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Energy storage performance of CuO as a cathode material for aqueous zinc ion battery
Materials Today Energy 15 (2020) 100370
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Energy storage performance of CuO as a cathode material for aqueous zinc ion battery Jinlei Meng a, b, Zhanhong Yang a, *, Linlin Chen a, b, Haigang Qin a, b, Fan Cui a, b, Yinan Jiang a, b, Xiao Zeng a, b a Hunan Province Key Laboratory of Chemical Power Source, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China b Innovation Base of Energy and Chemical Materials for Graduate Students Training, Central South University, Changsha, 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2019 Received in revised form 24 November 2019 Accepted 24 November 2019 Available online xxx
Rechargeable aqueous zinc ion batteries (ZIBs) with high specific capacity appear promising to meet the increasing demand for low cost and sustainable energy storage devices. Because the investigation of aqueous ZIBs is still in the incipient stage, the exploration of cathode materials with high specific capacity is necessary. Herein, the CuO nanorods were prepared by a simple liquid phase method and for the first time the CuO/Zn system with high and stable voltage platform was established successfully. The discharge platform is at 0.82 V and remains stable throughout the charge and discharge process. The constant current charge-discharge test shows that the CuO/Zn battery within the voltage of 0.4e1.1 V delivers high reversible capacity of 219 mA h g1 at 0.3 A g1. The cyclic voltammogram analysis shows that the Zn-ion storage in CuO is a diffusion-controlled kinetic process. Meanwhile, the phase evolution study during the first charge-discharge cycle reveals that the energy storage mechanism of CuO cathode is conversion reaction. The results demonstrate the feasibility of a conversion reaction energy storage mechanism for zinc ion batteries. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Copper oxide Nanorods Cathode materials Zinc ion battery
1. Introduction Due to environmental degradation and resource shortages, the massive use of renewable energy is an inevitable choice for sustainable development [1,2]. However, wind energy, solar energy and geothermal energy, due to their intermittent nature, need to be used in conjunction with energy storage equipment. Therefore, the development of energy storage equipment is the key to the efficient use of energy. Since the establishment of the battery research in 1600 year by Gilbert, the battery has become an indispensable energy storage device in people's daily life while improving its structure and function. Because potential safety issues, high cost and limited lithium (Li) supply pose a major challenge to lithium ion battery who dominates the current energy market, the development of new batteries is imminent. Aqueous zinc-ion batteries (ZIBs) are receiving more and more attention in view of the increasing lightness, portability, long battery life and green safety of
* Corresponding author. E-mail address: [email protected] (Z. Yang). https://doi.org/10.1016/j.mtener.2019.100370 2468-6069/© 2019 Elsevier Ltd. All rights reserved.
batteries. ZIBs use metal zinc as anode material. Compared with metals Na, Mg and Al, metal Zn delivers a high volumetric energy density (5851 mA h ml1) and has an outstanding theoretical capacity (819 mA h g1) [3e9]. Meanwhile, the high oxidationreduction potential of the zinc anode (0.76 V) can effectively ensure the stability of the electrode in the aqueous electrolyte. In addition, in near-neutral or weakly acidic electrolytes (such as ZnSO4, Zn(CF3SO3)2 solution), zinc dendrites are less likely to occur and there is no phenomenon of low coulombic efficiency and short life due to the production of ZnO in alkaline zinc batteries. There have been studies on ZIBs electrolytes. Zhi et al. reported that zinc ion batteries based on sol-gel transition electrolytes have extremely high safety [10]; Zhou et al. first used guar gum as ZIBs electrolyte and showed good electrochemical performance [11]. Combining with the advantages of rich zinc resources, environmental protection and easy transportation, it means that there is great potential for the commercialization of ZIBs [12]. The research on aqueous zinc ion batteries is still in initial stage and mainly focused on the development of cathode materials. Mnbased materials, vanadium-based materials and Prussian blue analogues have been applied for aqueous ZIBs. Manganese-based
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materials, including a-, b-, d-MnO2, Mn2O3, Mn3O4 and manganate have been investigated as Zn-insertion cathode materials. Although different crystal forms of MnO2 exhibit considerable specific capacity at low current densities, their cycle stability is poor and the capacity decays quickly after several cycles. Elaborative electrochemical mechanism illustrates that the crystal form of MnO2 is transformed during the discharge process and Mn is partially dissolved, resulting in a decline in stability [13e18]. Vanadium-based materials have become a research hot spot due to their high specific capacity at high rate discharge and good cycle stability. Such as porous V2O5 microspheres [19] and Na3V2(PO4)3/C [20]. The Prussian blue analogues cathode materials exhibit excellent cycling performance but low capacity (typically about 50 mA h g1) which limits its application [21,22]. Thus the exploration of cathode materials with high capacity and long cycle life has been considered as one key for the development of ZIBs. Copper oxide, a p-type semiconductor material, has been used in catalyst, solar energy storage and lithium ion battery anode materials because of its low toxicity and low cost [23e25]. In this work, the CuO/Zn system was first designed in 3 M ZnSO4 electrolyte. The CuO electrode exhibits stable charge and discharge platforms and a discharge specific capacity of up to 219 mA h g1 at 0.3 A g1. Moreover, electrochemical investigations, ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were carried out to understand the zinc storage mechanism. The low cost of the raw materials and the expected environmental protection suggest that the battery assembled here is significant for further exploration of copper based materials.
