Trans. Nonferrous Met. Soc. China 30(2020) 3347-3355

A simple physical mixing method for MnO₂/MnO nanocomposites with superior Zn²⁺ storage performance

Xiao-bei ZANG¹, Ling-tong LI¹, Zhi-xin SUN¹, Rabah BOUKHERROUB², Jia-xin MENG¹, Kun-peng CAI¹, Qing-guo SHAO¹, Ning CAO¹

1. School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao 266580, China;

2. University Lille, CNRS UMR 8520-IEMN, Lille, F-59000, France

Received 18 February 2020; accepted 28 September 2020

Abstract: MnO₂/MnO cathode material with superior Zn²⁺ storage performance is prepared through a simple physical mixing method. The MnO₂/MnO nanocomposite with a mixed mass ratio of 12:1 exhibits the highest specific capacity (364.2 mA·h/g at 0.2C), good cycle performance (170.4 mA·h/g after 100 cycles) and excellent rate performance  (205.7 mA·h/g at 2C). Analysis of cyclic voltammetry (CV) data at various scan rates shows that both diffusion- controlled insertion behavior and surface capacitive behavior contribute to the Zn²⁺ storage performance of MnO₂/MnO cathodes. And the capacitive behavior contributes more at high discharge rates, due to the short paths of ion diffusion and the rapid transfer of electrons.

Key words: zinc-ion battery; MnO₂/MnO cathode material; physical mixing method; reaction kinetics

1 Introduction

At present, lithium-ion batteries (LIBs) play a dominant role in the rechargeable battery market, due to the high energy density and long cycle life. LIBs are widely used in the fields of portable mobile devices, electric vehicles, and power storage aspects (wind, hydraulic and solar power) [1-3]. Unfortunately, the further development of LIBs is seriously restrained by the limited lithium resources, safety issues caused by organic electrolytes and strict assembly conditions [4]. Therefore, it is urgent to find alternative rechargeable batteries featuring high safety, abundant raw materials, low cost and great capability. Zinc-ion batteries (ZIBs), which consist of Zn anode, mild aqueous electrolyte and cathode material, could well overcome the afore-said drawbacks, and therefore are attracting more and more attention. The energy storage of ZIBs is based on the insertion/extraction of Zn²⁺ into/from cathode material [5]. Zn anode has high theoretical capacity (820 mA·h/g, 5855 mA·h/cm³) and low redox potential (-0.76 V vs SHE) [6], so exploring potential satisfactory cathode materials for Zn²⁺ insertion is key to the development of ZIBs.

So far, Mn-based materials [7-9], V-based materials [10-12], and Prussian blue analogues [13] have been mainly applied in cathode materials of ZIBs. Among them, Mn-based materials receive extensive attention due to their high theoretical capacity and suitable discharge voltage [14]. The reported Mn-based cathodes are mainly concentrated on MnO₂ with a single Mn valence (+4). Unfortunately, MnO₂ has poor conductivity and large volume change during charge and discharge process, which affects rate performance and cycle life of ZIBs. Many methods have been developed to improve the performance of MnO₂ cathodes, normally by fabricating nanostructured forms (nanoflakes [7], nanorods [8], nano-spheres [15], etc) and doping with conductive materials (carbon nanotubes [16], graphene [17], etc). Although these methods have greatly improved the rate capability and cycle life of MnO₂ cathode, the complicated preparation process and high cost make them unsuitable for mass production.

As known to all, Mn has multiple oxidation states (+2, +3, +4, +6, and so on). Recently, some researchers have studied the properties of mixed valence manganese oxides. It was found that the coexistence of Mn in different valence states can result in good catalytic oxidation activity [18,19], anomalously high specific capacitance, excellent power density and long cycling life [20,21].

Herein, we prepared a high performance MnO₂/MnO cathode using a simple physical mixing method. The morphology, phase structure and storage mechanism of MnO₂/MnO cathode were analyzed through scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunauer- Emmett-TellerInstrument (BET) characterization and electrochemical measurements.

2 Experimental

2.1 Materials

Various manganese oxides used in this experiment were purchased from Shanghai Macklin Biochemical Technology Co., Ltd, China. The purities of MnO₂, MnO, Mn₂O₃ and Mn₃O₄ are 99.95%, 99%, 98% and 97%, respectively.

