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Enhanced cycling stability of Li-rich nanotube cathodes by 3D graphene hierarchical architectures for Li-ion batteries
Acta Materialia 112 (2016) 11e19
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Enhanced cycling stability of Li-rich nanotube cathodes by 3D graphene hierarchical architectures for Li-ion batteries Dingtao Ma 1, Yongliang Li 1, Maosheng Wu, Libo Deng, Xiangzhong Ren, Peixin Zhang* School of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518060, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2016 Received in revised form 4 April 2016 Accepted 4 April 2016
A hybrid composite of Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) wrapped with reduced graphene oxide (RGO) nanosheets (LMNCO@RGO) was prepared as cathode for lithium-ion batteries. The discharge capacity of the LMNCO@RGO composite is only reducing 3.5% after 100 cycles at 1 C. Such composite which simultaneously combines a high surface area of LMNCO NTs with shorten ionic diffusion pathway and high conductivity of 3D graphene hierarchical architectures as well as structural protection layers, displaying a good cycling stability. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes Reduced graphene oxide nanosheets Electrospinning Cathode
1. Introduction The development of portable electronic devices has led to the increase in the demand for secondary batteries with light weight and high capacity. Under these circumstances, the novel electrodes of Li-ion batteries (LIBs) with better rate capability and cyclic stability must be developed to meet such demand [1,2]. Recent years, the solid solutions of Li2MnO3 and LiMO2 with high capacity as cathode materials for LIBs have captured a great deal of research attention. After the first report of this material, various kinds of the transition metals such as nickel, cobalt and others have been introduced to instead of M sites to improve the electrochemical performance. Such cathode materials can deliver high capacities (>250 mAh g1) and exhibit good electrochemical and thermal stability. However, they still suffer from low rate performance and poor cycle life, which are resulted from inferior electronic conductivity of the active material and phase transformation induced from side reactions, hindering their practical applications [3e5]. Several strategies were performed to improve rate capability and cycling stability. Among them, one-dimensional (1D) nanostructured cathodes, which minimize the Liþ ion transport distance
* Corresponding author. E-mail address: [email protected] (P. Zhang). 1 DM and YL contributed equally to this work. http://dx.doi.org/10.1016/j.actamat.2016.04.010 1359-6454/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
and increase the number of active sites, have been developed [6e8]. The 1D nanostructure could maintain their electrochemical activities for many cycles due to their high surface-to-volume ratios, supporting the effective transport of electrons and Liþ ions [9]. Although the shorten Liþ diffusion pathway benefit the high rate performance, the nanostructures with high specific area would also bring side reactions with the liquid electrolyte, leading to structural collapse and capacity fading during cycling. Recently, our group focused on the electrospinning research to prepare electrode materials of LIBs [10e12]. On one hand, we used ex-situ method to synthesize the LiMn0.54Ni0.13Co0.13O2/C composite. Compared with the pristine particle, the amorphous carbon layer of the composite could restrain the manganese dissolution and suppress the side reactions with electrolyte. On the other hand, we also used in-situ electrospinning method to synthesize the pristine tube-like LiMn0.54Ni0.13Co0.13O2 materials. Compared with the particle materials, such tube-like materials displayed excellent rate capability due to their high surface area and short ionic diffusion pathways. However, the large contact area also leads to the undesirable cycling stability. It has been reported that the conductivity and other properties of cathode materials could usually be modified by employing of carbon materials, such as carbon nanotubes (CNTs) [13e15], carbon coating [16e18] and conducting polymers [19,20], etc. Additionally, graphene has been demonstrated to use as a conducting additive to build a three-dimensional (3D) conducting network as well as structural protection layers [21e23].
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Herein, the Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) employed with reduced graphene oxide (RGO) nanosheets as 3D hierarchical architectures were synthesized to form LMNCO@RGO composite. To the best of our knowledge, this is the first time that LMNCO@RGO composite has been applied as a cathode for LIBs. Such LMNCO@RGO composite shows good cycling stability and improved electrochemical performance, which could be contributed to: 1) 1D LMNCO NTs with high surface area, well-guided charge transfer kinetics with short ionic diffusion pathways; 2) 2D RGO nanosheets with high specific area to greatly improve the conductivity of LMNCO NTs as well as suppresses the side reaction at high voltage. 2. Experimental 2.1. Synthesis of Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) The 1D LMNCO NTs were prepared by a combination of electrospinning process and subsequent heat-treatment. Polyacrylonitrile (PAN) and a stoichiometric amount of lithium acetate, manganese acetate, nickel acetate and cobalt acetate were respectively dissolved in dimethyl-formamide (DMF) and stirring for at least 10 h at room temperature. Then the PAN and LMNCO precursor solutions were mixed and stirred for another 24 h to form an electrospinning solution. The electrospinning process was carried out with a 13 kV high voltage, 1 mL h1 flowing rate, 170 mm needle-to-collector distance, 25% environmental humidity. The electrospun nanofibers were collected on aluminum foil. For heattreatment, the electrospun nanofibers were initially stabilized at 280 C for 4 h and subsequently treated at 850 C for 7 h in air, under a heating rate of 5 C min1. 