The effects of sodium fluoride as the electrolyte additive on the electrochemical performances of magnesium–8lithium–0.5zinc electrode in sodium chloride solution

Abstract

The electrochemical performance of Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution with different concentrations of NaF (0.0, 0.2, 0.5, 0.8, 1.0, 1.2 and 1.5 mmol L⁻¹) was investigated by means of potentiodynamic polarization, potentiostatic current–time curves, electrochemical impedance spectroscopy, scanning electron microscopy and utilization efficiencies. The findings indicate that Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution with 1.0 mmol L⁻¹ NaF has a higher discharge current density than that in 0.7 mol L⁻¹ NaCl solution with the other concentrations of NaF that were investigated. The addition of 1.0 mmol L⁻¹ NaF to the NaCl electrolyte solution loosens the product film and the oxidation products on the alloy surfaces of the electrode, which produces deeper and larger channels, as observed by the SEM investigation, compared with those in 0.7 mol L⁻¹ NaCl solution without any NaF. The addition of 1.0 mmol L⁻¹ NaF in 0.7 mol L⁻¹ NaCl solution improves the continuous discharging utilization efficiency of Mg–8Li–0.5Zn electrode by more than 18.80%, and enhances the interval discharge utilization efficiency of Mg–8Li–0.5Zn electrode by more than 27.90%.

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1. Introduction

Due to the lowest density among metallic structural materials and good formability, Mg–Li based alloys are attracting increasing research interest.^1–4 Additionally, they have potential applications in aerospace, automobile, 3C products, anodes and biomaterials because of their high specific strength, low density, high electrode potential, high Faradic capacity and good biocompatibility.^5–10 More importantly, the combination of these advantages, as well as use of seawater as an electrolyte, make a magnesium-hydrogen peroxide semi-fuel cell an attractive undersea power source. The system has high specific energy, stable discharging ability, short mechanical recharge time, long dry storage life, ability to work at ambient pressure, environmental acceptability, reliability, safety and low cost.² In addition, the Mg-H₂O₂ semi-fuel cell has a theoretical voltage of 4.14 V, which is higher than that of an Al–H₂O₂ semi-fuel cell^11,12 or an Al–AgO battery. For instance, Medeiros et al.^2,13 investigated the Mg-H₂O₂ semi-fuel cell using magnesium alloy AZ61 as the anode and carbon fiber-supported Pd–Ir as the cathode catalyst. Using this arrangement, they found that the cell has a voltage of above 1.7 V at 25 mA cm⁻² at room temperature. The magnesium efficiency is around 80%. The specific energy of the system ranges from 500 to 520 W h kg⁻¹ based on the weights consumed during the discharge of the magnesium anode, hydrogen peroxide and acid.

According to the thermodynamic analysis, magnesium anodes should exhibit very negative potentials. On the contrary, these magnesium electrodes operate at significantly less negative potentials in practical applications, resulting from the fact that magnesium is normally covered by passive oxide films, which delays the steady-state, and thus reduce the discharging rate. Moreover, magnesium anodes undergo a parasitic corrosion reaction, or self-discharge, resulting in the reduction of coulombic efficiency (less than 100% utilization of the metal) and evolution of hydrogen.

To improve the electrochemical properties of Mg alloys, some researchers have doped pure magnesium with other elements, such as lithium, aluminum, silver, yttrium, cerium, and rare earth (RE) elements.^14,15 Udhayan et al. reported that magnesium alloy AP65 (Al: 6–7%, Zn: 0.14–1.5%, Mn: 0.15–1.3% and Pb: 4.5–5%) has a hydrogen evolution rate of 0.15 mL min⁻¹ cm⁻² and a utilization efficiency of 84.6%. The open circuit potential of this alloy is −1.803 V (vs. SCE) measured in seawater.¹⁶ Sivashanmugam et al. investigated Mg–Li alloy with 13 wt% Li for their possible use in magnesium primary reserve batteries.¹⁷ They found that this Mg–Li alloy exhibited higher anodic efficiencies (81%) corresponding to the high current density (8.6 mA cm⁻²). The other method of increasing the electrochemical properties of Mg–Li based alloys is the addition of additives to electrolytes. The oxides of gallium, indium, calcium and zinc, and stannates as well as citrates, were found to be effective electrolyte additives for inhibiting corrosion and/or boosting discharge current density.^18–21 Recently, our group²² has studied the electrochemical performance of Mg–8Li–1Y electrode in 0.7 mol L⁻¹ NaCl solutions containing different concentrations of Na₂SnO₃. It was found that the peak power density of the semi-fuel cell with Mg–8Li–1Y alloy as the anode in 0.7 mol L⁻¹ NaCl solution with 0.3 mmol L⁻¹ Na₂SnO₃ (112 mW cm⁻²) is higher than that in 0.7 mol L⁻¹ NaCl solution without Na₂SnO₃ (83 mW cm⁻²).

