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Improved electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode material by double-layer coating with graphene oxide and V2O5 for lithium-ion batteries
Accepted Manuscript Title: Improved electrochemical performance of LiNi0.5 Co0.2 Mn0.3 O2 cathode material by double-layer coating with graphene oxide and V2 O5 for lithium-ion batteries Authors: Wenbin Luo, Baolin Zheng PII: DOI: Reference:
S0169-4332(17)30223-4 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.200 APSUSC 34995
To appear in:
APSUSC
Received date: Revised date: Accepted date:
22-12-2016 18-1-2017 19-1-2017
Please cite this article as: Wenbin Luo, Baolin Zheng, Improved electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode material by double-layer coating with graphene oxide and V2O5 for lithium-ion batteries, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.200 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 proof before it is published in its final 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.
Improved
electrochemical
performance
of
LiNi0.5Co0.2Mn0.3O2
cathode material by double-layer coating with graphene oxide and V2O5 for lithium-ion batteries Wenbin Luo*, Baolin Zheng School of Chemical Engineering, Fuzhou University, Fuzhou 350116, People’s Republic of China *Corresponding author. E-mail address: [email protected]
1
GRAPHICAL ABSTRACT
Highlights Citric acid assisted sol-gel method was used for synthesizing LiNi0.5Co0.2Mn0.3O2. The pristine LiNi0.5Co0.2Mn0.3O2 was surface-modified by double-layer coating. The double coating layer consists of graphene oxide and V2O5. Electrochemical performance was improved by double-layer coating.
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Abstract LiNi0.5Co0.2Mn0.3O2 cathode material synthesized by a sol-gel method was surface-modified by double-layer coating. The results of X-ray diffraction (XRD) confirm that the intrinsic structure was no change after surface modification. A double-layer structure consisting of an inner V2O5 (VO) layer and an outer conductive graphene oxide (GO) layer was coated on the surface of active material, as confirmed by transmission electron microscopy (TEM). The results of field emission scanning electron microscope (FE-SEM) equipped with an energy dispersive spectroscope (EDS) show that both graphene oxide and V2O5 uniformly covered LiNi0.5Co0.2Mn0.3O2 cathode material. The double-layer-coated LiNi0.5Co0.2Mn0.3O2 cathode material shows improved electrochemical performance with a capacity retention of 74.2% after 50 cycles in a range of 2.5-4.5 V at 55°C, compared with only 67.8% capacity retention for the pristine material. In addition, the double-layer-coated LiNi0.5Co0.2Mn0.3O2 releases 116.6 mAh·g-1 under a high current rate, while the pristine material only remains at 105.7 mAh·g-1. The results can be ascribed to the double coating layer not only avoids the side reaction between electrolyte and active material but also promotes Li+ and electronic conductivity. Differential capacity (dQ/dV) and electrochemical impedance spectroscopy (EIS) measurements reveal that the double coating layer effectively suppresses the increase of the electrode polarization during cycling.
Keywords: Lithium-ion batteries; LiNi0.5Co0.2Mn0.3O2; Graphene oxide; V2O5; Double-layer coating; Electrochemical performance
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1. Introduction Recently, energy crisis have motivated the development of energy storage equipments, such as, lithium-ion batteries [1-3], sodium-ion batteries [4], supercapacitors [5, 6], et cetera. Among these energy storage equipments, lithium-ion batteries have been widely used as a new power sources for hybrid electric vehicles (HEVs), electrical vehicles (EVs) and portable electronic devices [7-11]. Layered LiNixCoyMn1-x-yO2 materials with high specific capacity, high working voltage and environmental benignancy have attracted many researchers’ attention [12]. In these materials, the oxidation state of manganese keep in the tetravalent form during electrochemical cycling and the electrochemically inactive manganese play a stabilizing role to prevent a structural collapse during cycling [13]. The cobalt will work as a buffer against the local distortion and decrease the change in the structure of the cathode during cycling [14]. The amount of nickel is strongly related with the specific capacity. With the increase of nickel, although the theoretical capacity was improved, the electrochemical stability became poor. With the modest content of nickel, LiNi0.5Co0.2Mn0.3O2 has been studied widely as one of the promising cathode materials. However, the electrochemical stability of the layered LiNi0.5Co0.2Mn0.3O2 material is not good at high cut-off voltage and rate especially when the material is cycled at high temperature because of side reactions between cathode material and liquid electrolyte [15]. Some efforts such as partial substitution and coating of active material have been proved to be effective for improving electrochemical stability [16-18]. Among the above mentioned approaches, surface modification with inorganic materials, such as Al2O3 [19], MgO [20], ZnO2 [21], TiO2 [22], SiO2 [23], AlF3 [24], has been extensively reported to improve the electrochemical performance of cathode materials. These inorganic materials can act as a protective shell to suppress the side reactions from electrolyte. However, they usually have low lithium ion diffusivities and increased electrochemical resistance, resulting in the limited improvement of electrochemical performance. V2O5 can be a good choice, since V2O5 has a relatively higher ionic conductivity than most metal oxides and metal fluorides [25]. Moreover, when NH4VO3 was chosen as a coating precursor, ionized VO3- can react with Li+ ions from LiOH and Li2CO3 on the particle surface of nickel-rich layered oxide [26]. In addition, conductive carbon has commonly been used for surface-modification of cathode materials to 4
improve their cycling stability and rate performance. However, the conductive carbon is hard for LiNi0.5Co0.2Mn0.3O2 since an air atmosphere is need for LiNi0.5Co0.2Mn0.3O2 to avoid too many oxygen vacancies in the crystal, while carbon source need an inert atmosphere to carbonize at high temperature [27]. Graphene oxide or graphene can act as an alternative material to improve the electronic conductivity of LiNi0.5Co0.2Mn0.3O2 cathode material. In light of these previous studies, we are interested in the construction of a double-layer structure consisting of an inner V2O5 layer and an outer graphene oxide layer. The inner V2O5 not only promotes Li+ ion diffusivities but also suppresses the HF attack from the electrolyte. The outer graphene oxide layer contributes to the fast electron transportation on the surface of cathode materials. In this paper, layered LiNi0.5Co0.2Mn0.3O2 cathode material was synthesized by a citric acid assisted sol-gel method. The pristine LiNi0.5Co0.2Mn0.3O2 was double-layer-modified with V2O5 and graphene oxide. Then, we demonstrated that combined use of both graphene oxide and V2O5 coating significantly improved the electrochemical performance of the LiNi 0.5Co0.2Mn0.3O2 cathode material. In addition, the possible reaction mechanism was discussed in this work.
