Layered (NH4)2V10O25·8H2O coupled with Two-Electron iodide redox achieving exponential energy density enhancement of NH4+ supercapacitors

• Layered (NH 4 ) 2 V 10 O 25 ·8H 2 O was fabricated via one step by hydrothermal method. • Exponential energy density increase through electrolyte modification. • A new pair of REDOX reactions contribute to the energy density. Supercapacitors are considered as potential candidates for future efficient energy storage owing to their advantages of high power density, long-term cycle stability, and environmental friendliness. However, constructing supercapacitors with high energy density is still challenging up to now. Current researches on supercapacitors mainly focus on exquisite nanostructure design and complex combination and modification of electrode materials, ignoring the synergistic effect between electrodes and electrolytes. Herein, we propose a strategy of layered (NH 4 ) 2 V 10 O 25 ·8H 2 O coupling two-electron redox to enable an ultra-high energy density supercapacitor. Layered (NH 4 ) 2 V 10 O 25 ·8H 2 O is prepared by hydrothermal reaction and used as a basic site of electrochemical reaction. Meanwhile, two-electron redox is constructed by adding NH 4 I to the electrolyte for modification, and a method to improve the specific capacity of supercapacitor through electrode-electrolyte synergy was proposed for the first time. As expected, the synergistic effect of electrodes and electrolytes results in an exponential increase in specific capacity (from 200.83 F g -1 to 562.50 F g -1 ), while the energy density is enhanced from 40.2 Wh kg -1 to 112.5 Wh kg -1 . The (NH 4 ) 2 V 10 O 25 ·8H 2 O// active carbon asymmetric supercapacitor exhibits a high energy density of 50.55 Wh kg -1 . The specific capacity of (NH 4 ) 2 V 10 O 25 ·8H 2 O//active carbon hybrid supercapacitor still has 140.00 F g -1 after 5000 cycles at 10 A g -1 . This work provides a facile and efficient way to improve the energy density of supercapacitors.