Implementation of phosphonium salt for high-performance supercapacitors from room to ultra-low temperature conditions

Conventional supercapacitors encounter limitations in operating voltage and performance at low temperatures due to poor ionic conductivity and diminished interfacial dynamics in electrolytes. In this study, we synthesized the phosphonium salt 5-phosphinaspiro[4.4] nonane tetrafluoroborate (PSNBF₄) for the first time. We extensively characterized the physical and electrochemical properties of PSNBF₄ in acetonitrile (AN) and propionitrile (PN) as electrolytes, assessing their performance in supercapacitors at room temperature and − 40 °C. The results demonstrate PSNBF₄ electrolytes exhibit high solubility, outstanding ionic conductivity (1 M PSNBF₄/AN: 49.8 mS cm⁻¹; 1 M PSNBF₄/PN: 27.5 mS cm⁻¹), and high electrochemical stability, contributing to good capacitance retention after 500 h of floating tests at 25 °C. Supercapacitors using PSNBF₄/AN retained 78 % of their capacitance at 3.1 V, whereas those with PSNBF₄/PN maintained 68 % at 3.2 V. Impressively, these supercapacitors performed exceptionally well at −40 °C, displaying excellent cycle stability and high capacitance retention at elevated voltages. Supercapacitors with PSNBF₄/AN retain 96.6 % of their capacitance at 3.2 V, while PSNBF₄/PN retained 93.4 % of their capacitance at 3.4 V after floating for 500 h. These results demonstrate PSNBF₄-based supercapacitors can operate effectively at high voltages in both room and extremely low temperatures, addressing a significant challenge in commercial supercapacitor applications.