Achieving electrochemical capacitor functionality from nanoscale LiMn2O4 coatings on 3-D carbon nanoarchitectures

Abstract

Conformal nanoscale coatings of Na⁺-birnessite manganese oxide (MnOx) produced via redox reaction between aqueous permanganate (NaMnO₄·H₂O) and the carbon surfaces of fiber-paper-supported carbon nanofoams are converted to LiMn₂O₄ spinel through topotactic exchange of Na⁺ for Li⁺ in the as-deposited lamellar birnessite, followed by mild thermal treatments to complete the transformation to LiMn₂O₄. The evolution of the birnessite-to-spinel conversion is verified with X-ray diffraction, solid-state nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The mild conditions used to convert birnessite to spinel ensure that the conformal nanoscale nature of the oxide coating is retained throughout the macroscopically thick (170 μm) carbon nanofoam substrate during the conversion process. The architecture of the LiMn₂O₄–carbon nanofoam facilitates rapid ion/electron transport, enabling the LiMn₂O₄ to insert and extract Li⁺ from aqueous electrolytes at scan rates as high as 25 mV s⁻¹, and with a relaxation time of 37 s as derived from electrochemical impedance. This architectural expression of nanoscale LiMn₂O₄ delivers full theoretical capacity (148 mA h g⁻¹) at 2 mV s⁻¹.