Intercalation of sulfate anions into a Zn–Al layered double hydroxide: their synthesis and application in Zn–Ni secondary batteries

Abstract

ZnAl–SO₄-LDH was prepared via an anion-exchange process using ZnAl–CO₃-LDH as a precursor. Based on the analysis for the main characteristic peaks and intensity of the Fourier transform infrared (FT-IR) spectra, CO₃²⁻ was successfully replaced with SO₄²⁻. The powder X-ray diffraction (XRD) patterns indicated that all the products were well-crystallized and the scanning electron microscopy (SEM) images showed the clear layered structure of ZnAl–SO₄-LDH. This means during the anion exchange process, the framework of LDH was not affected. The electrochemical performance of ZnAl–SO₄-LDH was examined by galvanostatic cycling and cyclic voltammetry (CV) measurements, and the results demonstrate a novel active material ZnAl–SO₄-LDH for nickel–zinc secondary batteries with remarkable properties, including higher electrochemical activity than ZnAl–CO₃-LDH and desirable cycling stability over 200 cycles (retention rate 93.2%).

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Introduction

Nickel–zinc secondary batteries have stimulated extensive research interest for their superior performance. To the best of our current knowledge, Ni–Zn batteries have a high open circuit voltage, which is higher than most batteries on the market at the present aside from lithium batteries such as nickel–cadmium, super-iron batteries¹ and nickel–hydrogen batteries. In addition, Ni–Zn batteries display features, such as wide working temperature ranges (discharge smoothly between −20 °C and 50 °C), low cost and environmental friendliness (zinc resources are very abundant and both zinc and nickel have low toxicity). However, there are still some drawbacks to zinc electrodes, such as shape change, zinc dendrite formation, surface passivation and zinc self-corrosion, which severely limit their discharge and commercial applications.^2,3 Therefore, novel materials or strategies to improve the performance of zinc electrode materials are needed. During the past several years, most research has focused on improving the characteristics of the electrolyte or modification of the ZnO electrode.^4,5 In particular, the electrochemistry properties of LDHs as modified electrodes have attracted a lot of attention.^6,7

Layered double hydroxides (LDHs) are a class of anionic clays with laminated structures. A typical LDH is MgAl–CO₃ hydrotalcite, which can be expressed as Mg₆Al₂(OH)₁₆CO₃·4H₂O (MgAl-LDH). The structure of MgAl-LDH based on brucite (Mg(OH)₂)-like layers, wherein the Mg²⁺ coordinated octahedrally with hydroxyls via covalent bonds and some of the Mg²⁺ have been isomorphously replaced by Al³⁺, giving positively charged sheets. Therefore, the function of CO₃²⁻ in the interlayer region was to compensate the positive charge from the sheets. The Mg²⁺ and Al³⁺ in the slab layers can be isomorphously replaced with other divalent and trivalent metal ions as long as their ionic radius are in close proximity. Consequently, a large class of LDHs materials will be obtained and we can represent them with the general formula [M(II)_1−xM(III)_x(OH)₂]^x+[(Aⁿ⁻)_x/n·yH₂O] (where M(II) and M(III) represent divalent and trivalent metallic ions, A = NO₃⁻, Cl⁻, CO₃²⁻, and SO₄²⁻). The sheets are connected to anions and water molecules in the interlayer by electrostatic forces, hydrogen-bonding and van der Waals interactions. Therefore, the interlayer anions are exchangeable due to the weak interactions between the anions and slab layers.^8,9 Because of their highly tunable composition, LDH materials have been widely used as catalysts,¹⁰ anion exchangers,¹¹ absorbents,¹² heat stabilizers,¹³ polymer composites and bioactive materials.^14–16 Moreover, within a limited potential range in alkaline medium, LDH is capable of undergoing an inner redox reaction, so we have been investigated it as an anode active material in Ni–Zn secondary batteries.¹⁷ In our previous studies, ZnAl–CO₃-LDH¹⁷ and its derivative materials^18–21 have been investigated as an anode material in nickel–zinc secondary cells, showing good reversibility, superior electrochemical cycling stability and more excellent utilization in the alkaline solution when compared with the ZnO electrode because of their alkalescence, layered structure and stability in alkaline solution. However, the interlayer anion makes no contribution to the discharge capacity in LDHs with higher molecular weights. To make the anion play a positive role, we tried to study the impact of different anions on the properties of the nickel–zinc secondary cell. In our previous study, Xie et al.²² found that dodecyl sulfate (DS) intercalated into the ZnAl-LDH interlayer and exhibited better electrochemistry performance than the ZnAl-LDH precursor. DS has special characteristics as an anion surfactant, which will improve the surface properties and lead to a uniform distribution of LDH. This illustrates that the interlayer anion of LDH has an effect on the electrode electrochemistry performance.

