Dual-functional tris(trimethylsilyl) phosphate for simultaneous passivation and moisture scavenging in high-voltage electrochemical double-layer capacitors

Electrochemical double-layer capacitors (EDLCs) are critical components for frequency regulation in power systems owing to their long cycle life, high power density, inherent safety, and wide operating temperature range. Widely commercialized EDLCs that use tetrabutylammonium tetrafluoroborate (SBPBF₄) / propylene carbonate (PC) electrolytes often suffer from performance degradation because of the side reactions at the negative electrode, primarily caused by residual moisture and operating voltages exceeding 2.7 V. To address this issue, the present study proposes a novel strategy that employs tris(trimethylsilyl) phosphate (TMSP) as a dual-functional electrolyte additive. TMSP simultaneously forms a stable passivation layer on the negative electrode in situ while also eliminating residual moisture. First-principles calculations combined with experimental characterization demonstrated that TMSP underwent preferential reduction at the negative electrode compared to PC, forming a protective passivation layer. Furthermore, the TMSP-water reaction effectively suppressed moisture-induced side reactions. The TMSP-containing electrolyte exhibited excellent stability under a 3.5 V floating charge, maintaining 80 % capacitance retention after 500 h, representing a 2.78-fold improvement over conventional electrolytes. This work provides guidelines for rational electrolyte design in high-voltage EDLCs and valuable references for machine learning-assisted screening of electrolyte additives.