Interlayer Engineering of MnO₂ with High Charge Density Bi³⁺ for High Rate and Stable Aqueous Supercapacitor

Two‐dimensional δ‐MnO 2 has attracted considerable attention as supercapacitor electrode due to its ability to incorporate foreign ions into its interlayer spacing which suggests the possibility of capacitance enhancement through interlayer engineering. Incorporating high charge density cation such as Bi 3+ into the MnO 2 interlayer becomes an attractive strategy that not only serves to stabilize the layer structure, it is also an important prelude towards the weakening of the chemical bonding between the O from MnO 2 and the intercalating electrolyte species. This in turn leads to higher reversibility while ensuring excellent structural stability that is portrayed in enhanced cyclic stability and rate performance. Herein, the Bi 3+ ‐modified δ‐MnO 2 delivered a reversible specific capacity of 421 F g −1 at 1 A g −1 , which is higher than that of the previously reported mono/divalent cation‐intercalant modified MnO 2 systems. Using the N‐doped carbon nanosheets as the negative electrode, the assembled asymmetric supercapacitor achieved a high capacitance of 77 F g −1 at 1 Ag −1 . Furthermore, the high energy density of 20 Wh kg −1 was recorded at a high‐power density of 39 kW kg −1 , while simultaneously demonstrating excellent cycle performance of 82 % capacitance retention after 40 000 cycles. This work has shown that interlayer engineering of layered MnO 2 could potentially provide new insight into the development of high‐performance supercapacitor electrode.