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Effect of adding various carbon additives to porous zinc anode in rechargeable hybrid aqueous battery
Journal of Alloys and Compounds 658 (2016) 119e124
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of adding various carbon additives to porous zinc anode in rechargeable hybrid aqueous battery Haisheng Tao b, *, Xiang Tong a, Lu Gan a, Shuqiong Zhang a, Xuemei Zhang a, Xuan Liu b a b
College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China College of Environmental Science and Engineering, Anhui Normal University, Wuhu 241000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2015 Received in revised form 13 October 2015 Accepted 23 October 2015 Available online 27 October 2015
Anodic behavior of porous Zn in rechargeable hybrid aqueous battery is investigated. The battery comprises porous Zn anode and LiMn2O4 cathode. The electrolytes are Liþ and Zn2þ of sulfate salt in aqueous solution. The initial discharge capacity of pure porous Zn is 114 mAh g1, and the capacity decreases to less than 59 mAh g1 after 36 cycles. Then, the effects of various carbon additives on the battery performance are studied. The results show that the adding of active carbon increases the initial discharge capacity to 150 mAh g1, and the capacity maintains above 60 mAh g1 even after 210 cycles. The reasons for improvement for the battery are investigated by Tafel, scanning electron microscopy, and X-ray Diffraction. © 2015 Elsevier B.V. All rights reserved.
Keywords: Carbon additive Anode Zinc Hybrid Aqueous battery
1. Introduction With the continuous deterioration of the environment and excessive consumption of fossil fuels, the development of renewable energy sources of power like wind and solar has attracted considerable attention [1,2]. The most vexing issues of renewable energy are the sheer variability and intermittent of wind and solar power. Thus, it is significant to develop an energy storage system to solve the issues. All the kinds of energy storage systems, lithium ion batteries are remarkable, because they show high energy density, stable discharge voltage and long shelf life [3e6]. However, lithium ion batteries also suffer from some limitations, such as its toxicity, high cost and combustible organic solvents, which may cause some safety problems especially when it is out of control such as overcharging or short-circuiting [7e9]. As a result, the drawbacks above limit their application in large-scale batteries. In 1994, Dahn et al. proposed a new type of lithium ion battery [10] with aqueous electrolytes, VO2 anode and LiMn2O4 cathode (denoted as VO2// LiMn2O4). Compared with the conventional flammable electrolytes, it could operate in environmentally friendly aqueous electrolytes as well as possess highly ion conductivity. Afterwards, a large number
* Corresponding author. Tel.: þ86 553 5910728; fax: þ86 553 5910724. E-mail address: [email protected] (H. Tao). http://dx.doi.org/10.1016/j.jallcom.2015.10.225 0925-8388/© 2015 Elsevier B.V. All rights reserved.
of aqueous lithium ion batteries have been proposed such as LiV3O8//LiCoO2 [11], LiTi2(PO4)3//LiMn2O4 [12], LixV2O5-PPy// LiMn2O4 [13], PPy//LiCoO2 [14] and Zn//LiMnPO4 [15]. Recently, Yan et al. have proposed a novel rechargeable hybrid aqueous battery system [16] using two weak acidic electrolytes (Liþ and Zn2þ of chloride salt in aqueous solution). The function mechanism is that Liþ inserts into LiMn2O4 cathode and Zn2þ deposits on Zn anode at charge, then back to the solution of Liþ and Zn2þ at discharge, respectively. Subsequently, similar battery systems have been reported with a longer cycling life, such as Zn// Na0.95MnO2 [17] and PbSO4//LiMn2O4 [18], while the aqueous battery has a small capacity and has a low energy density in usual. In addition, Zn//Na0.95MnO2 has the inherent safety issue, such as dendrite formation on the Zn anode at charge, and PbSO4//LiMn2O4 uses heavy metal of Pb. To overcome these shortages, it is necessary to improve the performance of anode materials. Zn is an attractive candidate of anode material [19] for aqueous battery because of its low equilibrium potential (0.762 V vs. standard hydrogen electrode, SHE), abundance, low cost, low toxicity, and high specific energy. Compared with planar Zn, porous Zn material as anode is more widely used for battery [20,21], because it possesses higher surface area and could be easy to contact to the electrolyte. High surface area makes the passivation of Zn in charge process, especially when the hydrogen evolution on Zn makes the passivation growing
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intensity [22]. In order to solve this problem, some additives are added to the porous Zn anode. In the selection of the additive material, the requirements are taken into account, such as enhancement of the network at low additive concentration, excellent inter-particle contact, electrolyte access optimization, inert materials. And then, a series of oxides are used to weaken the passivation of Zn and hydrogen gas evolution, such as HgO [23], PbO [24], CdO [25], V2O5 [26], B2O3 [27], etc. From the viewpoint of environmental protection and economic benefit, the additives above can not meet the requirements. So it is still urgent to develop some novel and efficient additive materials. Carbon materials are applied widely in lithium ion battery, fuel cell and chemical sensor for its advantages such as high conductivity, high chemical stability, low price, non-toxicity, etc. For example, M. N. Masri et al. investigate [28] the addition of carbon black into the porous Zn anode in a Zn-air battery and the electrochemical performance is improved significantly. In this work, acetylene black (AB), carbon nanotube (CNT) and active carbon (AC) added to the porous Zn anode is investigated, respectively. Then the behaviors of the rechargeable hybrid aqueous battery (Zn//LiMn2O4) are studied by electrochemical method, scanning electron microscopy, and X-ray Diffraction. 2. Experimental 2.1. Preparation of anode and rechargeable hybrid aqueous battery fabrication Zn anode slurry are prepared by mixing 80 wt% Zn powder (99.99%, Aldrich), 10 wt% carbon material (AB, CNT and AC, respectively), and 10 wt% Polyvinylidene Fluoride (PVDF) binder blending in N-methyl-2-pyrrolidone (NMP). Then the slurry is coated on a black paper current collector (surface is 1.5 cm2) and dried in a vacuum at 60 C for 5 h. The cathode is prepared by the same method as that of the anode described above. The cathode consists of 80 wt% LiMn2O4, 10 wt% acetylene black, and 10 wt% PVDF. The LiMn2O4 is synthesized by the similar method according to our previous work. At first, MnCO3 is first prepared by a facile hydrothermal reaction from an aqueous solution of KMnO4 and glucose in a molar ratio of 10:1 at 180 C. Then, the prepared oval MnCO3 precursor is mixed with Li2CO3 in acetic acid under ultrasonic for 30 min. The mixture is vacuum-dried at 90 C. Then the mixture is heat-treated at 300 C for 3 h and 750 C for 4 h to get LiMn2O4, respectively. The electrolyte is prepared by dissolving 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 in deionized water. All of the segments are assembled to fabricate a rechargeable hybrid aqueous battery.
