Novel Insights into Energy Storage Mechanism of Aqueous Rechargeable Zn/MnO2 Batteries with Participation of Mn2+
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Pourbaix diagram of Mn–Zn–H2O system was used to analyze the charge–discharge processes of Zn/MnO2 batteries.
Electrochemical reactions with the participation of various ions inside Zn/MnO2 batteries were revealed.
A detailed explanation of phase evolution inside Zn/MnO2 batteries was provided.
KeywordsZinc-ion battery MnO2 cathode Energy storage mechanism Phase evolution
Lithium-ion batteries (LIBs) have been widely used in consumer electronics due to high energy density, portability, and some other merits [1, 2, 3, 4], whereas their security concerns and high cost restrict their large-scale applications in stationary grid storage and electric vehicles [5, 6]. Therefore, much attention has been paid to seek safe, eco-friendly, low-cost, and high-performance battery systems [6, 7]. Aqueous rechargeable zinc-ion batteries (ZIBs) are developed as a battery system, in which low-cost, non-toxic, and naturally abundant zinc metal is used as an anode and environment-friendly neutral aqueous Zn2+-containing solution serves as electrolyte . In recent years, a series of high-performance cathode materials for ZIBs have also been studied such as Prussian blue analog [9, 10, 11], vanadium oxides [12, 13, 14, 15, 16, 17, 18, 19], manganese oxides [20, 21, 22, 23, 24, 25, 26, 27, 28], and some metal sulfides [29, 30, 31]. Among these materials, MnO2 is particularly concerned for its high theoretical specific capacity, low cost, eco-friendliness, and diverse crystallographic polymorphs (e.g., α-MnO2, δ-MnO2, and γ-MnO2) [27, 28, 32].
Many efforts have been made to reveal the energy storage mechanisms of Zn/MnO2 ZIBs. Up to now, three types of energy storage mechanisms were proposed, including (i) Zn2+ insertion/extraction into/from MnO2 [8, 33, 34, 35, 36], (ii) conversion between MnO2 and MnOOH with the participation of H+ , and (iii) co-insertion of H+ and Zn2+ . Mechanisms (i) and (ii) explain the formation of ZnMn2O4 and MnOOH as discharging products on MnO2 cathode in ZIBs, respectively, while cannot explain that there are two redox processes during one charge/discharge cycle of ZIBs. Mechanism (iii) seems to be capable of explaining the coexistence of ZnMn2O4 and MnOOH as discharging products on MnO2 cathode, but deeper analysis will find that it is not accurate: The mechanism deems that potential of Zn2+ insertion is lower than that of H+ insertion (this means that MnOOH forms before ZnMn2O4 once the battery discharge process begins), being conflicted to the experimental result that MnOOH appears latter than ZnMn2O4. In short, the current mechanisms are unsatisfactory to explain genuine charge/discharge process in ZIBs, mainly because they were proposed based on a simplistic view that the insertion of Zn2+ and H+ and the phase change from MnO2 to ZnMn2O4 or MnOOH are highly reversible. Furthermore, to achieve satisfactory cyclic stability and rate performance of the Zn/MnO2 ZIBs, Mn2+ ions are always introduced in the electrolyte . However, electrochemical reactions inside the ZIBs become more complicated in such cases, thus corresponding energy storage mechanism has not been clearly revealed. Therefore, it is necessary to re-examine the thermodynamic and kinetic characteristics of Zn/MnO2 ZIBs to propose a reasonable Zn2+ storage mechanism.
In fact, for the active materials in aqueous ZIBs and some other rechargeable aqueous batteries, their structure and phase generally undergo complex changes during charge/discharge processes (e.g., the active materials can interact with not only metal ions, but also H+, OH−, and water molecules) . This is an important reason why the energy storage mechanism of MnO2 cathode in ZIBs is still inconclusive [40, 41, 42, 43]. Besides general experimental techniques such as cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) tests, Pourbaix diagram (E-pH diagram) has been widely used to study electrochemical reactions in aqueous solution [44, 45, 46, 47]. The electrochemical reductive products of active materials can be predicted according to the thermodynamics, which is beneficial for us to understand the charge/discharge process. Therefore, we combined experimental methods with the E-pH diagram of the Mn–Zn–H2O system together to comprehensively analyze the charge/discharge processes of MnO2 cathode in ZIBs and tried to reveal the authentic energy storage mechanism.
