Recently, water-in-salt electrolytes have been widely reported because of their ability in broadening the potential window of aqueous based energy storage devices.

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Inadequate capacity and poor durability of MnO 2 based pseudocapacitive electrodes have long been stumbling blocks in the way of their commercial use. Though layered δ-MnO 2 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-MnO 2 based electrode through a simple incorporation of exotic cations, hydrated Zn 2+ , 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-MnO 2 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-MnO 2 during cycling; DFT computation was used to estimate the energetics and vibration modes associated with the hydrated Zn 2+ . The performance enhancement is attributed largely to the preaccommodation of [Zn (H 2 O) n ] 2+ , which effectively suppresses the interlayer spacing change during cycling and thus benefits the stability. KEYWORDS: pseudocapacitor, layered δ-MnO 2 /Na 0.55 Mn 2 O 4 , tunnel structure modification/preaccommodation of exotic ions, in situ Raman, DFT computation