A Hollow-Structured Manganese Oxide Cathode for Stable Zn-MnO₂ Batteries

Abstract

Aqueous rechargeable zinc-manganese dioxide (Zn-MnO₂) batteries are considered as one of the most promising energy storage devices for large scale-energy storage systems due to their low cost, high safety, and environmental friendliness. However, only a few cathode materials have been demonstrated to achieve stable cycling for aqueous rechargeable Zn-MnO₂ batteries. Here, we report a new material consisting of hollow MnO₂ nanospheres, which can be used for aqueous Zn-MnO₂ batteries. The hollow MnO₂ nanospheres can achieve high specific capacity up to ~405 mAh g⁻¹ at 0.5 C. More importantly, the hollow structure of birnessite-type MnO₂ enables long-term cycling stability for the aqueous Zn-MnO₂ batteries. The excellent performance of the hollow MnO₂ nanospheres should be due to their unique structural properties that enable the easy intercalation of zinc ions.

1. Introduction

Lithium-ion batteries (LIBs) have predominantly held a significant share of the energy storage market for portable electronics and electric vehicles since the 1990s, due to their high energy/power density and long cycling life. However, with the rapid development of renewable energy plants, there is an extensive and urgent demand for energy storage technologies for large-scale smart grid applications, which require rechargeable battery systems with good cycling performance, low cost, high safety, and environmental friendliness. In searching for new chemistry beyond lithium-ion batteries, multivalent secondary batteries (Mg, Ca, Zn, and Al) have attracted tremendous research efforts, which could, in principle, deliver a higher energy density based on their multi-electron reaction mechanisms [1,2]. Among the multivalent batteries based on intercalation chemistries, aqueous rechargeable zinc ion batteries are considered as a promising candidate for large-scale energy storage applications because of their low cost and the large abundance of Zn [3]. In addition, the aqueous electrolytes in zinc ion batteries provide better safety compared to other battery systems with flammable organic electrolytes. However, the development of aqueous zinc ion batteries is significantly hindered by the limited choice of positive electrode materials, which usually suffer from low specific capacity and poor cycling stability [4]. Many failure mechanisms are associated with phase transformations and the formation of irreversible products [5,6]. Only a few positive electrodes coupled with suitable electrolytes have been demonstrated to be able to achieve stable long-term cycling for aqueous zinc ion batteries [7,8,9,10,11,12].

Despite their low cost and high abundance, manganese oxides have a variety of advantages including tunable crystal structure and a scalable manufacturing process, which have been widely used for many energy storage applications including lithium-ion batteries, supercapacitors, and zinc-air batteries [13,14,15]. Manganese oxides possess a variety of polymorphs, including α-, β-, γ-, δ-, λ-, and ε-types, which form different structures such as tunnel, layered, and spinel structures, and can be used as positive electrode materials for aqueous zinc manganese dioxide (Zn-MnO₂) batteries [16,17,18,19]. Birnessite-type manganese dioxide (δ-MnO₂) is featured with a layered structure, which is considered as a favorable host for the intercalation of various cations [20,21]. Considerable efforts have been made to verify this layered structure materials for reversible zinc ion intercalation [22]. It was observed that the birnessite-type manganese dioxide is not stable as a positive electrode material under the long-term cycling of a secondary Zn-MnO₂ battery [23]. In order to deliver a two-electron capacity for a long cycling life, the structure of δ-MnO₂ needs to be maintained by structure-stabilizing agents. For example, it was reported that the birnessite-type MnO₂ could achieve a full two-electron capacity for over 6000 cycles when mixed with bismuth oxide (Bi₂O₃), called Bi-birnessite (Bi-δ-MnO₂), intercalated with Cu²⁺ ions [24]. Also, we note that hollow nanostructures offer promising potentials for energy storage applications because of their favorable properties in terms of hierarchical structure complexity and fast ion transport pathway [25,26].

Herein, without stabilizing agents, we tackle the stability issue of δ-MnO₂ in aqueous Zn-MnO₂ batteries by tuning the nanostructure of this materials. A hollow spherical structure of δ-MnO₂ is developed to enable a robust architecture and a high specific capacity of the positive electrode for an aqueous Zn-MnO₂ battery. The hollow manganese oxide cathode exhibits high capacity and stable cycling performance with an aqueous electrolyte.

2. Materials and Methods

2.1. Synthesis of Hollow Spherical MnO₂ Particles

SiO₂ spherical particles were prepared by a sol-gel method and used as a template. In a typical synthesis procedure, 4.0 mL of tetrapropyl orthosilicate was added into the mixture of ethanol (50.0 mL), water (10.0 mL), and ammonia (1.0 mL, 25–28%) at room temperature under stirring. After 14 h, the obtained SiO₂ suspension was centrifuged, rinsed with distilled water, and re-dispersed in 30 mL H₂O to form a SiO₂ white suspension.

