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A new doping element to improve the electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2 materials for Li-ion batteries
Author’s Accepted Manuscript A new doping element to improve the electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2 materials for Li-ion batteries Yingying Sun, Qing Wu, Li Zhao www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)31599-2 https://doi.org/10.1016/j.ceramint.2018.06.154 CERI18598
To appear in: Ceramics International Received date: 1 June 2018 Revised date: 15 June 2018 Accepted date: 18 June 2018 Cite this article as: Yingying Sun, Qing Wu and Li Zhao, A new doping element to improve the electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2 materials for Li-ion batteries, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.06.154 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new doping element to improve the electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2 materials for Li-ion batteries
Yingying Sun 1 · Qing Wu 1 · Li Zhao 1*
1
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of
Chemistry and Chemical Engineering, Harbin Institute of Technology, West Dazhi Street 92#, Harbin 150001,
People’s Republic of China
Abstract: In this study, vanadium (V) was firstly used to dope alone or with molybdenum (Mo) in Li[Li
0.2Mn0.54Co0.13Ni0.13]O2
to enhance its electrochemical performance. A series of V-doped
lithium-rich materials Li1.2Mn0.54-xNi0.13Co0.13VxO2 (0≤x≤0.04) and V/Mo co-doped lithium-rich materials Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 (0.02≤x≤0.03, 0≤y≤0.03) with varied doping ratios have been successfully synthesized through a sol-gel method. The XRD results indicate that both Li1.2Mn0.54-xNi0.13Co0.13VxO2 (0≤x≤0.04) and Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 (0.02≤x≤0.03, 0≤y≤0.03) materials have a typical hexagonal α-NaFeO2 structure. The electrochemical performance of the original and doped Li[Li 0.2Mn0.54Co0.13Ni0.13]O2 samples was evaluated. The results present that cycling performance of Li1.2Mn0.54-xNi0.13Co0.13VxO2 materials is significantly enhanced in comparison to the original material. When the doping amount of V is 0.02, the discharge capacity of the Li1.2Mn0.54-xNi0.13Co0.13VxO2 materials after 300 cycles at 1C (142.0 mAh/g) is slightly higher than the initial value (140.3 mAh/g) and that of undoped materials (112.0 mAh·g-1 after 150 cycles at 1C). The
*
Corresponding author
Email: [email protected]
1
rate performance of V and Mo co-doped materials is significantly improved. When x=0.03 and y=0.03, the discharge capacity of the Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 materials at 5 C is 52.7% of that at 0.1 C while the discharge capacity ratio of the single-doped V materials is only 39.8%. The doping of V and Mo can effectively inhibit the transformation of Li[Li
0.2Mn0.54Co0.13Ni0.13]O2
materials from
layered structure to spinel structure in the charging-discharging process, and increase the ionic conductivity of the Li[Li0.2Mn0.54Co0.13Ni0.13]O2 materials, thus the electrochemical properties of the materials are obviously improved. Keywords: Sol-gel; V-doping; V and Mo co-doping; Electrochemical properties
1. Introduction With the continuous exploitation of traditional non-renewable resources, the fossil energy on the earth has been exhausting. In order to fundamentally solve the increasingly serious energy crisis, secondary batteries have been broadly utilized as the mutual transferable devices between chemical energy and electric energy[1-3]. Lithium-ion batteries have aroused tremendous attention by reason of their long cycle life, high operating voltage and high discharge capacity[4,5]. However, because of their rare element reserves, environmental pollution or crystal transformation, the application of traditional Li-ion battery materials (such as LiCoO2 and LiMnO4) has been seriously limited[6]. The lithium-rich manganese-based cathode materials xLi2MnO3 (1-x) LiMO2 (M = Ni, Co, Mn) can be identified as the solid solution system, which are consisted of Li2MnO3 and LiMO2 (M = Ni, Co, Mn) phases. Li2MnO3 phase belongs to monoclinic system and C2/m space group, and LiMO2 phase is a hexagonal system belonging to R-͵തm space group with a typical α-NaFeO2 layered structure[7-9]. The actual specific capacity of the lithium-rich laminated cathode materials from 2.0 V to 4.8 V is approximately twice the actual specific capacity of the currently used cathode material, so they are 2
regarded as the most promising lithium ion cathode materials[7,8,10]. Although Li-rich laminated cathode materials xLi2MnO3(1-x)LiMO2 (M = Ni, Co, Mn) have prominent advantages, including high specific capacity and long cycle life[11,12], there are still many drawbacks restricting their development, such as the large loss of first irreversible capacity, poor electronic conductivity, voltage drop during cycling[13-17]. The approaches for increasing the electrochemical performance of lithium-rich laminated cathode materials mainly include surface modification[18-20], surface coating[21-24], element doping[25-27], and preparation of electrode materials with special morphology[28,29]. Studies have shown that the doping of element can effectively enhance the structural stability of the active materials, the electronic and ionic conductivity of the materials, thereby improving the rate performance and cycle stability. Generally, there are four methods to modify the lithium-rich material: (1) Mg doped at Li site[30]; (2) metal cations doped at the transition metal site[25,31,32]; (3) F substitutes for O[33]; (4) B and other elements doped in the transition metal layer to form polyanion (BO4)3-[34] and so on. Q. Ma et al[26] successfully synthesized Se-doped Li1.2[Mn0.7Ni0.2Co0.1]0.8O2 via co-precipitation. The doping of Se can enhance the rate performance of materials by enhancing the crystalline degree of Li2MnO3 and reducing the voltage decay process of Li1.2[Mn0.7Ni0.2Co0.1]0.8O2 during cycling. According to Laisa et al.[35] , the substitution of Ca for Li in Li 1.2-2xCaxCo0.13Ni0.13Mn0.54O2 (x = 0.005) increased interlayer distance and improved structural stability, the initial coulombic efficiency and the capacity retention significantly improved.
M
Lei
et
al.[36]
successfully
synthesized
the
Nb
and
F
co-doped
Li1.2Mn0.54−xNbxCo0.13Ni0.13O2−6xF6x (x = 0, 0.01, 0.03, 0.05) via a solid-phase method. The capacity retention of Li1.2Mn0.54−xNbxCo0.13Ni0.13O2−6xF6x is 98.1% after 200 cycles at rate of 1C. In this work, the Li1.2Mn0.54-xNi0.13Co0.13VxO2 (x=0, 0.01, 0.02, 0.03, 0.04) is successfully 3
compound via traditional sol-gel method, and the Mn site of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 is effetively doped by V ion. The effects of V-doping on the crystal structure and cycle performance were studied in detail. For purpose of further improving their rate performance, the Mo element was doped following the V doped materials. The results indicate that the V and Mo co-doping can keep the stability of the crystal structure, and thus enhancing rate performance.