To study the effects of SO₄²⁻ inserted into LDHs will have on the properties of Zn–Ni secondary batteries, ZnAl–SO₄-LDH was prepared via an anion-exchange process using ZnAl–CO₃-LDH as the precursor in this study. For comparison, the electrochemical performances of the ZnAl–CO₃-LDH and ZnAl–SO₄-LDH electrodes were investigated.

Results and discussion

FT-IR analysis of the ZnAl-hydrotalcites

The FT-IR spectra of ZnAl–CO₃-LDH and ZnAl–SO₄-LDH are shown in Fig. 1(a). According to our previous studies,²² the broad peak around 3445 cm⁻¹ can be ascribed to the stretching of OH groups attached to the metal ions. The band at 1650 cm⁻¹ is due to the bending mode of water molecules. The lower wavenumber bands at 400–700 cm⁻¹ are due to the LDH lattice vibrations (Zn–O, Al–O). The bands of ZnAl–SO₄-LDH at 1112.62 cm⁻¹ can be assigned to SO₄²⁻.²³ However, there is still a weak absorption peak at 1354.17 cm⁻¹, which belongs to CO₃²⁻. Therefore, it can be concluded that CO₃²⁻ and SO₄²⁻ have been successfully exchanged, but not completely. The reason for this phenomenon is carbonate has a stronger affinity with the LDH layers than other anions; the affinity of the LDH towards various anions is in the following order:²⁴

CO₃²⁻ > SO₄²⁻ > OH⁻ > F⁻ > Cl⁻ > Br⁻ > NO₃⁻ > I⁻

Fig. 1 (a) The FT-IR spectra of ZnAl–CO₃-LDH and ZnAl–SO₄-LDH. (b) A schematic of the LDH intercalation process.

In addition, during the synthesis process, a small amount of carbon dioxide still exists in the synthesis reaction system even though it was under the protection of nitrogen. Therefore, ZnAl–CO₃-LDH cannot be easily converted to other anions by ion-exchange processes directly.²⁵ According to the previous report, the smaller the ionic radius, the easier the transport of anions into the interlayer would be and vice versa.²⁶ Therefore, in this study, an indirect synthesis method was employed. First, the CO₃²⁻ located in the interlayer region is vulnerable to attack by hydrochloric acid. Then, chloride ion is a monoatomic anion with a smaller ionic radius and therefore it can be diffused into the interlayered structure of ZnAl–CO₃-LDH and replace the position of CO₃²⁻ in the highly concentrated chloride ion solution. Therefore, ZnAl–Cl-LDH can be obtained as a precursor. As we all know, the lower the charge density, the weaker the electrostatic force between the anion and sheets.²⁴ Subsequently, with the help of sulfate, the ZnAl–Cl-LDH will be converted into ZnAl–SO₄-LDH via an anion exchange process. Fig. 1(b) shows a schematic of ZnAl–CO₃-LDH being transformed into ZnAl–SO₄-LDH. In a word, first, ZnAl–CO₃-LDH was converted to ZnAl–Cl-LDH and then the Cl⁻ exchanged with SO₄²⁻ using an ion-exchange method.