Then, 0.98 g of KMnO₄ was added to the SiO₂ suspension and followed by ultrasonic treatment for 30 min. The suspension was then transferred to a Teflon-lined autoclave and heated at 150 °C for 48 h. The brown product with a silica/manganese oxide core-shell structure (SiO₂@MnO₂) was obtained and then etched in the 2.00 M of NaCO₃ solution at 60 °C for 24 h.

After the removal of the SiO₂ core, the final products of the hollow spherical MnO₂ particles were collected by centrifugation, washed with deionized water, and freeze-dried.

2.2. Cell Assembly and Test

To prepare the cathode electrode, the slurry was prepared with 70 wt % MnO₂, 20 wt % KB (Ketjenblack), and 10 wt % PVDF (Polyvinylidene Fluoride) binder and casted onto a Ti foil current collector. The electrode was dried at 60 °C in a vacuum oven for 24 h. The loading of MnO₂ on the electrodes was around 0.5 mg/cm². The CR2032 coin cells were assembled with zinc metal as anodes and MnO₂ as cathodes. The electrolyte was 1.0 M ZnSO₄ with 0.2 M MnSO₄ as an additive and glass fiber was used as the separator. Galvanostatic measurements were carried out between 1.0 and 1.8 V on a Land CT2001A system (LANHE, Wuhan, China). The cyclic voltammetry (CV) experiments were performed with a CHI600E electrochemical workstation (CH, Shanghai, China) at a scanning rate of 0.1 mV s⁻¹ between 0.8 and 1.9 V. The electrochemical impedances spectroscopy (EIS) of the active material was recorded on an electrochemical workstation (Solartron) using the frequency response analysis with a range from 100 kHz to 0.01 Hz.

2.3. Materials Characterization

The dimensions and morphologies were examined using scanning electron microscopy (SEM, JSM-2100F, JEOL, Tokyo, Japan. The crystallographic structures were investigated by powder XRD (X-ray diffraction) measurements on a Rigaku D/max-TTR III diffractometer with Cu Kα radiation (Rigaku Corporation, Shibuya-ku, Japan), 40 kV, 200 mA. The nanostructures of hollow spherical samples were characterized by high-resolution transmission electron microscopy (HRTEM, JEOL, Tokyo, Japan, 2010).

3. Results

The hollow MnO₂ nanospheres were synthesized using a template approach. The synthesis process of hollow MnO₂ nanospheres is illustrated schematically in Figure 1. First, the SiO₂ nanospheres were prepared through a sol-gel method. To form the core-shell structure of SiO₂@MnO₂, the as-synthesized SiO₂ nanospheres were used as templates for a hydrothermal process with a KMnO₄ solution. After being etched in an aqueous Na₂CO₃ solution, the SiO₂ core was removed and the hollow MnO₂ nanospheres were obtained for characterization and electrochemical tests.

As shown in Figure 2a, the prepared monodisperse SiO₂ nanospheres show a uniform sphere morphology with a size ranging from 200 to 250 nm. After the reaction with aqueous KMnO₄ solution by a hydrothermal process at 150 °C for 48 h, the core-shell structure of SiO₂@MnO₂ was formed (Figure 2b). It was clearly shown that the SiO₂ nanospheres were fully covered with MnO₂ and no aggregation was observed. The uniform coating on SiO₂ nanoparticles was due to the surface-induced nucleation and growth of manganese oxide species. To remove the SiO₂ core materials, the core-shell SiO₂@MnO₂ particles were etched in an aqueous 2 M Na₂CO₃ solution for 24 h. After the etching process, very little silica is remained based on EDX (Energy Dispersive X-Ray Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy) measurements. Figure 2c shows the typical morphology of hollow spherical MnO₂ particles after the etching treatment. It is clearly seen that the spherical morphology is completely maintained and almost no damage was observed on the shell structure of MnO₂. Powder X-ray diffraction (XRD) measurement was used to examine the crystallographic structure phase in the as-synthesized hollow MnO₂ spheres. Figure 2d shows the XRD pattern of the as-synthesized hollow MnO₂ nanospheres, which shows peaks at 2θ around 12.4°, 24.8°, 36.8°, and 65.8°. These peaks can be indexed to birnessite-type MnO₂. The peaks lack the long-range order of layers and a tail toward higher angle two-theta, demonstrating common features of the birnessite structure [27].

