Cryptomelane-type MnO₂/carbon nanotube hybrids as bifunctional electrode material for high capacity potassium-ion full batteries

Potassium-ion batteries (PIBs) have been receiving a great deal of attention owing to abundant and cost-effective potassium resources. However, the key issue is to explore suitable electrode materials for accommodating the large size K-ions reversibly. In this report, K_1.06Mn₈O₁₆/CNT hybrids are systematically studied for the first time as bifunctional PIBs-electrodes, in which Mn-ions play as the redox center. As the cathode, it exhibits excellent electrochemical kinetics and reversibility, as well as highly structure stability. The cathode displays an unprecedented capacity of 309.4 mA h g⁻¹ at 20 mA g⁻¹ with the energy density of 733.3 Wh kg⁻¹ and outstanding rate performance (187.1 mA h g⁻¹ at 500 mA g⁻¹). In addition, the high capacity of 636.6 mA h g⁻¹ at 20 mA g⁻¹, ultra-long cycling lifespan over 500 cycles and remarkable rate capability can also be acquired when the material was tested as the anode. The extraordinary electrochemical properties are ascribed to the well-developed conductive network and robust microstructure stability. Meanwhile, the symmetrical K-ions full cell exhibits high discharge capacity of 241.0 mA h g⁻¹ at 100 mA g⁻¹. This work reveals the K-ions storage mechanism in cryptomelane bifunctional electrodes in depth and the fundamental understanding help us open up a new direction for PIBs.