**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.

**Author Contributions:** R.C. conceived and designed the experiments; X.G., Y.H., Z.L., and S.W. performed most of the experiments; L.Q. and S.J. analyzed the data; J.L., X.J., and Y.L. contributed reagents/materials/analysis tools; X.G. and R.C. wrote the paper.

**Funding:** This work was funded by the National Key Research and Development Program of China (Nos. 2017YFA0402800, 2017YFA0206700), the National Natural Science Foundation of China (No. 21776265), and the R&D Department of PetroChina.

**Conflicts of Interest:** The authors declare no conflict of interest.