2.2. Preparation of reduced graphene oxide (RGO) nanosheets Firstly, graphene oxide (GO) was prepared using the modified Hummers method as reported [24]. Then the as-synthesized GO powder was dispersed in de-ionized water and ultrasonicated for 8 h to obtain a homogenized suspension. After that, hydrazine hydrate was added into the suspension and the mixture was heated at 80 C for 24 h with continuous reflux with cold water. Finally, the RGO was collected by centrifuging and heated at 80 C for another 24 h. 2.3. Preparation of LMNCO@RGO Firstly, the as-prepared LMNCO NTs and RGO nanosheets with a mass ratio of 20 to 1 were respectively dispersed in absolute ethyl alcohol. Then the RGO solution was ultrasonicated for 2 h and added into the LMNCO NTs dispersion. After moderate stirring at 65 C for 1 h, the final product was obtained subsequently dried at 85 C to remove the residual solvent. 2.4. Materials characterization X-ray diffraction (XRD, Bruker, D8 Advance) with CuKa radiation operated at 40 kV and 200 mA, scanning electron microscope (SEM, JSM-7800F&TEAM Octane Plus) observation coupled with Energy dispersive x-ray spectroscope (EDX), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30) were used to characterize the crystal structure and morphologies, respectively. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) analysis was conducted to identify the chemical valence states of the main metal elements. Furthermore, the chemical composition of LMNCO NTs sample was measured quantitatively by inductively coupled plasma
spectroscopy (ICP, OPTIMA2100DV, USA), and the experimental result (nLi:nMn:nNi:nCo ¼ 1.195:0.543:0.127:0.132) confirms the successfully synthesis as designed (nLi:nMn:nNi:nCo ¼ 1.2:0.54:0.13: 0.13). With regard to the preparation of TEM samples, trace amount of these samples were respectively added into ethanol solution with ultrasonic dispersion for 1 h, and then dropped to the micro Cu grid and dried at 60 C for another 0.5 h. 2.5. Electrochemical measurements The electrochemical properties of the LMNCO NTs and LMNCO@RGO composite as cathode materials, with Li metal anode in 1 M LiPF6 in EC:DMC (1:1 v/v) electrolyte, were tested using CR2032 coin cells, which were assembled in a glove box (MBRAUN, UNILab2000) filled with high purity argon. Both of the two samples were mixed with 10 wt% of PVDF and 10 wt% of acetylene black. The mass loading of the samples were about 1.4 mg/cm2. Cyclic tests were conducted by using battery station (LAND, CT2001A) between a voltage range of 2.0e4.8 V (1 C ¼ 250 mA g1). Cyclic voltammetry (CV) was recorded on electrochemical workstation (CHI660A) with a scanning of rate of 1 mV s1 and electrochemical impedance spectroscopy (EIS) were performed using the electrochemical workstation by applying a voltage of 5 mV in the frequency range from 100 kHz to 0.1 Hz. All of the electrochemical tests were carried out at room temperature. 3. Results and discussion The overall preparation process of the LMNCO@RGO composite is presented in Fig. 1. Firstly, the LMNCO precursor fibers were synthesized by using electrospinning method. Secondly, the LMNCO NTs were obtained from the precursor fibers by calcinations under air. Finally, the LMNCO NTs and RGO nanosheets were then converted to LMNCO@RGO composite through ultrasonicating the mixture solution of LMNCO NTs and RGO nanosheets. The as-electrospun LMNCO precursor fibers exhibit smooth surface with diameter of ca. (600 ± 50) nm and lengths of hundreds of micrometer as shown in Fig. 2a. With the calcinations, the polymer and acetate salts are decomposed, resulting in a reduction in the diameter and length to (250 ± 50) nm and several micrometers, respectively (Fig. 2b). The RGO displays a sheet-like morphology after exfoliating from the GO (Fig. 2c), which is in line with previous reports [25,26]. After the hybridization of LMNCO NTs and RGO nanosheets in a mixed solution, the LMNCO@RGO composites in a stacked structure were prepared (Fig. 2d), and the LMNCO NTs are relatively uniform distributed over the RGO nanosheets. Fig. 3 shows the EDX mapping of LMNCO@RGO composite. It is obvious that the mapping of carbon element has a similar distribution with Mn, Co and Ni elements. This indicates that RGO nanosheets are uniformly mixed with LMNCO NTs. XRD characterization was conducted to investigate the crystal structure of these two samples. Fig. 4a displays the XRD patterns of the LMNCO NTs and LMNCO@RGO composite. Both of the patterns can be indexed to a hexagonal a-NaFeO2 structure with the R3m space group [27,28]. The splits of (006)/(012) and (018)/(110) peaks indicate a well-layered structure and good ordering [11,12]. In addition, the weak peaks observed around 20e23 should be considered as a superstructure similar to the monoclinic Li2MnO3 section [29]. As can be seen, the RGO nanosheets introduced sample does not contain any other peaks than those of the LMNCO NTs. These results indicate that the RGO nanosheets (Fig. S1) employing procedure does not affect the crystal structure of the cathode material. The Raman spectra of the LMNCO NTs and LMNCO@RGO composite are shown in Fig. 4b. Obviously, the bands
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Fig. 1. Illustration of the preparation process of LMNCO NTs@RGO composites.