Magnesium as the anode of magnesium hydrogen peroxide semi-fuel cell system has the advantages of high faradaic capacity, high specific energy, more negative standard electroreduction potential and high discharge performance in a seawater electrolyte. However, as an anode material, pure magnesium suffers a drawback of low current efficiency due to its severe parasitic corrosion reaction that results in the evolution of hydrogen and the “negative difference effect”. In addition, the passive film formed on magnesium surface leads to a cell voltage delay. Doping magnesium with other alloy elements is an effective way to improve the magnesium anode performance. Elemental lithium is the most active metal anode; doping magnesium with lithium can improve the electrochemical activity of the electrode. When the content of lithium exceeds 5.7% in Mg–Li alloys, the alloy adopts a body-centered cubic structure, improving the six-party phase plasticity of Mg at room temperature, which simplifies the preparation of the electrode with various shapes. However, the corrosion resistance of the electrode is reduced with increasing lithium content. Zinc can make the corrosion potential of the magnesium alloy move towards a positive value, thus reducing the parasitic corrosion reactions and evolution of hydrogen reaction of magnesium alloys, which facilitates the improvement of the electrode performance. Moreover, the addition of Zn can weaken the influence of Fe and Ni impurity on the magnesium alloy. However, if the content of Zn is high, it will lower the electrochemical activity of the electrode. From the above discussion, the content of lithium and zinc was selected to be 8% and 0.5%, respectively.

In this paper, Mg–8Li–0.5Zn alloy was prepared and the sodium fluoride (NaF) was selected as the electrolyte additive to investigate the electrochemical performances of Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution. The purpose of this study is to investigate the effects of the different concentrations of sodium fluoride as the electrolyte additive on the electrochemical oxidation performance of Mg–8Li–0.5Zn electrode (NaF was used as an activating agent in our experiment).

2. Experimental

2.1 Preparation of Mg–8Li–0.5Zn alloy

In this work, Mg–8Li–0.5Zn alloy was prepared from the ingots of pure magnesium (99.99%), pure lithium (99.99%) and pure zinc (99.99%) using a vacuum induction melting furnace. The ingots of the alloying components were charged into an induction furnace and melted under the protection of ultrahigh purity argon. The molten alloys were then poured through a tundish into a stainless steel cylinder with a height of 18 cm and an inner diameter of 6 cm. In the subsequent step, the alloy was cooled to ambient temperature under an argon atmosphere. The composition of the resulting alloys is given in Table 1.

Table 1 Nominal composition of the alloys (wt%)

Alloys	Mg	Li	Zn
Mg–8Li–0.5Zn	91.5	8	0.5

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The alloy ingots were machined to 10 × 10 × 5 mm to serve as the working electrode for electrochemical measurements. Before each experiment, the alloy surface facing the electrolyte was mechanically polished with 180^#, 800^#, 1200^# and 2000^# metallographic emery papers, degreased with acetone, washed with deoxygenated ultrapure water (Milli-Q), and immediately assembled in an electrochemical cell.