XRD and SEM analysis of the ZnAl-hydrotalcites

Fig. 2 presents the XRD patterns of ZnAl–CO₃-LDH, ZnAl–Cl-LDH and ZnAl–SO₄-LDH. From Fig. 2, the characteristic diffraction peaks of ZnAl–CO₃-LDH were found at 2θ = 11.63°, 23.39°, 34.55°, 39.13°, and 60.11°, which correspond to the reflections of (003), (006), (012), (015) and (110) ​planes, respectively. The XRD patterns of these samples were very similar, indicating that the ZnAl–SO₄-LDH share a hydrotalcite structure, i.e., the anion exchange process was almost not in relation to the layered internal structure and the product still maintains the hexagonal system characteristics. It can be found that the location of the characteristic diffraction peaks of ZnAl–Cl-LDH and ZnAl–SO₄-LDH were shifted to lower 2 theta values when compared with ZnAl–CO₃-LDH. This result revealed the increase in crystal layer spacing of ZnAl–Cl-LDH and ZnAl–SO₄-LDH. The d(003) were calculated to be 0.7644 nm, 0.7997 nm and 0.8505 nm for ZnAl–CO₃-LDH, ZnAl–Cl-LDH and ZnAl–SO₄-LDH, respectively, based on the Bragg equation. The Bragg equation is as follows:

nλ = 2d sin θ (1)

Fig. 2 The XRD patterns of ZnAl–CO₃-LDH, ZnAl–Cl-LDH and ZnAl–SO₄-LDH.

The increase in the basal spacing by converting CO₃²⁻ to Cl⁻ can ascribe to the weak interactions between Cl⁻ and the sheets due to the low negative charge density of Cl⁻. SO₄²⁻ has a tetrahedral structure, whereas CO₃²⁻ has a planar triangular structure. The effective radius of SO₄²⁻ (r = 244 pm) is bigger than CO₃²⁻ (r = 164 pm); therefore, the interlayer distance will be expanded if SO₄²⁻ is successfully intercalated into the LDH interlayer. This simultaneously suggests that the expansion of the channel between the layers caused by the SO₄²⁻ exceeds the constriction observed from the electrostatic attractions between SO₄²⁻ and the sheets. These diffraction peaks are sharp, narrow and symmetrical, which illustrates that the sample is well-crystallized. The SEM images of ZnAl–CO₃-LDH and ZnAl–SO₄-LDH are presented in Fig. 3(a) and (b). The images clearly show that the samples contain lamellar particles with a hexagon layer structure, which is the typical structure of hydrotalcite-like material.⁹ The results agree well with the XRD experimental results and the basic structure of the LDHs does not change in the anion exchange process. In addition, EDS spectrum measurements (Fig. 3(c)) confirmed the presence of S in ZnAl-LDH; further analysis showed that CO₃²⁻ and SO₄²⁻ have been successfully exchanged.

Fig. 3 (a) The SEM image of ZnAl–CO₃-LDH. (b) The SEM image of ZnAl–SO₄-LDH. (c) The EDS results for ZnAl–SO₄-LDH.

Cyclic voltammetry performance of the zinc electrodes

The recorded CV curves for the ZnAl–CO₃-LDH (code a) and ZnAl–SO₄-LDH (code b) electrodes are shown in Fig. 4(a). From the comparison between electrode a and b, we found that the anodic peak potential of the electrode b decreased −1.169 V (a), −1.197 V (b) and the cathodic peaks were −1.487 V (a) and −1.490 V (b), which means the potential interval between anodic and cathodic peak of the ZnAl–SO₄-LDH electrode became smaller (a Δϕ = 0.318 V, b Δϕ = 0.293 V). This phenomenon reveals the reversibility of ZnAl–SO₄-LDH was better. The better reversibility was attributed to the higher electrochemical activity of the ZnAl–SO₄-LDH electrode. Electrode b also shows a higher anodic peak current, larger anodic peak area and lower anodic peak potential. The anodic process reflects the discharge process of a Zn–Ni secondary battery. The increase in anodic peak area and anodic peak current means that electrode b possesses higher electrochemical activity, and can deliver a greater discharge capacity than electrode a, which was attributed to the SO₄²⁻ intercalated into the LDH. During the charge and discharge process, SO₄²⁻ will combine with Zn²⁺ and OH⁻ to form a zinc sulfate hydroxide hydrate (Zn₄(OH)₆(SO₄)·nH₂O) side product and it will be coated on the LDH surface or doped to the electrode, the side reaction is shown as follows:²⁷