ammonia (25%e28%, 50 ml) and deionized water (50 ml). The solution was maintained at 60 C for 1 h. Then the solution was poured to deionized water under vigorous stirring. The volume of the solution versus water was at 1:20. In a short time, the solution was full of Cu(OH)2. The Cu(OH)2 were filtered, washed by water and absolute ethanol, and then dispersed again in water. Annealing at 60 C for 20 min transformed the Cu(OH)2 into CuO nanorods. Finally, the precipitate was filtered, washed, and dried in a vacuum oven at 60 C to obtain CuO nanorods sample. XRD pattern was tested using a Japanese science ultima iv x-ray diffractometer with Cu Ka radiation. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was used to observe the morphology of the sample. X-ray photoelectron spectroscopy (XPS) was tested on a K-Alpha þ X-ray photoelectron spectrometer using a monochromated Al Ka source (Mono Al Ka). The CuO/Zn aqueous battery uses 3 M ZnSO4 as the electrolyte, the glass fiber membrane as the separator and the zinc foil with a thickness of 0.1 mm as the negative electrode. The cathode is prepared by grinding a CuO sample, conductive nanocarbon super P and polytetrafluoroethylene (PTFE) in a weight ratio of 7:2:1, preparing a sheet, and finally ballasting on a titanium mesh. Electrochemical performance was tested on an assembled CR2025 battery. A constant current charge and discharge test and Galvanostatic intermittent titration technique (GITT) were carried out on a Neware battery test system (CT-4008-5V10mA-164). Cyclic voltammetry (CV) were carried out on CHI 660E electrochemical workstation. The electrochemical impedance spectroscopy (EIS) test was carried out in the frequency range of 102 to 105 Hz with amplitude of 5 mV.
2. Experimental section 3. Results and discussion CuO nanorods were prepared by a simple liquid phase method. As reported in the previous literature [26], Cu(NO3)2$3H2O (0.005 mol) was weighed into a beaker and dissolved with aqueous
The crystal structure of CuO sample was first characterized by Xray diffraction (XRD). As shown in Fig. 1a, the XRD pattern is
Fig. 1. Morphology and structural characterization of CuO (a) XRD pattern of the CuO sample. (b) Crystal structure model diagram. (c) EDX spectrum of the synthesized CuO. (d)SEM image of CuO (e) TEM image of CuO.
J. Meng et al. / Materials Today Energy 15 (2020) 100370
dominated by (002) and (111) reflections, corresponding to a highly preferred orientation owing to its rod-liked crystals. The remaining weaker diffraction peaks fit well with the standard card (PDF# 450937) of CuO, indicating that high purity CuO has been synthesized. Fig. 1b shows the crystal structure model of CuO. CuO is built up of parallel quadrilateral stacks connected by OeO bonds. Each onedimensional CuO parallelogram consists of a central copper atom and four oxygen atom vertices, in which the copper atoms adopt a four-coordination configuration. The large crystal spacing of 3.48 Å belonged to (111) is far greater than the radii of Zn2þ ions (0.74 Å), which provides a large space for chemical conversion reaction and effectively reduces the volume expansion caused by the conversion process. As can be seen from the EDX result of CuO sample (Fig. 1c), the atomic ratio of Cu to O is about 1, further illustrates the successful preparation of CuO. The morphology of the obtained sample was then characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As seen in Fig. 1d, the sample CuO was found to be a rod-like structure with a length of
3
about 100e200 nm and a diameter of about 10 nm. The nanorods piled up irregularly. Fig. 1d shows a TEM image of the intersection of two separate CuO nanorods, indicating that the nanorods have good dispersion. To investigate the electrochemical performance of the CuO electrode, CuO/Zn cells with CuO sample as working electrodes and zinc foil as counter electrodes were constructed and tested. Cyclic voltammetry (CV) curves of CuO versus Zn at a scan rate of 0.1 mV s1 in the range of 0.4e1.1 V (Fig. 2a) show only a pair of redox peaks at 1.0 V and 0.67 V, manifesting the phase transition electrochemical reaction of CuO cathode. After the first cycle, the CV curves of reduction peak shifts to the high potential, suggesting the activation process driven by zinc ions. Electrode activation is usually accompanied by an increase in capacity which is frequently observed in vanadium-based and manganese-based electrode [27e31]. It is a kinetics-promoting process that can be caused by structural transformations and crystal defects. However, this activation can be interpreted as a conversion for CuO electrode. Since
Fig. 2. Electrochemical performance test of CuO electrode (a)CV curves at 0.1 mV s1. (b) Charge and discharge curves at 0.05 A g1. (c) Rate performance of CuO. (d) Chargedischarge curves at different current densities. (e) Cycle performance at 300 mA g1 in the voltage of 0.4e1.1V.
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the charge cut-off voltage is set to 1.1 V, the CuO electrode cannot be fully reversibly restored to the initial state after the first cycle. Fig. 2b shows the charge and discharge curves of the battery under different cycles. It can be seen that the discharge platform raised from the first 0.62 V to 0.8 V and was very stable during subsequent discharges. Generally, the potential difference for discharge/charge potential plateaus is relevant to the electrode polarization in the electrochemical reaction processes [32]. The reason for the lower discharge platform of the 1st discharge may be attributed to the large impedance of the fresh CuO/Zn battery. The strong polarization of the system makes the discharge platform drop significantly. CuO provides an initial discharge capacity of 356 mA h g1 at 0.3 A g1 (Fig. 2e) with a coulomb efficiency of 70%. The initial coulombic efficiency of the prepared CuO electrode is a little low but reaches to nearly 100% in subsequent several cycles. The low initial efficiency may be related to parasite reactions such as electrolyte decomposition and irreversible conversion of CuO, which is consistent with the previous results [25,26,33]. Except for the first cycle, a coulomb efficiency of about 100% and the CV curves are similar in shape, declaring that the electrode reactions become more reversible. This is consistent with the CV test results. In the rate performance test, the CuO delivers high average capacities of 255, 230, 200 and 190 mA h g1 at current densities of 0.05, 0.1, 0.3 and 0.5 A g1 (Fig. 2c), respectively. When the current density drops back to the initial 0.05 A g1, the average discharge specific capacity of 250 mA h g1 is recovered, corresponding to about 100% of the initial capacity. This indicates that the prepared CuO possesses considerable rate capability which due to that the crystal structure with large interplanar spacing provides easy and fast kinetics for the conversion reaction. As can be seen from Fig. 1c, the void formed by the irregular accumulation of CuO nanorods facilitates the contact between the active material and the electrolyte, which is also advantageous for excellent rate performance. Fig. 2d corresponds to the charge and discharge curves which have significant charge and discharge platforms at different densities.
