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Nanoplate and mulberry-like porous shape of CuO as anode materials for secondary lithium ion battery
Electrochimica Acta 206 (2016) 217–225
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Nanoplate and mulberry-like porous shape of CuO as anode materials for secondary lithium ion battery Subhalaxmi Mohapatra, Shantikumar V. Nair, Dhamodaran Santhanagopalan, Alok Kumar Rai* Amrita Centre for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Amrita University, Kochi 682041, Kerala, India
A R T I C L E I N F O
Article history: Received 19 February 2016 Received in revised form 11 April 2016 Accepted 21 April 2016 Available online 27 April 2016 Keywords: CuO Nanoplate Mulberry Porous Lithium ion batteries
A B S T R A C T
Facile hydrothermal synthesis of nanoplate and mulberry-like porous shape of CuO nanostructures was developed as anode materials for application in lithium ion batteries. The powder X-ray diffraction patterns of both the samples were indexed well to a pure monoclinic phase of CuO with no impurities. The CuO sample synthesized at different pH and reaction temperature exhibited nanoplate with average width and length of 150–300 nm and 300–700 nm and mulberry-like porous shape of CuO with average length of 300–400 nm. Electrochemical tests show that the lithium storage performances of both the nanoplate and mulberry-like samples are influenced more closely to its structural aspects than their morphology and size factors. The CuO nanoplate electrode exhibits high reversible charge capacity of 279.3 mAh g1 at 1.0C after 70 cycles, and a capacity of 150.2 mAh g1 even at high current rate of 4.0C during rate test, whereas the mulberry-like porous shape of CuO anode delivers only 131.4 mAh g1 at 1.0C after 70 cycles and 121.7 mAh g1 at 4.0C. It is believed that the nanoplate type architecture is very favorable to accommodate the volume expansion/contraction and aggregation of particles during the cyclic process. In contrast, the mulberry-like porous morphology could not preserve the integrity of the structure and completely disintegrated into nanoparticles during Li+ ion insertion/deinsertion due to the loose contact between the particles. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Non-carbon anodes (mainly silicon, tin, and transition metal oxides) are very promising for use in lithium ion batteries owing to their high theoretical capacity and better rate property in comparison with current commercial graphite anode. It is wellknown that the main characteristics of graphite anode such as low theoretical capacity and poor rate capability are not enough to satisfy the demand for future energy storage [1]. Despite many efforts over the last decades, the use of silicon and tin based anode materials in lithium ion batteries is still limited due to their large volume changes (>300%) during the alloying/dealloying process with Li+ ions, which leads to severe mechanical stress of the electrode and pulverization of the particles [2]. As a result, the capacity fades rapidly during the course of use. Fortunately, transition metal oxides with conversion mechanism of lithium storage (MO þ 2Liþ þ 2e $Li2 O þ M0 ) were regarded as a new hope for wide application in lithium ion batteries because of their
* Corresponding author. +91-4842-858750 (Ext. 5643). E-mail addresses: [email protected], [email protected] (A.K. Rai). http://dx.doi.org/10.1016/j.electacta.2016.04.116 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
low-cost, environmental benignity and high theoretical specific capacity [3,4]. Among all the proposed transition metal oxides, CuO is considered as a promising anode candidate for lithium ion batteries due to its abundance, low-cost, easy preparation, chemical stability, high theoretical capacity (674 mAh g1) and environmental friendliness [5]. Apart from all these advantages, the practical application of CuO is still hampered due to its poor cyclic performances, low conductivity, poor ion transport kinetics and severe capacity fading [5]. In order to avoid all these issues, synthesis of CuO with various unique nanostructure and porous morphologies [6–14], and fabrication of hybrid nanocomposite with conductive matrixes (such as carbon, carbon nanotubes and graphene nanosheets etc) are main typical approaches [5,15–18]. It is widely reported that the shape controlled morphologies highly affect the properties of the nanomaterials, suggesting that the morphological modification is a better key to improve poor cyclic retention of anode materials [19]. In addition, the nanostructure electrode can also offer easy Li+ ion diffusion, faster reaction kinetics, more stable structure and accommodation of large strain without severe pulverization [20]. Hence, it is believed that the novel architecture with porous morphology would be the feasible
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approach to solve the above issues of CuO anode material for lithium ion battery. Therefore, in the present work, first time we have successfully synthesized mulberry-like porous shape of CuO nanostructure through facile and cost-effective hydrothermal synthesis method. For comparison, CuO with nanoplate structure was also synthesized under different condition by the same method. In contrast to the belief, it was found that the nanoplate morphology remains well-preserved and stable even after long term cycling process, whereas the mulberry-like porous shape of CuO had strong capacity fading. This is due to the complete distortion of the mulberry-like porous shape and loss of contact between the particles upon cycling. Hence, the CuO nanoplate electrode has potential to be a high-performance anode material. 2. Experimental 2.1. Materials preparation Both nanoplate and mulberry-like porous shape of CuO were prepared by facile hydrothermal synthesis method. In a typical synthesis of CuO nanoplate sample, 0.2 M of copper acetate anhydrous [Cu(CH3COO)2H2O, Alfa Aesar, 98%] was dissolved in 30 ml of deionized water under continuous stirring to form a transparent solution. Then, a separately prepared 40 mL aqueous solution of sodium hydroxide (2 M) [NaOH, 99.99%, Sigma Aldrich] was slowly added into the above solution. More importantly, a few drops of liquor ammonia [Fisher Scientific, 25%] were also added into the solution to maintain the pH 13-14. After maintaining the pH, the solution was vigorously stirred for 24 h. Consequently, the resultant solution was then transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was then heated to 180 C for 15 h. After cooling to the room temperature, the black color precipitate was collected by centrifugation, washed with deionized water and ethanol for several times and dried at 80 C for 6 h. The obtained as-prepared powder was then annealed at 300 C for 3 h in air to obtain the crystalline CuO nanoplate sample. On the other hand, in order to synthesize mulberry-like porous shape of CuO, the whole procedure was same except few minor changes. First, 0.2 M copper acetate anhydrous was dissolved in 40 ml of deionized water followed by addition of 30 mL of aqueous solution of sodium hydroxide (0.2 M) under continuous stirring. Then, a few drops of liquor ammonia were also added to adjust the pH 10-11. Afterward, the obtained solution was vigorously stirred for 24 h at room temperature. The solution was then transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 120 C for 12 h. The black color precipitate at the end of the reaction was washed with deionized water and alcohol several times and finally dried at 80 C for 6 h. Finally, the obtained as-prepared powder was annealed at 300 C for 3 h in air to obtain the crystalline CuO mulberry sample.
Brunauer-Emmett-Teller method (BET, ASAP2010 Instrument Company, Norcross, GA).
