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Article Cite This: ACS Omega 2018, 3, 4620−4630
Synthesis and Application of Phosphorus/Co3O4−CuO Hybrid as High-Performance Anode Materials for Lithium-Ion Batteries Navid Zamani,*,† Ali Reza Modarresi-Alam,*,† and Meissam Noroozifar† †
Department of Chemistry, University of Sistan and Baluchestan, Zahedan 9816745785, Iran S Supporting Information *
ABSTRACT: The present study reports a novel red phosphorus (RP)/Co3O4−CuO hybrid as a high-performance anode material for lithium ion battery that was successfully synthesized by simple sol gel method and followed by facile ball milling of red phosphorus. Herein, we outstandingly improved practical application of RP anode (with its natural insulation property and rapid capacity decay in during the lithiation process) in lithium-ion batteries (LIBs) by confining nanosized amorphous RP into the Co3O4−CuO nanoparticle while RP can improve the electrochemical capacity returning and increased capacity of composite in high current density. This bonding can help maintain electrical contact, prevent to escape RP from the electrode and confirm the solid electrolyte interphase upon the large volume change of RP during cycling. As a result, by judicious usage of components in the RP/Co3O4− CuO hybrid nanostructured anode was achieved an initial Coulombic efficiency of 99.8% at a current density of 50 mA g−1 and an enhanced cycling stability (683.63 and 470.11 mAh g−1 after 60 cycles at a density of 0.1 and 1 A g1−) with interesting cycling capacity at high current density of 3 Ag1− (333.81 mAh g−1). Moreover, the composite electrode can still deliver a specific capacity of about 97.4% of initial capacity after cycling at high rates and returning to the initial current density of 0.1 A g1−.
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INTRODUCTION Looking forward to the energy future of portable electronics and hybrid electric vehicles (HEV), renewable energy will likely be the most tolerable way to solve many feasibility and environmental problems. Massive efforts have been made for satisfying the great challenge of find a cheap and efficient way of storing the variable renewable energy. In the past two decades, LIBs have achieved great attention because of their relatively high energy density and long cycle lifetime.1−6 Therefore, the study of various electrode materials will be great significant to develop LIBs with higher energy density, practical reliability, and long cycle stability.7−10 Much research has been focused novel anode materials for LIBs, involving carbon-based materials 11,12 (because of their moderate high surface area, excellent electrical conductivity, and appropriate power density), metal oxide anode materials 13,14 (because of their high specific capacity), and intermetallic metal anode materials.15,16 Despite some encouraging progress, the specific capacity of these materials is still low (less than 1000 mAh g−1) and the cycling life is poor and also kinetics of Li-ion and electron transport in electrodes and the interface of electrode/electrolyte is slow.17−19 One of the strategies to overcome these limitations may be using of hybrid nanomaterials. Combination of chemically distinguished materials with their frank characteristics can be a hybrid nanostructure. Separate restrictions or benefits of hybrid nanomaterial dependent to how to control the assembly, fabrication of the constitution, and type of architecture. Hybrid © 2018 American Chemical Society
nanocomposite, because of its synergistic effects of components and multifunctionality, can present desired properties and enhanced performance. Recently, elemental phosphorus (P) has been demonstrated as a particularly attractive anode material for LIBs due to its high theoretical capacity (2595 mAh g−1, assuming a complete reaction).20,21 Between different allotropes of P, red P (RP) received much interest as a good candidate for anode materials in LIBs because of its chemical stability at room temperature and atmosphere. In comparison with other allotropes, RP is commercially available with ease, safe, and abundant.22−24 Nevertheless, this battery performance is still unsatisfactory due to its volume expansion effects (∼300% during Li+ insertion/ extraction that may cause poor electrical contact, pulverization of particles and continuous growth of the solid electrolyte interphase (SEI)) and low electron conductivity (∼1 × 10−14 S/cm making the electrochemical redox reaction difficult) the experimental capacity is limited.25−,28 Lee’s group was one doing pioneering research on P anodes through the high-energy BM of red phosphorus and carbon black.22 Apparently, the results showed that electrochemical performance in LIBs can be improved through a composite of electrically conductive carbon matrix with phosphorus. However, different strategies presented for generating robust Received: August 8, 2017 Accepted: January 5, 2018 Published: April 26, 2018 4620
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density of 3 A g−1. Figure 1 shows a schematic illustration of RP/Co3O4−CuO hybrid that was prepared through a facile and simple BM process.