Besides, the long-term stability of cathode also has been evaluated. The steady discharge capacity of 170 mA h g1 at 0.3 A g1 between 25 and 125 cycles was observed (Fig. 2e). Moreover, electrochemical investigations and structural analysis were coupled to better understand the electrode behaviors and reaction mechanism. Considering the possibility of intercalation pseudo capacitance on the surface, kinetic analysis based on CV curves at different scan rate (Fig. 3a) was employed to differentiate the contribution. To ensure the accuracy of the kinetic analysis, the CV tests at different scan rate were based on one battery after being fully activated. The relationship between scan rate and peak current can be described by the following formula:
i ¼ anb
(1)
logðiÞ ¼ b logðvÞ þ logðaÞ
(2)
The electrochemical reaction process can be reflected by the b value. In particular, the b-value of 1.0 represents a capacitive behavior, while 0.5 indicates a total diffusion driven process. From the linear fit of log (v) and log (i) (Fig. 3b), it can be concluded that the b values of peak1 and peak2 are 0.531 and 0.516, suggesting the kinetics of a diffusion-dominated process. The galvanostatic intermittent titration technique (GITT) was used to determine the diffusion coefficient of Zn2þ (DZn). Fig. 3c presents the quasiequilibrium redox potential of the CuO electrode at 20 mA g1. It can be found that the voltage difference of charge-discharge platform is only 0.1 V and the GITT curve is very similar to the chargedischarge curve of the above continuous cycle (Fig. 2b), indicating that the material with a small polarization and the material is very close to the equilibrium even in the continuous charge-discharge process, which due to the good ionic and electronic conductivities of the CuO [33]. The calculated diffusion coefficient Dzn (1016e1015 cm2 s1) can be seen in Fig. 3d. The smaller diffusion coefficient indicates the dynamics of diffusion control, which is
Fig. 3. Electrochemical investigations of CuO electrode (a) CV curves with different scan rate. (b) log (i) versus log (v) plots of cathodic current response at two peaks shown in (a). (c) GITT profile of CuO electrode at 20 mA g1 . (d) The corresponding diffusivity coefficient (D) of Zn2þ in the discharge and charge processes. (e) Nyquist plots of CuO//Zn battery at different cycles. (f) Nyquist plots at different potential of the second charge-discharge process; inset shows the equivalent circuit.
J. Meng et al. / Materials Today Energy 15 (2020) 100370
consistent with the CV analysis results at different sweep rates. According to the calculation results of the diffusion coefficient, the DZn at the initial stage of discharge is much larger than that at the later stage of the discharge and the sudden change of the diffusion coefficient when the voltage drops to about 0.7 V leads to temporary accumulation of the reactants. This may be attributed to the fact that the electrochemical reaction can be divided into two steps: reduction of CuO to Cu2O and Cu and reduction of Cu2O to Cu. This is shown in the charge-discharge curves (Fig. 2b and c), where the voltage first decreases and then rises, and the turning point is around 0.7V. This phenomenon was also observed in the recently reported a-MnO2 material [13]. Electrochemical impedance spectroscopy (EIS) was used to gain more insights into the impedance changes of the cycling process at 300 mA g1. Fig. 3d and e shows the fitted Nyquist plots in which the high frequency semicircle and the low frequency line are associated with charge transfer resistance (Rct) and Zn2þ transfer (Warbug impedance), respectively. It can be noted in Fig. 3e that before the cycle the diameter of the depressed semicircles representing Rct is very large. After the first cycle, the semicircle becomes smaller and the Rct of 1st, 10th, 20th cycle are 25 U, 13 U, 12 U. This is consistent with the trend that the discharge specific capacity decreases first and then stabilizes. Fig. 3e shows the impedance plot at different voltages for the second cycle, a significant decrease in charge transfer resistance (Rct) in the fully discharged and charged state indicates a good charge transfer capability of the battery during charge and discharge. For the purpose of identifying the phase evolution of CuO during the electrochemical reaction, ex situ XRD were employed at different charge and discharge states in the first cycle of the CuO electrode (Fig. 4a). The initial state shows the peak of CuO. After discharging to 0.79 V, the peaks of Cu2O and Cu appeared and the peak of CuO weakened, indicating that CuO was converted. When fully discharged to 0.4 V, the CuO peak disappeared and the Cu peak appeared, demonstrating complete conversion of CuO. During the charging process, the peak of CuO appears again. However, the peak of CuO in the fully charged state is still weak and the peak of Cu2O is
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strong, which because Cu and Cu2O in the discharged state cannot be oxidized to CuO completely due to the low charging cut-off voltage. The low coulomb efficiency of the first cycle is also based on this reason, indicating that increasing the reversibility of the transition between Cu2O and CuO can effectively increase the battery capacity. Meanwhile, it can be noted that the ZnSO4[Zn(OH)2]3$xH2O at 30.5 exists in the whole process of charging and discharging, which can be attributed to the interaction between ZnSO4 electrolyte and the OH: produced during the conversion of Cu, Cu2O and CuO [27,34]. The XPS spectra were used to analysis the valence state of Cu in charge and discharge process. The high resolution XPS spectrum of Zn2p (Fig. 4b) demonstrates the presence of ZnSO4[Zn(OH)2]3$xH2O during charge and discharge. For the initial electrode (Fig. 4c), the copper element valence is mainly þ2 valence. After discharging to 0.4V (Fig. 4d), Cu2þ is reduced to Cuþ and Cu0. When charged to 1.1V (Fig. 4e), the copper element is mostly Cuþ and a small part is Cu2þ [35]. The peak of Cu0 which is not oxidized coincides with the peak of Cuþ. The XPS spectrum is consistent with the XRD test results. Based on the above results, the electrochemical reaction of the CuO/Zn battery within 0.4e1.1V can be described as below: First discharge: 3CuO þ 4Hþ þ 4e / Cu þ Cu2O þ 2H2O
(3)
Subsequent charge and discharge: Cu2O þ 2Hþ þ 2e42Cu þ H2O
(4)
Overall:Cu2O þ Zn þ (x/3)H2O þ (1/3)ZnSO442Cu þ (1/3) ZnSO4[Zn(OH)2]3$(x-1)H2O
(5)
The energy storage is completed by the transformation between different oxidation states of transition metal. After the CuO is reduced to Cu and Cu2O in the first discharge process, although some CuO was formed and decomposed during the subsequent cycle, the conversion reaction of Cu2O and Cu contributed most of the battery capacity.
