Electrochemical Study of Polymorphic MnO₂ in Rechargeable Aqueous Zinc Batteries

Abstract

Manganese dioxide is regarded as a promising energy functional material due to its open tunnel structure with enormous applications in energy storage and catalysis. In this paper, α-MnO₂ with a 2 × 2 tunnel structure and β-MnO₂ with a 1 × 1 tunnel structure were hydrothermally synthesized, which possess characteristic tunnel structures formed by the interconnected unit structure of [MnO₆] octahedrons. With regards to their different tunnel dimensions, the specific mechanism of ion intercalation in these two phases and the effect on their performance as aqueous Zn-MnO₂ battery cathodes are explored and compared. Comprehensive analyses illustrate that both α-MnO₂ and β-MnO₂ provide decent capacity in the aqueous battery system, but their intrinsic stability is poor due to the structural instability upon cycling. At the same time, experiments show that α-MnO₂ has a better rate performance than β-MnO₂ under larger currents, thus implying that the former has a broader application in this aqueous battery system.

1. Introduction

At present, the existing secondary battery market is populated by lithium-ion batteries (LIBs). However, drawbacks such as the insecurity of active lithium metal and the limitations of lithium resources hinder their application in the large energy storage devices of LIBs [1,2,3,4]. There is thus an urgent need to develop alternative, new secondary batteries. Aqueous zinc batteries have attracted a lot of attention because of their good safety, high theoretical capacity, and low cost [5,6,7]. When a given anode is zinc metal, the open-circuit voltage and zinc-ion storage capacity are almost entirely dependent on the cathode material of the aqueous zinc batteries [5,8,9,10]. The ion and electron transport properties and redox reaction kinetics of the cathode and cathode–electrolyte interface will affect the gravimetric energy and power density of zinc batteries [11]. Therefore, the design and development of cathode materials with a large storage capacity and robust crystalline structure has become a major challenge in the field [12]. So far, the cathode materials that have been developed and utilized mainly include vanadium-based materials, Prussian blue with its analogs, and manganese-based materials [13,14,15,16,17,18]. For example, vanadium-based materials have received extensive attention as promising cathode materials for multivalent batteries due to their huge theoretical capacity, multifunctional crystal structure, abundant resources, and low cost. Commonly used vanadium-based materials include vanadium oxide, metal vanadate, vanadium phosphate, etc. [19,20,21]. However, the structural instability and chemical dissolution of vanadium lead to fast capacity fading which means poor cycling stability [22]. As the tenth most abundant element in the earth’s crust, manganese is a typical transition metal element and a valence-variable metallic element, which is easily oxidized into manganese oxide or manganese hydroxide minerals. Manganese-based oxides have long been used as electrode materials for energy storage and conversion applications because of their good electrochemical performance, low cost, natural abundance, and low toxicity [23]. Common manganese-based oxides are mainly: MnO, MnO₂, Mn₂O₃, Mn₃O₄, etc. [24,25,26]. As a commonly used cathode material, manganese dioxide has many different crystal forms, such as α-MnO₂ [27], γ-MnO₂ [28], β-MnO₂ [29,30], todorokite-MnO₂, [31,32], and so on. They are all connected by the unit structure of MnO₂ octahedrons in various ways to form the rich polymorphism [32]. The microstructure and morphology of MnO₂ will directly affect the reaction mechanism and electrochemical performance of aqueous Zn-MnO₂ batteries [33]. Among various forms of MnO₂, α-MnO₂ with a 2 × 2 tunnel structure, β-MnO₂ with a 1 × 1 tunnel, and δ-MnO₂ with a layer structure are widely explored as electrode candidates for aqueous Zn-MnO₂ batteries [34,35,36].

In this work, we synthesized 2 × 2 tunnel-structued α-MnO₂, 1 × 1 tunnel-structued β-MnO₂, and layer-structured δ-MnO₂ by a simple hydrothermal method. The energy storage behaviors of the three samples were systematically compared by electrochemical analysis. The experimental results show that α-MnO₂ exhibits better energy storage performance than β-MnO₂ and δ-MnO₂, which could be understood by considering both their tunnel dimension differences and structural stability under battery cycling conditions.