phosphorus−carbon (P−C) bonds between RP and a variety of carbon materials, including single-walled carbon nanotube composite,29 graphite,30 graphene,31 and reduced graphene oxide.32 Recently, Yu and co-workers have designed a selfsupported and flexible electrode by loading crystalline red P into the porous carbon nanofibers for LIBs with high specific capacity and excellent electrochemical cycling (∼2030 mAh g−1 at 0.1C rate after 100 cycles). The improvement in electrochemical performance of this design is attributed to the special structure design, in which the carbon nanofibers matrix has a distinctive rule due to its good conductivity and easy access of ions.33 On the basis of our extensive research, we believed that to fabricate a novel structure as an anode material that can show good performance, the nanoporosity and downsizing of introducing active materials to the electrode is critical. On the other side, in an interesting study, Zhang and co-workers reported the advantages of reinforced concrete RP/TiO2 composites for LIBs in consideration of achieving complementary effects.34 In such a composite, TiO2 with high safety and outstanding stability but low capacity act as concrete, whereas RP with high capacity plays the role steel. Their designed unique reinforced concrete structure due to synergistic effect of RP and A-TiO2 presented an enhanced cycling capacity of 369 mAh g−1 over 100 cycles as well as an acceptable rate capacity of 202 mAh g−1 at the current density of 1 A g−1. However, these pioneering results inspired us to combine the above-mentioned strategies and use RP accompanying a combination of metal oxides (Co3O4 and CuO) to acquire a novel structure with appropriate electric connection and improved electrochemical performance. In this work, we attempted to propose a novel RP/Co3O4−CuO hybrid nanostructure for LIBs to unexpectedly achieve highly capacity in high current density through incorporation a reasonable value of RP inside Co3O4−CuO nanoparticles during BM (ball milling) process. However, to the best of our knowledge, there have not been any reports on RP/Co3O4−CuO hybrid nanomaterial as anode for LIBs. Particularly, the facility of the provision process and high capacity returning after cycling at high rates (97.4%), which is based on the composite (RP + Co3O4−CuO), are likely to make a great impact in the research on LIBs. On the one hand, rational value of RP is not an obstacle for electric conductive material and this logical amount effectively enhanced the power density and capacity of lithium ions. It means RP reinforcement stored local effect of Co3O4. Seemingly, this enhanced capacity in high current density especially is because of presence RP with different electronegativity that can break the electroneutrality of Co3O4−CuO to create the relative charged sites which are favorable and improve the discharge and charge capacity of Co3O4−CuO in LIBs.35,36 Furthermore, CuO nanoparticles act as robust conductive backrest to maintain and boost the overall conductivity in the total hybrid and also plays a barrier role for preventing particle aggregation and enhancing the material’s tolerance of the large volume change of phosphorus and Co3O4 during volume expansion/contraction. The electrochemical performances of the Co3O4−CuO and RP/Co3O4−CuO nanocomposites were assessed as an anode material for LIBs. The result of unique compositions (RP/Co3O4−CuO hybrid structure) showed the offered strategy could provide an effective and facile technique for improving the cycling stability (470.11 mAh g−1 after 60 cycles at a density of 1.0 Ag−1) with a high rate discharge capacity of 333.81 mAh g−1 at a current
Figure 1. Schematic illustration of the RP/Co3O4−CuO nanohybrid.
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RESULTS AND DISCUSSION The phase’s distinction and also crystallinity of samples were studied by XRD. Figure 2 presented XRD patterns of Co3O4−
Figure 2. XRD pattern of Co3O4−CuO and RP/Co3O4−CuO samples.
CuO and RP/Co3O4−CuO hybrid. In the XRD pattern of RP/ Co3O4−CuO, the characteristic (111), (220), (311), (222), (400), (422), (511), and (440) reflections of crystal faces of the cubic phase Co3O4 (JCPDS no. 42−1467, space group: Fd3m) and also the diffraction peaks due to CuO [indexed to (002), (111), (20−2) and (11−3) planes (JPCDS: 48−1548)] are apperceived. After infiltrating RP into Co3O4−CuO, in comparison to Co3O4−CuO, a diffraction peak at 15° of RP/ Co3O4−CuO appears, which is correspond to RP structure.37 Also, the average crystallite sizes corresponding to RP/Co3O4− CuO sample, increasing from 14.73 to 21.02 nm compared to Co3O4−CuO after introduction of RP. Because the average particle size corresponding to the RP/Co3O4−CuO sample, after introduction of RP, increased only 6.29 nm compared to Co3O4−CuO, it can be deduced that nanosized RP has been successfully introduced into the lattice of the Co3O4−CuO nanoparticle. In the other words, if the particle size of RP was not nanosized, the lattice of the Co3O4−CuO nanoparticle would be more expansive after incorporation with RP during the BM (ball milling) process. However, RP was sustained in our Co3O4−CuO nanocomposite by application of BM, which is well-known as a controlled technique that produces nanosized particles at room temperature. However, we could not directly determine the particle size of RP, but by using the Scherrer equation (D = kλ/ βcos θ, where D is the particle size in nanometers, λ is the 4621