Fig. 4. Ex situ XRD patterns and XPS spectrum of CuO in the first discharge-charge cycle at 0.3 A g1. (a) Ex situ XRD pattern in the first cycle at 0.3 A g1. (b) Zn2p region. (c), (d), (e) Cu2p region of the XPS spectra in the pristine and fully discharged/charged states.
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Fig. 5. (a) Comparison of the Ragone plot of the CuO//Zn cell with that of Mo6S8//Zn, NiHCF//Zn, ZnHCF//Zn, ZnVO Array//Zn, V2O5//Zn and Na3V2(PO4)3@C//Zn cells. (b) Average discharge voltage chart.
In order to measure the electrochemical performance of CuO electrodes, several other cathode materials were used for comparison. Fig. 5a compares the rate performance of CuO electrodes and other zinc ion battery cathode materials. Our CuO//Zn system shows better rate performance than the reported Mo6S8//Zn [36], NiHCF//Zn [37], ZnVO Array//Zn [38], Na1.1V3O7.9@rGO//Zn [39], V2O5//Zn [40] and Na3V2(PO4)3@C//Zn [41] system. As the power density increases, the energy density decreases slightly. Fig. 5b show the average discharge voltage of CuO is 0.82V, which is substantially the same as that of V2O5 (0.85 V) [40] and V2O5∙nH2O (0.8 V) [42] and higher than some vanadium-based materials such as H2V3O8 (0.69 V) [43] and Zn2V2O7 (0.65V) [27]. The average discharge voltage is calculated as following:
1 U¼ Qt
Q ðt
UdQ
(6)
0
where U and Q are the voltage and the discharge capacity respectively, and Qt is the total capacity. High discharge voltage and specific capacity make CuO a promising cathode material. 4. Conclusion In conclusion, CuO nanorods were prepared by a simple liquid phase method and for the first time CuO was evaluated as a cathode material for aqueous ZIBs. The CuO/Zn battery exhibits a long and stable discharge platform at 0.82 V, which is close to or slightly higher than the vanadium-based material. The CuO cathode delivers an average capacity as high as 170 mA h g1 at 0.3 A g1 during 200 cycles. Unlike the reported manganese-based, vanadium-based and prussian blue cathode materials, the CuO nanorods cathode exhibits a unique conversion reaction energy storage mechanism and it has been explained as conversion reaction between CuO, Cu2O and Cu, as evidenced by ex situ XRD and XPS measurements. This work not only broadens the choice of high specific capacity cathode materials for zinc ion batteries, but also provides a basis for the subsequent modification of copper-based materials. Author contributions section Jinlei Meng: Conceptualization, Validation, Investigation, Resources, Writing, Review & Editing. Zhanhong Yang: Project administration, Supervision, Resources. Linlin Chen: Resources. Haigang Qin: Resources.
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[30] B. Lee, C.S. Yoon, H.R. Lee, K.Y. Chung, B.W. Cho, S.H. Oh, Electrochemicallyinduced reversible transition from the tunneled to layered polymorphs of manganese dioxide, Sci. Rep. 4 (2014) 6066. [31] J. Lee, J.B. Ju, W.I. Cho, B.W. Cho, H.O. Si, Todorokite-type MnO2 as a zinc-ion intercalating material, Electrochim. Acta 112 (2013) 138e143. [32] Y. Fu, Q. Wei, G. Zhang, X. Wang, J. Zhang, Y. Hu, D. Wang, L. Zuin, T. Zhou, Y. Wu, S. Sun, Adv. Energy Mater. 8 (2018) 1801445. [33] Y. Shuang, X. Huang, D. Ma, H. Wang, F. Meng, X. Zhang, Electrodes: engraving copper foil to give large-scale binder-free porous CuO arrays for a highperformance sodium-ion battery anode, Adv. Mater. 26 (2014) 2273e2279. [34] L. Ma, S. Chen, H. Li, Z. Ruan, Z. Tang, Z. Liu, Z. Wang, Y. Huang, Z. Pei, J.A. Zapien, C. Zhi, Initiating a mild aqueous electrolyte Co3O4/Zn battery with 2.2 V-high voltage and 5000-cycle lifespan by a Co(III) rich-electrode, Energy Environ. Sci. 11 (2018) 2521e2530. [35] F. Xiong, Y. Fan, S. Tan, L. Zhou, Y. Xu, C. Pei, Q. An, L. Mai, Magnesium storage performance and mechanism of CuS cathode, Nano Energy 47 (2018) 210e216. [36] S.C. Munseok, W.H. Jongwook, Sung-Chul Lim, Seung-Tae Hong, Electrochemical zinc-ion intercalation properties and crystal structures of ZnMo6S8 and Zn2Mo6S8 chevrel phases in aqueous electrolytes, Inorg. Chem. 55 (2016) 3294e3330. [37] S.C. Munseok, W.H. Jongwook, H.K. Hunho, L. Hochun, Seung-Tae Hong, Organic electrolyte-based rechargeable zinc-ion batteries using potassium nickel hexacyanoferrate as a cathode material, J. Power Sources 337 (2017) 204e211. [38] C. Liang, C. Zhu, M. Song, L. Pei, X. Zhang, N.H. Tiep, F. Zhao, J. Wang, R. Wang, H. Zhang, J. Fan, A high-rate and stable quasi-solid-state zinc-ion battery with novel 2D layered zinc orthovanadate Array, Adv. Mater. 