4Zn²⁺ + 6OH⁻ + SO₄²⁻ + nH₂O → Zn₄(OH)₆(SO₄)·nH₂O (2)

Fig. 4 (a) The cyclic voltammogram for ZnAl–CO₃-LDH and ZnAl–SO₄-LDH. (b) A schematic of the ZnAl–SO₄-LDH reaction process.

Fig. 4(b) shows a schematic of this process. Zn₄(OH)₆(SO₄)·nH₂O has the similar structure with LDHs, all of them are layered structures. Nevertheless, the main reaction is still the redox reaction of LDH. The aluminum ion within ZnAl-LDH cannot be reduced to aluminium and still exists as an ion, and will provide a stable framework for the active material in the process of sedimentation. Therefore, the structure of ZnAl-LDH is stable in the discharge process. Zn₄(OH)₆(SO₄)·nH₂O has good electrochemical activity²⁸ and therefore ZnAl–SO₄-LDH shows the better electrochemical properties.

The electrochemical impedance spectroscopy of the zinc electrodes

To explore the effect of changing the charge transfer resistance of ZnAl–SO₄-LDH on the performance of the zinc electrode, EIS measurements were carried out. The Nyquist plots are shown in Fig. 5(a). Electrochemical impedance spectroscopy (EIS) or ac impedance methods have a tremendous increase in popularity in recent years. This method studies the systems response to the application of a periodic small-amplitude ac signal. The measurements are carried out at different ac frequencies. In general, Nyquist plots include a high-frequency capacitive semicircular loop and a sloping tail in the low-frequency region. The equivalent circuit for the electrode is shown in Fig. 5(b). R_s represents the ohmic resistance, which is the summation of the resistance of electrolyte, and current collector. R_ct is the charge transfer resistance and it can be calculated by the diameter of the semicircle in Fig. 4(a). In addition, CPE denotes the constant-phase element and Z_w is the Warburg impedance. The element parameters can be calculated using ZSimpWin software and are shown in Table 1. According to Table 1, ZnAl–SO₄-LDH has smaller R_e and R_ct values when compared to ZnAl–CO₃-LDH. The improved electrochemical properties could be attributed the change in the hydrotalcite structure because of the insertion of SO₄²⁻. Zn₄(OH)₆(SO₄)·nH₂O has a sheet-like structure (Fig. 3(b)) in which the modified octahedral sheets of Zn(OH)₂ alternate along the c-axis with layers of water molecules.²⁹ The two oxygen atoms of the octahedra are the apices of two sulfate tetrahedra of which one projects above and the other below the octahedral sheets. Tetrahedrally coordinated Zn atoms are located above and below the empty octahedra and corner-share their basal anions (hydroxides) with six octahedra. There are two types of zinc coordination and therefore Zn₄(OH)₆(SO₄)·nH₂O has better conductivity than that of LDH. Therefore, the improvement in conductivity was caused by the formation of a small amount of the Zn₄(OH)₆(SO₄)·nH₂O phase coated on the ZnAl–SO₄-LDH surface or doped to the electrode.

Fig. 5 (a) Nyquist plots of the electrodes with ZnAl–CO₃-LDH and ZnAl–SO₄-LDH. (b) The equivalent circuit for the electrode.