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of Co3O4−CuO and RP/Co3O4−CuO nanoparticles in the 450−2500 cm−1. As shown in Figure 3b, the peaks for the RP/ Co3O4−CuO hybrid in the region of 564 and 660 cm−1 shows characteristic absorption bonds of Co−O and Cu−O vibration which is attributed to Co3O4 and CuO, respectively.38 Interestingly, after impregnation of RP nanopowders into Co3O4−CuO nanoparticles, the peaks intensity at 564 and 660 cm−1 that related to Co3O4 and CuO were decreased. From these intensity changes of peaks in the RP/Co3O4−CuO composites can be understood RP nanopowders exist in pores of Co3O4−CuO nanocomposite. Also, when RP/Co3O4−CuO hybrid exposed to air, a thin layer of phosphorus−based oxides because of oxidation is obviously observed.23 This layer is acknowledged by reflection of a absorption peak around 1619 cm−1 is corresponded to the PO and P−O.39 Moreover, another new peak centered at 1097 cm−1 appears in the FT-IR spectra of the RP/Co3O4−CuO nanohybrid in comparison with Co3O4−CuO nanostructure, suggesting the formation of M (Co/Cu)-O-RP bonds during BM. Nevertheless, because of the detection of some peaks related to the several types of phosphate at this binding energy, it cannot be referred to certainly represented M (Co/Cu)-O-RP bonding. But, it can be expected that M-O-RP bonding occurs around this region.40,41 This chemical interaction induced by bonding between Co3O4−CuO nanoparticles and RP nanopowders increases the capacitance and prevents the loss of electrical contact during electrochemical performance. The morphologies of Co3O4−CuO and RP/Co3O4−CuO nanopowders were investigated by FE-SEM as shown in
wavelength of the radiation (1.54056 Å for CuKa radiation), k is a constant equal to 0.94, β is the peak width at half-maximum intensity and θ is the peak position) and the difference between the particle size of RP/Co3O4−CuO and Co3O4−CuO, we could assess the particle size of RP, which is about 6.3 nm. FT-IR was utilized to further investigate interaction between RP nanopowders and Co3O4−CuO nanostructures and also to analyze bonding of materials. Figure 3 shows the FT-IR spectra
Figure 3. FT-IR spectra of (a) Co3O4−CuO and (b) RP/Co3O4− CuO nanocomposites.
Figure 4. FE-SEM micrographs of (a) Co3O4−CuO and (b) RP/Co3O4−CuO composite. TEM micrograph of (c) Co3O4−CuO and (d) RP/ Co3O4−CuO composite. 4622
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Figure 5. EDS spectra of (a) Co3O4−CuO nanostructure and (b) RP/Co3O4−CuO nanohybrid.
Figure 6. (a) N2 adsorption/desorption isotherms and (b) pore size distribution for Co3O4−CuO nanostructure. (c) N2 adsorption/desorption isotherms and (d) pore size distribution for RP/Co3O4−CuO nanohybrid.
respectively. It can be seen that the particles size of Co3O4− CuO nanostructures were about 10 nm, whereas RP/Co3O4− CuO was up to about 25 nm in size. The particle sizes obtained by TEM were in good agreement with SEM results. TEM images showed that the surface of RP/Co3O4−CuO nanohybrids are rougher than the surface of Co3O4−CuO nanoparticles. Because, after BM, the particle size of Co3O4− CuO did not change significantly in RP/Co3O4−CuO, it can be deduced that most RP has been infiltrated in Co3O4−CuO nanocomposites. This kind of impregnated structure in RP/ Co3O4−CuO enhances capacity value by using of Co3O4−CuO and RP and generates numerous sites to Li-insertion/ extraction, whereas it remains electrically conductive because of the presence of CuO between Co3O4 and RP. By EDX analysis, the distribution of individual elements in the nanocomposites can be demonstrated. As shown in Figure 5a, the EDX spectrum shows the presence of Co (56.03 wt %), Cu (29.93 wt %) and O (14.04 wt %) elements in Co3O4−CuO nanostructure and the chemical elements of Co (43.52 wt %), Cu (32.96 wt %), O (11.29 wt %), and P (12.23 wt %) can be identified in Figure 5b, which are in good accordance with used weight ratio in their synthesis. The EDX results approved the successful synthesis of the Co3O4−CuO and RP/Co3O4−CuO nanomaterials as anode in lithium-ion battery.
