Preparation and electrochemical performance of In-doped ZnO as anode material for Ni–Zn secondary cells

Electrochimica Acta 56 (2011) 4075–4080

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Preparation and electrochemical performance of In-doped ZnO as anode material for Ni–Zn secondary cells Dongqing Zeng, Zhanhong Yang ∗ , Shengwei Wang, Xia Ni, Dengjun Ai, Qingqing Zhang College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

a r t i c l e

i n f o

Article history: Received 7 October 2010 Received in revised form 11 January 2011 Accepted 24 January 2011 Available online 17 February 2011 Keywords: In-doped ZnO Ni–Zn secondary cells Physical mixture In2 O3

a b s t r a c t In-doped ZnO (IZO) samples were synthesized by a simple co-precipitation method. X-ray diffraction (XRD) patterns, Raman spectra and scanning electron microscopy (SEM) images show that IZO with 2.5 wt% In2 O3 has a pure wurtzite structure and a plate-like morphology. IZO with 16.3 wt% In2 O3 (theoretical value) mainly shows a wurtzite structure. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge measurement were utilized to examine the electrochemical performances of IZO with 2.5 wt% In2 O3 as anode material for Ni–Zn simulated cells. Compared with the physical mixture of ZnO with In2 O3 , IZO increases the charge-transfer resistance of zinc electrode. Furthermore, the initial discharge capacity of IZO is 569 mAh g−1 , and the discharge capacity decays slightly with the capacity retention ratio of 95.2% over 73 cycles, which is much higher than that of the physical mixture of ZnO with In2 O3 . © 2011 Elsevier Ltd. All rights reserved.

1. Introduction There has been a rising interest in the researches about Ni–Zn alkaline secondary system for its remarkable advantages of high specific energy, high specific power, good low-temperature performance, low cost and environmental toxicity. These features also make it a promising candidate for the new generation of green power sources used in electric vehicle (EV) or hybrid electric vehicle (HEV). However, the pace of the widespread commercialization of Ni–Zn secondary batteries is disturbed by a poor cycle life of this system, which mainly results from the drawbacks of zinc electrode, such as shape change, zinc dendrite growth and zinc self-corrosion, in charge–discharge processes. These problems derive from a high solubility of the discharge products of zinc electrode in alkaline electrolyte. Hence, any way to limit the mass-transport of active material in zinc electrode is of great value to the industrial application of Ni–Zn alkaline secondary system. For decades, many attempts have been made to conquer the problems mentioned above. A large variety of organic additives, such as quinine derivatives, water-soluble polymers, hetero-cyclic aldehydes, were applied in zinc secondary batteries in the purpose of obtaining a beneficial effect on zinc morphology [1–5]. Meanwhile, various inorganic species, namely, Ca(OH)2 [6,7], Bi2 O3 [8], PbO [9], SnO2 [10], BaO [11], were adopted in zinc electrode to overcome the difficulties mentioned. Among all these inorganic

∗ Corresponding author. Tel.: +86 731 88879616. E-mail address: [email protected] (Z. Yang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.119

additives, compounds of indium are thought to be the important component of electrode because these compounds have a suppressive effect on H2 formation, decrease shape change and self-discharge, resulting in a more uniform current density distribution and improved cell lifetimes [12–14]. Unfortunately, great concern has been taken on the physical mixture of In-compounds with ZnO. The disadvantage of the physical mixture process is that the efficiency of the additives is relatively low because it is difficult to achieve sufficient contact between ZnO and additives by the simple physical mixture. Consequently, it is necessary to develop an effective way to utilize the valuable indium metal oxide for economical interest in industrial processes. The surface modification technique has been intensively investigated and proved to be an effective method for the improvement of the electrochemical performance of active material [15–18]. But there seems to be no reports on the application of In-doped ZnO (IZO) as anode material in zinc rechargeable system, which has drawn a lot of attention in gas sensors, antireflective coating, photocatalyst and solar cells [19]. Thereby, IZO was prepared and investigated in the present study. 2. Experimental 2.1. Preparation and characterization of IZO IZO with 2.4 wt% In2 O3 (theoretical value) was prepared via a simple co-precipitation method and the typical experimental operation was as follows: 0.20 g In2 O3 (analytical grade) was dissolved in 10 ml concentrated HCl solution and 13.62 g ZnCl2 (analyti-