2.3. Physical characterizations The morphologies and structures of the prepared Zn anode are characterized by scanning electron microscopy (SEM, Hitachi S4800, 25 kV) and X-ray diffraction (XRD, Shimadzu 6000, Nifiltered Cu Ka radiation). 3. Results and discussion CVs of Zn and LiMn2O4 electrodes in 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 aqueous electrolyte at the scan rate of 1 mV s1 are shown in Fig. 1. In the case of Zn, there is one couple of clear redox peaks located at 0.20/0.15 V (vs. Zn2þ/Zn), which are due to the reversible dissolution and the plating of Zn. In the case of LiMn2O4, there are two couples of reversible redox peaks at 1.70/1.75 V and 1.80/1.91 V (vs. Zn2þ/Zn), respectively, which attribute to the twostep lithium intercalation/extraction of LiMn2O4 in the aqueous electrolyte. Since there is a potential difference between Zn and LiMn2O4 in 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 aqueous electrolyte, they can be assembled into a battery. The system is schematically shown in Scheme 1. The proposed cathodic and anodic reactions can be described as follows: charge
x=2Zn2þ þ LiMn2 O4 )
* x=2Zn þ Li1x Mn2 O4 þ xLiþ
discharge
(1)
The performance of the rechargeable hybrid aqueous batteries with pure porous Zn, Zn/CNT, Zn/AC and Zn/AB electrode are shown in Fig. 2, respectively. Fig. 2A presents the typical discharge characteristics of the batteries at the first cycle. The discharge plateau voltage is ca. 1.8 V, which is consistent with the CV data in Fig. 1. This is due to the intercalation of Liþ ions into the host. The initial discharge capacity of pure Zn electrode is 114 mAh g1. When acetylene black, carbon nanotube and active carbon is added to the pure Zn anode system, the discharge capacity increases to 130, 140 and 150 mAh g1, respectively. It suggests that the introduction of carbon material in the porous Zn anode improves the discharge capacity. Among them, active carbon is the best one due to its highest specific surface area that provides more sufficient reaction channels. Fig. 2B shows the cycling characteristics of the pure porous Zn, Zn/CNT, Zn/AC and Zn/AB anode at the current density of
2.2. Electrochemical characterizations Cyclic voltammetry (CV) is carried out on a CHI 660C electrochemical workstation (Shanghai, China) with a three-electrode-cell at a scan rate of 1 mV s1 in the potential range of 0.2e2.2 V (vs. Zn2þ/Zn). A LiMn2O4 and Zn is used as the working electrode, respectively, and each electrode is about 1 mg with an active area of 0.8 cm2. A platinum wire is used as the counter electrode, and a Zn foil is used as the reference electrode. The galvanostatic chargeedischarge tests are characterized on a battery test system TC5.X (Neware, China) at room temperature. Tafel polarization measurements are performed on the CHI 660C electrochemical system. A three-electrode cell includes a saturated calomel electrode (SCE) reference electrode, a platinum counter electrode and a porous Zn working electrode. Tafel polarization is recorded from 0.90 V to 1.25 V in 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 aqueous electrolyte at the scan rate of 0.5 mV s1.
Fig. 1. CVs of Zn and LiMn2O4 in the aqueous electrolyte of 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 at the scan rate of 1 mV s1, which is tested by using platinum and Zn as the counter and reference electrode, respectively.
H. Tao et al. / Journal of Alloys and Compounds 658 (2016) 119e124
Scheme 1. Schematic illustration of the redox reactions for Zn//LiMn2O4 rechargeable hybrid aqueous battery during charge and discharge processes.
