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A versatile nitrogen-doped carbon coating strategy to improve the electrochemical performance of LiFePO4 cathodes for lithium-ion batteries
Journal of Alloys and Compounds 810 (2019) 151889
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A versatile nitrogen-doped carbon coating strategy to improve the electrochemical performance of LiFePO4 cathodes for lithium-ion batteries Yingying Wang, Xinlu Wang, Ao Jiang, Guixia Liu, Wensheng Yu, Xiangting Dong, Jinxian Wang* School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, 130022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2018 Received in revised form 10 May 2019 Accepted 14 August 2019 Available online 15 August 2019
To improve the structural stability and electrochemical performance of a LiFePO4 cathode, a nitrogendoped carbon (N-C) coating strategy is exploited using a simple spray-drying method with dopamine as an N-C coating source. Transmission electron microscopy shows a uniform N-C coating layer on the surface of the LiFePO4 spheres. The introduction of the N-C layer can improve the electrical conductivity and structural stability of the electrode, which results in N-C@LiFePO4 spherical cathode materials with excellent electrochemical properties. The 5% N-C@LiFePO4 spherical composite delivers a specific discharge capacity of 156 mAh g1, with a capacity retention rate of 86% after 150 cycles at 0.1C. More importantly, because the nitrogen-doped carbon coating can significantly improve the electrochemical performance of the electrode material through excellent electron conductivity, a remarkable rate performance can be achieved by virtue of the thin N-C coated layer, which is superior to that of uncoated LiFePO4. The results from electrochemical impedance spectroscopy further verify that 5% N-C@LiFePO4 has an excellent Liþ conductivity and low charge transfer resistance. This efficient and scalable strategy offers a facile approach to develop high performance LiFePO4-based cathode materials for large-scale lithium-ion storage systems. © 2019 Elsevier B.V. All rights reserved.
Keywords: LiFePO4 cathode Nitrogen-doped carbon coating Li-ion battery Spray-drying method
1. Introduction Lithium-ion batteries have become one of the most attractive energy storage systems due to a long cycle life, and are used in smart grids, electric vehicles, and renewable power stations [1e3] etc. The most important requirements for lithium-ion batteries are good safety, high energy density and low cost [4e7]. Commercialized cathode materials for lithium-ion batteries, such as LiFePO4 [8], LiMn2O4 [9], LiCoO2 [10], and ternary [11] cathode materials, have been generally equipped in plug-in hybrid electric vehicles (PHEV), electric vehicles (EVs). Among lithium-ion battery cathode materials, olivine-structured LiFePO4 has attracted widespread attention as a cathode material due to its potential application value. However, inherent limitations of LiFePO4, including low electronic conductivity (~1010 S cm1) and slow diffusion of lithium ions (~1014 cm2 s1), require further commitment for improvements
* Corresponding author. E-mail addresses: [email protected], [email protected] (J. Wang). https://doi.org/10.1016/j.jallcom.2019.151889 0925-8388/© 2019 Elsevier B.V. All rights reserved.
[12e16]. Researchers have recently tried to solve these problems by employing a variety of strategies, for instance surface coating, elemental doping [17e24], controlling particle size and morphology [25], etc. Doping of a heteroatom can influence the crystal structure and create Li-ion vacancies, leading to an improvement in the ionic and electronic conductivity. Controlling the particle size can increase its specific surface area and reduce the diffusion path of lithium ions, hence improving the electrochemical performance of LiFePO4. In particular, conductive carbon coatings [26] (organic carbon sources: dopamine, sucrose, etc.) have been developed as a most successful strategy for enhancing the electrochemical performance of LiFePO4 [20]. The carbon coating not only improves the electrical conductivity but also retains the active material mass load. In addition, the carbon coating may act as a barrier between the active material and the electrolyte, thus preventing unwanted side reactions during the charging/discharging process. Moreover, carbon layers effectively limit volume expansion, thereby increasing structural stability, resulting in enhanced
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circulation. However, the traditional carbon coating method is complicated, and the thickness of the carbon layer is difficult to control: when the carbon coating is too thick, Li ions are blocked, and the performance of the LiFePO4 cathode is impaired. In addition, high carbon content reduces energy density and power density [27e29]. Compared with traditional LiFePO4-coated carbon, the spray drying method has the following advantages:
Precursor powders were calcined at 700 C for 5 h under argon atmosphere to form 5%N-C@LiFePO4. In contrast, the LiFePO4, 2%NC@LiFePO4, and 10%N-C@LiFePO4 were synthesized under the same conditions with dopamine mass fractions of 0%, 2%, and 10%, respectively.
(1) Spray drying allows granulation and drying to be completed in one step. (2) When spray drying, the material liquid is sprayed into an atomized dispersion under constant stirring, and drying is completed instantaneously. (3) In the early stages of the spray drying process, the vaporization of the solvent is rapid, which results in good dispersion and solubility that can be obtained without further treatment. (4) Since spray drying is a continuous and closed production process, the purity of the product is high, and the opportunity for exposure in the production environment and contact with the operator is eliminated. Environmental pollution is also reduced. (5) Spray drying to form an N-doped C source precursor on the surface of LiFePO4.
The structures, morphologies, and size of the as-prepared materials were characterized using a powder X-ray diffractometer (XRD, Dandong Tongda TD-3000) with Cu Kɑ radiation (l ¼ 1.5406 Å) in the range of 10e90 at a scanning speed of 0.02 s1 (operated at a voltage of 40 kV and current of 30 mA), fieldemission scanning electron microscope (SEM, JMS-7610F, JEOL, Japan) and a transmission electron microscope (TEM, FEI Tecnai G2 S-Twin, 200 kV). Element mapping images were obtained using an energy dispersive X-ray spectrometer (EDS, Oxford X-MaxN 80, 20 kV) and used to clarify the elemental distribution.