2. Experimental 2.1. Preparation of materials The pristine LiNi0.5Co0.2Mn0.3O2 (NCM) cathode material was prepared by a citric acid assisted
sol-gel
method.
Lithium acetate
(CH3COOLi·2H2O,
99.5%),
nickel
acetate
((CH3COO)2Ni·4H2O, 99.5%), cobalt acetate ((CH3COO)2Co·4H2O, 99.5%) and manganese acetate ((CH3COO)2Mn·4H2O, 99.5%) was chosen as the starting materials. The stoichiometric ratio of Li: Ni: Co: Mn = 1.05: 0.5: 0.2: 0.3 were dissolved in distilled water. The excess 5% Li+ was to supplement the loss of Li+ at high temperature calcine. Citric acid, chelating agent, with a molar ratio of transition metal: citric acid = 1: 1 was added drop by drop into the above solution with constant stirring. The resulting solution was evaporated at 80°C until gel was formed. The obtaining gel was presintered at 450°C for 5 h. The precursor was ground uniformly and then the result sample was annealed at 900°C for 12 h in air. Graphene oxide was prepared from natural graphite powder by a modified hummer’s method [28]. Graphene-oxide-V2O5-coated LiNi0.5Co0.2Mn0.3O2 (GO-VO-NCM) material was prepared by 5
a wet chemical method. Fig. 1 shows the graphical abstract. Firstly, NH4VO3 was dissolved in distilled water with concentration of 1 mg·ml -1. Then the as-prepared LiNi0.5Co0.2Mn0.3O2 powders were mixed with above solution with moderate stirring. The resulting mixture was heated at 80°C to evaporate remaining water to obtain the NH 4VO3-coated LiNi0.5Co0.2Mn0.3O2. The NH4VO3-coated LiNi0.5Co0.2Mn0.3O2 was annealed at 600°C for 3 h to obtain V2O5-coated LiNi0.5Co0.2Mn0.3O2 (VO-NCM). Then, Graphene oxide was dispersed in absolute alcohol solution with concentration of 0.5 mg·ml-1. The solution was under ultrasonic treatment to assure that the graphene oxide was dispersed completely. The obtained VO-NCM was added in above solution with moderate striring. The resulting mixture was heated at 70°C with constant stirring to evaporate remaining alcohol. The obtaining sample was dried at 150°C in a vacuum overnight. 2.2. Characterization X-ray diffraction (XRD) patterns of products were recorded on a Rigaku Ultima IV using the Cu Kα radiation (1.5406 Å). Scanning electron microscopy (SEM, Nova NanoSEM 230) equipped with an energy dispersive spectroscope (EDS, Nova NanoSEM) and transmission electron microscopy (TEM, TECNAI G2F20) were applied for the structural characterization and elements distribution of the samples. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was carried out to investigate the surface chemical compositions of the products. 2.3 Electrochemical measurements The electrochemical performance was tested using 2025-type coin cell with lithium metal as an anode. The cathode slurry was prepared by active material, carbon black and polyvinylidene fluoride (PVDF) at a weight ratio of 8: 1: 1 dispersed in N-methyl-2-pyrrolidone (NMP). The slurry was cast on Al foil and dried at 120°C in a vacuum for 12 h [29]. The electrolyte was 1 mol·L-1 LiPF6 in a 1: 1: 1 (volume ratio) mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylene methyl carbonate (EMC). A Celgard 2400 microporous polypropylene membrane was used as the separator. The cells were assembled in a glove box filled with highly pure argon gas (O2 and H2O level < 1 ppm) [30]. Electrochemical impedance spectroscopy (EIS) was performed on an IM6 electrochemical workstation (CHI660D, China) in the frequency range from 10 mHz to 100 kHz. Galvanostatic charge-discharge tests were performed in the voltage 6
range of 2.5 to 4.5 V (Li+/Li) at different current densities on a Land automatic batteries tester (CT 2001A, China).
3. Results and discussion
The XRD patterns of the pristine NCM, VO-NCM and GO-VO-NCM powders are shown in Fig. 2. All the diffraction patterns were confirmed to be well-defined hexagonal α-NaFeO2 structure with a space group of R-3m [31]. The splitting of (006)/(102) and (108)/(110) peaks in all the powers indicate the formation of a ordered layered structure [32]. No peaks corresponding to other phase were observed in the XRD patterns because the coated amount is too small to be observed in the XRD patterns or the coating materials were coated on NCM particles as amorphous. Because the ionic radius of Ni2+ (0.69 Å) is similar to Li+ (0.76 Å), the occupancy of the sites by the Ni2+ and Li+ are likely to exchange partially. The I003/I104 value is sensitive to the degree of cation mixing of the powers. As widely accepted, when the value of I 003/I104 above 1.2, implying that the material has a good layered structure with small cation mixing [33]. From the XRD patterns of pristine NCM, VO-NCM and GO-VO-NCM powers, the I003/I104 values were calculated to be 1.35, 1.33 and 1.32 respectively, implying all samples have low degree of cation mixing. The lattice constants of those powers were shown in Table 1. The values of the modified materials were close to those of the pristine material, indicating that the host structure was not changed after surface modification. The SEM images of the pristine NCM and GO-VO-NCM samples are shown in Fig. 3. Both samples present homogeneous primary grains in a few hundred nanometers size with a little agglomeration. The difference between the pristine NCM and GO-VO-NCM can hardly be observed for the coating amount is too small. The homogeneity of GO-VO-coating layer is demonstrated by EDS elemental mapping analysis shown in Fig. 4. It can be clearly seen that Ni, Co, Mn, V, C are uniformly distributed in the GO-VO-NCM material. The TEM images of the pristine NCM and GO-VO-NCM samples are shown in Fig. 5. The pristine NCM presents a nice crystallization, with lattice fringe extending to the particle boundary, as shown in Fig. 5(a). Fig. 5(b) shows an amorphous modification layer with thickness of about 10-14 nm on the surface of GO-VO-NCM sample. The inner modification layer, with a thickness 7
