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Superior electrochemical properties of Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 cathode material with hierarchical micro−nanostructure for lithium ion batteries
Journal of Alloys and Compounds 805 (2019) 673e679
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Superior electrochemical properties of Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 cathode material with hierarchical micronanostructure for lithium ion batteries Jian Chen*, Na Zhao**, KeJian Ban, YaNan Wang, XiaoYing Zhang Department of Materials Science and Engineering, Luoyang Institute of Science and Technology, Luoyang, 471023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 April 2019 Received in revised form 28 June 2019 Accepted 14 July 2019 Available online 15 July 2019
Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 electrode material with hierarchical micro‒nanostructure is prepared via a porous polypropylene membrane as the hard template. The X‒ray diffraction (XRD) spectrum shows that the doping of Mg2þ does not affect the crystal structure of the sample, and there is no diffraction peaks related to any impurities. The scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) mapping deliver that the sample composed of nanoparticles has a porous sheet‒like structure, and the elements (i.e., Mn, Ni, Mg and O) are uniformly distributed in the viewed regions. The X‒ray photoelectron spectroscopy (XPS) shows that the valence states of the Mg, Mn and Ni elements in the sample are þ2, þ4 and þ 2, respectively, and a small amount of Mn3þ and Ni3þ is also present for the valence degeneracy effect. Electrochemical properties show that the Mg2þ‒doped electrode material compared with pristine sample has lower initial irreversible capacity loss, higher charge/discharge specific capacity, better rate performance and capacity retention. The outstanding electrochemical performance of the Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 could be attributed to the enhanced structure stability, improved the purity for the decrease of valence degeneracy, reduced the charge transfer resistance and improved the diffusion coefficient of lithium ions during prolonged cycling. © 2019 Elsevier B.V. All rights reserved.
Keywords: Energy storage materials Chemical synthesis Microstructure Electronic properties
1. Introduction Lithium ion batteries have been become the mainstream energy storage devices on the market due to their high specific capacity and working voltage, light weight, low selfedischarge rare and more environmentally friendly. Simultaneously, lithium ion batteries have been widely used in electric vehicles and plugein hybrid vehicles as the only energy supply device. However, the short cruising range, poor safety performance, unsatisfactory fast charge/ discharge capability and high price seriously affect the development and widespread application of the lithium ion batteries. Therefore, many electrode materials with higher energy density and power density, better use safety and lower cost have been deeply studied by researchers. In recent years, the Li [Li0.2Ni0.2Mn0.6]O2 electrode material denoted as
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Chen), [email protected] (N. Zhao). https://doi.org/10.1016/j.jallcom.2019.07.136 0925-8388/© 2019 Elsevier B.V. All rights reserved.
0.5Li2MnO3$0.5LiNi0.5Mn0.5O2 has attracted much attention for high specific capacity, relatively low cost, high voltage platform and good safety performance compared with the conventional cathode materials, such as LiCoO2, LiMn2O4, LiFePO4, LiNi1exeyCoxMnyO2 (NCM), LiNi1exeyCoxAlyO2 (NCA). Unfortunately, the electrochemical properties of the Li[Li0.2Ni0.2Mn0.6]O2 cathode materials are directly limited by the poor electronic conductivity for the noneconductive Li2MnO3 component, the thick solideelectrolyte interfacial (SEI) layer and structural evolution from the layered phase to spinel phase and/or rockesaltetype phase during the repeated cycling [1]. The topographical structures of the electrode materials with special microenanostructures and the doping of metal ions have a significant effect on improving the dynamic properties of the active materials (e.g. interface charge transfer resistance, diffusion coefficient of lithium ions, electronic conductivity, electrode reaction reversibility). On the one hand, doping of metal ions helps to stabilize the crystal structure of the electrode material, reduce the polarization of the electrode reaction, and improve the intrinsic conductivity of the electrode materials. Thus, some metal ions have
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been applied to improve the electrochemical properties of the Li [Li0.2Ni0.2Mn0.6]O2 active materials, such as Kþ [2], B3þ [3], Cr3þ [4], Fe3þ [5], Sn4þ [6], Mo6þ [7]. Wang et al. [8] reported a Mg2þedoped Li[Li0.2Ni0.2Mn0.6]O2 cathode material prepared by a coeprecipitation process and ballemilling treatment, and the Li [Li0.2Ni0.195Mn0.595Mg0.01]O2 showed better cycle stabilities and a high discharge specific capacity of 226.5 mAh g1 at 0.1C after 60 cycles. However, the charge/discharge specific capacities and cycle stability at high Cerates need to be further improved. On the other hand, limited Lierich Mnebased Li[Li0.2Ni0.2Mn0.6]O2 with various morphologies have been synthesized, which exhibit superior electrochemical performance. Li et al. [9] synthesized a 3D reticular Li[Li0.2Ni0.2Mn0.6]O2 electrode material using KITe6 as the hard template, and the aseobtained sample exhibited superior rate capability and better cycling stability. The discharge capacity of the sample was 195.6 mAh g1 with 95.6% capacity retention after 50 cycles at 1C and 135.7 mAh g1 even at 1000 mA g1. Rodelike Li [Li0.2Ni0.2Mn0.6]O2 active materials with hierarchical micro‒nanostructures were prepared by a hydrothermal method, and the sample showed a high discharge capacity of 212.5 mAh g1 after 30 cycles at 1C and an initial discharge specific capacity of 198 mAh g1 at 2C [10]. The Li[Li0.2Ni0.2Mn0.6]O2 spheres with about 10 mm size composed of submicron scaled flakeeshaped primary particles were synthesized by a coprecipitation method, and the volume energy density of the electrode material could still reach 1290 Wh L1 at 1C after 300 cycles [11]. However, there are few reports on the synthesis of Li[Li0.2Ni0.2Mn0.6]O2 electrode materials with the special topographical features and doped with metal ions. In this work, the Mg2þedoped Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 microsheets with porous structures comprised of nanosized particles are prepared via a carbon gelecombustion process and a porous polypropylene (PP) membrane as hard template. The carbon gelecombustion method is an effective and practical method for preparing highepurity, multiecomponent electrode materials, and could effectively limit the occurrence of particle agglomeration [12]. Nanoesized particle size and porous structural features increase the contact area of the electrode material with the electrolyte, increase the number of active sites, shorten the migration path of lithium ions and electrons, and thus help to improve the dynamics of the electrode material. The influence of the doping of Mg2þ on the electrochemical performances of the Li[Li0.2Ni0.2Mn0.6] O2 active material is systematically studied in this work.
