Comparative Study on the Morphology-Dependent Performance of

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A Comparative Study on the Morphology Dependent Performance of Various CuO Nanostructures as Anode Materials for Sodium-ion Batteries Purna Chandra Rath, Jagabandhu Patra, Diganta Saikia, Mrinalini Mishra, Chuan-Ming Tseng, Jeng-Kuei Chang, and Hsien-Ming Kao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02159 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Sustainable Chemistry & Engineering

A Comparative Study on the Morphology Dependent Performance of Various CuO Nanostructures as Anode Materials for Sodium-ion Batteries Purna Chandra Rath†‡, Jagabandhu Patra‡, Diganta Saikia†, Mrinalini Mishra#, Chuan-Ming Tseng§, Jeng-Kuei Chang‡* and Hsien-Ming Kao†*

†

Department of Chemistry, National Central University, 300, Zhongda Road, Chung-Li,

32054, Taiwan, R.O.C. ‡

Institute of Materials Science and Engineering, National Central University, 300, Zhongda

Road, Chung-Li, 32054, Taiwan, R.O.C. #

Department of Chemical and Materials Engineering, National Central University, 300,

Zhongda Road, Chung-Li, 32054, Taiwan, R.O.C. §

Department of Materials Engineering, Ming Chi University of Technology, 84, Gungjuan

Road, Taishan District, New Taipei City, 24301, Taiwan, R.O.C.

*Corresponding authors: Hsien-Ming Kao Tel: +886-3-4275054 Fax: +886-3-4227664 E-mail: [email protected] Jeng-Kuei Chang E-mail: [email protected]

KEYWORDS: Morphology; CuO nanostructures; structure directing agent; sodium-ion battery

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ABSTRACT In this work, CuO with three different nanostructures, i.e., nanoflakes, nano-ellipsoids and nanorods, are successfully synthesized by a facile and environment friendly hydrothermal approach based on the use of different structure directing agents. The morphological influence on the anodic electrochemical performances, such as capacity, cycling stability, rate capability, and diffusion coefficient measurements, of these different CuO nanostructures is comparatively investigated for sodium-ion batteries. The capacity and cycling stability are higher for the CuO nanorods (CuO-NRs) based electrode as compared to the cases of CuO nano-ellipsoids (CuO-NEs) and CuO nanoflakes (CuO-NFs). At a low current density of 25 mA g-1, the CuO-NRs based electrode exhibits an excellent reversible capacity of 600 mA h g−1. It also exhibits a capacity of 206 mA h g−1 after 150 cycles with a capacity retention of 73% even at a higher current density of 1000 mA g-1. The exceptional performance of CuONRs is attributable to its slim nanorod morphology with a smaller particle size that provides a short diffusion path and the maximized surface area facilitating good diffusion in electrolytes, ensuring good electronic conductivity and cycling stability. The comparative analysis of these materials can provide valuable insights to design hierarchical nanostructures with distinct morphology to achieve the better materials designed for sodium-ion batteries.


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INTRODUCTION The quest for energy due to endless consumption of non-renewable energy sources and severe environmental pollution pushes the researchers to find the alternatives in terms of sustainable, clean and green energy.1 Among these, lithium-ion batteries (LIBs) exhibit high energy density, extended cycle life, and environment friendliness. Hence LIBs have been successfully applied in consumer electronics and powerful electric vehicles.2,3 However, concerns on limited availability of lithium resources and uneven geographical distribution severely restrict a large-scale application of LIBs.4 Sodium-ion batteries (SIBs), on the other hand, are the promising candidates to potentially substitute LIBs due to the cost effectiveness, virtually inexhaustible and ubiquitous resources of sodium around the world. However, the kinetic limitations in SIBs owing to the larger ionic radius and heavier weight of the sodium ion still hamper the widespread execution of SIBs, although the working principles of SIBs are similar to those of LIBs.4-6 Thus, some difficulties still need to be resolved in order to adopt the well-advanced LIB strategies for development of high energy electrode materials with fast and stable sodiation/de-sodiation kinetics for SIBs. Exploration of stable and low-cost anode materials for excellent performance in sodium-ion batteries has been restricted to carbonaceous materials, metal oxides, binary metal oxides, metal sulphides etc.7-18 To date, an efficient anode material for SIBs with promising electrochemical performances is yet to be identified. As a strategy, transition metal oxides with conversion reactions can be the materials of interest.7-15 Rahman et al. prepared Co3O4, Co3O4/CNT and Co3O4-Fe2O3/C composites and achieved reversible capacities of 447, 403 and 440 mA h g-1 after 50, 100 and 100 cycles, respectively.7,10,11 Lu et al. synthesized micro-nanostructured CuO/C spheres and obtained a capacity of 402 mA h g-1 after 600 cycles.8 Fan et al. synthesized the CuO nanosheets based electrode using carboxymethyl cellulose as the binder and obtained a high reversible capacity of 627 mAh g-1 3 ACS Paragon Plus Environment