of about 5 nm, belongs to V2O5 layer; and the outer modification layer, with a thickness of about 5-9 nm, is the part of graphene oxide layer. The double coating layer not only contributes to the fast Li+ and electron transportation, but also suppresses the side reaction between active material and electrolyte. Since transition metal (M = Ni, Co, Mn) was easily reduced to low oxidation state, leading to low specific capacity and bad electrochemical performance; possible oxidation states of transition metal ions in the GO-VO-NCM material were clarified using X-ray photoelectron spectroscopy (XPS). The XPS spectra of Ni 2p, Co 2p, Mn 2p, V 2p and C 1s for the sample are shown in Fig. 6. The electron binding energies of Co 2p1/2 and Mn 2p3/2 appear at 795.3 and 642.5 eV are in agreement with those reported for Co and Mn in similar transition metal oxide [34], which indicate that Co and Mn are presented as Co3+ and Mn4+, respectively. The peaks with electron binding energies of 855.0 and 861.3 eV are assigned to Ni 2p3/2 and corresponding satellite peak, which suggest that the nickel ion maintain at high oxidation state [35]. The result is also in conjunction with the small cation mixing suggested by the XRD results, since only the Ni 2+ ion can move to the Li+ layer owing to the similar ionic radii of Ni2+ and Li+. The peaks with binding energies of 517.2 and 524.7 eV are assigned to V 2p3/2 and V 2p1/2, which are very close to those reported for V in vanadium oxide [36]. This result indicates that the V element on the material surface is pentavalent state. Typically, the spectrum of graphene oxide C 1s showed four different curves. The peaks with binding energies of 284.6, 285.6, 286.6 and 288.2 eV are assigned to the carbon skeleton (C-C/C=C), hydroxyl group (C-OH), epoxide group (-C-O-C-) and carboxyl group (-O-C=O), respectively [37], as shown in Fig. 6(f). The binding energies of 289.4 eV is assigned to the carbonate (CO2-3 ) [26]. It is well known that the Ni-rich layered transition metal oxide always contain some Li compounds such as Li2CO3 and LiOH. The surface reaction mechanism can be described as follows [38]: O2-active + CO2/H2O → CO2-3 /2OH2Li+ + CO2-3 /2OH- → Li2CO3/LiOH Although we chose NH4VO3 as a coating precursor and ionized VO3- can reactive with Li impurities, there still remain a small number of Li impurities [26]. Fig. 7 shows the initial charge-discharge curves of the Li/pristine NCM, VO-NCM and GO-VO-NCM cells at a constant current of 0.1 C (1 C = 270 mAh·g-1) between 2.5 and 4.5 V 8
versus Li/Li+ at room temperature. All the cells display very smooth and stable curves even up to the cutoff voltage of 4.5 V. The GO-VO-NCM shows a discharge capacity of 182.6 mAh·g-1, which is very close to that of 183.3 mAh·g-1 for the pristine one. And the VO-NCM exhibits a discharge capacity of 179.7 mAh·g-1, since the V2O5 coating layer is electrochemically inactive within the voltage ranges and has a low electronic conductivity. Fig. 8 presents the discharge capacity of the Li/pristine NCM, VO-NCM and GO-VO-NCM cells over 50 cycles at a constant current density of 0.2 C in a range of 2.5-4.5 V at room temperature. Distinctly, the pristine NCM showed gradual capacity decay with a capacity retention of only 71.4% after 50 cycles. Meanwhile, the VO-NCM showed enhanced capacity retention of 75.1% after the same cycling period. However, the discharge capacity of VO-NCM showed lower than that of the pristine sample for the first few cycles. This result may be due to the increase of interfacial resistances because of the moderate conductivity of the V2O5 coating layer. In contrast, the GO-VO-NCM not only exhibits discharge capacities as high as pristine for the first few cycles, but also has the highest capacity retention of 76.5% after the same cycling period. This result suggests that the presence of a V2O5 coating layer along with a graphene oxide coating layer on the surface of NCM was favorable to maintain the specific capacity and improve cycling stability. To further investigate the effect of the coating on the cycling performance at high temperature, the Li/pristine NCM, VO-NCM and GO-VO-NCM cells were cycled at 55°C with a constant current density of 0.2 C. The corresponding results are showed in Fig. 9. The pristine NCM exhibits an obvious capacity fade from 175.6 mAh·g-1 to 119.0 mAh·g-1 with a capacity retention of 67.8%; the GO-VO-NCM, however, shows a high initial discharge capacity of 177.8 mAh·g-1 with 131.9 mAh·g-1 reserved in the 50th cycle, leading to a capacity retention of 74.2%. In contrast, the VO-NCM presents a moderately enhanced capacity retention of 72.4% but has a slightly low initial capacity of 175.1 mAh·g-1. Compared to cycle at room temperature, the improvement of the cycle performance at high temperature is more significant. It has been known that HF was produced by thermal decomposition and hydrolysis of LiPF 6 at high temperature [39]. Obviously, the double-layer-modified is more effective to avoid the transition metals from the active material on account of HF attack. Fig. 10 shows the rate capacity of the Li/pristine NCM, VO-NCM and GO-VO-NCM cells with the current density increasing from 0.1 to 1 C between 2.5 and 4.5 V every five cycles. It was 9
clearly observed that the discharge capacity reduced with the increase of current density due to the polarization. As shown in Fig. 10, the GO-VO-NCM shows enhanced rate performance compared to the pristine NCM at high rates. Although the discharge capacity of GO-VO-NCM is similar to that of the pristine NCM at low current density, the former still releases 116.6 mAh·g-1 under a high current rate of 1 C, while the pristine material only remains at 105.7 mAh·g-1. This result can be ascribed to the double coating layer can promotes Li+ conductivity and electron transport. In contrast, surface modification with V2O5 sample slightly decreased the discharge capacities under high current rates, indicating that the single V2O5 coating layer is difficult to enhance the rate performance. In order to investigate the evolution of the voltage platform during cycling, differential capacity (dQ/dV) profiles of the pristine NCM and GO-VO-NCM samples are performed for the first and 50th cycles at 0.2 C. As shown in Fig. 11, the anodic/cathodic peaks of the pristine NCM for the first cycle are centered at about 3.752/3.728 V, corresponding to the oxidation/reduction of Ni2+/Ni4+ couple. For the pristine NCM, the cathodic peak shifts from about 3.752 to 3.780 V and the anodic peak