Figures 4a, b. Both samples have almost similar morphology and show nearly spherical particles. Interestingly, they also synthesized with a narrow size distributions of diameters ranging from 18 to 22 nm (Figures 4a, b). But in the appearance, the external surface of RP/Co3O4−CuO nanohybrids are rougher than the external surface of Co3O4−CuO nanostructures. After impregnation of RP, the size distribution of diameters of RP/Co3O4−CuO (Figure 4b) remains unchanged in compared with Co3O4−CuO, indicating that most of RP has been successfully encapsulated in pores of Co3O4−CuO. By limiting RP into Co3O4−CuO, diminishes deterioration on electrode materials and enhancements volume capacity. It is believed CuO nanoparticles have a vital role to preserve electric contact between particles and also intercept restacking of Co3O4 nanoparticles during electrochemical behavior. On the other hand, pores of Co3O4−CuO prevent excessive volume expansion of RP during cycling. As a result, this new nanohybrid supplies flexibility to accommodate the volume change of RP particles, reduce the direct contact between RP and electrolyte during Li-insertion/extraction reactions and consequence delivers a limited SEI construction. TEM images were employed to get a deep understanding into the detail nanostructure and also further investigate the morphology of nanocomposites. Figure 4c, d shows the TEM images of Co3O4−CuO and RP/Co3O4−CuO nanoparticles, 4623
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Figure 7. (a) Comparative cyclic voltammograms of Co3O4−CuO nanostructure and RP/Co3O4−CuO nanohybrid electrodes; (b) first-third CV curves of the RP/Co3O4−CuO electrode, CVs were recorded at a potential scan rate of 0.1 mV/s versus a Li+/Li reference electrode in 1 M LiClO4 electrolyte solution. Initial three charge−discharge curves of (c) Co3O4−CuO nanostructure and (d) RP/Co3O4−CuO nanohybrid at a current density of 50 mAg−1.
peaks of RP/Co3O4−CuO, and the potential anodic and cathodic peaks for RP/Co3O4−CuO shift to the side potential, interestingly, the potential separation between the anodic and cathodic peaks for the RP/Co3O4−CuO hybrid composite (about 0.68 V) is lower than those of Co3O4−CuO (about 0.79 V). These broad redox peaks of RP/Co3O4−CuO hybrid composite can be assigned to the presence of RP and its contribution to enhancing the capacity. So, all three components (Co3O4, CuO, and RP) can be contributed to improvement of total capacity. It is expected that added RP (although has insulator property) due to its reasonable amount in composite, reinforce the Co3O4 with increasing capacity through produce different sites and also amplified electrochemical behavior of Co3O4−CuO composites. Apparently, can be said the synergies and strengthening effect of conductive network and porous architecture of Co3O4−CuO and RP powders, are very important to improvement of electrochemical behavior of RP/Co3O4−CuO hybrid composite. Figure 7b shows the first-third CV curves of the RP/Co3O4− CuO electrode within the potential window of 0.02−3 V (vs Li+/Li). In the cathodic process, there is one broad peak located at 0.58−1.1 V, belonging to the reduction of Co3O4 to Co, CuO to Cu and the formation of Li2O and a SEI layer.43−45 Of course, the broad of the peak at 1.2−1.6 V range was decreased in the latter scans, and is most likely due to the electrochemical decomposition of electrolyte during the formation of a SEI film on the RP/Co3O4−CuO electrode surface.40,46 This observation implies that SEI formation occurs mainly during the first cycle and is the irreversible capacity loss. Of course, in the cathodic scan in addition to reaction of Co3O4, CuO, and SEI, there are reactions corresponding to lithium ion insertion and formation of LixP (x = 1−3).47 In the
To determine the BET surface area and the corresponding BJH pore size distribution curves, nitrogen adsorption and desorption were measured. Figure 6 demonstrates the nitrogen adsorption/desorption isotherm and pore size distribution of the two composite samples. The adsorption isotherm of both Co3O4−CuO (Figure 6a) and RP/Co3O4−CuO (Figure 6c) are basically similar in shape, which are a typical type IV with a hysteresis loop, showing the mesoporous structure.42 In comparison RP/Co3O4−CuO with Co3O4−CuO, adsorbed less N2 indicating less specific surface and less pore volume. However, BET surface area and total pore volume are decreased from 32.1 m2/g and 0.1644 cm3/g in Co3O4−CuO nanostructure to 5.42 m2/g and 0.0264 cm3/g in RP/Co3O4− CuO hybrid nanocomposite (Figures 6a and 6c). The results suggest that nonporous RP powder filled into the pores of Co3O4−CuO nanoparticles and blocked partially those pores. The infiltration and intimation contact of RP with Co3O4− CuO nanoparticles during BM leads to the decrease of BET surface area and total pore volume. Also, the BJH pore size (Figures 6b, d) distribution confirmed typical mesopores structures of these composites with shown pores ranges from 2 to 40 nm. All the above-mentioned findings are in good agreement with SEM and TEM results. To scrutinize the influence of using RP on the behavior of electrochemical rate capability of the Co3O4−CuO, we carried out CV. The comparison of typical CV curves (at the 3st) of Co3O4−CuO and RP/Co3O4−CuO electrodes was performed (Figure 7a). As can be seen, both electrodes have similar electrochemical behavior, which confirmed the electrochemical reduction/oxidation reactions in any two electrodes accompany lithiation and delithiation. Although the potential anodic and cathodic peaks for Co3O4−CuO are sharper and narrower than 4624
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The Coulombic efficiency of the RP/Co3O4−CuO composite electrode for third-cycle is 99.7% whereas the Coulombic efficiency of third-cycle for Co3O4−CuO composite is 96%. For comparison, the initial Coulombic efficiency of some electrodes such as nanostructured phosphorus composite,23 P/SWCNT mixture,29 crystalline red phosphorus incorporated with porous carbon nanofibers,33 and P@CMK-3,47 are shown in Figure 8.
