High‐Energy‐Density and High‐Rate Membraneless Ni−Fe Battery Enabled by a 1‐Butyl‐3‐Methylimidazolium Bromide‐Based Gel Electrolyte

Abstract The aqueous Ni−Fe batteries offer great opportunities for energy storage and conversion devices and systems owing to their ultralong lifetime, high safety and reliability and eco‐friendly characteristics. However, their widespread applications are hindered due to the passivation, hydrogen evolution and self‐discharge of electrodes in aqueous electrolyte, which result in rapid decay in both capacity and charge‐discharge rate. Here we present a membraneless Ni−Fe battery by using 1‐butyl‐3‐methylimidazolium bromide‐based gel electrolyte in combination with strongly coupled self‐assembled nanostructure composites to address these issues. We show that the composite electrodes with stereotaxically‐constructed and functionally complementary components enable to achieve fast redox kinetics and large capacitive effects. Meanwhile, the Ionic liquids‐based electrolyte effectively suppresses the passivation, hydrogen evolution and self‐discharge, hence significantly enhances the kinetic reversibility, structural stability and rate performance. As a result, the membraneless Ni−Fe battery delivers an ultrahigh energy density of 192 Wh kg −1 along with a power density of 0.3 kW kg −1 , which competes with LiFePO 4 batteries. The Ni−Fe battery maintains 91.4 % of the initial capacity and 98 % coulombic efficiency for 9000 cycles at a 0.5 A g −1 current density.