Redox Active Cation Intercalation/Deintercalation in Two-Dimensional Layered MnO₂ Nanostructures for High-Rate Electrochemical Energy Storage

Two-dimensional (2D) layered materials with a high intercalation pseudocapacitance have long been investigated for Li + -ion-based electrochemical energy storage. By contrast, the exploration of guest ions other than Li + has been limited, although promising. The present study investigates intercalation/deintercalation behaviors of various metal ions in 2D layered MnO 2 with various interlayer distances, K-birnessite nanobelt (K-MnO 2 ), its protonated form (H-MnO 2 ), and a freeze-dried sample of exfoliated nanosheets. Series of metal ions, such as monovalent Li +, Na +, and K + and divalent Mg 2+, exhibit reversible intercalation during charge/discharge cycling, delivering high-rate pseudocapacitances. In particular, the freeze-dried MnO 2 of exfoliated nanosheets restacked with the largest interlayer spacing and a less compact 3D network exhibits the best rate capability and a stable cyclability over 5000 cycles. Both theoretical calculation and kinetic analysis reveal that the increased interlayer distance facilitates the fast diffusion of cations in layered MnO 2 hosts. The results presented herein provide a basis for the controllable synthesis of layered nanostructures for high-rate electrochemical energy storage using various single- and multivalent ions.