Micromeritics
2.3. Electrochemical testing The electrochemical behavior of the obtained products were examined by coin-type half-cell (CR-2032) assembled in an argon filled glove box (O2 and H2O maintained less than 0.5 ppm). The samples obtained above were used as active materials. For the anode preparation, the active material, carbon black as a conducting agent and poly (vinyl difluoride) (PVDF) as a binder was mixed together with a weight ratio of 70:20:10 in N-methyl-2pyrrolidinone (Sigma Aldrich) as the solvent to form a slurry. The resulting slurry was casted onto Cu foil current collector uniformly by the doctor blade technique and dried in a vacuum oven at 80 C for 12 h. Subsequently the slurry was pressed between stainless steel twin rollers to improve the adhesion between the Cu foil and active materials. Lithium foil was used as the counter electrode. 1 M LiPF6 dissolved in an ethylene carbonate and dimethyl carbonate (molar ratio 1:1, in volume) was used as the nonaqueous electrolyte. A glass fiber was also used as a separator. Cyclic voltammetry was obtained using an Autolab potentiostat (PGSTAT 302 N) with the scan rate of 0.1 mV s1 within the range of 0–3.0 V. The discharge/charge measurements were performed at room temperature using Arbin (BT 2000) in the voltage range between 0.01 V to 3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were also carried out using a Autolab potentiostat (PGSTAT 302 N) to measure the impedance of the assembled cells. Before the measurements, the cells were cycled for 9 cycles and then measured in the frequency range of 0.01 Hz to 100 MHz.
2.2. Materials characterization The composition and phase purity of both the annealed samples were analyzed by powder X-ray diffraction (XRD; Shimadzu X-ray diffractometer) using Cu Ka radiation (l = 1.5406 Å). The particles size and surface morphology of the samples were measured by field-emission scanning electron microscopy (FE-SEM; S4800Hitachi) and field-emission transmission electron microscopy (FE-TEM, FEI 20 FEG electron microscopy instrument operating at 200 kV). The samples were first soaked in ethanol and dispersed by ultrasonication before drop casting onto copper grids for FE-TEM examination. The surface area measurement was based on the nitrogen adsorption and desorption isotherms using the
Fig. 1. X-ray powder diffraction patterns of annealed CuO (a) nanoplate and (b) mulberry-like porous samples.
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3. Results and discussion 3.1. Structural and morphological analysis X-ray diffraction patterns of both the nanoplate and mulberrylike porous shape of CuO samples are shown in Fig. 1 (a) and (b), respectively. It can be seen that both the annealed samples demonstrate strong diffraction peaks, which can be clearly indexed with well-crystalline monoclinic phase of CuO (JCPDS Card No. 050661). No impurity peaks were detected, revealing the high purity of the final samples. It is also worth to notice that the diffraction peaks of mulberry-like porous shape of CuO sample are relatively wide in comparison to nanoplate CuO sample, which clearly indicates relatively smaller particles. In order to investigate the influences of different conditions mainly pH value and reaction temperature on the surface morphology of the CuO samples, FE-SEM analysis was carried out. Fig. 2 (a), (b) and (c) shows the FE-SEM images of various regions of the CuO sample at different magnification, which clearly demonstrate the plate-like morphology of the sample with width and length of 150–300 nm and 300–700 nm, respectively. It is well-known that the 2D nanoplate morphology can provide high diffusion coefficient and short ion/electron diffusion path during Li+ ions insertion and extraction, which improves the capacity retention over prolonged discharge/charge cycles [21]. In contrast, in order to synthesize mulberry-like shape of CuO, the pH was maintained at 10–11. It can be observed from Fig. 2 (d), (e) and (f) obtained at different regions that the whole sample is entirely composed of uniformly dispersed mulberry-like porous shape with a typical length of 300–400 nm. More precisely, the high CuO magnified image of Fig. 2 (f) clearly shows that the individual mulberry-like shape of CuO sample is highly porous and composed by several small primary nanoparticles, which are in the range of 20–40 nm. It is also worth noting that the mulberry-like porous shape was formed via an assembly way of tiny nanoparticles and the tiny nanoparticles involved in the forming assembly are not tightly held with each other. However, as for the lithium ion battery, such architectures with the pores can not only provide a plenty of channels for Li+ ion insertion, but also short diffusion
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length for Li+ ions, leading to a high lithium storage capacity and enhanced rate capability. The detailed morphological investigations of both the samples were further examined by FE-TEM analysis. Consistent with FESEM observation, FE-TEM analysis also confirmed that nanoplate and mulberry-like porous shape of CuO was uniformly formed with controlled pH of 13–14 and 10–11, respectively. The FE-TEM images of the CuO nanoplate sample at different regions are shown in Fig. 3 (a) and (b). It shows that the sample is composed of nonuniform nanoplates and the dark region in the middle of Fig. 3 (b) is resulted from the stacking of few nanoplates. In addition, high magnification FE-TEM image demonstrate that the free-standing single-crystal structure are thin enough to be transparent and thus provide stronger structural stability during the lithiation/delithiation processes. It can be also observed that the surface of nanoplate is very smooth without having any defect or strain, as shown in insert of Fig. 3 (a). A typical FE-TEM image of mulberry-like porous shape of CuO was also illustrated in Fig. 3 (c) and (d) at different regions, which clearly confirm that the mulberry-like shape of CuO was assembled by a large quantity of tiny nanoparticles and this can be also verify by the magnified inset image of Fig. 3 (d). N2 adsorption-desorption studies were also performed to determine the specific surface area of both the nanoplate and mulberry-like porous shape of CuO samples. The isotherms of both the samples are presented in Fig. 4 (a) and (b). It can be observed that both the samples show type IV isotherms with a distinct hysteresis loop. The mulberry-like porous shape of CuO has higher surface area (11.3 m2 g1) than the nanoplate CuO structure (3.9 m2 g1). Obviously, the BET specific surface area of the mulberry-like sample is higher than that of the nanoplate sample, which is due to the small range of nanoparticles. Large surface area means more active sites and large contact area with the electrolyte. 3.2. Electrochemical performance Fig. 5 (a) and (b) shows the first five cycles of cyclic voltammetry curves of the nanoplate and mulberry-like porous shape of CuO electrode, respectively at a scan rate of 0.1 mV s1. In
Fig. 2. FE-SEM images of (a, b and c) CuO nanoplate and (d, e and f) mulberry-like porous shape of CuO samples at different magnifications annealed at 300 C for 3 h.