Herein, based on comprehensive analysis methods including electrochemical analysis and E-pH diagram, etc., we provide novel insights into the energy storage mechanism of Zn/MnO2 batteries with the co-participation of Zn2+, H+, Mn2+, SO42−, and OH−. During the first discharge process, co-insertion of Zn2+ and H+ promotes the transformation of MnO2 into ZnxMnO4, MnOOH, and Mn2O3, accompanying with increased electrolyte pH and the formation of ZnSO4·3Zn(OH)2·5H2O (noted as “BZSP”). During the subsequent charge process, ZnxMnO4, MnOOH, and Mn2O3 revert to α-MnO2 with the extraction of Zn2+ and H+, while BZSP reacts with Mn2+ to form ZnMn3O7·3H2O. In the following charge/discharge processes, besides aforementioned electrochemical reactions, Zn2+ reversibly inserts into/extract from α-MnO2, ZnxMnO4, and ZnMn3O7·3H2O hosts, and BZSP, Zn2Mn3O8, and ZnMn2O4 convert mutually with the participation of Mn2+. This work is believed to provide theoretical guidance for further research on high-performance ZIBs.
2.1 Material Synthesis
MnO2 cathode material was synthesized through a chemical co-precipitation method. One hundred and fifty milliliters of 0.1 M MnSO4 aqueous solution was dropped into 100 mL of KMnO4 (0.1 M) solution under magnetic stirring, followed by continuous stirring for 6 h at room temperature. The resulting precipitate was filtered, washed repeatedly with deionized water, and dried at 80 °C for 12 h. The obtained sample was thoroughly ground in an agate mortar and then annealed at 400 °C for 5 h in air atmosphere. Note that we applied the heat treatment to improve the crystalline of MnO2 because this makes it easier for us to use X-ray diffraction (XRD) and transmission electron microscopy (TEM) to detect the phase evolution of MnO2 cathode during charge/discharge processes.
2.2 Electrochemical Characterizations
The cathode was prepared by coating a mixture paste of 70 wt% of MnO2 powder, 20 wt% of acetylene black, and 10 wt% of LA133 binder on a stainless steel foil and dried overnight under vacuum conditions at 80 °C. In the prepared cathode, the mass loading of MnO2 is around 1 mg cm−2. Zn/MnO2 ZIBs were assembled based on MnO2 cathode, metallic Zn foil anode, air-laid paper separator, and zinc salt solution electrolyte (2 M ZnSO4 or 2 M ZnSO4 +0.5 M MnSO4 solution).
The assembled ZIBs were kept more than 4 h before electrochemical measurements. The CV and GCD tests were performed on a Bio-logic VMP3 multichannel electrochemical station and a Land CT2001 battery tester, respectively. CV tests of the prepared MnO2 cathode were also carried out in a three-electrode system, in which a platinum plate as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and 50 mL electrolyte was applied.
2.3 Material Characterizations
Microstructure and composition were characterized by XRD (Rigaku 2500) with Cu-Kα radiation operating at 40 kV and 100 mA within an angle range of 10° to 70° at a scan speed of 5° min−1. Micro-morphology was observed by field emission scanning electron microscopy (SEM, Zeiss Supra 55) and TEM (Tecnai G2 F30). Element content in electrodes and electrolytes was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES).
