**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 urgen<sup>t</sup> 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 [7–12].

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 [13–15]. 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 [16–19]. 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<sup>−</sup><sup>1</sup> 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).
