Capacitive Behavior of Sodium Ion Pre-Intercalation Manganese Dioxide Supported on Titanium Nitride Substrate
Yibing Xie
Abstract
The sodium ion pre-intercalation manganese dioxide (Na[Formula: see text]MnO[Formula: see text] is supported on titanium nitride (TiN) substrate to form electroactive Na[Formula: see text]MnO 2 /TiN electrode through an electrodeposition process in Mn(CH 3 COOH) 2 /Na 2 SO 4 precursors with high Mn/Na ratio. MnO 2 has a tiled leaf-like structure with a wrinkling morphology. Na[Formula: see text]MnO 2 has a cross-linking nanorod structure with a nanoporous morphology, which is beneficial for electrolyte ion diffusion. The density functional theory (DFT) calculation results indicate that Na[Formula: see text]MnO 2 reveals the enhanced density of states (DOS) and the lowered band gap than MnO 2 , which is consistent with higher cyclic voltammetry current response due to superior electroactivity of Na[Formula: see text]MnO 2 . The Faradaic process involves Na[Formula: see text] adsorption/desorption on the surface of MnO 2 by contributing to electrochemical capacitance and Na[Formula: see text] intercalation/deintercalation on the deep interlayer of pre-intercalation Na[Formula: see text]MnO 2 by contributing to pseudocapacitance. Concerning the electrolyte ion size effect, both MnO 2 /TiN and Na[Formula: see text]MnO 2 /TiN electrodes have higher capacitive performance in Li 2 SO 4 electrolyte than that in Na 2 SO 4 and K 2 SO 4 electrolyte due to more feasible Li[Formula: see text] diffusion. When MnO 2 is converted into Na[Formula: see text]MnO 2 , the capacitance at 2.5 mA cm[Formula: see text] increases from 351.3 mF cm[Formula: see text] to 405.6 mF cm[Formula: see text] in Na 2 SO 4 electrolyte and from 376.3 mF cm[Formula: see text] to 465.1 mF cm[Formula: see text] in Li 2 SO 4 electrolyte. The conductive TiN substrate leads to high rate capacity retention ratio of 50.7% for MnO 2 /TiN and 49.5% for Na[Formula: see text]MnO 2 /TiN when current density increases from 0.5 mA cm[Formula: see text] to 5 mA cm[Formula: see text]. So, Na[Formula: see text]MnO 2 /TiN with sodium ion pre-intercalation exhibits the improved capacitive performance in Li 2 SO4 electrolyte to act well as the promising supercapacitor electrode.