2. Experimental Methods

2.1. Sample Synthesis

In this paper, the hydrothermal synthesis method is used to prepare α-MnO₂, β-MnO₂, and δ-MnO₂. All chemicals were all purchased from Aladdin and used directly without any further treatment. The specific operation dosage is as follows:

The synthesis of α-MnO₂: 23.5 mmol KMnO₄ (3.725 g) and 9.38 mmol MnSO₄·H₂O (1.585 g) were dissolved in 150 mL of deionized water, stirred for 10 min to fully dissolve, mixed (slowly pour MnSO₄·H₂O into KMnO₄) and set aside. In a 500 mL hydrothermal reactor, the reaction was stirred at room temperature for 30 min and finally placed in an oven at 160 °C for 12 h to obtain anhydrous α- MnO₂.

The synthesis of β-MnO₂: After dissolving 7.2 mmol KMnO₄ (1.138 g) and 12 mmol MnSO₄·H₂O (2.028 g) in 40 mL of deionized water, respectively, the solution was fully stirred for 10 min. Then, the two (slowly pouring KMnO₄ into MnSO₄·H₂O) were mixed into a 250 mL hydrothermal reactor, the reaction was stirred at room temperature for 30 min and then placed in an oven at 160 °C for 12 h to obtain anhydrous β-MnO₂.

The synthesis of δ-MnO₂: 100 mL 0.3 M Mn(Cl)₂.4H₂O solution and 1.58 g KMnO₄ were added into 100 mL 3.0 M NaOH, the mix solution was left in an ice bath for 15 min until the color turned to brown. Then, the mixture was moved from the ice bath, aged over 7 days, and finally deionized water and ethanol were used to wash the product during the filtration until the filtrate was nearly neutral.

2.2. Sample Characterization

The morphology and microstructure were observed by field emission scanning electron microscopy (SEM, FEI, Nova 200 NanoSEM) and by a transmission electron microscope (TEM, JEOL, JEM-2100F). X-ray powder diffraction (XRD, Bruker, D8ADVANCE) was carried out to illustrate the crystallographic structure.

2.3. Electrochemical Measurement

A slurry was prepared for cathodes with 70 wt% active materials, 20 wt% Ketjen black, and 10 wt% polyvinylidene fluoride binder in N-methyl-2-pyrrolidone (NMP). The mixed slurry was casted on stainless steel foil, and dried at 80 °C for 12 h in a vacuum oven. CR2032 coin-type cells were manufactured to estimate the electrochemical performance of pristine α-MnO₂, β-MnO₂, and δ-MnO₂ cathodes in aqueous Zn-MnO₂ batteries, with zinc metals as the anode. The loading of active materials on the cathodes (r = 12 mm) was 1.5 mg cm⁻². 2M ZnSO₄ was used as electrolyte, and the separator was glass fiber. All the electrochemical tests were performed between 1.0 and 1.8 V. Cycling performance and Galvanostatic charge–dicharge were tested in Neware. EIS (0.01 Hz to 100,000 Hz) and CV profiles were measured by electrochemical workstation (CHI660E, Shanghai Chenhua, China).

3. Results and Discussion

The X-ray powder diffraction (XRD) patterns (Figure 1a) show that the as-synthesized samples can be indexed with α-MnO₂ (JCPDS file #42-1348) and β-MnO₂ (JCPDS file #24-0735) and δ-MnO₂ (JCPDS file # 80-1089), individually. Figure 1b,c depict the ball and stick theoretical models of α-MnO₂ and β-MnO₂, respectively. β-MnO₂ has no cations in 1 × 1 tunnel frames owing to its smaller space, while α-MnO₂ with a 2 × 2 tunnel structure usually possesses large cations (pink spheres) to avoid structure collapse. Figure 1d shows the model of layered δ-MnO₂. Typically, the MnO₂ synthesized by a simple hydrothermal method are generally one-dimensional structures, such as nanowires and nanorods. The SEM images and TEM images of the obtained samples are similar to those recorded in the related literature [3,37,38], as shown in Figure 2 and Figure 3. Figure 2a shows α-MnO₂ nanowires with a length of around 1.5 μm and a width of around 100 nm, and Figure 2b shows the β-MnO₂ with a length of around 1.5 μm and a width of around 200 nm, indicating that β-MnO₂ has a larger size. The α-MnO₂ nanowires are mainly irregular, but some will gather in a flower shape, while β-MnO₂ nanorods are uniformly dispersed. SEM images show layered δ-MnO₂ mainly presenting irregular sheet shapes in Figure 2c.