Fig. 2. SEM images of a) as-electrospun precursor fibers, b) LMNCO NTs, c) RGO nanosheets, d) LMNCO@RGO composite.
of Eg and A1g located at 487.5 and 598 cm1 should be attributed to the LiMO2 layered structure [30,31]. Compared with LMNCO NTs, the LMNCO@RGO composite exists another two bands at 1345 and 1593 cm1, which should be attributed to the D band and G band of graphene, respectively [32]. Such results are also consistent with the pristine RGO spectrum shown in Fig. S2. The detailed structure of the LMNCO NTs and LMNCO@RGO composite were examined by TEM/HRTEM analysis. As shown in Fig. 5a, the contrast between the dark edges and the light centre indicates the formation of hollow structure. Due to the different
decomposition rate of the acetates in the core and PAN at the surface, the produced gases in the core gradually increase the pressure inside the fiber, pushing the metal oxide nanoparticles toward the surface and forming such tubular structure. In addition, the individual LMNCO NTs consisted of numerous LMNCO nanoparticles, also in accordance with the XRD results. And for the LMNCO@RGO composite, the LMNCO was wrapped by the RGO, forming a 3D conducting network which increases the ionic and electronic conductivities (Fig. 5b). The image of Fig. 5c shows many tiny holes (light regions) throughout the LMNCO NTs, and such holes originate
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Fig. 3. EDX elemental mapping of LMNCO@RGO composites.
Fig. 4. a) XRD patterns and b) Raman spectra of the LMNCO NTs and LMNCO@RGO composite.
from the inter-nanoparticle pores and should ensure a high electrode-electrolyte contact area with enhanced electrochemical activity. The observed lattice fringes with d spacing of ca.0.468 nm in the HRTEM image (Fig. 5d), which is in agreement with the (003) interplane spacing of the Li-rich cathode materials [33,34]. To investigate the surface chemical composition and the oxidation states of the C, Mn, Co and Ni atoms in the LMNCO@RGO composites, XPS measurement was performed. The XPS survey spectra of the LMNCO@RGO composite is presented in Fig. S3. As shown in Fig. 6a, the C1s peaks could be divided into four subpeaks: two main peak at binding energy of 284.6 eV and 285.5 eV should
belong to the non-oxygenated carbon species (CeC) and (C]C), whereas the other two minor peaks at binding energies of 287.7 eV and 289.5 eV should be assigned to oxidized carbon species (C]O) and (OeC]O), respectively [35,36]. Such unsaturated bonds should favor the electron's fast transportation and thus improving the facial electronic conductivity of the pristine. In the Mn2p spectrum (Fig. 6b), these two peaks at binding energies of 643 eV (Mn2p3/2) and 654.5 eV (Mn2p1/2) could be attributed to existence of Mn4þ ions. The Co2p spectrum (Fig. 6c) consisted of two spin-orbit split peaks at binding energy of 780.8 eV (Co2p3/2) and 796 eV (Co2p1/2), demonstrating the presence of Co3þ. Similar to the Co2p spectrum,
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Fig. 5. TEM images (a,c) for LMNCO NTs, and b) for LMNCO@RGO composites, and d) HRTEM images for LMNCO NTs.
the Ni2p spectrum (Fig. 6d) contained two main spin-orbit split peaks of Co2p3/2 at 855.3 eV and Co2p1/2 at 872.5 eV, corresponding to the presence of Ni2þ [37]. Fig. 7a shows the discharge cycling performance of the LMNCO NTs and LMNCO@RGO composite at a current density of 250 mA g1 for 100 cycles. The capacity of the LMNCO NTs decrease from 197 to 145 mAh g1 with capacity retention of 73.6%. In a sharp contrast, the LMNCO@RGO composite electrode maintains a high reversibility and only fades 3.5% capacity after 100 cycles, exhibits excellent cyclic stability. Although the unique tube-like structure of LMNCO, which shorten the diffusion pathway for both electrons and Liþ ions, offers more active sites for electrochemical reactions. However, such high electrode-electrolyte contact area may also brings more undesirable side reactions, like Mn3þ ions with liquid electrolyte, and thus leading structural collapse and capacity decay (Figs. S4 and S5) [38]. Fig. 7b shows a rate test at incremental rate from 0.05 to 5 C, and then back to 1 C. When at lower rate for both samples, similar discharge capacity can be observed, while differences increase gradually with the increasing rate. As can be seen, the rate capability for LMNCO@RGO composite has been significantly enhanced compared to the pristine sample. Furthermore, different charge-discharge curves of these two samples were also shown in Fig. 7ced. To compare these two samples, the LMNCO NTs encounter obvious voltage decay after 100 cycles, which leads to undesirable capacity retention due to the serious polarization phenomenon. Such contrast might be related to existence of 3D graphene network, this flexible interleaved structure favors fast electron and ion transport, as well as effectively limiting the volume expansion and detachment of LMNCO NTs during cycling. To evaluate the electrochemical properties of the two samples, the CV tests are shown in Fig. 8a. It is quite clearly that both of them exhibit analogous plateau-like voltage profiles. One distinguishable