anodic scan, there are reactions that correspond to the stepwise delithiation process from lithium insertion state to extraction state of P. It is worth noting that the peaks of the lithium/ delithiation process related to reaction of RP with Li ion also were detected in other RP/carbon material electrodes.47,48 In addition to this, in the broad peak located at around 1.7− 2.4 V, there are reactions corresponding to the reversible oxidation of metallic Co to Co3O4, Cu to CuO, and decomposition of SEI.49,50 However, distinguishing between the anodic/cathodic peaks of Co3O4, CuO and RP is difficult because of their overlap. All of these observed peaks can be associated with the lithiation and delithiation of components, showing that each of them has a relative contribution to the total capacity. Moreover, the anodic/cathodic peak locations were also detected in subsequent scans and almost fixed in latter cycles, revealing that the RP/Co3O4−CuO composite has appropriate redox reversibility, small polarization, and good electron conductivity. It should be noted that, more proper ion diffusion pathway and better contact area is obtained by linking CuO to Co3O4 in Co3O4−CuO. In other words, CuO facilitates the charge transfer and conduction from the electrode/ electrolyte. Furthermore, RP has a synergistic effect on the electrochemical properties of the hybrid composites by improving site access for Li ion. So, we can expect a superiorly of RP/Co3O4−CuO to Co3O4−CuO on the other electrochemical behaviors. The electrochemical lithium storage properties of samples were evaluated by galvanostatic charge/discharge measurements. Figure 7 c, d shows the first, second and third-cycle galvanostatic charge/discharge profile of Co3O4−CuO and RP/ Co3O4−CuO composites at a current density of 50 mAg−1 in the voltage range of 0.02−3.0 V (versus Li+/Li). The Co3O4− CuO sample delivers initial discharge and charge capacity of 1067 and 1043.21 mAhg−1 respectively with a Coulombic efficiency of 97.7%, meanwhile, the RP/Co3O4−CuO sample shows initial discharge and charge capacity 1024.17 and 1022.12 mAhg−1 respectively with an extraordinary specifically Coulombic efficiency of 99.8%. These interesting observations confirmed the CV cures (Figure 7 b). However, can be illustrated the mechanism of Li ion storage for Co3O4−CuO and RP/Co3O4−CuO during the first discharge process as shown below: The first: For Co3O4:46 Co3O4 + 8Li+ + 8e− ⇋ 4Li 2O + 3Co
Figure 8. Coulombic efficiency comparison of various electrodes in literature.
More importantly, the Coulombic efficiency of the RP/Co3O4− CuO for all three galvanostatic charge/discharge cycles is almost equal. Nevertheless, the relatively less discharge and charge capacity for ever cycle of RP/Co3O4−CuO compared with Co3O4−CuO, can be related to the straight formation of SEI film on RP surface which happens in some points of RP/ Co3O4−CuO. But in general, high specific Coulombic efficiency, high sustainability of Coulombic efficiency, relative decline of hysteresis (ΔEV) between the discharge and charge plateaus of RP/Co3O4−CuO nanoarray compared with Co3O4−CuO nanocomposite can be attributed to effective and reasonable presence of RP and created sites with different electronegativity, sensible link of RP and Co3O4 with CuO in the electrode to keep electrical connection between particles and presumably more uniform formation of the SEI layer on the RP/Co3O4−CuO electrode. Figure 9a, b presents the cycling performance and discharge capacity versus cycle number of the Co3O4−CuO and RP/ Co3O4−CuO composites in the voltage range of 0.02 and 3 V at a current density of 0.1 and 1 A g−1. Of course for better attitude, cycling performance at a current density of 1 A g−1 for Co3O4, CuO, and Co3O4−CuO nanostructure was investigated in Figure S1. It can be seen that the capacity drop of Co3O4 is very high, but for CuO, there is less capacity with more relative stability. Compared with single Co3O4 and CuO nanoparticles, Co3O4−CuO nanocomposite exhibited the highest stability and appropriate capacity that attributed to a synergetic contribution from CuO and Co3O4 nanoparticles and surface modification of the nanocomposite. As depicted in Figure 9a, accompanied by the cycle number increasing, the Co3O4−CuO electrode
(1)
For CuO:45 CuO + 2Li+ + 2e− ⇋ Li 2O + Cu
(2)
And for RP20 that is in RP/Co3O4−CuO: P + 3Li+ + 3e− ⇋ Li3P
(3)
And the second is formation of a SEI layer with electrolyte decomposition. The significant Coulombic efficiency and remain stable of Coulombic efficiency of RP/Co3O4−CuO compared with Co3O4−CuO for first discharge and charge capacity is a promising sign of impregnated RP into Co3O4− CuO. As can be seen, the second and third cycles of RP/ Co3O4−CuO in Figure 7b followed the same way, which confirmed the good stability and reversibility of our new nanostructure for LIBs. 4625
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Figure 9. Cycling performance at a current density of 0.1 Ag1− and 1 Ag−1 of (a) Co3O4−CuO nanostructure and (b) RP/Co3O4−CuO nanohybrid. (c) Rate capability of Co3O4−CuO nanostructure and RP/Co3O4−CuO nanohybrid at various current densities.