In order to further investigate the structure of the as-synthesized hollow MnO₂ nanospheres, we carried out high-resolution TEM analysis. Figure 3a clearly shows the hollow structure of MnO₂ nanospheres without aggregation observed. The MnO₂ shell is around 15 nm thick and its diameter is around 200 nm. Almost no damage was observed under TEM analysis, indicating that the shell structure is robust enough to tolerate the harsh etching process. Detailed analysis shows that the shell structure consists of very thin nanosheets of MnO₂, which form interconnected wrinkle structures (Figure 3b). The wrinkled structure was confirmed by HAADF-STEM (High-Angle Annular Dark Field Scanning Transmission Electron Microscopy) image (Figure 3c). Moreover, elemental compositions of the hollow MnO₂ structure were mapped by electron energy loss spectroscopy (EELS), confirming the uniform dispersion of elemental Mn and O (Figure 3d,e). A N₂ adsorption/desorption analysis of hollow MnO₂ nanospheres was conducted to analyze the surface area of the wrinkled hollow structure. The BET (Brunauer-Emmett-Teller) surface area of as-synthesized hollow MnO₂ nanosphere was ~200 m²/g with a pore size distribution at ~1.6 nm (Figure 4), indicating that the hollow MnO₂ nanosphere also featured a microporous structure.

The electrochemical performance of hollow MnO₂ nanospheres was evaluated in aqueous Zn-MnO₂ batteries. The Zn-MnO₂ cell was assembled with zinc foil as an anode and 1.0 M Zn(SO₄)₂ aqueous solution with 0.2 M MnSO₄ as an electrolyte. Figure 5a shows the cyclic voltammetry scan results of the Zn-MnO₂ cell with hollow MnO₂ nanospheres as cathode materials. The sweep range was between 1.9 V and 0.8 V vs. Zn/Zn²⁺, and the sweep rate was 0.1 mV/s. During the first cycle, a low cathodic peak at around 1.36 V and a sharp cathodic peak at around 1.22 V were observed, while only one anodic peak at around 1.58 V was observed when sweeping back. In the following scan cycles, the cathodic peak at 1.36 V increased gradually, indicating an activation process of hollow MnO₂ nanospheres during discharge. Figure 5b shows the typical galvanostatic discharge/charge profiles of the Zn-MnO₂ cell at a 1 C rate. The discharge curve in first cycle exhibited a flat plateau at around 1.26 V, which is consist with the CV results. Two plateaus, ~1.38 V and ~1.26 V, were observed during the second discharge process, which are related to two distinct cathodic peaks in the second sweep of CV curves, indicating a two-step intercalation process of zinc ions into the birnessite structure. Upon charge process, two plateaus at ~1.50 V and ~1.58 V were observed. Previously, the two-step intercalation process was also observed in other Zn-MnO₂ batteries based on birnessite-type materials [23,24]. The discharge capacity of hollow MnO₂ nanospheres reached up to ~270 mAh g⁻¹ at a 1 C rate.

Figure 5c shows the typical charge/discharge profiles of Zn-MnO₂ batteries at different current densities. At rates of 0.5, 1, 2, 5, and 10 C, specific discharge capacities of ~405, ~265, ~166, ~85, and ~40 mAh g⁻¹ were obtained, respectively, indicating a good rate performance of the hollow MnO₂ nanospheres. The long-term cycling performance of the Zn-MnO₂ batteries in terms of discharge capacity and coulombic efficiency was also investigated at 1 C. As shown in Figure 5d, we compared the cycling performance of nanosheets, nanorods, and hollow spherical structure of MnO₂ in aqueous Zn-MnO₂ batteries. The morphologies of MnO₂ nanosheets and nanorods are shown in Figure 6. The initial discharge capacity for hollow MnO₂ nanospheres was ~168 mAh g⁻¹. After the activation process, the discharge capacity of the second cycle was reached at ~270 mAh g⁻¹. Notably, after 100 cycles, the discharge capacity was stabilized at ~305 mAh g⁻¹ with a coulombic efficiency over 97%. However, the MnO₂ nanorods showed a quickly fading capacity. The MnO₂ nanosheets performed a low discharge capacity and poor cycling performance. The excellent rate capability and cycling stability of the Zn-MnO₂ cell should be due to the hollow structure of the birnessite-type MnO₂ cathode materials.

EIS measurements were performed to evaluate the impedance difference between before cycle and after the first discharge/charge cycle. As depicted in Figure 7a, the charge transfer impedance decreased after the first cycle, which indicated that the intercalation of Zn²⁺ ions into the MnO₂ structure became easier after the structure transformation. An ex situ XRD analysis was conducted for the cathode after the first cycle. As shown in Figure 7b, the representative birnessite structure peaks, (002) and (212), significantly decreased in intensity, especially compared to the mixed indices (161) peak. This selective loss suggests a loss of long-range order in the direction of the layers, perhaps due to a structural transformation to another polymorph with similar building blocks but not layered.

4. Conclusions

In summary, hollow MnO₂ nanospheres were synthesized through a facile hydrothermal approach and used as cathode materials in aqueous Zn-MnO₂ batteries. The hollow birnessite-type MnO₂ cathode achieved a relatively high discharge capacity and stable cycling performance with an aqueous electrolyte in a Zn-MnO₂ battery. The excellent electrochemical performance was ascribed to the unique hollow structure, which favors the intercalation process of zinc ions and enables a stable cycling of the Zn-MnO₂ battery.