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Fig. 3. FE-TEM images of (a and b) CuO nanoplate and (c and d) mulberry-like porous shape of CuO samples at different magnifications.
the first reduction sweep, three cathodic peaks were appeared at 1.66 V (weak), 0.97 V (strong), and 0.77 V (medium) for the nanoplate CuO electrode, and at 1.62 V (weak), 1.07 V (strong) and 0.72 V (medium) for the mulberry-like porous CuO electrode. These peaks can be ascribed to the multistep of electrochemical reaction processes such as (i) the limited amount of Li+ ion insertion, involving a formation of an intermediate solid solution phase (CuII1x CuIx O1x=2 ) (0 x 0.4), (ii) the phase transformation from CuII1x CuIx O1x=2 to Cu2O phase and (iii) finally the decomposition of Cu2O into Cu and amorphous Li2O (Cu2O + 2Li+ + 2e ! 2 Cu + Li2O), along with the growth of SEI layer [22–24]. On the other hand, during the subsequent oxidation process, the main anodic peak at 2.49 V for the nanoplate CuO electrode and at 2.46 V for the
mulberry-like porous electrode is due to the oxidation of Cu to Cu2O and then further oxidation to CuO. In the subsequent cycles, the cathodic and anodic peaks are slightly shifted and fixed at around 1.27 V for the nanoplate electrode and 2.18 V, 1.28 V and 0.81 V for the mulberry-like electrode. More importantly, it can be seen that the cyclic voltammetry curves of the nanoplate electrode overlap very well with the subsequent scan numbers, indicating good electrochemical reversibility of the CuO nanoplate electrode, whereas the mulberry-like porous CuO electrode clearly shows the poor reversibility in the subsequent cycles. In addition, it is also important to notice that the gap between the cathodic and anodic peaks is broader for the mulberry-like porous CuO electrode, exhibiting more polarization than the nanoplate CuO electrode.
Fig. 4. N2 adsorption/desorption isotherms of (a) nanoplate and (b) mulberry-like porous shape of CuO samples.
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Fig. 5. Cyclic voltammograms of (a) nanoplate and (b) mulberry-like porous shape of CuO electrodes.
The cyclic voltammetry peaks and their variations are consistent with the galvanostatic charge/discharge profile. Fig. 6 (a) is the first discharge and charge curves of nanoplate and mulberry-like porous shape of CuO electrode at a constant current rate of 1.0 C (1 C = 674 mA g1). The voltage profile of the first discharge reaction of both the electrodes showed a long voltage plateau at 1.1 V and a short voltage plateau at about 0.8 V, followed by a sloppy profile down to the cutoff voltage of 0.01 V, which mainly correspond to the multi-step electrochemical reaction process such as reduction of CuO into intermediate composite copper oxide phase, further to Cu2O phase and finally the decomposition of Cu2O into Cu nanoparticles and Li2O matrix, respectively [25]. On the other hand, first charge curve of both the electrodes display sloppy profile with a small plateau near 2.4 V, which is due to the reformation of CuO. Moreover, the initial discharge and charge capacity was measured to be 966.2 mAh g1 and 353.4 mAh g1 for the nanoplate CuO electrode and 929.4 mAh g1 and 546.4 mAh g1 for the mulberry-like shape of CuO electrode, respectively. However, the irreversible capacity loss of 612.8 mAh g1 and 383.0 mAh g1 in the first cycle could be assigned to the formation of inevitable solid electrolyte interface (SEI) layer on the electrodes surface, which generally occur for both 3d transition metals oxides and carbon electrodes [26,27]. Fig. 6 (b) and (c) shows the 2nd, 25th and 50th discharge/charge curves of nanoplate and mulberry-like porous shape of CuO electrodes at a current rate of 1.0C, respectively. It can be seen that all the slopes become narrow and the plateau slightly upward during the following discharge process. It can be observed from Fig. 6 (b) that after the 1st cycle, the curves of each cycle are similar in shape, indicating that the electrode reactions become more reversible. In addition, the reversible discharge and charge capacities of 354.4 mAh g1 and 329.1 mAh g1 in the 2nd cycle, 331.4 mAh g1 and 322.3 mAh g1 in the 25th cycle and 309.3 mAh g1 and 303.9 mAh g1 in the 50th cycles were delivered by CuO nanoplate electrode, which clearly shows small capacity loss in the further cycles. In contrast, the mulberry-like shape of CuO electrode in Fig. 6 (c) could retain only 550.2 mAh g1 and 511.4 mAh g1 and 254.2 mAh g1 and 257.3 mAh g1 and 164.7 mAh g1 and 164.3 mAh g1 during 2nd, 25th and 50th discharge/charge cycles, respectively. More importantly, the clear plateau observed at 1.32 V in 2nd cycle of mulberrylike porous shape of CuO electrode is no more available in the subsequent cycles of 25th and 50th cycles, which also indicates the significant loss of reversible capacity in the further cycling. It is believed that the nanoplates preserve its morphology and integrity of the electrode during Li+ ion insertion/deinsertion and utilizes efficient particle-particle contacts, which leads to improve the electrochemical performances. In contrast, the mulberry-like
Fig. 6. (a) Initial discharge/charge voltage profiles of both CuO nanoplate and mulberry-like porous shape of CuO electrodes; (b) 2nd, 25th and 50th discharge/ charge voltage profiles of CuO nanoplate electrode; (c) 2nd, 25th and 50th discharge/ charge voltage profiles of mulberry-like porous CuO electrode.
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Fig. 7. Cycling performance of (a) CuO nanoplate and (b) mulberry-like porous shape of CuO electrodes at constant current rate of 1.0 C.
porous shape of CuO structure could not maintain the integrity of the electrode during large volume expansion/contraction resulted from Li+ ion insertion/deinsertion due to the aggregation of the primary nanoparticles and the loose contact between the particles, thus lead to the pulverization of the electrode from the current collector. This hypothesis was confirmed by the post-cycling FETEM morphology as discussed later and shown in Fig. 9. Fig. 7 (a) and (b) compares the cycling performance of nanoplate and mulberry-like porous shape of CuO electrodes cycled at a constant current rate of 1.0 C. It is apparent to notice that the CuO nanoplate electrode exhibits better cyclic retention capacity than that of the mulberry-like porous shape of CuO electrode. The CuO nanoplate electrode maintains reversible charge capacity of 279.3 mAh g1 after 70 cycles, whereas the mulberry-like porous shape of CuO electrode capacity decays much faster and could retain only 131.4 mAh g1 after 70 cycles. The obtained results demonstrate that the CuO nanoplate electrode has better electrochemical reversibility and structural stability. More importantly, in order to clearly evaluate the performance of CuO nanoplate electrode, a detailed comparison
Fig. 8. Comparison of the C-rate capability of (a) CuO nanoplate and (b) mulberrylike porous shape of CuO electrodes at various currents rates between 0.5 C to 4.0 C.
between the present nanoplate electrode and the previous CuO electrodes based on different synthesis methods and various nanostructures is shown in Table 1 [7–9,12,22,25,28–31]. On the other hand, the mulberry-like porous shape of CuO electrode shows high capacity in initial few cycles due to the porous nature of the electrodes, which may trap more Li+ ions during electrochemical cycling. The charge capacities of the nanoplate and mulberry-like porous shape of CuO electrode cycled between 0.01 and 3.0 V under various C rates are also reported in Fig. 8 (a) and (b), respectively. As can be seen, the charge capacity decreases with increasing current rates. At a high current rate of 4.0 C, the CuO
Table 1 Comparison of specific capacities of the current work with previous CuO electrodes reported in the literatures. Materials
Synthesis Method
Current Rate
Cycle number
Specific Capacity (mAh g1)
Ref.