oxidation peaks at approximately 4 V, which can be attributed to the oxidation of Ni2þ and Co3þ to Ni3þ and Co4þ, respectively [39]. Correspondingly, a reduction peak at about 3.7 V is present due to the reduction of the Ni3þ and Co4þ. Oxidation during the charge process and reduction during the discharge process for LMNCO@RGO sample begins at lower (3.966 V) and higher (3.745 V) voltages, respectively, compare to those of the LMNCO NTs sample (4.029 and 3.668 V, respectively). The potential difference between the oxidation peak and the reduction peak for LMNCO@RGO is smaller than that of the LMNCO NTs sample, which indicates that the electrode polarization for the LMNCO@RGO sample can be reduced by incorporating the graphene nanosheets into LMNCO NTs and thus improving the electrochemical performance. Fig. 8b shows the cycling stability comparison of LMNCO@RGO with the Li-rich particles which were previously reported [40e42]. As can be seen, the LMNCO@RGO composite sample displays better cycling stability and higher discharge capacity than others. On one hand, the higher capacity should be contributed to high active surface area with short ionic diffusion pathways. On the other hand, the better cycling stability could be benefited from the porous architecture as well as the RGO nanosheets introduction. As shown in Fig. 8c, for the particle-like cathode material which was composed of LMNCO particles (model one), the Liþ movement during the electrochemical reactions would lead to the structural expansion and contraction, and the produced stress will bring to the structural collapse and thus influencing the cycling stability. Unlike model one, the model two which describing for LMNCO@RGO composite shows a better structural substrate to ensure the reaction stability. The synergistic effect of porous architecture (as shown in Fig. 5c) with RGO nanosheets wrapping can help to alleviate the structure volume changes caused by the lithium insertion/extraction. Besides, RGO nanosheets with high specific area could also suppresses
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Fig. 6. XPS spectra of the a) C1s, b) Mn2p, c) Co2p, and d) Ni2p of the LMNCO@RGO composites.
the side reaction at high voltage, which significantly improves the cyclability of the LIBs. To confirm our analysis and further understand the effect of RGO nanosheets on enhancing electrochemical performance, EIS were collected in the frequency range of 100 kHz to 0.1 Hz. The impedance spectra can be interpreted on the basis of an equivalent circuit (inset of Fig. 9). The small interrupt in the high frequency range is related to the electrolyte impedance (RU). Rs represents the impedance of Liþ diffusion in the surface layer of the composite cathode and corresponding to the semicircle in the high frequency. Rct is the charge-transfer resistance and corresponding to the semicircle in the intermediate frequency region. The sloped straight line in the low frequency range stands for the Warburg impedance Zw [43,44]. The values of RU, Rs and Rct after 50 cycles were fitted and summarized in Table S1. Compared with the LMNCO NTs and LMNCO@RGO composite electrode, there was a huge reduction of Rct for the LMNCO@RGO composite electrode upon cycling. After 50 cycles, the Rct value of LMNCO NTs sample was two times higher than that of the LMNCO@RGO composite. The remarkable decrease in the value of Rct should be attributed to increase in electronic conductivity and reduction of polarization, which were benefit from the 3D RGO nanosheet hierarchical architectures. In addition, the slope of LMNCO@RGO sample is larger than that of LMNCO sample, indicating that the introduction of graphene nanosheets enhances rapid Liþ ion transport during the lithium insertion/extraction reactions, therefore, increases the electrochemical performance [45].
Such results are also consistent with the cyclic voltammetry tests shown in Fig. 7c.
4. Conclusions In summary, LMNCO@RGO composite cathode materials were prepared by a simple hybridization of one-dimensional Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) and twodimensional reduced graphene oxide (RGO) nanosheets. The structural characterization confirmed the as-prepared LMNCO NTs possess a well layered structure. The LMNCO@RGO composite electrode exhibited excellent cyclic stability and better rate capability compared with the bare LMNCO NTs electrode. Such enhanced electrochemical properties are related to the unique structural features obtained through the combination of LMNCO NTs and RGO nanosheets. On one hand, such one-dimensional LMNCO NTs could shorten the Liþ ions diffusion path, accelerate electrochemical behavior and enhance the rate performance. On the other hand, the adding two-dimensional RGO nanosheets can form a highly conducting network to enhance the electron transports rates, more importantly; they can buffer the grain volume change, reducing the side reactions between the active material and electrolyte, protecting the 1D nanotube structure during cycling. Therefore, the superior electrochemical performance and facile synthesis procedure expect the LMNCO@RGO composite as promising cathode material for lithium-ion batteries.
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Fig. 7. a) Cycling performance, b) Rate performance and c)-d) charge-discharge curves of the LMNCO@RGO composite and LMNCO NTs.
Fig. 8. a) CV curves of the LMNCO NTs and LMNCO@RGO composites, b) Cycling stability comparison with other previous reports at 1 C during 100 cycles and c) the structure change illustration of particle-like sample and LMNCO@RGO sample.
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Fig. 9. Impedance test results of the LMNCO NTs and LMNCO@RGO composite a) before electrochemical cycling and b) after 50 cycles.
Acknowledgements We are grateful for the financial support provided by the National Natural Science Foundation of China (Grant #51374146, 50874074, 51502177), the Natural Science Foundation of Guangdong (Grant #2014A030310323), Shenzhen Government's Plan of Science and Technology (Grant #JCYJ20120613173950029, JCYJ20140418095735619, JCYJ20140418182819158), the Special Fund of the Central Finance for the Development of Local Universities (Grant #000022070149).