Table 1 The data for the EIS curves for the ZnAl-LDH samples, (a) ZnAl–CO₃-LDH and (b) ZnAl–SO₄-LDH

Sample	Rs (Ω)	Rct (Ω)
a	5.464	4.069
b	5.363	3.312

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The galvanostatic charge and discharge properties

Typical galvanostatic charge/discharge measurements were performed to evaluate the application of the samples as Zn/Ni secondary materials. The curves for the Zn/Ni secondary cells using ZnAl–CO₃-LDH and ZnAl–SO₄-LDH as the electrodes tested at the 40th cycle are displayed in Fig. 6(a) and (b). The overall electrode reaction can be described using eqn (3) and (4).^30,31

Fig. 6 (a) The charge voltage plateau of ZnAl–CO₃-LDH and ZnAl–SO₄-LDH. (b) The discharge voltage plateau of ZnAl–CO₃-LDH and ZnAl–SO₄-LDH. (c) The electrochemical cycle behavior of ZnAl–CO₃-LDH and ZnAl–SO₄-LDH.

Charging process

Zn(OH)₄²⁻ + 2e → Zn + 4OH⁻ (3)

Discharge process

Zn + 4OH⁻ → Zn(OH)₄²⁻ + 2e (4)

As can be observed in Fig. 6(a) and (b), the cell employed with the ZnAl–SO₄-LDH presents a lower charge plateau and higher discharge plateau than that found for ZnAl–CO₃-LDH. The decrease in the charge plateau voltage was conducive to suppress H₂ formation and improve the charge efficiency. Moreover, the higher discharge plateau voltage represents the higher discharge potential and better electrochemical properties of the discharge process. At the same time, the cell employed with ZnAl–SO₄-LDH shows a longer discharge time, reaching 47 min. Therefore, when compared with ZnAl–CO₃-LDH, ZnAl–SO₄-LDH as an electrode material for nickel–zinc secondary cells will obtain a higher utilization ratio and specific discharge capacity.

The discharge capacity with number of cycles for the electrode containing ZnAl–CO₃-LDH and ZnAl–SO₄-LDH are depicted in Fig. 5(c). The cell was charged at 1C rate for 1 h and discharge at 1C down to 1.2 V. A clear enhancement in the discharge capacity was observed during the first several cycles. After these cycles, the discharge capacity reached an almost constant capacitance of 325.2 mA h g⁻¹ for ZnAl–SO₄-LDH. This observation is likely due to the incomplete activation of the active material in the electrodes. During the subsequent cycles, the structure of the active material will be activated gradually. As can be observed in Fig. 5(c), the ZnAl–CO₃-LDH has higher maximum discharge capacity than ZnAl–SO₄-LDH. The LDH with a lower zinc mass ratio after the intercalation of SO₄²⁻ can be considered the reason for the decreased specific capacity. After 200 continuous cycles, the capacitance of ZnAl–SO₄-LDH decreased to 93.2% of its initial value, about 303.09 mA h g⁻¹; moreover, ZnAl–CO₃-LDH decreases rapidly to 247.85 mA h g⁻¹ from the maximum capacitance 328 mA h g⁻¹ with a retention rate of 75.6%. In general, zinc ions are reduced to the zinc metals using aluminium species as the deposition crystal nucleus during the charging process, whereas the reverse reaction will occur and restore the structure of Zn–Al hydrotalcite in the discharge process. In our previous study,²² through detects the reaction products when using ZnAl-LDH as electrode material, we found that the ZnO by-product was formed in the process and the corresponding proportion of the interlayer anion will escape from the LDH interlayer. When there are Zn²⁺, ZnO and SO₄²⁻ in the electrode at the same time, the chemical reaction that occur was,³²

Zn²⁺ + 3ZnO + SO₄²⁻ + nH₂O → Zn₄(OH)₆(SO₄)·nH₂O (5)

