J. Cent. South Univ. Technol. (2010) 17: 930-935

DOI: 10.1007/s11771-010-0579-0

Effects of dispersant on performance of Ni-Zn batteries

YANG Zhan-hong(杨占红), LIAO Jian-ping(廖建平), WANG Sheng-wei(王升威),

WANG Su-qin(王素琴), HU Jun(胡俊)

School of Chemistry and Chemical  Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: A new additive of sodium hexametaphosphate (SHMP) was introduced to the paste of zinc electrode, with the purpose of preventing the zinc active materials from agglomerating and improving the stability of batteries. The properties of the zinc electrodes were characterized by scanning electron microscopy (SEM), constant current charge/discharge measurement, self-discharge test and hydrogen collection experiment. The photographs of zinc electrode show that SHMP can significantly break up the agglomeration, uniformize the particle distribution and increase the surface area, which are advantageous to improve the electrochemical performance of zinc electrode. The experimental battery shows a 97 times cycling life and a 30.2% remaining capacity after 4 d storage. The hydrogen collection experimental results indicate that the SHMP can decrease the ratio of hydrogen evolution. Therefore, the corrosion of zinc electrode is suppressed and the charge/discharge efficiency is enhanced.

Key words: sodium hexametaphosphate; zinc electrode; Ni-Zn battery; uniformity

1 Introduction

Ni-Zn secondary battery has many advantages of high specific energy density (55-59.8 W·h/kg), high specific power (140-200 W/kg), high open-circuit voltage (1.75 V), low cost and environmental friendliness [1-3]. It is considered as one of the most potential power sources in the 21st century for mobile applications. Nevertheless, the secondary zinc battery is usually limited in widespread commercialization due to the degradation of the zinc anode material and the carbonation of the KOH electrolyte. Shape change, dendrite shorting, passivation or zinc densification, and hydrogen evolution reaction of the zinc electrode, are identified as the main causes for its poor cycle life [4-8]. Shape change refers to the reduction of the active surface area of the zinc electrode during cycling of the battery, as a result of the redistribution of active material over the electrode. Generally, these problems can be largely solved by using various additives such as Bi³⁺ [9], Ca(OH)₂ [10], In(OH)₃ [11], SnO₂ [12] in the zinc electrode, adding KTPS [13] directly into the electrolyte, using ZnO nanoparticles [14] or undergoing surface treatment [15] to enhance the electrochemical performance.

Furthermore, extensive commercialization of zinc secondary batteries is baffled by the poor uniformity of the zinc electrode. Even if the batteries are made in the same batch and process, both the cycle life and the self-discharge performances may be different from each other greatly. In conventional pasted zinc electrode, all the materials are stirred in a vessel. However, the paste is lack of fluidness, which is harmful to the mixing of the compositive materials. Therefore, zinc oxide and additives cannot be completely dispersed in the paste. This problem may be one of the main reasons for the poor uniformity of the zinc electrode. Dip coating or brush coating of polyaniline on the porous zinc electrode was found to be useful in stabilizing the capacity of the electrode on repeated cycling [16]. Polypyrrole-coated zinc electrodes were prepared and the polymer-coated electrodes displayed high electrochemical stability in alkaline electrolyte [17].

Zinc has a more negative reduction potential than hydrogen, which will cause a hydrogen evolution reaction. Many attempts were made to suppress hydrogen evolution. Mercury is reported to be one of the most effective additives for suppressing hydrogen evolution, but it causes pollution to the environment. Calcium zincate [18], V₂O₅ [19] and surfactant additives [20-21] were proposed. YUAN [22] reported that zinc oxide particles modified with Sn₆O₄(OH)₄ also suppressed hydrogen evolution.

In this work, a new additive of sodium hexametaphosphate (SHMP) was introduced to the paste of zinc electrode, and the properties of Ni-Zn batteries with zinc pasted electrodes using SHMP were evaluated.

2 Experimental

2.1 Preparation of zinc electrodes and experimental batteries

The negative electrode was prepared by rolling the electrode paste into a film under continuous stirring for 30 min and then pressing the film onto both sides of screened copper mesh and making it dry. Afterwards, the paste was pressed into a thickness of 0.35 mm. The obtained zinc electrodes had a composition (mass fraction) of ZnO (85%), zinc dust (5%), graphite (3.5%), polytetrafluoroethylene (PTFE, 4%), sodium carboxy- methylcellulose (CMC, 1%), In₂O₃ (1%) and SHMP (0.4%). The capacity ratio of zinc and nickel electrodes in all the experimental batteries was 2:1.