2. Experimental 2.1. Powder preparation and treatment All chemical reagents were of analytical grade, and were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). For the Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 sample, the specific preparation process was as follow: LiNO3 (1.303 g), Ni(NO3)2$6H2O (0.785 g), Mn(CH3COO)2$4H2O (2.206 g) and Mg(NO3)2$6H2O (0.077 g) were accurately weighed and dispersed into 30 mL of absolute ethanol, respectively. It should be noted that the amount of lithium salt added was generally 5% in excess to supplement the volatile of lithium nitrate under high sintering temperature. Then, 2.2 g C6H6O2 and 3.0 mL formaldehyde solution (37.0%) were added into the above solution and dissolved under magnetic stirring for 30 min. The obtained solution and a PP membrane (Celgard 2400) were transferred to a stainless steel autoclave together. The autoclave was heated at 85 C for 48 h, and the PP membrane was taken out after the reaction was completed. The Mg2þ‒doped Li [Li0.2Ni0.18Mn0.6Mg0.02]O2 microsheets with porous structures were obtained by sintering the membrane at 850 C for 10 h, and the as‒ obtained sample was denoted as LNMO‒Mg. In order to illustrate the effect of Mg2þ‒doping on the electrochemical performances of the electrode materials, the pristine Li [Li0.2Ni0.2Mn0.6]O2 sample denoted as LNMO was also synthesized under the same preparation conditions in addition to the addition of Mg(NO3)2∙6H2O. 2.2. Structural characterization Phase compositions and structures of the samples were investigated by Xeray diffraction (XRD, Rigaku D/Max 2550) recorded on a using Cu Ka radiation with wavelength of 1.5406 Å. The morphology and particle size of the samples were tested by the field emission scanning electron microscopy (FESEM, JEOL JSMe6700F) and transmission electron microscopy (TEM, JEM‒ 2010). Energy dispersive spectroscopy (EDS) mapping with the JEM‒2010 microscope was measured to characterize the elemental composition and distribution. The valence states of the elements on the surface of the samples were examined by X‒ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD) equipped with an Mg Ka X‒ ray source (E ¼ 1253.6 eV).
Fig. 1. Xray diffraction patterns of the LNMO and LNMOMg samples.
J. Chen et al. / Journal of Alloys and Compounds 805 (2019) 673e679
2.3. Electrochemical measurements The working electrode was prepared by mixed the sample, polyvinylidene fluoride and acetylene black in a weight ratio of 80:10:10. Then, a slurry was obtained after adding a certain amount
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of 1‒methyl‒2‒pyrrolidone (NMP) and grinding for 30 min. The slurry was spread on an Al foil, and the foil was dried in a vacuum oven at 100 C for 24 h to remove the NMP and absorbed moisture. The assembly of the coin cells (CR2025) was carried out in a standard glove box, wherein oxygen and moisture content were less
Fig. 2. SEM (aec), TEM (d,e), HRTEM (f) and the related EDS mapping (g) images of the LNMOMg sample.
Fig. 3. XPS spectra of full range (a), C1s (b), O1s (c), Mn2p (d), Ni2p (e) and Mg2s (f) of the LNMO and LNMOMg samples.
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than 1 ppm. The electrolyte was 1 M LiPF6 solution, and the solvent was a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1. Cyclic voltammetry curves and electrochemical impedance spectroscopy were performed on an electrochemical workstation (CHI 660E) in the range of 2.0e4.8 V and a frequency range of 0.01 Hze100000 Hz (amplitude of 5 mV), respectively. The charge/discharge performance was performed on a battery test device (LAND 2001A) with a voltage range of 2.0e4.8 V. All the electrochemical tests were performed at room temperature. 3. Results and discussion The XRD patterns of the assynthesized LNMO and LNMOMg samples are shown in Fig. 1, and there are no diffraction peaks related to any impurities. All the diffraction peaks of the samples can be indexed to aNaFeO2 structure with a layered hexagonal structure (space group: R3 m), and the weak reflection peaks located on the range of 20 and 23 indicate the presence of Li2MnO3 phase with monoclinic structure (space group: C2/m) for the shortrange ordering of Li ions and Mn ions in the transition metal layers [13]. The clear splitting of the (006)/(102) and (108)/ (110) peaks observed from the XRD patterns suggests the well layered structures of the samples. Fig. 2(ac) show the SEM images of the LNMOMg sample with different magnifications, and the LNMOMg sample has a microsized sheetlike porous structure. The LNMOMg sample is comprised with nanoscaled particles as shown in Fig. 2(d and e).
The porous structural features and the nanosized particles contribute to increase the contact area between the sample and the electrolyte, shorten the migration path of lithium ions and electrons and reduce the polarization of the electrode for the limited lithium ions diffusion rate. Fig. 2f shows the HRTEM image of the LNMOMg sample, and the interplanar spacing of 0.47 nm corresponds to the (003) plane of the Li[Li0.2Ni0.2Mn0.6]O2 active material [14]. EDS mapping is employed to provide evidence of elemental distribution in the LNMOMg sample, and all of the selected elements (i.e., Mn, Ni, Mg and O) showing with different colors are uniformly distributed in the viewed regions as shown in Fig. 2g. The XPS analysis technique is used to further analyze the chemical compositions and valence states of the surface elements of the samples. The full spectra of the LNMO and LNMOMg samples are shown in Fig. 3a, and the peaks shape and position of the resulting samples remain consistent except for the spectrum of the magnesium element in the LNMOMg sample. The C1s peak at binding energy of 284.5 eV is performed to calibrate the other peaks as shown in Fig. 3b. The high resolution spectra of the O1s for the samples are shown in Fig. 3c, and the spectra contain two main peaks. Among them, the peaks located at 529.4 eV for LNMOMg sample and 529.5 eV for LNMO sample represent the characteristic peaks of the metal oxygen bonds [9,15], and the peaks located at 531.5 eV for LNMOMg sample and 531.6 eV for LNMOMg sample are consistent with the early reports on oxidized lattices One (n < 2) in Lirich Mnbased active materials [14,16,17]. The high resolution spectra of Mn2p for LNMOMg sample are
Fig. 4. The typical charge/discharge curves of the LNMO sample (a) and LNMOMg sample (b) at 0.2C in the voltage range of 2.0e4.8 V; Coulombic efficiency and capacity retention of the LNMO and LNMOMg samples at 0.2C cycling 50 times (c).