As cathode materials for LIBs, an N-C layer coated with LiFePO4 exhibits excellent electrochemical properties, including excellent capacity retention and high rate performance due to a nanosphere structure and N-C layer coating. In addition, it has been demonstrated that carbon materials doped by heteroatoms (particularly nitrogen atoms) not only have higher electrical conductivity but also further increase the reactive sites and reduce the energy barrier for ion permeation [30,31]. Currently, dopamine is widely used to prepare N-C with the help of its self-polymerization process [17,32]. In the current work, we apply dopamine as an N-C source for coating LiFePO4 spheres by a spray-drying approach. The nitrogendoped carbon can provide more electron carriers in the conduction band and improve the electrochemical performance of the electrode material through good electron conductivity. A nitrogendoped carbon coated layer can also provide extrinsic defects for use as additional lithium-ion storage sites [33]. Consequently, the synthesized core-shell N-C@LiFePO4 composite displayed excellent specific capacity, delivering 91.7% (156 mAh g1) of the theoretical capacity at 0.1C. After 60 cycles, the reversible capacity of 5% NC@LiFePO4 at 0.1 C was as high as 147 mAh g1 (94.8% relative to the 0.1C initial discharge capacity). This method is therefore effective for increasing the performance of LiFePO4. 2. Experimental section 2.1. Material preparation Quantitative amounts of LiOH$H2O, NH4H2PO4, and Fe(NO3)3$9H2O at a molar ratio of 1:1:1 were weighed as the lithium source, phosphorus source, and iron source, respectively. An appropriate amount of polyethylene glycol was also used as surfactant. LiOHH2O and NH4H2PO4 were completely dissolved in 500 mL distilled water to obtain solution A. Fe(NO3)39H2O, 5 wt% dopamine and 2.5 g polyethylene glycol were dissolved in 500 mL distilled water to obtain solution B. Then, the prepared solution B was slowly added to solution An under stirring to form a precursor solution. The precursor solution was treated to obtain the corresponding solid powders through a spray-drying process with a spray rate of 900 mL h1 and a reaction temperature of 180 C.
2.2. Materials characterization
2.3. Electrochemical testing The electrochemical characterization of the prepared material was carried out by assembling 2032-type coin cells. For fabrication of the cathode electrode, the active material powders were mixed with super-P carbon black and polyvinylidene fluoride (80:10:10 in weight) in N-methyl pyrrolidone (NMP). The obtained homogeneous slurry was uniformly coated onto Al foil and dried in a vacuum oven at 120 C for 12 h. The electrode was pressed and punched into round disks of 15 mm in diameter with a loading weight of approximately 1.5 mg cm2. Electrochemical cells consisted of an active material working electrode and a lithium foil counter electrode, which were separated by a Celgard 2400 microporous membrane. The electrolyte solution was 1 mol L1 LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylene methyl carbonate (EMC) in a 1:1:1 vol ratio. The cells were assembled in an argon box with oxygen and moisture levels below 1 ppm. The coin cells were preliminarily charged and discharged by using a battery testing system (NEWARE Co. Ltd., Shenzhen, China) for testing cycling and rate performance with a voltage window of 2.5e4.2 V ns Li/Liþ. Electrochemical impedance spectroscopy (EIS) measurements were conducted using an electrochemical workstation (CHI760D, Shanghai Chenhua Co. Ltd). 3. Results and discussion Fig. 1 shows TEM images of the prepared 5%N-C@LiFePO4 sample. The morphology of the amorphous N-C layer with a thickness of 20 nm coated on the surface of the LFP can be observed. When the N-C coating amount reaches 10%, the coating layer becomes too thick, which will block lithium-ion migration and weaken the performance of the LiFePO4 cathode. Dopamine is a synthetic compound containing catechol and an amino functional group. It is oxidized to form a dopamine quinone compound with a catechol structure. A disproportionation reaction occurs between dopamine and dopamine oxime to produce a semiquinone radical, which is then coupled to form a crosslink bond [34e36]. The dopamine containing an amino group is immersed in a polyethylene glycol solution, and a layer of PEG is grafted onto the surface of the polydopamine coating by a Michael addition reaction, as shown in Fig. 2. Therefore, the presence of a coating layer was observed on the surface of the spherical particles obtained by spraying. The nitrogen-doped carbon layer can be obtained by high-temperature calcination. To further determine the result of the N-C coating on the surface of LiFePO4, the elemental distribution in a 5%N-C@LiFePO4 particle
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Fig. 1. TEM images of 5%N-C@LiFePO4 spheres.
Fig. 2. Cross-linking process of dopamine in aqueous solution.