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.actamat.2016.04.010.
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Full length article
Enhanced cycling stability of Li-rich nanotube cathodes by 3D graphene hierarchical architectures for Li-ion batteries Dingtao Ma 1, Yongliang Li 1, Maosheng Wu, Libo Deng, Xiangzhong Ren, Peixin Zhang* School of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518060, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2016 Received in revised form 4 April 2016 Accepted 4 April 2016
A hybrid composite of Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) wrapped with reduced graphene oxide (RGO) nanosheets (LMNCO@RGO) was prepared as cathode for lithium-ion batteries. The discharge capacity of the LMNCO@RGO composite is only reducing 3.5% after 100 cycles at 1 C. Such composite which simultaneously combines a high surface area of LMNCO NTs with shorten ionic diffusion pathway and high conductivity of 3D graphene hierarchical architectures as well as structural protection layers, displaying a good cycling stability. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes Reduced graphene oxide nanosheets Electrospinning Cathode
1. Introduction The development of portable electronic devices has led to the increase in the demand for secondary batteries with light weight and high capacity. Under these circumstances, the novel electrodes of Li-ion batteries (LIBs) with better rate capability and cyclic stability must be developed to meet such demand [1,2]. Recent years, the solid solutions of Li2MnO3 and LiMO2 with high capacity as cathode materials for LIBs have captured a great deal of research attention. After the first report of this material, various kinds of the transition metals such as nickel, cobalt and others have been introduced to instead of M sites to improve the electrochemical performance. Such cathode materials can deliver high capacities (>250 mAh g1) and exhibit good electrochemical and thermal stability. However, they still suffer from low rate performance and poor cycle life, which are resulted from inferior electronic conductivity of the active material and phase transformation induced from side reactions, hindering their practical applications [3e5]. Several strategies were performed to improve rate capability and cycling stability. Among them, one-dimensional (1D) nanostructured cathodes, which minimize the Liþ ion transport distance
* Corresponding author. E-mail address: [email protected] (P. Zhang). 1 DM and YL contributed equally to this work. http://dx.doi.org/10.1016/j.actamat.2016.04.010 1359-6454/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
and increase the number of active sites, have been developed [6e8]. The 1D nanostructure could maintain their electrochemical activities for many cycles due to their high surface-to-volume ratios, supporting the effective transport of electrons and Liþ ions [9]. Although the shorten Liþ diffusion pathway benefit the high rate performance, the nanostructures with high specific area would also bring side reactions with the liquid electrolyte, leading to structural collapse and capacity fading during cycling. Recently, our group focused on the electrospinning research to prepare electrode materials of LIBs [10e12]. On one hand, we used ex-situ method to synthesize the LiMn0.54Ni0.13Co0.13O2/C composite. Compared with the pristine particle, the amorphous carbon layer of the composite could restrain the manganese dissolution and suppress the side reactions with electrolyte. On the other hand, we also used in-situ electrospinning method to synthesize the pristine tube-like LiMn0.54Ni0.13Co0.13O2 materials. Compared with the particle materials, such tube-like materials displayed excellent rate capability due to their high surface area and short ionic diffusion pathways. However, the large contact area also leads to the undesirable cycling stability. It has been reported that the conductivity and other properties of cathode materials could usually be modified by employing of carbon materials, such as carbon nanotubes (CNTs) [13e15], carbon coating [16e18] and conducting polymers [19,20], etc. Additionally, graphene has been demonstrated to use as a conducting additive to build a three-dimensional (3D) conducting network as well as structural protection layers [21e23].
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Herein, the Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) employed with reduced graphene oxide (RGO) nanosheets as 3D hierarchical architectures were synthesized to form LMNCO@RGO composite. To the best of our knowledge, this is the first time that LMNCO@RGO composite has been applied as a cathode for LIBs. Such LMNCO@RGO composite shows good cycling stability and improved electrochemical performance, which could be contributed to: 1) 1D LMNCO NTs with high surface area, well-guided charge transfer kinetics with short ionic diffusion pathways; 2) 2D RGO nanosheets with high specific area to greatly improve the conductivity of LMNCO NTs as well as suppresses the side reaction at high voltage. 2. Experimental 2.1. Synthesis of Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) The 1D LMNCO NTs were prepared by a combination of electrospinning process and subsequent heat-treatment. Polyacrylonitrile (PAN) and a stoichiometric amount of lithium acetate, manganese acetate, nickel acetate and cobalt acetate were respectively dissolved in dimethyl-formamide (DMF) and stirring for at least 10 h at room temperature. Then the PAN and LMNCO precursor solutions were mixed and stirred for another 24 h to form an electrospinning solution. The electrospinning process was carried out with a 13 kV high voltage, 1 mL h1 flowing rate, 170 mm needle-to-collector distance, 25% environmental humidity. The electrospun nanofibers were collected on aluminum foil. For heattreatment, the electrospun nanofibers were initially stabilized at 280 C for 4 h and subsequently treated at 850 C for 7 h in air, under a heating rate of 5 C min1. 