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Electrochemical behavior of Mg electrode in sodium salt electrolyte system

A suitable electrolyte is crucial to enhancing the electrochemical performance of magnesium (Mg) batteries. Here, the influence of Na2SiO3 on the electrochemical behavior of AZ31B Mg alloy in the Na2SO4-NaNO3 composite electrolyte was investigated. The results revealed that the activation potential of the AZ31B Mg alloy first represented a negative shift and then a positive shift with the increase in Na2SiO3. The most negative activation potential (−1.51 V) and the lowest polarization (−3.20 V) were found when 6 mM of Na2SiO3 was added; no discharge hysteresis was observed, and the polarization resistance value (R 1) was 3,806 Ω. After 24 h immersion in the composite electrolyte with Na2SiO3, more and wider cracks appeared on the alloy surface, where a thick, dense film was formed, showing excellent discharge performance and corrosion resistance.


Introduction
In view of the limited nature and high price of lithium resources, the research on electrochemical energy storage devices, such as non-lithium batteries (Zhang J. L. et al., 2021;Liu et al., 2021;Wan et al., 2022) and supercapacitors Wei et al., 2021) is increasing. Magnesium (Mg) holds a promising application in anode materials for the first-generation Mg battery owing to its abundance, small density (1.74 g cm −3 ), low cost, and excellent electrical conductivity (Bertasi et al., 2016;Yang et al., 2022). In addition, located on the diagonal of the periodic table, Mg and Li share many similar chemical properties. The standard electrode potential of metallic Mg is −2.36 V (vs. SHE), allowing the formation of large opencircuit voltage and working voltage (Deng et al., 2019;Maddegalla et al., 2021). Mg has an electrochemical equivalent of 0.454 g Ah −1 and a theoretical specific capacity up to 2,202 mAh g −1 , enabling it to be applied to long-time discharge (Kékedy-Nagy et al., 2021). However, shortcomings also remain unresolved, such as the lessened battery storage capacity after discharge, voltage hysteresis, severe inferior corrosion resistance of the electrode, and poor current efficiency, forming the main obstructors of the commercial availability of Mg battery (Shao et al., 2015;Horia et al., 2022;Zhang et al., 2022). In the Mg battery, "voltage lag" is the essence of the passivation of the Mg electrode in the electrolyte. Passivation film hinders the reaction during battery discharge, and its breakdown promotes a smooth reaction. Therefore, a Mg battery is required for the normal voltage output over a period of time. The "hysteresis" in the Mg battery is mainly relevant to the coverage degree of surface passivation film and the rate and relaxation time of film breakdown (Gong et al., 2022;Wei et al., 2022).
Recently, employing a suitable additive into electrolytes has been proved to be simple and effective in changing the surface membrane structure of the Mg electrode, thus reducing voltage lag . The presence of both 0.005 M EDTA-ZnNa 2 and 0.01 M C 6 H 11 NaO 7 has been demonstrated to remarkably inhibit corrosion and improve the performance of Mg battery for pure Mg in 3.5 wt% NaCl electrolyte (Qu et al., 2022). Zhao et al. (2016) investigated the discharge performance of an oxyanion corrosion inhibitor (Li 2 CrO 4 ) as an electrolyte additive in 3.5 wt% NaCl electrolyte for Mg-air battery, verifying enormously reduced corrosion current density of AZ31B Mg alloys in the presence of 0.1 wt% Li 2 CrO 4 , which is beneficial to the intermittent discharge performance of the Mg-air battery.
Sodium metasilicate (Na 2 SiO 3 ) is relatively cheap, environment-friendly, and non-toxic, often used as an anionic corrosion inhibitor (Kong et al., 2022). This thesis mainly explored the electrochemical performance and corrosion behavior of AZ31B Mg alloy in Na 2 SO 4 -NaNO 3 and Na 2 SiO 3 composite electrolyte to pick out the matching electrolyte, thus ensuring the activation of the electrode and inhibiting the hydrogen evolution-induced self-corrosion.