was investigated by EDS mapping analysis, as shown in Fig. 3. The homogeneous distribution of C, N, Fe, P, and O elements demonstrates that N-C is successfully coated onto the surface of LiFePO4. This N-C shell is beneficial for improving the electrical conductivity. On the other hand, the N-C layer is thin, thus inducting good transference of ions between the active material and the electrolyte. During the spray-drying treatment process, the internal components of the sample particles contain an iron source, a phosphorus source, and a lithium source, and the particles are coated with a layer of polydopamine throughout the sample. After calcination, the LiFePO4 is synthesized inside the particles. Additionally, owing to the fact that dopamine contains N and C elements, the chain of polydopamine evolves into N-C compounds during the annealing process. As a result, the N-C layer can be uniformly constructed on the surface of the LiFePO4 spheres. XPS was carried out to examine the detailed atomic state of the N-C layer in N-C@LiFePO4, as shown in Fig. 4. The high-resolution N1s spectrum (Fig. 4a) displays three different states of N element: graphitic N (402.2 eV), pyridinic N (398.0 eV), and pyrrolic N (near 400 eV) [32]. The existence of electron deficiencies and vacancies in pyridinic N will facilitate Li-ion penetration, thus contributing to enhanced reversible capacity. The high-resolution C1s spectrum of N-C@LiFePO4 spheres (Fig. 4b) can be deconvoluted into three peaks. Two weak peaks at 286.3 eV and 288.6 eV correspond to N-C and C¼O, respectively, and a strong peak at
284.6 eV is assigned to the C-C bond [21,37]. It is confirmed that a nitrogen-doped carbon layer was successfully coated onto the surface of LiFePO4 spheres, which is conducive to improve the electrical conductivity of the cathode. The X-ray diffraction patterns for the pristine LiFePO4 and 2%, 5%, 10%N-C@LiFePO4 are illustrated in Fig. 5. All diffraction peaks indicated the orthorhombic space group of Pnma (JCPDS card number 40-1499), which corresponds to olivine LiFePO4. The distinct and sharp diffraction peaks indicate that the LFP and NC@LiFePO4 spheres have good crystallinity, indicating that the N-C coated layer cannot affect the crystal structure of LiFePO4. In addition, no extra peaks are observed in the N-C@LiFePO4 pattern, intimating that the N-C@LiFePO4 particles still have higher purity after coating the N-C layer and that the N-C layer content is rather low. High-resolution TEM (HRTEM) (Fig. 5b.) characterization for randomly selected particles shows that the product is highly crystalline. The clear lattice spacing of 0.396 nm corresponds to the (120) lattice plane of LFP, which is consistent with the XRD results (Fig. 5a.). The particle size and morphological features of LiFePO4 and X% N-C@LiFePO4 were investigated by SEM (Fig. 6). The X%NC@LiFePO4 (Fig. 6bed) samples are roughly sphere-like particles. After N-C coating, the morphology of LiFePO4 is not influenced, and no obvious agglomeration of particles is observed. Both X%NC@LiFePO4 and LiFePO4 have a particle size distribution ranging
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Fig. 3. EDS elemental mapping of 5%C-N@LiFePO4.
Fig. 4. High-resolution XPS date for the N-C@LiFePO4 sphere composite: (a) C1s spectrum and (b) N1s spectrum.
from hundreds of nanometres to several micrometres. The wide distribution of particle size is beneficial for incrementing the packing density of the cathode. The small particles fill in the gaps between large particles in addition to reducing the cell volume. This result shows that the spray-drying method is conducive to the formation of spherical particles, and the final sample has a very
high quality. Fig. 7a shows the initial charge/discharge curves for LiFePO4 and X%N-C@LiFePO4 at a 0.1 C rate in the voltage window of 2.5e4.2 V. Across a wide voltage window, the sample clearly indicates a flat voltage plateau between 3.4 and 3.5 V vs. Li/Liþ, which is the main characteristic of the two-phase LiFePO4 / FePO4 þ Liþ þ e
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Fig. 5. (a) XRD patterns for the N-C@LiFePO4 and LiFePO4 spheres; (b) HRTEM images of 5%N-C@LiFePO4 spheres.
Fig. 6. SEM images of LiFePO4 with varying N-C layer content: (a) 0% N-C, (b) 2% N-C, (c) 5% N-C, and (d) 10% N-C.
Fig. 7. (a) Initial charge/discharge profiles for LiFePO4 and X%N-C@LiFePO4 samples at 0.1C. (b) Charge/discharge profiles for a 5%N-C@LiFePO4 sample at 0.1 C for the first three cycles.
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Fig. 8. Cycling performances and coulombic efficiency for LiFePO4 and X% NC@LiFePO4 samples at 0.1C.
reaction [38]. The initial discharge capacity of 5% N-C@LiFePO4, 10% N-C@LiFePO4, and 2% N-C@LiFePO4 is 154.8 mAh g1, 145 mAh g1, and 107 mAh g1, which are higher than that of pristine LiFePO4 (93 mAh g1), the first cycle coulombic efficiency of four samples is 93.634%, 90.977%, 89.499% and 84.888%, respectively. 5%NC@LiFePO4 has a relatively high capacity and the most stable discharge plateau. Thus, the reaction kinetics for electron transport are indeed improved and a better electrochemical reversibility can be realized. The charge/discharge profiles for the first three cycles at a rate of 0.1C in the voltage window of 2.5e4.2 V are shown in Fig. 7b. At the rate of 0.1 C, the discharge capacity of the first three laps of 5%N-C@LiFePO4 is 154.8 mAh g1, 155.1 mAh g1, and 156.1 mAh g1, corresponding to 91.1%, 91.2%, and 91.8% of the theoretical capacity of LiFePO4, respectively, and the coulombic efficiency is 93.634%, 95.495% and 96.756%. Since the electrode material has an activation process during the initial charge and discharge, the capacity is highest at the third cycle. These results indicate that the NC coating can effectively promote the capacity of a LiFePO4 cathode. The cycle performance and coulombic efficiency of LiFePO4 and X%N-C@LiFePO4 are shown in Fig. 8. The cycle measurement was performed at 0.1C with a potential range of 2.5e4.2 V. It can be seen that the 5%N-C coated LiFePO4 shows the highest initial discharge specific capacity of 154 mAh g1. At the beginning of the cycle test, four samples showed an increase in capacity due to the passive electrode effect [39]. After 150 cycles, the capacity of 5%NC@LiFePO4 drops from the original 156 mAh g1 to 135 mAh g1, with a capacity retention rate as high as 86%. In contrast, LiFePO4,
