• Incorporating 4OT redox mediator into the electrolyte boosts activated carbon specific capacity up to 263 mAh·g−1 at 2 A·g−1, addressing the low-capacity bottleneck of carbon electrodes.
• The Ni3S2/CoNi2S4 positive electrode achieved a high specific capacity of 415 mAh·g−1, enabled by superior conductivity and abundant active sites.
• Redox-mediated capacity and potential window matching between electrodes enabled a maximum energy density of 55 Wh·kg−1, surpassing previously reported ASC values.
• This electrolyte-based strategy offers a mild, efficient, and non-destructive alternative to structural or heteroatom doping, preserving conductivity and rate performance.