Graphene effectively activating “dead” water molecules between manganese dioxide layers in potassium-ion battery

Aqueous potassium-ion batteries (APIBs), recognized as safe and reliable new energy devices, are considered as one of the alternatives to traditional batteries. Layered MnO 2 , serving as the main cathode, exhibits a lower specific capacity in aqueous electrolytes compared to organic systems and operates through a different reaction mechanism. The application of highly conductive graphene may effectively enhance the capacity of APIBs but could complicate the potassium storage environment. In this study, a MnO 2 cathode pre-intercalated with K + ions and grown on graphene (KMO@rGO) was developed using the microwave hydrothermal method for APIBs. KMO@rGO achieved a specific capacity of 90 mA h g −1 at a current density of 0.1 A g −1 , maintaining a capacity retention rate of >90% after 5000 cycles at 5 A g −1 . In-situ and ex-situ characterization techniques revealed the energy-storage mechanism of KMO@rGO: layered MnO 2 traps a large amount of “dead” water molecules during K + ions removal. However, the introduction of graphene enables these water molecules to escape during K + ions insertion at the cathode. The galvanostatic intermittent titration technique and density functional theory confirmed that KMO@rGO has a higher K + ions migration rate than MnO 2 . Therefore, the capacity of this cathode depends on the interaction between dead water and K + ions during the energy-storage reaction. The optimal structural alignment between layered MnO 2 and graphene allows electrons to easily flow into the external circuit. Rapid charge compensation forces numerous low-solvent K + ions to displace interlayer dead water, enhancing the capacity. This unique reaction mechanism is unprecedented in other aqueous battery studies.