CuO microsphere
Hydrothermal Method Self-assembled Synthesis Self-assembled Synthesis Template free Method Electrospinning –
1.0 C
100
203.6
[7]
50
340
[8]
0.5 C
50
374
[8]
0.1 C
50
320
[9]
2.22 C 0.1 C
50 55
167 231.9
[12] [22]
Hydrothermal Method Template free one step synthesis Simple liquid Chemical reduction Method Simple Solution Route Hydrothermal Method Hydrothermal Method
0.1 C
50
392.4
[25]
0.1 C
275
608
[28]
2.5 C 0.2 C
30 50
Null 440
[29] [30]
0.1 C
30
470
[31]
1.0 C
70
279.3
Current Study
4.0 C
20
150.2
Current Study
Dandelion CuO Caddice-clew CuO Pillow shaped Porous CuO CuO nanofibers Oatmeal CuO Structures CuO-flower like CuO Nanoribbons Array CuO urchin-like Octahedral CuO Hollow Crystals CuO bowknot Structure CuO Nanoplate CuO Nanoplate
0.5
C
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nanoplate electrode delivers a charge capacity of 150.2 mAh g1, which is still higher than the mulberry-like porous shape of CuO electrode capacity (121.7 mAh g1 at 4.0 C). It is also important to notice that the CuO nanoplate electrode shows very stable capacity during cycling at different current rates, whereas the mulberry-like porous shape of CuO electrode capacities are rapidly faded at all current rates. In addition, as long as the current rate reversed back to the low current rates of 0.5 C and 1.0 C, the capacity of CuO nanoplate electrode recovered to the original values of 364.71 mAh g1 and 285.71 mAh g1 respectively, which indicates that the integrity of CuO nanoplate electrode has been preserved even after high rate cycling. In contrast, when the current rate returns back to the low current rates of 0.5 C and 1.0 C, the mulberry-like porous shape of CuO electrode could retain only 50% capacity of the original capacity such as 328.04 mAh g1 at 0.5 C and 243.38 mAh g1 at 1.0 C. It is worth to suggest that the nanoplate type architecture is very stable and favorable to accommodate the volume expansion/contraction and maintain contact between particles during the cyclic process, whereas the mulberry-like
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porous shape is completely deformed during Li+ ion insertion/ deinsertion process. To support the above hypothesis, a post-cycling FE-TEM investigation was carried out on the electrodes which give us an in-depth understanding on the structure and electrochemical performance correlation. In order to do ex-situ FE-TEM studies, the cycled electrodes were initially dissociated from the cell in an argon filled glove box. The electrodes were thoroughly washed with dimethyl carbonate to remove the electrolyte. After drying, the electrode materials were scrapped off from the Cu-substrate to check the ex-situ FE-TEM analysis. Fig. 9(a) and (b) shows the exsitu FE-TEM images of CuO nanoplate electrode at different regions after 70 cycles. It is clearly observed that the nanoplate morphology has not been changed after repeated cycling processes. Fig. 9 (c) shows the high magnification image of a CuO nanoplate electrode along with the corresponding HR-TEM image in Fig. 9 (d). From the HR-TEM image, the lattice plane distance was measured to be 0.252 nm, and this is in good agreement with the (111) plane of monoclinic phase of CuO.
Fig. 9. (a, b) Low magnification ex-situ FE-TEM images of CuO nanoplate electrode at different magnifications. (c) The high-magnification image of a CuO nanoplate; of which the corresponding HR-TEM image is shown in (d). (e, f) mulberry-like porous shape of CuO electrode at different magnifications.
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collector during cycling. On the other hand, post-cycling FE-TEM images confirmed that the mulberry-like porous shape of CuO electrode is not capable enough to hold its morphology. Consequently, the mulberry porous shape was completely disintegrated into nanoparticles leading to the structural degradation of the electrode upon cycling. Acknowledgements A.K. Rai is grateful to Department of Science and Technology (DST), New Delhi, Government of India for the award of Ramanujan Fellowship (SB/S2/RJN-044/2015). The authors express their gratitude to Dr. Shaheer Akhtar, new and renewable energy materials development centre, Chonbuk National University, Jeonju, South Korea for helping with the FE-SEM characterizations. We are also thankful to Amrita Centre for Nanosciences for providing the infrastructure. Fig. 10. EIS spectra of CuO nanoplate and mulberry-like porous shape of CuO electrodes after 9 cycles at room temperature.