2.2 Material characterization

The crystal structure of samples was characterized by XRD (D8 Advance, Bruker) employing Cu K_α radiation from 10° to 80°. The micromorphology was observed by SEM (SU8100, Hitachi). The specific surface area and pore size were obtained by BET (ASAP 2460, Micro- meritics).

2.3 Electrochemical measurement

The CR2032 coin cells were assembled with Zn foil as anode, glass fiber as separator, and aqueous solution containing 2 mol/L ZnSO₄ and 0.1 mol/L MnSO₄ as electrolyte. The cathode was composed of 80 wt.% active materials (manganese oxides), 10 wt.% conductive material (acetylene black), and 10 wt.% binding agent (polyvinylidene fluoride, PVDF) in N-methyl-2-pyrrolidone (NMP). The slurry was cast onto stainless steel foil after being stirred evenly. The electrode was dried at 80 °C for 10 h and then cut into disks of 12 mm in diameter (the typical active material loading was about 0.8-1 mg/cm²).

The rate and cycle performances were measured on LAND battery measurement system (CT3001K) with a voltage window from 1 to 1.9 V at various current rates (0.2-2)C. For Zn-MnO₂ batteries, 1C is approximately equal to 617 mA/g. CV measurements were carried out using an electrochemical workstation (ModuLab XM) under the voltage ranging from 1 to 1.9 V at different scan rates (0.5-3 mV/s).

3 Results and discussion

3.1 Characterizations of manganese oxides

As shown in Fig. 1, the XRD diffractions display the crystal structures of various manganese oxides used in this study. The diffraction peaks of MnO, MnO₂, Mn₂O₃ and Mn₃O₄ can be well indexed to cubic MnO (JCPDS No. 07-0230; space group: ), tetragonal β-MnO₂ (JCPDS No. 24-0735; space group: P42/mnm), cubic Mn₂O₃ (JCPDS No. 41-1442; space group: Ia-3), and tetragonal Mn₃O₄ (JCPDS No. 24-0734; space group: I4₁/amd). They are vastly different in their morphology, as observed by SEM. Except for the irregular bulk of MnO₂, MnO, Mn₂O₃ and Mn₃O₄ are respectively presented as angular nanospheres (~1 μm in diameter), small nanoparticles (~100 nm in diameter) and slender nanorods (~1 um in length) (Fig. 2).

The electrochemical performance of the four manganese oxides is illustrated in Fig. 3(a). The initial discharge specific capacities of MnO₂, MnO, Mn₂O₃ and Mn₃O₄ respectively correspond to 81.3, 42.3, 31.6 and 11.7 mA·h/g at 2C. Three mixtures of MnO₂/MnO, MnO₂/Mn₂O₃ and MnO₂/Mn₃O₄ nanocomposites were prepared as cathode active materials of ZIBs at a mass ratio of 1:1. From the electrochemical results in Fig. 3(b), MnO₂/MnO nanocomposite (1:1), MnO₂/Mn₂O₃ nanocomposite (1:1) and MnO₂/Mn₃O₄ nanocomposite (1:1) achieve a reversible capacity of 122.2, 87.5 and 55.6 mA·h/g at 2C, respectively. It can be easily found that the mixture of different manganese oxides could improve the specific capacity of ZIBs, especially the mixture of MnO₂ and MnO. In order to further improve the electrochemical properties, the MnO₂/MnO mass ratios are adjusted.

Fig. 1 XRD patterns of MnO₂ (a), MnO (b), Mn₂O₃ (c) and Mn₃O₄ (d)

Fig. 2 SEM images of MnO₂ (a), MnO (b), Mn₂O₃ (c) and Mn₃O₄ (d)

Fig. 3 Initial charge and discharge performance of cathodes

3.2 Effect of mass ratio on performance of MnO₂/MnO nanocomposites

In MnO₂/MnO nanocomposites, MnO₂ nano- particles are distributed on the surface of angular MnO nanospheres (Figs. 4(a, b)), and the mixture of the two manganese oxides is further confirmed by XRD (Fig. 5). As a result, the architecture of angular nanospheres and small nanoparticles forms the mesoporous and macroporous structure of MnO₂/MnO nanocomposite with a concentrated pore size distribution of 2-100 nm (Fig. 6), and this porous structure is very likely to facilitate the transfer of Zn²⁺. In addition, the introduction of MnO enhances the adhesion between electrode materials and current collector, and relieves the phenomenon of MnO₂ detachment.