240 mA g1, respectively. All the anodes show excellent discharge capacity in the first cycle, but the capacity decreases gradually after several cycles. The capacity of the pure Zn anode decreases to 59 mAh g1 at the 36th cycle, which implies that the cyclic performance of the pure Zn is poor. It is mainly due to the passivation of the Zn particles surface. In comparison with the pure Zn anode, the cyclic behavior and discharge capacity of the batteries with Zn/ AB, Zn/CNT and Zn/AC anodes improve significantly. The discharge capacity of Zn/AB and Zn/CNT electrodes decrease to 65 and 69 mAh g1 over 80 cycles, respectively. It is ascribed to the carbon (acetylene black, carbon nanotube) coating on Zn particle surface that prevents the direct contact of the Zn anode with electrolyte. So the corrosion of the active Zn particle can be restrained. Simultaneously, it is seen that the Zn/AC anode delivers the discharge capacity of 60 mAh g1 even after 210 cycles. It illustrates that the cycle life and discharge capacity of the Zn/AC anode are remarkably superior to the corresponding performance of Zn/AB and Zn/CNT. The excellent cycle performance can be mainly ascribed to the following reasons. It is possible that the active carbon has high over-potential of hydrogen evolution. It can effectively suppress the generation of hydrogen and prevent the passivation of Zn particle.
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On the other hand, the active carbon with high specific surface area provides the growth substrate for Zn electrodeposits. As a result, the porous Zn with 10 wt% content of the carbon additive contributes to improve the electrode efficiency and reversibility. The polarization curves of the pure porous Zn, Zn/CNT, Zn/AC and Zn/AB electrodes are present in Fig. 3. The corresponding results are listed in Table 1, where the corrosion potential (Ecorr) and the corrosion current density (jcorr) are obtained by extrapolation of the anodic and cathodic Tafel lines. As compared with the pure Zn anode, the Ecorr values of several Zn electrodes composed of the additive of different carbon materials into pure porous Zn anode have few movements, whereas the jcorr values have shifted in a negative direction. It indicates that the carbon additive materials can prevent the corrosion of Zn particle in weak acidic electrolytes. In the case of the introduction of AC in the porous Zn anode has the minimum jcorr value. It is possibly explained by a large amount of AC particles covering the Zn surfaces, which hinders the diffusion reaction of the electrolyte throughout the Zn particle, which is in accordance with the SEM result (Fig. 5C). And the stability of the AC has the highest over-potential of hydrogen evolution. Hence, it could be concluded that the AC is the most suited additive from the suppress corrosion point of view, which is in accordance with the proposed analysis. The XRD patterns before (curve a, b, c, d) and after (curve a′, b′, c′, d′) discharge of Zn, Zn/AB, Zn/AC and Zn/CNT electrodes are shown in Fig. 4. Curve a in Fig. 4A shows the pattern for Zn. It is
Fig. 3. Tafel plots of pure Zn, Zn/CNT, Zn/AC and Zn/AB electrodes.
Fig. 2. First discharge curves (A) and cycling performance (B) of the batteries with pure porous Zn, Zn/CNT, Zn/AC, and Zn/AB anode, respectively. Current density is 240 mA g1 based on the cathode between 0 and 2.0 V.
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Table 1 The data for Tafel curves with pure Zn, Zn/CNT, Zn/AC and Zn/AB. Sample
Ecorr/V
jcorr/A cm2
Zn Zn/CNT Zn/AC Zn/AB
1.044 1.041 1.047 1.042
1.29 1.09 4.4 8.1
102 102 103 103
seen that the Zn peaks dominate the pattern at 2q angles of 36.29 , 38.99 and 43.23 , which are indexed to the planes (002), (100) and (101), respectively. Curve b e d shows the XRD patterns with respect to the effect of adding AB, CNT and AC into the porous Zn anode system. The diffraction patterns are similar to that of pure Zn (Curve a). The intensity of Zn/AB, Zn/AC and Zn/CNT is lower than that of pure Zn because the carbon material covering the entire Zn particles leads to a drop in the proportion of Zn. Curve a′ e d′ shows the XRD patterns after discharge of pure Zn, Zn/AB, Zn/AC and Zn/CNT samples post 36 cycles. It can be seen from the Curve a′ that the intensity of all the diffraction peaks of pure Zn sample (Curve a) is significantly reduced post 36 cycles. It may be the solubility of Zn particle in the electrolyte or the formations of new phase. By comparison, the Zn/AB, Zn/AC and Zn/CNT samples post 36 cycles are also tested. Comparing the intensity of these diffraction peaks of pure Zn, the intensity of the after discharge samples (Zn/AB, Zn/AC and Zn/CNT) are weaker than that of the before discharge. Then, the peak of Zn/AC sample has the highest intensity and the shape is the smallest change. The intensity of the peaks (locating at 36.29 , 38.99 and 43.23 )
of Zn, Zn/AB, Zn/AC and Zn/CNT electrode after cycling is weaker than that of no-using, which maybe come from the revolution of Zn (curve a′, b′, c′ and d′). The intensity maintaining of Zn/AC is the highest, and the shape change is the smallest than that of Zn, Zn/AB and Zn/CNT. These results indicate that AC is the best addition to stable the Zn in chargeedischarge process. The intensity of the 26.6 peak belonging to carbon additives is stronger after cycling, just because of the revolution of Zn, then the ratio of carbon in the electrode becomes higher. In order to compare the charges of all anode compositions before and after discharge, the morphology of the pure Zn, Zn/AB, Zn/AC, and Zn/CNT electrodes before discharge and post 36 cycles are tested by SEM. The SEM images shown in Fig. 5 display the morphologies of the (A) pure Zn, (B) Zn/AB, (C) Zn/AC, and (D) Zn/CNT electrodes before discharge, respectively. Obvious differences exist in these anode materials. Fig. 5A reveals the rough surface of the bare Zn particles, and the particle size is about 20e40 mm. It can be seen that the bare Zn particles are almost separate. Fig. 5B shows the AB flood the Zn powders with cobweb-like particle. Fig. 5C shows that the AC is located between the Zn particles. The Zn particles intervals are filled by AC, then the Zn particles are tended to link together. Fig. 5D shows the dispersion of CNT on the surface of Zn particle even starting to cover the Zn surface. The Fig. 5BeD also shows the retained roughness appearance of the Zn powder, which confirms that these carbon additives can not change the surface morphologies of Zn powders. The morphology of the pure Zn, Zn/AB, Zn/AC, and Zn/CNT electrodes post 36 cycles is shown in Fig. 6. The SEM micrograph
Fig. 4. XRD patterns of porous Zn anode before and post discharge: pure Zn (A), Zn/AB (B), Zn/AC (C) and Zn/CNT (D).
