In-situ characterization of MnO2-based zinc-ion batteries: understanding the role of by-products in impedance increase

In this study, we investigated the electrochemical behavior of MnO2 cathodes in zinc-ion batteries (ZIBs) using in-situ electrochemical impedance spectroscopy (EIS) analysis during cycling. Our results show that the impedance of the MnO2 cathode increases irreversibly during cycling, which is attributed to the generation of by-products at the cathode surface.


Introduction
The development of science and technology affects people's lives, and some new concepts such as artificial intelligence and digital currency have entered people's field of vision [1][2][3].The realization of advanced technology depends on the development of energy [4].The development of rechargeable batteries has revolutionized the way we store and use energy [5][6][7].With the increasing demand for portable electronics, electric vehicles, and renewable energy sources, there is a growing need for highperformance, cost-effective, and eco-friendly energy storage systems [8][9][10].One promising solution is the zinc-ion batteries (ZIBs), which has attracted considerable attention due to its high theoretical energy density, low cost, and abundant raw materials [11].
The key components of ZIBs are the cathode, anode, and electrolyte [12].Among these, the cathode plays a critical role in determining the performance and durability of the battery.MnO2 has been identified as a promising cathode material for ZIBs due to its high theoretical capacity (308 mAh/g), good cycling stability, and low cost.However, the performance of MnO2 cathodes is still far from optimal, and further research is needed to understand the underlying mechanisms and improve their electrochemical properties [13].
One useful tool for studying the electrochemical behavior of MnO2 cathodes is electrochemical impedance spectroscopy (EIS).EIS is a non-destructive technique that measures the impedance of a system over a range of frequencies.By analyzing the impedance spectra, researchers can gain insights into the electrochemical processes occurring at the electrode-electrolyte interface, such as charge transfer, ion diffusion, and surface reactions [14].
In this study, we used EIS to investigate the electrochemical behavior of MnO2 cathodes during cycling in a ZIBs.Our aim was to understand the changes in the impedance spectra and identify the factors contributing to the irreversible impedance increase observed during cycling.We hypothesized that the generation of by-products at the cathode surface would lead to the formation of a passivating by-products and reduce the effective surface area available for electrochemical reactions, resulting in an increase in the charge transfer resistance and overall impedance of the system.
Our results showed that the impedance of the MnO2 cathode increased irreversibly during cycling, consistent with previous reports.The impedance spectra were found to be complex, with multiple time constants and phase angles indicating the presence of various electrochemical processes.The analysis of the impedance spectra using equivalent circuit models revealed that the charge transfer resistance (Rct) increased significantly during cycling, suggesting that the formation of a passivating by-product at the cathode surface was the main cause of the impedance increase.
These findings have important implications for the design and optimization of MnO2 cathodes for ZIBs.The irreversible increase in impedance observed during cycling could limit the capacity and cycling stability of the battery, as well as its overall energy efficiency.The identification of the byproducts generated during cycling provides insights into the underlying electrochemical reactions and highlights the importance of electrolyte composition and operating conditions in mitigating the formation of passivating by-products at the cathode surface.
Previous studies have proposed various strategies to improve the electrochemical performance of MnO2 cathodes in ZIBs, such as surface modification, doping, and hybridization with other materials [15][16][17].Our results suggest that in addition to these approaches, it may be necessary to optimize the electrolyte composition and cycling conditions to minimize the formation of by-products and reduce the irreversible impedance increase.Further research is needed to explore the interactions between the cathode, electrolyte, and by-products during cycling and develop effective strategies to enhance the performance and durability of MnO2 cathodes in ZIBs.
This study demonstrates the use of EIS as a powerful tool for investigating the electrochemical behavior of MnO2 cathodes in ZIBs.Our findings provide new insights into the factors contributing to the irreversible impedance increase observed during cycling and highlight the importance of understanding the complex interplay between the cathode, electrolyte, and by-products in battery systems.These results have important implications for the development of high-performance and reliable ZIBs for a wide range of energy storage applications.

Results and Discussion
Figure 1a shows the scanning electron microscopy (SEM) image of the MnO2 cathode before cycling, revealing a uniform surface morphology with a flake-shaped particle morphology.The particles appear to have a nano-scale size, which is beneficial for the electrochemical performance of the cathode.The small size and flake shape of the particles provide a large surface area for electrochemical reactions to occur, allowing for efficient transport of ions and electrons.However, Figure 1b shows the SEM image of the MnO2 cathde after cycling, indicating the formation of a large number of by-products, zinc hydroxide sulfate (ZHS), on the cathode surface.After the cathode is taken out and dried, the electrolyte evaporates and leaves cracks on the surface of the material.The formation of these by-products is likely due to the reaction of the active material with the electrolyte during cycling, which can lead to the formation of passivating by-product that hinder the electrochemical reaction.The presence of these byproducts on the cathode surface is expected to increase the resistance of the cathode, leading to a decrease in battery performance.Thus, optimizing the electrolyte composition and operating conditions to prevent the formation of these by-products is crucial for improving the performance and stability of MnO2-based ZIBs.2a and b show that impedance of the second full charge was greater than that of the first full charge, indicating poor reversibility of the ZHS reaction.As shown in Figure 2a, in the case of full charge, the EIS spectrum of shows a typical charge transfer process without showing a diffuse area.Poor reversibility of the ZHS reaction suggests that the formation of passivating layers or irreversible reactions may occur during cycling, which can greatly affect the electrochemical reactivity and energy efficiency of the battery.These irreversible reactions or passivating layers can lead to a decrease in the number of active sites available for the electrochemical reactions, resulting in a reduction in battery capacity and efficiency.Thus, it is important to investigate the factors leading to poor reversibility of the ZHS reaction and optimize the battery design and operating conditions to mitigate the formation of passivating layers or irreversible reactions.Future research can focus on improving the cathode and electrolyte materials to enhance the electrochemical performance and stability of ZIBs.

Conclusion
In conclusion, our study demonstrated the importance of in-situ EIS analysis for understanding the underlying electrochemical reactions and performance of MnO2-based ZIBs during cycling.The results showed that the poor reversibility of the ZHS reaction could greatly affect the electrochemical reactivity and energy efficiency of the battery.The impedance of the byproducts to different charge and discharge processes during the battery cycle was tested in situ, which is of reference significance for further research on the mechanism of ZHS and the improvement of the reversibility of by-products.Future research can focus on optimizing the battery design and operating conditions to mitigate the formation of passivating layers or irreversible reactions and improve the electrochemical performance and stability of ZIBs for energy storage applications.

Figure 1 .
Figure 1.SEM images of MnO2 cathode before (a) and after (b) cycling.The in-situ EIS analysis of MnO2-based ZIBs during cycling provides insights into the underlying electrochemical reactions and battery performance.Figures2a and bshow that impedance of the second full charge was greater than that of the first full charge, indicating poor reversibility of the ZHS reaction.As shown in Figure2a, in the case of full charge, the EIS spectrum of shows a typical charge transfer process without showing a diffuse area.Poor reversibility of the ZHS reaction suggests that the formation of passivating layers or irreversible reactions may occur during cycling, which can greatly affect the electrochemical reactivity and energy efficiency of the battery.These irreversible reactions or passivating layers can lead to a decrease in the number of active sites available for the electrochemical

Figure 2 .
Figure 2. In-situ EIS of MnO2-based ZIBs of first (a) and second (b) cycles.