Binder chemistry sets the interfacial balance constant in CsPbBr${}_3$ nanocrystal supercapacitor electrodes

Abstract

Previous work on lead-free tin halide perovskites showed that the binder instead sets the electrolyte concentration at which capacitance is maximised, following the relationship $100\times [{\mathrm{Li}}^{+}{]}_{\mathrm{opt}}+{\mathrm{PVDF}}_{\mathrm{wt}\%}={\xi }_{\mathrm{Int}}$ with ${\xi }_{\mathrm{Int}}=25\pm 2.5$, established by varying the loading of a single polymer. Whether ${\xi }_{\mathrm{Int}}$ is universal or specific to that polymer has not been tested. Here, four binders spanning fluorinated (PVDF), carboxylic (PAA), cellulosic (CMC) and sulfonic (PEDOT:PSS) chemistry are compared on $\mathrm{CsPbB}{\mathrm{r}}_3$ nanocrystal electrodes at four LiTFSI concentrations in acetonitrile, with binder loading fixed at 15 wt% and all other formulation variables held constant. The relationship applies to $\mathrm{CsPbB}{\mathrm{r}}_3$: PVDF at 15 wt% gives an optimum at 0.10--0.15 M and 112 F g${}^{-1}$, against 126 F g${}^{-1}$ reported for $\mathrm{CsSnC}{\mathrm{l}}_3$ under the same conditions, extending the result to a different B-site cation, halide and crystal system. PVDF and CMC optimise at 0.10 M, giving ${\xi }_{\mathrm{Int}}=25$, while PAA and PEDOT:PSS optimise at 0.15 M, giving ${\xi }_{\mathrm{Int}}=30$, with maximum values of 188 F g${}^{-1}$ for PAA and 146 mF cm${}^{-2}$ for PEDOT:PSS. The two binders showing the shift carry ionisable acid groups at high density, indicating that ${\xi }_{\mathrm{Int}}$ expressed in weight percent requires a binder-specific value. In all four electrodes, $\mathrm{CsPbB}{\mathrm{r}}_3$ converts to $\mathrm{PbB}{\mathrm{r}}_2$ and CsBr during electrochemical characterisation, and the surface lead content of the PEDOT:PSS electrode decreases.