Therefore, the CuO nanoplate electrode with original structure and morphology after the long cycles indicates that the plate type structure may relieve the strain caused by severe volume change and prevent the agglomeration during continuous Li+ ion intake/ uptake. On the contrary, the mulberry-like porous shape of CuO electrode could not buffer the large volume expansion/contraction during cycling and disintegrated into nanoparticles, which leads to the degradation of the electrode materials, as shown in Fig. 9 (e) and (f). EIS test was also carried out to further understand the advantage of the CuO nanoplate electrode over mulberry-like porous shape of CuO electrode. Fig. 10 shows the Nyquist plot of both the electrodes after 9 cycles at 1.0 C. Both electrodes exhibit a high frequency semicircle and a long low-frequency line. It is wellknown that the semicircle in the high frequency region corresponds to the SEI resistance, and the charge-transfer resistance at the electrode/electrolyte interface and the straight line in a low frequency region is the Li+ ion diffusion resistance in the solid electrode material (Warburg impedance) [1]. It can be seen that the CuO nanoplate electrode exhibits much smaller semicircle compared with the mulberry-like CuO electrode, clearly indicating lower impedance in the former electrode. Hence, the improved cyclability, high rate performance and the depressed polarization could be attributed to the significant decrease in the impedance of nanoplate electrode. 4. Conclusion In summary, we reported a facile and cost-effective hydrothermal method for the synthesis of nanoplate and mulberry-like porous shape of CuO followed by annealing at low temperature of 300 C for 3 h. The X-ray diffraction patterns show no crystalline impurities in the annealed sample. FE-SEM and FE-TEM images confirmed that the CuO sample synthesized at different pH and reaction temperature exhibited nanoplate with average width and length of 150–300 nm and 300–700 nm and mulberry-like porous shape of CuO with average length of 300–400 nm. The CuO nanoplate structure exhibits good cycling performance and the high rate capability than the mulberry-like porous shape of CuO electrode. Such better electrochemical performances of the nanoplate electrode are likely to originate from the novel structure, which maintains the integrity of the electrode material and prevents its self-aggregation and detachment from current
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nanoplate and mulberry-like porous shape of CuO as anode materials for secondary lithium ion battery Subhalaxmi Mohapatra, Shantikumar V. Nair, Dhamodaran Santhanagopalan, Alok Kumar Rai* Amrita Centre for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Amrita University, Kochi 682041, Kerala, India
A R T I C L E I N F O
Article history: Received 19 February 2016 Received in revised form 11 April 2016 Accepted 21 April 2016 Available online 27 April 2016 Keywords: CuO Nanoplate Mulberry Porous Lithium ion batteries
A B S T R A C T
Facile hydrothermal synthesis of nanoplate and mulberry-like porous shape of CuO nanostructures was developed as anode materials for application in lithium ion batteries. The powder X-ray diffraction patterns of both the samples were indexed well to a pure monoclinic phase of CuO with no impurities. The CuO sample synthesized at different pH and reaction temperature exhibited nanoplate with average width and length of 150–300 nm and 300–700 nm and mulberry-like porous shape of CuO with average length of 300–400 nm. Electrochemical tests show that the lithium storage performances of both the nanoplate and mulberry-like samples are influenced more closely to its structural aspects than their morphology and size factors. The CuO nanoplate electrode exhibits high reversible charge capacity of 279.3 mAh g1 at 1.0C after 70 cycles, and a capacity of 150.2 mAh g1 even at high current rate of 4.0C during rate test, whereas the mulberry-like porous shape of CuO anode delivers only 131.4 mAh g1 at 1.0C after 70 cycles and 121.7 mAh g1 at 4.0C. It is believed that the nanoplate type architecture is very favorable to accommodate the volume expansion/contraction and aggregation of particles during the cyclic process. In contrast, the mulberry-like porous morphology could not preserve the integrity of the structure and completely disintegrated into nanoparticles during Li+ ion insertion/deinsertion due to the loose contact between the particles. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Non-carbon anodes (mainly silicon, tin, and transition metal oxides) are very promising for use in lithium ion batteries owing to their high theoretical capacity and better rate property in comparison with current commercial graphite anode. It is wellknown that the main characteristics of graphite anode such as low theoretical capacity and poor rate capability are not enough to satisfy the demand for future energy storage [1]. Despite many efforts over the last decades, the use of silicon and tin based anode materials in lithium ion batteries is still limited due to their large volume changes (>300%) during the alloying/dealloying process with Li+ ions, which leads to severe mechanical stress of the electrode and pulverization of the particles [2]. As a result, the capacity fades rapidly during the course of use. Fortunately, transition metal oxides with conversion mechanism of lithium storage (MO þ 2Liþ þ 2e $Li2 O þ M0 ) were regarded as a new hope for wide application in lithium ion batteries because of their
* Corresponding author. +91-4842-858750 (Ext. 5643). E-mail addresses: [email protected], [email protected] (A.K. Rai). http://dx.doi.org/10.1016/j.electacta.2016.04.116 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
low-cost, environmental benignity and high theoretical specific capacity [3,4]. Among all the proposed transition metal oxides, CuO is considered as a promising anode candidate for lithium ion batteries due to its abundance, low-cost, easy preparation, chemical stability, high theoretical capacity (674 mAh g1) and environmental friendliness [5]. Apart from all these advantages, the practical application of CuO is still hampered due to its poor cyclic performances, low conductivity, poor ion transport kinetics and severe capacity fading [5]. In order to avoid all these issues, synthesis of CuO with various unique nanostructure and porous morphologies [6–14], and fabrication of hybrid nanocomposite with conductive matrixes (such as carbon, carbon nanotubes and graphene nanosheets etc) are main typical approaches [5,15–18]. It is widely reported that the shape controlled morphologies highly affect the properties of the nanomaterials, suggesting that the morphological modification is a better key to improve poor cyclic retention of anode materials [19]. In addition, the nanostructure electrode can also offer easy Li+ ion diffusion, faster reaction kinetics, more stable structure and accommodation of large strain without severe pulverization [20]. Hence, it is believed that the novel architecture with porous morphology would be the feasible
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approach to solve the above issues of CuO anode material for lithium ion battery. Therefore, in the present work, first time we have successfully synthesized mulberry-like porous shape of CuO nanostructure through facile and cost-effective hydrothermal synthesis method. For comparison, CuO with nanoplate structure was also synthesized under different condition by the same method. In contrast to the belief, it was found that the nanoplate morphology remains well-preserved and stable even after long term cycling process, whereas the mulberry-like porous shape of CuO had strong capacity fading. This is due to the complete distortion of the mulberry-like porous shape and loss of contact between the particles upon cycling. Hence, the CuO nanoplate electrode has potential to be a high-performance anode material. 2. Experimental 2.1. Materials preparation Both nanoplate and mulberry-like porous shape of CuO were prepared by facile hydrothermal synthesis method. In a typical synthesis of CuO nanoplate sample, 0.2 M of copper acetate anhydrous [Cu(CH3COO)2H2O, Alfa Aesar, 98%] was dissolved in 30 ml of deionized water under continuous stirring to form a transparent solution. Then, a separately prepared 40 mL aqueous solution of sodium hydroxide (2 M) [NaOH, 99.99%, Sigma Aldrich] was slowly added into the above solution. More importantly, a few drops of liquor ammonia [Fisher Scientific, 25%] were also added into the solution to maintain the pH 13-14. After maintaining the pH, the solution was vigorously stirred for 24 h. Consequently, the resultant solution was then transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was then heated to 180 C for 15 h. After cooling to the room temperature, the black color precipitate was collected by centrifugation, washed with deionized water and ethanol for several times and dried at 80 C for 6 h. The obtained as-prepared powder was then annealed at 300 C for 3 h in air to obtain the crystalline CuO nanoplate sample. On the other hand, in order to synthesize mulberry-like porous shape of CuO, the whole procedure was same except few minor changes. First, 0.2 M copper acetate anhydrous was dissolved in 40 ml of deionized water followed by addition of 30 mL of aqueous solution of sodium hydroxide (0.2 M) under continuous stirring. Then, a few drops of liquor ammonia were also added to adjust the pH 10-11. Afterward, the obtained solution was vigorously stirred for 24 h at room temperature. The solution was then transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 120 C for 12 h. The black color precipitate at the end of the reaction was washed with deionized water and alcohol several times and finally dried at 80 C for 6 h. Finally, the obtained as-prepared powder was annealed at 300 C for 3 h in air to obtain the crystalline CuO mulberry sample.
Brunauer-Emmett-Teller method (BET, ASAP2010 Instrument Company, Norcross, GA).