The microscopic morphology of the α-MnO₂ nanowires, β-MnO₂ nanorods, and layered δ-MnO₂ were further characterized in Figure 3. In most cases, the two ends of α-MnO₂ are irregular sections, and β-MnO₂ has the regular tips of two ends showing a shuttle-like shape, while δ-MnO₂ are mainly distributed in sheets (Figure 3a,d,g). The high-resolution transmission electron microscope (HRTEM) images of α-MnO₂, β-MnO₂ and δ-MnO₂ are shown in Figure 3c,f,g, respectively. HRTEM images show that both α-MnO₂ nanowires and β-MnO₂ nanorods grow along their individual [001] directions and are uniformly dispersed. The typical characteristic crystal planes of α-MnO₂, β-MnO_2, and δ-MnO₂ are shown in Figure 3c,f,i, respectively. The (110) interplanar spacing of α-MnO₂ is 6.95 Å, while the (110) interplanar spacing of β-MnO₂ is 3.28 Å, which is much smaller than the (110) crystal plane of α-MnO₂, indicating that the tunnel size of β-MnO₂ is smaller than that of α-MnO₂, which is in line with the existing literature reports [27,39,40]. At the same time, the edge of α-MnO₂ exhibits a certain degree of lattice disorder and a relatively thick amorphous layer; on the contrary, the edge of β-MnO₂ exhibits a regular crystalline layer. The above observations can further show that β-MnO₂ has better thermodynamic stability despite its smaller tunnel size. The crystal lattice of δ-MnO₂ was observed in the HRTEM image (Figure 3i), corresponding to the planes of (−111) and (002), for which the spacing is 0.24 and 0.34 nm, which takes no account of the direaction of the crystalline domins.

The galvanostatic charge–discharge (GCD) curves of α-MnO₂, β-MnO₂, and δ-MnO₂ were obtained at the rate of 0.2 C (1 C = 300 mA h g⁻¹), and the three MnO₂ with different crystal forms exhibit completely different performances in the initial stage (Figure 4).

All MnO₂ samples exhibit similar charge–discharge profiles with a gradient plateau corresponding with previously reported α-MnO₂, β-MnO₂, and δ-MnO₂ electrodes (Figure 4a–c) [27,39,41,42]. The MnO₂ samples have similar first discharge profiles with different plateau voltages. The single plateau voltage of α-MnO₂ in the first discharge profiles is around 1.25 V (Figure 4a), while those of β-MnO₂ and δ-MnO₂ are around 1.3 V (Figure 4b) 1.32 V (Figure 4c), respectively. The first charge profiles of all MnO₂ samples present two identical plateaus at 1.55 V and 1.6 V. Through the first cycle of the discharge–charge reaction, the δ-MnO₂||Zn battery shows a smaller overpotential than the other two MnO₂||Zn batteries, indicating that the α-MnO₂||Zn and β-MnO₂||Zn batteries need a larger applied voltage in the initial reaction. In other words, it is more difficult for α-MnO₂ and β-MnO₂ to react. However, in the subsequent cycles, the overpotential plateaus of α-MnO₂||Zn batteries in charge–discharge profiles gradually tend to be consistent with β-MnO₂||Zn batteries, further demonstrating the reason for the different cycle stability of the two batteries. It is notable that the structure of α-MnO₂ remains good, but its initial specific capacity is poor.

During the first discharge process, the specific capacities of α-MnO₂, β-MnO₂, and δ-MnO₂ are 310 mA h g⁻¹, 224 mA h g⁻¹, and 173 mA h g⁻¹, respectively, indicating that the larger tunnel size of α-MnO₂ is beneficial for charge storage. It is worth mentioning that the specific capacity of α-MnO₂ decays rapidly during 1–3 cycles and the declining trend becomes slower in the subsequent cycling process (Figure 4e). However, the specific capacity of β-MnO₂ has an upward trend in the first few cycles, and a high specific capacity up to 290 mA h g⁻¹ is obtained at the 4th cycle. The specific capacity declines rapidly and then slows down in the following cycling process. In addition, the cycling process of the β-MnO₂ cathode is aborted after 90 cycles due to its poor structure stability. It is easy to be seen that δ-MnO₂ shows the best capacity retention owing to its layered structure. Cyclic comparisons manifest that α-MnO₂ has better performance benefitting from its larger tunnel size, and the structural stability of α-MnO₂ is significantly better than that of β-MnO₂. Moreover, the activation process of β-MnO₂ in the initial stage of the cycling results in the material pulverizing rapidly, further leading to the battery breakdown after 90 cycles. Meanwhile, α-MnO₂ still maintains the constant current charge and discharge state until after 100 cycles. Figure 4d depicts the rate performance of α-MnO₂, β-MnO₂ and δ-MnO₂ electrodes. At the small current density of 0.1 C, β-MnO₂ delivers a higher specific capacity than α-MnO₂ and δ-MnO₂. When the current density increases to 0.2 C, the specific capacity of α-MnO₂ exceeds that of β-MnO₂ and δ-MnO₂, while at the high rate of 1 C, β-MnO₂ provides almost no capacity. Notably, when the charge–discharge rate goes back to 0.1 C, the capacity retention of α-MnO₂ and β-MnO₂ was around 50%, yet δ-MnO₂ still retained about 70%, whereas δ-MnO₂ maintains good capacity retention. The overall capacity of δ-MnO₂ decreases slightly during charging/discharging and this can be attributed to its layered structure. However, a higher capacity of α-MnO₂ during the entire process attracts more attention.