Fig. 4 SEM images of MnO₂/MnO (1:1) (a) and MnO₂/MnO (12:1) (b) nanocomposites

Fig. 5 XRD patterns of MnO₂/MnO nanocomposites

Then, the electrochemical properties are tested. By performing five charge-discharge cycles at different rates, the highest specific capacities of MnO₂/MnO nanocomposites with different mixing mass ratios are compared in Fig. 7(a). It can be clearly seen that the discharge specific capacity of the MnO₂/MnO nanocomposite with a mass ratio of 12:1 is much higher than that of nanocomposites with other ratios, with specific capacity values of 364.2, 325.8, 314.9, 282.9, 205.7 mA·h/g at a rate of 0.2C, 0.5C, 0.7C, 1C and 2C, respectively(Fig. 7(b)). Compared with both MnO₂ cathode and MnO cathode, the electrochemical performance of MnO₂/MnO (12:1) cathode is greatly improved. Surprisingly, the addition of MnO can nearly double the discharge specific capacity of the commonly used MnO₂ electrode (Fig. 7(c)). With the increase of the number of cycles, the discharge specific capacity of cathode materials firstly increases and then decreases (Fig. 7(d)). This phenomenon is commonly found in transition metal oxide electrodes, which is mainly caused by the slow electrochemical activation process [22]. Besides, MnO₂/MnO shows better electrochemical activity than MnO₂ and MnO, which is probably related to the high catalytic activity of mixed valence compounds [23]. The MnO₂/MnO cathode displays good cycling stability, while the discharge specific capacity decreases to 170.4 mA·h/g after 100 cycles.

Fig. 6 N₂ adsorption/desorption isotherm of MnO₂/MnO nanocomposite (12:1) (Insert is corresponding pore size distribution)

Although MnO₂/MnO cathode exhibits better electrochemical performance than other typical cathode materials (Fig. 8) [17,24-28], it is also of great practical significance to conduct more research to extend the cycle life.

Fig. 7 Discharge specific capacity of MnO₂/MnO electrodes with different mixing ratios at different rates (a), charge and discharge curves of MnO₂/MnO (12:1) electrode at different rates (b), rate performance of MnO₂/MnO (12:1), MnO₂ and MnO electrodes (c), and cycle performance of MnO₂/MnO (12:1), MnO₂ and MnO electrodes (d)

Fig. 8 Comparison between MnO₂/MnO cathode and other cathode materials reported

3.3 Energy-storage mechanism of MnO₂/MnO cathode

Fig. 9 CV curves of MnO₂/MnO electrode at 0.5 mV/s (a) and XRD patterns at redox peaks (b)

Fig. 10 Schematic illustration of ZIBs based on MnO₂/MnO cathode

To further explore the energy-storage mechanism, CV curve of MnO₂/MnO electrode at a scan rate of 0.5 mV/s was used to analyze the transformation of MnO₂/MnO (Fig. 9(a)). In scans of the cathode, two reduction peaks are observable at 1.36 and 1.21 V, which can be ascribed to the insertion of Zn²⁺ into MnO₂/MnO hosts. In anodic scans, one oxidation peak is observed at 1.59 V, corresponding to the Zn²⁺-extraction from MnO₂/ MnO cathode. This is supported by the XRD analysis in Fig. 9(b). ZnMn₂O₄ is formed at the two reduction peaks and the composition is similar to the original one at the oxidation peak. The reactions of ZIBs during charge and discharge process can  be summarized by Eqs. (1)-(3) and depicted as Fig. 10.