Micromeritics
2.3. Electrochemical testing The electrochemical behavior of the obtained products were examined by coin-type half-cell (CR-2032) assembled in an argon filled glove box (O2 and H2O maintained less than 0.5 ppm). The samples obtained above were used as active materials. For the anode preparation, the active material, carbon black as a conducting agent and poly (vinyl difluoride) (PVDF) as a binder was mixed together with a weight ratio of 70:20:10 in N-methyl-2pyrrolidinone (Sigma Aldrich) as the solvent to form a slurry. The resulting slurry was casted onto Cu foil current collector uniformly by the doctor blade technique and dried in a vacuum oven at 80 C for 12 h. Subsequently the slurry was pressed between stainless steel twin rollers to improve the adhesion between the Cu foil and active materials. Lithium foil was used as the counter electrode. 1 M LiPF6 dissolved in an ethylene carbonate and dimethyl carbonate (molar ratio 1:1, in volume) was used as the nonaqueous electrolyte. A glass fiber was also used as a separator. Cyclic voltammetry was obtained using an Autolab potentiostat (PGSTAT 302 N) with the scan rate of 0.1 mV s1 within the range of 0–3.0 V. The discharge/charge measurements were performed at room temperature using Arbin (BT 2000) in the voltage range between 0.01 V to 3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were also carried out using a Autolab potentiostat (PGSTAT 302 N) to measure the impedance of the assembled cells. Before the measurements, the cells were cycled for 9 cycles and then measured in the frequency range of 0.01 Hz to 100 MHz.
2.2. Materials characterization The composition and phase purity of both the annealed samples were analyzed by powder X-ray diffraction (XRD; Shimadzu X-ray diffractometer) using Cu Ka radiation (l = 1.5406 Å). The particles size and surface morphology of the samples were measured by field-emission scanning electron microscopy (FE-SEM; S4800Hitachi) and field-emission transmission electron microscopy (FE-TEM, FEI 20 FEG electron microscopy instrument operating at 200 kV). The samples were first soaked in ethanol and dispersed by ultrasonication before drop casting onto copper grids for FE-TEM examination. The surface area measurement was based on the nitrogen adsorption and desorption isotherms using the
Fig. 1. X-ray powder diffraction patterns of annealed CuO (a) nanoplate and (b) mulberry-like porous samples.
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3. Results and discussion 3.1. Structural and morphological analysis X-ray diffraction patterns of both the nanoplate and mulberrylike porous shape of CuO samples are shown in Fig. 1 (a) and (b), respectively. It can be seen that both the annealed samples demonstrate strong diffraction peaks, which can be clearly indexed with well-crystalline monoclinic phase of CuO (JCPDS Card No. 050661). No impurity peaks were detected, revealing the high purity of the final samples. It is also worth to notice that the diffraction peaks of mulberry-like porous shape of CuO sample are relatively wide in comparison to nanoplate CuO sample, which clearly indicates relatively smaller particles. In order to investigate the influences of different conditions mainly pH value and reaction temperature on the surface morphology of the CuO samples, FE-SEM analysis was carried out. Fig. 2 (a), (b) and (c) shows the FE-SEM images of various regions of the CuO sample at different magnification, which clearly demonstrate the plate-like morphology of the sample with width and length of 150–300 nm and 300–700 nm, respectively. It is well-known that the 2D nanoplate morphology can provide high diffusion coefficient and short ion/electron diffusion path during Li+ ions insertion and extraction, which improves the capacity retention over prolonged discharge/charge cycles [21]. In contrast, in order to synthesize mulberry-like shape of CuO, the pH was maintained at 10–11. It can be observed from Fig. 2 (d), (e) and (f) obtained at different regions that the whole sample is entirely composed of uniformly dispersed mulberry-like porous shape with a typical length of 300–400 nm. More precisely, the high CuO magnified image of Fig. 2 (f) clearly shows that the individual mulberry-like shape of CuO sample is highly porous and composed by several small primary nanoparticles, which are in the range of 20–40 nm. It is also worth noting that the mulberry-like porous shape was formed via an assembly way of tiny nanoparticles and the tiny nanoparticles involved in the forming assembly are not tightly held with each other. However, as for the lithium ion battery, such architectures with the pores can not only provide a plenty of channels for Li+ ion insertion, but also short diffusion
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length for Li+ ions, leading to a high lithium storage capacity and enhanced rate capability. The detailed morphological investigations of both the samples were further examined by FE-TEM analysis. Consistent with FESEM observation, FE-TEM analysis also confirmed that nanoplate and mulberry-like porous shape of CuO was uniformly formed with controlled pH of 13–14 and 10–11, respectively. The FE-TEM images of the CuO nanoplate sample at different regions are shown in Fig. 3 (a) and (b). It shows that the sample is composed of nonuniform nanoplates and the dark region in the middle of Fig. 3 (b) is resulted from the stacking of few nanoplates. In addition, high magnification FE-TEM image demonstrate that the free-standing single-crystal structure are thin enough to be transparent and thus provide stronger structural stability during the lithiation/delithiation processes. It can be also observed that the surface of nanoplate is very smooth without having any defect or strain, as shown in insert of Fig. 3 (a). A typical FE-TEM image of mulberry-like porous shape of CuO was also illustrated in Fig. 3 (c) and (d) at different regions, which clearly confirm that the mulberry-like shape of CuO was assembled by a large quantity of tiny nanoparticles and this can be also verify by the magnified inset image of Fig. 3 (d). N2 adsorption-desorption studies were also performed to determine the specific surface area of both the nanoplate and mulberry-like porous shape of CuO samples. The isotherms of both the samples are presented in Fig. 4 (a) and (b). It can be observed that both the samples show type IV isotherms with a distinct hysteresis loop. The mulberry-like porous shape of CuO has higher surface area (11.3 m2 g1) than the nanoplate CuO structure (3.9 m2 g1). Obviously, the BET specific surface area of the mulberry-like sample is higher than that of the nanoplate sample, which is due to the small range of nanoparticles. Large surface area means more active sites and large contact area with the electrolyte. 3.2. Electrochemical performance Fig. 5 (a) and (b) shows the first five cycles of cyclic voltammetry curves of the nanoplate and mulberry-like porous shape of CuO electrode, respectively at a scan rate of 0.1 mV s1. In
Fig. 2. FE-SEM images of (a, b and c) CuO nanoplate and (d, e and f) mulberry-like porous shape of CuO samples at different magnifications annealed at 300 C for 3 h.