2. Experimental 2.1 Preparation of materials (1)Preparation of V-Doped Materials As shown in Fig. 1, Li1.2Mn0.54-xNi0.13Co0.13VxO2 (x = 0, 0.01, 0.02, 0.03, 0.04) was prepared by a sol-gel method, denoted as LMNC, LMNCV1, LMNCV2, LMNCV3, LMNCV4. According to the stoichiometric ratio, Mn(CH3COO)2·4H2O, Li(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, NH4VO3 and Ni(CH3COO)2·4H2O were weighed and dissolved in distilled water to prepare 2.0 mol·L-1 metal salt solution, and the solution was marked as A. Then a appropriate amount of glycolic acid was dissolved in distilled water to prepare 2.0 mol·L-1 solution, marked as Solution B. Drop solution B into the stirred solution A, the pH value was adjusted to approximately 7.0 with ammonia water, then the solution was evaporated at 90 °C until the fuchsia aquogel was formed. The gel was dried at 100 °C for 24 h, then sintered at 450 °C for 5 h in air, then cooled to indoor temperature, and calcined at 900 °C in the air for 12 h. (2)Preparation of materials co-doped with V and Mo Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 (x=0.02, 0.03; y=0.01, 0.02, 0.03) was prepared by a sol-gel method, denoted as LMNC-V2My and LMNC-V3My (y=0, 0.01, 0.02, 0.03), respectively. According to the stoichiometric ratio, Mn(CH3COO)2·4H2O, Li(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, 4
H8MoN2O4, NH4VO3 and Ni(CH3COO)2·4H2O were weighed and dissolved in distilled water to prepare 2.0 mol·L-1 metal salt solution, and the solution was marked as A. Then a appropriate amount of glycolic acid was dissolved in distilled water to prepare 2.0 mol·L-1 solution, marked as Solution B. Drop solution B into the stirred solution A, the pH value was adjusted to approximately 7.0 with ammonia water, and the solution was evaporated at 90 °C until the fuchsia aquogel was formed. The gel was dried at 100 °C for 24 h, then sintered at 450 °C for 5 h in air, then cooled to indoor temperature, and calcined at 900 °C in the air for 12 h. 2.2 Characterization The elemental composition, morphologies and distribution of all materials were analyzed by field emission scanning electron microscope (SEM, Hitachi SU8010) equipped with an energy dispersive X-ray detector (EDS). The phase composition and lattice parameters of all materials were measured by X-ray diffraction (XRD, Rigaku Ultima D/max-RB 12 KW). XRD spectrum was recorded in the range of 2θ values between 10o and 90o at a scan rate of 5o min-1. 2.3 Electrochemical measurement The cathodes were prepared by coating a slurry onto Al foil (acetylene black: PVDF: the samples = 1: 1: 8, grinding to a uniform viscous material with an certain amount of N-methyl-2-pyrrolidone (NMP) ), then vacuum drying under 100 ° C for 15 h. In a glove box filled with argon gas, a CR2025 coin cell battery wes assembled by a prepared electrode plate which used as a cathode and a lithium metal plate which used as an anode, the electrolyte used 1 mol/L LiPF6 dissolved in EC-DMC (1:1,in weight). Charge-discharge cycle tests were evaluated at room temperature under galvanostatic method within a voltage range of 2.0-4.8V. The cyclic voltammetry (CV) of the cell was recorded at a scan rate of 0.1 mV·s -1 over a voltage 5
range between 2.0V to 4.8V using an electrochemical workstation (CHI660E, Shanghai, China). The electrochemical impedance spectroscopy (EIS) measurements of the batteries were carried out by an electrochemical station (CHI660E, Shanghai, China) with a frequency range of 0.01-100000 Hz and an AC voltage amplitude of 5 mV.
3. Results and discussion 3.1 Effects of V doping on the properties of material Fig. 2 shows the XRD patterns of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples. It can be seen from Fig. 2 that all materials possess a typical α-NaFeO2 structure at space group R-͵തm[37,38]. Few weak diffraction peaks within 2θ=20-25o are ascribed to the LiMn6 cation arrangement in the
Li2MnO3 phase with a monoclinic cell structure in C2/m space group[39-41]. The structural characteristics of the lithium-rich material can be judged from the peak splitting degree of (006)/(012) and (018)/(110). The complete splitting of these diffraction peaks represents all materials have a homogeneous layered structure[42-44]. It can be seen from Fig. 2 that the (006)/(012) and (018)/(110) diffraction peaks of the synthesized materials obviously split, indicating that these materials have complete layered structures. The lattice parameters of V-doped materials were listed in Table 1. It can be seen from Table 1 that different V doping results in a slight change of the a-axis and c-axis of the materials. The c/a values of the materials were greater than 4.899, which shows that the layered structure of the materials is unbroken[45]. After V doping, the c/a values of the materials were decreased, indicating that the V element replaces the Mn element not to affect the structure of the materials. With the increase of the doping amount of V, the values of I(003)/I(104) was increased initially and then decreased, and the values of (I(006) + I(012) )/I(101) was decreased at first and then increased, showing that the cationic mixing of the materials is higher than the undoped materials, V doping will 6
increase the lithium-rich material cationic mixing. for the doping materials, when x = 0.02 and 0.03, the cationic mixing is lowest, the performance should be better. The microstructure and crystal size of the materials have important effects on their structural and electrochemical properties. Fig. 3 shows the SEM images of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples. It can be found that the materials are composed of irregularly shaped particles with sizes between 100 nm and 300 nm. With the increase of the doping amount of V, the agglomeration of the materials increases. Therefore, some active materials cannot be fully utilized, and the migration distance required for intercalation and delamination of Li ions becomes longer, resulting in decrease in specific capacity and rate performance. The initial charge/discharge curves of V-doped materials at a current of 0.1 C within 2.0 V to 4.8 V were shown in Fig. 4(a). It can be seen from Fig. 4(a) that the initial discharge capacity and the initial coulombic efficiency of V-doped materials are decreased. When x = 0.04, the initial discharge capacity is the lowest. When x = 0.01, the initial coulombic efficiency is 68.0%, lower than 69.0% of the undoped materials. When x = 0.03, the initial discharge capacity is 210.7 mAh·g -1, which is the highest. These findings indicate that the doping of V element does not improve the initial coulombic efficiency of the materials, which may be related to the increased cationic mixing of the V-doped materials. Fig. 4(b) and Fig. 4(c) show the cycle performance of V-doped materials. As can be seen from Fig. 4(b), the initial discharge capacity of the materials is decreased at 0.2 C after V doping, but the capacity remains unchanged after 50 cycles. When the doping amounts of V are 0, 0.01, 0.02, 0.03 and 0.04, the capacity retention rates of the materials are 69.8%, 86.7%, 96.0%, 99.8% and 95.3%, respectively. As can be seen from the Fig. 4(c), the discharge capacity of undoped materials decays to 112.0 mAh·g -1 7