shifts from about 3.728 to 3.660 V after 50 cycles. For the GO-VO-NCM, however, the cathodic peak shifts from 3.747 to 3.768 V and the anodic peak shifts from 3.723 to 3.680 V after the same cycling period. It is widely accepted that potential interval between the cathodic peak and anodic peak in connection with electrode polarization [40]. The potential interval of GO-VO-NCM increases from 0.024 to 0.088 V after 50 cycles, which is smaller than that of the pristine NCM (from 0.024 to 0.120 V). This result indicates that the double coating layer effectively suppresses the increase of the electrode polarization during cycling. In order to get insight into the reason for the improved electrochemical properties of the GO-VO-NCM sample, electrochemical impedance spectroscopy (EIS) was performed. Fig. 12 shows the Nyquist plots of the pristine NCM and GO-VO-NCM samples before cycling and after 50 cycles at 0.2 C between 2.5 and 4.5 V. There consist of a semicircle in the high to medium frequency region and a slope straight line in the low frequency region. Generally, the semicircle in the high to medium frequency region is assigned to the interfacial charge transfer resistance (Rct) [41], and the slope line related to Warburg impedance (W) which is associated with the impedance of lithium ion diffusion [42]. The corresponding equivalent circuit is shown in Fig. 12(c); Rs stands for the solution resistance, CPE (constant phase element) refers to the electric double-layer 10
capacitance, W1 indicates the Warburg impedance. The values of Rs and Rct were obtained by fitting ZSimpWin software. The corresponding results are showed in Table 2. The Rs values for both samples have no obvious change during cycling, indicating that slight decomposition of electrolyte is incapable to influence the value of solution resistance. In addition, the membrane separator maintain unimpeded during cycling. The increase of the Rct value is ascribed to the formation of the solid electrolyte interphase (SEI) film on the electrode surface. The Rct value of the pristine NCM electrode increases from 67.4 to 138.7 Ω after 50 cycles; the GO-VO-NCM electrode, however, has a slightly high initial Rct value of 73.7 Ω and only reaches up to 105.1 Ω after the same cycling period. Obviously, the GO-VO-NCM electrode presents a smaller resistance increase than that of the pristine electrode. It can be predicted that the GO-VO-NCM coating layer suppressed the side reaction of the active material with electrolyte at high voltage so as to avoid the over-rapid thickening of SEI film which contains some ionic and electronic insulators. In addition, the double coating layer plays an important role to promote Li+ and electronic conductivity so as to restrain the increase of resistance.
4. Conclusions In conclusion, LiNi0.5Co0.2Mn0.3O2 cathode material was synthesized by a citric acid assisted sol-gel method and the pristine NCM material was surface-modified with double-layer which consists of an inner V2O5 layer and an outer conductive graphene oxide layer. The double-layer-coated NCM cathode material delivered better cycling stability and rate performance than the pristine NCM cathode material or the single-layer-coated NCM cathode material. Presence of the double coating layer on the surface of the LiNi0.5Co0.2Mn0.3O2 not only suppresses the side reaction of the active material with electrolyte, but also promotes the Li + and electronic conductivity. So the double-layer coating is an effective strategy to overcome the drawbacks of the LiNi0.5Co0.2Mn0.3O2 material.
Acknowledgments The authors thank the National Natural Science Foundation of China (No. 51304052), the Doctoral Program of Higher Education of China (No. 20133514120002), the Natural Science 11
Foundation of Fujian Province (No. 2014J05059) and Fuzhou University Test Fund of Precious Apparatus (2016T038) for funding this work.
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dopamine,
Biomacromolecules 13 (2012) 4236-4246. [38] H. Liu, Z. Zhang, Z. Gong, Y. Yang, Origin of deterioration for LiNiO2 cathode material during storage in air, Electrochem. Solid-State Lett. 7 (2004) A190-A193. [39] J. Gnanaraj, E. Zinigrad, L. Asraf, H. Gottlieb, M. Sprecher, M. Schmidt, W. Geissler, D. Aurbach, A detailed investigation of the thermal reactions of LiPF6 solution in organic carbonates using ARC and DSC, J. Electrochem. Soc. 150 (2003) A1533-A1537. [40] D. Aurbach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, U. Heider, On the use of vinylene carbonate (VC) as an additive to electrolyte solutions for Li-ion batteries, Electrochim. Acta 47 (2002) 1423-1439. [41] G. Appetecchi, F. Croce, L. Persi, F. Ronci, B. Scrosati, Transport and interfacial properties of composite polymer electrolytes, Electrochim. Acta 45 (2000) 1481-1490. [42] J. Bisquert, Theory of the impedance of electron diffusion and recombination in a thin layer, J. Phys. Chem. B 106 (2002) 325-333.
16
Figure and table captions Fig. 1. Coating process of LiNi0.5Co0.2Mn0.3O2 with V2O5 and graphene oxide. Fig. 2. XRD patterns of the pristine NCM, VO-NCM and GO-VO-NCM powers. Fig. 3. SEM images of (a) the pristine NCM and (b) GO-VO-NCM powers. Fig. 4. (a) SEM image of GO-VO-NCM and (b-f) EDS elemental mappings of GO-VO-NCM corresponding to (a). Fig. 5. TEM images of (a) the pristine NCM and (b) GO-VO-NCM samples. Fig. 6. X-ray photoelectron spectroscopy (XPS) spectra of: (a) all elements (b) Ni, (c) Co, (d) Mn, (e) V, (f) fitting result of C for GO-VO-NCM sample. Fig. 7. The initial charge-discharge curves of the pristine NCM, VO-NCM and GO-VO-NCM samples. Fig. 8. Cycle performance of the pristine NCM, VO-NCM and GO-VO-NCM samples at room temperature under 0.2 C. Fig. 9. Cycle performance of the pristine NCM, VO-NCM and GO-VO-NCM samples at 55°C under 0.2 C. Fig. 10. Comparison of rate performance for the pristine NCM, VO-NCM and GO-VO-NCM samples. Fig. 11. Differential capacity (dQ/dV) vs. voltage profiles of the pristine NCM and GO-VO-NCM for the first and 50th cycles at 0.2 C. Fig. 12. Impedance spectra of (a) the pristine NCM and (b) GO-VO-NCM samples; (c) the used equivalent circuit.
17
Fig. 1.
18
Fig. 2.
19
Fig. 3.
20
Fig. 4.
21
Fig. 5.