30 (2018) 1803181. [39] Y. Cai, F. Liu, Z. Luo, G. Fang, J. Zhou, A. Pan, S. Liang, Pilotaxitic Na1.1V3O7.9 nanoribbons/graphene as high-performance sodium ion battery and aqueous zinc ion battery cathode, Energy Storage Mater. 13 (2018) 168e174. [40] P. Hu, M. Yan, T. Zhu, X. Wang, X. Wei, J. Li, L. Zhou, Z. Li, L. Chen, L. Mai, Zn/ V2O5 aqueous hybrid-Ion battery with high voltage platform and long cycle life, ACS Appl. Mater. Interfaces 49 (2017) 42717e42722. [41] G. Li, Z. Yang, Y. Jiang, C. Jin, W. Huang, X. Ding, Y. Huang, Towards polyvalent ion batteries: a zinc-ion battery based on NASICON structured Na3V2(PO4)3, Nano Energy 25 (2016) 211e217. [42] M. Yan, P. He, Y. Chen, S. Wang, Q. Wei, K. Zhao, X. Xu, Q. An, Y. Shuang, Y. Shao, K.T. Mueller, L. Mai, J. Liu, J. Yang, Water-lubricated intercalation in V2O5$nH2O for high-capacity and high-rate aqueous rechargeable zinc batteries, Adv. Mater. 30 (2018) 1703725. [43] P. He, Y. Quan, X. Xu, M. Yan, W. Yang, Q. An, L. He, L. Mai, High-performance aqueous zinceion battery based on layered H2V3O8 nanowire cathode, Small 13 (2017) 1702551.
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Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/
Energy storage performance of CuO as a cathode material for aqueous zinc ion battery Jinlei Meng a, b, Zhanhong Yang a, *, Linlin Chen a, b, Haigang Qin a, b, Fan Cui a, b, Yinan Jiang a, b, Xiao Zeng a, b a Hunan Province Key Laboratory of Chemical Power Source, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China b Innovation Base of Energy and Chemical Materials for Graduate Students Training, Central South University, Changsha, 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2019 Received in revised form 24 November 2019 Accepted 24 November 2019 Available online xxx
Rechargeable aqueous zinc ion batteries (ZIBs) with high specific capacity appear promising to meet the increasing demand for low cost and sustainable energy storage devices. Because the investigation of aqueous ZIBs is still in the incipient stage, the exploration of cathode materials with high specific capacity is necessary. Herein, the CuO nanorods were prepared by a simple liquid phase method and for the first time the CuO/Zn system with high and stable voltage platform was established successfully. The discharge platform is at 0.82 V and remains stable throughout the charge and discharge process. The constant current charge-discharge test shows that the CuO/Zn battery within the voltage of 0.4e1.1 V delivers high reversible capacity of 219 mA h g1 at 0.3 A g1. The cyclic voltammogram analysis shows that the Zn-ion storage in CuO is a diffusion-controlled kinetic process. Meanwhile, the phase evolution study during the first charge-discharge cycle reveals that the energy storage mechanism of CuO cathode is conversion reaction. The results demonstrate the feasibility of a conversion reaction energy storage mechanism for zinc ion batteries. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Copper oxide Nanorods Cathode materials Zinc ion battery
1. Introduction Due to environmental degradation and resource shortages, the massive use of renewable energy is an inevitable choice for sustainable development [1,2]. However, wind energy, solar energy and geothermal energy, due to their intermittent nature, need to be used in conjunction with energy storage equipment. Therefore, the development of energy storage equipment is the key to the efficient use of energy. Since the establishment of the battery research in 1600 year by Gilbert, the battery has become an indispensable energy storage device in people's daily life while improving its structure and function. Because potential safety issues, high cost and limited lithium (Li) supply pose a major challenge to lithium ion battery who dominates the current energy market, the development of new batteries is imminent. Aqueous zinc-ion batteries (ZIBs) are receiving more and more attention in view of the increasing lightness, portability, long battery life and green safety of
* Corresponding author. E-mail address: [email protected] (Z. Yang). https://doi.org/10.1016/j.mtener.2019.100370 2468-6069/© 2019 Elsevier Ltd. All rights reserved.
batteries. ZIBs use metal zinc as anode material. Compared with metals Na, Mg and Al, metal Zn delivers a high volumetric energy density (5851 mA h ml1) and has an outstanding theoretical capacity (819 mA h g1) [3e9]. Meanwhile, the high oxidationreduction potential of the zinc anode (0.76 V) can effectively ensure the stability of the electrode in the aqueous electrolyte. In addition, in near-neutral or weakly acidic electrolytes (such as ZnSO4, Zn(CF3SO3)2 solution), zinc dendrites are less likely to occur and there is no phenomenon of low coulombic efficiency and short life due to the production of ZnO in alkaline zinc batteries. There have been studies on ZIBs electrolytes. Zhi et al. reported that zinc ion batteries based on sol-gel transition electrolytes have extremely high safety [10]; Zhou et al. first used guar gum as ZIBs electrolyte and showed good electrochemical performance [11]. Combining with the advantages of rich zinc resources, environmental protection and easy transportation, it means that there is great potential for the commercialization of ZIBs [12]. The research on aqueous zinc ion batteries is still in initial stage and mainly focused on the development of cathode materials. Mnbased materials, vanadium-based materials and Prussian blue analogues have been applied for aqueous ZIBs. Manganese-based
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materials, including a-, b-, d-MnO2, Mn2O3, Mn3O4 and manganate have been investigated as Zn-insertion cathode materials. Although different crystal forms of MnO2 exhibit considerable specific capacity at low current densities, their cycle stability is poor and the capacity decays quickly after several cycles. Elaborative electrochemical mechanism illustrates that the crystal form of MnO2 is transformed during the discharge process and Mn is partially dissolved, resulting in a decline in stability [13e18]. Vanadium-based materials have become a research hot spot due to their high specific capacity at high rate discharge and good cycle stability. Such as porous V2O5 microspheres [19] and Na3V2(PO4)3/C [20]. The Prussian blue analogues cathode materials exhibit excellent cycling performance but low capacity (typically about 50 mA h g1) which limits its application [21,22]. Thus the exploration of cathode materials with high capacity and long cycle life has been considered as one key for the development of ZIBs. Copper oxide, a p-type semiconductor material, has been used in catalyst, solar energy storage and lithium ion battery anode materials because of its low toxicity and low cost [23e25]. In this work, the CuO/Zn system was first designed in 3 M ZnSO4 electrolyte. The CuO electrode exhibits stable charge and discharge platforms and a discharge specific capacity of up to 219 mA h g1 at 0.3 A g1. Moreover, electrochemical investigations, ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were carried out to understand the zinc storage mechanism. The low cost of the raw materials and the expected environmental protection suggest that the battery assembled here is significant for further exploration of copper based materials.
ammonia (25%e28%, 50 ml) and deionized water (50 ml). The solution was maintained at 60 C for 1 h. Then the solution was poured to deionized water under vigorous stirring. The volume of the solution versus water was at 1:20. In a short time, the solution was full of Cu(OH)2. The Cu(OH)2 were filtered, washed by water and absolute ethanol, and then dispersed again in water. Annealing at 60 C for 20 min transformed the Cu(OH)2 into CuO nanorods. Finally, the precipitate was filtered, washed, and dried in a vacuum oven at 60 C to obtain CuO nanorods sample. XRD pattern was tested using a Japanese science ultima iv x-ray diffractometer with Cu Ka radiation. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was used to observe the morphology of the sample. X-ray photoelectron spectroscopy (XPS) was tested on a K-Alpha þ X-ray photoelectron spectrometer using a monochromated Al Ka source (Mono Al Ka). The CuO/Zn aqueous battery uses 3 M ZnSO4 as the electrolyte, the glass fiber membrane as the separator and the zinc foil with a thickness of 0.1 mm as the negative electrode. The cathode is prepared by grinding a CuO sample, conductive nanocarbon super P and polytetrafluoroethylene (PTFE) in a weight ratio of 7:2:1, preparing a sheet, and finally ballasting on a titanium mesh. Electrochemical performance was tested on an assembled CR2025 battery. A constant current charge and discharge test and Galvanostatic intermittent titration technique (GITT) were carried out on a Neware battery test system (CT-4008-5V10mA-164). Cyclic voltammetry (CV) were carried out on CHI 660E electrochemical workstation. The electrochemical impedance spectroscopy (EIS) test was carried out in the frequency range of 102 to 105 Hz with amplitude of 5 mV.
2. Experimental section 3. Results and discussion CuO nanorods were prepared by a simple liquid phase method. As reported in the previous literature [26], Cu(NO3)2$3H2O (0.005 mol) was weighed into a beaker and dissolved with aqueous
The crystal structure of CuO sample was first characterized by Xray diffraction (XRD). As shown in Fig. 1a, the XRD pattern is
Fig. 1. Morphology and structural characterization of CuO (a) XRD pattern of the CuO sample. (b) Crystal structure model diagram. (c) EDX spectrum of the synthesized CuO. (d)SEM image of CuO (e) TEM image of CuO.
J. Meng et al. / Materials Today Energy 15 (2020) 100370
dominated by (002) and (111) reflections, corresponding to a highly preferred orientation owing to its rod-liked crystals. The remaining weaker diffraction peaks fit well with the standard card (PDF# 450937) of CuO, indicating that high purity CuO has been synthesized. Fig. 1b shows the crystal structure model of CuO. CuO is built up of parallel quadrilateral stacks connected by OeO bonds. Each onedimensional CuO parallelogram consists of a central copper atom and four oxygen atom vertices, in which the copper atoms adopt a four-coordination configuration. The large crystal spacing of 3.48 Å belonged to (111) is far greater than the radii of Zn2þ ions (0.74 Å), which provides a large space for chemical conversion reaction and effectively reduces the volume expansion caused by the conversion process. As can be seen from the EDX result of CuO sample (Fig. 1c), the atomic ratio of Cu to O is about 1, further illustrates the successful preparation of CuO. The morphology of the obtained sample was then characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As seen in Fig. 1d, the sample CuO was found to be a rod-like structure with a length of
3
about 100e200 nm and a diameter of about 10 nm. The nanorods piled up irregularly. Fig. 1d shows a TEM image of the intersection of two separate CuO nanorods, indicating that the nanorods have good dispersion. To investigate the electrochemical performance of the CuO electrode, CuO/Zn cells with CuO sample as working electrodes and zinc foil as counter electrodes were constructed and tested. Cyclic voltammetry (CV) curves of CuO versus Zn at a scan rate of 0.1 mV s1 in the range of 0.4e1.1 V (Fig. 2a) show only a pair of redox peaks at 1.0 V and 0.67 V, manifesting the phase transition electrochemical reaction of CuO cathode. After the first cycle, the CV curves of reduction peak shifts to the high potential, suggesting the activation process driven by zinc ions. Electrode activation is usually accompanied by an increase in capacity which is frequently observed in vanadium-based and manganese-based electrode [27e31]. It is a kinetics-promoting process that can be caused by structural transformations and crystal defects. However, this activation can be interpreted as a conversion for CuO electrode. Since
Fig. 2. Electrochemical performance test of CuO electrode (a)CV curves at 0.1 mV s1. (b) Charge and discharge curves at 0.05 A g1. (c) Rate performance of CuO. (d) Chargedischarge curves at different current densities. (e) Cycle performance at 300 mA g1 in the voltage of 0.4e1.1V.