after 150 cycles at 1 C current. When x = 0.02, the material has a capacity of 142.0 mAh·g -1 after 300 cycles, which is higher than the initial discharge capacity. The material cycle stability is greatly improved after V doping, probably because the V element can effectively prevent the materials from transforming from the layered structure to the spinel structure during the cycles. The rate performance of the V-doped materials is shown in Fig. 4(d). It can be seen from Fig. 4(d) that with the increase of V element doping, the rate performance of the materials is gradually increased. The discharge capacity is even lower than that of the undoped materials at a small discharge rate. The discharge capacity of the doped materials is 85 mAh·g-1 at 5 C, higher than that of that of the undoped materials. This shows that the V element has an impact on the rate performance. CV testing of the V-doped materials was conducted to investigate their electrochemical behavior as shown in Fig. 5. As shown in Fig. 5 there are two distinct oxidation peaks in the initial charge curve. The one near 4.2 V corresponds to the first discharge platform in the first charge-discharge curve, during which Li ions release from LiMO 2 accompanied by the oxidation reactions of Ni2+/Ni4+ and Co3+/Co4+. The other at about 4.6 V corresponds to the second discharge platform in the first charge-discharge curve, in which Li2MnO3 is activated and produces Li ions and MnO2, accompanied by the loss of oxygen. This peak disappears in the subsequent cycles, indicating that the process is irreversible. A new reduction peak appears near 3.25 V in the initial discharge curve. This is because Mn4+ are partially reduced in the initial discharge and the initial discharge specific capacity of the materials is also increased. When the lithium-rich material is charged to 4.4 V and above, the oxidation peak at 4.6 V in the first cycle curve gradually increases with the increase of the amount of the doped V, indicating the decrease of the initial charge-discharge efficiency of the materials. With the increase of the scanning numbers, the first oxidation peak moves to the negative direction, and the negative 8
potential shift gradually decreases, indicating that the cathode has a good reversibility. After V doping, the coincidence degree of the second lap and the third lap is better, indicating that V doping improves the cycle reversibility of the materials. This is consistent with the charge-discharge test results. To investigate the effects of V doping on the electrochemical performance of the doped materias, electrochemical impedance spectra (EIS) were measured. Fig. 6 shows EIS of the V-doped materials. The impedance spectra of the Li-rich manganese-based materials generally consist of three parts: the semicircle in the high frequency region is formed by solid electrolyte interface (SEI) resistance; the semicircle in the intermediate frequency region is caused by the charge-transfer resistance (Rct), and the slope of the low frequency region is connected with the diffusion of Li+ within the materials[46,47]. As shown in Fig. 6, compared with Rct, Rs of different materials could be ignored. Rct of the undoped materials is the largest. These indicate that the charge transfer resistance is decreased and the electrical conductivity of cathode materials is improved when the materials was doped with V element. As a result, the cycle performance is enhanced. 3.2 Effects of V and Mo co-doping on the properties of the materials Fig. 7 shows the XRD pattern of Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2. It can be seen from Fig. 7 that the (006)/(012) and (118)/(110) diffraction peaks of the synthesized materials have good splitting peaks, indicating that the layered structures of these materials are relatively ideal. The Lattice parameters of the V and Mo co-doped materials are listed in Table 2. When the doping amounts of V and Mo are different, the a-axis and c-axis of the materials change slightly. From the Table 2, the c/a values of the materials are greater than 4.899, indicating that the materials have an unbroken layered structure. When x=0.02, the value of I(003)/I(104) of the materials doped with Mo is higher than that of the materials doped with V, and the value of (I(006)+I(012))/I(101) is decreased, indicating that the degree of cation mixing is 9
reduced after Mo doping. This reduces the resistance of Li+ de-intercalation and improves the electrochemical performance of the materials. The SEM images of the V and Mo co-doped materials are shown in Fig. 8 and Fig. 9. As can be seen from the figure, after Mo doping, the particle sizes of the materials are increased from 100-200 nm to about 1 μm, which can lead to smaller the contact areas between the electrode material and the electrolyte. As a result, the polarization phenomenon in the electrochemical process of the electrode is increased, leading to the decrease in electrochemical performance. The initial charge/discharge curves of V and Mo co-doped materials at a current of 0.1 C within 2.0 and 4.8 V are shown in Fig. 10(a) and Fig. 10(b). As can be seen, the initial discharge capacity of the materials after double doping of V and Mo has no obvious changes compared with the single V doped materials. When x=0.02 and y=0.02, the initial charge-discharge capacities were 294.2 mAh·g-1 and 199.3 mAh·g-1 at 0.1 C, respectively. The maximum charge-discharge coulombic efficiency was 67.8%. All the co-doped materials are better in terms of the charge-discharge coulombic efficiency than the single V-doped materials, and their irreversible specific capacity is reduced. The rate performance of V and Mo co-doped materials was shown in Fig. 10(c) and Fig. 10(d). It can be seen that V and Mo co-doped materials has a higher discharge capacity at 0.5 C and above than V-doped materials, indicating that the doping of Mo can improve the rate performance of the materials. At 5 C, the V and Mo co-doped materials have a discharge capacity of 100 mAh·g-1, which is about 20 mAh·g-1 higher than that of the V-doped materials. It is calculated that the discharge capacity at 5 C of the materials doped with 0.03 Mo is 52.7% of that at 0.1C, while the discharge capacity ratio of the single V-doped materials is only 39.8%. This shows that the rate performance of Mo-doped materials is obviously improved. The possible reason is that the radius of Mo ion is larger than that of Mn ion, 10
broadening the deionization path of Li ions to increase the de-intercalation rate. Fig. 10(e) and Fig. 10(f) show EIS of materials co-doped with V and Mo. The impedance spectra of the Li-rich layered materials generally consist of three parts: the semicircle in the high-frequency region is related to SEI resistance, the semicircle in the intermediate frequency region is caused by Rct, and the slope of the low frequency region is related to the diffusion of Li + within the materials. As can be seen, when the doping amount of V unchanged, Rct of the materials firstly decreases and then increases with the increase of Mo content. The decrease of Rct is favorable for the migration of Li +, which can improve rate performance. CV testing of the materials co-doped with V and Mo was conducted to understand their electrochemical behavior, as shown in Fig. 11 and Fig. 12. As can be seen from the figures, the oxidation peak near 4.2 V in the initial charge curve is due to the release of Li+ from LiMO2 and oxidation of Ni2+ and Co3+. The removal of Li+ from Li2MnO3 and the generation of oxygen result in the formation of the second oxidation peak near 4.6 V. The peak disappears in the subsequent cycles, indicating that the process is irreversible. When the lithium-rich material is charged to 4.4 V and above, the oxidation peak at 4.6 V in the first cycle curve gradually increases with the increase of the amount of the doped Mo, indicating the decrease in the initial charge-discharge efficiency of the materials. With the increase of the scanning numbers, the first oxidation peak moves to the negative direction, and the negative potential shift gradually decreases, indicating that the cathode has good reversibility. The coincidence degree of the second lap and the third lap increases with the increase of Mo doping amount, indicating that the doping of Mo can reduce the electrochemical polarization and improve the cycle reversibility of the materials.