22
Fig. 6.
23
Fig. 7.
24
Fig. 8.
25
Fig. 9.
26
Fig. 10.
27
Fig. 11.
28
Fig. 12.
29
Table 1. Structural parameters of the pristine NCM, VO-NCM and GO-VO-NCM powers Table 1 Sample
a (Å)
NCM VO-NCM GO-VO-NCM
2.8727 2.8754 2.8765
c (Å) 14.2448 14.2510 14.2629
30
Table 2. Rs and Rct date of the pristine NCM and GO-VO-NCM samples. Table 2 Sample 1st 50th
NCM
GO-VO-NCM
Rs (Ω)
Rct (Ω)
Rs (Ω)
Rct (Ω)
1.7 2.4
67.4 138.7
3.4 5.2
73.7 105.1
31
S0169-4332(17)30223-4 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.200 APSUSC 34995
To appear in:
APSUSC
Received date: Revised date: Accepted date:
22-12-2016 18-1-2017 19-1-2017
Please cite this article as: Wenbin Luo, Baolin Zheng, Improved electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode material by double-layer coating with graphene oxide and V2O5 for lithium-ion batteries, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.200 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 proof before it is published in its final 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.
Improved
electrochemical
performance
of
LiNi0.5Co0.2Mn0.3O2
cathode material by double-layer coating with graphene oxide and V2O5 for lithium-ion batteries Wenbin Luo*, Baolin Zheng School of Chemical Engineering, Fuzhou University, Fuzhou 350116, People’s Republic of China *Corresponding author. E-mail address: [email protected]
1
GRAPHICAL ABSTRACT
Highlights Citric acid assisted sol-gel method was used for synthesizing LiNi0.5Co0.2Mn0.3O2. The pristine LiNi0.5Co0.2Mn0.3O2 was surface-modified by double-layer coating. The double coating layer consists of graphene oxide and V2O5. Electrochemical performance was improved by double-layer coating.
2
Abstract LiNi0.5Co0.2Mn0.3O2 cathode material synthesized by a sol-gel method was surface-modified by double-layer coating. The results of X-ray diffraction (XRD) confirm that the intrinsic structure was no change after surface modification. A double-layer structure consisting of an inner V2O5 (VO) layer and an outer conductive graphene oxide (GO) layer was coated on the surface of active material, as confirmed by transmission electron microscopy (TEM). The results of field emission scanning electron microscope (FE-SEM) equipped with an energy dispersive spectroscope (EDS) show that both graphene oxide and V2O5 uniformly covered LiNi0.5Co0.2Mn0.3O2 cathode material. The double-layer-coated LiNi0.5Co0.2Mn0.3O2 cathode material shows improved electrochemical performance with a capacity retention of 74.2% after 50 cycles in a range of 2.5-4.5 V at 55°C, compared with only 67.8% capacity retention for the pristine material. In addition, the double-layer-coated LiNi0.5Co0.2Mn0.3O2 releases 116.6 mAh·g-1 under a high current rate, while the pristine material only remains at 105.7 mAh·g-1. The results can be ascribed to the double coating layer not only avoids the side reaction between electrolyte and active material but also promotes Li+ and electronic conductivity. Differential capacity (dQ/dV) and electrochemical impedance spectroscopy (EIS) measurements reveal that the double coating layer effectively suppresses the increase of the electrode polarization during cycling.
Keywords: Lithium-ion batteries; LiNi0.5Co0.2Mn0.3O2; Graphene oxide; V2O5; Double-layer coating; Electrochemical performance
3
1. Introduction Recently, energy crisis have motivated the development of energy storage equipments, such as, lithium-ion batteries [1-3], sodium-ion batteries [4], supercapacitors [5, 6], et cetera. Among these energy storage equipments, lithium-ion batteries have been widely used as a new power sources for hybrid electric vehicles (HEVs), electrical vehicles (EVs) and portable electronic devices [7-11]. Layered LiNixCoyMn1-x-yO2 materials with high specific capacity, high working voltage and environmental benignancy have attracted many researchers’ attention [12]. In these materials, the oxidation state of manganese keep in the tetravalent form during electrochemical cycling and the electrochemically inactive manganese play a stabilizing role to prevent a structural collapse during cycling [13]. The cobalt will work as a buffer against the local distortion and decrease the change in the structure of the cathode during cycling [14]. The amount of nickel is strongly related with the specific capacity. With the increase of nickel, although the theoretical capacity was improved, the electrochemical stability became poor. With the modest content of nickel, LiNi0.5Co0.2Mn0.3O2 has been studied widely as one of the promising cathode materials. However, the electrochemical stability of the layered LiNi0.5Co0.2Mn0.3O2 material is not good at high cut-off voltage and rate especially when the material is cycled at high temperature because of side reactions between cathode material and liquid electrolyte [15]. Some efforts such as partial substitution and coating of active material have been proved to be effective for improving electrochemical stability [16-18]. Among the above mentioned approaches, surface modification with inorganic materials, such as Al2O3 [19], MgO [20], ZnO2 [21], TiO2 [22], SiO2 [23], AlF3 [24], has been extensively reported to improve the electrochemical performance of cathode materials. These inorganic materials can act as a protective shell to suppress the side reactions from electrolyte. However, they usually have low lithium ion diffusivities and increased electrochemical resistance, resulting in the limited improvement of electrochemical performance. V2O5 can be a good choice, since V2O5 has a relatively higher ionic conductivity than most metal oxides and metal fluorides [25]. Moreover, when NH4VO3 was chosen as a coating precursor, ionized VO3- can react with Li+ ions from LiOH and Li2CO3 on the particle surface of nickel-rich layered oxide [26]. In addition, conductive carbon has commonly been used for surface-modification of cathode materials to 4
improve their cycling stability and rate performance. However, the conductive carbon is hard for LiNi0.5Co0.2Mn0.3O2 since an air atmosphere is need for LiNi0.5Co0.2Mn0.3O2 to avoid too many oxygen vacancies in the crystal, while carbon source need an inert atmosphere to carbonize at high temperature [27]. Graphene oxide or graphene can act as an alternative material to improve the electronic conductivity of LiNi0.5Co0.2Mn0.3O2 cathode material. In light of these previous studies, we are interested in the construction of a double-layer structure consisting of an inner V2O5 layer and an outer graphene oxide layer. The inner V2O5 not only promotes Li+ ion diffusivities but also suppresses the HF attack from the electrolyte. The outer graphene oxide layer contributes to the fast electron transportation on the surface of cathode materials. In this paper, layered LiNi0.5Co0.2Mn0.3O2 cathode material was synthesized by a citric acid assisted sol-gel method. The pristine LiNi0.5Co0.2Mn0.3O2 was double-layer-modified with V2O5 and graphene oxide. Then, we demonstrated that combined use of both graphene oxide and V2O5 coating significantly improved the electrochemical performance of the LiNi 0.5Co0.2Mn0.3O2 cathode material. In addition, the possible reaction mechanism was discussed in this work.