H. Tao et al. / Journal of Alloys and Compounds 658 (2016) 119e124
Fig. 5. SEM micrographs of pure porous Zn (A), Zn/AB (B), Zn/AC (C), and Zn/CNT (D) anode prior to discharge.
Fig. 6. SEM micrographs of the pure porous Zn (A), Zn/AB (B), Zn/AC (C), and Zn/CNT (D) anode post to discharge.
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clearly shows the morphologic changes of the Zn anode. Fig. 6A shows that the pure Zn is completely covered with the flower-like structure post discharging. The SEM image of Fig. 6B displays that the surface of spherical Zn is partly corroded and aggregated into some irregular lamellae structures. For the anode of Zn/AC (Fig. 6C), the morphology of Zn particles is changed hardly and the surface shows that the formation of the flower-like structure is thinner than that of Zn and Zn/AB. The particle morphology of the Zn in Zn/ CNT is still observed post 36 cycles shown in Fig. 6D. These results indicate that the carbon additives offset the morphology changing of pure Zn particles and prolong the cycling life of the aqueous battery. 4. Conclusions In this work, acetylene black, carbon nanotube, and active carbon is added to the porous Zn anode composite for Zn/ ZnSO4þLi2SO4/LiMn2O4 rechargeable hybrid aqueous battery, respectively. Then the electrochemistry is used to study the anodic behavior of the various Zn-based anodic composite. The X-ray diffraction and microscopy methods are used to investigate the anodic composite before and after discharge. The results show that the carbon additions can improve the discharge capacity and the cyclic performance of the Zn anode. The cyclic performance of the battery with pure porous Zn is poor. The porous Zn anode with active carbon displays an initial discharge specific capacity 150 mAh g1 compared to 114 mAh g1 delivered by the pure Zn anode. The aqueous battery with active carbon addition increases the cycle life from 36 to 210, which indicates that the incorporation of carbon additives with porous Zn could be developed the rechargeable hybrid aqueous battery. Acknowledgments This work was financially supported by the Anhui Provincial Natural Science Foundation (1508085MB35).
References [1] Y. Zhou, P. Luckow, S.J. Smith, L. Clarke, Environ. Sci. Technol. 46 (2012) 7857e7864. [2] S. Linic, P. Christopher, D.B. Ingram, Nat. Mater. 10 (2011) 911e921. [3] J. Liu, X.W. Liu, Adv. Mater. 24 (2012) 4097e4111. [4] B. Scrosati, J. Hassoun, Y.K. Sun, Energy Environ. Sci. 4 (2011) 3287e3295. [5] M.Y. Ge, G.P. Rong, X. Fang, C.W. Zhou, Nano Lett. 12 (2012) 2318e2323. [6] M. Wagemaker, F.M. Mulder, Acc. Chem. Res. 46 (2013) 1206e1215. [7] N.S. Choi, Z.H. Chen, S.A. Freunberger, X.L. Ji, Y.K. Sun, K. Amine, Angew. Chem. Int. Ed. 51 (2012) 9994e10024. [8] S.Y. Hong, Y.J. Kim, Y.W. Park, A. Choi, N.S. Choi, Energy Environ. Sci. 6 (2013) 2067e2081. [9] Z. Chang, Y.Q. Yang, M.X. Li, X.W. Wang, Y.P. Wu, J. Mater. Chem. A 2 (2014) 10739e10755. [10] W. Li, J.R. Dahn, D. Wainwright, Science 264 (1994) 1115e1118. [11] G.J. Wang, N.H. Zhao, L.C. Yang, Y.P. Wu, H.Q. Wu, R. Holze, Electrochim. Acta 52 (2007) 4911e4915. [12] J.Y. Luo, W.J. Cui, P. He, Y.Y. Xia, Nat. Chem. 2 (2010) 760e765. [13] H. Wang, Y. Zeng, K. Huang, S. Liu, L. Chen, Electrochim. Acta 52 (2007) 5102e5107. [14] G.J. Wang, L.C. Yang, Q.T. Qu, B. Wang, Y.P. Wu, R. Holze, J. Solid State Electrochem. 14 (2010) 865e869. [15] M. Minakshi, P. Singh, S. Thurgate, K. Prince, Electrochem. Solid State Lett. 9 (2006) A471eA474. [16] J. Yan, J. Wang, H. Liu, Z. Bakenov, D. Gosselink, P. Chen, J. Power Sources 216 (2012) 222e226. [17] B.H. Zhang, Y. Liu, X.W. Wu, Y.Q. Yang, Z. Chang, Z.B. Wen, Y.P. Wu, Chem. Commun. 50 (2014) 1209e1211. [18] Y. Liu, Z.B. Wen, X.W. Wu, X.W. Wang, Y.P. Wu, R. Holze, Chem. Commun. 