Academic Journal of Science and Technology ISSN: 2771-3032 | Vol. 5, No. 2, 2023 244 High Performance Electrolyte for Iron‐Ion batteries Gongchuan You1, Liang He1, 2, * 1 School of Mechanical Engineering, Sichuan University, Chengdu 610065, China 2 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China * Corresponding author: Liang He (Email: hel20@scu.edu.cn) Abstract: Aqueous rechargeable batteries have received widespread attention due to their excellent power density, simple manufacturing process, and inexpensive electrolyte. Iron-ion batteries are expected to meet the goals of high safety, low cost, and non-toxicity pursued in the field of rechargeable batteries. However, passivation, parasitic hydrogen evolution reaction (HER), and low electroplating efficiency (50%-70%) limit the improvement of electrochemical performance, which greatly restricts their practical application. In this study, a high-performance electrolyte for iron-ion batteries was prepared, and the effect of zinc chloride (ZnCl2) additives on inhibiting HER and the improvement of coulomb efficiency in ferrous chloride (FeCl2) electrolyte was explored. Additionally, the effect of the addition of complexing agents in the electrolyte on the coulomb efficiency of the electrodes was studied. It’s demonstrated that the electrode can still obtain a coulomb efficiency of nearly 100% after 20 hours cycling in the electrolyte containing ZnCl2 additive and FeCl2, while in FeCl2 electrolyte, its coulomb efficiency after 20 hours of cycling is only 65%. Keywords: Coulomb efficiency, Aqueous batteries, Electrolyte, HER. 1. Introduction With the increasing demand for renewable energy and clean energy, energy conversion technologies based on solar, wind, and tidal energy have developed rapidly. However, most renewable energy sources are intermittent. Therefore, it is necessary to develop efficient and low-cost energy storage systems. Rechargeable batteries are one of the most potential energy storage devices. In addition, with the large-scale popularization of new energy vehicles and portable electronic products, it is particularly urgent to develop high- performance and low-cost rechargeable batteries. So far, the field of rechargeable batteries has been committed to using lithium, sodium, and potassium ions as charging carriers, as well as non-aqueous electrolyte batteries. However, lithium resources are relatively scarce and expensive, while sodium- ion batteries and potassium-ion batteries require a large amount of battery additives to achieve the expected energy density. On the other hand, the organic electrolytes used in these alkali metal ion batteries are expensive and toxic, which has a negative impact on the environment. Among the alternatives to lithium-ion batteries, zinc-ion batteries (with a theoretical mass specific capacity of 820 mAh g-1 and a theoretical volume specific capacity of 5851 mAh cm-3) [1] are undoubtedly the focus of attention. However, the content of zinc in the earth's crust is not high, resulting in high costs for zinc-based batteries. In addition, zinc-ion batteries also suffer serious zinc dendrites during the cycling. Therefore, the research on aqueous batteries with other metal anodes has received increasing attention. Among the few candidates, iron-ion batteries present great potential. As the second metal in the earth's crust, iron has a price of about 1/40 of zinc, and its theoretical energy density (theoretical mass specific capacity of 960 mAh g-1, theoretical volume specific capacity of 7557 mAh cm-3) [1] is significantly higher than the theoretical energy density of zinc. In addition, there is no obvious preferred crystal orientation in the growth of iron, which means that there is no serious dendritic growth in the iron anode. However, at present, there are few relevant reports and research work in this field, and there are many problems to be solved. In a few reported research achievements, Fe-I [2], Fe-Prussian blue [1], Fe-S [3] batteries, and Fe-LiFePO4 [1] dual ion batteries have been developed. Fe-S batteries have achieved an energy density of 1050 mAh g-1 and a coulomb efficiency of 90.7%. Moreover, due to the insolubility of iron sulfides, they can also inhibit the shuttle effect of polysulfides. Due to the low price of iron salts, the cost of this type of battery (less than $20 KWH-1) [4] is far lower than that of other batteries. In this paper, the modification of its electrolyte was studied to improve the electrochemical performance of iron-ion batteries, especially for the improvement of their coulomb efficiency. ZnCl2 is used as an additive in the electrolyte of iron-ion batteries. During the working process of the battery, Fe and Zn will co-deposit, forming a Fe-Zn alloy layer on the electrode surface. This alloy layer can effectively inhibit the occurrence of HER, which improves the coulombic efficiency. 2. Materials and Methods 2.1. Preparation of mixed electrolytes with different ZnCl2 concentrations Firstly, introduce 1 M FeCl2 powder and x ml ZnCl2 (x=5,10,15,20,25) saturated solution into a beaker, and then add deionized water. After sufficient stirring and constant volume, 30 ml electrolyte was obtained. 2.2. Preparation of mixed electrolytes with different FeCl2 concentrations Firstly, introduce x M FeCl2 (x=0.1,0.2,0.4,0.5,1,2,3) powder and x ml ZnCl2 (x=5,10,15,20,25) saturated solution into a beaker, and then add deionized water. After sufficient stirring and constant volume, 30 ml electrolyte was obtained. 245 2.3. Preparation of mixed electrolytes with different trisodium citrate concentrations Firstly, introduce 0.5 M FeCl2 powder, 5 ml ZnCl2 saturated solution and x M trisodium citrate (C6H5O7Na3) (x=0.005,0.01,0.02,0.05) into a beaker, and then add deionized water. After sufficient stirring and constant volume, 30 ml electrolyte was obtained. 2.4. Preparation of electrolytes with different ZnCl2 concentrations Introduce x ml ZnCl2 saturated solution into a beaker and then add deionized water. After sufficient stirring and constant volume, 30 ml electrolyte was obtained. 