J. Chen et al. / Journal of Alloys and Compounds 805 (2019) 673e679
decomposed into two main spineorbit lines of 2p3/2 at 642.5 eV and 2p1/2 at 654.0 eV with separation of 11.5 eV as shown in Fig. 3d, which indicates the dominant Mn (IV) cation [14,18,19]. However, a couple of less prominent peaks located at 641.6 eV and 653.2 eV attribute to the Mn2p3/2 and Mn2p1/2 of the Mn(III) cation, respectively [20]. In the Ni2p spectrum for LNMOMg sample (Fig. 3e), the dominant peaks at 854.7 eV for Ni2p3/2 accompanied by a shakeeup peak at about 861.1 eV and 872.3 eV for Ni2p1/2 are characteristics of Ni(II) cation, which is agreement with early reports for Ni2þ in Lierich electrode materials [9]. Meanwhile, a less prominent peak at about 855.8 eV (Ni2p3/2) is also observed, indicating the presence of a small amount of trivalent nickel ions in the LNMOMg sample [15]. The binding energies of metal elements for LNMO and LNMOMg samples are similar. However, compared to the LNMO sample, the contents of Ni3þ and Mn3þ in the LNMOMg sample are lower for the smaller peak area as shown in Fig. 3(d and e). The minor contribution of Ni3þ and Mn3þ could be attributed to the valence degeneracy during the electron transfer between Mn4þ and Ni2þ pairs [9,15,21]. This indicates that the doping of Mg ions helps to reduce the effects of valence
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degeneracy, improve the purity of the electrode materials and reduce the degree of ions mixing. In the Mg2s spectra for the LNMOMg sample (Fig. 3f), the symmetrical peak located at 1303.0 eV attributes to Mg(II) cation, which is consistent with that reported in the literatures [22e26]. The typical charge/discharge curves of the LNMO and LNMOMg samples at 0.2C are shown in Fig. 4(a and b), and the initial charge curves of the samples contain two voltage plateaus corresponding to different deintercalation reactions of Li ions, which is consistent with the characteristics of the Lirich Mnbased electrode materials [27,28]. The comparison of the electrochemical properties of the LNMO and LNMOMg samples can be seen in Table 1. The LNMOMg sample has higher initial coulombic efficiency, lower first irreversible capacity loss, higher discharge specific capacities and better cycle stability than that of LNMO sample. Fig. 5a shows the specific capacities of the LNMO and LNMOMg samples at different Cerates from 0.2C to 5C. It should be noted that the charging mode of the assembled coin cells adopts the constant current/constant voltage mode. For LNMOMg sample, the average discharge specific capacities are 224.0, 201.5,
Table 1 The comparison of the electrochemical performance of the LNMO and LNMOMg samples. Electrode Materials
Charge Capacity
Discharge Capacity
Coulombic Efficiency
Discharge Capacity
1st
50th
1st
50th
1st
50th
50 cycles
LNMO LNMO-Mg
286.6 271.0
172.4 210.0
189.7 196.0
169.8 209.0
66.2 72.3
98.5 99.5
89.5 106.6
Fig. 5. The rate performance (a), the typical charge/discharge curves (b,c) and the Midpoint voltage of the discharge curves at various Cerates (d) for LNMO and LNMOMg samples, respectively.
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189.3, 171.5 and 134.3 mAh g1 at 0.2C, 0.5C, 1C, 2C and 5C rates, respectively. In contrast, the average discharge specific capacities of the LNMO sample are only 211.5, 177.3, 157.0, 130.8 and 72.9 mAh g1. As the charge/discharge rate increases, the difference in specific capacities between the samples gradually increases. The typical charge/discharge profiles at different rates are shown in Fig. 5(b and c). Fig. 5d shows the Midpoint voltage of discharge curves at various rates in the range of 0.2Ce5C. The values of the Midpoint voltage for the samples gradually decrease with the Crate increase, however, the Midpoint voltage of the LNMOMg is higher than that of the LNMO sample, indicating that the doping of Mg ions helps to reduce the polarization of the electrodes caused for the high internal resistance and limited diffusion rate of lithium ions. The kinetic characteristics of the LNMO and LNMOMg samples are further investigated by CV and EIS measurements. Fig. 6(a and b) show the cyclic voltammetry curves of the LNMO and LNMOMg samples at different scan rates with scan ranging from 0.1 to 1.0 mV s1, respectively. Fig. 6c shows the relationship between the anode peak current (mA) and the square root of the scan rates (V0.5 s0.5). According to previous reported literatures, the diffusion coefficient of lithium ions (DLi) in electrode materials is calculated by the following formula [15]: 1/2 ip ¼ 0.4463n2/3F3/2CLiAR1/2T1/2D1/2 Li y
where ip is the peak current value of the redox peaks (A), n is the
number of electrons transferred during charge/discharge process, F is the Faraday constant (96485 C mol1), CLi is Li ion concentration in the pristine Li[Li0.2Ni0.2Mn0.6]O2 material (0.0228 mol cm3), A is the electrode area (1.13 cm2), R is the gas constant (8.314 J mol1 K1), T is the absolute temperature (298 K), and y is the scan rate (V s1). The value of the DLi can be calculated from the slop ip vs. y1/2, and the values of DLi for the LNMO and LNMO‒Mg samples are 1.41 1011 cm2 s1 and 4.81 1011 cm2 s1, respectively. Fig. 6d shows the Nyquist curves of the LNMO and LNMO‒Mg samples after 50 cycles at 0.2C. RSEI and Rct in the equivalent circuit represent SEI resistance and charge transfer resistance, and the intercept of the curves on the Z axis represents ohmic resistance, such as the resistance of the electrolyte and separator and various contact resistance [9,13,14,29,30]. The diagonal line in the low frequency region is related to the Warburg impedance (Zw). The resulting Nyquist curves are fitted by the corresponding equivalent circuit using the Zsimpwin 3.1 software, and the values of Rct for LNMO and LNMO‒Mg samples are 164.6 U and 127.7 U, respectively. Meanwhile, the exchange current density (jo ¼ RT/nFRct) is often used to measure the ability of an electrode to gain or lose electrons. It can be seen from the formula that the exchange current density (jo) is inversely proportional to the charge transfer resistance, and the LNMO‒Mg sample shows the higher value of jo. Thus, the low charge transfer resistance and high exchange current density and diffusion coefficient of the Liþ indicate that the kinetic characteristics of the Li[Li0.2Ni0.2Mn0.6]O2 electrode material are well improved after doped with Mg2þ.
Fig. 6. The cyclic voltammetry curves of the samples at various scan rates in the range of 0.1e1.0 mV s1 after 50 cycles at 0.2C: (a) LNMO sample, (b) LNMOMg sample; (c) The plots of peak currents (ip) vs. square root of the scan rates (y0.5) for the samples; (d) Nyquist curves of the LNMO and LNMOMg samples after 50 cycles at 0.2C (the inset shows the equivalent circuit for EIS results fitting).