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the charge cut-off voltage is set to 1.1 V, the CuO electrode cannot be fully reversibly restored to the initial state after the first cycle. Fig. 2b shows the charge and discharge curves of the battery under different cycles. It can be seen that the discharge platform raised from the first 0.62 V to 0.8 V and was very stable during subsequent discharges. Generally, the potential difference for discharge/charge potential plateaus is relevant to the electrode polarization in the electrochemical reaction processes [32]. The reason for the lower discharge platform of the 1st discharge may be attributed to the large impedance of the fresh CuO/Zn battery. The strong polarization of the system makes the discharge platform drop significantly. CuO provides an initial discharge capacity of 356 mA h g1 at 0.3 A g1 (Fig. 2e) with a coulomb efficiency of 70%. The initial coulombic efficiency of the prepared CuO electrode is a little low but reaches to nearly 100% in subsequent several cycles. The low initial efficiency may be related to parasite reactions such as electrolyte decomposition and irreversible conversion of CuO, which is consistent with the previous results [25,26,33]. Except for the first cycle, a coulomb efficiency of about 100% and the CV curves are similar in shape, declaring that the electrode reactions become more reversible. This is consistent with the CV test results. In the rate performance test, the CuO delivers high average capacities of 255, 230, 200 and 190 mA h g1 at current densities of 0.05, 0.1, 0.3 and 0.5 A g1 (Fig. 2c), respectively. When the current density drops back to the initial 0.05 A g1, the average discharge specific capacity of 250 mA h g1 is recovered, corresponding to about 100% of the initial capacity. This indicates that the prepared CuO possesses considerable rate capability which due to that the crystal structure with large interplanar spacing provides easy and fast kinetics for the conversion reaction. As can be seen from Fig. 1c, the void formed by the irregular accumulation of CuO nanorods facilitates the contact between the active material and the electrolyte, which is also advantageous for excellent rate performance. Fig. 2d corresponds to the charge and discharge curves which have significant charge and discharge platforms at different densities.
Besides, the long-term stability of cathode also has been evaluated. The steady discharge capacity of 170 mA h g1 at 0.3 A g1 between 25 and 125 cycles was observed (Fig. 2e). Moreover, electrochemical investigations and structural analysis were coupled to better understand the electrode behaviors and reaction mechanism. Considering the possibility of intercalation pseudo capacitance on the surface, kinetic analysis based on CV curves at different scan rate (Fig. 3a) was employed to differentiate the contribution. To ensure the accuracy of the kinetic analysis, the CV tests at different scan rate were based on one battery after being fully activated. The relationship between scan rate and peak current can be described by the following formula:
i ¼ anb
(1)
logðiÞ ¼ b logðvÞ þ logðaÞ
(2)
The electrochemical reaction process can be reflected by the b value. In particular, the b-value of 1.0 represents a capacitive behavior, while 0.5 indicates a total diffusion driven process. From the linear fit of log (v) and log (i) (Fig. 3b), it can be concluded that the b values of peak1 and peak2 are 0.531 and 0.516, suggesting the kinetics of a diffusion-dominated process. The galvanostatic intermittent titration technique (GITT) was used to determine the diffusion coefficient of Zn2þ (DZn). Fig. 3c presents the quasiequilibrium redox potential of the CuO electrode at 20 mA g1. It can be found that the voltage difference of charge-discharge platform is only 0.1 V and the GITT curve is very similar to the chargedischarge curve of the above continuous cycle (Fig. 2b), indicating that the material with a small polarization and the material is very close to the equilibrium even in the continuous charge-discharge process, which due to the good ionic and electronic conductivities of the CuO [33]. The calculated diffusion coefficient Dzn (1016e1015 cm2 s1) can be seen in Fig. 3d. The smaller diffusion coefficient indicates the dynamics of diffusion control, which is
Fig. 3. Electrochemical investigations of CuO electrode (a) CV curves with different scan rate. (b) log (i) versus log (v) plots of cathodic current response at two peaks shown in (a). (c) GITT profile of CuO electrode at 20 mA g1 . (d) The corresponding diffusivity coefficient (D) of Zn2þ in the discharge and charge processes. (e) Nyquist plots of CuO//Zn battery at different cycles. (f) Nyquist plots at different potential of the second charge-discharge process; inset shows the equivalent circuit.