Zinc sulfate hydroxide hydrate is a type of electrode material with good cycle performance as a Zn–Ni battery anode, which has been reported previously.²⁸ The existence of sulfate in the LDH has a favorable impact on the cycle performance; it can capture the discharge by-product and then raise the utility of negative material and improve the cycle performance of the battery. Fig. 7 shows the XRD pattern of the ZnAl–SO₄-LDH electrode after 50 charge/discharge cycles. The peak for ZnAl–SO₄-LDH has the highest intensity, indicating that the main content of the electrode was still ZnAl–SO₄-LDH with a layered structure during the charge/discharge cycling process, i.e., ZnAl–SO₄-LDH is stable in the simulated cell. The XRD pattern also contains the diffraction peaks, corresponding to Zn₄(OH)₆(SO₄)·5H₂O (JCPDS no. 39-0688). The abovementioned results show that Zn₄(OH)₆(SO₄)·5H₂O was formed during the discharging process.

Fig. 7 XRD pattern of ZnAl–SO₄-LDH electrode after 50 cycles.

The rate performance analysis of ZnAl–SO₄-LDH

The rate performance is an important factor in meeting the needs of high-storage applications. Galvanostatic charge–discharge measurements were performed at various rates and the results are shown in Fig. 8(a). Fig. 8(b) shows the corresponding discharge curves for ZnAl–SO₄-LDH. The results show that the discharge capacity increases with an increase in the rate from 1C to 2C, when the discharge rate continues to increase, the discharge capacity gradually decreases. We can observe that the ZnAl–SO₄-LDH electrode exhibits discharge capacities of 347 mA h g⁻¹, 398 mA h g⁻¹, 304 mA h g⁻¹, 200 mA h g⁻¹, 139 mA h g⁻¹, 118 mA h g⁻¹, and 98 mA h g⁻¹ at current rates of 1C, 2C, 4C, 6C, 8C, 10C and 12C, respectively. At a lower rate, with an increase in current rate, the hydrogen evolution over-potential will increase, which is conducive to improve the suppression of hydrogen evolution.²¹ Therefore, the charge efficiency of the ZnAl–SO₄-LDH electrode will be enhanced. However, when the current rate was higher than 2C, the intensification of the electrode polarization plays the most important role during the cycle performance, and the inhibition of hydrogen evolution could be ignored. High polarization leads to a significant decrease in charge–discharge efficiency, thereby reducing the discharge capacity of the ZnAl–SO₄-LDH electrode.

Fig. 8 (a) The discharge curves for the ZnAl–SO₄-LDH electrode at various charge–discharge rates from 1C to 12C. (b) The rate capability of ZnAl–SO₄-LDH.

Consequently, the reason for the improvement in the electrochemical activity and cycle stability can be concluded as follows: SO₄²⁻ in the interlayer leads to the formation of sulfate hydroxide hydrate during the charge and discharge process, which is beneficial to improve the electrochemical activity of the electrode. The redox reaction of the ZnAl–SO₄-LDH electrode is more sufficient and shows improved reversibility. Furthermore, SO₄²⁻ can capture the discharge by-product and enhance the utilization ratio of active material, which is beneficial for prolonging the battery's service life.

Experimental section

Synthesis of Zn–Al–CO₃-LDH

ZnAl–CO₃-LDH was synthesized using a constant pH co-precipitation method and hydrothermal method. This method combines the advantages of the co-precipitation method and hydrothermal method; the hydrothermal operation can make it easier to form uniform grains.

Under continuous stirring, Zn(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O (Zn/Al = 4 : 1) were dissolved in distilled water and mixed together to obtain an aqueous solution with a concentration of 0.25 M. Then, the abovementioned aqueous mixture and a mixed solution, including NaOH (2.5 M) and Na₂CO₃ (0.5 M), were simultaneously added to the vessel with 50 mL distilled water. The resultant solution was mechanically stirred for 30 min at 65 °C at a constant pH value of 10 and then transferred to an autoclave. The autoclave was heated in a drying oven for 12 h at 120 °C. Subsequently, the obtained slurry was filtered and washed several times with distilled water and ethanol. Finally, the precipitate was dried and ground into a fine powder.