2.2. Preparation of reduced graphene oxide (RGO) nanosheets Firstly, graphene oxide (GO) was prepared using the modified Hummers method as reported [24]. Then the as-synthesized GO powder was dispersed in de-ionized water and ultrasonicated for 8 h to obtain a homogenized suspension. After that, hydrazine hydrate was added into the suspension and the mixture was heated at 80 C for 24 h with continuous reflux with cold water. Finally, the RGO was collected by centrifuging and heated at 80 C for another 24 h. 2.3. Preparation of LMNCO@RGO Firstly, the as-prepared LMNCO NTs and RGO nanosheets with a mass ratio of 20 to 1 were respectively dispersed in absolute ethyl alcohol. Then the RGO solution was ultrasonicated for 2 h and added into the LMNCO NTs dispersion. After moderate stirring at 65 C for 1 h, the final product was obtained subsequently dried at 85 C to remove the residual solvent. 2.4. Materials characterization X-ray diffraction (XRD, Bruker, D8 Advance) with CuKa radiation operated at 40 kV and 200 mA, scanning electron microscope (SEM, JSM-7800F&TEAM Octane Plus) observation coupled with Energy dispersive x-ray spectroscope (EDX), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30) were used to characterize the crystal structure and morphologies, respectively. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) analysis was conducted to identify the chemical valence states of the main metal elements. Furthermore, the chemical composition of LMNCO NTs sample was measured quantitatively by inductively coupled plasma
spectroscopy (ICP, OPTIMA2100DV, USA), and the experimental result (nLi:nMn:nNi:nCo ¼ 1.195:0.543:0.127:0.132) confirms the successfully synthesis as designed (nLi:nMn:nNi:nCo ¼ 1.2:0.54:0.13: 0.13). With regard to the preparation of TEM samples, trace amount of these samples were respectively added into ethanol solution with ultrasonic dispersion for 1 h, and then dropped to the micro Cu grid and dried at 60 C for another 0.5 h. 2.5. Electrochemical measurements The electrochemical properties of the LMNCO NTs and LMNCO@RGO composite as cathode materials, with Li metal anode in 1 M LiPF6 in EC:DMC (1:1 v/v) electrolyte, were tested using CR2032 coin cells, which were assembled in a glove box (MBRAUN, UNILab2000) filled with high purity argon. Both of the two samples were mixed with 10 wt% of PVDF and 10 wt% of acetylene black. The mass loading of the samples were about 1.4 mg/cm2. Cyclic tests were conducted by using battery station (LAND, CT2001A) between a voltage range of 2.0e4.8 V (1 C ¼ 250 mA g1). Cyclic voltammetry (CV) was recorded on electrochemical workstation (CHI660A) with a scanning of rate of 1 mV s1 and electrochemical impedance spectroscopy (EIS) were performed using the electrochemical workstation by applying a voltage of 5 mV in the frequency range from 100 kHz to 0.1 Hz. All of the electrochemical tests were carried out at room temperature. 3. Results and discussion The overall preparation process of the LMNCO@RGO composite is presented in Fig. 1. Firstly, the LMNCO precursor fibers were synthesized by using electrospinning method. Secondly, the LMNCO NTs were obtained from the precursor fibers by calcinations under air. Finally, the LMNCO NTs and RGO nanosheets were then converted to LMNCO@RGO composite through ultrasonicating the mixture solution of LMNCO NTs and RGO nanosheets. The as-electrospun LMNCO precursor fibers exhibit smooth surface with diameter of ca. (600 ± 50) nm and lengths of hundreds of micrometer as shown in Fig. 2a. With the calcinations, the polymer and acetate salts are decomposed, resulting in a reduction in the diameter and length to (250 ± 50) nm and several micrometers, respectively (Fig. 2b). The RGO displays a sheet-like morphology after exfoliating from the GO (Fig. 2c), which is in line with previous reports [25,26]. After the hybridization of LMNCO NTs and RGO nanosheets in a mixed solution, the LMNCO@RGO composites in a stacked structure were prepared (Fig. 2d), and the LMNCO NTs are relatively uniform distributed over the RGO nanosheets. Fig. 3 shows the EDX mapping of LMNCO@RGO composite. It is obvious that the mapping of carbon element has a similar distribution with Mn, Co and Ni elements. This indicates that RGO nanosheets are uniformly mixed with LMNCO NTs. XRD characterization was conducted to investigate the crystal structure of these two samples. Fig. 4a displays the XRD patterns of the LMNCO NTs and LMNCO@RGO composite. Both of the patterns can be indexed to a hexagonal a-NaFeO2 structure with the R3m space group [27,28]. The splits of (006)/(012) and (018)/(110) peaks indicate a well-layered structure and good ordering [11,12]. In addition, the weak peaks observed around 20e23 should be considered as a superstructure similar to the monoclinic Li2MnO3 section [29]. As can be seen, the RGO nanosheets introduced sample does not contain any other peaks than those of the LMNCO NTs. These results indicate that the RGO nanosheets (Fig. S1) employing procedure does not affect the crystal structure of the cathode material. The Raman spectra of the LMNCO NTs and LMNCO@RGO composite are shown in Fig. 4b. Obviously, the bands
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Fig. 1. Illustration of the preparation process of LMNCO NTs@RGO composites.