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consistent with the CV analysis results at different sweep rates. According to the calculation results of the diffusion coefficient, the DZn at the initial stage of discharge is much larger than that at the later stage of the discharge and the sudden change of the diffusion coefficient when the voltage drops to about 0.7 V leads to temporary accumulation of the reactants. This may be attributed to the fact that the electrochemical reaction can be divided into two steps: reduction of CuO to Cu2O and Cu and reduction of Cu2O to Cu. This is shown in the charge-discharge curves (Fig. 2b and c), where the voltage first decreases and then rises, and the turning point is around 0.7V. This phenomenon was also observed in the recently reported a-MnO2 material [13]. Electrochemical impedance spectroscopy (EIS) was used to gain more insights into the impedance changes of the cycling process at 300 mA g1. Fig. 3d and e shows the fitted Nyquist plots in which the high frequency semicircle and the low frequency line are associated with charge transfer resistance (Rct) and Zn2þ transfer (Warbug impedance), respectively. It can be noted in Fig. 3e that before the cycle the diameter of the depressed semicircles representing Rct is very large. After the first cycle, the semicircle becomes smaller and the Rct of 1st, 10th, 20th cycle are 25 U, 13 U, 12 U. This is consistent with the trend that the discharge specific capacity decreases first and then stabilizes. Fig. 3e shows the impedance plot at different voltages for the second cycle, a significant decrease in charge transfer resistance (Rct) in the fully discharged and charged state indicates a good charge transfer capability of the battery during charge and discharge. For the purpose of identifying the phase evolution of CuO during the electrochemical reaction, ex situ XRD were employed at different charge and discharge states in the first cycle of the CuO electrode (Fig. 4a). The initial state shows the peak of CuO. After discharging to 0.79 V, the peaks of Cu2O and Cu appeared and the peak of CuO weakened, indicating that CuO was converted. When fully discharged to 0.4 V, the CuO peak disappeared and the Cu peak appeared, demonstrating complete conversion of CuO. During the charging process, the peak of CuO appears again. However, the peak of CuO in the fully charged state is still weak and the peak of Cu2O is
5
strong, which because Cu and Cu2O in the discharged state cannot be oxidized to CuO completely due to the low charging cut-off voltage. The low coulomb efficiency of the first cycle is also based on this reason, indicating that increasing the reversibility of the transition between Cu2O and CuO can effectively increase the battery capacity. Meanwhile, it can be noted that the ZnSO4[Zn(OH)2]3$xH2O at 30.5 exists in the whole process of charging and discharging, which can be attributed to the interaction between ZnSO4 electrolyte and the OH: produced during the conversion of Cu, Cu2O and CuO [27,34]. The XPS spectra were used to analysis the valence state of Cu in charge and discharge process. The high resolution XPS spectrum of Zn2p (Fig. 4b) demonstrates the presence of ZnSO4[Zn(OH)2]3$xH2O during charge and discharge. For the initial electrode (Fig. 4c), the copper element valence is mainly þ2 valence. After discharging to 0.4V (Fig. 4d), Cu2þ is reduced to Cuþ and Cu0. When charged to 1.1V (Fig. 4e), the copper element is mostly Cuþ and a small part is Cu2þ [35]. The peak of Cu0 which is not oxidized coincides with the peak of Cuþ. The XPS spectrum is consistent with the XRD test results. Based on the above results, the electrochemical reaction of the CuO/Zn battery within 0.4e1.1V can be described as below: First discharge: 3CuO þ 4Hþ þ 4e / Cu þ Cu2O þ 2H2O
(3)
Subsequent charge and discharge: Cu2O þ 2Hþ þ 2e42Cu þ H2O
(4)
Overall:Cu2O þ Zn þ (x/3)H2O þ (1/3)ZnSO442Cu þ (1/3) ZnSO4[Zn(OH)2]3$(x-1)H2O
(5)
The energy storage is completed by the transformation between different oxidation states of transition metal. After the CuO is reduced to Cu and Cu2O in the first discharge process, although some CuO was formed and decomposed during the subsequent cycle, the conversion reaction of Cu2O and Cu contributed most of the battery capacity.
Fig. 4. Ex situ XRD patterns and XPS spectrum of CuO in the first discharge-charge cycle at 0.3 A g1. (a) Ex situ XRD pattern in the first cycle at 0.3 A g1. (b) Zn2p region. (c), (d), (e) Cu2p region of the XPS spectra in the pristine and fully discharged/charged states.
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Fig. 5. (a) Comparison of the Ragone plot of the CuO//Zn cell with that of Mo6S8//Zn, NiHCF//Zn, ZnHCF//Zn, ZnVO Array//Zn, V2O5//Zn and Na3V2(PO4)3@C//Zn cells. (b) Average discharge voltage chart.
In order to measure the electrochemical performance of CuO electrodes, several other cathode materials were used for comparison. Fig. 5a compares the rate performance of CuO electrodes and other zinc ion battery cathode materials. Our CuO//Zn system shows better rate performance than the reported Mo6S8//Zn [36], NiHCF//Zn [37], ZnVO Array//Zn [38], Na1.1V3O7.9@rGO//Zn [39], V2O5//Zn [40] and Na3V2(PO4)3@C//Zn [41] system. As the power density increases, the energy density decreases slightly. Fig. 5b show the average discharge voltage of CuO is 0.82V, which is substantially the same as that of V2O5 (0.85 V) [40] and V2O5∙nH2O (0.8 V) [42] and higher than some vanadium-based materials such as H2V3O8 (0.69 V) [43] and Zn2V2O7 (0.65V) [27]. The average discharge voltage is calculated as following:
1 U¼ Qt
Q ðt
UdQ
(6)
0
where U and Q are the voltage and the discharge capacity respectively, and Qt is the total capacity. High discharge voltage and specific capacity make CuO a promising cathode material. 4. Conclusion In conclusion, CuO nanorods were prepared by a simple liquid phase method and for the first time CuO was evaluated as a cathode material for aqueous ZIBs. The CuO/Zn battery exhibits a long and stable discharge platform at 0.82 V, which is close to or slightly higher than the vanadium-based material. The CuO cathode delivers an average capacity as high as 170 mA h g1 at 0.3 A g1 during 200 cycles. Unlike the reported manganese-based, vanadium-based and prussian blue cathode materials, the CuO nanorods cathode exhibits a unique conversion reaction energy storage mechanism and it has been explained as conversion reaction between CuO, Cu2O and Cu, as evidenced by ex situ XRD and XPS measurements. This work not only broadens the choice of high specific capacity cathode materials for zinc ion batteries, but also provides a basis for the subsequent modification of copper-based materials. Author contributions section Jinlei Meng: Conceptualization, Validation, Investigation, Resources, Writing, Review & Editing. Zhanhong Yang: Project administration, Supervision, Resources. Linlin Chen: Resources. Haigang Qin: Resources.
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