at 50 mA g-1.9 This electrode also delivered a reversible capacity of 210 mA h g-1 after 300 cycles at a high current density of 2000 mA g-1. Komaba et al. prepared nanocrystalline Fe3O4 and -Fe2O3 with capacity retention of 160 and 170 mA h g-1, respectively.12 Hariharan et al. synthesized -MoO3 anode with capacity retention of 100 mA h g-1 over 500 cycles.13 Among them, CuO is the one to be considered because it bears important characteristics such as high specific capacity (674 mA h g−1), global opulence, and ease of production which allows it to be economically affordable.19,20 However, several drawbacks, for example, significant volume expansion (173%), weak electronic conductivity, and relatively low ion diffusion kinetics, can cause serious mechanical strain and inadequate cycling performance, and thus impede its practical applications.21 An effective approach is to construct novel architectures based on morphological designs to overcome the problems in the case of SIBs and enhance their electrochemical storage performance. This strategy has been successful in some LIBs to a certain extent.22-25 Recent development demonstrates that morphology optimization enhances the electrochemical and energy storage performances in LIBs by using different electrode materials, for example, CuO, Fe2O3, SnO2, and mesoporous carbons.25-28 Since transportation of sodium ions is a crucial factor in determining the performance of SIBs, morphological designs of materials with various nanostructures to provide effective diffusion pathways could be an attractive approach to achieve high energy capacity and excellent cycle stability of SIBs. The high specific surface area of nanosized materials is believed to lessen the sodium-ion diffusion length and ease the sodium-ion transport during the charge-discharge process. Moreover, transportation of electrons in nanosized materials is significantly different from their bulk counterparts. Different CuO nano-architectures have found to be crucial for understanding the morphology-dependent electrochemical properties.29,30 For example, porous CuO nanowires synthesized by Wang et al. showed a reversible capacity of 303 mA h g−1 after 50 cycles at 50 mA g−1.31 The hollow


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octahedron shaped CuO/Cu2O composite exhibited a reversible capacity of 100 mA h g−1 at 200 mA g−1 after 40 cycles.21 Yuan et al. demonstrated that the binder-free porous CuO arrays developed by engraving copper foil in-situ, showed superior electrochemical performance and reached a capacity of 542 mA h g−1 after 30 cycles at 50 mA g−1.32 Zhang et al. showed that porous CuO/Cu2O composite prepared by annealing Cu in metal-organic frameworks (MOFs) delivered a reversible capacity of 415 mA h g−1 after 50 cycles at a current density of 50 mA g−1.33 Recently, Chen et al. reported that three-dimensional network structured binder free CuO anode exhibited a third cycles reversible capacity of 680 mA h g−1 at 50 mA g−1.34 However, a comparative study of CuO based materials with various morphologies has not yet been done in the case of SIBs. It is anticipated that the results of this type of study will be beneficial for better designing of hierarchical nanostructures with desirable structural features, and thus paves the way to develop a facile route for controlling the morphology of electrode materials for enhancement of their electrochemical properties. Herein, we report the morphology-dependent electrochemical behavior of different nanostructured CuO synthesized by a facile hydrothermal route with different structure directing agents. The morphologies of CuO investigated in this work are nanoflakes (NFs), nano-ellipsoids (NEs) and nanorods (NRs). The effect of different structure directing agents on the morphology and dimensions of the CuO nanostructures are systematically studied as well. Further, the crystalline phase, surface area, and electronic conductivity are considered as the key factors for Na+-ion diffusion. The sodium storage behavior is highly dependent on the morphology of the CuO nanostructures. Hence, the comparative electrochemical behavior of the CuO nanostructures in correlation to their unique morphologies gives deeper insights into the sodium storage behavior in SIBs.

EXPERIMENTAL SECTION



Material Synthesis Nanostructured CuO with NRs and NEs morphologies were synthesized by using salicylic acid and citric acid as structure directing agents, respectively.35,36 First, 0.5 g of CuSO4·5H2O was dissolved in 10 mL water and either 0.2 g of salicylic acid or citric acid was dissolved in 10 mL ethanol separately. Then, the two solutions were mixed under vigorous stirring followed by addition of 15 mL NaOH solution (2.5 M). Thereafter the mixture was poured into a Teflon-lined steel autoclave, and held at 120 °C for 6 h. The black precipitate was removed from the bottom of the Teflon cup and washed by centrifuging with water-ethanol mixed solvent and dried for 12 h in an oven at 70 °C. Drying was followed by grinding and then calcination was carried out for 2 h at 400 °C to obtain CuO nanostructures. When the structure directing agent was not added in the synthesis mixture, the CuO nanostructure with NFs morphology was obtained. The CuO nanostructures with three types of morphologies (denoted as CuO-NFs, CuO-NEs, and CuO-NRs) were comparatively studied for their electrochemical performances in SIBs. Cell Assembly The electrode slurry prepared by mixing 75 wt.% of active material (different nanostructured CuO), 15 wt.% carbon black and 10 wt.% poly(vinylidene fluoride) in N-methyl-2pyrrolidone solution was coated on a copper foil with the help of a doctor blade, and then vacuum-dried for 6 h at 100 °C. Circular disk of the requisite dimension for a CR2032 coin cell were then punched out from dried coated foil after roll-pressing. In the setup of the battery, the counter and reference electrode is sodium metal foil and the separator is a glass fiber membrane. The average mass loading of CuO on the Cu foil was ~1.5 mg cm-2. The electrolyte was composed of 1 M NaClO4 salt in propylene carbonate (PC)/fluoroethylene carbonate (95:5 v/v). Assembling of the coin cell was done in an Ar filled glove box (Innovation Technology Co. Ltd.) with moisture and oxygen contents kept below 0.1 ppm. 6 ACS Paragon Plus Environment

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Materials and electrochemical characterizations Wide angle X-ray diffraction (XRD) patterns were obtained using a X-ray diffractometer (Bruker D8 Advance) with Cu K radiation ( = 1.54178 Å). A UniRaman spectrometer (Protrustech Co. Ltd.) equipped with a 532 nm laser source was employed to obtain the Raman spectra. Field emission scanning electron microscope (FESEM, FEI Nova Nano SEM 230) and ultra-high resolution transmission electron microscope (HRTEM, JEOL JEMARM200F) were used to analyze the morphology and microstructure of samples. The surface chemical composition of different nanostructured CuO was analyzed by X-ray photoelectron spectroscopy (XPS) (Thermo VG Scientific Sigma Probe spectrometer. A Quantachrome Autosorb iQ-2 analyzer was used to record the N2 adsorption-desorption isotherms at 77 K. The specific surface areas were obtained in the relative pressure (P/P0) range of 0.05-0.3 by using the Brunauer-Emmett-Teller (BET) method. A Biologic VSP-300 potentiostat was used to perform cyclic voltammetry (CV) between 0.01 and 3.0 V at a sweep rate of 0.1 mV s -1. The electrodes’ charge-discharge performances (capacity, rate capability, and cyclic stability) were assessed using an Arbin BT-2043 battery tester. Electrochemical impedance spectroscopy (EIS) measurements were performed as well. The frequency range was 106-10-2 Hz and the AC amplitude was 10 mV. Subsequent to battery cycling tests, the cycled electrode was removed from the coin cell in an Ar filled glove box, rinsed with PC electrolyte and dried. Post characterizations were carried out in order to investigate the structural and morphological changes of the CuONFs, CuO-NEs and CuO-NRs based electrodes after 100 cycles at a current density of 100 mA g−1. The morphology of the powder scrapped off from the cleaned electrodes was studied by SEM and HRTEM. Further, the structural changes upon sodiation/de-sodiation were clarified by ex-situ XRD, which was provided by the Beamline 17A of the National Synchrotron Radiation Research Center in Taiwan. To characterize the phases developed 7 ACS Paragon Plus Environment


through-out the charge-discharge at various potentials, the electrodes were set in advance at several potentials at a current density of 25 mA g−1 during charging and discharging cycles and then removed from the cells. The high photon flux offered by the synchrotron radiation source gives better detection sensitivity and accuracy compared to a conventional in-house Xray device.

RESULTS AND DISCUSSION The XRD analysis of the CuO-NFs, CuO-NEs, and CuO-NRs materials (Figure 1) ̅11), (111), (𝟐 ̅02), (020), (202), (𝟏 ̅13), (𝟑 ̅11) and (220) clearly show characteristic (110), (𝟏 diffraction peaks that can be indexed to standard monoclinic CuO (JCPDS file No. 45-0937). All the nanostructured CuO-NFs, CuO-NEs and CuO-NRs exhibit peaks of similar intensities and well matched with the JCPDS data of CuO, suggesting the absence of impurities in the synthesized samples.

Figure 1. Wide angle XRD patterns of different CuO nanostructures (a) CuO (JCPDS 450937) (b) CuO-NFs (c) CuO-NEs and (d) CuO-NRs (with peak assignments). 8 ACS Paragon Plus Environment

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The average crystal size, D, of nanostructured CuO-NFs, CuO-NEs and CuO-NRs was calculated according to the Scherrer equation,37 D=

k.λ

(1)

B.cosθ

where k is a shape factor with the value of unity, λ is the X-ray wavelength of Cu K ( = 1.5405 Å), B is the full-width at half-maximum (FWHM) of the peak in radians and θ is the Bragg diffraction angle. The average crystal sizes calculated by the Scherrer equation were approximately 63, 55 and 45 nm for CuO-NFs, CuO-NEs and CuO-NRs, respectively. For additional confirmation of materials purity, Raman spectra of CuO-NFs, CuO-NEs and CuO-NRs are measured and shown in Figure S1 (Supporting Information, SI). All the spectra are of similar intensities and exhibit three peaks at 281, 331 and 614 cm-1.38,39 The peak at 331 cm-1 is comparatively weaker and the peak at 614 cm-1 is broad. While the peak at 281 cm-1 is assigned to the Ag mode, the peaks at 331 and 614 cm-1 are associated with the Bg modes of CuO. The presence of only characteristic peaks of CuO in all three different nanostructured samples indicates that materials are phase-pured. Figure 2 shows the morphologies of different CuO nanostructured samples as observed by HRTEM. The SEM images of all the CuO nanostructures are displayed in Figure S2 (SI). Figure 2(a) shows the CuO-NFs with irregular morphology with length in the range of 5001000 nm and thickness in the range of 80-100 nm (as shown in Figure S3, SI) in the absence of the capping agent. On the other hand, nano-ellipsoid CuO with regular size morphology was obtained with a dimension of 100-150 nm when citric acid was used as a structure directing agent as displayed in Figure 2(c). Moreover, when the structure directing agent was salicylic acid, the CuO structure exhibited nanorods of an average length of 100 nm and of uniform diameter of 10‒20 nm as shown in Figure 2(e). Based on these observations, it can be inferred that the coordination ability of the copper ion to different ligands (provided by the structure directing agent) to form the copper-ligand complex might be essential in 9 ACS Paragon Plus Environment


Figure 2. TEM, SAED pattern and d-spacings of CuO-NFs (a,b), CuO-NEs (c,d) and CuONRs (e,f).

ascertaining the ultimate morphology of the CuO nanoparticles. Both SEM and TEM images confirm the nanostructures of different morphologies of the present CuO samples. As shown in the insets of Figure 2(b,d,f), the d-spacing of all the CuO nanostructures is prominent and ̅ 11) plane of the matches to the lattice spacing of 0.25 nm, which corresponds to the (𝟏 monoclinic CuO. As shown in the insets of Figure 2(b,d,f), the selected area electron ̅11), diffraction (SAED) patterns, clearly exhibit distinct ring patterns corresponding to the (𝟏 ̅ 11) planes for CuO-NFs and (110), (111), (𝟐 ̅ 02), (202), (𝟑 ̅ 11) planes for (111), (202), (𝟑 10 ACS Paragon Plus Environment

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CuO-NEs and CuO-NRs, which are related to monoclinic CuO for all the nanostructures and corroborate the highly crystalline nature of all the CuO materials. The probable mechanism of the formation of different nanostructured CuO can be ascribed to the structure directing agents, i.e., salicylic acid and citric acid, and has been discussed in the literature. In the absence of a structure directing agent, orthorhombic Cu(OH)2 consisting of chains in the (001) planes oriented along (100), first precipitates after the addition of OH– (from NaOH). Square-planar coordination of the Cu2+ ions with strong H-bonds is an important feature of these chains. Two dimensional layers of distorted Cu(OH)6 octahedra parallel to the (010) plane, held by H-bonds are then manifested by edgesharing. The speed of growth of such crystal is generally proportional to 1/dhkl, according to the Bravais-Friedel-Donnay-Harker analysis.40 Owing to relatively loose hydrogen linkage the inter-planar distance of (010) is the longest (10.60 Å), whereas the inter-planar distance of (100) is the shortest (2.952 Å). The nanoflake structure is resulted by the growth of CuO nanoparticles in the (100) direction upon stacking and scrolling of the sheets parallel to (010). Consequent to hydrothermal treatment of orthorhombic Cu(OH)2, dehydration reaction resulted in flake-like, monoclinic CuO nanostructures. Xiao et al. suggested that the appearance of flake-like CuO without structure directing agent is regulated by the internal crystallographic structure of the Cu(OH)2 precursor.41 When citric acid is used as structure directing agent, the scenario becomes different. CuO ellipsoid nanoparticles are formed majorly due to the process of Ostwald ripening. The crystal growth depends on the relative intrinsic surface energy despite the formation of copper citrate complex. The Cu2+ ion densities for the (001), (100), and (010) planes are 12.5 ions per nm2, 11.5 ions per nm2, and 8.5 ions per nm2, respectively, for a monoclinic CuO unit cell.42 The trend of relative surface energy of bare and passivated surfaces are unalike. 43 The plane which has the lowest surface energy owing to its low surface charge density is the



bare (010) plane. Whereas, the planes that have higher surface energies are the bare (100) and (001) planes. Hence, selective adsorption of citrate molecules on the surface of primary crystals is favored during the growth and assembly by the Ostwald ripening mechanism. This phenomenon facilitates the formation of anisotropic nano-ellipsoids structure with the (100) and (001) plane as the shortest dimension. When salicylic acid is used as a structure directing agent, on addition of OH– ion into the copper solution, Cu2+ ions from precursor can coordinate with the salicylic acid to provide copper salicylate complex. The copper salicylate complex controls the releasing of Cu2+ ion, and retards the growth of the Cu(OH)2 moiety in all other directions except for the (010) direction perpendicular to the principal plane of the complex. The amount of primary nanocrystals assembled in (001), (100), and (010) directions is substantially different owing to the variance in aggregation potential and rate. Thermodynamically, the most stable surface for CuO is (001) whereas (010) is the least.44 Polycrystalline Cu(OH)2 nanorods are formed by the preferred one-dimensional growth of Cu((OH)2 nanorods in (010) direction together with coordination self-assembly. The Cu(OH)2 nanorods dehydrate to form CuO nanorods upon hydrothermal treatment. To support the formation mechanism, the plane orientation and direction of individual CuO-NF, CuO-NE and CuO-NR crystals were investigated by electron diffraction and HRTEM images as shown in Figure S4 (SI). From the SAED patterns, it was observed that all the nanostructures were monoclinic phase of CuO. The SAED pattern of CuO-NF single crystal showed the diffraction spots along [001] zone axis with the crystal planes indexed to (020) and (200). The lattice spacing of the CuO-NF single crystal measured from the HRTEM image (Figure S4(c), CuO-NF) was found to be 0.23 nm corresponding to the (200) crystal plane of CuO. The SAED pattern of the CuO-NE single crystal showed the diffraction spots along [101] zone axis with the crystal planes indexed to (020), (11̅1)and (20̅2).


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Figure 3. XPS spectra of Cu 2p and O 1s of CuO-NFs (a,b), CuO-NEs (c,d) and CuO-NRs (e,f).

The d-spacing was measured from the lattice plane and found to be 0.17 nm corresponding to the (020) plane of CuO-NE (Figure S4(c), CuO-NE). Similarly, for the CuO-NR single crystal, the SAED pattern showed the diffraction spots along [1̅10] zone axis with the crystal planes listed to (002), (111) and (220). The measured d-spacing of 0.25 nm was attributed to the (002) crystal plane of CuO-NR (Figure S4(c), CuO-NR). Therefore, the orientation planes and the direction obtained from the HRTEM images and SAED patterns clearly suggested



that the CuO-NF, CuO-NE and CuO-NR single crystals were formed with different structure directing agents, and thus gave support for the formation mechanism. Figure 3 displays the Cu 2p and O 1s XPS spectra of all the CuO nanostructures under investigation. For all the CuO nanostructures, the Cu 2p XPS spectra show two distinct peaks located around 935 eV and 955 eV, corresponding to the Cu 2p3/2 and Cu 2p1/2 states due to the oxidized copper (II) species. The presence of the shake-up satellite peaks around 943 eV and the single satellite peak at 963 eV are the evidence of 3d9 shell of the Cu2+ state. The difference in spin-orbit coupling energy of the Cu 2p3/2 and Cu 2p1/2 states was found to be around 20 eV, indicative of Cu2+ oxidation state.20,25 Further, the O 1s XPS peaks were deconvoluted using a fitting procedure. The main peak of O 1s was observed around 530 eV, corresponds to the binding energy value of Cu‒O, while the other peak observed around 532 eV was attributed to the O‒H bonds, may be due to the physically adsorbed H2O on the CuO surface.45 The peak positions of all the samples are almost similar. This confirms the presence of CuO in all the nanostructures. The electrochemical properties of all the three nanostructured CuO were evaluated by CV and galvanostatic charge-discharge tests. The first five representative CV curves, which are the results of electrochemical redox reactions, are shown in Figure 4. All the CuO nanostructures exhibited similar types of CV curves. In the first cathodic scan (sodiation process), the broad peak observed at 0.94/0.92 V for different CuO nanostructures were attributed to the decomposition of electrolyte and formation of a solid electrolyte interphase (SEI) layer.17,46,47 The SEI layer is one of the key components of battery cycle life. It is formed on the electrode surfaces from the decomposition of electrolytes. The electrolyte decomposition results in the formation of different oxygen-containing products (e.g., Na-carbonate, Naalkoxide, etc.). The SEI layer is responsible for initial loss of capacity. The addition of fluoroethylene carbonate (FEC) in the electrolyte can modify the structural order and surface


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Figure 4. Cyclic voltammetry curves of (a) CuO-NFs (b) CuO-NEs (c) CuO-NRs.

chemical composition of the passivation layer of the cycled electrode, leading to formation of chemically and mechanically stable, and structurally robust SEI film. The formed SEI film stabilizes the electrodes, suppresses the continuous consumption of electrolytes, prevents cointercalation of solvent molecules, facilitates higher reversible capacities, enhances capacity retention and improves rate capability. From the second cycle onwards, the peak at 0.94/0.92 V disappears and three reduction peaks at 1.85/1.86, 0.52/0.48 and 0.1 V appear for CuONFs, CuO-NEs and CuO-NRs. These three peaks are related to three electrochemical reversible reactions, namely (i) formation of transitional copper oxide phase CuII1-xCuxIO1-x/2 (1.85/1.86 V), (ii) the generation of the Cu2O phase (0.52/0.48 V), and (iii) further reduction of the Cu2O phase into Cu and Na2O (0.1 V).8,16,31,32 These reactions can be summarized by the equations as follows.31 𝐼𝐼 𝐶𝑢𝑂 + 𝑥𝑁𝑎+ + 𝑥𝑒 − → 𝐶𝑢1−𝑥 𝐶𝑢𝑥𝐼 𝑂1−𝑥/2 + 𝑥/2𝑁𝑎2 𝑂

(2)

𝐼𝐼 2𝐶𝑢1−𝑥 𝐶𝑢𝑥𝐼 𝑂1−𝑥/2 + (2 − 2𝑥)𝑁𝑎+ + (2 − 2𝑥)𝑒 − → 𝐶𝑢2 𝑂 + (1 − 𝑥)𝑁𝑎2 𝑂

(3)



𝐶𝑢2 𝑂 + 2𝑁𝑎+ + 2𝑒 − → 2𝐶𝑢 + 𝑁𝑎2 𝑂

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(4)

During the desodiation process (anodic scan), the oxidations peaks observed at 1.23/1.22 V and 2.25/2.27 V for these CuO-NFs, CuO-NEs and CuO-NRs can be ascribed to the generation of the Cu2O phase and further oxidation of the Cu2O to CuO phase, respectively.32,48 The desodiation process can be summarized as 2𝐶𝑢 + 𝑁𝑎2 𝑂 → 𝐶𝑢2 𝑂 + 2𝑁𝑎 + + 2𝑒 −

(5)

𝐶𝑢2 𝑂 + 𝑁𝑎2 𝑂 → 2𝐶𝑢𝑂 + 2𝑁𝑎+ + 2𝑒 −

(6)

In later cycles, the CV curves basically overlapped, indicating a stable electrochemical reaction. Further, the cathodic and anodic peaks for all the CuO nanostructures slightly shifted to higher potentials, indicating excellent reversibility in the redox reactions. The morphological dependence on electrochemical properties of all the CuO nanostructures was explored. Figure 5(a-c) shows the first, second and third galvanostatic charge-discharge (desodiation-sodiation) curves of CuO-NFs, CuO-NEs, and CuO-NRs at a current density of 25 mA g−1 in the voltage range of 0.01 to 3.0 V vs. Na/Na+. Three sloping potentials ranging from 2.5-1.5 V, 1.4-0.8 V, and 0.8-0.01 V were observed in the first discharge (sodiation) curve of CuO-NFs. These corresponded to the multistep electrochemical reaction of CuO pertaining to the creation of a Cu II1-xCuxIO1-x/2 solid solution, a Cu2O phase, reduction to Cu and Na2O, and development of the organic layers of the electrolytes. Similar behavior was also observed for CuO-NEs and CuO-NRs, which corroborated well with the CV results as well as the literature reported elsewhere.8,16,32,47 The initial discharge capacities of CuO-NFs, CuO-NEs and CuO-NRs were 941, 817 and 1035 mA h g−1, respectively. The charge capacities were 507, 442 and 569 mA h g-1 with 53%, 54% and 55% coulombic efficiencies, respectively. All the CuO based electrodes exhibited large irreversible capacities, which were significant for the conversion type of anodes.49,50


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Figure 5. Charge–discharge profiles of (a) CuO-NFs, (b) CuO-NEs and (c) CuO-NRs in the range of 0.01 V and 3.0 V at a current density of 25 mA g−1. (d) Ex-situ XRD patterns of the CuO-NRs based electrode at various voltages.

The initial loss in the first cycle capacity as well as efficiency is associated with the formation of unstable SEI, decomposition of electrolytes, and irreversible trapping of Na+ ions in the oxide lattice (interfacial sodium storage). Afterwards, the reversible capacities nearly overlapped, indicating improved contact between electrode-electrolyte and stable SEI layer. Further, the coulombic efficiency swiftly goes beyond ~90% after the second cycle. After three activation cycles, the discharge capacities of CuO-NFs, CuO-NEs, and CuO-NRs were measured and found to be 537, 469, and 600 mA h g−1, respectively, at a current density of 25 mA g−1. The discharge capacity of CuO-NRs (600 mA h g−1) was larger in comparison to both CuO-NFs and CuO-NEs.



The ex-situ XRD patterns for the samples at various potentials, as shown in Figure 5(d), gave further insights into the reaction mechanism and phase transformation at the electrode during the charge/discharge process. In the discharge process, at open circuit potential (OCP), the CuO-NRs based electrode displayed all the CuO peaks along with current collector (metallic Cu) peaks. When the discharge voltage was decreased to 1.8 and 0.6 V, there was a progressive reduction of the CuO peaks at the 2θ values of 35.5° and 38.7° and a new peak at 36.3° was observed, which could be assigned to the intermediate Cu2O phase.20 The Cu2O diffraction peaks were preserved at the charging potential of 1.5 V. However, upon further charging to 3.0 V, two peaks appeared at 2θ values of 35.5° and 38.7° which corroborate the formation of the CuO phase. A close examination of the ex-situ XRD patterns indicated that there were no obvious XRD peaks of the Na2O phase due to its low crystalline nature. The poor crystalline phase of Na2O is beneficial to ameliorate the kinetics of the Na+ insertion/extraction. This also helpful for the in-situ evolution of a conducting metallic Cu matrix, which serves as a uniform and contiguous contact with the active component, and thereby enhances the electrochemical performance.16,32 The rate capability of various electrodes at current densities varying from 25-2000 mA g-1 is illustrated in Figure 6(a). Among all the nanostructured CuO based electrodes, the CuONRs based electrode possesses the best rate performance. The discharge capacities of CuONRs are 600, 515, 447, 407, 367, 305, 277, and 196 mA h g−1 at current densities of 25, 50, 100, 250, 500, 1000, 1500 and 2000 mA g−1, respectively. These values are all exceeding those of CuO-NEs and CuO-NFs based anodes. Upon raising the current density further to 2000 mA g−1, the discharge capacity obtained for CuO-NRs is 196 mA h g−1 which is much higher than those of CuO-NEs (110 mA h g−1) and CuO-NFs (157 mA h g−1). In contrast, both CuO-NEs and CuO-NFs display mediocre performances. It can be attributed to the lack of order in the disorganized structure of CuO-NFs, and the submicrometer particle size of the


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Figure 6. (a) Comparative rate performances of CuO-NFs, CuO-NEs and CuO-NRs; (b) cycling performances of CuO-NFs, CuO-NEs and CuO-NRs at a current density of 100 mA g−1. (c) Nyquist plot of CuO-NFs, CuO-NEs and CuO-NRs (d) ZRe vs. -1/2 plot at the low frequency region.

CuO-NEs where aggregation lowers the surface area, possibly hindering the sodium-ion diffusion.51 Additionally, the inter-particle contact resistance might have increased, causing sluggish sodium-ion diffusion for both CuO-NFs and CuO-NEs. The cycle performances of different nanostructured CuO electrodes at a current density of 100 mA g−1 are evaluated as shown in Figure 6(b) (after three activation cycles). It was observed that the CuO-NRs based electrode exhibited better cyclic stability than CuO-NFs and CuO-NEs based electrodes. The discharge capacities of 261, 188 and 326 mA h g−1 were



Page 20 of 36

observed for CuO-NFs, CuO-NEs and CuO-NRs based electrodes after 100 cycles with capacity retention of 66, 52, and 74% respectively, indicating the superior cycle performances by the CuO-NRs based electrode. In contrast, both CuO-NFs and CuO-NEs displayed relatively low performances. The coulombic efficiency approached 98.5, 98.5, and 99% at 100th cycle for CuO-NFs, CuO-NEs and CuO-NRs based electrodes, respectively. To further understand the electrochemical performances, the cycling behavior of the CuO-NFs, CuO-NEs and CuO-NRs based electrodes at a high current density of 1000 mA g−1 were performed (Figure S5). After 150 cycles, the CuO-NRs based electrode demonstrated a capacity of 206 mA h g−1, the best among these three materials. At the end of the 150th cycle, the capacity retained by CuO-NFs, CuO-NEs and CuO-NRs are 65, 33 and 73%, respectively. The capacities of the electrodes fabricated with CuO-NFs and CuO-NEs fade much faster than that with CuO-NRs. A lower specific capacity is often obtained when the electrode is cycled at a higher current density, and a larger localized strain is induced due to concentration

polarization.52

The

volume

expansion

and

contraction

during the

charge/discharge steps could cause the loss of electronic continuity upon disintegration of the particles and lead to a major capacity loss in the disorganized nanoflake and sub-micrometer nano-ellipsoid CuO particles.52 On the other hand, the nanostructure of CuO-NRs can provide particularly a short diffusion path and accommodates the strain, and hence circumvents the capacity loss upon cycling.45 Superior specific capacitance, outstanding rate capacity, and exceptional cycling stability synergistically make the CuO-NRs a promising architecture for energy storage. Figure 6(c) shows the impedance characteristics of different CuO nanostructures after 50 cycles obtained from EIS analysis as revealed in the Nyquist plot. At high frequency, the EIS spectra comprises of a semicircle whereas at low frequency it shows a sloping line. The corresponding equivalent circuit is shown in the inset of the Figure 6(c), where Re, Rct, Cdl,


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and W are the electrolyte resistance, charge transfer resistance, surface double-layer capacitance, and the Warburg impedance associated with Na+ diffusion in the electrode, respectively. As seen from the Figure 6(c), there is no significant difference in the Re values of all the CuO electrodes (NRs: 8 Ω, NEs: 7 Ω and NFs: 9 Ω). However, a substantial difference in the charge transfer values was observed for these CuO nanostructures. The values of Rct obtained were 240, 288, and 220 Ω for CuO-NFs, CuO-NEs and CuO-NRs, respectively. The diameter of the semicircle in the plot for CuO-NRs is smaller than that for CuO-NEs

and

CuO-NFs,

indicating

that

the

charge-transfer

resistance

at

the

electrode/electrolyte interface of the CuO-NRs is significantly lower than that of CuO-NEs and CuO-NFs structures. The Warburg impedance and the Na+ ion diffusion coefficient are evaluated from the low-frequency impedance plot (Warburg part) by fitting ZRe vs. -1/2 as shown in Figure 6(d) and using equations (7) and (8) 𝑍𝑅𝑒 = 𝑅𝑒 + 𝑅𝑐𝑡 + 𝜎𝜔 −1⁄2 𝐷𝑁𝑎+ =

(7)

𝑅2 𝑇 2

(8)

2𝐴2 𝑛4 𝐹 4 𝐶02 𝜎 2

where ZRe stands for the real part of impedance,  denotes the angular frequency,  represents the Warburg coefficient, R denotes gas constant, T stands for the absolute temperature, A is the overall contact area, n denotes the number of the electrons transfer per mole, F is the Faraday constant, and C0 is the concentration of sodium-ion in the electrodes. The Na+-ion diffusion coefficient for the CuO-NRs based electrode was 2.69  10−14 cm2 s−1. It is 1.5 times higher compared to CuO-NFs (1.75  10−14 cm2 s−1) and 2 times greater than CuO-NEs (1.34  10-14 cm2 s−1) The higher Na+-ion diffusion coefficient of CuO-NRs implies good electrochemical kinetics of the nanorod structure. Hence, a NR structure with smaller dimension is favorable for the shortened diffusion length in comparison to the NEs and NFs structure. The nanorod structure endowed with nanosized dimensions, for example, 21 ACS Paragon Plus Environment


10-20 nm thick and length ~100 nm, adequately lessen the diffusion path of sodium ions and partially buffer volume change during the charge-discharge process. Based on the experimental results, the high specific capacity, cycling stability and superior rate performances may therefore be associated to the slim nanorod morphology with small particle size. The CuO-NFs and CuO-NEs show decent electrochemical performances as well. Apparently, morphology is an essential controlling factor for electrochemical performance. The CuO-NRs synthesized by the hydrothermal pathway had very narrow diameter distribution (10-20 nm) as evidenced by SEM and HRTEM. According to previous reports, the nanorods with smaller diameter demonstrate enhanced performances owing to large surface-to-volume ratio, which is related to the surface electrochemical reactivity of a material.53-55 Researchers have indicated that the reversible capacity increases remarkably with decreasing particle size.46-58 One-dimensional nanostructures like nanorod can offer high specific surface area, resistance to self-agglomeration and well-guided charge transfer kinetics to enhance the electrode perfromances.55,59 The slim nanorod morphology lessens the diffusion distance by maximizing the electrode/electrolyte contact area, and thus capacitates fast electron transportation. It can lead to facile insertion and extraction of Na+ within the electrolyte thereby enhancing the capacity, especially at high current density. According to the adsorption–desorption isotherms, (as shown in Figure S6, SI) the BET surface area of the CuO nanostructures with NFs, NEs and NRs morphology was found to be in the range of 1020 m2 g-1. Close stacking of the nanoflakes from the disorganized aggregates lowers the effective surface area for CuO-NFs. Since the BET surface area of CuO-NEs is higher than those of CuO-NFs and CuO-NRs, it is expected that CuO-NEs could outperform both CuONFs and CuO-NRs in electrochemical performances. However, in reality, the nanostructured CuO-NRs exhibits better activity, an observation reasonably confirming a morphology dependent behavior. Another important observation is that the particle size of CuO-NEs and


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CuO-NRs is quite similar. Hence, a closer look at the intrinsic crystal size of these materials is necessary. The crystal sizes calculated by the Scherrer equation for CuO-NFs, CuO-NEs and CuO-NRs were 62.5, 54.5 and 44.5 nm, respectively. The smaller crystal size for CuONRs morphology may plausibly explain why the nanostructured CuO-NRs outperforms the counterparts of CuO-NFs and CuO-NEs. Moreover, there are reports on higher observed rate capability owing to the reduced crystal size as well.60-63 As evident from the high Rct value as well as slowest diffusion rate coefficient derived from the Nyquist plot, nanostructure of CuO-NEs probably hinders the sodium-ion diffusion into the SEI layer, which consequences the poorest electrochemical performance. The nanostructures of CuO-NEs and CuO-NFs have long diffusion distance (> 50-100 nm), whereas the nanostructure of CuO-NRs bears narrow diffusion path (< 20 nm) for the electrolyte to penetrate.47 Hence, it has fast electron transport resulting in the enhancement of the cycling stability. Further, the polycrystalline CuO-NRs structure possesses the optimal crystalline imperfection and grain boundaries,52,53 which provide additional Na+ intercalation positions. In contrast to the CuO-NRs based electrode, the discharge/charge capacities of both the CuO-NFs and CuO-NEs based electrodes rapidly decrease as a function of the cycle number. The weak performance of CuO-NFs and CuO-NEs can be attributed to limited interaction of surface and electrolyte owing to agglomeration of crystallites as well as increased diffusion distance. The onedimensional nanorod structure may enable proficient transport of sodium ions because of the reduced diffusion length, thereby alleviating the performance loss. Furthermore, the ample space inside nanorod renders an effective elastic buffer to relax the volume expansion occurred during sodium-ion insertion/de-insertion and the strain accrued in the CuO particles. The remarkable difference in the capacity and cycle stability of the CuO-NRs based electrode is a clear demonstration of the beneficial effects of the one-dimensional nanostructures on the electrochemical performances over the other nanostructured CuO materials. Indeed, several



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Figure 7. (a) TEM image (b) SAED patterns and (c) d-spacing of CuO-NRs based electrode after 100 cycles.

metal oxides such as Fe2O3, SnO2, Mn3O4 and NiO with one-dimensional nanostructure displayed

exceptional

electrochemical

performances

in

lithium-ion

battery

and

supercapacitors applications as suggested by different researchers.53-57,59,64-67 The morphology of the structure of CuO-NRs after 100 charge-discharge cycles (terminated at 3.0 V) is shown in Figure 7(a). In comparison to the original morphology of the nanorod structure, no substantial change occurred after 100 cycles, although a slight agglomeration, a few broken rods and some spherical microstructures were formed in certain area. The SAED pattern, shown in Figure 7(b), depicts that the nanostructure of CuO-NRs retains crystalline nature after 100 cycles of charge-discharge. The lattice fringes (Figure 7(c) of CuO with a distinct d-spacing value of 0.25 nm (CuO (1̅ 11)) demonstrates that the conversion reaction of CuO can be reversed even after 100 charge-discharge cycles. After 24 ACS Paragon Plus Environment

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100 charge-discharge cycles, the CuO-NFs and CuO-NEs based electrodes pulverized and transformed to smaller nanoparticles with aggregations of particles due to the strong volume expansion/contraction upon cycling, as shown in Figure S7(a,c) (SI). Meanwhile, the corresponding SAED patterns (Figure S7(b,d), SI) exhibited well resolved diffraction rings, indicating that the resultant particles were substantially crystalline. Moreover, the SEM images of all the CuO nanostructures after 100 charge-discharge cycles are shown in Figure S8 (SI). With nanostructured CuO-NFs and CuO-NEs, the electrode materials became porous as well as agglomerated with relatively thick surface SEI film causing reduction in accessible sites. On the contrary, the CuO-NRs based electrode showed a smoother surface with stable SEI film. The results establish that the nanostructured CuO-NRs can maintain its structural and morphological integrity after cycling without serious particle pulverization, which helps to achieve the better cycling performance than CuO-NFs and CuO-NEs. The performance of CuO based nanostructures can be further improved by carbon coating, optimizing binder, and electrolyte formulation, which requires further investigation.68-70

CONCLUSION In this work, a systematic comparative analysis of anode materials with different CuO morphologies for applications in SIBs is presented. Different CuO nanostructures, such as nanoflakes, nano-ellipsoids and nanorods were prepared by a simple, environment friendly hydrothermal approach. The CuO materials with nanorods and nano-ellipsoids were obtained when salicylic acid and citric acid were used as the structure directing agents, respectively. The influence of different morphology on electrochemical behavior was investigated in details based on specific capacity, rate capability, cyclic stability, EIS spectra, diffusion coefficient and ex-situ XRD analysis. The CuO-NRs based electrode exhibits excellent electrochemical behavior, whereas CuO-NEs and CuO-NFs based electrodes show decent



performances. The CuO-NRs based electrode delivers an excellent reversible capacity of 600 mA h g−1 at 25 mA g−1. It delivers a reversible capacity of 206 mA h g−1 at 1000 mA g−1 after 150 cycles with a capacity retention of 73%. The remarkable enhancement in the electrochemical performance is attributed to the small size, polycrystalline one dimensional CuO nanorods, which provide efficient transport of sodium ions by shortening the diffusion distances of ions and electron during the redox process. The present results demonstrate that the CuO nanorods based electrode can be a promising anode material for SIBs.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxxx. Raman spectra and SEM images of CuO-NFs, CuO-NEs and CuO-NRs; SEM image of width of CuO-NFs; HRTEM images, SAED patterns of single crystal and growth direction of CuO-NF, CuO-NE and CuO-NR; Cycle performances of CuO-NFs, CuO-NEs and CuO-NRs at a current density of 1000 mA g-1; BET surface area of CuO-NFs, CuO-NEs and CuO-NRs; TEM and SEM images, and SAED patterns of CuO-NFs and CuO-NEs based electrodes after 100 cycles.

Author Information Corresponding Author Hsien-Ming Kao, E-mail: [email protected] Jeng-Kuei Chang, E-mail: [email protected] ORCID Hsien-Ming Kao: 0000-0002-4144-3890


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Jeng-Kuei Chang: 0000-0002-8359-5817 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The financial support for this work from the Ministry of Science and Technology of Taiwan (Grant number: MOST 105-2119-M-008-012) is gratefully acknowledged.

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TOC/Graphical Abstract

SYNOPSIS- Morphology based correlation of environmentally benign and economically affordable nanostructured CuO materials for sustainable energy storage.


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Comparative Study on the Morphology-Dependent Performance of

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