Low-Crystalline Akhtenskite MnO₂-Based Aqueous Magnesium-Ion Hybrid Supercapacitors with a Superior Energy Density Boosted by Redox Bromide-Ion Additive Electrolytes

Abstract

Recently, as a new type of hybrid supercapacitors, aqueous magnesium-ion hybrid supercapacitors (MHSs) have triggered continuous attention. Benefiting from the insertion/extraction processes of bivalent magnesium ions in the battery-type electrode, MHS offers the advantage of charging two electrons per ion into the battery-type material. However, the low energy density of reported MHSs is still unsatisfying for their practical applications. Herein, a novel redox bromide-ion additive aqueous MHS (B-MHS) has been designed via introducing the Br3 –/Br– redox additive in 1 M MgSO4 electrolyte to promote their energy density. The optimally designed B-MHS exhibits the highest specific capacity of 268.1 mA h g–1 at a current density of 2 A g–1 in a wide voltage range of 2.6 V (0–2.6 V). Also, the maximum energy density of 262.3 W h kg–1 can be achieved at a power density of 1956.8 W kg–1, which is better than that of MHS. Most importantly, the energy storage mechanism and interactive correlation between magnesium ion insertion/extraction and redox reaction (Br3 –/Br–) have been detailedly investigated. The proposed strategy provides a new route in promoting the energy density of MHS, which should be helpful in designing and constructing high-energy-density storage devices.

Non-lithium-based metal ion capacitors: recent advances and perspectives

“…96 Mg-OMS-1/graphene//CMS capacitor using 0.5 M Mg(NO 3 ) 2 as the electrolyte depicted good electrochemical performance in the voltage window of 0–1.8 V. 79 Han et al reported the design of redox bromide-ion additive aqueous MgICs by introducing the Br 3 − /Br − redox additive in 1 M MgSO 4 . 97 Tian and co-workers developed printable MgICs using MgSO 4 –polyacrylamide (PAM) gel electrolyte. 98 The as-prepared gel electrolyte can return to its initial state after various deformations, such as flattening, bending, and/or stretching.…”

MoS2 nanosheets with expanded interlayer spacing for ultra-stable aqueous Mg-ion hybrid supercapacitor

“…[14][15][16][17] Most studies on supercapacitors have focused on improving the specific capacity and cycling stability by making full use of the EDLs and pseudocapacitance storage mechanisms. 18,19 EDL charge storage is a very rapid and reversible process, providing high power capability and a stable, long cycle life. However, the relatively small capacity seriously limits the practical application of EDL supercapacitors.…”

“…20 Pseudocapacitance can provide additional capacity due to the ion-intercalation/ deintercalation mechanism or interfacial faradic reactions, but the corresponding cycling stability is poor. 7,18,21 Therefore, previous studies have mainly focused on two issues: developing electrode materials with a high specific surface area 20,[22][23][24][25][26][27] and developing electrolyte carriers with better charge transport performance to improve cycling stability and energy storage. [28][29][30][31][32][33][34][35][36] Most of the reported aqueous ion supercapacitors, such as Li + , Na + and Zn 2+ , have low specific capacity, poor cycling performance and dendrite growth.…”

Coordination Polymer Derived Porous Carbon Activated in Situ by the ZnCl2 Dot: Capacitances Greatly Enhanced by Redox-Activity Additives in Electrolytes

“…Besides, sponge-like hierarchical pores on the carbon substrate smooth the transformation of [Fe(CN) 6 ] 3− / [Fe(CN) 6 ] 4− , including the ion diffusion, charge transfer, and ion intercalation/deintercalation, further enhancing the longterm recycling. 34 Following, the Coulombic efficiency η is counted from the formula below:…”

“…Even after 5000 cycles, 89.0% of initial capacitance is still reserved, which demonstrates remarkable the reversibility of the redox couple of [Fe­(CN) 6 ] 3– /[Fe­(CN) 6 ] 4– on the AC-Zn-CP electrode (Figure d). Besides, sponge-like hierarchical pores on the carbon substrate smooth the transformation of [Fe­(CN) 6 ] 3– /[Fe­(CN) 6 ] 4– , including the ion diffusion, charge transfer, and ion intercalation/deintercalation, further enhancing the long-term recycling …”