11
4. Conclusion In summary, the cathode materials of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 doped with V were synthesized by the sol-gel method. Both original and V-doped materials show a typical hexagonal α-NaFeO2 structure. When the doping amount of V is 0.03, the capacity retention of the V-doped materials after 50 cycles at 0.2 C is 99.8%. When the doping amount of V is 0.02, the discharge capacity (142.0 mAh·g-1) of the materials after 300 cycles is slightly higher than the initial discharge capacity (140.3 mAh·g-1), however the discharge capacity of the undoped materials decays to 112.0 mAh·g-1 after 150 cycles. The doping of V can significantly enhance the cyclic performance of materials, but the rate performance is not significantly changed. Mo element was doped following the V doping, the results show that rate performance of Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 (0.02≤x≤0.03, 0≤y≤0.03) is significantly
enhanced.
When
x=0.03
and
y=0.03,
the
discharge
capacity
of
the
Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 materials at 5 C is 52.7% of that at 0.1 C while the discharge capacity ratio of the single-doped V materials is only 39.8%.
Acknowledgements We would like to acknowledge Shenzhen EPT Battery Co., LTD for the support of this work
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cathode material by fluorine incorporation, Electrochim. Acta 105 (2013) 200-208. [42] W. He, D. Yuan, J. Qian, X. Ai, H. Yang, Y. Cao, Enhanced high-rate capability and cycling stability of Na-stabilized layered Li1.2[Co0.13Ni0.13Mn0.54]O2 cathode material, J. Mater. Chem. A 1 (2013) 11397-11403. [43] X. Liu, J. Liu, T. Huang, A. Yu, CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials for Li-ion batteries, Electrochim. Acta 109 (2013) 52-58. [44] Y.J. Lee, J.M. Lee, J.E. Lee, K.B. Lee, E.S. Lee, J. Yoon, M.H. Yu, J.H. Baek, C.I. Shin, J.K. Han, B.I. Choi, MR Elastography for Noninvasive Assessment of Hepatic Fibrosis: Reproducibility of the Examination and Reproducibility and Repeatability of the Liver Stiffness Value Measurement, J. Magn. Reson. Imaging. 39 (2014) 326-331. [45] L. Sun, X. Yi, X. Ren, P. Zhang, J. Liu, Synthesis and Electrochemical Performances of Y-Doped Lithium-Rich Layered Li[Li0.2Ni0.2Mn0.6]O2 Cathode Material, J Electrochem. Soc. 163 (2016) A766-A772. [46] Y. Bai, X. Wang, X. Zhang, H. Shu, X. Yang, B. Hu, Q. Wei, H. Wu, Y. Song, The kinetics of Li-ion deintercalation in the Li-rich layered Li1.12[Ni0.5Co0.2Mn0.3]0.89O2 studied by electrochemical impedance spectroscopy and galvanostatic intermittent titration technique, Electrochim. Acta 109 (2013) 355-364. [47] C. Yang, Q. Zhang, W. Ding, J. Zang, M. Lei, M. Zheng, Q. Dong, Improving the electrochemical performance of layered lithium-rich cathode materials by fabricating a spinel outer layer with Ni 3+, J. Mater. Chem. A 3 (2015) 7554-7559.
18
Captions of Figures and Tables Fig. 1 Flow diagram of the synthesis of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples via a sol-gel method Fig. 2 XRD patterns of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples Fig. 3 SEM images of the LMNC-Vx samples (a, b) x=0.01; (c, d) x=0.02; (e, f) x=0.03; (g, h) x=0.04 Fig. 4 Electrochemistry performance of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples: (a) Initial charge/discharge curves; (b)cycling performance at 0.2 C; (c) cycling performance at 1 C; (d) rate performance Fig. 5 CV curves of the LMNC-Vx (a) x=0.01; (b) x=0.02; (c) x=0.03; (d) x=0.04 Fig. 6 Electrochemical impedance spectra of the LMNC-Vx (The inset is the equivalent circuit for the impedance spectra) Fig. 7 XRD patterns of the LMNC-V2My and LMNC-V3My (y=0, 0.01, 0.02, 0.03). Fig. 8 SEM images of the LMNC-V2My (a, b) y=0.00, (c, d) y=0.01, (e, f) y=0.02, (g, h) y=0.03 Fig. 9 SEM images of the LMNC-V3My (a, b) y=0.00, (c, d) y=0.01, (e, f) y=0.02, (g, h) y=0.03 Fig. 10 Initial charge/discharge curves: (a) LMNC-V2My; (b) LMNC-V3My. Rate performance: (c) LMNC-V2My; (d) LMNC-V3My. Electrochemical impedance spectra: (e) LMNC-V2My; (f) LMNC-V3My 19
Fig. 11 CV curves of the LMNC-V2My (a) y=0.00; (b) y=0.01; (c) y=0.02; (d) y=0.03 Fig. 12 CV curves of the LMNC-V3My (a) y=0.00; (b) y=0.01; (c) y=0.02; (d) y=0.03 Table 1 Lattice parameters of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples Table 2 Lattice parameters of the LMNC-V2My and LMNC-V3My (y=0, 0.01, 0.02, 0.03).
