**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.

**Figure 1.** Schematic illustration of the synthetic process of hollow MnO2 nanospheres.

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.

**Figure 2.** SEM images of SiO2 nanospheres (**a**); SiO2@MnO2 core-shell structure (**b**); and hollow MnO2 nanospheres (**c**); (**d**) XRD patterns of the hollow MnO2 nanospheres.

**Figure 3.** High- (**a**) and low-magnification (**b**) HRTEM images of hollow MnO2 nanospheres; (**c**) HAADF-STEM image of hollow MnO2 nanospheres. Elemental mapping of hollow MnO2 nanospheres: (**d**) Mn and (**e**) O.

*Nanomaterials* **2018**, *8*, 301

**Figure 4.** BET measurement of hollow MnO2 nanospheres. (**a**) Nitrogen adsorption/desorption isotherms of as-synthesized hollow MnO2 nanospheres; (**b**) the pore size distribution of hollow MnO2 nanospheres, as calculated using a BJH (Barrett-Joyner-Halenda) method.

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<sup>−</sup><sup>1</sup> 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<sup>−</sup><sup>1</sup> 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<sup>−</sup>1. After the activation process, the discharge capacity of the second cycle was reached at ~270 mAh g<sup>−</sup>1. Notably, after 100 cycles, the discharge capacity was stabilized at ~305 mAh g<sup>−</sup><sup>1</sup> 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.

*Nanomaterials* **2018**, *8*, 301

**Figure 5.** Electrochemical performance of Zn-MnO2 batteries: (**a**) CV profiles; (**b**) typical charge–discharge curves; (**c**) rate performance; and (**d**) long-term cycling stability of hollow MnO2 nanospheres, MnO2 nanosheets, and MnO2 nanorods at 1 C with an electrolyte of 1.0 M Zn(SO4)2 and 0.2 M MnSO4.

**Figure 6.** SEM images of (**a**) MnO2 nanosheets and (**b**) MnO2 nanorods.

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.

**Figure 7.** (**a**) Electrochemical impedance spectra of the Zn/MnO2 cells before any cycles and after the first cycle; (**b**) XRD patterns of the cathode after the first cycle.