2. Experimental 2.1. Preparation of materials The pristine LiNi0.5Co0.2Mn0.3O2 (NCM) cathode material was prepared by a citric acid assisted
sol-gel
method.
Lithium acetate
(CH3COOLi·2H2O,
99.5%),
nickel
acetate
((CH3COO)2Ni·4H2O, 99.5%), cobalt acetate ((CH3COO)2Co·4H2O, 99.5%) and manganese acetate ((CH3COO)2Mn·4H2O, 99.5%) was chosen as the starting materials. The stoichiometric ratio of Li: Ni: Co: Mn = 1.05: 0.5: 0.2: 0.3 were dissolved in distilled water. The excess 5% Li+ was to supplement the loss of Li+ at high temperature calcine. Citric acid, chelating agent, with a molar ratio of transition metal: citric acid = 1: 1 was added drop by drop into the above solution with constant stirring. The resulting solution was evaporated at 80°C until gel was formed. The obtaining gel was presintered at 450°C for 5 h. The precursor was ground uniformly and then the result sample was annealed at 900°C for 12 h in air. Graphene oxide was prepared from natural graphite powder by a modified hummer’s method [28]. Graphene-oxide-V2O5-coated LiNi0.5Co0.2Mn0.3O2 (GO-VO-NCM) material was prepared by 5
a wet chemical method. Fig. 1 shows the graphical abstract. Firstly, NH4VO3 was dissolved in distilled water with concentration of 1 mg·ml -1. Then the as-prepared LiNi0.5Co0.2Mn0.3O2 powders were mixed with above solution with moderate stirring. The resulting mixture was heated at 80°C to evaporate remaining water to obtain the NH 4VO3-coated LiNi0.5Co0.2Mn0.3O2. The NH4VO3-coated LiNi0.5Co0.2Mn0.3O2 was annealed at 600°C for 3 h to obtain V2O5-coated LiNi0.5Co0.2Mn0.3O2 (VO-NCM). Then, Graphene oxide was dispersed in absolute alcohol solution with concentration of 0.5 mg·ml-1. The solution was under ultrasonic treatment to assure that the graphene oxide was dispersed completely. The obtained VO-NCM was added in above solution with moderate striring. The resulting mixture was heated at 70°C with constant stirring to evaporate remaining alcohol. The obtaining sample was dried at 150°C in a vacuum overnight. 2.2. Characterization X-ray diffraction (XRD) patterns of products were recorded on a Rigaku Ultima IV using the Cu Kα radiation (1.5406 Å). Scanning electron microscopy (SEM, Nova NanoSEM 230) equipped with an energy dispersive spectroscope (EDS, Nova NanoSEM) and transmission electron microscopy (TEM, TECNAI G2F20) were applied for the structural characterization and elements distribution of the samples. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was carried out to investigate the surface chemical compositions of the products. 2.3 Electrochemical measurements The electrochemical performance was tested using 2025-type coin cell with lithium metal as an anode. The cathode slurry was prepared by active material, carbon black and polyvinylidene fluoride (PVDF) at a weight ratio of 8: 1: 1 dispersed in N-methyl-2-pyrrolidone (NMP). The slurry was cast on Al foil and dried at 120°C in a vacuum for 12 h [29]. The electrolyte was 1 mol·L-1 LiPF6 in a 1: 1: 1 (volume ratio) mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylene methyl carbonate (EMC). A Celgard 2400 microporous polypropylene membrane was used as the separator. The cells were assembled in a glove box filled with highly pure argon gas (O2 and H2O level < 1 ppm) [30]. Electrochemical impedance spectroscopy (EIS) was performed on an IM6 electrochemical workstation (CHI660D, China) in the frequency range from 10 mHz to 100 kHz. Galvanostatic charge-discharge tests were performed in the voltage 6
range of 2.5 to 4.5 V (Li+/Li) at different current densities on a Land automatic batteries tester (CT 2001A, China).