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4. Conclusions Mg2þ‒doped Li[Li0.2Ni0.18Mn06Mg0.02]O2 microsheets with porous structures comprised of nano‒particles are obtained by a PP membrane with rich pore structure as hard template. The doping of Mg2þ does not change the crystal structure of the Li[Li0.2Ni0.2Mn0.6] O2 material, and helps to improve the structure stability and the purity of the active material for the decrease of valence degeneracy. The aseobtained Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 electrode material delivers the initial discharge specific capacity of 196.0 mAh g1 and 209.0 mAh g1 after 50 cycles at 0.2C. For even higher Cerates, a discharge specific capacity of 134.3 mAh g1 at 5 C is reached for this sample. The superior electrochemical performance could be attributed to the special topographical features, increased the migration speed of lithium ions, and reduced the polarization and charge transfer resistance during repeated charge/discharge process. Acknowledgements This work was financially supported by the Education Department and Science and Technology Department of Henan Province (15A430033, 182102210425) and Research Foundation of Luoyang Institute of Science and Technology (2018YZ05). References [1] W. Hua, M. Chen, B. Schwarz, M. Knapp, M. Bruns, J. Barthel, X. Yang, F. Sigel, R. Azmi, A. Senyshyn, A. Missiul, L. Simonelli, M. Etter, S. Wang, X. Mu, A. Fiedler, J.R. Binder, X. Guo, S. Chou, B. Zhong, S. Indris, H. Ehrenberg, Lithium/oxygen incorporation and microstructural evolution during synthesis of Lierich layered Li[Li0.2Ni0.2Mn0.6]O2 oxides, Adv. Energy Mater. 9 (2019) 183094. [2] M. Yang, B. Hu, F. Geng, C. Li, X. Lou, B. Hu, Mitigating voltage decay in highecapacity Li1.2Ni0.2Mn0.6O2 cathode material by surface Kþ doping, Electrochim. Acta 291 (2018) 278e286. [3] D. Uzun, Boronedoped Li1.2Mn0.6Ni0.2O2 as a cathode active material for lithium ion battery, Solid State Ion. 281 (2015) 73e81. [4] L.F. Jiao, M. Zhang, H.T. Yuan, M. Zhao, J. Guo, W. Wang, X.D. Zhou, Y.M. Wang, Effect of Cr doping on the structural, electrochemical properties of Li [Li0.2Ni0.2x/2Mn0.6x/2Crx]O2 (x ¼ 0, 0.02, 0.04, 0.06, 0.08) as cathode materials for lithium secondary batteries, J. Power Sources 167 (2007) 178e184. [5] X. Liu, T. Huang, A. Yu, Fe doped Li1.2Mn0.6ex/2Ni0.2ex/2FexO2(x0.1) as cathode materials for lithiumeion batteries, Electrochim. Acta 133 (2014) 555e563. [6] Y. Zhao, M. Xia, X. Hu, Z. Zhao, Y. Wang, Z. Lv, Effects of Sn doping on the structural and electrochemical properties of Li1.2Ni0.2Mn0.8O2 Lierich cathode materials, Electrochim. Acta 174 (2015) 1167e1174. [7] Y. Zang, C.eX. Ding, X.eC. Wang, Z.eY. Wen, C.eH. Chen, Molybdenumedoped lithiumerich layeredestructured cathode material Li1.2Ni0.2Mn0.6O2 with high specific capacity and improved rate performance, Electrochim. Acta 168 (2015) 234e239. [8] D. Wang, Y. Huang, Z. Huo, L. Chen, Synthesize and electrochemical characterization of Mgedoped Lierich layered Li[Li0.2Ni0.2Mn0.6]O2 cathode material, Electrochim. Acta 107 (2013) 461e466. [9] L. Li, L. Wang, X. Zhang, Q. Xue, L. Wei, F. Wu, R. Chen, 3D reticular Li1.2Ni0.2Mn0.6O2 cathode material for lithiumeion batteries, ACS Appl. Mater. Interfaces 9 (2017) 1516e1523.
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[10] L. Zhang, B. Wu, N. Li, D. Mu, C. Zhang, F. Wu, Rodelike hierarchical nano/ micro Li1.2Ni0.2Mn0.6O2 as high performance cathode materials for lithiumeion batteries, J. Power Sources 240 (2013) 644e652. [11] P. Oh, S. Myeong, W. Cho, M.eJ. Lee, M. Ko, H.Y. Jeong, J. Cho, Superior longeterm energy retention and volumetric energy density for Lierich cathode materials, Nano Lett. 14 (2014) 5965e5972. [12] N. Zhao, J. Chen, Z.eQ. Liu, K.eJ. Ban, W.eJ. Duan, Porous LiNi1/3Co1/3Mn1/3O2 microsheets assembled with single crystal nanoparticles as cathode materials for lithium ion batteries, J. Alloy. Comp. 768 (2018) 782e788. [13] K. Mu, Y. Cao, G. Hu, K. Du, H. Yang, Z. Gan, Z. Peng, Enhanced electrochemical performance of Lirich cathode Li1.2Ni0.2Mn0.6O2 by surface modification with WO3 for lithium ion batteries, Electrochim. Acta 273 (2018) 88e97. [14] S. Zhao, B. Sun, K. Yan, J. Zhang, C. Wang, G. Wang, Aegis of LithiumRich cathode materials via heterostructured LiAlF4 coating for HighPerformance LithiumIon batteries, ACS Appl. Mater. Interfaces 10 (2018) 33260e33268. [15] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, Uniform LiNi1/3Co1/3Mn1/3O2 hollow microspheres: designed synthesis, topotactical structural transformation and their enhanced electrochemical performance, Nano Energy 2 (2013) 1249e1260. [16] G. Assat, J.eM. Tarascon, Fundamental understanding and practical challenges of anionic redox activity in Lieion batteries, Nat. Energy 3 (2018) 373e386. [17] X. Li, Y. Qiao, S. Guo, Z. Xu, H. Zhu, X. Zhang, Y. Yuan, P. He, M. Ishida, H. Zhou, Direct visualization of the reversible O2e/Oe redox process in Lierich cathode materials, Adv. Mater. 30 (2018) 1705197. [18] S. Chong, Y. Wu, Y. Chen, C. Shu, Y. Liu, A strategy of constructing spherical coreeshell structure of [email protected] cathode material for high performance lithiumeion batteries, J. Power Sources 356 (2017) 153e162. [19] R. Yu, X. Zhang, T. Liu, L. Yang, L. Liu, Y. Wang, X. Wang, H. Shu, X. Yang, Spinel/ layered heterostructured lithiumerich oxide nanowires as cathode material for higheenergy lithiumeion batteries, ACS Appl. Mater. Interfaces 9 (2017) 41210e41223. [20] J. Chen, N. Zhao, J. Zhao, J. Li, F.eF. Guo, G.eD. Li, Facile synthesis of LiMn2O4 microsheets with porous microenanostructure as higherate cathode materials for Lieion batteries, J. Solid State Electrochem. 