Synthesis of ZnAl–Cl-LDH

NaCl (0.500 mol) and Zn–Al–CO₃-LDH (0.5035 g) were dispersed in 100 mL of distilled water and ultrasonically mixed together. Then, the mixture was transferred to a 1000 mL volumetric flask containing 400 mL distilled water. Using a pipetting gun takes 200 μL of concentrated hydrochloric acid was added to the volumetric flask with the protective aeration of nitrogen for 20 min. Subsequently, the volumetric flask was placed into a constant temperature water bath oscillator for 10 h at 25 °C. Finally, the precipitate was filtered, washed, dried and ground into a fine powder.

Synthesis of ZnAl–SO₄-LDH

Na₂SO₄ (0.100 mol) and ZnAl–Cl-LDH (0.4000 g) were dispersed in 100 mL of distilled water and the remaining steps repeated as described in the process used to synthesis ZnAl–Cl-LDH.

Material characterization

The structures of the obtained materials were characterized by X-ray diffraction (XRD) (Siemens) using Cu Kα radiation (λ = 0.15418 nm) at 36 kV and 30 mA. The samples were scanned in the 2θ range from 5° to 80°. The morphologies and structural properties of the samples were examined using a Nova Nano SEM 230 scanning electron microscope (SEM) operated at 10 kV. Fourier transform infrared (FT-IR) spectra were obtained using an AVATAR-360 (Nicolet Magna) infrared spectrophotometer (as KBr discs, with wavenumber 400–4000 cm⁻¹, resolution 0.09 cm⁻¹).

The preparation and electrochemical measurements of the zinc electrodes

The zinc electrodes were prepared by incorporating slurries containing LDH, acetylene black and polytetrafluoroethylene (PTFE, in diluted emulsion) with a mass ratio of 8 : 1: 1 and the mixture was sprayed on a copper mesh substrate (1.2 cm × 1.2 cm in size). The galvanostatic charge–discharge tests were conducted on a BST-5 V/10 mA Battery Program-control Test System (Neware, China) at room temperature, the tests were performed at 1C for 1 h and discharged at 1C to a cutoff voltage of 1.2 V with a counter electrode of sintered nickel hydroxide (β-Ni(OH)₂). A solution of 6 M KOH saturated with ZnO was used as the electrolyte. Cyclic voltammetry (CV) measurements were performed on a CHI1660B electrochemical workstation at a scanning rate of 10 mVs⁻¹, shifting from −0.95 V to −1.65 V at room temperature. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (CHI660B) with a frequency range from 100 kHz to 0.01 Hz and the AC potential perturbation was 10 mV. The data was fitted using Zview software. CV and EIS measurements were carried out in a conventional three-electrode system with the zinc electrode (after activation at 1C charge–discharge for 20 cycles) as a working electrode, a sintered nickel electrode as the counter electrode and Hg/HgO as the reference electrode; the electrolyte was 6 M KOH solution.

Conclusions

ZnAl–CO₃-LDH (Zn/Al = 4) was prepared using a hydrothermal method and then converted to ZnAl–SO₄-LDH via anion-exchange processes. The FT-IR and EDS results indicate that SO₄²⁻ was successfully inserted into the LDH interlayer with only a small quantity of residual CO₃²⁻. The XRD pattern and the SEM images illustrate that the ZnAl–SO₄-LDH sample maintains the well-crystallized hexagonal lamellar structure of LDH. Electrochemical investigations demonstrate remarkable performance, including improved electro-chemical activity, higher cycling stability and utilization ratio of active material when ZnAl–SO₄-LDH was used as electrode material for nickel–zinc secondary cells. We can ascribe these intriguing electrochemical behaviors to the SO₄²⁻ inserted into the LDH interlayer, which leads to sulfate hydroxide hydrate formation and capture of the discharge by-product. In a word, the electrochemical activity and cycling performance of the material for 200 continuous cycles demonstrates that ZnAl–SO₄-LDH is suitable as a promising electrode material for Zn–Ni secondary batteries.

Acknowledgements

This study was supported by the Natural Science Foundation of China (No. 21371180) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130162110018).

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