3 Results and Discussion
3.1 Characterization of MnO2
3.2 Electrochemical Analysis
There is a dip and a platform in the initial GCD curve (Fig. 2c) and the reaction type of R1 (at about 1.2 V) and R2 (at about 1.4 V) are further studied by the constant voltage discharge test (Fig. 2d). The current changes greatly when the battery is discharged at 1.2 V at which R1 will happen, and it keeps almost flat when discharged at 1.4 V at which R2 will occur. This indicates that a heterogeneous reaction occurs during R1 and a homogeneous reaction occurs during R2. Such a heterogeneous reaction between solid phases accompanying with nucleation process and electro-crystallization process will cause the formation of the dip and steep curve in the discharge curve in Fig. 2c . With the increasing CV cycles (Fig. 2b), the peak current intensity of R1 and O1 is getting weaker, while the peak current intensity of R2 and O2 becomes stronger. Therefore, the redox reactions R1/O1 and R2/O2 are more likely to be independent of each other. That the initial process differs from the subsequent one can also be seen from Fig. 2c (the red circle). R2 is weaker in the initial discharge process than that in the second one, which indicates that a new phase may generate as active materials.
3.3 Phase Evolution of Cathode in the Initial Discharge Process
ex situ XRD tests of the cathodes at different charge/discharge states support were performed. As shown in Fig. S3, when the cathode is initially discharged from 1.9 to 1.4 V, no new phase produces, while the cathode is further discharged to 1.0 V, several new diffraction peaks occur, indicating the appearance of new phases. In the charging process, some diffraction peaks cannot be detected, which means the disappearance of some phases. After 100 charges/discharge cycles, the XRD pattern is not in conformity with the XRD patterns of the cathode at the original state and fully charged state in the 1st charge process. These demonstrate that the cathode undergoes a complicated phase evolution. In the following, phase evolution of MnO2 cathodes during the 1st discharge process, the 1st charge process and subsequent discharge/charge processes were investigated in detail.
3.4 Phase Evolution of Cathode in the First Charge Process
3.5 Phase Evolution of Cathode in Subsequent Discharge/Charge Processes
3.6 Thermodynamic Analysis
Overall, we combined electrochemical analysis, phase identification with E-pH diagram of the Mn–Zn–H2O system together to analyze charge/discharge processes of aqueous rechargeable Zn//MnO2 batteries and revealed complicated phase evolution of the cathode (i.e., what new phases will form and how can they form in different charge/discharge stages). We obtained some different conclusions from previous literature. For example, Sun et al. thought that the conversion of H+ occurs before Zn2+ insertion . But we find that Zn2+ insertion occurs before the conversion of H+ in the first discharge process, and this is confirmed by thermodynamic analysis. Besides, previous literature deemed that the disappearance of BZSP is always caused by the change in electrolyte pH , but we find that BZSP can react with Mn2+ in the electrolyte to form a new phase of ZnMn3O7.
Based on experimental results and theoretical analysis of Zn/MnO2 ZIBs with the mixture electrolyte of ZnSO4 + MnSO4 aqueous solution, we found that the mechanism in ZIBs is dynamic and the phase transformation at MnO2 cathode is irreversible during charge/discharge processes. Not only H+ and Zn2+ but also Mn2+ in the electrolyte take part in the reactions. In the first discharge process, ZnxMnO2, MnOOH, Mn2O3, and by-product BZSP generate, and then in the first charge process, α-MnO2 and ZnMn3O7·3H2O appear. In the following charge/discharge processes, ZnMn2O4 and ZnMn3O8 are further generated on the surface of MnO2 and serve as the hosts for Zn2+ insertion. The mechanism becomes dynamic and complex because of the co-participation of the insertion process, conversion reaction, and oxidation reactions. The aforementioned phase changes inside ZIBs are well explained by the Mn–Zn–O phase diagram and the E-pH diagram. This work can provide guidance for continual research from the following aspects. (i) The research method combining electrochemical analysis and phase identification with E-pH diagram together can be used to analyze charge/discharge processes of other electrochemical energy storage systems, such as aqueous rechargeable Zn//V2O5 batteries. (ii) According to the proposed energy storage systems in this work, at least two approaches can be applied to enhance cycling performance of ZIBs: One is adding Mn2+ to promote the disappearance of BZSP, and the other one is adding pH buffer into the electrolytes or preparing solid electrolytes to prohibit the generation of OH– and BZSP.
The authors appreciate the financial support from the International Science & Technology Cooperation Program of China (No. 2016YFE0102200), Shenzhen Technical Plan Project (No. JCYJ20160301154114273), National Key Basic Research (973) Program of China (No. 2014CB932400), and Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N111).
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