22 (2018) 331e338. [21] Z.D. Huang, X.M. Liu, S.W. Oh, B.A. Zhang, P.C. Ma, J.K. Kim, Microscopically porous, interconnected single crystal LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion batteries, J. Mater. Chem. 21 (2011) 10777e10784. [22] H. Xu, S. Deng, G. Chen, Improved electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2 by Mg doping for lithium ion battery cathode material, J. Mater. Chem. A 2 (2014) 15015e15021. [23] H. Zhao, S. Liu, Z. Wang, Y. Cai, M. Tan, X. Liu, Enhanced elevatedetemperature performance of LiAlxSi0.05Mg0.05Mn1.90exO4 (0 x 0.08) cathode materials for higheperformance lithiumeion batteries, Electrochim. Acta 199 (2016) 18e26. [24] M. Lao, S. Qian, H. Yu, L. Yan, P. Li, X. Lin, N. Long, M. Shui, J. Shu, Enhanced electrochemical properties of Mg2þ doped Li2Na2Ti6O14 anode material for lithiumeion batteries, Electrochim. Acta 196 (2016) 642e652. [25] H. Zhang, J. Shi, X. Xu, L. Zhu, Y. Luo, D. Li, Q. Meng, Mgedoped TiO2 boosts the efficiency of planar perovskite solar cells to exceed 19%, J. Mater. Chem. A 4 (2016) 15383e15389. [26] Y. Weng, D. Deng, L. Zhang, Y. Su, Y. Lv, A cataluminescence gas sensor based on mesoporous Mgedoped SnO2 structures for detection of gaseous acetone, Anal. Methods 8 (2016) 7816e7823. [27] Y. Liu, X. Fan, X. Huang, D. Liu, A. Dou, M. Su, D. Chu, Electrochemical performance of Li1.2Ni0.2Mn0.6O2 coated with a facilely synthesized Li1.3Al0.3Ti1.7(PO4)3, J. Power Sources 403 (2018) 27e37. [28] S. Hy, F. Felix, J. Rick, W.N. Su, B.J. Hwang, Direct in situ observation of Li2O evolution on LiRich HighCapacity cathode material, Li[NixLi(12x)/3Mn(2x)/ 3]O2 (0 x 0.5), J. Am. Chem. Soc. 136 (2014) 999e1007. [29] X. Ding, L.‒N. Xiao, Y.‒X. Li, Z.‒F. Tang, J.‒W. Wan, Z.‒Y. Wen, C.‒H. Chen, Improving the electrochemical performance of Li‒rich Li1.2Ni0.2Mn0.6O2 by using Ni‒Mn oxide surface modification, J. Power Sources 390 (2018) 13e19. [30] Y. Liu, Q. Wang, Z. Zhang, A. Dou, J. Pan, M. Su, Investigation the electrochemical performance of layered cathode material Li1.2Ni0.2Mn0.6O2 coated with Li4Ti5O12, Adv. Powder Technol. 27 (2016) 1481e1487.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Superior electrochemical properties of Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 cathode material with hierarchical micronanostructure for lithium ion batteries Jian Chen*, Na Zhao**, KeJian Ban, YaNan Wang, XiaoYing Zhang Department of Materials Science and Engineering, Luoyang Institute of Science and Technology, Luoyang, 471023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 April 2019 Received in revised form 28 June 2019 Accepted 14 July 2019 Available online 15 July 2019
Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 electrode material with hierarchical micro‒nanostructure is prepared via a porous polypropylene membrane as the hard template. The X‒ray diffraction (XRD) spectrum shows that the doping of Mg2þ does not affect the crystal structure of the sample, and there is no diffraction peaks related to any impurities. The scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) mapping deliver that the sample composed of nanoparticles has a porous sheet‒like structure, and the elements (i.e., Mn, Ni, Mg and O) are uniformly distributed in the viewed regions. The X‒ray photoelectron spectroscopy (XPS) shows that the valence states of the Mg, Mn and Ni elements in the sample are þ2, þ4 and þ 2, respectively, and a small amount of Mn3þ and Ni3þ is also present for the valence degeneracy effect. Electrochemical properties show that the Mg2þ‒doped electrode material compared with pristine sample has lower initial irreversible capacity loss, higher charge/discharge specific capacity, better rate performance and capacity retention. The outstanding electrochemical performance of the Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 could be attributed to the enhanced structure stability, improved the purity for the decrease of valence degeneracy, reduced the charge transfer resistance and improved the diffusion coefficient of lithium ions during prolonged cycling. © 2019 Elsevier B.V. All rights reserved.
Keywords: Energy storage materials Chemical synthesis Microstructure Electronic properties
1. Introduction Lithium ion batteries have been become the mainstream energy storage devices on the market due to their high specific capacity and working voltage, light weight, low selfedischarge rare and more environmentally friendly. Simultaneously, lithium ion batteries have been widely used in electric vehicles and plugein hybrid vehicles as the only energy supply device. However, the short cruising range, poor safety performance, unsatisfactory fast charge/ discharge capability and high price seriously affect the development and widespread application of the lithium ion batteries. Therefore, many electrode materials with higher energy density and power density, better use safety and lower cost have been deeply studied by researchers. In recent years, the Li [Li0.2Ni0.2Mn0.6]O2 electrode material denoted as
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Chen), [email protected] (N. Zhao). https://doi.org/10.1016/j.jallcom.2019.07.136 0925-8388/© 2019 Elsevier B.V. All rights reserved.