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Fig. 3. FE-TEM images of (a and b) CuO nanoplate and (c and d) mulberry-like porous shape of CuO samples at different magnifications.
the first reduction sweep, three cathodic peaks were appeared at 1.66 V (weak), 0.97 V (strong), and 0.77 V (medium) for the nanoplate CuO electrode, and at 1.62 V (weak), 1.07 V (strong) and 0.72 V (medium) for the mulberry-like porous CuO electrode. These peaks can be ascribed to the multistep of electrochemical reaction processes such as (i) the limited amount of Li+ ion insertion, involving a formation of an intermediate solid solution phase (CuII1x CuIx O1x=2 ) (0 x 0.4), (ii) the phase transformation from CuII1x CuIx O1x=2 to Cu2O phase and (iii) finally the decomposition of Cu2O into Cu and amorphous Li2O (Cu2O + 2Li+ + 2e ! 2 Cu + Li2O), along with the growth of SEI layer [22–24]. On the other hand, during the subsequent oxidation process, the main anodic peak at 2.49 V for the nanoplate CuO electrode and at 2.46 V for the
mulberry-like porous electrode is due to the oxidation of Cu to Cu2O and then further oxidation to CuO. In the subsequent cycles, the cathodic and anodic peaks are slightly shifted and fixed at around 1.27 V for the nanoplate electrode and 2.18 V, 1.28 V and 0.81 V for the mulberry-like electrode. More importantly, it can be seen that the cyclic voltammetry curves of the nanoplate electrode overlap very well with the subsequent scan numbers, indicating good electrochemical reversibility of the CuO nanoplate electrode, whereas the mulberry-like porous CuO electrode clearly shows the poor reversibility in the subsequent cycles. In addition, it is also important to notice that the gap between the cathodic and anodic peaks is broader for the mulberry-like porous CuO electrode, exhibiting more polarization than the nanoplate CuO electrode.
Fig. 4. N2 adsorption/desorption isotherms of (a) nanoplate and (b) mulberry-like porous shape of CuO samples.
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Fig. 5. Cyclic voltammograms of (a) nanoplate and (b) mulberry-like porous shape of CuO electrodes.
The cyclic voltammetry peaks and their variations are consistent with the galvanostatic charge/discharge profile. Fig. 6 (a) is the first discharge and charge curves of nanoplate and mulberry-like porous shape of CuO electrode at a constant current rate of 1.0 C (1 C = 674 mA g1). The voltage profile of the first discharge reaction of both the electrodes showed a long voltage plateau at 1.1 V and a short voltage plateau at about 0.8 V, followed by a sloppy profile down to the cutoff voltage of 0.01 V, which mainly correspond to the multi-step electrochemical reaction process such as reduction of CuO into intermediate composite copper oxide phase, further to Cu2O phase and finally the decomposition of Cu2O into Cu nanoparticles and Li2O matrix, respectively [25]. On the other hand, first charge curve of both the electrodes display sloppy profile with a small plateau near 2.4 V, which is due to the reformation of CuO. Moreover, the initial discharge and charge capacity was measured to be 966.2 mAh g1 and 353.4 mAh g1 for the nanoplate CuO electrode and 929.4 mAh g1 and 546.4 mAh g1 for the mulberry-like shape of CuO electrode, respectively. However, the irreversible capacity loss of 612.8 mAh g1 and 383.0 mAh g1 in the first cycle could be assigned to the formation of inevitable solid electrolyte interface (SEI) layer on the electrodes surface, which generally occur for both 3d transition metals oxides and carbon electrodes [26,27]. Fig. 6 (b) and (c) shows the 2nd, 25th and 50th discharge/charge curves of nanoplate and mulberry-like porous shape of CuO electrodes at a current rate of 1.0C, respectively. It can be seen that all the slopes become narrow and the plateau slightly upward during the following discharge process. It can be observed from Fig. 6 (b) that after the 1st cycle, the curves of each cycle are similar in shape, indicating that the electrode reactions become more reversible. In addition, the reversible discharge and charge capacities of 354.4 mAh g1 and 329.1 mAh g1 in the 2nd cycle, 331.4 mAh g1 and 322.3 mAh g1 in the 25th cycle and 309.3 mAh g1 and 303.9 mAh g1 in the 50th cycles were delivered by CuO nanoplate electrode, which clearly shows small capacity loss in the further cycles. In contrast, the mulberry-like shape of CuO electrode in Fig. 6 (c) could retain only 550.2 mAh g1 and 511.4 mAh g1 and 254.2 mAh g1 and 257.3 mAh g1 and 164.7 mAh g1 and 164.3 mAh g1 during 2nd, 25th and 50th discharge/charge cycles, respectively. More importantly, the clear plateau observed at 1.32 V in 2nd cycle of mulberrylike porous shape of CuO electrode is no more available in the subsequent cycles of 25th and 50th cycles, which also indicates the significant loss of reversible capacity in the further cycling. It is believed that the nanoplates preserve its morphology and integrity of the electrode during Li+ ion insertion/deinsertion and utilizes efficient particle-particle contacts, which leads to improve the electrochemical performances. In contrast, the mulberry-like
Fig. 6. (a) Initial discharge/charge voltage profiles of both CuO nanoplate and mulberry-like porous shape of CuO electrodes; (b) 2nd, 25th and 50th discharge/ charge voltage profiles of CuO nanoplate electrode; (c) 2nd, 25th and 50th discharge/ charge voltage profiles of mulberry-like porous CuO electrode.
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Fig. 7. Cycling performance of (a) CuO nanoplate and (b) mulberry-like porous shape of CuO electrodes at constant current rate of 1.0 C.
porous shape of CuO structure could not maintain the integrity of the electrode during large volume expansion/contraction resulted from Li+ ion insertion/deinsertion due to the aggregation of the primary nanoparticles and the loose contact between the particles, thus lead to the pulverization of the electrode from the current collector. This hypothesis was confirmed by the post-cycling FETEM morphology as discussed later and shown in Fig. 9. Fig. 7 (a) and (b) compares the cycling performance of nanoplate and mulberry-like porous shape of CuO electrodes cycled at a constant current rate of 1.0 C. It is apparent to notice that the CuO nanoplate electrode exhibits better cyclic retention capacity than that of the mulberry-like porous shape of CuO electrode. The CuO nanoplate electrode maintains reversible charge capacity of 279.3 mAh g1 after 70 cycles, whereas the mulberry-like porous shape of CuO electrode capacity decays much faster and could retain only 131.4 mAh g1 after 70 cycles. The obtained results demonstrate that the CuO nanoplate electrode has better electrochemical reversibility and structural stability. More importantly, in order to clearly evaluate the performance of CuO nanoplate electrode, a detailed comparison
Fig. 8. Comparison of the C-rate capability of (a) CuO nanoplate and (b) mulberrylike porous shape of CuO electrodes at various currents rates between 0.5 C to 4.0 C.
between the present nanoplate electrode and the previous CuO electrodes based on different synthesis methods and various nanostructures is shown in Table 1 [7–9,12,22,25,28–31]. On the other hand, the mulberry-like porous shape of CuO electrode shows high capacity in initial few cycles due to the porous nature of the electrodes, which may trap more Li+ ions during electrochemical cycling. The charge capacities of the nanoplate and mulberry-like porous shape of CuO electrode cycled between 0.01 and 3.0 V under various C rates are also reported in Fig. 8 (a) and (b), respectively. As can be seen, the charge capacity decreases with increasing current rates. At a high current rate of 4.0 C, the CuO
Table 1 Comparison of specific capacities of the current work with previous CuO electrodes reported in the literatures. Materials
Synthesis Method
Current Rate
Cycle number
Specific Capacity (mAh g1)
Ref.
