context: Firstly, the effect of the KMnO4 concentration on the MnO2 loading and the capacity of the battery was studied. As shown in Fig. 3e, the loading of MnO2 on the electrode gradually increases with the increase of KMnO4 concentration. When the concentration of KMnO4 is 0.1 mM, a low MnO2 loading of 4.9 mg cm−3 (weight percentage of 21.1%) is obtained. And the capacity of the corresponding battery can be as high as 369.7 mA h g−1 (based on the mass of the active material of MnO2), superior to that of most of the earlier reported Zn–MnO2 batteries and aqueous zinc batteries (see Table S1†), such as Zn–MnO2 batteries (225 mA h g−1), Zn–ZnMn2O4 batteries (150 mA h g−1), Zn–PTO batteries (336 mA h g−1), Zn–NiCo2O4 batteries (183.1 mA h g−1), Zn–MoS2 batteries (202.6 mA h g−1), Ni–Zn batteries (265 mA h g−1), and Zn–V2O5@PEDOT batteries (360 mA h g−1). When the concentration of KMnO4 increases to 10 mM, the mass loading of MnO2 reaches 51.0 mg cm−3 (weight percentage of 79.1%), and the corresponding capacity remains at 306.7 mA h g−1. Meanwhile, the volumetric capacity of the battery increases accordingly at the beginning, and reaches its highest value (15.6 mA h cm−3) when the concentration of KMnO4 is about 10 mM, which is chosen as the optimal concentration for the preparation of cathode materials. Notably, a high loading of MnO2 seems to have a negligible effect on the ion diffusion as elucidated from the Nyquist plots in Fig. S9.† The linear slopes for the two electrodes are nearly the same, indicative of their similar ion diffusion resistance. High mass loading and high volumetric capacity are desirable in energy storage devices because of the advantages in maximizing the packing density of the electrode materials while lowering the manufacturing cost by reducing the number of inactive material layers.