2.2 Testing procedure

A specifically designed home-made three-electrode electrochemical cell was used to carry out the electrochemical measurements on Mg–8Li–0.5Zn electrode. A blackened platinum wire mesh was used as the auxiliary electrode, and the saturated calomel electrode (SCE) with the Luggin capillary positioned close to the alloy surface served as the reference electrode (in this article, all the potentials were referred to the saturated calomel electrode without special indication).⁶ The exposure area of the metal alloy electrode was 0.502 cm², which was used to calculate the current density. The electrochemical experiments were carried out at room temperature in 0.7 mol L⁻¹ NaCl aqueous solution containing different concentrations of NaF. All solutions were made with analytical grade chemical reagents and Millipore Milli-Q water (resistivity > 18 MΩ cm), and were purged with N₂ gas for 15 minutes before measurements to remove the O₂ gas dissolved in the solution.

The measurements of potentiodynamic polarization curves (5 mV s⁻¹, −2.2 V to −0.8 V vs. SCE), potentiostatic current–time curves (−1.4 V, −1.2 V, −1.0 V and −0.8 V vs. SCE), as well as electrochemical impedance spectra (0.01 Hz to 100 kHz, 5 mV, open circuit potential) were performed using an eight channel VMP3/Z potentiostat (Princeton Applied Research) controlled by EC-lab software. The morphology of the alloy surface was examined using a scanning electron microscope (SEM; JEOL JSM-6480) equipped with an energy-dispersive spectroscope (EDS) unit. Images were acquired using a 20 kV accelerating voltage.

Furthermore, to investigate the utilization efficiency of Mg–8Li–0.5Zn alloy, the weighted alloy samples were discharged at constant potential (−1.0 V for 2 h) while recording the current–time curves. The reaction products that remained attached to the alloy surfaces were removed after discharge. Then, the cleaned remaining alloy coupons were dried and weighed. The alloy utilization efficiency (η) was calculated using eqn (1).

n = Σx_iz_i (2)

M_a = Σx_iM_i (3)

where Q is the charge in Coulombs obtained by the integration of current–time curve; F is Faraday constant (96 485 C mol⁻¹); m₀ and m_t are the weights of alloy sample in grams before and after discharge, respectively; n is the average number of electrons per discharge reaction assuming the oxidation states of the products are 2+ for Mg, 1+ for Li and 2+ for Zn. M_a is the average atomic mass (g mol⁻¹) of the sample. n and M_a were calculated by eqn (2) and (3), respectively, where x_i is the mole fraction; z_i is the oxidation state; and M_i is the atomic mass of component i.

3. Results and discussion

3.1 Potentiodynamic polarization measurements

Fig. 1 presents the potentiodynamic polarization curves of Mg–8Li–0.5Zn electrode measured in 0.7 mol L⁻¹ NaCl solution with different concentrations of the electrolyte additive NaF (0.0 mmol L⁻¹, 0.2 mmol L⁻¹, 0.5 mmol L⁻¹, 0.8 mmol L⁻¹, 1.0 mmol L⁻¹, 1.2 mmol L⁻¹ and 1.5 mmol L⁻¹).

Fig. 1 The potentiodynamic polarization curves for Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution containing different concentrations of NaF, (a) 0.0 mmol L⁻¹; (b) 0.2 mmol L⁻¹; (c) 0.5 mmol L⁻¹; (d) 0.8 mmol L⁻¹; (e) 1.0 mmol L⁻¹; (f) 1.2 mmol L⁻¹; (g) 1.5 mmol L⁻¹.

It can be seen that all the polarization curves have similar shapes and exhibit Tafel behaviors in both anodic and cathodic branches. The corrosion potentials of Mg–8Li–0.5Zn electrode in the solution with different concentrations of NaF are slightly more negative than those of Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution (without NaF), demonstrating that the presence of NaF as an electrolyte additive can obviously enhance the electrochemical activity of Mg–8Li–0.5Zn electrode. However, the change of the corrosion potential is not a simple linear relationship in accordance with the increase in the concentration of the electrolyte additive. It may be that certain interaction effects occur among NaF, the oxidation products and Mg–8Li–0.5Zn electrode, and thus the different concentrations of the NaF may show different effects on the interaction relationship. Furthermore, it can be noted that the anodic current of Mg–8Li–0.5Zn electrode in the solution containing 1.0 mmol L⁻¹ NaF in the region close to the corrosion potential is the lowest than that of Mg–8Li–0.5Zn electrode in the solution containing other concentrations of NaF (0.0, 0.2, 0.5, 0.8, 1.2 and 1.5 mmol L⁻¹). This implies that Mg–8Li–0.5Zn electrode in the solution with 1.0 mmol L⁻¹ NaF is considerably more corrosion resistant than the Mg–8Li–0.5Zn electrode in the solution containing other concentrations of NaF. This demonstrates that 1.0 mmol L⁻¹ NaF shows the best corrosion resistant ability on the electrode among the NaF concentrations that were investigated.⁴

