<p>The widespread deployment of electrochemical capacitors in energy-intensive technologies is fundamentally limited by their low energy density and severe self-discharge. The search of high-voltage supercapacitors has the appeal of an effective solution to increase the energy density, but suffers from risk of electrolyte decomposition and self-discharge. We herein address this challenge through a synergistic electrode/electrolyte co-design that integrates a lignin-derived porous carbon electrode with a tailored Li⁺-based weakly solvating electrolyte containing a functional fluorinated diluent. The porous carbon features sub-nanometer pores that are geometrically matched to the weakly solvated Li⁺ ions, enabling stable operation at an unprecedented 4.0&#xa0;V with a high energy density of 77.4 Wh kg⁻¹ and over 90% capacitance retention after 10,000 cycles. Mechanistic analysis reveals that the sub-nanometer pores precisely accommodate solvated ions to facilitate high capacitance, while the fluorinated diluent suppresses electrolyte degradation and mitigates parasitic reactions under elevated potentials.</p> Graphical Abstract <p></p>