2%N-C@LiFePO4, and 10%N-C@LiFePO4 have a lower specific discharge capacity and capacity retention. Moreover, comparing the coulombic efficiency of the four samples (the ratio of charge capacity to discharge capacity), the coulombic efficiency for 5%NC@LiFePO4 remains at approximately 98.879% over all the 150 cycles. A proper amount of the N-C layer can effectively reduce the side reaction between the electrode and the electrolyte and mitigate erosion due to HF. High rate performance is one of the most important electrochemical properties of lithium-ion batteries in high-power applications. To further evaluate the N-C effect, the rate performance of LiFePO4 and X% N-C@LiFePO4 cells was compared, as revealed in Fig. 9. The assembled cells were tested for charging and discharging from 0.1 C to 5 C, respectively, and cycled 10 times at each rate. When the rate was increased from 0.1 C to 5 C, the discharge specific capacity of the cell gradually decreased; however, the capacity still reached 121 mAh g1. After 60 cycles at an unequal rate, the high reversible capacity of 5%N-C@LiFePO4 at 0.1 C was 147 mAh g1 (94.8% relative to the 0.1 C initial discharge capacity), indicating excellent reversibility. Fig. 9b intuitively compares the rate performance of the four samples. With the introduction of the N-C layer, the capacity loss of 5%N-C@LiFePO4 is smaller at large rate, indicating that the N-C with optimum content can enhance utilization of the LiFePO4 cathode material at high charge/discharge current density. The excellent cycling performance and high rate capability of N-C@LiFePO4 is attributed to the effective N-C layer coating on the LiFePO4 spheres, which effectively improves the conductivity and reduces the electrode polarization. The high capacity at a high rate illustrates that the N-C coating is a promising modification method for achieving admirable electrochemical performance. To further explore the impact of the N-C layer on the LiFePO4 electrode performance, the AC spectra for the assembled cells were tested (Fig. 10). The impedance spectrum of each N-C@LiFePO4 electrode includes three regions. The intercept of the Z0 axis in the high frequency region is attributed to the electrolyte resistance (Re), and the semicircle in the intermediate frequency region reflects the charge transfer resistance (Rct). The oblique part of the low frequency region (Zw) represents the solid-state diffusion of lithium ions inside the active material [40e42]. As presented in Fig. 10, the semi-circular diameters of the other samples are larger than that of 5%N-C@LiFePO4, indicating that the charge transfer of the 5% N-C-coated LiFePO4 electrode is significantly improved. In short, the N-C layer coating facilitates the improvement of the electrode reaction kinetics and elevates the cycle and rate performance of cells during charge and discharge [43,44].
Fig. 9. (a) Rate performance of LiFePO4 and X% N-C@LiFePO4 samples, (b) comparison of the rate capacity between LiFePO4 and X%N-C@LiFePO4.
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Fig. 10. Nyquist plots of LiFePO4 and X%N-C@LiFePO4 samples at the open-circuit voltage.
4. Conclusion We successfully modified LiFePO4 with N-C by means of a simple spray-drying technique. This method involves a fast drying process, high yield, and high production efficiency. After hightemperature calcination, the N-C layer is successfully coated onto the surface of LiFePO4, and the thickness of the coating layer is approximately 20 nm. The synthetic 5%N-C@LiFePO4 has a specific capacity of 156 mAh g1 at a rate of 0.1C. After 60 cycles at an unequal rate, the high reversible capacity of 5%N-C@LiFePO4 at 0.1 C was 147 mAh g1. We confirm that the N-C layer can stabilize the material surface and protect against HF attack. More importantly, the N-C coating layer can improve the electron transport and lithium-ion migration during electrochemical reaction in the battery. These results demonstrate that a facile strategy can be easily used to construct an advanced cathode material for a Li-ion battery, which can also be expanded to other electrode materials. Acknowledgements This research was supported by the Natural Science Foundation of Jilin Province (No. 20170101128JC), the Science and Technology Research Project of the Education Department of Jilin Province during the 13th five-year-plan period (No. JJKH20190558KJ), the Industrial Technology Research and Development Project of Jilin Province Development and Reform Commission (2017C052-4), and the Science and Technology Planning Project of Changchun City (no. 2013064). References [1] F. Cheng, J. Liang, Z. Tao, J. Chen, Functional materials for rechargeable batteries, Adv. Mater. 23 (2011) 1695e1715. [2] Q. Zhang, E. Uchaker, S.L. Candelaria, G. Cao, Nanomaterials for energy conversion and storage, Chem. Soc. Rev. 42 (2013) 3127e3171. [3] X.-P. Gao, H.-X. Yang, Multi-electron reaction materials for high energy density batteries, Energy Environ. Sci. 3 (2010) 174e189. [4] K. Du, J. Huang, Y. Cao, Z. Peng, G. Hu, Study of effects on LiNi0.8Co0.15Al0.05O2 cathode by LiNi1/3Co1/3Mn1/3O2 coating for lithium ion batteries, J. Alloy. Comp. 574 (2013) 377e382. [5] S. Hwang, W. Chang, S.M. Kim, D. Su, D.H. Kim, J.Y. Lee, K.Y. Chung, E.A. Stach, Investigation of changes in the surface structure of LixNi0.8Co0.15Al0.05O2 cathode materials induced by the initial charge, Chem. Mater. 26 (2014) 1084e1092. [6] V. Augustyn, S. Therese, T.C. Turner, A. Manthiram, Nickel-rich layered LiNi1xMxO2 (M ¼ Mn, Fe, and Co) electrocatalysts with high oxygen evolution reaction activity, J. Mater. Chem. 3 (2015) 16604e16612.