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cal grade) was dissolved with deionized water. Then, these two solutions were mixed thoroughly and diluted into 200 ml with deionized water. Under unstopped strong mechanical stirring, a solution of 14 wt% NH3 ·H2 O was added dropwise to the mixed solution until the pH of this solution was adjusted to 7.0. After continuous agitation for another 15 min and aging for 1 h, the resulted precipitate was filtrated and washed with deionized water and absolute ethyl alcohol, dried at 60 ◦ C for 4 h, subsequently calcined at 500 ◦ C for 2 h. The as-grown product was characterized by Xray diffraction (XRD) (Rigaku D/max III with CuK␣ radiation) and scanning electron microscopy (SEM, FEI Quanta-200). Raman spectra were obtained in a Raman spectroscope meter (Lab RAM HR, France) with the 488 nm line of an Ar-ion laser. The In-doped content was calculated according to the results of energy dispersive X-ray spectrum (EDS, FEI Quanta-200). Disk-shaped IZO specimens in thickness about 1.4 mm were made by pressing IZO powder in a stainless steel die (diameter 15 mm) under a pressure of 30 MPa and then covered by Ag electrodes. The resistance of the specimens was measured at 25 ± 1 ◦ C to calculate the resistivity. The resistivity of the common ZnO specimens was also obtained by the same method. IZO with 16.3 wt% In2 O3 (theoretical value) was prepared by the same co-precipitation method and characterized by XRD. 2.2. Electrochemical measurements In-doped zinc electrodes were prepared by incorporating slurries containing IZO with 2.5 wt% In2 O3 (measured value), graphite and polytetrafluoroethylene (PTFE, 60 wt%, in diluted emulsion) to a copper mesh substrate (1.0 × 1.0 cm in size), which served as the current collector. The weight ratio of IZO, graphite and PTFE was 10:1:1. Afterwards, the pasted electrodes were dried at 80 ◦ C and pressed to a thickness of 0.28 mm. For comparison, zinc electrodes with the physical mixture of ZnO with In2 O3 (2.5 wt%) were also fabricated. A solution of 4.5 M KOH, 1.0 M NaOH, 0.5 M LiOH, saturated with ZnO, was used as the electrolyte. A cell consisted of zinc electrode and the commercial sintered Ni(OH)2 electrode was assembled and placed in a simple cell container made of Perspex. It should be emphasized that the ratio of the capacity of the cathode and anode was about 5:1 in the aim of making full use of the active material in zinc electrodes. All the cells were pre-activated for 10 times by the following operations: the cells were circularly charged at 62.5 mA g−1 for 10 h, and then discharged at 125 mA g−1 to 1.2 V cut-off. The galvanostatic charge–discharge tests were conducted on a BTS-5V/100mA battery-testing instrument (Neware, China) at room temperature (25 ± 5 ◦ C). The cells were circularly charged at 125 mA g−1 for 5 h, and then discharged at the same current to 1.2 V cut-off. Electrochemical impedance spectroscope (EIS) measurements were performed on a RST 5000-type electrochemical system (SHIRUISI, China) at room temperature (25 ± 1 ◦ C). The applied frequency range was between 0.01 Hz and 100 kHz, and the amplitude of AC signal was set at 5 mV. A three-electrode cell assembly was used in EIS tests with a Hg/HgO electrode as reference electrode, a commercial sintered Ni(OH)2 electrode as counter electrode, and a pre-activated zinc electrode as working electrode. Over a shifting range from −0.95 V to −1.65 V, cyclic voltammetry (CV) was also carried out on RST 5000-type electrochemical workstation at a scanning rate of 20 mV/s. The cell assembly used was the same as that in EIS measurements. 2.3. Determination of the dissolution of zinc electrode in alkaline solution The dissolution of zinc electrode with IZO with 2.5 wt% In2 O3 was determined as follows: the discharged pre-activated zinc electrode was immersed in an alkaline solution of 4.5 M KOH, 1.0 M

Fig. 1. XRD patterns of the common ZnO and as-prepared IZO samples.

NaOH, and 0.5 M LiOH (50.00 ml). Then, this system was sealed and placed in a drier at 25 ◦ C for 15 days. Afterwards, zinc electrode was taken out and the remained alkaline solution was utilized to determine the concentration of Zn2+ . After the alkaline solution (10.00 ml) being firstly adjusted to pH 5.0–6.0 by a 3.0 M HCl solution, the concentration of Zn2+ was determined by a complexometric titration method [20,21], in which the standard solution of disodium ethylene diamine tetraacetic acid (EDTA-2Na, 0.005018 M) was used as titration agent, xylenol orange solution (XO, 0.5 wt%) as indicator, a solution of hexamethylene tetramine (HT, 1.2 M)–HCl with pH 5.0-6.0 as buffer solution. And the color change of the solution from purplish red to yellow was chosen as the titration end point. The dissolution of zinc electrode with the physical mixture of ZnO with In2 O3 was also determined by the same method. 3. Results and discussion Fig. 1 shows the XRD patterns of the common ZnO and asprepared IZO samples. Comparing these XRD patterns with the XRD standard spectra of ZnO (PDF 36-1451), the common ZnO and IZO with 2.4 wt% In2 O3 (theoretical value) have a pure wurtzite structure, which belongs to the hexagonal crystal system [space group P63mc]. IZO with 16.3 wt% In2 O3 (theoretical value) mainly has the wurtzite structure and some impurities can also be detected. The impurities such as In2 O3 (ZnO)7 (PDF 20-1441), In2 O3 (ZnO)19 (PDF 43-0622) are a clear indication of the existence of IZO with a special structure [22]. Although the diffraction peaks of common ZnO are sharper than that of IZO with 2.4 wt% In2 O3 , there seems to be no other differences in their XRD patterns. Fig. 2 shows a comparison of the common ZnO and IZO with 2.4 wt% In2 O3 in the range of 31–37◦ (2) corresponding to (1 0 0), (0 0 2) and (1 0 1) peaks of ZnO with wurtzite structure. It is clearly shown that all the three peaks of IZO slightly shift towards the smaller angles as compared with that of the common ZnO. This interesting phenomenon is attributed to the lattice expansion of ZnO, which can be understood as the ionic radius of the In3+ (0.081 nm) dopant is larger than that of Zn2+ (0.074 nm) when the substitution of In atoms is into the ZnO lattice [23]. In order to further confirm the fact that In is successfully doped in the ZnO lattice in IZO with 2.4 wt% In2 O3 , Fig. 3 shows the Raman spectra of the common ZnO and IZO. From Fig. 3, two intensive peaks located at 98 cm−1 and 438 cm−1 are ascribed to Elow 2 high

and E2

, respectively, which are the characteristic bands of ZnO

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Fig. 2. Refined XRD patterns of the common ZnO and IZO sample.

with the wurtzite structure. The peaks at 381 cm−1 , 412 cm−1 and 578 cm−1 are respectively indicated to A1 (TO), E1 (TO) and A1 (LO), which are well known as the first-order optical modes of wurtzite ZnO. The peaks appeared at 202 cm−1 and 331 cm−1 can be assigned high and E2 –Elow to 2Elow 2 2 , respectively, which are thought as the second-order features caused by multi-phonon processes [24,25]. It should be noted that there is an additional phonon mode (AM) at 633 cm−1 , which is not presented in the common ZnO. This AM has been founded in ZnO doped with other elements, which is ascribed to the local vibrational mode due to host defects [26,27]. Moreover, the enhancement in the peak intensity of the A1 (LO) mode has also been reported in other doped ZnO materials [28,29]. According to the results of EDS, the In-doped content is actually 2.5 wt% In2 O3 for IZO. Based on the above XRD, Raman and EDS results, we are safe to conclude that IZO with 2.5 wt% In2 O3 is obtained. In order to avoid the interference from impurities, we focus on the electrochemical properties of IZO with 2.5 wt% In2 O3 , which has the same pure wurtzite structure as the common ZnO. And all the following mentioned IZO refer to IZO with 2.5 wt% In2 O3 . Fig. 4a–d, f and h present the SEM morphologies of the common ZnO and IZO. As shown in Fig. 4a, the common ZnO particles mainly show a prismatic structure with the sizes about 0.05–0.5 ␮m. From Fig. 4b, d and f, it can be clearly seen that the IZO sample is irreg-

ular plate-like with a dimension of wide range. The thickness of these plates is about 0.1–0.2 ␮m, and there are some particles with chip-like and cotton-like structures distributed on the surface of the large plates. Interestingly, some small agglomerates can also be observed. As betrayed from Fig. 4c and h, these agglomerates are staked by numbers of plates with a small size. This is due to the absence of sufficient dispersive procedures in doping process. From Fig. 4e, g and i, these particles with different structures are all composed of IZO. Fortunately, these agglomerates do not exert any unfavorable influence on the electrochemical properties of IZO, which is verified by the following electrochemical tests. In order to understand the effects of In-doping technique on the electrochemical performance of ZnO, CV studies were performed and the recorded CV curves are given in Fig. 5. As shown in Fig. 5, the cathodic peaks of zinc electrodes containing the physical mixture of ZnO with In2 O3 and IZO appear at −1.580 V and −1.594 V, respectively. Although a more negative potential, generally speaking, means a lower electrochemical kinetics of reduction process, In-doped zinc electrode shows a much higher peak current than In2 O3 -intermingled zinc electrode, suggesting that the reduction of In-doped zinc electrode is not hampered but favorably enhanced. The possible reason is that the superficial In3+ ions, which distribute on the surface of IZO particle, incline to be reduced into indium metal. This indium metal could create an excellent electrical contact between active materials and the copper mesh and aid electron transfer in zinc electrode. During the anodic process, a clear distinction can be drawn from the fact that there is only one anodic peak located at −1.087 V for In2 O3 -intermingled zinc electrode and there are two anodic peaks for In-doped zinc electrode, which are around −1.084 V and −1.000 V, respectively. A much higher anodic current of In-doped zinc electrode implies that the anodic process of Indoped zinc electrode is easier. Moreover, this “two anodic peaks” phenomenon is an important characteristic for anodic dissolution of zinc and was ever deeply depicted by previous works [15,30,31]. The two anodic peaks occurring at In-doped zinc electrode correspond to two processes of anodic dissolution. It is well established that the reaction at EpA1 , the anodic peak potential labeled A1, gives birth to the zincate ion, Zn(OH)4 2− , by a probable three-step process described by the over-all reaction [30,31] Zn + 4OH− = Zn(OH)4 2− + 2e

(1)

The second peak A2 appears at a more positive potential when there has been an inadequate contact between ZnO and OH− ion, which is caused by the pre-proceeding reaction (1) for some time. And a possible reaction corresponding to peak A2 can be represented by [15] Zn + 3OH− = Zn(OH)3 − + 2e

Fig. 3. Raman spectra of the common ZnO and IZO sample.

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(2)

As mentioned in the cathodic process, this inadequate contact between ZnO and OH− ion is also induced by the coating of IZO particle by the metal indium. In order to investigate the influence of In-doping process on the impedance of pasted zinc electrode, EIS measurements were carried out and the impedance diagrams are presented in Fig. 6A. Both Nyquist plots involve a high-frequency capacitive semicircular loop and a low-frequency straight line. The high-frequency capacitive semicircular loop can be ascribed to the charge-transfer resistance in parallel with the double-layer capacitance, and the slope in the low-frequency region is most probably caused by the diffusion of zincate in zinc electrode. The equivalent circuit used to fit the EIS spectra is reported in Fig. 6B, where CPE represents the constant phase element for a porous electrode, Re designates the total ohmic resistance, which includes the resistance of the electrolyte, current collector, electrode materials, etc., Rct is the charge-transfer resistance and Zw is the Warburg impedance.

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Fig. 4. SEM images for (a) the common ZnO and (b–d, f, h) IZO and EDS spectra for (e, g, i) IZO.

According to the equivalent circuit, the fitted Rct of In-doped zinc electrode and the common zinc electrode are 15.52  and 6.468 , respectively. A larger Rct always means a more difficult electrochemical reaction and leads to an increase in the electrochemical polarization. The most possible reason is that after the reduction of the superficial In3+ into indium metal at charging process, this indium metal exists on the surface of ZnO particle, producing a sufficient separation of ZnO from alkaline electrolyte as revealed in CV results. The fitted Re for In-doped zinc electrode and the common zinc electrode are 0.1329  and 0.1534 , respectively. A smaller

Re generally bears a sign of a better electrical conduction and a weaker ohmic polarization. Two factors may partly contribute to the smaller Re . The first, the indium metal reduced at charging in Indoped zinc electrode can endow a better electrical contact among active particles and between the copper mesh and active material. Secondly, IZO is known as an n-type semiconductor, which exhibits excellent property of controllable low resistivity [32]. In our study, the resistivity of IZO was determined to be 5.1 × 104  cm, which is much lower than that of the common ZnO (2.7 × 106  cm). Hence, as the electrode material, the bulk IZO survived from

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Fig. 5. Cyclic voltammograms for zinc electrodes with In2 O3 -intermingled ZnO and IZO.

the electrochemical reaction would decrease the resistance of electrode. Fig. 7 displays the typical galvanostatic charge–discharge curves of Ni–Zn cells with IZO and In2 O3 -intermingled ZnO at the 20th cycle. From Fig. 7A, it can be seen that the Ni–Zn cell with IZO shows a lower charge plateau voltage. This may be explained by the sufficient electrical contact resulted from the indium metal on the surface of ZnO particles. Meanwhile, as shown in Fig. 7B, a decrease in discharge plateau voltage can be observed for Ni–Zn cell with IZO, which indicates that In-doped modification gives a suppressive effect on the electrochemical oxidized reactions of Zn. This suppressive effect would be related to the decrease in the direct contact between ZnO and the electrolyte. All these results match well with the CV and EIS results. However, it should be noted that a decrease in discharge plateau voltage has no undesirable influence on the discharge capacity of Ni–Zn cells. This may lie in the fact that In-doped modification only suppresses electrochemical reac-

Fig. 7. Typical galvanostatic charge–discharge curves of Ni–Zn cells with In2 O3 intermingled ZnO and IZO at the 20th cycle.

tion rate to a certain extent and leads to a lower electrochemical kinetics, which is favorable to keep the electrochemical stability of zinc electrode [15]. And this anticipation is confirmed by the following cyclic tests. Fig. 8 illustrates the variation of specific discharge capacity with cycle number for zinc electrodes with IZO and In2 O3 -intermingled

Fig. 6. (A) Nyquist plots for zinc electrode with In2 O3 -intermingled ZnO and IZO at 50% state of charging and (B) equivalent circuit model for the EIS spectra.

Fig. 8. Cyclic behavior of Ni–Zn cells with In2 O3 -intermingled ZnO and IZO.

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ZnO. It can be found that IZO delivers an initial discharge capacity of 569 mAh g−1 with the capacity retention ratio of 95.2% over 73 cycles. And the initial discharge capacity is 530 mAh g−1 for In2 O3 -intermingled ZnO with the capacity retention ratio of 72.8%. These results demonstrate that IZO has a higher discharge capacity and more stable electrochemical cycle performance than In2 O3 intermingled ZnO. Several factors could be responsible for this improvement. As to IZO, indium metal on the surface of ZnO particle may handicap the direct contact of the active material with the alkaline electrolyte and thus suppress the dissolution of ZnO in the electrolyte. The concentration of Zn2+ dissolved from In-doped zinc electrode was determined to be 0.008319 M, which is lower than that of zinc electrode with In2 O3 -intermingled ZnO (0.01265 M). This indicates that In-doped zinc electrode is more stable in alkaline solution. Besides, the plate-like morphology of IZO helps to the electrochemical stability of zinc electrode. For the plate-like IZO, the relatively loose arrangement of the active materials in zinc electrode, which results from some plates standing erect to the current collector, could facilitate the deep penetration of the electrolyte into zinc electrode and thus enhance the active material utilization [33]. Furthermore, the rapidest growth direction 0 0 0 1 determined by the crystal growth habit of ZnO is vertical or inclined to the accelerated growth direction induced by concentration polarization which is aroused by liquid-side mass transfer [33,34]. Hence, the zinc dendrite is suppressed effectively and a beneficial zinc electrodeposition is obtained.

Innovation Fund for Technology Based Firms of China in 2010 (No. 10C26214104497) and Production, Teaching and Research Integrated Project of Guangdong Province and Ministry of Education (No. 2010B090400341). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

4. Conclusion IZO samples were prepared by a co-precipitation method. The electrochemical investigation of IZO with 2.5 wt% In2 O3 shows that In-doped modification is a novel method to improve the electrochemical performances of ZnO. The improvement in electrochemical behavior lies in the following reasons: (1) the superficial In3+ ions, during charging, could be reduced into indium metal, which may decrease the direct contact between ZnO and alkaline electrolyte and enhance the retainment of ZnO in the electrode; (2) the bulk IZO serves as the electrode material with low resistivity; and (3) the as-grown sample has a plate-like morphology. Although an increased charge-transfer resistance is presented in EIS plot for IZO, it may be safe to conclude that In-doped modification is more effective than the physical mixture process. Acknowledgements The authors gratefully acknowledge the financial supports from National Natural Science Foundation of China (No. 91023031),

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