Fig. 2. SEM images of a) as-electrospun precursor fibers, b) LMNCO NTs, c) RGO nanosheets, d) LMNCO@RGO composite.
of Eg and A1g located at 487.5 and 598 cm1 should be attributed to the LiMO2 layered structure [30,31]. Compared with LMNCO NTs, the LMNCO@RGO composite exists another two bands at 1345 and 1593 cm1, which should be attributed to the D band and G band of graphene, respectively [32]. Such results are also consistent with the pristine RGO spectrum shown in Fig. S2. The detailed structure of the LMNCO NTs and LMNCO@RGO composite were examined by TEM/HRTEM analysis. As shown in Fig. 5a, the contrast between the dark edges and the light centre indicates the formation of hollow structure. Due to the different
decomposition rate of the acetates in the core and PAN at the surface, the produced gases in the core gradually increase the pressure inside the fiber, pushing the metal oxide nanoparticles toward the surface and forming such tubular structure. In addition, the individual LMNCO NTs consisted of numerous LMNCO nanoparticles, also in accordance with the XRD results. And for the LMNCO@RGO composite, the LMNCO was wrapped by the RGO, forming a 3D conducting network which increases the ionic and electronic conductivities (Fig. 5b). The image of Fig. 5c shows many tiny holes (light regions) throughout the LMNCO NTs, and such holes originate
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Fig. 3. EDX elemental mapping of LMNCO@RGO composites.
Fig. 4. a) XRD patterns and b) Raman spectra of the LMNCO NTs and LMNCO@RGO composite.
from the inter-nanoparticle pores and should ensure a high electrode-electrolyte contact area with enhanced electrochemical activity. The observed lattice fringes with d spacing of ca.0.468 nm in the HRTEM image (Fig. 5d), which is in agreement with the (003) interplane spacing of the Li-rich cathode materials [33,34]. To investigate the surface chemical composition and the oxidation states of the C, Mn, Co and Ni atoms in the LMNCO@RGO composites, XPS measurement was performed. The XPS survey spectra of the LMNCO@RGO composite is presented in Fig. S3. As shown in Fig. 6a, the C1s peaks could be divided into four subpeaks: two main peak at binding energy of 284.6 eV and 285.5 eV should
belong to the non-oxygenated carbon species (CeC) and (C]C), whereas the other two minor peaks at binding energies of 287.7 eV and 289.5 eV should be assigned to oxidized carbon species (C]O) and (OeC]O), respectively [35,36]. Such unsaturated bonds should favor the electron's fast transportation and thus improving the facial electronic conductivity of the pristine. In the Mn2p spectrum (Fig. 6b), these two peaks at binding energies of 643 eV (Mn2p3/2) and 654.5 eV (Mn2p1/2) could be attributed to existence of Mn4þ ions. The Co2p spectrum (Fig. 6c) consisted of two spin-orbit split peaks at binding energy of 780.8 eV (Co2p3/2) and 796 eV (Co2p1/2), demonstrating the presence of Co3þ. Similar to the Co2p spectrum,
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Fig. 5. TEM images (a,c) for LMNCO NTs, and b) for LMNCO@RGO composites, and d) HRTEM images for LMNCO NTs.
the Ni2p spectrum (Fig. 6d) contained two main spin-orbit split peaks of Co2p3/2 at 855.3 eV and Co2p1/2 at 872.5 eV, corresponding to the presence of Ni2þ [37]. Fig. 7a shows the discharge cycling performance of the LMNCO NTs and LMNCO@RGO composite at a current density of 250 mA g1 for 100 cycles. The capacity of the LMNCO NTs decrease from 197 to 145 mAh g1 with capacity retention of 73.6%. In a sharp contrast, the LMNCO@RGO composite electrode maintains a high reversibility and only fades 3.5% capacity after 100 cycles, exhibits excellent cyclic stability. Although the unique tube-like structure of LMNCO, which shorten the diffusion pathway for both electrons and Liþ ions, offers more active sites for electrochemical reactions. However, such high electrode-electrolyte contact area may also brings more undesirable side reactions, like Mn3þ ions with liquid electrolyte, and thus leading structural collapse and capacity decay (Figs. S4 and S5) [38]. Fig. 7b shows a rate test at incremental rate from 0.05 to 5 C, and then back to 1 C. When at lower rate for both samples, similar discharge capacity can be observed, while differences increase gradually with the increasing rate. As can be seen, the rate capability for LMNCO@RGO composite has been significantly enhanced compared to the pristine sample. Furthermore, different charge-discharge curves of these two samples were also shown in Fig. 7ced. To compare these two samples, the LMNCO NTs encounter obvious voltage decay after 100 cycles, which leads to undesirable capacity retention due to the serious polarization phenomenon. Such contrast might be related to existence of 3D graphene network, this flexible interleaved structure favors fast electron and ion transport, as well as effectively limiting the volume expansion and detachment of LMNCO NTs during cycling. To evaluate the electrochemical properties of the two samples, the CV tests are shown in Fig. 8a. It is quite clearly that both of them exhibit analogous plateau-like voltage profiles. One distinguishable