3. Results and discussion
The XRD patterns of the pristine NCM, VO-NCM and GO-VO-NCM powders are shown in Fig. 2. All the diffraction patterns were confirmed to be well-defined hexagonal α-NaFeO2 structure with a space group of R-3m [31]. The splitting of (006)/(102) and (108)/(110) peaks in all the powers indicate the formation of a ordered layered structure [32]. No peaks corresponding to other phase were observed in the XRD patterns because the coated amount is too small to be observed in the XRD patterns or the coating materials were coated on NCM particles as amorphous. Because the ionic radius of Ni2+ (0.69 Å) is similar to Li+ (0.76 Å), the occupancy of the sites by the Ni2+ and Li+ are likely to exchange partially. The I003/I104 value is sensitive to the degree of cation mixing of the powers. As widely accepted, when the value of I 003/I104 above 1.2, implying that the material has a good layered structure with small cation mixing [33]. From the XRD patterns of pristine NCM, VO-NCM and GO-VO-NCM powers, the I003/I104 values were calculated to be 1.35, 1.33 and 1.32 respectively, implying all samples have low degree of cation mixing. The lattice constants of those powers were shown in Table 1. The values of the modified materials were close to those of the pristine material, indicating that the host structure was not changed after surface modification. The SEM images of the pristine NCM and GO-VO-NCM samples are shown in Fig. 3. Both samples present homogeneous primary grains in a few hundred nanometers size with a little agglomeration. The difference between the pristine NCM and GO-VO-NCM can hardly be observed for the coating amount is too small. The homogeneity of GO-VO-coating layer is demonstrated by EDS elemental mapping analysis shown in Fig. 4. It can be clearly seen that Ni, Co, Mn, V, C are uniformly distributed in the GO-VO-NCM material. The TEM images of the pristine NCM and GO-VO-NCM samples are shown in Fig. 5. The pristine NCM presents a nice crystallization, with lattice fringe extending to the particle boundary, as shown in Fig. 5(a). Fig. 5(b) shows an amorphous modification layer with thickness of about 10-14 nm on the surface of GO-VO-NCM sample. The inner modification layer, with a thickness 7
of about 5 nm, belongs to V2O5 layer; and the outer modification layer, with a thickness of about 5-9 nm, is the part of graphene oxide layer. The double coating layer not only contributes to the fast Li+ and electron transportation, but also suppresses the side reaction between active material and electrolyte. Since transition metal (M = Ni, Co, Mn) was easily reduced to low oxidation state, leading to low specific capacity and bad electrochemical performance; possible oxidation states of transition metal ions in the GO-VO-NCM material were clarified using X-ray photoelectron spectroscopy (XPS). The XPS spectra of Ni 2p, Co 2p, Mn 2p, V 2p and C 1s for the sample are shown in Fig. 6. The electron binding energies of Co 2p1/2 and Mn 2p3/2 appear at 795.3 and 642.5 eV are in agreement with those reported for Co and Mn in similar transition metal oxide [34], which indicate that Co and Mn are presented as Co3+ and Mn4+, respectively. The peaks with electron binding energies of 855.0 and 861.3 eV are assigned to Ni 2p3/2 and corresponding satellite peak, which suggest that the nickel ion maintain at high oxidation state [35]. The result is also in conjunction with the small cation mixing suggested by the XRD results, since only the Ni 2+ ion can move to the Li+ layer owing to the similar ionic radii of Ni2+ and Li+. The peaks with binding energies of 517.2 and 524.7 eV are assigned to V 2p3/2 and V 2p1/2, which are very close to those reported for V in vanadium oxide [36]. This result indicates that the V element on the material surface is pentavalent state. Typically, the spectrum of graphene oxide C 1s showed four different curves. The peaks with binding energies of 284.6, 285.6, 286.6 and 288.2 eV are assigned to the carbon skeleton (C-C/C=C), hydroxyl group (C-OH), epoxide group (-C-O-C-) and carboxyl group (-O-C=O), respectively [37], as shown in Fig. 6(f). The binding energies of 289.4 eV is assigned to the carbonate (CO2-3 ) [26]. It is well known that the Ni-rich layered transition metal oxide always contain some Li compounds such as Li2CO3 and LiOH. The surface reaction mechanism can be described as follows [38]: O2-active + CO2/H2O → CO2-3 /2OH2Li+ + CO2-3 /2OH- → Li2CO3/LiOH Although we chose NH4VO3 as a coating precursor and ionized VO3- can reactive with Li impurities, there still remain a small number of Li impurities [26]. Fig. 7 shows the initial charge-discharge curves of the Li/pristine NCM, VO-NCM and GO-VO-NCM cells at a constant current of 0.1 C (1 C = 270 mAh·g-1) between 2.5 and 4.5 V 8
versus Li/Li+ at room temperature. All the cells display very smooth and stable curves even up to the cutoff voltage of 4.5 V. The GO-VO-NCM shows a discharge capacity of 182.6 mAh·g-1, which is very close to that of 183.3 mAh·g-1 for the pristine one. And the VO-NCM exhibits a discharge capacity of 179.7 mAh·g-1, since the V2O5 coating layer is electrochemically inactive within the voltage ranges and has a low electronic conductivity. Fig. 8 presents the discharge capacity of the Li/pristine NCM, VO-NCM and GO-VO-NCM cells over 50 cycles at a constant current density of 0.2 C in a range of 2.5-4.5 V at room temperature. Distinctly, the pristine NCM showed gradual capacity decay with a capacity retention of only 71.4% after 50 cycles. Meanwhile, the VO-NCM showed enhanced capacity retention of 75.1% after the same cycling period. However, the discharge capacity of VO-NCM showed lower than that of the pristine sample for the first few cycles. This result may be due to the increase of interfacial resistances because of the moderate conductivity of the V2O5 coating layer. In contrast, the GO-VO-NCM not only exhibits discharge capacities as high as pristine for the first few cycles, but also has the highest capacity retention of 76.5% after the same cycling period. This result suggests that the presence of a V2O5 coating layer along with a graphene oxide coating layer on the surface of NCM was favorable to maintain the specific capacity and improve cycling stability. To further investigate the effect of the coating on the cycling performance at high temperature, the Li/pristine NCM, VO-NCM and GO-VO-NCM cells were cycled at 55°C with a constant current density of 0.2 C. The corresponding results are showed in Fig. 9. The pristine NCM exhibits an obvious capacity fade from 175.6 mAh·g-1 to 119.0 mAh·g-1 with a capacity retention of 67.8%; the GO-VO-NCM, however, shows a high initial discharge capacity of 177.8 mAh·g-1 with 131.9 mAh·g-1 reserved in the 50th cycle, leading to a capacity retention of 74.2%. In contrast, the VO-NCM presents a moderately enhanced capacity retention of 72.4% but has a slightly low initial capacity of 175.1 mAh·g-1. Compared to cycle at room temperature, the improvement of the cycle performance at high temperature is more significant. It has been known that HF was produced by thermal decomposition and hydrolysis of LiPF 6 at high temperature [39]. Obviously, the double-layer-modified is more effective to avoid the transition metals from the active material on account of HF attack. Fig. 10 shows the rate capacity of the Li/pristine NCM, VO-NCM and GO-VO-NCM cells with the current density increasing from 0.1 to 1 C between 2.5 and 4.5 V every five cycles. It was 9
