Enhanced ionic conductivity of gel polymer electrolyte comprising black phosphorus nanosheets for supercapacitors

Gel polymer electrolytes are considered a promising alternative for liquid electrolytes and solid-state electrolytes. However, improvements in ion diffusion and rate capability are needed to meet the requirements for their practical use. This study investigated the effect of incorporating black phosphorus nanosheets (BPNs) in gel polymer electrolytes on capacitive performance. BPNs were dispersed in an electrolyte composed of polyvinylidene difluoride (PVDF) as a framework, TEABF₄ as a salt, and propylene carbonate as a solvent. BPNs suppressed the formation of ion pairs, leading to a substantial decrease and increase in the population of solvent-separated ion pairs and free BF₄⁻ ions, respectively. Additionally, BPNs retarded the nucleation of PVDF crystals and reduced PVDF crystallinity. The combined effects resulted in a noticeable increase in the ionic conductivity of the gel polymer electrolyte. When assembled into supercapacitors, the gel polymer electrolyte containing BPNs preserved capacitance at high scan rates and exhibited a capacitance retention of 92.2 % after 5000 charge–discharge cycles. Incorporating too many BPNs in the electrolyte caused BPN aggregation, which increased the solvation of electrolyte ions and subsequently increased charge transfer resistance. This study demonstrated a strategy of incorporating BPNs into gel polymer electrolytes to effectively promote ion dissociation and improve the capacitive performance of supercapacitors.