oxidation peaks at approximately 4 V, which can be attributed to the oxidation of Ni2þ and Co3þ to Ni3þ and Co4þ, respectively [39]. Correspondingly, a reduction peak at about 3.7 V is present due to the reduction of the Ni3þ and Co4þ. Oxidation during the charge process and reduction during the discharge process for LMNCO@RGO sample begins at lower (3.966 V) and higher (3.745 V) voltages, respectively, compare to those of the LMNCO NTs sample (4.029 and 3.668 V, respectively). The potential difference between the oxidation peak and the reduction peak for LMNCO@RGO is smaller than that of the LMNCO NTs sample, which indicates that the electrode polarization for the LMNCO@RGO sample can be reduced by incorporating the graphene nanosheets into LMNCO NTs and thus improving the electrochemical performance. Fig. 8b shows the cycling stability comparison of LMNCO@RGO with the Li-rich particles which were previously reported [40e42]. As can be seen, the LMNCO@RGO composite sample displays better cycling stability and higher discharge capacity than others. On one hand, the higher capacity should be contributed to high active surface area with short ionic diffusion pathways. On the other hand, the better cycling stability could be benefited from the porous architecture as well as the RGO nanosheets introduction. As shown in Fig. 8c, for the particle-like cathode material which was composed of LMNCO particles (model one), the Liþ movement during the electrochemical reactions would lead to the structural expansion and contraction, and the produced stress will bring to the structural collapse and thus influencing the cycling stability. Unlike model one, the model two which describing for LMNCO@RGO composite shows a better structural substrate to ensure the reaction stability. The synergistic effect of porous architecture (as shown in Fig. 5c) with RGO nanosheets wrapping can help to alleviate the structure volume changes caused by the lithium insertion/extraction. Besides, RGO nanosheets with high specific area could also suppresses
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Fig. 6. XPS spectra of the a) C1s, b) Mn2p, c) Co2p, and d) Ni2p of the LMNCO@RGO composites.
the side reaction at high voltage, which significantly improves the cyclability of the LIBs. To confirm our analysis and further understand the effect of RGO nanosheets on enhancing electrochemical performance, EIS were collected in the frequency range of 100 kHz to 0.1 Hz. The impedance spectra can be interpreted on the basis of an equivalent circuit (inset of Fig. 9). The small interrupt in the high frequency range is related to the electrolyte impedance (RU). Rs represents the impedance of Liþ diffusion in the surface layer of the composite cathode and corresponding to the semicircle in the high frequency. Rct is the charge-transfer resistance and corresponding to the semicircle in the intermediate frequency region. The sloped straight line in the low frequency range stands for the Warburg impedance Zw [43,44]. The values of RU, Rs and Rct after 50 cycles were fitted and summarized in Table S1. Compared with the LMNCO NTs and LMNCO@RGO composite electrode, there was a huge reduction of Rct for the LMNCO@RGO composite electrode upon cycling. After 50 cycles, the Rct value of LMNCO NTs sample was two times higher than that of the LMNCO@RGO composite. The remarkable decrease in the value of Rct should be attributed to increase in electronic conductivity and reduction of polarization, which were benefit from the 3D RGO nanosheet hierarchical architectures. In addition, the slope of LMNCO@RGO sample is larger than that of LMNCO sample, indicating that the introduction of graphene nanosheets enhances rapid Liþ ion transport during the lithium insertion/extraction reactions, therefore, increases the electrochemical performance [45].
Such results are also consistent with the cyclic voltammetry tests shown in Fig. 7c.
4. Conclusions In summary, LMNCO@RGO composite cathode materials were prepared by a simple hybridization of one-dimensional Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) and twodimensional reduced graphene oxide (RGO) nanosheets. The structural characterization confirmed the as-prepared LMNCO NTs possess a well layered structure. The LMNCO@RGO composite electrode exhibited excellent cyclic stability and better rate capability compared with the bare LMNCO NTs electrode. Such enhanced electrochemical properties are related to the unique structural features obtained through the combination of LMNCO NTs and RGO nanosheets. On one hand, such one-dimensional LMNCO NTs could shorten the Liþ ions diffusion path, accelerate electrochemical behavior and enhance the rate performance. On the other hand, the adding two-dimensional RGO nanosheets can form a highly conducting network to enhance the electron transports rates, more importantly; they can buffer the grain volume change, reducing the side reactions between the active material and electrolyte, protecting the 1D nanotube structure during cycling. Therefore, the superior electrochemical performance and facile synthesis procedure expect the LMNCO@RGO composite as promising cathode material for lithium-ion batteries.
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Fig. 7. a) Cycling performance, b) Rate performance and c)-d) charge-discharge curves of the LMNCO@RGO composite and LMNCO NTs.
Fig. 8. a) CV curves of the LMNCO NTs and LMNCO@RGO composites, b) Cycling stability comparison with other previous reports at 1 C during 100 cycles and c) the structure change illustration of particle-like sample and LMNCO@RGO sample.
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Fig. 9. Impedance test results of the LMNCO NTs and LMNCO@RGO composite a) before electrochemical cycling and b) after 50 cycles.
Acknowledgements We are grateful for the financial support provided by the National Natural Science Foundation of China (Grant #51374146, 50874074, 51502177), the Natural Science Foundation of Guangdong (Grant #2014A030310323), Shenzhen Government's Plan of Science and Technology (Grant #JCYJ20120613173950029, JCYJ20140418095735619, JCYJ20140418182819158), the Special Fund of the Central Finance for the Development of Local Universities (Grant #000022070149).
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.actamat.2016.04.010.
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