0.5Li2MnO3$0.5LiNi0.5Mn0.5O2 has attracted much attention for high specific capacity, relatively low cost, high voltage platform and good safety performance compared with the conventional cathode materials, such as LiCoO2, LiMn2O4, LiFePO4, LiNi1exeyCoxMnyO2 (NCM), LiNi1exeyCoxAlyO2 (NCA). Unfortunately, the electrochemical properties of the Li[Li0.2Ni0.2Mn0.6]O2 cathode materials are directly limited by the poor electronic conductivity for the noneconductive Li2MnO3 component, the thick solideelectrolyte interfacial (SEI) layer and structural evolution from the layered phase to spinel phase and/or rockesaltetype phase during the repeated cycling [1]. The topographical structures of the electrode materials with special microenanostructures and the doping of metal ions have a significant effect on improving the dynamic properties of the active materials (e.g. interface charge transfer resistance, diffusion coefficient of lithium ions, electronic conductivity, electrode reaction reversibility). On the one hand, doping of metal ions helps to stabilize the crystal structure of the electrode material, reduce the polarization of the electrode reaction, and improve the intrinsic conductivity of the electrode materials. Thus, some metal ions have
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been applied to improve the electrochemical properties of the Li [Li0.2Ni0.2Mn0.6]O2 active materials, such as Kþ [2], B3þ [3], Cr3þ [4], Fe3þ [5], Sn4þ [6], Mo6þ [7]. Wang et al. [8] reported a Mg2þedoped Li[Li0.2Ni0.2Mn0.6]O2 cathode material prepared by a coeprecipitation process and ballemilling treatment, and the Li [Li0.2Ni0.195Mn0.595Mg0.01]O2 showed better cycle stabilities and a high discharge specific capacity of 226.5 mAh g1 at 0.1C after 60 cycles. However, the charge/discharge specific capacities and cycle stability at high Cerates need to be further improved. On the other hand, limited Lierich Mnebased Li[Li0.2Ni0.2Mn0.6]O2 with various morphologies have been synthesized, which exhibit superior electrochemical performance. Li et al. [9] synthesized a 3D reticular Li[Li0.2Ni0.2Mn0.6]O2 electrode material using KITe6 as the hard template, and the aseobtained sample exhibited superior rate capability and better cycling stability. The discharge capacity of the sample was 195.6 mAh g1 with 95.6% capacity retention after 50 cycles at 1C and 135.7 mAh g1 even at 1000 mA g1. Rodelike Li [Li0.2Ni0.2Mn0.6]O2 active materials with hierarchical micro‒nanostructures were prepared by a hydrothermal method, and the sample showed a high discharge capacity of 212.5 mAh g1 after 30 cycles at 1C and an initial discharge specific capacity of 198 mAh g1 at 2C [10]. The Li[Li0.2Ni0.2Mn0.6]O2 spheres with about 10 mm size composed of submicron scaled flakeeshaped primary particles were synthesized by a coprecipitation method, and the volume energy density of the electrode material could still reach 1290 Wh L1 at 1C after 300 cycles [11]. However, there are few reports on the synthesis of Li[Li0.2Ni0.2Mn0.6]O2 electrode materials with the special topographical features and doped with metal ions. In this work, the Mg2þedoped Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 microsheets with porous structures comprised of nanosized particles are prepared via a carbon gelecombustion process and a porous polypropylene (PP) membrane as hard template. The carbon gelecombustion method is an effective and practical method for preparing highepurity, multiecomponent electrode materials, and could effectively limit the occurrence of particle agglomeration [12]. Nanoesized particle size and porous structural features increase the contact area of the electrode material with the electrolyte, increase the number of active sites, shorten the migration path of lithium ions and electrons, and thus help to improve the dynamics of the electrode material. The influence of the doping of Mg2þ on the electrochemical performances of the Li[Li0.2Ni0.2Mn0.6] O2 active material is systematically studied in this work.
2. Experimental 2.1. Powder preparation and treatment All chemical reagents were of analytical grade, and were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). For the Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 sample, the specific preparation process was as follow: LiNO3 (1.303 g), Ni(NO3)2$6H2O (0.785 g), Mn(CH3COO)2$4H2O (2.206 g) and Mg(NO3)2$6H2O (0.077 g) were accurately weighed and dispersed into 30 mL of absolute ethanol, respectively. It should be noted that the amount of lithium salt added was generally 5% in excess to supplement the volatile of lithium nitrate under high sintering temperature. Then, 2.2 g C6H6O2 and 3.0 mL formaldehyde solution (37.0%) were added into the above solution and dissolved under magnetic stirring for 30 min. The obtained solution and a PP membrane (Celgard 2400) were transferred to a stainless steel autoclave together. The autoclave was heated at 85 C for 48 h, and the PP membrane was taken out after the reaction was completed. The Mg2þ‒doped Li [Li0.2Ni0.18Mn0.6Mg0.02]O2 microsheets with porous structures were obtained by sintering the membrane at 850 C for 10 h, and the as‒ obtained sample was denoted as LNMO‒Mg. In order to illustrate the effect of Mg2þ‒doping on the electrochemical performances of the electrode materials, the pristine Li [Li0.2Ni0.2Mn0.6]O2 sample denoted as LNMO was also synthesized under the same preparation conditions in addition to the addition of Mg(NO3)2∙6H2O. 2.2. Structural characterization Phase compositions and structures of the samples were investigated by Xeray diffraction (XRD, Rigaku D/Max 2550) recorded on a using Cu Ka radiation with wavelength of 1.5406 Å. The morphology and particle size of the samples were tested by the field emission scanning electron microscopy (FESEM, JEOL JSMe6700F) and transmission electron microscopy (TEM, JEM‒ 2010). Energy dispersive spectroscopy (EDS) mapping with the JEM‒2010 microscope was measured to characterize the elemental composition and distribution. The valence states of the elements on the surface of the samples were examined by X‒ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD) equipped with an Mg Ka X‒ ray source (E ¼ 1253.6 eV).
Fig. 1. Xray diffraction patterns of the LNMO and LNMOMg samples.
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2.3. Electrochemical measurements The working electrode was prepared by mixed the sample, polyvinylidene fluoride and acetylene black in a weight ratio of 80:10:10. Then, a slurry was obtained after adding a certain amount
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of 1‒methyl‒2‒pyrrolidone (NMP) and grinding for 30 min. The slurry was spread on an Al foil, and the foil was dried in a vacuum oven at 100 C for 24 h to remove the NMP and absorbed moisture. The assembly of the coin cells (CR2025) was carried out in a standard glove box, wherein oxygen and moisture content were less
Fig. 2. SEM (aec), TEM (d,e), HRTEM (f) and the related EDS mapping (g) images of the LNMOMg sample.
Fig. 3. XPS spectra of full range (a), C1s (b), O1s (c), Mn2p (d), Ni2p (e) and Mg2s (f) of the LNMO and LNMOMg samples.
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than 1 ppm. The electrolyte was 1 M LiPF6 solution, and the solvent was a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1. Cyclic voltammetry curves and electrochemical impedance spectroscopy were performed on an electrochemical workstation (CHI 660E) in the range of 2.0e4.8 V and a frequency range of 0.01 Hze100000 Hz (amplitude of 5 mV), respectively. The charge/discharge performance was performed on a battery test device (LAND 2001A) with a voltage range of 2.0e4.8 V. All the electrochemical tests were performed at room temperature. 3. Results and discussion The XRD patterns of the assynthesized LNMO and LNMOMg samples are shown in Fig. 1, and there are no diffraction peaks related to any impurities. All the diffraction peaks of the samples can be indexed to aNaFeO2 structure with a layered hexagonal structure (space group: R3 m), and the weak reflection peaks located on the range of 20 and 23 indicate the presence of Li2MnO3 phase with monoclinic structure (space group: C2/m) for the shortrange ordering of Li ions and Mn ions in the transition metal layers [13]. The clear splitting of the (006)/(102) and (108)/ (110) peaks observed from the XRD patterns suggests the well layered structures of the samples. Fig. 2(ac) show the SEM images of the LNMOMg sample with different magnifications, and the LNMOMg sample has a microsized sheetlike porous structure. The LNMOMg sample is comprised with nanoscaled particles as shown in Fig. 2(d and e).