50 (2014) 13714e13717. [19] X.G. Zhang, J. Power Sources 163 (2006) 591e597. [20] H.F. Li, C.J. Xu, C.P. Han, Y.Y. Chen, C.G. Wei, B.H. Li, F.Y. Kang, J. Electrochem. Soc. 162 (2015) A1439eA1444. [21] M. Minakshi, D.A. ppadoo, E.M. Danielle, Electrochem. Solid State Lett. 13 (2010) A77eA80. [22] B. Sharifi, M. Mojtahedi, M. Goodarzi, K.J. Vahdati, Hydrometallurgy 99 (2009) 72e76. [23] R. Shivkumar, K.G. Paruthimal, T. Vasudevan, J. Power Sources 75 (1998) 90e100. [24] W.L. Chang, K. Sathiyanarayanan, E.S. Wook, H.S. Kima, M.S. Yun, J. Power Sources 160 (2006) 161e164. [25] R. Renuka, L. Srinivasan, S. Ramamurthy, A. Veluchamy, J. Appl. Electrochem. 31 (2001) 655e661. [26] S. Bindiya, S. Basavanna, N.Y. Arthoba, J. Mater. Eng. Perform. 21 (2012) 1879e1884. [27] M. Minakshi, D.R. Mitchell, Electrochim. Acta 53 (2008) 6323e6327. [28] M.N. Masri, A.A. Mohamad, J. Electrochem. Soc. 160 (2013) A715eA721.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of adding various carbon additives to porous zinc anode in rechargeable hybrid aqueous battery Haisheng Tao b, *, Xiang Tong a, Lu Gan a, Shuqiong Zhang a, Xuemei Zhang a, Xuan Liu b a b
College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China College of Environmental Science and Engineering, Anhui Normal University, Wuhu 241000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2015 Received in revised form 13 October 2015 Accepted 23 October 2015 Available online 27 October 2015
Anodic behavior of porous Zn in rechargeable hybrid aqueous battery is investigated. The battery comprises porous Zn anode and LiMn2O4 cathode. The electrolytes are Liþ and Zn2þ of sulfate salt in aqueous solution. The initial discharge capacity of pure porous Zn is 114 mAh g1, and the capacity decreases to less than 59 mAh g1 after 36 cycles. Then, the effects of various carbon additives on the battery performance are studied. The results show that the adding of active carbon increases the initial discharge capacity to 150 mAh g1, and the capacity maintains above 60 mAh g1 even after 210 cycles. The reasons for improvement for the battery are investigated by Tafel, scanning electron microscopy, and X-ray Diffraction. © 2015 Elsevier B.V. All rights reserved.
Keywords: Carbon additive Anode Zinc Hybrid Aqueous battery
1. Introduction With the continuous deterioration of the environment and excessive consumption of fossil fuels, the development of renewable energy sources of power like wind and solar has attracted considerable attention [1,2]. The most vexing issues of renewable energy are the sheer variability and intermittent of wind and solar power. Thus, it is significant to develop an energy storage system to solve the issues. All the kinds of energy storage systems, lithium ion batteries are remarkable, because they show high energy density, stable discharge voltage and long shelf life [3e6]. However, lithium ion batteries also suffer from some limitations, such as its toxicity, high cost and combustible organic solvents, which may cause some safety problems especially when it is out of control such as overcharging or short-circuiting [7e9]. As a result, the drawbacks above limit their application in large-scale batteries. In 1994, Dahn et al. proposed a new type of lithium ion battery [10] with aqueous electrolytes, VO2 anode and LiMn2O4 cathode (denoted as VO2// LiMn2O4). Compared with the conventional flammable electrolytes, it could operate in environmentally friendly aqueous electrolytes as well as possess highly ion conductivity. Afterwards, a large number
* Corresponding author. Tel.: þ86 553 5910728; fax: þ86 553 5910724. E-mail address: [email protected] (H. Tao). http://dx.doi.org/10.1016/j.jallcom.2015.10.225 0925-8388/© 2015 Elsevier B.V. All rights reserved.