3.2 Potentiostatic oxidation measurements

Fig. 2A–D show the potentiostatic oxidation curves of Mg–8Li–0.5Zn electrode measured at the constant potentials of −1.4, −1.2, −1.0 and −0.8 V in 0.7 mol L⁻¹ NaCl solution containing different concentrations of NaF (0.0, 0.2, 0.5, 0.8, 1.0, 1.2 and 1.5 mmol L⁻¹), which was explored to evaluate the discharging performances of Mg–8Li–0.5Zn electrode. The testing time is 1000 s under each potential. Overall, the current–time profiles are similar for the four tested potentials, and the discharging currents are increased when the polarization potentials are increased from −1.4 to −0.8 V. Moreover, the anodic oxidation current densities rapidly increase in the initial discharging stage due to the double layer charging. They then decrease to an approximately constant value, which can be attributed to the formation and accumulation of oxidation products that prevent the Mg–8Li–0.5Zn electrode surface from coming into contact with the electrolyte in the subsequent discharge period, thus leading to a decrease in the active electrode surface area on the alloy surfaces. The oxidation current density of Mg–8Li–0.5Zn electrode is enhanced after an appropriate concentration of the electrolyte additive is added to 0.7 mol L⁻¹ NaCl solution. The discharging current density of the Mg–8Li–0.5Zn electrode was the highest in 0.7 mol L⁻¹ NaCl solution containing 1.0 mmol L⁻¹ NaF at the discharging potentials of −1.4, −1.2, −1.0 and −0.8 V. It is interesting to note that the larger vibration of the discharging currents occurs at a lower anodic polarization potentials, such as −1.4 V and −1.2 V, indicating that the formation and dissolution of the surface film occurs sharply.²²

Fig. 2 Current–time curves measured at (A) −1.4 V, (B) −1.2 V, (C) −1.0 V and (D) −0.8 V for the Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution containing different concentrations of NaF, (a) 0.0 mmol L⁻¹; (b) 0.2 mmol L⁻¹; (c) 0.5 mmol L⁻¹; (d) 0.8 mmol L⁻¹; (e) 1.0 mmol L⁻¹; (f) 1.2 mmol L⁻¹; (g) 1.5 mmol L⁻¹.

As illustrated in Fig. 2A, the oxidation current density of the electrode in 0.7 mol L⁻¹ NaCl solution with 1.0 mmol L⁻¹ NaF (9.12 mA cm⁻²) is 19.2% higher than that in 0.7 mol L⁻¹ NaCl solution without NaF (7.65 mA cm⁻²) when the discharging potential is −1.4 V. At the discharging potential of −1.2 V (Fig. 2B), the discharging current densities of the electrode in the solution with NaF are all boosted compared with those in the solution without NaF. The discharging performances of Mg–8Li–0.5Zn electrode are improved best at the NaF concentration of 1.0 mmol L⁻¹. The oxidation current density of the Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution with 1.0 mmol L⁻¹ NaF (16.12 mA cm⁻²) is 14.40% higher than that in 0.7 mol L⁻¹ NaCl solution without NaF (14.09 mA cm⁻²). At the discharging potential of −1.0 V (Fig. 2C), the oxidation current density of the Mg–8Li–0.5Zn electrode in the solution with 1.0 mmol L⁻¹ NaF is 22.53 mA cm⁻², which is 6.22% higher than that in 0.7 mol L⁻¹ NaCl solution without NaF (21.21 mA cm⁻²). Fig. 2D illustrates that at the discharging potential of −0.8 V, the oxidation current density of Mg–8Li–0.5Zn electrode is increased to 29.75 mA cm⁻² when the NaF concentration is 1.0 mmol L⁻¹, which is 8.10% higher than that in 0.7 mol L⁻¹ NaCl solution (27.52 mA cm⁻²). At a constant potential of −0.8 V, the discharge current density of Mg–8Li–0.5Zn electrode decreases in the following order: 1.0 mmol L⁻¹ > 0.0 mmol L⁻¹ ≈ 0.2 mmol L⁻¹ ≈ 0.5 mmol L⁻¹ ≈ 0.8 mmol L⁻¹ ≈ 1.2 mmol L⁻¹ > 1.5 mmol L⁻¹. The potentiostatic current–time measurements lead to the conclusion that the optimal concentration of the electrolyte additive is 1.0 mmol L⁻¹, which best promotes the electrochemical activity of Mg–8Li–0.5Zn electrode.

3.3 Electrochemical impedance spectroscopy

Fig. 3 displays the Nyquist plots of Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution with 0.0 mmol L⁻¹ and 1.0 mmol L⁻¹ NaF. As seen in Fig. 3, the Nyquist plots of Mg–8Li–0.5Zn electrode display a high frequency capacitive loop.⁷ According to the impedance studies on conventional magnesium alloys, such as AZ91 and AZ61,^23–26 the high frequency capacitive loop observed in the impedance spectra of Mg–8Li–0.5Zn electrode in the electrolyte solution with electrolyte additive of NaF (0.0 mmol L⁻¹ and 1.0 mmol L⁻¹) may result from charge transfer.

Fig. 3 The impedance spectra of the Mg–8Li–0.5Zn electrode recorded in 0.7 mol L⁻¹ NaCl solution (a) without NaF and (b) with 1.0 mmol L⁻¹ NaF after discharging at −1.0 V for 1000 s.

The resistance of the electron transfer for the Mg–8Li–0.5Zn electrode in the solution with 1.0 mmol L⁻¹ NaF is ca. 165 Ω cm⁻², which is smaller than that for the Mg–8Li–0.5Zn electrode in the electrolyte solution without NaF (ca. 235 Ω cm⁻²), suggesting that Mg–8Li–0.5Zn electrode in the solution containing 1.0 mmol L⁻¹ NaF is more electrochemically active than that in 0.7 mol L⁻¹ NaCl solution. It may be that the presence of 1.0 mmol L⁻¹ NaF in the electrolyte solution can accelerate the electron transfer process or decrease the activation energy of the reaction of Mg–8Li–0.5Zn electrode. This result is consistent with the above-mentioned results of the current–time measurements (Fig. 2).

3.4 Surface morphologies of the electrode after discharging

The surface morphologies of the Mg–8Li–0.5Zn electrode after discharging at −1.0 V for 20 minutes in the electrolyte solution without NaF and with 1.0 mmol L⁻¹ NaF are obtained by SEM investigation. Fig. 4 shows the SEM images. By comparing Fig. 4a and b, it can be found that the addition of NaF into the electrolyte solution markedly changes the morphology of the oxidized surfaces of Mg–8Li–0.5Zn electrode. Fig. 4a indicates that the oxidized products attached to the Mg–8Li–0.5Zn alloy surface after discharging in 0.7 mol L⁻¹ NaCl solution are relatively finer. On the contrary, the oxidation products on the Mg–8Li–0.5Zn electrode (after discharging in the electrolyte solution containing 1.0 mmol L⁻¹ NaF at −1.0 V for 20 minutes) are presented as relatively larger lumps. In particular, the larger channels are observed in Fig. 4b, which allows the electrolyte to penetrate more easily, leading to significant improvement in the discharging current density of the Mg–8Li–0.5Zn electrode.²² These results are in good agreement with the results obtained from the potentiostatic current–time curves (Fig. 2) and the electrochemical impedance spectroscopy (Fig. 3).

Fig. 4 SEM micrographs of the Mg–8Li–0.5Zn electrode after the samples are discharged at −1.0 V for 20 minutes in (a) 0.7 mol L⁻¹ NaCl solution and (b) 0.7 mol L⁻¹ NaCl solution containing 1.0 mmol L⁻¹ NaF.

3.5 EDS analysis of electrode with and without NaF addition after discharge

Near-surface elemental analyses of Mg–8Li–0.5Zn electrodes after discharging with and without sodium fluoride are carried out by EDS analysis. It can be seen from Fig. 5 that the oxygen content decreases after the addition of sodium fluoride in the electrolyte solution, indicating that the arrangement of the oxidation product on alloy surface becomes more loose and it easily falls off, resulting in the decrease of the elemental oxygen on the electrode surface after the electrolyte additive is added to the solution. This allows the electrolyte solution to come more in contact with the alloy, thus increasing the effective reaction area. Moreover, the content of magnesium is also decreased, indicating that the utilization of magnesium is improved; more magnesium is converted to magnesium ions, which is beneficial for the anodic reaction and can improve the discharge activity. Thus, it may be inferred that the resulting battery has a higher current efficiency.

Fig. 5 The EDS analysis for Mg–8Li–0.5Zn electrode after discharge in 0.7 mol L⁻¹ NaCl solution containing different concentrations of NaF: (a) 0.0 mmol L⁻¹; (b) 1.0 mmol L⁻¹.

3.6 Utilization efficiencies

The continuous discharging utilization efficiency and interval discharging utilization efficiency of the Mg–8Li–0.5Zn electrode discharged for 30 minutes in the electrolyte solution without NaF and with 1.0 mmol L⁻¹ NaF were measured using a weight loss method. The utilization efficiency is defined as the ratio of the mass loss responsible for the generation of the discharging current to the total mass loss within the discharging period. The continuous discharging utilization efficiency and interval discharging utilization efficiency are presented in Tables 2 and 3, respectively.

Table 2 Continuous discharging utilization efficiency of the Mg–8Li–0.5Zn electrode

NaF/mmol L^−1	m0/g	mf/g	Q/C	Utilization efficiency η (%)
0.0	4.0304	3.9757	133.8197	28.8680
1.0	2.1278	2.1003	128.6632	47.6850

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Table 3 Interval discharging utilization efficiency of the Mg–8Li–0.5Zn electrode

NaF/mmol L^−1	m0/g	mf/g	Q/C	Utilization efficiency η (%)
0.0	3.9419	3.8568	130.3372	18.0730
1.0	2.5953	2.1526	120.7403	46.0070

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As shown in Tables 2 and 3, the continuous discharging utilization efficiency and interval discharging utilization efficiency of the Mg–8Li–0.5Zn electrode (the electrolyte solution containing 1.0 mmol L⁻¹ NaF) are around 18.82% and 27.93%, higher than that of the Mg–8Li–0.5Zn electrode (the electrolyte solution without NaF). These results indicate that the utilization efficiency of the Mg–8Li–0.5Zn electrode can be enhanced by adding 1.0 mmol L⁻¹ NaF to the electrolyte solution. The reason for this enhancement is that 1.0 mmol L⁻¹ NaF prevents the formation of passive coatings on the surface or destroys the dense structure of the surface films to facilitate their peeling off from the alloy surfaces. Either way, it leads to an enhancement of the discharging performance; in addition, the presence of NaF in the electrolyte solution may lead to a decrease of the self-discharging rate and an increase of hydrogen evolution overpotential. Moreover, comparing Table 2 with Table 3, it can be concluded that the difference value between the continuous discharging utilization efficiency and interval discharging utilization efficiency of the electrode is decreased after 1.0 mmol L⁻¹ NaF is added to the 0.7 mol L⁻¹ NaCl solution. The reason for this phenomenon is that the rate for the self-corrosion reactions occurring on the Mg–8Li–0.5Zn electrode are reduced or decreased by the presence of 1.0 mmol L⁻¹ NaF in the electrolyte solution under open circuit potential during interval discharging period.

To verify whether there is a “negative difference effect” on the Mg–8Li–0.5Zn electrode containing different concentrations of NaF (0.0, 0.2, 0.5, 0.8, 1.0, 1.2 and 1.5 mmol L⁻¹) in the electrolyte solution, the utilization efficiencies were successively obtained at the discharging potentials of −1.4, −1.2, −1.0 and −0.8 V for 1 h; the results are given in Table 4.

Table 4 Utilization efficiency of the Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution containing different concentrations of NaF: (a) 0.0 mmol L⁻¹; (b) 0.2 mmol L⁻¹; (c) 0.5 mmol L⁻¹; (d) 0.8 mmol L⁻¹; (e) 1.0 mmol L⁻¹; (f) 1.2 mmol L⁻¹; (g) 1.5 mmol L⁻¹ at various potentials

NaF/mmol L^−1	Utilization efficiency η (%)
	−0.8 V	−1.0 V	−1.2 V	−1.4 V
0.0	32.9173	27.8812	15.7853	14.2725
0.2	36.3321	33.1165	25.3635	18.5050
0.5	41.5583	36.9734	33.1287	23.1817
0.8	47.9915	40.8878	37.5862	26.3218
1.0	53.7654	47.9032	43.8713	30.7830
1.2	50.1187	46.5511	39.8897	24.1143
1.5	46.8812	43.1715	37.5813	22.7865

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It can be found that the utilization efficiencies of Mg–8Li–0.5Zn electrode in the electrolyte solution with the different concentrations of NaF (0.0, 0.2, 0.5, 0.8, 1.0, 1.2 and 1.5 mmol L⁻¹) increase with the increment of the potential (from −1.4 V to −0.8 V), indicating that the “negative difference effect” does not exist on the Mg–8Li–0.5Zn electrode in the presence of different concentrations of NaF.²⁷ Moreover, the utilization efficiency at the same discharging potential first increased with the increase of the concentration of the electrolyte additive, and then decreased with the increase of the concentration of the electrolyte additive. The optimum concentration of the additive was found to be 1.0 mmol L⁻¹, which is consistent with the other experimental results.

4. Conclusions

Casting ingots of Mg–8Li–0.5Zn alloy were prepared by the induction melting method and its electrochemical performances in 0.7 mol L⁻¹ NaCl solution containing different concentrations of NaF (0.0, 0.2, 0.5, 0.8, 1.0, 1.2 and 1.5 mmol L⁻¹) were studied. The main results are summarised as follows:

1. The Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution with 1.0 mmol L⁻¹ NaF has higher electrochemical activity than that of the Mg–8Li–0.5Zn electrode in 0.7 mol L⁻¹ NaCl solution with other concentrations of NaF (0.0, 0.2, 0.5, 0.8, 1.2 and 1.5 mmol L⁻¹).

2. The addition of 1.0 mmol L⁻¹ NaF to the NaCl electrolyte solution can markedly alter the morphology of the oxidation products of the Mg–8Li–0.5Zn electrode to be deeper and larger channels, which facilitates the electrolyte penetration, and then leads to the high discharge activity of Mg–8Li–0.5Zn electrode.

3. The continuous discharging utilization efficiency and interval discharging utilization efficiency of the Mg–8Li–0.5Zn electrode are increased by around 18.82% and 27.93% in 0.7 mol L⁻¹ NaCl solution with 1.0 mmol L⁻¹ NaF, compared with that in 0.7 mol L⁻¹ NaCl solution without NaF, respectively.

4. Mg–8Li–0.5Zn electrode has no “negative difference effect” in 0.7 mol L⁻¹ NaCl solution and in 0.7 mol L⁻¹ NaCl solution with 1.0 mmol L⁻¹ NaF.

Acknowledgements

We gratefully acknowledge the Natural Science Foundation of Heilongjiang Province of China (B201201), China Postdoctoral Science Foundation (Grant no. 2014M561357), the National Natural Science Foundation of China (21203040, 21301038, 51108111), and the Fundamental Research Funds for the Central Universities (HEUCF201403018).

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