2.2 Charge/discharge and self-discharge behavior of Ni-Zn batteries

Sealed AA Ni-Zn batteries consisting of a nickel electrode, a zinc electrode and a separator were assembled. A solution of 7 mol/L KOH was used as the electrolyte. The charge/discharge tests of experimental batteries were performed using a BS-9300 cell-testing instrument at room temperature of (298±2) K. The batteries were charged at a constant current of 500 mA for 60 min and discharged at the same current to a cut-off voltage of 1.4 V. The designed capacity of AA batteries is 500 mA·h.

The self-discharge behaviors of sealed AA batteries were evaluated by charging at a current of 500 mA for  60 min and then putting into an oven at 50 ℃. After 4 d, the batteries were removed and cooled to room temperature, and then discharged at a constant current of 250 mA to a cut-off voltage of 1.4 V to calculate the remaining capacity.

2.3 Hydrogen collection experiments

In order to evaluate the restraining effect of SHMP on the hydrogen evolution reaction of zinc electrode, the hydrogen collection experiment was performed. In the experiment, 1.0 g zinc dust was immersed in 100 mL   7 mol/L KOH electrolyte with 0.7 g SHMP or without SHMP at 50 ℃.

3 Results and discussion

3.1 Effect of SHMP on morphology changes of zinc electrode films

The viscosity of negative pastes (77% solid content) containing various contents of dispersant is shown in Fig.1. The paste shows a viscosity of 0.34 Pa?s at 0.1% of SHMP. The viscosity of the paste decreases marginally with further addition of SHMP and reaches the minimum value of 0.032 Pa?s at 0.4% of SHMP. This indicates that the best-dispersed paste of the powder can be prepared at a minimum content of 0.4% SHMP, and further increase of the content of SHMP (up to 0.55%) cannot reduce the viscosity any more.

Fig.1 Viscosity vs mass fraction of SHMP of negative powder paste

The paste was the mixture of zinc, zinc oxide, graphite and additives, which was obtained by continuously stirring for 30 min under the same operating conditions. To illuminate the agglomeration of particles, SEM images of particle dispersion in the paste are shown in Fig.2. The agglomeration of zinc oxide and graphite powder can be easily observed, as shown in Figs.2(a) and (c). By adding 0.4% SHMP, the zinc oxide and graphite particles are well dispersed, as shown in Figs.2(b) and (d).

The enlarged photographs of the pressed zinc electrode surfaces without and with SHMP are illustrated in Figs.3(a) and (b), respectively. It can be found that there are numerous black spots and stripy traces on the pressed zinc electrode surface. The black spots are caused by the agglomeration of graphite particles, while the stripy traces result from the scrape of the agglomerated zinc oxide when the electrode passes through the scraper. From Fig.3, it can be observed that the addition of SHMP can effectively make the particles well dispersed, and a uniformly gray film appears.

Dispersion of particles with SHMP can be explained based on its properties. The molecular structure of SHMP ((NaPO₃)₆) is shown in Fig.4. SHMP is one of polyphosphate compounds, which can be adsorbed on the surface of the particle to form a molecular layer and significantly enhance the surface potential. The electrostatic repulsion arising from the electric double layer can effectively prevent the particles from agglomerating. Furthermore, SHMP can be hydrolyzed

Fig.2 SEM images of paste and surface of zinc electrode without and with SHMP: (a) Paste without SHMP; (b) Paste with 0.4% SHMP; (c) Electrode without SHMP; (d) Electrode with 0.4% SHMP

Fig.3 Enlarged photographs of zinc electrode surface at six times: (a) Zinc electrode surface without SHMP; (b) Zinc electrode surface with 0.4% SHMP

Fig.4 Molecular structure of SHMP

into PO₄^3- and increase the wetting effect. Therefore, it is often used as the dispersant.