Experiment Chemicals and materials
In this study, Na 2 SO 4 , NaNO 3 , and Na 2 SiO 3 of analytical grade (≥99%) were utilized. The electrolyte was obtained by a mixture of 2 M Na 2 SO 4 and 2 M NaNO 3 (volume ratio = 1:9), and the Na 2 SiO 3 concentration ranged from 0.2 to 1.0 mM.
According to our previous work (Xu et al., 2017), the AZ31B Mg alloy (3.0 wt% Al, 1.0 wt% Zn, 0.2 wt% Mn, and 96.8 wt% Mg) was purchased from Wuxi Xinbiao Metal Material Co. Ltd. in China. The Mg alloy was sheared into an appropriate size of 1 × 1 cm with a thickness of 0.6 cm, then embedded with electric conductive copper wire, and encapsulated with epoxy resin in quick succession. Subsequently, these prepared samples were used as the working electrode for the electrochemical analysis.

Electrochemical characterization
The electrochemical characterization was performed on an electrochemical measurement system (CHI660E, China) using a standard three-electrode system comprising the graphite rod as the counter electrode, the saturated calomel electrode as the reference electrode, and the AZ31B Mg alloy as the working electrode.
The influence of Na 2 SiO 3 additive on the electrochemical behavior of AZ31B Mg alloy electrode in the Na 2 SO 4 -NaNO 3 composite electrolyte was investigated by the linear sweep voltammetry (LSV) at a sweep rate of 1 mV s −1 . The discharge curves were explored at a discharge current density of 3 mA cm −2 . Moreover, the electrochemical impedance spectroscopy (EIS) was conducted at open circuit potentials in the range of 10 -2~1 0 5 Hz with an amplitude of 5 mV.

Surface morphology analysis
The effect of the Na 2 SiO 3 additive on the morphology of the corrosive film layer on the Mg alloy electrode surface immersed into the Na 2 SO 4 -NaNO 3 composite electrolyte with or without Na 2 SiO 3 (0.6 mM) for 24 h was revealed by the scanning electron microscopy (SEM, HITACHI S 4800) at an operating voltage of 15 kV.