The porous structural features and the nanosized particles contribute to increase the contact area between the sample and the electrolyte, shorten the migration path of lithium ions and electrons and reduce the polarization of the electrode for the limited lithium ions diffusion rate. Fig. 2f shows the HRTEM image of the LNMOMg sample, and the interplanar spacing of 0.47 nm corresponds to the (003) plane of the Li[Li0.2Ni0.2Mn0.6]O2 active material [14]. EDS mapping is employed to provide evidence of elemental distribution in the LNMOMg sample, and all of the selected elements (i.e., Mn, Ni, Mg and O) showing with different colors are uniformly distributed in the viewed regions as shown in Fig. 2g. The XPS analysis technique is used to further analyze the chemical compositions and valence states of the surface elements of the samples. The full spectra of the LNMO and LNMOMg samples are shown in Fig. 3a, and the peaks shape and position of the resulting samples remain consistent except for the spectrum of the magnesium element in the LNMOMg sample. The C1s peak at binding energy of 284.5 eV is performed to calibrate the other peaks as shown in Fig. 3b. The high resolution spectra of the O1s for the samples are shown in Fig. 3c, and the spectra contain two main peaks. Among them, the peaks located at 529.4 eV for LNMOMg sample and 529.5 eV for LNMO sample represent the characteristic peaks of the metal oxygen bonds [9,15], and the peaks located at 531.5 eV for LNMOMg sample and 531.6 eV for LNMOMg sample are consistent with the early reports on oxidized lattices One (n < 2) in Lirich Mnbased active materials [14,16,17]. The high resolution spectra of Mn2p for LNMOMg sample are
Fig. 4. The typical charge/discharge curves of the LNMO sample (a) and LNMOMg sample (b) at 0.2C in the voltage range of 2.0e4.8 V; Coulombic efficiency and capacity retention of the LNMO and LNMOMg samples at 0.2C cycling 50 times (c).
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decomposed into two main spineorbit lines of 2p3/2 at 642.5 eV and 2p1/2 at 654.0 eV with separation of 11.5 eV as shown in Fig. 3d, which indicates the dominant Mn (IV) cation [14,18,19]. However, a couple of less prominent peaks located at 641.6 eV and 653.2 eV attribute to the Mn2p3/2 and Mn2p1/2 of the Mn(III) cation, respectively [20]. In the Ni2p spectrum for LNMOMg sample (Fig. 3e), the dominant peaks at 854.7 eV for Ni2p3/2 accompanied by a shakeeup peak at about 861.1 eV and 872.3 eV for Ni2p1/2 are characteristics of Ni(II) cation, which is agreement with early reports for Ni2þ in Lierich electrode materials [9]. Meanwhile, a less prominent peak at about 855.8 eV (Ni2p3/2) is also observed, indicating the presence of a small amount of trivalent nickel ions in the LNMOMg sample [15]. The binding energies of metal elements for LNMO and LNMOMg samples are similar. However, compared to the LNMO sample, the contents of Ni3þ and Mn3þ in the LNMOMg sample are lower for the smaller peak area as shown in Fig. 3(d and e). The minor contribution of Ni3þ and Mn3þ could be attributed to the valence degeneracy during the electron transfer between Mn4þ and Ni2þ pairs [9,15,21]. This indicates that the doping of Mg ions helps to reduce the effects of valence
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degeneracy, improve the purity of the electrode materials and reduce the degree of ions mixing. In the Mg2s spectra for the LNMOMg sample (Fig. 3f), the symmetrical peak located at 1303.0 eV attributes to Mg(II) cation, which is consistent with that reported in the literatures [22e26]. The typical charge/discharge curves of the LNMO and LNMOMg samples at 0.2C are shown in Fig. 4(a and b), and the initial charge curves of the samples contain two voltage plateaus corresponding to different deintercalation reactions of Li ions, which is consistent with the characteristics of the Lirich Mnbased electrode materials [27,28]. The comparison of the electrochemical properties of the LNMO and LNMOMg samples can be seen in Table 1. The LNMOMg sample has higher initial coulombic efficiency, lower first irreversible capacity loss, higher discharge specific capacities and better cycle stability than that of LNMO sample. Fig. 5a shows the specific capacities of the LNMO and LNMOMg samples at different Cerates from 0.2C to 5C. It should be noted that the charging mode of the assembled coin cells adopts the constant current/constant voltage mode. For LNMOMg sample, the average discharge specific capacities are 224.0, 201.5,
Table 1 The comparison of the electrochemical performance of the LNMO and LNMOMg samples. Electrode Materials
Charge Capacity
Discharge Capacity
Coulombic Efficiency
Discharge Capacity
1st
50th
1st
50th
1st
50th
50 cycles
LNMO LNMO-Mg
286.6 271.0
172.4 210.0
189.7 196.0
169.8 209.0
66.2 72.3
98.5 99.5
89.5 106.6
Fig. 5. The rate performance (a), the typical charge/discharge curves (b,c) and the Midpoint voltage of the discharge curves at various Cerates (d) for LNMO and LNMOMg samples, respectively.