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stability, Sci. Rep. 6 (2016) 25942. [41] X. Xia, Y. Wang, D. Wang, Y. Zhang, Z. Fan, J. Tu, H. Zhang, H.J. Fan, Atomiclayer-deposited iron oxide on arrays of metal/carbon spheres and their application for electrocatalysis, Nano Energy 20 (2016) 244e253. [42] D.P. Abraham, J.R. Heaton, S.H. Kang, D.W. Dees, A.N. Jansen, Investigating the low-temperature impedance increase of lithium-ion cells, J. Electrochem. Soc. 155 (2008) A41. [43] X. Xia, Z. Ku, D. Zhou, Y. Zhong, Y. Zhang, Y. Wang, M.J. Huang, J. Tu, H.J. Fan, Perovskite solar cell powered electrochromic batteries for smart windows, Materials Horizons 3 (2016) 588e595. [44] Y. Zhong, X. Xia, J. Zhan, X. Wang, J. Tu, A CNT cocoon on sodium manganate nanotubes forming a core/branch cathode coupled with a helical carbon nanofibre anode for enhanced sodium ion batteries, J. Mater. Chem. 4 (2016) 11207e11213.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A versatile nitrogen-doped carbon coating strategy to improve the electrochemical performance of LiFePO4 cathodes for lithium-ion batteries Yingying Wang, Xinlu Wang, Ao Jiang, Guixia Liu, Wensheng Yu, Xiangting Dong, Jinxian Wang* School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, 130022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2018 Received in revised form 10 May 2019 Accepted 14 August 2019 Available online 15 August 2019
To improve the structural stability and electrochemical performance of a LiFePO4 cathode, a nitrogendoped carbon (N-C) coating strategy is exploited using a simple spray-drying method with dopamine as an N-C coating source. Transmission electron microscopy shows a uniform N-C coating layer on the surface of the LiFePO4 spheres. The introduction of the N-C layer can improve the electrical conductivity and structural stability of the electrode, which results in N-C@LiFePO4 spherical cathode materials with excellent electrochemical properties. The 5% N-C@LiFePO4 spherical composite delivers a specific discharge capacity of 156 mAh g1, with a capacity retention rate of 86% after 150 cycles at 0.1C. More importantly, because the nitrogen-doped carbon coating can significantly improve the electrochemical performance of the electrode material through excellent electron conductivity, a remarkable rate performance can be achieved by virtue of the thin N-C coated layer, which is superior to that of uncoated LiFePO4. The results from electrochemical impedance spectroscopy further verify that 5% N-C@LiFePO4 has an excellent Liþ conductivity and low charge transfer resistance. This efficient and scalable strategy offers a facile approach to develop high performance LiFePO4-based cathode materials for large-scale lithium-ion storage systems. © 2019 Elsevier B.V. All rights reserved.
Keywords: LiFePO4 cathode Nitrogen-doped carbon coating Li-ion battery Spray-drying method
1. Introduction Lithium-ion batteries have become one of the most attractive energy storage systems due to a long cycle life, and are used in smart grids, electric vehicles, and renewable power stations [1e3] etc. The most important requirements for lithium-ion batteries are good safety, high energy density and low cost [4e7]. Commercialized cathode materials for lithium-ion batteries, such as LiFePO4 [8], LiMn2O4 [9], LiCoO2 [10], and ternary [11] cathode materials, have been generally equipped in plug-in hybrid electric vehicles (PHEV), electric vehicles (EVs). Among lithium-ion battery cathode materials, olivine-structured LiFePO4 has attracted widespread attention as a cathode material due to its potential application value. However, inherent limitations of LiFePO4, including low electronic conductivity (~1010 S cm1) and slow diffusion of lithium ions (~1014 cm2 s1), require further commitment for improvements
* Corresponding author. E-mail addresses: [email protected], [email protected] (J. Wang). https://doi.org/10.1016/j.jallcom.2019.151889 0925-8388/© 2019 Elsevier B.V. All rights reserved.
[12e16]. Researchers have recently tried to solve these problems by employing a variety of strategies, for instance surface coating, elemental doping [17e24], controlling particle size and morphology [25], etc. Doping of a heteroatom can influence the crystal structure and create Li-ion vacancies, leading to an improvement in the ionic and electronic conductivity. Controlling the particle size can increase its specific surface area and reduce the diffusion path of lithium ions, hence improving the electrochemical performance of LiFePO4. In particular, conductive carbon coatings [26] (organic carbon sources: dopamine, sucrose, etc.) have been developed as a most successful strategy for enhancing the electrochemical performance of LiFePO4 [20]. The carbon coating not only improves the electrical conductivity but also retains the active material mass load. In addition, the carbon coating may act as a barrier between the active material and the electrolyte, thus preventing unwanted side reactions during the charging/discharging process. Moreover, carbon layers effectively limit volume expansion, thereby increasing structural stability, resulting in enhanced
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circulation. However, the traditional carbon coating method is complicated, and the thickness of the carbon layer is difficult to control: when the carbon coating is too thick, Li ions are blocked, and the performance of the LiFePO4 cathode is impaired. In addition, high carbon content reduces energy density and power density [27e29]. Compared with traditional LiFePO4-coated carbon, the spray drying method has the following advantages:
Precursor powders were calcined at 700 C for 5 h under argon atmosphere to form 5%N-C@LiFePO4. In contrast, the LiFePO4, 2%NC@LiFePO4, and 10%N-C@LiFePO4 were synthesized under the same conditions with dopamine mass fractions of 0%, 2%, and 10%, respectively.
(1) Spray drying allows granulation and drying to be completed in one step. (2) When spray drying, the material liquid is sprayed into an atomized dispersion under constant stirring, and drying is completed instantaneously. (3) In the early stages of the spray drying process, the vaporization of the solvent is rapid, which results in good dispersion and solubility that can be obtained without further treatment. (4) Since spray drying is a continuous and closed production process, the purity of the product is high, and the opportunity for exposure in the production environment and contact with the operator is eliminated. Environmental pollution is also reduced. (5) Spray drying to form an N-doped C source precursor on the surface of LiFePO4.