Table 1 Lattice parameters of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) sample x
a/Å
c/Å
c/a ratio
I(003)/I(104)
(I(006)+I(012))/I(101)
0
2.8488
14.2293
5.0006
1.5875
0.2988
0.0
2.8477
14.1930
4.9840
1.3127
0.3630
0.0
2.8525
14.2226
4.9860
1.5132
0.3371
0.0
2.8490
14.2199
4.9911
1.3916
0.3232
0.0
2.8510
14.2000
4.9808
1.3838
0.4376
1 2 3 4
Table 2 Lattice parameters of the LMNC-V2My and LMNC-V3My (y=0, 0.01, 0.02, 0.03). x
y
a/
c/Å
Å 0
0
.02
ratio 2.8
477 0 .01
c/a
2.8 735
14.1 930 14.2 057
4.98 40 4.94 38
I(003) /I(104) 1.31
(I(006)+I(012))/ I(101) 0.3630
27 1.57 18
20
0.3410
0 .02
551 0
.03 0
2.8
2.8 525
0
.03
2.8 490
0 .01
530 0
.02
2.8 525
0 .03
2.8
2.8 510
14.1 516 14.2 488 14.2 199 14.2 186 14.2 193 14.2 000
4.95 66 4.99 53 4.99 11 4.98 38 4.98 48 4.98 08
1.81
0.3337
28 1.62
0.3538
63 1.38
0.3232
16 1.51
0.3366
21 1.76
0.3432
10 1.39 38
21
0.3376
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
PII: DOI: Reference:
S0272-8842(18)31599-2 https://doi.org/10.1016/j.ceramint.2018.06.154 CERI18598
To appear in: Ceramics International Received date: 1 June 2018 Revised date: 15 June 2018 Accepted date: 18 June 2018 Cite this article as: Yingying Sun, Qing Wu and Li Zhao, A new doping element to improve the electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2 materials for Li-ion batteries, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.06.154 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new doping element to improve the electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2 materials for Li-ion batteries
Yingying Sun 1 · Qing Wu 1 · Li Zhao 1*
1
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of
Chemistry and Chemical Engineering, Harbin Institute of Technology, West Dazhi Street 92#, Harbin 150001,
People’s Republic of China
Abstract: In this study, vanadium (V) was firstly used to dope alone or with molybdenum (Mo) in Li[Li
0.2Mn0.54Co0.13Ni0.13]O2
to enhance its electrochemical performance. A series of V-doped
lithium-rich materials Li1.2Mn0.54-xNi0.13Co0.13VxO2 (0≤x≤0.04) and V/Mo co-doped lithium-rich materials Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 (0.02≤x≤0.03, 0≤y≤0.03) with varied doping ratios have been successfully synthesized through a sol-gel method. The XRD results indicate that both Li1.2Mn0.54-xNi0.13Co0.13VxO2 (0≤x≤0.04) and Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 (0.02≤x≤0.03, 0≤y≤0.03) materials have a typical hexagonal α-NaFeO2 structure. The electrochemical performance of the original and doped Li[Li 0.2Mn0.54Co0.13Ni0.13]O2 samples was evaluated. The results present that cycling performance of Li1.2Mn0.54-xNi0.13Co0.13VxO2 materials is significantly enhanced in comparison to the original material. When the doping amount of V is 0.02, the discharge capacity of the Li1.2Mn0.54-xNi0.13Co0.13VxO2 materials after 300 cycles at 1C (142.0 mAh/g) is slightly higher than the initial value (140.3 mAh/g) and that of undoped materials (112.0 mAh·g-1 after 150 cycles at 1C). The
*
Corresponding author
Email: [email protected]
1
rate performance of V and Mo co-doped materials is significantly improved. When x=0.03 and y=0.03, the discharge capacity of the Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 materials at 5 C is 52.7% of that at 0.1 C while the discharge capacity ratio of the single-doped V materials is only 39.8%. The doping of V and Mo can effectively inhibit the transformation of Li[Li
0.2Mn0.54Co0.13Ni0.13]O2
materials from
layered structure to spinel structure in the charging-discharging process, and increase the ionic conductivity of the Li[Li0.2Mn0.54Co0.13Ni0.13]O2 materials, thus the electrochemical properties of the materials are obviously improved. Keywords: Sol-gel; V-doping; V and Mo co-doping; Electrochemical properties
1. Introduction With the continuous exploitation of traditional non-renewable resources, the fossil energy on the earth has been exhausting. In order to fundamentally solve the increasingly serious energy crisis, secondary batteries have been broadly utilized as the mutual transferable devices between chemical energy and electric energy[1-3]. Lithium-ion batteries have aroused tremendous attention by reason of their long cycle life, high operating voltage and high discharge capacity[4,5]. However, because of their rare element reserves, environmental pollution or crystal transformation, the application of traditional Li-ion battery materials (such as LiCoO2 and LiMnO4) has been seriously limited[6]. The lithium-rich manganese-based cathode materials xLi2MnO3 (1-x) LiMO2 (M = Ni, Co, Mn) can be identified as the solid solution system, which are consisted of Li2MnO3 and LiMO2 (M = Ni, Co, Mn) phases. Li2MnO3 phase belongs to monoclinic system and C2/m space group, and LiMO2 phase is a hexagonal system belonging to R-͵തm space group with a typical α-NaFeO2 layered structure[7-9]. The actual specific capacity of the lithium-rich laminated cathode materials from 2.0 V to 4.8 V is approximately twice the actual specific capacity of the currently used cathode material, so they are 2
regarded as the most promising lithium ion cathode materials[7,8,10]. Although Li-rich laminated cathode materials xLi2MnO3(1-x)LiMO2 (M = Ni, Co, Mn) have prominent advantages, including high specific capacity and long cycle life[11,12], there are still many drawbacks restricting their development, such as the large loss of first irreversible capacity, poor electronic conductivity, voltage drop during cycling[13-17]. The approaches for increasing the electrochemical performance of lithium-rich laminated cathode materials mainly include surface modification[18-20], surface coating[21-24], element doping[25-27], and preparation of electrode materials with special morphology[28,29]. Studies have shown that the doping of element can effectively enhance the structural stability of the active materials, the electronic and ionic conductivity of the materials, thereby improving the rate performance and cycle stability. Generally, there are four methods to modify the lithium-rich material: (1) Mg doped at Li site[30]; (2) metal cations doped at the transition metal site[25,31,32]; (3) F substitutes for O[33]; (4) B and other elements doped in the transition metal layer to form polyanion (BO4)3-[34] and so on. Q. Ma et al[26] successfully synthesized Se-doped Li1.2[Mn0.7Ni0.2Co0.1]0.8O2 via co-precipitation. The doping of Se can enhance the rate performance of materials by enhancing the crystalline degree of Li2MnO3 and reducing the voltage decay process of Li1.2[Mn0.7Ni0.2Co0.1]0.8O2 during cycling. According to Laisa et al.[35] , the substitution of Ca for Li in Li 1.2-2xCaxCo0.13Ni0.13Mn0.54O2 (x = 0.005) increased interlayer distance and improved structural stability, the initial coulombic efficiency and the capacity retention significantly improved.