2.5. Electrochemical measurements The electrochemical performance of the electrode in the prepared electrolyte was evaluated by the CV curve tested at CHI760E and the cycling test at CT3001A 1U with three electrode system. In detail, Pt is served as the counter and working electrode, and Ag/AgCl is applied as the reference electrode in three electrode system. 3. Results and Discussion As shown in Figure 1a, it could be found that after adding 25 ml of ZnCl2 saturated solution, the coulomb efficiency of the electrode remained 98.5% after 45 cycles, while in an electrolyte without zinc chloride addition, the coulomb efficiency of the electrode was only 65% after 45 cycles. Furthermore, the coulomb efficiency of the electrode decreased to 10% during 30 cycles in 25 ml ZnCl2 saturated solution. Notably, the function of C6H5O7Na3 was to stabilize the electrolyte as a complexing agent to prevent the oxidation of Fe2+, but the addition of this substance will reduce electrode coulomb efficiency (Figure 1b). Figure 1. (a) Effect of mixed electrolyte on coulomb efficiency; (b) Effect of C6H5O7Na3 on coulomb efficiency of mixed electrolyte It was speculated that C6H5O7Na3 will participate in the electrode reaction, which attracted some redox electrons. In addition, this side reaction will become more serious as the concentration of C6H5O7Na3 increases. As shown in Figure 2, as the concentration of C6H5O7Na3 increases, the coulomb efficiency of the electrode gradually decreases, and this downward trend is increasingly evident. Figure 2. Effect of C6H5O7Na3 concentration on coulomb efficiency In order to deeply explore the effect of zinc chloride additives on the performance of electrolytes, electrolytes with different amounts of ZnCl2 were prepared. As shown in Figure 3a, with the content of zinc chloride increasing, the 246 coulomb efficiency of the electrode became higher and higher. It could be speculated that “the salt in water structure” formed by high concentration of ZnCl2, reduced the free water molecules in the electrolyte, thereby inhibiting HER. Considering the performance and cost of the electrolyte, 5ml saturated ZnCl2 solution was selected as the optimal addition amount [5-6]. In Figure 3b, the effect of Fe2+ concentration on the performance of the electrolyte at the optimal amount of ZnCl2 was studied. It can be observed that when the concentration of Fe2+ is between 0.1 M and 1 M, the coulomb efficiency of the electrode will increase with the increase of concentration, while its coulomb efficiency decreased significantly between 2 M and 3 M. Among them, 1 M of Fe2+ had the best coulomb efficiency performance. Figure 3. (a) Effect of ZnCl2 concentration on coulomb efficiency; (b) Effect of FeCl2 concentration on coulomb efficiency Eventually, to explore the working mechanism of electrodes in mixed electrolyte, CV tests were carried out at different Fe2+ concentrations (Figure 4). In Figure 4a, it could be clearly observed that as the concentration of Fe2+ increases, the oxidation peak of the electrode shifts towards a higher potential. This could be due to the increasing concentration of Fe2+ in the electrolyte, that resulted in a decrease of Fe-Zn alloy deposition resistance, thereby reducing its deposition potential. Even when the concentration of Fe2+ exceeded 3 M, this trend was most obvious. Combined with Figure 4b, it could be found that when the concentration of Fe2+ was between 0.1 M and 2 M, the concentration of Fe2+ in the electrolyte became lower than the concentration of Zn2+, making the deposition behavior of the electrode closer to the Zn2+ deposition behavior. But, while the concentration of Fe2+ was up to 3 M in the electrolyte, which was higher than that of Zn2+, the deposition behavior of the electrode got closer to that of Fe2+. In addition, it was worth noting that the higher the concentration of Fe2+, the greater the probability of oxidation reaction occurring. This could also explain that when the concentration of Fe2+ exceeded 2 M, the coulomb efficiency of the electrode decreased with the Fe2+ concentration increasing. Thus, considering cost and performance, a mixed electrolyte of 1 M FeCl2 and 5 ml saturated ZnCl2 was the best electrolyte for iron-ion batteries. Figure 4. (a) CV curves at different concentrations of FeCl2 in mixed electrolytes; (b) CV curves in ZnCl2 electrolytes with different concentrations 247 4. Conclusion In this paper, a high-performance mixed electrolyte for iron-ion batteries was prepared by adding ZnCl2 to the electrolyte of commonly used iron-ion batteries. Due to the co-deposition phenomenon of zinc and iron ions, this will change the deposition behavior of iron ions. The deposition potential barrier of co deposited Fe-Zn alloy is lower, which is more conducive to the deposition of metal ions. Moreover, the high concentration of ions in the electrolyte can inhibit the content of free water, thereby reducing HER. In summary, the high-performance mixed electrolyte for iron ion batteries prepared in this paper can effectively inhibit HER and improve the coulomb efficiency of the battery, providing a new strategy for the development of iron-ion batteries. Acknowledgment The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding: This work was supported by the Fundamental Research Funds for the Central Universities (No. 20822041F4045), and the Science and Technology Project of Yibin Sanjiang New Area (No. 2023SJXQSXZJ003). References [1] X. Wu, A. Markir, Y. Xu, et al. A Rechargeable Battery with an Iron Metal Anode [J]. Adv. Funct. Mater. 2019. 29(20). [2] C. Bai, H. Jin, Z. Gong, et al. A high-power aqueous rechargeable Fe-I2 battery [J]. Energy Storage Mater. 2020. 28247-254. [3] X. Wu, A. Markir, Y. Xu, et al. Rechargeable Iron–Sulfur Battery without Polysulfide Shuttling [J]. Adv. Energy Mater. 2019. 9(40). [4] Z. He, F. Xiong, S. Tan, et al. Iron metal anode for aqueous rechargeable batteries [J]. Materials Today Advances. 2021. 11. [5] C. Zhang, J. Holoubek, X. Wu, et al. 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