The structures, morphologies, and size of the as-prepared materials were characterized using a powder X-ray diffractometer (XRD, Dandong Tongda TD-3000) with Cu Kɑ radiation (l ¼ 1.5406 Å) in the range of 10e90 at a scanning speed of 0.02 s1 (operated at a voltage of 40 kV and current of 30 mA), fieldemission scanning electron microscope (SEM, JMS-7610F, JEOL, Japan) and a transmission electron microscope (TEM, FEI Tecnai G2 S-Twin, 200 kV). Element mapping images were obtained using an energy dispersive X-ray spectrometer (EDS, Oxford X-MaxN 80, 20 kV) and used to clarify the elemental distribution.
As cathode materials for LIBs, an N-C layer coated with LiFePO4 exhibits excellent electrochemical properties, including excellent capacity retention and high rate performance due to a nanosphere structure and N-C layer coating. In addition, it has been demonstrated that carbon materials doped by heteroatoms (particularly nitrogen atoms) not only have higher electrical conductivity but also further increase the reactive sites and reduce the energy barrier for ion permeation [30,31]. Currently, dopamine is widely used to prepare N-C with the help of its self-polymerization process [17,32]. In the current work, we apply dopamine as an N-C source for coating LiFePO4 spheres by a spray-drying approach. The nitrogendoped carbon can provide more electron carriers in the conduction band and improve the electrochemical performance of the electrode material through good electron conductivity. A nitrogendoped carbon coated layer can also provide extrinsic defects for use as additional lithium-ion storage sites [33]. Consequently, the synthesized core-shell N-C@LiFePO4 composite displayed excellent specific capacity, delivering 91.7% (156 mAh g1) of the theoretical capacity at 0.1C. After 60 cycles, the reversible capacity of 5% NC@LiFePO4 at 0.1 C was as high as 147 mAh g1 (94.8% relative to the 0.1C initial discharge capacity). This method is therefore effective for increasing the performance of LiFePO4. 2. Experimental section 2.1. Material preparation Quantitative amounts of LiOH$H2O, NH4H2PO4, and Fe(NO3)3$9H2O at a molar ratio of 1:1:1 were weighed as the lithium source, phosphorus source, and iron source, respectively. An appropriate amount of polyethylene glycol was also used as surfactant. LiOHH2O and NH4H2PO4 were completely dissolved in 500 mL distilled water to obtain solution A. Fe(NO3)39H2O, 5 wt% dopamine and 2.5 g polyethylene glycol were dissolved in 500 mL distilled water to obtain solution B. Then, the prepared solution B was slowly added to solution An under stirring to form a precursor solution. The precursor solution was treated to obtain the corresponding solid powders through a spray-drying process with a spray rate of 900 mL h1 and a reaction temperature of 180 C.
2.2. Materials characterization
2.3. Electrochemical testing The electrochemical characterization of the prepared material was carried out by assembling 2032-type coin cells. For fabrication of the cathode electrode, the active material powders were mixed with super-P carbon black and polyvinylidene fluoride (80:10:10 in weight) in N-methyl pyrrolidone (NMP). The obtained homogeneous slurry was uniformly coated onto Al foil and dried in a vacuum oven at 120 C for 12 h. The electrode was pressed and punched into round disks of 15 mm in diameter with a loading weight of approximately 1.5 mg cm2. Electrochemical cells consisted of an active material working electrode and a lithium foil counter electrode, which were separated by a Celgard 2400 microporous membrane. The electrolyte solution was 1 mol L1 LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylene methyl carbonate (EMC) in a 1:1:1 vol ratio. The cells were assembled in an argon box with oxygen and moisture levels below 1 ppm. The coin cells were preliminarily charged and discharged by using a battery testing system (NEWARE Co. Ltd., Shenzhen, China) for testing cycling and rate performance with a voltage window of 2.5e4.2 V ns Li/Liþ. Electrochemical impedance spectroscopy (EIS) measurements were conducted using an electrochemical workstation (CHI760D, Shanghai Chenhua Co. Ltd). 3. Results and discussion Fig. 1 shows TEM images of the prepared 5%N-C@LiFePO4 sample. The morphology of the amorphous N-C layer with a thickness of 20 nm coated on the surface of the LFP can be observed. When the N-C coating amount reaches 10%, the coating layer becomes too thick, which will block lithium-ion migration and weaken the performance of the LiFePO4 cathode. Dopamine is a synthetic compound containing catechol and an amino functional group. It is oxidized to form a dopamine quinone compound with a catechol structure. A disproportionation reaction occurs between dopamine and dopamine oxime to produce a semiquinone radical, which is then coupled to form a crosslink bond [34e36]. The dopamine containing an amino group is immersed in a polyethylene glycol solution, and a layer of PEG is grafted onto the surface of the polydopamine coating by a Michael addition reaction, as shown in Fig. 2. Therefore, the presence of a coating layer was observed on the surface of the spherical particles obtained by spraying. The nitrogen-doped carbon layer can be obtained by high-temperature calcination. To further determine the result of the N-C coating on the surface of LiFePO4, the elemental distribution in a 5%N-C@LiFePO4 particle
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Fig. 1. TEM images of 5%N-C@LiFePO4 spheres.
Fig. 2. Cross-linking process of dopamine in aqueous solution.
was investigated by EDS mapping analysis, as shown in Fig. 3. The homogeneous distribution of C, N, Fe, P, and O elements demonstrates that N-C is successfully coated onto the surface of LiFePO4. This N-C shell is beneficial for improving the electrical conductivity. On the other hand, the N-C layer is thin, thus inducting good transference of ions between the active material and the electrolyte. During the spray-drying treatment process, the internal components of the sample particles contain an iron source, a phosphorus source, and a lithium source, and the particles are coated with a layer of polydopamine throughout the sample. After calcination, the LiFePO4 is synthesized inside the particles. Additionally, owing to the fact that dopamine contains N and C elements, the chain of polydopamine evolves into N-C compounds during the annealing process. As a result, the N-C layer can be uniformly constructed on the surface of the LiFePO4 spheres. XPS was carried out to examine the detailed atomic state of the N-C layer in N-C@LiFePO4, as shown in Fig. 4. The high-resolution N1s spectrum (Fig. 4a) displays three different states of N element: graphitic N (402.2 eV), pyridinic N (398.0 eV), and pyrrolic N (near 400 eV) [32]. The existence of electron deficiencies and vacancies in pyridinic N will facilitate Li-ion penetration, thus contributing to enhanced reversible capacity. The high-resolution C1s spectrum of N-C@LiFePO4 spheres (Fig. 4b) can be deconvoluted into three peaks. Two weak peaks at 286.3 eV and 288.6 eV correspond to N-C and C¼O, respectively, and a strong peak at
284.6 eV is assigned to the C-C bond [21,37]. It is confirmed that a nitrogen-doped carbon layer was successfully coated onto the surface of LiFePO4 spheres, which is conducive to improve the electrical conductivity of the cathode. The X-ray diffraction patterns for the pristine LiFePO4 and 2%, 5%, 10%N-C@LiFePO4 are illustrated in Fig. 5. All diffraction peaks indicated the orthorhombic space group of Pnma (JCPDS card number 40-1499), which corresponds to olivine LiFePO4. The distinct and sharp diffraction peaks indicate that the LFP and NC@LiFePO4 spheres have good crystallinity, indicating that the N-C coated layer cannot affect the crystal structure of LiFePO4. In addition, no extra peaks are observed in the N-C@LiFePO4 pattern, intimating that the N-C@LiFePO4 particles still have higher purity after coating the N-C layer and that the N-C layer content is rather low. High-resolution TEM (HRTEM) (Fig. 5b.) characterization for randomly selected particles shows that the product is highly crystalline. The clear lattice spacing of 0.396 nm corresponds to the (120) lattice plane of LFP, which is consistent with the XRD results (Fig. 5a.). The particle size and morphological features of LiFePO4 and X% N-C@LiFePO4 were investigated by SEM (Fig. 6). The X%NC@LiFePO4 (Fig. 6bed) samples are roughly sphere-like particles. After N-C coating, the morphology of LiFePO4 is not influenced, and no obvious agglomeration of particles is observed. Both X%NC@LiFePO4 and LiFePO4 have a particle size distribution ranging
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Fig. 3. EDS elemental mapping of 5%C-N@LiFePO4.
Fig. 4. High-resolution XPS date for the N-C@LiFePO4 sphere composite: (a) C1s spectrum and (b) N1s spectrum.
from hundreds of nanometres to several micrometres. The wide distribution of particle size is beneficial for incrementing the packing density of the cathode. The small particles fill in the gaps between large particles in addition to reducing the cell volume. This result shows that the spray-drying method is conducive to the formation of spherical particles, and the final sample has a very
high quality. Fig. 7a shows the initial charge/discharge curves for LiFePO4 and X%N-C@LiFePO4 at a 0.1 C rate in the voltage window of 2.5e4.2 V. Across a wide voltage window, the sample clearly indicates a flat voltage plateau between 3.4 and 3.5 V vs. Li/Liþ, which is the main characteristic of the two-phase LiFePO4 / FePO4 þ Liþ þ e
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Fig. 5. (a) XRD patterns for the N-C@LiFePO4 and LiFePO4 spheres; (b) HRTEM images of 5%N-C@LiFePO4 spheres.
Fig. 6. SEM images of LiFePO4 with varying N-C layer content: (a) 0% N-C, (b) 2% N-C, (c) 5% N-C, and (d) 10% N-C.
Fig. 7. (a) Initial charge/discharge profiles for LiFePO4 and X%N-C@LiFePO4 samples at 0.1C. (b) Charge/discharge profiles for a 5%N-C@LiFePO4 sample at 0.1 C for the first three cycles.
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Fig. 8. Cycling performances and coulombic efficiency for LiFePO4 and X% NC@LiFePO4 samples at 0.1C.
reaction [38]. The initial discharge capacity of 5% N-C@LiFePO4, 10% N-C@LiFePO4, and 2% N-C@LiFePO4 is 154.8 mAh g1, 145 mAh g1, and 107 mAh g1, which are higher than that of pristine LiFePO4 (93 mAh g1), the first cycle coulombic efficiency of four samples is 93.634%, 90.977%, 89.499% and 84.888%, respectively. 5%NC@LiFePO4 has a relatively high capacity and the most stable discharge plateau. Thus, the reaction kinetics for electron transport are indeed improved and a better electrochemical reversibility can be realized. The charge/discharge profiles for the first three cycles at a rate of 0.1C in the voltage window of 2.5e4.2 V are shown in Fig. 7b. At the rate of 0.1 C, the discharge capacity of the first three laps of 5%N-C@LiFePO4 is 154.8 mAh g1, 155.1 mAh g1, and 156.1 mAh g1, corresponding to 91.1%, 91.2%, and 91.8% of the theoretical capacity of LiFePO4, respectively, and the coulombic efficiency is 93.634%, 95.495% and 96.756%. Since the electrode material has an activation process during the initial charge and discharge, the capacity is highest at the third cycle. These results indicate that the NC coating can effectively promote the capacity of a LiFePO4 cathode. The cycle performance and coulombic efficiency of LiFePO4 and X%N-C@LiFePO4 are shown in Fig. 8. The cycle measurement was performed at 0.1C with a potential range of 2.5e4.2 V. It can be seen that the 5%N-C coated LiFePO4 shows the highest initial discharge specific capacity of 154 mAh g1. At the beginning of the cycle test, four samples showed an increase in capacity due to the passive electrode effect [39]. After 150 cycles, the capacity of 5%NC@LiFePO4 drops from the original 156 mAh g1 to 135 mAh g1, with a capacity retention rate as high as 86%. In contrast, LiFePO4,