M
Lei
et
al.[36]
successfully
synthesized
the
Nb
and
F
co-doped
Li1.2Mn0.54−xNbxCo0.13Ni0.13O2−6xF6x (x = 0, 0.01, 0.03, 0.05) via a solid-phase method. The capacity retention of Li1.2Mn0.54−xNbxCo0.13Ni0.13O2−6xF6x is 98.1% after 200 cycles at rate of 1C. In this work, the Li1.2Mn0.54-xNi0.13Co0.13VxO2 (x=0, 0.01, 0.02, 0.03, 0.04) is successfully 3
compound via traditional sol-gel method, and the Mn site of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 is effetively doped by V ion. The effects of V-doping on the crystal structure and cycle performance were studied in detail. For purpose of further improving their rate performance, the Mo element was doped following the V doped materials. The results indicate that the V and Mo co-doping can keep the stability of the crystal structure, and thus enhancing rate performance.
2. Experimental 2.1 Preparation of materials (1)Preparation of V-Doped Materials As shown in Fig. 1, Li1.2Mn0.54-xNi0.13Co0.13VxO2 (x = 0, 0.01, 0.02, 0.03, 0.04) was prepared by a sol-gel method, denoted as LMNC, LMNCV1, LMNCV2, LMNCV3, LMNCV4. According to the stoichiometric ratio, Mn(CH3COO)2·4H2O, Li(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, NH4VO3 and Ni(CH3COO)2·4H2O were weighed and dissolved in distilled water to prepare 2.0 mol·L-1 metal salt solution, and the solution was marked as A. Then a appropriate amount of glycolic acid was dissolved in distilled water to prepare 2.0 mol·L-1 solution, marked as Solution B. Drop solution B into the stirred solution A, the pH value was adjusted to approximately 7.0 with ammonia water, then the solution was evaporated at 90 °C until the fuchsia aquogel was formed. The gel was dried at 100 °C for 24 h, then sintered at 450 °C for 5 h in air, then cooled to indoor temperature, and calcined at 900 °C in the air for 12 h. (2)Preparation of materials co-doped with V and Mo Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 (x=0.02, 0.03; y=0.01, 0.02, 0.03) was prepared by a sol-gel method, denoted as LMNC-V2My and LMNC-V3My (y=0, 0.01, 0.02, 0.03), respectively. According to the stoichiometric ratio, Mn(CH3COO)2·4H2O, Li(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, 4
H8MoN2O4, NH4VO3 and Ni(CH3COO)2·4H2O were weighed and dissolved in distilled water to prepare 2.0 mol·L-1 metal salt solution, and the solution was marked as A. Then a appropriate amount of glycolic acid was dissolved in distilled water to prepare 2.0 mol·L-1 solution, marked as Solution B. Drop solution B into the stirred solution A, the pH value was adjusted to approximately 7.0 with ammonia water, and the solution was evaporated at 90 °C until the fuchsia aquogel was formed. The gel was dried at 100 °C for 24 h, then sintered at 450 °C for 5 h in air, then cooled to indoor temperature, and calcined at 900 °C in the air for 12 h. 2.2 Characterization The elemental composition, morphologies and distribution of all materials were analyzed by field emission scanning electron microscope (SEM, Hitachi SU8010) equipped with an energy dispersive X-ray detector (EDS). The phase composition and lattice parameters of all materials were measured by X-ray diffraction (XRD, Rigaku Ultima D/max-RB 12 KW). XRD spectrum was recorded in the range of 2θ values between 10o and 90o at a scan rate of 5o min-1. 2.3 Electrochemical measurement The cathodes were prepared by coating a slurry onto Al foil (acetylene black: PVDF: the samples = 1: 1: 8, grinding to a uniform viscous material with an certain amount of N-methyl-2-pyrrolidone (NMP) ), then vacuum drying under 100 ° C for 15 h. In a glove box filled with argon gas, a CR2025 coin cell battery wes assembled by a prepared electrode plate which used as a cathode and a lithium metal plate which used as an anode, the electrolyte used 1 mol/L LiPF6 dissolved in EC-DMC (1:1,in weight). Charge-discharge cycle tests were evaluated at room temperature under galvanostatic method within a voltage range of 2.0-4.8V. The cyclic voltammetry (CV) of the cell was recorded at a scan rate of 0.1 mV·s -1 over a voltage 5
range between 2.0V to 4.8V using an electrochemical workstation (CHI660E, Shanghai, China). The electrochemical impedance spectroscopy (EIS) measurements of the batteries were carried out by an electrochemical station (CHI660E, Shanghai, China) with a frequency range of 0.01-100000 Hz and an AC voltage amplitude of 5 mV.
