 10 h, and at 900 °C for 10 h in air atmosphere. The structure of the products was characterized by XRD analysis using a D8-ADVANCE (Bruker, Germany) diffractometer. The morphologies of the sample were investigated with a scanning electron microscope (SEM, JSM-6490). Electrochemical measurements were carried out using CR2016 coin cells. Working electrodes were prepared by mixing 80 wt% active materials, 15 wt% Super P, and 5 wt% polyvinylidene fluorides dissolved in N-methylpyrrolidinone. In a glove box with an
L. Zhang et al. / Materials Letters 185 (2016) 100–103
101
argon atmosphere, test cells were assembled with Li foil as counter electrode, Celgard 2400 as separator, and 1 M LiPF6 solution (1:1 mixture of ethylene carbonate and dimethyl carbonate by volume) as electrolyte. Charge/discharge tests were conducted on NEWER system. Cyclic voltammetry analysis (CV; 2.0–4.8 V, versus Li þ /Li) and electrochemical impedance spectroscopy (EIS) were carried out using an electrochemical workstation (CHI660C). All tests were conducted at room temperature.
3. Results and discussion 3.1. Characterization XRD patterns of Li1.2Mn0.54NixCoyO2 with Ni/Co ratios are shown in Fig. 1. All the diffraction lines can be indexed as a layered structure based on α-NaFeO2, with an R-3 m space group [13]. The splitting between (006)/(012) and (018)/(110) indicates a highly crystalline layered structure. In addition, c/a ratio is an index for characterization of the layered structure, and high c/a ratio also indicates less cation disorder [14]. The lattice parameters of the samples and values of c/a and I(003)/I(104) are shown in Table 1. The c/a ratios are close to and larger than 4.9, which suggest a highly layered structure. Moreover, the values of I(003)/I(104), as indicators of cation mixture intensity are higher than 1.2 indicating less confusion of Li þ and Ni2 þ [15]. The sample Li1.2Mn0.54Ni0.195Co0.065O2 (x:y ¼3:1) showed the lightest cation mixture with a value of 1.82, which could lead to excellent electrochemical performance. The SEM images of Li1.2Mn0.54NixCoyO2 are shown in Fig. 2. All samples with different Ni/Co ratios exhibit an aspheric morphology with approximately uniform particle size. The small particle size and high calcination temperature led to a certain degree of aggregation. Li1.2Mn0.54NixCoyO2 with x:y ¼ 3:1 displays less aggregation than the other samples. 3.2. Electrochemical performance Fig. 3 shows electrochemical performance of Li1.2Mn0.54NixCoyO2 with different x/y. The initial charge/discharge curves of Li1.2Mn0.54NixCoyO2 at 20 mA g 1 are presented in Fig. 3a. The plateaus at 4.0–4.5 V represent the shift of Co3 þ Co4 þ , Ni2 þ - Ni4 þ [16]. The platform at 4.5 V represents the activation of the Li2MnO3-like region, accompanied by irreversible prolapse of Li2O [17]. The sample Li1.2Mn0.54NixCoyO2 with x: y¼ 3:1 exhibits an excellent initial discharge capacity of 313 mA h g 1, which corresponds to the XRD and SEM results. Fig. 3b presents the cyclic performance curves of different Ni/ Co ratios in Li1.2Mn0.54NixCoyO2 samples and the charge/discharge curves of sample Ni/Co ¼3:1 in inset. The initial discharge capacities of samples with different Ni/Co ratios at 100 mA g 1 corresponded to 198, 149, 21,784, 235, and 272 mA h g 1, and the 50 cycle discharge capacities corresponded to 206, 192, 217, 216, and 246 mA h g 1. The capacities of samples with Ni/Co¼ 1:3 and 1:2 showed certain degrees of growth during the first few cycles, which could be due to the activation of Li2MnO3 [18,19]. When Ni/ Co ¼3:1, the capacity retention rate was 90.59% after 50 cycles. As previously mentioned, high layered structure, less cation mixture, and even uniform particle, exhibit better cycle stability is consistent. Therefore, the sample Ni/Co¼ 3:1 exhibited high charge and discharge specific capacity. Figs. 3c and 3d present the CV curves of Li1.2Mn0.54Ni0.195Co0.065 O2 (x:y¼ 3:1) and Li1.2Mn0.54Ni0.13Co0.13O2 (x:y¼ 1:1) during the first three cycles. The CV curves are similar, indicating that both samples have similar electrochemistry reaction mechanisms. The first cycle has two high potential oxidation peak: one in the
Fig. 1. XRD patterns of Li1.2Mn0.54NixCoyO2 with different Ni/Co ratios.
Table 1 XRD performance parameters of Li1.2Mn0.54NixCoyO2. Ni/Co
c (Å)
a (Å)
c/a
I(003)/I(104)
1:3 1:2 1:1 2:1 3:1
14.21 14.20 14.05 14.21 14.17
2.84 2.84 2.83 2.85 2.83
4.99 4.99 4.96 4.98 4.99
1.52 1.58 1.65 1.52 1.82
vicinity of 4.1 V, which could be attributed to the reduction of Ni4 þ and Co4 þ [20]; and another close to 4.7 V, which is associated with the reduction of Mn4 þ triggered by electrochemical activation of Li2MnO3 and decomposition of the electrolyte [17]. The CV curve features of the next cycle significantly differed from those in the first cycle. The strongest oxidation peak at 4.7 V gradually weakened and disappeared during the third cycle. The reduction peak in the first cycle occurred mainly in the vicinity of 3.75 V, which corresponds to Ni3 þ , Ni4 þ , and Co4 þ reductions. During the second and third cycles, the emergence of Mn4 þ /Mn3 þ reduction peak near 3.25 V could be due to Li2MnO3 decomposition; this process produced Mn4 þ , which was electrochemically active during the first cycle and demonstrated capacity growth. Fig. 3d reveals that the CV curves of the Li1.2Mn0.54Ni0.195Co0.065O2 electrode have a larger area than those of Li1.2Mn0.54Ni0.13Co0.13O2 electrodes. Thus, Li1.2Mn0.54Ni0.195Co0.065O2 exhibits a larger capacity. Meanwhile, the CV curves of Li1.2Mn0.54Ni0.195Co0.065O2 show better consistency, indicating that the sample of Li1.2Mn0.54Ni0.195Co0.065O2 is electrochemically stable. Fig. 3e displays the EIS curves of Li1.2Mn0.54NixCoyO2 after 50 cycles at 100 mA g 1 and equivalent circuit with parameters. The Nyquist plots contain a semicircle at high frequency, a second semicircle in the middle frequency, and a straight line in the low frequency. The intercept at high frequency is ohmic resistance (Rs). The first semicircle is related to Li þ diffusion through the SEI layer (R1), which is indicated by the first semicircle. The second semicircle is related to charge transfer process occurring at the electrode-electrolyte interface (R2), and the straight line is attributed to Warburg impedance (W) [21,22]. Although the ohmic resistance and Warburg impedance are close, the values of R1 and R2 the sample with Ni/Co ¼3:1 were the smallest. Therefore, the sample with Ni/Co¼ 3:1 showed low electrode polarization and a fast electrode/interfacial reaction, which lead to satisfactory good charge/discharge performance.
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Fig. 2. SEM images of Li1.2Mn0.54NixCoyO2 with different x and y (a, b, c, d, e referring to samples of x:y ¼1:1, 1:2 , 1:1, 2:1, 3:1).
Fig. 3. Electrochemical tests of the Li1.2Mn0.54NixCoyO2: (a) first charge/discharge profiles at 20 mA g 1; (b) cycling performance at 100 mA g 1 and inset is the charge/ discharge curves of sample Ni/Co ¼ 3:1; (c) and (d) CV curves; and (e) EIS. curves of Li1.2Mn0.54NixCoyO2 after 50 cycles at 100 mA g 1 and inset is equivalent circuit with parameters.
4. Conclusions A Li-rich cathode material, namely, Li1.2Mn0.54NixCoyO2, was synthesized by aerogel with urea as chelating agent. The results showed Li1.2Mn0.54Ni0.195Co0.065O2 (x:y¼3:1) had high layered
structure, less cation mixture, and even uniform particle. Li1.2Mn0.54Ni0.195Co0.065O2 exhibited excellent rate capability and cycle performance after 50 cycles, with a capacity of 247 mA h g 1 at 100 mA g 1.
L. Zhang et al. / Materials Letters 185 (2016) 100–103
Acknowledgements This work is supported by foundation of Henan educational committee (No. 13A530366), foundation for young teachers of 2012 Henan Province colleges and universities (ggjs116), foundation for young teachers of Zhengzhou University of Light Industry (2011XGGJS005), and scientific research foundation of Zhengzhou University of Light Industry in 2015 (No. 2015XJJZ036).
References [1] [2] [3] [4] [5] [6] [7] [8]
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