2%N-C@LiFePO4, and 10%N-C@LiFePO4 have a lower specific discharge capacity and capacity retention. Moreover, comparing the coulombic efficiency of the four samples (the ratio of charge capacity to discharge capacity), the coulombic efficiency for 5%NC@LiFePO4 remains at approximately 98.879% over all the 150 cycles. A proper amount of the N-C layer can effectively reduce the side reaction between the electrode and the electrolyte and mitigate erosion due to HF. High rate performance is one of the most important electrochemical properties of lithium-ion batteries in high-power applications. To further evaluate the N-C effect, the rate performance of LiFePO4 and X% N-C@LiFePO4 cells was compared, as revealed in Fig. 9. The assembled cells were tested for charging and discharging from 0.1 C to 5 C, respectively, and cycled 10 times at each rate. When the rate was increased from 0.1 C to 5 C, the discharge specific capacity of the cell gradually decreased; however, the capacity still reached 121 mAh g1. After 60 cycles at an unequal rate, the high reversible capacity of 5%N-C@LiFePO4 at 0.1 C was 147 mAh g1 (94.8% relative to the 0.1 C initial discharge capacity), indicating excellent reversibility. Fig. 9b intuitively compares the rate performance of the four samples. With the introduction of the N-C layer, the capacity loss of 5%N-C@LiFePO4 is smaller at large rate, indicating that the N-C with optimum content can enhance utilization of the LiFePO4 cathode material at high charge/discharge current density. The excellent cycling performance and high rate capability of N-C@LiFePO4 is attributed to the effective N-C layer coating on the LiFePO4 spheres, which effectively improves the conductivity and reduces the electrode polarization. The high capacity at a high rate illustrates that the N-C coating is a promising modification method for achieving admirable electrochemical performance. To further explore the impact of the N-C layer on the LiFePO4 electrode performance, the AC spectra for the assembled cells were tested (Fig. 10). The impedance spectrum of each N-C@LiFePO4 electrode includes three regions. The intercept of the Z0 axis in the high frequency region is attributed to the electrolyte resistance (Re), and the semicircle in the intermediate frequency region reflects the charge transfer resistance (Rct). The oblique part of the low frequency region (Zw) represents the solid-state diffusion of lithium ions inside the active material [40e42]. As presented in Fig. 10, the semi-circular diameters of the other samples are larger than that of 5%N-C@LiFePO4, indicating that the charge transfer of the 5% N-C-coated LiFePO4 electrode is significantly improved. In short, the N-C layer coating facilitates the improvement of the electrode reaction kinetics and elevates the cycle and rate performance of cells during charge and discharge [43,44].
Fig. 9. (a) Rate performance of LiFePO4 and X% N-C@LiFePO4 samples, (b) comparison of the rate capacity between LiFePO4 and X%N-C@LiFePO4.
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Fig. 10. Nyquist plots of LiFePO4 and X%N-C@LiFePO4 samples at the open-circuit voltage.
4. Conclusion We successfully modified LiFePO4 with N-C by means of a simple spray-drying technique. This method involves a fast drying process, high yield, and high production efficiency. After hightemperature calcination, the N-C layer is successfully coated onto the surface of LiFePO4, and the thickness of the coating layer is approximately 20 nm. The synthetic 5%N-C@LiFePO4 has a specific capacity of 156 mAh g1 at a rate of 0.1C. After 60 cycles at an unequal rate, the high reversible capacity of 5%N-C@LiFePO4 at 0.1 C was 147 mAh g1. We confirm that the N-C layer can stabilize the material surface and protect against HF attack. More importantly, the N-C coating layer can improve the electron transport and lithium-ion migration during electrochemical reaction in the battery. These results demonstrate that a facile strategy can be easily used to construct an advanced cathode material for a Li-ion battery, which can also be expanded to other electrode materials. Acknowledgements This research was supported by the Natural Science Foundation of Jilin Province (No. 20170101128JC), the Science and Technology Research Project of the Education Department of Jilin Province during the 13th five-year-plan period (No. JJKH20190558KJ), the Industrial Technology Research and Development Project of Jilin Province Development and Reform Commission (2017C052-4), and the Science and Technology Planning Project of Changchun City (no. 2013064). References [1] F. Cheng, J. Liang, Z. Tao, J. Chen, Functional materials for rechargeable batteries, Adv. Mater. 23 (2011) 1695e1715. [2] Q. Zhang, E. Uchaker, S.L. Candelaria, G. Cao, Nanomaterials for energy conversion and storage, Chem. Soc. Rev. 42 (2013) 3127e3171. [3] X.-P. Gao, H.-X. Yang, Multi-electron reaction materials for high energy density batteries, Energy Environ. Sci. 3 (2010) 174e189. [4] K. Du, J. Huang, Y. Cao, Z. Peng, G. Hu, Study of effects on LiNi0.8Co0.15Al0.05O2 cathode by LiNi1/3Co1/3Mn1/3O2 coating for lithium ion batteries, J. Alloy. Comp. 574 (2013) 377e382. [5] S. Hwang, W. Chang, S.M. Kim, D. Su, D.H. Kim, J.Y. Lee, K.Y. Chung, E.A. Stach, Investigation of changes in the surface structure of LixNi0.8Co0.15Al0.05O2 cathode materials induced by the initial charge, Chem. Mater. 26 (2014) 1084e1092. [6] V. Augustyn, S. Therese, T.C. Turner, A. Manthiram, Nickel-rich layered LiNi1xMxO2 (M ¼ Mn, Fe, and Co) electrocatalysts with high oxygen evolution reaction activity, J. Mater. Chem. 3 (2015) 16604e16612.
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