3. Results and discussion 3.1 Effects of V doping on the properties of material Fig. 2 shows the XRD patterns of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples. It can be seen from Fig. 2 that all materials possess a typical α-NaFeO2 structure at space group R-͵തm[37,38]. Few weak diffraction peaks within 2θ=20-25o are ascribed to the LiMn6 cation arrangement in the
Li2MnO3 phase with a monoclinic cell structure in C2/m space group[39-41]. The structural characteristics of the lithium-rich material can be judged from the peak splitting degree of (006)/(012) and (018)/(110). The complete splitting of these diffraction peaks represents all materials have a homogeneous layered structure[42-44]. It can be seen from Fig. 2 that the (006)/(012) and (018)/(110) diffraction peaks of the synthesized materials obviously split, indicating that these materials have complete layered structures. The lattice parameters of V-doped materials were listed in Table 1. It can be seen from Table 1 that different V doping results in a slight change of the a-axis and c-axis of the materials. The c/a values of the materials were greater than 4.899, which shows that the layered structure of the materials is unbroken[45]. After V doping, the c/a values of the materials were decreased, indicating that the V element replaces the Mn element not to affect the structure of the materials. With the increase of the doping amount of V, the values of I(003)/I(104) was increased initially and then decreased, and the values of (I(006) + I(012) )/I(101) was decreased at first and then increased, showing that the cationic mixing of the materials is higher than the undoped materials, V doping will 6
increase the lithium-rich material cationic mixing. for the doping materials, when x = 0.02 and 0.03, the cationic mixing is lowest, the performance should be better. The microstructure and crystal size of the materials have important effects on their structural and electrochemical properties. Fig. 3 shows the SEM images of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples. It can be found that the materials are composed of irregularly shaped particles with sizes between 100 nm and 300 nm. With the increase of the doping amount of V, the agglomeration of the materials increases. Therefore, some active materials cannot be fully utilized, and the migration distance required for intercalation and delamination of Li ions becomes longer, resulting in decrease in specific capacity and rate performance. The initial charge/discharge curves of V-doped materials at a current of 0.1 C within 2.0 V to 4.8 V were shown in Fig. 4(a). It can be seen from Fig. 4(a) that the initial discharge capacity and the initial coulombic efficiency of V-doped materials are decreased. When x = 0.04, the initial discharge capacity is the lowest. When x = 0.01, the initial coulombic efficiency is 68.0%, lower than 69.0% of the undoped materials. When x = 0.03, the initial discharge capacity is 210.7 mAh·g -1, which is the highest. These findings indicate that the doping of V element does not improve the initial coulombic efficiency of the materials, which may be related to the increased cationic mixing of the V-doped materials. Fig. 4(b) and Fig. 4(c) show the cycle performance of V-doped materials. As can be seen from Fig. 4(b), the initial discharge capacity of the materials is decreased at 0.2 C after V doping, but the capacity remains unchanged after 50 cycles. When the doping amounts of V are 0, 0.01, 0.02, 0.03 and 0.04, the capacity retention rates of the materials are 69.8%, 86.7%, 96.0%, 99.8% and 95.3%, respectively. As can be seen from the Fig. 4(c), the discharge capacity of undoped materials decays to 112.0 mAh·g -1 7
after 150 cycles at 1 C current. When x = 0.02, the material has a capacity of 142.0 mAh·g -1 after 300 cycles, which is higher than the initial discharge capacity. The material cycle stability is greatly improved after V doping, probably because the V element can effectively prevent the materials from transforming from the layered structure to the spinel structure during the cycles. The rate performance of the V-doped materials is shown in Fig. 4(d). It can be seen from Fig. 4(d) that with the increase of V element doping, the rate performance of the materials is gradually increased. The discharge capacity is even lower than that of the undoped materials at a small discharge rate. The discharge capacity of the doped materials is 85 mAh·g-1 at 5 C, higher than that of that of the undoped materials. This shows that the V element has an impact on the rate performance. CV testing of the V-doped materials was conducted to investigate their electrochemical behavior as shown in Fig. 5. As shown in Fig. 5 there are two distinct oxidation peaks in the initial charge curve. The one near 4.2 V corresponds to the first discharge platform in the first charge-discharge curve, during which Li ions release from LiMO 2 accompanied by the oxidation reactions of Ni2+/Ni4+ and Co3+/Co4+. The other at about 4.6 V corresponds to the second discharge platform in the first charge-discharge curve, in which Li2MnO3 is activated and produces Li ions and MnO2, accompanied by the loss of oxygen. This peak disappears in the subsequent cycles, indicating that the process is irreversible. A new reduction peak appears near 3.25 V in the initial discharge curve. This is because Mn4+ are partially reduced in the initial discharge and the initial discharge specific capacity of the materials is also increased. When the lithium-rich material is charged to 4.4 V and above, the oxidation peak at 4.6 V in the first cycle curve gradually increases with the increase of the amount of the doped V, indicating the decrease of the initial charge-discharge efficiency of the materials. With the increase of the scanning numbers, the first oxidation peak moves to the negative direction, and the negative 8
potential shift gradually decreases, indicating that the cathode has a good reversibility. After V doping, the coincidence degree of the second lap and the third lap is better, indicating that V doping improves the cycle reversibility of the materials. This is consistent with the charge-discharge test results. To investigate the effects of V doping on the electrochemical performance of the doped materias, electrochemical impedance spectra (EIS) were measured. Fig. 6 shows EIS of the V-doped materials. The impedance spectra of the Li-rich manganese-based materials generally consist of three parts: the semicircle in the high frequency region is formed by solid electrolyte interface (SEI) resistance; the semicircle in the intermediate frequency region is caused by the charge-transfer resistance (Rct), and the slope of the low frequency region is connected with the diffusion of Li+ within the materials[46,47]. As shown in Fig. 6, compared with Rct, Rs of different materials could be ignored. Rct of the undoped materials is the largest. These indicate that the charge transfer resistance is decreased and the electrical conductivity of cathode materials is improved when the materials was doped with V element. As a result, the cycle performance is enhanced. 3.2 Effects of V and Mo co-doping on the properties of the materials Fig. 7 shows the XRD pattern of Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2. It can be seen from Fig. 7 that the (006)/(012) and (118)/(110) diffraction peaks of the synthesized materials have good splitting peaks, indicating that the layered structures of these materials are relatively ideal. The Lattice parameters of the V and Mo co-doped materials are listed in Table 2. When the doping amounts of V and Mo are different, the a-axis and c-axis of the materials change slightly. From the Table 2, the c/a values of the materials are greater than 4.899, indicating that the materials have an unbroken layered structure. When x=0.02, the value of I(003)/I(104) of the materials doped with Mo is higher than that of the materials doped with V, and the value of (I(006)+I(012))/I(101) is decreased, indicating that the degree of cation mixing is 9
reduced after Mo doping. This reduces the resistance of Li+ de-intercalation and improves the electrochemical performance of the materials. The SEM images of the V and Mo co-doped materials are shown in Fig. 8 and Fig. 9. As can be seen from the figure, after Mo doping, the particle sizes of the materials are increased from 100-200 nm to about 1 μm, which can lead to smaller the contact areas between the electrode material and the electrolyte. As a result, the polarization phenomenon in the electrochemical process of the electrode is increased, leading to the decrease in electrochemical performance. The initial charge/discharge curves of V and Mo co-doped materials at a current of 0.1 C within 2.0 and 4.8 V are shown in Fig. 10(a) and Fig. 10(b). As can be seen, the initial discharge capacity of the materials after double doping of V and Mo has no obvious changes compared with the single V doped materials. When x=0.02 and y=0.02, the initial charge-discharge capacities were 294.2 mAh·g-1 and 199.3 mAh·g-1 at 0.1 C, respectively. The maximum charge-discharge coulombic efficiency was 67.8%. All the co-doped materials are better in terms of the charge-discharge coulombic efficiency than the single V-doped materials, and their irreversible specific capacity is reduced. The rate performance of V and Mo co-doped materials was shown in Fig. 10(c) and Fig. 10(d). It can be seen that V and Mo co-doped materials has a higher discharge capacity at 0.5 C and above than V-doped materials, indicating that the doping of Mo can improve the rate performance of the materials. At 5 C, the V and Mo co-doped materials have a discharge capacity of 100 mAh·g-1, which is about 20 mAh·g-1 higher than that of the V-doped materials. It is calculated that the discharge capacity at 5 C of the materials doped with 0.03 Mo is 52.7% of that at 0.1C, while the discharge capacity ratio of the single V-doped materials is only 39.8%. This shows that the rate performance of Mo-doped materials is obviously improved. The possible reason is that the radius of Mo ion is larger than that of Mn ion, 10
broadening the deionization path of Li ions to increase the de-intercalation rate. Fig. 10(e) and Fig. 10(f) show EIS of materials co-doped with V and Mo. The impedance spectra of the Li-rich layered materials generally consist of three parts: the semicircle in the high-frequency region is related to SEI resistance, the semicircle in the intermediate frequency region is caused by Rct, and the slope of the low frequency region is related to the diffusion of Li + within the materials. As can be seen, when the doping amount of V unchanged, Rct of the materials firstly decreases and then increases with the increase of Mo content. The decrease of Rct is favorable for the migration of Li +, which can improve rate performance. CV testing of the materials co-doped with V and Mo was conducted to understand their electrochemical behavior, as shown in Fig. 11 and Fig. 12. As can be seen from the figures, the oxidation peak near 4.2 V in the initial charge curve is due to the release of Li+ from LiMO2 and oxidation of Ni2+ and Co3+. The removal of Li+ from Li2MnO3 and the generation of oxygen result in the formation of the second oxidation peak near 4.6 V. The peak disappears in the subsequent cycles, indicating that the process is irreversible. When the lithium-rich material is charged to 4.4 V and above, the oxidation peak at 4.6 V in the first cycle curve gradually increases with the increase of the amount of the doped Mo, indicating the decrease in the initial charge-discharge efficiency of the materials. With the increase of the scanning numbers, the first oxidation peak moves to the negative direction, and the negative potential shift gradually decreases, indicating that the cathode has good reversibility. The coincidence degree of the second lap and the third lap increases with the increase of Mo doping amount, indicating that the doping of Mo can reduce the electrochemical polarization and improve the cycle reversibility of the materials.