the effect of different amount of RP (different mass ratio of Co3O4−CuO to RP) on cycling stability at a current density of 0.1 A g−1 upon 60 cycles. Because of the low electrical conductivity of RP, the low and rational amount of it was used in composite as anode material in LIBs. According to Figure S2, when amount of RP increases (or decreases the mass ratio of 20 until 6.7), capacity is also increased in cycling performance after 60 cycles. But, in more mass ratio of 6.7, because of high amount of RP and its negative effective on electrochemical behavior, capacity is reduced remarkably. For the material that is used as electrode, the rate capability is a major factor when assessments for practical application. So, the rate performance of Co3O4−CuO and RP/Co3O4−CuO electrodes were examined at different current density (0.1 to 3 A g−1). As shown in Figure 9c, with the gradually increased densities, Co3O4−CuO electrode delivered an average discharge capacities based on the mass of the composite of 1060.79, 911.05, 790.19, 497.78, and 196.07 mAh g−1 at the current densities of 0.1, 0.2, 0.5, 1.0, and 3 A g−1, respectively. Although for RP/Co3O4−CuO, the discharge capacity based on composite were obtained 1035.99, 925.05, 857.39, 594.53, and 333.81 mAh g−1 when the current densities increase from 0.1, 0.2, 0.5, 1.0 to 3.0 A g−1. For each stage, the discharge process of the samples is compared for 10 cycles. It should be noted that the capacities faded rapidly at higher currents. It is clear that the RP/Co3O4−CuO hybrid displays good long cycling behavior and also a much better discharge capability than Co3O4−CuO electrode at high current rate. In addition, nearly 97.4% of the initial discharge capacity can be still recovered once the rate is restored to the initial 0.1 A g−1 for RP/Co3O4−CuO, which is higher than capacity
delivers capacity retention around 60% of the initial capacity, which can remain over 60 cycles at 0.1 A g−1 (637.35 mAh g−1 at the 60th cycle). In contrast, the RP/Co3O4−CuO hybrid electrode shows a high capacity of 683.63 mAh g−1 after 60 cycles while still keep near 70% initial capacity (Figure 9b). It is interesting that at a current density of 1 A g−1, the RP/Co3O4− CuO hybrid electrode can also deliver a high discharge capacity of 470.11 mAh g−1 even after 60 cycles, whereas the capacity retention remains about 62.7%. Although, the initial discharge capacity of Co3O4−CuO at 1 A g−1 in the first cycles was higher than the initial discharge capacity of RP/Co3O4−CuO, but after 60 cycles, the capacity reduced with a higher rate and reaches about 338.66 mAh g−1 (44.8% capacity retention upon 60 cycles at 1 A g−1). However, Co3O4−CuO nanocomposite shows higher initial discharge capacity in the first cycles (especially in low current density) while the discharge capacity of RP/Co3O4−CuO hybrid electrode is more stable with by the cycle number increasing. This phenomenon indicates the enhanced effect of RP incorporating on the improved performance. The stable capacity is likely due to RP, which creates a large number of topological sites onto the Co3O4−CuO surface, which directs the formation of a disordered nanostructure that better boosts the discharge and charge capacity. It is worth noting that, after RP nanopowder impregnation into Co3O4−CuO nanostructure, electrical contact has been preserved on the electrode. To the best of our knowledge, for first time RP/Co3O4−CuO composite is used as anode in lithium ion battery and shows a good long-term cycling performance (>62% within 60 cycles) with a high reversible capacity (>470.11 mAh g−1) that is an indication of promising application prospect. Figure S2 shows 4626
DOI: 10.1021/acsomega.7b01153 ACS Omega 2018, 3, 4620−4630
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ACS Omega returning of Co3O4−CuO (93.7% of initial discharge capacity) when recursion to the initial 0.1 A g−1. This capacity retention after cycling at high rates and possess high discharge capacity for RP/Co3O4−CuO hybrid electrode demonstrating the excellent Li storage properties and appropriate conductive electrode between components and also indicating the novel structure by effective embedding RP in Co3O4−CuO, which is a promising strategy. It is believed that in RP/Co3O4−CuO electrode, Co3O4−CuO nanoarray limits nanosized RP into its pores, leading to protect RP from fracture and separation and also remain transport length of ions/electrons, which eventually directed to higher storage capacities, good Coulombic efficiencies, and suitable cycling stability. We first studied the conductivity of samples by four-probe technique. The results are shown that conductivity of RP/ Co3O4−CuO is about 1.21 S cm−1, which is a little less conductive than Co3O4−CuO (1.6 S cm−1). As expected, the addition of a reasonable amount of RP cannot dramatically change the conductivity. However, addition of a small amount RP can effectively increase capacity. Interestingly, the conductivity of Co3O4 and CuO were low alone (0.04 and 0.044 S cm−1 respectively), whereas for the Co3O4−CuO nanocomposite, the conductivity was improved because of the effective synergetic effect. Then, the EIS measurements were conducted to further investigate the kinetic properties, the electronic conductivity and to better understand the electrochemical behavior of RP/ Co3O4−CuO nanohybrid electrode. For comparison, EIS measurements of the Co3O4−CuO and RP/Co3O4−CuO were carried out and their Nyquist impedance plots were recorded (Figure S3). Nyquist plot as EIS displays the frequency response of the electrode/electrolyte cell and in this diagram the real part of the impedance is plotted on the X axis and the imaginary part is on the Y axis. The impedance curves of the electrode are consisted of a single semicircle in the high-medium frequency region and an inclined line at low frequency. In fact, in an equivalent circuit, the semicircle at the Z real axis (Z′) reflects ohmic resistance of the electrolyte and electric contact resistance (Rs), whereas the semicircle in the high-middle frequency range is related charge-transfer resistance at the interface between the electrode and electrolyte (Rct), a constant phase element (Qc) denoted the capacitance of the double layer and the willing line at low frequency can be attributed to the Warburg impedance (Zw), which is represented by the ionic diffusion process within the electrode.51 Figure S3 shows that, as expected, the semicircle diameter of the Co3O4−CuO and RP/Co3O4−CuO nanocomposite are almost similar, but the slope line of the Co3O4−CuO electrode is somewhat more vertical than the RP/Co3O4−CuO electrode. However, because of presence of RP, this low difference of Warburg impedance is natural for RP/Co3O4−CuO. In addition, the simulated Nyquist impedance plots and a suitable equivalent circuit of RP/Co3O4−CuO nanocomposite in different cycles was demonstrate in Figure 10. As shown in Figure 10, remaining low value of semicircles of the hybrid composite after several cycles at high frequency, stating suitable Li ion reaction and good electron transport rate during the cycle test. From the intercepts at the highest frequency of the curves on the Z′, the Rs of the electrode can be received. The Rs values for RP/Co3O4−CuO hybrid are about 10.0−20.0 Ω after 10, 20, and 30 cycles, indicating that the electrode keeps its structure
Figure 10. Nyquist plot and equivalent circuit model of the RP/ Co3O4−CuO nanohybrid after 10, 20, and 30 cycles. The frequency range and pulse amplitude applied were 100 kHz to 0.01 Hz and 5 mV, respectively; and inset at top shows the equivalent circuit to fit the impedance data.
and morphology after cycling. Rct of cells with the nanohybrids after 10, 20, and 30 cycles are 312, 347, and 368 Ω, respectively. The Rct in the Nyquist plots corresponds with high frequency semicircle. The resistance is slowly increased, indicates relatively stable SEI layer and well-maintained electrical conductivity during the initial cycles which is nearly stable after subsequent cycles.28,52 The formation of stable SEI can prevent capacity fading when nanohybrid sustains consecutive expansion/contraction during cycling. On the other hand, maintenance of the good electrical contact is beneficial to rapid charge/discharge of composite, which confirmed by the good rate performance as shown in Figure 9c.
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CONCLUSIONS In conclusion, we for the first time successfully prepared a RP/ Co3O4−CuO nanohybrid as an anode material in LIBs through low cost sol−gel process and followed by simple BM method. Due to the synergistic effect of Co3O4 and RP particles, the capacity of this electrode enhanced via increasing different locations for adsorption Li ion in high current density. On the other hand, CuO nanoparticles with their essential roles behave as conductive matrix to sustain electrical contact between compounds during insertion/extraction, buffer the enormous stress from the volume expansion of the Co3O4 nanoparticles and also assisted retain a stable SEI layer. This novel composite (RP/Co3O4−CuO) is given a Coulombic efficiency of 99.8% for the first discharge and charge capacity at a current density of 50 mA g−1, and shows good rate performance of 683.63 and 470.11 mAh g−1 at current density of 0.1 and 1.0 A g−1 after 60 cycles, respectively. Even at high current density of 3 A g−1, the cycling capacity is obtained 333.81 mAh g−1. Our results suggest that because of the facile and low-cost synthesis, this novel nanocomposite would be a promising candidate for practical application as anode material of LIBs.
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EXPERIMENTAL SETUP Fabrication of Co3O4−CuO Nanocomposite. The materials used in the synthesis of Co3O4−CuO nanoparticles were of analytical grade and were used without any further purification. A typical sol−gel synthesis of Co3O4−CuO with 4627
DOI: 10.1021/acsomega.7b01153 ACS Omega 2018, 3, 4620−4630
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ACS Omega
an excitation potential of 5 mV between 100 kHz to 0.01 Hz at a scan rate 0.1 mV s−1.
citric acid (as a chelating agent) was performed the following way. First, cobalt acetate and copper nitrate (Aldrich) (0.1/0.08 molar ratio) were dissolved in ethylene glycol and citric acid (4:1, v/v) with the aid of ultrasonication to obtain a sol then sonication was continued for 1 h to obtain a gel. The resulting gelatin was heated at 160 °C for 12 h to form a gel precursor. The obtain powder was calcined at 350 °C in air then the product cooled in the furnace to room temperature. Fabrication of RP/Co3O4−CuO Hybrid. Co3O4−CuO nanopowder (prepared in former step) with Commercial RP (%99, Sigma-Aldrich) (1:0.15, weight ratio) were placed in a stainless steel BM and sealed in a glovebox under argon protection as protection gas, performed at 400 rpm for 20 h. BM is recently well introduced as a powerful and effective method for the preparation of materials in nanoscale particles. It is believed that in the BM process, large bulk RP is ground into nanopowder and then confined in the pores of Co3O4− CuO nanostructure. From this point of view, this new design of Co3O4−CuO nanoarray with its pores embedded by nanosized RP can modify the expansion of RP after lithiation without cracking Co3O4−CuO nanostructure. Furthermore, all the construction is electrochemically active and provides good transport kinetics for both electrons and ions. Sample Characteristics. Powder X-ray diffraction (XRD) was performed by an Equinox 3000 with Cu Kα radiation (λ = 0.15418 nm). The Fourier transform infrared (FT-IR) spectra of the products was studied using a JASCO FTIR spectrophotometer. The morphology and structure of the powder samples were successfully studied by a field-emission scanning electron microscopy (FE-SEM, Mira 3-XMU) equipped with energy dispersive X-ray microanalysis (EDS or EDX) to check the chemical composition of the powders, and a transmission electron microscope (TEM, Zeiss EM 900). The N2 adsorption/desorption isotherms were obtained by a porosimetry system and an accelerated surface area (Belsorp mini II sorption analyzer) for determining the pore volume and surface areas. The pore volume was calculated by Barrett− Joyner−Halenda (BJH) method and the surface area was calculated by Brunauer−Emmett−Teller (BET) method. Electrochemical Measurement. The electrochemical measurements of Co3O4−CuO and RP/Co3O4−CuO composites as anode materials in LIBs were conducted by galvanostatic charge/discharge technique. The working electrode was prepared by coating slurry containing 85 wt % active materials (RP/Co3O4−CuO or Co3O4−CuO), 10 wt % ultrapure graphite (conductive agent, purity >99.99 wt %) and 5 wt % PVDF (polyvinylidine fluride, as a binder) in Nmethyl-2-pyrrolidone (NMP) as the dispersant on copper foil. Then, the electrode was dried in a vacuum oven in 60 °C overnight before test. The typical active materials loading of the electrodes sheet was about 2.5 mg/cm2. The coin cells were assembled in an argon-filled glovebox, using lithium foil as the counter and reference electrode, the polypropylene membrane (Celgard 2400) as separator and a solution of 1 mol L−1 LiClO4 (battery grade, Aldrich) in propylene carbonate. The galvanostatic charge/discharge tests were conducted on a battery test system (Kimia stat-5 V/10 mA, kimia pardaz rayane, Iran) at various densities with a cutoff voltage window of 0.05−3.0 V (vs Li+/Li). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurement tests were performed by a Galvanostat/Potantiostat Autolab (PGSTAT 302N) and the CV test was carried out by applying
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01153. Comparisons of cycle performance and cycling stability and Nyquist plots from EIS (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (N.Z.). *E-mail:
[email protected] (A.R..M.-A.). ORCID
Ali Reza Modarresi-Alam: 0000-0003-4055-4633 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Graduate Council of University of Sistan and Baluchestan and National Nanotechnology Initiative funded by the Iranian government.
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REFERENCES
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