Anode: ZnZn²⁺+2e                     (1)

Cathode: MnO₂/MnO+Zn²⁺+2eZnMn₂O₄    (2)

Overall: MnO₂/MnO+ZnZnMn₂O₄         (3)

To better understand fundamental mechanisms underlying the improved electrochemical performance of MnO₂/MnO electrode, the reaction kinetics were investigated by CV measurements. The CV measurements of MnO₂/MnO electrode (Fig. 11(a)) were carried out at various scan rates ranging from 0.5 to 3 mV/s. CV curves at various scan rates can help to understand the reversibility of the electrode reactions. Obviously, as the scan rate increases, the peaks on CV curves gradually become broader, but the shapes of CV curves remain consistent. The CV data at various scan  rates are analyzed according to the following equations [22]:

I=av^b                                   (4)

where the current I conforms to a power law relationship with the sweep rate v. a and b are   two adjustable parameters, and b values can be determined from the slope of the plot of lg I versus lg v, as described in the following equation:

lg I=blg v+lg a                           (5)

where the coefficient b usually varies from 0.5 to 1. The b value of 0.5 indicates the diffusion-controlled insertion behavior, while the b value of 1 represents the surface capacitive behavior. As shown in Fig. 11(b), the b values of three redox peaks are calculated to be 1.09 (Peak 1), 0.534 (Peak 2) and 0.93 (Peak 3), respectively. It is suggested that both diffusion-controlled behavior and capacitive behavior contribute to the electrochemical kinetic of MnO₂/MnO electrode, but the capacitive behavior plays more important role. The specific contributions of capacitive behavior to the total capacity at various scan rates can be determined by

I(v)=k₁v+k₂v^1/2                               (6)

where k₁v and k₂v^1/2 respectively represent the current contributions from surface capacitive behavior and diffusion-controlled insertion behavior. The calculation results in Fig. 11(c) show that the contribution of capacitive behavior increases gradually with the incremental scan rate. Specifically, the diffusion-controlled insertion behavior holds a dominant position in the total contribution of MnO₂/MnO electrode at low discharge rates, while capacitive behavior contributes more at high discharge rates. In conclusion, one of the reasons for the outstanding performance of MnO₂/MnO electrode at high rates can be attributed to the short paths of ion diffusion and the rapid transfer of electrons.

Fig. 11 CV curves of MnO₂/MnO electrode at different scan rates (a), lg I and lg v plots at specific peak currents (b), and contribution ratios of capacitive behavior to MnO₂/MnO electrode (c)

4 Conclusions

(1) The MnO₂/MnO nanocomposites have been successfully fabricated via a simple physical mixing. The SEM analyses reveal that MnO₂ nanoparticles are distributed on the surface of angular MnO nanospheres.

(2) The MnO₂/MnO cathode displays improved specific capacity values of 364.2 mA·h/g at the rate of 0.2C and excellent cycle stability versus commonly used MnO₂ cathode. And the optimized mass ratio of MnO₂/MnO is found to be 12:1.

(3) The mechanism of improved electro- chemical performance is investigated by CV curve and reaction kinetics. The outstanding performance of MnO₂/MnO cathode at high rates is attributed to the short paths of ion diffusion and the rapid transfer of electrons.

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简单物理混合方法提高MnO₂/MnO纳米复合材料的储Zn²⁺性能

臧晓蓓¹，李灵桐¹，孙志欣¹，Rabah BOUKHERROUB²，孟佳欣¹，蔡鲲鹏¹，邵庆国¹，曹 宁¹

1. 中国石油大学(华东) 材料科学与工程学院，青岛 266580；

2. University Lille, CNRS UMR 8520-IEMN, Lille, F-59000, France

摘  要：通过简单的物理混合方法制备具有优异储Zn²⁺性能的MnO₂/MnO正极材料。混合质量比为12:1的MnO₂/MnO纳米复合材料具有最高的比容量(0.2C时达364.2 mA·h/g)，良好的循环性能(100次循环后达170.4 mA·h/g)和倍率性能(2C时达205.7 mA·h/g)。通过探究储能机理，充放电过程中扩散控制的插入行为和表面电容行为均有助于MnO₂/MnO正极的储Zn²⁺性能。由于离子扩散的短路径和电子的快速转移，在高放电倍率下，电容行为发挥更大的作用。

关键词：锌离子电池；MnO₂/MnO复合正极材料；物理混合方法；反应动力学

(Edited by Bing YANG)

Foundation item: Project (21905304) supported by the National Natural Science Foundation of China; Project (ZR2019BEM031) supported by the Natural Science Foundation of Shandong Province, China; Projects (18CX02158A, 19CX05001A) supported by the Fundamental Research Funds for the Central Universities, China

Corresponding author: Ning CAO; Tel: +86-15376713767; E-mail: caoning@upc.edu.cn

DOI: 10.1016/S1003-6326(20)65466-8