3.2 Effect of SHMP on uniformity of sealed Ni-Zn batteries

The cycle number and the capacity retention rates of two batches sealed AA batteries are presented in Tables 1 and 2, respectively. It can be seen that, in the case of the batteries with SHMP, the average cycle number is 71.4 and the variance is 140.84, while those of the batteries without SHMP are 56.0 and 180.00, respectively. Another two batches of batteries were stored at 50 ℃ for 4 d to examine the self-discharge performance, and the capacity retention rates are shown in Table 2. For the

Table 1 Cycle numbers of sealed Ni-Zn batteries

Table 2 Capacity retention rates (%) of sealed Ni-Zn batteries

batch of batteries with SHMP, the capacity retention rates range from 17.5% to 30.2%. Whereas for the zinc electrode without SHMP, only two batteries have better self-discharge performance and one is completely self-discharged. The fluctuation of the batteries with SHMP is much lower than that without SHMP. This demonstrates that the SHMP can improve the uniformity of the batteries, which is certainly relative to the uniformity of negative paste.

3.3 Effect of SHMP on charge/discharge performance of sealed Ni-Zn batteries

Fig.5 illustrates typical charge/discharge voltage curves for Ni-Zn batteries at the 5th cycle (at 1C rate). For battery A, the potential rise is observed obviously during the later stage of the charge process, which reaches 2.10 V, whereas for battery B, the potential rises very slowly, which reaches only 2.02 V. Furthermore,

Fig.5 Charge/discharge voltage curves of Ni-Zn batteries (A—Without SHMP; B—With 0.4% SHMP)

battery B has been discharged for 53.9 min, which is longer than that of battery A (52.3 min). Lower charging voltage will help to improve the efficiency of the charge process and reduce the gas evolution to maintain the battery sealed and increase the safety of the batteries. The improvement of discharge time for the secondary battery has an important significance to promote the industrial application.

The plot of discharge capacity vs cycle number for sealed Ni-Zn batteries is presented in Fig.6. The batteries were charged at the current of 500 mA for 60 min and then discharged at the same current to the cut-off voltage of 1.4 V. The results demonstrate that the discharge capacity for the 97th cycle of battery with SHMP still remains 90.8% of the initial capacity. And it does not decay very much compared with that of Ni-Zn battery without SHMP in zinc electrode, which only reaches 74 cycles if 450 mA·h is set as the cut-off capacity.

Fig.6 Plot of capacity vs cycle number for sealed Ni-Zn batteries (A—Without SHMP; B—With 0.4% SHMP)

Furthermore, it can be observed from Fig.6 that the discharge capacity of Ni-Zn battery without SHMP in zinc electrode fluctuates violently with cycle number, and the amplitude of fluctuation becomes more and more strenuous with the increase of cycle number. Nevertheless, the discharge plot of battery with SHMP is located above that of battery without SHMP and is smoother, which indicates that battery with dispersant in zinc electrode has higher discharge capacity and better stability of capacity. The results suggest that the secondary batteries with SHMP in negative electrode exhibit good performance of cycleability and stability.

3.4 Hydrogen collection experiments

The hydrogen collection experiments were carried out in order to investigate the improvement of cycleability, stability and uniformity of Ni-Zn batteries. Fig.7 illustrates the amount of hydrogen evolution for zinc dust in electrolyte. The zinc dust immersed in the electrolyte without SHMP generates 5.30 mL hydrogen after 40 h, while the volume of evolved hydrogen is only 3.90 mL in the electrolyte with 0.4% SHMP. This indicates that the hydrogen evolution of zinc dust is suppressed by SHMP, which may be caused by the chelate complexes of SHMP and Zn²⁺ in the batteries. The chelate complexes deposit on the cathode. This deposit layer has a dense membrane, which can reduce the hydrogen evolution reaction. The suppressed hydrogen evolution will improve the efficiency of the charge process, restrain the self-discharge and maintain the stability of the battery system, which has an important significance to promote cycleability, stability and uniformity of Ni-Zn batteries.

Fig.7 Effect of SHMP on hydrogen evolution of zinc dust (A— Without SHMP; B—With 0.4% SHMP)

4 Conclusions

(1) SHMP can prevent ZnO and graphite particles from agglomerating in zinc electrode effectively by forming molecular layer on the particle surface and enhancing the surface potential significantly.

(2) The zinc electrodes with SHMP have better electrochemical performance of cycleability, stability and uniformity because the SHMP disperses the particles and suppresses the hydrogen evolution of zinc dust in the alkaline electrolyte.

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(Edited by CHEN Wei-ping)

Foundation item: Project(2006BAE03B03) supported by the National Key Technologies Research and Development Program of China

Received date: 2009-11-21; Accepted date: 2010-03-22

Corresponding author: YANG Zhan-hong, PhD, Professor; Tel: +86-731-88879616; E-mail: zhanhongyang@126.com