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

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.


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-MnO 2 ) batteries [16][17][18][19]. Birnessite-type manganese dioxide (δ-MnO 2 ) 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-MnO 2 battery [23]. In order to deliver a two-electron capacity for a long cycling life, the structure of δ-MnO 2 needs to be maintained by structure-stabilizing agents. For example, it was reported that the birnessite-type MnO 2 could achieve a full two-electron capacity for over 6000 cycles when mixed with bismuth oxide (Bi 2 O 3 ), called Bi-birnessite (Bi-δ-MnO 2 ), intercalated with Cu 2+ 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-MnO 2 in aqueous Zn-MnO 2 batteries by tuning the nanostructure of this materials. A hollow spherical structure of δ-MnO 2 is developed to enable a robust architecture and a high specific capacity of the positive electrode for an aqueous Zn-MnO 2 battery. The hollow manganese oxide cathode exhibits high capacity and stable cycling performance with an aqueous electrolyte.

Synthesis of Hollow Spherical MnO 2 Particles
SiO 2 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 SiO 2 suspension was centrifuged, rinsed with distilled water, and re-dispersed in 30 mL H 2 O to form a SiO 2 white suspension.
Then, 0.98 g of KMnO 4 was added to the SiO 2 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 (SiO 2 @MnO 2 ) was obtained and then etched in the 2.00 M of NaCO 3 solution at 60 • C for 24 h.
After the removal of the SiO 2 core, the final products of the hollow spherical MnO 2 particles were collected by centrifugation, washed with deionized water, and freeze-dried.

Cell Assembly and Test
To prepare the cathode electrode, the slurry was prepared with 70 wt % MnO 2 , 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 MnO 2 on the electrodes was around 0.5 mg/cm 2 . The CR2032 coin cells were assembled with zinc metal as anodes and MnO 2 as cathodes. The electrolyte was 1.0 M ZnSO 4 with 0.2 M MnSO 4 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.

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).

Results
The hollow MnO 2 nanospheres were synthesized using a template approach. The synthesis process of hollow MnO 2 nanospheres is illustrated schematically in Figure 1. First, the SiO 2 nanospheres were prepared through a sol-gel method. To form the core-shell structure of SiO 2 @MnO 2 , the as-synthesized SiO 2 nanospheres were used as templates for a hydrothermal process with a KMnO 4 solution. After being etched in an aqueous Na 2 CO 3 solution, the SiO 2 core was removed and the hollow MnO 2 nanospheres were obtained for characterization and electrochemical tests. material was recorded on an electrochemical workstation (Solartron) using the frequency response analysis with a range from 100 kHz to 0.01 Hz.

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).

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 As shown in Figure 2a, the prepared monodisperse SiO 2 nanospheres show a uniform sphere morphology with a size ranging from 200 to 250 nm. After the reaction with aqueous KMnO 4 solution by a hydrothermal process at 150 • C for 48 h, the core-shell structure of SiO 2 @MnO 2 was formed ( Figure 2b). It was clearly shown that the SiO 2 nanospheres were fully covered with MnO 2 and no aggregation was observed. The uniform coating on SiO 2 nanoparticles was due to the surface-induced nucleation and growth of manganese oxide species. To remove the SiO 2 core materials, the core-shell SiO 2 @MnO 2 particles were etched in an aqueous 2 M Na 2 CO 3 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 MnO 2 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 MnO 2 . Powder X-ray diffraction (XRD) measurement was used to examine the crystallographic structure phase in the as-synthesized hollow MnO 2 spheres. Figure 2d shows the XRD pattern of the as-synthesized hollow MnO 2 nanospheres, which shows peaks at 2θ around 12.4 • , 24.8 • , 36.8 • , and 65.8 • . These peaks can be indexed to birnessite-type MnO 2 . 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 MnO 2 nanospheres, we carried out high-resolution TEM analysis. Figure 3a clearly shows the hollow structure of MnO 2 nanospheres without aggregation observed. The MnO 2 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 MnO 2 , 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 MnO 2 structure were mapped by electron energy loss spectroscopy (EELS), confirming the uniform dispersion of elemental Mn and O (Figure 3d,e). A N 2 adsorption/desorption analysis of hollow MnO 2 nanospheres was conducted to analyze the surface area of the wrinkled hollow structure. The BET (Brunauer-Emmett-Teller) surface area of as-synthesized hollow MnO 2 nanosphere was~200 m 2 /g with a pore size distribution at~1.6 nm (Figure 4), indicating that the hollow MnO 2 nanosphere also featured a microporous structure. 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 m 2 /g with a pore size distribution at ~1.6 nm (Figure 4), indicating that the hollow MnO2 nanosphere also featured a microporous structure.  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 m 2 /g with a pore size distribution at ~1.6 nm (Figure 4), indicating that the hollow MnO2 nanosphere also featured a microporous structure.     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/Zn 2+ , 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.  Figure 5a shows the cyclic voltammetry scan results of the Zn-MnO 2 cell with hollow MnO 2 nanospheres as cathode materials. The sweep range was between 1.9 V and 0.8 V vs. Zn/Zn 2+ , 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 MnO 2 nanospheres during discharge. Figure 5b shows the typical galvanostatic discharge/charge profiles of the Zn-MnO 2 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-MnO 2 batteries based on birnessite-type materials [23,24]. The discharge capacity of hollow MnO 2 nanospheres reached up to~270 mAh g −1 at a 1 C rate. Figure 5c shows the typical charge/discharge profiles of Zn-MnO 2 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 MnO 2 nanospheres. The long-term cycling performance of the Zn-MnO 2 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 MnO 2 in aqueous Zn-MnO 2 batteries. The morphologies of MnO 2 nanosheets and nanorods are shown in Figure 6. The initial discharge capacity for hollow MnO 2 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 MnO 2 nanorods showed a quickly fading capacity. The MnO 2 nanosheets performed a low discharge capacity and poor cycling performance. The excellent rate capability and cycling stability of the Zn-MnO 2 cell should be due to the hollow structure of the birnessite-type MnO 2 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 Zn 2+ 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.  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 Zn 2+ 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. 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 Zn 2+ ions into the MnO 2 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.

Conclusions
In summary, hollow MnO 2 nanospheres were synthesized through a facile hydrothermal approach and used as cathode materials in aqueous Zn-MnO 2 batteries. The hollow birnessite-type MnO 2 cathode achieved a relatively high discharge capacity and stable cycling performance with an aqueous electrolyte in a Zn-MnO 2 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-MnO 2 battery.