of aqueous lithium ion batteries have been proposed such as LiV3O8//LiCoO2 [11], LiTi2(PO4)3//LiMn2O4 [12], LixV2O5-PPy// LiMn2O4 [13], PPy//LiCoO2 [14] and Zn//LiMnPO4 [15]. Recently, Yan et al. have proposed a novel rechargeable hybrid aqueous battery system [16] using two weak acidic electrolytes (Liþ and Zn2þ of chloride salt in aqueous solution). The function mechanism is that Liþ inserts into LiMn2O4 cathode and Zn2þ deposits on Zn anode at charge, then back to the solution of Liþ and Zn2þ at discharge, respectively. Subsequently, similar battery systems have been reported with a longer cycling life, such as Zn// Na0.95MnO2 [17] and PbSO4//LiMn2O4 [18], while the aqueous battery has a small capacity and has a low energy density in usual. In addition, Zn//Na0.95MnO2 has the inherent safety issue, such as dendrite formation on the Zn anode at charge, and PbSO4//LiMn2O4 uses heavy metal of Pb. To overcome these shortages, it is necessary to improve the performance of anode materials. Zn is an attractive candidate of anode material [19] for aqueous battery because of its low equilibrium potential (0.762 V vs. standard hydrogen electrode, SHE), abundance, low cost, low toxicity, and high specific energy. Compared with planar Zn, porous Zn material as anode is more widely used for battery [20,21], because it possesses higher surface area and could be easy to contact to the electrolyte. High surface area makes the passivation of Zn in charge process, especially when the hydrogen evolution on Zn makes the passivation growing
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intensity [22]. In order to solve this problem, some additives are added to the porous Zn anode. In the selection of the additive material, the requirements are taken into account, such as enhancement of the network at low additive concentration, excellent inter-particle contact, electrolyte access optimization, inert materials. And then, a series of oxides are used to weaken the passivation of Zn and hydrogen gas evolution, such as HgO [23], PbO [24], CdO [25], V2O5 [26], B2O3 [27], etc. From the viewpoint of environmental protection and economic benefit, the additives above can not meet the requirements. So it is still urgent to develop some novel and efficient additive materials. Carbon materials are applied widely in lithium ion battery, fuel cell and chemical sensor for its advantages such as high conductivity, high chemical stability, low price, non-toxicity, etc. For example, M. N. Masri et al. investigate [28] the addition of carbon black into the porous Zn anode in a Zn-air battery and the electrochemical performance is improved significantly. In this work, acetylene black (AB), carbon nanotube (CNT) and active carbon (AC) added to the porous Zn anode is investigated, respectively. Then the behaviors of the rechargeable hybrid aqueous battery (Zn//LiMn2O4) are studied by electrochemical method, scanning electron microscopy, and X-ray Diffraction. 2. Experimental 2.1. Preparation of anode and rechargeable hybrid aqueous battery fabrication Zn anode slurry are prepared by mixing 80 wt% Zn powder (99.99%, Aldrich), 10 wt% carbon material (AB, CNT and AC, respectively), and 10 wt% Polyvinylidene Fluoride (PVDF) binder blending in N-methyl-2-pyrrolidone (NMP). Then the slurry is coated on a black paper current collector (surface is 1.5 cm2) and dried in a vacuum at 60 C for 5 h. The cathode is prepared by the same method as that of the anode described above. The cathode consists of 80 wt% LiMn2O4, 10 wt% acetylene black, and 10 wt% PVDF. The LiMn2O4 is synthesized by the similar method according to our previous work. At first, MnCO3 is first prepared by a facile hydrothermal reaction from an aqueous solution of KMnO4 and glucose in a molar ratio of 10:1 at 180 C. Then, the prepared oval MnCO3 precursor is mixed with Li2CO3 in acetic acid under ultrasonic for 30 min. The mixture is vacuum-dried at 90 C. Then the mixture is heat-treated at 300 C for 3 h and 750 C for 4 h to get LiMn2O4, respectively. The electrolyte is prepared by dissolving 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 in deionized water. All of the segments are assembled to fabricate a rechargeable hybrid aqueous battery.
2.3. Physical characterizations The morphologies and structures of the prepared Zn anode are characterized by scanning electron microscopy (SEM, Hitachi S4800, 25 kV) and X-ray diffraction (XRD, Shimadzu 6000, Nifiltered Cu Ka radiation). 3. Results and discussion CVs of Zn and LiMn2O4 electrodes in 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 aqueous electrolyte at the scan rate of 1 mV s1 are shown in Fig. 1. In the case of Zn, there is one couple of clear redox peaks located at 0.20/0.15 V (vs. Zn2þ/Zn), which are due to the reversible dissolution and the plating of Zn. In the case of LiMn2O4, there are two couples of reversible redox peaks at 1.70/1.75 V and 1.80/1.91 V (vs. Zn2þ/Zn), respectively, which attribute to the twostep lithium intercalation/extraction of LiMn2O4 in the aqueous electrolyte. Since there is a potential difference between Zn and LiMn2O4 in 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 aqueous electrolyte, they can be assembled into a battery. The system is schematically shown in Scheme 1. The proposed cathodic and anodic reactions can be described as follows: charge
x=2Zn2þ þ LiMn2 O4 )
* x=2Zn þ Li1x Mn2 O4 þ xLiþ
discharge
(1)
The performance of the rechargeable hybrid aqueous batteries with pure porous Zn, Zn/CNT, Zn/AC and Zn/AB electrode are shown in Fig. 2, respectively. Fig. 2A presents the typical discharge characteristics of the batteries at the first cycle. The discharge plateau voltage is ca. 1.8 V, which is consistent with the CV data in Fig. 1. This is due to the intercalation of Liþ ions into the host. The initial discharge capacity of pure Zn electrode is 114 mAh g1. When acetylene black, carbon nanotube and active carbon is added to the pure Zn anode system, the discharge capacity increases to 130, 140 and 150 mAh g1, respectively. It suggests that the introduction of carbon material in the porous Zn anode improves the discharge capacity. Among them, active carbon is the best one due to its highest specific surface area that provides more sufficient reaction channels. Fig. 2B shows the cycling characteristics of the pure porous Zn, Zn/CNT, Zn/AC and Zn/AB anode at the current density of
2.2. Electrochemical characterizations Cyclic voltammetry (CV) is carried out on a CHI 660C electrochemical workstation (Shanghai, China) with a three-electrode-cell at a scan rate of 1 mV s1 in the potential range of 0.2e2.2 V (vs. Zn2þ/Zn). A LiMn2O4 and Zn is used as the working electrode, respectively, and each electrode is about 1 mg with an active area of 0.8 cm2. A platinum wire is used as the counter electrode, and a Zn foil is used as the reference electrode. The galvanostatic chargeedischarge tests are characterized on a battery test system TC5.X (Neware, China) at room temperature. Tafel polarization measurements are performed on the CHI 660C electrochemical system. A three-electrode cell includes a saturated calomel electrode (SCE) reference electrode, a platinum counter electrode and a porous Zn working electrode. Tafel polarization is recorded from 0.90 V to 1.25 V in 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 aqueous electrolyte at the scan rate of 0.5 mV s1.
