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.