clearly observed that the discharge capacity reduced with the increase of current density due to the polarization. As shown in Fig. 10, the GO-VO-NCM shows enhanced rate performance compared to the pristine NCM at high rates. Although the discharge capacity of GO-VO-NCM is similar to that of the pristine NCM at low current density, the former still releases 116.6 mAh·g-1 under a high current rate of 1 C, while the pristine material only remains at 105.7 mAh·g-1. This result can be ascribed to the double coating layer can promotes Li+ conductivity and electron transport. In contrast, surface modification with V2O5 sample slightly decreased the discharge capacities under high current rates, indicating that the single V2O5 coating layer is difficult to enhance the rate performance. In order to investigate the evolution of the voltage platform during cycling, differential capacity (dQ/dV) profiles of the pristine NCM and GO-VO-NCM samples are performed for the first and 50th cycles at 0.2 C. As shown in Fig. 11, the anodic/cathodic peaks of the pristine NCM for the first cycle are centered at about 3.752/3.728 V, corresponding to the oxidation/reduction of Ni2+/Ni4+ couple. For the pristine NCM, the cathodic peak shifts from about 3.752 to 3.780 V and the anodic peak shifts from about 3.728 to 3.660 V after 50 cycles. For the GO-VO-NCM, however, the cathodic peak shifts from 3.747 to 3.768 V and the anodic peak shifts from 3.723 to 3.680 V after the same cycling period. It is widely accepted that potential interval between the cathodic peak and anodic peak in connection with electrode polarization [40]. The potential interval of GO-VO-NCM increases from 0.024 to 0.088 V after 50 cycles, which is smaller than that of the pristine NCM (from 0.024 to 0.120 V). This result indicates that the double coating layer effectively suppresses the increase of the electrode polarization during cycling. In order to get insight into the reason for the improved electrochemical properties of the GO-VO-NCM sample, electrochemical impedance spectroscopy (EIS) was performed. Fig. 12 shows the Nyquist plots of the pristine NCM and GO-VO-NCM samples before cycling and after 50 cycles at 0.2 C between 2.5 and 4.5 V. There consist of a semicircle in the high to medium frequency region and a slope straight line in the low frequency region. Generally, the semicircle in the high to medium frequency region is assigned to the interfacial charge transfer resistance (Rct) [41], and the slope line related to Warburg impedance (W) which is associated with the impedance of lithium ion diffusion [42]. The corresponding equivalent circuit is shown in Fig. 12(c); Rs stands for the solution resistance, CPE (constant phase element) refers to the electric double-layer 10
capacitance, W1 indicates the Warburg impedance. The values of Rs and Rct were obtained by fitting ZSimpWin software. The corresponding results are showed in Table 2. The Rs values for both samples have no obvious change during cycling, indicating that slight decomposition of electrolyte is incapable to influence the value of solution resistance. In addition, the membrane separator maintain unimpeded during cycling. The increase of the Rct value is ascribed to the formation of the solid electrolyte interphase (SEI) film on the electrode surface. The Rct value of the pristine NCM electrode increases from 67.4 to 138.7 Ω after 50 cycles; the GO-VO-NCM electrode, however, has a slightly high initial Rct value of 73.7 Ω and only reaches up to 105.1 Ω after the same cycling period. Obviously, the GO-VO-NCM electrode presents a smaller resistance increase than that of the pristine electrode. It can be predicted that the GO-VO-NCM coating layer suppressed the side reaction of the active material with electrolyte at high voltage so as to avoid the over-rapid thickening of SEI film which contains some ionic and electronic insulators. In addition, the double coating layer plays an important role to promote Li+ and electronic conductivity so as to restrain the increase of resistance.
4. Conclusions In conclusion, LiNi0.5Co0.2Mn0.3O2 cathode material was synthesized by a citric acid assisted sol-gel method and the pristine NCM material was surface-modified with double-layer which consists of an inner V2O5 layer and an outer conductive graphene oxide layer. The double-layer-coated NCM cathode material delivered better cycling stability and rate performance than the pristine NCM cathode material or the single-layer-coated NCM cathode material. Presence of the double coating layer on the surface of the LiNi0.5Co0.2Mn0.3O2 not only suppresses the side reaction of the active material with electrolyte, but also promotes the Li + and electronic conductivity. So the double-layer coating is an effective strategy to overcome the drawbacks of the LiNi0.5Co0.2Mn0.3O2 material.
Acknowledgments The authors thank the National Natural Science Foundation of China (No. 51304052), the Doctoral Program of Higher Education of China (No. 20133514120002), the Natural Science 11
Foundation of Fujian Province (No. 2014J05059) and Fuzhou University Test Fund of Precious Apparatus (2016T038) for funding this work.
12
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Figure and table captions Fig. 1. Coating process of LiNi0.5Co0.2Mn0.3O2 with V2O5 and graphene oxide. Fig. 2. XRD patterns of the pristine NCM, VO-NCM and GO-VO-NCM powers. Fig. 3. SEM images of (a) the pristine NCM and (b) GO-VO-NCM powers. Fig. 4. (a) SEM image of GO-VO-NCM and (b-f) EDS elemental mappings of GO-VO-NCM corresponding to (a). Fig. 5. TEM images of (a) the pristine NCM and (b) GO-VO-NCM samples. Fig. 6. X-ray photoelectron spectroscopy (XPS) spectra of: (a) all elements (b) Ni, (c) Co, (d) Mn, (e) V, (f) fitting result of C for GO-VO-NCM sample. Fig. 7. The initial charge-discharge curves of the pristine NCM, VO-NCM and GO-VO-NCM samples. Fig. 8. Cycle performance of the pristine NCM, VO-NCM and GO-VO-NCM samples at room temperature under 0.2 C. Fig. 9. Cycle performance of the pristine NCM, VO-NCM and GO-VO-NCM samples at 55°C under 0.2 C. Fig. 10. Comparison of rate performance for the pristine NCM, VO-NCM and GO-VO-NCM samples. Fig. 11. Differential capacity (dQ/dV) vs. voltage profiles of the pristine NCM and GO-VO-NCM for the first and 50th cycles at 0.2 C. Fig. 12. Impedance spectra of (a) the pristine NCM and (b) GO-VO-NCM samples; (c) the used equivalent circuit.
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Fig. 1.
18
Fig. 2.
19
Fig. 3.
20
Fig. 4.
21
Fig. 5.
22
Fig. 6.
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Fig. 7.
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Fig. 8.
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Fig. 9.
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Fig. 10.
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Fig. 11.
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Fig. 12.
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Table 1. Structural parameters of the pristine NCM, VO-NCM and GO-VO-NCM powers Table 1 Sample
a (Å)
NCM VO-NCM GO-VO-NCM
2.8727 2.8754 2.8765
c (Å) 14.2448 14.2510 14.2629
30
Table 2. Rs and Rct date of the pristine NCM and GO-VO-NCM samples. Table 2 Sample 1st 50th
NCM
GO-VO-NCM
Rs (Ω)
Rct (Ω)
Rs (Ω)
Rct (Ω)
1.7 2.4
67.4 138.7
3.4 5.2
73.7 105.1
31