11
4. Conclusion In summary, the cathode materials of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 doped with V were synthesized by the sol-gel method. Both original and V-doped materials show a typical hexagonal α-NaFeO2 structure. When the doping amount of V is 0.03, the capacity retention of the V-doped materials after 50 cycles at 0.2 C is 99.8%. When the doping amount of V is 0.02, the discharge capacity (142.0 mAh·g-1) of the materials after 300 cycles is slightly higher than the initial discharge capacity (140.3 mAh·g-1), however the discharge capacity of the undoped materials decays to 112.0 mAh·g-1 after 150 cycles. The doping of V can significantly enhance the cyclic performance of materials, but the rate performance is not significantly changed. Mo element was doped following the V doping, the results show that rate performance of Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 (0.02≤x≤0.03, 0≤y≤0.03) is significantly
enhanced.
When
x=0.03
and
y=0.03,
the
discharge
capacity
of
the
Li1.2Mn0.54-x-yNi0.13Co0.13VxMoyO2 materials at 5 C is 52.7% of that at 0.1 C while the discharge capacity ratio of the single-doped V materials is only 39.8%.
Acknowledgements We would like to acknowledge Shenzhen EPT Battery Co., LTD for the support of this work
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18
Captions of Figures and Tables Fig. 1 Flow diagram of the synthesis of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples via a sol-gel method Fig. 2 XRD patterns of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples Fig. 3 SEM images of the LMNC-Vx samples (a, b) x=0.01; (c, d) x=0.02; (e, f) x=0.03; (g, h) x=0.04 Fig. 4 Electrochemistry performance of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples: (a) Initial charge/discharge curves; (b)cycling performance at 0.2 C; (c) cycling performance at 1 C; (d) rate performance Fig. 5 CV curves of the LMNC-Vx (a) x=0.01; (b) x=0.02; (c) x=0.03; (d) x=0.04 Fig. 6 Electrochemical impedance spectra of the LMNC-Vx (The inset is the equivalent circuit for the impedance spectra) Fig. 7 XRD patterns of the LMNC-V2My and LMNC-V3My (y=0, 0.01, 0.02, 0.03). Fig. 8 SEM images of the LMNC-V2My (a, b) y=0.00, (c, d) y=0.01, (e, f) y=0.02, (g, h) y=0.03 Fig. 9 SEM images of the LMNC-V3My (a, b) y=0.00, (c, d) y=0.01, (e, f) y=0.02, (g, h) y=0.03 Fig. 10 Initial charge/discharge curves: (a) LMNC-V2My; (b) LMNC-V3My. Rate performance: (c) LMNC-V2My; (d) LMNC-V3My. Electrochemical impedance spectra: (e) LMNC-V2My; (f) LMNC-V3My 19
Fig. 11 CV curves of the LMNC-V2My (a) y=0.00; (b) y=0.01; (c) y=0.02; (d) y=0.03 Fig. 12 CV curves of the LMNC-V3My (a) y=0.00; (b) y=0.01; (c) y=0.02; (d) y=0.03 Table 1 Lattice parameters of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) samples Table 2 Lattice parameters of the LMNC-V2My and LMNC-V3My (y=0, 0.01, 0.02, 0.03).
Table 1 Lattice parameters of the LMNC-Vx (x = 0, 0.01, 0.02, 0.03, 0.04) sample x
a/Å
c/Å
c/a ratio
I(003)/I(104)
(I(006)+I(012))/I(101)
0
2.8488
14.2293
5.0006
1.5875
0.2988
0.0
2.8477
14.1930
4.9840
1.3127
0.3630
0.0
2.8525
14.2226
4.9860
1.5132
0.3371
0.0
2.8490
14.2199
4.9911
1.3916
0.3232
0.0
2.8510
14.2000
4.9808
1.3838
0.4376
1 2 3 4
Table 2 Lattice parameters of the LMNC-V2My and LMNC-V3My (y=0, 0.01, 0.02, 0.03). x
y
a/
c/Å
Å 0
0
.02
ratio 2.8
477 0 .01
c/a
2.8 735
14.1 930 14.2 057
4.98 40 4.94 38
I(003) /I(104) 1.31
(I(006)+I(012))/ I(101) 0.3630
27 1.57 18
20
0.3410
0 .02
551 0
.03 0
2.8
2.8 525
0
.03
2.8 490
0 .01
530 0
.02
2.8 525
0 .03
2.8
2.8 510
14.1 516 14.2 488 14.2 199 14.2 186 14.2 193 14.2 000
4.95 66 4.99 53 4.99 11 4.98 38 4.98 48 4.98 08
1.81
0.3337
28 1.62
0.3538
63 1.38
0.3232
16 1.51
0.3366
21 1.76
0.3432
10 1.39 38
21
0.3376
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure