Abstract
Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness. However, their applications have been limited by a narrow potential window (∼1.23 V), beyond which the hydrogen and oxygen evolution reactions occur. Here we report the formation of layered Mn5O8 pseudocapacitor electrode material with a well-ordered hydroxylated interphase. A symmetric full cell using such electrodes demonstrates a stable potential window of 3.0 V in an aqueous electrolyte, as well as high energy and power performance, nearly 100% coulombic efficiency and 85% energy efficiency after 25,000 charge–discharge cycles. The interplay between hydroxylated interphase on the surface and the unique bivalence structure of Mn5O8 suppresses the gas evolution reactions, offers a two-electron charge transfer via Mn2+/Mn4+ redox couple, and provides facile pathway for Na-ion transport via intra-/inter-layer defects of Mn5O8.
Highlights
Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness
X-ray and neutron pair distribution function (PDF) analyses, shown in Fig. 1a,b, point to the formation of monoclinic Mn5O8 (Supplementary Tables 1 and 2), which consists of two-dimensional octahedral sheets of [Mn34 þ O8] in the bc plane separated by Mn2 þ layers, giving a compositional formula of Mn2 þ 2Mn4 þ 3O8
In order to elucidate the mechanisms of this high-voltage and high-rate performance found in the Mn5O8 system, we first provide synchrotron-based soft X-ray spectroscopy (sXAS) results with its inherent surface and elemental sensitivities, followed by the Density functional theory (DFT) calculations
Summary
Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness Their applications have been limited by a narrow potential window (B1.23 V), beyond which the hydrogen and oxygen evolution reactions occur. Suo et al reported a water-in-salt electrolyte, by dissolving concentrated Li-bis(trifluoromethane sulfonyl)imide salt in water This electrolyte system introduces a desirable SEI that enables an aqueous Li-ion full cell operation at 2.3 V for more than 1,000 cycles[4]. These excellent works open up the opportunities for improving the potential window, the intrinsic limitation of the ionic conductivity in Li-ion-based aqueous systems has hindered high-rate performance of the cell, especially for the pseudocapacitive storage. The 3.0 V aqueous symmetric full cell exhibits a high energy density, 23 Wh kg À 1 at a rate of 550 C, with nearly 100% coulombic efficiency and 85% energy efficiency after 25,000 charge–discharge cycles
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