Simple and Cost-Effective Approach To Dramatically Enhance the Durability and Capability of a Layered δ-MnO₂ Based Electrode for Pseudocapacitors: A Practical Electrochemical Test and Mechanistic Revealing

Inadequate capacity and poor durability of MnO2 based pseudocapacitive electrodes have long been stumbling blocks in the way of their commercial use. Though layered δ-MnO2 has higher potential to be used due to its proton-free energy storage reactions, its durability is still far away from carbon based electrodes associated with structure deformation caused by interlayer spacing change and Jahn–Teller effect. Here we report an effective approach to dramatically enhance not only the stability but also the capacity of δ-MnO2 based electrode through a simple incorporation of exotic cations, hydrated Zn2+, in the tunnel of the material. Even at a very fast charge/discharge rate (50 A g–1), the capacity of the electrode is gradually increased from 268 to 348 F g–1 after ∼3,000 cycles and then remains relatively constant in the subsequent ∼17,000 cycles, which means ∼128% of the initial capacity is maintained after 20,000 cycles. In contrast, the capacity of bare δ-MnO2 electrode without modification is degraded gradually along the cycling, retaining only ∼74% of the initial value after 20,000 cycles. To reveal the basic chemistry between them, synchrotron X-ray diffraction and Raman spectroscopy were performed to explore the structural evolution of the modified δ-MnO2 during cycling; DFT computation was used to estimate the energetics and vibration modes associated with the hydrated Zn2+. The performance enhancement is attributed largely to the preaccommodation of [Zn (H2O)n]2+, which effectively suppresses the interlayer spacing change during cycling and thus benefits the stability.