The CV curves of different MnO₂ cathodes at a scan rate of 0.1 mV s⁻¹ between the voltage range of 1.0–1.8 V were shown in Figure 5a–c. In the first cycle, there is one obvious peak at 1.14 V for α-MnO₂ and δ-MnO₂, and one at 1.21 V for the β-MnO₂. These peaks can be attributed to the intercalation of H⁺ into the different lattice positions of MnO₂ tunnel structure, with the reaction of Mn⁴⁺ to Mn³⁺ at the same time. During the following cycle, two reduction peaks were viewed at 1.22 V and 1.36 V for α-MnO₂, β-MnO₂ and δ-MnO₂. In addition, one obvious oxidation peak was observed around 1.6 V and 1.67 V for these MnO₂ samples. With the cycling progress, the intensity of the redox peaks of α-MnO₂ decrease slightly, while the peaks of β-MnO₂ increase apparently, indicting increased polarization of the battery for α-MnO₂ and the activation reaction for β-MnO₂. By comparison, α-MnO₂ and β-MnO₂ present a smaller enclosed region of the CV profiles in the following process, yet δ-MnO₂ is likely to remain unchanged, demonstrating that the tunnel structure has a lower capacity retention than the layer structure. Figure 5d illustrates the AC impedance spectroscopy curves of different MnO₂ electrodes. According to the EIS curves of α-MnO₂, β-MnO₂ and δ-MnO₂, apparently, all the Nyquist plots have a similar shape with a semicircle in the high–medium frequency region and a straight line in the low-frequency region, demonstrating that the electrochemical behavior of the three samples is controlled by both charge transfer and ion diffusion. The EIS data can be well fitted by a typical equivalent electrical circuit (the inset of Figure 5d), where R₁ is the ohmic resistance, R₂ is the faradic charge-transfer resistance, and W_o1 is the Warburg impedance. Notably, the lower the values of these parameters, the better the performance of the electrode. Figure 5d shows that the R₂ value of α-MnO₂ is much smaller than that of β-MnO₂ and δ-MnO₂, indicating a higher electric conductivity of α-MnO₂. The higher slope of δ-MnO₂ indicates the better Zn-ion diffusion in the internal electrodes owing to its layered structure.

4. Conclusions

In this work, the characterization of the microscopic morphology and structure of α-MnO₂ nanowires, β-MnO₂ nanorods, and layered δ-MnO₂ was carried out using HRTEM and XRD, etc. Compared with 2 × 2 tunnel-structured α-MnO₂, 1 × 1 tunnel-structured β-MnO₂ possesses better thermodynamic stability and a more regular morphological structure. However, when used as the cathode of zinc ion batteries, α-MnO₂ with a larger tunnel size is superior in both cycling and rate performance compared with β-MnO₂, although layered δ-MnO₂ exhibits higher capacity retention and poor initial capacity. This shows that the larger tunnel structure of α-MnO₂ can accommodate more carriers and has a broader application prospect. This work provides electrochemical insights into the energy storage of representative Zn-MnO₂ aqueous batteries and points to future strategies to enhance cycling stability, such as face modification and modified electrolytes to suppress Mn dissolution. Furthermore, the electrochemical behaviors of α-MnO₂, β-MnO₂, and δ-MnO₂ have been compared, which may motivate the development of more sustainable systems other than Zn-MnO₂ batteries in which the performance of aqueous electrolytes is enhanced.