Results and discussion
The AZ31B Mg alloy electrodes were soaked in the Na 2 SO 4 -NaNO 3 composite electrolytes of different Na 2 SiO 3 concentrations (C Na2SiO3 ) for 24 h. Then, the LSV curves of the electrodes were measured at a sweep rate of 1 mV s −1 , and the results are shown in Figure 1. The inflection point on the curve is generally referred as the activation electrode potential (E act ). Generally, negative values of E act imply a low self-corrosion rate and a high discharge activity, under which the Mg alloy is strongly corrosion-resistant (Wang et al., 2014). Based on the curves, Na 2 SiO 3 addition led to negative shifts in E act , indicating the ability of Na 2 SiO 3 to improve the discharge behavior of the Mg alloy electrode in the composite electrolyte. With the increase in C Na2SiO3 , E act values shifted negative first and then positive. E act reached to lowest point (−1.51 V) when C Na2SiO3 was 6 mM, 1.1 V poorer than the original value without Na 2 SiO 3 additive. A plausible reason was that the Na 2 SiO 3 addition affected the ionization Frontiers in Chemistry frontiersin.org 02 equilibrium by accelerating the release of Mg 2+ ions from the Mg alloy, consequently improving the ionic conductivity and the mass transfer of Mg 2+ ions . The shift of E act values might also be closely related to the microstructural change of the Mg alloy in the composite electrolyte added with Na 2 SiO 3 , as verified by the SEM images (vide infra). Figure 2 illustrates the discharge curves of AZ31B Mg alloy electrodes in the composite electrolytes with varying C Na2SiO3 at a discharge current density of 3 mA cm −2 . In the absence of Na 2 SiO 3 , the discharge potential reached its maximum rapidly at the initial discharge stage and then slowly recovered to a stable state, which was ascribed to the activation process. This period lasted for 2.7 s and was attributed to the "hysteresis time." Surprisingly, the "hysteresis time" disappeared after the addition of Na 2 SiO 3 , and the discharge curves were rapidly stabilized. Na 2 SiO 3 , as a type of surfactant, loosened the passive film on the electrode surface, promoting detachment and hence eliminating the hysteresis time required for the current to penetrate the passive film and enhancing the discharge activity of Mg alloy electrodes.
Notably, the Na 2 SiO 3 addition reduced the discharge potential, which was −1.93 V in the composite electrolyte without Na 2 SiO 3 at 3 mA cm −2 . In particular, when C Na2SiO3 was 6 mM, the discharge voltage occurred at −3.21 V, corresponding to a negative shift of 1,380 mV. This facilitated the increase in the output voltage of a battery cell. However, the discharge potential shifted positively with C Na2SiO3 > 6 mM. It is speculated that at high Na 2 SiO 3 concentrations, the excess SiO 3 2− ions may react with Mg 2+ ions in the electrolyte to form deposits on the electrode surface (Ge et al., 2013), potentially thickening the passive film on the electrode surface and hindering the discharge of the electrode, thus shifting the discharge potential to the positive direction.
The EIS spectra of the AZ31B Mg alloy electrodes after 24 h immersion in the composite electrolytes of varying Na 2 SiO 3 concentrations are shown in Figure 3. EIS was conducted at an open-circuit potential of 10 -2~1 0 5 Hz, with a sinusoidal disturbance amplitude of 5 mV. The insert illustrated the equivalent circuit of the EIS system. In the equivalent circuit diagram, R s was induced by the solution resistance; 下标 corresponded to the high-frequency chargetransfer resistance arising from the alloy surface and corrosive film layer. CPE 1 denoted the double-layer capacitance induced by irregular oxide film on the alloy surface.
As shown in Figure 3, the EIS plots exhibited capacitive loops of similar shapes at low and high frequencies, and only some changes took place in the radii of the capacitive loops, indicating that the corrosion mechanism of the AZ31B Mg alloy was free from the influence of the addition amount of Na 2 SiO 3 . At a Na 2 SiO 3 concentration of 6 mM, the capacitive reactance diameter reached the maximum. According to the curves, R 1 was 957 Ω in the absence of Na 2 SiO 3 . With the Na 2 SiO 3 addition, the resistance value was enhanced significantly, indicating the prohibited corrosion of the AZ31B Mg alloy in the Na 2 SO 4 -NaNO 3 composite electrolyte. When C Na2SiO3 was 6 mM, the R 1 value reached the maximum of 3,886 Ω, increasing by more than four times and representing a stronger corrosion resistance.
The surface morphology images of AZ31B Mg alloy electrodes after 24 h immersion in the Na 2 SO 4 -NaNO 3 composite electrolyte are shown in Figure 4. Figures 4A,B show the SEM images of electrodes soaked in the absence of Na 2 SiO 3 . After 24 h of soaking, a thick, dense, and smooth film was formed on the Mg alloy electrode. Despite their large number, the cracks appearing on the surface were very narrow (generally <3 μm in width). As a result of hydrogen evolution from the electrode during the early soaking stages, a few small holes and pits were formed on the film. After 24 h of soaking in the presence of 6 mM Na 2 SiO 3 ( Figures 4C,D), the alloy surface exhibited more and wider cracks (the widest cracks exceeded 10 μm in width). A thick and dense film was formed, effectively protecting the Mg alloy electrode and enhancing its corrosion resistance.

FIGURE 2
Discharge curves of AZ31B Mg alloy electrode in the Na 2 SO 4 -NaNO 3 composite electrolyte with different C Na2SiO3 at a current density of 3 mA cm −2 .

FIGURE 3
Nyquist curves of EIS for AZ31B Mg alloy electrodes in the Na 2 SO 4 -NaNO 3 composite electrolyte with different concentrations of Na 2 SiO 3 .

Conclusion
The influence of Na 2 SiO 3 on the electrochemical and corrosion behavior of the AZ31B Mg alloy electrode in the Na 2 SO 4 -NaNO 3 composite electrolyte (the volume ratio of 2 M Na 2 SO 4 to 2 M NaNO 3 was 1:9) was investigated in this work. When C Na2SiO3 was 6 mM, E act values reached −1.51 V, 1.1 V lower than that without the Na 2 SiO 3 additive. In particular, the discharge voltage occurred at −3.21 V, shifting negative to 957 mV, and the discharge curves were rapidly stabilized. Moreover, the resistance value reached a maximum value of 3,886 Ω, increasing by more than four times. The composite electrolyte with 6 mM Na 2 SiO 3 was appropriate for Mg alloy, loosening the passive film on the electrode surface, facilitating the ionic conductivity, and eliminating the hysteresis time. This can realize not only the excellent discharge activity but also the high corrosion resistance of Mg alloy electrodes. Therefore, the present work offers a new electrolyte formulation to enhance the electrochemical behavior and lifespan of Mg batteries.

Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors without undue reservation.