Fig. 1. CVs of Zn and LiMn2O4 in the aqueous electrolyte of 0.5 mol L1 Li2SO4 and 1 mol L1 ZnSO4 at the scan rate of 1 mV s1, which is tested by using platinum and Zn as the counter and reference electrode, respectively.
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Scheme 1. Schematic illustration of the redox reactions for Zn//LiMn2O4 rechargeable hybrid aqueous battery during charge and discharge processes.
240 mA g1, respectively. All the anodes show excellent discharge capacity in the first cycle, but the capacity decreases gradually after several cycles. The capacity of the pure Zn anode decreases to 59 mAh g1 at the 36th cycle, which implies that the cyclic performance of the pure Zn is poor. It is mainly due to the passivation of the Zn particles surface. In comparison with the pure Zn anode, the cyclic behavior and discharge capacity of the batteries with Zn/ AB, Zn/CNT and Zn/AC anodes improve significantly. The discharge capacity of Zn/AB and Zn/CNT electrodes decrease to 65 and 69 mAh g1 over 80 cycles, respectively. It is ascribed to the carbon (acetylene black, carbon nanotube) coating on Zn particle surface that prevents the direct contact of the Zn anode with electrolyte. So the corrosion of the active Zn particle can be restrained. Simultaneously, it is seen that the Zn/AC anode delivers the discharge capacity of 60 mAh g1 even after 210 cycles. It illustrates that the cycle life and discharge capacity of the Zn/AC anode are remarkably superior to the corresponding performance of Zn/AB and Zn/CNT. The excellent cycle performance can be mainly ascribed to the following reasons. It is possible that the active carbon has high over-potential of hydrogen evolution. It can effectively suppress the generation of hydrogen and prevent the passivation of Zn particle.
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On the other hand, the active carbon with high specific surface area provides the growth substrate for Zn electrodeposits. As a result, the porous Zn with 10 wt% content of the carbon additive contributes to improve the electrode efficiency and reversibility. The polarization curves of the pure porous Zn, Zn/CNT, Zn/AC and Zn/AB electrodes are present in Fig. 3. The corresponding results are listed in Table 1, where the corrosion potential (Ecorr) and the corrosion current density (jcorr) are obtained by extrapolation of the anodic and cathodic Tafel lines. As compared with the pure Zn anode, the Ecorr values of several Zn electrodes composed of the additive of different carbon materials into pure porous Zn anode have few movements, whereas the jcorr values have shifted in a negative direction. It indicates that the carbon additive materials can prevent the corrosion of Zn particle in weak acidic electrolytes. In the case of the introduction of AC in the porous Zn anode has the minimum jcorr value. It is possibly explained by a large amount of AC particles covering the Zn surfaces, which hinders the diffusion reaction of the electrolyte throughout the Zn particle, which is in accordance with the SEM result (Fig. 5C). And the stability of the AC has the highest over-potential of hydrogen evolution. Hence, it could be concluded that the AC is the most suited additive from the suppress corrosion point of view, which is in accordance with the proposed analysis. The XRD patterns before (curve a, b, c, d) and after (curve a′, b′, c′, d′) discharge of Zn, Zn/AB, Zn/AC and Zn/CNT electrodes are shown in Fig. 4. Curve a in Fig. 4A shows the pattern for Zn. It is
Fig. 3. Tafel plots of pure Zn, Zn/CNT, Zn/AC and Zn/AB electrodes.
Fig. 2. First discharge curves (A) and cycling performance (B) of the batteries with pure porous Zn, Zn/CNT, Zn/AC, and Zn/AB anode, respectively. Current density is 240 mA g1 based on the cathode between 0 and 2.0 V.