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189.3, 171.5 and 134.3 mAh g1 at 0.2C, 0.5C, 1C, 2C and 5C rates, respectively. In contrast, the average discharge specific capacities of the LNMO sample are only 211.5, 177.3, 157.0, 130.8 and 72.9 mAh g1. As the charge/discharge rate increases, the difference in specific capacities between the samples gradually increases. The typical charge/discharge profiles at different rates are shown in Fig. 5(b and c). Fig. 5d shows the Midpoint voltage of discharge curves at various rates in the range of 0.2Ce5C. The values of the Midpoint voltage for the samples gradually decrease with the Crate increase, however, the Midpoint voltage of the LNMOMg is higher than that of the LNMO sample, indicating that the doping of Mg ions helps to reduce the polarization of the electrodes caused for the high internal resistance and limited diffusion rate of lithium ions. The kinetic characteristics of the LNMO and LNMOMg samples are further investigated by CV and EIS measurements. Fig. 6(a and b) show the cyclic voltammetry curves of the LNMO and LNMOMg samples at different scan rates with scan ranging from 0.1 to 1.0 mV s1, respectively. Fig. 6c shows the relationship between the anode peak current (mA) and the square root of the scan rates (V0.5 s0.5). According to previous reported literatures, the diffusion coefficient of lithium ions (DLi) in electrode materials is calculated by the following formula [15]: 1/2 ip ¼ 0.4463n2/3F3/2CLiAR1/2T1/2D1/2 Li y
where ip is the peak current value of the redox peaks (A), n is the
number of electrons transferred during charge/discharge process, F is the Faraday constant (96485 C mol1), CLi is Li ion concentration in the pristine Li[Li0.2Ni0.2Mn0.6]O2 material (0.0228 mol cm3), A is the electrode area (1.13 cm2), R is the gas constant (8.314 J mol1 K1), T is the absolute temperature (298 K), and y is the scan rate (V s1). The value of the DLi can be calculated from the slop ip vs. y1/2, and the values of DLi for the LNMO and LNMO‒Mg samples are 1.41 1011 cm2 s1 and 4.81 1011 cm2 s1, respectively. Fig. 6d shows the Nyquist curves of the LNMO and LNMO‒Mg samples after 50 cycles at 0.2C. RSEI and Rct in the equivalent circuit represent SEI resistance and charge transfer resistance, and the intercept of the curves on the Z axis represents ohmic resistance, such as the resistance of the electrolyte and separator and various contact resistance [9,13,14,29,30]. The diagonal line in the low frequency region is related to the Warburg impedance (Zw). The resulting Nyquist curves are fitted by the corresponding equivalent circuit using the Zsimpwin 3.1 software, and the values of Rct for LNMO and LNMO‒Mg samples are 164.6 U and 127.7 U, respectively. Meanwhile, the exchange current density (jo ¼ RT/nFRct) is often used to measure the ability of an electrode to gain or lose electrons. It can be seen from the formula that the exchange current density (jo) is inversely proportional to the charge transfer resistance, and the LNMO‒Mg sample shows the higher value of jo. Thus, the low charge transfer resistance and high exchange current density and diffusion coefficient of the Liþ indicate that the kinetic characteristics of the Li[Li0.2Ni0.2Mn0.6]O2 electrode material are well improved after doped with Mg2þ.
Fig. 6. The cyclic voltammetry curves of the samples at various scan rates in the range of 0.1e1.0 mV s1 after 50 cycles at 0.2C: (a) LNMO sample, (b) LNMOMg sample; (c) The plots of peak currents (ip) vs. square root of the scan rates (y0.5) for the samples; (d) Nyquist curves of the LNMO and LNMOMg samples after 50 cycles at 0.2C (the inset shows the equivalent circuit for EIS results fitting).
J. Chen et al. / Journal of Alloys and Compounds 805 (2019) 673e679
4. Conclusions Mg2þ‒doped Li[Li0.2Ni0.18Mn06Mg0.02]O2 microsheets with porous structures comprised of nano‒particles are obtained by a PP membrane with rich pore structure as hard template. The doping of Mg2þ does not change the crystal structure of the Li[Li0.2Ni0.2Mn0.6] O2 material, and helps to improve the structure stability and the purity of the active material for the decrease of valence degeneracy. The aseobtained Li[Li0.2Ni0.18Mn0.6Mg0.02]O2 electrode material delivers the initial discharge specific capacity of 196.0 mAh g1 and 209.0 mAh g1 after 50 cycles at 0.2C. For even higher Cerates, a discharge specific capacity of 134.3 mAh g1 at 5 C is reached for this sample. The superior electrochemical performance could be attributed to the special topographical features, increased the migration speed of lithium ions, and reduced the polarization and charge transfer resistance during repeated charge/discharge process. Acknowledgements This work was financially supported by the Education Department and Science and Technology Department of Henan Province (15A430033, 182102210425) and Research Foundation of Luoyang Institute of Science and Technology (2018YZ05). References [1] W. Hua, M. Chen, B. Schwarz, M. Knapp, M. Bruns, J. Barthel, X. Yang, F. Sigel, R. Azmi, A. Senyshyn, A. Missiul, L. Simonelli, M. Etter, S. Wang, X. Mu, A. Fiedler, J.R. Binder, X. Guo, S. Chou, B. Zhong, S. Indris, H. Ehrenberg, Lithium/oxygen incorporation and microstructural evolution during synthesis of Lierich layered Li[Li0.2Ni0.2Mn0.6]O2 oxides, Adv. Energy Mater. 9 (2019) 183094. [2] M. Yang, B. Hu, F. Geng, C. Li, X. Lou, B. Hu, Mitigating voltage decay in highecapacity Li1.2Ni0.2Mn0.6O2 cathode material by surface Kþ doping, Electrochim. Acta 291 (2018) 278e286. [3] D. Uzun, Boronedoped Li1.2Mn0.6Ni0.2O2 as a cathode active material for lithium ion battery, Solid State Ion. 281 (2015) 73e81. [4] L.F. Jiao, M. Zhang, H.T. Yuan, M. Zhao, J. Guo, W. Wang, X.D. Zhou, Y.M. Wang, Effect of Cr doping on the structural, electrochemical properties of Li [Li0.2Ni0.2x/2Mn0.6x/2Crx]O2 (x ¼ 0, 0.02, 0.04, 0.06, 0.08) as cathode materials for lithium secondary batteries, J. Power Sources 167 (2007) 178e184. [5] X. Liu, T. Huang, A. Yu, Fe doped Li1.2Mn0.6ex/2Ni0.2ex/2FexO2(x0.1) as cathode materials for lithiumeion batteries, Electrochim. Acta 133 (2014) 555e563. [6] Y. Zhao, M. Xia, X. Hu, Z. Zhao, Y. Wang, Z. Lv, Effects of Sn doping on the structural and electrochemical properties of Li1.2Ni0.2Mn0.8O2 Lierich cathode materials, Electrochim. Acta 174 (2015) 1167e1174. [7] Y. Zang, C.eX. Ding, X.eC. Wang, Z.eY. Wen, C.eH. Chen, Molybdenumedoped lithiumerich layeredestructured cathode material Li1.2Ni0.2Mn0.6O2 with high specific capacity and improved rate performance, Electrochim. Acta 168 (2015) 234e239. [8] D. Wang, Y. Huang, Z. Huo, L. Chen, Synthesize and electrochemical characterization of Mgedoped Lierich layered Li[Li0.2Ni0.2Mn0.6]O2 cathode material, Electrochim. Acta 107 (2013) 461e466. [9] L. Li, L. Wang, X. Zhang, Q. Xue, L. Wei, F. Wu, R. Chen, 3D reticular Li1.2Ni0.2Mn0.6O2 cathode material for lithiumeion batteries, ACS Appl. Mater. Interfaces 9 (2017) 1516e1523.
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