CuO microsphere
Hydrothermal Method Self-assembled Synthesis Self-assembled Synthesis Template free Method Electrospinning –
1.0 C
100
203.6
[7]
50
340
[8]
0.5 C
50
374
[8]
0.1 C
50
320
[9]
2.22 C 0.1 C
50 55
167 231.9
[12] [22]
Hydrothermal Method Template free one step synthesis Simple liquid Chemical reduction Method Simple Solution Route Hydrothermal Method Hydrothermal Method
0.1 C
50
392.4
[25]
0.1 C
275
608
[28]
2.5 C 0.2 C
30 50
Null 440
[29] [30]
0.1 C
30
470
[31]
1.0 C
70
279.3
Current Study
4.0 C
20
150.2
Current Study
Dandelion CuO Caddice-clew CuO Pillow shaped Porous CuO CuO nanofibers Oatmeal CuO Structures CuO-flower like CuO Nanoribbons Array CuO urchin-like Octahedral CuO Hollow Crystals CuO bowknot Structure CuO Nanoplate CuO Nanoplate
0.5
C
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nanoplate electrode delivers a charge capacity of 150.2 mAh g1, which is still higher than the mulberry-like porous shape of CuO electrode capacity (121.7 mAh g1 at 4.0 C). It is also important to notice that the CuO nanoplate electrode shows very stable capacity during cycling at different current rates, whereas the mulberry-like porous shape of CuO electrode capacities are rapidly faded at all current rates. In addition, as long as the current rate reversed back to the low current rates of 0.5 C and 1.0 C, the capacity of CuO nanoplate electrode recovered to the original values of 364.71 mAh g1 and 285.71 mAh g1 respectively, which indicates that the integrity of CuO nanoplate electrode has been preserved even after high rate cycling. In contrast, when the current rate returns back to the low current rates of 0.5 C and 1.0 C, the mulberry-like porous shape of CuO electrode could retain only 50% capacity of the original capacity such as 328.04 mAh g1 at 0.5 C and 243.38 mAh g1 at 1.0 C. It is worth to suggest that the nanoplate type architecture is very stable and favorable to accommodate the volume expansion/contraction and maintain contact between particles during the cyclic process, whereas the mulberry-like
223
porous shape is completely deformed during Li+ ion insertion/ deinsertion process. To support the above hypothesis, a post-cycling FE-TEM investigation was carried out on the electrodes which give us an in-depth understanding on the structure and electrochemical performance correlation. In order to do ex-situ FE-TEM studies, the cycled electrodes were initially dissociated from the cell in an argon filled glove box. The electrodes were thoroughly washed with dimethyl carbonate to remove the electrolyte. After drying, the electrode materials were scrapped off from the Cu-substrate to check the ex-situ FE-TEM analysis. Fig. 9(a) and (b) shows the exsitu FE-TEM images of CuO nanoplate electrode at different regions after 70 cycles. It is clearly observed that the nanoplate morphology has not been changed after repeated cycling processes. Fig. 9 (c) shows the high magnification image of a CuO nanoplate electrode along with the corresponding HR-TEM image in Fig. 9 (d). From the HR-TEM image, the lattice plane distance was measured to be 0.252 nm, and this is in good agreement with the (111) plane of monoclinic phase of CuO.
Fig. 9. (a, b) Low magnification ex-situ FE-TEM images of CuO nanoplate electrode at different magnifications. (c) The high-magnification image of a CuO nanoplate; of which the corresponding HR-TEM image is shown in (d). (e, f) mulberry-like porous shape of CuO electrode at different magnifications.
224
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collector during cycling. On the other hand, post-cycling FE-TEM images confirmed that the mulberry-like porous shape of CuO electrode is not capable enough to hold its morphology. Consequently, the mulberry porous shape was completely disintegrated into nanoparticles leading to the structural degradation of the electrode upon cycling. Acknowledgements A.K. Rai is grateful to Department of Science and Technology (DST), New Delhi, Government of India for the award of Ramanujan Fellowship (SB/S2/RJN-044/2015). The authors express their gratitude to Dr. Shaheer Akhtar, new and renewable energy materials development centre, Chonbuk National University, Jeonju, South Korea for helping with the FE-SEM characterizations. We are also thankful to Amrita Centre for Nanosciences for providing the infrastructure. Fig. 10. EIS spectra of CuO nanoplate and mulberry-like porous shape of CuO electrodes after 9 cycles at room temperature.
Therefore, the CuO nanoplate electrode with original structure and morphology after the long cycles indicates that the plate type structure may relieve the strain caused by severe volume change and prevent the agglomeration during continuous Li+ ion intake/ uptake. On the contrary, the mulberry-like porous shape of CuO electrode could not buffer the large volume expansion/contraction during cycling and disintegrated into nanoparticles, which leads to the degradation of the electrode materials, as shown in Fig. 9 (e) and (f). EIS test was also carried out to further understand the advantage of the CuO nanoplate electrode over mulberry-like porous shape of CuO electrode. Fig. 10 shows the Nyquist plot of both the electrodes after 9 cycles at 1.0 C. Both electrodes exhibit a high frequency semicircle and a long low-frequency line. It is wellknown that the semicircle in the high frequency region corresponds to the SEI resistance, and the charge-transfer resistance at the electrode/electrolyte interface and the straight line in a low frequency region is the Li+ ion diffusion resistance in the solid electrode material (Warburg impedance) [1]. It can be seen that the CuO nanoplate electrode exhibits much smaller semicircle compared with the mulberry-like CuO electrode, clearly indicating lower impedance in the former electrode. Hence, the improved cyclability, high rate performance and the depressed polarization could be attributed to the significant decrease in the impedance of nanoplate electrode. 4. Conclusion In summary, we reported a facile and cost-effective hydrothermal method for the synthesis of nanoplate and mulberry-like porous shape of CuO followed by annealing at low temperature of 300 C for 3 h. The X-ray diffraction patterns show no crystalline impurities in the annealed sample. FE-SEM and FE-TEM images confirmed that the CuO sample synthesized at different pH and reaction temperature exhibited nanoplate with average width and length of 150–300 nm and 300–700 nm and mulberry-like porous shape of CuO with average length of 300–400 nm. The CuO nanoplate structure exhibits good cycling performance and the high rate capability than the mulberry-like porous shape of CuO electrode. Such better electrochemical performances of the nanoplate electrode are likely to originate from the novel structure, which maintains the integrity of the electrode material and prevents its self-aggregation and detachment from current
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