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Table 1 The data for Tafel curves with pure Zn, Zn/CNT, Zn/AC and Zn/AB. Sample
Ecorr/V
jcorr/A cm2
Zn Zn/CNT Zn/AC Zn/AB
1.044 1.041 1.047 1.042
1.29 1.09 4.4 8.1
102 102 103 103
seen that the Zn peaks dominate the pattern at 2q angles of 36.29 , 38.99 and 43.23 , which are indexed to the planes (002), (100) and (101), respectively. Curve b e d shows the XRD patterns with respect to the effect of adding AB, CNT and AC into the porous Zn anode system. The diffraction patterns are similar to that of pure Zn (Curve a). The intensity of Zn/AB, Zn/AC and Zn/CNT is lower than that of pure Zn because the carbon material covering the entire Zn particles leads to a drop in the proportion of Zn. Curve a′ e d′ shows the XRD patterns after discharge of pure Zn, Zn/AB, Zn/AC and Zn/CNT samples post 36 cycles. It can be seen from the Curve a′ that the intensity of all the diffraction peaks of pure Zn sample (Curve a) is significantly reduced post 36 cycles. It may be the solubility of Zn particle in the electrolyte or the formations of new phase. By comparison, the Zn/AB, Zn/AC and Zn/CNT samples post 36 cycles are also tested. Comparing the intensity of these diffraction peaks of pure Zn, the intensity of the after discharge samples (Zn/AB, Zn/AC and Zn/CNT) are weaker than that of the before discharge. Then, the peak of Zn/AC sample has the highest intensity and the shape is the smallest change. The intensity of the peaks (locating at 36.29 , 38.99 and 43.23 )
of Zn, Zn/AB, Zn/AC and Zn/CNT electrode after cycling is weaker than that of no-using, which maybe come from the revolution of Zn (curve a′, b′, c′ and d′). The intensity maintaining of Zn/AC is the highest, and the shape change is the smallest than that of Zn, Zn/AB and Zn/CNT. These results indicate that AC is the best addition to stable the Zn in chargeedischarge process. The intensity of the 26.6 peak belonging to carbon additives is stronger after cycling, just because of the revolution of Zn, then the ratio of carbon in the electrode becomes higher. In order to compare the charges of all anode compositions before and after discharge, the morphology of the pure Zn, Zn/AB, Zn/AC, and Zn/CNT electrodes before discharge and post 36 cycles are tested by SEM. The SEM images shown in Fig. 5 display the morphologies of the (A) pure Zn, (B) Zn/AB, (C) Zn/AC, and (D) Zn/CNT electrodes before discharge, respectively. Obvious differences exist in these anode materials. Fig. 5A reveals the rough surface of the bare Zn particles, and the particle size is about 20e40 mm. It can be seen that the bare Zn particles are almost separate. Fig. 5B shows the AB flood the Zn powders with cobweb-like particle. Fig. 5C shows that the AC is located between the Zn particles. The Zn particles intervals are filled by AC, then the Zn particles are tended to link together. Fig. 5D shows the dispersion of CNT on the surface of Zn particle even starting to cover the Zn surface. The Fig. 5BeD also shows the retained roughness appearance of the Zn powder, which confirms that these carbon additives can not change the surface morphologies of Zn powders. The morphology of the pure Zn, Zn/AB, Zn/AC, and Zn/CNT electrodes post 36 cycles is shown in Fig. 6. The SEM micrograph
Fig. 4. XRD patterns of porous Zn anode before and post discharge: pure Zn (A), Zn/AB (B), Zn/AC (C) and Zn/CNT (D).
H. Tao et al. / Journal of Alloys and Compounds 658 (2016) 119e124
Fig. 5. SEM micrographs of pure porous Zn (A), Zn/AB (B), Zn/AC (C), and Zn/CNT (D) anode prior to discharge.
Fig. 6. SEM micrographs of the pure porous Zn (A), Zn/AB (B), Zn/AC (C), and Zn/CNT (D) anode post to discharge.
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clearly shows the morphologic changes of the Zn anode. Fig. 6A shows that the pure Zn is completely covered with the flower-like structure post discharging. The SEM image of Fig. 6B displays that the surface of spherical Zn is partly corroded and aggregated into some irregular lamellae structures. For the anode of Zn/AC (Fig. 6C), the morphology of Zn particles is changed hardly and the surface shows that the formation of the flower-like structure is thinner than that of Zn and Zn/AB. The particle morphology of the Zn in Zn/ CNT is still observed post 36 cycles shown in Fig. 6D. These results indicate that the carbon additives offset the morphology changing of pure Zn particles and prolong the cycling life of the aqueous battery. 4. Conclusions In this work, acetylene black, carbon nanotube, and active carbon is added to the porous Zn anode composite for Zn/ ZnSO4þLi2SO4/LiMn2O4 rechargeable hybrid aqueous battery, respectively. Then the electrochemistry is used to study the anodic behavior of the various Zn-based anodic composite. The X-ray diffraction and microscopy methods are used to investigate the anodic composite before and after discharge. The results show that the carbon additions can improve the discharge capacity and the cyclic performance of the Zn anode. The cyclic performance of the battery with pure porous Zn is poor. The porous Zn anode with active carbon displays an initial discharge specific capacity 150 mAh g1 compared to 114 mAh g1 delivered by the pure Zn anode. The aqueous battery with active carbon addition increases the cycle life from 36 to 210, which indicates that the incorporation of carbon additives with porous Zn could be developed the rechargeable hybrid aqueous battery. Acknowledgments This work was financially supported by the Anhui Provincial Natural Science Foundation (1508085MB35).
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